Effect of the type of precursor and the synthesis method on thiophene hydrodesulfurization activity of activated carbon supported Fe-Mo, Co-Mo and Ni-Mo carbides

Abstract

In this work it is studied the effect of the type of precursor (sulfate vs. nitrate of promotor) and the synthesis method (conventional vs. carbothermal carbiding) on the thiophene hydrodesulfurization (HDS) activity of activated carbon supported Fe-Mo, Co-Mo and Ni-Mo carbides. Catalytic precursors were prepared by co-impregnation of the support with solutions of ammonium heptamolybdate and the promotor salt. Conventional carbiding consists of a temperature-programmed treatment under a CH 4/H 2 (1:4) atmosphere, while the carbothermal method employs pure H 2. The passivated carbided solids were characterized by XRD and XPS. The presence of metals and Fe 3Mo 3C or M 6Mo 6C 2 phases (M = Co or Ni) in samples from sulfate precursors and of β-Mo 2C and metals in those from nitrate precursors was verified by XRD, whereas XPS showed the presence at the surface of Mo δ+ (0 ≤ δ ≤ 2), Mo 4+, Mo 6+, Fe 3+, Co 2+, Ni 0 and Ni 2+ species. The bimetallic carbides obtained from sulfate precursors retained sulfur on the surface, as shown by signals in the S 2p region, at 169 and 162 eV, assigned to S 2− and SO 4 2−, respectively. Prior to the catalytic reaction, the passivated carburized forms of carbon supported catalysts were presulfided in situ. The catalytic activity of carbides was strongly influenced by the type of precursor and slightly by the synthesis method. The activity of carbides obtained by the carbothermal method and with sulfate precursors was greater than that of the solids obtained by the conventional method and with nitrate precursors. The carbides obtained from nitrates showed increased catalytic activity when presulfided, suggesting that the carbides with sulfided surfaces or mixed carbo-sulfide species could be the active phase in HDS on carbide catalysts. The catalysts derived from sulfates showed higher activity than those obtained from nitrates, even without presulfiding, suggesting that sulfide from the precursor sulfate results in more active species than those obtained after presulfiding.