CeO2–ZrO2 binary oxides for NOx removal by sorption

Abstract

Removal of NO_x (NO in the presence of O₂) by CeO₂–ZrO₂ binary oxides was investigated. CeO₂–ZrO₂ prepared by a modified sol–gel method [CZ(SG)] showed high NO_x sorption capacity, while CeO₂–ZrO₂ prepared by a coprecipitation method [CZ(CP)] was not effective. In both cases, NO_x was found to be removed by adsorption on the surface but not by absorption into the bulk. In situ diffuse reflectance Fourier transform infrared measurements demonstrated that NO_x is adsorbed as several types of nitrates. X-ray diffraction measurements revealed the formation of a complete solid solution, Ce_0.5Zr_0.5O₂, for CZ(SG) and the presence of separate phases of CeO₂ and ZrO₂ for CZ(CP). Homogeneous mixing of Ce ions and Zr ions in the solid solution was considered to be one of the important factors for the high NO_x sorption capacity of CZ(SG). The high NO_x sorption capacity was also accounted for by the presence of a large amount of basic sites and high catalytic activity for NO oxidation to NO₂, which is the first step in the NO_x removal process.