Hydrothermal Synthesis of Shape/Size-Controlled Cerium-Based Oxides

Abstract

A low energy band gap between Ce³⁺ and Ce⁴⁺ states in cerium oxide along with its high oxygen mobility and high oxygen storage capacity are properties that qualify it as one of the most widely used heterogeneous catalysts and catalyst oxide supports. This thesis report is an account of studies that were carried out on the synthesis and catalytic properties of pure, metal-doped and noble-metal impregnated cerium oxide nanoparticles. Our results revealed that synthesis temperature, during hydrothermal reactions, plays a critical role in controlling the shape, size, oxygen vacancy concentration, and low temperature reducibility in CeO₂ nanoparticles. In addition, OH⁻ ion concentration was found to play an important role in engineering the lattice constants and oxygen vacancy concentrations of ceria nanoparticles within the same particle morphology and synthesis temperature. Secondly, our studies demonstrated that hydrothermal synthesis is a facile one-step approach to the preparation of compositionally homogeneous Ce[subscript x]Zr[subscript 1-x]O₂ (0dxd1) nanocrystals, in which CeO₂-ZrO₂ mixed oxides present a superior low-temperature oxygen release capability compared to pure CeO₂. The Ce[subscript 0.5]Zr[subscript 0.5]O₂ system proved to have good thermal stability up to 1000°C under reducing and oxidizing atmosphere. We have also seen that at above 1000°C, phase transformation occurs from psudocubic to cation ordered pyrochlore or tetragonal phase under reducing and oxidizing atmosphere, respectively. This method may be easily extended to other cerium-based mixed oxides or synthesis of analogous mixed oxides. Lastly, our results established that the impregnation of 1 wt. % platinum and gold on CeO₂ nanorods and on nanocubes causes an enhanced reduction on their surface reduction temperatures with negligible effect on their bulk reducibility. It was also shown that both pure and impregnated CeO₂ nanorods have a lower surfac