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

Abstract

We present results of our recent investigations detailing the growth and properties of oxygenphases prepared on Pt(111) and Pt(100) surfaces in ultrahigh vacuum using oxygen atombeams. Our studies reveal common features in the oxidation mechanisms of Pt(111) andPt(100). On both surfaces, oxygen atoms initially populate a chemisorbed phase, and thenincorporate into intermediate phases prior to the growth of bulk-like oxide. The bulk oxidegrows on both surfaces as three-dimensional particles with properties similar to those ofPtO2 and decomposes explosively during heating, exhibiting higher thermal stability than theintermediate oxygen phases. Our results suggest that kinetic barriers stabilize the oxideparticles against decomposition, thereby producing explosive desorption, and hence alsohinder Pt oxide growth at low coverages. We also find that the kinetics of bulk oxideformation on Pt(100) measured as a function of the O atom incident flux and surfacetemperature is quantitatively reproduced by a model based on a precursor-mediatedmechanism. The model assumes that oxygen atoms adsorbed on top of a surface oxidephase act as a precursor species that can either associatively desorb or react with thesurface oxide to produce a bulk oxide particle. Similarities in the development ofintermediate oxygen phases on Pt(100), Pt(111) and other transition metal surfaces suggestthat 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 oxygenphases on Pt(111) and Pt(100) toward the oxidation of CO, and that the reactionexhibits autocatalytic kinetics that can be explained with a model that treats thereaction as occurring within a dilute oxygen phase that coexists with oxide particles.