CeO2-ZrO2 binary oxides for NOx removal by sorption

Removal of NOx (NO in the presence of O-2) by CeO2-ZrO2 binary oxides was investigated. CeO2-ZrO2 prepared by a modified sol-gel method [CZ(SG)] showed high NOx sorption capacity, while CeO2-ZrO2 prepared by a coprecipitation method [CZ(CP)] was not effective. In both cases, NOx 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 NOx is adsorbed as several types of nitrates. X-ray diffraction measurements revealed the formation of a complete solid solution, Ce0.5Zr0.5O2, 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 NOx sorption capacity of CZ(SG). The high NOx 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 NOx removal process.