Synergistic Facet Engineering and Axial Asymmetric Coordination of Single-Atom Cu Polarize Nitric Oxide Intermediates to Boost Photocatalytic Nitrate Reduction to Ammonia

Abstract Sluggish charge dynamics and poor *NO polarization at active sites hinder the efficiency of photocatalytic nitrate reduction to ammonia (PcNRA). To address these issues, an atomic-level regulation strategy synergizing facet engineering with axial coordination was developed. Specifically, isolated Cu sites were anchored onto an electron-rich, ultrathin BiVO4 substrate exposing (040) facets. Functionalized ligands featuring distinct donor heteroatoms (S, N, O) were then systematically introduced, yielding a series of single-atom catalysts (S/N/O-Cu/BVO) with tailored axial microenvironments. A comprehensive methodology combining multidimensional spectroscopy and computational chemistry was established to verify this single-atom axial coordination. Detailed analyses of characteristic binding energies and absorption edge shifts via X-ray photoelectron spectroscopy (XPS) and synchrotron X-ray absorption fine structure (XAFS) identified localized coordination field effects. Crucially, these effects are distinct from conventional electronic metal-support interactions (EMSI), providing spectroscopic confirmation of successful axial ligand incorporation and guiding the prediction of their configurations. Building upon these spectral findings, density functional theory (DFT) models were constructed and optimized, conclusively validating the asymmetric axial coordination configurations within the system, specifically Cu-S2O1, Cu-N2O1, and Cu-O4. Catalytic evaluations demonstrated that S-Cu/BVO achieves the highest ammonia yield at 435 μmol·h–1·g–1. In-situ spectroscopy and DFT calculations elucidated the multidimensional synergistic mechanism driving this enhanced activity: (1) The prominently exposed (040) facets act as an interfacial electron sink, efficiently directing photogenerated electrons toward the Cu sites; (2) The axial S ligands reconstruct the electronic structure of the Cu centers via strong orbital coupling, thereby improving effective orbital overlap with nitrate; (3) The resulting asymmetric Cu-NO-S bridging configuration polarizes the critical *NO intermediate, substantially lowering the kinetic barrier for the rate-determining step.