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, E_total (E_f + E_d), which couples adatom formation (E_f) and diffusion (E_d) energies, serves as a descriptor of ripening kinetics, and (ii) the E_d, used to evaluate diffusion rate constants and characteristic diffusion times, τ. The results show that Pt consistently exhibits the largest E_total 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 E_total 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 E_total 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.