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 Cu(2)O/ZnO mixtures under three different atmospheres of synthetic air (SA), SA + CO(2) and CO(2). The system milled in CO(2) shows disproportionation of Cu(2)O into Cu(0), Cu(1+) (cuprite Cu(2)O-type phase) and Cu(2+) (tenorite CuO-type phase), while most of the Zn(2+) 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 CO(2) atmosphere, ball milling results in the oxidation of nearly half the Cu(1+) into Cu(2+) with no Cu metal formed. The copper phase in this material is almost entirely amorphous. In SA, a significant amount of Cu(2+)- and Zn(2+)-based phases appears to react to form a nanocrystalline/amorphous Cu(1-x)Zn(x)O (x approximately 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 Zn(2+) in the Cu fcc sublattice of the cuprite Cu(2)O matrix and the concomitant oxidation of Cu(1+) into Cu(2+). The formation of such a highly mixed Cu(1-x)Zn(x)O 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.