H₂O-assisted O₂ reduction by H₂ on Pt and PtAu bimetallic nanoparticles: Influences of composition and reactant coverages on kinetic regimes, rates, and selectivities

Hydrogen peroxide (H₂O₂) can replace hazardous oxidants in industrial processes but is currently too expensive for many such applications. While direct synthesis of H₂O₂ (H₂ + O₂ → H₂O₂) may reduce costs in comparison to incumbent technology, current catalysts lack the requisite stability and selectivity. Here, we examine the direct synthesis of H₂O₂ on bimetallic Pt₁Au_x (0 ≤ x ≤ 230) and Pt catalysts at steady-state in pure water and relate kinetic parameters for H₂O₂ and H₂O formation to possible active site structures informed by complementary characterization methods. X-ray photoelectron spectra show significant Pt surface enrichment compared to the bulk composition. Analysis of infrared spectra of mixed monolayers of ¹²CO* and ¹³CO* indicate that Pt and Au form substitutional surface alloys. The Pt₁Au_x nanoparticles with the greatest mole fractions of Au predominantly expose Pt monomers (i.e., isolated Pt atoms), yet Pt atoms exposed upon all these nanoparticles possess electronic structures distinct from bulk Pt. Despite these differences, rate measurements are consistent with product formation through proton-electron transfer pathways for all Pt₁Au_x catalysts. In situ XAS indicate that Pt remains metallic during H₂O₂ synthesis. Under the most oxidizing conditions, selectivities toward H₂O₂ increase strongly with the Au to Pt ratio from 2% for monometallic Pt to 85% for Pt₁Au₁₇₀. However, selectivities are similar among all catalysts within reducing conditions. Comparisons of apparent activation enthalpies for the formation of H₂O₂ and H₂O across these catalysts and the range of conditions suggest that Pt monomers within Au provide the greatest selectivities for H₂O₂ formation, because these active sites present high barriers for O-O bond rupture. Selectivities decrease with increasing ratios of H₂ to O₂ pressures, because Pt atoms aggregate and form oligomers that readily dissociate dioxygen intermediates. The combined use of spectroscopy, kinetics, and concepts employed in reaching these conclusions take inspiration from the legacy of Prof. Michel Boudart, and specifically his elegant methods for interrogating bimetallic catalysts.