Unconventional Carrier Transfer Enabled Direct O2 Reduction by Oxygen Vacancies for Efficient Photooxidation of CH4

Photocatalytic oxidation of methane (CH₄) to high value-added liquid oxygenates with oxygen (O₂) as an oxidant has long been a promising process. In addition to the activation of CH₄, the selective reduction of O₂ to hydroperoxyl (^·OOH) is also a crucial factor in determining the efficiency and selectivity of the overall reaction. In thermal catalysis, strong interactions between oxygen vacancies (OVs) on the catalyst surface and O₂ are widely adopted as a means of enhancing O₂ activation. However, reports of this phenomenon are limited in photocatalysis; this is attributable to the fact that the reduction of O₂ is dependent upon its reaction with photogenerated electrons (e^–). For the majority of photocatalysts, the transfer of e^– to metal cocatalysts results in the spatial separation of e^– from OVs. Therefore, the intrinsic affinity between OVs and O₂ is not fully exploited. In this study, we discovered that H₂ treatment at elevated temperatures on ZIF-8-derived ZnO led to the formation of an OV-enriched amorphous layer on the surface. After the introduction of the Ag cocatalyst, the as-prepared Ag_x-OVs/ZnO exhibited an unconventional carrier transfer direction, whereby photogenerated e^– and holes (h⁺) are transferred to OVs and Ag, respectively. As a result, the preferential O₂ adsorption and activation sites can be spatially combined together on the OVs and thus drive the reduction of O₂ to realize efficient and directed generation of ^·OOH. In parallel, the transfer of h⁺ to the Ag sites weakens its oxidation capability and avoids overoxidation while ensuring CH₄ activation. The optimized defect-rich Ag_0.5-OVs/ZnO-z photocatalyst reaches a high liquid oxygenate yield of 10148 μmol g_cat^–1 h^–1 with 95% selectivity and a high TOF of 2.4 × 10⁶ μmol g_Ag^–1 h^–1. This work sheds new light on the design of photocatalytic CH₄ oxidation catalysts via manipulation of carrier transfer direction.