Anion (Fluoride)-Doped Ceria Nanocrystals: Synthesis, Characterization, and Its Catalytic Application to Oxidative Coupling of Benzylamines

Fluoride doping in the CeO₂ lattice has been achieved by a simple, reliable, reproducible, and safe solution-based method. F-containing CeO₂ has retained the fluorite structure, and its effect has been confirmed from various analytical techniques such as powder X-ray diffraction, Fourier transform IR, Raman, UV–visible diffuse reflectance, photoluminescence (PL), and X-ray photoelectron spectroscopy (XPS), scanning electron microscopy–energy-dispersive X-ray (EDX) and transmission electron microscopy–EDX analysis. The concentration of fluoride in the CeO₂ lattice has been determined from chemical analysis and core-level XPS analysis. The concentration of Ce³⁺ in the F-doped and undoped CeO₂ samples have been determined both from XPS analysis as well as from variable-temperature magnetic susceptibility measurements. The characteristic Ce³⁺ emission in the PL spectrum indicated the increase of Ce³⁺ ion concentration in the F-doped sample, conforming to the results from XPS and magnetic measurements. F-doped CeO₂ nanocrystals showed moderate monodispersity as determined from particle-size measurements using dynamic light scattering experiments and high surface area of 106.1 m²/g. Optical band gap of CeO₂ has narrowed upon doping with fluoride ions from 3.05 to 2.95 eV. The formation of extrinsic oxygen vacancy complexes upon F-doping has been observed in the Raman spectrum (at 1097 cm^–1) in addition to fingerprint bands of CeO₂. The UV-shielding property and photocatalytic inactivity toward aqueous dye degradation process of F-doped CeO₂ has suggested its potential use in cosmetic applications. Both F-doped CeO₂ and CeO₂ have been used as catalysts for oxidative coupling of benzylamines to imines in the presence of molecular oxygen under solvent-free conditions. F-doped CeO₂ exhibited better catalytic efficiency than CeO₂. The oxidation procedure using these catalysts is simple, environmentally benign, and solvent-free, and the catalysts are reusable