Ind. Eng. Chem. Res. 1996, 35, 3067-3073
3067
Effect of Adding Co to MoS2/Al2O3 upon the Kinetics of the Water-Gas Shift Carl R. F. Lund Department of Chemical Engineering, Box 604200, SUNYsBuffalo, Buffalo, New York 14260-4200
A microkinetic model for the kinetics of the water-gas shift over sulfided CoMo/Al2O3 catalysts was developed starting from a similar model for unpromoted sulfided Mo/Al2O3 catalysts. Co was found to promote the catalyst’s activity only at low CO/H2O ratios; at high ratios the Mo catalyst was marginally more active than the CoMo catalyst. The most important difference between the two models was the strength of interactions between the surface and hydroxyl groups. The addition of Co increased the stability of hydroxyl groups relative to sulfhydryl groups, and at higher H2O concentrations this allowed oxidized surface sites to more readily participate in both steam adsorption and hydrogen desorption steps. The results are most easily reconciled in terms of a promotional model where the Co and Mo are in close proximity and the active sites are similar to sites on unpromoted Mo catalysts. A microkinetic model has been developed for the kinetics of the water-gas shift (WGS), reaction 1, over
CO + H2O a CO2 + H2
(1)
sulfided Mo/Al2O3 (Lund, 1995). It is based upon a simple regenerative scheme similar to that proposed by Hou et al. (1983), reactions 2-4. The mechanism fits – S2– S Mo5+ + H2O
– S2– O
+ H2S
(2)
– S2– O
S2– Mo4+ + CO2
(3)
Mo5+
+ CO
S2– Mo4+ + H2O
Mo5+
S2– O– Mo5+
+ H2
(4)
the experimental data (5-15 atm, 250-300 °C, 1% H2S) well, it extrapolated to the range (1 atm, 450 °C, 2500 ppm H2S) of other data with good accuracy, and the coverages of surface species it predicted were consistent with spectroscopic and other data in the literature. In the present investigation catalysts with Co added have been studied to assess its effect upon the kinetics of the reaction. Since sulfided CoMo/Al2O3 catalysts are used extensively for hydrodesulfurization (HDS) of crude oils, insights into the effect of Co gained from the study of sulfur-tolerant WGS may also be relevant for HDS systems. A 1980 review of WGS (Newsome) discusses the performance of alkali-promoted, supported CoMo sulfide catalysts for WGS. In the work considered in that review, the kinetics were described using a rate expression that was first-order (reversible) in CO. In the range of conditions studied, such an expression was satisfactory. Typical values of the apparent activation energy were in the range 11.7-21.3 kcal mol-1. XPS analysis (Xie et al., 1991) has revealed that the Mo-O-Al bonding is weakened when Co is added to the catalyst, making sulfidation more facile. During startup of a reactor containing aluminasupported CoMo catalyst, it was observed that H2 production was always greater than CO2 production during the transient period preceding steady state (Hakkarainen et al., 1993). While this is consistent with a pure redox scheme, it was also observed that hydrogen S0888-5885(95)00785-8 CCC: $12.00
was not evolved during water pretreatment. Hydrogen appeared only when CO was added to the feed. It should be noted that the catalyst used in these studies was sulfided when loaded into the reactor, but under the conditions employed it converted to an oxide. The kinetics of sulfur-tolerant WGS were also measured in another series of investigations (Srivatsa, 1987; Spillman, 1988; Srivatsa and Weller, 1988; Ramaswamy, 1990). A “spinning basket” fully mixed reactor was used at temperatures ranging from 200 to 300 °C, pressures from 5 to 27 atm, and gas hourly space velocities from 4800 to 24 000. Mo/Al2O3 was prepared by incipient wetness impregnation of Davison γ-alumina (20-60 mesh) with an aqueous solution of ammonium heptamolybdate to produce a loading of 15 wt % (as MoO3). The impregnated catalyst was vacuum dried for 3 h and subsequently calcined at 500 °C in air for 6 h. This is the catalyst for which the microkinetic model mentioned previously (Lund, 1995) was developed. A CoMo/Al2O3 catalyst, used in 87 experimental runs, was then prepared using a second impregnation of the Mo/Al2O3 catalyst with an aqueous solution of cobalt nitrate to produce a loading of 14.6 wt % MoO3 and 3 wt % CoO. After the second impregnation the catalyst was again vacuum dried and calcined. In addition, 88 kinetic runs were made using a similar commercial CoMo/Al2O3 catalyst (Amocat 1A). In the present study the data from this series of investigations have been reevaluated using the microkinetic analysis method (Dumesic et al., 1987, 1993; Lund, 1995). Experimental Methods The reaction mechanism used to describe the kinetics of WGS over sulfided Mo/Al2O3 is given in eqs 5-15.
H2S + * + *-S a 2 *-SH
(5)
2*-SH a 2*-S + H2
(6)
H2O + * + *-S a *-SH + *-OH
(7)
H2O + * + *-O a 2*-OH
(8)
2*-OH a 2*-O + H2
(9)
*-OH + *-SH a *-O + *-S + H2
(10)
H2S + * + *-O a *-SH + *-OH
(11)
© 1996 American Chemical Society
3068 Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996
*-S + *-OH a *-SH + *-O
(12)
CO + * a *-CO
(13)
*-CO + *-O a *-CO2 + *
(14)
*-CO2 a * + CO2
(15)
The same mechanistic description has been used in the present study, and the modeling methodology is the same as that described previously (Lund, 1995). Briefly, gas phase heats of formation were estimated for all species. Surface bond strengths (for S, SH, O, OH, CO, and CO2) were introduced as adjustable model parameters. From these, heats of reaction could be calculated for each step in the mechanism. The activation energies were then estimated through linear Evans-Polanyitype correlations between the heat of reaction and the activation energy. One correlation was used for steps 5-12, and a second correlation was used for steps 1315; this introduced four additional model parameters. Forward preexponential factors were estimated from transition state theory, and reverse preexponential factors were calculated to maintain thermodynamic consistency using a reference temperature of 500 K. The resulting set of kinetic expressions was fit to experimental data using a perfectly mixed CSTR reactor model by minimizing an objective function, Ψ, defined as the sum of the squares of the errors between the measured outlet CO mole fraction and that predicted by the model. The reactor design equations were solved using a globally convergent Newton-Raphson method, and fitting was accomplished using a downhill simplex method (Press et al., 1992). Sensitivity analyses were conducted using the procedure previously described (Lund, 1995); this produced a sensitivity factor for each model parameter and for each kinetic parameter. The sensitivity factor, Sj, was defined such that Sj will approach 0 when parameter j has little impact upon the value of the sum of the squares of the errors, and it will approach unity when the quality of the fit is highly sensitive to parameter j. Model Formulation. It is reasonable to use the same basic mechanism, eqs 5-15, for the sulfided CoMo/ Al2O3 catalysts as that developed for sulfided Mo/Al2O3 catalyst. First, power-law kinetic models for the two catalysts did not indicate gross changes in kinetic behavior (Spillman, 1988). In addition, infrared spectroscopy of adsorbed species indicates that the same surface species are present on the two catalysts, though bond strengths may differ. Most “active” hydrogen is present as sulfhydryl groups (Stuchly and Bera´nek, 1987). The addition of Co does affect the interaction between Mo and OH groups of the support, but the spectra for the two catalysts in the sulfided state are very similar (Topsøe, 1980; Topsøe and Topsøe, 1993). CO adsorption is weak (most desorbs upon evacuation at room temperature), and the absorption band for the promoted catalyst is observed at frequencies between those for Mo alone and Co alone (Mauge´ and Lavalley, 1992). Again, no new species are reported upon promotion with Co. Oxygen uptakes are reported to be approximately equal for promoted and unpromoted catalysts (Zmierczak et al., 1982). NO chemisorption can be used to distinguish between Mo and Co sites and suggests that Co is located at edges of MoS2 crystallites in substitutional or interstitial sites (Topsøe and Topsøe, 1983). The addition of Co to the catalyst does promote WGS activity, though the magnitude of the effect is much less
than the 30-fold increase reported for HDS (e.g., Prins et al., 1989). The literature on HDS using sulfided CoMo/Al2O3 catalysts suggests several ways in which the addition of Co may promote HDS activity (Topsøe et al., 1981; Duchet et al., 1983; Wivel et al., 1984; Harris and Chianelli, 1986; Prins et al., 1989; Niemann et al., 1990; Nørskov et al., 1992). One possibility is that Co is a textural promoter that creates more sites but is not involved in the catalysis itself. This might occur via decoration at the edges of MoS2 crystallites leading to a reconstruction and an increase in the number of sites. Alternatively, Co located in substitutional or interstitial sites at the edges of the crystallites might be the active site. A separate Co-containing phase might promote dissociation of hydrogen and thereby supply hydrogen atoms to the sulfided Mo via spillover. Adding Co may also increase the electron density at the Mo sites causing a change in activity. Finally, Co that is not associated with the Mo may act as the active site. These suggestions form the basis for the different kinetic models that were considered here for WGS with Co-promoted catalysts. The microkinetic model, eqs 5-15, was fit to the data using three different procedures. In the first case the kinetic parameters were all held constant at the values determined for the unpromoted catalyst. Only the total number of catalytic sites was allowed to vary under the postulate that the effect of Co is purely textural and leads to a larger number of sites that are catalytically the same as in the unpromoted catalyst. In the second case, the site density was held constant at the value measured by use of low-temperature chemisorption of oxygen. Additionally, the empirical model parameters corresponding to the Evans-Polanyi correlations were held constant at the values determined for the unpromoted catalyst. The strengths of all surface bonds were then allowed to vary for the purpose of fitting the model to the data. This case might correspond to a promotional model where the Co is located close enough to the Mo sites to affect them electronically or to a model where the Co is the active site, but being sited at edge substitutional or interstitial positions of the Mo phase, it is strongly affected by the Mo. In the third case, all the model parameters (Evans-Polanyi constants and bond strengths) were allowed to vary. If Co that is not associated with the Mo is the active site, or if an isolated Co phase provides hydrogen atoms via spillover, this model might lead to the best fit for the data. The experimental data used for modeling were discussed in the introduction (Srivatsa, 1987; Spillman, 1988; Srivatsa and Weller, 1988; Ramaswamy, 1990). No distinction was made between the data for the Amocat 1A catalyst and laboratory-prepared catalysts. Pretreatment procedures and activity measurement have been described previously (Lund, 1995) as well as in the original reports. All the catalysts contained nearly the same amount of Co and Mo. Results In the original studies where the data were collected, they were fit using power-law rate expressions. Each of these studies resulted in a set of power-law parameters determined from only the data collected in that study. These power-law expressions fit the associated data very well. In the present study a value was computed for the sum (over the data from all the studies) of the squares of the differences between the measured outlet CO mole fraction and the outlet CO
Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996 3069 Table 1. A Comparison of Bond Strengths and Their Sensitivity Factorsa for Promoted and Unpromoted Catalysts As Determined by Microkinetic Modeling unpromoted
promoted
surface species
bond strength (kcal mol-1)
sensitivity factor
bond strength (kcal mol-1)
sensitivity factor
S SH O OH CO CO2
73.75 49.60 126.09 71.57 8.63 3.21
0.62 0.88 0.99 0.93 7.3 × 10-4 0.04
78.22 47.00 125.62 72.93 8.11 3.14
0.05 0.59 0.99 0.94 3.9 × 10-4 8.8 × 10-3
a A sensitivity factor of 1.0 indicates that the quality of the fit is strongly affected by the value of the associated bond strength; a value of 0.0 indicates that the associated bond strength has no effect upon the quality of the fit.
mole fraction predicted by the power-law expression for that data point. The resulting value, Ψpower-law ) 0.012, was then used as a benchmark for the quality of the fit of each microkinetic modeling case. Because several power-law expressions were used, the total number of adjusted parameters associated with this benchmark value is greater than the number of adjusted parameters for any of the microkinetic models, and thus this value represents the lower limit of Ψ values to be expected for the microkinetic models. Before any fitting was performed, the microkinetic model for the unpromoted, sulfided Mo/Al2O3 catalyst was used to calculate a base case value of Ψ0 ) 0.047 (i.e., all parameters were held constant at values corresponding to the unpromoted catalyst while modeling the data for the promoted catalyst). In case one, where the only “parameter” allowed to vary during the fitting process was the total number of sites per gram, the best fit of the model to the data resulted in a value of Ψ1 ) 0.030. In obtaining this improvement, a fitted value of 1.90 × 1019 sites g-1 resulted, compared to the measured value of 1.34 × 1019 sites g-1. While the fit was better than that obtained using the model for the unpromoted catalyst, the objective function was still more than twice that of the power-law model. The second fitting case used the measured site density (1.34 × 1019 sites g-1) and additionally held the EvansPolanyi model parameters equal to the values for the unpromoted catalyst. Thus, only the bond strengths were varied. The best fit in this case resulted in a value Ψ2 ) 0.015. This is much better than case 1 and comparable to the power-law kinetics benchmark value. In the third case the measured site density was again used, but all the model parameters (Evans-Polanyi and bond strengths) were allowed to vary. In the fit which resulted, the Evans-Polanyi parameters were not significantly different from the unpromoted case, and the bond strengths which resulted were essentially equal to those of case 2. Since case 2 and case 3 resulted in virtually the same parameter values, it is not surprising that the value Ψ3 ) 0.015 was the same as for case 2. The finding that case 2 and case 3 resulted in essentially the same set of parameters is also consistent with the finding that the sensitivity factors for the EvansPolanyi model parameters were all quite small (between 5.9 × 10-6 and 7.2 × 10-4) indicating that the fit of the model is not very sensitive to their values. Table 1 lists the best values of the adjusted model parameters along with the associated sensitivity factors resulting from case 2. For comparison purposes, parameter values and their sensitivity factors are also
shown in the table for the unpromoted catalyst. The table reveals two important differences between the model for the unpromoted catalyst and that for the promoted catalyst. First, when the catalyst is promoted with Co, the model becomes much, much less sensitive to the value of the bond strength of adsorbed sulfur. Second, when the catalyst is promoted with Co, hydroxyl groups are stabilized relative to sulfhydryl groups. For both models the fits are not sensitive to the values of the CO or CO2 bond strengths. The kinetic parameters for the Co-promoted catalysts resulting from case 2 are presented in Table 2. The preexponential factors were held constant and equal to those for the unpromoted catalyst; the activation energies are, of course, different. The table also presents sensitivity factors for each of the kinetic parameters. These show that the model is not sensitive to the values of the preexponential factors and in part justify not varying them during the fitting process. Discussion As already noted, Co does not cause the rate of WGS to increase anywhere near as much as it increases HDS activity. In fact, Spillman (1988) noted that while the rate was higher for the promoted catalyst at some conditions, there were other conditions where the rate was higher over the unpromoted catalyst. Data from Spillman’s thesis illustrating this behavior are shown in Figure 1 where the rate for the promoted catalyst relative to the unpromoted catalyst is plotted versus feed CO/H2O ratio for conditions typical of the experimental studies. Measured values are plotted as squares whereas the circles represent computed values from the power-law rate expressions. It should be noted that while the total pressure was 15 atm for these experiments and calculations it appears that the amount of inert (nitrogen) was varied simultaneously with the CO/ H2O feed ratio. Further, for the experimental data the two catalysts were not run at exactly the same CO/H2O feed ratio; the data are plotted at the average of the values for the two catalysts. Nonetheless, the data illustrate the essential point: at high CO/H2O feed ratios the activity of the unpromoted catalyst is higher than the promoted catalyst. This observation explains why the fit in case 1 was not very good. In that case the only parameter that was adjusted was the total site density. Clearly, in such a situation if the site density is increased for the promoted catalyst then the activity will increase for that catalyst at all conditions; it is not possible to increase the activity at some conditions while simultaneously decreasing it at others if the only parameter being varied is the total site density. It is similarly difficult to imagine how Co could act solely as a textural promoter for WGS. One could imagine that the presence of Co might induce surface reconstruction and thereby vary the total site density. It could also be imagined that the extent to which surface reconstruction occurred might vary as the conditions (CO/H2O ratio) varied. It is more difficult to picture how such reconstruction could lead to a decrease in surface area (at high ratios). Furthermore, the behavior shown in Figure 1 is reversible, so if a purely textural explanation for the effect of Co is invoked then the associated surface reconstruction must be dynamic and must be able to cycle reversibly between one limit where the surface area is higher than the unpromoted catalyst and another where it is lower.
3070 Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996 Table 2. Kinetic Parameters for the Co-Promoted Catalyst reaction
Afora (s-1)
Areva (s-1)
5 6 7 8 9 10 11 12 13 14 15
1.0 × 1.0 × 1013 1.0 × 106 1.0 × 106 1.0 × 1013 1.0 × 1013 1.0 × 106 1.0 × 1013 1.0 × 106 1.0 × 1013 1.0 × 1013
2.55 × 5.17 × 108 5.44 × 1011 1.08 × 1012 1.30 × 108 2.59 × 108 5.08 × 1012 5.02 × 1012 2.16 × 1013 1.47 × 1014 2.35 × 105
106
1012
Efor (kcal mol-1)
sensitivity to Efor
Erev (kcal mol-1)
sensitivity to Erev
10.95 17.90 12.49 14.97 14.09 16.57 13.43 15.05 9.96 16.04 16.12
2.56 × 5.08 × 10-2 1.10 × 10-1 1.62 × 10-1 9.90 × 10-3 4.08 × 10-3 8.03 × 10-3 5.05 × 10-2 3.10 × 10-1 5.18 × 10-1 5.13 × 10-1
19.17 16.00 18.78 18.16 18.38 17.76 18.55 18.14 18.08 13.10 12.98
8.56 × 10-2 1.73 × 10-2 1.83 × 10-3 1.06 × 10-4 4.45 × 10-2 9.05 × 10-3 4.02 × 10-2 3.48 × 10-2 5.93 × 10-1 4.27 × 10-1 9.05 × 10-2
10-2
a Values are in terms of thermodynamic activities, if ideal gas behavior is assumed units become atm-1 s-1 for adsorption steps; the largest sensitivity factor for any preexponential factor was 6.8 × 10-3.
Figure 1. Rate of WGS for the promoted catalyst relative to an unpromoted catalyst at varying CO/H2O feed ratios. Conditions are 523 K, 15 atm total pressure, 2.0 g catalyst, and 1500 cm3(STP)/min total feed rate. For the simulations the feed contains 19% inerts and 1% H2S.
Figure 2. Comparison of the rates of CO2 production for promoted and unpromoted catalysts as the CO/H2O feed ratio is varied at constant total feed of 1500 cm3(STP)/min with 19% inerts and 1% H2S. Total pressure is 15 atm, temperature is 523 K, and catalyst mass is 2.0 g.
The finding that the same final model results whether all model parameters are varied or whether only the surface bond strengths are varied might suggest that promotional models involving a separate Co phase are less likely. For example, if hydrogen molecules were dissociated on a separate Co phase and then supplied to the Mo phase via spillover, a change in the EvansPolanyi parameters might be expected for the affected steps. Similarly, if the active site of the promoted catalysts was actually a separate Co phase it might be expected that the microkinetic model for the unpromoted (Mo phase) catalyst simply would not fit the promoted catalyst or, if it did fit, that gross changes would be observed for both the Evans-Polanyi parameters and the bond strengths. A promotional model where the active site involves Co and Mo in close proximity and where the nature of the site is quite similar for the two catalysts seems most consistent with the kinetic results. Still, kinetic results alone cannot disprove postulates involving a separate Co phase (but they do require kinetic similarity between that phase and the Mo phase of the unpromoted catalyst). It has been noted that the microkinetic model fits the experimental data about as well as the original powerlaw expressions. Importantly, the microkinetic model predicts that Co will only promote activity at low CO/ H2O feed ratios and that at high ratios the Co-containing catalyst will actually be less active than the unpromoted catalyst. This is shown in Figure 1 by the solid line which is a simulation using the microkinetic model. For the purposes of the simulation the total pressure was fixed at 15 atm and the inert pressure (including H2S) was held constant at 3 atm. (It has already been
noted that for the comparison data points shown in Figure 1 the inert pressure varied along with the CO/ H2O feed ratio.) All other conditions were the same for the two cases. When the individual rates of reaction are plotted as in Figure 2 instead of the ratio of the rates it can be seen that as the feed ratio increases the rates increase for both the unpromoted and the promoted catalyst, but the latter increases more steeply. Then, at higher feed ratios, both rates begin to decrease, but again the promoted catalyst displays a steeper trajectory. Kinetic network analyses on the promoted and unpromoted catalysts were conducted in an attempt to understand this behavior. Two sets of such analyses were conducted: one set at a CO/H2O feed ratio of 3.0 and another set at a ratio of 0.3. These are the two extremes of the data plotted in Figures 1 and 2. The general behavior of the reaction network was described previously (Lund, 1995) and remains valid for both catalysts at the conditions of Figure 2. No single step in the network, Figure 3, is rate-determining. At all conditions the surface is quasi-equilibrated with H2S (according to reactions 5, 11, and 12). In addition the adsorption of steam is irreversible for both oxidized sites, reaction 8, and sulfided sites, reaction 7. The network analyses further revealed that at high feed ratios (high CO partial pressures) the adsorption of CO via reaction 13 and its surface reaction via reaction 14 are also at quasiequilibrium for both catalysts. At the low feed ratio these steps were not quasi-equilibrated, but they remained highly reversible. When the steady-state surface coverage of O, CO, CO2, or vacant sites at a given feed ratio on the
Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996 3071
Figure 3. Reaction network used to model the water-gas shift reaction. Surface species are indicated by rounded rectangles. Locations where gaseous species enter the network are denoted by ovals, and equation numbers for reaction steps are given in parentheses.
Figure 4. Contribution of sulfide-vacancy pair sites (reaction 7) and oxide-vacancy pair sites (reaction 8) to the rate of H2O adsorption on promoted and unpromoted catalysts at high and low CO/H2O feed ratios.
unpromoted catalyst was compared to the same species coverage at the same feed ratio on a promoted catalyst, the values were comparable. This was not true for the remaining species: the promoted catalyst had OH and S coverages that were an order of magnitude or more larger than the coverages on the unpromoted catalyst at the same feed ratio. The SH coverages were an order of magnitude larger for the unpromoted catalyst. Perhaps the most striking difference between the promoted and unpromoted catalysts, however, is shown in Figures 4 and 5. Figure 4 shows that the sulfide-vacancy site for water adsorption (reaction 7) accounts for over 90% of the adsorption on the unpromoted catalyst regardless of the feed ratio. On the promoted catalyst this is also true at high CO/H2O feed ratios, but at low ratios (high steam partial pressures) the sulfide-vacancy site only accounts for half the adsorption while the oxidevacancy site accounts for the other half via reaction 8.
Figure 5. Contribution of sulfhydryl pair sites (reaction 6), hydroxyl pair sites (reaction 9), and mixed pair sites (reaction 10) to the rate of H2 desorption from promoted and unpromoted catalysts at high and low CO/H2O feed ratios.
Figure 5 shows a similar pattern for H2 desorption where surface hydroxyl groups make a major contribution to the rate for the promoted catalyst at high steam pressures. These observations, taken together with the changes in the bond strengths indicated in Table 1, allow an explanation for the promotional effect of Co as revealed in Figures 1 and 2. Recalling that the network analysis showed that there is no rate-determining step, the observed rate represents a kinetic balance between two or more steps. In the following discussion a step that affects the overall kinetic balance significantly will be described as offering significant “kinetic resistance.” The overall mechanism can be subdivided into an oxidative pathway wherein steam oxidizes the surface and a reductive pathway wherein carbon monoxide reduces the surface. (A sulfidation pathway can also be identified.) At all conditions studied, neither the oxidative pathway (reactions 6-10 and 12) nor the
3072 Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996
reductive pathway (reactions 13-15) is completely at quasi-equilibrium, though some steps within those pathways are. This means that one or more steps from each pathway contribute significant kinetic resistance to the overall reaction. As conditions vary, the relative amount of kinetic resistance offered by the different pathways may change and within either pathway the relative kinetic resistance offered by different mechanistic steps may change. The modeling results, including bond strengths, Evans-Polanyi parameters, and steady-state surface coverages, suggest that the kinetics of the reductive pathway are very similar over the two catalysts. That is, the kinetic resistance in the reductive pathway is approximately the same for the promoted catalyst as it is for the unpromoted catalyst. In contrast, the modeling results indicate some significant differences between the two catalysts in their oxidative pathways (Figures 4 and 5). Specifically, for the promoted catalyst, high steam pressures facilitate the adsorption of steam on oxide-vacant pair sites in addition to the path involving sulfide-vacant pair sites. Desorption of H2 from pair sites that include hydroxyl groups is similarly facilitated at these conditions. The availability of two oxidative pathways for the promoted catalyst will reduce the kinetic resistance in this pathway at high steam pressures. In contrast, the unpromoted catalyst uses predominantly the sulfide-vacant pair site at high steam pressures, so its kinetic resistance is higher under these conditions. At low steam pressures, both catalysts use predominantly the sulfide-vacant pair site. The model suggests that there is a little more kinetic resistance in the oxidative pathway of the promoted catalyst than in the unpromoted catalyst at these low steam conditions. This reasoning can be used to rationalize the behavior observed in Figure 2. It has already been noted that the reductive pathways for the two catalysts offer very similar kinetic resistances at all conditions studied. They therefore contribute little to the differences between the two catalysts. At the highest CO/H2O feed ratios studied, most of the kinetic resistance is expected to come from the oxidative pathway since two of the three steps in the reductive pathway are at quasiequilibrium (the CO adsorption and surface reaction steps). At these low steam conditions both catalysts use predominantly the sulfide-vacant pair site, and as already discussed the unpromoted catalyst offers slightly less kinetic resistance there. Thus, the unpromoted catalyst has a higher rate at high feed ratios (low steam pressures). As the CO/H2O feed ratio is decreased (steam pressure is increased) the two catalysts respond differently. First, the kinetic resistance in the reductive pathway for both catalysts increases (note the CO adsorption and surface reaction steps in that pathway move away from quasi-equilibrium). For the unpromoted catalyst this the only effect that is operative; as the feed ratio drops below ca. 1 the added kinetic resistance begins to cause a significant decrease in rate. In the case of the promoted catalyst two opposing effects are operative. Again starting from a high feed ratio, the first effect that is noticed is a decrease in the kinetic resistance in the oxidative pathway (as the oxide-vacancy pair sites begin to contributessee above). This leads to the observed increase in the reaction rate for the promoted catalyst. This effect continues as the feed ratio is further reduced and the rate for the promoted catalyst becomes greater than that of the unpromoted catalyst
(see Figure 2). Eventually, however (at feed ratios below ca. 1), the increasing kinetic resistance in the reductive pathway begins to dominate (in the same manner as just described for the unpromoted catalyst), and the rate stops increasing and begins to decrease. The primary effect of adding Co to sulfided Mo/Al2O3 catalysts can thus be explained kinetically by noting that the Co reduces the kinetic resistance of the oxidative pathway at high steam pressures. The most straightforward promotional model for accomplishing this involves sites that are like the sites on the unpromoted catalyst with the promoter acting to weaken surface SH bonds and strengthen surface OH bonds. As already noted though, a promotional model involving separate Co and Mo phases could also explain the results provided the mechanism is very similar for the two phases. Finally, it is probably not appropriate to even attempt to apply the results for WGS to promotion in HDS. The catalyst surface during WGS consists mostly of oxidized sites which would not be present during HDS. Still, the approach used here, namely, making a detailed comparison of the kinetics over the two catalysts (not just a simple activity comparison at standard conditions), might prove quite revealing in the case of HDS. Here the promoted and unpromoted catalysts differed in activity by 20-30%. In HDS the differences are 30-fold; it would be interesting to see if a single detailed mechanistic model could describe the full kinetic behavior of two catalysts with that large a difference in activity. If it could it would again lend support to a promotional model where the Co and Mo are both in close proximity to the active sites and where the promoter’s effect is to alter surface bond strengths and consequently reactivity (Nørskov et al., 1992; Topsøe and Topsøe, 1993). Conclusions The kinetics of WGS over Co-promoted, sulfided Mo/ Al2O3 catalysts are very well described by the same mechanistic scheme that describes the kinetics of the unpromoted catalyst. The promotional effect of Co is only observed when the steam partial pressure is high, and even at those conditions its magnitude is modest (20-30% increase in rate). According to the modeling, the most important effects of adding Co are an increase in the surface bond strength of hydroxyl groups relative to sulfhydryl groups and a very small decrease in the bond strength of surface oxide. (An increase in surface sulfide bond strength is also indicated, but the model is not particularly sensitive to this parameter.) At higher steam partial pressures where the promotional effect is most pronounced the Co-containing catalyst makes use of two oxidative pathways whereas the unpromoted catalyst uses predominantly one pathway, and this difference can be used to explain the difference in rate between the two catalysts. At lower steam partial pressures the promoted catalyst actually exhibits a lower rate than the unpromoted catalyst. Kinetic analysis alone is not sufficient to confirm any one particular promotional model, but it does impose limitations on proposed models. Two possibilities that have been debated in the case of HDS are, first, a site that is similar to the unpromoted catalyst with the promoter in substitutional or other adjacent sites and, second, a site on a separate Co phase that is not closely associated with the Mo phase. The striking similarity between the microkinetics of the promoted and unpro-
Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996 3073
moted catalysts is easily reconciled if the former promotional model is adopted. If the latter is held, the present results require that the mechanism and energetics of reaction for the Co phase must be very similar to those of unpromoted Mo catalysts. Acknowledgment is made to the donors of The Petroleum Research Fund, administered by the ACS, for support of this research. Literature Cited Duchet, J. C.; van Oers, E. M.; de Beer, V. H. J.; Prins, R. CarbonSupported Sulfide Catalysts. J. Catal. 1983, 80, 386. Dumesic, J. A.; Milligan, B. A.; Greppi, L. A.; Balse, V. R.; Sarnowski, K. T.; Beall, C. E.; Kataoka, T.; Rudd, D. F.; Trevino, A. A. A Kinetic Modeling Approach to the Design of Catalysts: Formulation of a Catalyst Design Advisory Program. Ind. Eng. Chem. Res. 1987, 26, 1399. Dumesic, J. A.; Rudd, D. F.; Aparicio, L. M.; Rekoske, J. E.; Trevino, A. A. The Microkinetics of Heterogeneous Catalysis; American Chemical Society: Washington, DC, 1993. Hakkarainen, R.; Salmi, T.; Keiski, R. L. Water-gas shift on a cobalt-molybdenum oxide catalyst. Appl. Catal. A 1993, 99, 195. Harris, S.; Chianelli, R. R. Catalysis by Transition Metal Sulfides: A Theoretical and Experimental Study of the Relation between the Synergic Systems and the Binary Transition Metal Sulfides. J. Catal. 1986, 98, 17. Hou, P.; Meeker, D.; Wise, H. Kinetic Studies with a SulfurTolerant Water Gas Shift Catalyst. J. Catal. 1983, 80, 280. Lund, C. R. F. The Microkinetics of Water-Gas Shift over Sulfided Mo/Al2O3 Catalysts. Submitted for publication in Ind. Eng. Chem. Res., 1995. Mauge´, F.; Lavalley, J. C. FT-IR Study of CO Adsorption on Sulfided Mo/Al2O3 Unpromoted or Promoted by Metal Carbonyls: Titration of Sites. J. Catal. 1992, 137, 69. Newsome, D. S. The Water-Gas Shift Reaction. Catal. Rev.-Sci. Eng. 1980, 21, 275. Niemann, W.; Clausen, B. S.; Topsøe, H. X-Ray Absorption Studies of the Ni Environment in Ni-Mo-S. Catal. Lett. 1990, 4, 355. Nørskov, J. K.; Clausen, B. S.; Topsøe, H. Understanding the trends in the hydrodesulfurization activity of the transition metal sulfides. Catal. Lett. 1992, 13, 1. Press, W. H.; Teukolsky, S. A.; Vettering, W. T.; Flannery, B. P. Numerical Recipes in C The Art of Scientific Computing; Cambridge University Press: New York, 1992. Prins, R.; De Beer, V. H. J.; Somorjai, G. A. Structure and Function of the Catalyst and the Promoter in Co-Mo Hydrodesulfurization Catalysts. Catal. Rev.-Sci. Eng. 1989, 31, 1.
Ramaswamy, A. N. Investigations on the Water-Gas Shift Reaction, Effect of Hydrogen and of Catalyst Acidity. M.S. Thesis, SUNY-Buffalo, Buffalo, NY, 1990. Spillman, D. M. An Investigation of the High Pressure Kinetics of the Water-Gas Shift Reaction over a Sulfided Molybdenum Oxide-Alumina Catalyst Promoted by Cobalt Oxide and an Alkali Metal or Rare Earth Oxide. M.S. Thesis, SUNY-Buffalo, Buffalo, NY, 1988. Srivatsa, N. R. Kinetic Studies of the Water-Gas Shift Reaction on a Sulfided Cobalt-Molybdena-Alumina Catalyst. Ph.D. Dissertation, SUNY-Buffalo, Buffalo, NY, 1987. Srivatsa, N. R.; Weller, S. W. Water-Gas Shift Kinetics over Sulfided Catalyst: Elevated Pressure. In Proceedings of the 9th International Congress on Catalysis, Calgary, Canada; Phillips, M. J., Ternan, M., Eds.; Chemical Institute of Canada: Ottawa, 1988. Stuchly, V.; Bera´nek, L. Activation of a Hydrorefining CobaltMolybdenum Catalyst II. Development of Polyfunctional Activity of the Catalyst During Activation. Appl. Catal. 1987, 35, 35. Topsøe, H.; Clausen, B. S.; Candia, R.; Wivel, C.; Mørup, S. In Situ Mo¨ssbauer Emission Spectroscopy Studies of Unsupported and Supported Sulfided Co-Mo Hydrodesulfurization Catalysts: Evidence for and Nature of a Co-Mo-S Phase. J. Catal. 1981, 68, 433. Topsøe, N. Infrared Study of Sulfided Co-Mo/Al2O3 Catalysts: The Nature of Surface Hydroxyl Groups. J. Catal. 1980, 64, 235. Topsøe, N.-Y.; Topsøe, H. Characterization of the Structure and Active Sites in Sulfided Co-Mo/Al2O3 and Ni-Mo/Al2O3 Catalysts by NO Chemisorption. J. Catal. 1983, 84, 386. Topsøe, N.-Y.; Topsøe, H. FTIR Studies of Mo/Al2O3-Based Catalysts II. Evidence for the Presence of SH Groups and Their Role in Acidity and Activity. J. Catal. 1993, 139, 641. Wivel, C.; Clausen, B. S.; Candia, R.; Mørup, S.; Topsøe, H. Mo¨ssbauer Emission Studies of Calcined Co-Mo/Al2O3 Catalysts: Catalytic Significance of Co Precursors. J. Catal. 1984, 87, 497. Xie, X.; Yin, H.; Dou, B.; Huo, J. Characterization of a potassiumpromoted cobalt-molybdenum/alumina water-gas shift catalyst. Appl. Catal. A 1991, 77, 187. Zmierczak, W.; MuraliDhar, G.; Massoth, F. E. Studies on Molybdena Catalysts XI. Oxygen Chemisorption on Sulfided Catalysts. J. Catal. 1982, 77, 432.
Received for review December 19, 1995 Revised manuscript received March 11, 1996 Accepted March 12, 1996X IE950785V X Abstract published in Advance ACS Abstracts, August 15, 1996.
