A gas–liquid switching reaction system for normothermic photo-driven chemical looping ammonia synthesis

Abstract

Chemical looping ammonia synthesis (CLAS) has recently emerged as a promising approach for sustainable ammonia synthesis. However, it requires a high operating temperature and carefully designed nitrogen carriers. Herein, we developed a gas–liquid switching reaction system (GLS) that enables metal oxides (TiO₂, MoO₃, NiO, ZrO₂, Y₂O₃, Gd₂O₃, Bi₂O₃, Co₃O₄, CuO, In₂O₃, and MgO) to act as nitrogen carriers for achieving photo-driven CLAS under ambient conditions. TiO₂ within the GLS demonstrated competitive activity with that of thermal-driven CLAS, achieving an ammonia production rate of 1.48 ± 0.03 mmol g⁻¹ h⁻¹, a 49.3-fold improvement over the single photocatalytic process, with an apparent quantum yield of 5.31% at 400 nm and a solar to ammonia (STA) efficiency of 0.40%. GLS is a two-step process, nitridation in the gaseous reaction (N₂ and water vapor) followed by surface renewal in the liquid reaction (dissolved N₂ and water). This design utilizes the reactant phase state to influence its surface accessibility by inducing periodic alternation between nitrogen and water coverage. GLS modulates the electron-withdrawing effect of N atoms, controls the formation and breaking of Ti–N bonds, and drives the N atoms in and out of the lattice for efficient surface nitriding and ammonia production. This work bridges the CLAS strategy with photocatalysis, charting a new course for sustainable ammonia synthesis using low-cost metal oxides under ambient conditions, and offers a fresh perspective to overcome the kinetic and thermodynamic hurdles in chemical looping reactions.