Time-resolved Studies for the Mechanism of Reduction of Copper Oxides with Carbon Monoxide: Complex Behavior of Lattice Oxygen and the Formation of Suboxides

Abstract

Temperature-programmed reduction (TPR), synchrotron-based time-resolved X-ray diffraction (TR-XRD), extended X-ray absorption fine-structure (EXAFS), and X-ray absorption near-edge structure (XANES) were employed in this work to systematically study the reaction of CuO and Cu2O with CO gas molecules. Both TPR and isothermal reduction results showed that CuO was easier to reduce than Cu2O under the same reaction conditions. In situ measurements of XRD and XANES showed a direct transformation pathway for CuO reduction (CuO → Cu) when there was a large supply of CO, while they showed a sequential step pathway involving one intermediate (CuO → Cu2O → Cu) with a limited supply of CO. An induction period for CuO reduction was seen and increased with decreasing temperature. O vacancies in CuO were observed during the induction period that could foster formation of a nonstoichiometric metastable copper-oxide species. The metastable species can either react rapidly with CO to form metallic copper during high CO fl...