Functionalisation of metal surfaces by N-heterocyclic carbenes : model studies and heterogeneous catalytic investigations

In this thesis, different nitrogen heterocyclic carbenes (NHCs) were used to modify a 5% Pt/Al₂O₃ catalyst to investigate changes in catalytic activity and selectivity on ethyl pyruvate hydrogenation after NHC functionalisation. A semi-batch reactor was used to provide realistic testing of the NHC functionalised 5% Pt/Al₂O₃ catalyst for ethyl pyruvate hydrogenation at 5 bar of hydrogen. Reflection-absorption Infrared spectroscopy (RAIRS) and X-ray photoelectron spectroscopy (XPS) were utilised to characterise the electronic structures and molecular adsorption geometry of NHC-modified Pt foils and their influence on ethyl pyruvate. Diffuse reflectance Infrared Fourier transform spectroscopy (DRIFTS) was used to obtain insitu spectra of the NHC-modified 5% Pt/Al₂O₃ catalysts as a function of changes in the reaction environment. Near-edge X-ray absorption fine structure (NEXAFS) was used to observe changes of general molecular arrangement and gas-phase water formation in the NHC modified 5% Pt/Al₂O₃ catalysts as the temperature was varied under 1 bar H₂. Ethyl pyruvate and a specific NHC were measured under ultra-high vacuum (UHV) conditions on a Cu(111) substrate. Scanning tunnelling microscopy (STM) and Temperature-programmed desorption (TPD) were used to investigate the desorption properties and the adsorption behaviour on the Cu(111) surface, respectively. Additionally, computational calculations were conducted to aid in the understanding of the experimental data. This study shows that the chemical structure of NHC is able to influence the activity and selectivity of the 5% Pt/Al₂O₃ catalyst. The same NHC might exhibit different behaviour with ethyl pyruvate on different metal surfaces or the same metal surface under different chemical conditions. The aromatic-carbonyl interaction might be the dominant intermolecular interaction between NHCs and ethyl pyruvate when the hydrogen-bonding donor effect is relatively weak, providing a new design strategy for molecular modification to realise enantioselective heterogeneous catalysis.