Construction of the Fe3+-O-Mn3+/2+ hybrid bonds on the surface of porous silica as active centers for efficient heterogeneous catalytic ozonation

The Fe3+-O-Mn3+/2+ hybrid bonds were constructed on the surface of the amorphous and porous silica molecular sieve (MCM-41) by a three-step method that involved impregnation, hydrothermal incorporation, and annealing. The obtained FeMn-MCM-41 has an amorphous, less-ordered mesoporous structure with uniformly distributed Fe and Mn and a large specific surface area. It exhibits high activity, good recyclability, and wide pH applicable range for the heterogeneous catalytic ozonation (HCO). The TOC removal of methyl orange at high concentration of 327.5 mg L-1 is 78% on FeMn-MCM-41, which is not only much higher than the value of single ozonation (43%) but also much higher than the value on mixed catalysts of Fe-MCM-41 and Mn-MCM-41 (52%). We demonstrate that large amounts of the Fe3+-O-Mn3+/2+ hybrid bonds were created on the surface of porous silica, and the partial electron migration occurs in the hybrid Fe3+-O-Mn2+ (or Fe3+-O-Mn3+) bonds. As is well known, Fe3+ and Mn3+/2+ are very strong and weak Lewis acid sites in their oxides, respectively, and the partial electron migration lowers the acidity of Fe3+ sites and in the meantime increases the acidity of Mn3+/2+ sites, making both sites more suitable for catalytic production of free hydroxyl radicals ((OH)-O-center dot). The synergetic effect of Fe3+ and Mn3+/2+ significantly improve the intrinsic activity of the FeMn-MCM-41 for HCO. This work may offer a new strategy for developing efficient mesoporous catalysts for catalytic ozonation.