Abstract
Reaction mechanism for the formation of ethylene on TiO2 and benzene on Au/TiO2 starting from ethoxide (CH3CH2O(a)) species. Other minor reactions are neglected. [Display omitted]
► The reactions of ethanol over TiO2 and Au/TiO2 catalysts is studied. ► Over TiO2 most reaction products desorbed in one single desorption domain at 660K. ► Over Au/TiO2 the reaction products desorbed in two temperature domains at 600 and 660K. ► The main reaction products were CH2CH2 over TiO2 and C6H6 over Au/TiO2. ► C6H6 was not formed from CH2CH2 but from the condensation of acetaldehyde.
The surface reactions of ethanol over TiO2 and Au/TiO2 nanoparticle catalysts were systematically investigated by temperature programmed desorption (TPD) and infra-red (IR) spectroscopic studies, in order to understand the effect of adding gold on the surface chemistry. Transmission Electron Microscopy (TEM) indicated that Au particles were mostly less than 10nm in size; TiO2 was of pure anatase form (XRD) of about 15–20nm in size. Ethanol TPD on H2-reduced TiO2 showed that most reaction products desorbed in one single desorption domain at ca. 660K. The main reaction product was ethylene (with a carbon selectivity of about 70%); other minor products were acetaldehyde, butene, and crotonaldehyde in decreasing order of yield. Ethanol TPD over H2-reduced Au/TiO2 was considerably different. First, a large fraction of the reaction products desorbed around 600K; second the main desorption product was observed to be benzene. Infrared spectroscopy indicated that at room temperature both ethanol (1262cm−1, 1310cm−1, and 1398cm−1; due to O–H bending, CH2 wagging, and CH3 symmetric deformation modes, respectively) and ethoxide species (1047cm−1, 1073cm−1, 1093cm−1, and 1122cm−1; due to the stretching modes of CO and CC) are present with the former disappearing faster than the latter. In addition bands at 1634 and 1658cm−1 attributed to ν(C–O), ν(CC) and ρ(CH3) of adsorbed crotonaldehyde are seen when the ethanol dosed surface was flashed to 570K. A scheme for the formation of the reaction products on TiO2 and Au/TiO2 is proposed in which benzene is formed on Au/TiO2 by successive condensation reactions.