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Al2O3
~ APPLE ID CATALYSIS A: GENERAL
ELSEVIER
Applied Catalysis A: General 144 (1996) 177-194
Chemisorption and TPD studies of hydrogen on Ni/AI203 Stefan S m e d s a Tapio Salmi a,* Lars Peter Lindfors b Outi Krause b ~ Laboratory of lndustrial Chemistry Abo Akademi Biskopsgatan 8, FIN-20500/~bo, Finland b Neste Oy, Technology Centre, PO Box 310, FIN-06101 Pom,oo, Finland
Received 20 October 1995; revised 5 February 1996; accepted 28 February 1996
Abstract
The activities of two supported nickel catalysts, a commercial (17 wt.-% Ni/AI~_O 3) and a non-commercial (10 wt.-% N i / A 1 2 0 3) catalyst, were investigated in gas phase toluene hydrogenation. Both catalysts were active in hydrogenation, exhibiting rate maxima at about 443 K. The catalysts were characterized using hydrogen chemisorption and temperature programmed desorption (TPD) techniques. Decreasing hydrogen adsorption capacity was generally found in the temperature interval 298-423 K, the capacity of both the commercial and the non-commercial Ni-catalysts being about 20 cm3/gNi at 423 K. No effect on the total adsorption capacity was tbund by increasing the pretreatment temperature from 503 K to 773 K on the commercial catalyst. Three adsorption states of hydrogen (I-III) were resolved from the TPD-spectra of both catalysts. Hydrogen desorption was modelled with peak shape analysis as a second order process with free readsorption, giving hydrogen adsorption enthalpies ranging from - 1 0 8 to - 1 2 4 kJ m o l - i for adsorption state I. The kinetic data and the TPD studies indicate that the decrease of the toluene hydrogenation activity at temperatures above 443 K is due to the decay of adsorption state I. Keywords: Hydrogen; Nickel; Chemisorption; TPD
1. I n t r o d u c t i o n
Nickel catalysts are frequently used in hydrogenation processes, like hydrogenation of aromatics, organic synthesis and hardening of fats. Special attention * Corresponding author. Tel.: (+358-21) 2654427; fax: (+ 358-21) 2654479: e-mail: [email protected]. 0926-860X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved, PII S0926-860X(96)00 103-2
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has recently been focused on the hydrogenation of aromatic compounds, because the aromatic content in fuels have to be suppressed in a near future [1]. In our previous studies [2,3] we considered the kinetics of the gas phase hydrogenation of a typical aromatic molecule, toluene. The hydrogenation rate was found to be strongly dependent on the concentration of hydrogen, and the dependence of the rate on the hydrogen concentration increased with increasing temperature: in an empirical rate law r = k H P nm P Tn the hydrogen exponent (m) was approximately 0.3 at 423 K measured on a non-commercial Ni/A120 3catalyst, but it increased to 1.9 at 473 K on the same catalyst. The reaction rate exhibited a prominent maximum at approximately 443 K. The increase of the hydrogen exponent in the rate law and the rate maximum might be caused by the shift of the adsorption equilibrium of hydrogen with temperature: less hydrogen is present on the surface at 473 K than at 423 K. This hypothesis should, however, be confirmed by a separate investigation of the adsorption-desorption behaviour of hydrogen, by studying hydrogen chemisorption and temperature programmed desorption (TPD) on the Ni catalysts close to the toluene hydrogenation temperatures. Many TPD and flash desorption studies on the H2-Ni system have been published earlier, of which we will only refer to a few [5-13]. Successful efforts [4-7,9-13] have been taken towards a quantitative interpretation of the TPD data (H2-Ni) in order to deduce the adsorption enthalpies and entropies from the TPD-data. Some studies are purely qualitative [8], characterizing the different adsorption states of H 2 on Ni. The aim of this work is to compare the activities of two different Ni catalysts in toluene hydrogenation and to interpret the hydrogenation kinetics by means of the results of chemisorption and TPD studies.
2. Experimental 2.1. Chemicals
The hydrogen gases (99.5% reduction; 99.999% H 2, < 4 adsorption measurements) was was purchased from J.T. Baker
H 2 and < 40 ppm H20 for pretreatment and ppm H20, < 4 ppm N 2, < 2 ppm 0 2 for obtained from AGA. Analytical grade toluene Co. The chemicals were used as received.
2.2. Catalyst preparation and activation
Two supported Ni-catalysts were investigated: a commercial Engelhard Ni/A1203-catalyst containing 17 wt.-% Ni (BET surface area 104 m2/g, specific pore volume 0.35 cm2/g) and a non-commercial Ni/A1203-catalyst containing 10 wt.-% Ni (BET surface area 145 m2/g, specific pore volume 0.45
S. Smeds et al, / Applied Catalysis A: General 144 (1996) 177-194
179
cm3/g). The non-commercial catalyst was prepared by the Atomic Layer Epitaxy-technique (ALE): the support material (Akzo 000-1.5E Alumina crushed and sieved to 0.15-0.35 mm particles) was exposed to a stream of a vaporized organometallic compound (nickel acetyl acetonate). The interaction between the gas and the catalyst was restricted to chemisorption by proper choice of the reaction temperature. Surface saturation was assured by using an excess of the organic reagent. More details of the catalyst preparation procedure are described elsewhere [14,15]. Prior to the experiments the catalysts were activated according to the following procedure. The catalyst samples were reduced under hydrogen flow (200 m l / m i n , 1 atm) while heating from ambient at a rate of 180 K / h until the temperature was 503 K. The prereduced and passivated commercial catalyst was activated during 3 h at 503 K according to the recipe of the manufacturer. For the non-commercial catalyst, heating was continued at the same rate until the temperature was 623 K. Here the reduction was continued 30 min, after which the temperature was increased with 180 K / h to 773 K, where the catalyst was reduced for 2 h. For the sake of comparison the commercial catalyst was also activated according to the procedure applied for the non-commercial catalyst. No decrease in the amount of hydrogen adsorbed was found as a function of the number of heating and cooling cycles.
2.3. Toluene hydrogenation The toluene hydrogenation experiments [2,3] were carried out in gas phase at 1 atm and at 393-483 K in a laboratory scale differential reactor containing typically 0.05 g Ni catalyst. The partial pressures of hydrogen and toluene were varied from 0.2 atm to 0.8 atm and from 0.1 atm to 0.5 atm, respectively. The products were analyzed on-line using a gas chromatograph (Hewlett-Packard 5890 A) equipped with a capillary column (40 m, Silica, J & W Scientific) and a flame ionization detector. The toluene conversion in the differential reactor was always below 5% and the reactor operated in the absence of mass transfer and thermodynamic limitations. Toluene, methylcyclohexane and trace amounts of methylcyclohexene and methylene cyclohexane were detected in the product stream. Very minor deactivation was observed; the measured activities can be regarded as initial ones.
2.4. Hydrogen chemisorption The hydrogen chemisorption experiments were commenced by degassing the reduced catalyst sample (0.1 g) in a sample burette (Rotaflow) at 503 K (the commercial catalyst) or at 718 K (the non-commercial catalyst). After 1 h of degassing (10 -6 mbar) the sample burette was cooled down ( - 200 K / h ) to the actual adsorption temperature. The hydrogen adsorption isotherms were obtained
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S. Smeds et al. / Applied Catalysis A: General 144 (1996) 177-194
at 298 K, 373 K and 423 K with a sorptometer (Sorptomatic 1900, Carlo Erba Instruments). As will be seen (Section 3.2.), no increase in the overall adsorption capacity (298 K) was achieved by increasing the reduction temperature of the commercial catalyst to 773 K, which justifies the low degassing temperature by showing that essentially all the hydrogen measurable by room temperature chemisorption was removed already at 503 K.
2.5. TPD of hydrogen The TPD-experiments of the reduced catalysts (0.1 g) were performed in vacuo in the same burette as used in the chemisorption experiments. After reduction of the catalyst, the sample burette was cooled ( - 200 K / m i n ) to room temperature under continuous flow of hydrogen (200 m l / m i n ) . At room temperature the sample was then (without prior evacuation) exposed to a hydrogen flow (200 m l / m i n ) during 1 h, after which the hydrogen chemisorption was considered to be complete. Now the burette was placed in a temperature programmable oven (TP 190, Carlo Erba Instruments), which was coupled to a quadropole mass spectrometer (QTMD, Carlo Erba Instruments) and a vacuum pump (Elettrorava Turbo Controller). Before the desorption measurement the sample was evacuated to a pressure of 10 - 6 mbar for 40 rain to remove all physisorbed hydrogen. The oven was heated from room temperature linearly to 1200 K and six mass numbers were monitored simultaneously during the TPD experiment ( 1(H + ), 2(H 2), 16(CH 4,0), 18(H 2O), 28(N 2 ,CO), 32(0 2). Different heating rates ([3) were tested (13 = 8, 15 and 25 K / m i n ) . Besides hydrogen, negligible traces of water and O 2 were detected during the experiment.
3. Results and discussion
3.1. Toluene hydrogenation Toluene hydrogenation rates for comparable gas phase conditions on the two Ni-catalysts are summarized from earlier studies [2,3] and listed in Table 1. The
Table l C o m p a r i s o n o f the toluene h y d r o g e n a t i o n rates on the c o m m e r c i a l and n o n - c o m m e r c i a l catalysts Catalyst
r (10 4 m o l / g ~ i s )
r * (mol/molNis)
T (K)
PT (atm)
PH (atm)
Commercial, ca. 17 w t - % Ni
2.4 a 2.9 b
0.094 0.11
448 448
0.30 0.24
0.7 0.6
N o n - c o m m e r c i a l , ca. 10 w t - % Ni
4.0
0.10
448
0.3
0,7
r * = the reaction rate per mol surface Ni. a H2 pretreatment at 503 K. b H z pretreatment at 773 K.
S. Smeds et al. /Applied Catalysis A: General 144 (1996) 177-194
181
measured hydrogenation rates, calculated per gram of Ni, were higher for the non-commercial catalyst than for the commercial catalyst over the entire temperature range studied, the difference between the catalyst activities increasing with temperature. Since the kinetic studies on the two catalysts were done with different pretreatment conditions, the activity difference could partly be caused by these differences (Table 1). An attempt to compare the catalyst activities using the same pretreatment conditions (2 h at 773 K in H 2 flow), although slightly different reactant pressure conditions, is also shown in Table 1. The main reason for the activity difference proved to be different Ni dispersions (Table 3), measured by the hydrogen uptake of the catalysts. Thus, as seen in column 3 of Table 1, we get approximately equal rates calculated per surface Ni atom. To summarize, Table 1 shows that the non-commercial catalyst makes better use of its nickel, whereas the commercial catalyst, if reduced at 773 K, shows slightly, but not significantly, higher turnover rates related to the surface nickel. The reaction rate shows a maximum at approximately 443 K for both catalysts. This rate maximum is a typical phenomenon reported earlier, e.g. by van Meerten and Coenen [16] for benzene hydrogenation on Ni. In our earlier studies of toluene [3] and ethylbenzene [17] hydrogenation on Ni catalysts, we concluded that the hydrogenation rate might be controlled by surface reaction steps between adsorbed aromatic component and hydrogen, and that the decrease of the reaction rate above 443 K is caused by the escape of hydrogen from the Ni surface at higher temperatures. This presumption can, however, only be confirmed by direct studies of hydrogen chemisorption on nickel.
3.2. Chemisorption of hydrogen As a measure of the hydrogen adsorption capacity the Langmuir isotherm with dissociative adsorption was used to calculate the monolayer volume (Vm) at normal conditions (1 atm, 273 K): V
--=0= V,o
( Kn P~)°5
(1)
1 + ( K . pH) °5
From Eq. (1) the parameters Vm and K H were estimated by non-linear regression. To suppress the correlation between the parameters, the isotherm was rewritten in the following form:
pO5 v -
(2)
al + a 2 p~5
where a I = 1 / ( V m K~/2) and a 2 1 / V m. The estimated adsorption parameters are listed in Table 2 and the calculated adsorption isotherms are shown in Fig. 1 (commercial catalyst) and Fig. 2 (non-commercial catalyst) for the experimental =
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S. Smeds et al. /Applied Catalysis A: General 144 (1996) 177-194
Table 2 The estimated hydrogen adsorption monolayer volumes (Vm) and the overall adsorption equilibrium constants (K n) for the commercial and non-commercial Ni-catalysts Temperature (K)
Vm (cm3/gNi)
K n (103 atm - I )
a. Commercialcam~st(fresh) 298 373 423
28 22 18
42 l0 3.3
b. Commercialcata~st(used) 298 373 423
20 27 11
2.1 0.39 1.7
c, Non-commerc~lcata~st(fresh) 298 373 423
43 30 22
1.6 0.65 0.31
Non-commercialcatalyst(used) 298 373 423
44 37 33
2.6 0.61 0.60
temperatures 298 K, 373 K and 423 K. The agreement between the experimental data and the Langmuir isotherm is good, and the adsorption constants generally exhibit an expected thermodynamic tendency: they decrease with increasing temperature. From the temperature dependence of the overall equilibrium constants the overall adsorption enthalpy was estimated to vary between - 1 5 and - 3 0 kJ mol-~. These values are somewhat low compared to the adsorption enthalpies of H 2 o n Ni reported hitherto [18]. One important reason for this is the nature of the Langmuir adsorption model, not being able to account for the expected decrease in adsorption enthalpy with increasing surface coverage, or any other kind of heterogeneity. The estimated values of the overall equilibrium constants also contain considerable uncertainty. Figs. 1 and 2 show that for most of the catalyst samples the amount of adsorbed hydrogen decreases with increasing temperature. The used commercial Ni catalyst was an exception: the adsorption capacity was unexpectedly higher at 373 K than at 298 K (Fig. lb). This could be due to hydrogenation of some fouling products from the toluene hydrogenation reaction, a hypothesis also supported by the much more moderate decrease in adsorption capacity with increasing temperature observed on the used non-commercial catalyst (Fig. 2b) compared to the fresh one (Fig. 2a). Hydrogenation and removal of these fouling products during pretreatment (773 K) is not expected since high temperatures make the hydrogenation reaction thermodynamically unfavoured. Therefore,
S. Smeds et al. / Applied Catalysis A: General 144 (1996) 177-194
183
cm3/g Ni 50
(a)
"10 ..(3 40 03 "O r"
298 K
30 D
o
t3)
£
373
K
423
K
n-.-
~>, 20
c"
-6 > 0
I
I
I
I
5
10
15
20
torr 25
equilibrium hydrogen pressure
cm3/g Ni 5O
(b)
"0 ~) 40 o9
C: cD
30
£ "~ r-
E
K
373 o
20
o
298 423
10 /
O >
_...---r-------r-~
I
0
0
5
,
K
K
•
[
1
I
10
15
20
Itorr 25
equilibrium hydrogen pressure Fig. 1. Chemisorption isotherms of a fresh (a) and a used (b) commercial Ni catalyst.
comparison between the adsorption capacities has to be done using the flesh samples. The irreversible hydrogen adsorption capacity - - calculated per gram of Ni in the catalyst - - of the non-commercial catalyst was always higher than the adsorption capacity of the commercial catalyst, as can be seen from comparison of Fig. 1 and Fig. 2. This could partly be due to the difference in the pretreatment conditions. However, preliminary tests applying the same pretreatment conditions (773 K) show no increase in the room temperature overall adsorption capacity of the commercial catalyst. Hence, the increase in reaction rate with reduction temperature (see Section 3.1) cannot be explained by an increase in the hydrogen adsorption capacity. It is also noted that the difference between the catalysts diminishes (Table 2a and c) at temperatures close to those
S. Smeds et al. / Applied Catalysis A: General 144 (1996) 177-194
184 cm3/g Ni 50 "O
(a) 298 K
~ 4o
Q
o
o
c- 30
373 K
£
o
o
o
~ 20 e-" ID
o
423
K
E lO o > 0
,,I
5
I
I
10
15
,
,
,
I
20
torr 25
equilibrium hydrogen pressure cm3/g Ni 5O -(b) ..Q
298 4o
a
"o ~-
~
373 K
30
£ "o
K
rL
23
K
20
e--
E lo o> 0
I 5
I 10
,
I 15
I 20
torr 25
equilibrium hydrogen pressure Fig. 2. Chemisorption isotherms of a fresh (a) and a used (b) non-commercial Ni catalyst.
in the hydrogenation experiments. Unfortunately the small amounts adsorbed did not permit reliable adsorption measurements at higher temperatures than 423 K. From the adsorbed monolayer capacity at room temperature the metal dispersion ( D ) of the catalyst was estimated according to
"VmM
D = - V,,ol
(3)
where v is the stoichiometric coefficient of chemisorption (here: v = 2) and M and Vmo~ denote the molar mass of nickel and the normal gas volume (1 atm, 273 K), respectively. The metal dispersions calculated from the adsorption capacity at 298 K are listed in Table 3. As can be seen from Table 3 the measured dispersion of Ni is clearly higher (23%) on the non-commercial
S. Smeds et al. / Applied Catalysis A: General 144 (1996) 177-194
185
Table 3 The metal dispersion of Ni (D) estimated from the maximum hydrogen chemisorption monolayer Catalyst
D (%)
T (K)
Commercial fresh Non-commercial fresh
15 23
298 298
catalyst than on the commercial catalyst (15%). This difference in dispersion may partly be explained by the lower metal content of the non-commercial catalyst, yielding a larger part of the Ni accessible for hydrogen at room temperature. Another reason is believed to lie in the method of preparation of the non-commercial catalyst [ 14,15], giving an optimal dispersion of the Ni. Any effect of different degree of reduction can be ruled out since, as already mentioned, high-temperature reduction (773 K) of the commercial catalyst gave no increase in adsorption capacity.
3.3. Temperatureprogrammed desorption of hydrogen (TPD) The chemisorption studies can only give a view of the overall adsorption capacities of the catalysts. To further characterize the nickel catalysts and to investigate the surface heterogeneity, three kinds of TPD-experiments were carried out: an experiment, where the commercial catalyst was activated according to the milder procedure (503 K) as well as experiments, where both catalysts were pretreated in a similar way (773 K). All the three TPD-curves, which were measured from 0.1 g catalyst samples and shown in Fig. 3, have a low-temperature peak up to about 600 K. The samples activated at 773 K also show a high-temperature peak above 600 K (maxima at 700-800 K), which may not originate from adsorbed hydrogen, but rather from decomposition of some hydrogen containing Ni-alumina compounds (possibly Ni-hydroaluminate) formed during the high-temperature treatment. A more probable explanation is that hydrogen spillover takes place only during the high temperature treatment and desorbs at extremely high temperatures, as shown by Kramer and Andre [19] for hydrogen on Ni/A1203. As an additional test, a subsequent TPD-spectrum was recorded immediately after the first, adsorbing hydrogen without prior repetition of the high-temperature pretreatment. The resulting spectra showed no desorption above 600 K. Several other authors [6-8,10,12] have also found hydrogen desorbing at high temperatures (very strong chemisorption, above 600 K) on supported Ni when the catalyst was exposed to hydrogen during the cooling from the reduction temperature to the adsorption temperature. When the nickel is held in inert gas during the cooling, only the lower temperature desorption is observed (strong chemisorption) [9,11,13]. Obviously high-temperature treatment in hydrogen
186
S. Smeds et al. /Applied Catalysis A: General 144 (1996) 177-194
1.5e-06 +
°:" "-. 13
1 e-06
•
N o
+(3
+o e~ O~
'-"1
~2
O O + +
-,~ ~
÷
\
ca o + o O÷
5e-07
o÷"
\
q,.
#-,
*+
I~l-o
300
350
400
450
500
i
I
I
i
J
i
J
550
600
650
700
750
800
850
900
temperature (K) Fig. 3. The TPD-spectra of 0.1 g commercial and non-commercial Ni catalysts. (~3) Non-commercial pretreated at 773 K. ( + ) Commercial pretreated at 503 K. (O) Commercial pretreated at 773 K.
causes very strongly bound adsorption sites to be filled up. Also a very weak form of molecular adsorption, desorbing below 300 K, have been detected on supported Ni [10]. This form is not considered in the present paper. For a quantitative interpretation of the desorption spectra, a line shape analysis procedure originally presented by Cvetanovi6 and Amenomiya [20] and modified for second order desorption by Konvalinka et al. [4] was applied. The procedure starts from the following assumptions: second order desorption [10], linear heating rate and freely occurring readsorption. For a detailed derivation we refer to the work of Konvalinka et al. [4]. The desorption kinetics of hydrogen adsorbed in one state is expressed by the change of the hydrogen surface coverage (0): dO
--
--- --kd Oz + k a P n (1 - O) 2
dt
(4)
Po
where k a and ka denote the rate constants of adsorption and desorption, respectively. During the course of the desorption the sample temperature is changed at a constant rate ([3): dT dt -
(51
S. Smeds et al. / Applied Catalysis A: General 144 (1996) 177-194
187
Equating the amount of hydrogen desorbing per unit time to the amount of hydrogen detected in the carrier gas per unit time transforms Eq. (4) into
(d°t
F C = -V,~12m "-~
= PmVskd 0 2 -
PmVskaC(1 - 0
(6)
where C is the mole fraction of hydrogen in the desorbing gas and F is the carrier gas flow rate, which in a vacuum system is equal to the continually evacuated (by pumping) amount of gas from the sample cell [21]. The assumption of free readsorption allows us to neglect the net rate of desorbing gas (FC) compared to the readsorption rate ( VsumkaC( 1 - O)2), giving
kd 02
1
02
C= ka(1-0) 2 = K (1-0) e
(7)
where K is the adsorption equilibrium constant defined as
=Aexp(
)exp(
Here the adsorption enthalpy ( A H ) and entropy (AS) are assumed to be temperature independent. By combining Eq. (4)-(8) we get dO dT-
F
1
02
/2mVs/3 K ( 1 - 0) 2
(9)
In order to determine the theoretical peak shape of a second order desorption peak, it is first convenient to introduce the following quantities: e = - A H / R T , ~M = - A H / R T M , Cn = C/CM and Tn = T / T M, where lower index M refer to the conditions at the peak maximum. The condition d C / d T = 0 (which also implies that d20/dT 2= 0) is valid at the peak maximum. Eq. (9) gives, after differentiation, an expression for the equilibrium constant at the peak maximum: 1
(1 - 0M) 3 - a n
=
2O,,
RT?,
( lO)
Calculation of TM is now possible if Eqs. (9) and (10) are combined to dO_ dT,,
02 (1-- 0M) 3 (--(~M) (1--0) 2 20 M eMexp(eM)ex p , ' ~
(11)
Integration of Eq. (11) between the limits 00 (initial surface coverage) and 0 M as well as T = 0 and T = TM yields the equation
(1--0M) 3
0M
00 +In
--(0 M-0o)
S. Smeds et aL / Applied Catalysis A: General 144 (1996) 177-194
188
from which 0 M can be solved numerically. When 0 M is known, it is possible to integrate Eq. (11) from 0 M to an arbitrary coverage 0 and Tn = 1 to Tn, yielding an implicit relation between the coverage 0 and the corresponding normalized temperature Tn:
(I--0M) 3
0
0M + l n
= , M e x p ( , M ) f r " e x p ( - eM ]aT.
(13)
L !
Eq. (7) in the normalized form (with Cn =
Cn-
(1-0) 2
02
exp e M - -
To
C/C M) (14)
represents the theoretical line shape for second order desorption with freely occurring readsorption as well as temperature independent enthalpy and entropy of adsorption. In Eq. (14), 0 M is calculated from Eq. (12) and 0 is calculated from Eq. (13). The line shape data fitting procedure was commenced with the assumption that individual adsorption states are completely isolated from each other, which allows the maximal value of 0-- 1 for each adsorption state at full coverage. The procedure of cooling the reduced Ni-catalyst in continuously flowing hydrogen prior to adsorption at room temperature was supposed to ensure almost complete saturation of all possible adsorption states. Since we are dealing with real heterogeneous catalysts, we did not find it convenient to try to measure the actual surface concentration of hydrogen for the different adsorption states. The vacuum TPD system used also made a calculation of the overall adsorption capacity not worthwhile because of the uncertainty concerning the measurement of the pumping speed ( F ) of the system. Therefore the treatment is restricted to normalized peak shape analysis with 00 = 0.95 as a reasonable initial coverage for all individual adsorption states, and the adsorption enthalpy ( A H ) as the only parameter in the calculations. The TPD curve was assumed to be dominated by the peak of the lowest adsorption state (I) until the peak maximum was attained. This first half of the first peak was fitted to the theoretical line shape (Eq. (14)) by adjusting the adsorption enthalpy (A HI) and minimizing the sum of residual squares (Q): N
O : E (Cnj,exp--fllj,rllode[)2
(15)
j=l
The minimization was carried out using the Marquardt-Levenberg method [22]. Using the calculated value of A H~, the rest of state I was simulated according to Eq. (14). The entire peak corresponding to state I was then
S. Smeds et al./Applied Catalysis A: General 144 (1996) 177-194
189
1.5e-06
c
1e-06
-.p 5e-07
g
. 300
{X 350
400
450
500
550
600
temperature (K) Fig, 4. The resolved TPD spectra of the non-commercial Ni catalyst (reduction 773 K).
subtracted from the total TPD curve, resulting in a curve made up of only the higher adsorption states. This curve was in the following steps treated exactly in the same way as the total curve was treated above, resulting in the resolution of adsorption state II. The resolved TPD curves, with three adsorption states (I-III) in the low temperature region below 600 K, of the fresh commercial and non-commercial catalysts are shown in Fig. 4Fig. 5Fig. 6. The fit of the experimental concentrations to the model, Eq. (14), was very good for state I, whereas greater uncertainty for the simulated peak shapes of states II and lII made the fit for these states less exact. Table 4 presents the calculated adsorption enthalpies with 95% confidence intervals for the three low temperature adsorption states on the catalyst samples studied, as well as calculated peak areas of the resulting desorption peaks. The calculated peak areas per mass unit of Ni for all the states I - I l I correlates reasonably well with the measured hydrogenation rates presented in Table 1: the non-commercial catalyst is most active, calculated per gram of Ni, and also shows a greater adsorption capacity for all the observed adsorption states (I-III). Also the overall adsorption capacity of hydrogen on the non-commercial catalyst (43 cm 3 gNi, Table 4b) compared to the overall capacity on the commercial catalyst (28 cm 3 gNU, Table 4a) correlates well with the difference in activity between the catalysts in Table 1. Hence no conclusion concerning the nature of
S. Smeds et al. / A p p l i e d Catalysis A: General 144 (1996) 177-194
190
1.5e-06
"E: 1e-06 -1
~d
cfi 5e-07
300
350
400
450
500
550
600
temperature (K) Fig. 5. The resolved TPD spectra of the commercial Ni catalyst (reduction 503 K).
1.5e-06
~" "E
le-06
c
-,~ 5e-07
300
350
400
450
500
550
temperature (K) Fig. 6. The resolved TPD spectra of the commercial Ni catalyst (reduction 773 K).
600
S. Smeds et a l . / Applied Catalysis A: General 144 (1996) 177-194
191
Table 4 The adsorption enthalpy of hydrogen (a) and calculated peak areas per mass unit of Ni (b) obtained from peak shape analysis of the TPD-data for the commercial (low- and high-temperature reduction) and non-commercial catalysts (high-temperature reduction) Commercial (503 K reduction)
Commercial (773 K reduction
a. Adsorption enthalpy, A H (kJ ~ mol) 1 - 108.5±0.4 II -201.8+_ 1.4 Ill -360+5
State
Non-commercial (773 K reduction)
- 124.3_+0.3 -222.5_+2.0 -413+10
- 124.3+_0.3 - 2 0 2 . 3 + 1.6 -407±5
h. Peakareas(a~itra~uni~) 1 4.1 I1 2.1 II1 1.0
2.2 0.9 0.3
2.2 1.2 0.5
the catalytically active hydrogen could be drawn from adsorption capaoty considerations. The reported adsorption enthalpies in Table 4 are in the range of - 110 to 120 kJ tool I for adsorption state I, which is higher than the calorimetrically measured hydrogen adsorption enthalpies from - 6 0 to - 7 5 kJ tool-~ at high coverages reported on Ni films [18], but in better accordance with enthalpies around - 1 0 0 kJ tool -1 calculated from TPD studies on low index Ni single crystals [13]. Concerning Ni/AlzO3, Weatherbee et al. [6] reported hydrogen adsorption enthalpies of - 7 0 kJ mol -~ for the lower-temperature (333 K) desorption and - 125 kJ tool-1 for the higher-temperature (603 K) desorption. For adsorption state II in this work, enthalpies around - 2 0 0 kJ tool-r were calculated. These values seem too high even for strongly bound adsorption states with interaction between the metal and the support. Konvalinka et al. [10] found the adsorption enthalpy - 1 7 0 kJ mol-~ for their strongest adsorption state at about 800 K on Ni/SiO2, which already corresponds to the states we did not consider (above 600 K) in our results. They also used peak shape analysis to find two adsorption states between 300 K and 600 K, compared to the three states we found in the same range. Adsorption enthalpies over - 3 5 0 kJ tool l calculated for the third adsorption state are probably too big even for strong interaction between metal and support. The values may be influenced by greater uncertainty than the lower states, because of the nature of the resolution procedure. Lower initial coverages in the peak shape analysis procedure were tested to give somewhat lower adsorption enthalpies, but no exact initial coverages could be estimated. In any case, no decisive difference between the adsorption properties of the catalyst samples, that could pin-point the catalytically active hydrogen, could be found with this method of peak shape analysis. Another consideration concerning the nature of the catalytically active hydrogen is the fact that state I, according to these results, is the only adsorption state -
S. Smeds et al. / A p p l i e d Catalysis A: General 144 (1996) 177-194
192
which looses a considerable part of its adsorbed hydrogen in the temperature region (440-450 K) of the hydrogenation rate maximum. If an Arrhenius type of dependence of the intrinsic surface rate on the temperature is assumed, the rate would have increased fivefold at 450 K compared to at 400 K. This statement is valid if the activation energy is around 50 kJ m o l - 1 and the frequency factor is independent of temperature. From 400 K to about 450 K, the coverage of state I decreases from 0.5 to around 0.1, whereas the corresponding decrease for state II is only from full coverage to about 0.8 times full coverage. The total desorption curve is also not able to account for the required decrease in coverage to compensate for the increase of the intrinsic rate constant, even if the observed hydrogen reaction orders ( > 2) are taken into account. With these considerations, we conclude that the amount of hydrogen lost from adsorption state I in the temperature region from 400 K to 450 K would be able to over-compensate the thermodynamically expected rise in the reaction rate constant.
4. Conclusions
Chemisorption experiments of hydrogen on the catalysts revealed that the hydrogen adsorption capacities (normal gas volume) of both catalysts were about 20 cm 3 g~i ~ at 423 K (Table 2). The TPD-studies demonstrated the surface heterogeneities of these catalysts: hydrogen exists in different adsorption states (I-III), state I being responsible for the main part of the adsorption. Peak shape analysis of the TPD curves suggested an adsorption enthalpy around 100-120 kJ m o l - ~ for the hydrogen in state I. The decay of this adsorption state at temperatures above 400 K may explain the temperature maximum observed in the toluene hydrogenation rate: the hydrogenation rate decreases rapidly at temperatures above 400 K as the catalytically active hydrogen (state I) escapes from the nickel surface. The toluene hydrogenation activity of the catalyst would then not be dependent on the overall adsorption capacity, but merely on the properties of adsorption state I.
5. Nomenclature al~ a 2
A C CM
c. D F AH J
parameters in Eq. (2) adsorption entropy parameter, A = exp(A S/R) mole fraction of hydrogen (TPD) mole fraction of hydrogen at TPD peak maximum normalized mole fraction, C n = C/C M dispersion of the Ni catalyst inert gas flow rate (TPD) adsorption enthalpy of hydrogen index for the data points in TPD experiments
S. Smeds et al./Applied Catalysis A: General 144 (1996) 177-194 ka
kd k"
K KH KM m n
M N PH PT Po
Q
r R AS t
T
rM V Um
V~ol
E EM v
OM Oo
193
adsorption rate constant of hydrogen (TPD) desorption rate constant of hydrogen (TPD) apparent rate constant in the toluene hydrogenation rate law adsorption equilibrium constant of hydrogen (TPD) overall adsorption equilibrium constant of hydrogen (chemisorption) adsorption equilibrium constant of hydrogen at TPD peak maximum hydrogen exponent in the toluene hydrogenation rate law toluene exponent in the toluene hydrogenation rate law molar mass of nickel number of data points in TPD experiments partial pressure of hydrogen partial pressure of toluene total pressure in the burette sum of residual squares toluene hydrogenation rate gas constant adsorption entropy of hydrogen time temperature temperature at TPD peak maximum adsorbed volume (chemisorption) volume specific amount of gas adsorbed at full coverage in a certain state adsorbed monolayer normal volume (NTP: 273 K, 1 atm) molar normal volume of an ideal gas (NTP) volume of catalyst bed in TPD experiments heating rate lumped variable, ~ = - A H / R T lumped variable, ~'M = -- A H / R T M stoichiometric coefficient of hydrogen chemisorption surface coverage of hydrogen at TPD peak maximum initial surface coverage of hydrogen (TPD) in a certain adsorbed state
Acknowledgements The authors express their sincere thanks to Mr. K. Er~inen (/kbo Akademi) for his valuable advice during the chemisorption and TPD studies. The financial support to one of the authors (S.S.) from Neste Oy is gratefully acknowledged. References [1] u s Clean Air Act. [2] L.P. Lindfors, T. Salmi and S. Smeds, Chem. Eng. Sci., 22 (1993) 3813.
194 [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
S. Smeds et al. / Applied Catalysis A: General 144 (1996) 177-194
L.P. Lindfors and T. Salmi, Ind. Eng. Chem. Res., 32 (1993) 34. J.A. Konvalinka, J.J.F. Scholten and J.C. Rasser, J. Catal. 48 (1977) 365. P.I. Lee and J,A. Schwarz, J. Catal., 73 (1982) 272. G.D. Weatherbee and C.H. Bartholomew, J. Catal., 87 (1984) 55. R. Spinicci and A. Tofanari, React. Kinet. Catal. Lett., 27 (1985) 65. Y. lkushima, M. Arai and Y. Nishiyama, Appl. Catal., 11 (1984) 305. P.I. Lee, Y.J. Huang, J.C. Heydweiller and J.A. Schwarz, Chem. Eng. Commun., 63 (1988) 205. J.A. Konvalinka, P.H. van Oeffelt and J.J.F. Scholten, Appl. Catal., 1 (1981) 141. D.M. Stockwell, G. Bertucco, G.W. Coulston and C.O. Bennett, J. Catal,, 113 (1988) 317. P.G, Glugla, K.M. Bailey and J.L. Falconer, J. Catal., 115 (1989) 24. K. Christmann, O. Schober, G. Ertl and M. Neumann, J. Chem. Phys., 60 (1974) 4528. L.P. Lindfors, E. Rautiainen and E. Lakomaa, U.S. Patent No. 5124293 (1992). M. Lindblad, L.P. Lindfors and T. Suntola, Catal. Lett., 27 (1993) 323. R.Z.C. van Meerten and J.W. Coenen, J. Catal., 37 (1975) 37. S. Smeds, D. Murzin and T. Salmi, Appl. Catal., 125 (1995) 271. G. Wedler, Chemisorption: An Experimental Approach, Butterworths, London, 1976, p. 39-41. R. Kramer and M. Andre, J. Catal., 58 (1979) 287. R.J. Cvetanovi6 and Y. Amenomiya, Adv. Catal., 17 (1967) 103. K. Christmann, Introduction to Surface Physical Chemistry, Steinkopff Verlag, Darmstadt, 1991, pp. 153-154. [22] MATLAB ® Optimization Toolbox, The Math Works Inc., Natick MA, USA, 1992.
ELSEVIER
Applied Catalysis A: General 144 (1996) 177-194
Chemisorption and TPD studies of hydrogen on Ni/AI203 Stefan S m e d s a Tapio Salmi a,* Lars Peter Lindfors b Outi Krause b ~ Laboratory of lndustrial Chemistry Abo Akademi Biskopsgatan 8, FIN-20500/~bo, Finland b Neste Oy, Technology Centre, PO Box 310, FIN-06101 Pom,oo, Finland
Received 20 October 1995; revised 5 February 1996; accepted 28 February 1996
Abstract
The activities of two supported nickel catalysts, a commercial (17 wt.-% Ni/AI~_O 3) and a non-commercial (10 wt.-% N i / A 1 2 0 3) catalyst, were investigated in gas phase toluene hydrogenation. Both catalysts were active in hydrogenation, exhibiting rate maxima at about 443 K. The catalysts were characterized using hydrogen chemisorption and temperature programmed desorption (TPD) techniques. Decreasing hydrogen adsorption capacity was generally found in the temperature interval 298-423 K, the capacity of both the commercial and the non-commercial Ni-catalysts being about 20 cm3/gNi at 423 K. No effect on the total adsorption capacity was tbund by increasing the pretreatment temperature from 503 K to 773 K on the commercial catalyst. Three adsorption states of hydrogen (I-III) were resolved from the TPD-spectra of both catalysts. Hydrogen desorption was modelled with peak shape analysis as a second order process with free readsorption, giving hydrogen adsorption enthalpies ranging from - 1 0 8 to - 1 2 4 kJ m o l - i for adsorption state I. The kinetic data and the TPD studies indicate that the decrease of the toluene hydrogenation activity at temperatures above 443 K is due to the decay of adsorption state I. Keywords: Hydrogen; Nickel; Chemisorption; TPD
1. I n t r o d u c t i o n
Nickel catalysts are frequently used in hydrogenation processes, like hydrogenation of aromatics, organic synthesis and hardening of fats. Special attention * Corresponding author. Tel.: (+358-21) 2654427; fax: (+ 358-21) 2654479: e-mail: [email protected]. 0926-860X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved, PII S0926-860X(96)00 103-2
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has recently been focused on the hydrogenation of aromatic compounds, because the aromatic content in fuels have to be suppressed in a near future [1]. In our previous studies [2,3] we considered the kinetics of the gas phase hydrogenation of a typical aromatic molecule, toluene. The hydrogenation rate was found to be strongly dependent on the concentration of hydrogen, and the dependence of the rate on the hydrogen concentration increased with increasing temperature: in an empirical rate law r = k H P nm P Tn the hydrogen exponent (m) was approximately 0.3 at 423 K measured on a non-commercial Ni/A120 3catalyst, but it increased to 1.9 at 473 K on the same catalyst. The reaction rate exhibited a prominent maximum at approximately 443 K. The increase of the hydrogen exponent in the rate law and the rate maximum might be caused by the shift of the adsorption equilibrium of hydrogen with temperature: less hydrogen is present on the surface at 473 K than at 423 K. This hypothesis should, however, be confirmed by a separate investigation of the adsorption-desorption behaviour of hydrogen, by studying hydrogen chemisorption and temperature programmed desorption (TPD) on the Ni catalysts close to the toluene hydrogenation temperatures. Many TPD and flash desorption studies on the H2-Ni system have been published earlier, of which we will only refer to a few [5-13]. Successful efforts [4-7,9-13] have been taken towards a quantitative interpretation of the TPD data (H2-Ni) in order to deduce the adsorption enthalpies and entropies from the TPD-data. Some studies are purely qualitative [8], characterizing the different adsorption states of H 2 on Ni. The aim of this work is to compare the activities of two different Ni catalysts in toluene hydrogenation and to interpret the hydrogenation kinetics by means of the results of chemisorption and TPD studies.
2. Experimental 2.1. Chemicals
The hydrogen gases (99.5% reduction; 99.999% H 2, < 4 adsorption measurements) was was purchased from J.T. Baker
H 2 and < 40 ppm H20 for pretreatment and ppm H20, < 4 ppm N 2, < 2 ppm 0 2 for obtained from AGA. Analytical grade toluene Co. The chemicals were used as received.
2.2. Catalyst preparation and activation
Two supported Ni-catalysts were investigated: a commercial Engelhard Ni/A1203-catalyst containing 17 wt.-% Ni (BET surface area 104 m2/g, specific pore volume 0.35 cm2/g) and a non-commercial Ni/A1203-catalyst containing 10 wt.-% Ni (BET surface area 145 m2/g, specific pore volume 0.45
S. Smeds et al, / Applied Catalysis A: General 144 (1996) 177-194
179
cm3/g). The non-commercial catalyst was prepared by the Atomic Layer Epitaxy-technique (ALE): the support material (Akzo 000-1.5E Alumina crushed and sieved to 0.15-0.35 mm particles) was exposed to a stream of a vaporized organometallic compound (nickel acetyl acetonate). The interaction between the gas and the catalyst was restricted to chemisorption by proper choice of the reaction temperature. Surface saturation was assured by using an excess of the organic reagent. More details of the catalyst preparation procedure are described elsewhere [14,15]. Prior to the experiments the catalysts were activated according to the following procedure. The catalyst samples were reduced under hydrogen flow (200 m l / m i n , 1 atm) while heating from ambient at a rate of 180 K / h until the temperature was 503 K. The prereduced and passivated commercial catalyst was activated during 3 h at 503 K according to the recipe of the manufacturer. For the non-commercial catalyst, heating was continued at the same rate until the temperature was 623 K. Here the reduction was continued 30 min, after which the temperature was increased with 180 K / h to 773 K, where the catalyst was reduced for 2 h. For the sake of comparison the commercial catalyst was also activated according to the procedure applied for the non-commercial catalyst. No decrease in the amount of hydrogen adsorbed was found as a function of the number of heating and cooling cycles.
2.3. Toluene hydrogenation The toluene hydrogenation experiments [2,3] were carried out in gas phase at 1 atm and at 393-483 K in a laboratory scale differential reactor containing typically 0.05 g Ni catalyst. The partial pressures of hydrogen and toluene were varied from 0.2 atm to 0.8 atm and from 0.1 atm to 0.5 atm, respectively. The products were analyzed on-line using a gas chromatograph (Hewlett-Packard 5890 A) equipped with a capillary column (40 m, Silica, J & W Scientific) and a flame ionization detector. The toluene conversion in the differential reactor was always below 5% and the reactor operated in the absence of mass transfer and thermodynamic limitations. Toluene, methylcyclohexane and trace amounts of methylcyclohexene and methylene cyclohexane were detected in the product stream. Very minor deactivation was observed; the measured activities can be regarded as initial ones.
2.4. Hydrogen chemisorption The hydrogen chemisorption experiments were commenced by degassing the reduced catalyst sample (0.1 g) in a sample burette (Rotaflow) at 503 K (the commercial catalyst) or at 718 K (the non-commercial catalyst). After 1 h of degassing (10 -6 mbar) the sample burette was cooled down ( - 200 K / h ) to the actual adsorption temperature. The hydrogen adsorption isotherms were obtained
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S. Smeds et al. / Applied Catalysis A: General 144 (1996) 177-194
at 298 K, 373 K and 423 K with a sorptometer (Sorptomatic 1900, Carlo Erba Instruments). As will be seen (Section 3.2.), no increase in the overall adsorption capacity (298 K) was achieved by increasing the reduction temperature of the commercial catalyst to 773 K, which justifies the low degassing temperature by showing that essentially all the hydrogen measurable by room temperature chemisorption was removed already at 503 K.
2.5. TPD of hydrogen The TPD-experiments of the reduced catalysts (0.1 g) were performed in vacuo in the same burette as used in the chemisorption experiments. After reduction of the catalyst, the sample burette was cooled ( - 200 K / m i n ) to room temperature under continuous flow of hydrogen (200 m l / m i n ) . At room temperature the sample was then (without prior evacuation) exposed to a hydrogen flow (200 m l / m i n ) during 1 h, after which the hydrogen chemisorption was considered to be complete. Now the burette was placed in a temperature programmable oven (TP 190, Carlo Erba Instruments), which was coupled to a quadropole mass spectrometer (QTMD, Carlo Erba Instruments) and a vacuum pump (Elettrorava Turbo Controller). Before the desorption measurement the sample was evacuated to a pressure of 10 - 6 mbar for 40 rain to remove all physisorbed hydrogen. The oven was heated from room temperature linearly to 1200 K and six mass numbers were monitored simultaneously during the TPD experiment ( 1(H + ), 2(H 2), 16(CH 4,0), 18(H 2O), 28(N 2 ,CO), 32(0 2). Different heating rates ([3) were tested (13 = 8, 15 and 25 K / m i n ) . Besides hydrogen, negligible traces of water and O 2 were detected during the experiment.
3. Results and discussion
3.1. Toluene hydrogenation Toluene hydrogenation rates for comparable gas phase conditions on the two Ni-catalysts are summarized from earlier studies [2,3] and listed in Table 1. The
Table l C o m p a r i s o n o f the toluene h y d r o g e n a t i o n rates on the c o m m e r c i a l and n o n - c o m m e r c i a l catalysts Catalyst
r (10 4 m o l / g ~ i s )
r * (mol/molNis)
T (K)
PT (atm)
PH (atm)
Commercial, ca. 17 w t - % Ni
2.4 a 2.9 b
0.094 0.11
448 448
0.30 0.24
0.7 0.6
N o n - c o m m e r c i a l , ca. 10 w t - % Ni
4.0
0.10
448
0.3
0,7
r * = the reaction rate per mol surface Ni. a H2 pretreatment at 503 K. b H z pretreatment at 773 K.
S. Smeds et al. /Applied Catalysis A: General 144 (1996) 177-194
181
measured hydrogenation rates, calculated per gram of Ni, were higher for the non-commercial catalyst than for the commercial catalyst over the entire temperature range studied, the difference between the catalyst activities increasing with temperature. Since the kinetic studies on the two catalysts were done with different pretreatment conditions, the activity difference could partly be caused by these differences (Table 1). An attempt to compare the catalyst activities using the same pretreatment conditions (2 h at 773 K in H 2 flow), although slightly different reactant pressure conditions, is also shown in Table 1. The main reason for the activity difference proved to be different Ni dispersions (Table 3), measured by the hydrogen uptake of the catalysts. Thus, as seen in column 3 of Table 1, we get approximately equal rates calculated per surface Ni atom. To summarize, Table 1 shows that the non-commercial catalyst makes better use of its nickel, whereas the commercial catalyst, if reduced at 773 K, shows slightly, but not significantly, higher turnover rates related to the surface nickel. The reaction rate shows a maximum at approximately 443 K for both catalysts. This rate maximum is a typical phenomenon reported earlier, e.g. by van Meerten and Coenen [16] for benzene hydrogenation on Ni. In our earlier studies of toluene [3] and ethylbenzene [17] hydrogenation on Ni catalysts, we concluded that the hydrogenation rate might be controlled by surface reaction steps between adsorbed aromatic component and hydrogen, and that the decrease of the reaction rate above 443 K is caused by the escape of hydrogen from the Ni surface at higher temperatures. This presumption can, however, only be confirmed by direct studies of hydrogen chemisorption on nickel.
3.2. Chemisorption of hydrogen As a measure of the hydrogen adsorption capacity the Langmuir isotherm with dissociative adsorption was used to calculate the monolayer volume (Vm) at normal conditions (1 atm, 273 K): V
--=0= V,o
( Kn P~)°5
(1)
1 + ( K . pH) °5
From Eq. (1) the parameters Vm and K H were estimated by non-linear regression. To suppress the correlation between the parameters, the isotherm was rewritten in the following form:
pO5 v -
(2)
al + a 2 p~5
where a I = 1 / ( V m K~/2) and a 2 1 / V m. The estimated adsorption parameters are listed in Table 2 and the calculated adsorption isotherms are shown in Fig. 1 (commercial catalyst) and Fig. 2 (non-commercial catalyst) for the experimental =
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S. Smeds et al. /Applied Catalysis A: General 144 (1996) 177-194
Table 2 The estimated hydrogen adsorption monolayer volumes (Vm) and the overall adsorption equilibrium constants (K n) for the commercial and non-commercial Ni-catalysts Temperature (K)
Vm (cm3/gNi)
K n (103 atm - I )
a. Commercialcam~st(fresh) 298 373 423
28 22 18
42 l0 3.3
b. Commercialcata~st(used) 298 373 423
20 27 11
2.1 0.39 1.7
c, Non-commerc~lcata~st(fresh) 298 373 423
43 30 22
1.6 0.65 0.31
Non-commercialcatalyst(used) 298 373 423
44 37 33
2.6 0.61 0.60
temperatures 298 K, 373 K and 423 K. The agreement between the experimental data and the Langmuir isotherm is good, and the adsorption constants generally exhibit an expected thermodynamic tendency: they decrease with increasing temperature. From the temperature dependence of the overall equilibrium constants the overall adsorption enthalpy was estimated to vary between - 1 5 and - 3 0 kJ mol-~. These values are somewhat low compared to the adsorption enthalpies of H 2 o n Ni reported hitherto [18]. One important reason for this is the nature of the Langmuir adsorption model, not being able to account for the expected decrease in adsorption enthalpy with increasing surface coverage, or any other kind of heterogeneity. The estimated values of the overall equilibrium constants also contain considerable uncertainty. Figs. 1 and 2 show that for most of the catalyst samples the amount of adsorbed hydrogen decreases with increasing temperature. The used commercial Ni catalyst was an exception: the adsorption capacity was unexpectedly higher at 373 K than at 298 K (Fig. lb). This could be due to hydrogenation of some fouling products from the toluene hydrogenation reaction, a hypothesis also supported by the much more moderate decrease in adsorption capacity with increasing temperature observed on the used non-commercial catalyst (Fig. 2b) compared to the fresh one (Fig. 2a). Hydrogenation and removal of these fouling products during pretreatment (773 K) is not expected since high temperatures make the hydrogenation reaction thermodynamically unfavoured. Therefore,
S. Smeds et al. / Applied Catalysis A: General 144 (1996) 177-194
183
cm3/g Ni 50
(a)
"10 ..(3 40 03 "O r"
298 K
30 D
o
t3)
£
373
K
423
K
n-.-
~>, 20
c"
-6 > 0
I
I
I
I
5
10
15
20
torr 25
equilibrium hydrogen pressure
cm3/g Ni 5O
(b)
"0 ~) 40 o9
C: cD
30
£ "~ r-
E
K
373 o
20
o
298 423
10 /
O >
_...---r-------r-~
I
0
0
5
,
K
K
•
[
1
I
10
15
20
Itorr 25
equilibrium hydrogen pressure Fig. 1. Chemisorption isotherms of a fresh (a) and a used (b) commercial Ni catalyst.
comparison between the adsorption capacities has to be done using the flesh samples. The irreversible hydrogen adsorption capacity - - calculated per gram of Ni in the catalyst - - of the non-commercial catalyst was always higher than the adsorption capacity of the commercial catalyst, as can be seen from comparison of Fig. 1 and Fig. 2. This could partly be due to the difference in the pretreatment conditions. However, preliminary tests applying the same pretreatment conditions (773 K) show no increase in the room temperature overall adsorption capacity of the commercial catalyst. Hence, the increase in reaction rate with reduction temperature (see Section 3.1) cannot be explained by an increase in the hydrogen adsorption capacity. It is also noted that the difference between the catalysts diminishes (Table 2a and c) at temperatures close to those
S. Smeds et al. / Applied Catalysis A: General 144 (1996) 177-194
184 cm3/g Ni 50 "O
(a) 298 K
~ 4o
Q
o
o
c- 30
373 K
£
o
o
o
~ 20 e-" ID
o
423
K
E lO o > 0
,,I
5
I
I
10
15
,
,
,
I
20
torr 25
equilibrium hydrogen pressure cm3/g Ni 5O -(b) ..Q
298 4o
a
"o ~-
~
373 K
30
£ "o
K
rL
23
K
20
e--
E lo o> 0
I 5
I 10
,
I 15
I 20
torr 25
equilibrium hydrogen pressure Fig. 2. Chemisorption isotherms of a fresh (a) and a used (b) non-commercial Ni catalyst.
in the hydrogenation experiments. Unfortunately the small amounts adsorbed did not permit reliable adsorption measurements at higher temperatures than 423 K. From the adsorbed monolayer capacity at room temperature the metal dispersion ( D ) of the catalyst was estimated according to
"VmM
D = - V,,ol
(3)
where v is the stoichiometric coefficient of chemisorption (here: v = 2) and M and Vmo~ denote the molar mass of nickel and the normal gas volume (1 atm, 273 K), respectively. The metal dispersions calculated from the adsorption capacity at 298 K are listed in Table 3. As can be seen from Table 3 the measured dispersion of Ni is clearly higher (23%) on the non-commercial
S. Smeds et al. / Applied Catalysis A: General 144 (1996) 177-194
185
Table 3 The metal dispersion of Ni (D) estimated from the maximum hydrogen chemisorption monolayer Catalyst
D (%)
T (K)
Commercial fresh Non-commercial fresh
15 23
298 298
catalyst than on the commercial catalyst (15%). This difference in dispersion may partly be explained by the lower metal content of the non-commercial catalyst, yielding a larger part of the Ni accessible for hydrogen at room temperature. Another reason is believed to lie in the method of preparation of the non-commercial catalyst [ 14,15], giving an optimal dispersion of the Ni. Any effect of different degree of reduction can be ruled out since, as already mentioned, high-temperature reduction (773 K) of the commercial catalyst gave no increase in adsorption capacity.
3.3. Temperatureprogrammed desorption of hydrogen (TPD) The chemisorption studies can only give a view of the overall adsorption capacities of the catalysts. To further characterize the nickel catalysts and to investigate the surface heterogeneity, three kinds of TPD-experiments were carried out: an experiment, where the commercial catalyst was activated according to the milder procedure (503 K) as well as experiments, where both catalysts were pretreated in a similar way (773 K). All the three TPD-curves, which were measured from 0.1 g catalyst samples and shown in Fig. 3, have a low-temperature peak up to about 600 K. The samples activated at 773 K also show a high-temperature peak above 600 K (maxima at 700-800 K), which may not originate from adsorbed hydrogen, but rather from decomposition of some hydrogen containing Ni-alumina compounds (possibly Ni-hydroaluminate) formed during the high-temperature treatment. A more probable explanation is that hydrogen spillover takes place only during the high temperature treatment and desorbs at extremely high temperatures, as shown by Kramer and Andre [19] for hydrogen on Ni/A1203. As an additional test, a subsequent TPD-spectrum was recorded immediately after the first, adsorbing hydrogen without prior repetition of the high-temperature pretreatment. The resulting spectra showed no desorption above 600 K. Several other authors [6-8,10,12] have also found hydrogen desorbing at high temperatures (very strong chemisorption, above 600 K) on supported Ni when the catalyst was exposed to hydrogen during the cooling from the reduction temperature to the adsorption temperature. When the nickel is held in inert gas during the cooling, only the lower temperature desorption is observed (strong chemisorption) [9,11,13]. Obviously high-temperature treatment in hydrogen
186
S. Smeds et al. /Applied Catalysis A: General 144 (1996) 177-194
1.5e-06 +
°:" "-. 13
1 e-06
•
N o
+(3
+o e~ O~
'-"1
~2
O O + +
-,~ ~
÷
\
ca o + o O÷
5e-07
o÷"
\
q,.
#-,
*+
I~l-o
300
350
400
450
500
i
I
I
i
J
i
J
550
600
650
700
750
800
850
900
temperature (K) Fig. 3. The TPD-spectra of 0.1 g commercial and non-commercial Ni catalysts. (~3) Non-commercial pretreated at 773 K. ( + ) Commercial pretreated at 503 K. (O) Commercial pretreated at 773 K.
causes very strongly bound adsorption sites to be filled up. Also a very weak form of molecular adsorption, desorbing below 300 K, have been detected on supported Ni [10]. This form is not considered in the present paper. For a quantitative interpretation of the desorption spectra, a line shape analysis procedure originally presented by Cvetanovi6 and Amenomiya [20] and modified for second order desorption by Konvalinka et al. [4] was applied. The procedure starts from the following assumptions: second order desorption [10], linear heating rate and freely occurring readsorption. For a detailed derivation we refer to the work of Konvalinka et al. [4]. The desorption kinetics of hydrogen adsorbed in one state is expressed by the change of the hydrogen surface coverage (0): dO
--
--- --kd Oz + k a P n (1 - O) 2
dt
(4)
Po
where k a and ka denote the rate constants of adsorption and desorption, respectively. During the course of the desorption the sample temperature is changed at a constant rate ([3): dT dt -
(51
S. Smeds et al. / Applied Catalysis A: General 144 (1996) 177-194
187
Equating the amount of hydrogen desorbing per unit time to the amount of hydrogen detected in the carrier gas per unit time transforms Eq. (4) into
(d°t
F C = -V,~12m "-~
= PmVskd 0 2 -
PmVskaC(1 - 0
(6)
where C is the mole fraction of hydrogen in the desorbing gas and F is the carrier gas flow rate, which in a vacuum system is equal to the continually evacuated (by pumping) amount of gas from the sample cell [21]. The assumption of free readsorption allows us to neglect the net rate of desorbing gas (FC) compared to the readsorption rate ( VsumkaC( 1 - O)2), giving
kd 02
1
02
C= ka(1-0) 2 = K (1-0) e
(7)
where K is the adsorption equilibrium constant defined as
=Aexp(
)exp(
Here the adsorption enthalpy ( A H ) and entropy (AS) are assumed to be temperature independent. By combining Eq. (4)-(8) we get dO dT-
F
1
02
/2mVs/3 K ( 1 - 0) 2
(9)
In order to determine the theoretical peak shape of a second order desorption peak, it is first convenient to introduce the following quantities: e = - A H / R T , ~M = - A H / R T M , Cn = C/CM and Tn = T / T M, where lower index M refer to the conditions at the peak maximum. The condition d C / d T = 0 (which also implies that d20/dT 2= 0) is valid at the peak maximum. Eq. (9) gives, after differentiation, an expression for the equilibrium constant at the peak maximum: 1
(1 - 0M) 3 - a n
=
2O,,
RT?,
( lO)
Calculation of TM is now possible if Eqs. (9) and (10) are combined to dO_ dT,,
02 (1-- 0M) 3 (--(~M) (1--0) 2 20 M eMexp(eM)ex p , ' ~
(11)
Integration of Eq. (11) between the limits 00 (initial surface coverage) and 0 M as well as T = 0 and T = TM yields the equation
(1--0M) 3
0M
00 +In
--(0 M-0o)
S. Smeds et aL / Applied Catalysis A: General 144 (1996) 177-194
188
from which 0 M can be solved numerically. When 0 M is known, it is possible to integrate Eq. (11) from 0 M to an arbitrary coverage 0 and Tn = 1 to Tn, yielding an implicit relation between the coverage 0 and the corresponding normalized temperature Tn:
(I--0M) 3
0
0M + l n
= , M e x p ( , M ) f r " e x p ( - eM ]aT.
(13)
L !
Eq. (7) in the normalized form (with Cn =
Cn-
(1-0) 2
02
exp e M - -
To
C/C M) (14)
represents the theoretical line shape for second order desorption with freely occurring readsorption as well as temperature independent enthalpy and entropy of adsorption. In Eq. (14), 0 M is calculated from Eq. (12) and 0 is calculated from Eq. (13). The line shape data fitting procedure was commenced with the assumption that individual adsorption states are completely isolated from each other, which allows the maximal value of 0-- 1 for each adsorption state at full coverage. The procedure of cooling the reduced Ni-catalyst in continuously flowing hydrogen prior to adsorption at room temperature was supposed to ensure almost complete saturation of all possible adsorption states. Since we are dealing with real heterogeneous catalysts, we did not find it convenient to try to measure the actual surface concentration of hydrogen for the different adsorption states. The vacuum TPD system used also made a calculation of the overall adsorption capacity not worthwhile because of the uncertainty concerning the measurement of the pumping speed ( F ) of the system. Therefore the treatment is restricted to normalized peak shape analysis with 00 = 0.95 as a reasonable initial coverage for all individual adsorption states, and the adsorption enthalpy ( A H ) as the only parameter in the calculations. The TPD curve was assumed to be dominated by the peak of the lowest adsorption state (I) until the peak maximum was attained. This first half of the first peak was fitted to the theoretical line shape (Eq. (14)) by adjusting the adsorption enthalpy (A HI) and minimizing the sum of residual squares (Q): N
O : E (Cnj,exp--fllj,rllode[)2
(15)
j=l
The minimization was carried out using the Marquardt-Levenberg method [22]. Using the calculated value of A H~, the rest of state I was simulated according to Eq. (14). The entire peak corresponding to state I was then
S. Smeds et al./Applied Catalysis A: General 144 (1996) 177-194
189
1.5e-06
c
1e-06
-.p 5e-07
g
. 300
{X 350
400
450
500
550
600
temperature (K) Fig, 4. The resolved TPD spectra of the non-commercial Ni catalyst (reduction 773 K).
subtracted from the total TPD curve, resulting in a curve made up of only the higher adsorption states. This curve was in the following steps treated exactly in the same way as the total curve was treated above, resulting in the resolution of adsorption state II. The resolved TPD curves, with three adsorption states (I-III) in the low temperature region below 600 K, of the fresh commercial and non-commercial catalysts are shown in Fig. 4Fig. 5Fig. 6. The fit of the experimental concentrations to the model, Eq. (14), was very good for state I, whereas greater uncertainty for the simulated peak shapes of states II and lII made the fit for these states less exact. Table 4 presents the calculated adsorption enthalpies with 95% confidence intervals for the three low temperature adsorption states on the catalyst samples studied, as well as calculated peak areas of the resulting desorption peaks. The calculated peak areas per mass unit of Ni for all the states I - I l I correlates reasonably well with the measured hydrogenation rates presented in Table 1: the non-commercial catalyst is most active, calculated per gram of Ni, and also shows a greater adsorption capacity for all the observed adsorption states (I-III). Also the overall adsorption capacity of hydrogen on the non-commercial catalyst (43 cm 3 gNi, Table 4b) compared to the overall capacity on the commercial catalyst (28 cm 3 gNU, Table 4a) correlates well with the difference in activity between the catalysts in Table 1. Hence no conclusion concerning the nature of
S. Smeds et al. / A p p l i e d Catalysis A: General 144 (1996) 177-194
190
1.5e-06
"E: 1e-06 -1
~d
cfi 5e-07
300
350
400
450
500
550
600
temperature (K) Fig. 5. The resolved TPD spectra of the commercial Ni catalyst (reduction 503 K).
1.5e-06
~" "E
le-06
c
-,~ 5e-07
300
350
400
450
500
550
temperature (K) Fig. 6. The resolved TPD spectra of the commercial Ni catalyst (reduction 773 K).
600
S. Smeds et a l . / Applied Catalysis A: General 144 (1996) 177-194
191
Table 4 The adsorption enthalpy of hydrogen (a) and calculated peak areas per mass unit of Ni (b) obtained from peak shape analysis of the TPD-data for the commercial (low- and high-temperature reduction) and non-commercial catalysts (high-temperature reduction) Commercial (503 K reduction)
Commercial (773 K reduction
a. Adsorption enthalpy, A H (kJ ~ mol) 1 - 108.5±0.4 II -201.8+_ 1.4 Ill -360+5
State
Non-commercial (773 K reduction)
- 124.3_+0.3 -222.5_+2.0 -413+10
- 124.3+_0.3 - 2 0 2 . 3 + 1.6 -407±5
h. Peakareas(a~itra~uni~) 1 4.1 I1 2.1 II1 1.0
2.2 0.9 0.3
2.2 1.2 0.5
the catalytically active hydrogen could be drawn from adsorption capaoty considerations. The reported adsorption enthalpies in Table 4 are in the range of - 110 to 120 kJ tool I for adsorption state I, which is higher than the calorimetrically measured hydrogen adsorption enthalpies from - 6 0 to - 7 5 kJ tool-~ at high coverages reported on Ni films [18], but in better accordance with enthalpies around - 1 0 0 kJ tool -1 calculated from TPD studies on low index Ni single crystals [13]. Concerning Ni/AlzO3, Weatherbee et al. [6] reported hydrogen adsorption enthalpies of - 7 0 kJ mol -~ for the lower-temperature (333 K) desorption and - 125 kJ tool-1 for the higher-temperature (603 K) desorption. For adsorption state II in this work, enthalpies around - 2 0 0 kJ tool-r were calculated. These values seem too high even for strongly bound adsorption states with interaction between the metal and the support. Konvalinka et al. [10] found the adsorption enthalpy - 1 7 0 kJ mol-~ for their strongest adsorption state at about 800 K on Ni/SiO2, which already corresponds to the states we did not consider (above 600 K) in our results. They also used peak shape analysis to find two adsorption states between 300 K and 600 K, compared to the three states we found in the same range. Adsorption enthalpies over - 3 5 0 kJ tool l calculated for the third adsorption state are probably too big even for strong interaction between metal and support. The values may be influenced by greater uncertainty than the lower states, because of the nature of the resolution procedure. Lower initial coverages in the peak shape analysis procedure were tested to give somewhat lower adsorption enthalpies, but no exact initial coverages could be estimated. In any case, no decisive difference between the adsorption properties of the catalyst samples, that could pin-point the catalytically active hydrogen, could be found with this method of peak shape analysis. Another consideration concerning the nature of the catalytically active hydrogen is the fact that state I, according to these results, is the only adsorption state -
S. Smeds et al. / A p p l i e d Catalysis A: General 144 (1996) 177-194
192
which looses a considerable part of its adsorbed hydrogen in the temperature region (440-450 K) of the hydrogenation rate maximum. If an Arrhenius type of dependence of the intrinsic surface rate on the temperature is assumed, the rate would have increased fivefold at 450 K compared to at 400 K. This statement is valid if the activation energy is around 50 kJ m o l - 1 and the frequency factor is independent of temperature. From 400 K to about 450 K, the coverage of state I decreases from 0.5 to around 0.1, whereas the corresponding decrease for state II is only from full coverage to about 0.8 times full coverage. The total desorption curve is also not able to account for the required decrease in coverage to compensate for the increase of the intrinsic rate constant, even if the observed hydrogen reaction orders ( > 2) are taken into account. With these considerations, we conclude that the amount of hydrogen lost from adsorption state I in the temperature region from 400 K to 450 K would be able to over-compensate the thermodynamically expected rise in the reaction rate constant.
4. Conclusions
Chemisorption experiments of hydrogen on the catalysts revealed that the hydrogen adsorption capacities (normal gas volume) of both catalysts were about 20 cm 3 g~i ~ at 423 K (Table 2). The TPD-studies demonstrated the surface heterogeneities of these catalysts: hydrogen exists in different adsorption states (I-III), state I being responsible for the main part of the adsorption. Peak shape analysis of the TPD curves suggested an adsorption enthalpy around 100-120 kJ m o l - ~ for the hydrogen in state I. The decay of this adsorption state at temperatures above 400 K may explain the temperature maximum observed in the toluene hydrogenation rate: the hydrogenation rate decreases rapidly at temperatures above 400 K as the catalytically active hydrogen (state I) escapes from the nickel surface. The toluene hydrogenation activity of the catalyst would then not be dependent on the overall adsorption capacity, but merely on the properties of adsorption state I.
5. Nomenclature al~ a 2
A C CM
c. D F AH J
parameters in Eq. (2) adsorption entropy parameter, A = exp(A S/R) mole fraction of hydrogen (TPD) mole fraction of hydrogen at TPD peak maximum normalized mole fraction, C n = C/C M dispersion of the Ni catalyst inert gas flow rate (TPD) adsorption enthalpy of hydrogen index for the data points in TPD experiments
S. Smeds et al./Applied Catalysis A: General 144 (1996) 177-194 ka
kd k"
K KH KM m n
M N PH PT Po
Q
r R AS t
T
rM V Um
V~ol
E EM v
OM Oo
193
adsorption rate constant of hydrogen (TPD) desorption rate constant of hydrogen (TPD) apparent rate constant in the toluene hydrogenation rate law adsorption equilibrium constant of hydrogen (TPD) overall adsorption equilibrium constant of hydrogen (chemisorption) adsorption equilibrium constant of hydrogen at TPD peak maximum hydrogen exponent in the toluene hydrogenation rate law toluene exponent in the toluene hydrogenation rate law molar mass of nickel number of data points in TPD experiments partial pressure of hydrogen partial pressure of toluene total pressure in the burette sum of residual squares toluene hydrogenation rate gas constant adsorption entropy of hydrogen time temperature temperature at TPD peak maximum adsorbed volume (chemisorption) volume specific amount of gas adsorbed at full coverage in a certain state adsorbed monolayer normal volume (NTP: 273 K, 1 atm) molar normal volume of an ideal gas (NTP) volume of catalyst bed in TPD experiments heating rate lumped variable, ~ = - A H / R T lumped variable, ~'M = -- A H / R T M stoichiometric coefficient of hydrogen chemisorption surface coverage of hydrogen at TPD peak maximum initial surface coverage of hydrogen (TPD) in a certain adsorbed state
Acknowledgements The authors express their sincere thanks to Mr. K. Er~inen (/kbo Akademi) for his valuable advice during the chemisorption and TPD studies. The financial support to one of the authors (S.S.) from Neste Oy is gratefully acknowledged. References [1] u s Clean Air Act. [2] L.P. Lindfors, T. Salmi and S. Smeds, Chem. Eng. Sci., 22 (1993) 3813.
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