347
Applied Catalysis, 3 (1982) 347-376 Elsevier Scientific Publishing Company, Amsterdam -Printed
in The Netherlands
STRUCTURE
AND CoMo/Si02
AND CATALYTIC
CATALYSTS:
ACTIVITY
OF CoMo/y-A1203
AN XPS AND ESR CHARACTERIZATION
A. MATHIEUX,
P. GAJARDOa,
Groupe de Physico-chimie
OF SULFIDED
HYDRODESULPHURIZATION
USED CATALYSTS
P. GRANGE and B. DELMON Mine'rale et de Catalyse,
Place Croix du Sud 1, B-1348
Louvain-la-Neuve,
Universit.6 Catholique
de Louvain,
Belgium.
adeceased
(Received
14 April
1981, accepted
1 May 1982)
ABSTRACT A series of sulfided Co, MO and CoMo supported on y-Al 0 and SiO2 catalysts has been investigated, after they had been used as cataly $S t for 8 h at 305°C and 30 bar pressure in contact with a mixture of thiophene, cyclohexene, cyclohexane and hydrogen. The catalytic activity in the hydrodesulphurization (HDS) of thiophene and hydrogenation of cyclohexene was measured. The sulphided and used catalysts were investigated by X-ray diffraction, diffuse reflectance spectroscopy, XPS and ESR. Samples were transferred from the catalytic reactor to the XPS and ESR equipments under controlled argon atmosphere. Several cobalt and molybdenum surface species were identified and their functions in HDS and hydrogenation reactions were evaluated. Cobalt is in its full sulphided state only if molybdenum is present in the catalyst. The high HDS and hydrogenation performance of CoMo supported on A1203 is basically due to a strong CoMo-support interaction; this leads to a high dispersion of the non-sulphided species (precursor) and, in consequence, of the active sulphided species.
INTRODUCTION In industrial
plant,
or, most probably, marked
changes
sulphur
if reaction
in the catalysts
reaction of MoS2
temperature (synergetic
the greater
catalytically encountered
conditions, catalyst
are altered.
drastically
depends
effect
catalysts
flow reactor.
With results
a direct and simple correlation structure
the amount of
ratio and also on the
the higher the H2 pressure
a considerable
under conditions catalyst,
namely
obtained between
which
number of published had not worked
very different
in
in such non-representative catalytic
may lead to wrong conclusions
0
from those
in steady state conditions
behaviour
and
about the nature of the
sites.
0166-9834/82/0000-0000/$02.75
may undergo
effect of cobalt on the HDS activity
have dealt with catalysts
had worked
work in a partially
catalysts
For instance,
on the H2 pressure;
[3]. However,
by the real industrial
surface
catalysts
on the H2S/H2
[1,2]. Also, the promotor
or which
a high pressure
CoMo/Y-A1203
state. These sulphided
conditions
depends
effect)
the promotor
papers on sulphided
active
hydrotreating
fully sulphided
1982 Elsevier Scientific Publishing Company
348
The present
paper is concerned
sulfided
catalysts
pressure
of 30 bar.
having worked
from the reactor
the specific
role of y-Al203
sulfided
on the catalyst
to cobalt and molybdenum
forms of the oxidic
previously
investigated
nature of the species
CoMo/y-A1203
two catalysts
to be an "inert"
[4, 61. The catalysts
and CoMo/Si02
present
in used sulfided
catalytic
catalysts
activities
are the
series of catalysts
[4,5,7 - IO]. In this paper, an attempt
with HDS and hydrogenation
EXPERIMENTAL
is considered
during
Also, to estimate
characteristics,
Silica
under a
from air contamination
to the XPS or ESR apparatus.
on SiO2 have been studied.
carrier with respect
of CoMo/y-A1203
in a flow reactor
The samples were protected
the transfer
supported
with XPS and ESR studies in HDS reaction
to identify
the
and their relationship
will be made.
METHODS
Catalysts The preparation CoMo/y-A1203
and characteristics
and CoMo/Si02
catalysts
elsewhere
[4, 81. All catalysts
supported
phases
of the oxidic
used in the present
supported
(Co3O4 + Mo03),
namely,
on y-A1203
study have been described
contain
the same amount of
The SiO2 supported
were sulfided
cedure estabiished (this procedure
prior to their activity
from a sr;udy performed
enabled
The preparation
flow quartz
reactor,
was 600°C and 4OO"C,
catalysts
a sulfidation
respectively,
on unsupported
oxides to Cogs8 and
form of the catalysts
of Ar-H2S
was carried
(15% H2S). The reaction
for the alumina
of the reactor was increased
in order to avoid the sublimation
temperature
on the surface,
the samples were subsequently
designated
Catalytic
value of the atomic
activity
Apparatus. automatic
evacuated
under outgassing.
In the (IOO'C h-')
of Mo03. Sulfidation adsorbed
Torr) for 2 h and
catalysts
have been
and silica series respectively,
ratio Co/(Co + MO) being indicated
by r.
measurements
Catalytic
high pressure
powder were deposited
activity
s 1.5 cm, for alumina
measurements
"Catatest"
in the middle
of 2.2 cm2 cross-section
for removing
(lOa
Sulfided
as "Al-S-r" and "Si-S-r" for the alumina
the approximate
progressively
lasted 6 h. To desorb the sulfur physically
then cooled at room temperature
out in a
temperature
and silica catalysts.
in the interval 4OO"C-6OO"C, at the ultimate
to a pro-
Co304 and Moo3 [ll, 121
of the unsupported
of the sulfide
using a mixture
test according
former case, the temperature
transfer
proport-
12.2 f 0.1 wt % (Co304 + Mo03) with r = 0.36 and 1.00.
All catalysts
MoS2).
of the sulfided
14.8 + 0.6 wt %, with variable
ions [r = Co/(Co + MO)] of the active elements. contain
precursors
apparatus
part of a continuous
and 85.5 cm length.
and silica catalysts,
the catalyst
from the reactor,
to the XPS apparatus
were performed (GeomCcanique).
Catalyst
or to the ESR tubes,
steel
Two grams of catalyst
fixed bed flow reactor
bed height was z 0.8 cm and
respectively. without
in a stainless
The special
box used
air contamination,
and for
has
previously
been described
349
c131. Charge feed, experimental a mixture
of cyclohexene
of thiophene
was added;
all catalysts, 30 bar.
conditions
the alumina
and silica
zation of thiophene
Analysis
was measured
space velocities
series,
(L.H.S.V.)
respectively.
of the catalysts
and hydrogenation
For of
were 34 and 18 h-' for
The liquid and gas phase products
were studied,
At reaction
temperature
namely
hydrodesulphuri-
(9 h on stream),
was performed
the reactor was allowed
of catalytic
will be defined
feed was changed
by the conversion
Hydrogenation
of cyclohexene
had reached
to the Ar feed
It was then isolated
off the inlet and outlet
activities.
started.
analysis
At the end of the experiment
to cool. When the temperature
to room temperature.
of 10 atm of Ar by turning
to hydrogen
and the reaction
every hour and qualitative
150°C (after g 30 min) the hydrogen-hydrocarbon and the reactor was cooled
of the reactor
(305"C), the argon was changed feed was established
of the gas phase was made after 7 h of reaction.
Expression
ratio was 600.
of cyclohexene.
IO-15 min, the hydrocarbon
of the liquid products
pressure
(liquid)
5,000 ppm
at 305°C under a pressure
Pure argon was fed into the reactor as the temperature
was increased. and, after
activity
The reactant was
by gas chromatography.
The two main functions
Procedure.
analysis.
(70 % wt) to which
H2 (gas, NTP) / hydrocarbon
the catalytic
The liquid hourly
were analysed
and product
(30 % wt) and cyclohexane
reactor
under a valves.
and HDS catalytic
and thiophene,
activities
respectively,
on
2 g of catalyst.
Physico-chemical
measurements
Surface area, X-ray Diffraction copy (DRS). The BET specific measured tests.
volumetrically.
alumina
reference
were analysed
DRS measurements
Reflectance
Spectros-
by N2 adsorption,
by XRD after the catalytic
with a Co target
(28 kV, 12 mA) with
(200-800 nm) of catalysts
(only the
out using a Beckman ACTA IV spectrophotometer.
support
pellets were obtained
of the catalysts
(XRD) and Diffuse
area was determinated
PW 1010 equipped
series) were carried
pellet of the catalyst
surface
The catalysts
A Philips-Norelco
a Fe filter, was used.
Analysis
(y-A1203) was used as reference. by powder
pressing
with air could not be avoided
C;talyst
at 1.7 ton cm
A
and
. The contact
in these physico-chemical
measure-
ments. XPS. (i) Apparatus a Vacuum Generator
and binding
ESCA 2.
improve the signal to noise ratio. employed spectra obtain
for analysis
energy
A Tracer
(BE) reference.
Northern Conditions
The XPS equipment
NS 560 signal averager of spectra
recording
are the same as those used in a previous
work
was
was used to and the method [7].
The
of the Cls, Si2s, A12s, S2s, S2p, Mojd and Co2p levels were recorded. reproducible
recording
was used.
results,
a strict
standardization
Thus, it was expected
to minimize
of the order and time of the effect of residual
To
350 gas deposition
on the surface of the catalyst
Spectra of catalyst was employed
were typically
on the XPS signal
during
14-18 h.
intensities.
Most of the time
to improve the signal to noise ratio of the CO
The electrical
and Mojd levels. 2P of the samples, which was periodically controll-
charge accumulation
ed, was low in these sulfided Preliminary indicated
recorded
measurements,
catalysts. in which Au was used as reference
(AURA 7,2 = 82.8 eV),
that the BE of the Ols and Alps lines were independent
composition.
Hence, the Ols line was taken as internal
value of 530.3 eV.
The advantages
AIEs have been discussed
reference,
of the use of internal
by Ogilvie et al [14].
of the catalyst using a BE
references
The accuracy
like Ols or
in the determin-
ation of the BE of single peaks was f 0.1 eV. X.P.S.
(ii) Deconvolution
by computer
(or "mirror") by gaussian
of composite
using a peak synthesis method.
lines.
out according
sequence: spectrum.
ii)
Background
substraction
This operation
was performed
that the XPS peaks are described
of the spectra
of the original
Smoothing
ground varied
In both cases it is assumed
The deconvolution
to the following
i)
spectra.
program or by hand using the peak symmetry
from a smoothed
linearly with the BE.
line.
by both methods
It was assumed
This assumption
was carried
that this back-
has been justified
elsewhere
c71. iii) Peak deconvolution.
In the method of deconvolution approximate
positions
at half maximum straints
(FWHM) must be initially
are indicated
in the subsections
the S2p, Mogd and Co2p peaks. interactive
optimization
after smoothing
be identified,
either
i)
included
intensities
and full width
in the input data.
corresponding
The deconvolution
by peak symmetry
and background
These con-
to the deconvolution
of the spectrum
of
is based on
only applies
subtraction,
if, from the original
a half of a gaussian
from the left or right side of the spectrum.
ution by the peak symmetry method general
the number of peaks, their
of the XPS parameters.
The method of deconvolution spectrum,
by peak synthesis,
in the BE scale, their relative
is usually
performed
according
can
The deconvol-
to the following
sequence:
The presence
is sought.
of a half gaussian
line in one of the sides of the spectrum
If it does exist, the other half of the gaussian
peak is drawn by
symmetry. ii)
This gaussian
of the resulting subtracted peaks cannot
peak is subtracted
curve
is described
from the spectrum.
from the envelope.
by a gaussian,
This operation
If one of the sides
a new peak is again built and
is repeated
until further
gaussian
be found.
With the above-mentioned of a half of a gaussian
imposed constraints
peak in the smoothed
the error of the deconvolution
and that relative
and background
by peak symmetry
to the presence
subtracted
is determinated
spectrum,
by the quality
of
351
the fitting between experimental very small differences definition
between
of the S2p, S2s - pi03~ and c02~
When necessary,
additional
gaussian
line. This general
of the background
deconvolution
and theoretical
curves were observed:
calculations,
peaks.
In this work, only
they basically
procedure
depend
on the
has been applied
to the
levels of all used catalysts.
specific
to each level, have been done.
In Figure 1 we present examples of original spectra of S2p, S2s - Flojd and c02~ levels
(al, bl and cl respectively).
(a and b) and Al-S-O.75 smoothing
These examples
(c) catalysts.
(al, bl, cl respectively)
deconvolution
procedure
correspond
The deconvolution
is presented
to used Al-S-O.50
of these spectra,
in Figure 2.
after
The details
of the
for each level are the following:
spectrum (Figure 2a). This spectrum was deconvoluted into three separate II (a) s2P I it has been assumed that and S2p '*I). In the deconvolution, peaks (S s2P ITP' the S2p and S2p 'I* peaks are separated by 3.2 eV (the S2p doublet appeared as one single
peak in our equipment).
between
theoretical
assuming
different
because
S2p
II
This value corresponds
and experimental BE distances
III
and S2p
below) and therefore,
separating
are attributed
the peaks. This assumption
to species
S
II 2P
and S2p
III
i)
The original
all catalysts.
spectrum
(bl) after background
to the general
taken from the S2s - Mo3d composite account
and S2p111),
the standard
of the corresponding mined by making ' species
s2P ording
S2p/S2s
of the 2p level. The other
to their relative
BE distances
molybdenum, ii)
of spectrum is presented
In all catalysts,
rum b3 had a gaussian to the half gaussian
(Sppl,
S
into
II
distaz:e
;BE)
of the three sulfur species were deter-
(S2s11 and S2s
and intensities
III
deduced
III
to the
) were drawn acc-
from the deconvolut-
peaks were then
subtracted
part of the spectrum,
to observe
from
coming only from
that the left side of the spect-
shape. Hence, one should assume
that this side corresponded
of a Mo3d 5,2 peak of the molybdenum
to the following
corresponding
hypotheses
by 3.2 eV and
c)
species
to this species,
and constraints:
lines have the same full width at half maximum lines are separated
was made taking
region
in b3.
it was possible
lowest BE. The F?o~~ doublet, according
in the S
species
2b. The remaining
shows
peaks in
at 225.4 eV (Szsl) correspond
ion of the 2p level. The S2s * , s2sII and S2s the envelope
of the
the S2s peaks were thus first
ratio and ChPe relative
intensity
peak located
support-
(b2) clearly
by gaussian
This operation
detected
levels. The positions
the gaussian
(see
Catalysts
of the deconvolution
subtraction
procedure,
spectra.
the number of sulfur species
is justified
with y-A1203
composition.
of S2s species which were well described According
deviation when
peaks.
- Mo3d spectrum (Figure 2b). The details (b) s2s 110~~ and S2s peaks were the following:
the contribution
associated
do not depend on the catalyst
ed on Si02 do not exhibit
to the minimum
in all series of catalysts,
curves.
(FWHM);
a) b)
the ratio of their
having the
has been constructed the Mo
3d 5/2 and M"3d 312 the maxima of these
intensities
(MOBS 5/2:
352
n
m
353
bl
.b2
16(10
P
l7M BE
H
b4 ,nIv:in 2210
FIGURE 2
Examples
226.0
of deconvolution
axis of Figure b indicate
2360
of the S2p(a, Al-S-0.50),
and Co2p 3,2 (c, Al-S-0.75)
Al-S-0.50)
, 231.0
spectra.
the BE values
S2s and Mojd (b,
Dotted vertical
of oxidic
lines on the BE
MO in oxidation
states
IV,
V
and VI (taken from [50]).
is 1.54. The values of 3.2 eV and 1.54 have been determined experimentM"3d 3/2) ally with pure MoD3. This doublet, labelled as Mo3z, is presented in b4 as a solid line. It corresponds The dotted
to a molybdenum
line is the remaining
species denoted
part of the spectrum
hereafter
resulting
as MO,.
after subtraction
of the Mosjd doublet. spectrum (Figure 2~). The deconvolution of this level was done by ('I "2~ 3/2 the peak synthesis method, according to hypotheses and constraints proposed by Declerck-GrimGe method,
et al [15] and further
this level, after smoothing
developed
by us [16]. According
and background
to this
is deconvoluted
subtraction
into three peaks: i)
A narrow peak, Co,, exhibiting
sulfided ii)
diamagnetic
A broader
cobalt
species,
peak, Co,, attributed
iii) A satellite The constraint
This peak, given by
has no satellite. to high spin cobalt
peak, Co, sat, corresponding
species.
to the Car peak.
imposed ias that (FWHM of Co,) / (FWHM of Co,) was equal to
0.74. These hypotheses chemical
the lowest BE value.
properties
and constraint
of the system,
based on XPS measurements
had permitted
the successful
and on the physicodeconvolution
of
354 the Co2p level of similar catalysts X.P.S.
(iii)
Quantitative
has been taken as line intensity. been calculated obtained
considering
normalized
each element
the response
signal
intensity
of molybdenum.
peaks having a satellite, the "2~ 3/2 The calibration
procedure
the calculation,
the sum of of the
species corresponding
to
was the sum of the intensity
of
model
compounds
as that used during the other measurements. were CoMo04, taking
CoA1204,
presence
influence
and
their stability
in the catalyst.
such as transformation
and the possible
using The
Na4(SiMo,2040)
into account
and their possible
side problems,
during analysis
under We
of the surface
of the matrix
of the solid
intensity. of the calculations
surface compositions
for the quantitative
are presented
we designate
intensities,
in the results section
as defined
were performed
The magnetic
corresponding
to the total molybdenum
to individual
field was modulated
of the analysis.
or cobalt
species. using a dual sample
at 100 kHz using an incident microwave
of the X-band
"Strong Pitch Varian" was used as a standard.
the depth of the resonant
on quantitative
on a Varian El2 spectrometer
power of 20 mW. The first derivative
tube (3 mm inner diameter)
analysis
above, of the Alps. S2p, Co2p 3,2 and Mojd lines.
The index s or r corresponds
ESR. Analyses
chemical
as IAl, Is, ICoT. IMoT, IMos, Ices and ICor the
The index T refers to intensities
cavity.
The thus
to the number of ions of
was done by employing
were chosen
of XPS analysis
The details
species.
intensity
used for calibration
thus to diminish
normalized
have
lines.
coso4. These model compounds
Hereafter,
for each element
of the apparatus.
To facilitate
of the spectrometer
the same experimental model substances
on the signal
coefficient
is proportional
the case of cobalt
In
the signal
and satellite
compounds
The area under each gaussian
intensities
of Mogd 5,2 and Mogd 3,2 peaks was taken for the calculation
signal intensity
the conditions
analysis.
The signal
"seen" by the apparatus.
the intensities
expected
[163.
chemical
(9.5 GHz) spectra were recorded.
With our set up, the sample quartz
was filled with the catalysts
to a height greater
than
cavity.
RESULTS Composition,
surface
area, XRD and DRS
Active phase content, areas of the sulfided were determined
atomic
catalysts
on the oxidic
(Co304 + Mo03) were expressed
composition
ratio
are reported
precursor
r = Co/(Co + MO) and BET surface
in Table 1. The Co and MO contents
form of the catalysts.
conventionally
by considering
denum were present as Co304 and Mo03, respectively.
Active
phase contents
that cobalt and molyb-
All sulfided
catalysts
are of
black colour. X-ray diffraction peaks, except
spectra,
recorded
after catalytic
in the case of the Al-S-l.00
catalyst,
test, did not show any
where
lines at d = 2.98 and
355 TABLE
1
Cobalt and molybdenum
contents,
atomic composition
r = Co/(Co + MO) and BET
surface areas of catalysts Co + MO contentlwt
Catalyst
%
Co/(Co
+ MO)
Surface area/m'
Al-S-O.00
14.80
0.00
114
Al-S-O.05
14.42
0.05
138
Al-S-O.25
14.54
0.27
120
Al-S-O.35
14.68
0.38
111
Al-S-O.50
15.61
0.52
115
Al-S-O.75
15.98
0.75
117
Al-S-l.00
14.46
1.00
105
Si-S-O.36
12.30
0.36
125
Si-S-1.00
12.10
1.00
115
1.75 1, corresponding The principal structure
in Al-S-l.00
Co(II),
between
catalyst.
8 h reaction,
The DRS method
of Co in the CoMo/A1203
indicate
of CoA1204
the absence
to CoA1204.
In the other samples,
impossible,
because
Catalytic
the presence
absorption
of a CoA1204-like
is a good tool to identify
catalysts
[7,18]. Measurements,
where molybdenum
of tetrahedral
made on Al-S-l.00
of the local environment
a DRS study is
to those of CoA1204
[19].
activity
Quantitative
analysis
supported
catalysts,
products:
I-butene,
of the gas phase carried
after 7 h of reaction, cis-2-butene,
and traces of isobutane.
Butadiene
activities,
respectively,
The activities
trans-2-butene,
butane,
propane,
Analogously,
of the Si-S-O.00
The dependence
supported
with respect
silica supported activities,
we present, and Si-S-O.36
of catalytic catalysts
measured
propene,
ethane
of the HDS and hydrogenation catalysts
to the activity
during
in Figure
the first 8 h.
of the catalyst
end of the first hour, et/c, x 100 (cl and ct are the conversions of reaction).
and silica
of the following
was not detected.
of the alumina
are expressed
out with alumina
showed the presence
In Figures 3 and 4 we report the evolution
activity
after
of Co corresponding
is present,
bands of MO'S2 superpose
the
[17]. A broad triple band
500 and 700 nm, due to ligand field transition
is characteristic
-1
peaks of CogS8, were observed.
aim of the DRS study was to detect
local configuration situated
to the principal
g
at the
at 1 and t hours
5, the evolution
of catalytic
catalysts.
activities
on atomic composition
is shown respectively
after 8 h, correspond
in Figures
in alumina
and
6 and 7. The reported
to steady state values.
356
160
Thiophene
I
conversion
Al-S-100
401 1
FIGURE 3
Variation
5
of the HDS activity
150
Cyclohexene
9
h
t
of Al-S-r catalysts
with time on stream.
I hydrogenation
I
I
I
AI-S-025
AI-S-100
301
h
t FIGURE 4
Variation
of the hydrogenation
I
9
5
1
I
activity
of Al-S-r catalysts
with time
on stream.
-XPS Spectra and BE. The original
smoothed
spectra of levels S2o. Mo3d - S2s and
corresponding to the series of catalysts supported on A1203 are presented co2py in Figures 8, 9 and 10 respectively. The BE of sulfur, molybdenum (MO,) and cobalt (Co,) species of catalysts Figure
11 presents
and unsupported
sulfides
are presented
the Si2s - S2p line of the Si-S-O.36
in Table 2.
catalyst.
It is worth-
357 120
8 I u*
80
40
0
FIGURE 5
Variation
for Si-S-O.00
FIGURE 6
Conversion
activities
with time on stream
catalysts.
I
I
Q5Cl
0
13
10
hours
t
of the HDS and hydrogenation
and Si-S-O.36
01
7
4
1
1 A Co
l
MO
vs. atomic
composition
of alumina
vs. atomic
composition
of silica
supported
catalysts
after
8 h of reaction.
FIGURE 7
Conversion
supported
catalysts
after
8 h of reaction. while noticing
the absence
Quantitative
analysis.
for the evaluation [4,5,7,21,23-261. dispersed
XPS signal
of the dispersion The intensity
on the surface
III
of the Sppl' and S2p intensities of active
peaks in this spectrum. have been successfully
phase on the surface
of an XPS signal of an element
of the carrier
(forming a monolayer)
employed
of supports
x, homogeneously is described
by the
358
AI-S-Q25
AI-S-Q35
FIGURE 8 S2p spectra of the Al-S-r series (binding energies in eV are indicated on the curves). following relationship [27]: (1 )
where 0 x(y) is the photoelectron cross section of element x for level y, Ax (y) is the mean free path of the corresponding photoelectron, nx is the concentration of element x on the surface of the carrier and k is a proportionality constant which includes factors such as detector efficiency, surface contamination, X-ray fl ux , etc., Using equation (1) and the appropriate calibration of the XPS apparatus for each element (see experimental methods), one may estimate the surface atomic fraction
359
AI-S-Q00
AI-S-005
AI-S-025
AI-S-035
/ /
Al-S-Q50
Al-S-Q75
I 775
I 780
I 786 BE
spectra of the Al-S-r
FIGURE 9
M0Sd-s2~
indicated
on the curves).
FIGURE
Copp S/2 spectra
10
indicated
=
Ix/$,,
+ ICo +
such as surface
An estimation
in eV are
roughness,
had a precision
The IAl/(IAl
error
etc., may partially
in our quantitative
has not
of the above-mentioned
Under these circumstances,
for similar
invalidate
analyses
of all XPS signals, when
data directly
Several
of dispersion.
that various
for the analyses.
et al. [25] have reported,
intensities
measurements
the intensities
of the intensity
(2) is only approximate.
contamination,
it is expected
degree
are followed
that variations
by equation
of the experimental
to a similar
procedures
(2)
(1) for absolute
been done. Nevertheless,
Okamoto
(binding energies
I#&
of dispersion
the use of equation
affect
series
in eV are
"seen" by the spectrometer:
The evaluation factors
of the Al-S-r
(binding energies
on the curves).
of the catalyst
Ix
series
(ev)
reflect
variations
catalysts,
factors
identical one may admit
of dispersion.
that their relative
XPS
of f 5%.
+ ICoT + Il,ioT),IMoT/(IAl
+ ICoT + IH~T) and ICoT/(IAl
+ ICoT + &T)
ratios give indications
about the degree of dispersion
on the T-Al203
These ratios are plotted
The dotted and cobalt Al-S-l.00
surface.
of cobalt and molybdenum
against
Co/(Co+ MO) in Figure
lines correspond
to the values which should be obtained
species
the same degree of dispersion
retained
12.
if molybdenum
as in Al-S-O.00
and
respectively.
To estimate the lS'(lMoT 'COS'(~A~
the degree of sulfidation
+ *COT
of our used catalysts,
we have plotted
) ratio (Figure 13) and the IMos/(IAl + IMoT + ICoT) and
+ 'MOT + 'COT
) ratios (Figure 14) against Co/(Co + MO).
-ESR The ESR spectra of the catalysts Three different
supported
signals are observed.
Signal
on y-Al203 I, present
are presented
in Figure
in all catalysts
15.
containing
TABLE 2 Binding energies species,
(BE) of c02~ 3,2, MOBS 5,2 and S2p levels of Co,, MO, and sulphur
and spin-orbit
Solid
"2~
splitting
3/21eV
(AE)
of the Co
M"3dMz/2'eV
co,
I s2P
S
Al-S-O.00
2P
160.5
227.2
level (Cos). AE/eV
S2IT /eV
III
s2P
s2P
coS
164.7
168.0
-
227.4
160.3
-
168.0
-
Al-S-O.25
777.2
227.6
160.5
164.5
167.8
15.2
Al-S-O.35
776.7
227.8
160.7
165.1
168.2
15.2
Al-S-O.50
777.1
227.6
160.4
165.1
168.4
15.2
Al-S-O.75
776.6
227.3
160.4
165.1
168.4
15.2
Al-S-l.00
776.6
160.4
164.6
167.9
15.4
160.6
-
160.7
-
Al-S-O.05
MOSER
227.8
"9'8 b
776.8
aFrom references bFrom reference molybdenum,
[20,21] [223
has a peak-to-peak
a g-value centered Signal
III
Al-S-O.75
15.1
at 2.0028.
width of 65 G and a g-value of 1.93. Signal It is visible
in all catalysts
except
is a broader signal observed only in cobalt rich catalysts, and Al-S-1.00;
its peak-to-peak
g = 2.14. In the case of the Al-S-l.00 to 2mW, otherwise
the "saturation"
The ESR spectrum
of catalyst
width
phenomenon
Si-S-O.00
namely
is 208 G and it corresponds
catalysts,
the microwave
II has
in Al-S-1.00. in
to
power was reduced
took place.
(Figure
16a) exhibits
a signal similar
to
361
A 1 lA_ Si2,
Si -S 436
z 8 l-4 5
145
150
155
160
166
170
eV
BE FIGURE
11
XPS spectra of Si2s and Spp lines of catalyst
signal
I found in the alumina
series.
This signal
but the shape is different;
we designate
presents
a "triple"
signal V, centered
Si-S-O.36
exhibits
rich alumina
signal, signal
supported
(Figure
III
catalysts.
it: this latter can be identified
Si-S-O.36
has the same g-value
it as signal
IV.
This catalyst
at g = 2.0028
16b), already
(1.93), also
(Figure 16a). Catalyst
observed
in the case of cobalt
A small signal, at g = 1.97, is superimposed with signal
IV observed
in catalyst
on
Si-S-0.00.
DISCUSSION Catalytic
activity
The results effect
reported
is the different supported
become
different
of HDS activities
with the reaction
manner
of evolution
catalysts
consequently,
time (Figures
and produce
(CoMo). Changes of activity of sulfur,
benzothiophene.
3 and 4). The HDS activities
with reaction
on the
may be associated
and presulfided,
with
our pure MO and
they have a poor hydro-
a low amount of H2S. Their surfaces,
these catalysts
situation
catalysts
time (Figure 3). The
In fact, because
to H2 rich atmosphere.
An inverse
fact to be noticed
in CoMo and in Co or MO
of both groups of catalysts
have been prereduced
activity
are exposed
decrease.
elimination
observed
of their surfaces.
the initial period of reaction, further
of a synergetic
CoMo on the one hand, and Co or MO supported
conditioning
desulphurization
the existence
[283. The interesting
higher and lower, respectively.
a progressive Co supported
of CoMo/A1203
evolution
catalysts
of fresh sulfided other,
in Figure 6 corroborate
in the HDS activity
prevails
of fresh sulfided
have been observed
Under such circumstances,
during
lose sulfur and their activities in catalysts
CoMo/A1203,
with high HDS activity
accompanied
by an
by Gissy et al [29] in the HDS of
362
FIGURE
12
Relative
intensities
of the Al, MOT and COT XPS signals
The effect of the support on the activity of the HDS and hydrogenation (Figure 5), whereas
activities
in Al-S-0.00,
is revealed
of MO supported
in HDS. These behaviours
may be explained
considerable
difference
of molybdenum
During catalyst Al203 [6,30].
preparation,
with H2 [8,31,32]
molybdenum
into account
with respect
forms a stable monolayer
at temperatures
does not react so extensively
it forms Moo3 and behaves
as unsupported
drop
is low in hydrogenation
taking
is that it may be only partially
and H2/2S [33] respectively,
500°C). Molybdenum ation,
of reactivity
(A consequence
by a conspicuous
on Si02 (Si-S-0.00)
the decay of activities
and moderate
(Al-S-r series).
the
to the carrier. on the surface of
reduced and sulfided as high as 400-
with Si02; during catalyst
prepar-
Moo3 when it is reduced with H2
[4,5,10,341. CoMo catalysts
keep or increase their HDS activity
3 and 5). In contrast, their hydrogenation
(except Al-S-0.25),
activity.
The different
with reaction
the same catalysts behaviour
time (Figures
undergo a decay of
of catalysts
in HDS and
363
FIGURE
13
Dependence
of the Is/(IMoT
+ ICoT ) ratio on catalyst
of the relative
intensities
composition
(Al-S-r series).
FIGURE
14
catalyst
Dependence
composition
hydrogenation different
and decrease concept, several
strongly
suggests
that these reactions
take place separately
sites. During the initial period,
of activities
for HDS and hydrogenation
one for HDS and another [20,35-451.
also take place on unsupported
[46,47].
hydrogenolysis,
hydrogenation
condit-
to increase The dual site
has been proposed
by
keep in mind that the two types of reactions
catalysts
the basal planes of MoS2 catalyze planes of MoS2 promote
respectively.
one for hydrogenation,
One must
on two
due to reaction
and death of both types of sites should occur leading
authors
Co and MO on
(Al-S-r series).
types of active
ions, genesis
of the sulphided
reactions.
In
Stevens
et al [20] suggest
whereas
the edge of structural
supported
that
catalysts,particularly
364
FIGURE
15
ESR spectra of Al-S-r catalysts.
by the corresponding
in CoMo/Al2OS,
the number of the different
due to the influence will help elucidate active phases
Signals
I,
II
and III
are indicated
number.
of the support.
types of active sites may be modified
XPS and ESR observations
the state of their surfaces
in the HDS of thiophene
of the used catalysts
and make suggestions
and hydrogenation
about of
of cyclohexene.
XPS Identification catalysts,
of surface
may be attributed
species, to sulfur
(i) Sulphur. in sulfides
BE with the BE of S2p in MoS2 and Co9SB [20-221. the presence
of Co9SB, detected
in Al-S-l.00
The S2p peak, present
in all
because of the similarity
This assignment
of its
is supported
(by XRD), and MoS2, previously
ed by analytical electron microscopy on the same used catalysts [91. II Peaks S2p and S2p "I, present in all alumina supported catalysts,
by
observ-
are absent
365
FIGURE 16
ESR spectra
are indicated
in Si-S-O.00
of Si-S-O.00
by the corresponding
and Si-S-0.36.
(a) and Si-S-O.36
Hence, we are led to assume
from species which are able to be adsorbed the presence to suppose adsorbed
of H2S and thiophene
that S2p
II
and Spp
III
on the one hand, and H2S-Na2S
in the reaction
S2p
more, the adsorption by other authors absence
II
system,
and S2p
[49-521.
assignment
of S2p
in our catalysts) experiment s2P CoMo/A1203 obtained
catalysts.
in both cases assignation
A possible catalysts
of peaks S2p
arises
II
leave unexplained
the
is also able to
(the same as that used
set for the accumulation in the analysis
The same negative
with pure thiophene introduced
The XPS of the of the
result was
at room temperature. by H2S or thiophene
in our XPS measurements.
Consequently,
an
and S2p 'I1 must be sought.
of the XPS differences
from the different
Further-
has been observed
to give an unambiguous
as those employed
that the amount of sulfur
explanation
in BE we measured
at 305°C under 10 bar overnight.
No S2p signal was observed.
is undetectable
would
on Si02, which
the pure A1203
with the XPS apparatus
when the A1203 was impregnated
alternative
on A1203 or catalysts
supported
, we treated
using the same conditions
Hence, one may conclude on A1203
from the differences
this interpretation
with a H2/H2S mixture
was performed
spectrum,
However,
III
in the thiophene-Na2S,
, which were 4.4 and 7.6 eV, respectively.
in catalysts
and Spp
logical
and H2S. respectively,
of BE of sulfur
and H2S at 250°C [53]. In an attempt
II
In view of
it seemed
on the other, are 2.5 and 5-8 eV, respective-
of H2S and thiophene
of these species
adsorb thiophene
I
IV and V
that these peaks originated
atmosphere,
arose from thiophene
ly [48]. These values are not too different (Table 2) between
III,
on A1203, but not on SiO2.
Indeed, the differences
in A1203.
(b). Signals
numbers.
catalytic
between
properties
Si02 and A1203 supported of these supports.
The
366 cracking
and isomerizing
silica [54]. Hence, a alumina
before
properties
reasonable
of alumina
hypothesis
it is desulfurized,
yielding
are much stronger
than those of
is that some thiophene a carbon deposit
is cracked
containing
species such as C-S-H and C-S-S-C which could be the origin of signals Szp
on
some sulfur S
III
II
and
2P
* Identification
Mogd spectrum
of surface
species,
MoS2 (Table 2). The component catalysts presence
(ii) Molybdenum.
(Figure 2b) has given a Mo3: doublet
containing
Mozd appears
MO. This assignment
in deconvolutions
[9]. The small differences inherent
The position
in BE values
taken from reference
on the BE axis of Figure 2b (bottom). exhibit
its position subtraction
to Mo(VI),
The molybdenum
in the BE axis, the dotted
sulfided molybdenum
in the VI,
to detect any molybdenum
electron
microscopy
due to the small
MO(V) and Mo(IV)
[50], are shown as vertical ions located
higher BE than in a sulfide environment
of the MoS2 contribution,
of all
by the
of the spectra.
of the Mojd 5,2 line corresponding
oxidic surroundings,
oundings
by analytical
(Table 2) are probably
to the decomposition
of the to BE of
oftheMo3dlevel
of Maid to MoS2 is supported
of MoS2 in these used catalysts,detected
uncertainties
The deconvolution
having a BE similar
line remaining
V and IV oxidation
in oxidic
[55]. Therefore,
in
lines surr-
due to
in b4 (Figure 2), after
may be attributed
to unsulfided
and semi-
In no case was it possible
states.
ion, in oxidic or sulfide
dotted
surrounding,
with oxidation
state lower than IV. Declerck-Grime'e et al. [15,56] found a "shoulder"
on the high energy
side of
the Mojd 3,2 level, which was attributed
to the presence
The authors
could not arise from unsulfided
species
estimated
because
that this shoulder
it was situated
In view of our more complete presence
of MO ions in oxygen
the visible
thiophene Patterson complete
at a BE which did not correspond
interpretation
have reported
HDS on oxidic
sulfided
[23,25,33,50],
by other authors
effect of A1203.
Hence,
surroundings
of alumina
it is possible
and attached
Mo(V1) oxidic monolayer
only represents
et al. [26], also using
of CoMo/A1203
catalysts
was not
at 400°C.
to MoS2 under the sulfiding
observed
supported
may be interpreted to consider
conditions
catalysts
employed
Identification
in
like ours, also
by the stablizing
the Mo(IV) and MO(V) species,
to the surface of A1203, as remainders
of the
created
after
on the surface of the catalyst
calcination.
deconvoluted
the
for 100 min. On the other hand,
to a H2S mixture
sulfiding
to Moo2 or Mo03.
one cannot exclude
the shoulder
Okamoto
molybdenum
of such species after having conducted
catalysts
et al. [50] found that the sulfidation after 10 h of exposure
Free Moo3 is easily
original
actually
spectrum.
the presence
CoMo/A1203
this work. The incomplete
oxidic
of the spectra,
surroundings;
part of a more complicated
XPS techniques,
of MO(V) ions in MoS2.
of surface species, (iii) Cobalt. The Co spectrum has been 2P into Co,, Car and Co, sat peaks. The assignment of these peaks was
in
367 made on the basis of XPS-ESR reported
elsewhere
The assignment
shape and spin-orbit
Cogs8
of Co, was carried
splitting
and used Co/Al203
(AE)
Co2p sulfur deficient
CogSB because
the existence
Cogs8 in Co/Al203
out taking
into account
of the shape of the XPS peaks
in our oxidic
to those of
to CoA1204
or
(typical of high spin
ESR signal corresponding
of CoA1204
Co, CoA1204
the BE, peak
of the 2p level, which are similar
of a strong
and the presence
and CoMo/Al203 to CogS8,
(r = 1.0) and to pseudo
in CogS8. The Car and Co, sat peaks were ascribed
observed
Co species),
of sulfided
to that study, Co, is ascribed
Car sat to sulfur deficient
and its satellite (r = 0.35).
studies
[16]. According
to sulfur deficient
CoMo/A1203
catalyst
precursors. These assignments Table 2 indicates similar;
unambiguous
assignment
the absence
and typical
cobalt will be denoted
CogS8-like
Quantitative
to Coo in our used catalysts
exhibit
species.
to CoA1204
or sulfur deficient
Cogs8
ive exploitation
discussed
together
of the XPS results.
of the XPS results are considerable
of the distribution
is especially similar
useful
catalysts
etc.,) are nearly cancelled
the outer
It
The difficulties [58]. However,
elements
in identical
in the catalyst
these XPS
series,
of a quantitatit has been
one can obtain
on the catalyst.
ways.
an
The method
study of a series of
Under these circumstances,
of the surface,
contamination,
thus facilitating
the inter-
IcoT/(IAl
+ ICoT + IMoT) and IAl/(IAl
+ ICoT +
of the concentration
layers of the catalyst
Co/(Co + MO) (Figure
present
(paramagnetic),
these
of results.
The Ip,oT/(IA1 + ICoT + IbT),
strong
form of
To decide whether
of the method,
(such as roughness
IFloT) ratios are representative
catalysts
form of cobalt ex-
this sulfided
is to make a comparative
and treated
the errors due to the sample
pretation
of the various
if the purpose
prepared
But.
(see later)
with ESR and DRS data.
shown [7,26] that, in spite of the limitations evaluation
due
the Co, and Car sat peaks and their XPS
of high spin cobalt
interpretation
an
species.
are typical
results will be further
However,
on the use of XPS is not possible
to Co in CogS8; hereafter,
similar
of all catalysts
peaks correspond
Co species.
the Co, peak to Cogs8 or to some sulfided
XPS properties
The spectra
in this work.
of Co2p in Coo and Cogs8 [23,26,33,57].
of any ESR signal ascribable
characteristics
of diamagnetic
of Co based entirely
of BE and AE values
us to assign
hibiting
studied
in Cogs8 and Co, in our catalysts are quite 2P splitting values and shape of the Co, peak (FWHM) in
are also similar
to the similarity
in the series of catalysts
that the BE of Co
the AE spin-orbit
all catalysts
allows
are also valid
precursor
in the surface
(compare the ICoT/(IA1
is worthwhile
to the corresponding
plots for the same
form [7]. As in the oxide form, there
dispersion + ICoT
ions in
[7,27]. The plots of these ratios vs.
12) are quite similar
in their oxide
increase
surface
of these respective
of the cobalt
is a
species when molybdenum
+ I,,,oT ) values at r = 1.00 and r = 0.75).
to notice that the atomic
fraction
of cobalt
"seen" by XPS,
is
) in the sulfided catalyst with r = 1.00 is higher than in 'CoT'(IAl + *MoT + ICoT the same catalyst in its oxidic precursor form, namely 0.07 and 0.03, respectively [73, suggesting
that sulfiding
on the bare v-A1203
surface.
signal for r < 0.5 is more overall
decrease
Co into v-A1203 the formation
important
taking
during
the curve is situated
of cobalt.
electron
microcrystallites crystallization
this statement.
in all catalysts
containing
to explain
in the Mo/A1203
reported
preparation,
Figure
theShigh
from the strong bringing
dispersion
Co-MO-Al
vents a full crystallization sulfidation
[25,26],
on the surface
all catalysts:
strongly
of cobalt
forms,
into account.
of the spectra are consider-
as these observed
in Figure
species
structure
and molybdenum
12.
basically
during
reduction
In
results
form and
[7]. This interaction
about a high dispersion
of catalysts.
the dotted
line would forming
Figure
pre-
[S] and
of the active
for unsupported
of unsulfided attached
represent
catalysts
species
a deficit
of sulfur
to A1203, e.g. the Mo(IV), of a deficiency
in
the IS/(IMoT + ICoT) value if all
sulfur deficit
[20,223),
cobalt and molybdenum,
Other evidence
13 indicates
Cogs8 and MoS2. The sulfur deficit
not only in terms of an intrinsic
(as is reported
surroundings.
when the total
taking place in the oxide precursor
of the bilayer
thus bringing
Co and MO ions were sulfided explained,
of Co and MO,
of the catalyst.
Degree of sulfidation
existence
components
are taken
of cobalt and molybdenum
interaction
about the formation
This applies
role in HDS reactions,
and IMo ), shows the same tendencies
our opihion,
of the
to a large extent after sulfiding
conditions.
14, where only the sulfide
and
et al. [23] and
or not) as well as when only the sulfided
to play the active
of
The partial
and reaction
by Dufresne
that the high dispersion
is retained
at 600°C and use for 8 h in the reaction
which appears
The presence
by
catalysts.
in this work demonstrate
catalyst
performed
the drop of the Ma/Al and the increase
and CoMo/A1203
of the
taking place
molybdenum.
sulfidation
of MoS2, has been also mentioned
as
due to
by the destruction
of molybdenum
of the same catalysts
taking place during
line which
of molybdenum
[9] corroborates
of molybdenum,
during
can be explained
by an agglomeration
step. Analysis
amount of Co and MO (sulfided
ed (Ice
of
about
placed above the
microscopy
Okamoto et al. [25,26]
Actually,
of the
retain the same dispersion
was observed
leading to formation
achieved
bringing
nearly on the dotted
the same line was clearly
The difference
accompanied
the reduction-sulfiding
Results
as a consequence
an increase of the degree of spreading
monolayer,
CO/MO ratios
of cobalt
of the cobalt
of catalysts,
to the case where this element would
line, showing
analytical
dispersion
intensity
This effect may be due to a migration
place during the preparation
In oxidic precursors,
in Al-S-0.00.
the presence
enhance
of CoA1204.
would correspond
molybdenum
reaction
than would be expected
of the cobalt content.
In the case of molybdenum,
dotted
and catalytic
The drop in the relative
can be
of the sulfided
phases
but it may also deal with the
for instance
CoA1204 and molybdenum
MO(V) and Mo(V1)
species
in oxide
in sulfur have been presented
by other
369 authors
[25,26,49].
Okamoto
et al. [25,26]
related
zation effect of the MO phase by the formation observed
on silica supported
on alumina vacancies
supported
catalysts
due to the deeper
views expressed remote control
catalysts
by the formation
by one of us [41-433
support
one explanation
several
effects
of higher amounts
induced by cobalt.
effect was
in explaining
the "synergy
at the exclusion
of sulfur
of the other ones.
ion
to the
by contact"
of these sites. Our present
come into play for decreasing
the effect
This corresponds
by Cogs8 on the number of reduced
MoS2 and on the extent of reduction
to the stabili-
No similar
[59]. Nag et al. [49] explained
reduction
effect exerted
this deficiency
of CoA1204.
by a
active
sites on
results
cannot
It is very likely that
the sulfur content.
-ESR The interpretation able ESR literature Signal
I.
sulfided
of the ESR signalswill on sulfided
This signal,
catalysts
when similar
catalysts
number of studies [60,61,62-651).
located
[60-621.
at g = 1.93 has been observed
It has been assigned
has been published
[60,63],
the avail-
concerning
by other authors
to oxo-MO(V);
exists
A
(see for example provided
it is stabili-
but it does not appear
like A1203,
in
it is also observed
at high temperature.
this signal
to remark that this species
carrier
into account
and our own results.
are reduced with H2 or outgassed
Suffice
zed by an active
be done taking
CoMo systems
in unsupported
MoS2 and Moo3 [60,62,64]. Signal
A single signal very similar
II.
to signal
by Dudzik et al. [66] (see also the discussion [67])in an ESR study of sulfur signal to polyatomic thought
sulfur
impregnated
biradicals;
to depend on the pore structure
and on the recording
temperature,
ic sulfur atom chains. O2 was present
Lo Jacono
in the system.
that our signal
in polyatomic
Signal
This broad signal
III.
containing element. which
cobalt.
containing (observed
into account
the triple
the Dudzik's
that it corresponds
this was
on the support
et al. [60], who attributed
it must be assigned
Because
II.
ions, Koningsetal. and NiMolA1203)
located on MoS2 and WS2 crystals. Co/A1203
Depending
signal became a triple one.
(AH = 208 G) at g = 2.14
to our signal
also in NiW/A1203
on sulfided
of these biradicals
of the support.
[65] found a signal
only promotor
in reference
assigned
A similar
it to paramagnet-
signal only if
results
and the fact
to paramagnetic
sul-
sulfur radicals.
Consequently,
Koningsetal.
is similar
the stability
II is single, we conclude
fur present
The authors
et al. [67] observed
Taking
15) has been reported
of the communication
zeolites.
the single
triple band has been found by Seshadri
II (Figure
and Co/Si02
appears
only in catalysts
to a species
in Cow/Al203
containing
this signal was not observed assumed
from paramagnetic
centers
have not reported
the spectra
in their paper [65]. The discrepancy (and therefore
in samples
that this broad signal
originated
In fact, the authors
studies with regard to the presence
this
(g = 2.09, AH = 320 G)
with the origin)
between of signal
both III
370 may be due to different
pretreatment
conditions
Lo Jacono et al. [61] found a similar CoMo/y-A1203.
These authors
suggest
Co(I) or by a low spin Co(I1)
(T < 140 K), in unsupported to our signal
in CoMo2S4).
et al. C68.691
MoS2-Co9S8
discussions
Cogs8
with Hz, which brings about a partial
was caused by a cobalt
species
XPS measurements
surface
species
on Al-S-l.00
of CogS8) which
(as corresponding
the dramatic
disappearance
indicates
of the Co, species
(Figure
on the determination in the catalysts
Because
compositions
strength
of signal
observed
species
of signal
this phenomenon
is not supported
appears at r = 0.50, whereas
is
from 1.00
of the amount
signal
role
is absent
III
present
is observed
Hagenbach
the maximum
remark
of the corresponding
composition,
depends on the preparation catalysts.
using unsupported we cannot
and suggest the dependence
and composition
is a reduced
surface
area developed
[41-431,
[41-433.
species on not only on
by CogS8.
in supported
explaining
the reduced
MoS2 +
at the moment
observation
and cannot be comparable
effect
[44].
signal should be dependent,
In the frame of the hypothesis
by a remote control
of cobalt
in presence
conditions
but also on the surface method
shows
of one of us concerning
is that, if the Car species
CogS8, the intensity
work which
in this work at r = 1.00 and dis-
et al. [68,69],
sites on reaction
of both In our
temperatures.
at r = 0.20. Although
in the frame of an hypothesis
of the number of HDS active
solubility
on the support.
by recent spectroscopic
in the y-A1203,even
de-
form [73. Lo
due to the presence
to an increasing
and calcination III
such as CoA1204,
(III)
these discrepancies , we can make a general
an explanation
the catalyst
a diminution
is hardly dissolved
for typical catalyst
The general
with an increase
in their oxide precursor
when they are simultaneously
this explanation
fully explain
on the
15) al-
in this work
when r decreases
III
shows
sulfur content low (Figure
fact observed
state of cobalt.
as in catalysts
They attributed
ions in the alumina,
Cogs8 mixtures,
signal
CogS8, but to other high Co(I1)
Jacono et al. [613 also observed
The maximum
is relatively
by
analysis
with r < 0.50, XPS peaks Co, and Car sat can no longer be ascribed
to sulfur deficient
that molybdenum
less sulfur than
is justified
14). Hence, one may infer that MO plays a decisive
of the sulfided
tected when r = 0.05-0.50,
of molybdenum.
having
that the corresponding
takes place concomitantly
that both
namely a cobalt associat-
to a species
The significant
of
It was concluded
(where the X-ray diffraction
of this strong
to 0.50; this phenomenon
supported
reduction.
to the Co, species)
that
from the interactions
on CogS8. This hypothesis
catalyst
though no Co is trapped as CoA1204.
contact"
resulting
of Cogs8 and corresponding
i.e. a reduced
opinion,
measurements
This latter signal may be similar
III and the XPS Car peak have the same origin,
ed with the surface
surface
Also, a broad signal at
[163, we were led to the simple hypothesis
III
the presence
be caused either by
in low temperature
catalysts.
signal
CogS8,
used.
(H2S/H2, 400°C)
III.
In our previous
ESR signal
in sulfided
that this signal might
(as cobalt
g = 2.17 was found by Hagenbach
of the samples
signal
the "synergy
surface
This
and unby
species would
be
formed
by some over-reduction
of the surface of Co9S8, when the catalyst
verge of going beyond the maximum the cobalt
content
are too reducing. formation
The proposed mechanism
of the highest concentration
"normal" working
conditions
ration should be observed, because
a catastrophic
is increased the maximum
beyond
The differences
sulfides
could explain in parallel
allows
for the
species for r = 1.00 in
that the high Car concent-
near the maximum
activity,
This would
reconcile
and unsupported
the facts that
catalysts
is ob-
with respect
supported
catalysts
to signal
III:
Al-S-O-35
with unsupported
the presence
of signal
with its presence
illustrat-
exhibits
does not. The SiO2 supported
of the interaction
behave essentially
are well
Si-S-O.36
catalysts CoMo-Si02
[44]. These have been
making
as a mere mixture
that cobalt and
of Co9S8 + MoS2. This
III in r = 0.36 catalysts
on unsupported
catalysts
catalysts
supported
on SiO2,
at r = 0.20 [68,69].
IV. A signal similar to signal IV was observed when the same catalyst,
Signal
its oxidic
form, was reduced at 400°C by H2 [4]. This signal was also found
duced molybdates
namely MO(V). is located Signal
[62,65]
that signal
in the same symmetry,
they correspond
have mentioned
as signal
(denoted
Si-S-0.00,
is situated
signal V to paramagnetic
catalysts,
is no evidence
located
II
in
as signal
molybdenum
I,
II,
III,
and tungsten [62,65], may
(g = 1.985), III and IV to paramagnetic
centers
and MO(V) ions should be
to attribute
signal
our signals
V (Figure
to Konings'
to these
16a), detected signal
ESR proof with which
of catalysts
suggesting
of the catalyst
in
II and IV. to assign our
on the edges of MoS2 crystals.
that if there are Mo(II1)
on the surface
observed
in spite of the fact that the presence
However,
in the XPS spectrum
Hence, one may conclude
their concentration
environment.
by Koningsetal.
by the authors
we have no additional centers
of signals
in which Mo(III)
at g values corresponding
Beside this similarity,
Mo(II1).
Signal
We have not found clear ESR evidence
in our Al203 supported
tetrahedral
and unsupported
I (see above).
of MoS2 on them is quite probable.
to
IV have the same origin,
by the authors
I, reported
(g = 1.995) of Konings' work were ascribed
species
in re-
It was ascribed
of 2.036, 2.0028 and 1.994. Koningsetal.
state. Signal
to our signal
in
in both types of catalysts,
in a distorted V an ensemble
to g-values
located at the edges of MoS2 crystals, involved.
[62,70-731.
I and signal
(Si02 and y-A1203)
in the sulfided
well correspond
namely
5 ESR signals,
IV and V) in supported catalysts
on silica
Abdo et al. [73] state that this MO(V),
V. We have designated
Si-S-0.00;
supported
and in Moo3
MO(V). We must thus conclude
there
HDS catalysts
III)
because
conditions
occurs when the Co concentration
in supported
its counterpart
by the weakness
molybdenum
[42,43].
SiO2 and y-A1203
observed
appear to have many analogies explained
value
of the signal
between
whereas
catalysts,
of the activity
either
or working
compositions.
ed by the differences
III,
for supported
of Co, (signal
on unsupported
decrease
intensity
activity,
the pretreatment
[41]. But it also predicts
the optimal
served for different
signal
hydrodesulphurization
is too high or because
is on the
Moreover,
the presence
species
of
in the catalysts,
is low and undetectable
by XPS.
372
The absence of Mo(III) in similar sulfided catalysts had been previously mentioned in work based on ESR [60J and XPS measurements [50,74J. An alternative assignment of signal V may be made assuming that the signal appearing at g = 2.0028 and signal II of the alumina supported catalysts have the same origin, namely paramagnetic sulfur in polyatomic sulfur radicals (see discussing of signal II). This assumption i~ quite reasonable in view of the fact that alumina and silica have been treated in the same way before ESR analysis. The small signal at g = 2.036 is situated at the same g-value as a signal observed by Peacock et al. [70J in Mo and Bi-Mo catalyst supported on silica; these authors attributed it to carbon (for carbon, g is usually 2.003). The signal at g = 1.99 may be considered as the result of the superposition of signals coming from Mo(V) and paramagnetic sulfur. These ESR results show how essential it is to withdraw the catalysts from the hiah pressure reactor without air contamination if valuable information about used catalysts is to be obtained. For example, if contamination by O2 should take place to a significant degree, signal III would be destroyed [61J and the triple signal mentioned by Lo Jacono [67J should appear in the ESR spectrum of CoMo/y-A1 203 catalysts. Also, it is worthwhile remarking that no signal due to cobalt metal was detected in any catalyst, which corroborates the results found by XPS. Surface structures and relations with catalytic properties The preceding interpretation of experimental data indicated the presence of various cobalt and molybdenum surface species in sulfided catalysts having worked for 8 h. Such a complex systemproducesamultifunctional catalyst. Reactions such as HDS, hydrodenitrogenation, hydrogenation, hydrodeoxygenation, isomerization and cracking take place on their surface under hydrotreating conditions. Having performed an approximate individualization of surface structures, it is worthwhile to make now an attempt to indicate the specific catalytic role of each structure in the above mentioned hydrotreating reactions, particularly in HDS thiophene and hydrogenation of cyclohexene. CoA1 204-like species. Its presence in sulfided catalysts is indirectly infered from the presence of an XPS signal corresponding to a high spin cobalt species (Cor)' not associated with the ESR signal III. It is present in all catalysts with atomic composition 0.05 < r < 0.75. This species is practically inactive for hydrodesulphurization and hydrogenation [75J. The formation of this inactive compound during the calcination stage can be considered as a ransom which must be paid for obtaining good cobalt dispersion [7J. This compound is not reduced nor sulfided with H2 and H2/H 2S, respectively, at temperatures as high as 50QoC [23,75J. Surface cobalt in sulfur deficient Co~ This species is observed in catalysts with r ~ 0.75. It is characterized by the association of an XPS signal corresponding to COr with ESR signal III. The catalytic activity of CoMo/y-A1 203 in HDS had been
373 previously
attributed
and discussion
to the species
of communication
CogS8. Considering
with sulfur deficient properties
of the catalysts,
HDS and hydrogenation the lowest reduced
in addition time
species,
The origin
MO(V) and Mo(IV).
detected
in HDS of thiophene,
4 and 5 suggest
of Si-S-O.00
dramatically
the hydrogenation
[4,5,34].
activity
In addition,
of catalyst
has
if a
Al-S-1.00,
rather than decrease
with
Lombard0
are active
assumed
that unsulfided
behaves
in
of cyclohexene.
at a higher activity
of Al-S-O.00
to CoMo/y-A1203
noticing
where the decay of activity
that Mo(IV)
by various
and MO(V) supported
of olefins.
in HDS catalysts
are
that the
3 and 5). The low hydrogenation
has been reported
sites in the hydrogenation
with
that these species
It is worthwhile
(Figures
the
In other
as in unsupported
behaviour
in HDS of thiophene,
Mo species
that these
observed
the same time.
one may assume
et al. [77] indicated
alumina
differences
is stabilized
this particular
are comparable
relative
effect
indicating
in the hydrogenation
of Al-S-O.00
of cyclohexene.
is not observed
molybdenum
to the stabilizing
in this work
by 93% during
MO(V) and Mo(VI),
and Si-S-O.00
of CoMo/Si02
[54,65,76].
in
which
decays only by 21% after 5 h reaction,
in which molybdenum
in the hydrogenation
same phenomenon
evidence
drops
Hence, matching
of Mo(IV),
of Al-S-O.00
increase,
but significant
of Al-S-O.00
level than that of Si-S-0.00,
activity
the activity
that they are involved
Indeed, while the activity
the presence
activities
of these oxided or semisulfided
is no experimental
Figures
involved
of this species.
by XPS and ESR, have been attributed
There
are active
catalysts
content
species of Co9S8 were active,
species
words,
that the catalytic
Cogs8 is very low. Al-S-1.00,
3 and 4).
of the support.
activity
[69]
our XPS and ESR results and the catalytic
to being high, should presumably
(Figures
Mo(VI),
has the highest
(reference
[67]). This signal has been identified
we must estimate
of sulfur deficient
activity,
surface
III
giving the ESR signal
in reference
authors on
De Beer et al. [76]
are active
for olefin
hydrog-
enation. E,.
This compound
in the catalysts. and operation
seems to be the only fully sulfided
We have already
under high pressure
of the catalyst
produces
separate
expressed
the opinion
conditions,
compound
[9,28] that, after sulfiding
the oxide bilayer
crystallites
of molybdenum
in the precursor
of MoS2 and cobalt
in the sulfided
state. Studies
performed
MoS2 exhibits association
with unsupported
some activity
with cobalt which
in both reactions
(Figures
and will not be discussed -Co9S, or Co&,-like unambiguously
detected
of Co2p peak) suggest
catalysts
in hydrogenation
have permitted
and HDS reactions,
gives CoMo/A1203
outstanding
confirmation
catalytic
properties
3, 4 and 6). This topic has been amply debated further
species
[28,44]
in this paper.
(without
sulfur deficit).
by X-ray diffraction the presence
that
but it is its
in Al-S-1.00.
of this cobalt
species
The Cogs8 structure XPS results
was
(BE and shape
on A1203 supported
374 catalysts. present
It can be reduced
on unsupported
and hydrogenation Co9S8-like catalytic
catalysts
activities
Hence,
13 and 14, yielding species
cobalt and molybdenum during their sulfiding
but it is the association
is not
of CoMo/A1203
of cobalt and molybdenum a great number of contacts
on the surface
of the catalyst.
precursor
catalysts,
its origin
in the strong
also observed
a too extensive
interaction
by Okamoto
of the active
between
excellent is given,
in our
species
as shown
sulfided
molyb-
The high dispersion
process.
in the oxidic
provoked
of
is preserved
This property
of
et al. [26], in our opinion,
of CoMo/A1203
drop of dispersion
sulfided
form of CoMo/v-A1203
and use in the hydrotreating
some HDS
of Cogs8 and/or
sulfide which gives CoMo/A1203
on the oxidic
CoMo/A1203
the formation
that Co9S8 per se exhibits
the high HDS activity
by the high dispersion
denum and cobalt
indicate
[28,44],
species with molybdenum properties.
in Figures
prevents
Co9S8 when molybdenum
in the catalyst.
Studies
opinion,
to a sulfur deficient
bilayer,
by crystallization
has
which
during
phase.
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