Pd–Au bimetallic catalysts have shown promising performance for a number of oxidative reactions. The present study utilizes reactive molecular beam scattering (RMBS), reflection–absorption infrared spectroscopy (RAIRS), temperature-programmed desorption (TPD), and density functional theory (DFT) techniques in an attempt to enhance the fundamental understanding of oxygen activation and reaction with CO on Pd–Au surfaces. Our results reveal that the presence of contiguous Pd sites is crucial for adsorption of oxygen molecules on Pd/Au(111) surfaces at 77 K. Upon heating, oxygen admolecules desorbed molecularly without detectable dissociation in O2-TPD measurements. CO-RMBS experiments indicate that at lower temperatures (77–150 K) oxygen admolecules were readily displaced by CO due to competitive adsorption. Oxygen admolecules can be thermally activated at higher temperatures (180–250 K) to react with CO to form CO2. DFT calculations show that the Pd–Au surface containing larger Pd ensembles favors dissociative CO oxidation, whereas associative CO oxidation and O2 desorption are the two main competing processes for the Pd–Au surface containing small Pd ensembles. An associative CO oxidation pathway was not experimentally observed, which is likely due to facile CO-induced O2 desorption. These results provide mechanistic insights into the interaction of oxygen with Pd–Au surfaces, which may prove informative for the rational design of Pd–Au catalysts for associated reactions involving O2 as a reactant.

Pd-Au catalysts have shown exceptional performance for selective hydrogen production via HCOOH decomposition, a promising alternative to solve issues associated with hydrogen storage and distribution. In this study, we utilized temperature-programmed desorption (TPD) and reactive molecular beam scattering (RMBS) in an attempt to unravel the factors governing the catalytic properties of Pd-Au bimetallic surfaces for HCOOH decomposition. Our results show that Pd atoms at the Pd-Au surface are responsible for activating HCOOH molecules; however, the selectivity of the reaction is dictated by the identity of the surface metal atoms adjacent to the Pd atoms. Pd atoms that reside at Pd-Au interface sites tend to favor dehydrogenation of HCOOH, whereas Pd atoms in Pd(111)-like sites, which lack neighboring Au atoms, favor dehydration of HCOOH. These observations suggest that the reactivity and selectivity of HCOOH decomposition on Pd-Au catalysts can be tailored by controlling the arrangement of surface Pd and Au atoms. The findings in this study may prove informative for rational design of Pd-Au catalysts for associated reactions including selective HCOOH decomposition for hydrogen production and electro-oxidation of HCOOH in the direct formic acid fuel cell.

Pd−Au bimetallic catalysts have shown promising performance in numerous reactions that involve hydrogen. Fundamental studies of hydrogen interactions with Pd−Au surfaces could provide useful insights into the reaction mechanisms over Pd−Au catalysts, which may, in turn, guide future catalyst design. In this study, the interactions of hydrogen (i.e., adsorption, absorption, diffusion, and desorption) with Pd/Au(111) model surfaces were studied using temperature-programmed desorption (TPD) under ultrahigh-vacuum conditions. Our experimental results reveal Pd−Au bimetallic surfaces readily dissociate H 2 and yet also weakly bind H adatoms, properties that could be beneficial for catalytic reactions involving hydrogen. The presence of contiguous Pd sites, characterized by reflection−absorption infrared spectroscopy using CO as a probe molecule (CO-RAIRS), was found to be vital for the dissociative adsorption of H 2 at 77 K. The H adatom binds to Pd−Au alloy sites more strongly than to Au(111) but more weakly than to Pd(111) as indicated by its desorption temperature (∼200 K). With hydrogen exposure at slightly higher temperatures (i.e., 100−150 K), extension of a low-temperature desorption feature was observed, suggesting the formation of subsurface H atoms (or H absorption). Experiments using deuterium indicate that H−D exchange over the Pd−Au bimetallic surface obeys Langmuir−Hinshelwood kinetics and that H/D adatoms are mobile on the surface at low temperatures.

In this study, we have combined ultra-high vacuum (UHV) experiments and density functional theory (DFT) calculations to investigate ethanol (EtOH) dehydrogenation on Pd-Au model catalysts. Using EtOH reactive molecular beam scattering (RMBS), EtOH temperature-programmed desorption (TPD), and DFT calculations, we show how different Pd ensemble sizes on Au(111) can affect the mechanism for EtOH dehydrogenation and H production. The Au(111) surface with an initial coverage of 2 monolayers of Pd (2 ML Pd-Au) had the highest H yield. However, the 1 ML Pd-Au catalyst showed the highest selectivity and stability, yielding appreciable amounts of only H and acetaldehyde. Arrhenius plots of H production confirm that the mechanisms for EtOH dehydrogenation differed between 1 and 2 ML Pd-Au, supporting the perceived difference in selectivity between the two surfaces. DFT calculations support this difference in mechanism, showing a dependence of the initial dehydrogenation selectivity of EtOH on the size of Pd ensemble. DFT binding energies and EtOH TPD confirm that EtOH has increasing surface affinity with increasing Pd ensemble size and Pd coverage, indicating that surfaces with more Pd are more likely to induce an EtOH reaction instead of desorb. Our theoretical results show that the synergistic influence of atomic ensemble and electronic effects on Pd/Au(111) can lead to different H association energies and EtOH dehydrogenation capacities at different Pd ensembles. These results provide mechanistic insights into ethanol's dehydrogenation interactions with different sites on the Pd-Au surface and can potentially aid in bimetallic catalyst design for applications such as fuel cells.

Partial oxidation of alcohols is a topic of great interest in the field of gold catalysis. In this work, we provide evidence that the partial oxidation of allyl alcohol to its corresponding aldehyde, acrolein, over oxygen-precovered gold surfaces occurs via multiple reaction pathways. Utilizing temperature-programmed desorption (TPD) with isotopically labeled water and oxygen species, reactive molecular beam scattering, and density functional theory (DFT) calculations, we demonstrate that the reaction mechanism for allyl alcohol oxidation is influenced by the relative proportions of atomic oxygen and hydroxyl species on the gold surface. Both atomic oxygen and hydroxyl species are shown to be active for allyl alcohol oxidation, but each displays a different pathway of oxidation, as indicated by TPD measurements and DFT calculations. The hydroxyl hydrogen of allyl alcohol is readily abstracted by either oxygen adatoms or adsorbed hydroxyl species on the gold surface to generate a surface-bound allyloxide intermediate, which then undergoes α-dehydrogenation via interaction with an oxygen adatom or surface hydroxyl species to generate acrolein. Mediation of a second allyloxide with the hydroxyl species lowers the activation barrier for the α-dehydrogenation process. A third pathway exists in which two hydroxyl species recombine to generate water and an oxygen adatom, which subsequently dehydrogenates allyloxide. This work may aid in the understanding of oxidative catalysis over gold and the effect of water therein.