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)(2)S, (NH4)(2)S-5) to iron, during the precipitation reaction. XPS characterization of the highly sulfided catalyst (20000 ppm) after calcination at 200 degrees C and 400 degrees C did not reveal the presence of surface sulfur (160-170 eV region). However, low temperature calcination (200 degrees C) followed by reduction (300 degrees C) revealed surface S2- (162.3 eV), while high temperature calcination (400 degrees C) gave rise to SO42- (169.4 eV) species. Use of a higher reduction temperature (400 degrees C) increased the surface sulfide concentration to 15.6 at% S2- as compared to 1.3 at% after reduction at 300 degrees C. At lower S loadings (2000, 5000 ppm) only sulfate species were observed under analogous conditions. The addition of polysulfide ((NH4)(2)S-5) 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. (C) 1997 Elsevier Science B.V.