Synergistic multi-electron/proton transfer in Cu₄POM@MOF composite boosts ammonia generation

Ammonia synthesis with highly catalytic efficiency remains a fundamental challenge due to the kinetically demanding multi-electron/proton transfer. We address this by designing cooperative Cu-substituted sandwich-type polyoxometalate embedded in photoactive MOF (Cu₄POM@NU1000) with varied loadings, which synergistically merges efficient light harvesting, multi-electron storage, transition metals and proton transfer in one architecture. Preliminary study reveals that higher Cu₄POM loading enhanced the photocatalytic N₂ fixation ability. Thus, 1.41-Cu₄POM@NU1000 composite was selected for systematic evaluation of photocatalytic performance. Under visible light irradiation in pure water, it exhibited an ammonia generation rate 1.8 times of the NU1000 alone while maintaining stability. In-situ spectroscopic and trapping experiments reveal that the embedded Cu₄POM acts as electron sponges and proton-coupled redox mediator, concurrently accelerating water photo-oxidation (providing H⁺) and promoting the sequential hydrogenation of adsorbed N₂. DFT calculations further proved and highlighted the advantage of Cu₄POM in driving hydrogenation of adsorbed N₂ and the desorption of NH₃ while NU1000 facilitates the initial N₂ adsorption process. Finally, a seven-day outdoor experiment utilizing natural sunlight and atmospheric nitrogen confirmed this promising strategy toward green ammonia production for agricultural use.