Surface Impregnation Combustion Method to Prepare Nanostructured Metallic Catalysts without Further Reduction: As-Burnt Co/SiO₂ Catalysts for Fischer–Tropsch Synthesis

A series of the as-burnt supported metallic catalysts (Co/SiO 2 ) were prepared by a novel surface impregnation combustion method using cobalt nitrates (noted as M) and citric acid (noted as CA), and burnt in the argon atmosphere. H 2 and CH 4 which were derived from the decomposition of the CA, acted as the reducing agents for synthesizing metallic Co from Co 2+ in the chelated compounds. The X-ray diffraction (XRD) patterns revealed that all the as-burnt catalysts with different M/CA molar ratios were converted into Co and CoO species supported on SiO 2 (Q-50). The as-burnt catalyst C 1.5, with M/CA = 1/1.5, was almost fully converted to metallic Co species and amorphous carbon supported on SiO 2 . With increase of the content of CA in the initial precursors, the reduction degree of the as-burnt catalysts which was measured by temperature-programmed reduction (TPR) and the dispersion of metallic cobalt increased gradually. The as-burnt catalysts were used directly in slurry-phase Fischer-Tropsch synthesis (FTS) without further reduction. The effects of M/CA molar ratios on the properties of the as-burnt catalysts were studied by TG-DTA measurement in the argon atmosphere, XRD, FT-IR, Raman spectrum, TPR, H 2 -chemisorption, and transmission electron microscopy (TEM). The amount of amorphous carbon left in the as-burnt catalysts was measured by energy dispersive spectroscopy (EDS) analysis and TG-DTA measurement in air. Comparing with the Co/SiO 2 catalyst prepared by the conventional impregnation method (noted as C N ) which was followed by 10 h reduction at 723 K in hydrogen, the CO conversion of the as-burnt Co/SiO 2 catalyst (without reduction) with M/CA = 1/1 was almost 3-fold of that of the catalyst C N . This surface impregnation autocombustion method may open a new way to prepare metallic catalysts without further reduction, especially the catalysts which were difficult to be reduced at higher temperature.