Efficient Visible-Light-Driven CO<sub>2</sub> Methanation with Self-Regenerated Oxygen Vacancies in Co<sub>3</sub>O<sub>4</sub>/NiCo<sub>2</sub>O<sub>4</sub> Hetero-Nanocages: Vacancy-Mediated Selective Photocatalysis
Maomao Ni, Yijia Zhu, Changfa Guo, De‐Li Chen, Jiqiang Ning, Yijun Zhong, Yong Hu
Abstract
Surface atomic vacancies in semiconductor photocatalysts 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 Co 3 O 4 /NiCo 2 O 4 hetero-nanocage (HNC) that accommodates three kinds of possible oxygen vacancies (V O s), a comprehensive study was performed to unravel the roles of vacancies and demonstrate the mechanism of efficient visible-light-driven carbon dioxide (CO 2 ) methanation. Upon light irradiation, efficient separation of charge carriers occurs in the Z-scheme Co 3 O 4 /NiCo 2 O 4 HNCs, leading to the transfer of an electron to NiCo 2 O 4 . It has been identified for NiCo 2 O 4 that only the vacancy V O 2 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 CH 4 from NiCo 2 O 4 -V O 2, the remaining oxygen (*O) favorably combines with protons and electrons to produce water molecules, and therefore, V O 2 vacancies are regenerated, which significantly improves the durability of the methanation process. Besides, Ni atoms are found to be critical in initiating the CO 2 methanation process by upshifting the d-band center of Co in NiCo 2 O 4 -V O 2 toward the Fermi level and reducing the energy barrier of the *CHO intermediate. As a result, the main product of CO 2 reduction is switched from CO for Co 3 O 4 to CH 4 for NiCo 2 O 4, and the optimized photocatalyst exhibits an impressive single-carbon (C 1 ) compound formation rate of 20.32 μmol g –1 h –1 and a high CH 4 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 Co 3 O 4 /NiCo 2 O 4 HNCs and opens a path for the development of robust CO 2 reduction photocatalysts.