COMBINED TEMPERATURE-PROGRAMMED DESORPTION AND FOURIER-TRANSFORM INFRARED-SPECTROSCOPY STUDY OF CO2, CO AND H-2 INTERACTIONS WITH MODEL ZNO/SIO2, CU/SIO2 AND CU/ZNO/SIO2 METHANOL SYNTHESIS CATALYSTS

This paper reports Fourier-transform infrared (FTIR) spectra and temperature-programmed desorption (TPD) studies of CO2, CO and H-2 adsorbed on ZnO/SiO2, Cu/SiO2 and Cu/ZnO/SiO2 catalysts. The adsorption of CO2 on ZnO/SiO2 catalyst at 295 K produced weakly held hydrogencarbonate species and chemisorbed carbonate structures. The comparable reaction on reduced Cu/Zn/SiO2 catalyst gave hydrogencarbonate and carbonate species on zinc oxide, and symmetric carbonate species on copper. However substantial quantities of carboxylate were produced only when the copper and zinc oxide components were intimately mixed. Oxygen ion defects were created on the zinc oxide surface in the presence of copper. In addition, carbonate structures were formed at interfacial sites between copper and zinc oxide. The adsorption of CO on reduced Cu/ZnO/SiO2 catalyst was similar to the adsorption of CO on Cu/SiO2 catalyst. There was no evidence to suggest that Cu+ species were stabilized by the zinc oxide component. Hydrogen desorbed recombinatively from a Cu/SiO2 catalyst at 313 K regardless of the adsorption temperature. In contrast hydrogen adsorption on ZnO/SiO2 catalyst was minimal. For Cu/ZnO/SiO2 catalyst spillover of hydrogen onto the zinc oxide surface was observed. Spillover hydrogen could diffuse into the bulk structure of ZnO.