Fischer-Tropsch synthesis over Ru-group Ib/SiO 2 catalysts

Abstract

Reaction studies probing the catalytic activity and selectivity of the Fischer-Tropsch synthesis reaction have been performed for a series of chlorine-free, silica-supported Ru-group Ib (Ag, Au, Cu) bimetallic catalysts. The goal of this work was to isolate ensemble effects, electronic effects, or the influence of the metal particle structure on catalytic behavior. The rate per surface ruthenium atom, or turnover frequency, was used to correlate catalytic activity with surface composition. Surface compositions were determined previously via nuclear magnetic resonance of chemisorbed hydrogen and strong hydrogen chemisorption experiments. The turnover frequencies for the series of RuCu catalysts declined by a factor of about two when the relative copper composition increased from 0% to 10% (at constant ruthenium loading). Increasing the copper loading beyond this level did not affect the turnover frequency. Similar behavior was noted for the RuAg catalysts where the turnover frequency declined until a silver loading of about 25% was obtained. Increased silver content did not further change the turnover frequency. The drop in turnover frequency with relatively small amounts of copper or silver is attributed to a structure sensitivity effect. As the segregating element (lb) preferentially goes to the low-coordination, defect-like sites, the observed turnover frequency declines. Once all low-coordination sites are filled, no variation in turnover frequency with composition was observed, indicating a lack of ensemble effects. This result contrasts with previously reported investigations of the RuCu system. The overall rate changed modestly and the turnover frequency did not change significantly with composition over the series of RuAu catalysts. The product distribution shifted to smaller molecules with increasing copper content in the RuCu series but remained constant over the entire composition range for the RuAg and RuAu catalysts. The ability of hydrogen to exchange between copper and ruthenium sites was correlated with enhanced chain termination rates.