Deactivation of nickel catalysts by sulfur and carbon for the pyrolysis-catalytic gasification/reforming of waste tires for hydrogen production

Hydrogen production from the two-stage pyrolysis-gasification/reforming of waste tires has been investigated using Ni/Al2O3 and Ni/dolomite catalysts in the relation to four cycles of use. The Ni/dolomite catalyst produced a higher hydrogen yield and the highest theoretical hydrogen potential to produce hydrogen gas compared to the Ni/Al2O3 catalyst. In addition, the used Ni/dolomite catalyst had the lower carbon deposition, as determined by temperature-programmed oxidation, being 18.2 wt % for the Ni/Al2O3 catalyst and 2.8 wt % for the Ni/dolomite catalyst. Detailed analysis of the reacted catalysts using transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDXS) was used to characterize the presence of nickel, sulfur, and carbon on the used catalysts. The results showed that, for the Ni/Al2O3 catalyst, sulfur was mainly present on the surface of the nickel particles. However, for the used Ni/dolomite catalyst, no sulfur peaks were detected whether in the bulk metal or on the surface of the nickel particle. In addition, carbon deposition was closely associated with the nickel particles of the used Ni/Al2O3 catalyst but not for the used Ni/dolomite catalyst. The results suggest that nickel of the Ni/Al2O3 catalyst becomes deactivated because of sulfur reaction and carbon deposition, preventing reaction of the various reactant species generated from the pyrolysis of the tires. However, using dolomite as a support, the catalytic deactivation process was less pronounced and there was negligible deactivation of nickel by sulphur.