AVS 50th International Symposium
    Surface Science Thursday Sessions
       Session SS-ThA

Paper SS-ThA7
Photo-oxidation of Trimethyl Acetic Acid on the Surface of TiO@sub 2@(110)

Thursday, November 6, 2003, 4:00 pm, Room 328

Session: Oxide Reactions and Catalysis
Presenter: M.A. Henderson, Pacific Northwest National Laboratory
Authors: M.A. Henderson, Pacific Northwest National Laboratory
J. Szanyi, Pacific Northwest National Laboratory
J.M. White, University of Texas, Austin
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TiO@sub 2@ has received considerable attention for its use as a photocatalyst in the destruction of organics. Complexities associated with microcrystalline TiO@sub 2@ (e.g., identification of surface active sites and intermediates) often hamper mechanistic study of the step-by-step conversion of organics into CO@sub 2@. In an attempt to address mechanistically the photo-oxidation of organics on TiO@sub 2@, we employ rutile TiO@sub 2@(110) as a model photocatalyst and trimethyl acetic acid (TMAA) as a model organic. A saturation exposure of TMAA at 300 K results in dissociation of the acid O-H bond to form bidentate trimethyl acetate (TMA) and OH groups at the bridging anion sites. In TPD, recombination of TMA and OH to form parent occurs between 300 and 550 K, along with water from OH combination and some TMA decomposition to isobutane and isobutene. However, the majority of the TMA adlayer decomposes to CO and isobutane/isobutene at 650 K. UV exposure was performed at 110 K in order to capture intermediates during photolysis, subsequently analyzed in TPD. UV exposure in the absence of O@sub 2@ results in rapid conversion of the species responsible for the 300-550 K TPD states into acetate groups. In contrast, the TMA species responsible for the 650 K TPD states are converted into isobutene/isobutene and CO@sub 2@ by cleavage of the C-COO bond. Inclusion of O@sub 2@ does not affect the rate of acetate formation, but accelerates the conversion of the 650 K TMA into isobutene/isobutane. Evidence has been found for the continued photo-oxidation of the C@sub 4@ daughter products into C@sub 3@ species. These reactions do not appear to be wavelength dependent for photons with energies in excess of the TiO@sub 2@ bandgap. Based on these results, we propose that products of hole trapping on TMA depend on the structural environment of the adlayer, and that a limited amount of electron trapping permits some degree of photochemistry in the absence of O@sub 2@.