Unveiling NOx-to-N2 Conversion Mechanisms over Nickel Single-Atom Catalysts under Ammonia Combustion Conditions: DFT and Microkinetic Study

Catalytic conversion of NOx to N2 under ammonia combustion conditions remains challenging because efficient catalysts must simultaneously achieve sufficient NO adsorption and rapid N–O activation, two requirements that are often intrinsically coupled. Herein, density functional theory calculations combined with microkinetic simulations were employed to investigate the conversion pathways of NO-derived species over single-atom catalysts anchored on N-coordinated vacancy graphene. A series of transition-metal SACs were evaluated using H2O adsorption free energy as a prescreening descriptor for competitive H2O occupation, after which Ni-based catalysts were selected for detailed coordination-dependent mechanistic investigation. Although Ni@Vc–N2–Gr does not exhibit the strongest NO adsorption, it delivers the lowest energy barrier by maximizing electron back-donation from Ni to the NO 2π* antibonding orbital. Electronic structure analyses demonstrate that dual-N coordination reconstructs the local electronic environment of the Ni center, enhancing antibonding orbital occupation while maintaining moderate adsorption strength. This feature enables a decoupling of NO adsorption and activation, significantly weakens the N–O bond, and lowers the barrier for N2O decomposition and N2 formation. Consequently, Ni@Vc–N2–Gr exhibits the lowest overall reaction barrier (0.53 eV), outperforming both other coordination environments and Ni3 cluster. Microkinetic analysis further shows that Ni@Vc–N2–Gr exhibits favorable elementary-step kinetics for N2O decomposition and N2 formation, consistent with the enhanced electron back-donation induced by the dual-N coordination environment. This work identifies electron-back-donation-driven adsorption–activation decoupling as a key descriptor governing NOx reduction and provides a coordination-engineering strategy for designing highly efficient single-atom catalysts for deep denitrification.