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

The reactions of ethanol on UO2(111) single crystal have been investigated by temperature programmed desorption (TPD) while the surface characteristic s were studied by Auger electron spectroscopy and low energy electron diffr action. Ethanol-TPD indicated products desorption in two temperature domain s (at 370 and 570 K), with a coincident desorption of ethanol, ethylene and acetaldehyde at the high temperature region. The sticking coefficient of e thanol 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+/-0.0 5 indicates 'equal' sites for dehydration and dehydrogenation reactions. Th is ratio was affected neither by changing ethanol coverage nor by pre-dosin g 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 indic ated an inverse linear relationship between the dehydrogenation/dehydration pathway over TiO2(001) [Kim and Barteau, J. Mol. Catal. 63 (1990) 103], Zn O(0001) [Vohs and Barteau, Surf. Sci. 221 (1989) 590], Cu2O(100) [Schulz an d 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.