No description available.

Los nanoclústeres (NCs) exhiben propiedades físicas y químicas muy novedosas sensibles a su tamaño y geometría. Además, resultan muy interesantes pues tienden un puente entre el comportamiento de los átomos y el del sólido. Por otro lado, reciben gran atención por sus aplicaciones tecnológicas, como por ejemplo, los NCs de metales de transición en el campo de la catálisis heterogénea. En particular, los clústeres de Pt depositados sobre óxidos tales como rutilo TiO2, han mostrado ser eficientes como catalizadores en reacciones tales como la oxidación de CO a CO2, paso intermedio importante de la reacción de WGS (water gas shift). Es por ello que en esta tesis se realizó una caracterización exhaustiva de clústeres de Pt aislados (Ptn) y clústeres de Pt soportados (Ptn/TiO2(110)), evaluando sus propiedades estructurales, cohesivas y electrónicas a través de un estudio teórico utilizando métodos de modelado ab initio basados en la Teoría de la Funcional Densidad (DFT). Primeramente, se consideraron los clústeres de Ptn (n = 2, 4, 13, 19, 55, 79, 85 y 147) aislados. Para cada clúster se optimizó su estructura, y a partir de ella se determinaron las demás propiedades. Las densidades de estados vibracionales obtenidas en el marco de la aproximación armónica muestran un comportamiento muy diferente al del sólido, con presencia de estados discretos, que dan lugar a desviaciones del modelo de Debye para el calor específico a volumen constante a bajas temperaturas. Entre los clústeres estudiados, el Pt13 fue abordado en detalle, en particular porque las típicas configuraciones de capa cerrada resultaron inestables. Este es un resultado inesperado, puesto que muchos trabajos proponen a las estructuras (Oh) e (Ih) como las más plausibles para este tamaño de clúster. Ante este resultado, se utilizó la técnica de dinámica molecular ab initio, que incorpora efectos térmicos, con el fin de analizar la evolución de los clústeres en el tiempo y en búsqueda de nuevas configuraciones estables de menor energía. Como resultado, un nuevo isómero de baja simetría con estructura de capas apiladas es predicho para el clúster Pt13. Otro tamaño de clúster estudiado en distintas configuraciones fue el Pt4. Se consideraron las geometrías bidimensionales planar (P), romboédrica (R) y la tridimensional tetraédrica (T). De manera aislada, el clúster (T) resultó más estable, aunque esta misma tendencia no se mantuvo al momento de depositar los NCs en el sustrato de rutilo estequiométrico (TiO2). Para analizar la potencialidad del sistema Ptn/TiO2 como catalizador, fueron depositados NCs de Pt4 (P y T) y Pt13(Oh) sobre la superficie TiO2(110) tanto estequiométrica como reducida (TiO2(110)+Vo). Para ambos sustratos se estudió la estabilidad relativa de las estructuras de los clústeres, determinando las geometrías de equilibrio, las energías de adsorción, los efectos de transferencia de carga y la densidad electrónica de estados para caracterizar los diferentes aspectos de la interacción metal-óxido. En particular se evaluó la factibilidad del uso de estos sistemas en las reacciones de oxidación de CO por un átomo de oxígeno de la superficie, como paso intermedio en la reacción de WGS. / Nanoclusters exhibit novel physcial and chemical properties sensitive to their size and geometry. Besides, they are of interest since they tend to stablish a bridge between the behaviour of the atoms and the solid. On the other side, they are receiving great attention due to their technological applications, such as for example, the transition metal nanoclusters in the field of heterogeneous catalyst. In particular, Pt clusters deposited on oxides such as rutile TiO2, have shown to be efficient as catalysts in reactions such as the oxidation of CO to CO2, an important intermediate step in the WGS (water gas shift) reaction. Due to this fact in this thesis an exhaustive characterization of isolated Pt clusters (Ptn) and suppported Pt clusters is performed, evaluating their structural, cohesive and electronic properties through a theoretical study using ab initio modelling methods based on Density Functional Theory,. Firstly, the isolated Ptn (n = 2, 4, 13, 19, 55, 79, 85 y 147) clusters were considered. For each cluster its structure was optimized, and with it the other properties were determined. The vibrational densities of states, calculated in the harmonic approximation, show a behaviour very different from the bulk one, with the presence of discrete states that give rise to deviations from the Debye model for the specific heat at constant volumen and low temperatures. Among the clusters studied, the Pt13 was treated in detail, in particular because the typical close shell configurations happen to be unstable. This is an unexpected result, since various works report (Oh) e (Ih) structures as the most plausible ones for this cluster size. Considering this, we applied the ab initio molecular dynamic technique, that incorporates explicitly thermal effects, with the aim to analyse the time evolution of the cluster, and to search new stable configurations of lower energy. As a result, a new layered low symmetry stable isomer of lower energy is predicted for the Pt13 cluster. Another cluster size studied in different configurations was Pt4. The two-dimensional planar geometry (P), rombohedrical (R) and the tridimensional tetrahedrical (T) one, were considered. For the isolated clusters, the (T) one was more stable, although this trend is changed for the supported NCs on stequiometric rutile (TiO2). To analyze the potential activity of the Ptn/TiO2 as a catalyst, NCs of Pt4 (P y T) and Pt13(Oh) were deposited on the TiO2(110) stoiquiometric and reduced (TiO2(110)+Vo) surfaces. For both substrates the relative stability of the structures was studied, determining the equilibrium geometries, the adsorption energies, the effects of charge transfer and the electronic density of states to characterize different aspects of the metal-oxide interaction. In particular the feasibility of the use of these systems for the oxidation of CO by one oxygen atom of the surface, as intermediate step in the WGS reaction, was evaluated.

On the basis of our investigation of the "Aldridge" catalyst, an alkali impregnated cobalt0-molybdate on an A1 203 support, for the water-gas shift, methanation, and ethanol dehydration reactions, we can make the following conclusions: 1. The cesium-impregnated "Aldridge" catalyst is highly active for the water-gas shift reaction under sulfur tolerant conditions. 2. The activity of this catalyst is strongly dependent upon the cesium:molybdenum molar ratio. The normalized first order rate constant increases with this ratio until an optimum is reached for the full strength and half strength catalyst. 3. The transition temperatures appeared only with the cesium-impregnated full and half strength catalysts, but not with the one-fifth catalysts. 4. The potassium-impregnated cobalt-molybdate catalyst is quite active, in Comparison to lithium- and sodium-impregnated versions. 5. The cesium-impregnated zinc-molybdate catalyst is not as active as the unimpregnated cobalt-molybdate. Its activity is approximately half that of catalyst "Z" at 400°C. 6. We don't believe that the "Aldridge" catalyst is a catalytic melt. / Master of Science

L'intérêt pour la synthèse de Fischer-Tropsch (FTS) est d'actualité. Elle permet la conversion de matière première (biomasse) en combustible liquide. Comparés aux catalyseurs à base de cobalt, ceux à base de fer présentent une désactivation rapide, une activité et une sélectivité faibles en produisant une quantité non désirable de CO2. Après plusieurs décennies d'études, l'origine de ces défauts reste méconnue. Les catalyseurs classiques sont généralement fortement chargés en fer (>70 wt.%) et composés de nombreuses phases empêchant l'établissement d'une relation structure-activité. Il est nécessaire de développer des catalyseurs contenant du fer plus actifs, plus sélectifs et plus stables par une approche rationnelle. La synthèse de nanoparticules de taille contrôlée (3.5 nm) encapsulées dans les murs d'une silicalite-1 creuse (Fe@hollow-silicalite-1) est présentée. L'encapsulation empêche le frittage pendant la synthèse de Fischer-Tropsch, permettant de garder une bonne dispersion du fer. Contrairement aux autres catalyseurs, le catalyseur Fe@hollow-silicalite-1actif ne produit pas de CO2. L'hydrophobicité de la silicalite-1 est très certainement à l'origine de la non-production de CO2 par inhibition de la réaction directe du gaz à l'eau. On démontre que le catalyseur Fe@hollow-silicalite-1convertit le CO2 en CO par réaction du gaz à l'eau inversée (R-WGS). Afin d'établir une relation structure-activité, des catalyseurs à base de fer de taille bien contrôlée sont synthétisés et caractérisés (MET, in-situ XANES, in-situ Mössbauer). Deux catégories de TOF suivant la taille des particules, ~10-2 s-1 pour les plus larges (>20 nm) et ~10-3 s-1 pour les plus petites, sont observées / Fischer-Tropsch synthesis (FTS) is gaining renewed interests as it allows converting alternative feedstocks (biomass) into liquid fuels. Compared to Co-based catalysts, state of the art Fe catalysts show lower activity, faster deactivation and lower selectivity as it produces an undesirable amount of CO2. Despite decades of studies, the origins of low activity and selectivity and fast deactivation are still unclear. Typical Fe based catalysts are highly metal loaded (>70 wt.%) and composed of many different phases, which strongly impedes the establishment of structure-activity relationships. There is a need to develop more active, more selective and more stable iron FTS catalysts by rational approaches.The synthesis of well-controlled 3.5 nm iron nanoparticles encapsulated in the walls of a hollow-silicalite-1 zeolite (Fe@hollow-silicalite-1) is presented. The encapsulation prevents particle sintering under FTS conditions leading to a high and stable Fe dispersion. The catalyst Fe@hollow-silicalite-1 is active and highly selective in FTS. Most importantly, Fe@hollow-silicalite-1 does not produce CO2 in contrast to all other Fe-based catalysts. The strong hydrophobicity of the silicalite-1 is likely the origin of the lack of CO2 production by inhibition of the forward WGS reaction. We demonstrated that Fe@hollow-silicalite-1converts CO2 into CO by the reverse WGS reaction. In order to establish a structure-activity relationship, a series of Fe-based catalysts with well-controlled particle sizes were synthesized and characterized (TEM, in-situ XANES, in-situ Mössbauer, XRD). We observed two distinct categories of TOFs depending on the particle size, ~10-2 s-1 for larger (>20 nm) and ~10-3 s-1 for smaller ones

No description available.

Le reformage des gaz primaires de gazéification a été étudié à des températures de 800-1000[degrés Celsius].Le reformage thermique à ces températures a permis de convertir jusqu'à 65 % du goudron et de réduire le ratio phénol/naphtalène de 99 %, ce qui représente un avantage certain pour le traitement des eaux de lavages. L'ajout d'un lit fixe contenant du char a permis d'augmenter la conversion du goudron à 85 % et de réduire les phénols sous le seuil de détection du GC-MS. Dans ces conditions, il a été démontré que le reformage thermique convertit tous les gaz C[indice inférieur 2] -C[indice inférieur 3] , à l'exception de l'éthane, pour lequel une cinétique a été établie, et d'une légère production d'acétylène. L'ajout d'un lit fixe contenant du char a augmenté la conversion des gaz C[indice inférieur 2] -C[indice inférieur 3] et a même permis la conversion de 30 % du méthane à une température aussi basse que 925[degrés Celsius]. Par contre, l'ajout de char transporté par le gaz de synthèse à des concentrations moindres n'a pas eu d'effet observable sur la conversion des gaz C[indice inférieur 1]-C[indice inférieur 3].Le char transporté a cependant augmenté le taux de la réaction de water gas shift , ce qui permet un meilleur ajustement du ratio H2 /CO et une synthèse optimale dans les étapes catalytiques subséquentes.

O titanato de estrôncio (SrTiO3) é um óxido de estrutura perovskita e tem sido intensamente estudado para uso em diversas aplicações, entre elas, como suporte catalítico. Entretanto, sua utilização especificamente na reação de deslocamento gás-água ainda é pouco explorada. Esta reação é de interesse para a produção de hidrogênio livre de CO, necessário para aplicações como o abastecimento de células de combustível. Este trabalho de tese teve como objetivo o estudo dos compostos SrTi1-xCuxO3, CuO/SrTiO3 e NiO/SrTiO3 como catalisadores para a reação de deslocamento gás-água, uma vez que, dentre os metais de baixo custo, Cu e Ni são altamente ativos para esta reação. As amostras SrTi1-xCuxO3 foram sintetizadas pelo método dos precursores poliméricos com calcinação em N2 e O2, possibilitando a obtenção de partículas de maior área superficial em comparação com a calcinação convencional em atmosfera ambiente. Para as amostras CuO/SrTiO3 e NiO/SrTiO3, o suporte SrTiO3, foi sintetizado pelo método de sol-precipitação e a impregnação com cobre e níquel foi realizada por via úmida. As técnicas de absorção e difração de raios-X in situ em condições de reação mostraram a estabilidade da estrutura e do estado de oxidação após o tratamento de redução. Imagens de microscopia eletrônica de varredura (MEV) e de transmissão (TEM) em conjunto com a espectroscopia de raios-X de energia dispersiva (EDX) foram utilizadas a fim de estabelecer uma relação entre a atividade catalítica e o teor a dispersão de fase ativa sobre o suporte. Todas as composições estudadas se mostraram ativas entre 250 e 350°C, entretanto, a composição NiO/SrTiO3 com 10% de Ni apresentou o melhor resultado, com uma conversão de CO a 350°C, próxima ao equilíbrio e estável por um período mínimo de10 h. / Strontium titanate (SrTiO3) is an oxide of perovskite structure and has been extensively studied for use in several applications, including as catalytic support. However, its use specifically in the water-gas shift reaction is still little explored. This reaction is of interest for the production of CO-free hydrogen, required for applications such as in fuel cell. This work aimed to study SrTi1-xCuxO3, CuO/SrTiO3 and NiO/SrTiO3 compounds to be applied as catalysts for the water-gas shift reaction, since, among the low-cost metals, Cu and Ni are highly active for this reaction. The SrTi1-xCuxO3 samples were synthesized by the polymeric precursor method with the samples submitted to a N2 and O2 calcination, making possible to obtain particles with a larger surface area compared to conventional calcination in ambient atmosphere. For the CuO/SrTiO3 and NiO/SrTiO3 samples, the SrTiO3 support was synthesized by the sol-precipitation method and the impregnation with copper and nickel on the support was performed by a wet method. The in situ X-ray absorption and diffraction techniques under reaction conditions showed the stability of the structure and the oxidation state after the reduction treatment. Scanning electron microscopy (SEM) and transmission (TEM) images in conjunction with energy dispersive X-ray spectroscopy (EDX) were used in order to establish a relationship between the catalytic activity and the content and dispersion of the active phase on the support. All the compositions studied were active at 250 to 350 °C, however, the NiO/SrTiO3 sample with 10% of Ni presented the best result, with a CO conversion at 350 °C, close to equilibrium and stable for a minimum of 10 h.

The search for environmentally benign energy sources is becoming increasingly urgent. One such technology is fuel cells, e.g., the polymer electrolyte membrane (PEM) fuel cell which uses hydrogen as a fuel and emits only H2O. However, reforming hydrocarbon fuels to produce the needed hydrogen yields reformate streams containing CO2 as well as CO, which is toxic to the PEM fuel cell at concentrations above 100ppm. As the amount of CO permitted to reach the fuel cell increases, the performance of the PEM fuel cell decreases until it ultimately stops functioning. The water-gas-shift (WGS) reaction, CO + H2O <-> H2 + CO2, provides a method for extracting the energy from the toxic CO by converting it into usable H2 along with CO2 which can be tolerated by the fuel cell. Although a well established industrial process, alternate catalysts are sought for fuel cell application. Catalyst selection for the WGS reaction has, until recently, been based on trial-and-error screening of potential catalysts due to a lack of fundamental understanding of the catalyst's functioning. For this reason, we embarked on a deeper understanding of the molecular events involved in the WGS reaction such that a more systematic and theory-guided approach may be used to design and select catalysts more efficiently, i.e., rational catalyst design. The goal of this research was to develop a comprehensive predictive microkinetic model for the WGS reaction which is based solely on a detailed mechanism as well as theories of surface-molecule interactions (i.e., the transition-state theory) with energetic parameters determined a priori. This was followed by a comparison of the experimental results of sample catalysts to validate the model for various metal-based catalysts of interest including Cu, Fe, Ni, Pd, Pt, Rh, and Ru. A comprehensive mechanism of the plausible elementary reaction steps was compiled from existing mechanisms in the literature. These were supplemented with other likely candidates which are derivatives of those identified in the literature. Using established theories, we predicted the kinetics of each of the elementary reaction steps on metal catalysts of interest. The Unity Bond Index-Quadratic Exponential Potential Method (UBI-QEP) was used to predict the activation energies in both the forward and reverse direction of each step based solely on heats of chemisorption and bond dissociation energies of the species involved. The Transition State Theory (TST) was used to predict the pre-exponential factors for each step assuming an immobile transition state; however, the pre-exponential factors were adjusted slightly to ensure thermodynamic consistency with the overall WGS reaction. In addition, we have developed a new and powerful theoretical tool to gain further insight into the dominant pathways on a catalytic surface as reactants become products. Reaction Route (RR) Graph Theory incorporates fundamental elements of graph theory and electrical network theory to graphically depict and analyze reaction mechanisms. The stoichiometry of a mechanism determines the connectivity of the elementary reaction steps. Each elementary reaction step is viewed as a single branch with an assumed direction corresponding to the assumed forward direction of the elementary reaction step. The steps become interconnected via nodes which reflect the quasi-steady state conditions of the species represented by the node. A complete RR graph intertwines a series of routes by which the reactants may be converted to products. Once constructed, the RR graph may be converted into an electrical network by replacing, in the steady-state case, each elementary reaction step branch with a resistor and including the overall reaction as a power source where rate and affinity correspond to current and voltage, respectively. A simplification and reduction of the mechanism may be performed based on results from a rigorous De Donder affinity analysis as it correlates to Kirchhoff's Voltage Law (KVL), akin to thermodynamic consistency, coupled with quasi-steady state conditions, i.e., conservation of mass, analyzed using Kirchhoff's Current Law (KCL). Hence, given the elementary reaction step resistances, in conjunction with Kirchhoff's Laws, a systematic reduction of the network identifies the dominant routes, e.g., the routes with the lowest resistance, along with slow and quasi-equilibrium elementary reaction steps, yielding a simplified mechanism from which a predictive rate expression may possibly be derived. Here, we have applied RR Graph Theory to the WGS reaction. An 18-step mechanism was employed to understand and predict the kinetics of the WGS reaction. From the stoichiometric matrix for this mechanism, the topological features necessary to assemble the RR graph, namely the intermediate nodes, terminal nodes, empty reaction routes and full reaction routes, were enumerated and the graph constructed. The assembly of the RR graph provides a comprehensive overview of the mechanism. After reduction of the network, the simplified mechanism, comprising the dominant pathways, identified the quasi-equilibrium and rate-determining steps, which were used to determine the simplified rate expression which predicts the rate of the complete mechanism for different catalysts. Experimental investigations were conducted on the catalysts of interest to validate the microkinetic model derived. Comparison of the experimental results from the industrially employed catalysts (e.g., Cu, Ni, Fe, etc.) shows that the simplified microkinetic model sufficiently predicts the behavior of the WGS reaction for this series of catalysts with very good agreement. Other catalysis tested (Pt, Pd, Rh and Ru), however, had sufficient methanation activity that a direct comparison with WGS kinetics could not be made. In summary, we have developed a comprehensive approach to unravel the mechanism and kinetics of a catalytic reaction. The methodology described provides a more fundamental depiction of events on the surface of a catalyst paving the way for rational analysis and catalyst design. Illustrated here with the WGS reaction as an example, we show that the dominant RRs may be systematically determined through the application of rigorous fundamental constraints (e.g. thermodynamic consistency and mass conservation) yielding a corresponding explicit a priori rate expression which illustrates very good agreement not only with the complete microkinetic mechanism, but also the experimental data. Overall, RR graph theory is a powerful new tool that may become invaluable for unraveling the mechanism and kinetics of complex catalytic reactions via a common-sense approach based on fundamentals.

Tese de mestrado integrado. Engenharia Química. Faculdade de Engenharia. Universidade do Porto. 2010

In this research project, adsorption is considered in conjunction with the reverse water gas shift reaction in order to convert CO2 to CO for synthetic fuel production. If the CO2 for this process can be captured from high emitting industries it can be a very good alternative for reduced fossil fuel consumption and GHG emission mitigation. CO as an active gas could be used in Fischer-Tropsch process to produce conventional fuels. Literature review and process simulation were carried out in order to determine the best operating conditions for reverse water gas shift (RWGS) reaction. Increasing CO2 conversion to CO requires CO2/CO separation downstream of the reactor and recycling unreacted CO2 and H2 back into the reactor. Adsorption as a viable and cost effective process for gas separation was chosen for the CO2/CO separation. This was started by a series of adsorbent screening experiments to select the best adsorbent for the application. Screening study was performed by comparing pure gas isotherms for CO2 and CO at different temperatures and pressures. Then experimental isotherm data were modeled by the Temperature-Dependent Toth isotherm model which provided satisfactory fits for these isotherms. Henry law’s constant, isosteric heat of adsorption and binary mixture prediction were determined as well as selectivity for each adsorbent. Finally, the expected working capacity was calculated in order to find the best candidate in terms of adsorption and desorption. Zeolite NaY was selected as the best candidate for CO2/CO separation in adsorption process for this project. In the last step breakthrough experiments were performed to evaluate operating condition and adsorption capacity for real multi component mixture of CO2, CO, H2 in both cases of saturated with water and dry gas basis. In multi components experiments zeolite NaY has shown very good performance to separate CO2/CO at low adsorption pressure and ambient temperature. Also desorption experiment was carried out in order to evaluate the working capacity of the adsorbent for using in industrial scale and eventually temperature swing adsorption (TSA) process worked very well for the regeneration step. Integrated adsorption system downstream of RWGS reactor can enhance the conversion of CO2 to CO in this process significantly resulting to provide synthetic gas for synthetic fuel production as well as GHG emission mitigation.