Effect of Pd Coordination and Isolation on the Catalytic Reduction of O2 to H2O2 over PdAu Bimetallic Nanoparticles

Abstract

The direct synthesis of hydrogen peroxide (H₂ + O₂ → H₂O₂) may enable low-cost H₂O₂ production and reduce environmental impacts of chemical oxidations. Here, we synthesize a series of Pd₁Au_x nanoparticles (where 0 ≤ x ≤ 220, ∼10 nm) and show that, in pure water solvent, H₂O₂ selectivity increases with the Au to Pd ratio and approaches 100% for Pd₁Au₂₂₀. Analysis of in situ XAS and ex situ FTIR of adsorbed ¹²CO and ¹³CO show that materials with Au to Pd ratios of ∼40 and greater expose only monomeric Pd species during catalysis and that the average distance between Pd monomers increases with further dilution. Ab initio quantum chemical simulations and experimental rate measurements indicate that both H₂O₂ and H₂O form by reduction of a common OOH* intermediate by proton–electron transfer steps mediated by water molecules over Pd and Pd₁Au_x nanoparticles. Measured apparent activation enthalpies and calculated activation barriers for H₂O₂ and H₂O formation both increase as Pd is diluted by Au, even beyond the complete loss of Pd–Pd coordination. These effects impact H₂O formation more significantly, indicating preferential destabilization of transition states that cleave O–O bonds reflected by increasing H₂O₂ selectivities (19% on Pd; 95% on PdAu₂₂₀) but with only a 3-fold reduction in H₂O₂ formation rates. The data imply that the transition states for H₂O₂ and H₂O formation pathways differ in their coordination to the metal surface, and such differences in site requirements require that we consider second coordination shells during the design of bimetallic catalysts.