title: Adsorption and reaction of SO2 with a polycrystalline UO2 film: promotion of S-O bond cleavage by creation of O-defects and Na or Ca coadsorption.
abstract: To characterize UO(2) for its possible use in desulfurization applications, the interactions of molecular sulfur dioxide (SO(2)) with a polycrystalline uranium dioxide film have been studied by means of X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and low-energy ion scattering (LEIS). The stoichiometric, oxygen-deficient, calcium-precovered and sodium-precovered UO(2) surfaces have been characterized. The changes in oxide reactivity upon creation of oxygen vacancies and coadsorption of sodium and calcium have been studied. After creation of a reduced UO(2-x) surface (x approximately 0.44) via Ar(+) sputtering, the U 4f XPS spectrum shows conspicuous differences that are good indicators of the surface stoichiometry. Molecular SO(x) formation (x = 2-4) is observed after SO(2) deposition onto stoichiometric UO(2) and onto UO(2) precovered with small amounts (<1 ML) of Na or Ca; complete dissociation of SO(2) is not observed. Heating leads to desorption of the SO(x) species and to transformation of SO(2) to SO(3) and SO(3) to SO(4). On oxygen-deficient UO(2) and on UO(2) precovered with large Na or Ca coverages (> or =4 ML), both the formation of SO(x)= species and complete dissociation of SO(2) are observed. A higher thermal stability of the sulfur components is observed on these surfaces. In all cases for which dissociation occurs, the XPS peak of atomic sulfur shows similar structure: three different binding states are observed. The reactivity of oxygen-deficient UO(2) and sodium- and calcium-precovered UO(2) (coverages > or = 4 ML) is attributed to charge transfer into the antibonding LUMO of the adsorbed molecule.
authors.affiliation: Laboratory for Surface Modification, Department of Physics and Astronomy, Rutgers-The State University of New Jersey, Piscataway, NJ 08854, USA.
authors.forename: T W
authors.initials: TW
authors.lastname: Schlereth
authors.forename: M N
authors.initials: MN
authors.lastname: Hedhili
authors.forename: B V
authors.initials: BV
authors.lastname: Yakshinskiy
authors.forename: T
authors.initials: T
authors.lastname: Gouder
authors.forename: T E
authors.initials: TE
authors.lastname: Madey