Balancing d‐π/d‐π* Interactions: Valence‐Engineered Pd/TiO 2 for Photocatalytic Acetylene Semi‐Hydrogenation

Photocatalytic acetylene (C 2 H 2 ) semihydrogenation is a green route for ethylene (C 2 H 4 ) production, yet the activity–selectivity trade‐off caused by imbalanced adsorption/activation of reactants and products remains a core challenge. Herein, we realize precise regulation of Pd oxidation states (Pd 0 , Pd δ + , and Pd 2+ ) on anatase TiO 2 nanocubes via a mild reduction strategy, fabricating a Pd δ + /TiO 2 photocatalyst with intermediate valence of Pd sites. Systematic characterizations and theoretical calculations reveal that Pd δ + optimizes the d‐π/d‐π* orbital interactions between Pd and C 2 H 2 /C 2 H 4 , achieving a moderate adsorption strength that balances efficient C 2 H 2 activation and rapid C 2 H 4 desorption. Pd 0 causes overadsorption/overhydrogenation, whereas Pd 2+ leads to weak substrate activation. Consequently, Pd δ + /TiO 2 achieves nearly 100% C 2 H 2 conversion and ∼90% C 2 H 4 selectivity, with a record‐high turnover number (TON) of ∼4.1 × 10 4 and excellent cycling stability. This work clarifies the electronic mechanism of Pd valence engineering in carrier dynamics and modulating orbital hybridization, providing a new strategy for the rational design of C 2 H 2 semihydrogenation photocatalysts.