Enantioselective properties of the intrinsically chiral Cu{531} surface

The focus of this study is to gain an improved understanding of enantioselective heterogeneous catalysis, by investigating the adsorption of both reactants and possible products on a well-defined chiral surface. This has been achieved by employing a selection of surface science techniques, principally Reflection-Absorption Infrared Spectroscopy (RAIRS) and Scanning Tunnelling Microscopy (STM). STM images of the clean substrate clearly show that the Cu{531} surface strongly departs from the bulk-terminated structure, instead exhibiting marked roughness. The surface can be considered a frozen lattice gas, where all atoms are on lattice sites. In order to fully understand surface reactions it is important to gain insight into how individual reactants interact with the surface. Therefore, the adsorption of pyruvic acid and ammonia, chosen with a view to producing alanine, were studied individually. At 300 K, pyruvic acid adsorption results in the formation of pyruvate species having two distinct adsorption geometries depending on coverage, whereas it was found that ammonia adsorbs intact below room temperature. Pyruvic acid exposure to an ammonia pre-covered surface at 100 K results in the formation of the pyruvate ion, the enol tautomer of pyruvate, and glyoxylic acid. No infrared bands observed in this system are assigned to nitrogen-containing species, indicating that ammonia is acting, not as a reactant, but as a catalyst on the surface. Dosing pyruvic acid on an ammonia pre-covered surface at 300 K results in the formation of two nitrogen-containing species: iminoacetic acid and aminoacrylic acid. However, no clear evidence of alanine, the target molecule, was observed. STM images of single enantiomer adsorption of alanine show massive restructuring of the surface, forming distinctly different facets depending on the chirality of the adsorbate. RAIRS experiments using racemic alanine revealed no preferential adsorption of a single handedness, indicating that Cu{531} is not able to perform enantiospecific separation of alanine.