Photohole-oxidation-assisted anchoring of ultra-small Ru clusters onto TiO2 with excellent catalytic activity and stability

Abstract

Ultra-small metal clusters with good activity and stability are of great significance for their practical applications in catalysis and materials science. Here we report a photohole-oxidation-assisted approach for anchoring ultra-small Ru clusters (∼1.5 nm) with an extremely high density (∼10¹⁷ m⁻²) onto TiO₂ support. The resulting clusters have good thermal stability and exhibit excellent long-term catalytic activity for the hydrogenation of CO₂ to methane (methanation). The anchoring process involves the oxidation of Ru³⁺ in solution by photogenerated holes on the TiO₂ surface to give tiny RuO₂ species (<0.8 nm) immobilized on the surface, followed by a H₂ reduction step to produce Ru⁰ clusters. Aberration-corrected high-resolution transmission electron microscopy (Cs-HRTEM) observations identify the Ru–Ru bond length contraction at the metal surface (relative to the interior of the particle) as well as bond length changes in the defect region at the metal–support interface. Density functional theory (DFT) calculations further demonstrate that the ultra-small Ru clusters are well stabilized and tightly anchored onto the TiO₂ substrate via Ru–O covalent bonding in the defect region of the metal–support interface. The high-dispersion of ultra-small Ru clusters as well as the strong chemical bonding at the interface account for their surprisingly high catalytic reactivity and excellent thermal/reaction stability. This synthetic method may open up a new way to fabricate thermally stable ultra-small metal clusters for practical industrial applications in catalysis.