Preparação e caracterização de catalisadores de níquel suportado em óxido de zircônio e aluminio.

AUTOR(ES)
DATA DE PUBLICAÇÃO

2005

RESUMO

Alumina-supported nickel has been extensively used to catalyze the methane steam reforming because of its low cost, as compared to noble metal-based catalysts. However, it is not stable during the reaction mainly due to phase transition and sinterization. This calls for the development of new catalysts. An attractive option would be the use of zirconia-based supports, which has several advantages such as thermal stability, hardness and amphoteric properties. By the other hand this solid has low specific surface area, when compared to conventional catalytic supports. A probable solution to overcome this problem is to combine the properties of alumina and zirconia. With this goal in mind, the preparation and characterization of zirconium and aluminum-supported nickel catalysts were studied in this work. The supports were prepared by precipitation methods at room temperature from zirconium oxychloride and aluminum nitrate aqueous solutions, followed by calcination at 500 oC. Solids with Zr/Al (molar) = 10; 5; 2; 1; 0.5; 0.2 and 0.1, besides pure alumina and zirconia were obtained. The catalysts were prepared by impregnating nickel nitrate aqueous solutions on the supports at room temperature, followed by calcination at 500 oC. Samples were characterized by energy dispersive of X-ray, thermogravimetry, differential thermal analysis, Fourier transformed infrared spectroscopy, X-ray diffraction, temperature-programmed reduction, specific surface area and porosity measurements and X-ray photoelectron spectroscopy. The effect of the temperature on the crystallization of the supports was studied in situ by X-ray diffraction using a heating chamber. It was found both tetragonal and monoclinic phases in pure zirconia and g-Al2O3 in pure alumina. The addition of small amounts of aluminum stabilized the tetragonal phase of zirconia while higher amounts of this metal led to poorly crystallized solids. The aluminum-rich samples (Zr/Al= 0.1; 0.2 and 0.5) also produced g-Al2O3. At high temperatures, the poorly crystallized solids also formed nickel aluminate. The specific surface area increased with aluminum addition, a fact which was assigned to the presence of this metal on the solid surface, where it acts as a spacer. The aluminum-poor sample (Zr/Al= 0.1) showed the highest specific surface area. However, the specific surface areas decreased due to nickel addition, probably due to the blocking of some pores of the solids. The samples showed pores with different shapes and sizes and produced Type II and III isotherms with hysteresis loops, typical of mesoporous materials. The addition of aluminum also affect the nickel reduction making this process more difficult; this effect increased with amount of nickel in solids. Among the aluminum and zirconium-based samples, the solid with the same amount of these metals (Zr/Al (molar)= 1) showed the highest amount of nickel on the surface. It has high specific surface area and can be easily reduced, allowing the production of active phase of catalyst in the methane steam reforming. Therefore, it is the most promising catalyst to this reaction.

ASSUNTO(S)

fisico-quimica reforma a vapor de metano óxido de zircônio catalisador de níquel. system zro2-al2o3 nickel catalysts. óxido de zircônio e alumínio zirconia methane steam reforming

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