Promoting the efficiency and selectivity of NO₃^--to-NH₃ reduction on Cu-O-Ti active sites via preferential glycol oxidation with holes.

Abstract

The combined reductive and oxidative reaction is the essence of a solar-driven photoredox system. Unfortunately, most of these efforts focus on the specific half-reactions, and the key roles of complete photoredox reactions have been overlooked. Taking the nitrate reduction reaction (NO₃^-RR) as a typical multiple-electrons involved process, the selective reduction of the NO₃^- into ammonia (NH₃) synthesis with high efficiency is still a grand challenge. Herein, a rational oxidative half-reaction is tailored to achieve the selective conversion of NO₃^- to NH₃ on Cu-O-Ti active sites. Through the coupled NO₃^-RR with glycol oxidation reaction system, a superior NH₃ photosynthesis rate of 16.04 ± 0.40 mmol g_cat^-1 h^-1 with NO₃^- conversion ratio of 100% and almost 100% of NH₃ selectivity is reached on Cu-O-Ti bimetallic oxide cluster-anchored TiO₂ nanosheets (CuO_x@TNS) catalyst. A combination of comprehensive in situ characterizations and theoretical calculations reveals the molecular mechanism of the synergistic interaction between NO₃^-RR and glycol oxidation pair on CuOx@TNS. The introduction of glycol accelerates the h⁺ consumption for the formation of alkoxy (•R) radicals to avoid the production of •OH radicals. The construction of Cu-O-Ti sites facilitates the preferential oxidation of glycol with h⁺ and enhances the production of e^- to participate in NO₃^-RR. The efficiency and selectivity of NO₃^--to-NH₃ synthesis are thus highly promoted on Cu-O-Ti active sites with the accelerated glycol oxidative half-reaction. This work upgrades the conventional half photocatalysis into a complete photoredox system, demonstrating the tremendous potential for the precise regulation of reaction pathway and product selectivity.