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 H-2 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% H-2/N-2 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 P-31 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 P-31 NMR shifts as Knight shifts, which enables them to be distinguished from those of diamagnetic phosphates. (C) 2002 Elsevier Science (USA).