In this work the performance of cerium-based oxygen carriers is investigated in a simulated chemical looping dry reforming system using methane and carbon dioxide as fuel and oxidizing gas respectively. The samples to be studied are pure cerium oxide and cerium oxide doped with zirconium, yttrium, samarium, and scandium more specifically: CeO2, Ce0.8Y0.2O1.9, Ce0.85Y0.05Zr0.1O1.975, Ce0.95Zr0.05O2, Ce0.9Sm0.05Zr0.05O1.975, and Ce0.9Sc0.05Zr0.05O1.975. Characteristics such as crystallography including lattice parameter and particle size of the samples are evaluated using X-ray diffraction (XRD) and particle size analysis. The oxygen transport capacity will also be measured using a thermogravimetric analyzer (TGA). This method of measurement also allowed for insight on oxygen release temperatures as well as recyclability of the samples. The particle size analysis showed that the synthesis method of precipitation-agglomeration resulted in samples with consistent particle size distribution indicating the method can be scaled up. The X-ray analysis of samples before and after the TGA tests show that all the materials tested had a cubic fluorite crystal structure which was maintained through the oxidation reduction cycles. The lattice parameter was found to increase slightly with a loss in oxygen content in the samples. The addition of trivalent dopants resulted in a decrease in the temperature of initiation of reduction in methane. While reduction of commercial ceria initiated near 800 oC. the addition of trivalent dopants resulted in a lowering of the initialization temperature between of 150 – 200oC. The activation energy of commercial ceria was 248.42 kJ/mol for reduction in methane, indicating that the rate controlling mechanism is chemical reaction rather than diffusion. The addition of trivalent dopants resulted in a significant lowering in the activation energy. The activation energies obtained in this study show that the addition of dopant increased the significance of diffusion through the solid and the controlling mechanisms were both diffusion and chemical reaction. Overall, the addition of trivalent dopants enhanced the extent of oxygen exchange in CLDR process.

Carbon dioxide reforming, or drying reforming, of methane can now be used in new applications such as landfill gas utilization where CO and CH need to be converted to a mixture of CO and H, called synthesis gas or syn-gas. A novel Ni-Co/AlMgO bimetallic powder catalyst was developed in previous research for dry reforming of methane (DRM) process which can eliminate carbon deposition. But it is difficult242x to apply this loose-powder catalyst in industrial scale. The procedure of making spherical Ni-Co/AlMgOx bimetallic catalyst supported on BASF CSS-350 alumina balls (BASF Catalysts LLC) using impregnation method with different impregnation steps and calcination steps is explained in this thesis. For every batch of preparation, the concentration of metal solution was calculated based on different impregnation steps. BET (Brunauer-Emmett-Teller) analysis, compressive strength test, XANES (X-ray Absorption Near-Edge Structure) measurement and ICP-MS (Inductively Coupled Plasma Mass Spectrometry) analysis are conducted to understand the physical and chemical properties of the catalyst. It is found that both impregnation steps and calcination steps have great influence on the performance of the prepared catalyst samples. Among all the catalysts prepared, BF-4-0.25(MgNiCo)-C, which was made by using 4 impregnation-calcination cycles, shows the best activity and stability for 160 h time-on stream (TOS) under the reaction condition of 0.10 g catalyst loading, 750 oC, ambient pressure, GHSV=100,000 ml/gc·h, and CH4/CO2/N2 = 1/1/1. The CH4 conversion started at 66.7% and slowly dropped to 52.8% after 160 hours. I I BF-4-0.25(MgNiCo)-C spherical catalyst shows lower reaction rate compared to the loose powder format but shows compatible or higher activity to other two reported catalysts in similar compositions. Most importantly, it is a shaped catalyst ready for industrial use.

The dry reforming of methane (DRM) has received critical attention because it converts two major greenhouse gases, methane and carbon dioxide, into molecular hydrogen and carbon monoxide, known as synthesis gas (syngas). Syngas is an important feedstock to produce various chemicals. A major drawback of the DRM process is the high deactivation rates of conventional nickel and cobalt catalysts. Experimental findings indicate that treating nickel and cobalt catalysts with boron reduces deactivation rates and enhances the catalytic activity. This study investigates the mechanism through which boron promotes catalytic stability using density functional theory calculations. First, the location of boron in nickel and cobalt catalysts is explored. Boron is found to be more stable occupying on-surface and substitutional sites in the catalysts. However, during DRM operation, carbon dioxide is able to oxidize on-surface and substitutional boron. The formed boron oxide units may react with each other and form diboron trioxide or react with hydrogen to form boric acid, and eventually leave the catalyst, which means they cannot have an effect on deactivation rates. This study argues that interstitial boron plays the major role since it is protected from getting oxidized by carbon dioxide. Geometric optimization indicates that interstitial boron leads to spontaneous surface reconstruction in both extended surfaces and nanoparticles. The effect of interstitial boron on the binding energies of methyl, hydrogen, carbon monoxide, and oxygen on extended surfaces and nanoparticles is studied and utilized using the Brønsted-Evans-Polanyi principle to give an insight about how boron reduces deactivation rates. Our analysis indicates that interstitial boron lowers the activation energies of methane and carbon dioxide. On (100) surfaces, boron lowers C–H activation energies in methane more than it lowers C=O activation energies in carbon dioxide, which means catalytic deactivation rates due to metal oxidation are lowered. On (111) surfaces, boron lowers carbon dioxide activation energies more than it lowers methane activation energies, which means catalytic deactivation rates due to coke formation are lowered. The computational study is consistent with experimental findings and gives an atomistic understanding of the beneficial role of boron on the DRM process catalyzed by nickel and cobalt.

Dry reforming of methane (DRM) is an attractive and promising process for the conversion of methane and carbon dioxide which are the most abundant carbon sources into valuable syngas. The produced syngas, which is a mixture of hydrogen and carbon monoxide, can be used as intermediates in the manufacture of numerous chemicals. To achieve high conversion, DRM reaction is operated at high temperatures (700-900 °C) that can cause major drawbacks of catalyst deactivation by carbon deposition, metal sintering or metal oxidation. Therefore, the primary goal is to develop a metal based catalyst for DRM that can completely suppress carbon formation by designing the catalyst composition. The strategy of this work was to synthesize Ni-based catalysts all of which prepared by homogeneous deposition precipitation method (HDP) to produce nanoparticles with narrow size distribution. In addition, control the reactivity of the metal by finely tuning the bimetallic composition and the reaction conditions in terms of reaction temperature and pressure. The highly endothermic dry reforming of methane proceeds via CH4 decomposition to leave surface carbon species, followed by removal of C with CO2-derived species to give CO. Tuning the reactivity of the active metal towards these reactions during DRM allows in principle the catalyst surface to remain active and clean without carbon deposition for a long-term. The initial attempt was to improve the resistance of Ni catalyst towards carbon deposition, therefore, a series of 5 wt.% bimetallic Ni9Pt1 were supported on various metal oxides (Al2O3, CeO2, and ZrO2). The addition of small amount of noble metal improved the stability of the catalyst compared to their monometallic Ni and Pt catalysts, but still high amount of carbon (> 0.1 wt.%) was formed after 24 h of the reaction. The obtained results showed that the catalytic performance, particle size and amount of deposited carbon depends on the nature of support. Among the tested catalysts, Ni9Pt1/ZrO2 showed high stability with the least carbon amount (0.55 wt.%). On the other hand, mono- and bimetallic Co-Ni/ZrO2 were then prepared following the same synthesis protocol. The ZrO2 support was chosen because of its high thermal stability and absence of mixed oxide formation with the active metals. It was demonstrated that on monometallic Co catalyst, the kinetic analysis showed first-order in CH4 and negative-order in CO2 on the DRM rate. The Co catalyst deactivated without forming carbon deposits. On contrary, on monometallic Ni catalyst, the DRM rate was proportional to CH4 pressure but insensitive to CO2 pressure. The Ni surface provides comparatively higher rates of CH4 decomposition and the resultant DRM than the Co catalyst but leaves some deposited carbon on the catalyst surface. In contrast, the bimetallic CoNi catalyst showed kinetics resembling the Co catalyst, i.e., the first-order with respect to CH4 pressure and the negative-order with respect to CO2 pressure on the DRM rate. Noticeably, the stability of CoNi catalyst was drastically improved over the monometallic counterparts and no deposited carbon was detected after the DRM reaction. The results suggest that for an appropriate Co/Ni ratio, the bimetallic CoNi/ZrO2 catalyst exhibits intermediate reactivity towards CH4 and CO2 between Co and Ni producing negligible carbon deposition by balancing CH4 and CO2 activation.

Many countries around the world have decided to play a positive role in combating climate change and reduce carbon dioxide in the atmosphere. In addition to reducing emissions, initiatives include the capture, storage and utilization of CO2. Converting it to valuable products through reforming of methane not only utilizes major greenhouse gasses, but can also be a means for energy from biogas. The main challenge hindering this process is developing a scalable active catalyst that can resist deactivation. To address this challenge, focus has shifted from simple metal oxides towards metal nanoparticles dispersed and organized in complex well defined structures. Oxide perovskites have the potential to contain metal and support in a single structure as the case of LaNiO3. Metal-organic frameworks are another type of materials that can be used as sacrificial agents to produce the type of complex metal oxides required. Three synthesis approaches were studied for the synthesis of La-Ni materials. Combustion synthesis is a cost and time efficient method. However, it becomes challenging to accurately predict the outcomes. Hydrothermally synthesized perovskites give pure phase materials but are sensitive to synthesis variables. MOF based materials showed conversions of 94% and 83% for CO2 and CH4, respectively, with stable performance for +100 hours and can be a promising future route in heterogeneous catalysis.

In light of global warming’s environmental implications, research is shifted towards potential processes that can utilize CO2 and reduce its emissions in the industrial sector. One of the promising processes is dry reforming of methane (DRM), which is capable of utilizing CO2 and producing valuable syngas (H2 and CO). The main challenge of DRM is the deactivation of catalysts under the reaction temperatures (above 700 °C) due to sintering of the active metal and coke formation. Ni-based catalysts are the most widely investigated catalysts in literature for DRM due to their cost efficiency and availability. This study is an extension of the work done by Saudi Basic Industries Corporation (SABIC) devoted to investigating Ni-Ce-In system for DRM reaction. Five catalysts were synthesized by dry impregnation method according to SABIC synthesis procedure (Ni/Al2O3, Ni-In/Al2O3, Ni/CeO2/Al2O3, Ni/In-CeO2/Al2O3, and Ni-In/CeO2/Al2O3). The metallic loading targets were 7.5 wt.%, 10 wt.%, and 0.8 wt.% for nickel, cerium, and indium, respectively. The addition of indium in combination with cerium resulted in the highest catalytic activity. Additionally, the co-impregnation of indium and cerium resulted in enhancing the catalytic activity more than subsequential impregnation (Ni/In-CeO2/Al2O3 compared to Ni-In/CeO2/Al2O3). The addition of cerium or indium separately with nickel did not seem to affect activity since Ni/Al2O3, Ni-In/Al2O3, and Ni/CeO2/Al2O3 exhibited similar conversion values. All catalysts were stable for more than two days under DRM conditions without deactivating. Therefore, deactivation behaviors of the catalysts were not covered in this study.

In this work the synthesis, characterization, and gas transport properties of hydrogen selective silica membranes were studied along with the catalytic reforming of CH₄ with CO₂ (CH₄ + CO z 2 CO + 2 H₂) in a hydrogen separation membrane reactor. The silica membranes were prepared by chemical vapor deposition (CVD) of a thin SiO₂ layer on porous supports (Vycor glass and alumina) using thermal decomposition of tetraethylorthosilicate (TEOS) in an inert atmosphere. These membranes displayed high hydrogen permeances (10⁻⁸ - 10⁷ mol m⁻² s⁻¹ Pa⁻¹) and excellent H₂ selectivities (above 99.9 %) over other gases (CH₄, CO, and CO₂). The membranes were characterized using Scanning Electron Microscopy and Atomic Force Microscopy, and the mechanism of gas transport was studied applying existing theories with a newly developed treatment. The catalytic reforming of CH₄ with CO₂ was carried out in a membrane reactor installed with a hydrogen separation ceramic membrane. The reaction was conducted at various pressures (1 - 20 atm) and temperatures (873 K and 923 K) at non-equilibrium conditions, and the results were compared with those obtained in a packed bed reactor in order to evaluate performance of the membrane reactor for the reaction. It was found that concurrent and selective removal of hydrogen from the reaction in the membrane reactor resulted in considerable enhancements in the yields of the reaction products, H₂ and CO. The enhancements in the product yields in the membrane reactor increased with pressure showing a maximum at 5 atm, and then decreased at higher pressures. This was due to a trade-off between a thermodynamic quantity (hydrogen production by the reaction) and transport property (hydrogen separation through the membrane). It was also found that the reverse water-gas shift (RWGS) reaction occurred simultaneously with the reforming reaction giving the detrimental effect on the reaction system by reducing the amount of hydrogen production in favor of water. This was particularly significant at high pressures. / Ph. D.

Afin de développer une économie basée sur l'hydrogène, il est souhaitable de pouvoir le produire à partir de biogaz (CH4 and CO2) ou de gaz à effet de serre (GES). Le reformage à sec (DRM) et le reformage oxydant du méthane (ODRM) sont des voies prometteuses pour produire H2 et CO à partir des GES et suscitent une grande attention en raison de préoccupations environnementales. Ces réactions ont été étudiées à basse température (600 -700 ° C) sur des oxydes mixtes CeNiX(AlZ)OY, NiXMg2AlOY, et des catalyseurs supportés Ni/SBA-15. Diverses techniques physico-chimiques ont été utilisées pour caractériser les catalyseurs, tels que DRX, XPS, TPR et Raman. L’influence de différents paramètres a été examinée, telles que la température de réaction, le prétraitement sous H2, la teneur en Ni, la masse de catalyseur et les concentrations en réactifs. En particulier, les réactions ont été étudiées à 600 °C, sans dilution des réactifs et sur 10 mg de catalyseur. Les meilleures activités catalytiques et sélectivités sont obtenues sur des catalyseurs partiellement réduits à température appropriée. L'addition d'O2 augmente la conversion du CH4 mais diminue la conversion du CO2, et O2/CH4 =0,3 apparaît comme la condition optimisée en raison de l'activité et de la sélectivité élevées et de la faible formation de carbone. Enfin, un site actif impliquant des espèces Ni en interaction forte avec d'autres cations est proposé. Il est obtenu sur un catalyseur partiellement réduit formé pendant le traitement in situ sous H2 ou sous flux de CH4, il implique des lacunes anioniques, des espèces O2- et des cations / In order to develop a sustainable hydrogen economy, it is desirable to produce hydrogen from biogas (CH4 and CO2) or greenhouses gases (GHG). Dry reforming (DRM) and oxidative dry reforming of methane (ODRM) are promising routes to produce H2 and CO from GHG and have received much attention due environment concerns. Herein, these reactions were studied at low temperatures (600 -700 °C) over CeNiX(AlZ)OY, NiXMg2AlOY mixed oxides and Ni/SBA-15 supported catalysts. Various physico-chemical techniques were employed to characterize the catalysts, such as XRD, XPS, H2-TPR and Raman. The influences of different parameters were examined, such as reaction temperature, pretreatment in H2, Ni content, mass of catalyst and reactants concentration, in particular, at 600°C in harsh conditions (feed gases without dilution) on low mass of catalyst (10 mg). The best catalytic activity and selectivity are obtained on partially reduced catalysts at appropriate temperature. The addition of O2 increases CH4 conversion but decreases CO2 conversion, and O2/CH4 = 0.3 could be the optimized condition due to high activity, selectivity and low carbon formation. Finally, an active site involving Ni species in close interactions with other cations is proposed. It is related to a partially reduced catalyst involving anionic vacancies, O2- species, and cations, which is formed during the in situ H2 treatment or CH4 flow

Biomass gasification offers the chance to produce carbon neutral, renewable fuels. One of the main problems facing the commercialization of biomass gasification technology is the presence of large quantities of methane and carbon dioxide in the biogas. Catalytic reforming of these wastes allows for effective utilization of biomass derived syngas. In most reforming studies, impregnation methods are the primary synthesis technique. Impregnation methods often lead to poor dispersion and are un-reproducible from batch to batch. In the development of a novel catalyst for reforming applications, another preparation method is implemented, controlled adsorption (CA). Ni/Al2O3 and Co/Al2O3 prepared by CA are compared against catalysts that were prepared by a more traditional method, dry impregnation (DI). It is found that controlling the metal deposition provides catalysts with higher dispersion and consequently higher activity for methane dry reforming. NiAl2O4 catalysts prepared by Pechini synthesis were also studied for catalytic conditioning of biomass derived syngas. Physicochemical characterization revealed unique structural properties, indicated a high degree of mobility of nickel in the aluminate structure, and demonstrated the regeneration properties of nickel aluminates under harsh reaction conditions, which will be important at extended reaction times when catalyst regeneration becomes necessary. Fourfold coordinated nickel species are believed to be responsible for high, stable methane dry reforming activity and metallic nickel is believed to be the active site that allows for high, stable conversion during methane dry reforming.

Made available in DSpace on 2016-06-02T19:56:41Z (GMT). No. of bitstreams: 1 3274.pdf: 3373226 bytes, checksum: 0bbe8c76b7e7419d9334c8f4a3cb3b96 (MD5) Previous issue date: 2010-07-29 / Universidade Federal de Minas Gerais / Catalysts obtained by thermal decomposition of the precursors with hydrotalcite-like structure [Mg1-xAlx (OH)2]x+ [CO3]x/n. mH2O and [Ni1-x Mg1-xAlx (OH)2]x+ [CO3]x/n. mH2O were prepared, characterized and tested in the reactions of dry and oxidative reforming of methane. The precursors were modified with the addition of nickel during the precipitation or by impregnation, in order to increase the thermal stability and dispersion of the active metal and the resistance against carbon deposition during the reaction. The catalyst precursors were characterized by X-ray diffraction, thermal and gravimetric analysis, atomic emission spectroscopy, surface area determination (BET method) and temperature programmed reduction. The results indicated that the precipitation method led the formation of the hydrotalcite-type structure and that the variation in the calcination temperature resulted in mixed oxides with the desired structure: at 650 °C the mixed oxides were formed, but when the calcination temperature was increased to 850 °C these oxides had become more stable. It was observed in the catalytic experiments that the addition of different amounts of magnesium influenced the activity and stability of the oxides, when compared with samples that contained only Ni/Al. The mixed oxide with composition 12.67% Ni-Mg1,35/Al was the more active and resistant to deactivation during the reaction. / Catalisadores obtidos através da decomposição térmica dos precursores com estrutura tipo hidrotalcitas [Mg1-xAlx(OH)2]x+ [CO3]x/n . mH2O e [Ni1-x + Mg1-xAlx(OH)2]x+ [CO3]x/n . mH2O, foram preparados, caracterizados e testados na reação de reforma seca e reforma oxidativa do metano. Os precursores foram modificados com a adição de níquel durante a precipitação e através do método de impregnação, visando o aumento da estabilidade térmica, dispersão do metal ativo e resistência à deposição de carbono durante a reforma seca do metano. Todos os óxidos com estrutura tipo hidrotalcita foram preparados pelo método de precipitação a pH constante, com exceção dos óxido impregnados, utilizando sais de nitratos dos metais como sais de partida. Os catalisadores e precursores foram caracterizados por Difração de Raios X, Analise Termogravimétrica, Espectroscopia de Emissão Atômica por Plasma Induzido, Medidas de Área Superficial Método B.E.T. e Redução a Temperatura Programada. Os resultados indicam que o método de precipitação proporcionou a formação da estrutura tipo hidrotalcita e as diferentes temperaturas de tratamento térmico, resultaram em óxidos mistos com a estrutura desejada e com propriedades interessantes do ponto de vista para a aplicação em catalise heterogênea. A partir de 650°C é observada a formação dos óxidos mistos, porém é observada a formação de espécies de níquel com diferentes interações com o suporte, assim quando a temperatura de calcinação foi aumentada para 850°C o óxido obtido tornou-se mais estável, o que determinaram novas propriedades físicas e estruturais a estes óxidos. Verificou-se, nos ensaios catalíticos, que a adição de diferentes quantidades de magnésio, influenciou na atividade e estabilidade dos óxidos formados, quando comparados com amostras que continham apenas Ni/Al. O óxido misto com composição 12,67% Ni-Mg1,35/Al foi a que se mostrou mais ativa e resistente a desativação durante o ensaio reacional, mas outros catalisadores também apresentaram resultados interessantes.