CeO_2–x Nanoparticles Heated to 900 °C Afford Nanomaterials without Thermally Induced Mismatches: Implications for Catalyst Carriers

Nowadays, CeO 2– x (0 < x < 0.5) nanoparticles are used for a wide range of applications, most of them using high temperatures. Since the phase diagram of nanoparticles differs from that of the corresponding bulk material and plays a pivotal role in many applications, the phase transition of CeO 2– x nanoparticles exposed to a CO atmosphere at 900 °C was probed by in situ XRD measurements. The results show the presence of the fluorite and Cu 2 Se-like phases at this temperature. During the cooling process to room temperature in the He atmosphere, the Cu 2 Se-like phase turns metastable and transforms back to the fluorite phase. The phase transition occurs in 1D, with the crystallization mechanism controlled by volume diffusion. The Cu 2 Se-like phase presents a negative thermal expansion coefficient (α) for the temperature range of 580–300 °C due to the O diffusion from the Cu 2 Se-like to the fluorite crystal structure. Consequently, tuning the amounts of the fluorite and the Cu 2 Se-like phase allows reaching a tunable thermal expansion coefficient of the CeO 2– x nanoparticles. In particular, the condition for a null thermal expansion coefficient was successfully achieved, which allows the use of smart CeO 2– x nanoparticles without inducing thermal mismatches for applications like catalyst carriers.