Effect of cobalt precursor and pretreatment conditions on the structure and catalytic performance of cobalt silica-supported Fischer-Tropsch catalysts

摘要

The effect of cobalt precursor and pretreatment conditions on the structure of cobalt species in silica-supported Fischer-Tropsch (FT) catalysts was studied with a combination of characterization techniques (X-ray diffraction,UV-visible,X-ray absorption,X-ray photoelectron spectroscopies,DSC-TGA thermal analysis,propene chemisorption,and temperature-programmed reduction combined with in situ magnetic measurements).The catalysts were prepared via aqueous impregnation of silica with solutions of cobalt nitrate or acetate followed by oxida-tive pretreatment in air and reduction in hydrogen.It was found that after impregnation and drying cobalt exists in octahedrally coordinated complexes in catalysts prepared from cobalt nitrate or cobalt acetate.Decomposition of the octahedral complexes results in the appearance of Co_3O_4 crystallites and cobalt silicate species.Cobalt repartition between crystalline Co_3O_4 and the cobalt silicate phase in the oxidized samples depends on the exothermicity of salt decomposition in air and the temperature of the oxidative pretreatment.Co_3O_4 crystallite is the dominant phase in the samples prepared via endothermic decomposition of supported cobalt nitrate.Significantly higher cobalt dispersion is found in the catalyst prepared via low-temperature cobalt nitrate decomposition.The uncovered enhanced cobalt dispersion is associated with lower cobalt reducibility.The high exothermicity of cobalt acetate decomposition leads primarily to amorphous,barely reducible cobalt silicate.A more efficient heat flow control at the stage of cobalt acetate decomposition significantly increases the concentration of easy reducible Co_3O_4 in the oxidized catalysts and the number of cobalt metal active sites after reduction.The catalytic measurements show that FT reaction rates depend on the number of cobalt surface metal sites;a higher concentration of cobalt metal sites in the catalysts prepared from cobalt nitrate or with the use of soft cobalt acetate decomposition results in higher catalytic activity in FT synthesis.