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), 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 PdAu 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 O-2-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 PdAu surface containing larger Pd ensembles favors dissociative CO oxidation, whereas associative CO oxidation and O-2 desorption are the two main competing processes for the PdAu surface containing small Pd ensembles. An associative CO oxidation pathway was not experimentally observed, which is likely due to facile CO-induced O-2 desorption. These results provide mechanistic insights into the interaction of oxygen with PdAu surfaces, which may prove informative for the rational design of PdAu catalysts for associated reactions involving O-2 as a reactant.