Study On The Controllable Preparation Of SnS₂ And SnS₂/CNT Composite For Photocatalytic Reduction Of Cr（Ⅵ） Wastewater

Hexavalent chromium is one of the most common heavy metal pollutants in the environment.It causes damage to the liver,kidney and DNA of the human body,and is highly carcinogenic.Its toxicity is about 100 times that of trivalent chromium,and reducing it from the hexavalent state to the trivalent state can greatly reduce its toxicity.Photocatalysis is an advanced technology that uses light energy to produce the electron-hole pairs by stimulating semiconductors to carry out redox reactions,so as to achieve the removal of pollutants or the generation of clean energy.Under normal circumstances,photocatalytic reactions are mostly heterogeneous reactions that occur at the water/solid interface.The photocatalytic performance of the catalyst depends on the rate of which photogenerated carriers migrate from the bulk phase to the surface and the rate of active species’migration such as electron-hole pairs and protons between the catalyst and the substrate surface.How to improve the migration rate of active species from the catalyst bulk to the surface and from the surface to the substrate is the key to enhancing the performance of the photocatalyst.This dissertation takes the chromium removal reaction by photocatalytic reduction as a model reaction,and develops a visible light-responsive metal sulfide SnS₂-based photocatalyst,and studies the influence and mechanism of semiconductor recombination and adjustment of acid sites on the catalyst surface for the catalyst performance.The specific research content as follows.（1）The SnS₂-CNT composite photocatalyst was synthesized by ball milling,and the carbon nanotubes in the catalyst were mainly used as interfacial electron transfer media.The decrease of the electron density of S element in the XPS characterization,and the red shift of the Sn-S vibration peak in the Raman characterization of SnS₂-CNT materials together explain the formation of S-C bonds.DRS characterization shows that SnS₂-CNT photocatalyst has a narrower band gap and stronger visible light absorption capacity than SnS₂.The results of transient photocurrent and photoluminescence spectra show that compared with pure SnS₂,the electron-hole separation efficiency of SnS₂-CNT catalyst is significantly improved.The reason is that the formation of the S-C bond makes the interface of SnS₂ and the CNT form a close contact,promotes the separation and migration of photo-generated carriers,and improves the photocatalytic reduction and chromium removal ability of the catalyst.The photocatalytic chromium removal experiment shows that when the catalyst dosage is 0.2gL^-1,the potassium dichromate concentration is 20mgL^-1,under the condition of pH 2.5 and visible light irradiation,SnS₂-CNT can remove hexavalent chromium within 40min completely.removal,The SnS₂ only has a 31%removal rate of hexavalent chromium under this condition.（2）By controlling the types of sulfur sources and mineralizers in the hydrothermal reaction,SnS₂ photocatalysts containing S-H or O-H type Bronsted acid sites on the surface were successfully prepared,and formation mechanism of two Bronsted acid sites were analyzed.Using in-situ infrared spectroscopy,contact angle,XPS and other characterization methods,the existence of S-H and O-H on the surface of the relevant catalyst was proved.The results of the Mott Schottky curve show that the presence of S-H makes the conduction band energy level of the catalyst surface lower,which is beneficial to the migration of electrons from the bulk phase to the surface.The characterization results of transient photocurrent,AC impedance and photoluminescence spectra show that compared with O-H type SnS₂,the SnS₂photocatalyst with S-H on the surface has significantly improved photogenerated carrier separation efficiency.The experimental results of isotope kinetics and sacrificial agent show that,compared with O-H type SnS₂,the proton migration rate on the surface of S-H type SnS₂ photocatalyst is significantly increased.The above two factors make the S-H type SnS₂ photocatalyst have better photocatalytic chromium removal potential.The photocatalytic chromium removal experiment shows that when the catalyst dosage is 0.13gL^-1,the potassium dichromate concentration is 10mgL^-1,under the condition of pH 2.5 and visible light irradiation,SnS₂-SH can remove hexavalent chromium within 8min completely.And its reaction kinetic constant is 16 times that of SnS₂-OH.