Besides oxygen defects mobile Ti³⁺-defects may be formed in TiO₂ so that TiO₂-bulk crystals may be self doped with higher concentrations of Ti³⁺ [1]. Accordingly the partly reduced system Ti(III)/Ti(IV) in Titana is an interesting redox pair in heterogeneous catalysis.  When adsorbing O₂ at Rutile-TiO₂(110), reactive species are formed at the surface. Depending on the density of Ti³⁺-defects and the adsorption temperature the formation of superoxide species O₂^d- or O_ad^d--atoms may be observed. They partially oxidize Ti³⁺ close to the surface to TiO₂-islands at temperatures above 360 K and fully above 410 K when bulk Ti³⁺ species diffuse towards the surface [2]. The high affinity of titanium ions towards oxygen also causes activation of oxygen containing organic molecules [3]. Mechanistic studies showed that oxidation and reduction path ways of benzaldehyde and aceton adsorbed at Rutile-TiO₂(110) probably depend on the presence of -CH-Atomen in the neighborhood of the keto-group. The C-C coupling reaction of benzaldehyde with subsequent formation of stilbene under reductive conditions is for example depending on the concentration of bulk Ti³⁺ defects [4]. This reaction path is quenched in the presence of coadsorbed oxygen. On the other hand first experiments indicate that the oxidation of acetone containing an -H in contrast to benzaldehyde not only results in the formation of an intermediate diolate but also undergoes a C-C coupling to a b-hydroxyketone intermediate and OH_ad with coadsorbed O_ad^d-. This is apparent from polarisation depending Fourier-Transform-Infrared-Reflexion-Absorption-Spectroscopy (FT-IRRAS) [5] and would correspond to an aldol-like equivalent surface reaction, in other words a steering of heterogeneous catalysed redox reactions via acidic hydrogen. The aim of the project is to elucidate this in more detail for thermal and photochemical reactions of ketones, aldehydes and amines with and without coadsorption of oxygen and water at single crystalline Rutile-TiO₂(110) and surfaces modified with cocatalysts as a function of the density of Ti³⁺-defects. Methods like FT-IRRAS, thermal desorption spectroscopy (TDS), x-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) will be used under ultra high vacuum conditions.

[1] M. Li, W. Hebenstreit, U. Diebold, A. M. Tyryshkin, M. K. Bowman, G. G. Dunham, M. Henderson, J. Phys. Chem. B, 2000,104, 4944-4950.

[2] M. A. Henderson, W. S. Epling, C. L. Perkins, Ch. H. F. Peden, U. Diebold, J. Phys. Chem. B, 1999, 103, 5328-5337; E. Lira, J. Ø. Hansen, P. Huo, R. Bechstein, P. Galliker, E. Lægsgaard, B. Hammer, St.Wendt, F.Besenbacher, Surface Science, 2010, 604, 1945-1960.

[3] U. Diebold, Surf. Sci. Reports, 2003, 48, 53-229.

[4] P. M. Clawin, C. M. Friend, K. Al-Shamery, Chem. Eur. J. 2014, 20, 7665-7669.

[5] P. Clawin, PhD thesis, Carl von Ossietzky University of Oldenburg, 2014.