AVS 45th International Symposium
    Surface Science Division Thursday Sessions
       Session SS2-ThM

Paper SS2-ThM8
Methanol Adsorption and Reactivity at U and UO@sub 2@ Surfaces

Thursday, November 5, 1998, 10:40 am, Room 309

Session: Oxide Surface Chemistry
Presenter: M.T. Paffett, Los Alamos National Laboratory
Authors: M.T. Paffett, Los Alamos National Laboratory
J.A. Lloyd, Los Alamos National Laboratory
Correspondent: Click to Email
In specific actinide materials storage situations it has been often observed that deleterious hydrocarbon interactions at actinide oxide surfaces have lead to a buildup of potentially explosive gases (usually H@sub 2@) arising from predominately radiolytic effects. Much less emphasis however, has been given to the deleterious consequences of unforeseen catalytic or reactive chemical events, arising from the storage of actinides and actinide oxides in mixed waste (actinides with organic solvents). In this vein we are seeking to unravel some of the fundamental details of reactive surface chemistry at model actinide oxide surfaces. In this study, the interaction of methanol-d (CH@sub 3@OD) with U and UO@sub 2@ surfaces has been studied using x-ray photoelectron spectroscopy (XPS), thermal desorption-mass spectrometry (TDMS), and secondary ion mass spectrometry (SIMS) over the temperature range of 90-500K. Low temperature (90 K) adsorption on uranium resulted in formation of methoxy species along with condensed phase adsorbed methanol. Room temperature (300K) adsorption on uranium produces only methoxy species, uranium carbide and uranium oxide. Heating to 400K completely decomposes the adsorbed methoxy species: 25% desorbing as methane and the remaining methoxide irreversibly converting to uranium oxycarbide (UO@sub 0.7@C@sub 0.3@). Concomitant with these C fragment conversion processes, desorption of hydrogen and deuterium is also seen over a wide temperature regime. Methanol-d adsorption on UO@sub 2@ also produces a methoxy surface species at surfaces temperatures << 150K and submonolayer exposure. Adsorbate decomposition following thermal desorption releases gaseous CH@sub 4@, H@sub 2@, HD, and D@sub 2@. Oxygen derived from the methanol molecule was incorporated into the UO@sub 2@ layer. The reactive adsorption and thermal decomposition of methanol at U and UO@sub 2@ surfaces is compared with that observed at other metal and metal oxide surfaces.