The effect of sulfur addition during the preparation of Co/Zn/TiO2 Fischer-Tropsch catalysts

The effect of sulfur on Co(10%)/Zn(x wt%)TiO2 (x = 0 and 10) catalysts was investigated by TPR, XPS and Fischer-Tropsch (FT) reactor studies. The catalysts were prepared by impregnating different amounts of sulfur on Co(10%)/Zn(x wt%)/TiO2(X = 0 and 10) catalysts, using ammonium sulfide (20% in water) as a sulfur precursor. The effect of changing the order of impregnating cobalt, zinc and the sulfur precursor was also investigated. Based on the XPS results, impregnation of different amounts of sulfur on Co/Zn(x wt%)/TiO2(X = 0 and 10) catalysts indicated that sulfur preferentially reacts with cobalt and zinc instead of titania and blocked the active sites for FT reactions. These catalysts were inactive towards FT synthesis. Impregnating sulfur onto TiO2 prior to Co and Zn addition gave a Co/Zn/S/TiO2 catalyst that showed lower FT activity when compared to the unsulfided Co/Zn/TiO2 catalyst. XPS data revealed that sulfur reacted with titania and zinc as well as cobalt species prior to FT synthesis. A drastic increase in the C-1-C-11 selectivity and a decrease in the C12+ selectivities were observed for the sulfided catalysts (Co/Zn/S/ TiO2 and Co/S/Zn/TiO2). The FT results correlated with the XPS and TPR results. The unsulfided catalysts with or without zinc produced higher molecular weight products (C12+,) and less methane when compared to the sulfided catalysts. A model is proposed that explains the reactions of sulfur with Zn, Co and TiO2 and is consistent with the characterisation and activity results. (C) 2008 Elsevier B.V. All rights reserved.