DFT insight into the stability of single metal atoms on Mo-based o-MXenes driven by the ligand effect

Abstract

MXenes, a novel class of two-dimensional transition metal carbides, nitrides or carbonitrides, have emerged as promising supports for single-atom catalysts due to their tunable structural and electronic properties. Using density functional theory calculations, we investigate ordered, termination-free Mo-based bimetallic MXenes (o-Mo2M"2C3 and o-Mo2M"C2 with M"= Ti, Zr, Hf, V, Nb and Ta) as materials to stabilize platinum-group-metal (PGM) single atoms at Mo vacancies. Compared with the corresponding Mo-only MXenes, introducing a second metal component M" strengthens PGM anchoring and increases sintering resistance, which we attribute to a ligand effect that enhances PGM-C covalency and reorganizes the PGM d states. This stabilization is further captured by a combined electronic descriptor δ = εp ‒ εd. Here, δ is defined as the difference between the p band center (εp) of the C atoms adjacent to the PGM and the d band center (εd) of the PGM atom. This descriptor δ shows a strong correlation with the binding energetics across both considered MXenes families. To assess reactivity trends, we use CO, CH3 and NH3 adsorption strength, highlighting a stability-reactivity trend. An XGBoost-based machine learning analysis further indicates that adsorption energies are governed by multiple electronic descriptors (beyond εd), with the average electronegativity of PGM-MXene system (χavg) contributing to the trends. This study provides new atomic-level understanding into the rational design principle for stabilizing atomically dispersed SACs on MXenes and provides guidelines for experimental synthesis.