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

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_2­) 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 Au_xPd 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?H^â?¡_H2O2) and H₂O (Î?H^â?¡_H2O) formation on Au₁Pd than on pure Pd clusters. Further addition of Au, however, does not continue to increase Î?H^â?¡_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 Au_xPd such an ideal catalyst will help guide the rational design of selective catalysts for DS by our group and others.