Efficient Visible-Light-Driven CO2 Methanation with Self- Regenerated Oxygen Vacancies in Co3O4/NiCo2O4 Hetero- Nanocages: Vacancy-Mediated Selective Photocatalysis

Surface atomic vacancies in semiconductor photo catalysts are highly attractive for improving catalysis efficiency and product selectivity, but the underlying mechanism of vacancy mediated selectivity still remains ambiguous. By constructing a type of direct Z-scheme Co3O4/NiCo2O4 hetero-nanocage (HNC) that accommodates three kinds of possible oxygen vacancies (VOs), a comprehensive study was performed to unravel the roles of vacancies and demonstrate the mechanism of efficient visible-light driven carbon dioxide (CO2) methanation. Upon light irradiation, efficient separation of charge carriers occurs in the Z-scheme Co3O4/NiCo2O4 HNCs, leading to the transfer of an electron to NiCo2O4. It has been identified for NiCo2O4 that only the vacancy VO2 over three cations (Co, Co, and Ni) at octahedral sites could facilitate the methanation process and possess the behavior of self-regeneration. Intriguingly, after the release of the product CH4 from NiCo2O4-VO2, the remaining oxygen (*O) favorably combines with protons and electrons to produce water molecules, and therefore, VO2 vacancies are regenerated, which significantly improves the durability of the methanation process. Besides, Ni atoms are found to be critical in initiating the CO2 methanation process by upshifting the d-band center of Co in NiCo2O4-VO2 toward the Fermi level and reducing the energy barrier of the *CHO intermediate. As a result, the main product of CO2 reduction is switched from CO for Co3O4 to CH4 for NiCo2O4, and the optimized photocatalyst exhibits an impressive single-carbon (C1) compound formation rate of 20.32 mu mol g-1 h-1 and a high CH4 selectivity of up to 96.3%, outperforming the Co-/Ni-based photocatalysts. This work offers an in-depth insight into the precise atomic-level regulation of the photocatalytic selectivity and stability of Co3O4/ NiCo2O4 HNCs and opens a path for the development of robust CO2 reduction photocatalysts.