Growth and properties of high-concentration phases of atomic oxygen on platinum single-crystal surfaces

We present results of our recent investigations detailing the growth and properties of oxygen phases prepared on Pt(111) and Pt(100) surfaces in ultrahigh vacuum using oxygen atom beams. Our studies reveal common features in the oxidation mechanisms of Pt(111) and Pt(100). On both surfaces, oxygen atoms initially populate a chemisorbed phase, and then incorporate into intermediate phases prior to the growth of bulk-like oxide. The bulk oxide grows on both surfaces as three-dimensional particles with properties similar to those of PtO2 and decomposes explosively during heating, exhibiting higher thermal stability than the intermediate oxygen phases. Our results suggest that kinetic barriers stabilize the oxide particles against decomposition, thereby producing explosive desorption, and hence also hinder Pt oxide growth at low coverages. We also find that the kinetics of bulk oxide formation on Pt(100) measured as a function of the O atom incident flux and surface temperature is quantitatively reproduced by a model based on a precursor-mediated mechanism. The model assumes that oxygen atoms adsorbed on top of a surface oxide phase act as a precursor species that can either associatively desorb or react with the surface oxide to produce a bulk oxide particle. Similarities in the development of intermediate oxygen phases on Pt(100), Pt(111) and other transition metal surfaces suggest that precursor-mediated kinetics may be a general feature in transition metal oxidation. Finally, we find that Pt oxide particles are less active than lower-coverage oxygen phases on Pt(111) and Pt(100) toward the oxidation of CO, and that the reaction exhibits autocatalytic kinetics that can be explained with a model that treats the reaction as occurring within a dilute oxygen phase that coexists with oxide particles.