The Crucial Role of Hydrogen Ligation in the Stability of Single Atoms on Rutile TiO2: A First-Principles Study

Abstract

Understanding the stability of TiO2-supported single-atom catalysts (SACs) under H-2 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, tau. The results show that Pt consistently exhibits the largest E-total and longest tau, 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 (*Rh-2) formation. Au is unstable on oxygen-deficient TiO2 but is kinetically stabilized upon hydroxylation, which significantly increases both E-total and tau. 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.