SSITKA INVESTIGATION OF PALLADIUM PRECURSOR AND SUPPORT EFFECTS ON COHYDROGENATION OVER SUPPORTED PD CATALYSTS

CO hydrogenation on supported Pd catalysts was investigated using stea dy-state isotopic transient kinetic analysis (SSITKA) in order to expl ore the differences in catalytic behavior which have been previously r eported by others to result from using different supports and Pd precu rsors. In this study, two different precursors [PdCl2 and Pd(NO3)(2)] and three supports [SiO2, Al2O3, and SiO2-Al2O3] Were investigated. Th e dispersion of the Pd particles on the prepared catalysts, as evaluat ed by CO chemisorption, was not affected by either the Pd precursor or the support used. In addition, for a given support the catalysts exhi bited almost the same selectivities during CO hydrogenation toward met hane, methanol [MeOH], and dimethyl ether [DME]. However, catalysts pr epared using PdCl2 showed higher overall activities than those prepare d using Pd(NO3)(2) for a given support. The nature of the support alte red significantly the activity and the selectivity of the Pd. The orde r of activities at steady-state for a given precursor was Al2O3 > SiO2 -Al2O3 > SiO2. The activity of the Pd/SiO2-Al2O3 was lower than expect ed probably due to the relatively high level of S (500 ppm) present in the support as an impurity. It was found that Pd/SiO2 produced essent ially only MeOH. The use of acidic supports [Al2O3 and SiO2-Al2O3] res ulted 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 si gnificant amounts of methane. SSITKA results showed [after considering readsorption effects] that the Pd precursor used, or more specificall y its anion [Cl- and NO3-], had a slight effect on the intrinsic activ ity of the sites producing methane (1/tau(methane)) [Cl- > NO3-] and a significant effect on the surface coverage of the intermediates leadi ng to methane (N-methane) [Cl- > NO3-]. On the other hand, the precurs or used had little or no effect on the intrinsic activity of the sites producing MeOH (1/tau(MeOH)) or the surface coverage of MeOH intermed iates (N-MeOH) The nature of the support altered only slightly the int rinsic activities of the sites producing methane and MeOH. However, th e number of surface intermediates leading to methane and MeOH were sig nificantly affected by the support used and were in the order Al2O3 > SiO2-Al2O3 and Al2O3 > SiO2-Al2O3 > SiO2, respectively. It is, thus, t he effect of the support on the concentration of active sites/intermed iates on Pd that determines the impact of the support on the reaction rate. (C) 1998 Academic Press.