Solvent Molecules Form Surface Redox Mediators in Situ and Cocatalyze O2 Reduction on Pd

Solvents influence catalysis by stabilizing reactive intermediates and facilitating low-barrier heterolytic reactions. Here, we demonstrate that solvents co-catalyze proton-electron transfer (PET) steps at the solid-liquid interface of Pd nanoparticles while covalently bound to the metal surface. These surface-bound redox mediators facilitate reactions between H₂ and O₂ during the formation of H₂O₂. Such co-adsorbates produce greater rates and selectivity than is achieved without these species. Moreover, the mediators (CH₂OH*) resemble homogenous (e.g., TEMPOH) and biological cofactors (e.g., NADH), which may inform the design of new co-catalytic moieties for other redox chemistries.

Herein, we elucidate the mechanisms of H₂O₂ formation in methanol and water using kinetic isotope effect (KIE) measurements and density functional theory (DFT) simulations. Methanol activates on Pd nanoparticles to form hydroxymethyl (CH₂OH*) species that enable low-barrier PET reactions with oxygen-derived intermediates. This reaction also forms CH₂O*, which reacts readily with H₂ to regenerate the active CH₂OH* species that turnover 20-90 times before desorbing as solution-phase formaldehyde. Water, in contrast, oxidizes H₂ on the Pd surface, which reduces O₂ through kinetically relevant electron transfer. The resulting hydronium ions then transfer protons to oxygen-derived species during H₂O₂ formation. Here, the water enables proton transfer but does not mediate electron transfer. In comparison, the hydroxymethyl co-catalyzes both paths because the organic fragment forms an enol-like complex on Pd, which promotes resonance-stabilized electron transfer while transferring protons to oxygen. , aqueous formaldehyde increases H₂O₂ selectivity (55-85%) and rates relative to pure water (25-55%) or methanol (5-35%). This understanding provides opportunities to leverage the advantages (greater selectivities and rates) and minimize disadvantages (deactivation, waste) of organic co-solvents.