Defect Enhanced Carbon Monoxide Oxidation at Elevated Oxygen Pressures on a Pt/Al2O3 Thin Film

Abstract

Carbon monoxide oxidation has been studied, using in situ soft X-ray techniques and ultrahigh vacuum (UHV) temperature-programmed reaction spectroscopy (TPRS), on a well-characterized supported platinum thin film over the100-1000 K temperature range and in pressures ranging from UHV to 0.01 Torr. In high oxygen pressures the reaction is enhanced by oxidation at defect sites. Temperature-programmed fluorescence yield near-edge spectroscopy (TP-FYNES) studies of preadsorbed CO coverages heated in pressures of flowing oxygen, ranging from I x 10 - 5 to I x 10 - 2 Torr, have been performed. Unlike with the Pt(111) surface, the onset temperature for oxidation of a saturated coverage of CO on the platinum thin film decreases dramatically with increasing oxygen pressure. The CO oxidation onset temperature decreased from 340 K in 1 x 10 - 5 Torr of flowing oxygen to 230 K in 1 x 10 - 2 Torr of flowing oxygen. Therefore, oxidation is not limited by CO desorption for high CO coverages, and reactions at defect sites control the rate of CO oxidation at high oxygen pressures. Isothermal oxidation experiments in 0.002 Torr of flowing oxygen have yielded an activation energy of 8.5 kcal/mol. TPRS of coadsorbed carbon monoxide and oxygen has indicated a primary oxidation channel at 350 K corresponding to the oxidation of CO on Pt( 111) terrace sites and minor oxidation channels above 400 K. The thin Pt/Al 2 O 3 film has been characterized using both chemical and structural methods. CO and O 2 temperature-programmed desorption (TPD) experiments indicate the Pt film surface is similar to Pt(111), but with a higher defect site concentration. Plane-view and cross-section transmission electron microscopy (TEM) experiments have confirmed a 100 A Pt film consisting of ∼450 A, well-ordered grains, as indicated by local area electron diffraction patterns, with the (111) orientation parallel to the substrate.