Utilising lone single atoms as model catalysts

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.