Prof. Dr. Sven Doye

Stellvertretender Sprecher

Prof. Dr. Rüdiger Beckhaus


The role of acidic α-hydrogen in the redox (photo)chemistry at titania

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 O2d- or Oadd--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 OHad with coadsorbed Oadd-. 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.

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