Promotion of methanol synthesis and the water-gas shift reactions by adsorbed oxygen on supported copper catalysts

Abstract

The surfaces of the copper metal crystallites of working Cu/ZuO/Al₂O₃ and other copper catalysts are partially oxidised in reaction mixtures for methanol synthesis and the water-gas shift reaction. Work with unsupported polycrystalline copper has confirmed earlier results that copper metal is the active phase in supported copper catalysts. The coverage of adsorbed oxygen, O(a), up to half-monolayer, was determined by reaction with N₂O and it was found to be controlled by the overall reaction CO₂(g)= CO(g)+ O(a). The free energy of formation of O(a) was calculated to be –240 kJ mol^–1 at 513 K. The induction period found in methanol synthesis from CO–CO₂–H₂ mixtures is consistent with the calculated rate of formation of O(a). The role of O(a) in the methanol synthesis and water-gas shift reactions is both as promoter and reaction intermediate. The dissociative chemisorption of hydrogen on copper is promoted by O(a) but this is not necessary for the reactions. Experiments with unsupported polycrystalline copper have shown that O(a) both increases the extent of physisorption of CO₂ and creates new chemisorbed states of CO₂, with desorption energies of 109, 113 and 125 kJ mol^–1. O(a) is also essential for the dissociative chemisorption of water on copper. A regenerative mechanism for the water-gas shift reaction on copper [involving the formation and reaction of O(a)] has been established by observation of the separate stages. The adsorbed formate intermediate, pivotal in methanol synthesis from carbon dioxide, is irrelevant to the water-gas shift reaction.