Thesis (Ph. D.)--West Virginia University, 2006. / Title from document title page. Document formatted into pages; contains x, 174 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 155-166).

The reforming of CH4 reaction with CO2 was studied over supported catalysts. From this study, we have recognized that the catalytic behavior of the copper catalysts were affected by many factors such as the reaction temperature, the categories of precursors of active metals, the additives, and the supports of the catalysts. In this study, it was found that the catalysts having carbon depositions not only performed higher activities but also became more stable. TPO results have shown that three types of carbonaceous species, C£\, C£] and C£^, were found to exist on the supported copper catalyst during CH4-CO2 reforming reaction. The mechanism of the carbon deposition was also investigated, and we propose a model to explain this process. The effects of various additives on the performance of the catalysts were also explored. We found that the Cu/Ni/SiO2 bimetallic catalysts exhibited high activities regardless of the ratio of Cu/Ni. These catalysts exhibited higher activity and stability than the monometallic catalyst at 800¢J. The deactivation rates of these catalysts were only about 0.15- 0.31% h-1 (after more than 20 hours on stream). Therefore, the Cu/Ni/SiO2 bimetallic catalysts are preferable for the carbon dioxide reforming of methane.

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.

京都大学 / 新制・課程博士 / 博士(人間・環境学) / 甲第24693号 / 人博第1066号 / 新制||人||250(附属図書館) / 2022||人博||1066(吉田南総合図書館) / 京都大学大学院人間・環境学研究科相関環境学専攻 / (主査)教授 吉田 寿雄, 教授 田部 勢津久, 教授 中村 敏浩, 教授 寺村 謙太郎 / 学位規則第4条第1項該当 / Doctor of Human and Environmental Studies / Kyoto University / DFAM

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CoordenaÃÃo de AperfeÃoamento de Pessoal de NÃvel Superior / Dry reforming of methane reaction was conducted in the presence of titanate nanotubes (TNTs) modified with Co, Ni and Pt. TNTs were synthesized by hydrothermal treatment and than these solids were either submitted to ion exchange for Ni and Co using hexahydrate nitrate solutions, or they were submitted to wet impregnation with H2Ptl6.6H2O (1% w/w of Pt) solution. The solids were characterized before and after the dry reforming of methane by elemental chemical analysis (CHN), X-ray diffraction (XRD), Raman spectroscopy, nitrogen adsorption-desorption isotherms, thermoprogrammed reduction (TPR), CO2 thermoprogrammed desorption (CO2-TPD), transmission electronic microscopy (TEM), scanning electronic microscopy (SEM-EDS) and X-ray photoelectron spectroscopy (XPS). Raman and XRD results showed the presence of Na2Ti3O7 phase to all sodic nanotubes, while that the nanotubes modified displayed peaks and vibrational modes relative to CoTi3O7, NiTi3O7 and PtOx/Na2Ti3O7 phases. TEM images exhibited tubular morphology composed by multi-walls, as observed by XRD and Raman. SEM-EDS results showed the nanotubes composition with M/Ti ratio lower than the theoretical (value of 0,33), due to the presence of structural water. The XPS results confirmed the presence of M(OH)2 phase (M=Co, Ni or Pt) present on nanotubes surface. TPR patterns suggested the formation of M0/MTiO3 (M = Co, Ni and Pt) after the reduction of the nanotubes at 650 ÂC. The nitrogen adsorption-desorption isotherms of sodic and modified TNTs showed isotherms type IV with an essentially mesoporous structure. CO2-TPD patterns suggested the presence of weak and moderate basic sites in all catalysts, indicating phase transformation due to the decomposition, in situ, of as-prepared nanotubes. The catalyst NiTNT exhibited the highest CO2 and methane conversion at 600 ÂC, with about 43 and 25%, respectively, and H2/CO ratio equal 1, without deactivation over time. PtTNT was lesser susceptible to coking, although sintering remarkably decreased the performance of this solid. On the other hand, PtTNT and CoTNT showed formation of coke over the PtOx/PtTiO3 and Co0/CoTiO3 active phase, respectively, so that the latter solid deactivated during the dry reforming of methane. / A reaÃÃo da reforma seca do metano foi conduzida na presenÃa de nanotubos de titanatos (TNTs) modificados com Co, Ni e Pt. Os TNTs foram sintetizados via tratamento hidrotÃrmico e, posteriormente, foram submetidos à troca iÃnica por Ni e Co, utilizando soluÃÃes de nitrato hexahidratado, ou foram submetidos à impregnaÃÃo via-Ãmida com soluÃÃo de H2PtCl6.6H2O (1% m/m de Pt). Os catalisadores foram caracterizados antes e apÃs reaÃÃo de reforma seca do metano por anÃlise quÃmica (CHN), difraÃÃo de raios-X (DRX), espectroscopia Raman, isotermas de adsorÃÃo-dessorÃÃo de nitrogÃnio, reduÃÃo termoprogramada (TPR), dessorÃÃo termoprogramada de CO2 (TPD-CO2), microscopia eletrÃnica de transmissÃo (TEM), microscopia eletrÃnica de varredura (MEV-EDS) e espectroscopia fotoeletrÃnica de raios-X (XPS). Os resultados de Raman e DRX evidenciaram a presenÃa da fase Na2Ti3O7 para os nanotubos sÃdicos, enquanto que para os nanotubos modificados foram identificados picos e modos vibracionais referentes Ãs fases CoTi3O7, NiTi3O7 e PtOx/Na2Ti3O7. As imagens de TEM exibiram morfologia tubular composta por multiparedes, corroborando com os resultados de DRX e Raman. Os resultados de MEV-EDS mostraram a composiÃÃo dos nanotubos com razÃo M/Ti menor que o teÃrico (0,33), devido à presenÃa de Ãgua estrutural. Os resultados de XPS confirmaram a existÃncia da fase M(OH)2 (M=Co, Ni ou Pt) presentes na superfÃcie dos nanotubos. As curvas de TPR sugeriram a formaÃÃo da fase M0/MTiO3 (M = Co, Ni e Pt), apÃs a reduÃÃo dos nanotubos a 650 ÂC. As isotermas de adsorÃÃo-dessorÃÃo de nitrogÃnio dos TNTs sÃdicos e modificados apresentaram isotermas do tipo IV com estrutura essencialmente formada por mesoporos. Os perfis de TPD-CO2 sugeriram a presenÃa de sÃtios bÃsicos fracos e moderados em todos os catalisadores, indicando mudanÃa de fase devido à decomposiÃÃo in situ dos nanotubos como sintetizados. O catalisador NiTNT apresentou os melhores resultados de conversÃo de CO2 e metano a 600 ÂC, com aproximadamente 43 e 25%, respectivamente, e razÃo H2/CO igual a 0,5, sem desativaÃÃo ao longo do tempo. PtTNT foi menos susceptÃvel à formaÃÃo de coque, embora o fenÃmeno de sinterizaÃÃo tenha desfavorecido o desempenho do sÃlido. Por outro lado, os sÃlidos PtTNT e CoTNT apresentaram formaÃÃo de coque sobre as fases ativas PtOx/PtTiO3 e Co0/CoTiO3, respectivamente, de modo que este Ãltimo sÃlido desativou durante a reaÃÃo da reforma seca do metano.

Dry reforming of methane is an important reaction to generate syngas from two greenhouse gases. The syngas can be used in Fishcher-Tropsch synthesis to produce valueded chemicals. Chapter I reviews the catalytic conversion of methane and carbon dioxide to syngas, including DRM reaction chemistry, catalysts used in this process, catalyst deactivation, and the kinetics of DRM reaction. Chapter II discusses the development of bimodal pore NiCeMgAl catalysts for DRM reaction. Bimodal pore NiCeMgAl catalysts were synthesized via the refluxed co-precipitation method and systematically investigated the influence of active metal loading, calcination temperature, reduction temperature and gas hourly space velocity (GHSV) on the catalytic performance of DRM reaction. The Ni15CeMgAl sample with 15 wt% NiO loading was found to be active enough at 750 °C with a high CH4 conversion of 96.5%. The proper reduction temperature for the NiCeMgAl catalyst is either 550–650 °C or 850 °C. Higher calcination temperature favors the formation of NiAl2O4 and MgAl2O4 spinel structures. Compared with non-bimodal pore NiCeMgAl catalyst, bimodal pore NiCeMgAl catalyst has a longer stability in the feed gas without dilution. In chapter III, the kinetic behavior of bimodal pore NiCeMgAl catalyst for DRM reaction was investigated after the elimination of external and internal diffusion effects in a fixed-bed reactor as a function of temperature and partial pressures of reactants and products. A Langmuir-Hinshelwood model was developed assuming that the carbon deposition is ignorable but the RWGS reaction is non-ignorable and the removal of adsorbed carbon intermediate is the rate-determining step. A nonlinear least-square method was applied to solve the kinetic parameters. The derived kinetic expression fits the experimental data very well with a R2 above 0.97, and predicts the products flow rate satisfactorily. Chapter IV documents the results of in situ XRD study on the NiMgAl catalyst for DRM reaction. The phase evolution of a NiMgAl oxide catalyst at the reduction stage was qualitatively analysed and quantitatively determined by employing the continuous changes in XRD intensity and TPR information. The stable crystallite size of both active metal and spinel support is responsible for the long stability of NiMgAl catalyst without carbon deposition during the DRM reaction.

This dissertation describes the preparation of a novel inorganic membrane for hydrogen permeation and its application in a membrane reactor for the study of the methane steam reforming reaction. The investigations include both experimental studies of the membrane permeation mechanism and theoretical modeling of mass transfer through the membrane and simulation of the membrane reactor with 1-D and 2-D models. A hydrothermally stable and hydrogen selective membrane composed of silica and alumina was successfully prepared on a macroporous alumina support by chemical vapor deposition in an inert atmosphere at high temperature. Before the deposition of the silica-alumina composite, multiple graded layers of alumina were coated on the alumina support with a mean pore size of 100 nm by the sequential application of three boehmite sols with gradually decreasing sol particle sizes of 630, 200 and 40 nm, respectively. The resulting supported composite alumina-silica membrane had high permeability for hydrogen in the order of 10-7 mol m-2 s-1 Pa-1 at 873 K with a H2 /CH4 selectivity of 940 and exhibited much higher stability to water vapor at the high temperature of 873 K. In addition, the same unusual permeance order of Heï¼ H2ï¼ Ne previously observed for the pure silica membrane was also observed for the alumina-silica membrane, indicating that the silica structure did not change much after introduction of the alumina. The permeation of hydrogen and helium through vitreous glass and silica membranes was modeled using ab initio density functional calculations. Comparison of the calculated activation energies to those reported for vitreous glass (20—40 kJ mol -1) indicated the presence of 5- and 6-membered siloxane rings, consistent with the accepted structure of glass as a disordered form of cristobalite. The experimental studies of the steam reforming of methane were examined at various temperatures (773-923 K) and pressures (1-20 atm) with a commercial Ni/MgAl2O4 catalyst in a hydrogen selective silica-alumina membrane reactor and compared with a packed bed reactor. One-dimensional and two-dimensional modeling of the membrane rector and the packed bed reactor were performed at the same conditions and their performances were compared with the values obtained in the experimental study. Improved methane conversions and hydrogen yields were obtained in the membrane reactor compared to the packed bed reactor at all temperatures and pressures. From the two modeling studies, it was also found out that the two-dimensional model performed better in the membrane reactor case especially at higher pressures. / Ph. D.

Devido a crescente preocupação com relação ao efeito estufa e com o objetivo de obter produtos de maior valor agregado a partir dos gases CH4 e CO2, este trabalho estudou catalisadores de níquel suportados em diferentes óxidos (Al2O3, CeO2, MgO e ZrO2) promovidos com ZnO aplicados na reação de reforma seca de metano para obter gás de síntese (H2 + CO). Primeiramente foram estudados catalisadores de níquel suportados em ZrO2 dopados com diferentes teores de zinco: 0%, 5%, 12,5% e 25% a fim de selecionar o catalisador mais promissor para estudos posteriores. Também foram utilizados dois métodos de preparo: impregnação e co-precipitação. Os resultados de DRX mostraram que a adição de zinco estabilizou a fase tetragonal da zircônia em todos os teores de zinco e em ambos os métodos de preparo. As análises de RTP mostraram que os catalisadores impregnados sofreram maior redução do que os co-precipitados e para maiores teores de zinco foram necessárias maiores temperaturas de redução. Os testes catalíticos mostraram que o aditivo zinco não exerceu efeito sobre os catalisadores impregnados, porém para as amostras co-precipitadas notou-se um teor ótimo com relação à conversão de reagentes na reação e o catalisador 5Ni12ZZc apresentou os melhores resultados. A deposição de carbono também foi menor nos catalisadores co-precipitados. No estudo dos diferentes suportes, as análises de DRX sugerem a dopagem de todos os óxidos com zinco, pois houve mudanças no parâmetro de rede em todas as amostras. As análises de RTP mostraram que a adição de zinco diminui a temperatura de redução do catalisador suportado em alumina e, no suporte CeO2 o efeito é o contrário. Nos testes catalíticos observaram-se efeitos distintos: nos catalisadores Ni-ZnO-CeO2 e Ni-ZnO-MgO o efeito da adição de zinco causou diminuição na conversão dos reagentes, por outro lado, causou diminuição na deposição de carbono e inibição da formação do carbono grafite; nos catalisadores Ni-ZnO-Al2O3 a conversão de reagentes não se alterou com a adição de zinco porém houve uma diminuição na deposição de carbono. Os melhores resultados, portanto, foram apresentados pelo catalisador 5NiZnAl que converteu acima de 80% dos reagentes e não sofreu desativação pela deposição de carbono. / Due to the increasing concern related to the greenhouse effect and aiming to obtain products with higher added-value from CH4 and CO2 gases, nickel catalysts supported on different oxides (Al2O3, CeO2, MgO and ZrO2) promoted with ZnO were studied and applied to the dry reforming of methane reaction to obtain syngas (H2+CO). Firstly, ZrO2 supported Nickel catalysts doped with different zinc content: 0, 5, 12,5 and 25% molar ratio were used to identify and select the best amount of zinc to further studies. Two preparation methods were investigated: impregnation and co-precipitation. The XRD results showed that the zinc addition stabilized the zircon tetrahedral phase of all catalysts from both preparation methods. The TPR analyses showed that the impregnated catalysts reduced more than the co-precipitated ones and higher reduction temperature was needed for the catalysts with higher zinc content. The catalytic tests indicated that the zinc content did not have any effect over the impregnated catalysts, whilst for the co-precipitated samples, a great content was noted and the 5Ni12ZZc catalyst showed the best results. The co-precipitated catalysts also had lower carbon deposition. In the study of different supports, the XRD analyses showed that the zinc interacts with all the used oxides, because there were changes of the cell parameter of all samples. The TPR analyses indicated that the zinc addition lowers the reduction temperature of the alumina supported catalyst and the effect is opposite for the CeO2 support. Different effects were observed during the catalytic tests: for the CeO2 and MgO supported catalysts the effect of zinc addition caused a decrease in the conversion of reactants, but it lowered the deposition of carbon and inhibited the formation of graffiti carbon; for the Al2O3 supported catalysts, the reactant conversion was the same with the addition of zinc, however there was a reduction on the carbon deposition. Therefore, the best results were obtained for the 5NiZnAl catalyst, which converted over 80% of the reactants and wasn\'t deactivated by carbon deposition.

Catalisadores ativos e estáveis, preparados a partir dos hidróxidos duplos lamelares (HDLs) contendo os cátions Ni2+, Zn2+, Al3+ e Zr4+(NiZn-Al, NiZn-AlZr e NiZn-Zr), foram avaliados na reação de reforma seca do metano. Os precursores foram caracterizados por difração de raios X, confirmando a formação das fases para as amostras de NiZn-Al e NiZn-AlZr. As espectroscopias no infravermelho/Raman elucidaram a ocorrência dos ânions (CO32-) e das moléculas de água na intercamada. A Fisissorção de nitrogênio ( método de BET) determinou a presença de poros e a área superficial específica. As formas das isotermas foram do tipo IV, de acordo com a IUPAC, e representa uma estrutura mesoporosa. A análise termogravimétrica foi utilizada para determinar a temperatura de calcinação dos precursores para obtenção dos catalisadores e as temperaturas utilizadas foram 550 °C, 650 °C e 750 °C. Os resultados de difração de raios X dos catalisadores apresentaram uma mistura de óxidos binários (NiO, ZnO, ZrO2 e Al2O3) e óxidos ternários (NiAl2O4 e ZnAl2O4). As fases de espinélios foram observadas nos catalisadores obtidos a 650°C (Ni/ZnAl-6 e Ni/ZnAlZr-6) e 750°C (Ni/ZnAl-7 e Ni/ZnAlZr-7). A redução a temperatura programada de hidrogênio (TPR-H2) forneceu informações sobre a redução do catalisador. Os resultados exibiram interações entre a fase ativa com as outras fases presentes no intervalo entre 350 - 800 °C, enquanto que a dessorção àtemperatura programada de CO2 (DTP-CO2) mostrou que os catalisadores possuem sítios básicos fracos. Ademais, os catalisadores foram caracterizados por fisissorção de nitrogênio, microscopia eletrônica de varredura (MEV), espectroscopia Raman e basicidade usando um indicador de Hammett. O desempenho dos catalisadores foi avaliado na reação de reforma seca do metano utilizando uma razão molar de alimentação de CH4: CO2 = 1,5:1,0; a fim de simular a composição do biogás. Os resultados mostraram uma maior conversão para o CO2 em todos os catalisadores investigados, o que pode ser atribuído ao maior teor de CH4 alimentado. Além disso, é importante ressaltar a contribuição dos sítios básicos presentes nas amostras, o que pode também contribuir para a conversão do CO2. A maior conversão de CH4 foi observada para os catalisadores tratados a 750 °C. Após as reações, foi observada a formação de carbono grafite sobre todos os catalisadores, com menor percentual para a amostra Ni/ZnAl, calcinadas a 550 °C e 650 °C, e Ni/ZnZr-7, calcinada a 750 °C. Finalmente, os catalisadores calcinados a 650 °C e 750 °C também foram ativados na temperatura de 750°C e os resultados reacionais mostraram que essa temperatura favoreceu o processo de aglomeração e, portanto, promoveu baixas conversões para o CH4 e CO2. / Active and stable catalysts, prepared from the lamellar double hydroxides (HDLs) containing the cations Ni2 +, Zn2 +, Al3 + and Zr4 + (NiZn-Al, NiZn-AlZr and NiZn-Zr) were evaluated in the dry reforming reaction of methane. The precursors were characterized by X-ray diffraction method, confirming the formation of phases for NiZn-Al and NiZn-AlZr samples. Infrared / Raman spectroscopies elucidated the occurrence of anions (CO32-) and water molecules in the interlayer. Nitrogen physisorption (BET method) determined the presence of pores and the specific surface area. The shape of the isotherms were type IV, according to the IUPAC, representing a mesoporous structure. The thermogravimetric analysis was used to determine the calcination temperature of the precursors to obtain the catalysts and the temperatures used were 550 ° C, 650 ° C and 750 ° C. The X-ray diffraction results of the catalysts showed a mixture of binary oxides (NiO, ZnO, ZrO2 and Al2O3) and ternary oxides (NiAl2O4 and ZnAl2O4). The spinel phases were observed in the catalysts obtained at 650 °C (Ni/ZnAl-6 and Ni/ZnAlZr-6) and 750°C (Ni/ZnAl-7 and Ni/ZnAlZr-7) Temperature- programmed reduction with hydrogen (H2-TPR) provided information on catalyst reduction. The results showed interactions between the active phase and the other phases present in the range of 350 - 800 ° C, while temperature- programmed desorption of CO2 (CO2-TPD) showed that the catalysts have weak basic sites. In addition, the catalysts were characterized by Nitrogen physisorption, scanning electron microscopy (SEM), Raman spectroscopy and basicity using a Hammett indicator. The performance of the catalysts was evaluated in the dry reforming reaction of the methane using a CH4: CO2 molar feed ratio = 1.5: 1.0; In order to simulate the biogas composition. The results showed a higher conversion to CO2 in all the catalysts investigated, which can be attributed to the higher content of CH4 fed. In addition, it is important to restate the contribution of the basic sites present in the samples, which may also contribute to the conversion of CO2.The higher conversion of CH4 was observed for the catalysts treated at 750 ° C. After the reactions, the formation of graphite carbon was observed on all the catalysts, with a lower percentage for Ni / ZnAl samples, calcined at 550 ° C and 650 ° C, and Ni / ZnZr-7, calcined at 750 ° C. Finally, the catalysts calcined at 650 ° C and 750 ° C were also activated at the temperature of 750 ° C. and the reaction results showed that this temperature favored the agglomeration process and, therefore, promoted low conversions for CH4 and CO2.