Ethanol reactions over the UO2(111) single crystal:: effect of the Madelung potential on the reaction selectivity

The reactions of ethanol on UO2(111) single crystal have been investigated by temperature programmed desorption (TPD) while the surface characteristics were studied by Auger electron spectroscopy and low energy electron diffraction. Ethanol-TPD indicated products desorption in two temperature domains (at 370 and 570 K), with a coincident desorption of ethanol, ethylene and acetaldehyde at the high temperature region. The sticking coefficient of ethanol at 300 K is found equal to 0.6+/-0.2 at zero coverage and decreases to ea. 10(-2) near saturation. The ratio acetaldehyde/ethylene of 0.8+/-0.05 indicates 'equal' sites for dehydration and dehydrogenation reactions. This ratio was affected neither by changing ethanol coverage nor by pre-dosing the surface with O-2. The comparison of the reaction pathways of ethanol over UO2(111), from this work, with that of other well-defined oxides indicated an inverse linear relationship between the dehydrogenation/dehydration pathway over TiO2(001) [Kim and Barteau, J. Mol. Catal. 63 (1990) 103], ZnO(0001) [Vohs and Barteau, Surf. Sci. 221 (1989) 590], Cu2O(100) [Schulz and Cox, J. Phys. Chem. 97 (1993) 647], ZrO2(110) [Dilara and Vohs, Surf. Sci. 321 (1994) 8] and UO2(111) single crystals and their dielectric constant as well as the corresponding Madelung potentials. (C) 2000 Elsevier Science B.V. All rights reserved.