Properties and Reactivity of Nanostructured CeO2 Powders: Comparison among Two Synthesis Procedures

Three nanostructured cerium(IV) oxide powders were synthesized by two different synthetic routes: two samples were obtained by precipitation from a basic solution of cerium nitrate and treated at 523 and 923 K, respectively, and the third one was prepared by a microwave-assisted heating hydrolysis method and treated at 523 K. The obtained samples were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron (XPS), and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopic techniques, thermal analysis, and Brunauer-Emmett- Teller (BET) surface area. A broad particle size distribution is observed for CeO2 obtained by precipitation (8.0-15.0 nm). Smaller particles (sizes around 3.3-4.0 nm) with a narrow particle size distribution characterize the ceria obtained by microwave irradiation. XPS and DRIFT outcomes show that (i) the CeO2 prepared by microwave-assisted heating hydrolysis method is more reduced than that obtained by precipitation and treated at the same temperature; (ii) the CeO2 obtained by precipitation and treated at 923 K is more reduced than that prepared by microwave irradiation. A higher amount of basic and acidic sites were noticed on the CeO2 obtained by microwave irradiation. Methanol interacts mainly dissociatively with the CeO2 surface regardless of the preparation procedure and reduction degree of surface; however, on the CeO2 prepared by microwave irradiation the interaction is quantitative: all the methanol introduced in the reaction chamber reacts with the surface. The CeO2 obtained by precipitation and treated at 523 K does not oxidize methanol, even at higher temperature, while traces of oxidation products are noted on the sample treated at 923 K. Methanol oxidation is favored on the CeO2 prepared by microwave irradiation: the main oxidation products are formate species and inorganic carboxylate. Moreover, the oxidation capability increases with increasing temperature.