Methanol adsorption and reactivity at uranium and UO2 surfaces

The interaction of methanol (CH3OD) with polycrystalline uranium and UO2 has been studied by X-ray photoelectron spectroscopy( XPS), thermal desorption-mass spectrometry (TDMS) and secondary-ion mass spectrometry (SIMS) over the temperature range 90-500 K. Low-temperature (similar to 90 K) adsorption on uranium resulted in the formation of methoxy species along with condensed-phase adsorbed methanol. Room-temperature adsorption of methanol (or annealing an adsorbed methanol adlayer to 300 K) on uranium produces a mixture of methoxy and a uranium oxycarbide. Further heating to 400 K completely decomposes the adsorbed methoxy species, with 10% desorbing as methane and the remaining methoxide irreversibly converting to an uranium oxycarbide overlayer of nominal composition UO0.66C0.34. Concomitant with these carbon-fragment-conversion processes, desorption of hydrogen and deuterium are also seen over a wide temperature regime (325-450 K). Methanol-d adsorption on UO2 also produces a methoxy surface species at surfaces temperatures <150 K. Adsorbate decomposition following thermal desorption releases gaseous CH4, H-2, HD, D-2 and CO. Most of the oxygen derived from the methanol molecule (>90%) was incorporated into the UO2 layer and all of the adsorbed carbon desorbs from the surface as one of the molecular species identified above. The reactive adsorption and thermal decomposition of methanol at uranium and UO2 surfaces is compared with that observed at other metal and metal oxide surfaces. (C) 1999 Elsevier Science B.V. All rights reserved.