(A) study on the characteristics in oxygen storage capacity of $CeO_2-ZrO_2$ mixed oxides and the characteristics of noble-metal loaded $CeO_2-ZrO_2$ catalysts for NO+CO reaction

Abstract

$CeO_2-ZrO_2$ mixed oxide is widely used as oxygen storage material, for example, in the three-way catalysts for the clean up of automotive gases. The environmental regulation to reduce the automotive emissions has become more stringent increasingly in many countries such as USA, EU, and Korea. Recently, near-zero emissions and high durability are of urgent requirement and accordingly, the oxygen storage material ($CeO_2-ZrO_2$ mixed oxide) with advanced thermal stability has been of great interest for catalyst preparation. A number of different synthetic methods have been applied to prepare $CeO_2-ZrO_2$ and related mixed oxides: solid state synthesis, co-precipitation, high energy ball-milling, use of different additives/gelification agents such as hydrazine, oxalic acid, citric acid, sol-gel methods using alkoxide precursors, formation of aerogels by supercritical drying of hydrogels, spray pyrolysis, and co-impregnation method using the support with high surface area such as γ-$Al_2O_3$. Continuous hydrothermal synthesis in supercritical water (supercritical synthesis) is a method to prepare metal oxide nanoparticles rapidly and continuously using supercritical water as anti-solvent. Highly crystallized nanoparticles of homogeneous complex metal oxides as well as single metal oxides could be produced easily by the supercritical synthesis. It would be valuable to compare the physicochemical properties of $CeO_2-ZrO_2$ mixed oxides prepared by the supercritical synthesis and the other conventional methods. In this work, $CeO_2-ZrO_2$ mixed oxides were prepared by the new supercritical synthesis method and also by the conventional co-precipitation method. Physicochemical properties of the $CeO_2-ZrO_2$ mixed oxides were investigated and characterized with TPR, $N_2$ adsorption, $O_2$-uptake, XRD, and SEM. Discussions were made on the differences in the OSC characteristics and thermal stability of $CeO_2-ZrO_2$ mixed oxides between the two preparat...