Porous Metal Oxides as Catalysts
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Porous Metal Oxides as Catalysts Boxun Hu1, Christopher Brooks2, Eric Kreidler2, Steven L. Suib1, 3 * 1: Institute of Materials Science, University of Connecticut, Storrs, CT 06269, USA. 2: Honda Research Institute USA, Inc. 1381 Kinnear Rd. Suite 116, Columbus, OH 43212, USA. 3: Department of Chemistry, University of Connecticut, Storrs, CT 06269, USA. ABSTRACT Porous CoO/Mn2O3 Fischer-Tropsch (F-T) catalysts have been studied in CO hydrogenation. These CoO/Mn2O3 catalysts have been synthesized by incipient wetness impregnation method. These mesoporous catalysts have pore diameters of 2-25 nm and a surface area of 9.0 m2/g. The gas and liquid products have been analyzed by an online gas chromatograph. The solid products were characterized by gas chromatograph-mass spectroscopy. These microsize cobalt catalysts exhibit good activity with 72.1% CO conversion and they are very stable in a 48 h stream test at 280ºC. The selectivity to paraffins is above 95%. Few wax products were synthesized with a yield of less than 2%. The size effects of the cobalt catalysts have been studied by scanning electron microscopy. INTRODUCTION Fischer-Tropsch (F-T) synthesis has provided pathways of syngas (CO/H2) to liquid fuel and useful chemicals. Existing catalysts (Co, Fe, Ru, and Ni) supported on inert silica, alumina, zeolites, and carbon nanotubes have been developed for F-T synthesis.1-3 Cobalt and iron F-T catalysts have been used for industrial F-T synthesis.4, 5 The products of these catalysts are mainly paraffins. Additional manganese oxides have been used as promoters to increase the production of olefins.6, 7 The particle size affects the catalyst stability and activity.8 Nanosize catalysts are favored due to their high surface area but sintering is a serious problem for using nanosize cobalt catalysts. In this study, microsize Mn2O3 has been selected as a new support for the study of the size effect. These catalysts only contain cobalt, manganese, and oxygen. The simple components make these catalysts as a standard for the comparison with other existing promoted catalysts (e.g. K). For these purposes, the product profiles of gas, liquid, and solid products have been analyzed. The catalyst structure has been characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). EXPERIMENTAL Catalyst Preparation Materials used in this study were purchased from Alfa Aesar if no specification and they were used as received without further purification. The Mn2O3 supported cobalt catalysts were synthesized by an incipient wetness impregnation (IWI) method. For a typical synthesis, Co(NO3)2 (98+ wt%, molar ratio Co/Mn: 0.1-0.3) were dissolved in deionized (DI) water, and
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then Mn2O3 (98+ wt%, bixbyite-C) was added into the solution under agitation at 80ºC until water was evaporated. The impregnated samples were dried at 120ºC, and they were finally calcined in air (120 sccm) at 350-450ºC in a tube furnace. Characterizations of Catalysts and F-T Products Powder XRD experiments were performed with a Scintag XDS 2000
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