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–TiO 2 composite, carbon monoxide, either supplied directly or produced in situ from CO 2 , generates oxygen vacancies on the heteropolyacid that adsorb and activate N 2 , 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 NH 3 solutions up to 1.3 wt%, which is significantly higher than the typically reported concentrations in the direct photocatalytic N 2 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.