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Activity, selectivity and coking of bimetallic Ni-Co-spinel catalysts in selective hydrogenation reactions
9 Elsevier Science B.V. All rights reserved Catalyst Deactivation 1997 C.H. Bartholomew and G.A. Fuentes, editors
183
Activity, Selectivity and C o k i n g o f Bimetallic N i - C o - S p i n e l Catalysts in Selective H y d r o g e n a t i o n Reactions. J.C. Rodriguez 1, C. Guimon 2, A.J. Marchi 3, A. Borgna 3 and A. Monz 6n 1. 1 Department of Chemical Engineering.Faculty of Science. University of Zaragoza. 50009 Zaragoza. Spain. e-mail: [email protected]. 2 L.P.C.M. URA-CNRS 474. Avenue de l'Universit6. 64000 Pau. France. 3 INCAPE. FIQ-UNL-CONICET. Santiago del Estero 2654. 3000 Santa Fe. Argentina. The influence of the calcination and reduction temperatures of a Ni-Co-ZnA1204 catalyst was correlated with its catalytic activity in the hydrogenation of acetylene. A well interdispersed Ni-Co catalyst supported in a ZnA1204 spinel-like structure was obtained by using a coprecipitation method. Cobalt apparently effects a dilution of Ni surface ensembles, increasing the selectivity to ethylene. The influence of the operating temperature on activity, coking rate and selectivity was analyzed using a deactivation kinetic model. 1. INTRODUCTION. NiO-A1203 oxides are, after a reduction step, the usual catalysts in reactions such as: steam reforming of methane or light hydrocarbons [ 1], production of synthesis gas [2], and selective hydrogenations [3,4]. Impregnation and coprecipitation are widely used in the preparation of these catalysts. The coprecipitation method typically produces a solid solution of mixed oxides with a spinel-like structure [5-7]. Moreover, the high temperature conditions of the steam reforming, syngas production and some hydrogenation processes requires the use of catalysts with good thermal stability such as spinels or other types of mixed oxides. Besides Ni +2, other transition metals including Zn 2+ or Co 2+, also can form aluminates with an inverse spinel structure, some at lower temperatures than Ni 2+ spinels (i.e. 773 K or lower) [7,8]. The final catalyst in some of these processes usually consists of a very well dispersed metal phase in a mixed spinel matrix. The attainment of a well-dispersed metal phase may be important in the case of reaction structure sensitivity. While acetylene hydrogenation is a structureinsensitive reaction, acetylene hydrogenolysis is a structure-sensitive reaction that occurs with the hydrogenation reaction to produce undesirable products like coke and methane. Thus, acetylene hydrogenation consists of a complex reaction network with apparent structure sensitivity [4]. The catalytic behavior of reduced NiO-AI203 mixed-oxide spinels for selective hydrogenation has been studied by our research group for several years [4-6]. These materials have been successfully modified with structural (Zn, Cr) or catalytic (Cu, Co) promoters in order to obtain higher activities and selectivities for the desired products and to decrease coking reactions [4]. This study focused on the effects of calcination and reduction temperatures of a bimetallic Ni-Co catalyst on composition and extent of reduction of its surface and bulk phases and on its catalytic activity in the hydrogenation of acetylene. Furthermore, the effects of operating temperature on activity, coking rate and selectivity were studied and analyzed by means of a kinetic model that simultaneously fits activity and deactivation rate data.
184 2. EXPERIMENTAL. The catalyst was prepared by coprecipitation at a constant pH of 7.2+0.2 and a temperature of 333 K. Decomposition and calcination of the dried precursor (hydrotalcite-like structure) was performed in a N2 atmosphere at 773 K or 873 K for 14 hours, giving a solid with a non-stoichiometric spinel-like structure of the following nominal composition: (NiO)0.5(CoO)0.5ZnA1204. XPS (X-ray Photoelectron Spectroscopy) spectra were recorded with a Surface Science Instrument (SSI) spectrometer, using A1Ktx radiation. The C 1s band at 248.9 eV was used as internal standard. The system was equipped with an in-situ chamber in which pretreatment (reduction at different temperatures) of samples was carried out before analysis. Temperature Programmed Reduction(TPR) experiments were carded out in a separate unit using a total flow rate of 20 cm3/mincontaining 5% H2 in N2, at a heating rate of 10 K/min. SEM (Scanning Electron Microscopy) micrographs of catalysts before and after reaction were obtained with a JEOL JSM 6400 microscope. Solids were covered by electrodeposition with a thin gold film to improve sample conductivity. Reaction tests were carried out in a thermogravimetric system, operated as a differential reactor. Prior to the kinetic tests, catalyst samples were reduced "in situ" for 3 hours at temperatures ranging from 673 K to 873 K. Hydrogenation of acetylene was performed under the following conditions: total flow rate = 700 cm'/min, mass of catalyst = 200 mg, pressure = 1 bar, feed composition (% vol.) (H2/C2H2/N2)= 45/15/40, temperature = 448-498 K. 3. RESULTS AND DISCUSSION. 3.1 Influence of Calcination and Reduction Temperatures on Catalytic Performance. Binding energies of the different metals contained in the catalyst after calcination at 773 or 873 K results are presented in Table 1. No fundamental differences were found at either temperature. The values of B.E. obtained for Ni and Co are those typically reported for NiO+NiA1204 and well-defined COA1204 structures, respectively [7,8]. Nevertheless, the results shown in Table 2 (first and fourth rows) clearly indicate that an increase in the calcination temperature preferentially favors the incorporation of Co +2 cations into the spinel lattice, increasing the relative amount of nickel in the catalyst surface. This can be related to the greater tendency of cobalt to form a well-defined bulk spinel phase [5].
Table 1 Bindinl~ energies of the metallic cations. Element T calc. = 773 K T calc. = 873 K B.E. (eV) (%) B.E. (eV) (%) A1 (2p) 74.40 100 74.40 100 856.03 62.50 Ni (2p3/2) 856.20 58.2 862.00 41.8 862.02 37.50 781.39 70.88 Co (2p3/2) 781.45 67.2 784.17 13.2 783.60 14.71 787.25 14.41 787.58 19.6 1022.37 100 Zn (2p~t2) 1022.48 100
After reduction at different temperatures, XPS and TPR results [4], showed equal percentages of reduction of Ni ~ and Co ~. The similarity of the reduction temperatures of Co and Ni, and their high degree of interdispersion in the original precursor of the catalyst apparently favors a strong synergy between metals. This is supported by a shift of-0.7 eV in the binding energy of Co ~ (778.1 eV), with respect to the value obtained for this element in a monometallic catalyst Co-Zn-A1 prepared for comparison purposes (778.8 eV). However, reaction tests revealed only minor differences in the performance of the samples of catalyst calcined or reduced at different temperatures (ethylene selectivities equal to 0.75-0.8, coke concentrations after 3 h of reaction equal to 7.5-9.5 mg/100 mg cat. (results not shown) and similar reactant conversions). In fact, XPS shows that, in spite of the increase in the
185 percentage of reduction of Co and Ni, as the temperature of reduction increases, sintering becomes more important (Table 2, first and second columns). The coupling of both processes apparently leads to similar quantities of available surface metal, and therefore to similar catalytic behavior. The only exception is the sample of catalyst calcined at 873 K and reduced at 673 K. The activity of this sample for the main reaction (ethane and ethylene production) is lower than that expected from the total amount of reduced metal in the surface (see Table 2, last column). Table 2 Not only the type of Surface atomic ratios of Ni and Co metals. Catalyst metal (cobalt or nickel), but calcined and reduced at several temperatures. its accessible fraction, Tcalc.= 773 K determine the catalytic Tred. NiT/A1 COT/A1 Ni~ ~ Ni~176 behavior. All the catalyst unreduced 0.213 0.153 0 samples, with the exception 673 K 0.133 0.106 1.51 0.066 of that reduced at 673 K, 773 K 0.080 0.054 1.79 0.075 have similar amounts of Tcalc.= 873 K metal and Ni~ ~ ratios in Tred. NiT/A1 COT/A1 Ni~ ~ Ni~176 the surface, and thus, exhibit similar activities for unreduced 0.187 0.076 0 the main and coking 673 K 0.126 0.078 0.32 0.011 reactions. The sample 773 K 0.101 0.058 1.92 0.084 reduced at 673 K, on the 873 K 0.083 0.047 1.92 0.082 contrary, is unusually enriched in cobalt leading to a near zero activity for the main reaction, but this 12 apparently does not affect 12 ' ' ' ' ' ' 1 ' ' ' ' ' ' m Tea =773 K, T ~ t = 6 7 3 K the coking reaction (7.5 mg 10 of coke/100 mg cat. after 3 10 9 Tc~=873K, T,=t=773K h of reaction) (results not o T cai=873K, Tract=873K R shown). This result, that is 8 fully analogous to the behavior observed with the )= monometallic catalyst (CoZn-A1), can be explained by 4 o considering the differences in the adsorption strength .~ Tqzr.=448 K 2 of reactant molecules on 2 {_ H2/C2H2=3/1 nickel and cobalt sites. On cobalt atoms, acetylene is 0 4000 8000 12000 0 4000 8000 12(~) '0 strongly adsorbed leading to Time (s) an almost stoichiometric reaction (this adsorption Figure 1. Influence of the reduction temperature on the could explain the non-zero ethane and ethylene yields. coke production when the Co-Zn-A1 catalyst is used). On the nickel atoms, reaction takes place, favored by the weaker reactant adsorption. Considering also that cobalt has a dilution effect on the Ni crystallites, the combination of both effects leads to facts leads to an increased selectivity to the desired product (ethylene) [9,10].
186 3.2 Influence of the Operating Temperature on Catalytic Performance. The influence of the operating temperature on the activity, selectivity and coking rate were studied for a sample of catalyst calcined and reduced at 773 K. An increase in the operating temperature from 428 K to 498 K led to a greater initial coking rate, a higher f'mal coke content, a lower initial conversion and a more selective catalyst (Figures 2, 3 and 4). In Figure 2, it is evident that an increase in temperature produces an increase in the coking rate, and thus, in the final coke concentration. Independent of temperature, coke content sharply increases during the initial stage of the reaction, and the coking rate progressively decreases with time, to reach a pseudo-steady state, in which coke grows at a con~ 15 stant rate. These two reaction O O regimes could be related to ,r o o o o two different kinds of coke on the catalyst surface. Thus, t.on the bare metallic surface, 8 coking progresses quickly, by ~ 5 hydrogenolysis of acetylene 1) 428 K 4) 488 K 8 molecules, and methane 2) 448 K 5) 498 K production (results not 3)468 K shown). During this stage, , 0100 ~ I , I ~ I , I , I 00 2 4000 60(X) 8000 1 0 0 0 0 12000 filaments are nucleated and "Sme (s) the metallic surface is covered with a layer of coke causing a Figure 2. Influence of the operating temperature on the pseudo equilibrium to be coke deposition. Line: experimental data ; Symbols: data reached that depends on the predicted by model. operating conditions (partial pressures of reactants and temperature). This fast deactivation also determines 14 the initial level of catalyst 12 activity. Thus, the higher the temperature, the lower the 0 0 10 initial activity of the catalyst A & (Figure 3) because of the v 8 -o higher initial coking rate ~) BB B ~ ~ m~ ~ ~ ~ F m (Figure 2). During the second 6 period of coking, that 9 428K #o 448K 4 constitutes most of the 9 468K reaction period, growing 488K 9 496K carbon filaments (see Figure 5) accumulate, while the , I , I , I , I , I 00 2000 4000 6000 8000 1 0 0 0 0 12000 modification of the metallic surface by acetylene ~me (s) hydrogenolysis and filamental nucleation is less Figure 3. Influence of the operating temperature on important. The negligible total gas yield (ethane + ethylene + methane). effect of filamentous coke on the activity of metallic 20
E
.
,
9
,
9
,
9
,
.
,
9
187 particles which remain at the filament tip is well established (11). In fact, no important decrease in catalyst activity with reaction time is evident within the temperature interval studied (see Figure 3) (even, after 30 h of reaction at 448 K, catalyst activity remained constant). It can be concluded that the first stages of the reaction determine the state of the metallic surface, i.e. the percentage of bare sites for the main reaction, and hence, the overall performance of the catalyst. Two types of coke 1.0 . . . , 9 , 9 , , , , , 1.0 deposits (filamentous carbon and amorphous 0.9 0.9 k coke) were evident from 0.8 0.8 SEM. The process of filament nucleation gives 0.7 0.7 methane as a secondary reaction product. The increasing rate of encap0.5 - - ~, 0.5 sulation of the metallic 0.4 I~ 0.4 crystallites by carbon during this stage is re9 o~ sponsible for the negative 0.2 0.2 value observed for the 0.1 activation energy for the 010 4000 8000 12000 0.00 0.05 0.10 0.15 0.20 main reaction (Figure 3). On Time (s) Coke Content (mg/mg cat) the other hand, as mentioned previously, coke Figure 4. Influence of the operating temperature on the deposition causes an ethylene selectivity. incremental increase in ethylene selectivity (Figure 4) and a parallel decrease in the selectivity to ethane and methane. Thus, more than activity, coke affects product distribution with time. The mechanism developed by Thomson and Webb [10] to explain the hydrogenation of acetylene is presently commonly accepted with slight modifications [9]. It is proposed that ethane can be produced by two different routes, i.e. direct hydrogenation of acetylene on the metallic bare sites, via the organometallic-likeintermediate called ethylidine, or in successive hydrogenations. Ethylene is easily formed, even on the metallic sites covered by coke, via a hydrogen transfer mechanism from the coke layer to the second adsorption layer. Methane is preferentially formed on the bare metallic surface, and is probably a by-product of the nucleation of carbon filaments. Taking into account this mechanism, the incremental increase in selectivity to ethylene with reaction time could be a result of the progressive change of the catalyst surface. The presence of a well-dispersed cobalt-nickel active phase, in which reactants are adsorbed more strongly on cobalt than on nickel, probably leads to the abrupt increase in the selectivity to ethylene, i.e. the reaction product that requires smaller metal ensembles to be produced [9,10]. This fact explains the results shown in Figure 4, where it can be seen that the ethylene selectivity and coke levels are almost independent of the operating temperature. Thus, coke concentration determines the product distribution.
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0.6
9
0.31 ~ ,
0.3
i
,
i
,
,
'
'
'
9
'
'
'
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'
3.3. Kinetic Modeling of Activity and Coke Coverage. A kinetic model based on other previously proposed models [12], was developed to simultaneously fit activity and coke coverage. From the above results, for the present catalystreaction system, coke apparently causes a progressive decrease in the hydrogenation capacity. Consequently, activity was calculated as the quotient between the rates of hydrogen consumption (to ethane, ethylene and methane) at a given time, and at zero time.
188 The model proposed assumes reversible formation of amorphous coke (Ccm) and simultaneous formation of whiskers (Ccw) on the catalytic surface. The amount of monolayer coke at infinite time (Ccmax) depends on the operating conditions (e.g. temperature) and is governed by values of the rate constant for
dCc m rcm -
- kS (Cc max - Ccm ) h _ k RCcm (Ccmax - Ccm ) s (Eq. 1)
dCc w rcw - ~ - k dt
[
(Eq. 2)
w
1m
Cc a= 1 - ~ Ccmax J
(Eq. 3)
k.l=k.lo[-mi(1- T~/I
; i=R,S,W
(Eq. 4)
Table 3. Kinetic parameters obtained in the fitting of the activity-time data. Parameter
kso (S-1) "103 kRo (s "1) "102 kwo (s'l)'106
Parameter
5.242 _+0.38 8.428 +_0.89 5.29+ 0.268
Parameter
Eas(cal/mol) EaR(cal/mol) Eaw (cal/mol) Ccm~ (mg/mg cat)
Figure 5. SEM micrograph of coke formed on the catalyst surface after 3h reaction time.(X 4,300). Temp. Reaction = 448 K.
Parameter
8271 + 776 4634 + 923 4887 + 435 0.129 + 0.004 monolayer coke formation (ks) and that for coke removal (kR) (Eqn. 1). Coke removal takes place by reaction with hydrogen, generating other nonanalyzed reaction products (e.g. C4+) [9]. The growth rate of filamentous coke is assumed to be constant (Eqn. 2), according to literature data [11 ] and in view of the lack of any noticeable effect upon the activity of the catalyst. The total coking rate was predicted as the sum of Eqns. 1 and 2; Eqn. 3 relates catalyst activity to the mono-layer coke content. A global view
189 of the reaction-coking path-way is shown in Figure 6. The parameters shown in Table 3 were calculated using a multivariable, nonlinear regression fitting algo-rithm. The best results were obtained for h=2, m=l and s=2, and the dependence on temp-erature of ks, kR and kw, was expressed by the Arrhenius equation in its reparametrized form (Tm=468 K) (Eqn. 4). Consistent with experimental data (Figure 7), the model predicts a residual activity (as) for the hydrogenation reaction. The value of as is given by the ratio kR/(ks+kR) (12). Thus, the value obtained for EaR which is 1.8 times the value of Eas, explains the diminution of the residual activity with the observed temperature. On the other hand, the value of kw0 is nearly 1000 times lower that ks0, which also is, qualitatively consistent with the higher proportion of the amorphous coke observed by SEM (Figure 5) [ 11 ]. It is also interesting to note the low value of the activation energy calculated for the process of filamental growth (Eaw), since generally values within the range 3040 kcal/mol are reported [11]. Nevertheless, these values are usually obtained once the filament growing process has reached the steady state, after the initial period of induction for nucleation and formation of the filament. In this case, as a consequence of the parameterized calculation method used, the value of Eaw does not correspond directly to the temperature dependence of the rate of filament lengthening, but rather the difference between this rate dependence and the activation energies for filament nucleation and formation, and for encapsulation of metallic particles during the induction period.
Rlament formation
C=H. (g)
Filament
I
I
growth
I
0.9
T
I
0.8
I I
0.7
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9 > Filament C,H= (g) - ~ C,H. la"ff) " - 14q ii -X~ ~TH=(g) I nucleation
I
Ij
adsorbed II layer of C=H. ,
~
iI
1.0
I
1
C,H, (g)~ I Polimerization , Deactivation .of the C=H layer I---> of the catalyst reversible monolayer coke formation
Figure 6. Scheme of the reactioncoking pathway for the hydrogenation of acetylene.
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|
!
9
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!
9 428 K o 448K 4~K
9
I
encapsu ~1 at"ion I J
9
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o
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0.4 0"30
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A X
,
I
2000
i
I
4000
i
I
6000 9 me
i
I
8000
i
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10000
,
12000
(s)
Figure 7. Comparison of the experimental activity data (symbols) and the data predicted by the model (continuous line)at different operating temperatures.
4. CONCLUSIONS. It is possible to obtain well-dispersed Ni-Co catalyst supported in a ZnAl204 nonstoichiometric inverse-spinel structure, by using an appropriate coprecipitation method. The close proximity of the reduction temperatures of Ni and Co produces a high degree of interdispersion and a synergetic increase in the activity of the active metals. Thus, catalytic
190 performance of the Ni-Co catalyst is better (higher ethylene selectivity and lower coke formation) than either of the Ni- or Co-based catalysts. The ratio of Ni~ ~ on the catalyst surface, after different calcination and reduction temperatures, determines catalyst activity and selectivity. Cobalt apparently effects a dilution of surface Ni ensembles, thus reducing the population of multiatomic nickel ensembles, leading to an increased selectivity to ethylene. Increasing operating temperature increases coking rate causing a lower initial conversion but a more selective catalyst. Within the temperature interval studied, the main reaction has a negative apparent activation energy which is a consequence of encapsulation of part of the metallic particles during the initial period of the reaction. During this stage, major changes occur on the active surface as a result of cracking and hydrogenolysis of the acetylene molecules which lead to higher methane production. Coke deposition is modeled as a reversible process. The activation energies calculated for the coke formation and coke removal reactions, are 8.3 kcal/mol and 4.6 kcal/mol respectively. These values explain the diminution of residual hydrogenation activity with increasing reaction temperature. Finally, a value of 4.9 kcal/mol is calculated for coke filament growth. This low value is explained by considering that it is the difference between the activation energy for the filament lengthening process and that corresponding to a combination of filament nucleation/formation, and encapsulation of metallic particles during the initial induction period.
Acknowledgments. The authors wish to acknowledge the financial support of DGICYT (Spain) for this work (Project PB94-0568).
References. J.R.H. Ross, M.C.F. Steel, and A. Zeini-Isfahani, Appl. CataL 52 (1978) 280. Y.G. Chen, and J. Ren, Catal. Lett. 29 (1994) 39. J.P. Jacobs, A. Maltha, J.G.H. Reintjes, J. Drimal, V. Ponec, and H.H. Brongersma, J. CataL 147 (1994) 294. 4. J.C. Rodriguez, E. Romeo, A. Monz6n, A., Borgna, and A.J. Marchi, Proc. of XV Iberoamerican Symposium on Catalysis, C6rdoba (Argentina), 1996, pp. 909-914. 5. F. Cavani, F. Trifir6, and A. Vaccari, Catal. Today, 11 (1991) 173. 6. J.A. Pefia, J. Herguido, C. Guimon, A. Monz6n, and J Santamaria, J. Catal. 159 (1996) 313. 7. G.R. Gavalas, C. Phichitkul, and G.E. Voecks, J. Catal., 88, (1984) 65. 8. G. Fomasari, S. Gusi, F. Trifir6, and A. Vaccari, Ind. Eng. Chem. Res., 26 (1987) 1500. 9. J. Margitfalvi, L. Guczi, and A.H. Weiss, J. Catal., 72 (1981) 85. 10. S.J. Thomson, and G. Webb, J. Chem. Soc. Chem. Comm. 526 (1976). 11. Figueiredo, J.L., Progress in Catalyst Deactivation. (J.L. Figueiredo, Ed.), NATO Adv. Stud. Inst. Ser.; Ser E: Appl. Sci., Vol. 54, 1981, pp 45-63. 12. J.C. Rodriguez, J.A. Pefia, A. Monz6n, R. Hughes, and K. Li, Chem. Eng. J., 58 (1995) 7. 1. 2. 3.
183
Activity, Selectivity and C o k i n g o f Bimetallic N i - C o - S p i n e l Catalysts in Selective H y d r o g e n a t i o n Reactions. J.C. Rodriguez 1, C. Guimon 2, A.J. Marchi 3, A. Borgna 3 and A. Monz 6n 1. 1 Department of Chemical Engineering.Faculty of Science. University of Zaragoza. 50009 Zaragoza. Spain. e-mail: [email protected]. 2 L.P.C.M. URA-CNRS 474. Avenue de l'Universit6. 64000 Pau. France. 3 INCAPE. FIQ-UNL-CONICET. Santiago del Estero 2654. 3000 Santa Fe. Argentina. The influence of the calcination and reduction temperatures of a Ni-Co-ZnA1204 catalyst was correlated with its catalytic activity in the hydrogenation of acetylene. A well interdispersed Ni-Co catalyst supported in a ZnA1204 spinel-like structure was obtained by using a coprecipitation method. Cobalt apparently effects a dilution of Ni surface ensembles, increasing the selectivity to ethylene. The influence of the operating temperature on activity, coking rate and selectivity was analyzed using a deactivation kinetic model. 1. INTRODUCTION. NiO-A1203 oxides are, after a reduction step, the usual catalysts in reactions such as: steam reforming of methane or light hydrocarbons [ 1], production of synthesis gas [2], and selective hydrogenations [3,4]. Impregnation and coprecipitation are widely used in the preparation of these catalysts. The coprecipitation method typically produces a solid solution of mixed oxides with a spinel-like structure [5-7]. Moreover, the high temperature conditions of the steam reforming, syngas production and some hydrogenation processes requires the use of catalysts with good thermal stability such as spinels or other types of mixed oxides. Besides Ni +2, other transition metals including Zn 2+ or Co 2+, also can form aluminates with an inverse spinel structure, some at lower temperatures than Ni 2+ spinels (i.e. 773 K or lower) [7,8]. The final catalyst in some of these processes usually consists of a very well dispersed metal phase in a mixed spinel matrix. The attainment of a well-dispersed metal phase may be important in the case of reaction structure sensitivity. While acetylene hydrogenation is a structureinsensitive reaction, acetylene hydrogenolysis is a structure-sensitive reaction that occurs with the hydrogenation reaction to produce undesirable products like coke and methane. Thus, acetylene hydrogenation consists of a complex reaction network with apparent structure sensitivity [4]. The catalytic behavior of reduced NiO-AI203 mixed-oxide spinels for selective hydrogenation has been studied by our research group for several years [4-6]. These materials have been successfully modified with structural (Zn, Cr) or catalytic (Cu, Co) promoters in order to obtain higher activities and selectivities for the desired products and to decrease coking reactions [4]. This study focused on the effects of calcination and reduction temperatures of a bimetallic Ni-Co catalyst on composition and extent of reduction of its surface and bulk phases and on its catalytic activity in the hydrogenation of acetylene. Furthermore, the effects of operating temperature on activity, coking rate and selectivity were studied and analyzed by means of a kinetic model that simultaneously fits activity and deactivation rate data.
184 2. EXPERIMENTAL. The catalyst was prepared by coprecipitation at a constant pH of 7.2+0.2 and a temperature of 333 K. Decomposition and calcination of the dried precursor (hydrotalcite-like structure) was performed in a N2 atmosphere at 773 K or 873 K for 14 hours, giving a solid with a non-stoichiometric spinel-like structure of the following nominal composition: (NiO)0.5(CoO)0.5ZnA1204. XPS (X-ray Photoelectron Spectroscopy) spectra were recorded with a Surface Science Instrument (SSI) spectrometer, using A1Ktx radiation. The C 1s band at 248.9 eV was used as internal standard. The system was equipped with an in-situ chamber in which pretreatment (reduction at different temperatures) of samples was carried out before analysis. Temperature Programmed Reduction(TPR) experiments were carded out in a separate unit using a total flow rate of 20 cm3/mincontaining 5% H2 in N2, at a heating rate of 10 K/min. SEM (Scanning Electron Microscopy) micrographs of catalysts before and after reaction were obtained with a JEOL JSM 6400 microscope. Solids were covered by electrodeposition with a thin gold film to improve sample conductivity. Reaction tests were carried out in a thermogravimetric system, operated as a differential reactor. Prior to the kinetic tests, catalyst samples were reduced "in situ" for 3 hours at temperatures ranging from 673 K to 873 K. Hydrogenation of acetylene was performed under the following conditions: total flow rate = 700 cm'/min, mass of catalyst = 200 mg, pressure = 1 bar, feed composition (% vol.) (H2/C2H2/N2)= 45/15/40, temperature = 448-498 K. 3. RESULTS AND DISCUSSION. 3.1 Influence of Calcination and Reduction Temperatures on Catalytic Performance. Binding energies of the different metals contained in the catalyst after calcination at 773 or 873 K results are presented in Table 1. No fundamental differences were found at either temperature. The values of B.E. obtained for Ni and Co are those typically reported for NiO+NiA1204 and well-defined COA1204 structures, respectively [7,8]. Nevertheless, the results shown in Table 2 (first and fourth rows) clearly indicate that an increase in the calcination temperature preferentially favors the incorporation of Co +2 cations into the spinel lattice, increasing the relative amount of nickel in the catalyst surface. This can be related to the greater tendency of cobalt to form a well-defined bulk spinel phase [5].
Table 1 Bindinl~ energies of the metallic cations. Element T calc. = 773 K T calc. = 873 K B.E. (eV) (%) B.E. (eV) (%) A1 (2p) 74.40 100 74.40 100 856.03 62.50 Ni (2p3/2) 856.20 58.2 862.00 41.8 862.02 37.50 781.39 70.88 Co (2p3/2) 781.45 67.2 784.17 13.2 783.60 14.71 787.25 14.41 787.58 19.6 1022.37 100 Zn (2p~t2) 1022.48 100
After reduction at different temperatures, XPS and TPR results [4], showed equal percentages of reduction of Ni ~ and Co ~. The similarity of the reduction temperatures of Co and Ni, and their high degree of interdispersion in the original precursor of the catalyst apparently favors a strong synergy between metals. This is supported by a shift of-0.7 eV in the binding energy of Co ~ (778.1 eV), with respect to the value obtained for this element in a monometallic catalyst Co-Zn-A1 prepared for comparison purposes (778.8 eV). However, reaction tests revealed only minor differences in the performance of the samples of catalyst calcined or reduced at different temperatures (ethylene selectivities equal to 0.75-0.8, coke concentrations after 3 h of reaction equal to 7.5-9.5 mg/100 mg cat. (results not shown) and similar reactant conversions). In fact, XPS shows that, in spite of the increase in the
185 percentage of reduction of Co and Ni, as the temperature of reduction increases, sintering becomes more important (Table 2, first and second columns). The coupling of both processes apparently leads to similar quantities of available surface metal, and therefore to similar catalytic behavior. The only exception is the sample of catalyst calcined at 873 K and reduced at 673 K. The activity of this sample for the main reaction (ethane and ethylene production) is lower than that expected from the total amount of reduced metal in the surface (see Table 2, last column). Table 2 Not only the type of Surface atomic ratios of Ni and Co metals. Catalyst metal (cobalt or nickel), but calcined and reduced at several temperatures. its accessible fraction, Tcalc.= 773 K determine the catalytic Tred. NiT/A1 COT/A1 Ni~ ~ Ni~176 behavior. All the catalyst unreduced 0.213 0.153 0 samples, with the exception 673 K 0.133 0.106 1.51 0.066 of that reduced at 673 K, 773 K 0.080 0.054 1.79 0.075 have similar amounts of Tcalc.= 873 K metal and Ni~ ~ ratios in Tred. NiT/A1 COT/A1 Ni~ ~ Ni~176 the surface, and thus, exhibit similar activities for unreduced 0.187 0.076 0 the main and coking 673 K 0.126 0.078 0.32 0.011 reactions. The sample 773 K 0.101 0.058 1.92 0.084 reduced at 673 K, on the 873 K 0.083 0.047 1.92 0.082 contrary, is unusually enriched in cobalt leading to a near zero activity for the main reaction, but this 12 apparently does not affect 12 ' ' ' ' ' ' 1 ' ' ' ' ' ' m Tea =773 K, T ~ t = 6 7 3 K the coking reaction (7.5 mg 10 of coke/100 mg cat. after 3 10 9 Tc~=873K, T,=t=773K h of reaction) (results not o T cai=873K, Tract=873K R shown). This result, that is 8 fully analogous to the behavior observed with the )= monometallic catalyst (CoZn-A1), can be explained by 4 o considering the differences in the adsorption strength .~ Tqzr.=448 K 2 of reactant molecules on 2 {_ H2/C2H2=3/1 nickel and cobalt sites. On cobalt atoms, acetylene is 0 4000 8000 12000 0 4000 8000 12(~) '0 strongly adsorbed leading to Time (s) an almost stoichiometric reaction (this adsorption Figure 1. Influence of the reduction temperature on the could explain the non-zero ethane and ethylene yields. coke production when the Co-Zn-A1 catalyst is used). On the nickel atoms, reaction takes place, favored by the weaker reactant adsorption. Considering also that cobalt has a dilution effect on the Ni crystallites, the combination of both effects leads to facts leads to an increased selectivity to the desired product (ethylene) [9,10].
186 3.2 Influence of the Operating Temperature on Catalytic Performance. The influence of the operating temperature on the activity, selectivity and coking rate were studied for a sample of catalyst calcined and reduced at 773 K. An increase in the operating temperature from 428 K to 498 K led to a greater initial coking rate, a higher f'mal coke content, a lower initial conversion and a more selective catalyst (Figures 2, 3 and 4). In Figure 2, it is evident that an increase in temperature produces an increase in the coking rate, and thus, in the final coke concentration. Independent of temperature, coke content sharply increases during the initial stage of the reaction, and the coking rate progressively decreases with time, to reach a pseudo-steady state, in which coke grows at a con~ 15 stant rate. These two reaction O O regimes could be related to ,r o o o o two different kinds of coke on the catalyst surface. Thus, t.on the bare metallic surface, 8 coking progresses quickly, by ~ 5 hydrogenolysis of acetylene 1) 428 K 4) 488 K 8 molecules, and methane 2) 448 K 5) 498 K production (results not 3)468 K shown). During this stage, , 0100 ~ I , I ~ I , I , I 00 2 4000 60(X) 8000 1 0 0 0 0 12000 filaments are nucleated and "Sme (s) the metallic surface is covered with a layer of coke causing a Figure 2. Influence of the operating temperature on the pseudo equilibrium to be coke deposition. Line: experimental data ; Symbols: data reached that depends on the predicted by model. operating conditions (partial pressures of reactants and temperature). This fast deactivation also determines 14 the initial level of catalyst 12 activity. Thus, the higher the temperature, the lower the 0 0 10 initial activity of the catalyst A & (Figure 3) because of the v 8 -o higher initial coking rate ~) BB B ~ ~ m~ ~ ~ ~ F m (Figure 2). During the second 6 period of coking, that 9 428K #o 448K 4 constitutes most of the 9 468K reaction period, growing 488K 9 496K carbon filaments (see Figure 5) accumulate, while the , I , I , I , I , I 00 2000 4000 6000 8000 1 0 0 0 0 12000 modification of the metallic surface by acetylene ~me (s) hydrogenolysis and filamental nucleation is less Figure 3. Influence of the operating temperature on important. The negligible total gas yield (ethane + ethylene + methane). effect of filamentous coke on the activity of metallic 20
E
.
,
9
,
9
,
9
,
.
,
9
187 particles which remain at the filament tip is well established (11). In fact, no important decrease in catalyst activity with reaction time is evident within the temperature interval studied (see Figure 3) (even, after 30 h of reaction at 448 K, catalyst activity remained constant). It can be concluded that the first stages of the reaction determine the state of the metallic surface, i.e. the percentage of bare sites for the main reaction, and hence, the overall performance of the catalyst. Two types of coke 1.0 . . . , 9 , 9 , , , , , 1.0 deposits (filamentous carbon and amorphous 0.9 0.9 k coke) were evident from 0.8 0.8 SEM. The process of filament nucleation gives 0.7 0.7 methane as a secondary reaction product. The increasing rate of encap0.5 - - ~, 0.5 sulation of the metallic 0.4 I~ 0.4 crystallites by carbon during this stage is re9 o~ sponsible for the negative 0.2 0.2 value observed for the 0.1 activation energy for the 010 4000 8000 12000 0.00 0.05 0.10 0.15 0.20 main reaction (Figure 3). On Time (s) Coke Content (mg/mg cat) the other hand, as mentioned previously, coke Figure 4. Influence of the operating temperature on the deposition causes an ethylene selectivity. incremental increase in ethylene selectivity (Figure 4) and a parallel decrease in the selectivity to ethane and methane. Thus, more than activity, coke affects product distribution with time. The mechanism developed by Thomson and Webb [10] to explain the hydrogenation of acetylene is presently commonly accepted with slight modifications [9]. It is proposed that ethane can be produced by two different routes, i.e. direct hydrogenation of acetylene on the metallic bare sites, via the organometallic-likeintermediate called ethylidine, or in successive hydrogenations. Ethylene is easily formed, even on the metallic sites covered by coke, via a hydrogen transfer mechanism from the coke layer to the second adsorption layer. Methane is preferentially formed on the bare metallic surface, and is probably a by-product of the nucleation of carbon filaments. Taking into account this mechanism, the incremental increase in selectivity to ethylene with reaction time could be a result of the progressive change of the catalyst surface. The presence of a well-dispersed cobalt-nickel active phase, in which reactants are adsorbed more strongly on cobalt than on nickel, probably leads to the abrupt increase in the selectivity to ethylene, i.e. the reaction product that requires smaller metal ensembles to be produced [9,10]. This fact explains the results shown in Figure 4, where it can be seen that the ethylene selectivity and coke levels are almost independent of the operating temperature. Thus, coke concentration determines the product distribution.
0.6
0.6
9
0.31 ~ ,
0.3
i
,
i
,
,
'
'
'
9
'
'
'
9
'
3.3. Kinetic Modeling of Activity and Coke Coverage. A kinetic model based on other previously proposed models [12], was developed to simultaneously fit activity and coke coverage. From the above results, for the present catalystreaction system, coke apparently causes a progressive decrease in the hydrogenation capacity. Consequently, activity was calculated as the quotient between the rates of hydrogen consumption (to ethane, ethylene and methane) at a given time, and at zero time.
188 The model proposed assumes reversible formation of amorphous coke (Ccm) and simultaneous formation of whiskers (Ccw) on the catalytic surface. The amount of monolayer coke at infinite time (Ccmax) depends on the operating conditions (e.g. temperature) and is governed by values of the rate constant for
dCc m rcm -
- kS (Cc max - Ccm ) h _ k RCcm (Ccmax - Ccm ) s (Eq. 1)
dCc w rcw - ~ - k dt
[
(Eq. 2)
w
1m
Cc a= 1 - ~ Ccmax J
(Eq. 3)
k.l=k.lo[-mi(1- T~/I
; i=R,S,W
(Eq. 4)
Table 3. Kinetic parameters obtained in the fitting of the activity-time data. Parameter
kso (S-1) "103 kRo (s "1) "102 kwo (s'l)'106
Parameter
5.242 _+0.38 8.428 +_0.89 5.29+ 0.268
Parameter
Eas(cal/mol) EaR(cal/mol) Eaw (cal/mol) Ccm~ (mg/mg cat)
Figure 5. SEM micrograph of coke formed on the catalyst surface after 3h reaction time.(X 4,300). Temp. Reaction = 448 K.
Parameter
8271 + 776 4634 + 923 4887 + 435 0.129 + 0.004 monolayer coke formation (ks) and that for coke removal (kR) (Eqn. 1). Coke removal takes place by reaction with hydrogen, generating other nonanalyzed reaction products (e.g. C4+) [9]. The growth rate of filamentous coke is assumed to be constant (Eqn. 2), according to literature data [11 ] and in view of the lack of any noticeable effect upon the activity of the catalyst. The total coking rate was predicted as the sum of Eqns. 1 and 2; Eqn. 3 relates catalyst activity to the mono-layer coke content. A global view
189 of the reaction-coking path-way is shown in Figure 6. The parameters shown in Table 3 were calculated using a multivariable, nonlinear regression fitting algo-rithm. The best results were obtained for h=2, m=l and s=2, and the dependence on temp-erature of ks, kR and kw, was expressed by the Arrhenius equation in its reparametrized form (Tm=468 K) (Eqn. 4). Consistent with experimental data (Figure 7), the model predicts a residual activity (as) for the hydrogenation reaction. The value of as is given by the ratio kR/(ks+kR) (12). Thus, the value obtained for EaR which is 1.8 times the value of Eas, explains the diminution of the residual activity with the observed temperature. On the other hand, the value of kw0 is nearly 1000 times lower that ks0, which also is, qualitatively consistent with the higher proportion of the amorphous coke observed by SEM (Figure 5) [ 11 ]. It is also interesting to note the low value of the activation energy calculated for the process of filamental growth (Eaw), since generally values within the range 3040 kcal/mol are reported [11]. Nevertheless, these values are usually obtained once the filament growing process has reached the steady state, after the initial period of induction for nucleation and formation of the filament. In this case, as a consequence of the parameterized calculation method used, the value of Eaw does not correspond directly to the temperature dependence of the rate of filament lengthening, but rather the difference between this rate dependence and the activation energies for filament nucleation and formation, and for encapsulation of metallic particles during the induction period.
Rlament formation
C=H. (g)
Filament
I
I
growth
I
0.9
T
I
0.8
I I
0.7
ICH,tg) Ii
9 > Filament C,H= (g) - ~ C,H. la"ff) " - 14q ii -X~ ~TH=(g) I nucleation
I
Ij
adsorbed II layer of C=H. ,
~
iI
1.0
I
1
C,H, (g)~ I Polimerization , Deactivation .of the C=H layer I---> of the catalyst reversible monolayer coke formation
Figure 6. Scheme of the reactioncoking pathway for the hydrogenation of acetylene.
|
|
!
9
|
!
9 428 K o 448K 4~K
9
I
encapsu ~1 at"ion I J
9
O
o
9
A 488 K x 498 K
0.6
o o
0.5 &
0.4 0"30
X
x
A X
,
I
2000
i
I
4000
i
I
6000 9 me
i
I
8000
i
I
10000
,
12000
(s)
Figure 7. Comparison of the experimental activity data (symbols) and the data predicted by the model (continuous line)at different operating temperatures.
4. CONCLUSIONS. It is possible to obtain well-dispersed Ni-Co catalyst supported in a ZnAl204 nonstoichiometric inverse-spinel structure, by using an appropriate coprecipitation method. The close proximity of the reduction temperatures of Ni and Co produces a high degree of interdispersion and a synergetic increase in the activity of the active metals. Thus, catalytic
190 performance of the Ni-Co catalyst is better (higher ethylene selectivity and lower coke formation) than either of the Ni- or Co-based catalysts. The ratio of Ni~ ~ on the catalyst surface, after different calcination and reduction temperatures, determines catalyst activity and selectivity. Cobalt apparently effects a dilution of surface Ni ensembles, thus reducing the population of multiatomic nickel ensembles, leading to an increased selectivity to ethylene. Increasing operating temperature increases coking rate causing a lower initial conversion but a more selective catalyst. Within the temperature interval studied, the main reaction has a negative apparent activation energy which is a consequence of encapsulation of part of the metallic particles during the initial period of the reaction. During this stage, major changes occur on the active surface as a result of cracking and hydrogenolysis of the acetylene molecules which lead to higher methane production. Coke deposition is modeled as a reversible process. The activation energies calculated for the coke formation and coke removal reactions, are 8.3 kcal/mol and 4.6 kcal/mol respectively. These values explain the diminution of residual hydrogenation activity with increasing reaction temperature. Finally, a value of 4.9 kcal/mol is calculated for coke filament growth. This low value is explained by considering that it is the difference between the activation energy for the filament lengthening process and that corresponding to a combination of filament nucleation/formation, and encapsulation of metallic particles during the initial induction period.
Acknowledgments. The authors wish to acknowledge the financial support of DGICYT (Spain) for this work (Project PB94-0568).
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