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Bailey, K. M.; Campbell, T. K.; Falconer, J. L. Potassium Promo- tion of Ni/Al2O3 Catalysts. Appl. Catal. 1989, 54, 159-175. Campbell, C. T.; Goodman,...
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Ind. Eng. Chem. Res. 1998, 37, 2397-2403

2397

Kinetics of CO Hydrogenation over Ni-Mo/Al2O3 Catalysts with and without K Promotion A. Erhan Aksoylu and Z. I4 lsen O 2 nsan* Department of Chemical Engineering, Bogˇ azic¸ i University, 80815 Bebek, Istanbul, Turkey

The effect of K promotion on the CO hydrogenation activity and C2-C4 hydrocarbons selectivity of the 15 wt % Ni-10 wt % Mo/Al2O3 catalyst was investigated in the 498-548 K range using unpromoted and 1-3 wt % K-promoted samples. CO2 methanation tests at 523 K were used as a probe to confirm activity trends. Intrinsic kinetic data were obtained in the initial rate region on both unpromoted and 1 wt % K-promoted Ni-Mo catalysts. K promotion of the bimetallic Ni-Mo catalyst decreases both the activity and the specific activity of the Ni sites while increasing the C2-C3 olefin-to-paraffin ratio. The surface carbide mechanism with dissociative adsorption of hydrogen as the rate-limiting step gives the most plausible kinetic model among the models tested for both Ni-Mo and Ni-Mo-K catalysts. 1. Introduction Suitably designed bimetallic catalysts utilize the beneficial effect of the interaction between the two metals for the enhancement of both catalytic activity and product selectivity (Guczi, 1990; Adesina, 1996). The present work is part of an extensive study conducted on the characterization of a series of Ni-Mo/ Al2O3 catalysts for the production of lower hydrocarbons by the hydrogenation of carbon oxides (Aksoylu, 1997). Nickel is the one of the most suitable COx hydrogenation metals with high methanation ability and limited C2C4 hydrocarbons selectivity (Janardanarao, 1990; Trimm, 1980; Aksoylu et al., 1996; Aksoylu and O ¨ nsan, 1997; Ishihara et al., 1988). Molybdenum was chosen as the second metal since it is a component of desulfurization catalysts and its sulfur resistance is a potential advantage for industrial applications involving syngas containing appreciable amounts of sulfur (Dry, 1996; Concha et al., 1984; Tatsumi et al., 1986). Although Mo addition to Ni-based catalysts enhances the total hydrocarbons production and the C2-C4 hydrocarbons selectivity, it does not better the olefin/ paraffin ratios of the C2-C4 hydrocarbons produced (Aksoylu, 1997). Alkali-metal ions are widely used as promoters to improve olefin selectivity in CO hydrogenation (Campbell and Goodman, 1982; Woo et al., 1993; Chai and Falconer, 1985; Bailey et al., 1989; Snel, 1989). In this study, the effect of alkali-metal (K) promotion on the CO hydrogenation activity (total hydrocarbons production), specific activity (activity per metal surface area), and C2-C4 hydrocarbons selectivity of bimetallic Ni-Mo catalysts was investigated. The 15 wt % Ni10 wt % Mo/Al2O3 catalyst showing optimum performance (Aksoylu, 1997) was promoted with 1 and 3 wt % K. CO hydrogenation experiments were conducted on the unpromoted and K-promoted catalyst samples in the 498-548 K temperature range. CO2 methanation tests at 523 K were used as a probe to confirm activity and specific activity trends. Finally, initial rate studies were conducted on the unpromoted and 1 wt % Kpromoted Ni-Mo catalysts to find out how the kinetics * Corresponding author. Telephone: (90) 212 263 15 40 ×1875. Fax: (90) 212 287 24 60. E-mail: [email protected].

and/or mechanism of CO hydrogenation are affected by Mo and K promotion. 2. Experimental Section Commercial γ-alumina (Alcoa; 218 m2/g TSA, 200250 µm) was selected as the support material for preparing the bimetallic catalyst containing 15 wt % Ni and 10 wt % Mo by co-impregnation, using the conventional incipient wetness technique described previously (Aksoylu and O ¨ nsan, 1997). Appropriate quantities of aqueous nickel nitrate and ammonium heptamolybdate solutions were used as precursors. Catalyst samples promoted with 1 and 3 wt % K were prepared by successive impregnation of the calcined bimetallic catalyst with a K2CO3 solution and were dried at 323 K for 5 h and at 393 K for 3 h, followed by recalcination at 623 K for 4 h. Before each adsorption or reaction experiment, the catalyst samples were reduced in situ in a stream of 30 mL/min of pure hydrogen at 623 K for 3 h and then flushed under 30 mL/min of He flow at the same temperature. The metal surface areas (MSA) of the reduced catalyst samples were determined by irreversible CO adsorption at 303 K by using a Hewlett-Packard 5890 Series II gas chromatograph fitted with a TCD and a 50-cm-long, 3-mm i.d. stainless steel column packed with catalyst particles (Aksoylu and O ¨ nsan, 1997). COx hydrogenation tests were conducted at 498-548 K and atmospheric pressure in a stainless steel flow system with a 3.5-mm i.d. fixed-bed downflow microreactor controlled to (0.5 K and containing 270 ( 2 mg of reduced sample. The details of the reaction system and the analytical method have been reported elsewhere (Aksoylu and O ¨ nsan, 1997). Blank tests showed that the reactor material and the support alone were not catalytically active under the reaction conditions used. The feed was diluted with 70 volume % He, and the total flow to the microreactor was kept constant at 100 cm3/min corresponding to a space velocity of 6.2 cm3/ g‚s, which guaranteed both low conversion levels and the absence of transport effects. All three catalyst samples were tested for their CO hydrogenation activity and selectivity in the 498-548 K range using H2/CO )

S0888-5885(97)00467-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/05/1998

2398 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 Table 1. Effect of K Promotion on Metal Surface Areas and Activation Energies of CO Hydrogenation catalyst

metal surface area ((3%) (m2/g)

EA ((2%) (kJ/mol)

1.87 1.34

91.1 91.8

0.59

89.3

15 wt % Ni-10 wt % Mo/Al2O3 15 wt % Ni-10 wt % Mo-1 wt % K/Al2O3 15 wt % Ni-10 wt % Mo-3 wt % K/Al2O3

Table 2. Effect of K Promotion on the CO Hydrogenation Activity of 15 wt % Ni-10 wt % Mo/Al2O3 in the 498-548 K Range (Space Velocity ) 6.2 cm3/g‚s) total hydrocarbons and methane production rates (µmol/g‚s) HC T (K) H2/CO CO mol % K wt % 0 1 3

498 2 10

CH4 498 2 10

HC 523 2 10

CH4 523 2 10

HC 548 2 10

CH4 548 2 10

HC 523 1/2 20

CH4 523 1/2 20

1.064 0.798 3.487 2.441 8.757 7.707 1.453 0.915 0.720 0.573 1.934 1.354 5.260 4.366 0.993 0.636 0.285 0.225 0.915 0.668 1.948 1.383 0.363 0.232

Figure 1. Effect of K promotion on the specific activity of the Ni-Mo catalysts in the 498-548 K range (space velocity ) 6.2 cm3/g‚s).

2 and 10 mol % CO in the feed. CO2 methanation tests were also conducted on all three catalyst samples under the same feed conditions but only at 523 K. The effect of lower hydrogen concentration was studied via CO hydrogenation experiments carried out at 523 K with H2/CO ) 1/2 and 20 mol % CO in the feed. Using the unpromoted and 1 wt % K-promoted NiMo catalysts, a total of 32 kinetic experiments were conducted at 523 K with four different CO concentrations and two different H2/CO ratios at four different space times each. Each reaction experiment was conducted on a freshly reduced catalyst sample. A timeon-stream study indicated that the decrease in activity was insignificant after 6 h of exposure to syngas. 3. Results and Discussion 3.1. Effect of K Promotion on Activity, Selectivity, and Specific Activity. The unpromoted 15 wt % Ni-10 wt % Mo/Al2O3 catalyst was selected as the optimum from a set of nine bimetallic catalysts whose Ni and Mo contents had been determined by experimental design, and selection was made on the basis of both CO hydrogenation and CO2 methanation studies (Aksoylu, 1997). The major aim in the promotion of this catalyst by potassium was to obtain higher olefin selectivity. A comparison of the metal surface areas of unpromoted and K-promoted Ni-Mo samples indicates that K promotion by successive impregnation has led to a decrease in the active metal surface area (Table 1). The results of the CO hydrogenation tests conducted on the three catalyst samples in the 498-548 K temperature range are given in Table 2 and Figure 1, which report the variations in activity and specific activity, respectively. These results show that (i) K promotion decreases the activity of the catalyst samples and (ii) the specific activity of the sites is also decreased as the K loading increases. K promotion not only lowers the activity of the catalysts via decreases in the MSA values but also decreases the specific activities of the Ni sites. The reason for the lower specific activities of the Ni sites may be the K promotion mechanism, which causes an increase in the metal-carbon bond strength and a simultaneous weakening of the C-O bond of adsorbed

Figure 2. Effect of K promotion on the C2-C4 olefin/paraffin ratios of 15 wt % Ni-10 wt % Mo/Al2O3 at different temperatures and H2/CO ratios in the feed (space velocity ) 6.2 cm3/g‚s).

CO molecules (Snel, 1987). This will lead to higher CO dissociation rates and hence a higher carbon concentration on the surface (Campbell and Goodman, 1982), which results in lower hydrogenation and methanation activities due to the high ratio of carbon to hydrogen adsorbed on the surface. The significant decrease in the methane production rates with the addition of K (Table 2) and the considerable increase in the C2-C3 olefinto-paraffin ratios shown in Figure 2 support the K promotion mechanism proposed. Figure 2 also indicates that the C2-C3 olefin-to-paraffin ratio decreases with increasing reaction temperature for all the catalysts tested under identical feed conditions. This is probably due to the increase in the hydrogenation ability of the catalyst to form paraffins from the corresponding olefinic hydrocarbons as the temperature increases. CO hydrogenation tests conducted at 523 K but with a lower H2/CO ratio in the feed show that the decrease in the relative hydrogen concentration also enhances the olefin-to-paraffin ratio drastically. The synergetic interaction between Ni and Mo in bimetallic Ni-Mo/Al2O3 catalysts leads to increases in both the methanation and the polymerization activity when compared with the performance of monometallic Ni/Al2O3. It was found that the CH4 formation activity of Ni-Mo catalysts dominates at both low and high

Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 2399 Table 3. Effect of K Promotion on the C2-C4 Hydrocarbons Selectivity of 15 wt % Ni-10 wt % Mo/ Al2O3 in CO Hydrogenation in the 498-548 K Range (Space Velocity ) 6.2 cm3/g‚s)

Table 5. Effect of Feed Composition and Space Time on the CO Hydrogenation Activity of Unpromoted and 1 wt % K-Promoted 15 wt % Ni-10 wt % Mo/Al2O3 (T ) 523 K, Wcat ) 270 ( 2 mg)

C2-C4 hydrocarbons selectivities T (K) H2/CO CO mol % K wt % 0 1 3

498 2 10

523 2 10

0.25 0.21 0.21

548 2 10

0.30 0.30 0.28

0.12 0.17 0.29

523 1/2 20 0.37 0.36 0.36

Table 4. Effect of K Promotion on the Activity and Specific Activity of 15 wt % Ni-10 wt % Mo/Al2O3 in CO2 Methanation (T ) 523 K; Space Velocity ) 6.2 cm3/g‚s) K wt %

CH4 production (µmol/g‚s)

CH4/MSA (µmol/m2‚s)

CO/CH4

0 1 3

4.05 2.93 0.87

2.17 2.18 1.48

0.04 0.15 1.59

temperatures due to the low C-C polymerization activity and the high methanation activity at these temperatures, respectively, which leads to a maximum in the C2-C4 hydrocarbons selectivity at intermediate temperatures such as 523 K (Aksoylu, 1997). This is also true for the Ni-Mo sample promoted with 1 wt % K (Table 3). In K-promoted bimetallic catalysts, the relatively lower MSA due to K addition leads to a further decrease in the polymerization activity, which is more significant than the drop in the methanation activity of the sites, and the C2-C4 hydrocarbons selectivity is slightly decreased at 498 K. The effect of K promotion on the C2-C4 selectivity is not significant at 523 K; however, higher C2-C4 hydrocarbons selectivities are obtained at 548 K due to the relatively higher polymerization activity of the Ni sites at higher temperatures as well as the lower methanation activity resulting from the high C/H ratio of adsorbed species on the surface (Table 3). The activation energies calculated on the basis of CO converted to products were calculated as 91.1, 91.8, and 89.3 kJ/mol ((2%) for the unpromoted, 1 wt % K-promoted, and 3 wt % Kpromoted bimetallic Ni-Mo samples, respectively (see Table 1). The results of CO2 methanation tests conducted as a probe reaction on all three samples at 523 K show that K promotion leads to lower CH4 production rates and higher CO/CH4 production ratios as the K loading is increased from 1 to 3 wt % K (Table 4). This observation also supports the K promotion mechanism proposed, i.e., the strengthening of the metal-carbon bond and the relatively lower surface hydrogen concentration compared to the unpromoted catalyst, leading to a decrease in CH4 production and, consequently, to an increase in the CO/CH4 production ratio. In agreement with the results reported previously for a series of Ni and Ni-Mo catalysts (Aksoylu and O ¨ nsan, 1997; Aksoylu, 1997), the specific activities of the metal sites of the Ni-Mo-K catalysts are also higher in CO2 methanation than in CO hydrogenation. The reason for this higher activity may be the presence of carbon species on the catalyst surface in CO hydrogenation which may relate to the formation of C2+ hydrocarbons that have a strong retarding effect on CO methanation especially at lower conversion levels, while the presence of surface carbon species in CO2 methanation is reported

set no.

H2/CO

CO mol %

space time (g‚s/µmol), ×1000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

1 1 1 1 2 2 2 2 1 1 1 1 2 2 2 2

15 15 15 15 10 10 10 10 25 25 25 25 20 20 20 20

7.39 3.70 2.46 1.85 7.39 3.70 2.46 1.85 7.39 3.70 2.46 1.85 7.39 3.70 2.46 1.85

CO conversion (mol %) Ni-Mo Ni-Mo-K 9.92 5.62 4.15 3.07 23.43 10.58 8.96 6.19 6.38 3.40 2.15 1.22 13.48 7.61 5.07 3.64

8.84 4.67 2.79 2.23 18.37 7.80 6.00 5.18 4.11 2.13 1.71 0.91 11.14 4.76 3.32 2.64

to be negligible compared to CO hydrogenation (Tomoyuki et al., 1980). 3.2. Effect of Mo and K Addition on the CO Hydrogenation Kinetics. 3.2.1. Kinetic Experiments and Model Equations. In a detailed report on the kinetics of the Fischer-Tropsch reaction by Wojciechowski (1988), it is mentioned that a limited body of data have gradually been accumulated with the intention of understanding the fundamentals of the reaction, and only a part of these deal with the quantitative formulation of rate expressions for CO conversion and product distribution. This remark is still valid for the unconventional bimetallic CO hydrogenation catalysts with or without alkali-metal promotion. Although kinetic modeling of the methanation reaction in CO hydrogenation over Co-Mo-K/SiO2 has been reported by Chen and Adesina (1994), the effect of K has not been investigated separately by comparing data obtained on both K-promoted and unpromoted Co-Mo catalysts. In the present work, the effects of feed composition and space velocity on the CO hydrogenation activities of the unpromoted and 1 wt % K-promoted 15 wt % Ni10 wt % Mo/Al2O3 catalysts were studied by experiments conducted at four different CO concentrations using two different H2/CO ratios and four different space velocities each at 523 K (Table 5). The effect of space velocity on the C2-C3 olefin-to-paraffin ratios are plotted in Figure 3, which clearly indicates that the olefin-to-paraffin ratio increases with increasing space velocity. For each H2/ CO ratio and CO concentration in the feed, the Kpromoted catalyst gives higher olefin-to-paraffin ratios. The initial rate data presented in Table 5 were used in studying the kinetics of CO consumption. The surface carbide and enolic mechanisms have been considered (Wojciechowski, 1988; Chen and Adesina, 1994). The main difference between these two mechanisms is the type of CO adsorption. In the carbide mechanism CO adsorbs dissociatively, whereas molecular adsorption of CO is considered in the enolic mechanism. The other reactant, hydrogen, is taken to be dissociatively adsorbed in both mechanisms. In the carbide mechanism (Table 6), the hydrogenation reaction occurs between the adsorbed carbon formed via the dissociative adsorption of CO and the dissociatively adsorbed hydrogen in a stepwise manner to give meth-

2400 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998

Figure 3. Effect of space velocity on the C2-C3 olefin/paraffin ratios of unpromoted Ni-Mo and 1 wt % K-promoted Ni-Mo-K catalysts at 523 K. Table 6. Reaction Steps in the Carbide Mechanism (S: Vacant Sites) KCO

1. CO + 2S S CS + OS KH

2. H2 + 2S S 2HS KC1

3. CS + HS S HCS + S KC2

4. HCS + HS S H2CS + S KO1

5. OS + HS S HOS + S KO2

6. HOS + HS S H2O + 2S Table 7. Reaction Steps in the Enolic Mechanism (S: Vacant Sites) KCO

1. CO + S S OCS KH

2. H2 + 2S S 2HS KOH1

3. OCS + HS S HOCS + S KC2

4. HOCS + HS S H2O + S KC1

5. CS + HS S HCS + S KC2

6. HCS + HS S H2CS + S KO1

7. OS + HS S HOS + S KO2

8. HOS + HS S H2O + 2S

ane and higher hydrocarbons. In the enolic mechanism (Table 7), on the other hand, the reaction starts between the dissociatively adsorbed hydrogen and molecularly adsorbed CO that yields to an oxygenated intermediate, HOC-S, with S representing the surface site, which reacts with another adsorbed hydrogen to form water and adsorbed carbon. The reaction between the ad-

sorbed carbon and the adsorbed hydrogen follows the same steps given in the carbide mechanism. Previous work on the kinetics of CO hydrogenation over impregnated Ni/Al2O3 catalysts indicates that data are best correlated by the carbide mechanism. Therefore, the six model equations based on the different ratedetermining steps of the carbide mechanism and the two model equations based on the surface reaction controlling steps of the enolic mechanism were taken into consideration. The rate-controlling steps and the corresponding rate equations derived on the basis of Langmuir-Hinshelwood-Hougen-Watson ideal surface kinetics are given in Table 8. The details on the derivation of model equations can be found in the literature (Wojciechowski, 1988; Chen and Adesina, 1994). The parameters that are given in the model equations of Table 8 are defined in Table 9. 3.2.2. Model Discrimination. The data reported in Table 5 were used for obtaining the CO consumption rates required for testing the eight models listed in Table 8 for both the unpromoted and the K-promoted Ni-Mo catalysts. The kinetic parameters were first estimated by using multivariable linear least squares with standard matrix solutions. A criterion commonly used in model discrimination requires that the parameters be positive. The two models having positive parameters as a result of these calculations were the Carbide 2 and the Enolic 1 models for both Ni-Mo and Ni-Mo-K catalysts. The parameters of the eight model equations for CO consumption on the two catalysts were also estimated by nonlinear regression from the concentration versus rate data by a hybrid method using the Gauss-Newton and Levenberg-Marquardt algorithms (in MATLAB toolbox optimization toolkit). In testing each model, iterations starting from several different initial values were continued until the global optimum was reached and the same parameter values were obtained. For Carbide 2 and Enolic 1 models, parameter values estimated by linear regression were also used as initial values. In all cases, the nonlinear parameter estimation method gave more adequate results in terms of the residuals when compared to the results obtained from linear regression. The nonlinear estimation indicated that the only model giving nonnegative parameters and the smallest residuals is the Carbide 2 model for both the Ni-Mo and the Ni-Mo-K catalysts. The Enolic 1 model was eliminated because of the negative parameters obtained in the nonlinear analysis. 3.2.3. Discussion. The parameters of the eight model equations estimated by linear and nonlinear regression are reported in Tables 10 and 11 for the unpromoted and 1 wt % K-promoted Ni-Mo catalysts, respectively. In the Carbide 2 model, which is indicated by CO hydrogenation data on the Ni-Mo and the NiMo-K catalysts, both H2 and CO are dissociatively adsorbed, and the rate-controlling process is the irreversible dissociative adsorption of hydrogen. The parameters appearing in the Carbide 2 model equation estimated by nonlinear regression are found to be K1 ) 164.77 (µmol/g‚s‚atm)2 and K2 ) 6.77 (atm)-1/2 for the unpromoted Ni-Mo catalyst. The residual is r2 ) 0.12 (µmol/g‚s‚atm)2. The parameters of the Carbide 2 model for the Ni-Mo-K catalyst promoted with 1 wt % K are calculated to be K1 ) 209.32 (µmol/g‚s‚atm)2 and K2 ) 9.68 (atm)-1/2 with a residual of r2 ) 0.19 (µmol/g‚s‚ atm)2. To demonstrate the goodness of fit, CO consump-

Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 2401 Table 8. Rate-Determining Steps and the Corresponding Rate Equations model

rate-controlling step

proposed rate equation Carbide Mechanism

C1

3

-rCO ) C2

k1PCO1/2PH21/2 (1 + K1PH21/2 + K2PCO1/2)2

2

-rCO ) C3

k1PH2 (1 + K2PCO1/2)2

1

-rCO ) C4

k1PCO 1/2

(1 + K1PH2

+ K2PCOPH-1/2)2

4

-rCO ) C5

k1PCO1/2PH23/4 (1 + K1PH21/2 + K2PCO1/2PH2-1/4 + K3PCO1/2PH21/4)2

3 and 6

-rCO ) C6

k1PCO1/2PH23/2 (1 + K1PH21/2 + K2PCO1/2PH2-1/4 + K3PCO1/2PH21/2)2

4 and 6

-rCO )

k1PCO1/2PH2 (1 + K1PH21/2 + K2PCO1/2 + K3PCO1/2PH21/2)2

Enolic Mechanism E1

3

-rCO ) E2

k1PH1/2PCO (1 + KCOPCO + K1PH21/2)2

4

-rCO )

Table 9. Parameters Defined in the Model Equations of Table 8 model

parameter

expression

C1 C1-6, E1,2 C1 C2 C2 C3 C3 C4 C4 C4 C5 C5 C5 C6 C6 E1 E2 E2

k1 K1 K2 k1 K2 k1 K2 k1 K2 K3 k1 K2 K3 k1 K2 k1 k1 K2

kC1kO1KCOKH [S0]2 KH1/2 1 + (kC1/kO1)(kO1KCO/kC1) kH[S0]2 KCO1/2[(kC1/kO1)1/2 + (kC1/kO1)-1/2] kCO[S0]2 (kCO/kO1)KH-1/2 (kC2kO2KCOKC1)1/2KH3/4[S0]2 (kO1KCO/kC2KC1KH1/2)1/2 [1 + (kC2/kO1)](kO1KCOKC1KH/kC2)1/2 (kC1kO2KCOKO1KH)1/2KH1/4[S0]2 (kC1KCO/kO2KO1KH1/2)1/2 [1 + (kC1/kO2)](kO2KCOKO1KH/kC1)1/2 (kC2KO2KCOKO1KC1)1/2KH[S0]2 [1 + (kC2KC1/kO2KO1)](kO2KCOKO1/kC2KC1)1/2 kOH1KCOKH1/2[S0]2 kOH2KCOKHKOH1[S0]2 KCOKOH1KH1/2

tion rates calculated by using these parameters are compared in Table 12 with the experimental rates obtained on both catalysts. It should be noted that the Carbide 3 model where dissociative adsorption of CO is the rate-controlling step gave the worst results on the basis of the residuals calculated, in addition to its negative parameter values. This clearly indicates that CO dissociation cannot be the rate-determining step and confirms the K promotion mechanism, leading to high CO dissociation rates as proposed in section 3.1. It has been proposed in the literature that the hydrogenation of surface carbon by the adsorbed hy-

k1PCOPH2 1/2

(1 + K1PH2

+ KCOPCO + K2PCOPH21/2)2

drogen, i.e., the surface reaction, is the rate-determining step in CO hydrogenation over monometallic Ni catalysts (Kester and Falconer, 1984; Hayes et al., 1985; Prokhorenko et al., 1988). It was also postulated that hydrogen and carbon monoxide compete for similar adsorption sites and that CO inhibits the reaction (Hayes et al., 1985). On the basis of the kinetic data obtained in this study, the only plausible mechanism is the carbide mechanism with irreversible dissociative adsorption of hydrogen as the rate-determining step, and a comparison with the rate-determining steps proposed for monometallic Ni catalysts in the literature shows that Mo addition as the second metal alters the behavior of the Ni catalyst. On the other hand, the results of this study also show that K promotion does not lead to a change in the behavior of the bimetallic Ni-Mo catalyst. This is also indicated by the activation energies reported in Table 1. The Carbide 2 model proposed for the bimetallic Ni-Mo and Ni-Mo-K catalysts investigated in this work is in agreement with the models proposed in the literature for Co-Ni (Ishihara et al., 1992) and Co-Mo-K (Chen and Adesina, 1994) bimetallic systems. A comparison of the K2 values of the Carbide 2 model for the Ni-Mo and Ni-Mo-K catalysts shows that CO adsorption on the K-promoted Ni-Mo is likely to be stronger than that over the unpromoted Ni-Mo. This will mean that the surface coverage of adsorbed carbon is greater on the K-promoted catalyst than on the unpromoted one, and hence a reduced H/C ratio is to be expected on the surface.

2402 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 Table 10. Model Parameters for CO Consumption on 15 wt % Ni-10 wt % Mo/Al2O3 at 523 K k1 C1 C2 C3 C4 C5 C6 E1 E2

K1

K2

K3

KCO

nonlinear

linear

nonlinear

linear

nonlinear

linear

nonlinear

linear

321.481 164.77 0.2091 6.6889 2.3520 0.3918 150.855 14.3080

518.88 164.77 9.138 6.6907 2.3526 0.3923 594.84 14.298

-2.9414

-3.633

-1.2102 -2.2584 -2.1938 -2.1351 -1.2746 -2.3531

-1.157 -2.2587 -2.1939 -2.1351 1.9561 -2.3527

12.9292 6.774 -1.0798 -1.2458 -1.0446 -2.5998

16.963 6.774 -1.402 -1.2460 -1.0446 -2.5998

4.9418 4.3850 4.68738

4.9488 4.3853 4.68738

11.7403

11.7391

nonlinear

linear

10.2810 -7.7360

15.439 -7.7342

Table 11. Model Parameters for CO Consumption on 15 wt % Ni-10 wt % Mo-1 wt % K/Al2O3 at 523 K k1 C1 C2 C3 C4 C5 C6 E1 E2

K1

nonlinear

linear

104470 209.32 0.1568 0.5835 0.2265 0.0320 129.087 0.4569

5739.1 1857.0 4.251 0.5836 0.2264 0.0323 1214.5 0.4565

nonlinear

K2 linear

50

14.68

-1.1339 -2.2458 -2.1914 -2.1382 -1.3920 -2.1945

-1.1339 -2.2459 -2.1914 -2.1382 2.620 -2.1945

K3

KCO

nonlinear

linear

nonlinear

linear

-300 9.6833 -1.227 -1.5975 -1.0880 -2.4465

-77.795 33.254 -1.227 -1.5977 -1.0880 -2.4631

4.3056 3.8596 4.9569

4.3057 3.8596 4.9568

11.7578

11.758

nonlinear

linear

11.3970 -6.0576

31.007 -6.0573

Table 12. Initial Rate Data on CO Consumption (T ) 523 K) (-RCO)0 (µmol/g‚s) catalyst

H2/CO

CO (atm)

H2 (atm)

experimental

calculated (C2)

15% Ni-10% Mo/Al2O3 15% Ni-10% Mo/Al2O3 15% Ni-10% Mo/Al2O3 15% Ni-10% Mo/Al2O3 15% Ni-10% Mo-1% K/Al2O3 15% Ni-10% Mo-1% K/Al2O3 15% Ni-10% Mo-1% K/Al2O3 15% Ni-10% Mo-1% K/Al2O3

1 1 2 2 1 1 2 2

0.15 0.25 0.10 0.20 0.15 0.25 0.10 0.20

0.15 0.25 0.20 0.40 0.15 0.25 0.20 0.40

2.120 2.163 3.161 3.779 1.804 1.418 2.429 2.905

1.882 2.140 3.338 4.059 1.391 1.533 2.537 2.946

4. Conclusion

Acknowledgment

The results of CO hydrogenation experiments conducted for the characterization of Ni-Mo and NiMo-K catalysts and the related kinetic studies indicate the following: (i) K promotion lowers the total hydrocarbons production of the catalysts and the specific activities of the nickel sites while enhancing the C2-C4 hydrocarbons selectivity at higher temperatures. (ii) The significant decrease in methane production rates caused by K promotion is accompanied by a considerable increase in the C2-C3 olefin-to-paraffin ratios at all temperatures. (iii) The above results support the K promotion mechanism which leads to higher CO dissociation rates and hence higher surface carbon concentrations by increasing the metal-carbon bond strength. (iv) CO2 methanation tests used as a probe confirm the K promotion mechanism and the activity/specific activity trends observed in CO hydrogenation. (v) The carbide mechanism with irreversible dissociative adsorption of hydrogen as the rate-limiting step gives the most plausible model for both the unpromoted Ni-Mo and the 1 wt % K-promoted Ni-Mo-K catalysts on the basis of intrinsic kinetic data obtained in the initial rate region. (vi) A comparison with the surface reaction controlling mechanisms proposed in the literature for monometallic Ni catalysts shows that the addition of Mo as the second metal alters the behavior of nickel catalyst, while K promotion of the bimetallic Ni-Mo catalysts does not lead to a change in the rate-determining step.

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Received for review July 7, 1997 Revised manuscript received February 19, 1998 Accepted February 22, 1998 IE970467R