STRUCTURAL EFFECTS IN FISCHER-TROPSCH SYNTHESIS OVER BIMETALLIC SUPPORTED CATALYSTS.

摘要

The effect of copper on the activity/selectivity of Ru/SiO(,2) catalysts in CO hydrogenation has been investigated in terms of geometric and/or electronic effects. The bimetallic catalysts of Cu to Ru atomic ratios ranging between zero to one were prepared by aqueous and non-aqueous impregnation techniques. Two series of catalysts were prepared: the aqueous impregnation method resulted in the Ru dispersion of 4.6%, while non-aqueous method gave Ru dispersion of 31%. The catalysts were characterized using x-ray line broadening, hydrogen chemisorption, temperature-programmed desorption, and transmission electron microscopy techniques. The catalysts were studied in a continuous flow, fixed-bed differential reactor. The catalyst behavior is markedly different for the low- and high-dispersion of Ru; the effect of Cu is dependent on this dispersion. The observed behavior permits us to separate the "geometric effects" from the "electronic effects."; For low-dispersion Ru-Cu/SiO(,2) catalysts, the effect of Cu has been observed to be primarily geometric through the blockage of the active Ru surface sites. The changes observed can be explained in terms of a reduction in the Ru surface area as measured by hydrogen chemisorption, as well as several other observations: (1) the methanation turnover numbers and the activation energy of methanation over Ru remain constant, (2) the hydrocarbon product distribution is unaffected by the presence of Cu, and (3) the CO bonding to Ru surface is unaffected by the presence of Cu.; For high-dispersion Ru-Cu/SiO(,2) catalysts, the changes observed are interpreted in terms of interactions between Ru and Cu of an "electronic" nature. This is based on the following observations: (1) the methantion turnover numbers and the activation energy of methantion over Ru, both change in the presence of Cu, (2) the product distribution, namely the higher hydrocarbons (C(,5)('+)/C(,1)), is influenced by the presence of Cu, and (3) the CO bonding undergoes a change due to Cu as shown by the temperature-programmed desorption study. Furthermore, these studies also shed some light on the intermediates involved in the Fischer-Tropsch reactions.