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 similar to 250 to 400 K, and (2) ethoxys which cannot react with surface water or hydroxyls below similar to 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 (Ti-OEt), protonated with a proton from a hydroxyl group formed by a "bridging oxygen" atom of the surface lattice, O-br. The second type is attributed to ethoxy groups bound at "bridging oxygen" vacancies in the surface lattice, O-br-Et. Such vacancies are formed when water is produced from two O-br-H species. The O-br-Et groups are removed at similar to 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 beta-hydride elimination. Ethylene evolution is, however, not concerted with beta-hydrogen elimination. The Ti-OEt 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.

Addresses:
UNIV WASHINGTON, DEPT CHEM, SEATTLE, WA 98195.

Publisher:
ELSEVIER SCIENCE BV, AMSTERDAM

IDS Number:
TY777

ISSN:
0039-6028