Evidence for the adsorption of molecules at special sites located at copper/zinc oxide interfaces. Part 3.—Fourier-transform infrared study of methyl formate adsorption on reduced and oxidised Cu/ZnO/SiO2 catalysts

Abstract

FTIR spectra are reported of methyl formate adsorbed at 295 K on ZnO/SiO₂, reduced Cu/ZnO/SiO₂ and on Cu/ZnO/SiO₂ which had been preoxidised by exposure to nitrous oxide. Methyl formate on ZnO/SiO₂ gave adsorbed zinc formate species and strongly physisorbed molecular methanol on silica. The comparable reaction of methyl formate with reduced Cu/ZnO/SiO₂ catalyst produced bridging formate species on copper and a diminished quantity of zinc formate relative to that formed on ZnO/SiO₂ catalyst. This effect is explained in terms of site blockage on the ZnO surface by small copper clusters. Addition of methyl formate to a reoxidised Cu/ZnO/SiO₂ catalyst produced a considerably greater amount of formate species on zinc oxide and methoxy groups on copper were detected. The increase in concentration of zinc formate species was rationalised in terms of rearrangement of unidentate copper formate species to become bonded to copper and zinc oxide sites located at the interface between these two components.