Application of in situ Mössbauer spectroscopy to investigate the effect of precipitating agents on precipitated iron Fischer–Tropsch catalysts

Abstract

The type of the precipitating agent used during the preparation of a precipitated iron-based Fischer–Tropsch (FT) catalyst affects the catalyst pore structure, crystallite size, phase composition and catalytic behavior. Catalysts prepared by using precipitating agents, that contain carbonate ions, have pores that are larger than those of catalysts prepared using precipitating agents that contain hydroxides. Precipitation at pH>8, using aqueous NH 3 solution as a precipitating agent, results in the formation of large crystallites of FeOOH, which are not observed when Na 2CO 3 and K 2CO 3 are used. Higher % CO conversion during FT synthesis was observed with the catalyst prepared by using aqueous NH 3 solution. However, this is correlated with a low selectivity for the formation of olefins. For all catalysts, in situ Mössbauer spectra recorded during FT synthesis show that the % CO conversion increases with the formation of iron carbides, viz. ε′-Fe 2.2C and χ-Fe 2.5C.