Preparation of Ni2P with a Surface Nickel Phosphosulfide Layer by Reduction of Mixtures of Na4P2S6 and NiCl2

Abstract

A series of physical mixtures of Na₄P₂S₆ and NiCl₂ (P-NiPS(x), where x represents the P/Ni molar ratio) were employed for the preparation of Ni₂P. For comparison, a sulfur-containing Ni₂P catalyst (Ni₂P-S) and a sulfur-free Ni₂P catalyst (Ni₂P-TPR) were prepared by reduction of Ni₂P₂S₆ and a nickel phosphate precursor, respectively. The reduction of the P-NiPS(x) precursors with P/Ni ratios above 2/3 yielded Ni₂P catalysts with a distinct nickel phosphosulfide layer (NiPS(x)), and the Ni₂P phase started to form at ca. 200 °C. The reduction of Ni₂P₂S₆ to Ni₂P most likely follows a disproportionation mechanism. The P³⁺ species in Ni₂P₂S₆ disproportionate to PH₃ and P⁵⁺ during the reduction, and PH₃ further reacts with nickel and sulfur species to form Ni₂P and the surface nickel phosphosulfide layer. The sulfur atoms in the nickel phosphosulfide phase were in the form of S²⁻. The introduction of sulfur to Ni₂P favored the hydrogenation pathway of the hydrodesulfurization (HDS) of dibenzothiophene (DBT), but hardly affected the direct desulfurization (DDS) pathway and inhibited the hydrogenation of biphenyl. The DDS pathway rate constants of DBT HDS over the Ni₂P-TPR and NiPS(x) catalysts were observed to increase linearly with the increase in their surface Ni atomic concentrations.