Entwicklung neuartiger, auf Eisen und Kobalt basierenden Fischer-Tropsch Katalysatoren

Due to economic and ecological challenges connected with the utilization of crude oil the Fischer Tropsch (FT) synthesis gains increasing attention as one promising process for the preparation of synthetic fuels from alternative feedstocks, such as coal, natural gas or biomass. In order to enhance the efficiency of the FT process, current FT research focuses on the development of new cobalt- (Co) and iron- (Fe) based catalysts to improve the catalytic performance. In the present work novel synthesis methods were applied for the preparation of Co- and Fe-based catalysts and their potential with respect to FT synthesis was tested. For every preparation method, the synthesis parameters were varied and the influence of each parameter on the catalyst structure was determined by means of extensive structural characterization before and after catalytic experiments. Finally, the results of the characterization of every catalyst were correlated to its FT performances obtained within this work. A focus of this work was the preparation of multi-component-FT-catalysts such as Co/Al2O3 and Pd-doped iron catalysts by the Flame Spray Pyrolysis (FSP) technique. FSP is a one step process providing multiple options to tune the properties of the resulting materials. Catalyst preparation by the conventional single-flame setup (SFSP) leads to an intensive intermixing of the different components. While this intensive intermixing had a positive effect on Fe-catalysts since a fine dispersion of Pd in the iron oxide matrix could be reached, it caused a pronounced Co-aluminate formation in case of the Co catalysts resulting in FT inactive catalysts. The use of a double-flame setup enabled the independent formation of cobalt oxide and alumina particles reducing the Co-aluminate formation and leading to FT active catalysts. By crossing the flames at an optimized distance, the two components were combined and the degree of metal-support interaction was decreased by changing of the intersection distance of the two flames. Another focus of this work was the preparation of novel Al2O3-stabilized Co- and Fe-Xerogels by the so-called Epoxide-Addition-Method (EAM) which is based on a sol-gel-approach. These xerogels exhibited FT activities comparable to conventional catalysts prepared by incipient wetness impregnation or precipitation. In contrast to these well-established preparation techniques however, the EAM provides potential for the design of materials with tunable composition and porosity. In addition, this method allows the preparation of catalytic materials in form of thin coatings. These coatings are of specific interest for potential applications in novel microreactor concepts. Using this method to prepare Co- and Fe-based catalysts, it was shown that for the latter the Fe/Al2O3-ratio could be varied over a wide range while the FT activity is maintained. However, if the Al2O3 content is too high the formation of irreducible Fe-aluminates is induced resulting in a loss of FT activity. Similarly, for the Co/Al2O3 catalysts a high amount of Co-aluminates was observed to result in diminished FT activities. Yet, these aluminates could be successfully reduced to metallic Co at high reduction temperatures (800 °C) without any significant sintering of the Co-particles. The activity increased significantly after the high temperature reduction up to values comparable to the reference catalyst prepared by the conventional impregnation method. This high stability against sintering depends however on the precursor employed for the catalyst preparation. While the application of nitrates resulted in catalysts that are highly stable, catalysts prepared from the corresponding chloride precursors showed significant sintering during the high temperature reduction accompanied by a decrease of the catalytic activity. The results summarized above reveal a strong relation between the synthesis method, the resulting catalyst structure and its performance as Fischer Tropsch (FT) catalysts. From the knowledge gained, guidelines were determined that may lead to further improved catalysts with respect to the application for FT synthesis

Development of novel iron and cobalt-based Fischer Tropsch catalysts

Due to economic and ecological challenges connected with the utilization of crude oil the Fischer Tropsch (FT) synthesis gains increasing attention as one promising process for the preparation of synthetic fuels from alternative feedstocks, such as coal, natural gas or biomass. In order to enhance the efficiency of the FT process, current FT research focuses on the development of new cobalt- (Co) and iron- (Fe) based catalysts to improve the catalytic performance. In the present work novel synthesis methods were applied for the preparation of Co- and Fe-based catalysts and their potential with respect to FT synthesis was tested. For every preparation method, the synthesis parameters were varied and the influence of each parameter on the catalyst structure was determined by means of extensive structural characterization before and after catalytic experiments. Finally, the results of the characterization of every catalyst were correlated to its FT performances obtained within this work. A focus of this work was the preparation of multi-component-FT-catalysts such as Co/Al2O3 and Pd-doped iron catalysts by the Flame Spray Pyrolysis (FSP) technique. FSP is a one step process providing multiple options to tune the properties of the resulting materials. Catalyst preparation by the conventional single-flame setup (SFSP) leads to an intensive intermixing of the different components. While this intensive intermixing had a positive effect on Fe-catalysts since a fine dispersion of Pd in the iron oxide matrix could be reached, it caused a pronounced Co-aluminate formation in case of the Co catalysts resulting in FT inactive catalysts. The use of a double-flame setup enabled the independent formation of cobalt oxide and alumina particles reducing the Co-aluminate formation and leading to FT active catalysts. By crossing the flames at an optimized distance, the two components were combined and the degree of metal-support interaction was decreased by changing of the intersection distance of the two flames. Another focus of this work was the preparation of novel Al2O3-stabilized Co- and Fe-Xerogels by the so-called Epoxide-Addition-Method (EAM) which is based on a sol-gel-approach. These xerogels exhibited FT activities comparable to conventional catalysts prepared by incipient wetness impregnation or precipitation. In contrast to these well-established preparation techniques however, the EAM provides potential for the design of materials with tunable composition and porosity. In addition, this method allows the preparation of catalytic materials in form of thin coatings. These coatings are of specific interest for potential applications in novel microreactor concepts. Using this method to prepare Co- and Fe-based catalysts, it was shown that for the latter the Fe/Al2O3-ratio could be varied over a wide range while the FT activity is maintained. However, if the Al2O3 content is too high the formation of irreducible Fe-aluminates is induced resulting in a loss of FT activity. Similarly, for the Co/Al2O3 catalysts a high amount of Co-aluminates was observed to result in diminished FT activities. Yet, these aluminates could be successfully reduced to metallic Co at high reduction temperatures (800 °C) without any significant sintering of the Co-particles. The activity increased significantly after the high temperature reduction up to values comparable to the reference catalyst prepared by the conventional impregnation method. This high stability against sintering depends however on the precursor employed for the catalyst preparation. While the application of nitrates resulted in catalysts that are highly stable, catalysts prepared from the corresponding chloride precursors showed significant sintering during the high temperature reduction accompanied by a decrease of the catalytic activity. The results summarized above reveal a strong relation between the synthesis method, the resulting catalyst structure and its performance as Fischer Tropsch (FT) catalysts. From the knowledge gained, guidelines were determined that may lead to further improved catalysts with respect to the application for FT synthesis

The application of synchrotron methods in characterizing iron and cobalt Fischer-Tropsch synthesis catalysts

This article reviews the use of synchrotron radiation techniques to confront issues associated with cobalt and iron catalysts. The main areas that are examined include: (1) the application of TPR-XANES and TPR-EXAFS to gain insight into the hydrogen reduction of cobalt catalysts and the carburization of iron catalysts; (2) the impact of promoters on these reduction/carburization rates; (3) the local atomic structure of promoters and its influence on catalyst selectivity; (4) exploring the deactivation of cobalt catalysts; (5) the influence of crystallite size in stabilizing cobalts catalyst against water effects; and (6) temperature effects in alkali promoted iron catalysts in retaining the iron carbide phase against reoxidation by water. © 2012 Elsevier B.V. All rights reserved

The Role of Palladium in Iron based Fischer-Tropsch Catalysts Prepared by Flame Spray Pyrolysis

Flame spray pyrolysis (FSP) is a novel technique for the fabrication of nanostructured catalysts with far-reaching options to control structure and composition even in cases where complex composites need to be prepared. In this study, we took advantage of this technique to synthesize highly dispersed pure and Pd-doped iron oxide nanoparticles and investigated them as Fischer-Tropsch (FT) catalysts. By systematically varying the Pd content over a large range from 0.1 to 10 wt %, we were able to directly analyze the influence of the Pd content on activity and selectivity. In addition to catalytic measurements, the structure and composition of the particles were characterized before and after these measurements, using transmission electron microscopy, adsorption measurements, X-ray diffraction, and EXAFS. The comparison revealed on the one hand that small Pd clusters (diameter: 1-2 nm) evolve from initially homogeneously distributed Pd and on the other hand that the iron o xide transforms into iron carbides depending on the Pd content. The presence of Pd influences the particle size in the pristine samples (8-11 nm) resulting in specific surface areas that increase as the Pd content increases. However, after activation and reaction the specific surface areas become similar due to partial agglomeration and sintering. In a fixed bed FT reaction test, enhanced FT activity was observed with increasing Pd content while the selectivity shifts to longer chain hydrocarbons, mainly paraffins. Mechanistic implications regarding the role of Pd for the performance of the catalysts are discussed