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

In this work the effect of composition of (ZrO(2))(1-x) (CeO(2))(x) catalysts on the ratio g = 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 g = E(c)/E(d) of the corresponding activation energies was found equal to g [asymptotic to] 1 for x = 0.0, meaning total lack of diffusion limitations, and converges to g [asymptotic to] 2 for x = 1.0, corresponding to classical internal diffusion control. For 0 < x < 1 the values of 1 < g < 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.