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Support and size effects in CO2 hydrogenation reactions over Fe-based catalysts

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Lopez-Luna,  Mauricio       
Interface Science, Fritz Haber Institute, Max Planck Society;

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Lopez-Luna, M. (2023). Support and size effects in CO2 hydrogenation reactions over Fe-based catalysts. PhD Thesis, Technische Universität, Berlin.


Cite as: https://hdl.handle.net/21.11116/0000-000F-9652-B
Abstract
Hydrocarbons such as gasoline and short olefins are among the most important chemical raw materials in industry. They are used as energy sources and to power most means of transport and machines worldwide. For a long time, these chemicals were produced almost exclusively in the petrochemical industry. However, the over-exploitation of these resources has led to various environmental problems and has been shown to play a crucial role in greenhouse gas emissions into the atmosphere. As a possible environmentally friendly alternative, hydrocarbons can also be synthesized through the catalytic reduction of CO2 with green H2. This process has the great advantage that the CO2 can be used as a raw material. It is particularly attractive if the CO2 is extracted directly from the atmosphere in order to generate a neutral CO2 cycle. This work deals with the characterization of iron-based catalysts for the hydrogenation of CO2 to hydrocarbons. In particular, a combination of real catalysts (powder catalysts) and flat model catalysts (planar catalysts) is used. The advantage of such an approach is that surface science tools and methods can be used in addition to the conventional methods commonly used to characterize catalysts. The reason for this work is the numerous contradictions regarding the parameters controlling the catalytic performance of CO2 hydrogenation with Fe-based catalysts. My goal is to gain a better understanding of the fundamental behavior of the active catalytic Fe phase for the CO22 hydrogenation reaction. For this purpose, a combination of operando, in situ and ex situ characterizations are carried out and correlated with the catalytic performance of the various catalysts produced. Particular attention was paid to in situ surface characterization using X-ray photoelectron spectroscopy (NAP-XPS) and operando characterization using X-ray absorption spectroscopy (XAS). The first part of this work addresses the role of the oxide support in the catalytic performance of Fe-based nanoparticles. It is shown how the traditionally considered inert substrates (Al2O3 and SiO2) alter the selectivity of small Fe nanoparticles during CO2 hydrogenation. The selectivity changes depending on the substrate are associated with the interaction and influence of the carrier on the Fe nanoparticles and how the carrier defines the chemical state of the active Fe phase. The second section focuses on how the size of Fe nanoparticles affects the catalytic performance in CO2 hydrogenation. In particular, the size of the Fe nanoparticles influences the reducibility of the Fe precursors during the activation of the catalyst and thus directly affects the chemical state of the Fe under the reaction conditions. In this way, it determines the observed catalytic performance. This work ends by linking both effects in Fe-based catalysts (size and support effects) as they are coupled with each other. In summary, my work shows that the catalytic performance of Fe-based materials is highly dependent on parameters such as their support and the size of the nanoparticles. This results in a deeper understanding of the complex behaviors of these Fe-based systems described in the literature.