Precisely Bonded Fe─Cu Diatomic Sites with Nitrogen-Bridged Coordination on Hollow C3N4 Spheres for Efficient C─N Coupling and Selective Photocatalytic Urea Synthesis

Abstract

The photocatalytic synthesis of urea from CO₂ and N₂ co-reduction presents a promising alternative to the conventional energy-intensive Haber–Bosch process. However, competitive adsorption on the catalyst surface often limits selectivity and yield. Here, we designed hollow graphitic carbon nitride (g-C₃N₄) spheres, which serve as a high surface area scaffold for precise anchoring of Fe─Cu diatomic sites. Hollow architecture enhances light harvesting via inner-scattering effects and charge separation. Each Fe─Cu site is coordinated with two nitrogen atoms, forming N₂─Fe₁─Cu₁─N₂ /C₃N_{4 DAC} (hereafter referred to as FeCu/CN), which enables cooperative activation of CO₂ and N₂, in contrast to monodispersed diatomic (Fe+Cu/CN), and single-atom catalysts (Fe/CN, Cu/CN). The FeCu/CN bonded pairs serve as highly efficient active centers, facilitating the synergistic adsorption and activation of multiple reactants. Specifically, during the co-reduction of CO₂ and N₂, the Fe₁ site preferentially adsorbs and activates CO₂, while bonded Cu₁ sites stabilize N₂ on FeCu/CN and enable synergistic C─N coupling through the formation of *NCON intermediates. As a result, the FeCu/CN achieves an exceptional urea yield of 7.40 mg·g_cat⁻¹·h⁻¹ with a 38.58% selectivity under visible light irradiation. Our findings highlight the crucial role of atomic-level coordination in multireactants and offer insights into the C─N coupling for value-added products using CO₂ and N₂ as feedstock.