Atomic Layer Deposition and Immobilised Molecular Catalysts Studied by In and Ex Situ Electron Spectroscopy

Popular Science
The growing need for green processes in industry makes scientists design and develop processes that are more energy efficient than current processes. Naturally, some reactions require a high energy and long time to occur. This will cause high energy consumption and, typically, even more waste production. Therefore, many efforts have been done to design materials that can help to change the path of a chemical reaction and thus result in a decrease of the required energy for a particular chemical process. Such materials are called catalysts. Catalysts change the reaction chemistry in a way that the reaction can happen with consumption of less energy and in shorter time by changing the reaction path.
Catalysis has great impact on our lives, so much indeed that today we would not be able to do without catalysts. As an example, new medicines with less severe side effects on the human health are highly desired for the treatment of many illnesses. The development of such medicine cannot be done without a change of chemistry of the medicine, to avoid harmful substances and to limit side reactions that can occur in human body. Such a change of chemistry requires changes in the synthesis and production process of the medicine. Removing a harmful molecule/group or inserting a useful molecule/group is sometimes a complicated chemical process. In addition, an inserted molecule/group in a chemical structure should often have an orientation with respect to the rest of the molecular structure of the medicine. In my thesis, I have analysed newly synthesised catalysts that are designed for activating carbon-hydrogen bonds in order to substitute hydrogen with other groups or atoms – something which is extremely important for the production of fine chemicals such as medicines.
Catalysts can be divided into two groups: homogeneous and heterogeneous catalysts. A homogeneous catalyst, on the one hand, is a catalyst that acts in the same phase as the starting material and products of the desired chemical reaction. For instance, if all starting materials of the reaction are in the liquid phase, the catalysts that will catalyse the reaction between these substances is also in the liquid phase. On the other hand, a heterogeneous catalyst is in a different phase than the starting material and products. Often, the starting materials and products are in the gas phase and the catalyst is in the solid phase; the reaction is catalysed by a heterogeneous catalyst. My thesis focuses on the analysis of new Pd based heterogeneous catalysts and the study of the fundamental surface chemistry behind the synthesis of these catalysts.
Chemical reactions often happen on a surface. This implies a gas/liquid, gas/solid or liquid/solid interaction. Therefore, it is important to use techniques that can probe surfaces in order to have more insight in such chemical reactions. This can help improving the chemical reaction or design better catalysts for the use in catalysed chemical processes. In this thesis I used X-rays photoelectron spectroscopy (XPS), which is a surface-sensitive technique, to investigate the structure of newly synthesised catalyst materials and, furthermore, study the chemistry of reactions at surfaces.
One interesting technique that can be used in the production of catalyst materials and is also widely used in electronics is atomic layer deposition (ALD). ALD is a thin film growth technique with high thickness precision, conformity and uniformity over the surface. In ALD of oxide materials one uses two different precursors in two separate half-cycles to grow thin films. Each half-cycle is followed by an evacuation or purge step. I have used XPS in ambient pressure conditions (i.e. I used ambient pressure XPS (APXPS)) to investigate the surface chemistry of ALD. Since this was done during the thin film growth, I use the term operando to describe the conditions of the investigation. The high time resolution that could be achieved helps in monitoring the surface reactions during the early stages of the precursor-solid interaction. I applied APXPS to the study of HfO2 growth on SiO2 and gained insight in surface chemical species and their interaction with the SiO2 surface. Extremely useful input is gained in the reaction mechanism.