Spatially Decoupled Cascade Oxygen Reduction on Bi Single-Atom and Cluster-Hybrid Sites for Hydroxyl Radical...

The two-electron oxygen reduction reaction (2e^- ORR) for H₂O₂ production and the subsequent in situ activation to ·OH are constrained by linear scaling relationships, making it difficult for a single active site to simultaneously optimize O₂ activation and H₂O₂ conversion. This work overcomes the trade-off by spatially decoupling the cascade reaction onto single-atom and cluster-hybrid sites and employing photoelectrocatalysis (PEC). Through a ligand-exchange and pyrolysis route derived from a metal-organic framework precursor, Bi single atoms and clusters are co-anchored on B-doped TiO₂. The hybrid interface thermodynamically optimizes the intermediate adsorption energies to ensure cascade selectivity, whereas PEC kinetically accelerates the H₂O₂-to-·OH conversion by supplying photogenerated electrons. Density functional theory and free-energy diagrams indicate that H₂O₂ forms at the single atoms and dissociates into ·OH at the clusters, with the relationship between the p-band center and ·OH yield following the Sabatier principle. The catalyst delivers a ·OH production rate of 250 µM h^-1 at neutral pH and remains active over a broad pH range (3-9). Moreover, it enables broad-spectrum removal of recalcitrant organic pollutants and retains stable performance over 50 h of operation in a flow-cell reactor. This work offers guidance for designing cascade catalysts at single-atom and cluster-hybrid interfaces.