SSITKA investigation of palladium precursor and support effects on CO hydrogenation over supported Pd catalysts

CO hydrogenation on supported Pd catalysts was investigated using steady-state isotopic transient kinetic analysis (SSITKA) in order to explore the differences in catalytic behavior which have been previously reported by others to result from using different supports and Pd precursors. In this study, two different precursors [PdCl(2) and Pd(NO(3))(2)] and three supports [SiO(2), Al(2)O(3), and SiO(2)-Al(2)O(3)] Were investigated. The dispersion of the Pd particles on the prepared catalysts, as evaluated by CO chemisorption, was not affected by either the Pd precursor or the support used. In addition, for a given support the catalysts exhibited almost the same selectivities during CO hydrogenation toward methane, methanol [MeOH], and dimethyl ether [DME]. However, catalysts prepared using PdCl(2) showed higher overall activities than those prepared using Pd(NO(3))(2) for a given support. The nature of the support altered significantly the activity and the selectivity of the Pd. The order of activities at steady-state for a given precursor was Al(2)O(3) > SiO(2)-Al(2)O(3) > SiO(2). The activity of the Pd/SiO(2)-Al(2)O(3) was lower than expected probably due to the relatively high level of S (500 ppm) present in the support as an impurity. It was found that Pd/SiO(2) produced essentially only MeOH. The use of acidic supports [Al(2)O(3) and SiO(2)-Al(2)O(3)] resulted in the formation of DME due to the secondary reaction of MeOH on acid sites of the supports. DME production was found to be limited by the amount of MeOH formed. Pd on the acidic supports also produced significant amounts of methane. SSITKA results showed [after considering readsorption effects] that the Pd precursor used, or more specifically its anion [Cl(-) and NO(3)(-)], had a slight effect on the intrinsic activity of the sites producing methane (1/tau(methane)) [Cl(-) > NO(3)(-)] and a significant effect on the surface coverage of the intermediates leading to methane (N(methane)) [Cl(-) > NO(3)(-)]. On the other hand, the precursor used had little or no effect on the intrinsic activity of the sites producing MeOH (1/tau(MeOH)) or the surface coverage of MeOH intermediates (N(MeOH)) The nature of the support altered only slightly the intrinsic activities of the sites producing methane and MeOH. However, the number of surface intermediates leading to methane and MeOH were significantly affected by the support used and were in the order Al(2)O(3) > SiO(2)-Al(2)O(3) and Al(2)O(3) > SiO(2)-Al(2)O(3) > SiO(2), respectively. It is, thus, the effect of the support on the concentration of active sites/intermediates on Pd that determines the impact of the support on the reaction rate. (C) 1998 Academic Press.