Utilising lone single atoms as model catalysts

催化，即使用催化剂加速化学反应，几乎用于我们与之互动的每一种制成品的生产。工业上使用的催化剂可以用更少的能源和更少的废物进行化学反应，催化剂几乎可以无休止地回收和重复使用。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。催化剂可分为两类，均相催化剂和多相催化。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)，而多相催化剂与反应物以不同的物理状态存在。例如，均相催化剂可以溶解在溶剂中，有助于在同一溶剂中将小分子连接在一起，而多相催化剂可以是用于帮助气相分子反应的固体金属块。均相催化剂通常以金属原子为大分子的一部分，整体分子形状和大小对它们作为催化剂的行为有巨大的影响。这些催化剂对反应物分子可能经历的许多特定反应路径具有高度的选择性，因此减少了不需要的路径中的废物。均相催化剂可以用很少的昂贵金属原子运行，但可能很难从最终产品中分离出来。这是有问题的，因为药品等消费品很难达到高纯度(催化剂被认为是一种杂质)，而且一些有价值的催化材料丢失了，无法在以后的批次中重复使用。多相催化剂使用的是分布在相对惰性、廉价的载体材料上的极少数金属原子组成的小团簇(比人头发的宽度小10000多倍)。这些催化剂的选择性较低，因此产生更多废物，并且需要大量昂贵的金属才能生产相同数量的产品。与同质类似物相比，其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5年里，一种使多相催化剂更具吸引力的新方法-单原子催化(SAC)-已经变得突出起来。在SACS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，彼此相距很远。这有双重好处：它确保了金属的最有效利用(每个单个金属原子都是可能的催化点)，并引入了高选择性(通常与均相催化剂有关)。我们的建议是，通过将小分子实体直接连接到金属原子上以控制其行为，可以类似于目前催化剂的均质程度来调整SACS。我们认为，通过将不同的分子连接到精心选择的囊中的金属原子上，可以改变它们的行为，并可以选择催化剂选择的反应路径。我们将使用超清洁的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)、程序升温脱附(TPD))，并辅之以更接近反应堆/环境的操作技术(常压X射线光电子能谱、常压XSW、常压PHD)。通过结合这些技术，我们可以了解(由SAC催化的)化学反应是如何发生的，空间精度小于传统催化剂中原子之间的距离。我们产生的基本见解将揭示如何定制SACS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。通过在这一细节水平上研究这些催化剂，我们将深入了解支撑所有多相催化的基本化学。

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.