Solvent molecules influence the reactions of molecular hydrogen and oxygen on palladium nanoparticles. Organic solvents activate to form reactive surface intermediates that mediate oxygen reduction through pathways distinct from reactions in pure water. Kinetic measurements and ab initio quantum chemical calculations indicate that methanol and water cocatalyze oxygen reduction by facilitating proton-electron transfer reactions. Methanol generates hydroxymethyl intermediates on palladium surfaces that efficiently transfer protons and electrons to oxygen to form hydrogen peroxide and formaldehyde. Formaldehyde subsequently oxidizes hydrogen to regenerate hydroxymethyl. Water, on the other hand, heterolytically oxidizes hydrogen to produce hydronium ions and electrons that reduce oxygen. These findings suggest that reactions of solvent molecules at solid-liquid interfaces can generate redox mediators in situ and provide opportunities to substantially increase rates and selectivities for catalytic reactions.

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.

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Catalytic functions at interfaces between oxides and Au nanoparticles assist the activation of O2 and H2O2 during selective oxidations. We use in situ surface-enhanced Raman spectroscopy to reveal the differences in the types and distributions of reactive oxygen species (ROS) derived from H2O2 on Au catalysts that reflect interactions at nanoparticle–support interfaces. The pristine Au(111) does not activate H2O2 to form detectable surface intermediates, whereas Au nanoparticles on SiO2 bind small amounts of diatomic oxygen intermediates. In comparison, nanoparticles of Au on γ-Al2O3 bind significant coverages of diatomic and monoatomic oxygen species formed by activation processes that appear to involve hydroxyl (OH*) functions present on the support. Electrochemically roughened Au(111) activates O–O bonds in H2O2 by interactions with OH* groups to produce high atomic oxygen coverages. These observations appear consistent with comparisons between oxidation rates and barriers for supported Au catalysts and provide direct evidence for the involvement of interfacial sites and OH* groups in elementary steps that determine the distribution of ROS upon surfaces.