PNNL

Abstract

The preparation and anaerobic ultraviolet photolysis of trimethyl acetate (TMA) on rutile TiO2(110) have been examined with an emphasis on reaction paths. Substrates for photolysis were prepared by dosing trimethyl acetic acid at 100, 300 and 550 K. The chemistry was characterized by mass spectrometry during dosing, and by H2O adsorption and temperature programmed desorption after dosing. Using temperature programmed desorption after photolysis and mass spectrometry during photolysis, the products ejected and retained during photolysis were sought. The photolysis results are interpreted using the following mechanistic model. Photons with energies exceeding 3 eV create electronhole pairs in the substrate. With probabilities of 10-5 or lower, the holes initiate TMA chemistry by extracting an electron from the p orbital of the carboxylate moiety. The accompanying electrons are trapped at the surface and inhibit this chemistry. The electron deficient intermediate, TMA*, decarboxylates to form CO2 and either chemisorbed t-butyl, (-C(CH3)3), or physisorbed i-butene. For photolysis at 100 or 200 K, the (-C(CH3)3) accumulates and there is a slow photon-driven secondary reaction that, with a source of H, hydrogenates adsorbed t-butyl to physisorbed i-butane. For photolysis at 300 K, (-C(CH3)3) thermally reacts to form and desorb i-butene and ibutane during photolysis.