Anomalous diffusion during isopropanol decomposition on (ZrO 2) 1− x (CeO 2) x catalysts

Abstract

In this work the effect of composition of (ZrO 2) 1− x (CeO 2) x catalysts on the ratio γ = E c/ E d between the activation energies for the isopropanol decomposition, estimated in the chemical reaction ( E c) and the diffusion ( E d) controlled regimes, was studied. The solid catalysts with x = 0.0; 0.2; 0.5; 0.8 and 1.0 were prepared by heating the precursors at 400, 600 and 800 °C and characterized by N 2 porosimetry, X-ray diffraction, SEM/EDS analysis and NH 3/TPD. Their catalytic activity for the decomposition of isopropanol in the temperature range 200–400 °C was probed in a tubular flow reactor. The estimated Arrhenius plots showed a distinct and well-defined change of slopes corresponding to alternation from the chemical to the internal diffusion-controlled regime of the process. The ratio γ = E c/ E d of the corresponding activation energies was found equal to γ ≈ 1 for x = 0.0, meaning total lack of diffusion limitations, and converges to γ ≈ 2 for x = 1.0, corresponding to classical internal diffusion control. For 0 < x < 1 the values of 1 < γ < 2 signify anomalous diffusion related linearly to some compositional property of the (ZrO 2) 1− x (CeO 2) x catalysts. The origin of this phenomenon is discussed in relation to the variable binding strength and friction of movement of the isopropanol species on the more active ZrO 2 and the less active CeO 2 nanoparticles of the solid catalysts.