Plasma Synthesis of Ni₂P from Mixtures of NiCl₂ and Hypophosphites

A series of mixtures of NiCl₂, hypophosphites (NaH₂PO₂ and NH₄H₂PO₂), and additives were used as precursors (denoted as x additive:y hypophosphite:z NiCl₂, where x, y, and z represents the molar ratios of the compounds) for the synthesis of Ni₂P both in N₂ and H₂ plasma. The catalytic performances of the obtained catalysts were evaluated by the hydrodenitrogenation (HDN) of quinoline (Q) and decahydroquinoline (DHQ). Ni₂P was stoichiometrically synthesized by the treatment of 1.0NH₄H₂PO₂:2.0NiCl₂ with N₂ plasma at a low total power input of 8 W (40 V × 0.2 A). In H₂ plasma, the precursor was over-reduced to a mixture of Ni and Ni₂P, with Ni₂P as the minor phase. The over-reduction can be inhibited by the addition of proper amount of alkali-metal chloride (NaCl or KCl) or P₂O₅. CaCl₂ facilitated the reduction of nickel salts to Ni. KCl was the best among the additives investigated. Both the surface and bulk over-reduction of 1.0NH₄H₂PO₂:2.0NiCl₂ was inhibited by KCl, while P₂O₅ mainly inhibited the bulk over-reduction. The addition of KCl strongly suppressed the hydrogenation of Q to DHQ, but facilitated the HDN of DHQ. On the other hand, not only the hydrogenation of Q but also the dehydrogenation of DHQ was substantially inhibited by P₂O₅.