Photochemical looping strategy for concentrated ammonia synthesis via carbon monoxide mediated ambient nitrogen fixation

Ammonia is a cornerstone of fertilizers and an emerging carbon-free energy carrier, yet its industrial synthesis via Haber–Bosch consumes ~1% of global energy. Photocatalytic nitrogen fixation provides a potential alternative, but conventional processes operate in aqueous media and yield trace concentrations of ammonia that are impractical to collect and use. Here, we introduce a photochemical looping strategy that integrates nitrogen activation, reduction, and solid-state capture in a cyclic process. Using a Pt–heteropolyacid–TiO2 composite, carbon monoxide, either supplied directly or produced in situ from CO2, generates oxygen vacancies on the heteropolyacid that adsorb and activate N2, while water supplies protons and regenerates the vacancies. The formed ammonia is stored as a stable ammonium salt, which can be released upon calcination to produce aqueous NH3 solutions up to 1.3 wt%, which is significantly higher than the typically reported concentrations in the direct photocatalytic N2 reduction in aqueous suspensions. By coupling photocatalysis with solid-state storage, this looping approach overcomes long-standing barriers in conversion, selectivity, and product recovery, establishing a scalable framework for solar-driven ammonia production under mild conditions.