Valence electron matching law for MXene-based single-atom catalysts

Single-atom catalysts (SACs) have attracted considerable interest in the fields of energy and environmental science due to their adjustable catalytic activity. In this study, we investigated the matching of valence electron numbers between single atoms and adsorbed intermediates (O, N, C, and H) in MXene-anchored SACs (M-Ti₂C/M-Ti₂CO₂). The density functional theory results demonstrated that the sum of the valence electron number (V_M) of the interface-doped metal and the valence electron number (V_A) of the adsorbed intermediates in M-Ti₂C followed the 10-valence electron matching law. Furthermore, based on the 10-valence electron matching law, we deduced that the sum of the valence electron number (k) and V_M for the molecular adsorption intermediate interactions in M-Ti₂CO₂ adhered to the 11-valence electron matching law. Electrostatic repulsion between the interface electrons in M-Ti₂CO₂ and H₂O weakened the adsorption of intermediates. Furthermore, we applied the 11-valence electron matching law to guide the design of catalysts for nitrogen reduction reaction, specifically for N₂ → NNH conversion, in the M-Ti₂CO₂ structure. The sure independence screening and sparsifying operator algorithm was used to fit a simple three-dimensional descriptor of the adsorbate (R² up to 0.970) for catalyst design. Our study introduced a valence electron matching principle between doped metals (single atoms) and adsorbed intermediates (atomic and molecular) for MXene-based catalysts, providing new insights into the design of high-performance SACs.