II. Electrical Properties of Ni/Silica Gel and Pt/γ-Alumina Catalysts in Relation to Catalytic Activity

Abstract

The catalytic activity of silica gel and silica gel—supported nickel (2, 5, and 8 wt% Ni) and γ-alumina—supported platinum (0.3 and 0.6 wt% Pt) was studied for n-hexane or n-pentane cracking and cyclohexane dehydrogenation adopting pulse technique. The results showed that Pt/γ-alumina could be a selective catalyst for cyclohexane dehydrogenation reaction. Ni/silica gel catalyst was a good cracking catalyst, the activity of which increased by increasing the metal content up to 8% Ni. For cyclohexane, dehydrogenation over Ni/silica gel, the activity increased by increasing the metal loading. N2 sorption characterizations showed that both silica gel- and γ-alumina—supported catalyst samples exhibited mesoporous structures. Silica gel and silica gel—supported samples have ink-bottle type pores while γ-alumina and γ-alumina—supported platinum samples have platelike pores. BET surface area and total pore volume decreased by increasing metal loading for both catalyst samples. Electrical properties of pure γ-alumina and silica gel supports were functions of metal content and frequency of the applied field. The increase of conductivity is considered a good indicator of the decrease in the activation energy (increase of catalytic activity). The observed increase in conductivity with the increase of metal loading may be due to increase in the mobility of the free charge carriers (i.e., ions and free radicals taking part in the mechanism of catalytic conversion of hydrocarbons over the catalyst polarized active centers that increased by increasing metal loading). These charge carriers diffuse in the bulk of pores and reach electrodes where they discharge, giving rise to diffusion impedance (Warburg impedance).