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
