Trace Amorphous FeO x Steers O 2 Activation From Charge Transfer to Energy Transfer for Singlet‐Oxygen‐Mediated Biomass Valorization

Controlling O 2 activation in air is central to selective aerobic photooxidation. Yet directing O 2 activation toward an energy‐transfer (ET) pathway producing mild singlet oxygen ( 1 O 2 ), rather than a charge‐transfer (CT) pathway generating radicals, remains difficult because the catalysts must favor triplet‐energy transfer to O 2 while avoiding electron transfer. We show that ultrasmall amorphous FeO x supported on porous carbon nitride (PCN) redirects O 2 photoactivation from a mixed CT/ET process to a predominantly ET pathway. Compared with the crystalline counterpart, trace ultrasmall amorphous FeO x (∼3.8 nm, 0.29 wt% Fe) localizes photoexcitation in a triplet‐favored excited state by increasing exciton binding energy (45.69 meV) and accelerating intersystem crossing ( τ 1 = 19.88 ps). It also promotes O 2 capture and suppresses electron transfer owing to weak interfacial coupling. This dual regulation shifts reactive oxygen species generation from mixed •O 2 − / 1 O 2 to predominantly 1 O 2 , achieving a 1 O 2 yield of 982.2 µmol L −1 , 2.5 times that of crystalline FeO x . Such a 1 O 2 ‐mediated pathway enables selective photooxidation of 5‐hydroxymethylfurfural (HMF) to 2,5‐furandicarboxylic acid (FDCA) under ambient air, delivering 92.1% FDCA selectivity and a formation rate of 461 µmol g −1 h −1 despite ultralow Fe loading, identifying ultrasmall amorphous oxides as atom‐efficient sites for route‐selective O 2 activation for advanced aerobic photocatalysis.