Preparation of Highly Active Co/SiO2 Catalyst with Chelating Agent for the Fischer-Tropsch Synthesis: Control of Co Species by Optimization of Preparation Conditions
Takehisa MOCHIZUKI, Hiroki OSANAI, Takuya SATO, Daichi HONGO, Naoto KOIZUMI, Muneyoshi YAMADA
To control the fine structure of Co species on calcined Co/SiO2 catalyst, especially to depress the formation of Co silicate-like species after the calcination, the present study has investigated effects of some preparation conditions (pH of the impregnating solution, and heating rate during calcination) on the fine structure of Co species on the calcined catalysts prepared with trans-1,2-diamino-cyclohexane-N,N,N',N'-tetraacetic acid (CyDTA) or nitrilotriacetic acid (NTA), and their Fischer-Tropsch synthesis (FTS) activities. Characterization results by Co K-edge EXAFS and TPR showed that the fine structure of Co species on calcined Co/SiO2 and NTA-Co/SiO2 is not affected by these preparation conditions, and Co3O4-like species is mainly formed after the calcination. On the contrary, the fine structure of Co species on calcined CyDTA-Co/SiO2 depended on these preparation conditions. An irreducible α-Co2SiO4-like species is formed after the calcinations when CyDTA-Co/SiO2 catalyst was prepared from the impregnating solution with higher pH (>9.1), and/or was calcined at the rate above 1.0 K min-1, whereas Co3O4-like species was formed when prepared from the impregnating solution with lower pH (<3.1), and/or calcined at lower heating rate (0.1 K min-1). TG-DTA measurements during the calcination of CyDTA-Co/SiO2 showed that the pH of the impregnating solution and heating rate during calcinations affect the decomposition temperature of the CyDTA-Co complex formed in the impregnating solution. The reducibility of Co species became 6-8 times greater when the complex was decomposed at lower temperatures. Therefore, it is suggested that the fine structure of Co species after the calcination is affected by the decomposition temperature (stability) of CyDTA-Co complex during the calcination. The FTS activity of CyDTA-Co/SiO2 was ca. two times greater than that of Co/SiO2 catalyst, when the catalyst was prepared from the impregnating solution with lower pH, and then calcined at a lower heating rate.
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