Surface characterisation of nickel boride and nickel phosphide catalysts by X-ray photoelectron spectroscopy
The surface characterisation of nickel boride (NiB) and nickel phosphide (NiP) catalysts has been undertaken with X.p.s. Boron combined with nickel metal in NiB catalysts was found to donate electrons to the nickel metal, resulting in electron rich metal, whereas phosphorus bonded to nickel metal in NiP catalysts accepted electrons from the nickel metal, causing electron deficient metal. Preparation methods (different solvent, precursor salt and temperature) affected the surface concentrations of boron and phosphorus bonded to the nickel metal, surface areas and specific hydrogenation activities of the catalysts. The specific activities for hydrogenation reactions increased with increasing surface boron content in NiB catalysts, whereas they decreased with increasing surface phosphorus content in NiP catalysts. These positive and negative promoting effects of boron and phosphorus are explained in terms of the electron density of the nickel metal. Co-, Fe-, Pd- and PtB catalysts were also investigated for comparison.