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

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 Cu_1−xZn_xO 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.