Single-atom molybdenum immobilized on photoactive carbon nitride as efficient photocatalysts for ambient nitrogen fixation in pure water

Abstract

A series of Mo single-atom catalysts were prepared by calcining a low-cost primary material of urea with various amounts of Na₂MoO₄·2H₂O. Isolated Mo centers are immobilized on in situ formed polymeric carbon nitride via coordinating with two N donors to form two-coordinated MoN₂ species. The low-coordinated Mo centers can serve as active sites for N₂ chemisorption and activation, achieving high photocatalytic activity for NH₃ evolution with a rate of 50.9 μmol g_cat⁻¹ h⁻¹ in pure water. In the presence of ethanol as the electron scavenger, the NH₃ evolution rate can reach 830 μmol g_cat⁻¹ h⁻¹, and the catalyst shows a quantum efficiency of 0.70% at 400 nm. This is the first single-atom catalyst that can drive photocatalytic N₂ fixation in pure water with comparable performance to recent reports for photocatalytic N₂ reduction. Experimental investigations and density functional theory calculations demonstrate that the coordinatively unsaturated metal center in the single-atom catalysts can strongly adsorb N₂via an end-on configuration to elongate the NN bond from 1.11 Å to 1.15 Å, and thus photoexcited electrons can transfer to the weakened NN bond for efficient nitrogen fixation under ambient conditions. These findings provide new insight for solar-driven N₂ fixation by atomically dispersing low-coordination metal centers on photoactive supports.