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Title: Investigations into chiral adsorption systems relevant to asymmetric heterogeneous catalysis on metal surfaces
Author: Cornish, Alix
Awarding Body: University of Reading
Current Institution: University of Reading
Date of Award: 2011
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Two chiral adsorption systems are investigated, adsorption of a known chiral modifier, alanine, onto Ni surfaces and the interaction of chiral homoserine on intrinsically chiral Cu {531}. Alanine is known to act as a chiral modifier in the enantioselective hydrogenation of ~-ketoesters, but the reaction mechanism has yet to be determined. Two approaches are employed to investigate how alanine acts as a chiral modifier. Adsorption onto single crystal Ni{111} and {11O} surfaces is studied by XPS and angle resolved NEXAFS. It was shown that in the chemisorbed layer, alanine adsorbs in an anionic state with a deprotonated carboxylate group on both surfaces. This arrangement was also observed for achiral glycine on Ni {Ill}. A triangular adsorption footprint is created with surface bonds between both carboxylate oxygen atoms and the amino nitrogen. The growth of zwitterionc multi layer alanine is also observed for both systems but on Ni{11O} the multi layer is shown to segregate into alanine crystallites with a Stranskiy-Krastanov-like morphology at 240 K. On Ni {111}, the tilt angle of the carboxylate group from the surface is 45' compared to 55' on the Ni {Il O}, determined by angle resolved NEXAFS. Decomposition of these amino acids is shown to proceed via initial cleavage of the Ca-COO bond to release CO2 followed by cleavage of the Ca-CH3 bond to ultimately produce (Hx)C=N and atomic carbon. On the Ni {11O} surface, these final decomposition products coexist until -700 K whereas on Ni {Ill} only atomic species exist on the surface after 450 K, for both alanine and glycine. Ultimately, the atomic N recombines to form N2 which is detected by TPD at mass 28 and 14 amu. The second approach involved alanine adsorption on polycrystalline Ni. Previous experiments to probe this system had either been performed in UHV on single crystal surfaces, or in solution based studies with high surface area catalyst particles. This complexity gap is a poignant problem in the attempt to understand the mechanistic details of this reaction. Polycrystals were seen as a bridge between these two systems since they offer several different surface terminations and boundaries, which are analogous to reactive, defect sites on catalyst particles. PEEM and LEEM were successfully used to identify the surface facets of polycrystalline Ni and allowed the application of several surface science techniques. The oxidation of poly crystalline Ni is successfully probed using PEEM and LEEM. For oxygen exposure at temperatures of 473 K and higher, spatially resolved NEXAFS spectra clearly identify NiO crystallites in the I urn range with areas of clean Ni surface between them. Certain boundary regions are devoid of crystalIites. For lower temperatures a uniform chemisorbed oxygen layer with a different spectroscopic signature is formed. The same technique was applied to study alanine adsorption. It is shown that alanine is capable of restructuring grain boundaries of polycrystalline Ni, possibly creating chiral facets, where the enantioselective reaction could occur. The adsorption of homoserine on chiral Cu{531} was performed to improve our understanding of the interactions between chiral molecules and chiral substrates, specifically, the influence of longer side chains. XPS and angle resolved NEXAFS experiments were performed to probe enantiomeric differences. Homoserine was found to exist on the surface in an anionic state involving a deprotonated carboxylate group and a neutral amino group. It was found that three surface bonds are always made, one from the amino nitrogen and one from each of the carboxylate oxygen atoms, creating a triangular footprint, typical for a-amino acids. Under certain conditions; 50% saturation coverage of D-homoserine and also 85 and 100% saturation coverage of L- homoserine, the alcohol side chain makes an additional bond to the surface via the oxygen atom, creating a rectangular footprint. Both enantiomers are shown to occupy both adsorption sites equally but a 12' rotational difference in adsorption between the two enantiomers is found to exist for low coverages.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available