The aim of this work has been to use atomistic computer simulation methods to calculate the thermal conductivity and investigate factors that will modify the behaviour when applied to three different oxide materials: MgO, SiO2 and SrTiO3. These were chosen as they represent distinct classes of materials and are substrates for thermoelectric devices, where one of the primary goals is to tailor the system to reduce the thermal conductivity. Chapter 1 introduces thermoelectric concepts, gives a background of the theory and a review of various important thermoelectric materials. In Chapter 2 an overview of the interatomic interactions is presented along with details on the implementation of these interactions in a simulation of a 3D periodic crystal. Chapter 3 outlines the importance of phonon processes in crystals and several approaches to the calculation of thermal conductivity are presented. MgO results are given in Chapter 4. Both the Green-Kubo and Boltzmann transport equation (BTE) methods of calculating thermal conductivity were used. The effect on thermal conductivity of two different grain boundary systems are then compared and finally extended to MgO nanostructures, thus identifying the role of surfaces and complex nanostructure architectures on thermal conductivity. In Chapter 5 two different materials with the formula unit SiO2 are considered. The two materials are quartz and silicalite which show interesting negative thermal expansion behaviour which may impact upon the thermal transport within the material. Chapter 6 presents results on the promising thermoelectric material STO. Once again the results from both Green-Kubo and BTE calculations are compared. Grain boundaries are also studied and the effect of inter-boundary distance and boundary type on the thermal conductivity is explored. Finally, a nanostructured STO system (assembled nanocubes) with promising thermoelectric applications is studied. Chapter 7 outlines the conclusions made from this work and suggests areas for future study.

"Water behavior in pure silicalite and Dealuminated Zeolite Y (DAY), two highly hydrophobic zeolites, was investigated at different temperatures in the range 100-600 K by molecular dynamics simulations. The Compass forcefield was used to carry out the study. A full flexibility of water molecules and of the zeolite framework was considered. The results show that water behavior is more complex in silicalite than in zeolite DAY. Three different activation energies for water diffusion were obtained in silicalite in the range 250-600 K compared to two for DAY. The values of these activation energies are discussed in detail and are related to the hydrogen bond‘s strength and the zeolite structure. Moreover, from the radial distribution functions (rdfs), it is shown that water mostly exists in the gas phase at room temperature in silicalite whereas liquid water is observed in DAY in agreement with previous experimental observations. The self-diffusion coefficients of water and the rdfs are obtained as a function of temperature in order to explain the behavior differences of water in the two all-silica zeolites. The loading influence on the self-diffusion coefficients is also investigated for both crystals. The results are compared with previous experimental and theoretical studies."

Zeolite membranes are considered to be a promising alternative to polymeric membranes and they have the potential to separate gases under harsh conditions. Silicalite-1 membranes in particular are easy to prepare and suitable for several industrial applications. In this research project, silicalite-1/ceramic composite membranes were prepared using the pore plugging hydrothermal synthesis method and supports with zirconium oxide and/or titanium oxide as active layers. The effect of the support’s pore size on the morphology and permeation performance of the prepared membranes was investigated using five supports with different active layer pore sizes in the range of 0.14 – 1.4 m. The prepared membranes were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), electron diffraction spectrometer (EDS), single gas and binary gas mixtures permeation tests. The results confirmed the presence of a typical silicalite-1 zeolite structure with a high internal crystalline order grown inside the pores of the active layer of the supports, with a dense film covering most of the supports active layers. Silicalite-1 crystals in the prepared membranes were preferably oriented with either a- or b-axes perpendicular to the support surface. Single gas permeation results illustrated that the observed permeances were not directly related to the kinetic diameter of permeants. Instead, the transport of the studied gases through the prepared membranes occurred by adsorption followed by surface diffusion mechanism. Binary gas tests performed with CO2 and N2 mixtures showed that the prepared membranes were selective and very permeable with CO2/N2 permselectivities up to 30 and a CO2 permeances in the order of 10-6 mol m-2 Pa-1 s-1. A model was developed, based on Maxwell−Stefan equations and Extended Langmuir adsorption isotherm, to describe the transport of binary CO2 and N2 mixtures through the prepared silicalite-1 membranes. The model results showed that the exchange diffusivities (D12 and D21) were less dependent on the feed pressure and feed composition compared to the permeances and the permselectivities. Hence, they are more appropriate to characterize the intrinsic transport properties of the prepared silicalite-1 membranes.

Thesis (Ph. D.)--Worcester Polytechnic Institute. / Keywords: Water; Adsorption; Molecular simulation; Nanoporous materials. Includes bibliographical references (p. 117-119).

Zeolite membranes are considered to be a promising alternative to polymeric membranes and they have the potential to separate gases under harsh conditions. Silicalite-1 membranes in particular are easy to prepare and suitable for several industrial applications. In this research project, silicalite-1/ceramic composite membranes were prepared using the pore plugging hydrothermal synthesis method and supports with zirconium oxide and/or titanium oxide as active layers. The effect of the support’s pore size on the morphology and permeation performance of the prepared membranes was investigated using five supports with different active layer pore sizes in the range of 0.14 – 1.4 m. The prepared membranes were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), electron diffraction spectrometer (EDS), single gas and binary gas mixtures permeation tests. The results confirmed the presence of a typical silicalite-1 zeolite structure with a high internal crystalline order grown inside the pores of the active layer of the supports, with a dense film covering most of the supports active layers. Silicalite-1 crystals in the prepared membranes were preferably oriented with either a- or b-axes perpendicular to the support surface. Single gas permeation results illustrated that the observed permeances were not directly related to the kinetic diameter of permeants. Instead, the transport of the studied gases through the prepared membranes occurred by adsorption followed by surface diffusion mechanism. Binary gas tests performed with CO2 and N2 mixtures showed that the prepared membranes were selective and very permeable with CO2/N2 permselectivities up to 30 and a CO2 permeances in the order of 10-6 mol m-2 Pa-1 s-1. A model was developed, based on Maxwell−Stefan equations and Extended Langmuir adsorption isotherm, to describe the transport of binary CO2 and N2 mixtures through the prepared silicalite-1 membranes. The model results showed that the exchange diffusivities (D12 and D21) were less dependent on the feed pressure and feed composition compared to the permeances and the permselectivities. Hence, they are more appropriate to characterize the intrinsic transport properties of the prepared silicalite-1 membranes.

No description available.

Les matériaux « coeur-coquille » composés d’une nanoparticule métallique encapsulée à l'intérieur de coquilles inorganiques (oxydes, carbone …) attirent de plus en plus l'attention par leurs propriétés particulières, en particulier dans le domaine de la catalyse. Les particules métalliques sont protégées par la coquille, qui empêche entre autres le frittage et la croissance des particules à haute température. Cependant, les coquilles sont généralement méso à macroporeuses et elles ne peuvent pas jouer le rôle de tamis moléculaire pour les molécules de taille nanométrique. En revanche, les zéolithes sont des solides cristallins microporeux dont les pores bien définis permettent une forte discrimination des réactifs basée sur la taille, la forme ou leur coefficient de diffusion. L’objectif de cette thèse visait à la synthèse de catalyseurs de type coeur-coquille dans lesquels la coquille est une zéolite microporeuse de structure MFI (silicalite-1 et ZSM-5), le coeur étant soit une particule de métal noble (Au, Ag, Pt, Pd), soit des alliages de ces différents métaux, soit enfin un métal de transition (Co, Ni, Cu). Ces catalyseurs ont été appliqués dans des réactions d'hydrogénation sélective (aromatiques substitués) et l'oxydation sélective de CO en présence d'hydrocarbures. Nous avons ainsi montré que la coquille zéolithique, tout en protégeant les particules du frittage, modifie la sélectivité des réactions en interdisant aux réactifs volumineux d’atteindre les sites catalytiques / Nanostructured yolk-shell materials, which consist of metal nanoparticle cores encapsulated inside hollow shells, attract more and more attention in material science and catalyst applications during the last two decades. Metal particles are usually highly mono-dispersed in size and isolated from each other by the shell, which prevents growth by sintering at high temperature. Because they are generally made of meso/macroporous oxides or amorphous carbon, shells cannot carry out molecular sieve-type separation of molecules at the nanometric scale. The aim of the present thesis was to synthesize yolk-shell catalyst with microporous zeolite shells (silicalite-1 and ZSM-5), containing noble (Au, Pt, Pd) transition (Co, Ni, Cu) and alloy metal nanoparticles. Zeolites are crystalline microporous solids with well-defined pores capable of discriminating nanometric reactants on the basis of size, shape and diffusion rate. Zeolite-based yolk-shell catalysts have been applied in selective hydrogenation (toluene and mesitylene) and oxidation (CO) reactions in the presence of hydrocarbons. Zeolite shells not only plaid a key role as membranes, thus changing selectivities as compared to conventional supported catalysts, but they also protected metal nanoparticles from sintering under reaction conditions

Zeolite membranes have great potential in gas separation applications because of their unique selective properties. The main challenge is in synthesizing defect-free zeolite membranes. In this study, we synthesized silicalite-1 zeolite membranes on ceramic supports composed of Al2O3 and TiO2 using the pore-plugging method. We investigated the effect of the fill-level in the autoclave during the synthesis on the membrane performance. In particular, we were interested in determining the conditions at which the defects' contribution to the total transport is minimized. We adopted and further developed the approach proposed by Carter (2019) to quantify the permeance contribution through defects. Comparing the membrane performance before and after calcination, we proposed several modifications to the original analysis of Carter (2019). Knowing the defect transport contribution, we determined the corrected diffusivity, an intrinsic property of zeolite crystals at a given temperature, of several adsorbed gases on silicalite-1 crystals. The defect's contribution decreased as the autoclave fill-level increased from 94 to 98%. A further increase in the autoclave fill-level introduced more defects and caused the autoclave lid to rupture. Despite the differences in the membranes' performance arising from the autoclave fill-level, the corrected diffusivities of CO2, CH4, and N2 in silicalite-1 showed minimal variation from membrane to membrane. This proves the validity of the proposed characterization method. Moreover, the reported corrected diffusivities are comparable to the literature's values, found using other characterization methods. However, none of the previously used methods is as simple and straightforward as the one we further developed in this study.

No description available.

Molecular simulations have become an important tool within the last few decades to understand physical processes in the microscale and customize processes in the macroscale according to the understanding developed at the molecular level. We present results from molecular simulations we performed to study the adsorption of hazardous organics in nanoporous materials. Adsorption of water in silicalite, a hydrophobic material, and the effect of defects were investigated by Monte Carlo simulations. Silanol nests were found to have a big impact on the hydrophobicity of silicalite. Even the presence of one silanol nest per unit cell caused a significant amount of water adsorption. We also investigated the effect of four different cations, H+, Li+, Na+, and Cs+. Their presence in silicalite increased the amount of water adsorbed. Monte Carlo and molecular dynamics simulations of MTBE adsorption in silicalite, mordenite, and zeolite beta with different Na+ cation loadings were carried out. The results revealed the importance of the pore structure on the adsorption of MTBE. Although these three zeolites have similar pore volumes, zeolite beta, with its pore structure which is mostly accessible to MTBE molecules, is predicted to adsorb significantly more MTBE than silicalite and mordenite. The Na+ cation loading, up to four cations does not have a significant effect on the adsorption capacity of the zeolites studied here, however, for silicalite and zeolite beta increasing the Na+ content increases the amount adsorbed at very low pressures. A new force field was developed by Monte Carlo simulations for 1,4-Dioxane, an important industrial solvent which has emerged as a potentially significant threat to human health. The objective was to develop reliable atom-atom interaction parameters to use in the simulations of the adsorption of 1,4-Dioxane in different adsorbent materials. Predictions of critical point data, liquid and vapour densities, heats of vaporization with our new force field were in good agreement with experimental data and outperformed predictions from simulations with other force field parameters available in literature. To obtain the isotherms of MTBE and 1,4-Dioxane adsorption from water in silicalite Monte Carlo simulations were performed. First we optimized the interaction parameters between the atoms of silicalite and the atoms of MTBE and 1,4-Dioxane. Using these optimized parameters we simulated the adsorption of MTBE and 1,4-Dioxane from water in silicalite. Despite the agreement of simulated and experimental isotherms of pure components, simulated isotherms of MTBE and 1,4-Dioxane adsorption from water in silicalite did not yield satisfactory results. Monte Carlo simulations were performed to investigate the affinity between two hazardous materials, PFOA and 1,1-DCE; and four different zeolites. Binding energies and Henry's constants were computed. For both PFOA and 1,1-DCE zeolite-beta had the highest affinity. The affinity between activated carbon with polar surface groups and water, and 1,4-Dioxane were investigated to shed light on why activated carbon is ineffective to remove 1,4-Dioxane from water. Results showed that presence of polar surface groups increased the affinity between water and activated carbon, while the affinity between 1,4-Dioxane and activated carbon was not effected by the presence of polar surface groups.