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In the common perception of catalysis, a catalyst should typically possess multiple functionalities in order to promote several individual reactions within a multi-step process. A promising way to approach this concept relies on functionalization of a catalytic surface with organic ligands, e.g. organic adsorbate or covalently bonded functional groups, to allow for a selective and controlled promotion of the desired reaction path via lateral reactant-ligand interactions. In such functionalized systems, the role of ligands can be multifold: it can either impose a geometrical constraint on the surface, hence affecting the interaction of the adsorbates with the underlying metal substrate, and/or undergo more specific intermolecular interaction, as in building a 1:1 complex with the reactant that affects its electronic structure or adsorption geometry. Despite ongoing vigorous progress in this field, the development of powdered heterogeneous catalysts of practical relevance, frequently relies on trial and error. For establishing a more rational approach towards synthesis of new advanced functionalized catalytic materials, the atomistic-level understanding of the mechanisms governing ligand-directed heterogeneous catalysis has to be significantly improved.
Towards this goal, a new experimental setup was designed and built which allows us to prepare and characterize ligand-modified model catalysts under well-controlled UHV conditions and investigate reactive surface processes across a broad pressure regime, from UHV and up to the near ambient. Integrative measuring techniques, capable of probing the mechanisms, kinetics and dynamics of such processes, namely molecular beams and infrared reflection absorption spectroscopy, were installed and applied in this work. Two case studies were performed by the newly built setup, aiming at exploring the lateral interactions between adsorbates on Pt(111) surface. Complementary, scanning tunneling microscopy was employed to obtain real space information on the distribution of the adsorbates. First, we show that a lateral interaction, mediated by hydrogen bonding between two adjacent acetophenone molecules, triggers the formation and stabilization of the normally unstable enol tautomer of acetophenone. This interaction is a crucial first step in the hydrogenation of the molecule. More importantly, we also provide a strong experimental evidence that confirms theoretical predictions that transformation of simple carbonyl compounds to their enol part enables low-barrier hydrogenation pathway of normally very stable C=O compounds. The second case reported in this dissertation involves a chiral modification of the surface with (R)-(+)-1-(1-naphthylethylamine) (R-NEA) and its interaction with CO that was utilized as a simple proxy for carbonyl compounds. The combination of spectroscopic and microscopic observations shows that the R-NEA molecules are self-assembled into directed short chains, clearly suggesting an intermolecular interaction between individual molecules. The individual molecules in the chains are also strongly inclined with respect to the surface plane as established spectroscopically. The adsorption geometry of R-NEA was found to be greatly affected by co-adsorption of CO, and the observed spectral changes induced in this process are indicative of a strong intermolecular interaction between CO and adsorbed R-NEA molecules, most likely the amino-group.
