Key Role of Surface Hydroxyl Groups in C-O Activation during Fischer-Tropsch Synthesis

C-O activation is a crucial step in Fischer-Tropsch synthesis (FTS). Several pathways have been proposed to activate CO, namely, direct C-O dissociation, activation via hydrogenation, and activation by insertion into growing chains. Invariably, very high barriers are calculated for both direct C-O dissociation and for hydrogenation at the O atom in CO* and RCO*, while hydrogenation at the C atom leads to oxygenates. We demonstrate that surface hydroxyl groups open a new pathway for CO* and RCO* activation via proton transfer to the O atom. In combination with the CO insertion mechanism, the calculated rate for this new pathway is consistent with the selectivity in FTS, and is in agreement with the kinetic effect of water. Hydroxyl group formation from O* is sufficiently fast to be quasi-equilibrated, and is much faster than CO2 formation. The role of surface hydroxyl groups as hydrogenating species is likely general, and involved in several oxygenate transformation reactions.