Synthesis Of Lithium Niobate-based Catalysts And Study On Photocatalytic Reduction Of Nitrate Nitrogen In Water

Photocatalytic reduction is considered one of the most promising technologies for efficient and friendly nitrate removal.As a non-linear optical material,Lithium niobate（LiNbO₃）had been proven to possess a good effect on photocatalytic reduction of nitrate.This was due to the following characteristics:First,the twisted perovskite structure endowed it with a unique physical property.Second,the sufficiently negative conduction band potential brought about photogenerated electrons with great reducibility.Finally,the electron-hole pairs were not easily recombined owing to the property of spontaneous polarization.Given these advantages,it was a suitable photocatalyst for efficient photocatalytic nitrate reduction in water.However,LiNbO₃still had some defects,such as higher oxidation potential,wider band gap,light absorption edge biased to the ultraviolet region and fewer photo-generated electrons to reduce nitrate.In order to improve these defects,several lithium niobate-based photocatalysts were successfully synthesized by surface modification and composite methods with other semiconductors.The specific research content and results were as follows:First,LiNbO₃catalysts with NH₄Cl surface modification were synthesized by hydrothermal method.With the increase of NH₄Cl content,the bulk LiNbO₃catalyst with a particle diameter of 600～700 nm gradually became a dispersed single-pore small sphere with a diameter of 400～500 nm,and then it became a small block catalyst with a particle size of150～200 nm.The 12 mol%NH₄Cl modified LiNbO₃（12%N-LNO）catalyst has the best photocatalytic performance and the removal rate of nitrate nitrogen reached 94%.First-principles calculations showed that LiNbO₃was an indirect bandgap semiconductor.Second,LiNbO₃/g-C₃N₄composite catalysts were synthesized by hydrothermal and calcination methods.It was found that complexing with g-C₃N₄could also make the bulk LiNbO₃into a single-pore small sphere.Among different catalysts,the catalyst with the mass ratio of LiNbO₃and g-C₃N₄of 1:0.5（1:0.5L-CN）had the best photocatalytic performance,the removal rate of nitrate nitrogen reached 99%and N₂selectivity reached 89%.The catalyst dosage of 0.4 g·L^-1and the acidic initial solution had the best photocatalytic performance.In this system,both electrons and carbon dioxide free radical(CO₂^·-)participated in the reaction,but electrons from the LiNbO₃conduction band played a key role in the nitrate reduction process.After three cycles,due to the decrease in active sites and the destruction of some heterojunctions during the cycle,the nitrate nitrogen removal rate and N₂selectivity decreased.LiNbO₃had a high charge separation efficiency and a sufficiently negative conduction band potential,and the effect of the Z-scheme heterojunction of the LiNbO₃/g-C₃N₄composite material resulted in higher charge separation efficiency,nitrate nitrogen removal rate and N₂selectivity.Third,LiNbO₃/ZnS composite catalysts were synthesized by precipitation and calcination methods.Under alkaline condition,the surface of LiNbO₃was negatively charged and electrostatically interacted with Zn²⁺,which made the bulk LiNbO₃self-assembled to form a single-pore small sphere,which significantly increased its specific surface area.Even under neutral conditions,the catalyst with the mass ratio of LiNbO₃and ZnS of 1:5（1:5L-ZS）had the best photocatalytic performance,the removal rate of nitrate nitrogen reached 99%and N₂selectivity reached 99%.In addition,in this system,both electrons and CO₂^·-participate in the reaction,but electrons from the ZnS conduction band played a major role in the nitrate reduction process.After the last three cycles,high photocatalytic activity and N₂selectivity were maintained.Combined with first-principles calculation,ZnS belonged to a direct bandgap semiconductor.LiNbO₃/ZnS composites formed a type II heterojunction,and the synergistic effect of LiNbO₃and ZnS resulted in higher charge separation efficiency,nitrate nitrogen removal rate and N₂selectivity.