In-situ soft X-ray studies of CO oxidation on the Pt(111) surface

Abstract

Temperature-programmed and isothermal carbon–K-edge fluorescence yield measurements have been used to characterize carbon monoxide oxidation on the Pt(1 1 1) surface in pressures of flowing oxygen ranging from 0.00002 to 0.02 Torr over the 200–600 K temperature range. Temperature programmed-fluorescence yield near edge spectroscopy (TP-FYNES) of the oxidation of pre-adsorbed CO coverages in O2 pressures up to 0.02 Torr has been used to directly characterize surface reaction rates and to focus on the effects of temperature and oxygen flux during oxidation. For a saturated CO coverage, oxidation does not begin until significant CO desorption from the surface occurs. This finding is supported by the observation that the onset temperature for oxidation remains at 305 K for the CO saturated surface even for O2 pressures up to 0.02 Torr. For large CO coverages below full saturation, the onset temperature for oxidation decreases with increasing oxygen pressure, indicating oxygen adsorption is not completely inhibited. After oxidation begins for large CO coverages, oxidation rapidly decreases the CO surface concentration, as the activation energy for CO oxidation is smaller than the activation energy for CO desorption. Isothermal oxidation experiments clearly show that the oxidation activation energies observed depend on both the CO coverage and the O2 pressure. Oxidation activation energies are larger for small CO coverages, with a clear increase below 70% of a saturated CO coverage. For instance, at an oxygen pressure of 0.002 Torr, the activation energy for oxidation increases from 11.9 to 16.8 kcal/mol as the CO coverage decreases below 70% of a saturated coverage. In addition, oxidation activation energies decrease with increasing oxygen pressure. For example, as the oxygen pressure is increased from 0.002 to 0.02 Torr, the activation energy for oxidation of CO coverages below 70% of a saturated coverage is decreased from 16.8 to 12.3 kcal/mol. For the fully saturated CO coverage, the inhibition of oxygen adsorption completely suppresses oxidation below the CO desorption temperature. For large CO coverages below saturation where oxidation proceeds at a measurable rate, the order in oxygen is approximately zero (0.08), suggesting that oxidation is generally not limited by oxygen adsorption. Taken together, these findings clearly confirm that CO inhibition of O2 adsorption is crucial for the saturated coverage of CO. For CO coverages below saturation, interactions within the adsorbed layer significantly alter both the activation energy and pre-factor for CO oxidation on the low-index Pt(1 1 1) surface.