(677a) Solvent Molecules Cocatalyze O2 Reduction through Proton-Electron Transfer Pathways on Pd Nanoparticles

Solvents influence catalysis by stabilizing reactive intermediates or introducing low-barrier reaction pathways. For instance, protic solvents assist the reduction of O₂ and the oxidation of H₂ on noble metal surfaces under an applied electrical potential. We find that solvent molecules (e.g., water and methanol) serve a similar purpose during thermochemical reactions of H₂ and O₂ into H₂O₂ on Pd nanoparticles. Prior studies propose that oxygen reduction involves homolytic reactions between chemisorbed oxygen and hydrogen species. However, we find that adsorbed organic solvents and solution-phase molecules facilitate oxygen reduction through paths reminiscent of electrochemical systems.

We determined the mechanisms for H₂O₂ and H₂O formation in methanol and water using a combination of kinetic isotope effects, quantum mechanical calculations, and steady-state reaction rates. We find that methanol readily activates on Pd nanoparticles to form hydroxymethyl surface intermediates, which facilitate low-barrier proton-electron transfer reactions with adsorbed oxygen. This reaction also generates CH₂O*, which subsequently reduces back to CH₂OH* through the heterolytic oxidation of hydrogen by solution-phase methanol. Water, in contrast, facilitates the heterolytic oxidation of hydrogen to produce hydronium ions and electrons. Here, the kinetically relevant step is the transfer of electrons from hydrogen through the Pd surface to oxygen, followed by spontaneous proton transfer to form H₂O₂ or H₂O. These findings guided us to add formaldehyde to the solution to intentionally generate the hydroxymethyl co-catalysts in an otherwise aqueous media. The resulting combination of inner and outer-sphere interactions results in H₂O₂ selectivity (54%) and rates that are significantly greater than those in pure water (25%) or methanol (12%). Such understanding presents new opportunities to design co-catalytic surface structures for liquid-phase reactions. We acknowledge the generous support from the Energy Biosciences Institute, Shell International Exploration and Production Inc., and the National Science Foundation Graduate Research Fellowship Program.