Impact of Cr, Mn and Zr addition on Fe Fischer–Tropsch synthesis catalysis: Investigation at the active site level using SSITKA

Recently, we reported that Cr, Mn, or Zr promotion of a precipitated Cu-promoted Fe Fischer–Tropsch synthesis (FTS) catalyst significantly improves its catalytic activity. In this study, we used steady-state isotopic transient kinetic analysis (SSITKA) of methanation to investigate the activity of these catalysts at the site level, providing insight into how this promotion enhances catalyst activity. We found that the activity of the base Fe catalyst was enhanced by promotion with Cr, Mn, and Zr in varying degrees, depending on the type and concentration of metal added. The addition of these third metals promoted the dispersion of Fe but did not significantly affect either the BET surface area or reducibility. The activities of the reaction sites (estimated by 1/τCH4=TOFITK1/τCH4=TOFITK) were found to be similar regardless of the type of third metal added, suggesting that the active sites for Fe catalysts with or without third-metal promotion were essentially identical. The higher catalyst activities observed for the Cr-, Mn-, and Zr-promoted Fe catalysts were found to be due primarily to an increased number of active surface intermediates leading to hydrocarbon products.