Development And Mechanism Study Of New Visible Reduction Nitrate Catalytic Materials

As the main source of drinking water,the nitrate concentration of groundwater increases year by year due to the fertilizers.Nitrate can be reduced to nitrite or nitrosamine by the bacteria in the human body.Therefore,it is urgent to study how to reduced the nitrate in drinking water into harmless N₂ efficiently.Photocatalytic reduction method is an ideal treatment method.However,in previous studies,most people are just interested on Ti O₂ based-photocatalysts and modification,and could not obtain catalytic materials with visible light response.So,the research in this field has been unable to truly break through to the practical application stage.The purpose of this paper is to find the new photocatalyst materials with visible light response,high stability and high catalytic efficiency to reduced nitrate and the modification of the catalyst and optimize the catalytic conditions.Based on the principle of photocatalysis,the polymetallic sulfide compounds were selected as the research object.And the synthesis the target compounds was realized with solid state synthesis method and hydrothermal synthesis methods.Combined with solid chemistry and first-principles calculation,the catalytic mechanism is studied,we also want to find some general methods or rules can be the guidance of finding and designing the new visible catalysts.The main contents of this paper are as follows:1.We synthesized ZnCr₂S₄ with spinel structure by solvotherm method solid phase method.And systematically investigated its catalytic performance.The intrinsic narrow bandgap energy(1.96 e V)allow a strong light harvesting ability and indeed the as-prepared Zn Cr₂S₄ behaved as an efficient catalyst for nitrate reduction under UV light irradiation.Cocatalysts,including Ru O_x,Ag,Au,Pd,Pt,were loaded to further improve the reduction efficiency.A synergic effect was observed when loading Pd and Pt,which offered a very high activity.Increasing the loading content would enhance the activity accordingly.The highest nitrate conversion rate of 45 mg N/h together with the N2selectivity of 89%was achieved upon Zn Cr₂S₄ loaded with 3 wt%Pd and 3 wt%Pt,under the following experimental conditions:inner UV-irradiation(125 W),sodium-formate/formic acid buffer solution.The AQY at 380 nm for this particular catalyst was estimated to be 15.46%(with the irradiation beam density of 0.63 W).Most importantly,the visible light activity was explored for Zn Cr₂S₄ loaded with three cocatalysts simultaneously,including Pd,Pt and Au nanoparticles.With the assistance of the SPR effect of Au nanoparticles,the optimal conversion rate of nitrate is 1.352 mg N/h under full arc Xe-lamp,and 0.452 mg N/h under pure visible light(?>400 nm)irradia-tion.Our work proved that metal sulfides with appropriate modifications are good candidates for photocatalytic nitrate reduction.2.We systematically investigated the performance of CuInS₂ in photocatalytic nitrate reduction under visible light irradiation by loading cocatalysts.Particularly,with the assistance of the LSPR effect of Au,the high record of nitrate conversion rate of 8.32mg N h^-1was achieved under pure visible light.For this particular catalyst,the observed AQY was as high as 23.85%at 500 nm.High stability was proved by powder XRD,long-term and cycling photocatalysis experi-ments.Moreover,there was completely no over-reduction problem and it remained effective when different sacrificial agents were used.Overall,CuInS₂ holds high potential in the application to photocatalytic nitrate removal under solar irradiation.3.Due to one mistake of experimental design,the response of visible light and ultraviolet light to the same metal are different.Under the visible light condition,the best double loading combination is Pt-Ag.It can be seen from the experimental results that under the condition of pure visible light(wavelength greater than 400nm)as the light source,the conversion rate can also reach 14.24mg/h,and the selectivity of nitrogen can reach 78.86%.Moreover,it can also be measured that the apparent quantum efficiency can reach 32.62%and 28.44%respectively under the irradiation of 500nm and 600nm wavelength light sources.However,in the long time catalytic experiment,the efficiency was found to decrease with time.After a long time of illumination,the catalyst can recover its activity by calcining under hydrogen atmosphere.The results showed that the activity could be restored to more than 90%after being calcined for 12h at 250?.Similarly,the Pt-Ag loading CuInS₂ catalyst can maintain considerable reactivity although it is also affected by the remaining impurity ions or different sacrificial agents.4.According to the literature investigation,the catalytic reaction usually occurs on the small Miller index planes.Combined with the symmetry of the crystal structure,the kinetic simulation of nitrate degradation was carried out on the(001)and(010)surfaces of CuInS₂.Based on the results of experiments,we propose that photocatalytic degradation of nitrate is an adsorption reaction,and catalysis can be carried out in two different ways:1.Nitrate can be directly reduced to nitrogen;2.First,nitrate can be reduced to nitrite,and then nitrite reduced to nitrogen.To verify this mechanism,we conducted the corresponding simulation design,for both(001)and(010),the barrier of the first path is higher than the second path.But the two barriers are in the same magnitude,which shows that two reaction can happen simultaneously.In addition,the process of nitrate over-reduction to ammonia on the crystal surface is also simulated.Compared with the reaction rate constant results of other reaction processes,the over-reduction is almost impossible.Therefore,from the point of view of theoretical calculation,the mechanism come from experiments is correct.