Mechanisms of H- and OH-assisted CO activation as well as C–C coupling on the flat Co(0001) surface – revisited

Abstract

The mechanisms of H- and OH-assisted CO activation and the consecutive C–C coupling on the flat Co(0001) surface have been computed at the level of periodic RPBE density functional theory. The potential energy surfaces show that the OH-assisted route with the formation of COH, CHOH and CHO is kinetically more preferred than the H-assisted route with CHO formation; however, both routes coincide at the same point with the CHO intermediate which leads to CH₃OH formation. Considering the rather low surface OH coverage under H-rich conditions, the OH-assisted route might not be operative. Along with the minimum energy path of CH₃OH formation [CO + 4H → CHO + 3H → CH₂O + 2H → CH₃O + H → CH₃OH], the dissociation and the consecutive C–C coupling [CHO → CH + O and CH + CO; CH₂O → CH₂ + O and CH₂ + CO; CH₃O → CH₃ + O and CH₃ + CO] are not competitive kinetically and thermodynamically. This indicates that the flat Co(0001) surface does not represent the active site for Co-based Fischer–Tropsch synthesis; this is supported by the experimentally observed surface reconstruction and carbide formation under reaction conditions. In addition, water-gas shift reaction [CO + OH → COOH → CO₂ + H; CO + O → CO₂] is also not operative on the Co(0001) surface.