Synthesis And Catalytic Property Of Heterogeneous Photo-Fenton Catalysts Based On α-Fe₂O₃

The removal of organic dyes from water is an important subject in the field of water pollution treatment.Heterogeneous Fenton technology,as an important advanced oxidation process(AOP),is considered to be a promising method to remove stubborn organic dyes.The technology mainly relies on the Fenton reaction between Fe2+ and H2O2 in iron-based solid catalysts to generate strong oxidizing hydroxyl radical(·OH)to degrade the organic contaminant.And at the same time,Fe2+ is oxidized to Fe3+.However,Fe2+is regenerated via the reaction between Fe3+and H2O2 at a very slow rate in the Fenton system,leading to Fe3+ accumulation and Fe2+depletion.The weak cycle ability of Fe3+/Fe2+ reduces the efficient utilization rate of H2O2 and hinders the removal of organic pollutants.Therefore,It has become the critical problem in the application of Fenton technology that how to solve the slow regeneration rate of Fe2+and the low utilization rate of H2O2.Among iron-based catalysts,α-Fe2O3 is one of the most promising commercial Fenton reagents due to its stable structure,low cost,strong absorption of visible light and environmental friendliness.However,the same as most heterogeneous Fenton systems,α-Fe2O3 Fenton systems also have some important problems such as the difficulty of Fe2+regeneration and low efficiency of H2O2.Heterogeneous photo-Fenton reaction has received the attention of researchers,due to light can enhance the Fenton reaction.In recent years,the introduction of photogenerated electrons has been proven to be able to optimize the Fenton reaction by accelerating the Fe3+/Fe2+ cycle.In addition,for iron-based semiconductors,such as α-Fe2O3,the construction of semiconductor heterojunction structures can efficiently improve the utilization of photo-generated electrons,which might contribute to the enhanced heterogeneous photo-Fenton activity.According to the above mechanisms,this paper designed and constructed AgBr/α-Fe2O3 heterojunction and SnO2/α-Fe2O3 heterojunction to improve the photo-Fenton activity of α-Fe2O3 and explore the possible catalytic mechanism.The main research contents of this paper are as follows:(1)The AgBr/α-Fe2O3 heterogeneous catalysts were successfully prepared by the solvothermal method and deposition method.And for the first time,this work introduced AgBr semiconductor into α-Fe2O3 photo-Fenton system.X-ray diffraction(XRD)and scanning electron microscopy(SEM)results show that the numerous spot-like AgBr nanoparticles are successfully grown on the surface of α-Fe2O3 particles;AgBr/α-Fe2O3 heterojunctions exhibits a strong absorption ability to visible light according to the UV-visible diffuse reflection spectra(UV-vis DRS);The photo-Fenton degradation experiments show that the excellent photo-Fenton catalytic activity of AgBr/α-Fe2O3.Specifically,the rhodamine B dye could be completely degrade within 25 minutes under the photo-Fenton catalysis of AgBr/α-Fe2O3,while it cost 50 minutes for the complete removal of dye in the presence of AgBr or α-Fe2O3.In addition,The rate constant of AgBr/α-Fe2O3 photo-Fenton reaction can reach to 0.1438 min-1,which is 1.7 times and 41 times larger than that of α-Fe2O3 photo-Fenton reaction(0.0834 min-1)and α-Fe2O3 photocatalytic reaction(0.0O35 min-1),respectively..(2)The excellent photo-Fenton degradation activity of AgBr/α-Fe2O3 composites is abscribed to the superior visible light absorption ability,and the synergistic effect of photocatalysis and photo-Fenton reaction.A possible AgBr/α-Fe2O3 photo-Fenton catalytic mechanism was proposed.Under the visible light irradiation,AgBr semiconductor is excited,and its electrons transition to the conduction band.Due to the structure of AgBr/α-Fe2O3 heterojunction,the CB electrons of AgBr could migrate to the α-Fe2O3 conduction band,and the valence band holes of α-Fe2O3 could migrate to the AgBr valence band,which contribute to the effective separation of photo-generated carriers.On the one hand,photo-generated electrons generated by AgBr/α-Fe2O3 react directly with H2O2 to generate hydroxyl radicals.On the other hand,these electrons are captured by Fe3+ on the surface of α-Fe2O3,which accelerates the regeneration of Fe2+.The produced Fe2+ further activated H2O2 to generate oxidizing hydroxyl radicals and increase photo-Fenton reaction activity.(3)The SnO2/α-Fe2O3 photo-Fenton catalysts were synthesized by a facile two step hydrothermal method.XRD and X-ray photoelectron spectroscopy(XPS)results show the successful synthesis of SnO2/α-Fe2O3 composites;SEM images show that many spot-like SnO2 nanoparticles uniformly grows on the surface of α-Fe2O3 particles in the form of small dots;UV-vis DRS exhibit the strong visible light absorption ability of SnO2/α-Fe2O3 catalysts;The photo-Fenton activity test show that the SnO2/oc-Fe2O3 could efficiently degrade the methylene blue dye.The introduction of SnO2 promote the degradation effeciency from 66%(α-Fe2O3)increased to 97%(SnO2/α-Fe2O3)within 60 minutes.And the photo-Fenton reaction rate of SnO2/α-Fe2O3 was 0.0537 min-1,which was not only 2.8 times larger than that of α-Fe2O3(0.0191 min-1),but also 28 times larger than the photocatalytic reaction rate of α-Fe2O3(0.0019 min-1).(4)The superior photo-Fenton degradation activity of SnO2/α-Fe2O3 composites is mainly attributed to the enhanced visible light absorption ability and the synergistic effect of dye一sensitization and photo-Fenton reaction.It is found that SnO2 semiconductor can also promote the photo-Fenton reaction.The role of SnO2 is mainly divided into the following three:First,SnO2 improve the visible light absorption ability of the catalysts;Second,SnO2 and α-Fe2O3 formed a heterojunction,which promote the effective separation of photo generated carriers,Third,SnO2 act as a "storage sink" for photo-generated electrons from excited α-Fe2O3 and dye molecules,which accelerates the regeneration of Fe2+ and promotes the decomposition of hydrogen peroxide.