Surface Oxide Species on Metal Cocatalysts Mediate Pathways in Water‐Oxidation‐Coupled CO 2 Photoreduction: Insights From Pd

ABSTRACT Photocatalytic overall CO 2 reduction using H 2 O 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 CO 2 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 CO 2 activation. In situ spectroscopic analyses combined with theoretical calculations reveal that PdO x domains preferentially adsorb and activate CO 2 , 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 H 2 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 CO 2 photoreduction.