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.

Characterization techniques beyond microscopy, scattering and spectroscopy approaches are needed to understand and improve sub-angstrom discrimination between penetrants in carbon molecular sieve (CMS) membranes. Here we use a method based on molecular scale gas diffusion probes to understand relevant membrane properties at the required level of detail. We further use this method to consider hypotheses about the evolution of structure responsible for fundamental properties of CMS materials derived from a high performance CMS precursor polymer, 6FDA:BPDA-DAM. While 6FDA:BPDA-DAM derived CMS membranes display a ~230 % improvement in CO 2 permeability when compared to Matrimid ® derived CMS formed under the same conditions, the diffusional selectivity for these two materials are very similar at 35 and 38.5, respectively. These results indicate a non-trivial connection between CMS precursor material structure and resulting performance. Linking hypotheses about structural changes likely to occur during pyrolysis with the probe data provides insights regarding transformation of the random coil polyimide into ultra-rigid CMS, with exquisite size and shape diffusion selectivity. The results provide a framework for understanding and tuning properties of this special class of materials with important technological advantages in energy-intensive gas separations.

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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We examine relationships between H 2 O 2 and H 2 O formation on metal nanoparticles by the electrochemical oxygen reduction reaction (ORR) and the thermochemical direct synthesis of H 2 O 2 . The similar mechanisms of such reactions suggest that these catalysts should exhibit similar reaction rates and selectivities at equivalent electrochemical potentials (μ ̅ i ), determined by reactant activities, electrode potential, and temperature. We quantitatively compare the kinetic parameters for 12 nanoparticle catalysts obtained in a thermocatalytic fixed-bed reactor and a ring− disk electrode cell. Koutecky−Levich and Butler−Volmer analyses yield electrochemical rate constants and transfer coefficients, which informed mixed-potential models that treat each nanoparticle as a short-circuited electrochemical cell. These models require that the hydrogen oxidation reaction (HOR) and ORR occur at equal rates to conserve the charge on nanoparticles. These kinetic relationships predict that nanoparticle catalysts operate at potentials that depend on reactant activities (H 2 , O 2 ), H 2 O 2 selectivity, and rate constants for the HOR and ORR, as confirmed by measurements of the operating potential during the direct synthesis of H 2 O 2 . The selectivities and rates of H 2 O 2 formation during thermocatalysis and electrocatalysis correlate across all catalysts when operating at equivalent μ ̅ i values. This analysis provides quantitative relationships that guide the optimization of H 2 O 2 formation rates and selectivities. Catalysts achieve the greatest H 2 O 2 selectivities when they operate at high H atom coverages, low temperatures, and potentials that maximize electron transfer toward stable OOH* and H 2 O 2 * while preventing excessive occupation of O−O antibonding states that lead to H 2 O formation. These findings guide the design and operation of catalysts that maximize H 2 O 2 formation, and these concepts may inform other liquid-phase chemistries.