Chemical reactions on surfaces - a SFG study

This thesis focused on the investigation of chemical reactions on surfaces by usage of Sum Frequency Generation (SFG) spectroscopy and Thermal Desorption Spectroscopy (TDS). Understanding of chemical reactions on surfaces is of critical importance for modification and optimization of a variety of environmental and industrial processes, but the complexity of the ongoing reactions under "realistic" conditions often hinders an understanding on a molecular level. The aim of this work was to get insight into these complex chemical reactions by application of appropriate model systems in UHV. These model systems were methane dissociation on a Ruthenium(0001) single crystal and the dissociation of deuterated water(D2O) on α-Alumina single crystals. Understanding the interaction of methane with various metal surfaces have shown to be of particular importance for industrial applications, such as the Steam reforming of methane or the Fischer-Tropsch synthesis. Investigating the methane decomposition pathway with our experimental approach allowed the identification of all occurring dissociation(CH, CH2) and coupling(CCH, CCH2, CCH3) products on Ru(0001) in UHV by identification of their characteristic vibrational modes. This was the first experimental observation of CH2, CCH and CCH3 during methane decomposition on Ru(0001). These observations provided information about the limiting factors which control the kinetics of stability and formation of the surface species like surface temperature and morphology, adsorbate coverage or co-adsorption of other molecules. Investigation of the CH2 radical, which is assumed to be important for the Fischer-Tropsch-Synthesis, allowed the determination of its dissociation barrier which is 65 kJ/mol. This is four times higher than previous theoretical studies predicted. By cooperation with the Theory Department of the Fritz-Haber-Institut it was possible to explain this deviation by stabilization effects of co-adsorbed hydrogen. Another focus of this work was the investigation of coupling processes between CH-radicals which lead to CCH2 formation. Setting up a reaction system by chemical rate equations enabled a qualitative simulation of the surface temperature formation process and provided insight into the underlying elementary reaction steps and their kinetics. The second part of this thesis focused on the investigation of water/α-Alumina interaction. Water adsorption on these surfaces changes the morphology of the substrate and further affect its chemical properties dramatically. An understanding of this interaction is therefore of importance for environmental chemistry and heterogeneous catalysis. Investigation of water adsorption on α-Al2O3(0001) and α-Al2O3(1102) in UHV allowed an insight in this interaction on a molecular level. The combination of in-situ water dissociation by applying a molecular beam source and characterization by high resolution vibration spectroscopy allowed the first experimental observations and therefore verification of the, so far only theoretically predicted, single molecular dissociation channels on both surfaces. Furthermore, probing the substrate specific surface phonon modes via SFG allowed the observation of morphological changes due to water adsorption. This could open new channels in the understanding of surface reactivity for water dissociation.