Dissertation (Ph.D.)--Worcester Polytechnic Institute. / Keywords: microkinetics, reaction routes, mechanism analysis, water-gas-shift. Includes bibliographical references (p.295-305).

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.

The objectives of this work are related to the kinetic study and characterization of air treatment biofilters with and without percolation which were packed with inert packing materials in order to treat methanol, ethanol and toluene vapours.The thesis is divided into three sections.The first section contains a bibliographic introduction to biofiltration and an experimental study.The review of experimental work shows that methanol, ethanol and toluene can be treated in biofilters with or without percolation. In the experimental study of this section, ethanol is treated in a biotrickling filter at low nitrogen concentrations in the nutrient solution and high removal efficiencies are obtained. In this study, experimental protocols for maintaining the biofilter and controlling the biomass content in the packing bed were developed.The second section is composed of two experimental studies for characterizing biofilters with and without percolation in order to treat methanol. A methodology for calculating the biomass accumulated in the packing bed of a biofilter is among the new experimental protocols developed in this study. In the case of biotrickling filter, methodologies for determining the partition coefficient of methanol and the biomass production rate were developed.The role of the biofilm and the nutrient solution on bioflter performance was also analyzed.The studies of this section lead to a better comprehension of methanol biodegradation in biofilters.The third section contains two kinetic studies for biofilters with and without percolation. In the first study, a new experimental methodology is proposed to calculate microkinetic parameters related to microbial growth in a biofilter. In the second study, the microkinetic and macrokinetic behaviors of methanol and toluene biodegradation are compared.The influence of operating conditions on microbial growth and elimination capacity is also analyzed. This study includes the identification of energy indicators of biofilters with and without percolation, which could be used in energy balances and for estimating the temperature of packing bed.

This thesis is based on the study of ammonia oxidation on platinum group metals. The objectives of this thesis are accept or discard the diverse mechanisms proposed. Even suggest the most appropriate according to the data obtained. To carry out this work is necessary to know the geometry of each species that may exist on the surface of the catalyst and the transition states of the reactions that lead from one species (or combination of species) to another. This is know the key points of a reaction (activation energy and reaction enthalpy). With all data obtained was proposed a microkinetic model of the process and analysis this to obtain a reduced model, equivalent to a mechanism. With this model it is possible to obtain a simulation of the temporal evolution of each species, both in gas phase on the surface, depending on initial conditions. All this information is useful to know how the mechanism works and the evolution of products depending on the temperature or the oxygen-ammonia ratio. To carry out this thesis has used the density functional theory (DFT) implemented in VASP code on a model of a periodic cell of 2 ¡Á 2 with four layers of metal where the two more superficial are entirely free, being able to deform and adapt the molecule adsorbed. The Encut and k-points used are 400 eV and 5 ¡Á 5 ¡Á 1, respectively.This thesis is divided in three chapters. The first examines and compares the dehydrogenation of ammonia on platinum in the faces 100 and 111. The second chapter examines and compares the dehydrogenation on platinum, palladium and rhodium on both sides, 100 and 111. And the third chapter examines the process of ammonia oxidation on Pt(100).The first part has been carried out a systematic study of adsorption and the relative stability of the ammonia and the species of dehydrogenation on the surfaces of Pt (111) and Pt (100). Different adsorption geometries and positions have been studied. The vibrational spectra of various fragments of ammonia have been calculated and were compared with the experimental data available. The adsorption of NH3 is on top position and for the NH2 is on bridge and it is the most stable on Pt (100) than on Pt (111). For the NH and N are adsorbed on the hollow site. There is a considerable difference in the energy of adsorption of NH2 on both sides. This difference is mainly explained by the geometry that takes the kind on both sides. Being much more stable on the 100 side than on the face 111. Accordingly, the platinum surface determines the most stable species NHx: On Pt(100) has more affinity NH2 species, whereas species prefer NH Pt(111).The second part extends the study of the dehydrogenation to other metals such as Palladium and Rhodium. The different adsorption geometries and positions have been studied for the intermediate of ammonia dehydrogenation (NHx, x=0-2). The six surfaces studied, the NH3 adsorbs preferably on the top position, the NH2 on bridge, NH and N on hollow. However, the adsorption energies of the fragments NHx fluctuate considerably from one surface to another. All species absorbs more strongly on the face 100 than on face 111. The Rh(100) is the surface that provides maximum stability for the different NHx species. The reaction energy, the activation energy and the geometry of the transition state for the successive of ammonia dehydrogenation (NHx ¡ú NHx-1) have been determined, which allows calculating the rate coefficients. Our results prove that the reaction is structure sensitive. As a general trend, the first step of dehydrogenation is the limiting step, especially for palladium. According to the experimental data Rhodium is a good catalyst for the decomposition of NH3 compared to Pt and Pd. It has also been observed a linear relationship between the potential energy of the transition state and the adsorption energy of the products. The third part studies the ammonia oxidation on Pt(100). The conversion of NH3 leading to NHx intermediate species that reacts with adsorbed oxygen species and ultimately the formation of the products (NO, N2O, N2 and H2O) that it has been systematically calculated. The reaction comes through an imine mechanism, while the classical mechanisms postulated by Bodenstein and Andrussow (nytroxyl and hydroxilamine, respectively) as reaction intermediates can be discarded. The activation energy for the oxidative ammonia dehydrogenation on Pt(100) has been drastically reduced compared to the non-oxidative ammonia dehydrogenation. The barriers of ammonia dehydrogenation are greatly favored by the O-assisted way than the OH-assisted way. The final products are formed by recombination of adsorbed Nitrogen with N (N2), O (NO) and NO (N2O). The water is formed through the recombination of two adsorbed OH, regenerating adsorbed oxygen. The limiting step in the oxidative ammonia dehydrogenation is the first step, abstraction of the first proton of ammonia (NH3¡úNH2+H). While the nitric oxide desorption is the rate determining step (rds) of the process. We calculated the reaction rate coefficients of elementary steps involved in the reaction mechanism allows doing a microkinetic analysis. The simulations carried out with the microkinetic model describe well the experimental distribution of products obtained at different temperatures, depending on the time and the ratio of initial NH3/O2. Getting a temporal distribution of each species in gas phase and on the surface. / Esta tesis se basa en el estudio de la oxidación de amoniaco sobre el grupo del platino. El objetivo de esta tesis es descartar o aceptar los diversos mecanismos propuestos. Incluso proponer el más correcto según los datos obtenidos. Para llevar a cabo esta acometida es necesario conocer cada geometría de las diferentes especies que pueden existir sobre la superficie del catalizador, así como los estados de transición entre las reacciones que lleven de una especie (o combinación de especies) a otras. Es decir conocer los puntos claves de una reacción (energía de activación y entalpía de reacción). Con los datos obtenidos se ha realizado la microcin¨¦tica del proceso completo y se ha realizado un análisis microcinético, llegando a obtener un modelo reducido, el equivalente a un mecanismo de reacción. Con este modelo es posible obtener una simulación de la evolución temporal de cada especie, tanto en fase gas como sobre la superficie, en función de unas condiciones iniciales. Toda esta información es de gran utilidad para conocer el funcionamiento del mecanismo y conocer la evolución de los productos en función de la temperatura, o de la relación de amoniaco-oxigeno. Para realizar esta tesis se ha usado la Teoría del funcional de la Densidad (DFT), el programa VASP usa esta teoría con ondas planas para realizar los cílculos sobre un modelo periódico de una celda de 2¡Á2 con cuatro capas de metal donde las dos más superficiales están totalmente libres, pudiéndose deformar y adaptar al adsorbato. El Encut y los k-points usados son de 400 eV y 5¡Á5¡Á1, respectivamente. La tesis se ha dividido en tres capítulos. En el primero se estudia y compara la deshidrogenación del amoniaco sobre Platino en las caras 100 y 111. En el segundo capitulo se estudia y compara la deshidrogenación sobre Platino, Paladio y Rodio en las dos caras, 100 y 111. Y en el tercer capítulo se estudia el proceso de la oxidación de amoniaco sobre Platino en la cara 100.En la primera parte se han llevado a cabo una estudio sistemático de la adsorción y la estabilidad relativa del amoniaco y de las especies de la deshidrogenación sobre las superficies de Pt (111) y Pt (100). Diferentes geometrías y posiciones de adsorción han sido estudiadas. Los espectros vibracionales de los diversos fragmentos de amoníaco se han calculado y se han comparado con los datos experimentales disponibles. La adsorción de NH3 se realiza sobre la posici¨®n top el NH2 sobre la posición bridge y es la más estable sobre Pt (100) que sobre Pt (111). Para el NH y el N se adsorben sobre el hollow. Existe una diferencia considerable en la energía de adsorción del NH2 sobre las dos caras. Esta diferencia se explica principalmente por la geometría que adopta la especie sobre las dos caras. Siendo mucho más estable sobre la cara 100 que sobre la cara 111. En consecuencia, la superficie de platino determina la especie NHx más estable: Sobre Pt(100) tiene más afinidad la especie NH2, mientras que la especie NH prefiere el Pt (111). En la segunda parte el estudio de la deshidrogenación se ha ampliado a otros metales como el Paladio y el Rodio. Diferentes geometrías de adsorción y posiciones han sido estudiados para NH3 y los intermedios de la deshidrogenación del amoniaco (NHx, x = 0 - 2). En las seis superficies investigadas, el NH3 adsorbe preferentemente sobre la posición top, el NH2 en bridge, el NH y el N lo hacen sobre el hollow. Sin embargo, las energías de adsorción los fragmentos NHx difieren considerablemente de una superficie a otra. Todas las especies de absorber con más fuerza en la cara 100 que en el la cara 111. El Rh(100) es la superficie que proporciona la máxima estabilidad para las diferentes especies. La energía de reacción, la geometría del estado de transición y la barrera de activación de los sucesivos pasos de reacción de la deshidrogenación (NHx ¡ú NHx-1) se han determinado, lo que permite calcular los coeficientes de las velocidades de reacción. Nuestros cálculos demuestran que la reacción es sensible a la estructura de la superficie. Como tendencia general, el primer paso de la deshidrogenación es el paso limitante, especialmente para Paladio. De acuerdo con los datos experimentales el Rodio es un buen catalizador para la descomposición de NH3 frente al Pt y el Pd. También se ha observado una relación lineal entre la energía potencial del estado de transición y la energía de adsorción de los productos. En la tercera parte se ha estudiado el proceso de oxidación de amoniaco sobre Pt(100). La conversión de NH3 que lleva a especies intermedias de NHx que reacciona con especies que contienen oxígeno adsorbido y en última instancia la formación de los productos de reacción (NO, N2O, N2 y H2O), han sido calculadas sistemáticamente. La reacción procede a través de un mecanismo de amina, mientras que los mecanismos clásicos postulados por Andrussow y Bodenstein (nitroxilo y hidroxilamina, respectivamente) como productos intermedios de reacción pueden ser descartados. Las barreras de activación para la deshidrogenación oxidativa del amoniaco sobre Pt(100) se han reducido drásticamente con respecto a la deshidrogenación no-oxidativa. La energía de activación de la deshidrogenación de amoniaco y de las subsiguientes deshidrogenaciones (NHx) son en gran medida favorecidas por el oxigeno adsorbido con respecto al hidróxido adsorbido. Los productos finales están formados por recombinación de N adsorbido con N (N2), O (NO) y NO (N2O). El agua se forma a través de la recombinación de OH adsorbido, regenerando un oxígeno.La etapa limitante en la deshidrogenación oxidativa del amoniaco es la primera etapa, la abstracción del primer protón del NH3. Mientras que desorción del NO es la etapa limitante del proceso en general. Se han calculado los coeficientes de velocidad de reacción de los pasos elementales que participan en el mecanismo de reacción, permitiendo obtener un análisis microcinético. Las simulaciones realizadas con el modelo microcinético describen bien la distribución de productos obtenidos experimentalmente a diferentes temperaturas, en función del tiempo y del ratio de NH3/O2 iniciales. Obteniendo una distribución temporal de cada especie, en fase gas y sobre la superficie.