In-Situ Soft X-Ray Studies of CO Oxidation on the Pt(111) Surface

Published

August 1, 2004

Author(s)

D J. Burnett, A T. Capitano, A M. Gabelnick, A L. Marsh, Daniel A. Fischer, J L. Gland

Abstract

Temperature programmed and isothermal fluorescence yield measurements have been used to characterize carbon monoxide oxidation on the Pt(111) surface for oxygen pressures from 0.027 Pa to 2.7 Pa over the 200 to 600 K temperature range. Carbon monoxide concentrations have been determined under reaction conditions using fluorescence intensities in the carbon continuum standardized using a saturated monolayer. Temperature Programmed-Fluorescence Yield Near Edge Spectroscopy (TP-FYNES) studies of oxidation of preadsorbed CO in oxygen pressures (up to 2.7 Pa) have been used to characterize the surface reaction rates. For CO saturated surfaces, oxidation is clearly limited by CO desorption. For the saturated CO monolayer, the onset for oxidation remains at 305 K for oxygen pressures up to 2.7 Pa, indicating that the adsorbed CO completely limits dissociative oxygen adsorption. Oxidation only begins after CO desorption begins. For partial monolayers of preadsorbed carbon monoxide, the oxidation initiation temperature decreases markedly with increasing oxygen pressure, indicating oxygen adsorption is not completely inhibited. After oxidation begins for both the saturated and partial CO monolayers we observe a precipitous drop in CO coverage with increasing temperature. This result indicates that the activation energy for surface oxidation is smaller than the activation energy for CO desorption over a wide range of coverages. Isothermal oxidation experiments reveal that the oxidation activation energies observed depend on both the CO coverage and the oxygen pressure. Oxidation activation energies are smaller for large CO coverages, with a clear increase below monolayer (ML) = 0.7 CO. Above ML = 0.7 CO, the activation energy for oxidation decreased from 50.0 to 38.2 kJ/mol as the oxygen pressure increased from 0.27 to 2.7 Pa. Below ML =0.7 CO, the activation energy for oxidation decreased from 70.6 to 51.6 kJ/mol as the oxygen pressure increased from 0.27 to 2.7 Pa. In large excesses of oxygen the order in oxygen was approximately zero (0.08), suggesting that the surface is nearly oxygen saturated. Together, these results indicate that interactions between coadsorbed CO and atomic oxygen significantly alter the activation energy for CO oxidation on the low index Pt(111) surface.

Citation

Surface Science

Volume

564

Issue

No. 1-3

Pub Type

Journals

Keywords

carbon dioxide, intermediates, oxidation, plantinum, soft x-ray, surface science

Citation

Burnett, D. , Capitano, A. , Gabelnick, A. , Marsh, A. , Fischer, D. and Gland, J. (2004), In-Situ Soft X-Ray Studies of CO Oxidation on the Pt(111) Surface, Surface Science (Accessed September 9, 2024)