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 Agx-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 Ag0.5-OVs/ZnO-z photocatalyst reaches a high liquid oxygenate yield of 10148 μmol gcat-1 h-1 with 95% selectivity and a high TOF of 2.4 × 106 μmol gAg-1 h-1. This work sheds new light on the design of photocatalytic CH4 oxidation catalysts via manipulation of carrier transfer direction.