Ethanol photo-oxidation on a rutile TiO_2(110) single crystal surface

摘要

The reaction of ethanol has been studied on the surface of rutile TiO_2(110) by Temperature Programmed Desorption (TPD), online mass spectrometry under UV excitation and photoelectron spectroscopy while the adsorption energies of the molecular and dissociative modes of ethanol were computed using the DFT/GGA method. The most stable configuration is the dissociative adsorption in line with experimental results at room temperature. At 0.5 ML coverage the adsorption energy was found equal to 80 kJ mol~(-1) for the dissociative mode (ethoxide, CH_3CH_2O(a) + H(a)) followed by the molecular mode (67 kJ mol~(-1)). The orientation of the ethoxides along the [001] or [110] direction had minor effect on the adsorption energy although affected differently the Ti and O surface atomic positions. TPD after ethanol adsorption at 300 K indicated two main reactions: dehydration to ethylene and dehydrogenation to acetaldehyde. Pre-dosing the surface with ethanol at 300 K followed by exposure to UV resulted in the formation of acetaldehyde and hydrogen. The amount of acetaldehyde could be directly linked to the presence of gas phase O_2 in the vacuum chamber. The order of this photo-catalytic reaction with respect to O_2 was found to be 0.5. Part of acetaldehyde further reacted with O_2 under UV excitation to give surface acetate species. Because the rate of photo-oxidation of acetates (acetic acid) was slower than that of ethoxides (ethanol), the surface ended up by being covered with large amounts of acetates. A reaction mechanism for acetaldehyde, hydrogen and acetate formation under UV excitation is proposed.