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

Abstract

A series of the as-burnt supported metallic catalysts (Co/SiO2) 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. H2 and CH4 which were derived from the decomposition of the CA, acted as the reducing agents for synthesizing metallic Co from Co2+ 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 SiO2 (Q-50). The as-burnt catalyst C1.5, with M/CA = 1/1.5, was almost fully converted to metallic Co species and amorphous carbon supported on SiO2. 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, H2-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/SiO2 catalyst prepared by the conventional impregnation method (noted as CN) which was followed by 10 h reduction at 723 K in hydrogen, the CO conversion of the as-burnt Co/SiO2 catalyst (without reduction) with M/CA = 1/1 was almost 3-fold of that of the catalyst CN. 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.