Photooxidation and Photodesorption in the Photochemistry of Isobutene on TiO2(110)

The photochemistry of isobutene was examine TiO2(110) surface as a function of the surface pretreatment condition and irradiation temperature using temperature programmed desorption (TPD) and photon stimulated desorption (PSD). Isobutene adsorbs molecularly on the clean TiO2(110) surface without detectable thermal decomposition. Preadsorption of oxygen, either as atoms of chemisorbed molecules, did not promote thermal reactions with..isobutene, but instead blocked isobutene adsorption sites. Ultraviolet (UV) light irradiation of isobutene adsorbed on the clean surface led to depletion through photodesorption, without significant photooxidation. Isobutene PSD yields increased with increasing surface temperature suggesting that photodesorbing molecules sampled the surface during photodesorption. Preadsorption of oxygen promoted partial photooxidation of adsorbed isobutene to acetone, methacrolein, and isobutanal. However, the form of oxygen available for isobutene photoxidation had a significant impact on the reaction pathway. Acetone was only detected When molecular oxygen was present, indicating that O-2 addition occurred across the C=C bond. In contrast, results from use of isobutene-d(6) indicated that coadsorption with either O adatoms or O-2 molecules led to photochemical production of methacrolein (and likely-isobntanal) through C-H bond cleavage on a methyl group. Irradiation of a surface comprised of isobutene adsorbed on 1 ML of preadsorbed O-2 at 20 K showed the most photoConversion. Of isobutene, which suggests that photoactivation of adsorbed O-2 is a key step in partial. photooiddation of isobutene. Comparison of the isobutene PSD and photooxidation product yields as a function of surface temperature between 20 and 120 K indicates a competition between photoxidation and photodesorption that varies with temperature. "Direct" charge transfer events between isobutene and the surface, favored at higher temperature; - competed with partial ,oxidation pathways initiated by "indirect" activation of isobutene by O-2, which is favored at low temperature. Access of O-2 to the surface is critical to achieving desired isobutene photooxidation rates and products, with isobutene photodesorption providing a means : of regulating the isobutene surface. coverage.