In Situ XRD, XAS, and Magnetic Susceptibility Study of the Reduction of Ammonium Nickel Phosphate NiNH4PO4·H2O into Nickel Phosphide

Abstract

The reduction of the ammonium nickel phosphate NiNH₄PO₄·H₂O precursor into nickel phosphide (Ni₂P), a highly active phase in hydrotreating catalysis, was studied using a combination of magnetic susceptibility and in situ X-ray diffraction and X-ray absorption spectroscopy (XAS) techniques. The transformation of NiNH₄PO₄·H₂O into Ni₂P could be divided into three distinguishable zones: (1) from room temperature to 250 °C, the NiNH₄PO₄·H₂O structure was essentially retained; (2) from 300 to 500 °C, only an amorphous phase was observed; (3) above 500 °C, a crystallization process occurred with the formation of Ni₂P. An in situ XAS study and magnetic susceptibility measurements clearly revealed for the first time that the amorphous region corresponds to the nickel pyrophosphate phase α-Ni₂P₂O₇. The phosphate reduction into phosphide did not start before 550 °C and led to the selective formation of Ni₂P at 650 °C.