A thorough understanding of microscopic mechanism governing chemical reactions at oxide surfaces requires precise knowledge about the nature of the molecular adsorbates and possible transition states. With regards to the economically very important applications in heterogeneous catalysis, where powder catalysts are being used, an ideal approach would be to first determine vibrational bands of molecular adsorbates on the surface of the powder particles, then to assign them by using a data-basis for the corresponding vibrations measured for single-crystal model substrates and finally relate the remaining bands to e.g. defect-related species using quantum chemistry calculations. Of course, these calculations should be validated by a comparison to the experimental results contained in the model substrates data base. Unlike the case of metal substrates, unfortunately, the corresponding experimental data base for metal oxide single crystal surfaces is very small, mostly a result of the rather severe technical difficulties in applying vibrational spectroscopies to these insulating, often defective surfaces.

In this talk the results of a systematic study carried out for ZnO and TiO2 single crystals is presented. In particular for ZnO substrates a rather thorough understanding has been reached, both experimentally and theoretically, about atomic and molecular adsorbates (H, CO, H2O, CO2, methanol,…) on ZnO single crystals as well as on powder particles [1]. This progress is largely based on being able to obtain high-quality experimental data using HREELS (high resolution electron energy spectroscopy) and IR-spectroscopy for single crystal surfaces and IR-spectroscopy for powder particles. The last part of the will focus on hydrogen atoms adsorbed on ZnO and TiO2, some of which are rather surprising [2] and a short glimpse on photochemistry with oxides [3].

 

[1] Ch. Wöll, The Chemistry and Physics of Zinc Oxide Surfaces, Prog.Surf.Sci. 82, 55, 2007

[2] X. Yin, M. Calatayud, H. Qiu, Y. Wang, A. Birkner, C. Minot, C. Wöll, ChemPhysChem 9, 253, 2008

[3] Ch. Rohmann, Y. Wang, M. Muhler, J. B. Metson, H. Idriss, and Ch. Wöll, Chem. Phys. Lett. 460, 10, 2008