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

Adams, J. S. - Presenter, University of Illinois, Urbana-Champaign
Chemburkar, A., University of Minnesota
Priyadarshini, P., University of Illinois, Urbana-Champaign
Lu, Y., Virginia Polytechnic Institute and State University
Karim, A. M., Virginia Polytechnic Institute and State University
Neurock, M., University of Minnesota
Flaherty, D., University of Illinois, Urbana-Champaign
Ricciurdulli, T., University of Illinois at Urbana-Champaign
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.