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

Abstract

The research work that I describe in my thesis deals with three different heterogenisation approaches for synthesising a heterogeneous transition metal catalyst used for direct C-H activation reactions. The three heterogenisation approaches considered in my research are: (1) heterogenisation of a molecular catalyst on a polymer support using covalent bonds, (2) heterogenisation of a catalyst on a reduced graphene oxide (rGO) support using non-covalent interactions and (3) immobilisation of a catalyst on an inorganic surface using covalent bonds and encapsulation in an inorganic matrix.

Catalytic Pd complexes with one or two anchoring branches were successfully embedded in a polymer matrix by polymerisation, i.e. using the first approach. The catalyst samples were analysed using ultrahigh vacuum X-ray photoelectron spectroscopy (UHV XPS), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). No sign of Pd in the metalic state (Pd0) or nanoparticles was observed after synthesis and polymerisation and after using the catalyst for several cycles of oxygenation and halogenation reactions.

In the second approach molecular Pd catalysts with one or two anthracene branches were immobilised on an rGO surface using π-stacking. These samples were used in oxygenation and halogenation reactions. XPS shows that the catalysts synthesis and immobilisation was successful, without any reduction in the oxidation state of the Pd. The results also show that the catalysts are stable and recycable.

The third approach aims at achieving a highly efficient and selective heterogeneous catalyst synthesis. The basic idea is to encapsulate a heterogenised molecular catalyst in an inorganic matrix, which is complementary to the target reaction product in size and shape. Since such a matrix conveniently can be produced by atomic layer deposition (ALD), I directed my focus on understanding the mechanism of oxide ALD. To this end, the ALD of HfO2 from tetrakis(dimethylamido)hafnium (TDMAHf) and water was on In situ-oxidised SiO2/Si(111) was studied using operando ambient pressure x-ray photoelectron spectroscopy (APXPS). APXPS provides the possibility to bridge the pressure gap between standard UHV XPS investigation and the pressure conditions of the ALD process carried out in the mbar regime. It also offers the option of high temporal resolution on the secon dtimescale, which made it possible to follow the precursor-surface and precursor-precursor interaction during the first two ALD half cycles. The operando study of ALD enable the study of the ALD reaction mechanisms by following the evolution of the different surface chemical species. Furthermore, combining the experimental data with density functional theory (DFT) calculations contributed to improving the understanding of the surface chemistry and reaction mechanism of HfO2 growth.

In addition, a vapour phase study of two widely used precursors, tetrakis(dimethylamido)titanium (TDMAT) and TDMAHf, is also reported in my thesis. Experimental results AP XPS are combined with DFT calculations for a detailed analysis of the electronic structure of these two precursors. DFT calculations explain the difference in XPS results for the two precursor as being due to π-donation interactions between metals and ligands.

Abstract (Swedish)

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.

• author: Shayesteh, Payam
• supervisor: Joachim Schnadt; Ola Wendt
• opponent: Professor Held, Georg, University of Reading, Reading, UK
• organization: Synchrotron Radiation Research; NanoLund: Centre for Nanoscience; MAX IV Laboratory
• publishing date: 2019-02
• type: Thesis
• publication status: published
• subject: Atom and Molecular Physics and Optics; Other Physics Topics
• keywords: Ambient pressure XPS, Catalysis, Heterogenisation, Atomic layer deposition, HfO2, Gas phase, TDMAHf, TDMAT, Fysicumarkivet A:2019:Shayesteh
• volume: 1
• pages: 104 pages
• defense location: Rydbergsalen, Fysiska institutionen, Professorsgatan 1, Lund
• defense date: 2019-03-15 09:15:00
• ISBN: 978-91-7753-968-1
• ISBN: 978-91-7753-969-8
• language: English
• LU publication?: yes
• id: 1709dde3-cb1b-4072-9b96-2fa5c85c2d83
• date added to LUP: 2019-02-18 10:08:09
• date last changed: 2022-04-27 13:56:43