Surface characterization of sulfided precipitated-iron Fischer-Tropsch catalysts by X-ray photoelectron spectroscopy

A series of novel sulfur-containing precipitated-iron catalysts were prepared by the addition of 2000–20000 ppm sulfur (Na2S, (NH4)2S, (NH4)2S5) to iron, during the precipitation reaction. XPS characterization of the highly sulfided catalyst (20000 ppm) after calcination at 200°C and 400°C did not reveal the presence of surface sulfur (160–170 eV region). However, low temperature calcination (200°C) followed by reduction (300°C) revealed surface S2− (162.3 eV), while high temperature calcination (400°C) gave rise to SO42− (169.4 eV) species. Use of a higher reduction temperature (400°C) increased the surface sulfide concentration to 15.6 at% S2− as compared to 1.3 at% after reduction at 300°C. At lower S loadings (2000, 5000 ppm) only sulfate species were observed under analogous conditions. The addition of polysulfide ((NH4)2S5) instead of S2−, resulted in the formation of a surface polysulfide and SO42− surface species. A mechanism for the surface segregation of sulfur after reduction in terms of site competition with oxygen is consistent with the data.