Effect of the CO coverage on the Fischer-Tropsch synthesis mechanism on cobalt catalysts

摘要

Fischer-Tropsch (FT) synthesis, the conversion of CO and H_2 to long-chain hydrocarbons, is performed at relatively low temperatures and high pressures. Density functional theory shows that the CO coverage on Co(0 0 0 1) increases gradually until a (√3 × √3)R30o-CO configuration (1/3 ML) is formed. This structure is stable over a relatively wide temperature and pressure range near FT conditions, until a phase transition to a (2√3 × 2√3)R30o-7CO structure occurs at high CO pressures. The 1/3 ML CO coverage reduces the H_2 binding enthalpy from -121 to -74 kJ/mol and reduces the hydrogen coverage to <0.3 ML. The 1/3 ML coverage reduces the stability of reaction intermediates by 10-30 kJ/mol. For the CO insertion mechanism, where RCH~- groups couple with CO, the reduced stabilities decrease the overall surface barrier from 175 to 111 kJ/mol. The reduced barrier increases the CO turnover frequency to 0.02 s ~(-1), close to experimental values and five orders of magnitude higher than the corresponding low-coverage value.