Paper SS2-ThA1
The Photochemistry of Acetaldehyde on TiO₂(110)

Thursday, October 23, 2008, 2:00 pm, Room 208

The ultraviolet (UV) photon induced decomposition of acetaldehyde adsorbed on the oxidized rutile TiO₂(110) surface was studied with photon stimulated desorption (PSD) and thermal programmed desorption (TPD). Saturation coverage of acetaldehyde adsorbed on a reduced TiO₂(110) surface with 15 % oxygen vacancies exhibited a molecular desorption peak at 235 K along with a higher temperature shoulder at > 260 K that accounts for 1/3^rd of the monolayer. Butene desorbing at 530 K was observed as a minor decomposition channel accounting for 1-2% of the adsorbed acetaldehyde. Saturation coverage of acetaldehyde adsorbed on oxidized TiO₂ (110) desorbs molecularly at 240 K with a minor decomposition channel (7%) forming adsorbed acetate. Surface bound acetate decomposes to form gas phase ketene in a broad desorption peak centered at 700 K. Acetaldehyde adsorbed on oxidized TiO₂ (110) undergoes a facile thermal reaction (E_a = 8 kJ/mol) to form a photoactive acetaldehyde-oxygen complex. Acetaldehyde adsorbed on reduced TiO₂ (110) was photo-inactive. UV excitation of substrate charge carriers initiated photofragmentation of the acetaldehyde-oxygen complex resulting in the ejection of methyl radical into gas phase and conversion of the surface bound fragment to formate. The identity of methyl radical was confirmed using isotopically labeled acetaldehyde. The presence of surface bound formate was confirmed by desorption of formate thermal decomposition product, CO, at 550 K. Ejection of reactive organic radical species in TiO₂ photoxidation of organics can potentially lead to alternate reaction pathways occurring away from the catalyst surface.