Theoretical Insights into the Selectivity of Single-Atom Fe–N–C Catalysts for Electrochemical NOx Reduction

Single-atom Fe-N-C catalysts have attracted significant attention in the NOx reduction reaction (NOxRR). However, the origin of their selectivity in the NOxRR remains unclear, impeding further advancements in application. Herein, we investigate the potential-driven competitive mechanism for NH₃ and NH₂OH production in the NOxRR over single-atom pyridinic-FeN₄ and pyrrolic-FeN₄ sites using constant-potential density functional theory calculations. The origin of selectivity in the NOxRR is linked to the switching of Fe 3d orbitals as they interact with intermediates. The selectivity between NH₃ and NH₂OH is determined by the applied potentials. The pyridinic-FeN₄ predominantly generates NH₃ at higher reduction potentials (-0.6 to -1.2 V, vs SHE), while NH₂OH is favored at lower reduction potentials (0.6 to -0.6 V). The pyrrolic-FeN₄ shows a similar potential-dependent product distribution, with a crossover potential of -1.0 V. The selectivity-determining intermediates (SDIs) in the NOxRR are *NH₂OH and *NH₂ + *OH. The potential-dependent selectivity is governed by the switching of Fe 3d orbitals interacting with SDIs, from dumbbell-shaped Fe 3dz2 to four-leaf clover-like Fe 3dxz, 3dyz, and 3dx2-y², which plays a crucial role in controlling product distribution based on applied potentials. These findings offer new insights into the product selectivity of single-atom catalysts for the NOxRR.