Oxygen Activation and Reaction on Pd−Au Bimetallic Surfaces

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), temperatureprogrammed 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 O2TPD 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.