Effects of Redox Environment on Mobility of Single-Atom Catalysts (Silver/Copper) Supported on Titania

Density functional theory (DFT) is used to explore the stability of silver and copper single-atom catalysts (SACs) dispersed on an anatase-titania (TiO₂) surface. SACs are potential catalysts due to their unique electronic and geometric properties, which enhance selectivity in catalytic processes. However, their practical application is limited by the inherent instability of single atoms, which promotes agglomeration and disrupts the balance between activity and selectivity. In this study, DFT calculations are used to examine Ag and Cu adatom mobility on anatase TiO₂ as a function of surface redox state. Ab initio thermodynamics was employed to elucidate the extent of surface reduction as a function of the reduction potential of the gaseous atmosphere. DFT potential energy surfaces are constructed for single-atom diffusion under various redox states of the catalyst. An inverse volcano effect is found between SAC mobility and surface reduction state. Under oxidizing conditions, adsorbed oxygen increases the activation barriers for adatom surface diffusion. As the surface becomes progressively reduced, these barriers decrease, reaching a minimum at an intermediate state, which facilitates metal atom mobility. However, with further reduction, the formation of surface hydroxyl groups and oxygen vacancies creates highly corrugated potential energy surfaces, ultimately increasing the hopping barriers once again.