Direct synthesis of H2O2 (DS, H2 + O2 → H2O2) may enable low-cost production of H2O2 and thus replacement of carcinogenic halogenated oxidants, which would reduce the environmental impacts of industrial selective oxidations. However, few catalysts have been demonstrated to be sufficiently selective and stable for commercial implementation. Many groups have reported that DS selectivity may be improved by alloying Pd with Au, but the reason why PdAu alloys are more selective than Pd is not known. We combine detailed kinetic analysis and orthogonal characterization techniques to identify the cause of selectivity improvements in PdAu alloys. PdAu nanoparticles with mean atomic Au/Pd ratios of 5 to 100 were synthesized by reduction of aqueous Pd(NO3)2 onto Au nanoparticles supported on SiO2, previously prepared by strong electrostatic adsorption of Au complexes and thermal reduction. Rates for H2O2 and H2O formation are independent of O2 pressure (40-400 kPa O2) and initially increase in proportion to H2 pressure (20-100 kPa H2) before approaching constant values at greater pressures (100-400 kPa H2). Such trends are consistent with formation of H2O2 and H2O by kinetically relevant proton-electron transfer. We observe that addition of Au to Pd results in selectivity increases and rate decreases caused by differences in apparent activation enthalpies (ΔH‡). Values of ΔH‡ for H2O2 formation increase from -3 to 19 kJ mol-1 across this series of materials but are constant for catalysts with larger Au (Au/Pd &gt; 25) contents. Yet, ΔH‡ for H2O formation monotonically increase from 8 to 68 kJ mol-1 with Au content. XAS obtained in situ show that Pd predominantly exists as isolated monomers in the Au matrix for materials with Au/Pd ratios &gt; 25. Analysis of vibrational stark effects (dipole coupling shifts) for adsorbed 12CO and 13CO, in conjunction with statistical arguments, show that the distance between Pd monomers increases systematically with catalyst Au content. Comparisons of kinetic and spectroscopic data show that the selectivity improvements in PdAu alloys arise from differences in active site requirements for H2O and H2O2 formation. On Pd atoms, H2O2 transition states are sensitive to adjacent metal atoms, whereas H2O transition states are affected by both the nearest and next-nearest neighbors. These results show that both the first and second coordination shells should be considered in the design of active sites on bimetallic nanoparticles.

Effect of Pd isolation on the reduction of O2 to H2O2 over PdAu bimetallic nanoparticles

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.