MECHANISM OF ADSORPTION OF CO-2+ AND NI-2+ IONS ON THE PURE AND FLUORINATED GAMMA-ALUMINA/ELECTROLYTE SOLUTION INTERFACE

The mechanism of adsorption, from aqueous suspensions, of Co2+ and Ni2+ ions on the surface of gamma-alumina has been studied using potentiometric titrations, microelectrophoresis, and adsorption experiments. Correlation of the Co2+ or Ni2+ uptake corresponding to surface saturation, as obtained from the adsorption isotherms, with the concentration of the deprotonated and neutral surface hydroxyls of gamma-alumina, regulated by doping the support with various amounts of F- ions or by changing the pH of the impregnating solution, demonstrated that the deprotonated surface hydroxyls are mainly responsible for the creation of the adsorption sites. The combined use of microelectrophoresis and potentiometric titrations as well as the mathematical analysis of the adsorption isotherms showed that the adsorbed Co2+ and Ni2+ ions are located at the inner Helmholtz plane (IHP) of the double layer developed between the surface of the gamma-alumina particles and the impregnating solution. This analysis of the adsorption isotherms led also to the conclusion that the adsorbed Co2+ and Ni2+ ions are located at energetically equivalent sites of the IHP and that between the adsorbed ions relatively weak lateral interactions exist, provided that the density of the adsorption sites is relatively high. Moreover, it was found that fluorination increased the extent of adsorption, though it decreased the values of the adsorption constant and consequently the "adsorbed ions-support" interactions. Finally, it was found that the extent of adsorption of Co2+ and Ni2+ ions also increased by increasing the pH of the impregnating solution, but this method was less effective as compared to the fluorination for obtaining Co or Ni supported gamma-alumina catalysts with high Co or Ni surface.