Preparation Of Photocatalytic Composite Based On TiO₂ Nanosheets And Its Degradation Behavior And Mechanism Of Nitrate Reduction

With the rapid development of industry and agriculture in China,a large amount of nitrogen was discharged into natural water along with industrial and agricultural wastewater,causing increasingly serious nitrate pollution problem.Nitrate pollution not only threatens aquatic ecological safety,but also causes a variety of human health problems,so it is urgent to solve it.In recent years,as a simple,efficient and environmentally friendly new technology,Ti O₂-based photocatalytic reduction of nitrate has become a research hotspot.However,the traditional system based on Ti O₂photocatalytic reduction of nitrate still needs to be further solved in the aspects of morphology optimization,catalytic activity,product selectivity and catalytic stability.Here,this paper focuses on the above issues.（1）Ti O₂-NS and Ti O₂-NS modified by metal co-catalyst（Pd,Pt,In,Ag）were prepared by hydrothermal method and photoreduction deposition.Compared with P25（A mature commercial titanium dioxide）,Ti O₂-NS showed better catalytic activity,with80%nitrate removal rate and 71%nitrogen selectivity within 100min.Different co-catalyst have different catalytic performance.Although the photocatalytic efficiency of Ti O₂ nanosheets supported by Pd and Pt is improved,the photocatalytic efficiency of nitrate reduction is inhibited due to the competition of hydrogen evolution reaction（HER）.2%Ag@Ti O₂-NS have the best catalytic effect with the 99%nitrate removal rate and 82%nitrogen selectivity within 60min.Comprehensive experimental analysis shows that the surface properties of co-catalyst has a decisive influence on the catalytic activity of nitrate’s reduction.What is more,carbon dioxide free radical itself can not reduce nitrate efficiently in the photocatalytic system.The cooperation of photo-generated electron and carbon dioxide free radical is necessary to achieve better catalytic activity of photocatalytic reduction of nitrate.（2）Although co-catalyst Pd possesses a good stability and excellent property of promoting the conversion of NO₂^-in the photocatalytic reduction of nitrate over titanium dioxide,the competition of HER（hydrogen evolution reaction）and the low reduction potential(E_CBM=-0.5 V)of titanium dioxide’s conduction band limit its application.In order to further study the effect of surface properties of catalytic materials on the activity of photocatalytic reduction of nitrate,the composite Pd₃In₁@Ti O₂-NS with high photocatalytic reduction activity and nitrogen selectivity was successfully synthesized by a common photo-deposition method.Compared with Ti O₂-NS,Pd@Ti O₂-NS and In@Ti O₂-NS,Pd₃In₁@Ti O₂-NS achieved higher removal rate of nitrate（100%）and selectivity to N₂（90%）.The kinetics constant of Pd₃In₁@Ti O₂-NS（0.290）was 20 times higher than that of Ti O₂-NS（0.017）.Even after six cycling runs,Pd₃In₁@Ti O₂-NS still remained high catalytic activity.Free radical capture,photoelectric current,photoluminescence spectra,modified load mode catalytic experiments and DFT calculations indicate that the improved photocatalytic reduction activity is attributed to the surface properties of Pd₃In₁ alloy.On the one hand,the co-catalyst Pd₃In₁ alloy provides a stronger NO₃^-binding site,which ensures the preferential adsorption of nitrate to the catalytic surface,so that the photogenerated electrons are preferred to be used for nitrate reduction.On the other hand,Pd₃In₁ alloy reduces the energy barrier of nitrate-to-nitrite reduction process,making it easier for the conduction electrons of Ti O₂ nanosheets to reduce nitrate to nitrite and breaking the limit of the rate-limiting step of nitrate to nitrite.In this study,we studied the degradation behavior of photocatalytic reduction of nitrate over M（Pd,Pt,In and Ag）@Ti O₂-NS.On this basis,a composite material（Pd₃In₁@Ti O₂-NS）with high catalytic activity,high nitrogen selectivity and high stability was developed.In addition,the catalytic mechanism was further studied,which provided a simple and effective new idea for the regulation of the interface reaction of photocatalytic reduction of nitrate in water/wastewater treatment process.