Insights from Computational Modeling on the Effect of Doping Ti3C2 MXene Surfaces for the NH3 Conversion Reaction

Density functional theory (DFT) calculations were performed to investigate the effect of doping on ammonia interaction and activation over pristine and modified Ti3C2 MXene surfaces. A Ti vacancy model was used to represent experimentally relevant surface defects, followed by systematic substitution with 14 nonmetal and 16 transition-metal dopants. Structural stability was evaluated using binding, formation, and cohesive energies, and electronic properties through Bader charge and density of states (DOS) analyses. For nonmetal dopants, stronger binding was associated with deeper p-band centers below -4 eV (O, B, F), whereas weaker binding occurred for higher p-band centers above -3 eV (As, Br, I). For transition-metal dopants, a volcano-type relationship was observed between binding energy and d-band center, with the strongest interactions for d-band centers of approximately -2.3 to -2.7 eV (Re, Ru, Rh, Ir). Dopants with 7-9 valence electrons generally exhibited stronger binding, while shorter dopant-carbon bond lengths of about 1.9-2.0 Å correlated with enhanced surface interaction. Second- and third-row transition metals also tended to bind more strongly, likely due to their more spatially extended d orbitals. An exception was Mn, which bound relatively weakly despite a favorable valence electron count, likely because of its longer bond length. To evaluate the catalytic relevance of the most stable systems, adsorption energies of NH3 and key reaction intermediates (H, H2, N, N2, NH, and NH2) were calculated. Among the screened surfaces, Re- and Mo-doped Ti3C2 exhibited the strongest NH3 adsorption (-1.15 and -1.22 eV, respectively), whereas Co- and O-doped surfaces showed more moderate interactions. These trends were interpreted using bond length, charge transfer, and electronic structure analyses. Overall, this work provides computational insight into how dopant selection influences the stability, electronic structure, and ammonia interaction of Ti3C2 MXene surfaces, offering guidance for the design of MXene-based catalysts for ammonia conversion.