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Temperature programmed analysis and its applications in catalytic systems
309
I.
INTRODUCTION
I.1 Background
The
interaction of
reactants with
the
catalyst surface is
of
primary
importance in heterogeneous reaction systems. .The very rapid advancement of our knowledge of the surface aspects of heterogeneous catalytic reaction systems and catalyst characterization can be ascribed to the combined use of some new
techniques which became available. These are:
Spectroscopy
MS).
X-Ray
Photoelectron Spectroscopy
Auger Electron
(XPS)
and
Flash
Desorption Spectroscopy (FDS). Generally, ultra-high vacuum is necessary in order to study the mechanism of bonding between adsorbates and the adsorbing surface. Catalysts cannot thereby be studied under the conditions of the catalytic
reaction
system
in
which
they
operate.
When
using
a
temperature-programmedanalysis method, the need for a vacuum falls away; the desorbed particles then enter into a flow of carrier gas. For a
linear
increase in temperature, the concentration of the desorbing particles is recorded as a function of the temperature. This method, which was first used by Amenomiya and Cvetanovic (11, has been widely applied to
industrial
catalysts since, primarily because it can be used under practical operating conditions. Over the last decade this technique has been developed further and has been applied in an ever wide range of temperature-programmedapplications: desorption (TPD), reduction (TPR), oxidation (TPO) and temperature-programmed reactions, like methanation. sulphidation and carburization. The application of TPD, TPR and related techniques was reviewed by Cvetanovic and Amenomiya in 1972 (21, by Falconer and Schwarz in 1983 (31, Hurst and coworkers in 1982 on TPR (41, Lemaitre in 1984 (51 and Jones and NcNicol in 1986 (61. Since that time, a number of reviews have appeared. These concern with work done on catalysts (such as supported metal catalysts) and reaction systems. These techniques (TPD, TPR, TPO and TPSR) were used in this work, which culminated in various modifications to general experimental procedure, particularly in the case of reaction studies. The review now presented covers the work done on 'Temperature-ProgrammedAnalysis' since 1983.
1.2
ADDliCatiOnS
Temperature programmed analysis technique has been used in a number of ways for the study of heterogeneous catalysts such as:
Characterization of reduction.
supported
catalyst
by
temperature-programmed
310
Determination of binding energies and binding states of adsorbed molecules (TPD).
Determination of the surface acidity of zeolites (temperature programmed ammoniationl.
Measurements of surface area, metal surface area, dispersion of metal and adsorption kinetics (TPD).
Catalytic reaction studies (temperature-programmedreaction studies).
Characterization
the
of
coke
species
in
catalysts
deactivated
(temperature-programmedoxidation).
Sulphidation of Molybdenum catalyst [temperature-programmedsulphidationl
The transient nature of a temperature-programmedanalysis technique, in which both the temperature and
the surface coverage vary with
time, has
the
advantage of providing information which is not available from steady-state kinetic
measurements.
temperature-programmed
It
is
analysis
possible unit,
to which
devise can
a be
multi-purpose used
for
adsorption/desorption, oxidation, reduction, sulphidation, methanation and ammonination and which would have a universal detection system such as a mass spectrometer. Automatic equipment for doing catalyst characterization by temperature-programmedreduction/desorption/oxidationhas been reported (71. Figure 1 shows a schematic picture of such desorption (TPDl and a typical reduction profile (TPR). In the TPR-studies, a solid catalyst, which has been allowed to reach equilibrium with an adsorbing gas beforehand, in a well defined condition, is subjected to a programmed temperature rise and the amount of desorbing gas is thereby continuously monitored.
Various catalysts (metallic, bi-metallic, metal oxides, etc.), reported on in literature in conjunction with these techniques, are examined here and useful information, thus gathered, is discussed. In addition, the following is presented: The theory which applies when interpreting TPD, TPR and TPSR patterns; mathematical models for the evaluation of important parameters; the methodology for
obtaining theoretical TPD
and
TPR-patterns by
computer
simulation and future uses and trends in this field. Apparatus used is briefly described and also to what extent such apparatus can be put to other uses. In the case of TPR, a catalyst precursor, containing oxygen, is exposed to a programmed temperature rise while passing a reducing gas over it (usually
311
SURFACE COVE=AGE
300
400
500
600
700
500
no0
TIME
1. K
Figure 1. Temperature-programmeddesorption (TPD) and temperature-programmed reduction (TPR) response diagrams. hydrogen diluted by an inert gas). The reduction rate is measured continuously by monitoring the composition of the reducing gas at the outlet of the reactor. In temperature-programmed surface reactions (TPSRI, one gas
is
adsorbed on to the catalyst surface and a second reactive gas (or a reactive gas/inert gas mixture) is used as the carrier. The typical reactions studied, using TPSR, are methanation (CO and CO2 hydrogenation), oxidation (propene, coke oxidation), ammonination (ammonia with
acid
sites), reduction
(NO
hydrogenation) and sulphidation (sulphur reaction with hydrogen).
II. EXPERIMENTAL TECHNIQUES
II.1 General Comments
An extensive review of the numerous experimental arrangements that have been used for performing TPD and TPR-studies, is not within the scope of this review. A detailed description of temperature-programmeddesorption (TPDI is given by Falconer and Schwarz (31, whereas a TPR arrangement is described by Jones and McNicol (6). Lemaitre (5) discussed the TPD/TPR-combinationas based on the Rogers - Amenomiya - Robertson arrangement. Recently, a comprehensive paper by Boer, Boersma and Wagstaff appeared (71, that described an apparatus
312
in which TPR, TPD and TPO could all be carried out. This apparatus is rather complex for temperature-programmedcatalyst studies. With the use of a more universal detection
system,
such
as
mass-spectrometer with
a
thermal
conductivity detector, one single temperature-programmedanalyser could deal with
all
forms
reduction,
of
oxidation,
sulphidation.
methanation,
ammonination and desorption. Such a comprehensive unit would operate quickly and
automatically. A
general description is
given
of
a
technique of
characterizing heterogeneous catalysts, using temperature-programmedanalysis. This is believed to be of interest to readers as well.
II.2 Procedure
In a typical temperature-programmedanalysis experiment (as shown in Figure 21, a small amount of catalyst is contained in a reactor that can be heated by a furnace.
The catalyst is heated under temperature-programmingin either a
reducing or an oxidizing atmosphere depending on the catalyst being a metal or a oxide. When an oxidized catalyst is heated under temperature programming in a stream of hydrogen containing inert gas it will adsorb hydrogen as a function of the temperature/reactivity relationship of
the oxide species
(TPR). When a reduced catalyst is cooled in a stream of hydrogen containing inert gas, it will absorb hydrogen.
Upon subsequent temperature-programmed
heating of the catalyst, this hydrogen will be desorbed as a function of the temperature/adsorptionstrength relationship of the hydrogen adduct (TPD). An inert carrier gas is usually helium, but argon and nitrogen have also been used.
When a reduced catalyst is heated under temperature programming in a
stream of oxygen containing inert gas, it will adsorb oxygen as a function of the temperature/reactivityrelationship of the reduced catalyst. The behaviour of the catalyst can be assessed by continuously monitoring the concentration of oxygen and hydrogen, respectively, in the effluent.
Downstream from the reactor, a suitable detector, such as thermal conductivity detector or mass spectrometer is used for analysing changes in concentrations in the carrier gas. A small thermocouple is usually inserted in the catalyst bed for measuring the temperature. In temperature-programmedsurface reaction studies (TPSR), the adsorbate may decompose or several adsorbates may produce reaction products;
the
detector should be
'sensitive towards specific
substances'. If a thermal conductivity detector (for the detection of low concentrations) is used, then the carrier gas composition should be chosen such, there will be significant differences in thermal conductivity which can be measured. This choice of composition is found varying the concentrations of He + 02, N2 + Hz (or Ar + H2). Examples of typical gas compositions in the
313
context are shown in table I.
TABLE I Composition of carrier gas and reactant gas
Composition
Technique
Nitrogen + 5% by volume of hydrogen
TPR
Nitrogen + 0.1% by volume of hydrogen
TPD
Helium + 5% by volume of oxygen
TPO
Figure 2. Schematic of the Rogers-Amenomiya-Robertsonarrangement forTPD and TPR studies.
Gl and G2, gas cylinders (1, pure gas;
2, reducing gas
mixture); SV, shutoff valve; Vi, four way valve for gas selection; FC, Flow controls;
V2, four way valve for shunting the reactor;
R. quartz reactor
with quartz thermowell; F, Furnace; TCP. temperature controller-programmer; K, thermal conductivity detector; XV, recorder; RM, T, cold trap; rotameters (From Ref. 51.
314 Corrosion must be avoided when the detector filaments are brought into contact with reactive gases, such as NO, CO, Hz0 and H2S_ The basic components of an apparatus suitable for temperature programmed analysis of catalyst are
(a)
catalyst sample tube (reactor), (b) a furnace with programmable temperature controller (cl a dosing system for carrier and adsorbate gases (dl a system for chemical analysis (usually a GC detector followed by a cold trap for retention of
the desorbed material) and a
(e) vacuum system.
A
mass
spectrometer can also be used instead of GC detector in order to obtain direct information about the desorbed particles. Although the basic concept of a temperature-programmed analysis
experiment
is
relatively
simple,
its
implementation can be much more difficult. Potential experimental problems and how to avoid them, are discussed by Falconer and Schwarz (3).
The important
parameters that have to be optimized in temperature programmed analysis are:
flow rate of carrier gas. reactapt gas/inert gas ratio. catalyst sample volume/mass. catalyst sample particle size. geometry of the reaction vessel (catalyst sample reactor). heating rate. signal intensity. system pressure.
,Most of the temperature-programmedanalyses are usually run at atmospheric pressure, though higher pressures can be used.
A typical temperature-
programmed analysis (TPDI measurement usually call for the following steps:
Pretreatment of the catalyst (desorption of adsorbed foreign. species and possibly reduction). Exposure to the reactant gas or adsorbate. Desorption of the physisorbed fraction by extensive evacuation. Heating of the catalyst sample using a linear temperature program.
The
desorbed particles are transported to the detector by a stream of inert gas (usually He, Ar or N3) and recorded. Analysis of the desorbed particles by gas chromatography or by mass spectroscopy.
In a TPR experiment, a dilute reducing gas (e.g. 5% Hg, 95% Ng) instead of an inert carrier gas is passed over the catalyst in a reactor tube.
Depending
upon the reducibility of the components present on the catalyst surface, one or more maxima are obtained for the reducing gas consumption at characteristic
315
temperature.
The same technique may be extended to include temperature-
programmed reactions in general e.g. temperature-programmed oxidation (81, temperature-programmedcarburization .(91, temperature-programmedmethanation (101,
temperature-programmed
sulfidation
(121
and
ammoniation
(111.
temperature-programmed
temperature-programmed reaction
concerned with catalyst poisoning (13,141.
including
studies
Table II shows the typical
experimental conditions for a temperature-programmedanalysis technique on a supported metal catalyst with low-weight loading.
III. THEORY
With a temperature-programmedanalysis apparatus, adsorption, desorption and surface reaction experiments can be carried out. Temperature-programmed desorption (TPD) has been used most commonly to adsorbates to catalytic surfaces.
study the binding of
The temperature of the desorption peak
(maximum) is indicative of the strength with which the adsorbate is bound to the surface.
The more strongly the adsorbate is bound to the surface, the
higher is the temperature of the desorption peak. Various studies (151 are reported with the quantitative analysis of temperature-programmeddesorption spectra i.e., determining the desorption kinetics.
Four basic cases can be
differentiated, namely
first order kinetics without readsorption. second order kinetics without readsorption. first order kinetics with readsorption. kinetics of the second order.
Surface heterogeneity, i.e.whether a surface contains more than one type of adsorption site, plays an important role in the analysis. Appropriate kinetic models have been developed to account for this phenomenon (161.
If the heats
of adsorption on these sites differ significantly, the TPD spectrum will contain multiple peaks, one for each type of adsorption site.
When this
happens, the heat of adsorption on each type of site can be determined separately.
Multiple peaks in a TPD spectrum do not always indicate the
presence of multiple adsorption sites on the catalyst surface. also
can
results from
Extra peaks
lateral interactions between adsorbate molecules
(induced heterogeneity) (17) and diffusion of adsorbate in the subsurface region (18). Quantitative analysis of TPD spectra, i.e. the determination of reliable kinetic parameters, is quite a magnitude of experimental errors.
demanding task because of
the
Depends
Pretreatment, if any
upon catalyst
spectrometer
lo-60 K/min
TCD/Mass
rate
Heating
Quartz tube fixed bed 6.4 mm x 210 mm
Type of Detector
type
size
Reactor
Particle
mm
1-5X
Weight
0.025-0.25
100-500 mg
Catalyst weight
loading
Hydrogen
Adsorbate/Reactant
Depends
TCD upon catalyst
4-60 KImin
Quartz tube fixed bed
0.025-0.25
1-5x mm
+ 5% v/v
100-500 mg
Nitrogen Hydrogen
15-30 cc/min
High purity Nitrogen/ Argon
(Atmospheric
TCD/Mass Depends
0.05-0.3 mm
1-5x
100-500 mg
upon catalyst
spectrometer
spectrometer Depends upon catalyst and reaction
TCD/FID/Mass
lo-60 K/min
tube
Depends upon reaction
30-60 cc/min
tube fixed bed Stainless/Quartz fixed bed
lo-60 K/min
Quartz
0.025-0.25
1-5x mm
+ 5% v/v
100-500 mg
Nitrogen Oxygen
30-90 cc/min
High purity Helium/ Nitrogen/Hydrogen
Temperature Programmed Surface Reaction (TPSR)
Pressure)
High purity Helium/ Nitrogen
Temperature Programmed Oxidation (TPO)
Analyser
High purity Helium/ Nitrogen/Argon
15-60 cc/min
gas
Programmed
Temperature Programmed Reduction (TPR)
for a Temperature
Temperature Programmed Desorption (TPD)
Conditions
Flow rate
Carrier
TABLE 11 Typical Experimental :
317 In temperature-programmedreduction (TPR), a generally applicable model for kinetic analysis of measured reduction profiles does not exist because of the complexity of the process of reduction. Thus the TPR method is usually used for qualitative analysis of the reducibility of the catalyst surface and is quite sensitive to chemical changes resulting from promoters or metal/support interactions.
The quantitative analysis of TPR spectra calls for prior
knowledge of the surface mechanisms.
Most of the work done on TPR
concerned with the reductions of oxides using hydrogen.
is
Supported metal
oxides and metal zeolites have been studied extensively, using TPR technique (19,201. McNicol and Jones (61 presented an analysis based on temperatureprogrammed techniques to obtain the kinetic parameters relating to
the
reduction process. Temperature-programmedanalysis has been used in study of phenomena such as hydrogen spillover, metal-metal interactions and strong metal support interactions (21,221. Hurst et al. (41 gave the analysis of temperature-programmed reduction, based
on
thermodynamics, kinetics
and
mechanism of reduction applicable to supported metal oxide catalysts. Effects of variation of
experimental parameters on non-isothermal reduction and
evaluation of kinetic parameters based on experimental observations uas also given (41.
Monti and Baiker (23) defined a characteristic number, K. to
select the values of the operating variables in order to obtain the optimum reduction profile.
The value of K
related the heating rate, hydrogen
concentration, total flow rate and the amount of reducible sample in such a way that the operating variables can easily be adjusted. The characteristic number K
was defined as:
0
K=_
si C
111 .FO
8
where
soi =
Initial concentration of reducible species i
c
=
Concentration of hydrogen at the inlet of reactor (mol/m21.
=
Total flow rate of the reducing gas (m3(NTP1/sI.
on the surface.
g0
F
They reported the minimum value of and the maximum value of of
K
K
K
is 55s for a heating rate of 0.1 K/s
as 140s for heating rate of 0.3K/s.
below 55s the sensitivity becomes too low; for values of
K
For values exceeding
14Os, the quantity of hydrogen consumed is too large.
In temperature-programmedanalysis (TPD/TPR/TPO). the shapes of the spectra
318 can be influenced by several factors.
It is important to separate these
factors/effects and the rate process which controls the resulting spectra must be identified and be incorporated into the analysis and modelling of the process.
The various factors which affect the shape and position of the
spectra are:
surface heterogeneity (multiple adsorption states of different energies). readsorption. mass transfer effects (diffusional resistances). subsurface diffusion and adsorption. desorption kinetics.
Some of the factors, like diffusional resistances, can be eliminated with proper design of experimental conditions. been devoted
A number of papers have recently
to theoretical analysis, mathematical modelling and simulation
of TPD and TPR data (24-281.
Several
studies
on
temperature-programmed reactions,
using
supported
catalysts, has been reported in the literature (29-321. Most of these studies provided the information about catalytic mechanism on the surface. There is a lack of quantitative treatment of temperature-programmedreaction studies in the literature.
III.1 Theoretical aspects of the analvsis
Analysis of
desorption spectra, when a
temperature-programmed method is
used, can provide:
adsorption-desorptionmechanisms and their associated kinetics.
kinetic parameters such as heat of adsorption, activation energy of the adsorption and desorption process, order of desorption process.
valence state of
metal
ions in
zeolites and
supported catalysts,
interaction between metal oxide and support, identification of alloys formation in bimetallic catalysts using temperature-programmedreduction (TPR).
characterization of
the nature of
the coke species in deactivated
catalysts (TPO), study of reactions in evaluating the role of an active site
in a catalytic reaction (TPSR).
319 The interpretation of temperature-programmeddesorptlon/reductlon spectra is usually confined to the discussion of peak maximum temperatures, the number of more
or
less
resolved peaks
or
to
the
determination of
the
total
adsorbate/reactantconsumption from which the rate of desorptlon/oxidation/ reduction/reactioncould be -determined.
The
evaluation
of
the
desorptlon/reductlon during
kinetic a
parameters
for
adsorption
and
temperature-programmed analysis experiment
requires consideration of the material balances for the surface and the fluid phase concentrations of the adsorbate. Various types of mathematical models, based on simple adsorption to multisite adsorption and subsurface diffusion theory, have been given in the literature 1181. Table III lists the various models which have been used analysis spectra. are
in
interpretation of
temperature-programmed
The various kinetic parameters which could be evaluated
also given in Table III.
It is clear from Table III that mathematical
models proposed for getting kinetic parameters neglect mass transport effects during temperature-programmedanalysis, Mass transport processes such as inter and lntrapartlcle diffusion alter the shape of the temperature-programmed desorptlon/reductlonspectra. Therefore it is important to assess the role of mass transport in the temperature-programmedanalysis. The influence of mass transfer on temperature-programmedanalysis techniques will be examined in the next section.
III.2 Mass Transfer Considerations in Temperature Programmed Analvsis
If the kinetic parameters derived from the analysis of temperature-programmed profile are to be meaningful, it is necessary to establish that mass transfer effects
are
absent.
The
influence
of
mass
transfer
effects
on
temperature-programmeddesorption (TPD) spectra from porous catalysts has been examined from the standpoints of non dimensional analysis (25) and numerical simulation
of
specific
models
(27,411.
Gorte
(25)
proposed
that
lntra-particle concentration gradients can be minimized by reducing catalyst particle size and reducing the gas flow, increases the film resistance. This criterion was obtained by non-dlmenslonalizationof the differential equations governing mass transfer during temperature-programmedanalysis. Dimensionless groups of easily measured catalyst parameters have been reported and provide the researcher with guidelines for assessing:
readsorption. concentration gradients in the particles and along the catalyst bed. lag-time due to diffusion and hold up in the sample's tube.
Order of adsorption is pn Non-dissociatSve adsorption n and p = I Dissociative adsorption
&sorption rate with respccc to adsorbed species Absence of intraparticle diffusion Linear adsorption isotherm
order
sites,
Desorption
nth
EnergeticalZy homogeneous surface
kinetic
Mathematical,
of
(l”(r)
S
_(8)e-$
N] -
dT
% ka(l49
k (l-t3)p a
PWcV,
kd(B)Bn
Readsorption
(3%‘~’ V cm
40 z?=g
Free
exp
+.*12f
)
. ..[6]
151
. ..I41
. . .I31 TH
Slope
-
--Ed r
vs %
I
,PLot of
Series
2
TM In(T)
Slope
ADd = 7
Xonvalinka et al. (33)
Cve tanovic and Amenomiya (I)
Reference
Aualysfs
af TPD curves Valve of n is for varLous initial equal to slope coversges or for (Equation 3) various heating rates Measure N at number of temperatures Plot In N vs In 0 at fixed T
vsTN
1
Measure peak Temperature Tn Plot In(+)
rate)
Series of TPD curves at fixed B. and variable 6 (heating
data required
Parameters Evaloated
in Temperature-Programmed
Experimental
parameters
* ~)-lm[ln0]
n
Rd (~1 M
- 2
RT
-Ed(o)
model/Eqwtion
and experimental
‘d
Ed ’ AXl = RT 2 M Oon-l
[ln
* ,A!?
Ad
No Readsorption
used in Evaluation
Theory and fts limitations
Models
TemperatureProgrammed
Process
TABLE IIT: Mathematical
Temperature Programmed Desorption
Process
Theory and its limitations
Desorption controlling . With no Readsorption Ammonia desorption from sites of different sttenath IntraIparticle diffusion absent
TABLE 11X (cont.)
ne-
[ (1-g)::
P
o
a
- 2
. . . t71
= exp
dT
* &
exp
Ed (5)
f
0
. ..[lO]
(2s) (-d)dT . ..191 RT B
cm
F
“Cc]
‘“d
where I = &T exp(-Ed/RT)dT
Aq In (9)
-i!p
[In N] - la(&)
+ ln
en
(l-e)p
model/Equation
And =w+lnx M
-AR, (0) 7)
2 TM In CT)
axp(
Mathematical
at
regression analysis (Eq. 9) using all experimental data.
fixed T. Equation [lo) solved by non-linear. regression procedure. Evaluation of Ed and Ad using nonlinear
In (($PlJ
Series of TPD curves for various coverages up to saturation coverage Measure N and coverage 8, at a number of temps. Plot [In N] vs
Value of p must. be known i.e. Adsorption kinetics be known.
Experimental data required
Ed ’
Ad
Slope - n Order of deaorption
Parameters Evaluated
Forni & Magni (34)
Reference
Temperature Programmed Desarpcion
Process
Theory and its limitations
Intra-crystalline diffusion controlling (Applicable to zeolices)
Free Re-adsorption
TABLE 1x7. (cont.)
) f A6o -A6
“*Hd dT ( z)
-$ n
-
Q,(Ti*)
‘5 NH3(mmol/gsolid)
Ea”:
Activation diffusion
energy
of
. ..[171
DeiRc2= Do exp(-Ea*/RT)
51 I 61
*.. I
*
1..
t
coming
out of zeolite du.K the time interval At,.
Amount of
-**[141
II2
1(14%
. ..I131
*If21
. ..[ll]
Q, = Q,(T)
+““’
-D
(QfTi)
ew ( L.iL)(T*)n2&i
[
0
= GT exp (-AHd/RT)dT
MAti:
nil
M$
I’
0
A exp
Aeo In (g
= -(i)
Curve” Curve”
model/Equation
LHS: “Experimental RHS: “Calculated - Aq) (Aqo d(Aq) Afra
Mathcntntical.
Equations and f 171.
1161
Calculated peak is compared with experimental one using nan-linear regression routine and to obtain value of Ha* & Aa* from
TPD data analyzed by calculating tire Tl’D peak usinu. equations [,I41 & [151.
Equation 1121 solved numerically using non-linear regression optimization pracedure
Experimental data required
Do
ea*
AHd
&
Parameters Evaluated
Porni & Magni (34)
Reference
z h3
323
. 3 -
UA
c
ID
d +
f
f
. ..
* . .
-s -I-
h
2 0
Temperature Programmed Reduction
Temperature Programmed Desorption
Process
TABLE III
Theory end its limitations
Mothematicnl.
SbS~il~
cEfects
log
a:
[
1 -
(l-a)
Q-$
fraction
[_log
T2 ( l-s) E _2.3RT
log
at
]= lo&$
] = log
reduced
l-s
[26],
[24]
[l-
[l-
?]
t
of TPR curves
r 2.3R T
‘.*[281
Plot Left hand side of Equation [ 271 or [ 281 against A 1
Series
Equation [22] is used to construct theoretical TPD curves using an iterative numerical procedure. Theoretical TPD curves are compared witb experimentel TPD curves to get best values of kfnex parameters.
Experimental data required
?I - -E-
. ..[27]
time
g
6 [25]:
. ..[26]
From equation3
- al8
g=k(I
. ..[25]
. ..[24]
. ..[23]
rcoction
dfS] dt
. ..[22]
dT S-z In excess of reducing gas, is independent of gas concentration.
= k Cg ls]s -d-; k F A*e-e/RT
Mass transfer
-d C
respect
with
r-1
-d C *---=
rEh order to Gas
For
g
F 4 VcV, ka(l-8))
kd(6)
with
respect
Rate =
F 13~
sth order solid
For the reaction + Products Gas f solid
-d6 -_dT
n=p-1
1
as
k Cgr[Sls
model/&quation
Equation [6] is expressed Shape of TPD curves under either diffusion _d6 kd(W 8“’ control or kinetic control for first order F + VcVm k,(l-DtP desorption model For First order desoprtion identical
(cont.)
Slope -E =2.3R
Parameters Evaluated (5)*
Jones and McNicoL (6)
Lemaltre
Reference
:
Temperature Programmed Reduction
Process
TARLE III
Theory and limitations
its
Catalyst placed in tubular reactor (Plug flow). Reaction rate expressed in terms of consumption of gas.
(cont.)
r-1
B
’
TM2[C1;
In
s=l
RTM2
-&+ln& M
R
s
A*[C
-aE
(29)
1;
[SIS
s-l
-0
E RT2
IS],
dC 2
1
d[S] dT
ISI 25 dT
s-l
maximum rate
g
gr iSI
[C gr
d(Rate)_O dT From equation
At
+PC
+qc
d(Rate) --k dT
(g
. ..[32]
. . .I317
exp
...
1
1301
. . . I29 concentration
modcl/Rquation
- F AC g Flow rate : Change in gas AC g Rate -kC gr LSlS
.
F.
Rate
Mathematical
)
“S
1
TM
-
Mean hydrogen c¢ration at the temperature of the maximum reduction rate 2c TM M Plot In 7
-c:
kinetics
A, s = 1
order
r -
of TPR curves a value of r.
[CglM - -“M
First
For
Series Assume
Experimental data required
-
g
E and A* determined.
Unique value of E obtained for all concentrations used.
Slope
Parameters Evaluated Monti Baiker
C
Reference
(23)
Temperature Programmed Reduction
L’roceiis
TABLE III.
Theory nnd limitativns
Its
Solid-gas rcac t ion S 4 G + R(Svlid) + B.&as). Ideal Tubular reactor
Reaction orders are unity (r - s - 1,) First order with respect to gas and solid phase.
(cont.)
nD I-’
I . . . [331
-c
gM
ore
generated.
Complex kinetic expressions can be solved using this technioue, mm da x ce “F m: specific molar consumption (mol/g) F: specific flow rate of the reacting mixture (cc/s.g)
TPR profiles
Numerical integration of [33) using normalized Tn and Ctlt
go
- Cgo _ CP
CC&
M
T/f*
C
temperature.
functions
and sample
Normalized
rate
Integration of [331 gives the relation between 112 consumption
B
2Cg F
1nod~l/6(~uatlo11
1,z [1 + Lr S A*exp@
Mathcaatical EVtllUCl~C!d
Experimental
TPR curve
Figure 3 shows TPR peak shape analysis for different values of E” II e EM = sn
CUlX(?.
Estimation of A and E was done by the best fit vf theoretical curve to the measured TPR
Experimental TPR curve
A and E
E and A
Parameters
required
Exporimcntnl datn
Sestnk (36)
et al.
Monti and Baiker (23)
Reference
III
Temperature Programmed Reduction
I’roCCSS
TAlILE
Reaction Controlled Nucleation
Solid Solution Reaction Reduction of transition metal ions in zeolites. Reaction rate will diminish due to depletion of reactive species.
oxidic supcatalyst
by
Three dimensional interface
Reduction of precursors of ported nickel
Cc)
Its
Phase Boundary Controlled Reaction Reaction rate is proportional to the surface area of the unreacted solid (contracting sphere model)
Theory :wd llmitatlons
(b)
(a)
(cont.)
6
F
=!!!!I*
2
:
+
da
-
da dT-
x0:
CI)“~
a)
surface reactant
$
area
cxp
9
Actual reactive
(1
-
exp
of ($)
particle [37.]
the
...
of
($)..[36]
. . . [351
. . . [341
Initial concentration active species x S A* O” (l-a) exp (2) B
of (mol/cc)
. ..[39]
in
the
..
first power in (1-a) Z/3 power in (l-a)
a)
area
concentration species
(37): (36):
surface
kinetic constant of nucleation process
kn h*(l
x - x0 (1 - a)
x:
z-kSox
a)
(1 -
specific starting
-
dT
s A*
Equation Equation
da dT-
h
da E’ kn:
n=3 da zso:
S = So(l
c
and
TPR curve.
[34]
Equation [391 same as 1371, therefore Model (b) and (c) cannot be distinguished on the basis of the shape of TPR patterns.
Figs. 4 and 5 show Theoretical TPR patterns obtained using the two models
Solution of 1371 gives theoretical
Solution of Eq. [341 & [351 and comparison with experimental curve gives values of A & E.
I:‘xpcrlmclltol data required
A*
6
ACE
E
1% I.-illnote rs Evoluatcd
(37)
Jacobs
Lema?tre
et
al.
( 5)
TPD/TPR
Process
TAlJLg lli
(CWC.)
and
LCS
Multisite adsorption model Catalyst bed is modelled as a continuous flow ‘stirred tank reactor (CSTR). Peclet no. is small MO intra-particle diffusional effects Surface contains two distinct adsorption sites of different
Figure 6 represents the process of re-adsorption and subsurface diffusion during temperature programmed Reduction/ Desorption to generate theoretical C”CV(?S.
Subsurface adsorption diffusion theory Multiple peak due to diffusion of sorbate into the subsurface layers of metal-supported catalyst
linitatioas
‘l%cory
2
n: order Gas phase adsorbate
d52 dtEnka
Site
de1 dtinka
of
. , . 1421 subsurface sites total surface sites
- kd (l-e)51
. . . (41)
‘..
roc~uircd
lbdUOt:cd
Parnmetcrs
t=o
E”O
O=B
T-T
0
0
using 4th order Runge Kutta technique. Figure 7 shows a simulation of the TPR spectrum for MO C catalyst us I ng subsurface diffusion model.
at
Seri.os of TPD curves with different heating rates. Value of M is assumed .’ Ed’ ED Eqs.[40], 1411 6 1421 E are solved for the P’ ‘d 1401 A conditions *D’ P
data
Expccimcntal
the adsorbate
Pre-exponential Experimental TPD factors Adl, curve5 at varying flow rate of carrier Ad2 and (J-61)n ‘g - n kd gll.. [43]gas and heatin rates-Activation 1 Eqs. 1411,[431& t:441 energies were solved numericEdl c Ed2* ally using 4th order (1-fJ2)n cg - n kd e2n Runge Kutta method. 2 . ..[441 of the process concentration of the C : g
Nu MI---El Ns
Mass balance Site I
where
C - n kdO” g f kD(L-B)f
[kp @(l-c)
f kp e (1-t)
n ka(l-B)”
dT z=g dE = i dt
gp
on the
model/Rquatlon
Mass balance equation surface regfon
Machcmatical
Leary (1S)
Stacy (38)
et al.
et al.
I(CE~rCllC~
53
x
N
TemperatureProgrammed Diffusion
TemperatureProgrammed Reduction
order
kinetics
o*
d In
Diffusion in Uniform spherical particles under temperatureprogrammed conditions. This method has been applied to zeolite A to study the kinetics of gas diffusion in zeolites.
where
-N - g
= -&
(-g)
T*)
OD C (-n2 n=l
ki(T)
Si(T)
D,” exp
f;L z
:= ki(T)
o I +wp*
wP*
TM _ _ ER ii-
(%)
g
y(T) - C
d (&) M First order E -ER desorp tion bands. R = AR exp (F) High temperature is RTM2 M composed of methanol, water and formaldehyde.2 ln TM _ In B ‘Homogeneous surface’ E =&+lnC M
Partial oxidation of methanol to formaldehyde over MOO-, catalyst.
First
mc WC)
PV
Bulk volume of catalyst Reactor volume
coefficient
v:
Vc:
where
TPD curves with varying heating
Series curves
and
E* and Dt from slope intercept.
to get
of TPD at varying
Plot LHS ot Eq. [51] I “S TV yieldsvalue of RR and AR
. . . [5211
. ..[51j
. ..[50]
***[4g1~~~~;re TM of Formaldehyde peak.
. . . !48]
Si(‘O
w: residence time y: hydrogen consumption, Cgo - Cg v: stoichiometric coefficient
D(t)
Do
= Do exp
and E were obtained to get diffusion coefficient
Activation parameters
first order dcsorption and rate of desorption not ‘controlled by simple surfaceadsorbate bond cleavage.
Low value of A,~ suggests
ER and AR
l’aramctcrs Evaluated
et al.
(2)
Fraenkel 6 Levy (40)
Farneth (30)
331
332
0
1
t 0.6
Figure 3.
I
I
0.8
1.0
1.2
Dimensionless TPR peak shape analysisfor differentvalues of En.
E*=li?.4, 12.7. 15.0; 27.3; 19.7. The peaks become sharperwith (FromRef.361.
increasing
En
a
b E
h
KJlmol
nm
L
i “E
3-
1
121
2
112
3
2 E m 8’
600
700
800
900
T(K)
Figure 4. Theoretical TPR patteins of2a three directional phase boundary controlled reaction [A =31.%x 10 g/(cm .!a); E = 121 kJ/_yl; h = 8800 nm corresponding to So = 10 cm /g; g = 0.2 K/s, m = 133 x 10 mol/g; F = 2.0 x 10' cm3/(S.g)l. (From Ref. 5.1
a 4
b
E
h nm
kJ/mol
500
600
700
1
0800
2
800
500
600
100 500
600
700
T(K)
TPR patterns of a nucleation controlled reaction LA-2 h = 10 nm, g = 0.4 K/s; m = 1.33 x 10
334
CARRIER
GAS
SURFACE 0
‘P v
I
rD
SUBSURFACE REGION 5
54 = n k, (1 - 8 )”C, 'd
--”
rP
--kpe
kden
U-5)
Figure 6. Process occurring during TPD/TPR when both readsorption and subsurface diffusion are important. (From Ref. 18.1
335
- 0.8
Temperature
(Kl
Figure 7. Simulated TPD spectrum using subsurface diffusion model for parameter values in Table IV. Also shown: changes in surface and subsurface coverages with temperature. (From Ref. 18:)
TABLE IV: Parameter Values Used in Subsurface Diffusion Model Ns = 3.0 x 10-6 n
mol
= 2.0
M
= 1.0
u
= 4.0 x lo8 cm2/mol (4.0 x lo4 m2/mol)
Ad = 1.0 x 1013 a-l = 79.5 kJ/mol (19.0 kcal/mol) Ed Ap = 1.0 x lo6 s-1 Ep = 62.8 kJ/mol (15.0 kcal/mol) An = 1.0 x 106 s-l ED = 58.6 kJ/mol (14.0 kcal/mol) E, = 0.0
336
cG ,
r
r
51
d2
al
i
type
1
2
sites
sites
‘dl
type
=
n kd,e,”
ral -- n k, (1 - e,)“CG
r
a2
=
n ka (1 - fIJ"CG
Figure 8. TPD from a catalyst containing two distinct adsorption sites of differing binding energies. (From Ref. 18.1
337
8C ‘x 64 do_ “, 48 5 E .z
32
s n
I6 0
400
300
Temperature
TABLE V:
600
500 (K)
Parameter Values Used in Multisite Model
Ns =
3.0 x 10-6
m
= 2.0
n
=
mol
2.0
so = 0.005 cl
=
4.0 x lo8 cm2/mol (4.0 x lo4 m*/mol)
x1 =
0.5
x
0.5
2
=
'dl= "d2= Edl=
1.0 x lo13 s-l 1.0 x lo13 s-l 75.3 kJ/mol (18.0 kcal/mol)
Ed = 96.2 kJ/mol (23.0 kcal/mol) 1
Figure 9. Effect of initial coverage on TPD spectrum simulated by multisite model. Fo = 100 cc/min, p = 1.0 K/s; other parameter values as in Table V. (From Ref. 18.1
a
a
a
a
flow rate
0.
Ratio of adsorption to diffusion rates for infinite carrier-gas flow rates
Effect of carrier-gas
Accumulation of gas in the catalyst pore
Determines when readsorption is important
Determines when concentration gradients are present
Log time for gas to diffuse out of the pore
Lag time for sample measurement
Average residence the sample cell
time for
Comments
Physical description
For spherical catalyst pellets, replace L by r
n2 De
CL PSlL2P e
De as
FL
t3L2c (Tf - TO) De
F(Tf - To)
“2
Parameter
TABLE VI.l: Summary of Catalyst Parameters and Their Effect for CSTK (28)
When >l, readsorption is important, even at high carrier-gas flow rates
When20, flow rate is essentially infinite and the concentration at the catalyst edge is eftectively zero.
Should be kept less than 0.01 for cell concentration to follow net rate of desorption
Should be kept less than 0.01 for cell concentration to follow net rate of desotption
Significance
:
339
340 Demmin and Gorte (261 gave six dimensionless groups to check the above factors in a packed bed and continuously stirred tank reactor configuration. Table IV represents these factors along with their definition, and the criteria for checking their values in a temperature-programmedanalysis experiment. have also found that equilibrium readsorption will
often occur
in
They the
desorption process, in order to avoid concentration gradients within the particle, low carrier gas flow rates are necessary but such conditions are To avoid
likely to lead to axial concentration gradients in the catalyst bed.
these, the Peclet number must be considered. The value of Peclet number, 2 $b--(Table IV1 determines the extent of backmixing in the packed bed and when CB the value of this number is less than 0.1, the bed can be adequately modelled in accordance with CSTB. low,
are
likely
to
The high flow rates necessary to keep this quantity introduce
intra-particle concentration gradients.
Intra-particle concentration gradients were tested, using different catalyst particle radii.
Changes in
the peak
temperature (ATM1 and
effective
desorption rate constant (keffl with particle radius were reported by Tronconi and Forzatti in a TPD study (281 (see Table VI1 and given as: r
‘0
(551
= 3.7 AHd loglo ($1
ATM
O2
for B = 0.17 K/s and 10 i
AHd
5 167 KJ/mol
[561
at given temperature T.
Equations [551
and
[561
provide the basis for experimental criteria to
determine whether significant internal mass transfer effects are present during temperature-programmed analysis.
A plot of TM and loglO(kefflT vs
(rol could also show when diffusional effects are negligible. log10 dimensionless group &
fr equivalent to e
which is es against TM and (kefflT for a given value of less than
1
Gorte's
p
F
and
can also be plotted W
and if the value is
further confirms the absence of diffusional effects.
341 The Weisz-Prater criterion of checking the absence of
an
intra-particle
gradient to a static system, has been applied to a flow reactor system. Recently, Xue et al.
(42) have applied this to study the existence of
intra-particle concentration gradients during TPD experiments. They defined a modulus 9, as:
n-1 l/2 eff ci D 1 .I e where
r
I571
: radius of spherical porous catalyst particle.
effective desorption rate constant. k:ff: De : effective diffusion coefficient.
Ci
: adsorbate concentration at the centre of the particle.
n
: order of the desorption/reaction.
The magnitude of #, is used to estimate the effectiveness factor n),a measure of the concentration gradient of the reactant within the particle.
They
solved equation [581 numerically to get a series of contour plots at closely spaced temperatures covering the range of TPD experiment which allow the experimentalist to determine whether or
not
intraparticle concentration
gradients are significant for first and second order desorption processes.
9 1)$5;ci
#s is
=
r2 N Do "ohs e c
(581
"pseudo modulus" because the transport properties of the gas phase are
temperature dependent. The value of
n
close to
of intraparticle concentration gradients. criteria for
the absence of
1
will confirm the absence
Ibok and Ollis (43) proposed a
intraparticle mass transfer effects during
temperature programmed desorption in the absence of readsorption for first order desorption as:
r< 0
where
De
C -
(1
cslTH
l/2
1 M
+Kl k6,'obslT
r
catalyst particle size, cm.
surface concentration of reactant (mol/ccI. c: observed reaction rate. r obs 6 specific adsorption sites, sites/cm3. m
[SSI
342 Equation [591 was derived by modification of Weiss-Prater criterion which was developed for isothermal reaction conditions and represents the criteria for the absence of intraparticlediffusion during TPD.
Equation [561 was also applied to check diffusion gradients within the pellet in TPR by checking a dimensionless number d
defined as
[601
where
Nn vC
rate of consumption of hydrogen at peak temperature. sample volume.
If the value determined for
#
is less than 0.3 it is concluded no
appreciable diffusion gradients exist within the pellet. The intraparticle diffusion can be avoided by choosing the proper size of catalyst particle during TPD experiment. Table VII represents the guidelines for the design of a TPD experiment in order to avoid intraparticlemass transfer effects.
TABLE VII: Impact of design Parameter on the Effectiveness factor (7))(421
Factor
Variation
Effectiveness
N
Increase
Decrease
Increase
Increase
Increase
Decrease
cS r
0
De vC
Increase
Increase
Increase
Depends on which
(Dilution with
dominating
Comment
(al Cs decreases
inert particles) B
Increase
W (Catalyst mass)
Increase
Depends on which
(al r increases
dominating
(bl Cs decreases
Increase
Huang and Schwarz (441 showed in a study that in the absence of intraparticle diffusion the proper description of mass transfer effects on TPD from a bed of catalyst perfused by a flow of carrier gas requires a nonsteady mass balance which includes both axial dispersion and convective transport. The equations that describe the concentration in the bulk fluid, C, and on the surface, 0,
343 are given by (451:
ac
8=D at
a2c A-
”
e az2
!J - vm ?+A
(?E,
[611
-ae = ka Cg (1 - 0)"- kdeP
[621
at
Where De and
u
are temperature dependent quant.ities.
Equations [611 and
1621 subjected to the boundary conditions.
ac
De2
I gIZ=O-cgin) z=o=u(C
ac
2
[641 Z’Lo
=
O
The initial condition is given by
Cg(z,
0) =
ecz,
0) = 0
Equations [611 and
[651 [621 were solved' numerically and
experimental results to obtain the kinetic parameters.
compared with
the
The above analyses
demonstrate that the position and shape of TPD spectra obtained from porous catalysts are sensitive functions of:
catalyst particle size. catalyst bed depth. carrier flow rate. carrier gas composition.
Intra-particle concentration gradients can be minimized by reducing catalyst particle size and carrier gas flow rate.
Axial gradients in the gas phase
concentrations of adsorbate can be minimized by the use of very shallow catalyst beds.
For small particles (e.g. r. = 0.08 cm) the TPD spectra for
nonuniform initial adsorption and uniform initial adsorption are similar.
quite
Reasonably accurate estimates of the enthalpy of adsorption (AHa)
344 and ratio of pre-exponential factors for adsorption and desorption can dbe obtained from equation 1661 if intra-particle and inter-particle gradients in adsorbate concentrations are absent (461.
I
In
B where
8M :
AH =d+ln RTM
“m
- AHd [FnAR
[661
1
Fractional coverage of adsorption sites at TM obtained by varying the heating rate.
n:
reaction order.
Axial dispersion within the reactor or during flow between catalyst and detector has been checked during TPR using the criterion (39):
Lt - >>- Rf ii 3.8ZDM where
[671
L:
Length of the connection line.
V:
Linear velocity within the tube.
t
R:
Tube radius.
D : M
Molecular diffusion coefficient.
t
Axial dispersion via molecular diffusion is important for the low value of the Peclet number and can be checked experimentally by examining the effect of flow rate variation on the shape of TPD spectra.
If the shape of TPR/TPD
spectra is not changed when changing the flow rate, the axial dispersion effect is less significant.
In TPD study of zeolites, diffusion can be the rate determining for gas appearance in TPD. zeolites using
Fraenkel and Levy (40) measured diffusion coefficients in
TPD.
They
incorporated the
effect
of
particle
size
distribution in the diffusion equation and predicted its effect on TPD curve. Their results showed an excellent agreement with the literature values.
Thus we find that by proper design of the temperature-programmedexperiments the problem of mass transfer effects can be avoided. Methods are available in the literature to demonstrate that a reasonably accurate estimation of the adsorption/desorption/reduction/reaction kinetic adsorbate/reactant can be obtained.
parameters
of
the
Table VIII presents the development and
application of various models used in the temperature-programmedanalysis with its limitations.
345
: i
VIII
Wichterlova et al. (49)
Farneth et al. (30 1
Fraenkel
Technique Studied
Methanol Oxidation over MOO3
HZSM-5 and Mordenite with different SilAl ratio
Temperature Programmed Desorption of NH 3
Zeolite A (300-900K) Na-A, Cs-A & Rb-A
System
TPSR
TPD (Temperature Programmed Diffusion)
(cont.)
(40)’
et al.
(cont.)
Mori
Author
Table
First order desorption of NH3
Not considered
Not considered
in
First order methan01
Mass Transfer Effects
Particle size effects considered
Used
Diffusion equation for Uniform Spherical particle converted to diffusion rate as
Kinetic
Readsorption of Nllj
Nil
Nil
Re.-adsorption
Y~5~:~7vt2:~Z~; fZr:og (HZSM-5) sites) Alid (Mordenito)= 145 kJ/mol (strong sitoh
Temperature peak maxima TM were found to be influenced by acid strength, no. of acid sites and zeolite structures. Heat of adsorption hHd of pIIt3 for individual zeolite obtained was in good agreement with ealori-
Activation energy E-20.6 kcallmol and factor pre-ex onential A=2xlO ‘; s-l matcb very well with literature values obtained from other techniques.
Pre-exponential factor and activation energy of diffusion correlated with Peak maximum temperature (T,)
Mordenite A=0.93expj1.1x10-4E]S-I
~=2.2~~~[1.2~10-4~1~-~
Silica Alumina A(E)=15exp[1.2x10-4E]~“’ Y zeolite
Remarks
347
TPD
TPD
Tronconi et al. [511
Rees and Chen (521
TPD
Leary (50)
et al.
Technique Studied
(cont.)
Author
TABLE VIII
ZSM-5. ZSM-11 and Theta-l in H+/Na+ form Desorption of p-xylene
Fresh and sodium poisoned y-Al203 Methanol and ethanol dehydration
9 wtX/Pd/Si02
system
Used
First order desorp tion kinetics
ReactionDeactivation Kinetics First Order Desorption
Subsurface diffusion: diffusion of H2 in PdfSiO2 catalyst Dissociative adsorption kinetics
Kinetic
Not considered
Absent
Not considered
Mass Transfer Effects
Nil
Considered
Nil
Re-adsorption
Activation energies of desorption wore calculated (Ed)ZSM-5 * 89 kJ/mol = 98 kJ/mol (Ed)zsM-ll (Ed)Theta-1’ 110 kJ/mol As Si./Al in pentasil type zeolite decreases the sorption capacity and activation energy for desorption decreases
Effect of poisoning on the selectivity has been reported.
The desorption activation energy values for methanol and ethanol were Ed = 70-115 J/m01 Ed = 80-120 J/m01
Parameter values are given Ad = 1x1013 2 -1 1 - 1.5x10 sAl, AD = 1.5x104s-1 Ed - 19.5 kcal/mol Elm= 16.1 kcallmol ED = 14.5 kcal/mol
Remarks
: Q,
TABLE VIII
TPD
TPD
Quanzh?. et al. (54)
Technique Studied
(cont.)
Rees and Chen (53)
Author
_
Desorption of NH3, Pyridine on HY
Desorption of n-hexane ZSM-5, ZSM-11, snd Theta-l in U+/Nn+ form
System
Used
First order desorption kinetics
First order desorption kinetics
Kinetic
Nil
Not considered
Mass Transfer Effects
Nil
Nil
Re-adsorption
(Uronsted acid sites) = 11.6 kJ/mol Ed3 (Lewis acid sites)
For pyridine m 13.5 kJ/mol EdL Ed2 = 25.9 kJ/mol
kJ/mol
of NH3 on DY there are two activation 11.9kJ/mol Ed L = and Rd2 = 43.8
CnSrgieS
Desorption show that desorption
Saturation capacities and Sd Values Were calculated (Rd)ZSM-5 = 80 kJ/mol Wd)ZS+L I= 74 kJ/nroI and (Rd)Theta = 69 kJ/moL
Remarks
350
IV. RESULTS OF EXPERIMENTAL OBSERVATIONS AND APPLICATIONS
IV.1 Overview
Temperature-programmedanalysis
has been applied to the study of supported
metal, oxides and zeolite catalysts since its inception by Cvetanovic and Amenomiya (11 in 1972.
This review concentrates on work done since 1983 on
supported metal and metal oxide catalysts.
Over the years, several reviews
have appeared in literature to cover TPD and TPR done before 1983 (2,61. Attempts have been made to put the technique on a quantitative basis, using various
mathematical models
and
evaluating kinetic
parameters.
The
information provided by temperature-programmedanalysis has been suplemented by other techniques such as x-ray diffraction, photoelectron spectroscopy and electron microscopy. This section has been arranged according to experimental observations and applications and to the techniques used.
To aid the reader,
and for easy reference, the following outline of the sections to follow is given:
Temperature-programmeddesorption and reduction:
-
Metal oxides.
-
Supported metal catalysts.
-
Metal zeolite catalysts.
Temperature-Programmedsurface reaction:
-
Hydrogenation.
-
Methanation.
-
Sulphiding.
-
Oxidation.
-
Other reactions.
Temperature-programmedgasification and carburization.
IV.2 Temoerature-DroFcrammed desorotion and reduction
IV.2.1 Metal oxides:
Temperature-programmedanalysis is a very useful procedure for investigating interactions between a test molecule and a catalyst surface, and it has been applied by several workers to studies of gas adsorption on metals and metal
351 oxides (32,541. Without doubt, O2 has been the most extensively used testing agent to study binding energies, populations and reactivities of metal oxides in the context of a large number of catalytic olefin oxidation reactions. The behaviour of metal oxide catalysts to catalyze complete oxidation or selective oxidation is related to the kinds of oxygen species involved, e.g. lattice oxygen and adsorbed oxygen.
In a TPD study the oxygen adsorption phenomena were seen to be different on different metal oxides.
Some oxides exhibited no oxygen adsorption whereas
others always gave relative large amounts of oxygen desorption.
Table IX
presents oxygen desorption behaviour from various metal oxides and classification based on TPD study.
TABLE IX Desorption of Oxygen from Various Metal Oxides (561
Group
A
TMa [Kl 0
'2'5 Moo3
0 0
Bi203
0
w"3 Bi203.2Mo03 B
Cr203 Mn02 Fe2o3 Go304 NiO cue
C
A1203 Si02
0
2.13 x lo-'
723 813
6.54 x 1O-2
323
543
633
328
623
758
4.05 x 1o-3
303
438
653
3.30 x 10-2
308
608
698
398
663
823
1.12 x 1o-2 1.42 x lo1 2.05 x 1O-4
338
2.99 x lO-5
373 593
5.52 x lO-5
Ti02
398
463
ZnO
463
593
2.45 x lo-4
Sn02
353
423
2.11 x lo-3
aThese values were obtained at 8=20K/min. b The overall amount of oxygen desorbed below 823K,after oxygen adsorption.
its
352 It is clear from Table IX that metal oxides could be classified into three groups: A.
oxides which exhibited no oxygen adsorption over the range 283-833K.
B.
oxides which give relative large amounts of oxygen desorption.
C.
oxides for which evacuation at high temperatures followed by oxygen adsorption at relatively low temperature is required for oxygen desorption to appear over the range 283-673K (except last 2 oxides).
The amounts of oxygen desorbed at 833
K
for group B and C oxides were only a
few percent of the sample coverage, suggesting that the adsorption sites are some sort of surface defect. There is fairly good correlation between the amount of oxygen desorbed at 833
K
and the oxide formation per g mol of
O2 (-AH;) as shown in Figure 10. The plot shows that, as an oxide is, less stable, so the more easily is the surface reduced to form surface defects for adsorption
o MnO2
A120,
0
I
I
260
I
I
LOO AH;
(kJ
I
600
g-1 atom
0)
Figure 10. Correlation of the amounts of oxygen desorbed at 833 K with the heat of formation of oxides per g-atom of oxygen. (From Ref. 57.1 It is confirmed in this study that group A oxides which are selective oxidation catalysts, show no significant oxygen desorption.
In contrast,
considerable amounts of oxygen adsorbed on group B oxides mainly catalyze the complete oxidation of olefins.
The group C oxides are in the intermediate
situation, adsorbing moderate amounts of 0
and catalyzing both the selective 2 and complete olefin oxidation to roughly the same extent. Thus these findings indicate that
the adsorbed oxygen is
strongly connected with
complete
oxidation, while the lattice oxygen is more important for selective oxidation.
353 Uda
et
al.
(8) used temperature-programmedre-oxidation to characterize the
redox properties of r.Bismuth molybdate catalyst (Bi2Mo06). Two peaks were observed, one at 431
and another at 613 K.
The low temperature peak was 4+ found to be a result of the re-oxidation of MO to Mo6+ and Bi" to Bi"+ 3+ where 0 < m < 3, whereas high temperature peak is the result of Bim+ to Bi . K
The activation energy for low temperature peak was determined to be 122 kJ/mole and the activation energy for high temperature peak was 265 kJ/mole using the method of Cvetanovic and Amenomiya (1).
The high temperature peak
appears to be related to the rate controlling step for propylene oxidation to acrolein at temperatures below 673 K. as detected by TPR.
Tagawa
et
al.
(58) studied temperature-programmeddesorption of CuO/ZnO/A1203
and CuO/ZnO/Cr203 catalysts for the reaction of methanol synthesis from CO2 and H2.
TPD spectra after the reaction on the copper containing catalysts
gave peaks characteristic of copper formate with
maxima at 473-483 K.This was This surface particle
identified as the intermediate point of the reaction.
is difficult to detect by infrared spectroscopy because of catalyst opaqueness (30% cu) whereas TPD spectra is proved to be useful in its detection for common catalyst.
Farneth et
al.
(30) studied Moo3 catalyst for partial oxidation of methanol,
using temperature programmed desorption.
TPD
spectra of methanol on Moo3
catalyst showed two distinct peaks, one at low temperature (Tmax = 393K) as dominated by loss of methanol and another at Tmax = 493 K consisting of formaldehyde, methanol and
water.
The
second peak
was
found
to
be
independent of the extent of coverage, indicating first order desorption kinetics. Activation energy for the formation of formaldehyde was obtained as 87 KJ/mol from TPD experiment confirming the redox mechanism proposed by Mars and Van Krevelen and the value obtained by independent reactor studies.
Thus,
form
these
works
it
is
demonstrated
the
utility
of
temperature-programmeddesorption studies under high vacuum with simultaneous microbalance
and
mass
spectrometry detection
for
obtaining
detailed
information about heterogeneous redox reactions. This method allows the chemisorption. reaction and
reoxidation stages of
the
mechanism to
be
separated in time and thereby examined independently.Changes in catalyst mass as a function of exposure, temperature and sample history were followed along with mass spectral intensity profiles of desorbed gases. It is remarkable how parameters obtained by TPD can be combined to predict heterogeneous reaction rates in excellent agreement with reactor data.
354 Similarly, metal sulfide catalysts used for the synthesis of Cl-C5 alcohols have been characterized by temperature-programmeddesorption technique (59). Alkalized
molybdenum
sulfide
and
unalkalized cobalt-molybdenum sulfide
catalysts showed four adsorption states for H2, four states for H2S and three for CO.
Alkali does not alter the total adsorption capacity of
these
catalysts for H2' CO and H2S but affects the distribution among the various adsorbed states.
Weakly adsorbed hydrogen and strongly a,dsorbedCO promote
selectivity to higher alcohols. Stacy et al. (38) studied mobility of oxygen in Mo2C using TPD and TPR . The catalytic properties depend very strongly on the MO/C ratio and the mobility of oxygen. TPR spectrum of oxygen adsorbed on Mo2C contains 2 water peaks, a narrow one at 479
K
and a second much broader peak near 573K. While the first
peak is produced by surface oxygen, the second peak is caused by O2 that diffuses into the subsurface region of the catalyst during the temperature ramp.
As the surface becomes depleted at higher temperatures, this oxygen
diffuses back to the surface, reacts with the gas phase hydrogen and desorbs as H20.
For carbon deficient catalysts, the two peaks merge into one
asymmetric peak at 510
K.
Thus the location of the peaks depend on the MO/C
ratio of the sample.
Malet and Munuera (60) used TPD to determine the energy distribution of acidic sites at the surface of ;r-A1203using water and NH3 as probe molecules.
The
heterogeneity of the surface can introduce changes in the shape of TPD curves depending on
the energetic distribution of the sites. In this work, a
logarithmic relationship between activation energy for desorption and surface coverage (Freundlich-Type isotherm) and TPD-curve shape analysis has been succesfully apply to
study the behavior of
real surfaces (a-A1203) in
adsorption-desorptionprocesses.
On the other hand, a number of (TPR) studies have appeared in literature on metal oxide catalysts where hydrogen was used (6). Supported metal oxides may be homogeneously distributed across the surface of the support or exist as islands of oxide separated by uncovered support.
Islands of metal oxides may
be expected to reduce in a similar manner to unsupported oxide, in which case the support may act purely as a dispersing agent and promote the reduction. Under these conditions the reduction kinetics observed for the supported oxides resemble those found for the bulk oxide. Metal atoms and crystallites are known to be mobile on the surface of supported metal oxides so that under appropriate conditions the reduction of a homogeneously supported metal oxide may proceed with the reduction of individual metal ions. Groups of metal ions,
355 can be subjected to surface diffusion to form metal crystallites which in turn may diffuse and combine to form particles of the reduced phase.
Vanadium oxide catalysts have been studied using TPR by Bosch et TPR profile of V205 catalyst is shown in Figure 11
a1.(471.
The
and under certain
conditions shows of a number of discrete peaks in the temperature range of 900-1100 K.
The results were interpreted as the stepwise reduction of V205 to
V203 through intermediates such as V6013 and V02.
They also reported high
activation energy of about 200 kJ/mole in the reduction process, indicating that solid state diffusion affects the reduction of V205.
Here TPR profiles have been studied as a function of flow rate, heating rate and sample weight. These conditions influenced not only the peak positions but also their forms. Then the authors explained the influence of sample weight and heating rate in terms of the formation of water in the sample during reduction. _
4.3mg,16K minv’
16 K min”
/
I
600
I
900
\t-l5mg,4.6Kmin~’
I
I
1000
1100
I 1200
TIK)
Figure 11. TPR profiles of V205 - influence of heating rate and sample (From Ref. 47.)
mass.
356 Howden et al. (611 studied the interaction of Mn with iron manganese oxides catalyst using TPR. The main conclusion of this work is that using TPR, it is possible to discriminate between various oxide phases on the basis of chemical reactivity and to elucidate the role of manganese in modifying the reduction behaviour of iron oxides. Their studies revealed that:
(al presence of Mn retards the reduction of Fe
in mixed iron-manganese
oxides.
(bl reduction of the mixed oxides occurs at considerably lower temperatures in CO than in Hz.
(cl the phases that remain after isothermal reduction of the mixed oxides depend on the pretreatment conditions.
Ehrhardt et a1.[391 studied the behaviour of supported transition metal oxide catalysts using TPR technique.
They studied Cr03/Si02 system, an important
catalysts for ethylene polymerization process.
The reduction behaviour of
CrOx varies with different supports, like a A1203 vs a SiO3 support.
In the
case of CrO supported over SiO2, they found that chromium is distributed 3' into at least three modifications, showing different reducibility. Further they reported that:
at Cr contents 25% wt, intermediate phases are present on the support resulting from the decomposition of pure Cr03.
at Cr contents (5% wt, Cr03 binds to the SiO3 surface mainly as dichromates and chromates to a minor extent.
at lower Cr contents
removal of water during reduction is crucial. A higher water removal rate 3+ goes together with higher Cr'+/Cr ratios.
From this work it is demonstrated the importance of the use of a mathematical model for the analysis of nonisothermal reduction, when we faced with complex reduction profiles, allowing the estimation of kinetics parameters for several overlapping single steps from one TPR experiment.
351
On another work, Arnoldy et
al.
(621 studied the reduction of
MoOB
and MoOa
using temperature-programmedreduction. The formation of MO catalytic sites depends on Ha0 pressure, concentration of surface defects and surface area. Reduction of MOO, to MoOa and MoOa to MO metal both can be catalyzed in which case the rate determining step is:
-
Either Ha dissociation when the formation of catalytic sites is limited.
-
or MO-O bond breaking when excess catalytic sites are present.
Reduction of the MoOa to MOO, can be also non-catalyzed, with oxygen diffusion determining the rate controlling step.
An analysis of this work show that TPR gives useful information on the reduction behavior of MoOB and MOO2, by examination of TPR patterns as well as by calculation of activation energy values for reduction via variation of the heating rate. It was found that experimental parameters such as water content of the reducing mixture, sample size, precalcination temperature and heating rate, influence the TPR patterns of Moo3 and Moo2 to a large extent. TPR profiles are modified by changes in operating conditions in a similar way found by Bosch et
al. (47).
Temperature-programmedreduction has been also used for the study of CoO/AlaOa catalysts (631.
Four different reduction regions are present in CoO/AlaOa
catalyst. These are:
-
-
Phase I consists of Co304 crystallites (600 Kl.
Phase II consists of Co3+ ions (750
K)
in crystallites of Coa/AlOe or
well dispersed surface particles.
-
Phase III consists of surface Co2+ ions (900 Kl.
-
Phase IV consists either of surface Co2+ ions or of subsurface Co2* ions, occurring in diluted Co2+ - A13+ spine1 structures in CoA1a04 (1150 Kl.
Here again, the conditions under which preparation proceeds (calcination flow rate and calcination temperature) influence the Co valency in CoO/A1a03, the extent of solid state diffusion and the dispersion.
Arnoldy et TPR .
al.
(641 studied the reduction of Co - Mo03/A1a03 catalysts using
These catalysts are used for hydrodesulfurizationand are more active
358
than CoO/AlaOa or MoOs/AlaOs catalysts. The reduction of M06* surface species (monolayer and bi-layer species) is not affected significantly by the presence of co, whereas the reduction of CO" presence of MO.
ions is strongly influenced by the
Strong interaction of CO-MO affects the reduction temperature
for Co'* ions from 1200 K in CoO/A120s to 800-850 K for Co0 - Mo0a/A120s catalyst. This study established the role of Co in HDS catalyst, which is the active
component, whereas
MO
promotes
Co
interaction between Co-ions and A1203 support.
activity
by
decreasing
the
Calcination of catalyst above
or below 800 K affects the phases present in the catalyst besides the Co - MO - P phase.
Figure 12 shows the TPR patterns of
9.1% CoO/A120s. whereas figure 13
represents the TPR patterns of Mo0a/A120s,and Co0 -
Mo03/A1203
catalysts as
a function of calcination temperature. Calcination of the samples affect the peak positions as shown in the figures 12 and 13.
From this work it can be seen that using TPR, information on the reduction of Co and MO in COO-Mo03/A1203 catalyst can be obtained. More important the combination of. TCD pattern (Hz consumption mainly) and FID pattern (CH4 production due to reduction of organic impurities) gives relevant information on the reduction temperatures for Co species.
1
I
Figure 12. TPR patterns of 9.1% CoO/A1203 catalysts calcined at various temperatures. Calcination temperature (K): 380(a). 575(b), 625(c). 675(d). 725(e), 775(f), 825(g), 875(h), 900(i), 925(j). The upper and lower part of each pattern represent the TCD and FID signal, respectively. (From Ref. 64.1
359
c: 4
==+(T(IC)
Figure 13. TPR patterns of Mo03/A1203 and COO-Mo03/A1203 catalysts calcined at various temperatures. Calcination temperature (K): (al 675 K, (bl 785 K, (cl 895 K, (dl 995 K, (e) 1125 K. The upper and lower part of each pattern represent the TCD and FID signal, respectively. (From Ref. 64.1
In another study of MoOa on SiOa Thomas
et
al.
(651 showed that the TPR peak
occurred at 750 K as opposed to 900 K for bulk Moos.
In the case of r-AlaOs
based catalysts reduced at higher temperatures than SiOa-based catalysts confirmed the
observations made
for
many
other
systems
that
stronger
interactions occur between a metals and r-AlaOs than between a metal and SiOa. At low loadings of Moos on SiOa, the support served to disperse the metal on the surface but as loading increased, this dispersion deteriorated.
The effect of support has also been reported in case of cobalt catalyst (661. Table X shows the effect of support on TPR peak temperatures.
Different
supports give different peak temperatures showing strong interaction of CO with the supports.
TPR was able to demonstrate how the interaction of different support with the metal could modify the reduction temperature.
360
Table X TPR of cobalt catalyst on various supports (66)
Remarks
Peak Temperature
catalyst
4% Co/A1203
5733 (Peak 11
673K (Peak III
Bulk Co304
593K (Peak II
663K (Peak III
Co/Ti02
5883 (Peak 11
686K (Peak II) Co304 to Co0 (588KI
co/sio2
473K
973K (complete) Higher temperature needed for reduction of the supported catalyst compared with bulk Co304
10% co304/sio2
693K
873K (complete)due to metal support interaction
Co/A1203
6OOK (Phase II
Phase I: Co304 crystallites
750K (Phase III
Phase II: Co3+
9OOK (Phase 111)
Phase III: Surface Co'* ion
1150K
co*+ in CoAl
coo
(Phase’
(686~)
IV)
to
coo
94
Temperature programmed reduction study of iron oxide (Fe2031, nickel oxide (NiOI and copper oxide (CuOI has been reported in the literature (6). Griffin and Robers (67) recently reported temperature programmed desorption of H
on various Cu/ZnO based catalysts.
Table XI shows the results
2
obtained with CuO-2nO catalyst.
These results indicate that for hydrogen adsorbed on reduced catalysts, a single desorption state which has an apparent activation energy of 20-21 kJmol-' is observed. When H2 is adsorbed on oxidized catalysts, the desorption -1 energy of this state increases to 26-27 kJmo1 .
The
main
conclusion drawn
from
this
work
is
that
a
combination of
temperature-programmed desorption (TPDI and FTIR spectroscopy was able to provide information related with both the concentration of adsorption sites and also their adsorption energy. If the adsorbed species has an infrared active vibrational mode, then it is also possible the direct monitoring of the conditions of the adsorbate as adsorption or desorption proceeds.
(PPQ
CUO~~O
1x cuo/sio2
CUD (ppt>
82% CuO/ZnO
33% CuO/ZnO
2% CuO/Kado x 25
ZnO (ppt)
1% cu0/s102
CUD (ppt)
82x
33x CuO/ZnO
2% CuD/Kado x 25
cw
Catalyst
"2
Hz
R2
H2
Hz
H2
H2
H2
H2
H2
H2
H2
Gas
387-403K 387-403K
15DK 15OK
Oxidized
Oxidized
-
367-403K
15OK
Oxidized
150K
-
305K
430K
Oxidized
150K
215K
Oxidized Oxidized
15DK 30%318K
30%318K
Reduced
30a-3laK
Reduced
308-318K
30%318lc
305K
Reduced
Reduced
15OK
215K
Reduced Reduced
Phases and Peak Temperatures
0.13
1.4
3.0
1.9
0.5
1.2
1.7
4.5
0.05
1.9
Amount of gas adsorbed, umol/g
adsorbed on reduced and oxidized Cu catalysts
Oxidized/ Reduced
TABLE XI: Temperature programmed desorption data of H
The work reviewed of the characterization of bulk oxidic materials and supported materials indicate that by using the temperature-programmed analysis technique, it is possible to identify:
compound formation between metal and support. determination of valency of metallic substances. identification of desorption sites and their energy position. identification of reaction sites in oxidation reactions.
The use of TPD and TPR in the study of oxide catalysts is not restricted to just studying the desorption and reaction behavior of molecules adsorbed on the surface of a catalyst. TPR has been used extensively in studying the reduction of the metal oxides to their metal state prior to their use as a catalysts. In a similar manner, TPD can be used to investigate the changes that can take place in the metal oxide catalyst during heating in an inert gas or in vacuum. Thus, TPD and TPR, can be used to guide the proper choice of pretreatment condition for a particular metal oxide system.
It was also found that metal oxides supported on inert carries likes alumina and
silica-alumina exhibited
temperature-programmed behaviors
different
compared with bulk oxidic materials. Desorption and reduction may be hindered or promoted depending on the nature of the oxide-support interaction.
In many cases, the intrincate character of the TPD/TPR patterns does not allow detailed
quantitative
treatment.
In
those
cases,
however,
temperature-programmedanalysis, can help the investigator in delineating the actual complexity of the system under study.
1v.2-2-
SUDDOrted
metal
catalysts
Temperature-programmedreduction (TPR) has in recent years become one of the most widely used physicochemical techniques for the characterization of metalsupported catalysts.
The main feature of the method is its capability of
continuously monitoring the consecutive reactions of reducible species at increasing temperatures.
It
can
thus provide
information about
the
dispersion states of the metallic component as well as the metal-support and metal-metal interactions in catalysis, since all of these affect reduction behaviour.
However, the reduction steps are sometimes so complicated, as
shown by many reduction peaks in the TPR profile, that it is not easy to identify each
of
them.
In
this regard, a
supplementary spectroscopy
technique (such as Mossbauer spectroscopy, etc) become useful, which can not
363
only determine the chemical states of various species in catalysts, but also work under 'in situ' conditions.
In general, a complex sample will produce a characteristic TPR pattern which can be used as a fingerprint. Of course, reproducing such a fingerprint will necessitate maintaining constant experimental conditions from one analysis to
Thus ( the catalytic researcher may use TPR analysis as a quality
another.
control test,
to
check
the
reproducibility of
a
catalysts precursor
preparation. Conversely, he may rapidly screen which preparative variables will actually affect the properties of a catalyst precursor.
The aim of this section is to present a few typical TPR and combined temperature program technique studies in order to illustrate the use of this technique in the field of catalyst characterization. An attempt will be made to interpret the result in the light of the theoretical concepts given in the preceding section.
IV. 2.2.1- Sunported monometallic catalVsts
Most work on the characterizationof materials using TPR has been carried out in relation to catalysts and predominantly on supported metal catalysts in the oxidic form.
In this section the work done is reviewed in the area of
mqnometallic supported catalysts using temperature program analysis such as TPR, T&
and TPO.
Supported catalysts often
have
very
high
metal
dispersions and
low
concentrations of active catalytic components. That is why it is difficult to characterize such catalysts by conventional methods.
Dispersed Pt/A1203 is extensively used as a catalyst, especially in the reforming process.
Treatments in oxygen and in hydrogen are established
procedures in the activation of fresh catalysts as well as in the regeneration of spent catalysts. Usually catalysts with a range of 0.3 to 0.6% platinum are used.
These are prepared by impregnation of alumina with platinum metal
salt (H2PtC16).drying in air at 393 K, calcined at 773 K and finally reduced in Hz at 773 K.
Therefore, to obtain a well dispersed catalyst, primary
attention to the pretreatment steps of drying, calcination and reduction
must
be given.
Lieske et al. (68) studied the formation of oxidized Pt species during treatments in oxygen at different temperatures using TPR.
Two surface oxides
364
a
and
and
P-[Pt02ls.
two
chloride-containing surface
complexes
rPtl"OxCLyls,that could act as a redispersing agents were found.
of
This is
supported by the fact that the temperature range where the agents can be formed,
is
the
same
as
the
known
temperature of
redispersion, i.e.
approximately 773 to 873 K.
They proposed that the TPR peak was due to reduction of a Pt-O-Cl containing surface complex stabilized by interaction with A1203. This was also supported by UV-visible spectroscopy studies. The importance of the presence of chlorine during reduction/dispersionof platinum can be assessed from the TPR results. The authors showed in fact that a Cl-redispersal step is essential in the regeneration of Pt/A1203 catalysts.
Similarly. Blanchard et a1.(69) using TPR, found in a Pt/A1202 catalyst that the interaction between the Pt particles and the alumina support came during He disagreed with Huizinga et
the drying step.
al.
(70) who claimed that it
depends on the time and the temperature, whether a well-dispersed catalyst is formed. McNicol (71) showed that the interaction between platinum particles and the alumina support takes place during the calcination step. However McNicol did not examine the TPR profile of catalysts that had not been calcined.
From these results, it is clear that, while the same general conclusions can be drawn about interactions with A1203, the TPR techniques enables the fine differences between catalyst prepared by different methods to be established. TPR
provides
a
method
for
studying
the
optimum
parameters
for
calcination/reduction treatments. TPR can be conducted over dried samples or over dried and calcined samples. Generally this type of experiment does not permit
the
study
of
chlorine evolution during
the
reduction process.
Consequently the residual chlorine content on the sample is only determined for reduced catalysts. Moreover, a TPR experiment may be quite different from a reduction conducted under isothermal conditions over several hours.
EUROPT-1 a platinum/silica containing 6.3% wt metal and prepared by Johnson Matthey for the Research Group on Catalysis of the Council of Europe, was characterized using
temperature-programmed desorption
(TPD)
(72).
The
analysis revealed the existence of four states of adsorbed hydrogen. The most strongly held
state
is
assigned to
spillover hydrogen treated, before
desorption, on the support. Most of the Hz is adsorbed dissociatively on platinum metal, providing a value for the degree of dispersion of the platinum of 65% (H:Pt = I:1 assumed).
365
The evidence of the number of adsorbed states found by TPD was corroborated by 'H nmr spectroscopy. Here again it is reinforced the importance of the use of supplementary techniques supporting temperature-programmedanalysis findings.
Rhenium catalysts have several interesting properties.
They show high
activity for metathesis, hydrodesulfurization and hydrodenitrification. It is known that Re also increases the stablitiy of Pt reforming catalysts.
Detailed
information
on
reducibiliy
as
such,
is
important
because
catalytically active Re sites are only created by reduction of the Re+7 ions, generally present in freshly prepared catalysts, to lower valencies as is the case when Re carbenes and
ReSz/Re metal are formed. Past studies mainly deal
with reforming catalysts and TPR methods have been used frequently (41 to provide information on the reducibility over a wide range of temperatures.
In an interesting work, TPR has been applied to characterize the reducibility of Rea07 supported on Al20a, SiOa and carbon catalysts (731. Figure 14 shows the TPR patterns of dried AlzOs-supported catalysts as a function of Re content.
At low Re contents, the TPR patterns are dominated by peaks which
are not related to reduction of Re ions. Differences in the concentration of additives such as chloride might explain these variations. A peak near 1100 K is found assigned to reduction of A120s impurities, such as iron, sulfide and sulfate.
The latter causes a (c.a.1 ZOO'K fall in for a higher Re content,
suggesting that this reduction is catalyzed by Re.
Figure 14 also shows the TPR patterns of dried SiOs-supported catalysts as a function of the Re content.
Besides Re'7 reduction, a reduction pattern is
observed between 800 K and 1200 K, which is typical for the SiOz support. As in the A120s case, it is assigned to the reduction of impurities. Again, one can conclude that Re catalyzes this reduction, since its presence causes the appearance of a second maximum in the SiO2 reduction pattern as well as a shift to lower reduction temperatures.
The pattern of dried carbon-supported catalysts as a function of Re content is also presented in Figure 14 for comparison.
The reduction peaks are small
compared with those caused by gasification of the support, occurring in the region of 500-1240 K.
In general it can be seen that dried catalysts are
found to contain a so-called monolayer-type Re+'lsurface phase as well as crystalline NHIReOl.
With respect to the detection of NHqRe04 crystallites
TPR has advantages over other techniques (such as XRDI. because TPR can used
for
quantitative analysis and
for
detection of
the
very
be
small
366 metal clusters.
Calcination at 575 K or 825 K resulted in decomposition of
the Re ammonia salt. formation of the Re+' surface phase and Rea07 clusters, and Re loss as a sublimation of Rea07.
WI-
-I_.1ur
Figure 14. TPR patterns (10 K/mini of dried Re207/A1203, p207/Si02
and
Re207/carbon catalysts having the following Re contents (at./nm'l : (a) 0, (bl 0.0080, (cl 0.040, (dl 0.20, (el 0.38, (f) 0.80. The upper and lower part of each pattern represent the TCD and FID signals, respectively. (From Ref. 73.1
Differences in reducibility of the various catalyst samples are ascribed to variations in interaction.
the
strength and
the heterogeneity of
the Re*? support
The strength of the interaction was found to depend on the
support material and
decreases
in
the
order:
AlaOs>SiOa>carbon.
heterogeneity was essentially the same for all three supports. varying literature data on
The
The largely
the reducibility of Rea07/AlaOs catalysts is
supposedly related with the presence of additives, such as chloride, which may +7 increase the Re support interaction.
367 From the clear
results
of
evidence
temperature
on
this the
and on the
metal clusters
it
is
shown that
structure
of of
the
and on the metal-oxygen
systematic
studies
reveal
the
work
reducibility
of
origin
of
they filled
their
goal.
In another
study,
the the
the
catalysts,
of
supported
inconsistencies
on TiOa,
with
TPR technique over e.g.
bond strength.
reduction
Pd supported
the
metal
a
is
wide
on the
The authors
of
presence
of
though that
ReROT catalyst
the
literature
given
range
might
date.
be
Already
AlaOa, SiOa and carbon was chosen as a
system (74). Palladium differs from other metal catalysts in its interaction with hydrogen because in addition to chemisorption, it absorbs hydrogen into the bulk (751.
This adsorption interferes with the chemisorption and may
cause an error in the determination of the dispersion of palladium catalysts. The reduction and hydrogen sorption for these catalysts were investigated by TPR and temperature-resolvedsorption techniques. Pd was found to be reduced at temperatures lower than 473 K.
Spillover of hydrogen from Pd to supports
occurred at higher temperatures. Hydrogen treatment at high temperature can also induce strong metal support interactions (SMSI) and sintering.
’ It was
found that under the same reduction conditions, the sintering of supported palladium catalysts follows the trend Pd/C>Pd/TiOa>Pd/AlaOa>Pd/SiOa.
TPR was able to demonstrate how the rate of sintering of the Pd metal could be related with the support employed.
The chemisorption of hydrogen on Pd has also been studied by Konvalinka et al. (761. Pulse-wise adsorption and temperature-programmeddesorption (TPD) on Pd samples and Pd on activated carbon catalysts showed how dissolved hydrogen can leave Pd via the surface, notwithstanding the fact that the chemisorber layer is more strongly bound kJ/moll.
recombination activation
(90-100 kJ/moll than the dissolved hydrogen (24
The energy trap of
a
enthalpy
good agreement
formed
subsurface
and
of desorption
correlation
by
is
between
a
the
surface
surface
easily
surmounted
hydrogen
atom
lowered.
The results
appreciably
the pulse
is
by
which
by the
show a
and TPO date.
Recently Leary et al. (501 presented evidence of the penetration of hydrogen into subsurface absorption sites of silica-supported palladium during TPD at conditions more closely resembles a catalytic reaction.
By TPD it can be
shown that penetration of an adsorbate into the subsurface region of an adsorbent
during
In Figure
15 it
of Ha is
TPD can produce is
shown that
a high
the high
temperature temperature
peak in the TPD spectrum. p3 peak in the TPD spectrum
not produced by desorption from a strongly binding energy adsorption
368
400
520 Tcmceroture
(K)
I .oo 0.9 5 0.7 5 d 0.6i) e 0.50 f 0.40 0.30 0.20
/
b
400
6GO
A 700
500 Temperature
600
700
(K)
Figure 15. Effect of initial coverage on the TPD spectra of (a) hydrogen and (b) deuterium on the Pd/Si02 catalyst for B s 0.5 K/s. Hydrogen spectra : catalyst dispersion 9% and helium flow rate 100 cc/min (STP). Deuterium spectra : dispersion 15% and Argon flow rate 50 cc/min (STPI. (From Ref. 50.)
369
site on the catalyst surface.
Instead, it is produced by hydrogen that
penetrates into subsurface adsorption sites in the palladium crystallites during the temperature ramp. When the surface becomes depleted by desorption, the subsurface hydrogen diffuses back to the surface and desorbs in the high temperature peak.
The observation that a significant amount of hydrogen can
penetrate into subsurface absorption sites of the Pd crystallites during TPD strongly suggests that hydrogen also is present in the palladium subsurface under typical reaction conditions.
It is also noted that these results are
consistent with the observation of subsurface absorption sites in Pd (1101 single crystals studies under ultra high-vacuum conditions.
In the past 15 years Rh has been gaining importance in catalytic chemistry. Not only is Rh widely recognized as the best catalyst to promote the reduction of NO in three way catalysts, but also takes a special place in the conversion of synthesis gas, since its product-range can comprise oxygenated products as well as hydrocarbons. Supports are often impregnated or exchanged with metal chloro-complexes. A great number of data collected in the literature concern with RhC13+H20 as a precursor of supported rhodium catalysts. To give an idea of the chloride evolution on supported rhodium precursors the TPR was studied by Marques da Cruz et
.Hydrogen-oxygentitration and specific rate as
al.(771.
well as turnover frequency have shown that total reduction may occur at relatively low
temperatures and
without
total dechlorination.
Strong
interactions between support and metallic complex control the initial and final temperatures of reduction and dechlorination. measuring
chloride
evolution
is
The technique used for
recommended
simultaneous
for
temperature-programmedreduction when hydrogen consumptions are measured.
Vis et
al.
(781 studied supported Rh/A1203 and Rh/Ti02 catalysts with varying
metal loadings by TPR and TPO and hydrogen chemisorption.
In the case of
hydrogen chemisorption, it was shown that all the Rh on A1203 was well dispersed, while the dispersion on Ti02 was much lower.
TPR/TPO showed that
this was due to the growth of two different kinds of Rh/Rh203 particles on Ti02;
one kind was easily reduced/oxidized,with a high dispersion, and the
other kind was harder to reduce/oxidize, with a low dispersion. that
in
this
study
of
catalyst
varying
metal
It is seems
loading,
the
temperature-programmedreduction technique was able to demonstrate how the reduction properties of the metal could be related to the dispersion of the metal.
Among the Group VIII metals Ru occupies an intermediate position between Fe,Co, Ni and noble metals as may be demonstrated by
the metal-oxygen
370 interactions for Pt. Ru and Ni in Table XII.
TABLE XII Metal-Oxygen interaction for Pt. Ru and Ni
Pt
Ru
Ni
-134
-220
-243
-109
-176
-235
293
433
773
Heat of formation of oxide (kJ mol-II
Heat of chemisorption of oxygen (kJ mol-l1 Reduction temperature of oxide (Kl
When exposed to oxygen at room temperature, a freshly formed surface of Ru becomes covered by
a
film
of
oxide, which will
thicken at
higher
temperatures. The formation of oxides on the Ru surface causes the induction phenomena as found in Ru-catalysed liquid-phase hydrogenation reactions (79).
Koopman et aJ.(SOI, using TPR, studied the oxidation and subsequent reduction of Ru. R&IO2
prepared by direct reduction of RuC13/Si02 showed surface
oxidation followed by bulk oxidation upon exposure to air at room temperature. The application of higher reduction temperatures (up to 973 Kl of RuC13/Si02 was found to increase the active metal surface area of the Ru/SiO formed due 2 to the removal of traces of chlorine from the Ru surface. Sucessful measurement of Hz desorption by means of the TPR gives rapid and useful information about the Ru metal dispersion.
The interaction between Ru metal and MgO support during catalyst preparation was studied by Bossi et
al.
(81). They also warning the possible danger of
doing TPR using Hz/N2 gas mixtures when the material being reduced contains components that might be catalytically active for the synthesis of ammonia.
Jones and McNicol (61, interpreted Bossi et. al. results in the light that repeated reduction-oxidation process leads to
aweakening in
the Ru-MgO
interaction originally formed since the final reduction peak is substantially lower in temperature than that of bulk Ru02.
Temperature-programmedreduction (TPR) experiments on copper on titania- and
371 silica-supported catalysts were studied by Delk et al. (821.
The results are
shown in Figure 16, in which the most obvious and significant difference is the lower reduction temperature of the titania-supportedcopper. and 4% Cu/Ti02 show reduction peaks at 403 K. 493 K in the 4% Cu/Ti02.
Both the 1%
There is a second peak at -
In the 1% Cu/Ti02 there is a shoulder at the same
temperature. In the case of the silica-supportedcopper. the reduction peaks are found at 541 K and 502 K for the 40% and 1% Cu/Si02 samples, respectively. The 1% Cu/Si02 also displays a shoulder at - 548K. The higher temperature +2 peak arises from reduction of Cu species that are not in such close contact with the support.
An interesting feature is consumption of hydrogen at
temperatures above 573 K. after all the Cu is reduced. This was interpreted as being due to the reduction of the titania support catalyzed by a copper metal.
0 0
TEMPERATURE,
C
Figure 16. Temperature-progzammed reduction of supported copper catalysts following calcination at 450 C (al 4% Cu/Ti02, (b) 1% Cu/Ti02. (cl 4% Cu/SiO2, (dl 1% Cu/Si02 (From Ref. 82.1
Temperature-programmedreduction studies suggest that there is an anomalous copper-support interaction involved by high temperature reduction. Correlation of TPR and chemisorption date suggest that this is directly related to
372
copper-catalyzed reduction of the titanla support to a lower oxide state. We believe that this is a good demonstration of strong-support interaction (SMSI) on Cu on titania supported catalyst by using TPR.
The wide range of applications of supported nickel catalysts in hydrogenation and various stabilization and hydro-treating processes has been the reason for a great deal of studies aimed at their description and characterization, and
better control of
catalysts silica
their activity and selectivity. TPR on NiLSi
showed that the amount of Ni in strong interaction with the
increased
with
decreasing
loading
and
increasing
calcination
temperatures. This suggests that a chemical rather than a transport effect is dominant. The influence of calcination can be shown in Figure 17, in which as calcination temperature increased the reduction becomes more difficult and metal particles of reduced species were smaller.
640 K
6 634 K
“‘; 746 K
I
602 K
610K \
667 K C 720 K
766 K I
683K
Figure 17. Effects of calcination temperature on the TPR profile of 1% nickel on micronized Gasil 35 silica. (Al 623 K. (BI 673 K, tC) 773 K, (II) 973 K. (From Ref. 83.1
373
Furthermore, increase in the metal loading led to an increased population of Ni arising from the low temperature reduction peaks and as a consequence larger Ni particle sizes, as can be seen in Figure 18. Reducibility of Ni on r-Al203 is much more difficult than on silica (83).
The interaction of Ni
with A1203 is clearly much stronger than the interactions that occur with silica. *
649 I(
B
Figure 18. Effects of nickel loading on TPR profiles. Standard conditions (A) 1% W, (B) 5% W, (C) 9.5% W, sensitivity from A to D. (From Ref. 83.)
Micronized Gasil 35. (D) 31% W. Reduced
Mile and Zammit (83) showed the capacity of TPR to reveal the characteristics of the nickel metal and nickel oxide phases that could not be able to detect by using another more complicated and expensive spectroscopy technique.
The location of Nickel oxide and nickel in silica supported catalysts was studied by Mile et al. (841.
As shown in Figure 19 the TPR of bulk nickel
oxide consisted of a single reduction peak at 673 K slightly skewed toward lower temperatures.
At 523 K the peak was found to be small.
Supported
nickel on silica shows the marked effect of the support in producing a number of peaks and broadening the whole profile to much higher temperatures. There are three reduction peaks at 523. 673 and 773 K and a chemisorption peak at 533 K obtained on cooling the sample from 873 K to ambient in the N2/H2 carrier gas stream.
374
Figure 19. TPR of unsupported NiO and TPR of NiO supported on miczonized Gasil 35 (8.8% w/w) loading). TPR conditions: 60 mg of sample, R = 12 C/rain,5% H2 - N2 calcination at 4OO'C for 16 h. (From Ref. 84.)
In conclusion, the authors were able to distinguish two different types of NiO using TPR with a temperature difference of _ 100 K, with the more reducible oxide is located mainly in the small pores and the less reducible oxide located in the large pores.
Iron has been used successfully in Fischer-Tropsch reactions. TPR was used by (851 to study the profiles for Fe/Si02. Reduction of cr-Fe203near equilibrium conditions should be quantitative to Fe304 before final reduction to metallic Fe.
Fe0 was not encountered since the temperature never exceeded 843
(Figure 20. Table XIII).
K
To identify reactions corresponding to the OL and /3
peaks, mass spectroscopy was recorded for TPR samples removed after each peak. The spectrum of the Q peak sample was well characterized by parameters for Fe304, the absence of any detectable aFe203 spectrum indicated complete conversion to Fe304. The spectrum of the j3peak sample was that of aFe metal. Yuan et
al. (861 also
studied Fe
catalysts using an
TPR-Mossbauer spectroscopy technique.
Two
10%
in situ
Fe/A1203
combined
catalysts are
375
+t
+I
*
-
.
0
-
m
In
.
376 With these combined techniques, various reactions occurring
identified.
during the TPR process of the catalysts were revealed (Table XIV).
NOMINAL PROGRVtlING RATE (oC/MIN) A.
17.0
6.
8.0
C*
5.5
c
\
0
100
200
300
4G0
500
600
TEEPERATURE('Cl Figure 20. Temperature programmed reduction of Fe/Si02.
(From Ref. 85.1
It was found that although the ion-concentrationwas high considerable strong metal
(oxide) support interactions occurred in the catalyst with higher
surface area, and the TPR of the sample consisted of three consecutive stages, namely at ground 733 K, 1063
K
and above 1123 K.
In the first stage from 573
to 873 K, mainly the reduction of Fe(II1) to Fe304 and then Fe304 to Fe(II1 aluminate took place.
It was amazing to find that over the temperature range
of 743-873 K, Fe304 was converted into Fe(I1) and FeCIIII aluminates without hydrogen being consumed. An increase in the amount of Fe(III1 was observed
377
378
when increasing the TPR temperature from 743 to 873 K.
In the meantime, the
Fe(II1) particles which remained unreduced, were also transformed to Fe(III1 aluminate at these temperatures.
In the second stage from 873 to 1123
K
Fe(II1) aluminate was reduced giving rise to the formation of Fe(II1 aluminate and Fe(O). Fe(O).
In the final stage above 1123 K, Fe(I1) aluminate was reduced to
It was found that the reduction of Fe(II1 was accompanied by a
migration of the Fe(II1 ions from the octahedral sites to the less stable tetrahedral ones, and thus facilitated the reduction. Chemical control of the reduction was present during the TPR over the temperature range of 743-973 K.
It can be seen from the above paper that complementary Mossbauer effect spectroscopy and X-ray diffraction studies largely support the TPR results and conclusions.
From the different papers reviewed in this section on monometallic supported catalysts we can conclude that the temperature-programmedanalysis technique has provided valuable information on the ease of reduction, valency state of metals, metal-support interactions and the influence of one component in the support on the reducibility of another component.
It was also found that many authors used TPD-TPR-TPO extensively as a diagnostic tool for changes caused by variables during the preparation stages of the catalysts. They were able to find that the temperature profiles can give an indication of the actual activity and selectiviy of the final reduced catalyst, measured using another techniques.
IV.2.2.2 -
Supported bimetallic catalysts:
For the past 20 years, bimetallic catalysts are a subject of considerable interest in catalyst research because their performance differs markedly from that of their components, and consequently.the constitution of supported bimetallic catalysts has been the object of a great deal of work.
In order to understand the catalytic behaviour of bimetallic catalysts, the most fundamental question is. whether the catalyst particles indeed contain atoms of both metals.
A direct experimental verification is difficult due to
the limitation of existing physical techniques.
The temperature-programmedreduction and oxidation techniques (TPR-TPO) may however be used to obtain evidence for the interaction between the atoms of the two metallic components.
379
In the case of Pt-Re/Al202 catalysts, there has been some controversy in the literature concerning the reducibility of rhenium in supported catalysts, in particular regarding the extent of its reduction.
Temperature-programmedreduction (TPR) has been used to examine the effects of drying on the reduction of Pt-Re/A1202 catalysts (871. effect on the reduction
of
Drying has little
the monometallic Pt/Al,O, and Re/Al,O, as can be
seen in Figures 21.
Pt
Figure
21.
Temperature programmed reduction of
Pt/A1203, Re/A1203
and
Pt/Re/A120g catalyst. (From Ref. 871. In constrast, both the number of TPR peaks and the peak maximum temperature of the Pt-Re/A1203 vary with drying temperature (Figure 211.
The TPR profile of
380
the Pt-Re/A1203 catalyst dried at 373 or 573 K consists of a single peak with a peak maximum temperature similar to that of monometallic Pt/A1203. The size of this peak corresponds to the reduction of both the Pt and the Re. other temperature extreme of temperature-characteristic
(773 K)
monometallic
of
temperature-characteristic of
At the
two TPR peaks are found, one at a and
Pt
monometallic Re.
at
one
a
interpreted their
They
results on the fact that water influenced the rate of Re207 migration to Pt centers, the mobility of Re207 and hence, its rate of diffusion to Pt centers was dependent on drying temperatures.
This is similar to the findings of
McNicol (711 who observed a TPR profile for Pt-Re/A1203, dried at 673 K identical to the sum of the monometallic profiles also dried at 673 K. The results suggested that water influences the rate of Re207 migration to Pt reduction centres.
Low-temperature drying below 273 K does not remove the
water adsorbed by the catalyst during storage and the Re207 is still hydrated. The suggestion is that this hydrated Re207 is mobile and is able to migrate to the Pt reduction centres.
Thus, the reduction of the two metal oxides is
essentially simultaneous, resulting in a single TPR peak, and an alloy is formed.
In contrast, high-temperature drying, 773 K,
dehydrates the Re207
which is no longer able to migrate to the Pt reduction centres.
The Re207
therefore reduces at the temperature of the monometallic Re/A1203 resulting in two TPR peaks and by implications no alloy is formed.
In another, work Wagstaff and Prins (881 used TPR to characterize the finely dispersed metal compound in a
catalysts. Strong series of Pt-Re/Al 0 23 evidence has been obtained that zero valent Pt and Re atoms are in intimate contact with each other after catalyst reduction. The formation of bimetallic clusters support the explanation of alloys causing the improved performance of the bimetallic system. Treatment of the reduced catalyst with oxygen above
473 K causes segregation of platinum and rhenium oxides. reduction of a platinum-rich species around rhenium-rich species around'523 K. 373
K
leaves
the
bimetal
273 K
Then a TPR showed
and reduction of a
Adsorption of oxygen at temperatures up to
clusters
largely
intact
but
subsequent
high-temperature treatment in the absence of extra oxygen leads to segregation of Pt and Re species.
Mieville (891 also observed in a TPR that an interaction occurs with Pt and Re when Pt-Re catalysts are preoxidized at 573 K or lower: re-reduced at lower temperatures compared to Re/A1203. Pt-Re
show no
interaction and
At 773 K preoxidation, TPR profiles of
are
similar to what
they would
be
if
superimposed Pt-Re catalysts, previously exhibiting catalytic reduction. Intimate mixture of Pt/A1203 and Re/A1203 particles preoxidized at 573 K, also
381
exhibit reductive interaction; however, loose mixtures of relatively large particles do not.
The explanation could be that H2 spillover initiates Re
oxide reduction by creating nuclei of Re, which then catalyze the reduction of the remaining oxide.
The overall mechanism is controlled by the rate of
spillover which in turn is determined by the degree of hydration of the alumina surface.
The implication of TPR results from the above results is, that the reduction step is of critical importance in the preparation and regeneration of a Pt-Re reforming catalyst, since the interaction between the metals apparently occurs during this step.
Another intriguing bimetallic catalyst system is that of Pt-Sn. Similar to rhenium in Pt-Re, tin is known to assist in maintaining the activity of platinum reforming catalysts. This still doesn't explain what happens when tin oxide, supported on alumina, is reduced. Dautzenber et
al.
have
(901
determined the oxygen which is consumed when tin oxide, supported on alumina, after first having been reduced, was oxidised again. The TPR-technique was used
thereby. They
concluded that the
tin oxide
is
fully
reduced to
zero-valent tin. They also report however, that the first 0.6% (wtl of Sn cannot be reduced, because it is bound chemically to the supporting alumina.
Contrary to this, results obtained by Burch (91) and Sexton et
al. (921 show
the tin-valency does not appear to drop below 2 (Sn II) for a wide range of tin contents. In fact, there is a sharp peak near 523 K in figure 22 in the Pt-Sn catalyst-spectra that must be associated with the reduction of platinum. Pt
(IV) is completely reduced to Pt
(01. When calculating the extend of
reduction in tin-valency for all series of the experiments concerned, it was found that the valency-drop of the tin from the original IV to 0 amounted to approximately 50% Z 10%. The presence of platinum did not have the effect of increasing this drop. Hence, the average valency after the reaction must have been Sn (111, as concluded from the TPR-data. Burch claimed that tin cannot exert a numbers of different effects on clusters of Pt-atoms present at a reacting surface. It was
suggested that the special properties of
the
catalysts are due to a change in the electronic properties of small particles.
This could be due to interaction with Sn(II)-ions on the support-surface - or - a small percentage of metallic tin particles could associate with the Pt in the form of a solid solution. It is interesting to note that Burch and Sexton on the one hand, and Dautzenber on the other, although they worked in different laboratories, used the same techniques and arrived at
totally
382 different conclusions.
It is seems that the differences can not be ascribed
to differences in the TPR techniques. The reason could be placed in the different catalyst preparation and pretreatment techniques adopted by both groups.
Snl,.A120,TPR lawa
rcgia)
R-Snll.A1203TPR i4
900 0
loo
200
300 TEMPERATURE
Km
SW
300
400
TEMPERATURE *C
600
.C
Figure 22. Temperature programmed reduction (TPR) profiles for Sn (upper) and Pt-Sn (lower) catalysts prepared by impregnating (al an acidic solution of Sn dissolved in aqua regia into r-A1203, (bl an acetone solution of Snc14.5H20 into I-A1203. The loading of Pt was 0.5 wt %.
(From Ref. 92.1
Here again, in a recent work (931, it was found that the average oxidation state of tin in the catalyst was Sn(II), as judged from TPR and TPD-profiles. The possibility of small amounts of zero-valent tin and/or alloys being present thereby exists. TPD and TPR profiles were used in conjunction with neutron activation analysis quantitative data.
that enables one
to
obtain
more
accurate
383
The inescapable conclusion from all those results is that it is important to understand that the complexities involved in catalyst preparation can affect the
ultimate
structure of
the
sample
and
the
interpretation of
the
temperature-programmedanalysis data.
The combination of platinum and iridium supported on alumina, has been studied by Wagstaff and Prins (941. They observed a single TPR peak (Figure 231 with a maximum at 378 K and ascribed this peak to the reduction of Pt(II1 oxide and They suggested that the
Ir(IV) oxide in intimate contact with each other.
appearance of the single peak indicated that the catalysts consisted of bimetallic clusters.
On the other hand, Foger and Jaeger (951 claimed that they believed it is dangerous to use TPR experiments in isolation when determining the structure of bimetallic catalysts, since, in the absence of additional data, TPR results are subject to ambiguity. The Pt-Ir/A1203 catalysts were studied by combining the
technique
of
TPR
with
transmission
microscopy
(TEM),
diffraction
(XRD).
electron
selected-area electron diffraction (SAED) and
x-ray
According to their results, only the iridium was oxidized when Pt-Ir supported alloys are heated in oxygen.
fi i
I i / !
,I ’
! ! I i i
i~
100
TR)
i
200
5m
Figure 23. TPR profiles for a prereduced 2 wt % Pt-Ir/r-A1203 catalyst reoxidized under the conditions indicated. (From Ref. 95.1
As illustrated in Figure 23, a single peak in the TPR profile with a maximum at 353-363 K was obtained if the oxidizing temperature was below 673 K, but a fairly sharp second peak with a maximum at 483-503 K appeared if the oxidizing temperature was above 673 K. The differences in the results can be explained from the point of view that Wagstaff and Prins worked with r-alumina as a support; whereas Foger and Jaeger were using Si02-supported catalysts.
It is clear that the interaction between the metal species and the different carriers plays an important role in the reduction temperature as found by the TPR results.
The technique of temperature-programmedreduction (TPR) has also been used to investigate the extent and rate of dehydration of the catalyst prior to reduction (961. To examine this effect, the maximum rates of reduction of Pt/A1203, Ir/A1203 and Pt-Ir/A1203 are plotted as a function of the percentage dehydration of r-alumina, and the results are shown in Figure 24. For monometallic
catalysts
there
is
linear
a
relationship,
while
the
co-impregnated catalyst shows a sharp deviation from a straight line at - 50% dehydration.
1
----apt -0 Ir -*.-a Pt/lr
-I 0
IO
20
Jo
40 PERCENT
50
60
70
80
90
100
DEHYDRATION
Figure 24. Effect of TPR spectra of Pt/A1203, Ir/A1203 and Pt/Ir/A1203 catalysts as a function of the percent of hydration of the r-A1203 support. (From Ref. 96.)
385
The reduction of Pt and Ir co-dispersed on A1203 is very sensitive to the pretreatment conditions, especially to the extent of dehydration. readsorbs strongly on
the A1203 support during TPR.
Water
Furthermore, its
readsorption properties increase with increasing degrees of pre-hydration. This is due
to the increase in the free-energy of
activation with an
increasing degree of dehydration. Such effects will broaden TPR spectra and shift them to higher temperatures.
A new system used in catalytic reforming is Pt-Ge/A1203 catalysts.
Again
here, the main question arises as to the real nature of the interaction between the metals in this catalyst.
Goldwasser et a1.(97) studied two series of Pt-Ge/Al203 catalysts, that were prepared using different calcinatlon procedures. Both series were calclned at 673 K after the addition of both components, while one of them was calcined prior to the addition of platinum at 873 K. The TPR showed that there was complete reduction of Pt(IV) to Pt(O) for all catalysts and partial reduction of
the germanium (IV) precursor in the case of high
catalysts for the series only calcined at 673 K.
loading germanium
It seems that this behaviour
can be explained by the electronic surface model (911, where the special catalytic properties of the bimetallic catalyst are due to a change in the electronic properties of
the platinum, presumably via electron withdrawal by
the reduced germanium ions. The results in the other sense (calclnatlon prior to the addition of Pt at 873 K) indicate that germanium. after calclnatlon at 873 K is stabilized in the alumina support in such a way that its reduction does not occur under TPR conditions; there is no catalytic effect in respect of the
reduction of germanium (IV) by the metallic platinum.
dilution or
geometric effect
would
be
almost
Evidently, a
impossible under
these
conditions.
Recently, De Miguel et al. (981, using TPR, showed that calclned Pt-Ge/A1203 would contain approximately 40-50X of Ge(O) after reduction at 1073 K, unlike the Ge/A1203 catalyst, where Ge was not reduced to the zero-valent state;
it
can be inferred that a fraction of Ge would be reduced to Get01 below 723 K in Pt-Ge/A1203 catalysts.
Assuming that Ge(IV) could be reduced only to the
zero-valent state up to 723 K in bimetallic catalysts, the percentage of Ge(O) calculated was approximately 25% of the total Ge content as shown in Table XV.
In order to calculate the relative quantities of the Ge species with different oxidation states present on calclned Pt-Ge/A1203 reduced up to 1073 K, they assumed two hypotheses.
According to hypothesis I it was supposed that
* Hypothesis
Ge/A1203
Pt/A1203
Nonometallic
(calcined)
Pt-Ge/A1203-C
(chlorided)
Pt-Ce/A1203-B
(calcined)
Pt-Ge/A1203-B
(calcined)
12.71
12.71
12.71
Ge(IV)
13.12
catalysts
I:
the
13.12
catalysts
Pf
in
is
directly
36.37
-
29.76
30.86
30.86
27.55
Ge
sample
metal
reduced
0.00
26.24
25.42
25.42
25.42
26.24
Pt(IV)+(Pt(O)
for
Theoretical
in TPR experiments
urn01 of
consumption
Pt-Ge/A1203-A
Bimetallic
Sample
Hydrogen
TABLE XV:
for
to
Ge(0); hypothesis
36.37
29.76
30.86
30.86
27.55
Ge(IV)+(Ge(II)
(pmol)
catalysts
H2 consumption
different
II:
(98) Experimental
Ge(IV)
0
26
41
35
40
40
723OK
(umol) ~
to
is
first
29
26
67
58
70
69
1073°K
up
H2 consumption
reduced
60
-
30
47
28
22
-Ge(IV)
I
to
40
-
70
53
72
78
to
Ge(I1)
G.=(O)
Hypothesis
Ge in different reduction up
Ge(IV)
and
20
-
0
0
0
0
then
80
_
60
94
56
45
to
Ge(I1)
after
0
40
6
44
55
Ge(0)
II
Ge(0).
states
Hypothesis
oxidation 1073%(X)* at
0
26
15
24
25
723°K
Percentage reduced to
of Ge Ge(0)
387 Ge(IV) was directly reduced to Ge(Ol according to:
Ge02 + 2H2 -
Ge
+ 2H20
1681
while for hypothesis II it was assumed that Ge(IV1 was reduced to Ge(II1 and after all the Ge(IV1 had been transformed to Ge(I1). this was reduced to Get01
These results and the fact that TPR profiles of bimetallic samples (below 723 Kl show both a broadening of the first reduction peak and an intermediate reduction zone, would indicate a certain catalytic effect of Pt on the Ge reduction.
The results are
in good agreement with
those published by
Goldwasser et a1.(97)and Beltramini et al. (991 who obtained values between 25-50X Ge reduced to the zero-valent state.
It is clear that in bimetallic reforming catalysts, the TPR profile is a useful indicator of catalyst conditions in the calcined and reduced state. This is important for the study of the redispersion process of the catalyst. TPR, then could be applied as a fairly rapid, reliable and cheap technique for the study of a particular catalyst preparation process.
It is known that MO-based hydrotreating catalysts are widely used in the oil industry for the removal of organo-sulfur and organo-nitrogen contaminants. The catalysts are sulphided before use and there is general agreement that the active component is a MoS2 crystallite promoted by Co or Ni.
There is still
some dispute however, concerning the nature of the active site and the way in which Co or Ni acts as a promoter.
A series of hydroprocessing catalysts Mo03/A1203 having various Moo3 contents, Co-Mo/A1203, NiMo/A1203 and Ni-W/A1203 were characterized by TPD with He, TPS with H2S in H2 and TPR with H2 (1001. The results show that a decreasing temperature of appearance of H2S peak in the TPR spectra of sulphided Mo/A1203 catalyst corresponds to an increasing MO content and increasing catalytic activity.
TPR results provide a rapid qualitative test for the presence of
incompletely sulf.ided Moo3 on
the catalyst surfaces. A
summary of
the
quantitative TPD and TPR results are presented in Table XVI. TPR associated with the formation of sulfur anion vacancies on the surface indicate that only a small fraction of the sulfur is removed by reaction with H2, this fraction being higher for the promoted catalyst Co-Mo/A1203 than for the less active, unpromoted catalyst Mo/A1203.
The succesful application of the modified TPR, TPD and TPS tecniques for
0.14 0.19 0.20 0.10 0.15 0.08
473 473 673 523 493 473 443
353
353
353
353
353
353
353
Ni-Mo/A1203
Ni-W/A1203
y-AI203
2.5% Mo03/A1203
9% Mo03/A1203
10% MoO~/A~~O,~
15% Mo03/A1203
to transition
d S/(Mo + Ni)/2
+ Co)/2
M refers
c S/MO
b
metal atom.
a S refers to sulfur desorbed
0.14
0.40
0.13
0.25
0.20
0.48
0.38
0.40
0.29
0.23
Remainder
as H2S in the first peak TPR.
0.17
473
19% W03/A1203(E)
358
0.10
503
12% Mo03/A1203(E)358
0.29
0.24
473
353
Co-Mo/Al203
Peak I
TPD
H2S desorbed
(100)
TPR
Onset temperature for H2S desorption ('R)
TPD and TPR Results
TPD
Sulfided catalyst
Summary of Quantitative
TABLE XVI:
g
0.80
0.25
0.60
0.45
0.80
0.24
0.30
0.67
1.20
0.11
Remainder
S/(Ni + W)/2
S/W
f S/MO
e
0.14
0.10
0.04
0.06
0.07
0.05
0.22
0.40
0.28
Peak I
TPR
(molecules/rim*)
0.06g
0.06f
O.Olf
0.02f
0.05f
0.07f
0.06e
0.04d
0.05=
S"lMb , atomic ratio (Peak I, TPR)
389 hydroprocessing clearly
catalysts,
demonstrated
Ni-Mo/A1203 catalysts prepared techniques
(101). does
adsorbed.
the
containing
by wet impregnation,
catalyst
of
which
the total
rather
Ni-MO catalysts
temperature
on
the
has shown that
the Ni
or MO catalysts.
model
(Figure
contains for
oxidic
and sulfidic
25)
shown that the
in
or
forms
it
is
is
other
The results it
is
were
in the stoichiometry
of
a constant
the
that
that
scheme was
in a
with
MO
from
terms
of
typical
presented
of such a crystallite
all
sulphided
differently
interpreted
H2S
fraction
associated
behave
envisaged
A reaction
Ni to a Mo/A1203
nature
hand TPR of
the promoted catalysts
MO were
TPD and TPR
was suggested
MO catalysts On the
and
using of
the
peak represents
Consequently,
support.
which
of Ni
the addition
quantity
desorption
33 MO atoms.
the changes
concentrations and characterised,
alumina-supported
than with
simple
accounts
is
either
either
crystallite
different
amount of H2S desorbed.
H2S adsorbed
sulphide
affect
The high
in both
sulphiding
From TPD it
not
exist
here.
a
MoS2 which
during
TPD
and TPR.
I
He1773
4 Hz1773
V
Figure
25.
Model
of
MoS2 crystallite
during
TPD and
TPR (0)
sulphide
(where this occurs on an edge only one of the two sulfide positions and bottom layers is occupied); (0) SH ion either in the top or V, layer; (e) SH ion in both top and bottom layer; (e) MO ion; both top and bottom layer positions. (From Ref. 101.)
ion
in the top the bottom vacancy at
390
Furthermore, Burch and Collins (101-1021 studied a catalysts, using TPR.
to obtain information on
series of
industrial
the behaviour of
these
catalysts before and after regeneration. Figure 26 shows the different TPR profiles of used and fresh catalysts. The fresh catalyst has a peak maximum at 487'C and a shoulder at 527'C, whereas the regenerated catalyst has a peak at 557OC and a shoulder at 547'C.
It is shown that the regenerated catalyst
has become more difficult to reduce at low temperatures. However, the long tail for the fresh catalyst at high temperatures is less apparent for the regenerated catalyst which could indicate a decrease in the amount of highly dispersed MO
present.
The
decrease in
the
case
of
reduction after
regeneration is consistent with the lower surface area and the lower HDS activity for this catalyst. From this paper it is important to conclude that TPR can provide a quick and easy way of checking the effect of regeneration on the HD5 catalysts.
T/K
Figure 26. TPR profiles for regenerated catalysts (al Fresh catalyst, (bl and (cl repeat TPRs on two samples from the same batch of regeneiated catalyst, (dl regenerated catalyst after heating for a further 40 h at 723 K. (From Ref. 102.1
391
The use of TPR
technique is likely to be fundamentally more reliable than the
currently used measurement of total surface area. However, the authors claimed that further work will be required to determine whether the method is of general applicability.
Van't Blik and Prins (103-104) studied the effect of support on bimetallic Co-Rh catalyst using TPR techniques. They concluded:
-
On Co-Rh/A1203 catalyst, TPR showed a reduction peak at a much lower
temperature than that of Co/A1203. This and the slight shift relative to the peak of Rh/A1203 indicates that cobalt and rhodium ions are not far apart after coimpregnation.
This explains why bimetallic particles are easily
formed during reduction.
Oxidation at room temperature of
the reduced
bimetallic catalyst leaves the structure of the bimetallic particles largely intact; but cobalt is oxidized to a large extent while rhodium remains metallic.
TPR analysis of these catalysts also suggest that in the reduced
state the bimetallic particles are already surface-enriched in cobalt.
-
On Co-Rh/TiO*, the addition of rhodium to cobalt caused the resulting
bimetallic catalyst to be more difficult to oxidize, while the reducibility of the catalyst depends on the oxidation temperature. When oxidized below 800 K, the reduction proceeded at low temperatures, indicating that Rh203 was present To complete the oxidation,
in the surface of the bimetallic particles.
however, higher temperatures were needed and under these circumstances cobalt rhodate, CoRh204, was formed.
The TPR reduction behaviour of a oxidized
catalyst reveals that only cobalt oxide was
exposed, demonstrating the
oxygen-induced surface enrichment by cobalt.
-
On Co-Rh/Si02 catalysts, the reduction proceeds at lower temperatures
than the reduction of Co/SiOB catalysts, indicating that Rh catalyzes the reduction of the cobalt metal salt and cobalt oxide.
From combined TPR and
EXAFS analysis it is concluded that the reduced catalyst contains bimetallic Co-Rh particles, the interior of which are enriched in rhodium, while the outer layers contain more cobalt.
From these studies become clear that TPR is able to supply useful information about the structure of the reduced bimetallic Co-Rh supported on different carries.
On another TPD study on Cu-Zn catalysts for methanol synthesis (105). it was showed that the spectra displayed two peaks.
Only low temperature peaks (-
480Kl which might correspond to a reoxidation stage of copper, may reasonably be correlated with catalytic activity at -483K.
High temperature peaks (-
600Kl correspond to hydrogen desorbing from zinc oxide, and do not seem to Results from this work
form part of the methanol synthesis (Figure 27).
demonstrated that TPD profile for H2 can provide pertinent information than direct chemisorption.
In this chapter some of the most important applications of TPR and TPD to the characterization of supported mono- and bi-metallic catalysts were reviewed already. The work done was mainly concerned with the characterization of the catalysts, since it
is one
of
the most
important applications of
temperature-programmetechniques in this area. readily identified.
the
A number of phenomena can be
These comprise metal dispersion, interaction between
metal-support, formation of cations and ion species.
alloys, soluble solutions, identification of
The later can help during the catalyst preparation
to improve catalyst performance of these materials. It has been clearly demonstrated
that
reproducibility of
temperature-programmed analysis results from
laboratory to
can
give
excellent
laboratory when
similar
conditions of experimental and catalyst preparation are used. In those where different workers have found results at variance with others, there has usually been a clear explanation for the difference, usually associated with different conditions and/or different catalyst preparations used by
the
workers concerned. The technique has been shown to be extremely sensitive, witness
the
excellent
temperature-programmed profiles
from
catalysts
containing only a few tenths of a percent of reducible material.
As a conclusion, TPR results can be used in conjunction with those obtained from other complementary techniques such as XPS, XRD, SIMS, etc. in order to quantify the use of the data and to get a clear identificationof the catalyst surface phenomena.
IV.2.3 Metal Zeolite Catalysts
Zeolites have been used as catalysts and separating agents in the chemical and petrochemical industries (106). The alkali metal ion present in the zeolite is generally exchanged with other cations by heating the zeolite with an appropriate aqueous solution of the salt containing the metal to be exchanged on the zeolite. The metal salt may be a transition metal salt or an ammonium salt.
The exchanged zeolite is subject to drying and calcining in order to
generate a framework, containing surface hydroxyl groups that are acidic and active for carbonium ion reactions. The transition metal, which may be used
393
.. .’
: . . .
:
:
.
: .
. . . . . .
T(K)
0
Figure 27.
(a) Thermodesorption
100 -r(Y)
50
profiles
for
H2
from
Cu/Zn/Al
catalysts(b) Correlation between the amounts of H2 thermodesorbed (I: low temperature peaks, II: high temperature peaks) and the activity in methanol synthesis. (From Ref. 105.)
394
for rendering a hydrogenation or other function in the catalyst, is then reduced to the metallic state.
It is important to know the positions of the
zeolitic cations in the framework to understand the catalytic functions. Temperature-programmedanalysis techniques are used for the characterization of metal zeolite catalysts. The analysis enables the characterization of:
the state of cations and in some cases, to say something about their location in zeolite (TPR).
surface acidity in regard to both the amount and the strength of acid sites (TPD).
sorption and diffusion studies (TPD).
IV.2.3.1
Reduction of Metal Zeolite Catalysts
Hurst et al.(4) reviewed reduction of metal zeolite systems using temperature -programmed reduction technique. They reported reduction of Cr-X. Cr-Y. Mn-X, Fe-X, Ni-X, Ni-Y, Cu-Y. Ru-X, Pd-X and Pd-Y zeolites. Table XVII presents the TPR data of various metal zeolites reported in the literature.
The reduction of metal ions in zeolite is dependent on the method of reduction and pretreatment.
The reduction of metal ions in zeolite is generally
achieved by using hydrogen and can be represented as:
+ Mn+ + ; H2 ---_) MO + nH
(691
The protons react with the zeolite lattice to produce the hydroxyl groups.
nZO- + nH+ --+ riZOH
; ZOH: zeolite
[701
The TPR peak positions are sensitive to experimental conditions. That is why one should be careful when comparing results obtained by different authors. However, the sequence obtained by each author, is in general agreement with the prediction.
395
TABLE XVII Temperature-programmedreduction data of metal zeolite catalysts (4,6l
Catalyst
Temperature
(Cr"+,Nal-X-20 200-1000K
Dehydration at 723K
Complete reduction of
in N2 or O2
Cr3-+
Cr2+
Cr3++
1.5 mol of H_ oer mol
(650K)
of Cr3+
Cr species is not completely reduced to the metallic state in both X and Y
570-870K
Cr3++
Mn-X
200-800K
No reduction peak
200-1000K
Cr" would need
Cr2+-+ Cr++ Cr+ * Cr" (metal) 3 Peaks Cr-Y
Fe2 -NaX
Remarks
Pretreatment,
Cr2+
I Peak (703K1,II Peak
1q2
' -
consumption is very
small showing that a
(923K)
small part of Fe species is reduced to zero valent state.
(Ni2+,Nal-X-20 200-1000K
Oxygen pretreated sample Below lOOOK, Ni2--+ Ni+ 83% reduction
Above lOOOK, Ni++
N2 pretreated sample
Ni2+ reduction depends
91% reduction
upon the pretreatment
Ni2+-
and its location in
Ni"
(220kJ/mol) 900K Peak I
Ni"
zeolite
(86 kJ/mol) 830K Peak II Ni2+-NaY
cu2+-NaX.50
200-1000K
200-1000K
Single peak of Ni"
Hydrogen consumed
at 773 K
correspond to about
No evidence of
40% reduction of
Ni2+-
Ni2+---+Ni"
Ni+ step
2 Peaks Cu2+.
Cu+ (550K)
Supercage (87kJ/mol) Cu2+--+ Cu+ (650Kl Sodalite (64kJ/moll Cu+-
Cu"(not observed)
Nitrogen pretreatment produced less reduction of cu2++
cu+ than
air pretreatment
396
TABLE XVII (Cont.1 (Cu2+-Nal-Y-68 400-800K lg sample 5 Wmin
Cu2+--+ Cu+(440Kl
Experimental conditions
(111 kJ/moll
affect the TPR profiles
Cu+ -
Cu'(520K)
of Cu+-
Cue peak
(70 kJ/moll oxidation of Cue to cu2+ ions possible
Ru-X
400-800K
(46% exchanged)
Difficult to
Three peaks: Ru3+--+ Ru2+
423K
distinguish between
Ru2+-+ Ru+
4931:
ruthenium ions present
Ru+ --_)Rue
563K
in different zeolite locations.
Pd-X
200-600K
Dehydrated in 02/N2 at
An extra reduction
723K prior to TPR
peak occurs at 410K
N2 dehydration
(broader) in both
Two reduction peaks 210K cases' and 240K, Two hydrogen desorption peaks at 320K and 340K 0, dehydration Single peak at 270K and desorption at 340 K
Pd-Y
Sachtler et al.
200-600K
Pretreatment in air
Two stage reduction
R-15 K/min
at 775K
process Pd2+-+Pd+--+
10% H2/N2
Pd2+.
20 ml/min
Pd2+ ions located in
Pd" (290K)
Pd" is not acceptable because Pd" is present
supercage and sodalite
during first reduction
cage
process
(1071 used TPR and TPD to study the influence of co-exchange
metal cations such as Fe2+, Ca2+, La3+ and H+ on the formation of Pt particles in Y-type zeolite. The co-exchanged ions can affect the reducibility of the catalytic metal and thereby favourably affect catalytic activity.
The TPR and TPD profiles for Pt/HNaY, Pt/CaNaY and Pt/LaNaY are shown Figure
397
28.
2+ ions in Pt/HNaY (633 The reduction temperature of Pt
(823 Kl
K)
and Pt/HNaY
increases with the calcination temperature.
: 1.60 = Z = 1.40 i <
L
1
d 1.00 = i g .600 r u
TPOICaNaY
_i’“\,
a?
I-
-30.0
110.
250.
mo.
9
630.
Tomoeratwo PC)
t
0.0
f10.
200.
190. Tempwatue (*C)
so.
Figure 28. TPR (curves, a, b, c, d, e) and TPD (curves f, g, h, i, j) of Pt/NaY, Pt/HNaY, Pt/CaNaY and Pt/LaNaY samples calcined at 360 and S5O'C. (a) PtA-R?aY (3601; (b) PtfHNaY (550); fc) Pt/CaNaY (5.501;fdf PWLaNaY (550); (e) Pt/NaY (5501; (f) Pt/HNaY (360) after complete reduction; (g) (hl Pt/HNaY (550) after Pt/HNaY (550) after complete reduction; interrupting reduction at 400°C; (i) PtKaNaY (550) after complete reduction (dashed curve) and after interrupting reduction at 4OO'C (dotted curve); (J) PtAaNaY (550) after complete reduction (dashed curve) and after interrupting reduction at 400°C (dotted curve). (From Ref. 107.1
The reduction temperature and profiles of all three catalysts calcined at 823 K
shown in fig 28 have H/Pt ratios 0.62 f 0.10, 0.15 + 0.10 and 0.28 + 0.10
respectively. They also observed that:
size and location of Pt particles in a zeolite matrix is strongly dependent on the distribution of Pt ions between the supercages and sodalite cages. This is controlled by the calcination temperature prior to reduction.
f
398
at a low calcination temperature (e.g. 633 Kl the majority of Pt2+ ions are in supercages, at a medium calcination temperature (e.g. 723 K) the Pt2+ ions are distributed between super-cagesand sodalite cages 2+ high calcination temperature (e.g. 823 Kl most Pt and at a ions migrate to
sodalite cages and require a high reduction
temperature.
the
presence of
co-exchanged multivalent cations e.g.
2+ Fe
can
effectively block sodalite cages and hexagonal prisms, thus forcing pt2+
ions to stay in supercages even at high calcination temperatures.
if the number of multivalent cations is not sufficient to block the small cages, virtually all Pt2+ ions will ultimately be located in sodalite cages at the high calcination temperature. A high reduction temperature is then required, and migration of the reduced platinum to the external surface of the zeolite crystals will lead to the formation of large Pt particles 2+ the number of Pt ions which have migrated to sodalite cages can be estimated semi-quantitativelyfrom the evolution of hydrogen in TPR and T = 723 K, after interruption of the temperature-programmedreduction max at 673 K.
Some general conclusions may be drawn from temperature-programmedreduction of metal zeolite catalyst systems:
The ease of reduction is dependent on the location of the ion supercage > sodalite cage > hexagonal prism.
The rate-determining step is generally associated with the migration of
ions to reduction sites.
Reduction is frequently hampered by the formation of electron-deficient clusters.
IV.2.3.2
Acidity characterization
The acid sites in zeolites play an important role in their utiiization as catalysts
for
various
transformations
of
hydrocarbons
(106).
Temperature-programmedesorption (TPD) of ammonia and pyridine has been used
399
(108) to characterize the acid strength and the number of acid sites by direct physico-chemical method. TPD of preadsorbed ammonia has become so popular to the extent that the Japanese Catalysis Society proposed this as a possible standard method for testing zeolites (109).
Forni et
al.
(110)
used TPD of ammonia to characterize the acid sites of
partially decationated Y zeolite.
They found that desorption of ammonia is
controlled by intracrystalline surface diffusion with values of the apparent activation energy of 114 - 130 KJ/mol for HNaY zeolite. On the other hand, they discussed the TPD results on the basis of a comparison between what is expected from the use of theorethical equations based on different mechanisms assumed (i.e. adsorption of the base or diffusion of the desorbed ammonia with the zeolite is controlling step) and what has been actually obtained experimentally. Thr results show a good reproducibility of the TPR data with the mechanism proposed.
Wichterlova et
al.
(49)
used TPD of NH3 to determine the number and acid
strength of acid sites in HEM-5
zeolites and mordenite with different Si:Al
ratios.Figure 29 depicts typical curves for TPD of NH3 for EM-5
and mordenite
Figure 29. TPD of ammonia curves for zeolites, sample weight 0.30 g, /3= 21 K/min, Helium flow rate 4.7 ml/min.
(From Ref. 49.)
400
The temperature peak maxima were found to be influenced not only
by the acid
strength but also by the number of acid sites and zeolite structure. The peaks with maximum temperatures above 600K correspond with the desorption of ammonia from
strong structural Bronsted sites whereas
low
temperature peak
is
connected with desorption of ammonia from weak Bronsted sites. Table XVIII shows the value of ammonia adsorbed on various samples of mordenite and HZSM-5 zeolites.
TABLE XVIII Si:Al ratios, the number of skeletal hydroxyl groups as calculated from the chemical composition of the zeolites, the amounts of adsorbed and desorbed ammonia obtained from the calorimetric and TPD measurements, respectively, and TPD temperature maxima for HZSM-5 zeolites and H-mordenites (491.
Zeolite
Si:Al
Number of
NH3 (inmol/gl T:(K)
AHd(kJ/molel
Calor. TPD
Strong Weak
OH groups (innlol/g1
sites
Sites
HMa
9.0
1.33
HMb
10.0
1.2s
1.08
1.11
803
145
77
HZSM-5 a
12.8
1.04
0.87
0.84
715
146
75
IZSM-5 b
13.6
1.01
0.88
723
HZSM-5 c
21.9
0.60
0.58
705
139
84
HZSM-5 d
23.2
0.54
0.52
707
HZSM-5 e
30.0
0.53
0.45
0.44
699
HZSM-5 f
40.0
0.40
0.29
0.33
688
0.12
0.10
1.12
644
HZSM-5 g
140
1.36
0.52
810
* TPD of ammonia was carried out for a zeolite weight of 0.30 g, a helium flow-rate of 4.7 ml/s and a heating rate of 21 K min. The amount of NH3 adsorbed with amount of heat of 80-70 kJ/mol is roughly the same as the amount of ammonia adsorbed on the strong Bronsted sites and accordingly, is proportional to the number of aluminium atoms in the zeolite skeleton.
The adsorption of NH3 connected with amounts of heat of 80-70
kJ/mol could reflect its interaction with NH4+ ions or basic skeletal oxygen atoms of the Si-O-Al type.
On the other hand Suzuki et al.(lll) used TPD of NH3 to characterize the acid sites of ZSM-5 zeolites. The TPD spectra of NH3 revealed the presence of two peaks at 420-470K (11 and 620-670K (II) corresponding to adsorption of weak and strong acid sites, respectively.
Impregnation of ZSM-5 with calcium
401 phosphate suppressed the strong acid sites and the total number of acid sites did not change very much as revealed by a TPD of NH3. KoJima et
al. (112) measured the amount and strength of acid sites of NaNH4
mordenite using TPD of pyridine.
The impression is that pyridine may give a
better indications of catalytically relevant acid sites which would be accesible to molecules considerably larger than ammonia. Table XIX presents the amount of pyridine adsorbed on mordenite calcined at 573K and 873K.
TABLE XIX A
Amount of Pyridine (PyI Adsorbed on Mordenite Calcined at 573K
Catalyst
Total Py
Py desorbed
Irreversibly
adsorbed
during TPD
adsorbed by
(mmol/gI
(mmol/gI (mmol/gI (X of total)
NaM
1.31
1.29
0.023
2
NH4(11INaM
1.15
1.11
0.038
NH4(55)NaM
1.05
0.86
0.19
18
NH4(64INaM
1.06
0.86
0.20
19
NH4(851NaM
1.12
0.90
0.22
20
NH4(971NaM
1.42
1.22
0.20
14
HM
0.43
0.30
0.13
30
B
3
Amount of Pyridine Adsorbed on Mordenite Calcined at 873K
Catalyst
Total Py
Py desorbed
Irreversibly
adsorbed
during TPD
adsrobed by
(mmol/g)
(mmol/gI (mmol/gI
(X of total)
NaM
0.88
0.84
0.034
4
NH4(llINaM
0.83
0.77
0.061
7
NH4(551NaM
0.57
0.38
0.17
30
NH4(641NaM
0.47
0.30
0.17
36
NH4(851NaM
0.31
0.18
0.13
42
NH4(97)NaM
0.27
0.18
0.095
35
HM
0.22
0.13
0.09
41
402 The amount of pyridine adsorbed on NaM decreased whereas the amount of pyridine adsorbed onto HM doubled as the calcination temperature was increased from 573K to 773K.
Steric factors play a dominant role in pyridine TPD from
sodium ammonium mordenite and the number of pyridine molecules adsorbed appeared to be limited by the volume of the main channel. There was only one peak in all the cases of mordenite samples which showed a different peak maximum temperature and desorption rate.
Quanzhi et al. (54) used TPD of NH3 and Pyridine on HY zeolite to characterize the acid sites of zeolite. these basic molecules on
They reported desorption activation energies of
In the case of -1 desorption of NH3, two desorption activation energies were 11.9 kJ mol and -1 43.8 kJ mol compared with the values of 13.5 kJ/mole and 43.8 kJ/mole for desorption of pyridine.
the different acidic sites.
In the case of NH3 they observed one desorption
activation energy value of 27.7 kJ/mol over the whole temperature range.
For
pyridine one activation energy of desorption for Lewis acid sites was observed as 11.6 kJ/mol.
(Ed)pyridine
<
In addition, they found that:
(Ed)NH 3
and
(Ed)Lewis acid
< (Ed)Bronsted acid
From the work reviewed, we can see that different approach is used for the analysis of TPD curves of base-type zeolite systems. The dependence on temperatures on the amount of base chemically held by the zeolite acid sites and on the apparent diffusion coefficients, has been discussed and related with the surface acid centers of Bronsted and /or Lewis type. Many authors preferred to use ammonia as a base type because may fully characterize all the acid sites; while Kojima studies used pyridine in which he believe to obtain better indication of catalytically relevant acid sites which can be accessed by molecules considerably larger than ammonia. Another advantage of using pyridine is unambiguous identificationof the nature of acid sites by means of a combined TPR-infrared analysis.
IV.2.3.3 Sorption and Diffusion in Zeolites Rees et al.
(52) reported temperature-programmeddesorption of p-xylene from
three high silica type zeolites ZSM-5, EM-11 H+/Na+ forms.
and Theta-l, in their mixed
They measured the activation energies and
desorption as a function of coverage using TPD.
entropies of
The p-xylene desorption
activation energy values for ZSM-5. ZSM-11 and Theta-l were 89, 98 and 110 kJ/mol
respectively.
The
activation energy of
desorption of
p-xylene
403
decreased with the coverage whereas the entropy of desorption increased with the coverage as shown in the figure 30.
The desorption activation energy also
decreases with an increase in the Sl/Al ratio in ZSU-5 zeolites. The sorption capacity of both ZSM-5 and ZSM-11 increased with increasing Si/Al ratios, as shown in Table XX.
TAHLEXX p-Xylene saturation capacities and Ed values (6 = 10 K min-'1 (52)
Zeolite
Amount adsorbed /molecules per
Ed _I /kJ mol
unit cell
ZSM-SC151
5.95
86
ZSM-5(451
6.80
78
ZSM-5(781
6.89
76
Silicalite-1
7.05
70
ZSM-ll(151
6.28
87
ZSM-ll(72.9)
6.9
80
Rees et al. (531 also studied desorption of n-hexane from the sodium and hydrogen forms of ZSM-5. ZSM-11 and Theta-l using TPD. TPD profile of n-hexane from Na-ZSM-5 and Na-ZSM-11 are shown
in Figure 31.
The activation energy values were 92 kJ/mol (NaZSM-51, 84 kJ/mol (NaHZSM-51, 91 kJ/mol (NaZSM-11) and 87 kJ/mol (NaHZSM-11) obtained from TPD analysis. The activation energy was found to decrease with coverage. Ed values for pure Na+ form of ZSM-5 and ZSM-11 were higher than for NaH-ZSM-5 and NaH-ZSM-11. This indicate that the electric field around each Na+
ion is adding a
significant electrostatic component to the total interaction energy. Table XXI lists the saturation capacities of all zeolites for n-hexane. The large decrease in the saturation capacity of the Na+ form over that of the H form must be the result of inefficient packing of n-hexane molecules in channel system of Theta-l.
However, the addition of Na+ into the
channel network of ZSM-5 and 11 decreases the saturation capacity slightly.
404 TABLE XXI Saturation capacities of n-hexane for various zeolites (8 = 10 K min-11 (53)
Zeolite
Amount of n-hexane adsorbed (m/u.cI
Na-Theta-l
2.30
Na-H Theta-l
2.72
H Theta-l
3.66
NaZSM-5
6.18
NaHZSM-5
6.80
NaZSM-11
6.42
NaHZSM-11
6.99
100 : ? = a0 2 G 60
.
0
I
0
1.0
2.0 oXcn&micoulcr
1.0
3.0
4.0 per unilccll
2.0 3.0 1.0 wvaapc/trolaulu pa unitcd
5.0
5.0
Figure 30. (a) Activation energy of desorption of p-xylene as a function ofcoverage. b) ActivaJtfonentropy of desorption of p-xylene as a function of coverage. 8 = 6 K min OZSM-5 o ZSM-11 and 0 Theta-l (From Ref. 52.)
405 The desorption energy was
found to be
closely related to the heat of
adsorption of n-hexane rather than to the activation energy for diffusion in these frameworks. The activation energy Ed values for ZSM-5 and 11 were higher than Ed values for Theta-l due to strong electrostatic interactions with higher Al concentrations in the framework.
1.0
1 0.5
0 -
T/K-
1.0
0
05
q
B
m q
03
0
Figure 31. (a) Temperature programmed desorption of n-hexane from NaiSM-5, I3 -1 = 10 K min ; (b) -Temperature programmed desorption of n-hexane from NaZSM-11, g = 10 K min . (From Ref. 53.1 From the results of this work it is seems that using TPD profiles the authors were able to obtain information related with peak temperature, peak width, maximum rates of adsorption and activation energies of desorption as a function of coverage. The saturation capacities of the zeolite for n-hexane were also determined.
Similarly, Fraenkel and Levy (40) used temperature-programmeddesorption to The measure the diffusion coefficient of different gases in zeolite A.
406
technique is useful when diffusion is a rate determining step rather than desorption. The diffusion coefficient was related to peak maximum temperature to a heating rate 6 and to half of the height/width of the TPD peak. The -54' effect of particle size distribution was incorporated into the TPD equation. The diffusion coefficient D was expressed as:
-Ed D = AD exp ( El
1711
The values of the diffusion coefficients of N2, Ar and CO2 in K-A zeolite were calculated and given by equations 17.21to [741.
N2 Ar
co2
2 D(F)
=
1.27 x lo-' exp (-%I
[721
2 D(T)
=
0.796 x lo-' exp (-%I
[731
2 D(F)
=
0.496 x lo-' exp ( *I
[741
The important conclusion from this work is that TPD demonstrated again that is a powerful tool in studying the kinetics of gas diffusion in zeolites. Then, employing its peak-shape expressions makes the derivation of
activation
parameters for diffusion much easier, and usually more reliable than using another adsorption technique.
The application of temperature-programmedanalysis is not limited to merely determinating of the state of the metal ion in the zeolite or acid size distribution, but
it is also a powerful tool in studying the sorption of
hydrocarbons in zeolite as well as determination of diffusivity of gases in the zeolite.
IV.3
Temperature Programmed Surface Reaction (TPSR)
Temperature-programmedsurface reactions (TPSRI referred to study of reactions under temperature programming.
In this type of
study, two gases are
coadsorbed. usually sequentially, on the catalyst and heating is done in a inert carrier. Another possibility is when a reactive carrier gas is used and the "adsorbed" species is the result of decomposition of another molecule; the "adsorbed" species cannot desorb under the conditions employed. A wealth of information can be obtained from TPSR experiments if all the reaction
407 products are detected.
Several sites of different activity can exist on the
catalyst surface which can increase or decrease with change in catalyst properties.
Such changes cannot be
predicted by
steady state kinetic
measurements and may be observed using temperature-programmedanalysis. This approach is very useful for the study of reactions such as hydrogenation, oxidation, methanation. sulfidation and carburization.
IV.3.1 Hvdronenation
CO
and CO hydrogenation on supported metal catalysts using 2 temperature-programminghas been reported by Zagli et al. (291, McCarty and
Wise (113) and Low and Bell (114).
They observed CH4 peaks at different
temperatures for different catalysts.
TPSR was also used for studying the
mechanism of CH4-formation under different initial coverage of CO or CO2 on the catalyst surface. Different catalysts were compared for their activities for hydrogenation of CO or C4J2. When using
and observing peak temperatures
of CH4-formation it is possible to study:
support structure sensitivity
influence of reaction product (HZ01 on methane formation.
kinet'icparameters of the reaction like order and activation energy.
rate determining step of the reaction.
Hydrogenation of CO over a nickel catalyst has been studied recently by Vandervell and Bouker (115) using TPSR.
The objective of the study was to
clarify the reaction mechanism i.e. whether methanation proceeds with water formation or CO formation as given below:
(a) H20 formation : CO + 3H2 ----f CH4 + H20
[751
(b) CO2 formation : 2C0 + 2H2 -
[761
CH4 + CO2
Figure 32 shows the results of the methanation experiment with a fully reduced nickel catalyst for two different levels of CO in the feed.
For low CO, gas
408
High
CO CO:H,=1’1.56
co
w3amu) /-/
\
L-
C0,144amu) --------___
H,O(lBamub ._.-.-. -.-
,._‘_
Heahng Begun
Temperature
Low
:. :.:: : : :
(K)
co co. ii,=,
: : \
-.-.-
24
I j\
:
1. ,
supported over the synthesis Temperature programmed Figure 32. nickelcatalyst. Flow rate: 40 cc/min at 1 bar pressure, gas composition: 4% CO in Hydrogen. (From Ref. 115.1
409
the methanation reaction takes off at 460K, reaches a maximum at -490K and finally settles down at a temperature invariant rate at 520K. This correspond with complete conversion of H20 formation as per scheme (a). For high CC gas experiments, the situation is quite different.
Methanation begins later at
around 510K and shows only a slight maximum at 550K.
The main oxidised
product is CO2 with only minor production of HzO, showing that reaction (bl is now
the dominant process.
The
TPSR
study
further
supports
thus
the
dissociative mechanism
methanation i.e. carbon atoms are directly hydrogenated to methane.
for The
results also show that there is a competition between hydrogen and CO for 2+ adsorbed/dissociated oxygen atoms. ions which act as The role of Ca promoters for the methanation reaction of a modified Ni surface would produce CO2 as a major oxidised product during the methanation reaction.
Schwarz and Huang (116) also studied CO hydrogenation over Ni/A1203 catalyst using TPSR to examine the relationship between catalyst preparation procedures and the structure, dispersion, activity and selectivity of the catalyst. They used TPD of H2 and a temperature-programmed reaction of CO to obtain the structural information related to the accessibility of the surface nickel sites to H2 and CO .
TPSR was used to assess the amount of carbon containing
residues left on the catalyst surface after steady state CO hydrogenation. Three different types of carbon have been observed for Ni/A1203 catalysts (McCarty &
Wise,
(113)).
These surface carbon species are
formed by
dissociation of CO at elevated temperatures and readily produce methane upon subsequent exposure to hydrogen.
Information about carbon containing species
on the surface can be used for catalyst design, regeneration or to obtain more insight into the methanation mechanism. For low-weight loading catalysts, two types of carbon-containing species were observed whereas at higher loadings three types of carbon-containingspecies are left on the surface. Table XXII summarizes the peak temperatures and the total carbon deposition for each catalyst. The average number of carbon-containing residues was about 4C/Ni, independent of weight loading.
Perhaps the most significant conclusion of this work was, meanwhile TPD of H2 and TPR of CO provide structural information related to the accessibility of the surface nickel, sites for H2 and CO adsorption; the TPSR demonstrated the difference in activity and selectivity for the different catalysts under study.
410
TABLE XXII Summary of TPSR results (1161
Peak temperature
Nickel weight
Total carbon depostion
loading/%
-1 /1O-4 mole (g catalyst1
/K
0.8
4.9
457
621
-
1.2
5.1
478
639
2.1
5.1
463
643
-
3.0
5.2
447
588
670
3.5
4.2
456
587
661
4.1
4.8
442
590
654
6.6
5.1
454
667 >7OO(shoulderl
7.3
5.5
452
664 >7OO(shoulderl
8.3
6.0
457
653 >7OO(shoulder
Recently, Falconer and Sen (1171 used TPSR along with TPD and TPR of H2 and CO on a Ru/A1203 catalyst for CO hydrogenation. They identified the existence of two distinct reaction sites for CO hydrogenation on Ru/A1203 catalyst similar to Ni/A1203 catalyst. The first peak observed was due to hydrogenation of
CO
adsorbed on Ru and the second peak resulted from the decomposition of the CO-H complex on alumina resulting in the formation of methane. The presence of on Ni/AlzOs is probably not required for two reaction sites to be present. A CO-H species with methoxy stoichiometry adsorbs on alumina probably at the Ru/AlzOs interface.
On another work, Margitfalvi et al. (118) used TPSR to study the formation of benzene from n-hexane over Pt/A1203 catalyst.
They treated the catalyst in
hydrogen gas at different temperatures prior to reaction. Upon increasing the temperature of the hydrogen treatment the peaks of benzene formed shifted to higher temperatures also. This phenomenum is due to reduced mobility of the overall hydrogen pool at higher temperatures and consequently suppressing the hydrogen consumption in the conversion of n-hexane.
Figure 33 shows the
results of the TPSR experiment. The shift of the benzene peak was about 350 K upon increasing the temperature of the hydrogen treatment from 673 to 773 K.
411
X0
200 t("cl
300
Figure 33. Formation of benzene from n-hexane studied by TPR. The shift of benzene peaks upon applying high temperature hydrogen treatment (H21. Temperature of H2 : O-400°C, x-500°C,
q-575'C
catalyst : 0.1 g Pt/A1203,
treated in oxygen at 4OO'C before final reduction. (From Ref. 118.1
IV.3.2 Methanation
The technique of temperature-programmedmethanation (TPM) is very similar to that of TPR.
The H2/Ar reduction gas used in TPR is replaced by a H2/C02
cl:91 mixture. Hydrogen is consumed according to the reaction: co2 + 4H2 +
CH4 + 2H2C
[771
Accordingly, the detector responds directly to the extent to which the reaction takes place. Typical TPM curves are shown in Figure 34 with 8 = 10 30K mix-r-1 and the final temperature need not exceed 873K (1191.
The shape of the curve as shown in Figure 34 (curve A1 is typical for a freshly-reduced unsintered catalyst where the catalytic activity is very high. After sintering. all catalysts lose their activity to some extent. shown by curves B-D.
This is
Then TPM may be used to obtain information on activation
energies and hydrogen consumption.
412 From this information
catalysts that achieve this value at 623 K or less are highly active those requiring a temperature of up to 673 K have an adequate activity
those requiring over 773 K are unlikely to be
sufficiently active for
commercial use. 3000
----___
Theoretical -.-N,
f
maximum
Theoreticalmaximum
- _ _ - . A
B
/\
400
200
600
TemperaturePC
TemperaturePC
Figure 34. Typical Temperature programmed methanation curves for nickel catalysts. A. Freshly reduced, unsintered catalyst B,C,D, different catalysts previously exposed to sintering conditions. (From Ref. 119.1
It will thus be clear that temperature-programmedmethanation techniques are are
a
useful
tool
for:
accelerating the work of
evaluating methanation catalyst activity, and developing catalysts that are
aimed at
being
commercially beneficial.
IV.3.3 Sulnhidation
Ni-Mo/A12/03 and
Co-Mo/A12/03 are
a
important class
of
catalysts for
hydrodesulphurization (HDS). The reactivity of these catalysts is correlated with the HDS activity of presulphided catalysts. For example, catalysts which can be reduce at lowest temperatures show the highest activities. Studies on the conversion of oxides into sulphides and the structure of sulphides is limited. Sulphiding is a necessary pretreating step for HDS catalysts in order to
limit
coke
formation.
The
temperature-programmed sulphiding
(TPSI
technique has been applied to investigate the reactivity of oxidic Mo/A12/03
413
catalyst by Arnoldy et
al.
(621.
In TPS, the gas mixture used for sulphiding
contains 3.3% H2S. 28.1% H2 and 68.6% Ar.
TPS is a sensitive technique for
describing sulphiding reactions in detail.
It gives information on
the
sulphiding rate and mechanism as a function of temperature. Sulphiding of Mo03/A1303 takes place at low temperatures when compared with bulk compounds (Mo03, Mo021.
The
sulphiding mechanism is dominated by
O-S
exchange
reactions. Elemental sulphur is formed by the rupture of metal sulphide bonds and is reduced subsequently by H2. influences sulphiding drastically.
The HZ0 content of Wet
the catalysts
catalysts sulphide at
very
low
temperatures (typically 400-500K1, while dried catalyst sulphide at much higher temperatures (typically 600-700Kl. sulphiding more than predrying. place at very
Further prereduction hinders
Sulphiding of Mo03/A1203 catalysts takes
low temperatures, in
comparison with
reduction of
these
catalysts in temperature-programmedreduction. A 4.5 atoms/nm2 catalyst can be sulphided completely even at ca 500K by selecting a sufficiently low heating rate. The S/MO ratio after TPS is always below 2 (1.4 - 1.9 depending on MO content) pointing to limited formation of MoS2.
An increase in MO
content leads to sulphiding at a somewhat lower temperature. However. the influence of
MO
content on
reduction (TPR) is
much
more
Sulphiding appears to be complete at Ca 1lOOK in all cases.
significant.
The product of
sulphiding up to Ca 500K might well be a monolayer similar to the oxidic precursor.
Thus it can be seen that TPS can be used to detect the reactivities of all the MO species because of different reactive catalytic sites sulphiding at different rates.
On another work temperature-programmedsulphiding (TPS) has been applied to study the sulphiding of oxidic ReaO,/AlaOscatalysts in a HaS/Ha medium (120). It was found that predominating oxidic Re+' monolayer species sulphide easily. The sulphiding temperature of these species is influenced significantly by their water content.
In that case wet sample sulphide around 400 K, whereas
dry ones sulphide already extensively at room temperature. Strongly adsorbed Ha0 probably prevents HaS adsorption and therefore sulphiding. At the Ha/Has pressure ratio applied
(c.a. 8.51,
the ultimate sulphiding product of
crystalline Re compounds depends on the sulphiding temperature. ReSa and Re metal are thermodynamically favoured below and above c.a. 950 K respectively but are pinned slowly and incomplete due to the hindrance of diffusion through dense product shells. results in
the
Sulphiding of (un) supported crystalline NHIRe04
formation of
surrounded by a Re Sa shell.
extreme well-dispersed Re
metal
particles
Microporosity of this shell wireless diffusion
414
of HsS. but not of Hz, leading to much higher Hs/HaS pressure ratios in the interior of the particles than the ratio in the bulk SQa phase, and therefore, to thermodynamic stability of Re metal far below 950
K.
IV.3.4 Oxidation
TPSR has been used to characterize bismuth molybdate catalyst for oxidation of propylene by Uda et al,(81.
The redox properties of ;r-Bi2Mo06catalyst was In this technique the
examined using temperature programmed reoxidation.
catalyst was reduced to predetermined degree of reduction and subject to reducing agent containing reactant e.g. 20% propylene in ultrapure nitrogen. The catalyst is reduced by reducing agent above 673
K.
After reduction the
catalyst is subjected to oxidation under temperature programming.
A typical
TPSR curve of the T-phase catalyst prereduced by propylene is shown in Figure 35. Two peaks are observed. The maximum of the low temperature peak occurs at 431
K
and the maximum of the high temperature peak occurs at 513 K.
The
activation energy for low temperature peak was determined to be 122 KJ/mole and the activation energy for the high temperature peak was determined to be 265 KJ/mole.
Figure 35. TPR chromatogram for 3% reduced r-phase by C3H6 at 420'C. (8 = 6OC min-'1.
(From Ref. 8.)
The
low temperature reoxidation peak was found to be a result of the 4+ reoxidation of MO to Mo6+ and Bi" to Bim+. where 0 C m < 3. The high temperature reoxidation peak was found to be result of reoxidation of Bim+ to
Bi3+.
The high temperature reoxidation process also appears to be related to
415
the rate limiting step for propylene oxidation to acrolein at temperatures below 673 K.
Farneth et a1.(30) studied partial oxidation of methanol over Moo3 using TPSR. The catalyst sample was exposed to methanol and TPSR spectra were obtained as depicted in Figure 36 .
The spectra show two distinct desorption bands.
The
low-temperature feature (Tmax = 393 K) is dominated by loss of methanol still intact and the high-temperaturepeak (Tmax = 493 K) is composed of CH20, CH30H and H20.
The second peak was found to be independent of coverage and
following first order kinetics with an apparent activation energy of 20.6 kcal/mol.
A
reaction mechanism of
methanol oxidation based
the
on
experimental findings was suggested.
0
m
64
90
I20
150
la0
2lo
240
1?0
so0
TfhufRrruRE (C)
Figure 36. Mass spectral intensity profiles for CH30H (m/e 32). CH20 (m/e 30) and H20 (m/e 18) during TPD.
(From Ref. 30.1
Anderson (321 studied oxidation and ammoxidation of 3-picoline over VzOs catalyst using TPSR. His results were related to the surface-structureof the catalyst deduced on
the basis of
metal-oxygen bond
strength. The
TPSR
technique has been useful to study the basic steps of the catalytic reactions. Three different sites for the formation of nitril over the catalyst were identified and a mechanism of ammoxidation of 3-plcoline uas proposed. Most of
416 the nitrite is formed in a reaction between:
- chemisorbed 3-picoline and weakly adsorbed or gaseous NHa.
- chemisorbed 3-picoline and NH3 adsorbed at a vanadyl oxygen vacancy.
- chemisorbed 3-picoline and NH3 more strongly adsorbed in the form of either -NH2 or =NH group.
IV.3.5 Other reactions
Tagawa et
al.
(581 reported methanol synthesis from CO2 and H2 over Cu, Zn,
chromium and aluminum oxide catalysts using TPSR.
Temperature-programmed
desorption carried out after the reaction on copper-containing catalysts gave peak
characteristics of
intermediate of
the
copper
reaction.
formate
which
was
identified as
different catalysts studied
The
the were
CuO/ZnO/Cr203, Cr203/A1203 and G-66A, a commercial catalyst. Figure 37 shows a TPD spectrum obtained with A1203 after the C02+HZ reaction was carried out at 623
K.
A1203 showed no activity for the synthesis reaction.
TPD z ii 5 c) .---_
5
50
I 100
!
I 150
200 TEMPERATURE
I 250
I 300
I 35
(7~)
Figure 37. Temperature programmed desorption on A1203 after CO2 and H2 reaction. (From Ref. 58.1
Totally different spectra of TPD were observed on the active catalyst 30% CuO/A1203 as represented in Figure 38. peak maximum at 473-483 K
A large peak of CO2 appeared with a
followed by a broad second peak at a higher
temperature. While a small peak of methanol appeared at 443 K, the desorption of CO increased consistently with temperature and no maximum was observed up
417 to 623 K at which the TPD was stopped. Figure 38 also shows TPD spectra of commercial catalyst G-66A.
The existence of for-mateduring C02+H2 reaction
over catalyst G-66A was detected by TPSR rather than by infrared spectroscopy due to its complete opaqueness. Thus TPSR proved to be a useful technique for detecting intermediates on the catalyst surface which cannot be detected by IR spectroscopy.
50
100
150
200
250
TEMPERATURE
300
350
PC,
Figure 38. TPD after CO2 + H2 reaction on (al G-66A and (bl 30% CuO/A1203 catalysts. (From Ref. 58.)
Forzatti et a1.(511 studied deactivation kinetics of fresh and sodium-poisoned r-A1203 catalyst for dehydration of ethanol and methanol using TPSR. The TPSR curve analysis technique was able to provide the complete energy and kinetic picture of the reacting system. This can in effect be related to deactivation if any of it is due to catalyst poisoning.
TPSR spectra of methanol and
ethanol from fresh and Na-poisoned r-A1203 are shown in Figure 39 and 40 respectively. The fresh catalyst shows two peaks for methanol as well as ethanol desorption. Two peaks are related to the heterogeniety of the ;r-A1203 catalyst surface with Bronsted and Lewis sites of different acidic strengths. These show the interaction of the alcohol hydroxyl with either a surface hydroxyl or Lewis site and an oxygen pair. Impregnation of the catalyst with sodium indicates a .preferential poisoning of
the more acidic Lewis and
Bronsted sites with higher energies of activation. The first peak is reduced in size whereas the second peak
(490K)
almost disappears when the catalyst is
418
In the case of ethanol desorption. Na enhances the amount of
poisoned.
ethylene desorbed
and
shifts
the
ethylene desorption peak
to
higher
temperatures.In case of methanol desorption, the value of desorption-energy Ed ranges approximately from 70 to 115 J/mol over the range 0.2 C 9 < 0.85 owing to the existence of non-uniform poisoning whereas for ethanol desorption Ed values range from 80-120 J/mole over the range 0.2 < 9 < 0.8. CXlO"
tishm’l
1 (Kl
Figure 39. Thermal desorption curves of methanol from r-A1203. Poison level 0 = 0% Na;
level 1 = 0.8% Na, level 2 = 1.5% Na.
(From Ref. 51.1
T IKI
Figure 40 Thermal desorption curves of ethanol and ethylene from r-A1203 poison level 0 = 0% Na.
----- poison level 2 = 1.5% Na.
(From Ref. 51.)
419
Here, it is worth to point out that the TPSR has proven to be a suitable experimental tool for investigating the separability of reaction-deactivation kinetics, as it is able in practice to provide a complete energetic and kinetics description on nonideal heterogeneous catalytic surfaces, allowing for a direct study on the effect of poisoning processes upon the separability of the rate form.
Although most of the conditions strictly refers to the system referred here, it is possible to achieve rationalization in terms of chemical evidence, so that it is reasonable to expect similar results when other reacting system exhibiting similar chemical behavior is considered.
In conclusion the technique of TPSR is very useful for the study of rate of reactions on the different sites which is generally not available from the more conventional steady-state measurements
IV.4 Temperature-ProgrammedGasification and Carburization
IV.4.1 Gasification
Carbon deposition on catalysts is an important problem both in industrial and fundamental research. Deactivation or even fragmentation of the catalyst can take place, and as a consequence regeneration or replacement is necessary. Therefore, much effort is applied to developing coke-resistant catalysts. order to get a better understanding of
the mechanism of
In
coke deposition,
different techniques are needed to characterize the deposited carbon.
One of
these
called
is
temperature-programmed gasification,
(TPG)
with
H2
temperature-programmed hydrogenation (TPH) or with 02 called temperatureprogrammed oxidation (TPO).
McCarthy et al. (1211 used temperature-programmedhydrogenation (TPH) for study the reactivities of the different form of carbon deposits on nickel catalysts for forming hydrogen.
They proposed the existence of seven types of carbon
after treatment of the catalyst with CO or ethylene: C-u and C-a, atomic carbon; C-S, polymeric carbon; C-r, metal carbide; C-6 and C-6, filamentous carbon; C-c, graphic carbon.
Figueiredo
(1221 shows
results concerning the
temperature-programmed
gasification (TPGl of carbon on commercial Ni/Al2OS, CO-MO, Si02-A1203 and Pt/A1203 catalysts. Different patterns were found in which gasification of carbon is a function of metal-carbon contact and carbon morphology.
420
Temperature-programmed gasification with 02, CO2 and H2 was used for the characterization of carbon deposits on NWA1203
and CO/A1203, based on the The
difference in reactivity of the several types of deposits (1231.
gasification patterns show the existence of mainly two types of carbon One form is attributed to filamentous carbon, the second form is
deposits.
believed to have a more graphitic structure.
The TPG patterns are strongly
influenced by the oxidation state and the catalytic activity of the metal on the gasification of carbon.
In recent work the same authors (1241 used TPO for the determination of the amount
and
the
chemical
hydrotreating catalysts.
state
of
carbon
and
sulfur
on
deactivated
From Figure 41 it can be seen that supported MoS2
oxidizes at considerable lower temperatures (Tmax = 6OOKl than bulk MoS2 (Tmax = 85OKl.
This is probably caused by limitations towards diffusion.
In the
case of bulk MoS2, a surface metal oxide layer might be formed in the early stage of oxidation. From the TPO patterns of Figure 41, it can be seen that for increasing Co-loading, an additional maximum emerges, so peak III is attributed to a Co-sulfide phase.
The maximum in the oxidation pattern of
MO-sulfide shifts to a lower temperature with increasing Co or Ni loadings. This agrees with the explanation that a mixed phase is present, although a catalytic effect of CoS on oxygen dissociation is not excluded. The patterns of the spent catalyts (Figure 421 clearly show a low and a high temperature SO2 production maximum.
In Table XXIII, the temperatures of the SO2 maxima of the spent catalysts and sulphided catalysts are listed.
TABLE XXIII Peak temperatures (SO2 production) of sulBhided and spent catalysts (124
Catalyst
Mo(10.41
Peak I
sulfided
Peak II
Peak III
600 K
Co(2.OlMo(10.41
sulfided
520 K
700 K
C0(4.01M0(10.41
sulfided
510 K
600-700 K
Ni(2.11Mo(15.51
sulfided
600 K
650-750 K
Co(3.8lMo(15.21
spent
540 K
750 K
Ni(2.11Mo(15.51
spent
580 K
730 K
765 K
421
Figure 41. (a) Temperature programmed oxidation (TPO) patterns of sulfided MOIA1203. CoMO/A1203 and NiMO/A1203. (b) TPO sulfided
patterns
(SO2 production) of
MO/Al203, CoMO/Al203 and NiMO/Al203. (From Ref. 124.)
-SO*
I
600 temperature
I
700 (K)
Figure 42. TPO patterns (CO2 and SO2 production) of spent CO (15.2)/A1203and Ni (2.1) MO (15.5)/A1203. (From Ref. 124.)
(3.8) MO
422 Quantifying the data of TPO of spent catalysts showed that much more 02 was consumed that could be accounted for by adding up the 02 used for SO2 for CO2 production and oxidation of the metal to metal oxide.
Oxidation of the major part of Ni or Co sulphide coincides with the oxidation of the carbon deposit.
The reactivity of the carbon deposit is low, despite
the fact that the deposit contains a high amount of hydrogen.
The catalytic
effect of a metal can be a significant factor which determines the temperature at which the carbon reacts with 02.
As a consequence a comparison between
carbon deposits, merely based on oxidation temperatures can lead to erroneous conclusions.
The oxidative regeneration of cobalt-molybdate catalyst was also studied by Yoshimura and Furimsky (125) using temperature-programmedgasification with air. SO2. CO and CO2 were analysed as the major products (Figure 43). these results, a
burn-off model for
From
spent hydrotreatment catalysts was
proposed.
Figure
43.Product distribution from
catalysts in air 0
SO2
m co 2'
temperature programmed burn-off of
(From Ref. 125.)
423 To understand the importance of deactivation and regeneration of naphtha reforming, the location composition and structure of carbon deposits on the bifunctional catalyst needs
to
be
clarified.
Barbier
(1261 studied
coke-gasification using a temperature-programmedgasification technique with oxygen.
The TPG of coked catalysts shows two oxidation peaks, one at around
573 K and the other at around 7.23K.
These two peaks are particularly well
defined when platinum is supported on non-acidic and non-microporous alumina. Nevertheless, if such is the case, a low oxygen pressure during the TPG allows a good resolution of the two peaks (Figure 441.
Figure 44
TPO
of
coked Pt/A1203 catalyst (a) CO2
production, (b) 02
consumption. (From Ref. 126.)
A study of these peaks shifting with increasing temperature shows that the activation energy of coke combustion is equal to 10 kcal for the first peak and 15 kcal for the second.
Many materials are known to catalyze the gasification of coke, and these include several metals.
So it has been found that the low-temperature
combustion is due to the presence of coke on the metallic phase (127-128). Proof of this is clear when Pt/Si02 catalyst and pure alumina are coked
424
together as a mixture; then analyzed separately by TPG experiments.
Coke
deposited on Pt can be oxidized at 533 K. whereas coke deposited on alumina
is
Such a difference can be
oxidized at 823 K, as can be seen in Figure 45.
explained by assuming either that platinum catalyzes the oxidation of carbon or that coke deposited on the metal is different from the coke deposited on the alumina.
I
It I /
\
\
\
\ \
Figure 45. TPO of coke (a) deposited on Pt, (b) deposited on A1203.
(From
Ref. 128.
The influence of the platinum metal on the burning temperature of the coke produced during pure-hydrocarbon reforming was studied using TPO by Parera et al. (129). They found that the differences in the coke oxidation temperatures could be due to the nature of the coke formed when using different catalysts not being the same.
A lower coke burning temperature can be explained by the
fact that, when higher amounts of Pt are present, coke can be polymerized less because of the hydrogenation of coke precursors produced during the reaction and catalyzed by Pt.
Similarly, Parera et al.
(130)
using TPO studied the evidence of spillover
phenomena on reforming catalysts.
By the TPO they proved the presence of
carbon on the support and measured the temperature of oxidation of that
425 carbon.
The higher the required oxidation temperatures, the more graphltlc
the carbon becomes.
In a series of temperature-programmed hydrogenations
(TPH), it was found that Al203 that was coked while passing naphtha over it for a short time, produced a coke that could be partially eliminated by hydrogenation and completely eliminated if hydrogenation was performed in the presence of Pt.
The experiments indicated that hydrogen spillover from Pt
increased the rate of coke removal by the hydrogenolysls of coke.
In another
set of TPO experiments, it was found that coked alumina oxidized at lower temperatures when mixed with Pt/A1203 catalyst. The explanation is that Pt causes oxygen to dissociate and migrate lnto coked A1203. thus oxidizing coke at a much lower temperature than molecular oxygen. would.
This series of TPO
and TPH experiments indicates that hydrogen splllover is playing an important role in naphtha reforming or coke removal.
The effect of a second metal added to platinum on coke gasification was examined using TPO by Beltramini and Trlnun (131) on coked catalysts.
Figure
46, shows that coke on Pt/A1203 produces a twin peak combustion pattern.
On
the Pt-Ir/A1203 catalysts however, TPO showed only one peak in which the maximum decreases as the amount of iridium increases.
T mar A
39L
B
L72 363
c
L15
100
f
1°C)
A
c
200
300
LOO
T PC) Figure 46. Temperature programmed oxidation of coke catalysts. (From Ref. 131)
426
The amount of coke deposited on the two metals, as judged by the area of the thermograms, appeared to be about the same, but since TPO is a measure of the heat released by coke deposition, the areas are not in the same proportion when compared with the peak produced by combustion of coke on alumina.
This
is
more
because
the
presence of
the
metal
during
coking produces
a
hydrogenated coke that burns at lower temperatures with a higher heat of combustion than coke deposited on A1203-Cl.
The gasification of coke on the
support by oxygen adsorbed and spilt over from the metal seems to be another good explanation.
The effect of coke on different bimetallic systems was Beltramini (132).
studied by
Coke on Pt/A1203 and Pt-Ir/A1203 oxidizes at
lower
temperatures (Figure 471. An increase in the metal content produce a decrease in the maximum temperature peak burning.
On Pt-Re/A1203, Pt-Ge/A1203 and
Pt-Sn/A1203 most of the coke is formed and localized on the alumina support and burnt at a much higher temperature.
IV.4.2 Carburization
The role of carbides is still not clear in the synthesis catalysts (1331 and a number of studies concerning the effects of support interaction;
alkali
promotion and alloying have apparently not taken the carburization effect into account.
The formation of carbide on a series of Fe/SiO2, Ni/SiO2 and 4Fe-Ni/SiO 2 catalysts has been investigated in a temperature programmed carburization (TPCl experiment in the range of 298-873 employed as reaction gases.
K
(1341. Both CO and 3H2:C0 were
With the exception of Ni/Si02 in 3H2:C0, carbide
formation was observed in all instances.
In the bimetallic system the
mixed-metal carbide formed (H2C to M3C) was stabilized to about 773 K (Figure 481. In the 3H2:C0 the rates of carbide formation and coking on 4Fe:Ni/Si02 are both enhanced relative to
Fe/Si02.
The activation energies for the carburization reaction were determined using the standard programmed analysis (1351. The TPC analysis results are reported in Table XXIV.
No activation energies were determined from the TPC in the coking regimes, since the temperature-programmedanalysis requires a finite reaction-product relationship. In addition, the temperature ranges here are well beyond those of interest in synthesis reaction applications.
A surface reaction model
“I- P!-Ir
-G
m Pt
.
-50
Figure 47. Ref. 132. )
-+I
Temperature programmed oxidation
-1
I
TEMPERATURE. -A
-cl
Q -cc
of bimetallic
catalysts.
(From
428 accounting for the observed carburization behaviour and which correspond with the activation energy measurements was also proposed.
0
200
4GO
600
800
0 m
TEHPER4TUPL(“Cl
2G0300403500600 TW'ERATW
7CKl
(OC)
Figure 48. Temperature Programmed Carburization (TPC) of Fe-Ni system in (al CO, (bl 3H2:CO. (From Ref. 134.1
TABLE XXIV Activation Energies for the Carburization Reaction (134)
Catalyst
Reaction
E (kcal/moll
120 f 20
Fe/Si02
Fe + CO -+ FexC Fe + CO + H2 -+ FexC
14+
4
Ni/Si02
Ni + CO -+ Ni3C
355
2
4Fe:Ni/Si02
4Fe:Ni + CO -+ 14Fe:NilxC
1st
7
4Fe:Ni + CO + H2 -+ (4Fe:NilxC
222
2
429
In general, temperature programmed gasification (TPC) furnishes information on the total amount of carbon, the type of carbon deposits and on the oxidation state of the metal.
Depending on the specific interest. one will choose the
appropriate combination of gasifying agents.
TPH is the most sensitive
technique but H2 is the least reactive gasifying agent.
This combination
makes TPH suitable for studying the reactive forms of carbon deposits, because small differences in reactivity can still be detected. detection of
less reactive forms of
On the other hand,
carbon deposit will
need
a
gasification temperature leading to structural changes of the catalyst.
high TPC
will find its application in the study of coke-formation and regeneration effects on catalysts. At relatively low temperatures all carbon deposits are gasified. and thus high-temperature induced changes of the catalysts avoided. This is
a
necessary condition for regeneration studies. A disadvantage of TPC
is the highly exothermlc character of the CO2 reaction, causing a temperature rise during gasification and as a different carbon deposits.
consequence a
Better results
bad
resolution of
the
are sometimes obtained by TPG
(C021. because the reaction is endothermic and the oxldatln of the metal during the process improves the resolution of carbon deposits.
V. CONCLUSION AND PUTURE APPLICATIONS
We have presented a summary of the procedures for obtaining and evaluating the experimental data
from
a
temperature-programmed technique with
special
application to a catalytic system.
We have also presented convincing evidence that this method
has already been
succesful for catalyst characterization, understanding reaction mechanism, determining kinetic data, rapid evaluation of catalyst activity, selectivity and stability and measuring surface area of adsorptlon/desorption.
There are of course some disadvantages using these techniques. They arise from both fundamental and practical considerations. From a fundamental point of view, the disadvantages come' from the inability to interpret the experimental results, resulting sometimes in an incorrect evalutlon of phenomenas like diffusional transport process,
readsorption, multiple bonding,
etc.
In
practice the problems are principally due to wrong experimental conditions design that not correspond with real industrial conditions in which the catalyst is working.
But comparing advantages and disadvantages, we can say that if proper care is taken
during
experimental design
and
quantitative
treatment
of
the
430 temperature-programmed technique, then this technique offers a versatile, cheap and precise tool for both fundamental and applies catalytic studies. In general the advantages appears to outweight the disadvantages.
It is our opinion that temperature programmed analysis will continue to find increasing use at both University and industrial research laboratories in the area of:
catalyst characterization like heterogeniety of the surface, acidity, activity, and coke formation.
as a diagnostic tool for large scale catalyst manufacture, qualitiy control and regeneration.
characterization of corrosion layers, reducible species in solids such as coal ash.
different reaction modes e.g. in monitoring the activity of catalysts for reduction reactions such as hydrogenations, activity of
catalysts
under sulphidation or poisoning, activity of catalyst for particular reaction such as oxidation, methanation, carburization or ammonination.
study of separability of reaction-deactivationkinetics.
sorption and diffusion in zeolites.
TPD and TPR at low pressures to understand readsorption.
TPR and TPD at high pressure to understand catalytic pretreatment and catalyst deactivation.
theoretical modelling of adsorbate distributions within the catalyst bed during reactions.
combining
in
situ
temperatures-programmed techniques
spectroscopic techniques such as
with
other
Mossbauer and infrared
Understanding reaction mechanisms and modelling on single crystals and metal supported catalysts.
431 Temperature-programmed apparatus Various for
with types
into
temperature
involved
information
with
by
that
photoelectron
spectroscopy,
etc.
conclusion,
tremendous techniques.
this
growth In our
pace to ever wider
the
by
that interest
opinion fields
catalyst
using
this
covered
from instrument
another
work
interest
of application.
will
of
analysis
of
spread
research
since
The
is
such
as
now x-ray
Mossbauer
microscopy,
done
(71.
requirements.
part
and catalysis.
techniques
electron
and application
a single
manufacturers
to suit
development
the
into
mass spectrometer
has become an integral
spectroscopy,
review
in
available
integrated
a
temperature-programmed
found
diffraction,
like
programming apparatus
analysis
in the work of
provided
complemented
have been
system
systems are
temperature-programmed
laboratories
In
techniques
detection
of detection
incorporation
Today,
analysis
a universal
1983,
shows
temperature-programmed at
an ever
increasing
a
432 NOMENCLATURE A
Preexponential factor for adsorption equilibrium constant
A'
Preexponential factor of solid gas reaction
Aa
Preexponential factor of adsorption constant Preexponential factor of desorption constant
2 AD AP C g (Cg)M (Cg)n =go cs E AC g De Do DP DB E Ea Ed Ed' ED Ei EP
Preexponential factor of chemical reaction -1 Preexponential factor for diffusion from subsurface, s Preexponential factor for penetration of adsorbate Gas phase concentration, mol/m3 Gas phase concentration at peak maximum temperature, mol/m3 Normalized gas phase concentration, dimensionless Initial gas phase concentration. mol/m3 Surface concentration of gas, mol/mJ Mean hydrogen concentration at the temperature of the 3 maximum reduction rate, mol/m Change in gas concentration, mol/m3 2 Effective diffusion coefficient (El Preexponential factor of diffusion coefficient Particle diffusion coefficient Bed dispersion coefficient Activation Energy, kJ/mol Activation Energy for adsorption, kJ/mol Activation Energy for desorption, kJ/mol Apparent activation energy for desorption. kJ/mol Activation energy for diffusion from subsurface to surface, kJ/mol Activation energy for reduction of species i, kJ/mol Activation energy for penetration of adsorbate into subsurface region, kJ/mol
En F1
Normalized activation energy, dimensionless Specific flow rate of the reacting mixture, m3/s.kg -4 Carrier gas volumetric flow rate at STP (1 atm. 273 K),> 3
F. F
Carrier gas volumetric flow rate (temperaturedependent), E
f(e)
General form of adsorption dependence on coverage
g(B)
General form of desorption dependence on coverage
AHd(e)
Heat of adsorption, kJ/mol -1
k
Reaction rate constant, s
ka
m3 Adsorption rate constant, m. -! Desorption rate constant, s
kd kD
Rate constant for diffusion from subsurface to surface, s-1
433
k
eff
Effective desorption rate constant, s-1 Rate constant for nucleation process
kn k P
Rate constant for penetration of adsorbate into subsurface region,
1
Length of catalyst pores, m
L
Width of catalyst slab, m
S-l
M
Adsorbate released from the zeolite crystals during time interval m mol At, g Ratio of total number of subsurface sites to surface sites
MAt
n
Reaction order for desorption
n'
Apparent desorption order
N
Net desorption rate (s-'1
NB
Total number of subsurface sites, mol Maximum desorption rate, s-1
NP NC NS P
Number of particles in the bed Total number of surface sites, mol
Aq
Reaction order for adsorption (RT l/2 (m 5%' set' Amount of adsorbate held on the surface
Qo>Qt.Q,
Adsorbate present in the zeolite pores at t = 0, t, and m (Fl
r
Order of reaction with respect to gas phase
P
r
Radius of the spherical particle, cm
0
Observed rate of reaction
r
Ohs R Ra Rd
Gas constant IJ/mol.K) Rate of adsorption (s-l1 -1 Rate of desorption (s 1 Radius of crystal, m
RC
Order of reaction with respect to solid (species on the surface)
S
Sticking coefficient
3
Surface concentration of species i (mole/m21
3 0
si
Initial concentration of species i on the surface (mole/m21
El,
Surface area of solid species on 2 temperature, f
t
time Is1
T
Temperature, OK
TO
TM Tn Tf u
the surface at peak maximum
Initial temperature, OK Maximum peak temperature, OK Normalized temperature, dimensionless Final temperature, K Fraction of gas diffused out at time t (temperatureTl
434 V
Volume of the sample chamber (m31
vc 'rn w W
Total solid volume in sample chamber (m3) Moles of surface sites per solid volume (mole/m31 Width of the TPD curve at half of the maximum height (KI Bed weight, kg
x1 x2 y(T)
Fraction of sites of type 1 Fraction of sites of type 2 Consumption of reducing adsorbate (mol/m3)
Greek Letters Fraction of metal/metal oxide interface area reduced at time t
a a
S
External surface area cm21
a
Active surface area cm21
Be
Heating rate (K/s)
e
Fractional surface coverage (dimensionless)
eO
@T 5 82
Initial fractional surface coverage (dimensionless) Total surface coverage in multisite model Fraction of type 1 sites filled in multisite model Fraction of type 2 sites Initial coverage of type 1 sites
e10
Initial coverage of type 2 sites e20 r
Stoichiometric coefficient
E
Porosity (cm3/cm31
"B
Bed porosity Particle porosity
EP Q
Surface area occupied by one mole of cm2/mol
E
Subsurface coverage of adsorbate
x
Actual concentration of the reactive species (mol/ccI
X0 0
Initial concentration in active species (mol/cc) Initial
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2 3 4 5
6 7 8 9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39
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114 115 116 117 118
Low. G.G. and Bell, A.T., J. Catal., 57, 397. 1979. Vandervell, D., and Booker, M.. Appl. Catal.. 30, 151, 1987. Schuarz, J.A. and Huang, Y.J., Appl. Catal. 30, 239. 1987. Falconer, J.L. and Sen., B., J. Catal., 113, 444, 1988. Margitfalvi. J., Szedlaczek, P., Hegedus. M. and Nagy, F., React. Kinet. Catal. lett.. 24, 304, 1984. 119 Kitchener, I.J., Parkyns, N.D. and Scott, S.B.J., Anal. Proceed., 22, 237, August 1985. 120 Arnoldy, P., Van den Heljkant. J.A.M., de Beer, V.H.S. and MouliJn, J.A. Applied Catalysis, 23, 81. 1986. 121 McCarthy, J.G., Hou, P.Y.. Sheridan, D. and Wise, H., Am. Chem. Sec.-Symp. Ser., 202. 253, 1982. 122 Figueriedo. J.L.. Fuel, 65. 1381, 1986. 123 Van Doorn, J.. Verheul. R.C.S., Singoredjo. L. and Moulijn. J.A., Fuel, 65, 1383, 1986. 124 Van Doorn, J., Bosch, J.L., Bakkum, R.J. and MouliJn, J.A., Catalyst Deactivation, B. Delmon and G.F. Froment (Eds.), 391, 1987. 125 Yoshimura, Y. and Furimsky, E., Fuel, 65, 1388, 1986. 126 Barbier, J. Catalyst Deactivation, B. Delmon and G.F. Froment (Eds.), 1, 1987. 127 Bacaud, R., Charcosset, H., Guemin, M., Torresea-Hidalgo and L. Tournayan, Appl. Catalysis, 1, 81, 1981. 128 Barbier. J. Corro, G, Zhang, Y., Bournoricille. J.P. and Franck. J.P.. Applied Catalysis, l3, 245, 1985. 129 Parera. J.M., Figoli, N.S. and Traffano, E.M., J. Catalysis, 79. 484, 1983. 130 Parera, J., Traffano, E.M., Musso, J. and Pieck, Spillover of adsorbed species, (G. Pajonk and S. Teichner, eds.). Elsevier Pub.,lOl, 1983. 131 Beltramini, J.N. and Trimm. Erdol and Kohle Erdgas Petrochemie, 9, 400, 1987. 132. Beltramini, J.N., Applied Catalysis, submitted for publication. 133 Raupp, G.B. and Delgass, W.N., J. Catalysis, 58, 337, 1979. 134 Unmuth, E.E., Schwartz, L.H. and Butt, J.B., J. Catal. 63, 404, 1980. 135 Amenomiya, Y. and Cvetanovic. R.J., J. Phys. Chem., 67. 2046. 1963.
I.
INTRODUCTION
I.1 Background
The
interaction of
reactants with
the
catalyst surface is
of
primary
importance in heterogeneous reaction systems. .The very rapid advancement of our knowledge of the surface aspects of heterogeneous catalytic reaction systems and catalyst characterization can be ascribed to the combined use of some new
techniques which became available. These are:
Spectroscopy
MS).
X-Ray
Photoelectron Spectroscopy
Auger Electron
(XPS)
and
Flash
Desorption Spectroscopy (FDS). Generally, ultra-high vacuum is necessary in order to study the mechanism of bonding between adsorbates and the adsorbing surface. Catalysts cannot thereby be studied under the conditions of the catalytic
reaction
system
in
which
they
operate.
When
using
a
temperature-programmedanalysis method, the need for a vacuum falls away; the desorbed particles then enter into a flow of carrier gas. For a
linear
increase in temperature, the concentration of the desorbing particles is recorded as a function of the temperature. This method, which was first used by Amenomiya and Cvetanovic (11, has been widely applied to
industrial
catalysts since, primarily because it can be used under practical operating conditions. Over the last decade this technique has been developed further and has been applied in an ever wide range of temperature-programmedapplications: desorption (TPD), reduction (TPR), oxidation (TPO) and temperature-programmed reactions, like methanation. sulphidation and carburization. The application of TPD, TPR and related techniques was reviewed by Cvetanovic and Amenomiya in 1972 (21, by Falconer and Schwarz in 1983 (31, Hurst and coworkers in 1982 on TPR (41, Lemaitre in 1984 (51 and Jones and NcNicol in 1986 (61. Since that time, a number of reviews have appeared. These concern with work done on catalysts (such as supported metal catalysts) and reaction systems. These techniques (TPD, TPR, TPO and TPSR) were used in this work, which culminated in various modifications to general experimental procedure, particularly in the case of reaction studies. The review now presented covers the work done on 'Temperature-ProgrammedAnalysis' since 1983.
1.2
ADDliCatiOnS
Temperature programmed analysis technique has been used in a number of ways for the study of heterogeneous catalysts such as:
Characterization of reduction.
supported
catalyst
by
temperature-programmed
310
Determination of binding energies and binding states of adsorbed molecules (TPD).
Determination of the surface acidity of zeolites (temperature programmed ammoniationl.
Measurements of surface area, metal surface area, dispersion of metal and adsorption kinetics (TPD).
Catalytic reaction studies (temperature-programmedreaction studies).
Characterization
the
of
coke
species
in
catalysts
deactivated
(temperature-programmedoxidation).
Sulphidation of Molybdenum catalyst [temperature-programmedsulphidationl
The transient nature of a temperature-programmedanalysis technique, in which both the temperature and
the surface coverage vary with
time, has
the
advantage of providing information which is not available from steady-state kinetic
measurements.
temperature-programmed
It
is
analysis
possible unit,
to which
devise can
a be
multi-purpose used
for
adsorption/desorption, oxidation, reduction, sulphidation, methanation and ammonination and which would have a universal detection system such as a mass spectrometer. Automatic equipment for doing catalyst characterization by temperature-programmedreduction/desorption/oxidationhas been reported (71. Figure 1 shows a schematic picture of such desorption (TPDl and a typical reduction profile (TPR). In the TPR-studies, a solid catalyst, which has been allowed to reach equilibrium with an adsorbing gas beforehand, in a well defined condition, is subjected to a programmed temperature rise and the amount of desorbing gas is thereby continuously monitored.
Various catalysts (metallic, bi-metallic, metal oxides, etc.), reported on in literature in conjunction with these techniques, are examined here and useful information, thus gathered, is discussed. In addition, the following is presented: The theory which applies when interpreting TPD, TPR and TPSR patterns; mathematical models for the evaluation of important parameters; the methodology for
obtaining theoretical TPD
and
TPR-patterns by
computer
simulation and future uses and trends in this field. Apparatus used is briefly described and also to what extent such apparatus can be put to other uses. In the case of TPR, a catalyst precursor, containing oxygen, is exposed to a programmed temperature rise while passing a reducing gas over it (usually
311
SURFACE COVE=AGE
300
400
500
600
700
500
no0
TIME
1. K
Figure 1. Temperature-programmeddesorption (TPD) and temperature-programmed reduction (TPR) response diagrams. hydrogen diluted by an inert gas). The reduction rate is measured continuously by monitoring the composition of the reducing gas at the outlet of the reactor. In temperature-programmed surface reactions (TPSRI, one gas
is
adsorbed on to the catalyst surface and a second reactive gas (or a reactive gas/inert gas mixture) is used as the carrier. The typical reactions studied, using TPSR, are methanation (CO and CO2 hydrogenation), oxidation (propene, coke oxidation), ammonination (ammonia with
acid
sites), reduction
(NO
hydrogenation) and sulphidation (sulphur reaction with hydrogen).
II. EXPERIMENTAL TECHNIQUES
II.1 General Comments
An extensive review of the numerous experimental arrangements that have been used for performing TPD and TPR-studies, is not within the scope of this review. A detailed description of temperature-programmeddesorption (TPDI is given by Falconer and Schwarz (31, whereas a TPR arrangement is described by Jones and McNicol (6). Lemaitre (5) discussed the TPD/TPR-combinationas based on the Rogers - Amenomiya - Robertson arrangement. Recently, a comprehensive paper by Boer, Boersma and Wagstaff appeared (71, that described an apparatus
312
in which TPR, TPD and TPO could all be carried out. This apparatus is rather complex for temperature-programmedcatalyst studies. With the use of a more universal detection
system,
such
as
mass-spectrometer with
a
thermal
conductivity detector, one single temperature-programmedanalyser could deal with
all
forms
reduction,
of
oxidation,
sulphidation.
methanation,
ammonination and desorption. Such a comprehensive unit would operate quickly and
automatically. A
general description is
given
of
a
technique of
characterizing heterogeneous catalysts, using temperature-programmedanalysis. This is believed to be of interest to readers as well.
II.2 Procedure
In a typical temperature-programmedanalysis experiment (as shown in Figure 21, a small amount of catalyst is contained in a reactor that can be heated by a furnace.
The catalyst is heated under temperature-programmingin either a
reducing or an oxidizing atmosphere depending on the catalyst being a metal or a oxide. When an oxidized catalyst is heated under temperature programming in a stream of hydrogen containing inert gas it will adsorb hydrogen as a function of the temperature/reactivity relationship of
the oxide species
(TPR). When a reduced catalyst is cooled in a stream of hydrogen containing inert gas, it will absorb hydrogen.
Upon subsequent temperature-programmed
heating of the catalyst, this hydrogen will be desorbed as a function of the temperature/adsorptionstrength relationship of the hydrogen adduct (TPD). An inert carrier gas is usually helium, but argon and nitrogen have also been used.
When a reduced catalyst is heated under temperature programming in a
stream of oxygen containing inert gas, it will adsorb oxygen as a function of the temperature/reactivityrelationship of the reduced catalyst. The behaviour of the catalyst can be assessed by continuously monitoring the concentration of oxygen and hydrogen, respectively, in the effluent.
Downstream from the reactor, a suitable detector, such as thermal conductivity detector or mass spectrometer is used for analysing changes in concentrations in the carrier gas. A small thermocouple is usually inserted in the catalyst bed for measuring the temperature. In temperature-programmedsurface reaction studies (TPSR), the adsorbate may decompose or several adsorbates may produce reaction products;
the
detector should be
'sensitive towards specific
substances'. If a thermal conductivity detector (for the detection of low concentrations) is used, then the carrier gas composition should be chosen such, there will be significant differences in thermal conductivity which can be measured. This choice of composition is found varying the concentrations of He + 02, N2 + Hz (or Ar + H2). Examples of typical gas compositions in the
313
context are shown in table I.
TABLE I Composition of carrier gas and reactant gas
Composition
Technique
Nitrogen + 5% by volume of hydrogen
TPR
Nitrogen + 0.1% by volume of hydrogen
TPD
Helium + 5% by volume of oxygen
TPO
Figure 2. Schematic of the Rogers-Amenomiya-Robertsonarrangement forTPD and TPR studies.
Gl and G2, gas cylinders (1, pure gas;
2, reducing gas
mixture); SV, shutoff valve; Vi, four way valve for gas selection; FC, Flow controls;
V2, four way valve for shunting the reactor;
R. quartz reactor
with quartz thermowell; F, Furnace; TCP. temperature controller-programmer; K, thermal conductivity detector; XV, recorder; RM, T, cold trap; rotameters (From Ref. 51.
314 Corrosion must be avoided when the detector filaments are brought into contact with reactive gases, such as NO, CO, Hz0 and H2S_ The basic components of an apparatus suitable for temperature programmed analysis of catalyst are
(a)
catalyst sample tube (reactor), (b) a furnace with programmable temperature controller (cl a dosing system for carrier and adsorbate gases (dl a system for chemical analysis (usually a GC detector followed by a cold trap for retention of
the desorbed material) and a
(e) vacuum system.
A
mass
spectrometer can also be used instead of GC detector in order to obtain direct information about the desorbed particles. Although the basic concept of a temperature-programmed analysis
experiment
is
relatively
simple,
its
implementation can be much more difficult. Potential experimental problems and how to avoid them, are discussed by Falconer and Schwarz (3).
The important
parameters that have to be optimized in temperature programmed analysis are:
flow rate of carrier gas. reactapt gas/inert gas ratio. catalyst sample volume/mass. catalyst sample particle size. geometry of the reaction vessel (catalyst sample reactor). heating rate. signal intensity. system pressure.
,Most of the temperature-programmedanalyses are usually run at atmospheric pressure, though higher pressures can be used.
A typical temperature-
programmed analysis (TPDI measurement usually call for the following steps:
Pretreatment of the catalyst (desorption of adsorbed foreign. species and possibly reduction). Exposure to the reactant gas or adsorbate. Desorption of the physisorbed fraction by extensive evacuation. Heating of the catalyst sample using a linear temperature program.
The
desorbed particles are transported to the detector by a stream of inert gas (usually He, Ar or N3) and recorded. Analysis of the desorbed particles by gas chromatography or by mass spectroscopy.
In a TPR experiment, a dilute reducing gas (e.g. 5% Hg, 95% Ng) instead of an inert carrier gas is passed over the catalyst in a reactor tube.
Depending
upon the reducibility of the components present on the catalyst surface, one or more maxima are obtained for the reducing gas consumption at characteristic
315
temperature.
The same technique may be extended to include temperature-
programmed reactions in general e.g. temperature-programmed oxidation (81, temperature-programmedcarburization .(91, temperature-programmedmethanation (101,
temperature-programmed
sulfidation
(121
and
ammoniation
(111.
temperature-programmed
temperature-programmed reaction
concerned with catalyst poisoning (13,141.
including
studies
Table II shows the typical
experimental conditions for a temperature-programmedanalysis technique on a supported metal catalyst with low-weight loading.
III. THEORY
With a temperature-programmedanalysis apparatus, adsorption, desorption and surface reaction experiments can be carried out. Temperature-programmed desorption (TPD) has been used most commonly to adsorbates to catalytic surfaces.
study the binding of
The temperature of the desorption peak
(maximum) is indicative of the strength with which the adsorbate is bound to the surface.
The more strongly the adsorbate is bound to the surface, the
higher is the temperature of the desorption peak. Various studies (151 are reported with the quantitative analysis of temperature-programmeddesorption spectra i.e., determining the desorption kinetics.
Four basic cases can be
differentiated, namely
first order kinetics without readsorption. second order kinetics without readsorption. first order kinetics with readsorption. kinetics of the second order.
Surface heterogeneity, i.e.whether a surface contains more than one type of adsorption site, plays an important role in the analysis. Appropriate kinetic models have been developed to account for this phenomenon (161.
If the heats
of adsorption on these sites differ significantly, the TPD spectrum will contain multiple peaks, one for each type of adsorption site.
When this
happens, the heat of adsorption on each type of site can be determined separately.
Multiple peaks in a TPD spectrum do not always indicate the
presence of multiple adsorption sites on the catalyst surface. also
can
results from
Extra peaks
lateral interactions between adsorbate molecules
(induced heterogeneity) (17) and diffusion of adsorbate in the subsurface region (18). Quantitative analysis of TPD spectra, i.e. the determination of reliable kinetic parameters, is quite a magnitude of experimental errors.
demanding task because of
the
Depends
Pretreatment, if any
upon catalyst
spectrometer
lo-60 K/min
TCD/Mass
rate
Heating
Quartz tube fixed bed 6.4 mm x 210 mm
Type of Detector
type
size
Reactor
Particle
mm
1-5X
Weight
0.025-0.25
100-500 mg
Catalyst weight
loading
Hydrogen
Adsorbate/Reactant
Depends
TCD upon catalyst
4-60 KImin
Quartz tube fixed bed
0.025-0.25
1-5x mm
+ 5% v/v
100-500 mg
Nitrogen Hydrogen
15-30 cc/min
High purity Nitrogen/ Argon
(Atmospheric
TCD/Mass Depends
0.05-0.3 mm
1-5x
100-500 mg
upon catalyst
spectrometer
spectrometer Depends upon catalyst and reaction
TCD/FID/Mass
lo-60 K/min
tube
Depends upon reaction
30-60 cc/min
tube fixed bed Stainless/Quartz fixed bed
lo-60 K/min
Quartz
0.025-0.25
1-5x mm
+ 5% v/v
100-500 mg
Nitrogen Oxygen
30-90 cc/min
High purity Helium/ Nitrogen/Hydrogen
Temperature Programmed Surface Reaction (TPSR)
Pressure)
High purity Helium/ Nitrogen
Temperature Programmed Oxidation (TPO)
Analyser
High purity Helium/ Nitrogen/Argon
15-60 cc/min
gas
Programmed
Temperature Programmed Reduction (TPR)
for a Temperature
Temperature Programmed Desorption (TPD)
Conditions
Flow rate
Carrier
TABLE 11 Typical Experimental :
317 In temperature-programmedreduction (TPR), a generally applicable model for kinetic analysis of measured reduction profiles does not exist because of the complexity of the process of reduction. Thus the TPR method is usually used for qualitative analysis of the reducibility of the catalyst surface and is quite sensitive to chemical changes resulting from promoters or metal/support interactions.
The quantitative analysis of TPR spectra calls for prior
knowledge of the surface mechanisms.
Most of the work done on TPR
concerned with the reductions of oxides using hydrogen.
is
Supported metal
oxides and metal zeolites have been studied extensively, using TPR technique (19,201. McNicol and Jones (61 presented an analysis based on temperatureprogrammed techniques to obtain the kinetic parameters relating to
the
reduction process. Temperature-programmedanalysis has been used in study of phenomena such as hydrogen spillover, metal-metal interactions and strong metal support interactions (21,221. Hurst et al. (41 gave the analysis of temperature-programmed reduction, based
on
thermodynamics, kinetics
and
mechanism of reduction applicable to supported metal oxide catalysts. Effects of variation of
experimental parameters on non-isothermal reduction and
evaluation of kinetic parameters based on experimental observations uas also given (41.
Monti and Baiker (23) defined a characteristic number, K. to
select the values of the operating variables in order to obtain the optimum reduction profile.
The value of K
related the heating rate, hydrogen
concentration, total flow rate and the amount of reducible sample in such a way that the operating variables can easily be adjusted. The characteristic number K
was defined as:
0
K=_
si C
111 .FO
8
where
soi =
Initial concentration of reducible species i
c
=
Concentration of hydrogen at the inlet of reactor (mol/m21.
=
Total flow rate of the reducing gas (m3(NTP1/sI.
on the surface.
g0
F
They reported the minimum value of and the maximum value of of
K
K
K
is 55s for a heating rate of 0.1 K/s
as 140s for heating rate of 0.3K/s.
below 55s the sensitivity becomes too low; for values of
K
For values exceeding
14Os, the quantity of hydrogen consumed is too large.
In temperature-programmedanalysis (TPD/TPR/TPO). the shapes of the spectra
318 can be influenced by several factors.
It is important to separate these
factors/effects and the rate process which controls the resulting spectra must be identified and be incorporated into the analysis and modelling of the process.
The various factors which affect the shape and position of the
spectra are:
surface heterogeneity (multiple adsorption states of different energies). readsorption. mass transfer effects (diffusional resistances). subsurface diffusion and adsorption. desorption kinetics.
Some of the factors, like diffusional resistances, can be eliminated with proper design of experimental conditions. been devoted
A number of papers have recently
to theoretical analysis, mathematical modelling and simulation
of TPD and TPR data (24-281.
Several
studies
on
temperature-programmed reactions,
using
supported
catalysts, has been reported in the literature (29-321. Most of these studies provided the information about catalytic mechanism on the surface. There is a lack of quantitative treatment of temperature-programmedreaction studies in the literature.
III.1 Theoretical aspects of the analvsis
Analysis of
desorption spectra, when a
temperature-programmed method is
used, can provide:
adsorption-desorptionmechanisms and their associated kinetics.
kinetic parameters such as heat of adsorption, activation energy of the adsorption and desorption process, order of desorption process.
valence state of
metal
ions in
zeolites and
supported catalysts,
interaction between metal oxide and support, identification of alloys formation in bimetallic catalysts using temperature-programmedreduction (TPR).
characterization of
the nature of
the coke species in deactivated
catalysts (TPO), study of reactions in evaluating the role of an active site
in a catalytic reaction (TPSR).
319 The interpretation of temperature-programmeddesorptlon/reductlon spectra is usually confined to the discussion of peak maximum temperatures, the number of more
or
less
resolved peaks
or
to
the
determination of
the
total
adsorbate/reactantconsumption from which the rate of desorptlon/oxidation/ reduction/reactioncould be -determined.
The
evaluation
of
the
desorptlon/reductlon during
kinetic a
parameters
for
adsorption
and
temperature-programmed analysis experiment
requires consideration of the material balances for the surface and the fluid phase concentrations of the adsorbate. Various types of mathematical models, based on simple adsorption to multisite adsorption and subsurface diffusion theory, have been given in the literature 1181. Table III lists the various models which have been used analysis spectra. are
in
interpretation of
temperature-programmed
The various kinetic parameters which could be evaluated
also given in Table III.
It is clear from Table III that mathematical
models proposed for getting kinetic parameters neglect mass transport effects during temperature-programmedanalysis, Mass transport processes such as inter and lntrapartlcle diffusion alter the shape of the temperature-programmed desorptlon/reductlonspectra. Therefore it is important to assess the role of mass transport in the temperature-programmedanalysis. The influence of mass transfer on temperature-programmedanalysis techniques will be examined in the next section.
III.2 Mass Transfer Considerations in Temperature Programmed Analvsis
If the kinetic parameters derived from the analysis of temperature-programmed profile are to be meaningful, it is necessary to establish that mass transfer effects
are
absent.
The
influence
of
mass
transfer
effects
on
temperature-programmeddesorption (TPD) spectra from porous catalysts has been examined from the standpoints of non dimensional analysis (25) and numerical simulation
of
specific
models
(27,411.
Gorte
(25)
proposed
that
lntra-particle concentration gradients can be minimized by reducing catalyst particle size and reducing the gas flow, increases the film resistance. This criterion was obtained by non-dlmenslonalizationof the differential equations governing mass transfer during temperature-programmedanalysis. Dimensionless groups of easily measured catalyst parameters have been reported and provide the researcher with guidelines for assessing:
readsorption. concentration gradients in the particles and along the catalyst bed. lag-time due to diffusion and hold up in the sample's tube.
Order of adsorption is pn Non-dissociatSve adsorption n and p = I Dissociative adsorption
&sorption rate with respccc to adsorbed species Absence of intraparticle diffusion Linear adsorption isotherm
order
sites,
Desorption
nth
EnergeticalZy homogeneous surface
kinetic
Mathematical,
of
(l”(r)
S
_(8)e-$
N] -
dT
% ka(l49
k (l-t3)p a
PWcV,
kd(B)Bn
Readsorption
(3%‘~’ V cm
40 z?=g
Free
exp
+.*12f
)
. ..[6]
151
. ..I41
. . .I31 TH
Slope
-
--Ed r
vs %
I
,PLot of
Series
2
TM In(T)
Slope
ADd = 7
Xonvalinka et al. (33)
Cve tanovic and Amenomiya (I)
Reference
Aualysfs
af TPD curves Valve of n is for varLous initial equal to slope coversges or for (Equation 3) various heating rates Measure N at number of temperatures Plot In N vs In 0 at fixed T
vsTN
1
Measure peak Temperature Tn Plot In(+)
rate)
Series of TPD curves at fixed B. and variable 6 (heating
data required
Parameters Evaloated
in Temperature-Programmed
Experimental
parameters
* ~)-lm[ln0]
n
Rd (~1 M
- 2
RT
-Ed(o)
model/Eqwtion
and experimental
‘d
Ed ’ AXl = RT 2 M Oon-l
[ln
* ,A!?
Ad
No Readsorption
used in Evaluation
Theory and fts limitations
Models
TemperatureProgrammed
Process
TABLE IIT: Mathematical
Temperature Programmed Desorption
Process
Theory and its limitations
Desorption controlling . With no Readsorption Ammonia desorption from sites of different sttenath IntraIparticle diffusion absent
TABLE 11X (cont.)
ne-
[ (1-g)::
P
o
a
- 2
. . . t71
= exp
dT
* &
exp
Ed (5)
f
0
. ..[lO]
(2s) (-d)dT . ..191 RT B
cm
F
“Cc]
‘“d
where I = &T exp(-Ed/RT)dT
Aq In (9)
-i!p
[In N] - la(&)
+ ln
en
(l-e)p
model/Equation
And =w+lnx M
-AR, (0) 7)
2 TM In CT)
axp(
Mathematical
at
regression analysis (Eq. 9) using all experimental data.
fixed T. Equation [lo) solved by non-linear. regression procedure. Evaluation of Ed and Ad using nonlinear
In (($PlJ
Series of TPD curves for various coverages up to saturation coverage Measure N and coverage 8, at a number of temps. Plot [In N] vs
Value of p must. be known i.e. Adsorption kinetics be known.
Experimental data required
Ed ’
Ad
Slope - n Order of deaorption
Parameters Evaluated
Forni & Magni (34)
Reference
Temperature Programmed Desarpcion
Process
Theory and its limitations
Intra-crystalline diffusion controlling (Applicable to zeolices)
Free Re-adsorption
TABLE 1x7. (cont.)
) f A6o -A6
“*Hd dT ( z)
-$ n
-
Q,(Ti*)
‘5 NH3(mmol/gsolid)
Ea”:
Activation diffusion
energy
of
. ..[171
DeiRc2= Do exp(-Ea*/RT)
51 I 61
*.. I
*
1..
t
coming
out of zeolite du.K the time interval At,.
Amount of
-**[141
II2
1(14%
. ..I131
*If21
. ..[ll]
Q, = Q,(T)
+““’
-D
(QfTi)
ew ( L.iL)(T*)n2&i
[
0
= GT exp (-AHd/RT)dT
MAti:
nil
M$
I’
0
A exp
Aeo In (g
= -(i)
Curve” Curve”
model/Equation
LHS: “Experimental RHS: “Calculated - Aq) (Aqo d(Aq) Afra
Mathcntntical.
Equations and f 171.
1161
Calculated peak is compared with experimental one using nan-linear regression routine and to obtain value of Ha* & Aa* from
TPD data analyzed by calculating tire Tl’D peak usinu. equations [,I41 & [151.
Equation 1121 solved numerically using non-linear regression optimization pracedure
Experimental data required
Do
ea*
AHd
&
Parameters Evaluated
Porni & Magni (34)
Reference
z h3
323
. 3 -
UA
c
ID
d +
f
f
. ..
* . .
-s -I-
h
2 0
Temperature Programmed Reduction
Temperature Programmed Desorption
Process
TABLE III
Theory end its limitations
Mothematicnl.
SbS~il~
cEfects
log
a:
[
1 -
(l-a)
Q-$
fraction
[_log
T2 ( l-s) E _2.3RT
log
at
]= lo&$
] = log
reduced
l-s
[26],
[24]
[l-
[l-
?]
t
of TPR curves
r 2.3R T
‘.*[281
Plot Left hand side of Equation [ 271 or [ 281 against A 1
Series
Equation [22] is used to construct theoretical TPD curves using an iterative numerical procedure. Theoretical TPD curves are compared witb experimentel TPD curves to get best values of kfnex parameters.
Experimental data required
?I - -E-
. ..[27]
time
g
6 [25]:
. ..[26]
From equation3
- al8
g=k(I
. ..[25]
. ..[24]
. ..[23]
rcoction
dfS] dt
. ..[22]
dT S-z In excess of reducing gas, is independent of gas concentration.
= k Cg ls]s -d-; k F A*e-e/RT
Mass transfer
-d C
respect
with
r-1
-d C *---=
rEh order to Gas
For
g
F 4 VcV, ka(l-8))
kd(6)
with
respect
Rate =
F 13~
sth order solid
For the reaction + Products Gas f solid
-d6 -_dT
n=p-1
1
as
k Cgr[Sls
model/&quation
Equation [6] is expressed Shape of TPD curves under either diffusion _d6 kd(W 8“’ control or kinetic control for first order F + VcVm k,(l-DtP desorption model For First order desoprtion identical
(cont.)
Slope -E =2.3R
Parameters Evaluated (5)*
Jones and McNicoL (6)
Lemaltre
Reference
:
Temperature Programmed Reduction
Process
TARLE III
Theory and limitations
its
Catalyst placed in tubular reactor (Plug flow). Reaction rate expressed in terms of consumption of gas.
(cont.)
r-1
B
’
TM2[C1;
In
s=l
RTM2
-&+ln& M
R
s
A*[C
-aE
(29)
1;
[SIS
s-l
-0
E RT2
IS],
dC 2
1
d[S] dT
ISI 25 dT
s-l
maximum rate
g
gr iSI
[C gr
d(Rate)_O dT From equation
At
+PC
+qc
d(Rate) --k dT
(g
. ..[32]
. . .I317
exp
...
1
1301
. . . I29 concentration
modcl/Rquation
- F AC g Flow rate : Change in gas AC g Rate -kC gr LSlS
.
F.
Rate
Mathematical
)
“S
1
TM
-
Mean hydrogen c¢ration at the temperature of the maximum reduction rate 2c TM M Plot In 7
-c:
kinetics
A, s = 1
order
r -
of TPR curves a value of r.
[CglM - -“M
First
For
Series Assume
Experimental data required
-
g
E and A* determined.
Unique value of E obtained for all concentrations used.
Slope
Parameters Evaluated Monti Baiker
C
Reference
(23)
Temperature Programmed Reduction
L’roceiis
TABLE III.
Theory nnd limitativns
Its
Solid-gas rcac t ion S 4 G + R(Svlid) + B.&as). Ideal Tubular reactor
Reaction orders are unity (r - s - 1,) First order with respect to gas and solid phase.
(cont.)
nD I-’
I . . . [331
-c
gM
ore
generated.
Complex kinetic expressions can be solved using this technioue, mm da x ce “F m: specific molar consumption (mol/g) F: specific flow rate of the reacting mixture (cc/s.g)
TPR profiles
Numerical integration of [33) using normalized Tn and Ctlt
go
- Cgo _ CP
CC&
M
T/f*
C
temperature.
functions
and sample
Normalized
rate
Integration of [331 gives the relation between 112 consumption
B
2Cg F
1nod~l/6(~uatlo11
1,z [1 + Lr S A*exp@
Mathcaatical EVtllUCl~C!d
Experimental
TPR curve
Figure 3 shows TPR peak shape analysis for different values of E” II e EM = sn
CUlX(?.
Estimation of A and E was done by the best fit vf theoretical curve to the measured TPR
Experimental TPR curve
A and E
E and A
Parameters
required
Exporimcntnl datn
Sestnk (36)
et al.
Monti and Baiker (23)
Reference
III
Temperature Programmed Reduction
I’roCCSS
TAlILE
Reaction Controlled Nucleation
Solid Solution Reaction Reduction of transition metal ions in zeolites. Reaction rate will diminish due to depletion of reactive species.
oxidic supcatalyst
by
Three dimensional interface
Reduction of precursors of ported nickel
Cc)
Its
Phase Boundary Controlled Reaction Reaction rate is proportional to the surface area of the unreacted solid (contracting sphere model)
Theory :wd llmitatlons
(b)
(a)
(cont.)
6
F
=!!!!I*
2
:
+
da
-
da dT-
x0:
CI)“~
a)
surface reactant
$
area
cxp
9
Actual reactive
(1
-
exp
of ($)
particle [37.]
the
...
of
($)..[36]
. . . [351
. . . [341
Initial concentration active species x S A* O” (l-a) exp (2) B
of (mol/cc)
. ..[39]
in
the
..
first power in (1-a) Z/3 power in (l-a)
a)
area
concentration species
(37): (36):
surface
kinetic constant of nucleation process
kn h*(l
x - x0 (1 - a)
x:
z-kSox
a)
(1 -
specific starting
-
dT
s A*
Equation Equation
da dT-
h
da E’ kn:
n=3 da zso:
S = So(l
c
and
TPR curve.
[34]
Equation [391 same as 1371, therefore Model (b) and (c) cannot be distinguished on the basis of the shape of TPR patterns.
Figs. 4 and 5 show Theoretical TPR patterns obtained using the two models
Solution of 1371 gives theoretical
Solution of Eq. [341 & [351 and comparison with experimental curve gives values of A & E.
I:‘xpcrlmclltol data required
A*
6
ACE
E
1% I.-illnote rs Evoluatcd
(37)
Jacobs
Lema?tre
et
al.
( 5)
TPD/TPR
Process
TAlJLg lli
(CWC.)
and
LCS
Multisite adsorption model Catalyst bed is modelled as a continuous flow ‘stirred tank reactor (CSTR). Peclet no. is small MO intra-particle diffusional effects Surface contains two distinct adsorption sites of different
Figure 6 represents the process of re-adsorption and subsurface diffusion during temperature programmed Reduction/ Desorption to generate theoretical C”CV(?S.
Subsurface adsorption diffusion theory Multiple peak due to diffusion of sorbate into the subsurface layers of metal-supported catalyst
linitatioas
‘l%cory
2
n: order Gas phase adsorbate
d52 dtEnka
Site
de1 dtinka
of
. , . 1421 subsurface sites total surface sites
- kd (l-e)51
. . . (41)
‘..
roc~uircd
lbdUOt:cd
Parnmetcrs
t=o
E”O
O=B
T-T
0
0
using 4th order Runge Kutta technique. Figure 7 shows a simulation of the TPR spectrum for MO C catalyst us I ng subsurface diffusion model.
at
Seri.os of TPD curves with different heating rates. Value of M is assumed .’ Ed’ ED Eqs.[40], 1411 6 1421 E are solved for the P’ ‘d 1401 A conditions *D’ P
data
Expccimcntal
the adsorbate
Pre-exponential Experimental TPD factors Adl, curve5 at varying flow rate of carrier Ad2 and (J-61)n ‘g - n kd gll.. [43]gas and heatin rates-Activation 1 Eqs. 1411,[431& t:441 energies were solved numericEdl c Ed2* ally using 4th order (1-fJ2)n cg - n kd e2n Runge Kutta method. 2 . ..[441 of the process concentration of the C : g
Nu MI---El Ns
Mass balance Site I
where
C - n kdO” g f kD(L-B)f
[kp @(l-c)
f kp e (1-t)
n ka(l-B)”
dT z=g dE = i dt
gp
on the
model/Rquatlon
Mass balance equation surface regfon
Machcmatical
Leary (1S)
Stacy (38)
et al.
et al.
I(CE~rCllC~
53
x
N
TemperatureProgrammed Diffusion
TemperatureProgrammed Reduction
order
kinetics
o*
d In
Diffusion in Uniform spherical particles under temperatureprogrammed conditions. This method has been applied to zeolite A to study the kinetics of gas diffusion in zeolites.
where
-N - g
= -&
(-g)
T*)
OD C (-n2 n=l
ki(T)
Si(T)
D,” exp
f;L z
:= ki(T)
o I +wp*
wP*
TM _ _ ER ii-
(%)
g
y(T) - C
d (&) M First order E -ER desorp tion bands. R = AR exp (F) High temperature is RTM2 M composed of methanol, water and formaldehyde.2 ln TM _ In B ‘Homogeneous surface’ E =&+lnC M
Partial oxidation of methanol to formaldehyde over MOO-, catalyst.
First
mc WC)
PV
Bulk volume of catalyst Reactor volume
coefficient
v:
Vc:
where
TPD curves with varying heating
Series curves
and
E* and Dt from slope intercept.
to get
of TPD at varying
Plot LHS ot Eq. [51] I “S TV yieldsvalue of RR and AR
. . . [5211
. ..[51j
. ..[50]
***[4g1~~~~;re TM of Formaldehyde peak.
. . . !48]
Si(‘O
w: residence time y: hydrogen consumption, Cgo - Cg v: stoichiometric coefficient
D(t)
Do
= Do exp
and E were obtained to get diffusion coefficient
Activation parameters
first order dcsorption and rate of desorption not ‘controlled by simple surfaceadsorbate bond cleavage.
Low value of A,~ suggests
ER and AR
l’aramctcrs Evaluated
et al.
(2)
Fraenkel 6 Levy (40)
Farneth (30)
331
332
0
1
t 0.6
Figure 3.
I
I
0.8
1.0
1.2
Dimensionless TPR peak shape analysisfor differentvalues of En.
E*=li?.4, 12.7. 15.0; 27.3; 19.7. The peaks become sharperwith (FromRef.361.
increasing
En
a
b E
h
KJlmol
nm
L
i “E
3-
1
121
2
112
3
2 E m 8’
600
700
800
900
T(K)
Figure 4. Theoretical TPR patteins of2a three directional phase boundary controlled reaction [A =31.%x 10 g/(cm .!a); E = 121 kJ/_yl; h = 8800 nm corresponding to So = 10 cm /g; g = 0.2 K/s, m = 133 x 10 mol/g; F = 2.0 x 10' cm3/(S.g)l. (From Ref. 5.1
a 4
b
E
h nm
kJ/mol
500
600
700
1
0800
2
800
500
600
100 500
600
700
T(K)
TPR patterns of a nucleation controlled reaction LA-2 h = 10 nm, g = 0.4 K/s; m = 1.33 x 10
334
CARRIER
GAS
SURFACE 0
‘P v
I
rD
SUBSURFACE REGION 5
54 = n k, (1 - 8 )”C, 'd
--”
rP
--kpe
kden
U-5)
Figure 6. Process occurring during TPD/TPR when both readsorption and subsurface diffusion are important. (From Ref. 18.1
335
- 0.8
Temperature
(Kl
Figure 7. Simulated TPD spectrum using subsurface diffusion model for parameter values in Table IV. Also shown: changes in surface and subsurface coverages with temperature. (From Ref. 18:)
TABLE IV: Parameter Values Used in Subsurface Diffusion Model Ns = 3.0 x 10-6 n
mol
= 2.0
M
= 1.0
u
= 4.0 x lo8 cm2/mol (4.0 x lo4 m2/mol)
Ad = 1.0 x 1013 a-l = 79.5 kJ/mol (19.0 kcal/mol) Ed Ap = 1.0 x lo6 s-1 Ep = 62.8 kJ/mol (15.0 kcal/mol) An = 1.0 x 106 s-l ED = 58.6 kJ/mol (14.0 kcal/mol) E, = 0.0
336
cG ,
r
r
51
d2
al
i
type
1
2
sites
sites
‘dl
type
=
n kd,e,”
ral -- n k, (1 - e,)“CG
r
a2
=
n ka (1 - fIJ"CG
Figure 8. TPD from a catalyst containing two distinct adsorption sites of differing binding energies. (From Ref. 18.1
337
8C ‘x 64 do_ “, 48 5 E .z
32
s n
I6 0
400
300
Temperature
TABLE V:
600
500 (K)
Parameter Values Used in Multisite Model
Ns =
3.0 x 10-6
m
= 2.0
n
=
mol
2.0
so = 0.005 cl
=
4.0 x lo8 cm2/mol (4.0 x lo4 m*/mol)
x1 =
0.5
x
0.5
2
=
'dl= "d2= Edl=
1.0 x lo13 s-l 1.0 x lo13 s-l 75.3 kJ/mol (18.0 kcal/mol)
Ed = 96.2 kJ/mol (23.0 kcal/mol) 1
Figure 9. Effect of initial coverage on TPD spectrum simulated by multisite model. Fo = 100 cc/min, p = 1.0 K/s; other parameter values as in Table V. (From Ref. 18.1
a
a
a
a
flow rate
0.
Ratio of adsorption to diffusion rates for infinite carrier-gas flow rates
Effect of carrier-gas
Accumulation of gas in the catalyst pore
Determines when readsorption is important
Determines when concentration gradients are present
Log time for gas to diffuse out of the pore
Lag time for sample measurement
Average residence the sample cell
time for
Comments
Physical description
For spherical catalyst pellets, replace L by r
n2 De
CL PSlL2P e
De as
FL
t3L2c (Tf - TO) De
F(Tf - To)
“2
Parameter
TABLE VI.l: Summary of Catalyst Parameters and Their Effect for CSTK (28)
When >l, readsorption is important, even at high carrier-gas flow rates
When
Should be kept less than 0.01 for cell concentration to follow net rate of desorption
Should be kept less than 0.01 for cell concentration to follow net rate of desotption
Significance
:
339
340 Demmin and Gorte (261 gave six dimensionless groups to check the above factors in a packed bed and continuously stirred tank reactor configuration. Table IV represents these factors along with their definition, and the criteria for checking their values in a temperature-programmedanalysis experiment. have also found that equilibrium readsorption will
often occur
in
They the
desorption process, in order to avoid concentration gradients within the particle, low carrier gas flow rates are necessary but such conditions are To avoid
likely to lead to axial concentration gradients in the catalyst bed.
these, the Peclet number must be considered. The value of Peclet number, 2 $b--(Table IV1 determines the extent of backmixing in the packed bed and when CB the value of this number is less than 0.1, the bed can be adequately modelled in accordance with CSTB. low,
are
likely
to
The high flow rates necessary to keep this quantity introduce
intra-particle concentration gradients.
Intra-particle concentration gradients were tested, using different catalyst particle radii.
Changes in
the peak
temperature (ATM1 and
effective
desorption rate constant (keffl with particle radius were reported by Tronconi and Forzatti in a TPD study (281 (see Table VI1 and given as: r
‘0
(551
= 3.7 AHd loglo ($1
ATM
O2
for B = 0.17 K/s and 10 i
AHd
5 167 KJ/mol
[561
at given temperature T.
Equations [551
and
[561
provide the basis for experimental criteria to
determine whether significant internal mass transfer effects are present during temperature-programmed analysis.
A plot of TM and loglO(kefflT vs
(rol could also show when diffusional effects are negligible. log10 dimensionless group &
fr equivalent to e
which is es against TM and (kefflT for a given value of less than
1
Gorte's
p
F
and
can also be plotted W
and if the value is
further confirms the absence of diffusional effects.
341 The Weisz-Prater criterion of checking the absence of
an
intra-particle
gradient to a static system, has been applied to a flow reactor system. Recently, Xue et al.
(42) have applied this to study the existence of
intra-particle concentration gradients during TPD experiments. They defined a modulus 9, as:
n-1 l/2 eff ci D 1 .I e where
r
I571
: radius of spherical porous catalyst particle.
effective desorption rate constant. k:ff: De : effective diffusion coefficient.
Ci
: adsorbate concentration at the centre of the particle.
n
: order of the desorption/reaction.
The magnitude of #, is used to estimate the effectiveness factor n),a measure of the concentration gradient of the reactant within the particle.
They
solved equation [581 numerically to get a series of contour plots at closely spaced temperatures covering the range of TPD experiment which allow the experimentalist to determine whether or
not
intraparticle concentration
gradients are significant for first and second order desorption processes.
9 1)$5;ci
#s is
=
r2 N Do "ohs e c
(581
"pseudo modulus" because the transport properties of the gas phase are
temperature dependent. The value of
n
close to
of intraparticle concentration gradients. criteria for
the absence of
1
will confirm the absence
Ibok and Ollis (43) proposed a
intraparticle mass transfer effects during
temperature programmed desorption in the absence of readsorption for first order desorption as:
r< 0
where
De
C -
(1
cslTH
l/2
1 M
+Kl k6,'obslT
r
catalyst particle size, cm.
surface concentration of reactant (mol/ccI. c: observed reaction rate. r obs 6 specific adsorption sites, sites/cm3. m
[SSI
342 Equation [591 was derived by modification of Weiss-Prater criterion which was developed for isothermal reaction conditions and represents the criteria for the absence of intraparticlediffusion during TPD.
Equation [561 was also applied to check diffusion gradients within the pellet in TPR by checking a dimensionless number d
defined as
[601
where
Nn vC
rate of consumption of hydrogen at peak temperature. sample volume.
If the value determined for
#
is less than 0.3 it is concluded no
appreciable diffusion gradients exist within the pellet. The intraparticle diffusion can be avoided by choosing the proper size of catalyst particle during TPD experiment. Table VII represents the guidelines for the design of a TPD experiment in order to avoid intraparticlemass transfer effects.
TABLE VII: Impact of design Parameter on the Effectiveness factor (7))(421
Factor
Variation
Effectiveness
N
Increase
Decrease
Increase
Increase
Increase
Decrease
cS r
0
De vC
Increase
Increase
Increase
Depends on which
(Dilution with
dominating
Comment
(al Cs decreases
inert particles) B
Increase
W (Catalyst mass)
Increase
Depends on which
(al r increases
dominating
(bl Cs decreases
Increase
Huang and Schwarz (441 showed in a study that in the absence of intraparticle diffusion the proper description of mass transfer effects on TPD from a bed of catalyst perfused by a flow of carrier gas requires a nonsteady mass balance which includes both axial dispersion and convective transport. The equations that describe the concentration in the bulk fluid, C, and on the surface, 0,
343 are given by (451:
ac
8=D at
a2c A-
”
e az2
!J - vm ?+A
(?E,
[611
-ae = ka Cg (1 - 0)"- kdeP
[621
at
Where De and
u
are temperature dependent quant.ities.
Equations [611 and
1621 subjected to the boundary conditions.
ac
De2
I gIZ=O-cgin) z=o=u(C
ac
2
[641 Z’Lo
=
O
The initial condition is given by
Cg(z,
0) =
ecz,
0) = 0
Equations [611 and
[651 [621 were solved' numerically and
experimental results to obtain the kinetic parameters.
compared with
the
The above analyses
demonstrate that the position and shape of TPD spectra obtained from porous catalysts are sensitive functions of:
catalyst particle size. catalyst bed depth. carrier flow rate. carrier gas composition.
Intra-particle concentration gradients can be minimized by reducing catalyst particle size and carrier gas flow rate.
Axial gradients in the gas phase
concentrations of adsorbate can be minimized by the use of very shallow catalyst beds.
For small particles (e.g. r. = 0.08 cm) the TPD spectra for
nonuniform initial adsorption and uniform initial adsorption are similar.
quite
Reasonably accurate estimates of the enthalpy of adsorption (AHa)
344 and ratio of pre-exponential factors for adsorption and desorption can dbe obtained from equation 1661 if intra-particle and inter-particle gradients in adsorbate concentrations are absent (461.
I
In
B where
8M :
AH =d+ln RTM
“m
- AHd [FnAR
[661
1
Fractional coverage of adsorption sites at TM obtained by varying the heating rate.
n:
reaction order.
Axial dispersion within the reactor or during flow between catalyst and detector has been checked during TPR using the criterion (39):
Lt - >>- Rf ii 3.8ZDM where
[671
L:
Length of the connection line.
V:
Linear velocity within the tube.
t
R:
Tube radius.
D : M
Molecular diffusion coefficient.
t
Axial dispersion via molecular diffusion is important for the low value of the Peclet number and can be checked experimentally by examining the effect of flow rate variation on the shape of TPD spectra.
If the shape of TPR/TPD
spectra is not changed when changing the flow rate, the axial dispersion effect is less significant.
In TPD study of zeolites, diffusion can be the rate determining for gas appearance in TPD. zeolites using
Fraenkel and Levy (40) measured diffusion coefficients in
TPD.
They
incorporated the
effect
of
particle
size
distribution in the diffusion equation and predicted its effect on TPD curve. Their results showed an excellent agreement with the literature values.
Thus we find that by proper design of the temperature-programmedexperiments the problem of mass transfer effects can be avoided. Methods are available in the literature to demonstrate that a reasonably accurate estimation of the adsorption/desorption/reduction/reaction kinetic adsorbate/reactant can be obtained.
parameters
of
the
Table VIII presents the development and
application of various models used in the temperature-programmedanalysis with its limitations.
345
: i
VIII
Wichterlova et al. (49)
Farneth et al. (30 1
Fraenkel
Technique Studied
Methanol Oxidation over MOO3
HZSM-5 and Mordenite with different SilAl ratio
Temperature Programmed Desorption of NH 3
Zeolite A (300-900K) Na-A, Cs-A & Rb-A
System
TPSR
TPD (Temperature Programmed Diffusion)
(cont.)
(40)’
et al.
(cont.)
Mori
Author
Table
First order desorption of NH3
Not considered
Not considered
in
First order methan01
Mass Transfer Effects
Particle size effects considered
Used
Diffusion equation for Uniform Spherical particle converted to diffusion rate as
Kinetic
Readsorption of Nllj
Nil
Nil
Re.-adsorption
Y~5~:~7vt2:~Z~; fZr:og (HZSM-5) sites) Alid (Mordenito)= 145 kJ/mol (strong sitoh
Temperature peak maxima TM were found to be influenced by acid strength, no. of acid sites and zeolite structures. Heat of adsorption hHd of pIIt3 for individual zeolite obtained was in good agreement with ealori-
Activation energy E-20.6 kcallmol and factor pre-ex onential A=2xlO ‘; s-l matcb very well with literature values obtained from other techniques.
Pre-exponential factor and activation energy of diffusion correlated with Peak maximum temperature (T,)
Mordenite A=0.93expj1.1x10-4E]S-I
~=2.2~~~[1.2~10-4~1~-~
Silica Alumina A(E)=15exp[1.2x10-4E]~“’ Y zeolite
Remarks
347
TPD
TPD
Tronconi et al. [511
Rees and Chen (521
TPD
Leary (50)
et al.
Technique Studied
(cont.)
Author
TABLE VIII
ZSM-5. ZSM-11 and Theta-l in H+/Na+ form Desorption of p-xylene
Fresh and sodium poisoned y-Al203 Methanol and ethanol dehydration
9 wtX/Pd/Si02
system
Used
First order desorp tion kinetics
ReactionDeactivation Kinetics First Order Desorption
Subsurface diffusion: diffusion of H2 in PdfSiO2 catalyst Dissociative adsorption kinetics
Kinetic
Not considered
Absent
Not considered
Mass Transfer Effects
Nil
Considered
Nil
Re-adsorption
Activation energies of desorption wore calculated (Ed)ZSM-5 * 89 kJ/mol = 98 kJ/mol (Ed)zsM-ll (Ed)Theta-1’ 110 kJ/mol As Si./Al in pentasil type zeolite decreases the sorption capacity and activation energy for desorption decreases
Effect of poisoning on the selectivity has been reported.
The desorption activation energy values for methanol and ethanol were Ed = 70-115 J/m01 Ed = 80-120 J/m01
Parameter values are given Ad = 1x1013 2 -1 1 - 1.5x10 sAl, AD = 1.5x104s-1 Ed - 19.5 kcal/mol Elm= 16.1 kcallmol ED = 14.5 kcal/mol
Remarks
: Q,
TABLE VIII
TPD
TPD
Quanzh?. et al. (54)
Technique Studied
(cont.)
Rees and Chen (53)
Author
_
Desorption of NH3, Pyridine on HY
Desorption of n-hexane ZSM-5, ZSM-11, snd Theta-l in U+/Nn+ form
System
Used
First order desorption kinetics
First order desorption kinetics
Kinetic
Nil
Not considered
Mass Transfer Effects
Nil
Nil
Re-adsorption
(Uronsted acid sites) = 11.6 kJ/mol Ed3 (Lewis acid sites)
For pyridine m 13.5 kJ/mol EdL Ed2 = 25.9 kJ/mol
kJ/mol
of NH3 on DY there are two activation 11.9kJ/mol Ed L = and Rd2 = 43.8
CnSrgieS
Desorption show that desorption
Saturation capacities and Sd Values Were calculated (Rd)ZSM-5 = 80 kJ/mol Wd)ZS+L I= 74 kJ/nroI and (Rd)Theta = 69 kJ/moL
Remarks
350
IV. RESULTS OF EXPERIMENTAL OBSERVATIONS AND APPLICATIONS
IV.1 Overview
Temperature-programmedanalysis
has been applied to the study of supported
metal, oxides and zeolite catalysts since its inception by Cvetanovic and Amenomiya (11 in 1972.
This review concentrates on work done since 1983 on
supported metal and metal oxide catalysts.
Over the years, several reviews
have appeared in literature to cover TPD and TPR done before 1983 (2,61. Attempts have been made to put the technique on a quantitative basis, using various
mathematical models
and
evaluating kinetic
parameters.
The
information provided by temperature-programmedanalysis has been suplemented by other techniques such as x-ray diffraction, photoelectron spectroscopy and electron microscopy. This section has been arranged according to experimental observations and applications and to the techniques used.
To aid the reader,
and for easy reference, the following outline of the sections to follow is given:
Temperature-programmeddesorption and reduction:
-
Metal oxides.
-
Supported metal catalysts.
-
Metal zeolite catalysts.
Temperature-Programmedsurface reaction:
-
Hydrogenation.
-
Methanation.
-
Sulphiding.
-
Oxidation.
-
Other reactions.
Temperature-programmedgasification and carburization.
IV.2 Temoerature-DroFcrammed desorotion and reduction
IV.2.1 Metal oxides:
Temperature-programmedanalysis is a very useful procedure for investigating interactions between a test molecule and a catalyst surface, and it has been applied by several workers to studies of gas adsorption on metals and metal
351 oxides (32,541. Without doubt, O2 has been the most extensively used testing agent to study binding energies, populations and reactivities of metal oxides in the context of a large number of catalytic olefin oxidation reactions. The behaviour of metal oxide catalysts to catalyze complete oxidation or selective oxidation is related to the kinds of oxygen species involved, e.g. lattice oxygen and adsorbed oxygen.
In a TPD study the oxygen adsorption phenomena were seen to be different on different metal oxides.
Some oxides exhibited no oxygen adsorption whereas
others always gave relative large amounts of oxygen desorption.
Table IX
presents oxygen desorption behaviour from various metal oxides and classification based on TPD study.
TABLE IX Desorption of Oxygen from Various Metal Oxides (561
Group
A
TMa [Kl 0
'2'5 Moo3
0 0
Bi203
0
w"3 Bi203.2Mo03 B
Cr203 Mn02 Fe2o3 Go304 NiO cue
C
A1203 Si02
0
2.13 x lo-'
723 813
6.54 x 1O-2
323
543
633
328
623
758
4.05 x 1o-3
303
438
653
3.30 x 10-2
308
608
698
398
663
823
1.12 x 1o-2 1.42 x lo1 2.05 x 1O-4
338
2.99 x lO-5
373 593
5.52 x lO-5
Ti02
398
463
ZnO
463
593
2.45 x lo-4
Sn02
353
423
2.11 x lo-3
aThese values were obtained at 8=20K/min. b The overall amount of oxygen desorbed below 823K,after oxygen adsorption.
its
352 It is clear from Table IX that metal oxides could be classified into three groups: A.
oxides which exhibited no oxygen adsorption over the range 283-833K.
B.
oxides which give relative large amounts of oxygen desorption.
C.
oxides for which evacuation at high temperatures followed by oxygen adsorption at relatively low temperature is required for oxygen desorption to appear over the range 283-673K (except last 2 oxides).
The amounts of oxygen desorbed at 833
K
for group B and C oxides were only a
few percent of the sample coverage, suggesting that the adsorption sites are some sort of surface defect. There is fairly good correlation between the amount of oxygen desorbed at 833
K
and the oxide formation per g mol of
O2 (-AH;) as shown in Figure 10. The plot shows that, as an oxide is, less stable, so the more easily is the surface reduced to form surface defects for adsorption
o MnO2
A120,
0
I
I
260
I
I
LOO AH;
(kJ
I
600
g-1 atom
0)
Figure 10. Correlation of the amounts of oxygen desorbed at 833 K with the heat of formation of oxides per g-atom of oxygen. (From Ref. 57.1 It is confirmed in this study that group A oxides which are selective oxidation catalysts, show no significant oxygen desorption.
In contrast,
considerable amounts of oxygen adsorbed on group B oxides mainly catalyze the complete oxidation of olefins.
The group C oxides are in the intermediate
situation, adsorbing moderate amounts of 0
and catalyzing both the selective 2 and complete olefin oxidation to roughly the same extent. Thus these findings indicate that
the adsorbed oxygen is
strongly connected with
complete
oxidation, while the lattice oxygen is more important for selective oxidation.
353 Uda
et
al.
(8) used temperature-programmedre-oxidation to characterize the
redox properties of r.Bismuth molybdate catalyst (Bi2Mo06). Two peaks were observed, one at 431
and another at 613 K.
The low temperature peak was 4+ found to be a result of the re-oxidation of MO to Mo6+ and Bi" to Bi"+ 3+ where 0 < m < 3, whereas high temperature peak is the result of Bim+ to Bi . K
The activation energy for low temperature peak was determined to be 122 kJ/mole and the activation energy for high temperature peak was 265 kJ/mole using the method of Cvetanovic and Amenomiya (1).
The high temperature peak
appears to be related to the rate controlling step for propylene oxidation to acrolein at temperatures below 673 K. as detected by TPR.
Tagawa
et
al.
(58) studied temperature-programmeddesorption of CuO/ZnO/A1203
and CuO/ZnO/Cr203 catalysts for the reaction of methanol synthesis from CO2 and H2.
TPD spectra after the reaction on the copper containing catalysts
gave peaks characteristic of copper formate with
maxima at 473-483 K.This was This surface particle
identified as the intermediate point of the reaction.
is difficult to detect by infrared spectroscopy because of catalyst opaqueness (30% cu) whereas TPD spectra is proved to be useful in its detection for common catalyst.
Farneth et
al.
(30) studied Moo3 catalyst for partial oxidation of methanol,
using temperature programmed desorption.
TPD
spectra of methanol on Moo3
catalyst showed two distinct peaks, one at low temperature (Tmax = 393K) as dominated by loss of methanol and another at Tmax = 493 K consisting of formaldehyde, methanol and
water.
The
second peak
was
found
to
be
independent of the extent of coverage, indicating first order desorption kinetics. Activation energy for the formation of formaldehyde was obtained as 87 KJ/mol from TPD experiment confirming the redox mechanism proposed by Mars and Van Krevelen and the value obtained by independent reactor studies.
Thus,
form
these
works
it
is
demonstrated
the
utility
of
temperature-programmeddesorption studies under high vacuum with simultaneous microbalance
and
mass
spectrometry detection
for
obtaining
detailed
information about heterogeneous redox reactions. This method allows the chemisorption. reaction and
reoxidation stages of
the
mechanism to
be
separated in time and thereby examined independently.Changes in catalyst mass as a function of exposure, temperature and sample history were followed along with mass spectral intensity profiles of desorbed gases. It is remarkable how parameters obtained by TPD can be combined to predict heterogeneous reaction rates in excellent agreement with reactor data.
354 Similarly, metal sulfide catalysts used for the synthesis of Cl-C5 alcohols have been characterized by temperature-programmeddesorption technique (59). Alkalized
molybdenum
sulfide
and
unalkalized cobalt-molybdenum sulfide
catalysts showed four adsorption states for H2, four states for H2S and three for CO.
Alkali does not alter the total adsorption capacity of
these
catalysts for H2' CO and H2S but affects the distribution among the various adsorbed states.
Weakly adsorbed hydrogen and strongly a,dsorbedCO promote
selectivity to higher alcohols. Stacy et al. (38) studied mobility of oxygen in Mo2C using TPD and TPR . The catalytic properties depend very strongly on the MO/C ratio and the mobility of oxygen. TPR spectrum of oxygen adsorbed on Mo2C contains 2 water peaks, a narrow one at 479
K
and a second much broader peak near 573K. While the first
peak is produced by surface oxygen, the second peak is caused by O2 that diffuses into the subsurface region of the catalyst during the temperature ramp.
As the surface becomes depleted at higher temperatures, this oxygen
diffuses back to the surface, reacts with the gas phase hydrogen and desorbs as H20.
For carbon deficient catalysts, the two peaks merge into one
asymmetric peak at 510
K.
Thus the location of the peaks depend on the MO/C
ratio of the sample.
Malet and Munuera (60) used TPD to determine the energy distribution of acidic sites at the surface of ;r-A1203using water and NH3 as probe molecules.
The
heterogeneity of the surface can introduce changes in the shape of TPD curves depending on
the energetic distribution of the sites. In this work, a
logarithmic relationship between activation energy for desorption and surface coverage (Freundlich-Type isotherm) and TPD-curve shape analysis has been succesfully apply to
study the behavior of
real surfaces (a-A1203) in
adsorption-desorptionprocesses.
On the other hand, a number of (TPR) studies have appeared in literature on metal oxide catalysts where hydrogen was used (6). Supported metal oxides may be homogeneously distributed across the surface of the support or exist as islands of oxide separated by uncovered support.
Islands of metal oxides may
be expected to reduce in a similar manner to unsupported oxide, in which case the support may act purely as a dispersing agent and promote the reduction. Under these conditions the reduction kinetics observed for the supported oxides resemble those found for the bulk oxide. Metal atoms and crystallites are known to be mobile on the surface of supported metal oxides so that under appropriate conditions the reduction of a homogeneously supported metal oxide may proceed with the reduction of individual metal ions. Groups of metal ions,
355 can be subjected to surface diffusion to form metal crystallites which in turn may diffuse and combine to form particles of the reduced phase.
Vanadium oxide catalysts have been studied using TPR by Bosch et TPR profile of V205 catalyst is shown in Figure 11
a1.(471.
The
and under certain
conditions shows of a number of discrete peaks in the temperature range of 900-1100 K.
The results were interpreted as the stepwise reduction of V205 to
V203 through intermediates such as V6013 and V02.
They also reported high
activation energy of about 200 kJ/mole in the reduction process, indicating that solid state diffusion affects the reduction of V205.
Here TPR profiles have been studied as a function of flow rate, heating rate and sample weight. These conditions influenced not only the peak positions but also their forms. Then the authors explained the influence of sample weight and heating rate in terms of the formation of water in the sample during reduction. _
4.3mg,16K minv’
16 K min”
/
I
600
I
900
\t-l5mg,4.6Kmin~’
I
I
1000
1100
I 1200
TIK)
Figure 11. TPR profiles of V205 - influence of heating rate and sample (From Ref. 47.)
mass.
356 Howden et al. (611 studied the interaction of Mn with iron manganese oxides catalyst using TPR. The main conclusion of this work is that using TPR, it is possible to discriminate between various oxide phases on the basis of chemical reactivity and to elucidate the role of manganese in modifying the reduction behaviour of iron oxides. Their studies revealed that:
(al presence of Mn retards the reduction of Fe
in mixed iron-manganese
oxides.
(bl reduction of the mixed oxides occurs at considerably lower temperatures in CO than in Hz.
(cl the phases that remain after isothermal reduction of the mixed oxides depend on the pretreatment conditions.
Ehrhardt et a1.[391 studied the behaviour of supported transition metal oxide catalysts using TPR technique.
They studied Cr03/Si02 system, an important
catalysts for ethylene polymerization process.
The reduction behaviour of
CrOx varies with different supports, like a A1203 vs a SiO3 support.
In the
case of CrO supported over SiO2, they found that chromium is distributed 3' into at least three modifications, showing different reducibility. Further they reported that:
at Cr contents 25% wt, intermediate phases are present on the support resulting from the decomposition of pure Cr03.
at Cr contents (5% wt, Cr03 binds to the SiO3 surface mainly as dichromates and chromates to a minor extent.
at lower Cr contents
removal of water during reduction is crucial. A higher water removal rate 3+ goes together with higher Cr'+/Cr ratios.
From this work it is demonstrated the importance of the use of a mathematical model for the analysis of nonisothermal reduction, when we faced with complex reduction profiles, allowing the estimation of kinetics parameters for several overlapping single steps from one TPR experiment.
351
On another work, Arnoldy et
al.
(621 studied the reduction of
MoOB
and MoOa
using temperature-programmedreduction. The formation of MO catalytic sites depends on Ha0 pressure, concentration of surface defects and surface area. Reduction of MOO, to MoOa and MoOa to MO metal both can be catalyzed in which case the rate determining step is:
-
Either Ha dissociation when the formation of catalytic sites is limited.
-
or MO-O bond breaking when excess catalytic sites are present.
Reduction of the MoOa to MOO, can be also non-catalyzed, with oxygen diffusion determining the rate controlling step.
An analysis of this work show that TPR gives useful information on the reduction behavior of MoOB and MOO2, by examination of TPR patterns as well as by calculation of activation energy values for reduction via variation of the heating rate. It was found that experimental parameters such as water content of the reducing mixture, sample size, precalcination temperature and heating rate, influence the TPR patterns of Moo3 and Moo2 to a large extent. TPR profiles are modified by changes in operating conditions in a similar way found by Bosch et
al. (47).
Temperature-programmedreduction has been also used for the study of CoO/AlaOa catalysts (631.
Four different reduction regions are present in CoO/AlaOa
catalyst. These are:
-
-
Phase I consists of Co304 crystallites (600 Kl.
Phase II consists of Co3+ ions (750
K)
in crystallites of Coa/AlOe or
well dispersed surface particles.
-
Phase III consists of surface Co2+ ions (900 Kl.
-
Phase IV consists either of surface Co2+ ions or of subsurface Co2* ions, occurring in diluted Co2+ - A13+ spine1 structures in CoA1a04 (1150 Kl.
Here again, the conditions under which preparation proceeds (calcination flow rate and calcination temperature) influence the Co valency in CoO/A1a03, the extent of solid state diffusion and the dispersion.
Arnoldy et TPR .
al.
(641 studied the reduction of Co - Mo03/A1a03 catalysts using
These catalysts are used for hydrodesulfurizationand are more active
358
than CoO/AlaOa or MoOs/AlaOs catalysts. The reduction of M06* surface species (monolayer and bi-layer species) is not affected significantly by the presence of co, whereas the reduction of CO" presence of MO.
ions is strongly influenced by the
Strong interaction of CO-MO affects the reduction temperature
for Co'* ions from 1200 K in CoO/A120s to 800-850 K for Co0 - Mo0a/A120s catalyst. This study established the role of Co in HDS catalyst, which is the active
component, whereas
MO
promotes
Co
interaction between Co-ions and A1203 support.
activity
by
decreasing
the
Calcination of catalyst above
or below 800 K affects the phases present in the catalyst besides the Co - MO - P phase.
Figure 12 shows the TPR patterns of
9.1% CoO/A120s. whereas figure 13
represents the TPR patterns of Mo0a/A120s,and Co0 -
Mo03/A1203
catalysts as
a function of calcination temperature. Calcination of the samples affect the peak positions as shown in the figures 12 and 13.
From this work it can be seen that using TPR, information on the reduction of Co and MO in COO-Mo03/A1203 catalyst can be obtained. More important the combination of. TCD pattern (Hz consumption mainly) and FID pattern (CH4 production due to reduction of organic impurities) gives relevant information on the reduction temperatures for Co species.
1
I
Figure 12. TPR patterns of 9.1% CoO/A1203 catalysts calcined at various temperatures. Calcination temperature (K): 380(a). 575(b), 625(c). 675(d). 725(e), 775(f), 825(g), 875(h), 900(i), 925(j). The upper and lower part of each pattern represent the TCD and FID signal, respectively. (From Ref. 64.1
359
c: 4
==+(T(IC)
Figure 13. TPR patterns of Mo03/A1203 and COO-Mo03/A1203 catalysts calcined at various temperatures. Calcination temperature (K): (al 675 K, (bl 785 K, (cl 895 K, (dl 995 K, (e) 1125 K. The upper and lower part of each pattern represent the TCD and FID signal, respectively. (From Ref. 64.1
In another study of MoOa on SiOa Thomas
et
al.
(651 showed that the TPR peak
occurred at 750 K as opposed to 900 K for bulk Moos.
In the case of r-AlaOs
based catalysts reduced at higher temperatures than SiOa-based catalysts confirmed the
observations made
for
many
other
systems
that
stronger
interactions occur between a metals and r-AlaOs than between a metal and SiOa. At low loadings of Moos on SiOa, the support served to disperse the metal on the surface but as loading increased, this dispersion deteriorated.
The effect of support has also been reported in case of cobalt catalyst (661. Table X shows the effect of support on TPR peak temperatures.
Different
supports give different peak temperatures showing strong interaction of CO with the supports.
TPR was able to demonstrate how the interaction of different support with the metal could modify the reduction temperature.
360
Table X TPR of cobalt catalyst on various supports (66)
Remarks
Peak Temperature
catalyst
4% Co/A1203
5733 (Peak 11
673K (Peak III
Bulk Co304
593K (Peak II
663K (Peak III
Co/Ti02
5883 (Peak 11
686K (Peak II) Co304 to Co0 (588KI
co/sio2
473K
973K (complete) Higher temperature needed for reduction of the supported catalyst compared with bulk Co304
10% co304/sio2
693K
873K (complete)due to metal support interaction
Co/A1203
6OOK (Phase II
Phase I: Co304 crystallites
750K (Phase III
Phase II: Co3+
9OOK (Phase 111)
Phase III: Surface Co'* ion
1150K
co*+ in CoAl
coo
(Phase’
(686~)
IV)
to
coo
94
Temperature programmed reduction study of iron oxide (Fe2031, nickel oxide (NiOI and copper oxide (CuOI has been reported in the literature (6). Griffin and Robers (67) recently reported temperature programmed desorption of H
on various Cu/ZnO based catalysts.
Table XI shows the results
2
obtained with CuO-2nO catalyst.
These results indicate that for hydrogen adsorbed on reduced catalysts, a single desorption state which has an apparent activation energy of 20-21 kJmol-' is observed. When H2 is adsorbed on oxidized catalysts, the desorption -1 energy of this state increases to 26-27 kJmo1 .
The
main
conclusion drawn
from
this
work
is
that
a
combination of
temperature-programmed desorption (TPDI and FTIR spectroscopy was able to provide information related with both the concentration of adsorption sites and also their adsorption energy. If the adsorbed species has an infrared active vibrational mode, then it is also possible the direct monitoring of the conditions of the adsorbate as adsorption or desorption proceeds.
(PPQ
CUO~~O
1x cuo/sio2
CUD (ppt>
82% CuO/ZnO
33% CuO/ZnO
2% CuO/Kado x 25
ZnO (ppt)
1% cu0/s102
CUD (ppt)
82x
33x CuO/ZnO
2% CuD/Kado x 25
cw
Catalyst
"2
Hz
R2
H2
Hz
H2
H2
H2
H2
H2
H2
H2
Gas
387-403K 387-403K
15DK 15OK
Oxidized
Oxidized
-
367-403K
15OK
Oxidized
150K
-
305K
430K
Oxidized
150K
215K
Oxidized Oxidized
15DK 30%318K
30%318K
Reduced
30a-3laK
Reduced
308-318K
30%318lc
305K
Reduced
Reduced
15OK
215K
Reduced Reduced
Phases and Peak Temperatures
0.13
1.4
3.0
1.9
0.5
1.2
1.7
4.5
0.05
1.9
Amount of gas adsorbed, umol/g
adsorbed on reduced and oxidized Cu catalysts
Oxidized/ Reduced
TABLE XI: Temperature programmed desorption data of H
The work reviewed of the characterization of bulk oxidic materials and supported materials indicate that by using the temperature-programmed analysis technique, it is possible to identify:
compound formation between metal and support. determination of valency of metallic substances. identification of desorption sites and their energy position. identification of reaction sites in oxidation reactions.
The use of TPD and TPR in the study of oxide catalysts is not restricted to just studying the desorption and reaction behavior of molecules adsorbed on the surface of a catalyst. TPR has been used extensively in studying the reduction of the metal oxides to their metal state prior to their use as a catalysts. In a similar manner, TPD can be used to investigate the changes that can take place in the metal oxide catalyst during heating in an inert gas or in vacuum. Thus, TPD and TPR, can be used to guide the proper choice of pretreatment condition for a particular metal oxide system.
It was also found that metal oxides supported on inert carries likes alumina and
silica-alumina exhibited
temperature-programmed behaviors
different
compared with bulk oxidic materials. Desorption and reduction may be hindered or promoted depending on the nature of the oxide-support interaction.
In many cases, the intrincate character of the TPD/TPR patterns does not allow detailed
quantitative
treatment.
In
those
cases,
however,
temperature-programmedanalysis, can help the investigator in delineating the actual complexity of the system under study.
1v.2-2-
SUDDOrted
metal
catalysts
Temperature-programmedreduction (TPR) has in recent years become one of the most widely used physicochemical techniques for the characterization of metalsupported catalysts.
The main feature of the method is its capability of
continuously monitoring the consecutive reactions of reducible species at increasing temperatures.
It
can
thus provide
information about
the
dispersion states of the metallic component as well as the metal-support and metal-metal interactions in catalysis, since all of these affect reduction behaviour.
However, the reduction steps are sometimes so complicated, as
shown by many reduction peaks in the TPR profile, that it is not easy to identify each
of
them.
In
this regard, a
supplementary spectroscopy
technique (such as Mossbauer spectroscopy, etc) become useful, which can not
363
only determine the chemical states of various species in catalysts, but also work under 'in situ' conditions.
In general, a complex sample will produce a characteristic TPR pattern which can be used as a fingerprint. Of course, reproducing such a fingerprint will necessitate maintaining constant experimental conditions from one analysis to
Thus ( the catalytic researcher may use TPR analysis as a quality
another.
control test,
to
check
the
reproducibility of
a
catalysts precursor
preparation. Conversely, he may rapidly screen which preparative variables will actually affect the properties of a catalyst precursor.
The aim of this section is to present a few typical TPR and combined temperature program technique studies in order to illustrate the use of this technique in the field of catalyst characterization. An attempt will be made to interpret the result in the light of the theoretical concepts given in the preceding section.
IV. 2.2.1- Sunported monometallic catalVsts
Most work on the characterizationof materials using TPR has been carried out in relation to catalysts and predominantly on supported metal catalysts in the oxidic form.
In this section the work done is reviewed in the area of
mqnometallic supported catalysts using temperature program analysis such as TPR, T&
and TPO.
Supported catalysts often
have
very
high
metal
dispersions and
low
concentrations of active catalytic components. That is why it is difficult to characterize such catalysts by conventional methods.
Dispersed Pt/A1203 is extensively used as a catalyst, especially in the reforming process.
Treatments in oxygen and in hydrogen are established
procedures in the activation of fresh catalysts as well as in the regeneration of spent catalysts. Usually catalysts with a range of 0.3 to 0.6% platinum are used.
These are prepared by impregnation of alumina with platinum metal
salt (H2PtC16).drying in air at 393 K, calcined at 773 K and finally reduced in Hz at 773 K.
Therefore, to obtain a well dispersed catalyst, primary
attention to the pretreatment steps of drying, calcination and reduction
must
be given.
Lieske et al. (68) studied the formation of oxidized Pt species during treatments in oxygen at different temperatures using TPR.
Two surface oxides
364
a
and
and
P-[Pt02ls.
two
chloride-containing surface
complexes
rPtl"OxCLyls,that could act as a redispersing agents were found.
of
This is
supported by the fact that the temperature range where the agents can be formed,
is
the
same
as
the
known
temperature of
redispersion, i.e.
approximately 773 to 873 K.
They proposed that the TPR peak was due to reduction of a Pt-O-Cl containing surface complex stabilized by interaction with A1203. This was also supported by UV-visible spectroscopy studies. The importance of the presence of chlorine during reduction/dispersionof platinum can be assessed from the TPR results. The authors showed in fact that a Cl-redispersal step is essential in the regeneration of Pt/A1203 catalysts.
Similarly. Blanchard et a1.(69) using TPR, found in a Pt/A1202 catalyst that the interaction between the Pt particles and the alumina support came during He disagreed with Huizinga et
the drying step.
al.
(70) who claimed that it
depends on the time and the temperature, whether a well-dispersed catalyst is formed. McNicol (71) showed that the interaction between platinum particles and the alumina support takes place during the calcination step. However McNicol did not examine the TPR profile of catalysts that had not been calcined.
From these results, it is clear that, while the same general conclusions can be drawn about interactions with A1203, the TPR techniques enables the fine differences between catalyst prepared by different methods to be established. TPR
provides
a
method
for
studying
the
optimum
parameters
for
calcination/reduction treatments. TPR can be conducted over dried samples or over dried and calcined samples. Generally this type of experiment does not permit
the
study
of
chlorine evolution during
the
reduction process.
Consequently the residual chlorine content on the sample is only determined for reduced catalysts. Moreover, a TPR experiment may be quite different from a reduction conducted under isothermal conditions over several hours.
EUROPT-1 a platinum/silica containing 6.3% wt metal and prepared by Johnson Matthey for the Research Group on Catalysis of the Council of Europe, was characterized using
temperature-programmed desorption
(TPD)
(72).
The
analysis revealed the existence of four states of adsorbed hydrogen. The most strongly held
state
is
assigned to
spillover hydrogen treated, before
desorption, on the support. Most of the Hz is adsorbed dissociatively on platinum metal, providing a value for the degree of dispersion of the platinum of 65% (H:Pt = I:1 assumed).
365
The evidence of the number of adsorbed states found by TPD was corroborated by 'H nmr spectroscopy. Here again it is reinforced the importance of the use of supplementary techniques supporting temperature-programmedanalysis findings.
Rhenium catalysts have several interesting properties.
They show high
activity for metathesis, hydrodesulfurization and hydrodenitrification. It is known that Re also increases the stablitiy of Pt reforming catalysts.
Detailed
information
on
reducibiliy
as
such,
is
important
because
catalytically active Re sites are only created by reduction of the Re+7 ions, generally present in freshly prepared catalysts, to lower valencies as is the case when Re carbenes and
ReSz/Re metal are formed. Past studies mainly deal
with reforming catalysts and TPR methods have been used frequently (41 to provide information on the reducibility over a wide range of temperatures.
In an interesting work, TPR has been applied to characterize the reducibility of Rea07 supported on Al20a, SiOa and carbon catalysts (731. Figure 14 shows the TPR patterns of dried AlzOs-supported catalysts as a function of Re content.
At low Re contents, the TPR patterns are dominated by peaks which
are not related to reduction of Re ions. Differences in the concentration of additives such as chloride might explain these variations. A peak near 1100 K is found assigned to reduction of A120s impurities, such as iron, sulfide and sulfate.
The latter causes a (c.a.1 ZOO'K fall in for a higher Re content,
suggesting that this reduction is catalyzed by Re.
Figure 14 also shows the TPR patterns of dried SiOs-supported catalysts as a function of the Re content.
Besides Re'7 reduction, a reduction pattern is
observed between 800 K and 1200 K, which is typical for the SiOz support. As in the A120s case, it is assigned to the reduction of impurities. Again, one can conclude that Re catalyzes this reduction, since its presence causes the appearance of a second maximum in the SiO2 reduction pattern as well as a shift to lower reduction temperatures.
The pattern of dried carbon-supported catalysts as a function of Re content is also presented in Figure 14 for comparison.
The reduction peaks are small
compared with those caused by gasification of the support, occurring in the region of 500-1240 K.
In general it can be seen that dried catalysts are
found to contain a so-called monolayer-type Re+'lsurface phase as well as crystalline NHIReOl.
With respect to the detection of NHqRe04 crystallites
TPR has advantages over other techniques (such as XRDI. because TPR can used
for
quantitative analysis and
for
detection of
the
very
be
small
366 metal clusters.
Calcination at 575 K or 825 K resulted in decomposition of
the Re ammonia salt. formation of the Re+' surface phase and Rea07 clusters, and Re loss as a sublimation of Rea07.
WI-
-I_.1ur
Figure 14. TPR patterns (10 K/mini of dried Re207/A1203, p207/Si02
and
Re207/carbon catalysts having the following Re contents (at./nm'l : (a) 0, (bl 0.0080, (cl 0.040, (dl 0.20, (el 0.38, (f) 0.80. The upper and lower part of each pattern represent the TCD and FID signals, respectively. (From Ref. 73.1
Differences in reducibility of the various catalyst samples are ascribed to variations in interaction.
the
strength and
the heterogeneity of
the Re*? support
The strength of the interaction was found to depend on the
support material and
decreases
in
the
order:
AlaOs>SiOa>carbon.
heterogeneity was essentially the same for all three supports. varying literature data on
The
The largely
the reducibility of Rea07/AlaOs catalysts is
supposedly related with the presence of additives, such as chloride, which may +7 increase the Re support interaction.
367 From the clear
results
of
evidence
temperature
on
this the
and on the
metal clusters
it
is
shown that
structure
of of
the
and on the metal-oxygen
systematic
studies
reveal
the
work
reducibility
of
origin
of
they filled
their
goal.
In another
study,
the the
the
catalysts,
of
supported
inconsistencies
on TiOa,
with
TPR technique over e.g.
bond strength.
reduction
Pd supported
the
metal
a
is
wide
on the
The authors
of
presence
of
though that
ReROT catalyst
the
literature
given
range
might
date.
be
Already
AlaOa, SiOa and carbon was chosen as a
system (74). Palladium differs from other metal catalysts in its interaction with hydrogen because in addition to chemisorption, it absorbs hydrogen into the bulk (751.
This adsorption interferes with the chemisorption and may
cause an error in the determination of the dispersion of palladium catalysts. The reduction and hydrogen sorption for these catalysts were investigated by TPR and temperature-resolvedsorption techniques. Pd was found to be reduced at temperatures lower than 473 K.
Spillover of hydrogen from Pd to supports
occurred at higher temperatures. Hydrogen treatment at high temperature can also induce strong metal support interactions (SMSI) and sintering.
’ It was
found that under the same reduction conditions, the sintering of supported palladium catalysts follows the trend Pd/C>Pd/TiOa>Pd/AlaOa>Pd/SiOa.
TPR was able to demonstrate how the rate of sintering of the Pd metal could be related with the support employed.
The chemisorption of hydrogen on Pd has also been studied by Konvalinka et al. (761. Pulse-wise adsorption and temperature-programmeddesorption (TPD) on Pd samples and Pd on activated carbon catalysts showed how dissolved hydrogen can leave Pd via the surface, notwithstanding the fact that the chemisorber layer is more strongly bound kJ/moll.
recombination activation
(90-100 kJ/moll than the dissolved hydrogen (24
The energy trap of
a
enthalpy
good agreement
formed
subsurface
and
of desorption
correlation
by
is
between
a
the
surface
surface
easily
surmounted
hydrogen
atom
lowered.
The results
appreciably
the pulse
is
by
which
by the
show a
and TPO date.
Recently Leary et al. (501 presented evidence of the penetration of hydrogen into subsurface absorption sites of silica-supported palladium during TPD at conditions more closely resembles a catalytic reaction.
By TPD it can be
shown that penetration of an adsorbate into the subsurface region of an adsorbent
during
In Figure
15 it
of Ha is
TPD can produce is
shown that
a high
the high
temperature temperature
peak in the TPD spectrum. p3 peak in the TPD spectrum
not produced by desorption from a strongly binding energy adsorption
368
400
520 Tcmceroture
(K)
I .oo 0.9 5 0.7 5 d 0.6i) e 0.50 f 0.40 0.30 0.20
/
b
400
6GO
A 700
500 Temperature
600
700
(K)
Figure 15. Effect of initial coverage on the TPD spectra of (a) hydrogen and (b) deuterium on the Pd/Si02 catalyst for B s 0.5 K/s. Hydrogen spectra : catalyst dispersion 9% and helium flow rate 100 cc/min (STP). Deuterium spectra : dispersion 15% and Argon flow rate 50 cc/min (STPI. (From Ref. 50.)
369
site on the catalyst surface.
Instead, it is produced by hydrogen that
penetrates into subsurface adsorption sites in the palladium crystallites during the temperature ramp. When the surface becomes depleted by desorption, the subsurface hydrogen diffuses back to the surface and desorbs in the high temperature peak.
The observation that a significant amount of hydrogen can
penetrate into subsurface absorption sites of the Pd crystallites during TPD strongly suggests that hydrogen also is present in the palladium subsurface under typical reaction conditions.
It is also noted that these results are
consistent with the observation of subsurface absorption sites in Pd (1101 single crystals studies under ultra high-vacuum conditions.
In the past 15 years Rh has been gaining importance in catalytic chemistry. Not only is Rh widely recognized as the best catalyst to promote the reduction of NO in three way catalysts, but also takes a special place in the conversion of synthesis gas, since its product-range can comprise oxygenated products as well as hydrocarbons. Supports are often impregnated or exchanged with metal chloro-complexes. A great number of data collected in the literature concern with RhC13+H20 as a precursor of supported rhodium catalysts. To give an idea of the chloride evolution on supported rhodium precursors the TPR was studied by Marques da Cruz et
.Hydrogen-oxygentitration and specific rate as
al.(771.
well as turnover frequency have shown that total reduction may occur at relatively low
temperatures and
without
total dechlorination.
Strong
interactions between support and metallic complex control the initial and final temperatures of reduction and dechlorination. measuring
chloride
evolution
is
The technique used for
recommended
simultaneous
for
temperature-programmedreduction when hydrogen consumptions are measured.
Vis et
al.
(781 studied supported Rh/A1203 and Rh/Ti02 catalysts with varying
metal loadings by TPR and TPO and hydrogen chemisorption.
In the case of
hydrogen chemisorption, it was shown that all the Rh on A1203 was well dispersed, while the dispersion on Ti02 was much lower.
TPR/TPO showed that
this was due to the growth of two different kinds of Rh/Rh203 particles on Ti02;
one kind was easily reduced/oxidized,with a high dispersion, and the
other kind was harder to reduce/oxidize, with a low dispersion. that
in
this
study
of
catalyst
varying
metal
It is seems
loading,
the
temperature-programmedreduction technique was able to demonstrate how the reduction properties of the metal could be related to the dispersion of the metal.
Among the Group VIII metals Ru occupies an intermediate position between Fe,Co, Ni and noble metals as may be demonstrated by
the metal-oxygen
370 interactions for Pt. Ru and Ni in Table XII.
TABLE XII Metal-Oxygen interaction for Pt. Ru and Ni
Pt
Ru
Ni
-134
-220
-243
-109
-176
-235
293
433
773
Heat of formation of oxide (kJ mol-II
Heat of chemisorption of oxygen (kJ mol-l1 Reduction temperature of oxide (Kl
When exposed to oxygen at room temperature, a freshly formed surface of Ru becomes covered by
a
film
of
oxide, which will
thicken at
higher
temperatures. The formation of oxides on the Ru surface causes the induction phenomena as found in Ru-catalysed liquid-phase hydrogenation reactions (79).
Koopman et aJ.(SOI, using TPR, studied the oxidation and subsequent reduction of Ru. R&IO2
prepared by direct reduction of RuC13/Si02 showed surface
oxidation followed by bulk oxidation upon exposure to air at room temperature. The application of higher reduction temperatures (up to 973 Kl of RuC13/Si02 was found to increase the active metal surface area of the Ru/SiO formed due 2 to the removal of traces of chlorine from the Ru surface. Sucessful measurement of Hz desorption by means of the TPR gives rapid and useful information about the Ru metal dispersion.
The interaction between Ru metal and MgO support during catalyst preparation was studied by Bossi et
al.
(81). They also warning the possible danger of
doing TPR using Hz/N2 gas mixtures when the material being reduced contains components that might be catalytically active for the synthesis of ammonia.
Jones and McNicol (61, interpreted Bossi et. al. results in the light that repeated reduction-oxidation process leads to
aweakening in
the Ru-MgO
interaction originally formed since the final reduction peak is substantially lower in temperature than that of bulk Ru02.
Temperature-programmedreduction (TPR) experiments on copper on titania- and
371 silica-supported catalysts were studied by Delk et al. (821.
The results are
shown in Figure 16, in which the most obvious and significant difference is the lower reduction temperature of the titania-supportedcopper. and 4% Cu/Ti02 show reduction peaks at 403 K. 493 K in the 4% Cu/Ti02.
Both the 1%
There is a second peak at -
In the 1% Cu/Ti02 there is a shoulder at the same
temperature. In the case of the silica-supportedcopper. the reduction peaks are found at 541 K and 502 K for the 40% and 1% Cu/Si02 samples, respectively. The 1% Cu/Si02 also displays a shoulder at - 548K. The higher temperature +2 peak arises from reduction of Cu species that are not in such close contact with the support.
An interesting feature is consumption of hydrogen at
temperatures above 573 K. after all the Cu is reduced. This was interpreted as being due to the reduction of the titania support catalyzed by a copper metal.
0 0
TEMPERATURE,
C
Figure 16. Temperature-progzammed reduction of supported copper catalysts following calcination at 450 C (al 4% Cu/Ti02, (b) 1% Cu/Ti02. (cl 4% Cu/SiO2, (dl 1% Cu/Si02 (From Ref. 82.1
Temperature-programmedreduction studies suggest that there is an anomalous copper-support interaction involved by high temperature reduction. Correlation of TPR and chemisorption date suggest that this is directly related to
372
copper-catalyzed reduction of the titanla support to a lower oxide state. We believe that this is a good demonstration of strong-support interaction (SMSI) on Cu on titania supported catalyst by using TPR.
The wide range of applications of supported nickel catalysts in hydrogenation and various stabilization and hydro-treating processes has been the reason for a great deal of studies aimed at their description and characterization, and
better control of
catalysts silica
their activity and selectivity. TPR on NiLSi
showed that the amount of Ni in strong interaction with the
increased
with
decreasing
loading
and
increasing
calcination
temperatures. This suggests that a chemical rather than a transport effect is dominant. The influence of calcination can be shown in Figure 17, in which as calcination temperature increased the reduction becomes more difficult and metal particles of reduced species were smaller.
640 K
6 634 K
“‘; 746 K
I
602 K
610K \
667 K C 720 K
766 K I
683K
Figure 17. Effects of calcination temperature on the TPR profile of 1% nickel on micronized Gasil 35 silica. (Al 623 K. (BI 673 K, tC) 773 K, (II) 973 K. (From Ref. 83.1
373
Furthermore, increase in the metal loading led to an increased population of Ni arising from the low temperature reduction peaks and as a consequence larger Ni particle sizes, as can be seen in Figure 18. Reducibility of Ni on r-Al203 is much more difficult than on silica (83).
The interaction of Ni
with A1203 is clearly much stronger than the interactions that occur with silica. *
649 I(
B
Figure 18. Effects of nickel loading on TPR profiles. Standard conditions (A) 1% W, (B) 5% W, (C) 9.5% W, sensitivity from A to D. (From Ref. 83.)
Micronized Gasil 35. (D) 31% W. Reduced
Mile and Zammit (83) showed the capacity of TPR to reveal the characteristics of the nickel metal and nickel oxide phases that could not be able to detect by using another more complicated and expensive spectroscopy technique.
The location of Nickel oxide and nickel in silica supported catalysts was studied by Mile et al. (841.
As shown in Figure 19 the TPR of bulk nickel
oxide consisted of a single reduction peak at 673 K slightly skewed toward lower temperatures.
At 523 K the peak was found to be small.
Supported
nickel on silica shows the marked effect of the support in producing a number of peaks and broadening the whole profile to much higher temperatures. There are three reduction peaks at 523. 673 and 773 K and a chemisorption peak at 533 K obtained on cooling the sample from 873 K to ambient in the N2/H2 carrier gas stream.
374
Figure 19. TPR of unsupported NiO and TPR of NiO supported on miczonized Gasil 35 (8.8% w/w) loading). TPR conditions: 60 mg of sample, R = 12 C/rain,5% H2 - N2 calcination at 4OO'C for 16 h. (From Ref. 84.)
In conclusion, the authors were able to distinguish two different types of NiO using TPR with a temperature difference of _ 100 K, with the more reducible oxide is located mainly in the small pores and the less reducible oxide located in the large pores.
Iron has been used successfully in Fischer-Tropsch reactions. TPR was used by (851 to study the profiles for Fe/Si02. Reduction of cr-Fe203near equilibrium conditions should be quantitative to Fe304 before final reduction to metallic Fe.
Fe0 was not encountered since the temperature never exceeded 843
(Figure 20. Table XIII).
K
To identify reactions corresponding to the OL and /3
peaks, mass spectroscopy was recorded for TPR samples removed after each peak. The spectrum of the Q peak sample was well characterized by parameters for Fe304, the absence of any detectable aFe203 spectrum indicated complete conversion to Fe304. The spectrum of the j3peak sample was that of aFe metal. Yuan et
al. (861 also
studied Fe
catalysts using an
TPR-Mossbauer spectroscopy technique.
Two
10%
in situ
Fe/A1203
combined
catalysts are
375
+t
+I
*
-
.
0
-
m
In
.
376 With these combined techniques, various reactions occurring
identified.
during the TPR process of the catalysts were revealed (Table XIV).
NOMINAL PROGRVtlING RATE (oC/MIN) A.
17.0
6.
8.0
C*
5.5
c
\
0
100
200
300
4G0
500
600
TEEPERATURE('Cl Figure 20. Temperature programmed reduction of Fe/Si02.
(From Ref. 85.1
It was found that although the ion-concentrationwas high considerable strong metal
(oxide) support interactions occurred in the catalyst with higher
surface area, and the TPR of the sample consisted of three consecutive stages, namely at ground 733 K, 1063
K
and above 1123 K.
In the first stage from 573
to 873 K, mainly the reduction of Fe(II1) to Fe304 and then Fe304 to Fe(II1 aluminate took place.
It was amazing to find that over the temperature range
of 743-873 K, Fe304 was converted into Fe(I1) and FeCIIII aluminates without hydrogen being consumed. An increase in the amount of Fe(III1 was observed
377
378
when increasing the TPR temperature from 743 to 873 K.
In the meantime, the
Fe(II1) particles which remained unreduced, were also transformed to Fe(III1 aluminate at these temperatures.
In the second stage from 873 to 1123
K
Fe(II1) aluminate was reduced giving rise to the formation of Fe(II1 aluminate and Fe(O). Fe(O).
In the final stage above 1123 K, Fe(I1) aluminate was reduced to
It was found that the reduction of Fe(II1 was accompanied by a
migration of the Fe(II1 ions from the octahedral sites to the less stable tetrahedral ones, and thus facilitated the reduction. Chemical control of the reduction was present during the TPR over the temperature range of 743-973 K.
It can be seen from the above paper that complementary Mossbauer effect spectroscopy and X-ray diffraction studies largely support the TPR results and conclusions.
From the different papers reviewed in this section on monometallic supported catalysts we can conclude that the temperature-programmedanalysis technique has provided valuable information on the ease of reduction, valency state of metals, metal-support interactions and the influence of one component in the support on the reducibility of another component.
It was also found that many authors used TPD-TPR-TPO extensively as a diagnostic tool for changes caused by variables during the preparation stages of the catalysts. They were able to find that the temperature profiles can give an indication of the actual activity and selectiviy of the final reduced catalyst, measured using another techniques.
IV.2.2.2 -
Supported bimetallic catalysts:
For the past 20 years, bimetallic catalysts are a subject of considerable interest in catalyst research because their performance differs markedly from that of their components, and consequently.the constitution of supported bimetallic catalysts has been the object of a great deal of work.
In order to understand the catalytic behaviour of bimetallic catalysts, the most fundamental question is. whether the catalyst particles indeed contain atoms of both metals.
A direct experimental verification is difficult due to
the limitation of existing physical techniques.
The temperature-programmedreduction and oxidation techniques (TPR-TPO) may however be used to obtain evidence for the interaction between the atoms of the two metallic components.
379
In the case of Pt-Re/Al202 catalysts, there has been some controversy in the literature concerning the reducibility of rhenium in supported catalysts, in particular regarding the extent of its reduction.
Temperature-programmedreduction (TPR) has been used to examine the effects of drying on the reduction of Pt-Re/A1202 catalysts (871. effect on the reduction
of
Drying has little
the monometallic Pt/Al,O, and Re/Al,O, as can be
seen in Figures 21.
Pt
Figure
21.
Temperature programmed reduction of
Pt/A1203, Re/A1203
and
Pt/Re/A120g catalyst. (From Ref. 871. In constrast, both the number of TPR peaks and the peak maximum temperature of the Pt-Re/A1203 vary with drying temperature (Figure 211.
The TPR profile of
380
the Pt-Re/A1203 catalyst dried at 373 or 573 K consists of a single peak with a peak maximum temperature similar to that of monometallic Pt/A1203. The size of this peak corresponds to the reduction of both the Pt and the Re. other temperature extreme of temperature-characteristic
(773 K)
monometallic
of
temperature-characteristic of
At the
two TPR peaks are found, one at a and
Pt
monometallic Re.
at
one
a
interpreted their
They
results on the fact that water influenced the rate of Re207 migration to Pt centers, the mobility of Re207 and hence, its rate of diffusion to Pt centers was dependent on drying temperatures.
This is similar to the findings of
McNicol (711 who observed a TPR profile for Pt-Re/A1203, dried at 673 K identical to the sum of the monometallic profiles also dried at 673 K. The results suggested that water influences the rate of Re207 migration to Pt reduction centres.
Low-temperature drying below 273 K does not remove the
water adsorbed by the catalyst during storage and the Re207 is still hydrated. The suggestion is that this hydrated Re207 is mobile and is able to migrate to the Pt reduction centres.
Thus, the reduction of the two metal oxides is
essentially simultaneous, resulting in a single TPR peak, and an alloy is formed.
In contrast, high-temperature drying, 773 K,
dehydrates the Re207
which is no longer able to migrate to the Pt reduction centres.
The Re207
therefore reduces at the temperature of the monometallic Re/A1203 resulting in two TPR peaks and by implications no alloy is formed.
In another, work Wagstaff and Prins (881 used TPR to characterize the finely dispersed metal compound in a
catalysts. Strong series of Pt-Re/Al 0 23 evidence has been obtained that zero valent Pt and Re atoms are in intimate contact with each other after catalyst reduction. The formation of bimetallic clusters support the explanation of alloys causing the improved performance of the bimetallic system. Treatment of the reduced catalyst with oxygen above
473 K causes segregation of platinum and rhenium oxides. reduction of a platinum-rich species around rhenium-rich species around'523 K. 373
K
leaves
the
bimetal
273 K
Then a TPR showed
and reduction of a
Adsorption of oxygen at temperatures up to
clusters
largely
intact
but
subsequent
high-temperature treatment in the absence of extra oxygen leads to segregation of Pt and Re species.
Mieville (891 also observed in a TPR that an interaction occurs with Pt and Re when Pt-Re catalysts are preoxidized at 573 K or lower: re-reduced at lower temperatures compared to Re/A1203. Pt-Re
show no
interaction and
At 773 K preoxidation, TPR profiles of
are
similar to what
they would
be
if
superimposed Pt-Re catalysts, previously exhibiting catalytic reduction. Intimate mixture of Pt/A1203 and Re/A1203 particles preoxidized at 573 K, also
381
exhibit reductive interaction; however, loose mixtures of relatively large particles do not.
The explanation could be that H2 spillover initiates Re
oxide reduction by creating nuclei of Re, which then catalyze the reduction of the remaining oxide.
The overall mechanism is controlled by the rate of
spillover which in turn is determined by the degree of hydration of the alumina surface.
The implication of TPR results from the above results is, that the reduction step is of critical importance in the preparation and regeneration of a Pt-Re reforming catalyst, since the interaction between the metals apparently occurs during this step.
Another intriguing bimetallic catalyst system is that of Pt-Sn. Similar to rhenium in Pt-Re, tin is known to assist in maintaining the activity of platinum reforming catalysts. This still doesn't explain what happens when tin oxide, supported on alumina, is reduced. Dautzenber et
al.
have
(901
determined the oxygen which is consumed when tin oxide, supported on alumina, after first having been reduced, was oxidised again. The TPR-technique was used
thereby. They
concluded that the
tin oxide
is
fully
reduced to
zero-valent tin. They also report however, that the first 0.6% (wtl of Sn cannot be reduced, because it is bound chemically to the supporting alumina.
Contrary to this, results obtained by Burch (91) and Sexton et
al. (921 show
the tin-valency does not appear to drop below 2 (Sn II) for a wide range of tin contents. In fact, there is a sharp peak near 523 K in figure 22 in the Pt-Sn catalyst-spectra that must be associated with the reduction of platinum. Pt
(IV) is completely reduced to Pt
(01. When calculating the extend of
reduction in tin-valency for all series of the experiments concerned, it was found that the valency-drop of the tin from the original IV to 0 amounted to approximately 50% Z 10%. The presence of platinum did not have the effect of increasing this drop. Hence, the average valency after the reaction must have been Sn (111, as concluded from the TPR-data. Burch claimed that tin cannot exert a numbers of different effects on clusters of Pt-atoms present at a reacting surface. It was
suggested that the special properties of
the
catalysts are due to a change in the electronic properties of small particles.
This could be due to interaction with Sn(II)-ions on the support-surface - or - a small percentage of metallic tin particles could associate with the Pt in the form of a solid solution. It is interesting to note that Burch and Sexton on the one hand, and Dautzenber on the other, although they worked in different laboratories, used the same techniques and arrived at
totally
382 different conclusions.
It is seems that the differences can not be ascribed
to differences in the TPR techniques. The reason could be placed in the different catalyst preparation and pretreatment techniques adopted by both groups.
Snl,.A120,TPR lawa
rcgia)
R-Snll.A1203TPR i4
900 0
loo
200
300 TEMPERATURE
Km
SW
300
400
TEMPERATURE *C
600
.C
Figure 22. Temperature programmed reduction (TPR) profiles for Sn (upper) and Pt-Sn (lower) catalysts prepared by impregnating (al an acidic solution of Sn dissolved in aqua regia into r-A1203, (bl an acetone solution of Snc14.5H20 into I-A1203. The loading of Pt was 0.5 wt %.
(From Ref. 92.1
Here again, in a recent work (931, it was found that the average oxidation state of tin in the catalyst was Sn(II), as judged from TPR and TPD-profiles. The possibility of small amounts of zero-valent tin and/or alloys being present thereby exists. TPD and TPR profiles were used in conjunction with neutron activation analysis quantitative data.
that enables one
to
obtain
more
accurate
383
The inescapable conclusion from all those results is that it is important to understand that the complexities involved in catalyst preparation can affect the
ultimate
structure of
the
sample
and
the
interpretation of
the
temperature-programmedanalysis data.
The combination of platinum and iridium supported on alumina, has been studied by Wagstaff and Prins (941. They observed a single TPR peak (Figure 231 with a maximum at 378 K and ascribed this peak to the reduction of Pt(II1 oxide and They suggested that the
Ir(IV) oxide in intimate contact with each other.
appearance of the single peak indicated that the catalysts consisted of bimetallic clusters.
On the other hand, Foger and Jaeger (951 claimed that they believed it is dangerous to use TPR experiments in isolation when determining the structure of bimetallic catalysts, since, in the absence of additional data, TPR results are subject to ambiguity. The Pt-Ir/A1203 catalysts were studied by combining the
technique
of
TPR
with
transmission
microscopy
(TEM),
diffraction
(XRD).
electron
selected-area electron diffraction (SAED) and
x-ray
According to their results, only the iridium was oxidized when Pt-Ir supported alloys are heated in oxygen.
fi i
I i / !
,I ’
! ! I i i
i~
100
TR)
i
200
5m
Figure 23. TPR profiles for a prereduced 2 wt % Pt-Ir/r-A1203 catalyst reoxidized under the conditions indicated. (From Ref. 95.1
As illustrated in Figure 23, a single peak in the TPR profile with a maximum at 353-363 K was obtained if the oxidizing temperature was below 673 K, but a fairly sharp second peak with a maximum at 483-503 K appeared if the oxidizing temperature was above 673 K. The differences in the results can be explained from the point of view that Wagstaff and Prins worked with r-alumina as a support; whereas Foger and Jaeger were using Si02-supported catalysts.
It is clear that the interaction between the metal species and the different carriers plays an important role in the reduction temperature as found by the TPR results.
The technique of temperature-programmedreduction (TPR) has also been used to investigate the extent and rate of dehydration of the catalyst prior to reduction (961. To examine this effect, the maximum rates of reduction of Pt/A1203, Ir/A1203 and Pt-Ir/A1203 are plotted as a function of the percentage dehydration of r-alumina, and the results are shown in Figure 24. For monometallic
catalysts
there
is
linear
a
relationship,
while
the
co-impregnated catalyst shows a sharp deviation from a straight line at - 50% dehydration.
1
----apt -0 Ir -*.-a Pt/lr
-I 0
IO
20
Jo
40 PERCENT
50
60
70
80
90
100
DEHYDRATION
Figure 24. Effect of TPR spectra of Pt/A1203, Ir/A1203 and Pt/Ir/A1203 catalysts as a function of the percent of hydration of the r-A1203 support. (From Ref. 96.)
385
The reduction of Pt and Ir co-dispersed on A1203 is very sensitive to the pretreatment conditions, especially to the extent of dehydration. readsorbs strongly on
the A1203 support during TPR.
Water
Furthermore, its
readsorption properties increase with increasing degrees of pre-hydration. This is due
to the increase in the free-energy of
activation with an
increasing degree of dehydration. Such effects will broaden TPR spectra and shift them to higher temperatures.
A new system used in catalytic reforming is Pt-Ge/A1203 catalysts.
Again
here, the main question arises as to the real nature of the interaction between the metals in this catalyst.
Goldwasser et a1.(97) studied two series of Pt-Ge/Al203 catalysts, that were prepared using different calcinatlon procedures. Both series were calclned at 673 K after the addition of both components, while one of them was calcined prior to the addition of platinum at 873 K. The TPR showed that there was complete reduction of Pt(IV) to Pt(O) for all catalysts and partial reduction of
the germanium (IV) precursor in the case of high
catalysts for the series only calcined at 673 K.
loading germanium
It seems that this behaviour
can be explained by the electronic surface model (911, where the special catalytic properties of the bimetallic catalyst are due to a change in the electronic properties of
the platinum, presumably via electron withdrawal by
the reduced germanium ions. The results in the other sense (calclnatlon prior to the addition of Pt at 873 K) indicate that germanium. after calclnatlon at 873 K is stabilized in the alumina support in such a way that its reduction does not occur under TPR conditions; there is no catalytic effect in respect of the
reduction of germanium (IV) by the metallic platinum.
dilution or
geometric effect
would
be
almost
Evidently, a
impossible under
these
conditions.
Recently, De Miguel et al. (981, using TPR, showed that calclned Pt-Ge/A1203 would contain approximately 40-50X of Ge(O) after reduction at 1073 K, unlike the Ge/A1203 catalyst, where Ge was not reduced to the zero-valent state;
it
can be inferred that a fraction of Ge would be reduced to Get01 below 723 K in Pt-Ge/A1203 catalysts.
Assuming that Ge(IV) could be reduced only to the
zero-valent state up to 723 K in bimetallic catalysts, the percentage of Ge(O) calculated was approximately 25% of the total Ge content as shown in Table XV.
In order to calculate the relative quantities of the Ge species with different oxidation states present on calclned Pt-Ge/A1203 reduced up to 1073 K, they assumed two hypotheses.
According to hypothesis I it was supposed that
* Hypothesis
Ge/A1203
Pt/A1203
Nonometallic
(calcined)
Pt-Ge/A1203-C
(chlorided)
Pt-Ce/A1203-B
(calcined)
Pt-Ge/A1203-B
(calcined)
12.71
12.71
12.71
Ge(IV)
13.12
catalysts
I:
the
13.12
catalysts
Pf
in
is
directly
36.37
-
29.76
30.86
30.86
27.55
Ge
sample
metal
reduced
0.00
26.24
25.42
25.42
25.42
26.24
Pt(IV)+(Pt(O)
for
Theoretical
in TPR experiments
urn01 of
consumption
Pt-Ge/A1203-A
Bimetallic
Sample
Hydrogen
TABLE XV:
for
to
Ge(0); hypothesis
36.37
29.76
30.86
30.86
27.55
Ge(IV)+(Ge(II)
(pmol)
catalysts
H2 consumption
different
II:
(98) Experimental
Ge(IV)
0
26
41
35
40
40
723OK
(umol) ~
to
is
first
29
26
67
58
70
69
1073°K
up
H2 consumption
reduced
60
-
30
47
28
22
-Ge(IV)
I
to
40
-
70
53
72
78
to
Ge(I1)
G.=(O)
Hypothesis
Ge in different reduction up
Ge(IV)
and
20
-
0
0
0
0
then
80
_
60
94
56
45
to
Ge(I1)
after
0
40
6
44
55
Ge(0)
II
Ge(0).
states
Hypothesis
oxidation 1073%(X)* at
0
26
15
24
25
723°K
Percentage reduced to
of Ge Ge(0)
387 Ge(IV) was directly reduced to Ge(Ol according to:
Ge02 + 2H2 -
Ge
+ 2H20
1681
while for hypothesis II it was assumed that Ge(IV1 was reduced to Ge(II1 and after all the Ge(IV1 had been transformed to Ge(I1). this was reduced to Get01
These results and the fact that TPR profiles of bimetallic samples (below 723 Kl show both a broadening of the first reduction peak and an intermediate reduction zone, would indicate a certain catalytic effect of Pt on the Ge reduction.
The results are
in good agreement with
those published by
Goldwasser et a1.(97)and Beltramini et al. (991 who obtained values between 25-50X Ge reduced to the zero-valent state.
It is clear that in bimetallic reforming catalysts, the TPR profile is a useful indicator of catalyst conditions in the calcined and reduced state. This is important for the study of the redispersion process of the catalyst. TPR, then could be applied as a fairly rapid, reliable and cheap technique for the study of a particular catalyst preparation process.
It is known that MO-based hydrotreating catalysts are widely used in the oil industry for the removal of organo-sulfur and organo-nitrogen contaminants. The catalysts are sulphided before use and there is general agreement that the active component is a MoS2 crystallite promoted by Co or Ni.
There is still
some dispute however, concerning the nature of the active site and the way in which Co or Ni acts as a promoter.
A series of hydroprocessing catalysts Mo03/A1203 having various Moo3 contents, Co-Mo/A1203, NiMo/A1203 and Ni-W/A1203 were characterized by TPD with He, TPS with H2S in H2 and TPR with H2 (1001. The results show that a decreasing temperature of appearance of H2S peak in the TPR spectra of sulphided Mo/A1203 catalyst corresponds to an increasing MO content and increasing catalytic activity.
TPR results provide a rapid qualitative test for the presence of
incompletely sulf.ided Moo3 on
the catalyst surfaces. A
summary of
the
quantitative TPD and TPR results are presented in Table XVI. TPR associated with the formation of sulfur anion vacancies on the surface indicate that only a small fraction of the sulfur is removed by reaction with H2, this fraction being higher for the promoted catalyst Co-Mo/A1203 than for the less active, unpromoted catalyst Mo/A1203.
The succesful application of the modified TPR, TPD and TPS tecniques for
0.14 0.19 0.20 0.10 0.15 0.08
473 473 673 523 493 473 443
353
353
353
353
353
353
353
Ni-Mo/A1203
Ni-W/A1203
y-AI203
2.5% Mo03/A1203
9% Mo03/A1203
10% MoO~/A~~O,~
15% Mo03/A1203
to transition
d S/(Mo + Ni)/2
+ Co)/2
M refers
c S/MO
b
metal atom.
a S refers to sulfur desorbed
0.14
0.40
0.13
0.25
0.20
0.48
0.38
0.40
0.29
0.23
Remainder
as H2S in the first peak TPR.
0.17
473
19% W03/A1203(E)
358
0.10
503
12% Mo03/A1203(E)358
0.29
0.24
473
353
Co-Mo/Al203
Peak I
TPD
H2S desorbed
(100)
TPR
Onset temperature for H2S desorption ('R)
TPD and TPR Results
TPD
Sulfided catalyst
Summary of Quantitative
TABLE XVI:
g
0.80
0.25
0.60
0.45
0.80
0.24
0.30
0.67
1.20
0.11
Remainder
S/(Ni + W)/2
S/W
f S/MO
e
0.14
0.10
0.04
0.06
0.07
0.05
0.22
0.40
0.28
Peak I
TPR
(molecules/rim*)
0.06g
0.06f
O.Olf
0.02f
0.05f
0.07f
0.06e
0.04d
0.05=
S"lMb , atomic ratio (Peak I, TPR)
389 hydroprocessing clearly
catalysts,
demonstrated
Ni-Mo/A1203 catalysts prepared techniques
(101). does
adsorbed.
the
containing
by wet impregnation,
catalyst
of
which
the total
rather
Ni-MO catalysts
temperature
on
the
has shown that
the Ni
or MO catalysts.
model
(Figure
contains for
oxidic
and sulfidic
25)
shown that the
in
or
forms
it
is
is
other
The results it
is
were
in the stoichiometry
of
a constant
the
that
that
scheme was
in a
with
MO
from
terms
of
typical
presented
of such a crystallite
all
sulphided
differently
interpreted
H2S
fraction
associated
behave
envisaged
A reaction
Ni to a Mo/A1203
nature
hand TPR of
the promoted catalysts
MO were
TPD and TPR
was suggested
MO catalysts On the
and
using of
the
peak represents
Consequently,
support.
which
of Ni
the addition
quantity
desorption
33 MO atoms.
the changes
concentrations and characterised,
alumina-supported
than with
simple
accounts
is
either
either
crystallite
different
amount of H2S desorbed.
H2S adsorbed
sulphide
affect
The high
in both
sulphiding
From TPD it
not
exist
here.
a
MoS2 which
during
TPD
and TPR.
I
He1773
4 Hz1773
V
Figure
25.
Model
of
MoS2 crystallite
during
TPD and
TPR (0)
sulphide
(where this occurs on an edge only one of the two sulfide positions and bottom layers is occupied); (0) SH ion either in the top or V, layer; (e) SH ion in both top and bottom layer; (e) MO ion; both top and bottom layer positions. (From Ref. 101.)
ion
in the top the bottom vacancy at
390
Furthermore, Burch and Collins (101-1021 studied a catalysts, using TPR.
to obtain information on
series of
industrial
the behaviour of
these
catalysts before and after regeneration. Figure 26 shows the different TPR profiles of used and fresh catalysts. The fresh catalyst has a peak maximum at 487'C and a shoulder at 527'C, whereas the regenerated catalyst has a peak at 557OC and a shoulder at 547'C.
It is shown that the regenerated catalyst
has become more difficult to reduce at low temperatures. However, the long tail for the fresh catalyst at high temperatures is less apparent for the regenerated catalyst which could indicate a decrease in the amount of highly dispersed MO
present.
The
decrease in
the
case
of
reduction after
regeneration is consistent with the lower surface area and the lower HDS activity for this catalyst. From this paper it is important to conclude that TPR can provide a quick and easy way of checking the effect of regeneration on the HD5 catalysts.
T/K
Figure 26. TPR profiles for regenerated catalysts (al Fresh catalyst, (bl and (cl repeat TPRs on two samples from the same batch of regeneiated catalyst, (dl regenerated catalyst after heating for a further 40 h at 723 K. (From Ref. 102.1
391
The use of TPR
technique is likely to be fundamentally more reliable than the
currently used measurement of total surface area. However, the authors claimed that further work will be required to determine whether the method is of general applicability.
Van't Blik and Prins (103-104) studied the effect of support on bimetallic Co-Rh catalyst using TPR techniques. They concluded:
-
On Co-Rh/A1203 catalyst, TPR showed a reduction peak at a much lower
temperature than that of Co/A1203. This and the slight shift relative to the peak of Rh/A1203 indicates that cobalt and rhodium ions are not far apart after coimpregnation.
This explains why bimetallic particles are easily
formed during reduction.
Oxidation at room temperature of
the reduced
bimetallic catalyst leaves the structure of the bimetallic particles largely intact; but cobalt is oxidized to a large extent while rhodium remains metallic.
TPR analysis of these catalysts also suggest that in the reduced
state the bimetallic particles are already surface-enriched in cobalt.
-
On Co-Rh/TiO*, the addition of rhodium to cobalt caused the resulting
bimetallic catalyst to be more difficult to oxidize, while the reducibility of the catalyst depends on the oxidation temperature. When oxidized below 800 K, the reduction proceeded at low temperatures, indicating that Rh203 was present To complete the oxidation,
in the surface of the bimetallic particles.
however, higher temperatures were needed and under these circumstances cobalt rhodate, CoRh204, was formed.
The TPR reduction behaviour of a oxidized
catalyst reveals that only cobalt oxide was
exposed, demonstrating the
oxygen-induced surface enrichment by cobalt.
-
On Co-Rh/Si02 catalysts, the reduction proceeds at lower temperatures
than the reduction of Co/SiOB catalysts, indicating that Rh catalyzes the reduction of the cobalt metal salt and cobalt oxide.
From combined TPR and
EXAFS analysis it is concluded that the reduced catalyst contains bimetallic Co-Rh particles, the interior of which are enriched in rhodium, while the outer layers contain more cobalt.
From these studies become clear that TPR is able to supply useful information about the structure of the reduced bimetallic Co-Rh supported on different carries.
On another TPD study on Cu-Zn catalysts for methanol synthesis (105). it was showed that the spectra displayed two peaks.
Only low temperature peaks (-
480Kl which might correspond to a reoxidation stage of copper, may reasonably be correlated with catalytic activity at -483K.
High temperature peaks (-
600Kl correspond to hydrogen desorbing from zinc oxide, and do not seem to Results from this work
form part of the methanol synthesis (Figure 27).
demonstrated that TPD profile for H2 can provide pertinent information than direct chemisorption.
In this chapter some of the most important applications of TPR and TPD to the characterization of supported mono- and bi-metallic catalysts were reviewed already. The work done was mainly concerned with the characterization of the catalysts, since it
is one
of
the most
important applications of
temperature-programmetechniques in this area. readily identified.
the
A number of phenomena can be
These comprise metal dispersion, interaction between
metal-support, formation of cations and ion species.
alloys, soluble solutions, identification of
The later can help during the catalyst preparation
to improve catalyst performance of these materials. It has been clearly demonstrated
that
reproducibility of
temperature-programmed analysis results from
laboratory to
can
give
excellent
laboratory when
similar
conditions of experimental and catalyst preparation are used. In those where different workers have found results at variance with others, there has usually been a clear explanation for the difference, usually associated with different conditions and/or different catalyst preparations used by
the
workers concerned. The technique has been shown to be extremely sensitive, witness
the
excellent
temperature-programmed profiles
from
catalysts
containing only a few tenths of a percent of reducible material.
As a conclusion, TPR results can be used in conjunction with those obtained from other complementary techniques such as XPS, XRD, SIMS, etc. in order to quantify the use of the data and to get a clear identificationof the catalyst surface phenomena.
IV.2.3 Metal Zeolite Catalysts
Zeolites have been used as catalysts and separating agents in the chemical and petrochemical industries (106). The alkali metal ion present in the zeolite is generally exchanged with other cations by heating the zeolite with an appropriate aqueous solution of the salt containing the metal to be exchanged on the zeolite. The metal salt may be a transition metal salt or an ammonium salt.
The exchanged zeolite is subject to drying and calcining in order to
generate a framework, containing surface hydroxyl groups that are acidic and active for carbonium ion reactions. The transition metal, which may be used
393
.. .’
: . . .
:
:
.
: .
. . . . . .
T(K)
0
Figure 27.
(a) Thermodesorption
100 -r(Y)
50
profiles
for
H2
from
Cu/Zn/Al
catalysts(b) Correlation between the amounts of H2 thermodesorbed (I: low temperature peaks, II: high temperature peaks) and the activity in methanol synthesis. (From Ref. 105.)
394
for rendering a hydrogenation or other function in the catalyst, is then reduced to the metallic state.
It is important to know the positions of the
zeolitic cations in the framework to understand the catalytic functions. Temperature-programmedanalysis techniques are used for the characterization of metal zeolite catalysts. The analysis enables the characterization of:
the state of cations and in some cases, to say something about their location in zeolite (TPR).
surface acidity in regard to both the amount and the strength of acid sites (TPD).
sorption and diffusion studies (TPD).
IV.2.3.1
Reduction of Metal Zeolite Catalysts
Hurst et al.(4) reviewed reduction of metal zeolite systems using temperature -programmed reduction technique. They reported reduction of Cr-X. Cr-Y. Mn-X, Fe-X, Ni-X, Ni-Y, Cu-Y. Ru-X, Pd-X and Pd-Y zeolites. Table XVII presents the TPR data of various metal zeolites reported in the literature.
The reduction of metal ions in zeolite is dependent on the method of reduction and pretreatment.
The reduction of metal ions in zeolite is generally
achieved by using hydrogen and can be represented as:
+ Mn+ + ; H2 ---_) MO + nH
(691
The protons react with the zeolite lattice to produce the hydroxyl groups.
nZO- + nH+ --+ riZOH
; ZOH: zeolite
[701
The TPR peak positions are sensitive to experimental conditions. That is why one should be careful when comparing results obtained by different authors. However, the sequence obtained by each author, is in general agreement with the prediction.
395
TABLE XVII Temperature-programmedreduction data of metal zeolite catalysts (4,6l
Catalyst
Temperature
(Cr"+,Nal-X-20 200-1000K
Dehydration at 723K
Complete reduction of
in N2 or O2
Cr3-+
Cr2+
Cr3++
1.5 mol of H_ oer mol
(650K)
of Cr3+
Cr species is not completely reduced to the metallic state in both X and Y
570-870K
Cr3++
Mn-X
200-800K
No reduction peak
200-1000K
Cr" would need
Cr2+-+ Cr++ Cr+ * Cr" (metal) 3 Peaks Cr-Y
Fe2 -NaX
Remarks
Pretreatment,
Cr2+
I Peak (703K1,II Peak
1q2
' -
consumption is very
small showing that a
(923K)
small part of Fe species is reduced to zero valent state.
(Ni2+,Nal-X-20 200-1000K
Oxygen pretreated sample Below lOOOK, Ni2--+ Ni+ 83% reduction
Above lOOOK, Ni++
N2 pretreated sample
Ni2+ reduction depends
91% reduction
upon the pretreatment
Ni2+-
and its location in
Ni"
(220kJ/mol) 900K Peak I
Ni"
zeolite
(86 kJ/mol) 830K Peak II Ni2+-NaY
cu2+-NaX.50
200-1000K
200-1000K
Single peak of Ni"
Hydrogen consumed
at 773 K
correspond to about
No evidence of
40% reduction of
Ni2+-
Ni2+---+Ni"
Ni+ step
2 Peaks Cu2+.
Cu+ (550K)
Supercage (87kJ/mol) Cu2+--+ Cu+ (650Kl Sodalite (64kJ/moll Cu+-
Cu"(not observed)
Nitrogen pretreatment produced less reduction of cu2++
cu+ than
air pretreatment
396
TABLE XVII (Cont.1 (Cu2+-Nal-Y-68 400-800K lg sample 5 Wmin
Cu2+--+ Cu+(440Kl
Experimental conditions
(111 kJ/moll
affect the TPR profiles
Cu+ -
Cu'(520K)
of Cu+-
Cue peak
(70 kJ/moll oxidation of Cue to cu2+ ions possible
Ru-X
400-800K
(46% exchanged)
Difficult to
Three peaks: Ru3+--+ Ru2+
423K
distinguish between
Ru2+-+ Ru+
4931:
ruthenium ions present
Ru+ --_)Rue
563K
in different zeolite locations.
Pd-X
200-600K
Dehydrated in 02/N2 at
An extra reduction
723K prior to TPR
peak occurs at 410K
N2 dehydration
(broader) in both
Two reduction peaks 210K cases' and 240K, Two hydrogen desorption peaks at 320K and 340K 0, dehydration Single peak at 270K and desorption at 340 K
Pd-Y
Sachtler et al.
200-600K
Pretreatment in air
Two stage reduction
R-15 K/min
at 775K
process Pd2+-+Pd+--+
10% H2/N2
Pd2+.
20 ml/min
Pd2+ ions located in
Pd" (290K)
Pd" is not acceptable because Pd" is present
supercage and sodalite
during first reduction
cage
process
(1071 used TPR and TPD to study the influence of co-exchange
metal cations such as Fe2+, Ca2+, La3+ and H+ on the formation of Pt particles in Y-type zeolite. The co-exchanged ions can affect the reducibility of the catalytic metal and thereby favourably affect catalytic activity.
The TPR and TPD profiles for Pt/HNaY, Pt/CaNaY and Pt/LaNaY are shown Figure
397
28.
2+ ions in Pt/HNaY (633 The reduction temperature of Pt
(823 Kl
K)
and Pt/HNaY
increases with the calcination temperature.
: 1.60 = Z = 1.40 i <
L
1
d 1.00 = i g .600 r u
TPOICaNaY
_i’“\,
a?
I-
-30.0
110.
250.
mo.
9
630.
Tomoeratwo PC)
t
0.0
f10.
200.
190. Tempwatue (*C)
so.
Figure 28. TPR (curves, a, b, c, d, e) and TPD (curves f, g, h, i, j) of Pt/NaY, Pt/HNaY, Pt/CaNaY and Pt/LaNaY samples calcined at 360 and S5O'C. (a) PtA-R?aY (3601; (b) PtfHNaY (550); fc) Pt/CaNaY (5.501;fdf PWLaNaY (550); (e) Pt/NaY (5501; (f) Pt/HNaY (360) after complete reduction; (g) (hl Pt/HNaY (550) after Pt/HNaY (550) after complete reduction; interrupting reduction at 400°C; (i) PtKaNaY (550) after complete reduction (dashed curve) and after interrupting reduction at 4OO'C (dotted curve); (J) PtAaNaY (550) after complete reduction (dashed curve) and after interrupting reduction at 400°C (dotted curve). (From Ref. 107.1
The reduction temperature and profiles of all three catalysts calcined at 823 K
shown in fig 28 have H/Pt ratios 0.62 f 0.10, 0.15 + 0.10 and 0.28 + 0.10
respectively. They also observed that:
size and location of Pt particles in a zeolite matrix is strongly dependent on the distribution of Pt ions between the supercages and sodalite cages. This is controlled by the calcination temperature prior to reduction.
f
398
at a low calcination temperature (e.g. 633 Kl the majority of Pt2+ ions are in supercages, at a medium calcination temperature (e.g. 723 K) the Pt2+ ions are distributed between super-cagesand sodalite cages 2+ high calcination temperature (e.g. 823 Kl most Pt and at a ions migrate to
sodalite cages and require a high reduction
temperature.
the
presence of
co-exchanged multivalent cations e.g.
2+ Fe
can
effectively block sodalite cages and hexagonal prisms, thus forcing pt2+
ions to stay in supercages even at high calcination temperatures.
if the number of multivalent cations is not sufficient to block the small cages, virtually all Pt2+ ions will ultimately be located in sodalite cages at the high calcination temperature. A high reduction temperature is then required, and migration of the reduced platinum to the external surface of the zeolite crystals will lead to the formation of large Pt particles 2+ the number of Pt ions which have migrated to sodalite cages can be estimated semi-quantitativelyfrom the evolution of hydrogen in TPR and T = 723 K, after interruption of the temperature-programmedreduction max at 673 K.
Some general conclusions may be drawn from temperature-programmedreduction of metal zeolite catalyst systems:
The ease of reduction is dependent on the location of the ion supercage > sodalite cage > hexagonal prism.
The rate-determining step is generally associated with the migration of
ions to reduction sites.
Reduction is frequently hampered by the formation of electron-deficient clusters.
IV.2.3.2
Acidity characterization
The acid sites in zeolites play an important role in their utiiization as catalysts
for
various
transformations
of
hydrocarbons
(106).
Temperature-programmedesorption (TPD) of ammonia and pyridine has been used
399
(108) to characterize the acid strength and the number of acid sites by direct physico-chemical method. TPD of preadsorbed ammonia has become so popular to the extent that the Japanese Catalysis Society proposed this as a possible standard method for testing zeolites (109).
Forni et
al.
(110)
used TPD of ammonia to characterize the acid sites of
partially decationated Y zeolite.
They found that desorption of ammonia is
controlled by intracrystalline surface diffusion with values of the apparent activation energy of 114 - 130 KJ/mol for HNaY zeolite. On the other hand, they discussed the TPD results on the basis of a comparison between what is expected from the use of theorethical equations based on different mechanisms assumed (i.e. adsorption of the base or diffusion of the desorbed ammonia with the zeolite is controlling step) and what has been actually obtained experimentally. Thr results show a good reproducibility of the TPR data with the mechanism proposed.
Wichterlova et
al.
(49)
used TPD of NH3 to determine the number and acid
strength of acid sites in HEM-5
zeolites and mordenite with different Si:Al
ratios.Figure 29 depicts typical curves for TPD of NH3 for EM-5
and mordenite
Figure 29. TPD of ammonia curves for zeolites, sample weight 0.30 g, /3= 21 K/min, Helium flow rate 4.7 ml/min.
(From Ref. 49.)
400
The temperature peak maxima were found to be influenced not only
by the acid
strength but also by the number of acid sites and zeolite structure. The peaks with maximum temperatures above 600K correspond with the desorption of ammonia from
strong structural Bronsted sites whereas
low
temperature peak
is
connected with desorption of ammonia from weak Bronsted sites. Table XVIII shows the value of ammonia adsorbed on various samples of mordenite and HZSM-5 zeolites.
TABLE XVIII Si:Al ratios, the number of skeletal hydroxyl groups as calculated from the chemical composition of the zeolites, the amounts of adsorbed and desorbed ammonia obtained from the calorimetric and TPD measurements, respectively, and TPD temperature maxima for HZSM-5 zeolites and H-mordenites (491.
Zeolite
Si:Al
Number of
NH3 (inmol/gl T:(K)
AHd(kJ/molel
Calor. TPD
Strong Weak
OH groups (innlol/g1
sites
Sites
HMa
9.0
1.33
HMb
10.0
1.2s
1.08
1.11
803
145
77
HZSM-5 a
12.8
1.04
0.87
0.84
715
146
75
IZSM-5 b
13.6
1.01
0.88
723
HZSM-5 c
21.9
0.60
0.58
705
139
84
HZSM-5 d
23.2
0.54
0.52
707
HZSM-5 e
30.0
0.53
0.45
0.44
699
HZSM-5 f
40.0
0.40
0.29
0.33
688
0.12
0.10
1.12
644
HZSM-5 g
140
1.36
0.52
810
* TPD of ammonia was carried out for a zeolite weight of 0.30 g, a helium flow-rate of 4.7 ml/s and a heating rate of 21 K min. The amount of NH3 adsorbed with amount of heat of 80-70 kJ/mol is roughly the same as the amount of ammonia adsorbed on the strong Bronsted sites and accordingly, is proportional to the number of aluminium atoms in the zeolite skeleton.
The adsorption of NH3 connected with amounts of heat of 80-70
kJ/mol could reflect its interaction with NH4+ ions or basic skeletal oxygen atoms of the Si-O-Al type.
On the other hand Suzuki et al.(lll) used TPD of NH3 to characterize the acid sites of ZSM-5 zeolites. The TPD spectra of NH3 revealed the presence of two peaks at 420-470K (11 and 620-670K (II) corresponding to adsorption of weak and strong acid sites, respectively.
Impregnation of ZSM-5 with calcium
401 phosphate suppressed the strong acid sites and the total number of acid sites did not change very much as revealed by a TPD of NH3. KoJima et
al. (112) measured the amount and strength of acid sites of NaNH4
mordenite using TPD of pyridine.
The impression is that pyridine may give a
better indications of catalytically relevant acid sites which would be accesible to molecules considerably larger than ammonia. Table XIX presents the amount of pyridine adsorbed on mordenite calcined at 573K and 873K.
TABLE XIX A
Amount of Pyridine (PyI Adsorbed on Mordenite Calcined at 573K
Catalyst
Total Py
Py desorbed
Irreversibly
adsorbed
during TPD
adsorbed by
(mmol/gI
(mmol/gI (mmol/gI (X of total)
NaM
1.31
1.29
0.023
2
NH4(11INaM
1.15
1.11
0.038
NH4(55)NaM
1.05
0.86
0.19
18
NH4(64INaM
1.06
0.86
0.20
19
NH4(851NaM
1.12
0.90
0.22
20
NH4(971NaM
1.42
1.22
0.20
14
HM
0.43
0.30
0.13
30
B
3
Amount of Pyridine Adsorbed on Mordenite Calcined at 873K
Catalyst
Total Py
Py desorbed
Irreversibly
adsorbed
during TPD
adsrobed by
(mmol/g)
(mmol/gI (mmol/gI
(X of total)
NaM
0.88
0.84
0.034
4
NH4(llINaM
0.83
0.77
0.061
7
NH4(551NaM
0.57
0.38
0.17
30
NH4(641NaM
0.47
0.30
0.17
36
NH4(851NaM
0.31
0.18
0.13
42
NH4(97)NaM
0.27
0.18
0.095
35
HM
0.22
0.13
0.09
41
402 The amount of pyridine adsorbed on NaM decreased whereas the amount of pyridine adsorbed onto HM doubled as the calcination temperature was increased from 573K to 773K.
Steric factors play a dominant role in pyridine TPD from
sodium ammonium mordenite and the number of pyridine molecules adsorbed appeared to be limited by the volume of the main channel. There was only one peak in all the cases of mordenite samples which showed a different peak maximum temperature and desorption rate.
Quanzhi et al. (54) used TPD of NH3 and Pyridine on HY zeolite to characterize the acid sites of zeolite. these basic molecules on
They reported desorption activation energies of
In the case of -1 desorption of NH3, two desorption activation energies were 11.9 kJ mol and -1 43.8 kJ mol compared with the values of 13.5 kJ/mole and 43.8 kJ/mole for desorption of pyridine.
the different acidic sites.
In the case of NH3 they observed one desorption
activation energy value of 27.7 kJ/mol over the whole temperature range.
For
pyridine one activation energy of desorption for Lewis acid sites was observed as 11.6 kJ/mol.
(Ed)pyridine
<
In addition, they found that:
(Ed)NH 3
and
(Ed)Lewis acid
< (Ed)Bronsted acid
From the work reviewed, we can see that different approach is used for the analysis of TPD curves of base-type zeolite systems. The dependence on temperatures on the amount of base chemically held by the zeolite acid sites and on the apparent diffusion coefficients, has been discussed and related with the surface acid centers of Bronsted and /or Lewis type. Many authors preferred to use ammonia as a base type because may fully characterize all the acid sites; while Kojima studies used pyridine in which he believe to obtain better indication of catalytically relevant acid sites which can be accessed by molecules considerably larger than ammonia. Another advantage of using pyridine is unambiguous identificationof the nature of acid sites by means of a combined TPR-infrared analysis.
IV.2.3.3 Sorption and Diffusion in Zeolites Rees et al.
(52) reported temperature-programmeddesorption of p-xylene from
three high silica type zeolites ZSM-5, EM-11 H+/Na+ forms.
and Theta-l, in their mixed
They measured the activation energies and
desorption as a function of coverage using TPD.
entropies of
The p-xylene desorption
activation energy values for ZSM-5. ZSM-11 and Theta-l were 89, 98 and 110 kJ/mol
respectively.
The
activation energy of
desorption of
p-xylene
403
decreased with the coverage whereas the entropy of desorption increased with the coverage as shown in the figure 30.
The desorption activation energy also
decreases with an increase in the Sl/Al ratio in ZSU-5 zeolites. The sorption capacity of both ZSM-5 and ZSM-11 increased with increasing Si/Al ratios, as shown in Table XX.
TAHLEXX p-Xylene saturation capacities and Ed values (6 = 10 K min-'1 (52)
Zeolite
Amount adsorbed /molecules per
Ed _I /kJ mol
unit cell
ZSM-SC151
5.95
86
ZSM-5(451
6.80
78
ZSM-5(781
6.89
76
Silicalite-1
7.05
70
ZSM-ll(151
6.28
87
ZSM-ll(72.9)
6.9
80
Rees et al. (531 also studied desorption of n-hexane from the sodium and hydrogen forms of ZSM-5. ZSM-11 and Theta-l using TPD. TPD profile of n-hexane from Na-ZSM-5 and Na-ZSM-11 are shown
in Figure 31.
The activation energy values were 92 kJ/mol (NaZSM-51, 84 kJ/mol (NaHZSM-51, 91 kJ/mol (NaZSM-11) and 87 kJ/mol (NaHZSM-11) obtained from TPD analysis. The activation energy was found to decrease with coverage. Ed values for pure Na+ form of ZSM-5 and ZSM-11 were higher than for NaH-ZSM-5 and NaH-ZSM-11. This indicate that the electric field around each Na+
ion is adding a
significant electrostatic component to the total interaction energy. Table XXI lists the saturation capacities of all zeolites for n-hexane. The large decrease in the saturation capacity of the Na+ form over that of the H form must be the result of inefficient packing of n-hexane molecules in channel system of Theta-l.
However, the addition of Na+ into the
channel network of ZSM-5 and 11 decreases the saturation capacity slightly.
404 TABLE XXI Saturation capacities of n-hexane for various zeolites (8 = 10 K min-11 (53)
Zeolite
Amount of n-hexane adsorbed (m/u.cI
Na-Theta-l
2.30
Na-H Theta-l
2.72
H Theta-l
3.66
NaZSM-5
6.18
NaHZSM-5
6.80
NaZSM-11
6.42
NaHZSM-11
6.99
100 : ? = a0 2 G 60
.
0
I
0
1.0
2.0 oXcn&micoulcr
1.0
3.0
4.0 per unilccll
2.0 3.0 1.0 wvaapc/trolaulu pa unitcd
5.0
5.0
Figure 30. (a) Activation energy of desorption of p-xylene as a function ofcoverage. b) ActivaJtfonentropy of desorption of p-xylene as a function of coverage. 8 = 6 K min OZSM-5 o ZSM-11 and 0 Theta-l (From Ref. 52.)
405 The desorption energy was
found to be
closely related to the heat of
adsorption of n-hexane rather than to the activation energy for diffusion in these frameworks. The activation energy Ed values for ZSM-5 and 11 were higher than Ed values for Theta-l due to strong electrostatic interactions with higher Al concentrations in the framework.
1.0
1 0.5
0 -
T/K-
1.0
0
05
q
B
m q
03
0
Figure 31. (a) Temperature programmed desorption of n-hexane from NaiSM-5, I3 -1 = 10 K min ; (b) -Temperature programmed desorption of n-hexane from NaZSM-11, g = 10 K min . (From Ref. 53.1 From the results of this work it is seems that using TPD profiles the authors were able to obtain information related with peak temperature, peak width, maximum rates of adsorption and activation energies of desorption as a function of coverage. The saturation capacities of the zeolite for n-hexane were also determined.
Similarly, Fraenkel and Levy (40) used temperature-programmeddesorption to The measure the diffusion coefficient of different gases in zeolite A.
406
technique is useful when diffusion is a rate determining step rather than desorption. The diffusion coefficient was related to peak maximum temperature to a heating rate 6 and to half of the height/width of the TPD peak. The -54' effect of particle size distribution was incorporated into the TPD equation. The diffusion coefficient D was expressed as:
-Ed D = AD exp ( El
1711
The values of the diffusion coefficients of N2, Ar and CO2 in K-A zeolite were calculated and given by equations 17.21to [741.
N2 Ar
co2
2 D(F)
=
1.27 x lo-' exp (-%I
[721
2 D(T)
=
0.796 x lo-' exp (-%I
[731
2 D(F)
=
0.496 x lo-' exp ( *I
[741
The important conclusion from this work is that TPD demonstrated again that is a powerful tool in studying the kinetics of gas diffusion in zeolites. Then, employing its peak-shape expressions makes the derivation of
activation
parameters for diffusion much easier, and usually more reliable than using another adsorption technique.
The application of temperature-programmedanalysis is not limited to merely determinating of the state of the metal ion in the zeolite or acid size distribution, but
it is also a powerful tool in studying the sorption of
hydrocarbons in zeolite as well as determination of diffusivity of gases in the zeolite.
IV.3
Temperature Programmed Surface Reaction (TPSR)
Temperature-programmedsurface reactions (TPSRI referred to study of reactions under temperature programming.
In this type of
study, two gases are
coadsorbed. usually sequentially, on the catalyst and heating is done in a inert carrier. Another possibility is when a reactive carrier gas is used and the "adsorbed" species is the result of decomposition of another molecule; the "adsorbed" species cannot desorb under the conditions employed. A wealth of information can be obtained from TPSR experiments if all the reaction
407 products are detected.
Several sites of different activity can exist on the
catalyst surface which can increase or decrease with change in catalyst properties.
Such changes cannot be
predicted by
steady state kinetic
measurements and may be observed using temperature-programmedanalysis. This approach is very useful for the study of reactions such as hydrogenation, oxidation, methanation. sulfidation and carburization.
IV.3.1 Hvdronenation
CO
and CO hydrogenation on supported metal catalysts using 2 temperature-programminghas been reported by Zagli et al. (291, McCarty and
Wise (113) and Low and Bell (114).
They observed CH4 peaks at different
temperatures for different catalysts.
TPSR was also used for studying the
mechanism of CH4-formation under different initial coverage of CO or CO2 on the catalyst surface. Different catalysts were compared for their activities for hydrogenation of CO or C4J2. When using
and observing peak temperatures
of CH4-formation it is possible to study:
support structure sensitivity
influence of reaction product (HZ01 on methane formation.
kinet'icparameters of the reaction like order and activation energy.
rate determining step of the reaction.
Hydrogenation of CO over a nickel catalyst has been studied recently by Vandervell and Bouker (115) using TPSR.
The objective of the study was to
clarify the reaction mechanism i.e. whether methanation proceeds with water formation or CO formation as given below:
(a) H20 formation : CO + 3H2 ----f CH4 + H20
[751
(b) CO2 formation : 2C0 + 2H2 -
[761
CH4 + CO2
Figure 32 shows the results of the methanation experiment with a fully reduced nickel catalyst for two different levels of CO in the feed.
For low CO, gas
408
High
CO CO:H,=1’1.56
co
w3amu) /-/
\
L-
C0,144amu) --------___
H,O(lBamub ._.-.-. -.-
,._‘_
Heahng Begun
Temperature
Low
:. :.:: : : :
(K)
co co. ii,=,
: : \
-.-.-
24
I j\
:
1. ,
supported over the synthesis Temperature programmed Figure 32. nickelcatalyst. Flow rate: 40 cc/min at 1 bar pressure, gas composition: 4% CO in Hydrogen. (From Ref. 115.1
409
the methanation reaction takes off at 460K, reaches a maximum at -490K and finally settles down at a temperature invariant rate at 520K. This correspond with complete conversion of H20 formation as per scheme (a). For high CC gas experiments, the situation is quite different.
Methanation begins later at
around 510K and shows only a slight maximum at 550K.
The main oxidised
product is CO2 with only minor production of HzO, showing that reaction (bl is now
the dominant process.
The
TPSR
study
further
supports
thus
the
dissociative mechanism
methanation i.e. carbon atoms are directly hydrogenated to methane.
for The
results also show that there is a competition between hydrogen and CO for 2+ adsorbed/dissociated oxygen atoms. ions which act as The role of Ca promoters for the methanation reaction of a modified Ni surface would produce CO2 as a major oxidised product during the methanation reaction.
Schwarz and Huang (116) also studied CO hydrogenation over Ni/A1203 catalyst using TPSR to examine the relationship between catalyst preparation procedures and the structure, dispersion, activity and selectivity of the catalyst. They used TPD of H2 and a temperature-programmed reaction of CO to obtain the structural information related to the accessibility of the surface nickel sites to H2 and CO .
TPSR was used to assess the amount of carbon containing
residues left on the catalyst surface after steady state CO hydrogenation. Three different types of carbon have been observed for Ni/A1203 catalysts (McCarty &
Wise,
(113)).
These surface carbon species are
formed by
dissociation of CO at elevated temperatures and readily produce methane upon subsequent exposure to hydrogen.
Information about carbon containing species
on the surface can be used for catalyst design, regeneration or to obtain more insight into the methanation mechanism. For low-weight loading catalysts, two types of carbon-containing species were observed whereas at higher loadings three types of carbon-containingspecies are left on the surface. Table XXII summarizes the peak temperatures and the total carbon deposition for each catalyst. The average number of carbon-containing residues was about 4C/Ni, independent of weight loading.
Perhaps the most significant conclusion of this work was, meanwhile TPD of H2 and TPR of CO provide structural information related to the accessibility of the surface nickel, sites for H2 and CO adsorption; the TPSR demonstrated the difference in activity and selectivity for the different catalysts under study.
410
TABLE XXII Summary of TPSR results (1161
Peak temperature
Nickel weight
Total carbon depostion
loading/%
-1 /1O-4 mole (g catalyst1
/K
0.8
4.9
457
621
-
1.2
5.1
478
639
2.1
5.1
463
643
-
3.0
5.2
447
588
670
3.5
4.2
456
587
661
4.1
4.8
442
590
654
6.6
5.1
454
667 >7OO(shoulderl
7.3
5.5
452
664 >7OO(shoulderl
8.3
6.0
457
653 >7OO(shoulder
Recently, Falconer and Sen (1171 used TPSR along with TPD and TPR of H2 and CO on a Ru/A1203 catalyst for CO hydrogenation. They identified the existence of two distinct reaction sites for CO hydrogenation on Ru/A1203 catalyst similar to Ni/A1203 catalyst. The first peak observed was due to hydrogenation of
CO
adsorbed on Ru and the second peak resulted from the decomposition of the CO-H complex on alumina resulting in the formation of methane. The presence of on Ni/AlzOs is probably not required for two reaction sites to be present. A CO-H species with methoxy stoichiometry adsorbs on alumina probably at the Ru/AlzOs interface.
On another work, Margitfalvi et al. (118) used TPSR to study the formation of benzene from n-hexane over Pt/A1203 catalyst.
They treated the catalyst in
hydrogen gas at different temperatures prior to reaction. Upon increasing the temperature of the hydrogen treatment the peaks of benzene formed shifted to higher temperatures also. This phenomenum is due to reduced mobility of the overall hydrogen pool at higher temperatures and consequently suppressing the hydrogen consumption in the conversion of n-hexane.
Figure 33 shows the
results of the TPSR experiment. The shift of the benzene peak was about 350 K upon increasing the temperature of the hydrogen treatment from 673 to 773 K.
411
X0
200 t("cl
300
Figure 33. Formation of benzene from n-hexane studied by TPR. The shift of benzene peaks upon applying high temperature hydrogen treatment (H21. Temperature of H2 : O-400°C, x-500°C,
q-575'C
catalyst : 0.1 g Pt/A1203,
treated in oxygen at 4OO'C before final reduction. (From Ref. 118.1
IV.3.2 Methanation
The technique of temperature-programmedmethanation (TPM) is very similar to that of TPR.
The H2/Ar reduction gas used in TPR is replaced by a H2/C02
cl:91 mixture. Hydrogen is consumed according to the reaction: co2 + 4H2 +
CH4 + 2H2C
[771
Accordingly, the detector responds directly to the extent to which the reaction takes place. Typical TPM curves are shown in Figure 34 with 8 = 10 30K mix-r-1 and the final temperature need not exceed 873K (1191.
The shape of the curve as shown in Figure 34 (curve A1 is typical for a freshly-reduced unsintered catalyst where the catalytic activity is very high. After sintering. all catalysts lose their activity to some extent. shown by curves B-D.
This is
Then TPM may be used to obtain information on activation
energies and hydrogen consumption.
412 From this information
catalysts that achieve this value at 623 K or less are highly active those requiring a temperature of up to 673 K have an adequate activity
those requiring over 773 K are unlikely to be
sufficiently active for
commercial use. 3000
----___
Theoretical -.-N,
f
maximum
Theoreticalmaximum
- _ _ - . A
B
/\
400
200
600
TemperaturePC
TemperaturePC
Figure 34. Typical Temperature programmed methanation curves for nickel catalysts. A. Freshly reduced, unsintered catalyst B,C,D, different catalysts previously exposed to sintering conditions. (From Ref. 119.1
It will thus be clear that temperature-programmedmethanation techniques are are
a
useful
tool
for:
accelerating the work of
evaluating methanation catalyst activity, and developing catalysts that are
aimed at
being
commercially beneficial.
IV.3.3 Sulnhidation
Ni-Mo/A12/03 and
Co-Mo/A12/03 are
a
important class
of
catalysts for
hydrodesulphurization (HDS). The reactivity of these catalysts is correlated with the HDS activity of presulphided catalysts. For example, catalysts which can be reduce at lowest temperatures show the highest activities. Studies on the conversion of oxides into sulphides and the structure of sulphides is limited. Sulphiding is a necessary pretreating step for HDS catalysts in order to
limit
coke
formation.
The
temperature-programmed sulphiding
(TPSI
technique has been applied to investigate the reactivity of oxidic Mo/A12/03
413
catalyst by Arnoldy et
al.
(621.
In TPS, the gas mixture used for sulphiding
contains 3.3% H2S. 28.1% H2 and 68.6% Ar.
TPS is a sensitive technique for
describing sulphiding reactions in detail.
It gives information on
the
sulphiding rate and mechanism as a function of temperature. Sulphiding of Mo03/A1303 takes place at low temperatures when compared with bulk compounds (Mo03, Mo021.
The
sulphiding mechanism is dominated by
O-S
exchange
reactions. Elemental sulphur is formed by the rupture of metal sulphide bonds and is reduced subsequently by H2. influences sulphiding drastically.
The HZ0 content of Wet
the catalysts
catalysts sulphide at
very
low
temperatures (typically 400-500K1, while dried catalyst sulphide at much higher temperatures (typically 600-700Kl. sulphiding more than predrying. place at very
Further prereduction hinders
Sulphiding of Mo03/A1203 catalysts takes
low temperatures, in
comparison with
reduction of
these
catalysts in temperature-programmedreduction. A 4.5 atoms/nm2 catalyst can be sulphided completely even at ca 500K by selecting a sufficiently low heating rate. The S/MO ratio after TPS is always below 2 (1.4 - 1.9 depending on MO content) pointing to limited formation of MoS2.
An increase in MO
content leads to sulphiding at a somewhat lower temperature. However. the influence of
MO
content on
reduction (TPR) is
much
more
Sulphiding appears to be complete at Ca 1lOOK in all cases.
significant.
The product of
sulphiding up to Ca 500K might well be a monolayer similar to the oxidic precursor.
Thus it can be seen that TPS can be used to detect the reactivities of all the MO species because of different reactive catalytic sites sulphiding at different rates.
On another work temperature-programmedsulphiding (TPS) has been applied to study the sulphiding of oxidic ReaO,/AlaOscatalysts in a HaS/Ha medium (120). It was found that predominating oxidic Re+' monolayer species sulphide easily. The sulphiding temperature of these species is influenced significantly by their water content.
In that case wet sample sulphide around 400 K, whereas
dry ones sulphide already extensively at room temperature. Strongly adsorbed Ha0 probably prevents HaS adsorption and therefore sulphiding. At the Ha/Has pressure ratio applied
(c.a. 8.51,
the ultimate sulphiding product of
crystalline Re compounds depends on the sulphiding temperature. ReSa and Re metal are thermodynamically favoured below and above c.a. 950 K respectively but are pinned slowly and incomplete due to the hindrance of diffusion through dense product shells. results in
the
Sulphiding of (un) supported crystalline NHIRe04
formation of
surrounded by a Re Sa shell.
extreme well-dispersed Re
metal
particles
Microporosity of this shell wireless diffusion
414
of HsS. but not of Hz, leading to much higher Hs/HaS pressure ratios in the interior of the particles than the ratio in the bulk SQa phase, and therefore, to thermodynamic stability of Re metal far below 950
K.
IV.3.4 Oxidation
TPSR has been used to characterize bismuth molybdate catalyst for oxidation of propylene by Uda et al,(81.
The redox properties of ;r-Bi2Mo06catalyst was In this technique the
examined using temperature programmed reoxidation.
catalyst was reduced to predetermined degree of reduction and subject to reducing agent containing reactant e.g. 20% propylene in ultrapure nitrogen. The catalyst is reduced by reducing agent above 673
K.
After reduction the
catalyst is subjected to oxidation under temperature programming.
A typical
TPSR curve of the T-phase catalyst prereduced by propylene is shown in Figure 35. Two peaks are observed. The maximum of the low temperature peak occurs at 431
K
and the maximum of the high temperature peak occurs at 513 K.
The
activation energy for low temperature peak was determined to be 122 KJ/mole and the activation energy for the high temperature peak was determined to be 265 KJ/mole.
Figure 35. TPR chromatogram for 3% reduced r-phase by C3H6 at 420'C. (8 = 6OC min-'1.
(From Ref. 8.)
The
low temperature reoxidation peak was found to be a result of the 4+ reoxidation of MO to Mo6+ and Bi" to Bim+. where 0 C m < 3. The high temperature reoxidation peak was found to be result of reoxidation of Bim+ to
Bi3+.
The high temperature reoxidation process also appears to be related to
415
the rate limiting step for propylene oxidation to acrolein at temperatures below 673 K.
Farneth et a1.(30) studied partial oxidation of methanol over Moo3 using TPSR. The catalyst sample was exposed to methanol and TPSR spectra were obtained as depicted in Figure 36 .
The spectra show two distinct desorption bands.
The
low-temperature feature (Tmax = 393 K) is dominated by loss of methanol still intact and the high-temperaturepeak (Tmax = 493 K) is composed of CH20, CH30H and H20.
The second peak was found to be independent of coverage and
following first order kinetics with an apparent activation energy of 20.6 kcal/mol.
A
reaction mechanism of
methanol oxidation based
the
on
experimental findings was suggested.
0
m
64
90
I20
150
la0
2lo
240
1?0
so0
TfhufRrruRE (C)
Figure 36. Mass spectral intensity profiles for CH30H (m/e 32). CH20 (m/e 30) and H20 (m/e 18) during TPD.
(From Ref. 30.1
Anderson (321 studied oxidation and ammoxidation of 3-picoline over VzOs catalyst using TPSR. His results were related to the surface-structureof the catalyst deduced on
the basis of
metal-oxygen bond
strength. The
TPSR
technique has been useful to study the basic steps of the catalytic reactions. Three different sites for the formation of nitril over the catalyst were identified and a mechanism of ammoxidation of 3-plcoline uas proposed. Most of
416 the nitrite is formed in a reaction between:
- chemisorbed 3-picoline and weakly adsorbed or gaseous NHa.
- chemisorbed 3-picoline and NH3 adsorbed at a vanadyl oxygen vacancy.
- chemisorbed 3-picoline and NH3 more strongly adsorbed in the form of either -NH2 or =NH group.
IV.3.5 Other reactions
Tagawa et
al.
(581 reported methanol synthesis from CO2 and H2 over Cu, Zn,
chromium and aluminum oxide catalysts using TPSR.
Temperature-programmed
desorption carried out after the reaction on copper-containing catalysts gave peak
characteristics of
intermediate of
the
copper
reaction.
formate
which
was
identified as
different catalysts studied
The
the were
CuO/ZnO/Cr203, Cr203/A1203 and G-66A, a commercial catalyst. Figure 37 shows a TPD spectrum obtained with A1203 after the C02+HZ reaction was carried out at 623
K.
A1203 showed no activity for the synthesis reaction.
TPD z ii 5 c) .---_
5
50
I 100
!
I 150
200 TEMPERATURE
I 250
I 300
I 35
(7~)
Figure 37. Temperature programmed desorption on A1203 after CO2 and H2 reaction. (From Ref. 58.1
Totally different spectra of TPD were observed on the active catalyst 30% CuO/A1203 as represented in Figure 38. peak maximum at 473-483 K
A large peak of CO2 appeared with a
followed by a broad second peak at a higher
temperature. While a small peak of methanol appeared at 443 K, the desorption of CO increased consistently with temperature and no maximum was observed up
417 to 623 K at which the TPD was stopped. Figure 38 also shows TPD spectra of commercial catalyst G-66A.
The existence of for-mateduring C02+H2 reaction
over catalyst G-66A was detected by TPSR rather than by infrared spectroscopy due to its complete opaqueness. Thus TPSR proved to be a useful technique for detecting intermediates on the catalyst surface which cannot be detected by IR spectroscopy.
50
100
150
200
250
TEMPERATURE
300
350
PC,
Figure 38. TPD after CO2 + H2 reaction on (al G-66A and (bl 30% CuO/A1203 catalysts. (From Ref. 58.)
Forzatti et a1.(511 studied deactivation kinetics of fresh and sodium-poisoned r-A1203 catalyst for dehydration of ethanol and methanol using TPSR. The TPSR curve analysis technique was able to provide the complete energy and kinetic picture of the reacting system. This can in effect be related to deactivation if any of it is due to catalyst poisoning.
TPSR spectra of methanol and
ethanol from fresh and Na-poisoned r-A1203 are shown in Figure 39 and 40 respectively. The fresh catalyst shows two peaks for methanol as well as ethanol desorption. Two peaks are related to the heterogeniety of the ;r-A1203 catalyst surface with Bronsted and Lewis sites of different acidic strengths. These show the interaction of the alcohol hydroxyl with either a surface hydroxyl or Lewis site and an oxygen pair. Impregnation of the catalyst with sodium indicates a .preferential poisoning of
the more acidic Lewis and
Bronsted sites with higher energies of activation. The first peak is reduced in size whereas the second peak
(490K)
almost disappears when the catalyst is
418
In the case of ethanol desorption. Na enhances the amount of
poisoned.
ethylene desorbed
and
shifts
the
ethylene desorption peak
to
higher
temperatures.In case of methanol desorption, the value of desorption-energy Ed ranges approximately from 70 to 115 J/mol over the range 0.2 C 9 < 0.85 owing to the existence of non-uniform poisoning whereas for ethanol desorption Ed values range from 80-120 J/mole over the range 0.2 < 9 < 0.8. CXlO"
tishm’l
1 (Kl
Figure 39. Thermal desorption curves of methanol from r-A1203. Poison level 0 = 0% Na;
level 1 = 0.8% Na, level 2 = 1.5% Na.
(From Ref. 51.1
T IKI
Figure 40 Thermal desorption curves of ethanol and ethylene from r-A1203 poison level 0 = 0% Na.
----- poison level 2 = 1.5% Na.
(From Ref. 51.)
419
Here, it is worth to point out that the TPSR has proven to be a suitable experimental tool for investigating the separability of reaction-deactivation kinetics, as it is able in practice to provide a complete energetic and kinetics description on nonideal heterogeneous catalytic surfaces, allowing for a direct study on the effect of poisoning processes upon the separability of the rate form.
Although most of the conditions strictly refers to the system referred here, it is possible to achieve rationalization in terms of chemical evidence, so that it is reasonable to expect similar results when other reacting system exhibiting similar chemical behavior is considered.
In conclusion the technique of TPSR is very useful for the study of rate of reactions on the different sites which is generally not available from the more conventional steady-state measurements
IV.4 Temperature-ProgrammedGasification and Carburization
IV.4.1 Gasification
Carbon deposition on catalysts is an important problem both in industrial and fundamental research. Deactivation or even fragmentation of the catalyst can take place, and as a consequence regeneration or replacement is necessary. Therefore, much effort is applied to developing coke-resistant catalysts. order to get a better understanding of
the mechanism of
In
coke deposition,
different techniques are needed to characterize the deposited carbon.
One of
these
called
is
temperature-programmed gasification,
(TPG)
with
H2
temperature-programmed hydrogenation (TPH) or with 02 called temperatureprogrammed oxidation (TPO).
McCarthy et al. (1211 used temperature-programmedhydrogenation (TPH) for study the reactivities of the different form of carbon deposits on nickel catalysts for forming hydrogen.
They proposed the existence of seven types of carbon
after treatment of the catalyst with CO or ethylene: C-u and C-a, atomic carbon; C-S, polymeric carbon; C-r, metal carbide; C-6 and C-6, filamentous carbon; C-c, graphic carbon.
Figueiredo
(1221 shows
results concerning the
temperature-programmed
gasification (TPGl of carbon on commercial Ni/Al2OS, CO-MO, Si02-A1203 and Pt/A1203 catalysts. Different patterns were found in which gasification of carbon is a function of metal-carbon contact and carbon morphology.
420
Temperature-programmed gasification with 02, CO2 and H2 was used for the characterization of carbon deposits on NWA1203
and CO/A1203, based on the The
difference in reactivity of the several types of deposits (1231.
gasification patterns show the existence of mainly two types of carbon One form is attributed to filamentous carbon, the second form is
deposits.
believed to have a more graphitic structure.
The TPG patterns are strongly
influenced by the oxidation state and the catalytic activity of the metal on the gasification of carbon.
In recent work the same authors (1241 used TPO for the determination of the amount
and
the
chemical
hydrotreating catalysts.
state
of
carbon
and
sulfur
on
deactivated
From Figure 41 it can be seen that supported MoS2
oxidizes at considerable lower temperatures (Tmax = 6OOKl than bulk MoS2 (Tmax = 85OKl.
This is probably caused by limitations towards diffusion.
In the
case of bulk MoS2, a surface metal oxide layer might be formed in the early stage of oxidation. From the TPO patterns of Figure 41, it can be seen that for increasing Co-loading, an additional maximum emerges, so peak III is attributed to a Co-sulfide phase.
The maximum in the oxidation pattern of
MO-sulfide shifts to a lower temperature with increasing Co or Ni loadings. This agrees with the explanation that a mixed phase is present, although a catalytic effect of CoS on oxygen dissociation is not excluded. The patterns of the spent catalyts (Figure 421 clearly show a low and a high temperature SO2 production maximum.
In Table XXIII, the temperatures of the SO2 maxima of the spent catalysts and sulphided catalysts are listed.
TABLE XXIII Peak temperatures (SO2 production) of sulBhided and spent catalysts (124
Catalyst
Mo(10.41
Peak I
sulfided
Peak II
Peak III
600 K
Co(2.OlMo(10.41
sulfided
520 K
700 K
C0(4.01M0(10.41
sulfided
510 K
600-700 K
Ni(2.11Mo(15.51
sulfided
600 K
650-750 K
Co(3.8lMo(15.21
spent
540 K
750 K
Ni(2.11Mo(15.51
spent
580 K
730 K
765 K
421
Figure 41. (a) Temperature programmed oxidation (TPO) patterns of sulfided MOIA1203. CoMO/A1203 and NiMO/A1203. (b) TPO sulfided
patterns
(SO2 production) of
MO/Al203, CoMO/Al203 and NiMO/Al203. (From Ref. 124.)
-SO*
I
600 temperature
I
700 (K)
Figure 42. TPO patterns (CO2 and SO2 production) of spent CO (15.2)/A1203and Ni (2.1) MO (15.5)/A1203. (From Ref. 124.)
(3.8) MO
422 Quantifying the data of TPO of spent catalysts showed that much more 02 was consumed that could be accounted for by adding up the 02 used for SO2 for CO2 production and oxidation of the metal to metal oxide.
Oxidation of the major part of Ni or Co sulphide coincides with the oxidation of the carbon deposit.
The reactivity of the carbon deposit is low, despite
the fact that the deposit contains a high amount of hydrogen.
The catalytic
effect of a metal can be a significant factor which determines the temperature at which the carbon reacts with 02.
As a consequence a comparison between
carbon deposits, merely based on oxidation temperatures can lead to erroneous conclusions.
The oxidative regeneration of cobalt-molybdate catalyst was also studied by Yoshimura and Furimsky (125) using temperature-programmedgasification with air. SO2. CO and CO2 were analysed as the major products (Figure 43). these results, a
burn-off model for
From
spent hydrotreatment catalysts was
proposed.
Figure
43.Product distribution from
catalysts in air 0
SO2
m co 2'
temperature programmed burn-off of
(From Ref. 125.)
423 To understand the importance of deactivation and regeneration of naphtha reforming, the location composition and structure of carbon deposits on the bifunctional catalyst needs
to
be
clarified.
Barbier
(1261 studied
coke-gasification using a temperature-programmedgasification technique with oxygen.
The TPG of coked catalysts shows two oxidation peaks, one at around
573 K and the other at around 7.23K.
These two peaks are particularly well
defined when platinum is supported on non-acidic and non-microporous alumina. Nevertheless, if such is the case, a low oxygen pressure during the TPG allows a good resolution of the two peaks (Figure 441.
Figure 44
TPO
of
coked Pt/A1203 catalyst (a) CO2
production, (b) 02
consumption. (From Ref. 126.)
A study of these peaks shifting with increasing temperature shows that the activation energy of coke combustion is equal to 10 kcal for the first peak and 15 kcal for the second.
Many materials are known to catalyze the gasification of coke, and these include several metals.
So it has been found that the low-temperature
combustion is due to the presence of coke on the metallic phase (127-128). Proof of this is clear when Pt/Si02 catalyst and pure alumina are coked
424
together as a mixture; then analyzed separately by TPG experiments.
Coke
deposited on Pt can be oxidized at 533 K. whereas coke deposited on alumina
is
Such a difference can be
oxidized at 823 K, as can be seen in Figure 45.
explained by assuming either that platinum catalyzes the oxidation of carbon or that coke deposited on the metal is different from the coke deposited on the alumina.
I
It I /
\
\
\
\ \
Figure 45. TPO of coke (a) deposited on Pt, (b) deposited on A1203.
(From
Ref. 128.
The influence of the platinum metal on the burning temperature of the coke produced during pure-hydrocarbon reforming was studied using TPO by Parera et al. (129). They found that the differences in the coke oxidation temperatures could be due to the nature of the coke formed when using different catalysts not being the same.
A lower coke burning temperature can be explained by the
fact that, when higher amounts of Pt are present, coke can be polymerized less because of the hydrogenation of coke precursors produced during the reaction and catalyzed by Pt.
Similarly, Parera et al.
(130)
using TPO studied the evidence of spillover
phenomena on reforming catalysts.
By the TPO they proved the presence of
carbon on the support and measured the temperature of oxidation of that
425 carbon.
The higher the required oxidation temperatures, the more graphltlc
the carbon becomes.
In a series of temperature-programmed hydrogenations
(TPH), it was found that Al203 that was coked while passing naphtha over it for a short time, produced a coke that could be partially eliminated by hydrogenation and completely eliminated if hydrogenation was performed in the presence of Pt.
The experiments indicated that hydrogen spillover from Pt
increased the rate of coke removal by the hydrogenolysls of coke.
In another
set of TPO experiments, it was found that coked alumina oxidized at lower temperatures when mixed with Pt/A1203 catalyst. The explanation is that Pt causes oxygen to dissociate and migrate lnto coked A1203. thus oxidizing coke at a much lower temperature than molecular oxygen. would.
This series of TPO
and TPH experiments indicates that hydrogen splllover is playing an important role in naphtha reforming or coke removal.
The effect of a second metal added to platinum on coke gasification was examined using TPO by Beltramini and Trlnun (131) on coked catalysts.
Figure
46, shows that coke on Pt/A1203 produces a twin peak combustion pattern.
On
the Pt-Ir/A1203 catalysts however, TPO showed only one peak in which the maximum decreases as the amount of iridium increases.
T mar A
39L
B
L72 363
c
L15
100
f
1°C)
A
c
200
300
LOO
T PC) Figure 46. Temperature programmed oxidation of coke catalysts. (From Ref. 131)
426
The amount of coke deposited on the two metals, as judged by the area of the thermograms, appeared to be about the same, but since TPO is a measure of the heat released by coke deposition, the areas are not in the same proportion when compared with the peak produced by combustion of coke on alumina.
This
is
more
because
the
presence of
the
metal
during
coking produces
a
hydrogenated coke that burns at lower temperatures with a higher heat of combustion than coke deposited on A1203-Cl.
The gasification of coke on the
support by oxygen adsorbed and spilt over from the metal seems to be another good explanation.
The effect of coke on different bimetallic systems was Beltramini (132).
studied by
Coke on Pt/A1203 and Pt-Ir/A1203 oxidizes at
lower
temperatures (Figure 471. An increase in the metal content produce a decrease in the maximum temperature peak burning.
On Pt-Re/A1203, Pt-Ge/A1203 and
Pt-Sn/A1203 most of the coke is formed and localized on the alumina support and burnt at a much higher temperature.
IV.4.2 Carburization
The role of carbides is still not clear in the synthesis catalysts (1331 and a number of studies concerning the effects of support interaction;
alkali
promotion and alloying have apparently not taken the carburization effect into account.
The formation of carbide on a series of Fe/SiO2, Ni/SiO2 and 4Fe-Ni/SiO 2 catalysts has been investigated in a temperature programmed carburization (TPCl experiment in the range of 298-873 employed as reaction gases.
K
(1341. Both CO and 3H2:C0 were
With the exception of Ni/Si02 in 3H2:C0, carbide
formation was observed in all instances.
In the bimetallic system the
mixed-metal carbide formed (H2C to M3C) was stabilized to about 773 K (Figure 481. In the 3H2:C0 the rates of carbide formation and coking on 4Fe:Ni/Si02 are both enhanced relative to
Fe/Si02.
The activation energies for the carburization reaction were determined using the standard programmed analysis (1351. The TPC analysis results are reported in Table XXIV.
No activation energies were determined from the TPC in the coking regimes, since the temperature-programmedanalysis requires a finite reaction-product relationship. In addition, the temperature ranges here are well beyond those of interest in synthesis reaction applications.
A surface reaction model
“I- P!-Ir
-G
m Pt
.
-50
Figure 47. Ref. 132. )
-+I
Temperature programmed oxidation
-1
I
TEMPERATURE. -A
-cl
Q -cc
of bimetallic
catalysts.
(From
428 accounting for the observed carburization behaviour and which correspond with the activation energy measurements was also proposed.
0
200
4GO
600
800
0 m
TEHPER4TUPL(“Cl
2G0300403500600 TW'ERATW
7CKl
(OC)
Figure 48. Temperature Programmed Carburization (TPC) of Fe-Ni system in (al CO, (bl 3H2:CO. (From Ref. 134.1
TABLE XXIV Activation Energies for the Carburization Reaction (134)
Catalyst
Reaction
E (kcal/moll
120 f 20
Fe/Si02
Fe + CO -+ FexC Fe + CO + H2 -+ FexC
14+
4
Ni/Si02
Ni + CO -+ Ni3C
355
2
4Fe:Ni/Si02
4Fe:Ni + CO -+ 14Fe:NilxC
1st
7
4Fe:Ni + CO + H2 -+ (4Fe:NilxC
222
2
429
In general, temperature programmed gasification (TPC) furnishes information on the total amount of carbon, the type of carbon deposits and on the oxidation state of the metal.
Depending on the specific interest. one will choose the
appropriate combination of gasifying agents.
TPH is the most sensitive
technique but H2 is the least reactive gasifying agent.
This combination
makes TPH suitable for studying the reactive forms of carbon deposits, because small differences in reactivity can still be detected. detection of
less reactive forms of
On the other hand,
carbon deposit will
need
a
gasification temperature leading to structural changes of the catalyst.
high TPC
will find its application in the study of coke-formation and regeneration effects on catalysts. At relatively low temperatures all carbon deposits are gasified. and thus high-temperature induced changes of the catalysts avoided. This is
a
necessary condition for regeneration studies. A disadvantage of TPC
is the highly exothermlc character of the CO2 reaction, causing a temperature rise during gasification and as a different carbon deposits.
consequence a
Better results
bad
resolution of
the
are sometimes obtained by TPG
(C021. because the reaction is endothermic and the oxldatln of the metal during the process improves the resolution of carbon deposits.
V. CONCLUSION AND PUTURE APPLICATIONS
We have presented a summary of the procedures for obtaining and evaluating the experimental data
from
a
temperature-programmed technique with
special
application to a catalytic system.
We have also presented convincing evidence that this method
has already been
succesful for catalyst characterization, understanding reaction mechanism, determining kinetic data, rapid evaluation of catalyst activity, selectivity and stability and measuring surface area of adsorptlon/desorption.
There are of course some disadvantages using these techniques. They arise from both fundamental and practical considerations. From a fundamental point of view, the disadvantages come' from the inability to interpret the experimental results, resulting sometimes in an incorrect evalutlon of phenomenas like diffusional transport process,
readsorption, multiple bonding,
etc.
In
practice the problems are principally due to wrong experimental conditions design that not correspond with real industrial conditions in which the catalyst is working.
But comparing advantages and disadvantages, we can say that if proper care is taken
during
experimental design
and
quantitative
treatment
of
the
430 temperature-programmed technique, then this technique offers a versatile, cheap and precise tool for both fundamental and applies catalytic studies. In general the advantages appears to outweight the disadvantages.
It is our opinion that temperature programmed analysis will continue to find increasing use at both University and industrial research laboratories in the area of:
catalyst characterization like heterogeniety of the surface, acidity, activity, and coke formation.
as a diagnostic tool for large scale catalyst manufacture, qualitiy control and regeneration.
characterization of corrosion layers, reducible species in solids such as coal ash.
different reaction modes e.g. in monitoring the activity of catalysts for reduction reactions such as hydrogenations, activity of
catalysts
under sulphidation or poisoning, activity of catalyst for particular reaction such as oxidation, methanation, carburization or ammonination.
study of separability of reaction-deactivationkinetics.
sorption and diffusion in zeolites.
TPD and TPR at low pressures to understand readsorption.
TPR and TPD at high pressure to understand catalytic pretreatment and catalyst deactivation.
theoretical modelling of adsorbate distributions within the catalyst bed during reactions.
combining
in
situ
temperatures-programmed techniques
spectroscopic techniques such as
with
other
Mossbauer and infrared
Understanding reaction mechanisms and modelling on single crystals and metal supported catalysts.
431 Temperature-programmed apparatus Various for
with types
into
temperature
involved
information
with
by
that
photoelectron
spectroscopy,
etc.
conclusion,
tremendous techniques.
this
growth In our
pace to ever wider
the
by
that interest
opinion fields
catalyst
using
this
covered
from instrument
another
work
interest
of application.
will
of
analysis
of
spread
research
since
The
is
such
as
now x-ray
Mossbauer
microscopy,
done
(71.
requirements.
part
and catalysis.
techniques
electron
and application
a single
manufacturers
to suit
development
the
into
mass spectrometer
has become an integral
spectroscopy,
review
in
available
integrated
a
temperature-programmed
found
diffraction,
like
programming apparatus
analysis
in the work of
provided
complemented
have been
system
systems are
temperature-programmed
laboratories
In
techniques
detection
of detection
incorporation
Today,
analysis
a universal
1983,
shows
temperature-programmed at
an ever
increasing
a
432 NOMENCLATURE A
Preexponential factor for adsorption equilibrium constant
A'
Preexponential factor of solid gas reaction
Aa
Preexponential factor of adsorption constant Preexponential factor of desorption constant
2 AD AP C g (Cg)M (Cg)n =go cs E AC g De Do DP DB E Ea Ed Ed' ED Ei EP
Preexponential factor of chemical reaction -1 Preexponential factor for diffusion from subsurface, s Preexponential factor for penetration of adsorbate Gas phase concentration, mol/m3 Gas phase concentration at peak maximum temperature, mol/m3 Normalized gas phase concentration, dimensionless Initial gas phase concentration. mol/m3 Surface concentration of gas, mol/mJ Mean hydrogen concentration at the temperature of the 3 maximum reduction rate, mol/m Change in gas concentration, mol/m3 2 Effective diffusion coefficient (El Preexponential factor of diffusion coefficient Particle diffusion coefficient Bed dispersion coefficient Activation Energy, kJ/mol Activation Energy for adsorption, kJ/mol Activation Energy for desorption, kJ/mol Apparent activation energy for desorption. kJ/mol Activation energy for diffusion from subsurface to surface, kJ/mol Activation energy for reduction of species i, kJ/mol Activation energy for penetration of adsorbate into subsurface region, kJ/mol
En F1
Normalized activation energy, dimensionless Specific flow rate of the reacting mixture, m3/s.kg -4 Carrier gas volumetric flow rate at STP (1 atm. 273 K),> 3
F. F
Carrier gas volumetric flow rate (temperaturedependent), E
f(e)
General form of adsorption dependence on coverage
g(B)
General form of desorption dependence on coverage
AHd(e)
Heat of adsorption, kJ/mol -1
k
Reaction rate constant, s
ka
m3 Adsorption rate constant, m. -! Desorption rate constant, s
kd kD
Rate constant for diffusion from subsurface to surface, s-1
433
k
eff
Effective desorption rate constant, s-1 Rate constant for nucleation process
kn k P
Rate constant for penetration of adsorbate into subsurface region,
1
Length of catalyst pores, m
L
Width of catalyst slab, m
S-l
M
Adsorbate released from the zeolite crystals during time interval m mol At, g Ratio of total number of subsurface sites to surface sites
MAt
n
Reaction order for desorption
n'
Apparent desorption order
N
Net desorption rate (s-'1
NB
Total number of subsurface sites, mol Maximum desorption rate, s-1
NP NC NS P
Number of particles in the bed Total number of surface sites, mol
Aq
Reaction order for adsorption (RT l/2 (m 5%' set' Amount of adsorbate held on the surface
Qo>Qt.Q,
Adsorbate present in the zeolite pores at t = 0, t, and m (Fl
r
Order of reaction with respect to gas phase
P
r
Radius of the spherical particle, cm
0
Observed rate of reaction
r
Ohs R Ra Rd
Gas constant IJ/mol.K) Rate of adsorption (s-l1 -1 Rate of desorption (s 1 Radius of crystal, m
RC
Order of reaction with respect to solid (species on the surface)
S
Sticking coefficient
3
Surface concentration of species i (mole/m21
3 0
si
Initial concentration of species i on the surface (mole/m21
El,
Surface area of solid species on 2 temperature, f
t
time Is1
T
Temperature, OK
TO
TM Tn Tf u
the surface at peak maximum
Initial temperature, OK Maximum peak temperature, OK Normalized temperature, dimensionless Final temperature, K Fraction of gas diffused out at time t (temperatureTl
434 V
Volume of the sample chamber (m31
vc 'rn w W
Total solid volume in sample chamber (m3) Moles of surface sites per solid volume (mole/m31 Width of the TPD curve at half of the maximum height (KI Bed weight, kg
x1 x2 y(T)
Fraction of sites of type 1 Fraction of sites of type 2 Consumption of reducing adsorbate (mol/m3)
Greek Letters Fraction of metal/metal oxide interface area reduced at time t
a a
S
External surface area cm21
a
Active surface area cm21
Be
Heating rate (K/s)
e
Fractional surface coverage (dimensionless)
eO
@T 5 82
Initial fractional surface coverage (dimensionless) Total surface coverage in multisite model Fraction of type 1 sites filled in multisite model Fraction of type 2 sites Initial coverage of type 1 sites
e10
Initial coverage of type 2 sites e20 r
Stoichiometric coefficient
E
Porosity (cm3/cm31
"B
Bed porosity Particle porosity
EP Q
Surface area occupied by one mole of cm2/mol
E
Subsurface coverage of adsorbate
x
Actual concentration of the reactive species (mol/ccI
X0 0
Initial concentration in active species (mol/cc) Initial
435 LITERATURE CITJXJ 1
2 3 4 5
6 7 8 9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39
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40 41 42 43 44 45 46 47 48
49 50 51 52 53 54
55 56 57 58 59 60
61 62 63 64 65 66 67 68 69
70 71 72 73 74 75 76
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