The work in this thesis demonstrates how to obtain an atomic-scale picture of a diverse set of complex surface structures observed using STM, under disparate conditions. Chapters 4−6 each represent... Show moreThe work in this thesis demonstrates how to obtain an atomic-scale picture of a diverse set of complex surface structures observed using STM, under disparate conditions. Chapters 4−6 each represent a different approach to answer the same question: How can we find out what a surface looks like at the atomic scale? By employing appropriate theoretical tools that complement the experimental conditions and measurement techniques, it is possible to compare the results from theory and experiment in an intuitive manner to obtain additional insights. Additionally, Chapter 4 shows that theoretical studies, which do not take experimental conditions into account appropriately, can lead to wrong conclusions. Show less
Organic molecules in interstellar space are important as they influence the structure of galaxies and star formations. Studying catalytic processes in space allows us to understand how molecular... Show moreOrganic molecules in interstellar space are important as they influence the structure of galaxies and star formations. Studying catalytic processes in space allows us to understand how molecular species are formed and chemically evolved in the interstellar medium and solar system objects. Quantum chemical methods, such as “Density Functional Theory” (DFT), can be employed to study the chemical pathways for the formation of molecular species, which is challenging with only observations and experiments. This thesis studies, with DFT methods, how polycyclic aromatic hydrocarbons (PAHs), the most abundant organic species in space, catalyze the formation of molecular hydrogen in the interstellar medium. Specifically, how linear PAHs become superhydrogenated and how the presence of Stone Wales defect in PAHs contributes to their catalytic activity for molecular hydrogen formation. In addition, this thesis reports the study of the catalytic activity of forsterite, a silicate mineral abundant in grains, asteroids, and meteorites. Specifically, the presence of Schottky MgO vacancy in forsterite can catalyze the C-H activation of PAHs as the first step to study the breakdown reaction of PAHs in asteroidal settings. The latter is indispensable to understand the formation of the so-called organic inventory of solar system objects. Show less
Finding a new catalyst is no easy task, especially since our understanding of catalysts at the atomic level is still lacking. In this thesis, a step is made to combine model catalysts that we do... Show moreFinding a new catalyst is no easy task, especially since our understanding of catalysts at the atomic level is still lacking. In this thesis, a step is made to combine model catalysts that we do understand with realistic industrial conditions. This methodology comprises both the preparation of complex model catalysts and the development of new instrumentation. The model catalysts under study were MoO3 on Al2O3/NiAl(110), MoS2 on Au(111) and AuOx/WO3/ReO3 on Au(111). For MoO3, it is shown that the O2 pressure during physical vapor deposition preparation affects the particle dispersion, allowing for tuning of the structural properties of the model catalyst. For MoS2, the aim was to image the atomic structure of the active sites during the hydrodesulfurization reaction. To achieve this, an in-house developped high-pressure scanning tunneling microscope was modified to increase its corrosion resistance. Thus, it was possible to show that hydrocarbons can play a key role in determining the dominant active site structure of the MoS2 catalyst. Using the same microscope, gold oxide particles were imaged on Au(111). From our images and simple thermodynamic considerations, we determined that these particles are suprisingly stable. Finally, new methodology was developped to provide chemical contrast to high-pressure scanning tunneling microscopy. Show less
Catalysis is one of the most important technical and scientific developments, on which present-day society is based. For example, it is crucial to the production of fertilizers or clean... Show more Catalysis is one of the most important technical and scientific developments, on which present-day society is based. For example, it is crucial to the production of fertilizers or clean fuels and needed for the abatement of exhaust gases. Frequently, the employed catalysts are being discovered in a very empirical way; by trial and error. However, designing catalysts based on detailed understanding is preferred. Obtaining understanding is very difficult, because catalysts are very complex materials. Furthermore, its properties often depend on the atmosphere surrounding the catalysts, i.e., the temperature and pressure of reactants and products, which they are exposed to, and these properties also change over time. The major part of this thesis focuses on structural changes of Pt model catalysts exposed to high oxygen pressures at elevated temperatures. The changes were measured with a ReactorSTM, a special version of a scanning tunneling microscope (STM) adapted to operate at high pressure and temperatures. These observations show that various surface oxide with a single-layer thickness form under reaction conditions. These oxides are structurally and chemically different from the Pt bulk oxides. The second part describes a set of experiments to understand the role of low-coordinated atoms and water in Au-catalyzed CO oxidation. Show less
Catalysis is the working horse of the chemical industry. In many cases, it is a poorly understood process taking place at the surfaces of nanoparticles under relatively harsh conditions, such as... Show moreCatalysis is the working horse of the chemical industry. In many cases, it is a poorly understood process taking place at the surfaces of nanoparticles under relatively harsh conditions, such as high pressures and high temperatures. This thesis focuses on new approaches to acquire atomic-scale information on catalytic processes on metal nanoparticles in high-pressure, high-temperature conditions. This thesis starts with a comprehensive approach to the development of novel instruments and methods for in-situ experiments on model catalysts under working conditions. We introduce the ReactorAFM, the world’s first high-pressure, high-temperature non-contact Atomic Force Microscope, and two software packages for data analysis. Next, we have applied several in-situ measurement techniques to study catalytic model systems at atmospheric pressures and elevated temperatures. We describe a study of the interaction of gas mixtures of nitric oxide and hydrogen on the Pt(110) surface, using surface X-ray diffraction. In the next chapter, we used similar mixtures but with a Pt nanoparticle model catalyst in a high-pressure reaction cell in a transmission electron microscope. Lastly, we have applied four in-situ techniques, including our new ReactorAFM, to investigate the role of thin oxide shells in spontaneous reaction oscillations on Pd nanoparticles during the catalytic oxidation of carbon monoxide Show less
This thesis describes the development of a combined high-pressure/ultrahigh-vacuum flow reactor for the study of model catalysts by means of surface x-ray diffraction and grazing incidence small... Show moreThis thesis describes the development of a combined high-pressure/ultrahigh-vacuum flow reactor for the study of model catalysts by means of surface x-ray diffraction and grazing incidence small angle scattering. The system was used to measure a stability diagram for the different oxide phases (surface oxide, bulk-like oxides) that exist on Pd(100) during catalytic CO oxidation at near ambient pressures. As soon as an oxide was present the reactivity of the surface was found to be mass transfer limited by the flux of CO molecules reaching the surface. Experiments on spontaneous reaction oscillations of the CO oxidation rate on Pd(100) reveal that a high density of steps strongly alters the stability of the thin, catalytically active palladium oxide film. It is shown that stabilization of the metal, caused by the steps and consequent destabilization of the oxide, is at the heart of the well-known reaction rate oscillations exhibited during CO oxidation at atmospheric pressure. Lastly reaction oscillations on supported Pd nanoparticles are shown to be accompanied by shape changes of the particles consistent with the formation and removal of a thin palladium oxide film on the particles. Show less
This thesis describes the construction of a second generation high-pressure, high-temperature scanning tunneling microscope, the ReactorSTM, with which the surfaces of catalysts can be studied... Show moreThis thesis describes the construction of a second generation high-pressure, high-temperature scanning tunneling microscope, the ReactorSTM, with which the surfaces of catalysts can be studied under relevant reaction conditions. Furthermore, the thesis describes three separate catalytic systems at ~ 1 bar, and elevated temperatures. Firstly, NO reduction on Pt(100), in which a mathematical model for the reaction mechanism, following Langmuir-Hinshelwood kinetics, is proposed. Secondly, CO oxidation, in which the Pt(110) surface is atomically resolved at high p, T. Thirdly, the thesis describes a successful pilot experiment about the hydrodesulphurization of thiophene, which is catalytically activated by molybdenum disulphide nano-crystallites on an Au(111) support. Show less
