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 ≤1 × 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...