Low-valent molybdenum sites as active hydrogen traps in TiO2 for facilitating high-selectivity photocatalytic nitrate synthesis from NO

The photocatalytic utilization of NO to synthesize NO₃^- has received extensive attention. However, the active hydrogen (H*) species produced during the process are capable of reducing NO and NO₃^- to ammonia with a relatively lower value, thereby leading to a reduction in activity and selectivity. Herein, we developed the H* traps strategy by constructed Mo doping TiO₂, where low-valence Mo⁴⁺ acts as an H* trap to inhibit the generation of H* and impede ammonia generation. Mo_(L)-TiO₂ (Mo doping TiO₂ with low-valence Mo⁴⁺) exhibits remarkable NO₃^- generation activity with a yield of 285.61 μmol‧g_cat⁻¹‧h⁻¹ and nearly 100 % selectivity, which is significantly higher than that of Mo_(H)-TiO₂ (Mo doping TiO₂ with high-valence Mo⁶⁺). Experimental and theoretical analyses reveal that low-valent molybdenum has strong hydrogen adsorption energy, which hinders the generation of H* (H⁺+e^-→H*) and causes photogenerated electrons to no longer drive the ammonia synthesis reaction. Instead, photogenerated electrons facilitate the activation of NO to sequentially generate NO^- and cis-N₂O₂^2- intermediates, and these intermediates subsequently react with •O₂^- to form NO₃^-. This research presents a novel approach for enhancing the efficiency of photocatalytic NO conversion and adjusting product selectivity via the low-valent molybdenum driven active hydrogen traps strategy.