Influence of pH of the impregnation solution on the phosphorus promotion in W/Al2O3 hydrotreating catalysts

W/Al2O3 and WP/Al2O3 catalysts, prepared by co-impregnation at different pH values (1.5, 7 and 9), were characterised by several techniques (XRD, TPR, Raman spectroscopy, SEM-EDX. XPS and HRTEM) and tested in the parallel hydrodesulfurization (HDS) of gas oil and hydrodenitrogenation (HDN) of pyridine. The results show that P addition, whichever the pH of the impregnation solution is, has a positive effect on HDS activity and, to a minor extent, on HDN activity. The promoter effect of P for gas oil HDS was clearly influenced by the impregnation pH, which increased substantially by decreasing the impregnation pH. This improvement in HDS activity is interpreted as a result of an increased homogeneous distribution of tungsten on alumina, formation of more polytungsten oxide species and incipient WO3 crystallites. and slightly greater sulfidation of the oxo-tungsten species, as well as a slight increase in WS2-like stacking. By contrast. the P promotion for pyridine HDN was not significantly affected by the impregnation pH. indicating that HDN activity is less sensitive to the changes in the W distribution than the HDS activity. The possible role of P in the catalytic properties of W/Al2O3 catalysts is discussed. (C) 2002 Elsevier Science B.V. All rights reserved.