Oxygen Activation and Reaction on Pd–Au Bimetallic Surfaces

Abstract

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.

Effect of annealing in oxygen on alloy structures of Pd–Au bimetallic model catalysts

“…52,[56][57][58] The growth of the Pd overlayer on the Au(111) surface at 77 K has been suggested to obey a layer-by-layer mechanism. 52,[56][57][58] The growth of the Pd overlayer on the Au(111) surface at 77 K has been suggested to obey a layer-by-layer mechanism.…”

“…7,36,40 As the annealing temperature is increased, the surface Pd morphology is transformed from a mixture of isolated and contiguous sites to a surface containing only isolated Pd sites. 52,57 The deposition rate of Pd was calibrated with a quartz crystal microbalance (QCM) controller (Maxtek Inc.) assuming a thickness of 1 ML Pd equals 0.274 nm. 52,57 The deposition rate of Pd was calibrated with a quartz crystal microbalance (QCM) controller (Maxtek Inc.) assuming a thickness of 1 ML Pd equals 0.274 nm.…”

“…52,56,57 The Pd−Au surface was first heated to 500 K at 1 K/s to desorb any surface contaminants such as CO. After the sample had cooled to 77 K, an IR background scan was taken. 52,56,57 The Pd−Au surface was first heated to 500 K at 1 K/s to desorb any surface contaminants such as CO. After the sample had cooled to 77 K, an IR background scan was taken.…”

“…52 The oxygen-precovered Pd−Au surface was then heated to 250 K prior to CO beam impingement. In O 2 -TPD measurements, oxygen was dosed by impinging an O 2 beam on the Pd−Au surface at 77 K until its saturation.…”

“…52 The influence of annealing in oxygen on atomic structure and catalytic activity of Pd−Au bimetallic surfaces was investigated by comparing the physicochemical and catalytic properties of Pd−Au surfaces that were annealed in an O 2 environment versus under UHV conditions. These annealed Pd/Au(111) surfaces were characterized and tested in a fashion similar to the approach employed in our previous study.…”