Adsorption and reaction of so ₂ with a polycrystalline UO ₂ film: Promotion of S - O bond cleavage by creation of O-defects and Na or Ca coadsorption

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 ∼ 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 (≥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 ≥ 4 ML) is attributed to charge transfer into the antibonding LUMO of the adsorbed molecule.