Interaction of Ni/SiO2 with thiophene

The interaction between Ni/SiO2 (18 wt%) and thiophene was studied by thermal gravimetric analysis (TGA) and in a fixed-bed reactor. The samples were prepared by incipient wetness impregnation and reaction was realized in hydrogen flow at 280-360 degrees C in the presence of 10-40 mbar of thiophene. It is found that sulfidation of Ni is not complete in TGA experiments and the maximum achievable transformation degree depends not only on the temperature and thiophene pressure, but also on the sample weight and the gas flow rate. The maximum transformation degree is shown to vary reversibly when conditions are changed. Analysis of the reaction products during sulfidation in a fixed-bed reactor showed that H2S is present in the gas phase from the beginning of the reaction and that catalytic activity of the solid decreases strongly when going from Ni-0 to Ni3S2. All these observations suggest that, except at the surface, the interaction between thiophene and Ni/SiO2 is not direct: thiophene is first desulfurized and then H2S reacts with Ni. As Ni sulfidation progresses the amount of produced H2S decreases (because of a lowering catalytic activity of the solid) and after certain time H2S partial pressure can drop to the equilibrium value for Ni/Ni3S2 system. In such conditions Ni sulfidation cannot be completed and the system achieves a steady state corresponding to the maximum transformation degree observed in TGA experiments. The present study shows that due to involvement of a catalytic reaction, the stability of metallic particles in the presence of complex sulfur-containing molecules can be determined by the desulfurization activity of the metal and operation conditions rather than by the thermodynamics of the metal-sulfur system. (C) 2008 Elsevier B.V. All rights reserved.