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

传统上，与多相催化相关的基础研究是用高度有序且在几平方毫米的面积上具有均匀表面的单晶进行的。多年来，已经开发了各种技术，其中包括光电子能谱和电子衍射，这些技术可以非常准确地表征这种表面及其上的反应。另一方面，真实的工业催化剂由纳米范围内的非常小的微晶（纳米颗粒）组成。虽然在许多方面，这种纳米颗粒可以被视为具有与大颗粒相同的表面性质的小单晶，但它们也显示出宏观晶体不一定看到的特定尺寸效应。因此，非常需要具有与可用于纳米范围内的微晶的大单晶类似的表征方法。这一空白被一种新型的显微镜所填补，这种显微镜将电子显微镜与光电子能谱和电子衍射相结合;它被称为光电发射/低能电子显微镜（PEEM/LEEM）。英国第一台这种仪器刚刚在新的钻石同步加速器光源上投入使用。它是多相催化相关研究的理想工具;如果用于研究多晶表面，它可以同时对所有微晶表面进行详细研究，这相当于大量的单晶实验，但耗时要少得多。我们计划使用这种新仪器来研究对药物合成特别重要的“对映选择性”多相催化剂。大多数在生物学中发挥重要作用的分子都是手性的，这意味着它们的镜像不能通过空间中的任何旋转与原始分子相匹配-就像我们的左手和右手一样。因此，这些分子可以作为“左手”或“右手”版本（或“对映体”）存在。虽然这两个版本在物理性质上是相同的，但地球上所有的生物体都只使用或产生一种生物分子，这被称为“生命的左右不对称”。这给药物制造商带来了很大的问题，因为通常只有一种药物分子的对映体具有所需的效果，而“错误”的对映体通常会导致不必要的副作用。当在实验室中合成手性分子时，除非使用“对映选择性催化剂”，否则会产生等量的左手和右手版本，该催化剂将反应导向两种对映异构体之一。这种催化剂利用了与酶相同的技巧，活生物体的催化剂：它们提供“立体选择性位点”，使反应中只产生一种对映体。立体选择性位点的作用就像手套一样，既可以戴在左手上，也可以戴在右手上;它们的形状只允许一种类型的分子形成稳定的化学键。与酶不同，工业过程中优选的多相催化剂通常由无机材料、金属和氧化物制成。目前，在这些材料中引入足够大量的立体选择性位点的唯一方法是用手性改性剂分子覆盖它们的表面。关于它们产生的立体选择性位点的确切性质和确切的反应机制知之甚少。这使得难以进一步改进此类催化剂并发现可催化其它反应的新的手性改性剂。因此，我们建议研究这种手性改性剂对多晶金属表面的影响，这些表面由纳米范围内的微晶组成，使用金刚石的光电发射/低能电子显微镜。其目的是从微观上理解改性剂、金属表面和反应物分子之间的相互作用。基于这些信息，将探索最终可能导致新的工业立体选择性催化剂设计的途径。

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.