Facile Alcohothermal Synthesis, Size-Dependent Ultraviolet Absorption, and Enhanced CO Conversion Activity of Ceria Nanocrystals

Abstract

Uniform ceria nanocrystals with good crystallinity and high surface areas were prepared by a facile alcohothermal method with the addition of bases (KOH or NaOH), using Ce(III) or Ce(IV) salt as a starting material. The as-prepared nanocrystals were characterized by means of powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), nitrogen adsorption, thermogravimetry and differential thermal analysis (TG-DTA), Fourier transformation infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and ultraviolet and visible spectroscopy (UV−vis). The ceria crystals had controllable sizes from 2.6 to 6.9 nm estimated by the PXRD line broadening analysis. TEM and HRTEM micrographs showed that the as-prepared ceria nanocrystals have a relatively high degree of crystallinity and low degree of conglomeration under high base concentrations. BET specific surface areas of the as-synthesized ceria nanocrystals were very high (103−238 m2 g-1). XPS spectra indicated that the cerium in the nanocrystals was predominantly tetravalent. UV−vis spectra revealed that both the direct and indirect band gap energies of the as-prepared ceria nanocrystals showed a pronounced blue-shifting due to the quantum confinement effect compared to bulk ceria. And the dielectric confinement effect on the band gap energies was also discussed. The as-prepared ceria nanocrystals supported on γ-Al2O3 exhibited a rather lower conversion temperature (559 K) for CO oxidation to CO2 than that of bulk catalysts prepared by the coprecipitation method. Finally, a hydrolytic alcohothermal mechanism for the preparation of ceria nanocrystals was forwarded.