Local investigations of catalysts at work: An insight into a catalytic reactor

Sebastian Pirel Fredsgaard Jespersen*

*Corresponding author for this work

Research output: Book/ReportPh.D. thesis

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Heterogeneous catalysis, where gas reactants chemically react on the surface of a catalyst, have had and still have a tremendous impact on humanity and the modern world. Without heterogeneous catalysis, the inhabitants of the world could not be fed and the estimated world population would only count 3.5 billion people, corresponding to half of the number of today. It is estimated that about 90% of all chemicals produced today have undergone a catalytic process, includingtheproductionofplastics,polymersandfuels. Ingeneral,the world has become a better place. However, a growing population and increasing living standards world-wide have led to new demands of natural resources and new world problems, climate changes included, which need to be solved. Here, heterogeneous catalysis is going to play a major role in finding sustainable solutions. The secret to the catalytic ability of the catalyst lies within the atomic configuration of the surface terminations. The key to developing new efficient and effective catalysts are hence to fully understand the relation between the atomic configuration of the catalyst surfaces and their activity. Transmission electron microscopy has the capability to visualize the nano-structure of the catalysts with a resolution on the atomicscale and can thus reveal insight of the atomic configurations. In this thesis, transmission electron microscopy is used to investigate the dynamics of catalysts. To do this, nano-scale reactors based on Silicon technology were employed to simulate the conditions found in real chemical reactors. These so-called nanoreactors allow the catalyst loaded in the reactor to be studied at a pressure of several bars and elevated temperatures. Such in situ studies are of utmost importance because the gas composition, the pressure and the temperature are all known to influence the catalyst structure. The thesis will address four topics, all related to the use of transmission electron microscopy and the nanoreactor with the perspective of visualizing the catalyst under its active state. The first topic will seek to address the limit for the attainable resolution while introducing gas to the nanoreactors. As the gas pressure increases in the reactor, the number of interactions between the electrons and gas molecules will rise, which will lead to a degradation of the resolution. Studies have, however, indicated how tuning of the electron beam can postpone the degradation to higher pressure levels. This topic will therefore address how the properties of the electron beam, the pressure as well as the gas type, affect the inherent resolution of the microscope while employing the nanoreactor system. The second topic will report on the integrity of the Silicon-based electron-transparent windows of the nanoreactor. Transmission electron microscopy can become destructive and manipulative if one is not fully aware of controlling the electron beam probably. How the electron beam affects the specimen depends strongly on the specimen itself, the operational conditions and the illumination conditions. The investigation will therefore study how the operational conditions (pressure and temperature) and the illumination conditions (primary electron energy and the electron dose rate) influence the nanoreactor integrity while exposed to an oxidizing environment. The third topic will address the catalytic oxidation of CO over Pt nanoparticles. The catalytic system is used to address how the local environment along the reactor varies from the overall conversion for the entire reactor, and how the local particle shapes relate to the local environment in which they sit compared to the global activity. To do so, electron energy-loss spectroscopy is utilized to probe the local environment, whereas mass spectroscopy is used to integrate over the entire reactor. Lastly, topic four will address an industrial-style Cu/ZnO catalyst used for methanol synthesis. Here, the structure dynamics of the catalyst is investigated under high pressure conditions. During this examination it will become clear how important it is to be fully aware of the electron beam and to control it. With these four topics, it will, hopefully, be transparent why transmission electron microscopy in combination with the nanoreactor system is a powerful tool in the research for understanding and developing new, efficient and effective catalysts to help solve the problems of today.
Original languageEnglish
Place of PublicationLyngby, Denmark
PublisherTechnical University of Denmark
Number of pages126
Publication statusPublished - 2019


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