Tilt the molecule – change the chemistry

The aim of this course of research was to further our knowledge of the mechanism of catalysts at the molecular level – thence, to form predictions and suggestions toward rational catalyst design. The first part of this work deals with catalysis on silver. Although well-known in selective oxidation, the potential of silver for chemoselective hydrogenation was only recently realised. Here, the hydrogenation of acrolein and crotonaldehyde was studied in ultra-high vacuum on the Ag(111) model catalyst. The delicate hydrogenation of both aldehydes to the desired unsaturated alcohol was demonstrated for the first time by temperature programmed reaction (TPR). X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) showed unambiguously that the orientation of the C=C and C=O bonds with respect to the surface critically influences the outcome of the hydrogenation reaction. Using a similar approach, the hydrogenation chemistry of crotonaldehyde was studied on sulphur-modified Cu(111), to rationalise why sulphur enhances the selectivity of practical copper catalysts in the hydrogenation of crotonaldehyde to crotyl alcohol. The presence of sulphur (normally a poison) transformed the otherwise inert Cu(111) surface into an excellent catalyst for crotyl alcohol formation. A combination of TPR, XPS and NEXAFS showed that once again, the geometry of the adsorbed aldehyde was crucial in determining selectivity. In this case, the chemoselective reaction itself was observed not just by TPR, but also in situ by XPS and NEXAFS. The final Chapter of this thesis describes a significant breakthrough in chemoselective hydrogenation: the first demonstration of chemoselective hydrogenation of an α,β-unsaturated ketone under ultra-high vacuum conditions. Here, results from ultra-high vacuum experiments are used to make a direct prediction of practical catalyst behaviour.EThOS - Electronic Theses Online ServiceGBUnited Kingdo