FT合成用Co/SiO_2触媒調製時におけるキレート剤の添加効果：調製条件の最適化によるCo種の制御

摘要

To control the fine structure of Co species on calcined Co/SiO_ 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/ SiO_2 and NTA-Co/SiO_2 is not affected by these preparation conditions, and Ca_3O_4-like species is mainly formed after the calcination. On the contrary, the fine structure of Co species on calcined CyDTA-Co/SiO_2 depended on these preparation conditions. An irreducible α -Co_2SiO_4-like species is formed after the calcination when CyDTA-Co/SiO_2 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 Co_3O_4-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/SiO_2 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/SiO_2 was ca. two times greater than that of Co/SiO_2 catalyst when the catalyst was prepared from the impregnating solution with lower pH, and then calcined at a lower heating rate.%Coは1930年代からFischer-Tropsch合成（FTS）触媒としrnて広く用いられ，近年，環境問題，エネルギー問題の観点かrnらその重要性は年々増している。Co触媒は通常，Co前駆rn体をSiO_2に含浸後，乾燥，焼成して調製され，FTS反応の直rn前にH_2還元処理が施される。