Methane Tri-reforming over nickel catalysts
- Autores: Jesús Manuel García Vargas
- Directores de la Tesis: Fernando Dorado Fernández (dir. tes.), Paula Sánchez Paredes (dir. tes.)
- Lectura: En la Universidad de Castilla-La Mancha ( España ) en 2014
- Idioma: inglés
- Tribunal Calificador de la Tesis: José Luis Valverde Palomino (presid.), L. J. Alemany (secret.), Anabel Chen de Castillo (voc.)
- Materias:
- Enlaces
- Tesis en acceso abierto en: RUIdeRA
- Resumen
The present work is part of a research program carried out in the Department of Chemical Engineering at the University of Castilla-La Mancha, focused in the preparation, characterization and evaluation of catalysts that can be applied in industrially relevant reactions. In this way, the PhD work reported here was aimed to study and improve nickel catalysts applied to the tri-reforming process, evaluating the role of support, precursor and promoter and optimizing the catalyst preparation. Furthermore, the influence of feed composition and temperature in the catalytic behaviour have been also analyzed, developing in a final step a model in order to fit the experimental data. The influence of different support materials (alumina, ceria, ¿-silicon carbide and yttria-stabilized zirconia) on the catalytic behaviour of Ni catalysts for the dry reforming reaction and the tri-reforming process has been studied in chapter 1. The influence on the catalytic performance of the composition of the atmosphere surrounding the Ni/YSZ catalyst during the calcination step was also analysed. Temperature-programmed reduction experiments showed remarkable differences in the reduction profile and the degree of reduction of the catalysts as a function of both the support used and the calcination conditions. Ni/YSZ-O2, the catalyst calcined under an oxygen-poor atmosphere, presented a higher reducibility as a consequence of the higher number of oxygen vacancies in the surface of the support. The Ni/Al2O3catalyst gave the lowest CH4 and CO2 reaction rates as a consequence of its low reducibility due to the formation of Ni aluminate. The Ni/CeO2 catalyst showed the lowest H2/CO molar ratio for the tri-reforming process. This result can be explained on considering the higher basicity of this catalyst, as shown by CO2-TPD experiments. Ni/YSZ-O2 showed the higher reaction rate of CH4 and CO2 in the dry reforming experiments, showing the Ni/CeO2 and Ni/¿-SiC slightly lower values. The CeO2 and ¿-SiC catalysts had the best characteristics as catalytic supports for the tri-reforming process. The aim of the work described in chapter 2 is to evaluate the catalytic performance in the tri-reforming process of Ni/CeO2 and Ni/¿¿SiC catalysts prepared by using four different nickel salts (nitrate, acetate, chloride and citrate).Metal particles supported over ceria had bigger particle sizes (leading to lower metal-support interactions) than those supported on ¿¿SiC. It was also demonstrated that the metal salt used in the preparation of Ni-based catalysts had a marked influence on the size of the nickel particles. Larger particles with a worse catalytic behaviour were obtained when nickel chloride and nickel citrate were used as the precursors of Ni supported species. Methane consumption rate and H2/CO ratio in the effluents were influenced by the type of support and salt precursor used in the preparation of the catalysts. CO2-TPD proved that catalysts based on ceria as the support presented more basic sites, which was related to a decrease of the H2/CO molar ratio in the effluents coming from the reactor. High methane consumption rate and good catalytic stability were obtained when nickel nitrate and nickel acetate were used to prepare Ni/¿¿SiC catalysts. The results showed that these latter catalysts can be considered as promising ones for the tri-reforming process. Tri-reforming of methane has proved to be a highly efficient process for obtaining synthesis gas suitable for use in the Fischer Tropsch process and methanol synthesis. In chapter 3 the influence of the feedstock composition on methane conversion, the H2/CO molar ratio of the synthesis gas obtained by tri-reforming of methane and the heat released or supplied to the system with a Ni/¿¿SiC catalyst are all described. Firstly, a factorial plus central composite design of experiments was chosen in order to optimize the independent variables selected. Then, using the experimental data obtained, a quadratic model was built. It was observed that the effect of both water and oxygen volume flow on the H2/CO molar ratio was positive while that of methane and carbon dioxide volume flow was negative. Finally, in order to obtain an energetic optimum inside the target region, the influence of the independent variables studied previously on the overall reaction heat was calculated. The influence of alkaline (Na, K) and alkaline earth (Mg, Ca) cocations on the behaviour of Ni/¿¿SiC catalyst for the tri-reforming of methane has been evaluated in chapter 4. The cocations were loaded by co-impregnation with Ni, using different cocation/Ni ratios. Catalysts were characterized by AAS, TPR, N2 adsorption, CO2-TPD and XRD after calcination, as well as by XRD and TPO after reaction. It was analyzed the effect of the cocations on the ¿¿SiC oxidation rate, which was increased when Na or K were loaded. The presence of Mg led to a high catalytic performance and stability (with a lower coke formation) since it provoked a decrease of Ni particle size and an increase of both the interaction between nickel and promoter and the catalyst basicity. Catalysts with Ni:Mg molar ratios of 2/1 and 1/1 showed the best performance in terms of activity and stability and formation of coke. These catalysts were considered good candidates for the tri-reforming of methane. The influence of the order of Ni and Mg impregnation has been analyzed in terms of catalytic activity and stability of ¿-SiC supported catalysts for the tri-reforming of methane in chapter 5. Catalysts were characterized using different techniques such as Temperature Programmed Reduction, X-Ray Diffraction, Transmission Electron Microscopy and Temperature Programmed Oxidation. The addition of Mg clearly changed the reduction profile, increasing the temperature required to obtain Ni0. Higher reduction temperatures were needed when Mg was firstly loaded or when both metals, Ni and Mg, were simultaneously loaded, which was attributed to the occurrence of interactions between Ni and Mg. Catalyst prepared by first Ni impregnation showed the worst catalytic behaviours, probably due to a poor interaction between Ni and Mg, a possible blockage of Ni particles by Mg ones and the occurrence of Ni2Si after reaction. Catalysts prepared with the highest Mg/Ni molar ratio (1/1) showed smaller Ni particle sizes, lower coke rate formation and higher basicity and Ni-Mg interaction. Ni-Mg/SiC 1/1 was selected as the best catalyst due to its high catalytic activity and stability and low coke generation. In chapter 6, the influence of the temperature and feed composition on the catalytic behaviour of a Ni-Mg/¿-SiC catalyst was analyzed and modelized. This catalyst was characterized by Atomic Absorption Spectrophotometry (AAS), Temperature Programmed Reduction (TPR), N2 adsorption, Temperature Programmed Desortion of CO2 (TPD) and X-Ray Diffraction (XRD). 36 catalytic experiments at different temperatures and feed compositions were performed. The predominance of each one of the reactions that took place during the tri-reforming process was evaluated as a function of the temperature, finding at low temperatures a higher contribution of both the steam reforming and the water gas shift reactions. On the contrary, at higher temperatures, a higher contribution of the dry reforming was detected. Finally, a kinetic model was raised and experimental data were fitted to it. Steam reforming, dry reforming and water gas shift reactions were considered as the kinetically relevant equations. A good agreement between experimental and predicted data was observed.
