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