Carbon nanotubes formation and its influence on steam reforming of toluene over Ni/Al2O3 catalysts: Roles of catalyst supports

The effects of catalyst supports (γ-Al2O3 and α-Al2O3) on hydrogen yield and carbon nanotubes formation were studied during steam reforming of toluene over Ni-based catalysts. Hydrogen yield and CNTs quality over Ni/γ-Al2O3 were better than those over Ni/α-Al2O3 at S/C ratio of 1 due to the high Ni dispersion of Ni/γ-Al2O3. With the increasing of steam addition, CNTs production remarkably increased over Ni/α-Al2O3, while it decreased over Ni/γ-Al2O3. The CNTs on Ni/γ-Al2O3 were corroded at S/C ratio of 3 by excess steam. Toluene conversion and hydrogen yield over Ni/α-Al2O3 were significantly larger than those over Ni/γ-Al2O3, which was attributed to the different effects of CNTs on catalytic activity. The strong interaction between Ni particle and γ-Al2O3 led to the base-growth mechanism of CNTs on Ni/γ-Al2O3 in which Ni particles located at the bottom of CNTs, thus the growth of CNTs covered Ni active sites and decreased catalytic activity of Ni/γ-Al2O3. By contrast, owing to weak interaction, CNTs grown on Ni/α-Al2O3 followed tip-growth mechanism that would increase Ni dispersion and promote reforming reaction.