CO Oxidation over AuPd(100) from Ultrahigh Vacuum to Near-Atmospheric Pressures: CO Adsorption-Induced Surface Segregation and Reaction Kinetics

Polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS) is used to study CO adsorption on AuPd(100) surfaces, one well-annealed such that contiguous Pd sites do not initially exist and a second ion sputtered such that contiguous Pd sites are exposed. At CO pressures e1 × 10-3 Torr, Pd segregation to the surface is found; however, it is not sufficient to form contiguous Pd sites on the well-annealed sample. At CO pressures higher than ∼0.1 Torr, Pd segregation becomes greatly enhanced such that contiguous Pd sites form. Isosteric heats of adsorption of CO on Au sites and isolated Pd sites are measured to be 69 and 84 kJ/mol, respectively. The CO heat of adsorption on contiguous Pd sites is found to lie between these values. The CO oxidation reaction, monitored using both PM-IRAS and reaction kinetic measurements, occurs only on surfaces with contiguous Pd sites, due to the fact that O2 dissociation takes place exclusively on these sites. The surface composition and distribution can be tuned by varying the sample temperature, reactant pressure, and compositions, thus affecting the rate of CO2 formation. The extremely high reaction rate at relatively low temperatures is attributed to the greatly reduced binding energy of CO with the surface such that CO inhibition, a factor that causes low reaction rate on pure Pd at these temperatures, does not exist with the alloy surface.