Electrical properties of Ni/silica gel and Pt/γ-alumina catalysts in relation to metal content in the frequency domain

AC electrical conductivity analysis for silica gel and silica gel-supported nickel (via 2, 5 and 8 wt.% Ni) and γ-alumina and γ-alumina-supported platinum (viz., 0.3 and 0.6 wt.% Pt) is carried out. Measurements were carried out in the frequency range from 0.1 Hz up to 100 kHz at room temperature (20 °C) and 1 V for estimating the effect of increasing Ni or Pt on its supports namely silica gel and γ-alumina. Electrical properties of such mesoporous materials reflect directly its ion exchange and interfacial catalytic activity. Assuming that Ni or Pt particles are optimally dispersed over the surface of its supports, this may give rise to an increase in the conductivity and dielectric constant of the studied system. Electrical properties of pure γ-Al2O3 and silica gel supports depend on metal content and frequency of the applied field. The observed increase in conductivity with the increase in metal loadings may be due to the increase in the mobility of the free charge carriers. These charge carriers diffuse in the bulk of pores and reach electrodes and discharge there giving rise to diffusion impedance (Warburg impedance). An equivalent circuit is deduced to describe the electrical behavior of the systems studied. It constitutes elements forming three mechanisms: ion exchange at interfaces, diffusion in the pores and high frequency effect (beyond the scope of this work). The study evidenced that the increase of metal content has a great effect on the electrical properties of the studied system.