Compensation behaviour in the hydrogenation of benzene, toluene and o-xylene over Ni/SiO2. Determination of true activation energies

Abstract

The vapour phase hydrogenation of benzene, toluene and o-xylene has been studied over an Ni/SiO₂ catalyst prepared by homogeneous precipitation/deposition. The hydrogenations were carried out in the absence of diffusion limitations, catalyst deactivation or secondary conversions. Turnover frequencies and reaction probabilities decrease in the order benzene > toluene > o-xylene. The appearance of well defined reversible maxima (T_max) in the rate vs. temperature plots is reported and discussed. The reaction orders with respect to the aromatic and hydrogen partial pressures are plotted as a function of temperature. The effect of temperature on the effective surface collisions is also considered. The temperature dependence of the rate constants was fitted to Arrhenius equations and generated positive (T⩽T_max) and negative (TT_max) activation energies. The derivation of true activation energies and heats of adsorption from the kinetic data is presented. The turnover frequencies and apparent activation energies obtained in this study are compared to similar and different systems reported in the literature. A compensation effect is established for the experimentally determined or apparent kinetic parameters. The effect is attributed to variations in the temperature dependences of the surface concentration of the reactive aromatic species.