New highly mixed phases in ball-milled Cu/ZnO catalysts as established by EXAFS and XANES

Abstract

AnXAFS investigation at both the Cu and Zn K-edge has allowed to unravel new highly mixed phases in Cu/ZnO catalysts prepared by ball-milling mixtures of Cu2O and ZnO under 3 different atmospheres of synthetic air (SA), SA+CO2 and CO2. The system milled in CO2 shows the disproportionation of Cu2O into Cu0, Cu1+ (cuprite Cu2O-type phase) and Cu2+ (tenorite CuO-type phase), while most of the Zn2+ is transformed into a nanocrystalline / amorphous ZnO-type zincite that forms a superficial mixture of oxide and carbonate phases. When synthetic air is added to the CO2 atmosphere, ball-milling results in the oxidation of part of the Cu1+ into Cu2+ with no Cu metal formed. In SA, a significant amount of Cu2+- and Zn2+-based phases react to form a nanocrystalline / amorphous Cu1-xZnxO solid solution that was never reported before. This distorted rock salt-like solid solution, in which Zn and Cu feature different octahedral environments, is formed by incorporation of Zn2+ in the Cu2O matrix and the concomitant oxidation of Cu1+ into Cu2+ and results from strong Cu/Zn interactions in the Cu/ZnO system.