Surface Oxide Species on Metal Cocatalysts Mediate Pathways in Water-Oxidation-Coupled CO₂ Photoreduction: Insights...

Photocatalytic overall CO₂ reduction using H₂O as an electron donor is hindered by sluggish reaction kinetics and poorly defined interfacial pathways. Here, we demonstrate that surface oxide species on metal cocatalysts act as intrinsic mediators of the reaction pathway in water-oxidation-coupled CO₂ photoreduction. Taking Pd as a model system, we construct a Pd@PdO_x cocatalyst on TiN, in which metal Pd is partially encapsulated by spontaneously formed surface PdO_x species. This pre-formed Pd/PdO_x interface establishes a bifunctional reaction landscape that spatially coordinates proton management and CO₂ activation. In situ spectroscopic analyses combined with theoretical calculations reveal that PdO_x domains preferentially adsorb and activate CO₂, while adjacent metal Pd sites function as proton reservoirs derived from water oxidation. Directional proton transfer across the Pd/PdO_x interface lowers the barrier for *COOH formation, suppresses the competing H₂ evolution reaction, and promotes selective CO release. Under full-spectrum irradiation, Pd@PdO_x/TiN achieves a CO yield rate of 200 µmol g^-1 h^-1 with 81% selectivity, substantially outperforming the counterparts dominated by either Pd or PdO_x. This study highlights surface oxide species as structural determinants of pathway selection and provides mechanistic insights for engineering metal cocatalysts for efficient and selective CO₂ photoreduction.