Bond Making and Breaking Processes at Surfaces: Fundamentals of Adsorption and Catalysis

从汽车排气系统中的铂铑催化转化器，到将大气中的氮转化为肥料的铁催化剂，高活性金属是推动现代世界发展的许多最重要化学反应的关键。内燃机大量产生的有毒气体，如一氧化碳或一氧化氮，只要有足够的时间，就会在大气中自然地转化为毒性较小的物质，但前提是在我们城市街道的地面造成严重的呼吸问题之后。同样，简单地将氮和氢在足够高的压力下混合，最终会产生农业所必需的氨，但速度慢得不可能。在每种情况下，以及在许多其他情况下，金属催化剂的作用是加速和/或重新引导反应，防止环境污染或从无趣的低价值化学品中制造重要的高价值化学品。毫不奇怪，在元素周期表中心的过渡区，所涉及的稀有金属往往是世界上最有价值的元素之一，也难怪人们热衷于了解和优化它们的影响。在许多情况下，催化中重要的化学反应发生在固体过渡金属颗粒的表面。经过的分子在表面上沉淀并粘住（吸附），在表面上移动（扩散），最终从表面分离并漂浮（解吸）；在这些基本步骤之间，分子可能会在表面分解（解离）或结合在一起形成新的分子（结合）。分子和金属表面之间详细的化学相互作用对于确定这五种基本过程的相对速率至关重要，这意味着每种不同的金属，实际上是金属晶体的每个不同的暴露面，可能具有不同的催化性能。在我们的工作中，我们在高度控制的条件下对这些过程进行了复杂的测量。通过将这些结果与我们最先进的理论计算结果进行比较，我们能够建立一个完整的表面化学图谱，从而预测未来工业和环境使用的更好的催化剂。越来越多地使用新型合金和纳米结构表面将是我们计划在这个方向上工作的一个特点。我们这种基础表面科学在现实生活中产生重大影响的潜力反映在我们从工业赞助商那里吸引到的资金上。近年来，我们一直在与丰田（Toyota）合作，研究用于去除汽车尾气中一氧化氮的铱金催化剂。我们刚刚开始与庄信万丰（Johnson Matthey）合作，研究在同样的汽车环境下对甲烷的催化活化。与此同时，我们计划与BP替代能源公司合作，优化氢燃料电池技术的生产，壳牌研究公司刚刚承诺与我们合作，开发乙烯环氧化的新路线。然而，除了目前表面催化的应用之外，我们也在所谓的不对称催化方向上投入了大量的努力。在活生物体中发现的生物分子通常具有手性这一特征，也就是说它们可以以两种不相等的镜像形式存在。通过大多数化学手段，这些镜像分子彼此无法区分，但在体内的行为却截然不同；因此，在许多制药环境中，以纯手性状态制备药物是至关重要的。我们的研究旨在提供一种有效的方法来实现这一目标，通过使用本征性金属表面，从它们的母晶体中切割出来，以诱导手性选择性表面化学。

From the platinum-rhodium catalytic converter in your car exhaust system, to the iron catalyst that turns atmospheric nitrogen into fertilizer, highly-reactive metals are key to many of the most important chemical reactions that drive the modern world. Noxious gases like carbon monoxide or nitric oxide, produced in quantity by the internal combustion engine, would naturally revert to less toxic materials in the atmosphere given enough time, but only after causing significant respiratory problems at ground level on our city streets. Similarly, simply mixing nitrogen and hydrogen at high enough pressures would eventually yield the ammonia essential for agriculture, but impossibly slowly. In each case, and in many, many others, the role of the metal catalyst is to speed up and/or re-direct the reaction, preventing environmental pollution or making important high-value chemicals out of uninteresting low-value ones. It is little wonder that the often rare metals involved, from the transition region in the centre of the periodic table, are amongst the most valuable elements in the world, nor that efforts to understand and to optimise their effects are keenly pursued.In many cases, the chemical reactions important in catalysis happen at the surfaces of solid transition metal particles. Passing molecules settle and stick upon the surface (adsorb), move around on the surface (diffuse) and eventually detach from the surface and float away (desorb); in between these basic steps, the molecules may fall apart on the surface (dissociate) or join together to make new molecules (associate). The detailed chemical interactions between the molecules and the metal surface are crucial in determining the relative rates of these five elementary types of process, meaning that each different metal, and indeed each different exposed facet of a metal crystal, may have different catalytic properties. In our work, we make sophisticated measurements of these processes on extremely well-characterised surfaces under highly-controlled conditions. By comparing these with results from our state-of-the-art theoretical calculations, we are able to build up a complete picture of the surface chemistry, and hence to predict better catalysts for future industrial and environmental use. Increasing use of novel alloys and nanostructured surfaces will be a characteristic of our planned work in this direction.The potential for our kind of fundamental surface science to make a significant impact in real-life situations is reflected in the funding we have attracted from industrial sponsors. In recent years, we have been working with Toyota on iridium-gold catalysts for removal of nitric oxide from automobile exhausts, and we have just begun a collaboration with Johnson Matthey looking at the catalytic activation of methane in the same automotive context. Meanwhile, our planned work with BP Alternative Energy is looking towards optimising the production of hydrogen for fuel cell technology, and Shell Research have just committed to work with us towards new routes for ethylene epoxidation.Looking beyond the current applications of surface catalysis, however, we are also focussing a substantial effort in the direction of so-called asymmetric catalysis. The biological molecules found in living organisms are often characterised by the fact that they are chiral, which is to say that they can exist in two inequivalent mirror-image forms. These mirror-image molecules are indistinguishable from each other by most chemical means, but can have radically different behaviour within the body; in many pharmaceutical contexts, therefore, it is vital that drugs be prepared in a pure chiral state. Our research aims to provide an efficient means to achieve this through the use of intrinsically-chiral metal surfaces, which are cut from their parent crystal in such a way as to induce chirally-selective surface chemistry.