The Crucial Role of Hydrogen Ligation in the Stability of Single Atoms on Rutile TiO ₂ : A First-Principles Study

Understanding the stability of TiO₂-supported single-atom catalysts (SACs) under H₂ reduction conditions, where hydrogen adsorption on the metal/TiO₂ surface influences metal-support interactions, diffusion, and aggregation, is important for their long-term applications. Using first-principles density functional theory (DFT) calculations, we investigate the thermodynamic and kinetic stability of Rh, Ag, Pt, and Au-based SACs on pristine, oxygen-defective, and hydroxylated rutile TiO₂ (110) surfaces with and without H adsorption on the metal adatom. The thermodynamic driving force for aggregation was assessed by calculating dimerization energies as proxy, while the kinetic stability was quantified in two ways: (i) the total activation energy, Etotal (Ef + Ed), which couples adatom formation (Ef) and diffusion (Ed) energies, serves as a descriptor of ripening kinetics, and (ii) the Ed, used to evaluate diffusion rate constants and characteristic diffusion times, τ. The results show that Pt consistently exhibits the largest Etotal and longest τ, reflecting exceptional resistance to sintering, whereas Ag has the smallest values and is intrinsically unstable. Rh presents a distinctive case: although dimerization is thermodynamically favored, its Etotal is dominated by the formation energy of two separated Rh atoms on support (*Rh*Rh), giving Rh longer lifetimes than expected from its low diffusion barrier for dimer (*Rh₂) formation. Au is unstable on oxygen-deficient TiO₂ but is kinetically stabilized upon hydroxylation, which significantly increases both Etotal and τ. Hydrogen adsorption further modulates stability in a metal-dependent manner─stabilizing Rh but accelerating the aggregation of Ag and Au. This combined thermodynamic-kinetic framework provides a quantitative basis for predicting SAC sintering behavior and guiding strategies for stabilizing late transition metals under hydrogenation conditions.