Synthesis of Mesoporous Iron Oxides by an Inverse Micelle Method and Their Application in the Degradation of Orange II under Visible Light at Neutral pH

Mesoporous iron oxides (2-line ferrihydrite, α-Fe2O3, γ-Fe2O3, and Fe3O4) are successfully synthesized by modifying the reaction temperatures and calcination atmospheres of the sol–gel-based inverse micelle method. Different characterization techniques, such as PXRD, N2 sorption, SEM, HRTEM, Raman spectroscopy, and XANES, are performed to determine the properties of the catalysts. Larger pore sizes can be obtained in mesoporous γ-Fe2O3 and Fe3O4 compared with mesoporous 2-line ferrihydrite and α-Fe2O3. The catalytic performance of mesoporous iron oxides are examined as Fenton catalysts in orange II degradation in the presence of oxidant H2O2 at neutral pH under visible light. Adsorption capacities of mesoporous iron oxides on orange II are greater than that of commercial Fe2O3. The greatest adsorption capacity is found to be 49.3 mg/g with mesoporous 2-line ferrihydrite. In addition, the degradation efficiency of orange II is found to be markedly improved by mesoporous iron oxides compared with the commercial catalyst. In the best case scenario, 2-line ferrihydrite shows the highest degradation rate constant (0.0258 min–1) among all the catalysts tested. The excellent performance of 2-line ferrihydrite is mainly attributed to the larger surface area but also related to surface hydroxyl groups, acidic products, and possible additional adsorption sites. The recyclability of mesoporous 2-line ferrihydrite catalyst can be achieved up to 3 times without performance decay. At last, a discussion regarding the possible mechanisms of degradation of orange II over mesoporous 2-line ferrihydrite is proposed, based on the previous literature work and the observed reaction intermediates monitored by ESI/MS in this study.