(779b) Mechanistic Studies on the Direct Synthesis of H2O2 on Pd and Aupd Clusters 

Authors: 
Wilson, N. M., University of Illinois Urbana-Champaign
Flaherty, D., University of Illinois, Urbana-Champaign
Hydrogen peroxide (H2O2), an environmentally benign oxidant, is currently produced by the auto-oxidation of anthraquinones (AO), yet direct synthesis (DS) is a greener alternative with the potential to make H2O2 using less energy than AO. However, DS currently suffers from low H2O2 selectivities (< 60% H2O) on Pd (identified as an effective catalyst for DS) in the absence of acids and halides.1 In order to achieve economically viable DS, higher selectivities (> 90% H2O2)must be obtained, especially in the absence of toxic or caustic additives. We aim to reach this goal by developing a fundamental understanding of the DS mechanism on Pd and AuPd in order to elucidate the manner in which AuPd catalysts give higher H2O2 selectivities than do Pd catalysts.2 This information will guide the rational design of alternative catalysts, made from more abundant materials, that meet or exceed selectivities achieved on AuPd.

Steady-state H2O2 and H2O formation rates were measured using a fixed bed, plug flow reactor as functions of H2 pressure (25-400 kPa), O2 pressure (25-400 kPa), and temperature (273-337 K) on silica-supported, 6-11 nm diameter Pd and AuPd clusters with varying Au to Pd ratio. These data are inconsistent with Langmuirian mechanisms for H2O2 formation and instead show that H2O2 formation likely occurs by kinetically relevant proton-electron transfer to chemisorbed hydroperoxy (OOH**) on Pd and AuPd. This hypothesis is consistent with results from pure Pd3 and AuxPd clusters (where x denotes the bulk molar ratio of Au to Pd), which show that H2O2 will readily form by DS only in protic solutions, regardless of the Au to Pd ratio. Rate data suggest that H2O forms by kinetically relevant O-O bond cleavage in OOH** surface intermediates. Changing the catalysts from pure Pd to Au1Pd increases rates of H2O2 formation, while subsequent addition of Au (up to Au12Pd) decreases rates of both H2O2 and H2O formation (with H2O rates decreasing more than H2O2 rates). In addition, clusters with high Au to Pd ratios (e.g., Au12Pd) show low rates of H2O2 hydrogenation, as measured directly by feeding H2O2 to the system in the presence of H2. These changes are due, in part, to greater activation enthalpies for H2O2 (Î?Hâ?¡H2O2) and H2O (Î?Hâ?¡H2O) formation on Au1Pd than on pure Pd clusters. Further addition of Au, however, does not continue to increase Î?Hâ?¡H2O values. This observation suggests that the five-fold increase in H2O2 selectivity on Au12Pd over pure Pd must also result from a decrease in the number of Pd ensembles that act as sites for O-O bond rupture during primary and secondary reaction pathways. The improved understanding of the mechanisms behind DS and the factors that make AuxPd such an ideal catalyst will help guide the rational design of selective catalysts for DS by our group and others.

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