The influence of surface defects on ethanol dehydrogenation versus dehydration on the UO2(111) surface

The decomposition of ethanol has been investigated on Ar+-sputtered surface s of a UO2(111) single crystal. X-ray photoelectron spectroscopy (XPS) of t he U 4f peaks after sputtering for 1 h showed the presence of two distinct oxidation states: U4+ (U 4f(7/2) at 380.2 eV) and U-0 (U 4f(7/2) at 377.4 e V). Upon ethanol exposure at room temperature, the peak at 377.4 eV was att enuated, indicating that U-0 sites were oxidized to Ux+i(x less than or equ al to 4). The presence of a mixture of oxidation states on the surface infl uenced the reaction products observed during temperature programmed desorpt ion (TPD). While ethylene and acetaldehyde desorbed in one temperature doma in (at 560 K) from stoichiometric UO2(111), an additional desorption domain (at 475 K) was observed over the substoichiometric surface. The ratio of a cetaldehyde to ethylene produced was different in the two temperature domai ns. While this ratio was near unity for the 560 K domain, it decreased to c a. 0.5 for the 475 K peaks on the substoichiometric surface. The lower temp erature reaction channel is likely associated with surface oxygen vacancies , as it leads to greater oxygen abstraction, forming ethylene from surface ethoxide species. (C) 2000 Elsevier Science B.V. All rights reserved.