Kinetics Study of Photocatalytic Activity of Flame-Made Unloaded and Fe-Loaded CeO₂Nanoparticles

Abstract

Unloaded CeO2and nominal 0.50, 1.00, 1.50, 2.00, 5.00, and 10.00 mol% Fe-loaded CeO2nanoparticles were synthesized by flame spray pyrolysis (FSP). The samples were characterized to obtain structure-activity relation by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), Brunauer, Emmett, and Teller (BET) nitrogen adsorption, X-ray photoelectron spectroscopy (XPS), and UV-visible diffuse reflectance spectrophotometry (UV-vis DRS). XRD results indicated that phase structures of Fe-loaded CeO2nanoparticles were the mixture of CeO2and Fe2O3phases at high iron loading concentrations. HRTEM images showed the significant change in morphology from cubic to almost-spherical shape observed at high iron loading concentration. Increased specific surface area with increasing iron content was also observed. The results from UV-visible reflectance spectra clearly showed the shift of absorption edge towards longer visible region upon loading CeO2with iron. Photocatalytic studies showed that Fe-loaded CeO2sample exhibited higher activity than unloaded CeO2, with optimal 2.00 mol% of iron loading concentration being the most active catalyst. Results from XPS analysis suggested that iron in the Fe3+state might be an active species responsible for enhanced photocatalytic activities observed in this study.

Aqueous and Surface Chemistries of Photocatalytic Fe-Doped CeO2 Nanoparticles

“…As the localized pH at the liquid-solid interface increases upon equilibration with its surrounding aqueous environment of pH 7.00, the concentration of the second major soluble species Ce(OH) 2+ increases. The same effect occurs for Ce(OH) 2 + and Ce(OH) 3 as the pH increases, although the concentrations of these species remain negligible. Therefore, these are the solutes that have the possibility of being present in the surrounding solution, which is at a nominal pH of~7.00.…”

“…It is clear that the same progression of hydrolysis occurs for the conversion of Ce 3+ to Ce(OH) 3 . The greater stability of Ce 3+ is attributed to the lower field strength of Ce 3+ relative to that of Ce 4+ , which provides a lower driving force for hydrolysis [44], leading to the formation of Ce(OH) 2+ , Ce(OH) 2 + , and finally Ce(OH) 3 , although the pH range over which this occurs is smaller than for the Ce 4+ species. Furthermore, the inclusion of solid Ce(OH) 3 is based on similar experimental observations for the Pourbaix diagram of Hayes et al [43], which showed that Ce(OH) 3 precipitated when the pH exceeded~10.00.…”

“…The greater stability of Ce 3+ is attributed to the lower field strength of Ce 3+ relative to that of Ce 4+ , which provides a lower driving force for hydrolysis [44], leading to the formation of Ce(OH) 2+ , Ce(OH) 2 + , and finally Ce(OH) 3 , although the pH range over which this occurs is smaller than for the Ce 4+ species. Furthermore, the inclusion of solid Ce(OH) 3 is based on similar experimental observations for the Pourbaix diagram of Hayes et al [43], which showed that Ce(OH) 3 precipitated when the pH exceeded~10.00. The data for trinuclear Ce 3 (OH) 5 4+ , which forms from Ce(OH) 2+ or Ce(OH) 2+ + Ce(OH) 2 + , were not available in the HYDRA database [42] but the K h was obtained from elsewhere [44].…”

“…When the pH reaches~2.60, the formation of a mixture of solid CeO 2 ·2H 2 O and gelled Ce(OH) 4 is facilitated, after which the concentrations of the remaining hydroxide species decrease. By pH~7.00, the only remaining solute being generated is Ce(OH) 3 + but its concentration is so low as to be negligible.…”

“…Ceria (CeO 2 ) is well known as a photocatalyst and Fe doping of CeO 2 improves its photocatalytic performance [1][2][3][4]. However, it is less well known that, in common with many rare earth oxides [5,6], CeO 2 is hygroscopic and hence water can be absorbed upon exposure to atmospheric water vapor.…”