Temperature programmed desorption characterization of oxidized uranium surfaces: Relation to some gas-uranium reactions

The chemisorption characteristics and surface composition of oxidation over layers developing on metals when exposed to oxidizing atmospheres are impor tant in determining the protective ability of these layers against certain gas-phase reactions (e.g., corrosion and hydriding). In the present study, a special setup of supersonic molecular beam-temperature-programmed desorpt ion was utilized to determine the different chemisorbed species present on oxidized uranium surfaces. The main identified species included water (in d ifferent binding forms) and hydrogen. The latter hydrogen originates from t he water-uranium oxidation reaction, which produces uranium dioxide and two types of hydrogen: a near surface hydride and a surface-chemisorbed form t hat desorbs at a lower temperature than that of the hydride. Assignments of the different water desorption peaks to different binding sites were propo sed. In general, four water desorption features were identified (labeled W- 0, W-1, W-2, and Wa, respectively, in the order of increasing desorption te mperatures). These features correspond to a reversibly chemisorbed molecula r form (W-0), a more tightly bound water (chemisorbed on different type of oxide sites) or hydroxyl clusters (W-1), and strongly bounded (possibly iso lated) hydroxyl groups (W-2). The highest temperature peak (W-3) is related to the formation of complex water-carbo-oxy compounds and is present only on oxidation overlayers, which contain proper chemisorbed carbo-oxy species . The relation of the water and hydrogen thermal release behavior to some p roblems addressed to certain effects observed in hydrogen-uranium and water -uranium reactions is discussed. For the latter, a microscopic mechanism is proposed.