Dual mechanisms in hydrogen reduction of copper oxide: surface reaction and subsurface oxygen atom transfer

The study of the reduction of copper oxide (CuO) by hydrogen (H₂) is helpful in elucidating the reduction mechanism of oxygen carriers. In this study, the reduction mechanism of CuO by H₂ and the process of oxygen atom transfer were investigated through the density functional theory (DFT) method and thermodynamic calculation. DFT calculation results showed that during the reaction between H₂ and the surface of CuO, Cu underwent a Cu²⁺ → Cu¹⁺ → Cu⁰ transformation, the Cu-O bond (-IpCOHP = 2.41) of the Cu₂O phase was more stable than that (-IpCOHP = 1.94) of the CuO phase, and the reduction of Cu₂O by H₂ was more difficult than the reduction of CuO. As the surface oxygen vacancy concentration increased, it was more likely that the subsurface O atoms transfer to the surface at zero H₂ coverage (no H₂ molecule on the surface), allowing the surface to maintain a stable Cu₂O phase. However, when the H₂ coverage was 0.25 monolayer (ML) (one H₂ molecule every four surface Cu atoms), the presence of H atoms on the surface made the upward transfer of O atoms from the subsurface more difficult. The rate of consuming surface O atoms in the reduction reaction was greater than the rate of subsurface O atom transfer induced by the reduction reaction and the surface Cu₂O phase could not be maintained stably. Through thermodynamic analysis, at high H₂ concentration, the reaction between H₂ and CuO was more likely to generate Cu, while at low H₂ concentration, it was more likely to generate Cu₂O. In summary, the valence state of Cu in the reaction process between CuO and H₂ depended on the concentration of H₂.