Frontier Orbital Engineering Enables Predictive Control of CO2 Reduction Selectivity in Single-Atom Catalysts

Abstract Single-atom catalysts (SACs) offer a promising route for electrochemical CO2 reduction, yet achieving high activity and selectivity toward post-CO products remains challenging. Here, we propose an orbital engineering strategy for porphyrin-based SACs by constructing isoelectronic systems using group VIII and IX metals coordinated with neutral (IMD, Py, PMe3) and anionic axial ligands (PhO–, PhS–, Ph2N–, and Ph2P–) to activate CO2 with high efficiency. Binding energy calculations at the HSE06 functional, combined with energy decomposition analysis, reveal that the trans-field effect modulates the binding strength of *CO, thereby governing the key step that determines selectivity. Grand canonical density functional theory (GC-DFT) calculations are performed to evaluate reaction Gibbs free energy profiles and construct activity volcano plots. We have benchmarked appropriate DFT+U functionals to match the accuracy of the HSE06 level. Among the investigated systems, group IX metal-doped SACs, particularly Rh@COFs and Ir@COFs with anionic axial ligands (PhO–, Ph2N–, and PhS–), exhibit the highest activity and selectivity toward methanol formation. Mean-field microkinetic analysis further indicates that Rh@COF-OPh, Rh@COF-NPh2, and Ir@COF-SPh outperform the Cu(211) surface, while Fe@COF with Py, IMD, Ir@COF-PPh2, and Rh@COF-SPh display comparable activity with favorable selectivity toward products beyond the two-electron CO pathway. Notably, a stronger thermodynamic preference for methanol formation compared to methane, in contrast to the Cu(211) surface, is observed. Pourbaix stability analysis reveals that active catalysts Fe, Rh, and Ir embedded COF frameworks exhibit good stability across a wide electrochemical window. This study establishes an orbital engineering strategy based on isoelectronic design and trans-axial interactions, providing a predictive framework to tune product selectivity in CO2 reduction.