Oxidation State and Dielectric Properties of Ceria-Based Catalysts by Complementary Microwave Cavity Perturbation and X-Ray Absorption Spectroscopy Measurements

Abstract

The three-way catalytic converter (TWC) has become an integral part of the modern exhaust gas aftertreatment for gasoline engines. Some years ago, radio frequency technology has been successfully tested for monitoring and controlling ceria based TWC oxygen storage by contactless measuring the dielectric catalyst properties inside a cavity resonator. Applying the cavity perturbation method, we present results on CeO2, the oxygen storage component of three-way catalysts, and on differently prepared Pt–ceria model catalysts and combine it with in situ X-ray absorption spectroscopic (XAS) measurements conducted at the Ce L3-edge. The dielectric properties of the oxidized and reduced Pt–ceria and pure ceria powders were determined at 1.2 GHz between 250 and 550 °C. As the experiments show, the reduction of the material changes both, the polarization and the occurring dielectric losses. The complementary in situ XAS measurements, which allow monitoring variations in the redox state of ceria, gave a precise evaluation of the ceria reduction and oxidation. In addition, a substantial improvement of the low-temperature reducibility of ceria in the presence of Pt was demonstrated by both measurement techniques, in agreement with earlier studies. Furthermore, time resolved XAS data obtained during reducing/oxidizing cycles at 250 °C and 350 °C unraveled the faster and more pronounced reduction of ceria at higher temperature. Both methods proved to be valuable methods to provide insight into the oxygen storage and release process of ceria based materials, and capable to discriminate the impact of the noble metal or the ceria surface area in great detail, even under dynamic conditions, which is highly relevant for studying the function of state-of-the art three-way catalysis and giving input for OSC modeling studies.