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

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(0001) increases gradually until a (root 3 x root 3)R30 degrees-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 root 3 x 2 root 3)R30 degrees-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. (c) 2012 Elsevier Inc. All rights reserved.