Adsorption Parameters and Surface Species in the Nickel—Molybdenum Alumina System

Abstract

Isotherms corresponding to the adsorption of ammonium heptamolybdate solutions on nickel–alumina supports have been studied. Such isotherms exhibit two distinct adsorption regions, with both the support and the adsorbed species contributing to their formation. Two types of site are available for adsorption on the support and, depending on the molybdenum concentration present in solution, either monomeric (at low concentrations) or polymeric molybdenum species (at high concentrations) are formed and adsorbed. Monomer adsorption occurs at low concentrations, whereas polyanion adsorption predominates at high concentrations; the latter is reflected in an abrupt increase in molybdenum concentration adsorbed on the support. The adsorption isotherms measured at 20°C and 50°C, and TPR and XPS studies indicate that the samples prepared at 50°C contain a lower quantity of octahedral molybdenum, although dispersion of the molybdenum on the surface is higher. In addition, a direct relationship exists between the dispersion of the active component and the hydrodesulphurization activity of the sample. The presence of nickel on the support does not affect the total number of sites occupied by molybdenum. However, when the nickel content on the solid is increased, the molybdenum species occupy a higher proportion of octahedral sites. XPS studies have shown that samples with a higher nickel content possess a better dispersion of molybdenum species and exhibit a higher intrinsic activity. Enhanced molybdenum dispersion and thiophene conversion was associated with samples in which molybdenum adsorption occurred before nickel impregnation. For all the series studies, the maximum value in the intrinsic acitivity occurred at the same value of the Ni/Mo atomic ratio. This provides convincing evidence that the precursor ratio is important in this catalyst system.