Fabrication Of Au/Fe₃O₄-Chitosan Composites For Catalytic Reduction Of Dyes

Au/Fe₃O₄-chitosan composites were synthesized by a green method without using any strong reducing or stabilizing agents. The catalytic activity of the synthesized Au/Fe₃O₄-chitosan composites was evaluated towards the degradation of dyes in aqueous solution. The main contents were as follows:（1） Firstly, Fe₃O₄ nanoparticles were prepared by coprecipitation of Fe2+ and Fe3+ with ammonia water and treated under hydrothermal. Secondly, magnetic chitosan composites were prepared in a sample manner using crosslinking method. Lastly, the Au/Fe₃O₄-chitosan composites were synthesized using chitosan as reducing and capping agents by hydrothermal treatment. The morphological properties and structural features of the composites were investigated by transmission electron microscopy（TEM）, X-ray diffraction patterns（XRD）, thermogravimetric（TG） and Fourier transform infrared（FT-IR） techniques.（2） Catalytic degradation of azo dyes（methyl orange and Congo red） in the presence of NaBH4 using the as-prepared composite as catalysts were studied. The Au/Fe₃O₄-chitosan composites showed a high catalytic activity when it was prepared with 0.5 wt% Au, pH 6 at 50 oC for 1 h. The results of the study indicated that the degradation of methyl orange by Au/Fe₃O₄-chitosan catalyst was consistent with the pseudo first order kinetics. The catalytic efficiency increased gradually with increasing the amount of catalyst and was not subject to the initial methyl orange concentration under normal temperature and pressure. Furthermore, the as-prepared catalyst also exhibited a high catalytic activity for the degradation of Congo red. The degradation rate of Congo red solution in 9 min was up to 94 % when the initial concentration of Congo red was 0.1 mmol/L.（3） Catalytic degradation of 4-nitrophenol in the presence of NaBH4 using the as-prepared composite as catalysts were studied. The degradation rate of 4-nitrophenol in 6 min was up to 90 % when the Au/Fe₃O₄-chitosan composites were synthesized with 1.5 wt% Au, pH 6 at 70 oC for 2 h. In addition, increase of the reaction temperature, the initial concentration of 4-nitrophenol and the catalyst dosage in certain range would enhance the reaction rate, and promote the degradation of 4-nitrophenol. The degradation of 4-nitrophenol by Au/Fe₃O₄-chitosan catalyst was consistent with the pseudo first order kinetics, and the apparent activation energy was about 7.85 kJ/mol.（4） The degradation rate of dyes（methyl orange and Congo red） was nearly unchanged after twelve cycles, which indicated that the catalyst had high stability and was uneasy to lose activity or poisoning in the system. Also, the catalyst could be completely recovered. Thus, the Au/Fe₃O₄-chitosan catalyst developed in this work exhibited excellent catalytic performance.