Solvent-deficient synthesis of cerium oxide: Characterization and kinetics

Abstract

The reaction mechanism and kinetics of CeO2 synthesis using a solvent-deficient method are investigated by simultaneous thermogravimetric analysis (TGA)/differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The decomposition process of the cerium(III) nitrate hexahydrate and ammonium bicarbonate precursor mixture with four observed stages is monitored using TGA/DSC measurements in a nonisothermal regime with heating rates of 5, 10, 15 and 20 °C min−1. The proposed mechanism indicates a complex synthesis with several parallel reactions, some of which occur at room temperature. A detailed kinetic analysis is performed using isoconversional (expanded Friedman, modified Coats-Redfern and Kissinger) and model fitting (Nth order and nucleation and growth models) methods. The first three stages are best described by the Nth order model with activation energy values of 21, 53 and 90 kJ mol−1. The last stage, during which ammonium nitrate decomposition occurs, is best fit by the nucleation and growth model and has an activation energy of 129 kJ mol−1. The proposed mechanism, supported by the kinetic analysis in our study, indicates that CeO2 has already formed before the reaction reaches 200 °C. The average crystallite size of CeO2 synthesized at 300 °C, which was calculated from the XRD measurements and observed in the SEM and TEM data, is between 10 and 20 nm.