Template-free synthesis of mesoporous CeO₂ powders by integrating bottom-up and top-down routes for acid orange 7 adsorption

A combined bottom-up and top-down route was developed for the template-free synthesis of mesoporous CeO₂ powders using Ce(NO₃)₃·6H₂O, NH₄HCO₃, H₂O₂, and H₂O as starting reagents. The flake-like Ce₂(CO₃)₃·8H₂O precursor was etched by H₂O₂, and CeO₂ nucleated in situ with built-in equiaxed particles. Pores formed on the flakes owing to the loss of by-products of H₂O and CO₂. The formation of the mesostructured CeO₂ could also be explained by the large change in volume induced as a result of the difference in density between Ce₂(CO₃)₃·8H₂O and CeO₂. Accordingly, the original flake-like morphology of Ce₂(CO₃)₃·8H₂O was not preserved upon pore formation and during continuous stirring in the synthesis. A subsequent hydrothermal treatment destroyed the loose aggregates of CeO₂ derived from the reaction between H₂O₂ and Ce₂(CO₃)₃·8H₂O. Rearrangement of the CeO₂ particles via a dissolution-recrystallization process occurred under certain temperatures and pressures. Consequently, CeO₂ particles with coarser sizes, smoother surfaces, and mesoporous structures were obtained. The specific surface area of the particles was 166.5 m² g^-1 after hydrothermal treatment at 200 °C for 24 h. The mesoporous CeO₂ particles possessed better adsorption capacities of acid orange 7 dye than basic orange 2 dye in the absence of pH pre-adjustments. The saturated adsorption amount of acid orange 7 dye was 510.2 mg g^-1 at 298 K based on Langmuir linear fitting of the experimental data.