Adsorbate-induced chiral reconstructions studied by surface X-ray diffraction

与多相催化有关的基础研究经常受到所谓压力间隙的阻碍，即大多数能够阐明催化剂表面反应的原子细节的实验表面科学技术只能在真空中应用，而实际的催化反应通常在高反应物压力下甚至在溶液中进行。表面x射线衍射（SXRD）在一定程度上弥补了这一差距，这是一种实验技术，可以提供有关表面原子排列的精确几何信息。x射线相对于大多数其他表面敏感的方法的优势在于，它们可以穿透气体大气层，甚至是一层薄薄的液体薄膜。因此，它们是研究非均相催化过程的理想材料，特别是那些发生在溶液中的催化过程。在溶液和催化剂的界面上，反应物和溶剂分子的相互作用非常密切，研究它们在反应温度或接近反应温度时的相互作用是很重要的，这在真空中是不可能的，因为所有的溶剂都蒸发了。一个多世纪以来，晶体学家一直使用x射线衍射来确定晶体材料中原子的确切位置，这对我们理解化学和生物过程至关重要，就像DNA中的信息传递一样复杂。现代同步加速器光源，如钻石，在过去的十年中成为可用的，提供非常强烈的x射线辐射，现在可以探测到由于晶体表面的原子而产生的微弱衍射信号，并确定它们的位置。我们计划用SXRD研究对映选择性非均相催化剂表面的原子排列，这对药物的合成具有特别重要的意义。在生物学中发挥重要作用的大多数分子都是手性的，这意味着它们的镜像不能通过空间中的任何旋转与原始的相匹配——就像我们的左手和右手一样。这些分子以“左旋”或“右旋”形式存在（“对映体”）。虽然这两种形式在物理性质上是相同的，但地球上的所有生物都只使用或产生每种生物分子中的一种。这给药物制造带来了挑战，因为通常药物分子中只有一种对映体具有期望的效果，而“错误的”对映体通常会导致不必要的副作用。当手性分子在实验室中合成时，除非使用“对映选择性催化剂”，否则两种对映体的生成量是相等的。这种催化剂提供了“立体选择位点”，其形状只允许一种分子形成稳定的化学键，类似于左手适合或右手适合的手套。与自然界的对映选择性催化剂、酶不同，工业过程中首选的多相催化剂通常由无机材料、金属或氧化物制成。目前，在这些材料中大量引入立体选择位点的唯一方法是通过在其表面吸附手性改性剂分子（例如，由酒石酸或丙氨酸修饰的Ni催化剂在β -酮酯的不对称氢化中显示出明显的对映体过量）。然而，由于所有这些都是在溶液中发生的，我们对它们引起的修饰的确切性质、溶剂的作用或确切的反应机制知之甚少。这使得进一步改进这种催化剂和寻找新的手性改性剂来催化其他反应变得困难。因此，我们建议利用新的表面x射线衍射光束线I07和相关实验来研究Ni表面在反应条件下的几何变化。其目的是微观地了解改性剂、金属表面和反应物分子之间的相互作用，从而最终降低重要药物的生产成本。

Fundamental research related to heterogeneous catalysis is often hampered by the so-called pressure gap , i.e. the fact that most experimental surface science techniques that are able to elucidate the atomic details of reactions at the catalyst surface can only be applied in vacuum, whereas the actual catalytic reactions usually take place at high reactant pressures or even in solution. This gap is bridged to some extent by surface X-ray diffraction (SXRD), an experimental technique that provides exact geometric information about the arrangement of surface atoms. The advantage of X-rays over most other surface-sensitive methods is that they can penetrate a gas atmosphere or even a thin film of liquid. Thus they are ideal for the study of heterogeneous catalytic processes, in particular those that take place in solution. At the interface between solution and catalyst, reactant and solvent molecules interact very closely and it is important to study their interaction at or near the reaction temperature, which is not possible in vacuum where all the solvent evaporates.Crystallographers have used X-ray diffraction for over a century to determine the exact positions of atoms in the bulk of crystalline material, which has been crucial for our understanding of chemical and biological processes, as complicated as information transfer in DNA. Modern synchrotron light sources, such as Diamond, which became available over the last decade, deliver very intense X-ray radiation and make it now possible to detect also the weak diffraction signal due to atoms at the surfaces of crystals and determine their positions. We plan to use SXRD to study the arrangement of atoms at the surfaces of '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. These molecules exist as 'left-handed' or 'right-handed' versions ('enantiomers'). Although both versions are identical in their physical properties, all living organisms on earth only use or produce one of each biomolecule. This poses a challenge for drug manufacturing because normally only one enantiomer of a drug molecule has the desired effect, whereas the 'wrong' enantiomer often causes unwanted side effects. When chiral molecules are synthesized in the laboratory both enantiomers are created in equal amounts unless 'enantioselective catalysts' are used. Such catalysts provide 'stereoselective sites', which are shaped in a way that only allows one type of molecule to form stable chemical bonds, similar to gloves that either fit the left or the right hand.Unlike nature's enantioselective catalysts, enzymes, heterogeneous catalysts that are preferred in industrial processes are usually made of inorganic material, metals or oxides. Currently, the only way of introducing stereoselective sites in these materials in large enough quantities is by adsorbing chiral modifier molecules on their surfaces (e.g. Ni catalysts modified by tartaric acid or alanine show significant enantiomeric excess in the asymmetric hydrogenation of beta-ketoesters). Since all this takes place in solution, however, we know very little about the exact nature of the modification they cause, the involvement of solvents or 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 therefore propose to study the geometrical modifications of Ni surfaces under reaction conditions using the new surface X-ray diffraction beamline I07 at Diamond and related experiments. The aim is a microscopic understanding of the interactions between modifiers, metal surfaces and reactant molecules, which could eventually lower the costs of the production of vital drugs.