Photocatalytic reaction mechanisms of ethanol and its oxidation intermediates on titanium dioxide

Transient, isothermal photocatalytic oxidation (PCO) was combined with isotope labeling and temperature-programmed desorption (TPD) and oxidation (TPO) to directly identify reaction pathways and intermediates for the room-temperature PCO of ethanol on TiO2. Part of the ethanol reacts on the surface through the pathway: acetaldehyde → acetic acid → formaldehyde → formic acid → CO2. The remaining ethanol oxidizes more slowly through a pathway that does not contain acetic acid as an intermediate: acetaldehyde → formic acid + formaldehyde → formic acid → CO2. The same surface species were present during steady state PCO, and their concentrations were measured under various ethanol, O2, and H2O gas phase concentrations.;Different selectivities for PCO of ethanol were observed for two adsorption sites on TiO2, and this identification led to the design of a photocatalyst with enhanced selectivity to acetaldehyde, a partial oxidation product. Weakly bound ethanol appears to adsorb on sites that preferentially form acetaldehyde, whereas the more strongly-bound ethoxide species preferentially produces CO 2. The TiO2 was modified with acetaldehyde TPD products, which preferentially poison the sites where ethanol is strongly bound so that selectivity to acetaldehyde increased during ethanol PCO.;A possible step in PCO, lattice oxygen extraction by organics, was studied, and lattice oxygen was shown to not be the source of oxygen involved in PCO, but such extraction might cause catalyst deactivation. Adsorbed HCOOH extracts lattice oxygen from UV illuminated TiO2 at room temperature and the lattice is replenished in the dark at room temperature by O2. Acetic acid decomposes photocatalytically without O2 through two parallel pathways on TiO2. In a pathway that does not extract lattice oxygen, acetic acid decomposes to CO2 and leaves hydrogen and methyl groups on the surface, which combine to form CH4. Another pathway extracts oxygen from TiO2 to form adsorbed H2O and gas phase CO2 and C2H6 through a bimolecular reaction that is not recombination of surface methyl groups. Gas phase O 2 reacts quickly with adsorbed methyl groups before they recombine with H to produce CH4. The oxidizing agent during PCO is different than that of photocatalytic decomposition and is most likely adsorbed oxygen. A Mars Van Krevlen mechanism for PCO appears unlikely.;The role of gas phase O2 was investigated by studying the UV-initiated oxygen isotope exchange between O2 and H2O adsorbed on TiO2. The exchange reaction competes with PCO for adsorbed oxygen adsorbed organics block sites for O2 adsorption.