Electron-Extracting Pd Single Atoms Arrest Solar Photooxidative Upcycling of Waste Polyesters at Glycolaldehyde

Selective solar oxidation of ethylene glycol (EG) derived from waste poly(ethylene terephthalate) (PET) to glycolaldehyde (GAld) is hindered by the high reactivity of the aldehyde intermediate, which readily undergoes overoxidation and C─C cleavage. Here, we report an exfoliated polymeric carbon nitride (PCN) photocatalyst with atomically dispersed Pd sites (Pd_SA/PCN) that arrests EG photooxidation at the C₂ aldehyde stage. In PET hydrolysate, optimized Pd_SA/PCN achieves a GAld formation rate of 2479 µmol g_cat ^-1 h^-1 with 92.8% selectivity; in gram-scale upgrading of real PET waste, the selectivity reaches 95.9%. Mechanistic studies suggest that Pd single atoms function as electron-extraction centers, removing electrons from long-lived, low-energy localized states in PCN and increasing surface-accessible holes for selective alcohol dehydrogenation. Meanwhile, Pd sites strengthen EG adsorption and lower the barrier for initial O─H activation, favouring alkoxy intermediates while limiting ·OH-type overoxidation and C─C scission. The catalyst converts diverse real polyester wastes under fluctuating natural sunlight while maintaining >90% GAld selectivity. Coupled techno-economic and geospatial analyses identify priority deployment regions with a median annual net profit of US$5.95 million, highlighting the potential of single-atom materials for solar plastic upcycling to C₂ platform molecules.