UCLA

Supported Pd-Au bimetallic nanoparticles are a promising class of catalysts used for hydrogenation and oxidation reactions. Recently, the role of dynamic restructuring of Pd regions at and near the nanoparticle surface in response to modulating gas (H2 and O2) concentration was highlighted for controlling the surface Pd oxide stoichiometry. Here we investigate the mechanism of formation and decomposition of Pd hydride (PdHx) at and near the bimetallic nanoparticle surfaces, a key species for controlling the activity, selectivity, and stability of Pd catalysts in many hydrogenation reactions. Density functional theory (DFT) calculations provide additional insights into the stability and energetics of PdHx formation under varying H fractions and Pd sub-structures. Our results reveal a complex interplay between Pd ensemble size, surface structure, and hydrogen environment in determining the kinetics and thermodynamics of PdHx formation. By elucidating the mechanisms underlying surface PdHx formation and decomposition, the rational design of dynamic catalysts with controlled Pd hydride stoichiometries becomes possible. Hydrogen dissociation is an essential step in hydrogenation catalysis and critical to catalyst performance. Recently, Au–Pd core-shell nanoparticle catalysts have emerged as a highly promising class of materials for hydrogenation catalysis. Yet, fundamental understanding of the origin of bimetallic synergy in these core-shell catalysts and their interaction with hydrogen remains largely unknown. Here, we investigate the kinetics of hydrogen dissociation and recombination over Au–Pd core-shell catalysts using the hydrogen-deuterium (HD) exchange reaction. Density functional theory calculations indicate that subsurface hydrogen plays a major role in accelerating the hydrogen-deuterium exchange reaction by providing a lower-energy pathway for recombinative desorption of hydrogen and deuterium, which is the rate-limiting step. Altogether, our combined experiment-theory approach revealed that lattice strain and hydride formation can significantly influence the hydrogenation behavior of Au–Pd core-shell nanoparticles and are relevant to further advance the design of core-shell materials for hydrogenation catalysis. The ethanol oxidation reaction (EOR) has attracted considerable attention because of the high energy density of ethanol and its low-cost, renewable production from biomass. However, EOR suffers from sluggish kinetics and generally does not proceed to complete oxidation. In this study, we employed first-principles calculations to investigate the stability of Ptn clusters on various graphite surfaces, determine the optimal hydroxyl coverages, and explore potential-dependent EOR pathways. Our results show that the formation of Ptn clusters on defective graphite is highly favorable, particularly when the initial kinetic energy imparted to the clusters during hard-landing experiments is taken into account. The calculations further indicate that complete ethanol oxidation requires low-to-intermediate potentials and may involve changes in cluster configuration. At low potentials, the EOR exhibits low reactivity but high selectivity, whereas at high potentials, its reactivity increases at the expense of selectivity. In addition, our DFT calculations reveal CO poisoning of the cluster catalysts, in agreement with experimental observations.