Hydrogenation of CO2 over a Rice Husk Ash Supported Nickel

(1) From the measured enthalpy values shown in Table 2, all the reactions except reaction 2 can be seen to be highly exothermic. This excessive heat w...
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Ind. Eng. Chem. Res. 1998, 37, 3838-3845

Hydrogenation of CO2 over a Rice Husk Ash Supported Nickel Catalyst Prepared by Deposition-Precipitation Feg-Wen Chang,* Tyng-Jer Hsiao, and Jui-Dar Shih Department of Chemical Engineering, National Central University, Chungli 32054, Taiwan

The rice husk ash supported nickel catalysts was prepared by deposition-precipitation and used for hydrogenation of CO2 from a H2/CO2 (4/1) mixture. The reaction exhibited high selectivity (80%) for CH4 formation when carried out at 673-873 K with the supported nickel. The effects of nickel loading, deposition-precipitation time, calcination, and reduction of temperature on the catalytic performance were also extensively studied. These results show that nickel loading increases with an increase in the deposition-precipitation time up to 24 h, while metal dispersion increases with a decrease in nickel loading. The conversion of CO2 and the yield of CH4 were found to be independent of the calcination temperature and time. Furthermore, the conversion and yield increase with an increasing reaction temperature up to 723 K, but decrease with a further increase in the reaction temperature. Moreover, rice husk ash has been found to be preferable over silica gel as a catalyst support as revealed by the temperature-programmed desorption techniques and the reaction test. Introduction In catalysis research, various well-dispersed catalysts are nowadays prepared by application of a catalytic precursor onto a support, having a high surface area. All the metals belonging to group VIIIB, with the only exception of osmium, have been studied on a variety of supports for carrying out hydrogenation of CO2,1,2 hydrogenation of fatty oils,3 a steam-reforming reaction, and hydrogenation of aromatic hydrocarbons.4,5 However, the metal of choice for a catalyst is nickel, due to its easy availability and lower cost as compared to other noble metals. In the catalytic system, high activity can be achieved only for a material possessing a high metal area or displaying a high degree of metal dispersion under various reaction conditions. Therefore, to promote catalytic activity, modification of the catalyst’s surface properties has been extensively studied. The support not only provides a sufficient surface area for the metal to disperse, but also prevents it from sintering. Its significant role in catalyzing the reactions emphasizes the importance of catalyst preparation. Impregnation due to incipient wetness often leads to poor dispersions. The ion-exchange method, on the other hand, frequently yields high and homogeneous dispersions, but also yields low metal contents. However, deposition-precipitation of metal precursors on supports, when accompanied by adsorption, is recognized to yield high and homogeneous dispersion, even at high metal contents. The present investigation has therefore developed a low-cost support material for nickel catalysts, with the aim of promoting its catalytic activity. For this purpose, rice husk ash was used as a support, and nickel-containing catalysts supported on rice husk ash were prepared by deposition-precipitation. The catalytic activity was evaluated by using hydrogenation of CO2 as the test reaction. In addition, silica gel was also chosen as another support. And the activities of two kinds of silica-supported nickel catalysts were also compared. * To whom correspondence should be addressed. Fax: 8863-4252296.

To examine the effect of preparation conditions on the catalyst’s activity, the effects of deposition-precipitation time, calcination temperature, calcination time, reaction temperature, reaction time, and nickel loading during catalyst preparation were investigated.

Experimental Section Raw Material. Amorphous silica, commonly referred to as rice husk ash, was extracted from rice husk, by acid leaching, pyrolysis, and carbon-removing processes. In this experiment, the raw rice husk was thoroughly washed with distilled water to remove the adhering soil and was then dried at 373 K in an air oven. The acid leaching, pyrolysis, and carbon-removing processes were carried out following our previous reports.6-9 The rice husk was refluxed with 3 N HCl in a round-bottomed glass flask, maintained at a temperature of around 373 K within a thermostat for 1 h. After leaching, the husk was thoroughly washed with distilled water and dried. The pyrolysis process was performed in a tubular quartz reactor of 1/4 in. i.d. under a nitrogen atmosphere at 1173 K for 1 h. After pyrolysis, the husk was further heated in an air furnace at 1173 K for 1 h; and the husk thus obtained contained more than 99% amorphous silica. This silica was then used as the support material for nickel catalysts. We therefore denote this rice husk ash supported system as SiO2-RHA. Nickel nitrate (Merck, 99%) and urea (Merck, 99.5%) were used for preparing the catalyst. Sample Preparation. Rice husk ash (125-132 m2/ g; named as SiO2-RHA) and silica gel (490 m2/g; named as SiO2 gel) were used as supports. The silicas were pretreated at 373 K for 12 h prior to depositionprecipitation. Ni/SiO2 materials were prepared by a controlled-pH urea deposition technique, developed by van Dillen et al.10 A slow and homogeneous increase in OH- concentration is required to carry out deposi-

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Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 3839

Figure 1. Schematic diagram of the reaction system for the hydrogenation of the CO2 system.

tion-precipitation, wherein hydrolysis of urea at 363 K forms the source of hydroxide ions. Thus, a series (3.47-37.9 wt %) of Ni/SiO2 catalysts were prepared by homogeneous deposition-precipitation of nickel ions onto the surface of the suspended support. The precipitates thus obtained were thoroughly washed with distilled water and subsequently dried in an air oven for 12 h and were further calcinated in a furnace. Finally, 50 ( 1-mg samples, after being dried and calcinated, were installed in the temperature-programmed reduction (TPR) apparatus and reduced in a H2/Ar (5/95) stream at 923 K for 3 h, at a heating rate of 10 K/min. The effects of variables at each stage on the resulting catalyst are discussed later in this paper. Catalytic Reaction. In the present work, gas-phase hydrogenation of CO2 to methane by a Ni/SiO2-RHA catalyst is studied. About 50 mg of catalysts were placed in a micro-fixed-bed reactor operating at atmospheric pressure and reduced in this quartz reactor of 1/ in. i.d. under continuous flow of H /Ar (5/95) at the 4 2 rate of 40 mL/min. Next, the reactor was adjusted to the desired reaction temperature under the same hydrogen flow. As the temperature stabilized, the flow rates of carrier gas nitrogen and a H2/CO2 (4/1) mixture were adjusted to 30 and 25 mL/min, respectively, and fed into the reaction system. The methane content of the product gas was analyzed by gas chromatography, equipped with a thermal conductivity detector (TCD). The experimental setup is depicted in Figure 1. Characterization. Inductively Coupled PlasmaMass Spectrometer (ICP-MS). The nickel content of the unreduced catalyst was determined by ICP-MS (PerkinElmer, SCIEX ELAN 5000). BET Method. Total surface area of the supports was determined by nitrogen adsorption using the BET method, in a volumetric apparatus. Scanning Electron Microscope (SEM). The scanning electron microscopic studies of the catalyst precursors, as well as the catalysts which reduced and passivated in a 40-mL/min nitrogen stream at 333 K for 24 h, were performed on a Hitachi S-2300 apparatus. X-ray Diffraction. An X-ray diffractometer was used for identification of the product, with Cu KR radiation as the source. Temperature-Programmed Reduction (TPR). The catalyst (50 mg) was placed in the quartz reactor after calcination and heated from room temperature to 1173 K at 10 K/min under a 40-mL/min H2/Ar (5/95) flow stream. Notably, the positions of H2 were consumed

Figure 2. Changes in pH during precipitation.

Figure 3. Effect of deposition-precipitation time on nickel contents of Ni/SiO2-RHA.

during the TPR procedure. In combination with other characterization techniques, the TPR profiles were used to confirm the metal-support interaction. Temperature-Programmed Desorption (TPD). The surface areas of the metal were measured by hydrogen chemisorption, by assuming a stoichiometry of one hydrogen molecule adsorbed on two surface nickel atoms. The average cross-sectional area of each surface nickel atom is 4.87 Å2 (considering the density of nickel as 8.91 g/cm3 and a face-center cubic lattice).11 The surface properties were determined, after H2 chemisorption, by removing the chemisorbed hydrogen with

3840 Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998

Figure 4. Scanning electron micrograph of specimen. (a) SiO2-RHA (×10 k); (b) Ni/SiO2-RHA after precipitation (×10 k); (c) Ni/SiO2 gel after precipitation (×10 k); (d) Ni/SiO2-RHA after calcination (×10 k); (e) Ni/SiO2 gel after calcination (×10 k); (f) Ni/SiO2-RHA after being reduced and passivated (×10 k).

an argon stream of 30 mL/min from room temperature to 823 K. The extent of metal dispersion was then calculated as the percentage of surface nickel atoms with respect to total nickel atoms in the catalysts. Results and Discussion Upon heating the suspension containing Ni(II)(NO3)2, urea, and SiO2-RHA to 363-368 K, ammonium hydroxide was found to be uniformly generated throughout the solution through hydrolysis of urea, thereby pre-

cipitating Ni2+ on the suspended support.12 A typical pH versus time curve is presented in Figure 2. This figure shows that the concentration of hydroxide ions in the solution increases rapidly with the duration of hydrolysis of urea. If the nucleating agent SiO2-RHA is present in the solution, then rapid growth of nickel hydrosilicate, Ni3(OH)4Si2O5,10,13 occurs during which the hydroxide ions are consumed so rapidly that the pH displays a transient decrease. Therefore, the pH in Figure 2 briefly reaches a value of 5.5 but soon levels off at 5.3, where it remains unchanged until the

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Figure 6. XRD spectra of raw materials and catalyst precursors. (a) SiO2-RHA; (b) SiO2 gel; (c) 17.4 wt % Ni/SiO2-RHA after precipitation; (d) 17.4 wt % Ni/SiO2-RHA after calcination; (e) 34.1 wt % Ni/SiO2-RHA after calcination.

Figure 5. TPR profiles of nickel supported on rice husk ash (ramp rate, 10 K/min).

precipitation is complete. This finding is similar to that report by Hermans and Geus.13 During the precipitation process, all the nickel ions in the solution cannot be adsorbed on the support surface. The extent of adsorption depends on the nature of support as well as the degree of interaction between nickel ions and SiO2-RHA. We have also examined here the effect of duration of deposition-precipitation on nickel loading. The corresponding result is presented in Figure 3. As can be seen from this figure, metal loading increases with an increase in the depositionprecipitation time up to 24 h, after which the metal loading remains constant. This may be due to coverage of the entire support surface by nickel ions. The scanning electron micrographs of the catalyst precursors and the reduced and passivated catalysts are presented in Figure 4. One can clearly notice the smooth surface of SiO2-RHA in Figure 4a. Figures 4b,c display the surface of supports after depositionprecipitation. It can be seen that the surface of the supports is covered with a layer, having a sheetlike structure. The surface profiles of the catalysts after calcination are presented in Figure 4d,e. By comparing the profiles before and after calcination, it was found that the layer was decomposed to form a network structure. The meshes of the network were found to be denser on the surface of the reduced and passivated Ni/SiO2-RHA (Figure 4f) than those before reduction. Temperature-programmed reduction (TPR) was used to characterize the Ni/SiO2-RHA catalyst precursors with respect to the degree of interaction with support

and nickel atom location, by monitoring variations in the precipitation, drying, and calcination stages of catalyst preparation. Figure 5 summarizes the TPR results obtained for 3.47-37.9 wt % nickel contents in the Ni/SiO2-RHA catalysts. Obviously, the TPR profiles for high metal loading (34.1 and 37.9 wt %) display a broad peak at high temperatures (723∼923 K) and a sharp H2 consumption peak at a low temperature (623 K). In our previous studies,9 the peak corresponding to the consumption of H2 by NiO species supported on SiO2-RHA was found to occur at 623 K. Therefore, the sharp H2 consumption peak at a low temperature (623 K) is attributed to the NiO species. On the other hand, the TPR profile for low metal loading display only a single peak at 723∼923 K, which indicates that the nickel catalysts supported on rice husk ash by deposition-precipitation technique may form a less reducible compound. There are two possibilities for this phenomena: (1) The peak at the high temperature in the TPR profile is due to the formation of Ni(III) compounds during deposition-precipitation. (2) The peak at the high temperature is due to the presence of less reducible Ni(II) species in the samples. Figure 6 presents the X-ray diffraction spectra of the raw materials (curves 6a and 6b) and different catalyst precursors before and after calcination (curves 6c-e). Notably, the traces of Ni(III)oxide or higher species are absent in these spectra. The two small and broad traces in spectra 6c, and 6d correspond to nickel hydrosilicate. These results are similar to those reported by Gil et al.14 In addition, curve 6e, which is the spectrum of 34.1 wt % Ni/SiO2-RHA after calcination, displays traces of NiO. This supports our earlier finding of the sharp H2 consumption peak at a low temperature in the TPR profile of the high-nickel-loaded catalysts. Thus, possibility (1) is improbable. Therefore, it may be con-

3842 Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 Table 1. Surface Properties of Ni/SiO2-RHA Catalysts with Various Loadings loading (%) amount of nickel per catalyst (µmol/g of cat) dispersion (%) surface area of metal (m2/g of Ni) CH4 selectivity (%)

3.47% 281.4 47.60 237.99 75.76

cluded that the only nickel species present on the SiO2RHA surface are Ni(II) compounds. Undoubtedly, the main reason for the peak at a high temperature lies in the strong metal-support interaction (SMSI) between Ni(II) and the support. van Dillen et al.10 and Hermans and Geus13 have reported that nickel hydroxide reacts with SiO2 to form nickel hydrosilicate during deposition-precipitation. Gil et al.14 and Coenen15 have further shown that nickel hydrosilicate is a nickel antigorite-like type. Consequently, the interaction between Ni(II) and rice husk ash certainly explains the phenomena in the TPR profiles. Besides, Coenen15 has also reported that the nickel antigoritelike species are less reducible than nickel hydroxide and nickel carbonate. Therefore, the metal support interaction between Ni(II) and rice husk ash results in the nucleation of nickel hydrosilicate on the surface of the support. Table 1 presents metal dispersion and CH4 selectivity for various loadings of Ni/SiO2-RHA catalysts, as obtained by the TPD technique. Clearly, the degree of dispersion of nickel on the support can be increased by decreasing the weight loading of nickel in the catalyst. The amount of nickel per catalyst can be increased by increasing the weight loading of nickel in the catalyst, but the surface area of the metal decreases by increasing the weight loading of nickel in the catalyst. On the other hand, adsorption of nickel ions on the support results in the formation of multilayers at a high nickel loading. The nickel ions in the outward layer cannot make contact with the support and consequently remain in the hydroxide form. This nickel hydroxide decomposes to nickel oxide during the calcination process. This can be clearly seen from the spectra 6d and 6e and the TPR profiles. With a supported metal catalyst, it is often desirable to be able to determine the metal surface area, in distinction with the total surface area, since the metal surface area usually provides vital information. The conventional method of determining the metal surface area by selective chemisorption depends on the prevailing conditions of temperature and pressure at which the gas chemisorbs and forms a monolayer on the metal, but not to any appreciable extent on the support. The most suitable experimental conditions, however, vary with the nature of the support. Table 1 presents the metal dispersion and metal surface areas of different Ni/SiO2-RHA catalysts, as obtained by the TPD technique. Clearly, the specific metal area of the 17.4 wt % Ni/SiO2-RHA catalyst is largest of all the catalysts. Therefore, the degree of dispersion of nickel on the support and CH4 selectivity of hydrogenation of CO2 (ratio of CH4 yield to CO2 conversion) at 773 K can be increased by decreasing the weight loading of nickel in the catalyst. At this juncture, the possible reactions with the support during precipitation, washing, and drying processes must be borne in mind. The oxide or other metal compounds that are active catalysts may form a compound with the support or may dissolve in the support to form a solid solution during the calcination process.

17.4% 1612.2 54.38 271.91 70.72

24.2% 1684.4 40.85 204.26 68.79

34.1% 1788.4 30.78 153.91 66.60

37.9% 1700.1 26.32 131.64 66.50

Figure 7. Effect of calcination temperature on conversion and yield of hydrogenation of CO2 with 17.4 wt % Ni/SiO2-RHA catalysts (reaction temperature, 773 K; reaction time, 90 min; calcination time, 3 h).

Therefore, calcination can improve the interaction between the metal and support. In addition, calcination can also eliminate the volatile and unstable anions and cations that are not desired in the final catalyst and thereby increase the strength of the final catalysts. However, an excessive calcination temperature will enlarge the size of the metal crystallites on the support and thus decrease the activity of the catalyst. Figures 7 and 8 present the effects of the calcination temperature and duration of calcination on the catalytic behavior of 17.4 wt % Ni/SiO2-RHA for the hydrogenation of CO2 at 773 K, respectively. These results show that conversion as well as yield are independent of calcination temperature and duration within the reaction temperature range. This may be due to the strong interaction between nickel and rice husk ash. Figures 9 and 10 display the effects of the duration of reaction on CO2 conversion and CH4 yield during the hydrogenation of CO2 at 773 K, for various loadings of Ni/SiO2-RHA catalysts. As these figures demonstrate, CO2 conversion and CH4 yield increase with increasing nickel loading in the catalysts. Table 1 confirms that higher loading implies a higher amount of active nickel per catalyst. Consequently, a greater amount of active nickel per catalyst implies a higher CH4 yield obtained. In addition, the above results suggest that CO2 conversion and CH4 yield decrease as a function of reaction time, implying deactivation of the catalytic behavior of Ni/SiO2-RHA. The temperature dependence of conversion of CO2 and CH4 yield over rice husk ash supported nickel catalysts

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Figure 8. Effect of calcination time on conversion and yield of hydrogenation of CO2 with 17.4 wt % Ni/SiO2-RHA catalysts (reaction temperature, 773 K; reaction time, 90 min; calcination temperature, 873 K).

Figure 9. Effect of reaction time on CO2 conversion of hydrogenation of CO2 with Ni/SiO2-RHA catalysts (reaction temperature, 773 K).

are shown in Figures 11 and 12. It can be seen that conversion and yield increase as the reaction temperature increases up to a maximum (723 K), while for further increase in the reaction temperature, the conversion decreases. Obviously, a reaction temperature of 723 K is the optimum condition for hydrogenation of CO2 with Ni/SiO2-RHA. On the other hand, increasing the reaction temperature promotes the reaction rate up

Figure 10. Effect of reaction time on CH4 yield of hydrogenation of CO2 with Ni/SiO2-RHA catalysts (reaction temperature, 773 K).

Figure 11. Comparison of conversion and yield of hydrogenation of CO2 with 17.4 wt % Ni/SiO2-RHA and 18.2 wt % Ni/SiO2 gel catalysts (reaction time, 90 min; calcination temperature, 873 K).

to 723 K. Proceeding of the reaction at too high of a temperature produces an increase in the extent of deactivation of the catalyst and consequently a decrease in the selectivity to methane formation. The thermodynamic values in Table 2 for CO2 hydrogenation also provide evidence to the effects of the reaction temperature on CO2 conversion and CH4 yield during the reaction. The phenomena in Figures 11 and 12 can be explained by the following possible reasons:

3844 Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 Table 3. SiO2 Gel

Comparison of Properties of SiO2-RHA and

property color surface areaa average pore sizea particle size purity b structurec c

SiO2-RHA white 125-132 m2/g 70 Å 100-270 mesh >99% SiO2 amorphous

SiO2 gel white 490 m2/g 40 Å 70-230 mesh ∼99% SiO2 amorphous

a Obtained from BET method. b Obtained from ICP-MS and EA. Obtained from XRD analysis, shown in Figure 6a,b.

Table 4. Surface Properties of Nickel Catalysts on SiO2-RHA and SiO2 Gel Ni/SiO2-RHA loading (wt %) amount of nickel per catalyst (µmol/g of cat) surface area of metal (m2/g of Ni) dispersion (%) a

Figure 12. Comparison of CH4 selectivity of hydrogenation of CO2 with 17.4 wt % Ni/SiO2-RHA and 18.2 wt % Ni/SiO2 gel catalysts (reaction time, 90 min; calcination temperature, 873 K). Table 2. Thermodynamic Values for CO2 Hydrogenation16 reactiona temp. (K) 600 700 800 900 1000 600 700 800 900 1000 600 700 800 900 1000

(1)

(2)

(3)

(4)

Heat of Reaction ∆Hf° (kcal/mol) -42.792 -9.292 -52.008 -61.376 -43.68 -9.05 -52.73 -61.78 -44.449 -8.799 -53.248 -62.047 -45.105 -8.549 -53.654 -62.203 -45.653 -8.304 -53.957 -62.261

(5) -41.46 -41.35 -41.19 -40.996 -40.729

Free Energy Change of Reaction ∆G° (kcal/mol) -13.347 -3.991 -17.338 -21.329 -15.836 -8.366 -3.127 -11.493 -14.62 -11.574 -3.269 -2.298 -5.567 -7.865 -7.332 1.921 -1.5 0.594 -1.079 -3.108 7.173 -0.729 6.444 5.715 1.09 Equilibrium Constant, log Kp 4.861 1.453 6.314 7.768 2.611 0.976 3.588 4.564 0.893 0.628 1.521 2.148 -0.466 0.364 -0.144 0.261 -1.568 0.159 -1.408 -1.248 a

(1)

CO + H2O f CO2 + H2

(2)

CO + 3H2 f CH4 + H2O

(3)

2CO + 2H2 f CH4 + CO2

(4)

2CO f C + CO2

(5)

CO2 + 4H2 f CH4 + 2H2O

5.768 3.613 2.003 0.755 0.238

(1) From the measured enthalpy values shown in Table 2, all the reactions except reaction 2 can be seen to be highly exothermic. This excessive heat will result in overheating and consequent inactivation of the catalyst. It can also make the temperatures rise to such a degree that hydrogenation becomes limited due to the thermodynamic equilibrium.

Ni/SiO2 gel

5 wt %a 49.54

17.4 wt % 1612.2

5 wt %a 16.16

18.2 wt % 1360.4

29.09

271.91

9.49

219.36

5.82

54.38

1.89

43.87

Data of these columns obtained from our previous studies.9

(2) From the observed changes in free energy values and equilibrium constants in the range 600-1000 K for hydrogenation reactions, methane yields can be seen to be affected by temperature. A higher reaction temperature results in a rapid decrease in the ratio of H2 and CO2 in the reactant, and therefore the conversion of CO2 to CH4 decreases.16 It can hence be concluded that a higher reaction temperature promotes other side reactions and implies a lower CH4 selectivity, as can be observed from these figures. Reaction under a suitable temperature will promote the production of methane. Overall, by the rice husk ash supported catalysts tested herein, CO2 was hydrogenated with a selectivity of above 80% to produce methane. The catalytic behaviors of Ni/SiO2-RHA catalysts and Ni/SiO2 gel catalysts toward the hydrogenation of CO2 at 673-873 K were also studied and the results are shown in Figures 11 and 12. It has to be noted here that the specific surface areas and pore sizes of these two types of catalysts are different, and in addition, it is also not possible to control the metal content during the preparation of these catalysts by depositionprecipitation. Therefore, the same concentrations of nickel nitrate and urea should be used during the preparation of Ni/SiO2 gel and 18.2 wt % nickel catalyst. Though the metal content of Ni/SiO2 gel is higher than that of Ni/SiO2-RHA, as these figures reveal, the selectivity of 17.4 wt % Ni/SiO2-RHA toward CH4 formation is higher than that with 18.2 wt % Ni/SiO2 gel during the reaction. The reason behind this phenomena is the differences in the surface properties of these two kinds of catalysts. In Table 3 we have compared the properties of these two supports, which shows that the surface area of SiO2 gel is 4 times as large as that of SiO2-RHA. As is generally known, the higher surface area implies a greater number of active sites, and consequently a higher dispersion of metal. However, the average pore size of SiO2 gel is smaller than that of SiO2-RHA. The small pore may get clogged by large crystallites, causing a decrease in the active surface area. The metal surface area and metal dispersion of these two types of supported nickel catalysts were determined by the TPD technique, which are presented in Table 4. As can be seen from this table, the degree of dispersion and metal surface area of the 17.4 wt % SiO2-RHA catalyst are larger than those of

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the 18.2 wt % SiO2 gel. This can also be seen by comparing Figure 4b,c, which shows that the layer of nickel antigorite-like present over the surface of Ni/SiO2 gel is thicker than that over the surface of Ni/SiO2RHA. This may be due to the interaction between nickel and SiO2-RHA being stronger than that between nickel and SiO2 gel. The surface properties of 5 wt % Ni/SiO2RHA and 5 wt % Ni/SiO2 gel, both prepared by impregnation in our previous studies,9 are also presented in Table 4. The metal surface area of Ni/SiO2-RHA prepared by impregnation is about 3 times as large as that of Ni/SiO2 gel prepared by the same method. As compared with the present work, the differences between Ni/SiO2-RHA and Ni/SiO2 gel by depositionprecipitation are smaller than those by impregnation. This may be due to the interaction between the metal and support of the catalysts prepared by depositionprecipitation being stronger than that in the catalysts prepared by impregnation. The methane turnover number (methane produced per nickel site per second) was introduced to compare quantitatively the catalyst activities of this work to those in the literature. It is found that the methane turnover number of 16.2 × 10-3s-1 measured in this work (CO2 hydrogenation on 3.47 wt % Ni/SiO2-RHA at 773 K) is consistent with the reported value of 5.0 × 10-3s-1 in the work of Weatherbee and Bartholomew1 (CO2 hydrogenation on 3.0 wt % Ni/SiO2 at 550 K). Conclusions The rice husk ash supported nickel catalysts were prepared by deposition-precipitation. These catalysts display high nickel surface area and high dispersion, even at high nickel loading. Ni/SiO2-RHA also exhibits high selectivity in CH4 formation and high activity in hydrogenation of CO2 from H2/CO2 (4/1). These results suggest that rice husk ash is a preferable catalyst support than silica gel. Acknowledgment The authors express their thanks to the National Science Council for its financial support under Project NSC86-2214-E008-015. Literature Cited (1) Weatherbee, G. D.; Bartholomew, C. H. Hydrogenation of CO2 on Group VIII Metals. I. Specific Activity of Ni/SiO2. J. Catal. 1981, 68, 67.

(2) Yesgar, P. W.; Sheintuch, M. Nickel-Catalyzed Methanation Reactions Studied with an in situ Magnetic Induction Method: Experiments and Modeling. J. Catal. 1991, 127, 576. (3) Hindustan Lever Ltd. Method for Preparing a Heterogeneous Highly Active Silica Supported Nickel Catalysts. Indian Patent No. 147090, 1979. (4) Lindfors, L. P.; Salmi, T. Kinetics of Toluene Hydrogenation on a Supported Ni Catalyst. Ind. Eng. Chem. Res. 1993, 32, 34. (5) Anderson, J. A.; Daza, L.; Damyanova, D. S.; Fierro, J. L. G; Rodrigo, M. T. Hydrogenation of Styrene over Nickel/Sepiolite Catalysts. Appl. Catal. A 1994, 113, 75. (6) Chen, J. M.; Chang, F. W. The Chlorination Kinetics of Rice Husk. Ind. Eng. Chem. Res. 1991, 30, 2241. (7) Liou, T. H.; Chang, F. W. The Nitridation Kinetics of Pyrolyzed Rice Husk. Ind. Eng. Chem. Res. 1996, 35, 3375. (8) Liou, T. H.; Chang, F. W.; Lo, J. J. Pyrolysis Kinetics of Acid-Leached Rice Husk. Ind. Eng. Chem. Res. 1997, 36, 568. (9) Chang, F. W.; Hsiao, T. J.; Chung, S. W.; Lo, J. J. Nickel Supported on Rice Husk Ash-Activity and Selectivity in CO2 Methanation. Appl. Catal. A 1997, 164, 225. (10) van Dillen, J. A.; Geus, J. W.; Hermans, L. A. M.; van der Meijen, J. Production of Support Copper and Nickel Catalysts by Deposition-Precipitation. Chem. Soc. 1976, 677. (11) Shackelford, J. F. Introduction to Materials Science for Engineers; Macmillan: New York, 1985; Chapter 3. (12) Sivaraj, C.; Kantarao, P. Characterization of Copper/ Alumina Catalysts Prepared by Deposition-Precipitation Using Urea Hydrolysis I. Nitrous Oxide Decomposition and Reaction of Ethanol. Appl. Catal. 1988, 45, 103. (13) Hermans, L. A. M.; Geus, J. W. Interaction of Nickel Ions with Silica Supported during Deposition-Precipitation. In Preparation of Catalyst II; Delmon, B., Grange, P., Jacobs, P., Poncelet, G., Eds; Studies in Surface Science Catalysis; Elsevier: Amsterdam, 1979; Vol. 3, p 113. (14) Gil, A.; Diaz, A.; Gandia, L. M.; Montes, M. Influence of the Preparation Method and the Nature of the Support on the Stability of Nickel Catalysts. Appl. Catal. A 1994, 109, 167. (15) Coenen, J. W. E. Characterization of the Standard Nickel/ Silica Catalyst Euro Ni-1 II. Chemical Aspects: Precipitation, Reduction and Chemical Analysis. Appl. Catal. 1989, 54, 65. (16) Mills, G. A.; Steffgen, F. W. Catalytic Methanation. Catal. Rev. 1974, 8, 159.

Received for review March 9, 1998 Revised manuscript received June 29, 1998 Accepted July 9, 1998 IE980152R