Hydrogen peroxide is a selective oxidant useful for epoxidations, bleaching, and disinfection, yet, its use is limited because current H2O2 production methods are viable only at very large scales. Direct synthesis of H2O2 (H2 + O2 --&gt; H2O2) could enable on-site, and even in situ, H2O2 production, which motivates searches for highly selective catalysts. H2O2 formation rates and selectivities are known to increase when Au or Sn are added to Pd clusters. However, the reasons for these changes are not understood. Here, we combine rate measurements on Pd and Pd- and Pt-based bimetallic clusters to determine the mechanism of this reaction and to understand the reasons why alloying Pd often increases H2O2 selectivities. The change in H2O2 and H2O formation rates with H2 and O2 pressures are not consistent with a Langmuirian mechanism, but instead suggest O2* species react with a liquid-phase intermediate. Additionally, H2O2 formation rates in protic solvents are 103 larger than those measured in aprotic liquids. These observations suggest that proton-electron transfer processes, reminiscent of the two electron oxygen reduction reaction, form H2O2 on metal clusters. These reactions occur on surfaces saturated with O2*- and OOH*-intermediates. In parallel, O-O bonds within chemisorbed intermediates cleave homolytically to form H2O. Consequently, the H2O2 selectivities are determined by competition between heterolytic and homolytic reactions involving O2 at liquid-solid interface. Proton-electron transfer is the dominant pathway for H2O2 formation also on monometallic and bimetallic clusters comprised of Pd, Au, Pt, Ag and other late transition metals. Incorporating other metals, into Pd or Pt clusters does not change the mechanism for H2O2 or H2O formation. These modifications do, however, change barriers for the formation of H2O (significantly) with lesser effects on barriers for steps that lead to H2O2. We show that these energetic differences result, in part, from consequential changes in the location and groupings of Pd/Pt at the surface of metal clusters that affect O2 adsorption and O-O bond rupture barriers. In select cases, the second metal stabilizes the withdrawal of charge from Pd/Pt and hinders O-O bond rupture. Both mechanisms can increase H2O2 selectivities significantly. Overall, our work presents a clear mechanism for H2O2 formation on metal clusters and explains the roles of solvent identity and cluster composition in determining H2O2 selectivities.

Direct synthesis of H2O2: Competing reaction pathways depend differently on surface structure

253rd National Meeting (2017)

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