Mechanistic aspects of the role of chelating agents in enhancing Fischer–Tropsch synthesis activity of Co/SiO2 catalyst: Importance of specific interaction of Co with chelate complex during calcination

• Published in: Journal of catalysis 2012-05, Vol.289, p.151-163
• Main Authors: Koizumi, Naoto, Ibi, Yukiya, Hongo, Daichi, Hamabe, Yusuke, Suzuki, Shigenobu, Hayasaka, Yasuhiko, Shindo, Takayoshi, Yamada, Muneyoshi
• Format: Article
• Language: English
• Subjects: Catalysis; Catalysts; Chelating agent; Chemical reactions; Chemical synthesis; Chemistry; Co/SiO2 catalyst; CyDTA; Exact sciences and technology; Fischer–Tropsch synthesis; General and physical chemistry; High temperature; Quick XAFS; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2012.02.003

Preparation of Co/SiO2 catalysts by a novel two-step impregnation method using a chelating agent results in better dispersion of the cobalt and retarded agglomeration at high calcination temperatures. [Display omitted] ► FTS activity of Co/CyDTA/SiO2 increased linearly with calcination temperature. ► Size reduction of Co in Co/CyDTA/SiO2 was observed during calcination at 450–520K. ► This size reduction was associated with interaction of CoxOy with chelate complexes. ► CyDTA also enabled to suppress sintering of Co during calcination at 520–720K. ► Formation of Co silicate species was observed in this calcination temperature range. Co/SiO2 catalysts (20mass%) with enhanced activities for Fischer–Tropsch synthesis (FTS) were prepared by a newly developed two-step impregnation method using several chelating agents, and the effects of the calcination temperature and chelating agent on their FTS activities were investigated to clarify the role of the chelating agent during preparation of the catalyst. Their effects upon coordination environments of Co species were also studied by in situ Co K-edge quick XAFS in conjunction with ex situ characterization techniques. The FTS activity of the catalysts increased with the calcination temperature, 473–543K, when the chelating agents with strong affinity to Co2+ such as trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid (CyDTA) were used for preparation. Such increase in the activity was accompanied with size reduction of Co species, while those in Co/SiO2 prepared in the absence of the chelating agents were simply agglomerated during calcination. In situ quick XAFS suggested that size reduction of Co species was associated with specific interaction between small Co oxide clusters and chelate complexes during calcination. It was also speculated that these complexes were converted to surface silicate species after combustion of carbonaceous moieties which would work as anchoring sites of Co oxide clusters, preventing agglomeration of small Co3O4 species originated from the clusters during calcination at high temperatures.