Direct synthesis of H2O2 (H2 + O2 → H2O2) could enable on-site, and even in situ, H2O2 production, which motivates searches for highly selective catalysts and process conditions. H2O2 formation rates and selectivities depend sensitively on the addition of other transition metals, adsorption of halides, and solvent identity. Rates that differ among different solvent mixtures have been attributed to changes in the solubility of the reactants H2 and O2 within the liquid-phase. Yet, significant differences persist when we perform experiments in which the chemical potentials of these species are demonstrated to be equal in the gas and liquid phases. To date, these difference are not completely understood and are difficult to explain. Rate measurements, X-ray absorption spectroscopy, and computation on Pd and Pd-based bimetallic clusters to determine the mechanism of this reaction and to understand the reasons why alloying Pd often increases H2O2 selectivities. In aqueous alcohols, the change in H2O2 and H2O formation rates with H2 and O2 pressures are not consistent with reactions of oxygen and hydrogen atoms on equivalent sites, but instead suggest O2* species react with the solvent. H2O2 formation rates in protic solvents are 103 larger than those measured in aprotic liquids and large kinetic isotope effects (kH/kD &gt;&gt; 1) strongly suggest that alcohols serve as cocatalysts and coreactants in the kinetically relevant steps for hydrogen peroxide formation. Persistent organic surface residues introduce low barrier reaction pathways to reduce O2* and increase those for O-O dissociation relative to reaction pathways in pure water. Rate measurements and computational predictions suggest that heterolytic processes form H2O2 when deionized water is used as a solvent, however, heterolytic H* oxidation and electron transfer provide the greatest barriers in this pathway. Aside from mechanistic differences that reflect the choice of the reaction solvent, in situ XAS measurements show that phase diagram for Pd nanoparticles and the boundary at which palladium hydrides form depends on the solvent and not only on the pressure of H2 and O2 reactants. These results show that long-standing observations that H2O2 forms in greater yields within alcoholic solvent are not explained by simple differences in the solubility of H2 in the liquid-phase.

When the solvent controls the phase and mechanism of reactions on metal nanoparticles: Direct synthesis of H2O2 on Pd

2019 Fall National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.