Promotional effects of Cu and K on precipitated iron-based catalysts for Fischer–Tropsch synthesis

Abstract

The effects of Cu and K promoters on precipitated iron-based Fischer–Tropsch synthesis (FTS) catalysts were investigated by using N 2 physical adsorption, temperature-programmed reduction/desorption (TPR/TPD) and Mössbauer effect spectroscopy (MES). The FTS performances of the catalysts were tested in a slurry-phase continuously stirred tank reactor (CSTR). The characterization results indicated that Cu promoter facilitates the high dispersion of Fe 2O 3, significantly promotes the reduction and H 2 adsorption, but severely suppresses CO adsorption and the carburization. However, K promoter severely retards the reduction and suppresses the H 2 adsorption, facilitates the CO adsorption and promotes the carburization. In the FTS reaction, it was found that Cu promoter decreases the FTS initial activity and water gas shift (WGS) reaction activity, promotes the oxidation of iron carbides to Fe 3O 4 and accelerates the deactivation of iron-based catalyst. However, K promoter improves the FTS activity and WGS reaction activity, suppresses the oxidation of iron carbide to Fe 3O 4 and significantly improves the stability of iron-based catalyst. As compared with individual promotion of Cu or K, the double promotions of Cu and K significantly improve the FTS and WGS activities and keep excellent stability. Due to weaker CO adsorption and stronger H 2 adsorption than the catalysts without Cu, Cu promoted catalysts have higher selectivity to light hydrocarbons and methane and lower selectivity to heavy hydrocarbons. However, the opposite result is obtained on the catalyst incorporated with K promoter.