Preparation and 31P NMR characterization of nickel phosphides on silica

We prepared Ni2P by reduction of an oxidic precursor consisting of nickel oxides and phosphates (P/Ni ratio=0.5) in a flow of H2 upon heating to 823 K. SiO2-supported Ni2P was prepared by reducing a supported oxidic precursor (P/Ni ratio=0.65) in a flow of 5% H2/N2 upon heating to 1023 K. Supported precursors with a P-to-Ni ratio lower than 0.65 yielded phosphides with a lower P content, such as Ni3P and Ni12P5. Furthermore, the flow rate of the reducing agent has a strong effect on which phosphide forms. From temperature-programmed reduction measurements, we concluded that the reduction starts with the formation of Ni metal from NiO at around 600 K. Phosphates are reduced at higher temperatures to volatile P compounds that react with the Ni to Ni2P, with Ni3P and Ni12P5 as possible intermediates. The formed products were characterized by powder X-ray diffractometry and 31P MAS NMR spectroscopy. NMR spectroscopy is a powerful tool for identifying the different Ni phosphides on the support. The metallic character of Ni3P, Ni12P5, and Ni2P classifies their large 31P NMR shifts as Knight shifts, which enables them to be distinguished from those of diamagnetic phosphates.