Visualization study on preparation of CeO₂ by pyrolysis method via microwave heating

Preparation of cerium oxide by conventional liquid phase method has the disadvantages of complex technological process and effluent discharge. Spray pyrolysis for making CeO₂ has the disadvantages of nozzle plugging, and this traditional heating method produces a significant temperature gradient that results in unevenly heated reactants. To prevent the above issues, this study proposed an effective and environmental experimental scheme. Cerium chloride heptahydrate and deionized water were utilized for the raw material. High-purity nano cerium oxide particles were prepared by jet-flow pyrolysis technology via microwave heating. Combining the technology of microwave heating with jet-flow pyrolysis, whose Venturi reactor served as the primary piece of equipment, can improve the mixing of gas and liquid, increase chemical reaction efficiency, and reduce carbon emissions. It was a new effort in the area of pyrolysis. To visually analyze the distribution of each physical field and substance, numerical simulation was combined with x-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy to characterize the products. Effects of the various technological conditions (pyrolysis temperature, gas velocity, and adding citric acid) on the content of residual chloride element and microstructure of the product were examined. Results demonstrated that the temperature error between the experiment and simulation was below 20 ℃ with the condition of the same microwave power. When the pyrolysis temperature was at 500 ℃, CeO₂ could be produced in a single phase, but the particle profile was unclear. The particles had sharp profiles when the temperature was 600 ℃. Nanoscale spherical CeO₂ particles appeared when the average temperature reached 700 ℃. The results of the study’s simulations and experiments indicated that higher temperatures were associated with more regular microcosmic morphology and a lower content of residual chloride element. Increasing the gas velocity caused an obvious decrease in the average temperature, which led to more content of residual chloride elements. However, the gas collided with the solution more fiercely, which improved the mixing of the two phases. Experimental and simulated results showed that when gas velocity reached 1.2 m∙s⁻¹, better dispersity and less agglomeration of the product were obtained. Additionally, the residual chloride content was less than 1%. Because a significant amount of CO₂ was produced during the burning of the citric acid, the spherical cerium oxide particles broke into irregular particles. Porous structures also appeared when citric acid was added. The residual chloride content decreased with the increase of citric acid concentration when citric acid concentration was greater than 0.05 mol·L⁻¹.