Evidence for the adsorption of molecules at special sites located at copper/zinc oxide interfaces. Part 2.—A fourier-transform infrared spectroscopy study of methanol adsorption on reduced and oxidised Cu/ZnO/SiO2 catalysts

Abstract

FTIR spectra are reported of methanol adsorbed at 295 K on ZnO/SiO₂, on reduced Cu/ZnO/SiO₂ and on Cu/ZnO/SiO₂ which had been preoxidised by exposure to nitrous oxide. Methanol on ZnO/SiO₂ gave methoxy species on ZnO and SiO, in addition to both strongly and weakly physisorbed methanol on SiO₂. The corresponding adsorption of methanol on reduced Cu/ZnO/SiO₂ also gave methoxy species on Cu and a small amount of bridging formate. Reaction of methanol with a reoxidised Cu/ZnO/SiO₂ catalyst resulted in an enhanced quantity of methoxy species on Cu. Heating adsorbed species on Cu/ZnO/SiO₂ at 393 K led to the loss of methoxy groups on Cu and the concomitant formation of formate species on both ZnO and Cu. The comparable reaction on a reoxidised Cu/ZnO/SiO₂ catalyst gave an increased amount of formate species on ZnO and this correlated with an increased quantity of methoxy groups lost from Cu. An explanation is given in terms of adsorption of formate and formaldehyde species at special sites located at the copper/zinc oxide interface.