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

Abstract

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.