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 CeO 2 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 CeO 2 prepared by microwave-assisted heating hydrolysis method is more reduced than that obtained by precipitation and treated at the same temperature; (ii) the CeO 2 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 CeO 2 obtained by microwave irradiation. Methanol interacts mainly dissociatively with the CeO 2 surface regardless of the preparation procedure and reduction degree of surface; however, on the CeO 2 prepared by microwave irradiation the interaction is quantitative: all the methanol introduced in the reaction chamber reacts with the surface. The CeO 2 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 CeO 2 prepared by microwave irradiation: the main oxidation products are formate species and inorganic carboxylate. Moreover, the oxidation capability increases with increasing temperature.