Selectivity of photocatalytic oxidation of gaseous ethanol over pure and modified TiO₂

The rates of the two main stages of photocatalytic ethanol destruction - oxidation of ethanol to acetaldehyde and oxidation of acetaldehyde to CO ₂ - were studied under varied concentrations of ethanol and acetaldehyde and photocatalyst irradiance, at different temperatures, and over different photocatalysts. The rates followed the semiempirical three sites Langmuir-Hinshelwood model that envisages sites for ethanol and acetaldehyde adsorption and additional sites for competitive adsorption of ethanol and acetaldehyde. The increase in irradiance gave rise to higher selectivity toward CO₂ via increased concentrations of gaseous intermediate acetaldehyde. However, the rate of ethanol oxidation rose faster than the rate of acetaldehyde oxidation. The selectivity toward CO₂ monotonically decreased with temperature over TiO₂ and the rate of oxidation reached a maximum at 80°C. Among platinum-doped catalysts, the best activity was found for 1.1% Pt/TiO₂. Platinum addition to TiO ₂ resulted in a 1.5- to 2-fold increased overall rate of oxidation. The selectivity to CO₂ over Pt/TiO₂ catalyst monotonically increased with temperature. Separate studies in a batch reactor demonstrated that addition of platinum changed the product distribution. Acetic acid, instead of carbon monoxide, was formed in copious quantities over the Pt/TiO₂ catalyst.