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 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 VO 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 CH4 from NiCo2O4-VO 2, the remaining oxygen (*O) favorably combines with protons and electrons to produce water molecules, and therefore, VO 2 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-VO 2 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 μmol 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.

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Designing all-solid heterogeneous catalysts with frustrated Lewis pairs (FLPs) has aroused great attentions recently because of its appealing low dissociation energy for H2 molecule and thus a promotion of hydrogenation reaction is expected. The sterically encumbered Lewis acid (metal site) and base (nitrogen site) in the cavity of single transition metal atom doped M/C2N sheet makes it potential candidate with FLP, while a comprehensive understanding of its intrinsic property and reactivity is still required. Calculations show that the complete dissociation of H2 molecule into two H* at the N sites requires two steps, i.e., heterolytic cleavage of H2 molecule and the transfer of H* from metal site to N site, which are highly related to the acidity of the metal site. The Ni/C2N and Pd/C2N, which outperform over the other 8 transition metal atom (M) anchored M/C2N candidates, possess low energy barriers for the complete dissociation of H2 molecule, with values of only 0.30 and 0.20 eV, respectively. Furthermore, both Ni/C2N and Pd/C2N catalysts can achieve semi-hydrogenation of C2H2 into C2H4, with overall barriers of 0.81 and 0.75 eV, respectively, lower than many reported catalysts. It is speculated that M/C2N catalysts with intrinsic FLPs may also find applications in other important hydrogenation reaction.

Improving the reaction selectivity and activity for challenging substrates such as nitroaromatics bearing two reducible functional groups is important in industry yet remains a great challenge using traditional metal nanoparticle...