Partial methanol oxidation over copper:: Active sites observed by means of in situ X-ray absorption spectroscopy

Polycrystalline copper is investigated by a new surface sensitive in situ X-ray absorption spectroscopy method at the oxygen K-edge and copper L-2,L-3-edges under, for the first time, dynamic steady-state methanol-oxidation reaction conditions close to those used in practice. It is found that the selective oxidation of methanol to formaldehyde is catalyzed by a copper suboxide phase with a stoichiometry of Cu(x greater than or equal to 10)O. The product gas-phase composition is monitored simultaneously during the gas/solid reaction by using X-ray absorption and conventional mass spectroscopy. Novel atomic oxygen species are observed at the copper surface and sub-/near-surface region under reaction conditions characterized by a chemical bonding that is significantly different from copper (sub) oxides found in the ex situ spectroscopic analysis which contain oxygen atoms interacting strongly with copper 3d states. The abundance of the novel oxygen species is quantitatively correlated with the catalyst activity for the selective oxidation of methanol to formaldehyde. Conclusions are reached about the chemical composition of the active sites for the selective and total oxidation, respectively.