Green Synthesis of Nanostructure CeO2 Using Tea Extract: Characterization and Adsorption of Dye from Aqueous Phase

Nanostructure CeO₂ powders were synthesized using tea waste extract as gel precursor. The as-prepared samples were characterized by thermogravimetric analyzer (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Based on the TGA/DTG analysis, the intermediates of cerium chloride hydrates (CeCl₃.4H₂O and CeCl₃.H₂O) and cerium anhydrous (CeCl₃) were produced, and the formation temperature of CeO₂ was estimated to be 773 K. The cubic fluorite structure of CeO₂ was detected to be the predominant species and was completely formed at the calcination temperature of 773K–1073 K with a crystal size between 8.8 and 11.4 nm based on the XRD measurement. Moreover, the main chemical state of ceria on the surface of the synthesized samples was confirmed to be tetravalent ceria by XPS. All samples show a strong Raman signal at a well-defined chemical shift of 463 cm⁻¹ and a significant symmetry feature was observed, suggesting that the tetravalent ceria is the dominant species throughout the bulk sample. All the synthesized CeO₂ calcined at different temperatures showed higher adsorption efficiency for Congo red (CR) compared with commercial CeO₂. The adsorption efficiency maintained a steady state of more than 95% when the concentration of CR and adsorption temperature were varied in this study. The kinetic analysis showed that the second-order model was the appropriate model to interpret the adsorption behavior of synthesized CeO₂. The calculated adsorption capacity derived from the second-order model is in good agreement with the experimental data. The isotherm analysis revealed that the Freundlich and D-R models fit well for the synthesized CeO₂ and represent physisorption with a multilayer mechanism. The thermodynamic parameters, including the changes in Gibb's free energy, enthalpy, and entropy, suggested that the adsorption of CR on the synthesized CeO₂ sample was a spontaneous and endothermic process.

A novel regeneration method for deactivated commercial NH3-SCR catalysts with promoted low-temperature activities

A novel route is developed for regeneration of deactivated commercial NH₃-SCR catalysts, which includes an initial in situ construction of anatase TiO₂ porous film, followed by loading of MnO_x, CeO_x, and Mn-Ce mixed oxides as active components. The regenerated catalysts present largely improved low-temperature denitrification performance due to the synergetic effect of MnO_x and CeO_x. The denitrification efficiency could reach a high value of 97% at 200 °C and 100% at 250 °C when the Ce-Mn mixed oxides are loaded at the optimized molar quantity ratio of 10:9 (Ce:Mn). Properties and reaction mechanisms of the regenerated catalysts are investigated with characterizations of X-ray photoelectron spectroscopy (XPS), NH₃ temperature-programmed desorption (NH₃-TPD), H₂ temperature-programmed reduction (H₂-TPR), and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Our results demonstrate that the adsorption and oxidation of NO plays a crucial role for these three catalysts even though a difference exists on the reaction pathways. Graphical abstract.

Comparison of Packed-Bed and Micro-Channel Reactors for Hydrogen Production via Thermochemical Cycles of Water Splitting in the Presence of Ceria-Based Catalysts

Hydrogen production via two-step thermochemical cycles over fluorite-structure ceria (CeO2) and ceria-zirconia (Ce0.75Zr0.25O2) materials was studied in packed-bed and micro-channel reactors for comparison purposes. The H2-temperature program reduction (H2-TPR) results indicated that the addition of Zr4+ enhanced the material’s reducibility from 585 µmol/g to 1700 µmol/g, although the reduction temperature increased from 545 to 680 °C. Ce0.75Zr0.25O2 was found to offer higher hydrogen productivity than CeO2 regardless of the type of reactor. The micro-channel reactor showed better performance than the packed-bed reactor for this reaction.