Highly mixed phases in ball-milled Cu/ZnO catalysts: An EXAFS and XANES study

Abstract

New highly mixed phases have been identified in Cu/ZnO systems by EXAFS and XANES at both the Cu and Zn K-edge. The phases were generated by ball-milling Cu2O/ZnO mixtures under three different atmospheres of synthetic air (SA), SA + CO2 and CO2. The system milled in CO2 shows disproportionation of Cu2O into Cu-0, 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 nearly half the Cu1+ into Cu2+ with no Cu metal formed. The copper phase in this material is almost entirely amorphous. In SA, a significant amount of Cu2+- and Zn2+-based phases appears to react to form a nanocrystalline/amorphous Cu1-xZnxO (x approximate to 0.3) solid solution. This distorted rock saltlike solid solution, in which Zn and Cu feature different octahedral environments, was never reported before. It is thought to be formed by incorporation of Zn2+ in the Cu fcc sublattice of the cuprite Cu2O matrix and the concomitant oxidation of Cu1+ into Cu2+. The formation of such a highly mixed Cu1-xZnxO phase indicates strong Cu/ Zn interaction in the Cu/ZnO system, which also suggests the presence of highly mixed phases in conventionally prepared activated catalysts.