Synergy between copper and zinc oxide during methanol synthesis. Transfer of activating species

The nature of the synergy between Cu and ZnO has been examined using temperature-programmed desorption and reaction techniques. Adsorption of CO–CO2–H2 on Cu/SiO2 and on ZnO/SiO2 results in the formation of surface intermediates which are distinguishable by different activation energies for their dissociation into CO2 and H2. A mixture of the two catalysts does not simply exhibit the characteristics of both components. After adsorption of the gas-feed on a physical mixture at 325 K, CO2 and H2 are desorbed separately. It is proposed that, during the dissociation of the species on the Cu surface, hydrogen can spill acoross the silica, causing the zinc oxide to become hydrided and hydroxylated. Further, if there is intimate contact between the phases, it could be envisaged that the hydroxyl species contribute to the creation of bidentate Cu-methanoates from CO2 and even CO. This would explain the observation that when CO–H2(at 101 325 Pa) is passed over the catalysts during programmed heating, a sample prepared by coprecipitation is the only one to generate methanol.