A density functional theory study on reduction-induced structural transformation of copper-oxide-based oxygen carrier

A clear understanding of the structural transformation of copper-oxide-based oxygen carriers accompanying their reduction by fuels helps to design more efficient oxygen carriers for chemical looping combustion. Herein, density functional theory calculations have been performed on the bulk CuO, CuO(111) surface, and (CuO)₃₂ cluster models with the same number of CuO molecular units to investigate structural transformation accompanying the reduction. The results showed that the averaged reaction energies of desorbing an oxygen molecule from the bulk and surface models are roughly the same [246.2 kJ/(mol O₂) and 245.9 kJ/(mol O₂), respectively]. The slab model does not significantly lower the overall reaction energy compared to the bulk model. In contrast, the averaged reaction energy using the cluster model is significantly lower [127.5 kJ/(mol O₂)] than that of bulk and slab models. The key structural difference is the obvious Cu-Cu bond formation in the cluster model, which would result in nucleation of a metallic Cu phase. The results also showed that different states can be reached by desorbing different number oxygen atoms in a single step, corresponding to different reaction rates, when the system reaches the same level of reduction. These results demonstrate the complexity of reactions involving solid state materials and are consistent with the structural diversity observed experimentally. This study illustrates the importance of particle sizes and reaction conditions in the formation of suboxides during CuO reduction.