Theoretical Insights into the Selectivity of Single-Atom Fe–N–C Catalysts for Electrochemical NO<i>_<i>x</i></i> Reduction

Single-atom Fe-N-C catalysts have attracted significant attention in the NO<i>_<i>x</i></i> reduction reaction (NO<i>_<i>x</i></i>RR). However, the origin of their selectivity in the NO<i>_<i>x</i></i>RR remains unclear, impeding further advancements in application. Herein, we investigate the potential-driven competitive mechanism for NH₃ and NH₂OH production in the NO<i>_<i>x</i></i>RR over single-atom pyridinic-FeN₄ and pyrrolic-FeN₄ sites using constant-potential density functional theory calculations. The origin of selectivity in the NO<i>_<i>x</i></i>RR is linked to the switching of Fe 3<i>d</i> 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 NO<i>_<i>x</i></i>RR are *NH₂OH and *NH₂ + *OH. The potential-dependent selectivity is governed by the switching of Fe 3<i>d</i> orbitals interacting with SDIs, from dumbbell-shaped Fe 3<i>dz</i>^<i>2</i> to four-leaf clover-like Fe 3<i>dxz</i>, 3<i>dyz</i>, and 3<i>dx</i>^<i>2</i><i>-y</i>², 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 NO<i>_<i>x</i></i>RR.