Temperature-programmed reaction of CO adsorbed on oxygen-covered Pt(100): Reactivity of high-coverage oxygen phases

We utilized temperature-programmed reaction spectroscopy (TPRS) to investigate the reactivity of oxygen-covered Pt(100) toward the oxidation of CO. The reaction is facile on oxygen phases that form below coverages of about 1 ML (monolayers), producing complicated CO2 desorption traces. However, the reaction is inefficient when three-dimensional (3D) oxide particles cover the surface. We observe CO2 production in roughly three temperature regimes during TPRS. Between 120 and 220 K, O atoms react with CO molecules adsorbed within oxygen phase domains. In this regime, the reactivity toward CO is highest for the two-dimensional (2D) surface oxide and decreases in the order of metastable O phase, O chemisorbed in a disordered (3 x 1) phase, and 3D oxide. Decreases in the CO and O binding strengths generally correlate with increasing reactivity of the oxygen phases below 220 K. At temperatures from about 225 to 335 K, reaction involves species at the boundaries of separate O and CO domains. As the temperature surpasses 335 K, CO2 production occurs between CO and disordered or isolated O adatoms. Within the second temperature regime, our data suggests that both Pt and O atoms migrate away from 2D and 3D oxide domains and into CO domains where reaction occurs. Kinetic barriers associated with the creation and transport of Pt adatoms likely inhibit CO oxidation by the 3D oxide. We also find that the generation of Pt adatoms facilitates significant surface defect formation and oxygen loss to the bulk during reaction. The results of this investigation clearly demonstrate that the surface oxygen phase distribution strongly influences the temperature-programmed reaction of coadsorbed CO and O on Pt(100).