Nickel exsolution effect on the catalytic behavior of ruddlesden-popper manganites in sofc conditions using colombian natural gas
[EN] Several major problems have to be solved before Solid Oxide Fuel Cells (SOFC) can operate continuously using hydrocarbon fuels such as natural gas. The risk of low catalytic behavior for fuel reforming, the carbon formation/deposition on the anode material at high operating temperatures and the...
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| Tipo de recurso: | tesis doctoral |
| Fecha de publicación: | 2020 |
| País: | España |
| Institución: | Universitat Politècnica de València (UPV) |
| Repositorio: | RiuNet. Repositorio Institucional de la Universitat Politécnica de Valéncia |
| Idioma: | inglés |
| OAI Identifier: | oai:riunet.upv.es:10251/149474 |
| Acceso en línea: | https://riunet.upv.es/handle/10251/149474 |
| Access Level: | acceso abierto |
| Palabra clave: | Exsolution Ruddlesden-Popper Nickel Natural Gas Carbon H2S Poisoning Steam Reforming |
| Sumario: | [EN] Several major problems have to be solved before Solid Oxide Fuel Cells (SOFC) can operate continuously using hydrocarbon fuels such as natural gas. The risk of low catalytic behavior for fuel reforming, the carbon formation/deposition on the anode material at high operating temperatures and the presence of impurities in the fuel (in particular sulfides) can dramatically reduce the performance and durability of the cells. Taking all this into account, new anode materials with adequate (electro)catalytic properties are required. Recently, manganite compounds with Ruddlesden-Popper (RP) structure have been studied as potential new anode materials in INTERFASE group at Universidad Industrial de Santander (UIS). Their electrochemical performance have been described in previous works with promising results, but a fundamental knowledge was missing concerning the catalytic properties of such materials and the way to improve them by the addition of nickel metallic particles on the electrode surface. The current Ph.D. thesis was focused on the synthesis, characterization and catalytic study for steam reforming in SOFC anode conditions (low steam content) of a new RP manganite (La1.5Sr1.5Mn1.5Ni0.5O7±δ), which, in reducing atmosphere at high operating temperatures promotes via an exsolution mechanism the formation of two phases, i.e. an RP manganite of composition LaSrMnO4±δ decorated with metallic active Ni nanoparticles embedded in the surface; such strategy can be viewed as an original way to improve the (electro)catalytic properties of the anode materials and then a promising option for future SOFC systems operating with Colombian natural gas. The first chapter deals with the synthesis and characterization of the RP n= 2 phase La1.5Sr1.5Mn1.5Ni0.5O7±δ using the Pechini method. In agreement with SOFC operating temperature, Ni exsolution has been studied in diluted H2 at different temperatures (750, 800 and 850 °C) and reduction times. Ni nanoparticles decorating an RP n= 1 manganite is confirmed by XRD, TEM-EDS analysis and the size of the metallic particles on the oxide surface, below 100 nm, is characterized as a function of the exsolution conditions. The second chapter presents the catalytic behavior for the methane steam reforming reaction of the exsolved material applying the Gradual Internal Reforming concept adapted to SOFC operation (i.e. low water content, steam to carbon ratio equal to 0.15) at different reaction temperatures (750, 800 and 850 °C). The catalytic properties of Ni impregnated samples using a similar (La,Sr)2MnO4±δ ceramic support are also presented for comparison. The exsolved material exhibits better performance than the impregnated manganite for the reaction, especially at 850 °C, with higher conversion, conversion rate, and H2 production rate. Concerning the steam reforming of light alkane gas mixtures (CH4-C2H6, and CH4-C3H8), the behavior is affected due to the competition between the molecules and low available metallic active sites, but without affecting the H2 production. In addition, at long reaction times, the activity over the exsolved material is stable even with 100 h of reaction, without formation of carbonaceous species on the Ni particles, as confirmed by TEM and TGA/MS analysis. In the third and last chapter, the possible coke formation and sulfide poisoning are presented. Despite the high and stable catalytic behavior for methane steam reforming reaction with considerable carbon formation resistance, the exsolved material exhibits a high level of sensitivity to H2S poisoning, similar to the case of state-of-the-art Ni/YSZ anodic cermet and or Ni impregnated catalyst, with a drop of the activity to almost zero. Nevertheless, the exceptional overall results obtained for the exsolution-based material are promising for a possible use as SOFC anode operating with sulfur-free Colombian natural gas. |
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