An Auger Electron Spectroscopy Study of the Activation of Iron Fischer-Tropsch Catalysts

Abstract

Activation procedures can have a dramatic effect on the activity of iron-based catalysts for Fischer-Tropsch (F-T) synthesis. CO conversion over a 100 Fe/3 Cu/0.2 K catalyst (parts by weight) can vary by nearly a factor of 3, depending on activation. In contrast, a 100 Fe/5 Cu/4.2 K/25 SiO2 catalyst displays only minor variations in activity with activation conditions. An ultra-high vacuum surface analysis chamber coupled to an atmospheric pressure reactor has been used to measure the surface compositions of these catalysts following various hydrogen activation procedures. Activation of the 100 Fe/3 Cu/0.2 K catalyst in H2 results in rapid reduction of iron to the metallic state, and segregation of sulfur to the catalyst surface. The sulfur arises from bulk sulfate impurities present in the metal nitrates used to prepare the catalyst. Sulfur coverage increases with both activation time and temperature, due to an increase in the rate of sulfur diffusion with temperature. F-T activity of this catalyst varies inversely with sulfur coverage, consistent with the well-known poisoning effect of sulfur on F-T synthesis. For the 100 Fe/5 Cu/4.2 K/25 SiO2 catalyst no significant variations in surface composition are observed as a function of hydrogen activation temperature, consistent with the absence of any variations in catalyst activity. Only partial reduction of iron to a mixture of FexO and Fe3O4 is observed for this catalyst for all activation conditions investigated. Using electron beam effects to remove potassium and silica shows that one or both of these components inhibits reduction of iron to the metallic state in the 100 Fe/5 Cu/4.2 K/25 SiO2 catalyst. The difference in sensitivity of the two catalysts to activation conditions is explained by differences in surface area and reducibility of the two catalysts. Reduction of the 100 Fe/3 Cu/0.2 K catalyst is accompanied by a severe loss of surface area, such that sulfur segregation can result in substantial sulfur coverages on the surface. The 100 Fe/5 Cu/4.2 K/25 SiO2 catalyst maintains high surface area during activation, and not enough sulfur is available to form significant coverages on the surface.