Oxidation and Reduction Reaction Kinetics of Mixed Cerium Zirconium Oxides

The oxidation and reduction of mixed cerium zirconium oxides, Ce_1-xZr_xO₂, with x varied in the range 0-0.3 in steps of 0.05, in a low-pressure oxygen atmosphere were experimentally investigated. The experiments utilized a novel method developed by the authors, in which the samples are reduced and oxidized in a sealed vacuum chamber by irradiating them with a xenon arc lamp. The changes in pressure due to the release or consumption of oxygen are used to quantify the extent and rates of the reactions. The system can achieve heating rates in excess of 100 °C s^-1, which is hundreds of times faster than is possible with a thermobalance. In addition, the gas phase transport is very rapid as it is driven by pressure gradients. The combined high heating rate and fast gas phase transport offers unique conditions in which to measure the actual kinetics of the reactions. The oxidation kinetics showed a strong dependence on the amount of Zr⁴⁺ in the samples, where the rate of oxidation decreased with increasing Zr⁴⁺ concentration. The effect of adding 2.5% Sm³⁺ or La³⁺ to the Ce_0.85Zr_0.15O₂ was also investigated with the hope of improving the kinetics. The addition of La³⁺ to Ce_0.85Zr_0.15O₂ was seen to offer an increase in the reaction rates, although the oxidation kinetics were still slower than that of pure CeO₂. Finally, the reduction kinetic data are presented, but even with the optimal conditions the reduction reaction still appeared to be limited by the heating rate; thus, it was concluded that the reduction kinetics were too rapid to quantify by this method.