Ni-doped γ-Al2O3 as secondary catalyst for bio-syngas purification: influence of Ni loading, catalyst preparation and gas composition on catalytic activity

In this work, Ni/γ-Al2O3 catalysts were prepared by sol-gel methods with different Ni loadings (10 to 50 wt.%) and used as secondary catalyst for the steam reforming of toluene. A sample prepared by wet impregnation with 10 wt.% of Ni was also synthesized and compared to the corresponding sol-gel sample. This study was divided in three main parts: the comparison of catalysts prepared by sol-gel process and impregnation, the influence of the gas composition on the catalytic performance of the sol-gel 10 wt.% Ni/γ-Al2O3 catalyst and the influence of Ni loading on the catalytic activity.
When sol-gel and impregnated samples are compared, the impregnated catalyst showed a high initial toluene conversion followed by a consequent and progressive deactivation. Contrarily, the sol-gel catalyst showed a stable catalytic activity and relatively low carbon deposit. Indeed, before the steam reforming of toluene at 650 °C, the sol-gel catalyst was only calcined and no reduction step was realized to reduce nickel oxide. So this sample was reduced during the catalytic test at 650 °C. Moreover, it was observed that, if toluene was withdrawn from the syngas mixture during the catalytic test, the sol-gel sample was progressively re-oxidized by CO2 and H2O, leading to higher deactivation.
As the Ni loading increased, the nickel oxide with strong interactions (NiAl2O4) was progressively joined by nickel oxide with low interactions (NiO/Al2O3) and bulk nickel oxide (NiO). This led to a high initial conversion of toluene, but also to a progressive loss of the catalytic activity throughout the catalytic test. It was shown that the sol-gel method developed throughout this work allowed preparing micro/mesoporous Ni/γ-Al2O3 catalysts with a high dispersion of Ni nanoparticles.