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Cu-based materials have been regarded as suitable oxygen carrier (OC) candidates in chemical looping combustion as a result of their high reactivity. Because Cu is a common reduction product in the fuel reactor and can be reoxidized to CuO in the air reactor, obtaining insights into the complete oxidation process of Cu at a microcosmic level is critical for exploring the intrinsic oxidation mechanism and kinetics. In this work, the detailed oxidation steps have been investigated by density functional theory calculations. First, the most likely dissociative adsorption pathways of oxygen molecules on the Cu(111) surfaces are determined. On the basis of the Mulliken charge analysis and partial density of state analysis, the oxygen uptake process would preferentially produce a CuO nano-island rather than Cu2O on the surface. Then, ions (O2– and Cu2+) diffusion pathways are examined to explore the details of oxide growth. Oxygen inward diffusion leads to the formation of Cu2O, however with quite high energy barriers. On the contrary, the horizontal diffusion of copper atoms on the surface is quite easy in kinetics and thermodynamics, corresponding to an epitaxial growth of the CuO oxide nano-island and then the formation of an exterior thin CuO layer. Furthermore, the continuous outward diffusion (replenishing copper atoms for the epitaxial growth) of copper atoms in the Cu or Cu2O bulk is also considered. Results show that the energy barrier of each diffusion step in the Cu(111) bulk is smaller than that in the Cu2O(111) bulk, indicating that the bulk Cu2O phase preferentially forms as a result of copper outward diffusion in the Cu(111) bulk and then is oxidized to generate the bulk CuO phase eventually. In such a way, a complete oxidation mechanism of Cu → Cu2O → CuO is elucidated, which has been validated by a well-designed thermogravimetric experiment of controllable oxidation of pure copper.
Zheng et al. (Mon,) studied this question.