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

Abstract

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.