Sulfidation and thiophene hydrodesulfurization activity of nickel tungstensulfide model catalysts, prepared without and with chelating agents

Silica-supported NiWS catalysts with a high activity for thiophene hydrodes ulfurization are obtained when chelating agents such as 1,2-cyclohexanediam ine-N,N,N',N'-tetraacetic acid (CyDTA), ethylenediaminetetraacetic acid, or nitrilotriacetic acid are added in the impregnation stage. X-ray photoelec tron spectroscopy has been used to follow the state of Ni and W during the temperature programmed sulfidation of Ni-W model catalysts prepared with an d without chelating agents on planar SiO2 films on silicon substrates. Full y sulfided catalysts have been tested in thiophene hydrodesulfurization. Th e activity increases with increasing Ni content and reaches a plateau at a Ni:W atomic ratio of 0.66. In NiW catalysts prepared without additives the sulfidation of Ni precedes that of W. However, Ni sulfide formed at low tem peratures changes its structure at high temperatures where WS2 is present, as indicated by the Ni XPS binding energy, which we tentatively attribute t o redispersion of sulfidic Ni over WS2. Chelating agents stabilize Ni again st sulfide formation at low temperature, the effect being strongest when Cy DTA. is applied. CyDTA retards the sulfidation of Ni to temperatures where all W has already been sulfided. This complete reversal in the order in whi ch the two elements convert to sulfides is seen as the key step in preparin g highly active NiWS HDS catalysts. (C) 2000 Academic Press.