Decomposition processes and structural transformations of cerium propionate into nanocrystalline ceria at different oxygen partial pressures

Abstract

The structural transformations that occur when thermal treatments turn cerium propionate into nanocrystalline ceria have been analysed with thermoanalytical techniques (TG, DTA and MS) and with structural and magnetic characterization (HRTEM, SQUID and XRD) of the final and intermediate products. Attention has been paid to what occurs during the decomposition of propionate and how the process is affected by the furnace atmosphere (oxidizing or inert). In an oxidizing atmosphere, the decomposition of cerium propionate is triggered by the oxidation of Ce³⁺ to Ce⁴⁺. This reaction entails the loss of large unoxidized propionate fragments of the propionate ligands. As decomposition proceeds, the carbonaceous residue makes the oxygen transport inside the material more difficult and decomposition becomes diffusion limited. At this point, extensive oxidation of the residue begins until it is completely removed. Crystallization of CeO₂ occurs simultaneously with decomposition. In these conditions, crystalline nanoparticles (diameter of 3–5 nm) can be obtained at a temperature as low as 300 °C. In an inert atmosphere, decomposition occurs in three steps. During the first step, one of the three propionate ligands is lost, with little oxidation of Ce³⁺, and is substituted by a hydroxyl group. The second step entails the loss of the remaining ligands with a substantial oxidation of Ce³⁺ to Ce⁴⁺. After this step, the intermediate product is, proposed as, a mixture of amorphous Ce(OH)₃ and Ce(OH)₄. Finally, the third step leads to conversion of the Ce hydroxide into crystalline CeO₂. In an inert atmosphere, the process is less reproducible than in air and a carbonaceous residue remains in the product.