Pd─N₄ Sites in MOFs Modulate Oxygen Reduction Pathways for 100% Selective Photocatalytic CO₂-to-CH₄ Conversion from...

Direct photocatalytic CO₂ reduction in flue gas is significantly challenged by the thermodynamically favored oxygen reduction reaction. While conventional approaches showed promise, the inherent O₂ affinity of transition and noble metals prevented full suppression of O₂ adsorption and activation, severely constraining the multi-step proton-coupled electron transfers required for the CO₂-to-CH₄ pathway. We therefore envisioned a CO-mediated oxygen scavenging mechanism by modulating oxygen reduction pathways. Via Pd─N₄ site engineering, the resulting Pd/Cu₃(HITP)₂/TiO₂ composite effectively suppressed competitive oxygen reduction reaction, enabling selective CO₂-to-CH₄ conversion under aerobic conditions. Control experiments and density functional theory calculations revealed that the Pd─N₄ sites steered oxygen reduction toward CO-mediated pathways-thermodynamically and kinetically favored over conventional oxygen reduction reaction, thereby mitigating competitive effects and simultaneously purifying the product. Consequently, such composite exhibited complete CH₄ selectivity at 6.7 µmol g^-1 h^-1 under simulated industrial flue gas conditions (15 vol% CO₂, 3 vol% O₂, 5 vol% H₂O, balanced N₂). Our work highlights catalytic site modulation and advances a new strategy for photocatalytic CO₂ reduction in oxygenated flue gas via pathway-selective oxygen reduction.