Bridging the Complexity Gap: Surface-specific Screening of Enantio-selective Model Catalysts using Photoemission and Low-energy Electron Microscopy

点击翻译按钮获取中文摘要

Traditionally, fundamental research related to heterogeneous catalysis is carried out with single crystals which are highly ordered and have uniform surfaces over an area of several mm2. Over the years a variety of techniques has been developed, among them photoelectron spectroscopy and electron diffraction, which allow very accurate characterization of such surfaces and reactions thereon. Real industrial catalysts, on the other hand, consist of very small crystallites in the nanometer range (nano-particles). Although, in many aspects, such nano-particles can be seen as small single crystals with the same surface properties as big ones, they also show size-specific effects which are not necessarily seen with macroscopic crystals. It is, therefore, highly desirable to have similar characterisation methods as for large single crystals available for crystallites in the nanometer range. This gap is filled by a new type of microscope which combines electron microscopy with photoelectron spectroscopy and electron diffraction; it is called photoemission/low-energy electron microscope (PEEM/LEEM). The first instrument of this kind in the UK has just been commissioned at the new Diamond synchrotron light source. It is the ideal tool for research related to heterogeneous catalysis; if used to study polycrystalline surfaces, it allows the detailed study of all crystallite surfaces at the same time, which is equivalent to a large number of single-crystal experiments, but much less time-consuming.We plan to use this new instrument to study 'enantioselective' heterogeneous catalysts which are of particular importance to the synthesis of drugs. Most molecules that play an important role in biology are chiral, meaning that their mirror images cannot be matched with the original by any rotation in space - just as our left and right hands. Therefore, these molecules can exist as 'lefthanded' or 'right-handed' versions (or 'enantiomers'). Although both versions are identical in their physical properties, all living organisms on earth only ever use or produce one of each biomolecule, which is known as the 'left-right asymmetry of life'. This poses a great problem for drug manufacturers because normally only one enantiomer of a drug molecule has the desired effect, whereas the 'wrong' enantiomer can often cause unwanted side effects. When chiral molecules are synthesized in the laboratory, left and right-handed versions are created in equal amounts, unless 'enantioselective catalysts' are used, which direct the reaction towards one of the two enantiomers. Such catalysts utilize the same trick as enzymes, the catalysts of living organisms: they provide 'stereoselective sites' that allow only one enantiomer to be produced in the reaction. Stereoselective sites act like gloves that either fit the left or the right hand; they are shaped in a way that only allows one type of molecule to form stable chemical bonds.Unlike enzymes, heterogeneous catalysts that are preferred in industrial processes are usually made of inorganic materials, metals and oxides. Currently, the only way of introducing stereoselective sites in these materials in large enough quantities is by covering their surfaces with chiral modifier molecules. There is little known about the exact nature of the stereoselective sites they create and the exact reaction mechanisms. This makes it difficult to further improve such catalysts and to find new chiral modifiers that could catalyse other reactions. We are, therefore, proposing to study the effect of such chiral modifiers on polycrystalline metal surfaces, which consist of crystallites in the nanometre range, using the photoemission/low-energy electron microscope at Diamond. The aim is a microscopic understanding of the interactions between modifiers, metal surfaces and reactant molecules. Based on this information, pathways will be explored that can eventually lead to the design of new industrial stereoselective catalysts.