EFFECT OF PHOSPHORUS CONCENTRATION AND METHOD OF PREPARATION ON THE STRUCTURE OF THE OXIDE FORM OF PHOSPHORUS-NICKEL-TUNGSTEN ALUMINA HYDROTREATING CATALYSTS/

Two series of phosphorus containing Ni-W/Al2O3 catalysts were prepared by different preparation methods and varying phosphorus content from 0 to 7.6 wt.% P2O5 The influence of the phosphorus concentration and t he preparation procedure on the structure and the dispersion of the co mpounds formed in the oxide form of the catalysts was studied. Diffuse reflectance spectroscopy (DRS), infrared spectroscopy (LR) and X-ray photoelectron spectroscopy (XPS) were used for the characterization of the catalysts. It was demonstrated that introduction of phosphorus in NiW/Al2O3 catalysts impedes the formation of NiAl2O4 and increases th e amount of Ni2+(Oh) ions in the oxide form of the samples. This effec t of phosphorus is better expressed when phosphorus is first introduce d to alumina, followed by co-impregnation with nickel and tungsten, Su rface AlPO4 is formed in the phosphorus containing samples irrespectiv e of the preparation procedure. Phosphorus is distributed as a monolay er up to concentrations of 1.3 P at/nm(2) in both series of catalysts. It was also found that the increased phosphorus content in the sample s leads to an increased degree of polymerization of the tungsten speci es via W-O-W bonds. The presence of phosphorus changes the dispersion of the active components. Evaluation of the average particle size of t ungsten species shows that it increases from 10 Angstrom for the phosp horus free sample up to 20-25 Angstrom for the samples with high phosp horus content. The relationship between the structure of the phosphoru s promoted NiW/Al2O3 catalysts and their hydrodesulfurization activity is discussed. (C) 1997 Elsevier Science B.V.