(617go) Elucidation of the Mechanisms for Direct Synthesis on Pd, Aupd, and Agpt 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 (i.e., less concentration and separation steps) 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 DS that can compete financially, 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 Additional work examined tunable octahedron AgPt clusters to demonstrate the effects of changing the coordination of Pt surface atoms. 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. Using Au₁Pd catalysts increases rates of H₂O₂ formation over pure Pd, 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. Work by our group has shown that undercoordinated Pd sites tend to form H₂O over H₂O₂.³ We intend to further probe this phenomenon by tuning AgPt octahedral clusters to have Pt preferentially migrate to either coordinated or undercoordinated surface sites. The improved understanding of the mechanisms behind DS, the factors that make Au_xPd such an ideal catalyst, and the importance of the position of active metal within bimetallic clusters will help guide the rational design of selective catalysts for DS.

(1) Samanta, C. Appl. Catal., A 2008, 350, 133.

(2) Landon, P. et al. Chem. Chem. Phys 2003, 5, 1917.

(3) Wilson, N. M.; Flaherty, D. W. J. Am. Chem. Soc. 2016, 138, 574.