Identifying Complex Molecular Adsorbates Using a Combined Experimental and Theoretical Approach

Conrad Becker, Jan Haubrich, Klaus Wandelt, Françoise Delbecq, David Loffreda, Philippe Sautet

Institut für Physikalische und Theoretische Chemie, Germany

The investigation of reaction mechanisms, selectivities or activities in catalytic processes requires a profound understanding of the interactions of the reactants, intermediates and products with the catalyst itself. Since the surface structures of industrial catalysts are usually highly complex, model systems with defined surface structures have to be used to perform such investigations. Besides using mono- and bimetallic single-crystalline samples, also alloying the surface of a monometallic substrate with a second metal can be used to create well-defined model catalyst surfaces. We employ high-resolution electron energy loss spectroscopy (HREELS) in conjunction with density functional theory (DFT) to gain deeper insights into molecule-surface interactions, surface processes and modifications by alloying effects. For simple systems (e.g. cyclopentene on Pt(111)) for which only a single adsorption mode exist on the surface this approach yields the atomic structure of the surface molecule complex [1]. When multifunctional molecules such as alpha,beta-unsaturated aldehydes (e.g. prenal and crotonaldehyde) are considered not only the vibrational spectra are by far more complex but also the number of possible adsorption configurations is multiplied [2]. This requires an additional effort in the calculations of the stable adsorption configurations and vibrational spectra. Our approach is to calculate all possible adsorption configurations of the molecules in question using DFT and comparing the derived theoretical vibrational spectra to the experimental ones. This allows us to identify the molecular species actually present on the surface. By comparing the adsorption on Pt(111) to that on two ordered Pt-Sn surface alloys the role of alloying on the adsorption of multifunctional molecules is derived. For alpha,beta-unsaturated aldehydes important changes in the bonding modes of the most stable adsorption complexes are found. The knowledge of the initially present adsorption modes on the surfaces open also the route to a complete thermodynamic description of the decomposition pathways.

[1] C. Becker, F. Delbecq, J. Breitbach, G. Hamm, D. Franke, F. Jäger K. Wandelt, J. Phys. Chem. B 108 (2004) 18960
[2] J. Haubrich, D. Loffreda, F. Delbecq, Y. Jugnet, P. Sautet, A. Krupski, C. Becker, K. Wandelt, Chem. Phys. Lett., 433, 188 (2006)