An eco-friendly route for template-free synthesis of high specific surface area mesoporous CeO2 powders and their adsorption for acid orange 7

Abstract

An eco-friendly route was developed for the synthesis of mesoporous CeO₂ powders without any additional template. The original cerium precursors were separated from Ce³⁺ aqueous solution by (NH₄)₂CO₃ or Na₂CO₃ via a chemical precipitation method, then H₂O₂ was introduced to induce the phase transformation from original cerium precursors to CeO₂ precursors with initial porous structures, finally the crystallinities of CeO₂ precursors were improved by a hydrothermal treatment, meanwhile the mesoporous structures of final CeO₂ powders were formed. The BET surface areas of mesoporous CeO₂ powders synthesized using (NH₄)₂CO₃ and Na₂CO₃ as precipitants were 106.1 and 76.9 m² g^-1, respectively. Moreover, a mesoporous CeO₂ sample with BET surface area of 100.0 m² g^-1 was also synthesized using commercial Ce₂(CO₃)₃·xH₂O as an existing cerium precursor under the same conditions as control, which could shorten experimental processes and reduce costs. The oxidation-induced phase transformation from original cerium precursors to CeO₂ precursors with initial porous structures was the precondition for further forming of mesoporous structures of final CeO₂ powders during the hydrothermal process. These mesoporous CeO₂ powders showed the rapid and effective adsorption for acid orange 7 dye from simulated wastewater without pH pre-adjustment at room temperature. Furthermore, the adsorption capacities of these mesoporous CeO₂ powders for removal of acid orange 7 dye were determined according to the Langmuir linear fits.

Fig. 1. Synthesis of mesoporous CeO₂ using (NH₄)₂CO₃, Na₂CO₃ as precipitants, and using commercial Ce₂(CO₃)₃·xH₂O as an existing precursor in the presence of H₂O₂.

Fig. 2. XRD patterns of (a) Precursor 1, (b) Precursor 2 and (c) commercial Ce₂(CO₃)₃·xH₂O.

Fig. 3. XRD patterns of (a) Precursor 1-1, (b) Precursor 2-1 and (c) Precursor 3-1.

Fig. 4. XRD patterns of (a) Sample 1, (b) Sample 2 and (c) Sample 3.

Fig. 5. TEM images of (a) Sample 1, (c) Sample 2 and (e) Sample 3 ((b), (d) and (f) show the corresponding high-magnification TEM images, respectively).

Fig. 6. Nitrogen adsorption–desorption isotherms of (a) Sample 1, (b) Sample 2 and (c) Sample 3 (the insets in (a–c) show the corresponding BJH pore size distribution curves).

Fig. 7. XRD patterns of (a) Sample 1, (b) Sample 2 and (c) Sample 3 synthesized in the absence of H₂O₂.

Fig. 8. TEM images of Precursor 1-1, Precursor 2-1 and Precursor 3-1 synthesized in the absence (a, c and e) and presence (b, d and f) of H₂O₂.

Fig. 9. Effects of calcination on the (a) grain sizes and (b) S_BET of the hydrothermally produced mesoporous CeO₂ powders: Sample 1, Sample 2 and Sample 3 in the presence of H₂O₂ (calcination condition: 500 °C; 2 h; in air).

Fig. 10. Time-dependence of adsorption profiles of AO7 dye on mesoporous CeO₂: (a) Sample 1, (b) Sample 2 and (c) Sample 3 synthesized in the presence of H₂O₂ (T = 25 °C; [AO7] = 40 mg L; [CeO₂] = 2.0 g L; V = 100 mL; in the dark; no pH pre-adjustments).

Fig. 11. Effects of AO7 initial concentration on the AO7 adsorption efficiency and adsorption amount measured in the dark and presence of mesoporous CeO₂: (a) Sample 1, (b) Sample 2 and (c) Sample 3 synthesized in the presence of H₂O₂.

Fig. 12. Langmuir linear fits of AO7 dye adsorbed onto mesoporous CeO₂: (a) Sample 1, (b) Sample 2 and (c) Sample 3 synthesized in the presence of H₂O₂.