Trace Amorphous FeO_x Steers O₂ Activation From Charge Transfer to Energy Transfer for Singlet-Oxygen-Mediated Biomass...

Controlling O₂ activation in air is central to selective aerobic photooxidation. Yet directing O₂ activation toward an energy-transfer (ET) pathway producing mild singlet oxygen (¹O₂), rather than a charge-transfer (CT) pathway generating radicals, remains difficult because the catalysts must favor triplet-energy transfer to O₂ while avoiding electron transfer. We show that ultrasmall amorphous FeO_x supported on porous carbon nitride (PCN) redirects O₂ 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 (τ₁ = 19.88 ps). It also promotes O₂ capture and suppresses electron transfer owing to weak interfacial coupling. This dual regulation shifts reactive oxygen species generation from mixed •O₂ ^-/¹O₂ to predominantly ¹O₂, achieving a ¹O₂ yield of 982.2 µmol L^-1, 2.5 times that of crystalline FeO_x. Such a ¹O₂-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 O₂ activation for advanced aerobic photocatalysis.