Effect of Pd Coordination and Isolation on the Catalytic Reduction of O to HO over PdAu Bimetallic Nanoparticles.

The direct synthesis of hydrogen peroxide (H + O → HO) may enable low-cost HO production and reduce environmental impacts of chemical oxidations. Here, we synthesize a series of PdAu nanoparticles (where 0 ≤ ≤ 220, ∼10 nm) and show that, in pure water solvent, HO selectivity increases with the Au to Pd ratio and approaches 100% for PdAu. Analysis of XAS and FTIR of adsorbed CO and CO 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. quantum chemical simulations and experimental rate measurements indicate that both HO and HO form by reduction of a common OOH* intermediate by proton-electron transfer steps mediated by water molecules over Pd and PdAu nanoparticles. Measured apparent activation enthalpies and calculated activation barriers for HO and HO formation both increase as Pd is diluted by Au, even beyond the complete loss of Pd-Pd coordination. These effects impact HO formation more significantly, indicating preferential destabilization of transition states that cleave O-O bonds reflected by increasing HO selectivities (19% on Pd; 95% on PdAu) but with only a 3-fold reduction in HO formation rates. The data imply that the transition states for HO and HO 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.