FTIR studies of dynamic surface structural changes in Cu-based methanol synthesis catalysts

FTIR studies of CO adsorption have been carried out on Cu supported on ZnO, Al2O3, and SiO2. The reducibility of Cu as well as the surface structures of the metallic Cu particles formed were found to differ on the various supports. The active state of copper was found to be metallic-like in all the systems. The relative ease of reduction of Cu and the specific methanol synthesis activity decrease according to: Cu/ZnO > Cu/Al2O3 much greater than Cu/SiO2. Changes in the reduction potential of the gas result in reversible surface structural changes for Cu/ZnO but not for Cu/Al2O3 or Cu/SiO2. Under moderate reducing condition, the CO adsorption results suggest that the Cu crystals on ZnO expose predominantly (110)facets. Under more severe reducing conditions, significant changes in the CO adsorption were observed and these cannot solely be explained by changes in the relative distribution of Cu surface planes. Instead, the IR transmission and CO adsorption results suggest that the higher reducing potential will lead to an increased number of oxygen vacancies at the ZnO surface and increased spreading of the Cu crystallites. Moreover, the large downward shifts in the CO band observed under severe reduction conditions suggest migration of zinc onto the surface of the Cu particles resulting in the formation of a Cu-Zn surface alloy. The proposal is seen to be in accord with recent surface science and theoretical investigations. The tendency to form the Cu-Zn surface alloy depends critically on the reduction potential of the gas. It is suggested that the recent observations of dynamic changes in the structural and catalytic properties of methanol synthesis catalysts is influenced by reversible reaction involving formation and destruction of the surface alloy. (C) 1999 Elsevier Science B.V. All rights reserved.