Utilising lone single atoms as model catalysts

催化作用，即使用催化剂加速化学反应，几乎用于我们接触的每一种制成品的生产。工业上使用的催化剂可以使化学反应以更少的能量发生，产生更少的废物，而且催化剂几乎可以无限地回收和再利用。因此，了解和改进催化剂材料显然对当前和未来的绿色经济至关重要。催化剂可分为两大类：均相催化剂和多相催化剂。均相催化剂与反应物具有相同的物理状态（固体、液体或气体），而非均相催化剂与反应物具有不同的物理状态。例如，均相催化剂可以溶解在溶剂中，并有助于将同一溶剂中的小分子连接在一起，而多相催化剂可以是固体金属块，用于帮助气相分子反应。均相催化剂通常以金属原子为大分子的一部分，整体分子的形状和大小对它们作为催化剂的行为有着巨大的影响。这些催化剂对许多反应物分子可以经历的特定反应途径具有高度选择性，因此减少了不必要途径的浪费。均相催化剂可以使用很少的昂贵金属原子，但很难从最终产物中分离出来。这是有问题的，因为像药品这样的消费品很难达到高纯度（催化剂被认为是杂质），而且一些有价值的催化材料会丢失，不能在以后的批次中重复使用。多相催化剂使用很小的（比人类头发的宽度还小10000倍以上）金属原子簇散布在相对惰性、廉价的支撑材料上。这些催化剂的选择性较差，因此产生更多的废物，并且需要更多的昂贵金属来生产同样数量的产品。与均相类似物相比，巨大的优势是催化剂很容易回收并从产品中分离出来，以便在以后的批次中重复使用。在过去的五年中，一种使多相催化剂更具吸引力的新方法——单原子催化（SAC）得到了突出的应用。在sac中，负责催化行为的昂贵金属材料的单个原子分散在固体载体上，彼此相去甚远。这是双重优势：它确保了金属的最有效利用（每个金属原子都是可能的催化位点），并引入了高选择性（通常与均相催化剂有关）。我们的建议是，SACs可以通过将小分子实体直接附着在金属原子上来控制其行为，从而与目前的均相催化剂类似。我们提出，通过将不同的分子附着在精心选择的sac中的金属原子上，可以改变它们的行为，并且可以选择催化剂选择的反应途径。我们将采用超净真空环境和其中的尖端技术（x射线驻波（XSW），光电子衍射（PhD），扫描隧道显微镜（STM），温度程序解吸（TPD）），辅以更接近反应堆/环境环境的技术（环境压力x射线光电子能谱，环境压力XSW，环境压力PhD）。通过结合这些技术，我们可以跟踪化学反应（由SAC催化）是如何发生的，其空间精度比传统催化剂中原子之间的距离要小。我们产生的基本见解将揭示如何定制sac的反应性，这是一种从最小的构建块设计催化剂的全新方法。通过在这个层面上研究这些催化剂的细节，我们将深入了解支撑所有多相催化的基本化学原理。

Catalysis, the acceleration of chemical reactions using a catalyst, is used in the production of almost every manufactured product we interact with. Catalysts used industrially allow chemical reactions to happen using less energy and producing less waste, and the catalyst can be retrieved and reused almost endlessly. Understanding and improving catalyst materials are clearly, therefore, vital for current and future green economies. Catalysts can be grouped in to two distinct categories, homogenous catalysts and heterogenous catalysis. A homogenous catalyst shares the same physical state (solid, liquid or gas) as the reactants while heterogeneous catalysts exist in a different physical state to reactants. For example, a homogeneous catalyst could be dissolved in a solvent and help to join together small molecules in the same solvent, while a heterogenous catalyst could be a solid block of metal used to help gas phase molecules react. Homogenous catalysts commonly feature metal atoms as part of larger molecules and overall molecular shape and size has huge implications for their behaviour as catalysts. These catalysts are highly selective for specific reaction pathways from many that reactant molecules can undergo, and as such reduce waste from the unwanted pathways. Homogenous catalysts can operate with very few expensive metal atoms but can be difficult to separate from the final products. This is problematic both because it is hard to achieve high purity for consumer goods likes pharmaceuticals (the catalyst is considered an impurity) and some valuable catalytic material is lost and cannot be reused for later batches. Heterogenous catalysts use small (over 10000 times smaller than the width of a human hair) clusters of very few metal atoms spread over a relatively inert, cheap support material. These catalysts are less selective, so produce more waste, and require larger quantities of expensive metals for the same amount of product. The huge advantage, compared with homogenous analogues, is that the catalyst is easily recovered and separated from the product for re-use in later batches. In the last 5 years, a new approach used to make heterogenous catalysts more attractive - single atom catalysis (SAC) - has become prominent. In SACs single atoms of the expensive metallic material responsible for the catalytic behaviour are spread out, far apart from each other, on a solid support. This is doubly advantageous: it ensures the most efficient utilisation of metals (every single metal atom is a possible catalysis site) and introduces high selectivity (usually associated with homogenous catalysts). Our proposition is that SACs could be tuned similarly to how homogenous catalysts currently are, by attaching small molecular entities directly to the metal atom to control its behaviour. We propose that by attaching different molecules to the metal atoms in carefully chosen SACs their behaviour can be altered, and the reaction pathways that the catalyst selects can be chosen. We will employ ultra-clean vacuum environments and cutting edge techniques housed within them (X-ray standing waves (XSW), photoelectron diffraction (PhD), scanning tunnelling microscopy (STM), temperature programmed desorption (TPD)), supplemented with techniques operating closer to reactor / ambient environments (ambient pressure X-ray photoelectron spectroscopy, ambient pressure XSW, ambient pressure PhD).By combining these techniques, we can follow how the chemical reaction (catalysed by the SAC) happens with spatial precision smaller than the distances between atoms in a conventional catalyst. The fundamental insight we produce will reveal how to tailor the reactivity of SACs, an entirely new method for designing catalysts from their smallest building blocks. By studying these kinds catalysts at this level of detail, we will provide insight into the fundamental chemistry that underpins all heterogenous catalysis.