Development of resonant inelastic x-ray scattering spectroscopy in single site catalysis

A catalyst is a material which increases the rate of a chemical reaction without being consumed itself and can direct the reaction to specifically form desired products. Catalytic processes are widely used in daily life: A catalyst in a car converts the toxic exhaust gases from the engine into more environmentally friendly gases. In the pharmaceutical and flavour and fragrance industry, catalysts are used to produce the required medicines or perfumes. A small deviation in the product contents or structure can result in the production of toxic compounds or stinking liquids, emphasising the importance of catalytic selectivity. In the industries as exemplified here, homogeneous catalysts are used, i.e. the catalysts and reactants are present in solution. These homogeneous catalysts are preferred because all catalyst molecules are the same, with one single catalytically active metal centre. The molecules can be synthesised in a controlled manner and the structural and electronic properties of the active centre modified by changing the type of atoms and the overall structure around it. One of the main problems is that all compounds are present in solution, making it very difficult to separate and recycle the catalyst. Many researchers are trying to make similar single active sites on or in a solid material, forming so-called heterogeneous catalysts. After reaction, these catalysts can be simply filtered out of the mixture and reused. Unfortunately, the performance of the heterogeneous catalysts is often changed in comparison to their homogeneous counterparts. The reasons are often poorly understood.Detailed information on the structural and electronic properties of the catalyst and how they change during reaction is required to understand the reaction mechanism. Like x-rays in a hospital can be used to visualise the bone structure of skeleton in a body, x-rays can be used to visualise the structure and electronics of molecules and materials. High energy x-rays expose the detailed structure providing information on the type of atoms, their position and geometry. This so-called x-ray absorption spectroscopy (XAS) can be applied while the catalysts are working, providing insights in how reactants bond to the active centre. So far, XAS has been mainly used to obtain a structural picture whereas the electronic structure is often poorly understood. The major disadvantage of XAS is that it determines an average of all the different structures present, as such blurring the image and making it more difficult to interpret. New developments in XAS using new instrumentation and data acquisition methods while selecting specific x-ray energies provide more detailed electronic information as has been feasible so far. The charges on and bonding between atoms can now be visualised and their distribution and orientation studied, creating a 3-dimensional electronic picture or movie of the material. Selectively probing certain x-ray energies will allow one to obtain information on atoms with specific properties only; for example the structure of atoms with only one specific charge (or oxidation state). These novel selective XAS techniques will be developed and applied to catalytic systems. The properties and catalytic performance of homogeneous and their heterogeneous analogues will be studied and compared. The influence of attaching the catalyst to a support structure on the structural and electronic properties of the active site will be investigated and optimised. The development of new XAS techniques and corresponding theory is challenging from both a physical and chemical point of view and likely to provide exciting fundamental insights in the XAS techniques as well as in material properties important in a broad field of science. The knowledge obtained on the catalytic systems can be used to optimise many industrially important processes, making them more efficient and environmentally sustainable.