| In recent years, heterogeneous Fenton process has made great progress in catalytic oxidation of toxic organic pollutants, and has been widely recognized as a highly promising environmental purification technology. In heterogeneous Fenton-like reaction, the decomposition of H2O2 to hydroxyl radical mainly depends on the solid catalysts, so the properties of catalysts have significant effect on the perspective of heterogeneous Fenton oxidation. As an effective way to improve the catalytic activity and stability of heterogeneous Fenton catalysts, different elements, such as transition metals, rare earth metals, and non-metallic have been doped into the catalysts. Nevertheless, how to further enhance the catalytic activity, widen the pH working range, and improve the visible light utilization is still a challenge. Based on these observations, we prepared a series of novel bimetallic solid acid catalysts in the view of acid-catalysis and photo-catalysis. These materials are possessed of the combining benefits of both metal ions doping and acid functionalization, which could greatly enhance the activity and stability of the catalysts. Moreover, the reaction mechanism of acid catalytic activation of H2O2 is further explored by various characterizations. The main research contents and results are as follow:Porous solid superacid SO42-/Fe2-xZrx03 and SO42-/ Fe2-xTixO3 bimetallic oxides was first synthesized by citric acid templated and sulfuric acid impregnating method. Separate experimental results indicated the highest catalytic activity was obtained with a molar rate of 5:1 for Fe/M (Zr and Ti) and calcinations of 773 K. Characterized by the TEM, BET, XRD, XPS, NH3-TPD, Py-IR, and UV-vis DRS techniques, it was found that the incorporation of transition metal ions and sulfur had a significant effect on the properties, crystal form and structure of the solid. Results demonstrated that the prepared catalysts exhibited hierarchical porosity:mesopores (2-50 nm) together with the macropores (> 50 nm).With transition metal doping and H2SO4 activation, an obvious increase in the BET surface areas and pore volume was observed for the samples, which would be benefit for the surface adsorption and mass transfer in the pore of the substrates. As noted, Ti doped catalyst presented larger pore size of mesopores (9.2 nm) than that of Zr doped. Because transition metal ions substituted the lattice Fe3+ or incorporated interstitially into the lattice of Fe2O3, the characteristic peak intensities of the samples decreased dramatically and the position of the α-Fe2O3 main peak (104) and (110) shifted slightly to the lower diffraction angles. Secondly, a large number of strong Lewis acid sites were produced on the surface due to a cooperative effect of transition metal and sulfur, but Br(?)nsted acid sites were only immobilized on the surface of sulfated Fe-Zr metal oxide. Moreover, it was found that the zeta potentials of samples can be ranked as pHIEP (SFZ-5)< pHIEP (SF)< pHIEP (FZ-5), which indicated the Zr doping and H2SO4 activation had a significant influence on the surface charge of solid. Meanwhile, the metal doping and generation of oxygen vacancies can narrow the band gap of sulfated bimetallic oxides and efficiently enhance visible light photocatalytic performance.The photo-Fenton activities of SO42-/Fe2-xZrxO3 and SO42-/Fe2-xTixO3 were evaluated via degradation of dyes active brilliant red X-3B. The results showed that two sulfated mixed metal oxides had remarkable catalytic activity for the degradation of organic pollutants in photo-Fenton reaction under visible light irradiation and exhibited excellent tolerance with respect to organic matters poisoning. This is attributed to the favorable synergetic effect of metal doping and sulfation. It should be noted that the leaching of Fe ions in all the cycles was very low remaining below 1.0 mg/L. Moreover, the kinetic of photo-Fenton catalytic oxidation was also discussed in the present work. In contrast to pseudo-first-order kinetics, the behavior observed for samples could be fitted better by using the combined pseudo-first-order kinetics model. Secondly, more details in the influence of operational variables, such as H2O2 concentration, catalyst loading, and the initial pH on the photo degradation of X-3B were investigated. Results demonstrated that the highest degradation efficiency was observed under the conditions of 9.8 mmol/L H2O2,1.0 g/L catalyst, and pH of 6.0. Two sulfated bimetallic oxides successfully widened the working pH range of Fenton reaction to strong alkaline (pH=10.0). Especially, the activity of sulfated Fe-Ti oxide was slightly affected by the pH values varied from 4.0 to 10.0. Thirdly, attempts have been made to characterize the properties and structure of solid superacid after incorporation of metal ions and sulfation, and we deduce the structure-activity relationship in solid acid catalysts and the effect of light irradiation on performance enhanced. Experimental data was clearly evidenced that the synergetic effect of acid sites and photo-induced electrons transfer played a crucial role in Fenton reaction. This may be attributed to that the appearance of acid sites, which not only provide better adsorption centers for H2O2 and substances, bur also be favorable for the separation of photo-induced electrons and holes and suppressed the charge recombination.Porous Fe-Ti bimetallic oxides were first doped with methionine and sodium thiosulfate. Characterized by the TEM, BET, XRD, XPS, NH3-TPD, Py-IR, and UV-vis DRS techniques, it was found that the incorporation of different sulfur sources had a significant effect on the properties, crystal form and structure of the solid. Results demonstrated that no macropores structure was observed on the surface of prepared catalysts, and some nanoparticulars aggregated each other. The characteristic peak intensities of the samples after methionine doping was lower than that of sample doped with sodium thiosulfate, but the introduction of sulfur can not change the α-Fe2O3 crystal form of iron oxides. Secondly, it was found that a decrease in the surface area of samples were obtained after sulfur doping, but the catalyst modified by methionine and sodium thiosulfate presented larger pore size of mesopores (16.6 nm) than the no sulfated Fe-Ti bimetallic oxide and SO42-/Fe2-xTixO3. The surface area and pore volume of sample after methionine doping was 41.9 m2/g and 0.19 cm3/g, respectively, much larger than that of sample modified with sodium thiosulfate. Thirdly, large numbers of S-O groups were generated on the surface of bimetallic oxide due to two kinds of sulfur species doping, which made Fe-Ti mixed oxides exhibiting the strong acidity and many acid sites. In comparison with sodium thiosulfate, the doing of methionine can not only make more S-O groups generated on the solid surface, but also greatly improve the hydrophilic of solid. Moreover, much stronger Lewis acid sites were occured on solid surface after methionine doping.The photo-Fenton activities of S doped Fe-Ti bimetallic oxides were examined by degradation of dyes active brilliant red X-3B. The results showed that the doping of sulfur had a positive effect on the activity enhanced of Fe-Ti bimetallic oxides. By analysis of the decoloration of dyes, TOC removal efficiency, and the decomposition rate of H2O2, it was found that the activity of catalyst doped with methionine was much higher than that of sample modified with sodium thiosulfate. This is attributed to the obviously enhanced acid property of catalyst after methionine doping, which can be benefit for the decomposition of H2O2 to hydroxyl radical. In the three consecutive catalytic reaction cycles, only methionine doped catalyst exhibited excellent stability for the degradation of dyes. Moreover, the influences of operational variables on the photo degradation of X-3B were investigated in details. Results demonstrated that the operation condition such as H2O2 concentration, catalyst loading, and the initial pH had less effect on the catalytic activity of Fe-Ti bimetallic oxides modified with the methionine, in comparison with the sodium thiosulfate doped catalyst. Importantly, the methionine doped bimetallic oxides also presented high activity in strong alkaline condition (pH= 10.0), similar to the SO42-/Fe2-xZrxO3 and SO42-/Fe2-xTixO3.By mean of investigating the effect of different sulfation methods and different sulfur sources on the properties, structure, and catalytic ability of Fe-Ti bimetallic oxides, we deduce the possible sulfation mechanism of bimetallic oxides. Results demonstrated that the introduction of S-O functional groups into the solid could enhance the catalytic activity and stability of catalyst, no matter using doping or impregnating method. Secondly, the sulfur doping method more easily created strong acid sites than sulfuric acid impregnating method, which was due to the FeⅢ-SO42-and TiⅣ-SO42- generated on the solid surface after methionine doping. Thirdly, by compared with different sulfur sources, it was found that the stronger acid sites were more easily produced by the organic sulfur doping. This was attributed to the tight connection of organic sulfur and citric acid enhancing the generation of covalent bonds between the sulfur groups and metal ions. |
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