CO hydrogenation on Ru/Al2O3: Selectivity under transient conditions

Abstract

Transient response and isotopic tracer techniques were used to investigate the surface coverages of the reactants and the selectivity of ruthenium/alumina catalysts in CO hydrogenation. Two forms of carbon--active carbidic carbon, C[alpha], and hydrogen-containing alkylic carbon, C[beta]--are found to be present on the catalyst surface. Approximately 0.1 monolayer of C[alpha] is deposited on the catalyst through the dissociative chemisorption of CO. C[beta] is formed only in the presence of hydrogen. Unlike C[alpha], the amount of C[beta] on the catalyst surface is found to increase continuously. Under varying partial pressures of CO and H2 the surface coverage of C[alpha] changes much less than the overall rate of reaction. In addition to C[alpha] and C[beta], the catalyst surface also contained approximately 0.7 monolayer of CO and 0.1 to 0.2 monolayer of inactive carbon. Measurements of product distributions of C[alpha] and C[beta] during the hydrogen titration transients show that for C[alpha] the Schultz-Flory distribution is obeyed. In contrast, C[beta] products do not follow the same distribution. The relative selectivities of the two species of carbon during hydrogen titration transients were very different. C[beta] titration produced normal and branched alkanes in comparable amounts. No alkene is produced from C[beta]. C[alpha] titration produced primarily normal alkenes and alkanes. Our results also show that the chain propagation step is much faster than the other steps in hydrocarbon synthesis (Fischer-Tropsch synthesis) and that CO dissociation is not the rate-limiting step.