Hydrogen peroxide (H2O2) is a selective, environmentally benign alternative oxidant, but is also prohibitively expensive. The direct synthesis (DS, H2 + O2 → H2O2) of hydrogen peroxide may decrease costs but few Pd-based catalysts have been demonstrated to be sufficiently selective and stable. The addition of Au to Pd has been known to improve H2O2 selectivity, but why such selectivity improvements are observed is less well understood. Here we demonstrate the critical role of in situ spectroscopic methods, performed at high pressures and at the liquid-solid interface, to directly link catalytic performance to catalyst structure. We have synthesized a series of highly uniform bimetallic Pd1Aux (molar basis; x = 10, 25, 50, 100) catalysts using a scalable synthesis technique. Selectivity towards H2O2 relative to H2O increases monotonically with increasing Au content ranging from 24% on monometallic Pd to &gt;99% on Pd1Au100 at moderate reaction conditions (55 kPa H2, 60 kPa O2, 276 K, DI water) in the absence of halide promoters. These differences reflect values of ΔH‡H2O2 that increase by ~20 kJ mol-1 while ΔH‡H2O values increase by &gt;50 kJ mol-1 across this series of materials. These differences reflect an increasing electronic modification of active sites that form H2O2 and H2O. Formation rates for each catalyst are consistent with previously proposed proton-electron transfer mechanisms on active sites that in situ Raman spectra show to be covered largely by mixtures of superoxide, hydroperoxide, and atomic oxygen under relevant conditions. FTIR spectra of CO adsorbed to these catalysts show that steady-state linear to bridging CO peak area ratios increase with increasing Au content, which indicates that the fractions of surface Pd atoms that exist as isolated monomers on the predominantly Au surface also increase. Isosteric desorption rates measured by 13CO-12CO exchange experiments and singleton frequency measurements show that the dilution of Pd sites among Au atoms leads to weaker adsorption and reduced electron exchange with the surface. These data suggest the activation enthalpies for reaction pathways depend on the coordination of Pd in bimetallic nanoparticles during direct synthesis, which we examine by comparisons with in situ X-ray absorption spectroscopy to relate Pd-Pd coordination numbers to observed selectivities and apparent barriers.

Role of in situ spectroscopy in elucidating the impact of Pd coordination on the catalytic reduction of O2 to H2O2 on bimetallic nanoparticles

2019 Fall National Meeting

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