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
In recent years it has become clear that the space in between the stars, contains a remarkable amount of highly diverse molecules, ranging from simple diatomics to large complex species.... Show moreIn recent years it has become clear that the space in between the stars, contains a remarkable amount of highly diverse molecules, ranging from simple diatomics to large complex species. Astronomical observations and dedicated laboratory experiments show that icy dust grains play a prominent role in the chemical enrichment of matter in space. Exotic solid state reactions taking place for temperatures as low as -260 degree merge small abundant species to larger and larger compounds until species are formed that are considered to be of relevance for life; sugars, fats and precursors of amino acids. Ultimately this material is embedded in matter from which stars and planets form. So, could it be possible, that the ingredients for life form everywhere, in space, following very similar chemical pathways? In this thesis a dedicated laboratory study is described that studies exactly these processes. With a new experimental setup the physical and chemical processes are characterized that are needed to interpret and guide astronomical observations and that yield parameters needed as input in astrochemical models. It is concluded that the chemical complexity in the solid state reaches out much further than assumed so far. 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
This thesis is devoted to the study of regular and deuterated water in ices and on surfaces against an interstellar background. A large network for the formation of regular water has been studied... Show moreThis thesis is devoted to the study of regular and deuterated water in ices and on surfaces against an interstellar background. A large network for the formation of regular water has been studied with the use of a Kinetic Monte Carlo model. A specific reaction has been investigated as well: H2 + O -> OH + H. Furthermore, in the light of deuterium fractionation, a thermal study on deuteron scrambling in the ice has been performed: H2O + D2O -> 2 HDO. Finally, two low-temperature routes relevant to HDO formation have been investigated: H2O + OD -> OH + HDO and D2O + OH -> OD + HDO. 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 uses the surface science approach to address questions regarding the interaction of oxygen with platinum and its subsequent reaction with carbon monoxide. A Pt(111) single crystal... Show moreThis thesis uses the surface science approach to address questions regarding the interaction of oxygen with platinum and its subsequent reaction with carbon monoxide. A Pt(111) single crystal surface is used as a model for the catalyst. Chapter 1 provides an overview of the literature on the subject. The description of employed experimental techniques and their backgrounds are presented in Chapter 2. Chapter 3 discusses the adsorption of oxygen on Pt(111) at various temperatures and its role in the oxidation of carbon monoxide. Chapter 4 gives an atomic scale insight into the reaction between adsorbed oxygen and carbon monoxide for different ratios of oxygen and carbon monoxide pressures. In Chapter 5, the reaction between CO and oxygen on Pt(111) was used to register noise in tunneling current due to diffusion and recombination of molecules on the catalytically active surface, to draw conclusions on the most likely rate-limiting step in the process Show less
The main theme of this thesis is the catalytic oxidation of CO, which we have investigated on several model catalyst surfaces at atmospheric pressures and elevated temperatures with the combination... Show moreThe main theme of this thesis is the catalytic oxidation of CO, which we have investigated on several model catalyst surfaces at atmospheric pressures and elevated temperatures with the combination of Scanning Tunneling Microscopy and Mass Spectrometry.The study of CO oxidation on low-index and vicinal palladium surfaces has shown that when exposed to ambient pressures of oxygen at elevated temperature, these surfaces oxidize irrespective of their orientation. In this pressure regime the oxides were shown to have a higher reactivity than the metallic surfaces.In a certain window of partial pressure combinations of O2 and CO reaction rate oscillations were observed on Pd(100) and on its vicinal surface Pd(1.1.17). ). CO adsorption on Pt(111) was found to lead to the formation of a regular overlayer structure, identified as (√19 x √19) R23.4°-13CO. The stability of this structure under different reaction conditions was discussed. These results were further used to illustrate the importance of temperature in a catalytic system.Spectacularly high conversion rates could be achieved during CO oxidation at atmospheric pressure on metallic Pt(100) surface. Show less
