Fischer-Tropsch synthesis enables the production of high quality diesel fuel from biomass derived synthesis gas. In order to increase the overall diesel yield, it is necessary to perform a subsequent hydrocracking of the long-chain linear paraffins. This work is focused on characterization and testing of catalysts for the hydrocracking reaction of Fischer-Tropsch waxes. In particular, noble metal catalyst based on Pt and Pd on amorphous silica-alumina support were tested. Palladium based catalysts performed nearly an ideal bifunctional mechanism, while platinum based catalysts performed another way of cracking: hydrogenolysis. Platinum based catalysts are more active than palladium ones, with the same metal loading. This is a consequence of the nature of the metal sites. The product distribution is similar for both platinum and palladium catalysts. However, due to the hydrogenolysis cracking mechanism performed by platinum based catalysts, the amount of light gases produced on platinum based catalysts is higher. Furthermore, the deactivation behavior of the Platinum and Palladium catalysts has been studied, and the results showed that the dispersion of the active phase decreased with deactivation and the average crystallite diameter increased. This means a decrease in activity. A regeneration program, temperature programmed oxidation (TPO), has been carried out demonstrating that the activity was not completely recovered.

The effects of noble metal (NM) promotion and deposition order (co-deposition of NM with the final Co deposition [co-dep] or sequential deposition of NM after Co deposition [seq-dep]) on surface area, pore size, metal retention, crystallite size, noble metal distribution and bonding in Co Fischer Tropsch (FT) catalysts were studied as were the resulting Co reducibility and Fischer Tropsch activity/selectivity properties. Catalysts containing nominally 25wt% Co with either 0.3 wt% Ru, 0.58 wt% Pt, 0.55wt% Re, or no NM on a La-stabilized-Al2O3 support were prepared by wet deposition. The Co, Pt, and Re were uniformly dispersed, but Ru distribution and retention were problematic and deposition-order dependent—85% was lost with co-dep, but it was uniformly distributed while 54% was lost with seq-dep and it was concentrated at the pellet edge. The co-dep catalysts all have smaller reduced Co crystallite size than their corresponding seq-dep catalysts. The average crystallite diameters for all 3 co-dep catalysts are between 4.1 and 4.3nm and ~90% of the crystallites are < 6nm. XAFS measurements showed that after reduction at 360°C, Pt is bonded with Co even with mild calcination between the final Co and the Pt deposition. On the other hand, neither Ru nor Re formed direct bonds with Co. Ru remained in a separate metal phase after reduction even at low loadings. Re remained as Re2O7 and still promoted Co reduction well (e.g. 42% reduced to Co metal compared to none for the unpromoted catalyst). By all measures of reducibility (TPR, EOR, H2 uptake), all NM promoted catalysts were more reducible than the unpromoted catalyst. The co-dep catalysts have lower TPR peak temperatures, but lower extents of reduction than their corresponding seq-dep catalysts. The NM type effect on overall extent of reduction trend was Co/Pt-seq>Co/Re-seq>Co/Ru-seq=Co/Pt-co>Co/Re-co>Co/Ru-co>Co. The Co/Pt-co catalyst was the most active of all the catalysts both on rate per mass and per site basis. The co-dep catalysts were all more active than the corresponding sequentially deposited catalysts. The co-dep Pt and Re catalyst activity is greater due to higher activity per site, while co-dep Ru activity is greater due to a higher abundance of active sites.

A series of silver-promoted, 20 wt% cobalt Fischer-Tropsch synthesis (FTS) catalysts supported on an alumina modified with 5 wt% silica were prepared using two methods: traditional incipient wetness impregnation (IWI) and a new solvent-deficient precipitation (SDP) technique. Catalysts containing silver promoter concentrations of 0.3, 0.6, 1.2, and 2.5 wt% were prepared using each of the two methods. Silver improved the reducibility of the cobalt significantly, lowering reduction temperatures by up to 100°C, and increasing the extent of reduction by up to 35%. Further, in both preparation methods, changing the silver loading altered the cobalt dispersion. The smallest Co crystallite size was achieved with 1.2 wt% Ag loading, which produced average cobalt crystallite sizes of about 7 nm. The Fischer-Tropsch CO consumption rate increased with decreasing crystallite size and thus was highest for 1.2 wt% Ag. Intrinsic CO consumption rates per Co site (CO turnover frequency) were also measured for each catalyst. A clear increase in the intrinsic turnover frequency (TOF) was observed as Ag loading was increased from 0.6% to 1.2 wt% for both preparation methods. Both 1.2 wt% Ag catalysts produced TOF’s of ~0.050 s-1 (equivalent to a CO consumption rate of ~65 mmol gcat-1 h-1), which are 20-30% higher than the catalysts containing 0.3, 0.6, and 2.5 wt% Ag and are comparable to reported rates for commercial FTS catalysts. Higher loadings of Ag (2.5 wt%) resulted in higher extents of reduction, but led to lower TOFs and larger Co crystallite diameters, which is assumed to be due to blockage of active sites and changes in the Ag-Ag and Ag-Co coordination ratio. Thus, silver promotion appears to improve catalytic performance by enhancing cobalt reduction, dispersion, and electronic properties. The SDP and IWI methods produced catalysts with the same properties, but the SDP method is a simpler, one-pot technique that offers many potential advantages.In another part of this work a series of Co Fischer-Tropsch synthesis catalysts with the same composition and preparation procedures, but supported on four different aluminas, were synthesized, characterized, and kinetically tested to investigate the effect of the supports on their performance. The results demonstrate that the most active catalyst, with a rate of 49 mmol CO gcat-1h-1 at 220°C and 20 atm, is obtained by using the Al-Si support, which has: high surface area and pore volume, high hydrothermal stability and lower number of hydroxyl groups on the surface.Finally, the effect of Co nanocrystal (NC) size on deactivation of Co FT catalysts by sintering was studied using surface chemical properties of the active metal phase and the support and their interactions. Values of surface energy, adhesion and cohesion energy, chemical potential, diffusivity, and energy required for sintering deactivation versus Co NC diameter were calculated. Effects of the FT reaction environment, such as water concentration and temperature, on sintering was explained and quantified. Co sintering via Ostwald Ripening was found to be a chemically driven phenomena. Also, adhesion energy and appropriate deposition of Co NCs inside support pores are the most important factors affecting the sintering rate via crystal migration and coalescence.

Esta tesis se inscribe en el marco del Programa para el De-sarrollo y Utilización del Recurso Gasífero de la Provincia de Santa Cruz cuyo principal objetivo es identificar y examinar alternativas de incorporación de valor agregado al gas natu-ral. En particular en la presente se estudia, a nivel prefac-tibilidad privada y social, la viabilidad de la radicación de una planta localizada el Puerto Punta Loyola, destinada a la pro-ducción de combustibles Gas to Liquids (GTL) mediante el pro-ceso FischerTropsch. Para la evaluación privada se utiliza la metodología propuesta por UNIDO, mientras que para el aná-lisis social la de OCDE. Debido a que los procesos FischerTropsch permiten valorizar reservas de gas natural localizadas en regiones alejadas de los centros de consumo y permiten alcanzar las especificaciones de calidad de los combustibles, estas tecnologías pueden jugar un rol fundamental en los pró-ximos años. En Argentina, luego de la recuperación de la eco-nomía tras la crisis de los años 1999-2003 y del año 2009, la demanda de gas oil ha mostrado una tendencia creciente. Sin embargo, la capacidad de refinación local no ha sufrido varia-ciones significativas en los últimos años, teniendo por tanto que recurrirse a la importación para garantizar su provisión. Este problema de abastecimiento junto con las normas relati-vas a la calidad de los combustibles representan un nicho de mercado para el proyecto. Desde el punto de vista técnico es factible transformar el gas natural en combustibles líquidos ya que la tecnología ha sido probada por muchos años. Sin embargo, cabe mencionar que el costo de capital de la planta bajo estudio se duplicaría respecto de los proyectos mundiales. Los productos a comercializar se valúan según sus precios internos regulados por la Resolución N 394/2007 haciendo que el proyecto no sea aceptable. Por lo tanto, se plantean algunos escenarios alternativos -amortización acelerada o la posibilidad de financiar la inversión inicial- que si bien mejoran la rentabilidad del proyecto no resulta suficientes para reco-mendar su realización. El proyecto solo es aceptable cuando se supone que la producción se coloca a precios internacio-nales. Sin embargo, se demuestra que leves cambios desfavo-rables en las variables críticas tornarían el proyecto inacepta-ble. Los indicadores de rentabilidad se calcularon a partir de la tasa de descuento informada por la industria petroquímica (18%). A pesar de considerarlo alto, se respeta este dato aunque, dado que se trata de una tasa de rentabilidad real habría que tomar como referencia la estimación realizada por Weston y Copeland (1995). Respecto de la evaluación social solo se realiza el análisis de eficiencia, sin tener en cuenta la distribución del ingreso. En este marco, dado que el proyecto es intensivo en el uso de un recurso escaso en el país, no re-sulta aceptable. En un análisis posterior sería interesante estudiar un proceso alternativo de producción (Synfuels) para comparar los resultados. Además resultaría provechoso incor-porar los efectos de multiplicación de empleo que se origina-rán a partir de la instalación de la planta y de las externalida-des del proyecto. / This thesis was elaborated in the framework of the Programa para el Desarrollo y Utilización del Recurso Gasífero de la Pro-vincia de Santa Cruz whose main objective is to identify and examine alternatives to add value to natural gas. The work was focused on the study of the viability of a plant for the production of fuels Gas to Liquids (GTL) using Fischer-Tropsch process, to be located in Puerto Punta Loyola. For the private evaluation the methodology proposed by UNIDO was used, while the one proposed by the OECD guided the social analy-sis. Since Fischer-Tropsch processes can both add value to natural gas reserves located in regions far from consumption centers and reach the quality specifications for fuels, these technologies can play a fundamental role in the near future. In Argentina, after the 1999-2003 and 2009 crisis, demand for gas oil has shown an increasing trend. However, the local refi-ning capacity has not changed significantly in recent years, inducing increased imports to ensure supply. Shortages of supply problems together with quality standards of fuel provi-de a market niche for the project.From a technical point of view it is possible to transform natural gas into liquid fuels since the technology has been proven for many years. Howe-ver, it is noteworthy that the capital cost of the plant under study would double the cost of similar projects over the world. Products manufactured in the plant to be traded in the market have to be valued according to their domestic prices regulated by Resolution No. 394/2007 and because of this fact the project becomes unacceptable. Some alternative scenarios - accelerated depreciation or the possibility of finan-cing the initial investment- were then presented. Though able to improve the profitability of the projects they are not enough to recommend its implementation. The project beco-mes acceptable only when it is assumed that the production is valued at international prices. Besides, it is shown that sli-ght adverse changes in critical variables would make it un-acceptable. The profitability indicators were calculated using the discount rate reported by the petrochemical industry (18%). Although considered high, this value was respected. However, since this is a real rate of return, estimates by Wes-ton and Copeland (1995) should be taken as a reference. With respect to social evaluation only the analysis of effi-ciency was performed, regardless of income distribution. In this context, since the project is intensive in the use of a scarce resource in the country, it becomes not acceptable. In a future research it would be interesting to study an alter-native process of production (Synfuels) in order to compare the results. Moreover, it would be helpful to incorporate the employment multiplier effects induced by the installation of the plant as well as the externalities of the project.

Vibrofluidized microreactor systems have been developed for studies of unsteady-state Fischer-Tropsch synthesis. This development is aimed at preventing carbon deposition on a fused-iron catalyst in a novel reactor called the “heat-tray.” This reactor involves a supernatant gas flowing over a shallow fluidized bed of catalyst particles. Three systems were built: (1) a vibrofluidized-bed microreactor system for obtaining baseline carbon deposition infonnation under industrially important reaction conditions; (2) a sliding-plug vibrofluidized-bed microreactor system for rapid switching of feed gases in the F-T synthesis; and (3) a cold-flow microreactor model for studying the gas mixing characteristics of the sliding-plug vibrofluidized-bed microreactor. The results show that catalyst defluidization occurred under steady-state synthesis conditions below 395°C using a feed gas of H₂/CO ratio of 2:1 or less. Above 395°C, the probability of hydrocarbon chain growth (α) on the fused-iron catalyst was low enough (α < 0.50) to prevent accumulation of high-molecular-weight species that cause defluidization. Carbon deposition was rapid above 395°C when a feed gas of H₂/CO ratio of 2:1 or less was used. Spent catalyst fractions in the form of free-flowing catalyst and "bugdust" were quantitatively analyzed for carbon and iron. Mössbauer spectroscopic analysis of free-flowing catalyst showed mainly Hägg carbide (x-Fe₅C₂) and magnetite (Fe₃O₄) with a smaller fraction present as α-Fe. Scanning electron microscopic analysis of the bugdust revealed a mass of highly porous, fine particles with a high carbon content (18-30 wt%). Cold-flow microreactor model studies show that rapid (on the order of seconds), quantitative switching of feed gases over a vibrofluidized-bed of catalyst could be achieved. Vibrofluidization of the catalyst bed induced little backmixing of feed gas over the investigated flow-rate range of 417 to 1650 actual mm³/s. Further, cold-flow microreactor model studies showed intense solid mixing when a -150+300 µ bed of fused-iron catalyst was vibrofluidized at 24 cycles per second with a peak-to-peak amplitude of 4 mm. The development of this microreactor system has provided an easy way of accurately determining integral fluid-bed kinetics in a laboratory reactor. Further, the unique ability of the microreactor system to rapidly switch feed gases over an intensely-mixed solid has important applications in chemical kinetics and reaction engineering. / Ph. D.

Tesis por compendio / La presente tesis doctoral está centrada en la investigación de los parámetros estructurales que determinan las propiedades catalíticas en la síntesis de Fischer-Tropsch (SFT) de catalizadores de cobalto soportados en TiO2. Por un lado, el estudio de la influencia del polimorfo de óxido de titanio (rutilo vs. anatasa) utilizado como soporte en catalizadores de Co promovidos con Ru ha permitido obtener correlaciones entre la estructura cristalina del soporte, la extensión del efecto SMSI (interacción fuerte metal-soporte) y los resultados catalíticos. Por otro lado, mediante la modificación de las propiedades texturales del soporte TiO2-anatasa con el objetivo de obtener catalizadores con baja, media y alta área superficial se ha podido avanzar en el conocimiento del efecto SMSI y su correlación con las propiedades texturales del soporte. Además, las consecuencias del aumento en área superficial del soporte en la actividad y selectividad de catalizadores CoRu/TiO2 para la SFT se han podido explicar en base a las relaciones establecidas entre estructura y efecto SMSI. Adicionalmente, el uso de técnicas de luz sincrotrón junto con caracterización espectroscópica in situ realizada a presiones superiores a la atmosférica, ha permitido explicar el papel de la adición y concentración de Ru como promotor en catalizadores CoRu/TiO2. Finalmente, se han estudiado tratamientos de reducción-oxidación-reducción (ROR) en catalizadores CoRu/TiO2 con el objetivo de mejorar su actividad catalítica. Como conclusión general, los conocimientos derivados de los resultados obtenidos en esta tesis doctoral pueden aportar estrategias adecuadas para el diseño de catalizadores de FT mejorados basados en Co empleando TiO2 como soporte. / The present doctoral thesis focused on the investigation of the structural parameters that can determine the ultimate catalytic properties for Fischer-Tropsch synthesis (FTS) of TiO2-supported cobalt catalysts. On the one hand, the study of the influence of the titania polymorph (rutile vs. anatase) as support for Ru-promoted Co and Ru nanoparticles (NPs) has allowed to identify some correlations between the TiO2 crystalline phase, the SMSI (strong metal-support interaction) effect, and the catalytic performance for FTS of the catalysts. On the other hand, by preparing CoRu catalysts supported on TiO2-anatase with low, medium, and high surface area, further insights into the SMSI effect and its dependence on the textural properties of the TiO2-anatase support have been gained. Besides, the consequences of increasing the surface area of the support on the activity and selectivity of the catalysts for FTS have been explained based on the established structure-SMSI relationships. Moreover, a detailed study involving the use of in situ synchrotron-based spectroscopic characterizations at pressures higher than the ambient pressure usually applied in most previous works, has been carried out aiming at explaining the role of Ru addition and concentration as promoter in Co/TiO2 catalysts. Finally, reduction-oxidation-reduction (ROR) treatments have been applied on CoRu/TiO2 catalysts to revert the SMSI effect as a feasible strategy to enhance their catalytic activity. Overall, the results reported in this thesis provide grounds for designing TiO2-supported Co catalysts with improved activity and selectivity for FTS. / La present tesi doctoral està centrada en la investigació dels paràmetres estructurals que poden tenir influència en les propietats catalítiques dels catalitzadors que s'han aplicat a la reacció de síntesi de Fischer-Tropsch (SFT). S'ha estudiat la influència del polimorf de titani (rutil o anatasa) utilitzat com a suport de nanopartícules (NPs) de Co i Ru, observant correlacions entre l'estructura cristal·lina del suport, l'efecte SMSI (forta interacció metall-suport) i els resultats catalítics. D'altra banda, es va fer un estudi modificant les propietats texturals de la anatasa amb l'objectiu d'obtenir catalitzadors amb diferent àrea superficial, i s'ha pogut establir un coneixement més profund de l'efecte SMSI i la seua correlació amb les propietats texturals del suport. A més, la influència de l'augment de l'àrea superficial del suport per a la reacció de SFT, en termes d'activitat i selectivitat, han sigut explicats d'acord a les relacions establides entre l'estructura i l'efecte SMSI. Addicionalment, fent ús de tècniques de llum sincrotró juntament amb caracterització in situ realitzada a altes pressions, ha sigut possible explicar el paper de l'addició i concentració de Ru com a promotor en catalitzadors CoRu/TiO2. Finalment, s'han estudiat els tractaments de reducció-oxidació-reducció (ROR) en catalitzadors CoRu/TiO2 amb l'objectiu de millorar la seua activitat catalítica. En resum, els coneixements derivats dels resultats obtinguts en esta tesi doctoral permeten establir estratègies per al disseny de catalitzadors millorats per a la síntesi de FT basats en cobalt utilitzant TiO2 com a suport. / Bertella, F. (2018). Structural, promotion and metal-support interaction effects in Co/TiO2 catalysts for Fischer-Tropsch synthesis [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/107952 / Compendio

La síntesis de Fischer-Tropsch (SFT) es en la actualidad, la ruta más eficiente para la producción de hidrocarburos a partir de gas natural, metano o carbón. El principal interés de la SFT, es la obtención de combustible sintético (diesel y gasolina) de alta calidad y medioambientalmente limpio. Para la producción de diesel sintético se utilizan catalizadores de cobalto soportado sobre SiO2 (Co/SiO2), con una elevada selectividad a hidrocarburos pesados, pero que sin embargo son poco activos debido a la baja dispersión de la fase de cobalto activa sobre SiO2. En la presente tesis se han ensayado diversas sílices mesoporosas ordenadas (MCM-41, MCM-48, SBA-15 y Me-MCM-41), como soportes alternativos para catalizadores de cobalto de FT. La elevada área superficial de estos materiales y su estructura de canales ordenada permite dispersar el cobalto con mayor eficacia, con respecto al soporte convencional SiO2. En esta serie, el mejor resultado de actividad y selectividad a hidrocarburos pesados, se ha obtenido para el catalizador de cobalto soportado sobre SBA-15. Por ello, se ha investigado la influencia de diversos parámetros (contenido de cobalto, sal precursora de cobalto, incorporación de promotores) en las propiedades catalíticas del catalizador Co/SBA-15. Por otra parte, se ha explorado el uso de las zeolitas deslaminadas ITQ-2 e ITQ-6 (de alta área externa y estabilidad hidrotermal), como soportes para catalizadores de cobalto de FT. Estos catalizadores también resultan más activos y selectivos a hidrocarburos pesados que el catalizador de referencia (Co/SiO2). Para la obtención de gasolina sintética se utilizan catalizadores de FT basados en hierro. No obstante, debido al bajo rendimiento a la fracción gasolina obtenida directamente del proceso de FT y a la baja calidad (octano) de ésta, es necesario llevar a cabo diversos procesos (craqueo, reformado, isomerización) sobre los hidrocarburos obtenidos de la reacción de FT, para aumentar el rendimiento y la calidad (octanaje) de la fracción gasolina. Por ello, en la última parte de esta tesis, se han preparado y ensayado catalizadores híbridos que combinan un catalizador de hierro y una zeolita capaz de promover reacciones (in-situ) de craqueo y aromatización de los productos primarios de FT. El estudio de la influencia de la estructura zeolítica, indica que la combinación del catalizador de hierro de FT con la zeolita ZSM-5 permite obtener el mayor rendimiento a la fracción de gasolina, y dentro de ésta, a isoparafinas y aromáticos. El estudio del efecto de la relación Si/Al, de la presencia de promotores (Pd, Ga) o del tamaño de cristal de la zeolita, indica diversos métodos para optimizar el rendimiento del catalizador híbrido hacia los productos de interés, así como su estabilidad en el proceso catalítico. / López Cruz, C. (2004). Materiales mesoporosos y zeolitas deslaminadas como soportes para catalizadores de Fischer-Tropsch basados en Cobalto [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/131736

A síntese de Fischer-Tropsch é um processo de conversão do gás de síntese (CO + H2) em hidrocarbonetos de cadeias longas. Os catalisadores clássicos para a hidrogenação do CO são, principalmente, o Fe e o Co suportados em diferentes óxidos. O desempenho catalítico do catalisador é influenciado pelo tamanho, dispersão e grau de redução das partículas metálicas. Estudos recentes mostram uma promissora aplicação de materiais à base de nanofibras de carbono na catálise heterogênea. Estes materiais apresentam algumas vantagens em relação aos suportes catalíticos tradicionais, tais como: uma baixa interação metal-suporte, elevada área superficial, ausência de poros fechados, alta condutividade térmica, elevada inércia química e hidrofobicidade. Neste trabalho foram fabricados suportes catalíticos macroscópicos à base de nanofibras de carbono, empregando o método de vapor deposição, a partir da decomposição do etano. Os catalisadores foram preparados pela impregnação incipiente do Co e de promotores na superfície do suporte. Foram empregados os metais nobres, Ir, Pt e Ru, como promotores catalíticos, com o objetivo de incrementar a redutibilidade e a atividade do catalisador. Todos os catalisadores foram caracterizados por Quimissorção de CO, Fisissorção de N2, Redução a Temperatura Programada (TPR), Espectroscopia Fotoeletrônica de Raios X (XPS) e Microscopia Eletrônica de Transmissão (MET). Os catalisadores foram, então, testados na síntese de Fischer-Tropsch, utilizando um reator de leito fixo e fluxo contínuo, com análise simultânea dos produtos gasosos e controle sistemático da temperatura, pressão e vazão dos reagentes. Finalmente, foram analisados os produtos líquidos obtidos na reação com objetivo de conhecer a influência dos promotores na seletividade dos hidrocarbonetos formados. / Fischer-Tropsch synthesis is a process of converting the syngas (CO + H2) to long-chain hydrocarbons. The traditional catalysts for the CO hydrogenation are Fe and Co supported on different oxides. Catalytic performance of the catalyst is influenced by size, dispersion and degree of reduction of metal particles. Recent studies show a promising application of materials based on carbon nanofibers in heterogeneous catalysis. These materials have some advantages compared to traditional catalyst supports, such as a low metal support interaction, high surface area, no closed pores, high thermal conductivity, high chemical resistance, and hydrophobicity. In this work, based on macroscopic carbon nanofiber catalyst supports have been manufactured by employing the method of chemical vapor deposition from ethane decomposition. Catalysts were prepared by incipient wetness impregnation of Co and promoters on the support surface. Noble metals, Ir, Pt and Ru were used as catalytic promoters, with the aim of increasing the reductibility and catalyst activity. All catalysts were characterized by CO Chemisorption, N2 Physisorption, Temperature Programmed Reduction (TPR), X-ray Photoelectron Spectroscopy (XPS), and Transmission Electron Microscopy (TEM). The catalysts were then tested in the Fischer-Tropsch synthesis using a fixed bed reactor, continuous flow, with simultaneous analysis of gaseous products and systematic temperature control, pressure, and flow rate of the reactants. Finally, the liquid products obtained in the reaction were analyzed in order to determine the influence of promoters on the selectivity of hydrocarbons formed.

La synthèse Fischer-Tropsch (FT) permet de convertir un mélange d’hydrogène et de monoxyde de carbone (gaz de synthèse) en hydrocarbures avec une distribution large de longueur de chaine. Le gaz de synthèse peut être produit à partir de différentes ressources comme le gaz naturel, le charbon et la biomasse. Afin de diversifier les sources d’énergie, la synthèse FT peut apporter une contribution pour la production de carburants liquides. Néanmoins, la formation de méthane pendant la réaction affecte la faisabilité économique du procédé. Cette étude a pour but de comprendre le mécanisme de formation du méthane sur des catalyseurs de FT. Pour atteindre cet objectif, une étude cinétique en régime transitoire couplée à la technique « SSITKA » a été mise en œuvre sur différents catalyseurs nickel et cobalt. Les données expérimentales sont ensuite utilisées pour alimenter un modèle microcinétique. En utilisant cette méthodologie, nous avons montré que deux intermédiaires distincts de surface conduisaient à la production de méthane. Le modèle microcinétique consiste en deux voies de production de méthane, l'une par dissociation directe de CO, l'autre par décomposition de CO assistée par hydrogène. Nous proposons que les proportions relatives de ces deux intermédiaires dépendent de la structure des particules métalliques, notamment la distribution des sites en sur les terrasses et les coins / The Fischer-Tropsch synthesis (FTS) converts a mixture of hydrogen and carbon monoxide (syngas) selectively into hydrocarbons with a large chain length distribution. Syngas can be produce from different resources such as natural gas, coal and biomass. In the light of energy resource diversification, FTS can make a contribution to the production of liquid fuels. However, methane formation as byproduct has a large impact on the process economic feasibility. This study aims at the understanding of the methane formation over syngas conversion catalysts, such as nickel and cobalt. To this purpose, Steady-State Isotopic Transient Kinetic Analysis (SSITKA) and step-transient experiments over different nickel and cobalt samples have been carried out and the data have been used to develop a microkinetic model describing methane formation. By using these methodologies, it was found that the CO conversion to methane proceeds through two different surface intermediate species. The microkinetic model is developed on the hypothesis of two reacting paths leading to methane: the unassisted CO dissociation and the H-assisted CO decomposition. It is proposed that these two reacting intermediates are related to the structure of the catalyst particle, specifically to the distribution of the catalyst surface sites on terraces and steps

Nous avons étudié des catalyseurs de cobalt utilisés pour la synthèse Fischer-Tropsch des hydrocarbures en suivant les processus d’activation, de fonctionnement et de désactivation par microscopie électronique en transmission in-situ environnementale. Une méthodologie operando a été développée en couplant ces études structurales dans une cellule environnementale à la mesure de gaz résiduels par spectrométrie de masse. Utilisant des nano-objets de morphologie contrôlée, nous avons mis en évidence des mécanismes de désactivation des catalyseurs par frittage des particules, ré-oxydation, encapsulation et carburation. Pour les catalyseurs supportés, leur activation se distingue par une densification des particules. Nous avons démontré que la réduction des particules est plus facile sur silice que sur alumine ainsi qu’en présence des atomes de Pt. Durant leur fonctionnement sous gaz de synthèse, nous avons détecté la formation d’hydrocarbures. Cette méthodologie a donné accès aux propriétés physico-chimiques d’une série de catalyseurs de cobalt, ouvrant ainsi la voie à des études approfondies de catalyseurs hétérogènes par microscopie operando. / In this work, we studied cobalt catalysts used in producing hydrocarbons through the Fischer-Trospch synthesis process, in which their activation, operation and deactivation mechanisms are recorded using in-situ and environmental transmission electron microscopy. An operando methodology was developed by combining electron microscopy-based structural studies in an environmental cell and mass spectrometry analysis of the reacting gases. Using controlled-morphology nanocatalysts we have demonstrated that deactivation occurs through particles sintering, re-oxidation, encapsulation and carburization. The activation of supported catalysts arises due to particles densification. It was clearly established that the reduction of the nanoparticles is better enhanced on a silica support than on an alumina one. Also, Pt doping of the catalysts greatly increases their reduction. By analysing the hydrocarbons formed during the Fischer-Tropsch reactions of a series of cobalt catalysts, we could obtain a comprehensive physico-chemical characterization, thus opening the way to in-depth studies of heterogeneous catalysts by operando microscopy.