L. Gamble, Linda S. Jung, C. Campbell
Mar 1, 1996
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2
Influential Citations
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Journal
Surface Science
Abstract
The decomposition and protonation of surface ethoxy groups on TiO2(110) has been studied as a function of surface hydroxyl or water coverage by TPD and XPS. Surface ethoxys were produced by dissociative adsorption of deuterated ethanol (EtOD), as well as tetraethoxysilane (TEOS). The effects of both preadsorption and post-adsorption of water (and hydroxyls) on the thermal reactions of these ethoxys are studied. Two different types of adsorbed ethoxy species are identified: (1) ethoxys which can readily be removed by combination with surface hydroxyl groups to desorb as ethanol gas from ∼250 to 400 K, and (2) ethoxys which cannot react with surface water or hydroxyls below ∼450 K in TPD, even when recooled, dosed with hydroxyls again and reheated. The first type is attributed to ethoxy groups bound to surface Ti atoms (TiOEt), protonated with a proton from a hydroxyl group formed by a “bridging oxygen” atom of the surface lattice, Obr. The second type is attributed to ethoxy groups bound at “bridging oxygen” vacancies in the surface lattice, Obr-Et. Such vacancies are formed when water is produced from two Obr-H species. The Obr-Et groups are removed at ∼ 650 K by decomposition, giving ethylene and ethanol in a 1:1 ratio. Product yields and a temperature shift found when using CD3CH2OH prove that this decomposition is initiated by β-hydride elimination. Ethylene evolution is, however, not concerted with β-hydrogen elimination. The TiOEt species also follows this path if insufficient hydroxyls are present to allow its full removal below 400 K. Migration of ethoxy and hydroxyl species between Ti4+ sites and surface oxygen vacancies is postulated to explain quantitative yields. Experiments on sputtered TiO2 support this model.