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

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 FeOx 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 FeOx (∼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 FeOx. 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.