Origin of selectivity switch in Fischer-Tropsch synthesis over Ru and Rh from first-principles statistical mechanics studies.

For its unique position in chemical industry, Fischer-Tropsch (FT) synthesis has been a hot subject in heterogeneous catalysis. Due to its great complexity in product distribution, it remains unclear how to maximally convert syngas to long-chain hydrocarbons. By combining extensive DFT calculations with grand canonical Monte Carlo simulations, this work examines the key elementary steps in FT synthesis over Ru and Rh surfaces, including CO dissociation, C/C coupling, and hydrogenations. The origin of the relationship between activity and selectivity of catalysts is revealed based on the calculated reaction rate at working temperatures, in which the catalytic role of surface steps as the center of accumulating surface CH(x) species is highlighted. This theoretical work demonstrates that the ability to dissociate CO under carbon-rich conditions is the key requirement for a good FT catalyst. The RC + C (R = alkyl or H) pathway occurring at surface steps may be a general mechanism for FT chain propagation on transition metals.