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.