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

A reação de eletro-oxidação de metanol (MEOR) desempenha um papel crucial na transição para um cenário energético sustentável, sendo aplicável em dispositivos como células a combustível de metanol direto e em processos de eletro-reforma para produção de hidrogênio limpo. Isso permitiria estabelecer um ciclo de energia sustentável, reduzindo a dependência de fontes de energia únicas. No entanto, a implementação em larga escala desses dispositivos enfrenta desafios, sendo essencial compreender o mecanismo da MEOR, identificar sítios ativos e compreender o impacto das condições experimentais na busca por catalisadores eficientes e seletivos. Apesar dos esforços dedicados ao estudo da MEOR, sua complexidade dificulta a correlação entre a resposta cinética eletroquímica e os processos na superfície do eletrodo. Nesse contexto, utilizando abordagens numéricas baseadas em modelagem microcinética e experimentos com eletrodos de platina policristalina e monocristalinos, o estudo foca em compreender o mecanismo de reação, determinar as vias predominantes e avaliar como estas são afetadas pela temperatura, um parâmetro crítico em todas as aplicações. O modelo microcinético proposto foi construído considerando aspectos mecanicistas relevantes e validado por comparação com dados experimentais. O modelo conseguiu simular a dinâmica não linear, incluindo o comportamento caótico, observado experimentalmente, juntamente com um perfil voltamétrico razoável. A análise de sensibilidade destacou a importância das espécies OHad e COad na origem das oscilações. Os experimentos com eletrodos monocristalinos revelaram que a taxa de fluxo de gás pode afetar significativamente a resposta em regime oscilatório, destacando a importância do controle desse parâmetro experimental. Além disso, insights valiosos foram obtidos em relação às oscilações de modo misto, anteriormente pouco compreendidas, que foram associadas ao restabelecimento periódico da concentração de metanol na dupla camada. A influência da temperatura na cinética da MEOR e nas vias de reação paralelas foi investigada usando um eletrodo de Pt(100). Os resultados indicam que medidas cronoamperométricas em estado estacionário fornecem valores mais confiáveis para as energias de ativação aparentes. Observou-se uma mudança dependente da temperatura na predominância de vias de oxidação, sugerindo um mecanismo de controle cinético e termodinâmico para evitar o envenenamento completo da superfície do eletrodo. Em conjunto, as descobertas oferecem informações cruciais sobre o mecanismo de reação, vias predominantes e sua sensibilidade à temperatura. Esses insights são fundamentais para orientar o desenvolvimento de materiais visando aumentar a eficiência da conversão e otimizar a temperatura operacional em dispositivos de conversão de energia, contribuindo assim à transição para um panorama energético mais sustentável.

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Fischer-Tropsch synthesis (FTS), developed in the early 1900's, is defined as the catalytic conversion of H2 and CO to hydrocarbons and oxygenates with the production of H2O and CO2. Accurate microkinetic modeling can in principle provide insights into catalyst design and the role of promoters. This work focused on gaining an understanding of the chemistry of the kinetically relevant steps in FTS on Fe catalyst and developing a microkinetic model that describes FTS reaction kinetics. Stable Al2O3-supported/promoted (20% Fe, 1% K, 1% Pt) and unsupported Fe (99% Fe, 1% Al2O3) catalysts were prepared and characterized. Transient experiments including temperature programmed desorption (TPD), temperature programmed hydrogenation (TPH), and isothermal hydrogenation (ITH) provided insights into the chemistry and energetics of the early elementary reactions in FTS on Fe catalyst. Microkinetic models of CO TPD, ITH, and FTS were developed for Fe catalyst by combining transition state theory and UBI-QEP formalism. These models support the conclusion that hydrocarbon formation occurs on Fe via a dual mechanism involving surface carbide and formyl intermediates; nevertheless, hydrocarbon formation is more favorable via the carbide mechanism. Carbon hydrogenation was found to be the rate determining step in the carbide mechanism. CO heat of adsorption on polycrystalline Fe at zero coverage was estimated to be -91.6 kJ/mol and -64.8 kJ/mol from ITH and FTS models respectively, while a mean value of -50.0 kJ/mol was estimated from the TPD model. Statistically designed steady-state kinetic experiments at conditions similar to industrial operating conditions were used to obtain rate data. The rate data were used to develop a microkinetic model of FTS. FTS and ITH appear to follow similar reaction pathways, although the energetics are slightly different. In both cases, hydrocarbon formation via the carbide mechanism was more favorable than via a formyl intermediate while carbon hydrogenation was the rate determining step. Promotion of Fe with K does not alter Fischer-Tropsch synthesis reaction pathways but it does alter the energetics for the steps leading to the formation of CO2. This phenomenon accounts for the CO2 selectivity of 0.3 observed for K-promoted Fe against 0.17 observed for un-promoted Fe. A Langmuir Hinshelwood rate expression derived from the microkinetic model was put into a fixed bed FTS reactor design code; calculated reactor sizes, throughput, temperature profiles and conversion are similar to those of pilot and demonstration FTS reactors with similar feed rates and compositions.