Oxidation-Induced and Hydrothermal-Assisted Template-Free Synthesis of Mesoporous CeO2 for Adsorption of Acid Orange 7

Abstract

Hydrogen peroxide (H₂O₂), an accessible and eco-friendly oxidant, was employed for the template-free hydrothermal synthesis of mesoporous CeO₂ based on a cerium carbonate precursor (Ce₂(CO₃)₃•xH₂O). Its microstructure and physicochemical properties were characterized by XRD, TEM and N₂ sorption techniques. The formation of the CeO₂ phase with a porous structure was strongly dependent on the presence of H₂O₂, while the values of the BET surface area, pore diameter and pore volume of CeO₂ were generally related to the amount of H₂O₂ in the template-free hydrothermal synthesis. The BET surface area and pore volume of the mesoporous CeO₂ synthesized hydrothermally at 180 °C with 10 mL H₂O₂ were 112.8 m²/g and 0.1436 cm³/g, respectively. The adsorption process had basically finished within 30 min, and the maximum adsorption efficiency within 30 min was 99.8% for the mesoporous CeO₂ synthesized hydrothermally at 140 °C with 10 mL, when the initial AO7 concentration was 120 mg/L without pH preadjustment. The experimental data of AO7 adsorption were analyzed using the Langmuir and Freundlich isotherm modes. Moreover, the mesoporous CeO₂ synthesized at 140 °C with 10 mL H₂O₂ was regenerated in successive adsorption-desorption cycles eight times without significant loss in adsorption capacity, suggesting that the as-synthesized mesoporous CeO₂ in this work was suitable as an adsorbent for the efficient adsorption of AO7 dye from an aqueous solution.

Keywords: CeO2; adsorption; azo dye; hydrothermal; mesoporous; template-free.

Figure 1

XRD patterns of (a) commercial Ce₂(CO₃)₃•xH₂O powders; The resulting precipitate synthesized hydrothermally (b) at 180 °C for 24 h without adding H₂O₂, (c) at 180 °C for 24 h with desired amounts H₂O₂ of 2–15 mL, and (d) at a set temperature of 120–200 °C for 24 h with 10 mL H₂O₂.

Figure 2

TEM images of (a) commercial Ce₂(CO₃)₃•xH₂O particles and (b) CeO₂ sample synthesized hydrothermally at 200 °C for 24 h with 10 mL H₂O₂.

Figure 3

N₂ adsorption–desorption isotherms of the mesoporous CeO₂ synthesized hydrothermally (a) at 180 °C for 24 h with a desired amounts H₂O₂ of 2, 5 and 10 mL, and (b) the mesoporous CeO₂ synthesized hydrothermally at a set temperature of 140 and 200 °C for 24 h with 10 mL H₂O₂.

Figure 4

(a) Time-dependence of adsorption profiles of AO7 dye without pH pre-adjustment onto mesoporous CeO₂ synthesized hydrothermally at 180 °C for 24 h with a desired amount H₂O₂ of 2–15 mL and (b) synthesized hydrothermally at a set temperature of 120–200 °C for 24 h with 10 mL H₂O₂. ([CeO₂] = 2.0 g/L; [AO7] = 120 mg/L; V = 100 mL; distilled water; 200 rpm; room temperature).

Figure 5

(a) Langmuir and (b) Freundlich linear fittings of AO7 molecule onto mesoporous CeO₂ synthesized hydrothermally at 140 °C for 24 h with 10 mL H₂O₂.

Figure 6

Adsorption histogram in successive adsorption–desorption cycles eight times. (Sample: mesoporous CeO₂ powders synthesized hydrothermally at 140 °C for 24 h with 10 mL H₂O₂; desorbing agents: 20 mL 0.5 mol/L NaOH; desorption time: 5 min; room temperature).