Effects of support on hydrodenitrogenation activity of nickel phosphide catalysts

The effects of the support on active site formation and hydrodenitrogenation(HDN) activity of nickel phosphide catalysts were examined, using TiO₂, Al₂O₃ and TiO₂-Al₂O₃, composite supports. A series of TiO₂-Al₂O₃ prepared by hydrolysis and deposition of tetrabutyl titanate on macropore Al₂O₃, and the supported nickel phosphide catalyst, were prepared by incipient wetness impregnation and in situ H₂ reduction method. The samples were characterized by X-ray diffraction(XRD), BET surface area, transmission electron microscopy(TEM) and H₂ temperature-programmed reduction(H₂-TPR).Their hydrodenitrogenation(HDN) performance were evaluated on a continuous-flow fixed-bed reactor by using quinoline as the model molecules. The results showed that the TiO₂-Al₂O₃ composite support still retained the pore properties of macropore Al₂O₃, and TiO₂ were well dispersed on the Al₂O₃ surface in the form of anatase. Different supports have great influence on the reduction behaviour of the oxidic precusors and HDN activity of phosphide catalysts. The main active phase after reduction was Ni₂P phase for the TiO₂ and TiO₂-Al₂O₃ supportd catalyst, but only Ni₁₂P₅ appeared for the Al₂O₃ supported catalyst. The order of HDN activities of nickel phosphide reduced at optimal reaction conditions was TiO₂-Al₂O₃ > Al₂O₃> TiO₂. TiO₂-Al₂O₃ supported catalyst with the Ti /Al atomic ratio of 1:8 exhibited the highest HDN activity among all catalysts. The presence of TiO₂ weakened the strong interaction between the Al₂O₃ and phosphate, and contributed to the formation of Ni₂P active phase and the improvement of HDN activity.