Pd Size-Controlled C-C Bond Cleavage-Recoupling in Photocatalytic Glycerol Selective Oxidation to Glycolaldehyde

Selective photocatalytic upgrading of glycerol to glycolaldehyde is intrinsically challenging because, under a conventional C-C cleavage scenario, the carbon-based selectivity of a C2 product is, in principle, capped at 66.67%. Here, a size-regulated Pd on TiO₂ platform as an efficient photocatalyst realized selective oxidation of glycerol into glycolaldehyde with selectivity up to 87.54%. As the Pd size increases from single atoms to nanoparticles, both glycerol conversion and glycolaldehyde selectivity increase monotonically, reaching 36.52% conversion and a glycolaldehyde formation rate of 12.05 mmol g^‒1 h^‒1. Combined in situ spectroscopy, isotope-labeling experiments, and density functional theory calculations reveal that increasing Pd size switches glycerol adsorption from a symmetric terminal-terminal mode to an asymmetric terminal-secondary mode, thereby promoting C-C activation and enabling a radical-mediated cleavage-recoupling route toward glycolaldehyde. Isotopic liquid chromatography-mass spectrometry verifies C-C recombination after bond cleavage, which accounts for the glycolaldehyde selectivity exceeding the conventional 66.67% carbon limit, water as a direct contributor to •OH generation, and oxygen incorporation into the products. This work establishes a mechanistic framework in which metal size alters the interfacial adsorption geometry and reaction energy, thereby providing a general strategy for the selective photocatalytic valorization of polyol biomass molecules.