Direct synthesis of H2O2 on Pd and AuxPd1 clusters: Understanding the effects of alloying Pd with Au

Abstract

Direct synthesis (H2 + O2 → H2O2) could produce H2O2 (an environmentally benign oxidant) more cost-effectively and sustainably than anthraquinone oxidation, enabling broader use of H2O2 for industrial oxidations. We examine direct synthesis on AuxPd1 clusters to better understand the reasons for the high H2O2 selectivities of these materials. Steady-state H2O2 and H2O formation rates were measured as functions of reactant pressure, temperature, and the protic or aprotic nature of the solvent. The analysis of these measurements indicates that H2O2 forms by consecutive proton-electron transfer steps on AuxPd bimetallic catalysts. Among similarly sized Pd and AuxPd1 catalysts, increases in the Au:Pd ratio lead to simultaneous but unequal increases in the activation enthalpies (ΔH‡) for both H2O2 and H2O formation, which must result from significant electronic changes to Pd by Au. Detailed comparisons of these changes in ΔH‡ for H2O2 and H2O production to H2O2 selectivities provide compelling evidence that these electronic effects are primarily responsible for the high H2O2 selectivities commonly reported on AuPd bimetallic catalysts. Additionally, these results lack any clear trends that suggest ensemble effects contribute to the increased preference to form H2O2 on AuPd bimetallics within the ranges of compositions typically reported. These findings provide useful information on the significance of electronic effects in these AuxPd1 clusters and may guide the design of increasingly selective bimetallic catalysts.