Chemisorption and Reaction of NO and N_2O on Oxidized and Reduced Ceria Surfaces Studied by Soft X-Ray Photoemission Spectroscopy and Desorption Spectroscopy

摘要

We have examined the chemisorption and reaction of NO and N_2O on ceria surfaces by soft X-ray photoelectron spectroscopy (SXPS),X-ray absorption spectroscopy,and isothermal and tem-perature programmed desorption (TPD) spectroscopy.Samples in-clude highly oriented CeO_2(100) and CeO_2(111) thin films with sur-faces of variable oxidation states.CeO_2(111) thin films were grown in situ allowing control over the initial oxidation state of the ceria suface,quantitatively determined by valence band,Ce 4d,and Ce 3d photoemission.Following NO exposure,various N-containing surface species were observed by N 1s photoemission,and the dis-tribution of these species depended upon surface oxidation state,exposure,and adsorption temperature.These species included N_2O,nitrite,NO~-,and three states believed to be associated with atomic or anionic forms of N.Nitrite and N_2O are seen on a fully oxidized surface while NO~-,N_2O,and dissociation products are observed on a reduced surface.The primary reaction of NO with reduced ceria is reoxidation of the ceria by the NO,both by NO~- formation and by NO dissociation,leading to immediate and thermally induced N_2 desorption.Adsorption of NO at 150 K is predominantly molec-ular while exposure to NO at 400 K leads to a thermally activated nitride which desorbs at temperatures above 500 K.Dissociative ad-sorption of NO leads to the displacement of N~(3-) at high exposures.