Reduction of CuO and Cu2O with H2:H embedding and kinetic effects in the formation of suboxides

Time-resolved X-ray diffraction, X-ray absorption fine structure, and first-principles density functional calculations were used to investigate the reaction of CuO and Cu2O with H-2 in detail. The mechanism for the reduction of CuO is complex, involving an induction period and the embedding of H into the bulk of the oxide. The in-situ experiments show that, under a normal supply of hydrogen, CuO reduces directly to metallic Cu without formation of an intermediate or suboxide (i.e., no Cu4O3 or Cu2O). The reduction of CuO is easier than the reduction of Cu2O. The apparent activation energy for the reduction of CuO is about 14.5 kcal/mol, while the value is 27.4 kcal/mol for Cu2O. During the reduction of CuO, the system can reach metastable states (MS) and react with hydrogen instead of forming Cu2O. To see the formation Of Cu2O, one has to limit the flow of hydrogen, slowing the rate of reduction to allow a MS Cu2O transformation. These results show the importance of kinetic effects for the formation of well-defined suboxides during a reduction process and the activation of oxide catalysts.