PHOSPHORUS INDUCED FORMATION OF A NIWO4 PHASE IN THE OXIDE FORM OF P-NI-W/AL2O3 CATALYSTS

The effect of the preparation method and of phosphorus concentration on the dispersion, the distribution and on the type of the surface species formed by the active components in a series of P-Ni-W/Al2O3 samples with varying phosphorus content was studied by X-ray diffraction, infrared, diffuse reflectance and X-ray photoelectron spectroscopies. The formation of a AlPO4 species is observed in all phosphorus-containing samples. The distribution of phosphorus in the co-impregnated P-Ni-W/Al2O3 system is in the form of a monolayer up to concentrations of 1 P at/nm2. The distribution of nickel in the co-impregnated samples is inhomogeneous and is accompanied by the formation of NiO crystallites. The state of nickel in the samples is strongly influenced by the presence of phosphorus. The increase of phosphorus concentration in the co-impregnated P-Ni-W/Al2O3 samples stimulates the formation of a NiWO4 phase. The distribution and the dispersion of tungsten in phosphorus-containing samples is inhomogeneous and WO3 and NiWO4 phases are observed depending on phosphorus concentration and on the presence of nickel in the samples.