Hydrothermal Synthesis of Bismuth-based Photocatalysts and Their Application in the Removal of Contaminants

Author:Qi Xue Mei

Supervisor:zhu xin yuan wu jiang

Database:Doctor

Degree Year:2016

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Pages:153

Size:5205K

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In our country,the problem of the water and atmospheric environmental pollution is becoming more and more serious,which causes extensive concern.The power plant emitted into the atmospheric environment about 500-700 tons of mercury per year by coal combustion in our country.Mercury is a kind of poisonous heavy metals,which have higher volatilization and bioaccumulation properties and could cause great harm to the ecological environment and human health.Catalytic oxidation using photocatalytic technology to remove elemental mercury(Hg0)emitted from coal-fired flue gas has been recognized by many researchers.The researchers usually adopted TiO2-based materials as the photocatalysts and the removal efficiency of Hg0 are generally low,therefore develop stable and possesses remarkable removal efficiency of Hg0 become the hot issues of this research field.The valence band of bismuth based compounds is made up of Bi 6s and O 2p states,which narrow the bandgap and therefore can absorb more light energy.What’s more,bismuth based layered material are composed of(Bi2O2)2+cationic and various kinds of ionic layers,which is beneficial to the transfer of charge between the layers and captured by the water molecules.These properties can effectively suppress the recombination of photogenerated holes and electrons and then improve the photocatalytic activity.Therefore,the layered structures of bismuth compounds possess remarkable photocatalytic performance and have become the research hot issue in the field of photocatalysis.In this dissertation,a series of bismuth-based photocatalysts were fabricated by optimizing the synthesis conditions.The physical and chemical properties of the as-prepared samples were characterized by various techniques.The photocatalytic activities were evaluated by the oxidation removal efficiency of gaseous elemental mercury and the degradation of organic compounds in aqueous under light irradiation.Meanwhile,the relationship between structures and photocatalytic performances was also investigated to research and develop the photocatalysts which were suitable for the oxidation removal of Hg0 emitted from flue gas.The main results are listed below:1.BiVO4 is one kind of bismuth based compounds which can absorb visible light.Mixed crystal structure is in favor of the effective separation of photogenerated hole and electron pair and thus improves the photocatalytic activity of the photocatalysts.In this chapter,the BiVO4 samples with tetragonal/monoclinic mixed crystal structure were prepared by hydrothermal process using ethylenediamine as chelating agent and alkali source.The influence of ethylenediamine on the morphology and crystal structure of BiVO4 was studied by controlling the addition amount of ethylenediamine.The as-prepared BiVO4 photocatalysts were well characterized by a series of measurements.The influence of crystal structure and morphology on the photocatalytic activity was investigated.It was shown that BiVO4 sample exhibited relatively higher photocatalytic performance for the degradation of Rhodamine B in aqueous under visible-light irradiation.However,the oxidation removal performance of Hg0 was very low and the removal efficiency was only ca.15%.This could be ascribed to that the Hg0 was mainly oxidized by the holes generated under the light irradiation.The valence band potential of BiVO4 was relative negative and therefore the oxidation ability of the holes was weak,which could not oxidize Hg0 effectively.2.Building composites with heterogeneous structure is a kind of effective method to improve the photocatalytic activity.A series of different proportion of Ag3PO4/BiVO4-x composites were prepared through in-situ deposition method to overcome the low photocatalytic activity for the BiVO4.The effect of Ag3PO4:BiVO4ratio on the photocatalytic activity was systemically studied.The prepared Ag3PO4/BiVO4-x composites showed excellent photocatalytic activity for the degradation of Rhodamine B and the 2,4-dichlorophenol under visible light irradiation.Compared to BiVO4,although the photocatalytic oxidation removal efficiency of Hg0 was enhanced,the oxidation removal efficiency of Hg0 was still low(ca.38%)and not suitable for the removal of Hg0 in the flue gas system.3.In order to further synthesis bismuth based photocatalysts,which can effectively photocatalytic oxidation removal of Hg0.Meanwhile,the layered structure bismuth based compounds own the internal electrostatic field,which was beneficial to the separation of photogenerated holes and electrons and then improve their photocatalytic activities.In this chapter,a series of Bi2O2CO3 photocatalysts were synthesized by hydrothermal synthesis method.The influences of precursor solution pH value and the carbon source on the crystal structure,morphology and photocatalytic properties of final Bi2O2CO3 products were systematically studied.The photocatalytic performance of the products was evaluated through the removal efficiency of elemental mercury in flue gas.The final products were pure Bi2O2CO3when the pH value of precursor was in the range of 6.65-9.06 using sodium citrate as carbon source,and three products with different dimensional hierarchical morphology were obtained.Carbon source had an important influence on the composition and morphology of the products.Compared with BiVO4 and Ag3PO4/BiVO4-x composites,the Bi2O2CO3 sample exhibited excellent photocatalytic oxidation activity for the removal of Hg0 and the removal efficiency could reach 74%under UV light irradiation 25min.4.BiOIO3 exhibited superior photocatalytic activity for the degradation of organic pollutants in the aqueous solution because of the heterolayered structure and internal polar field.Therefore,in this chapter,a series of BiOIO3 photocatalysts were prepared through a simple and mild hydrothermal synthesis method.The influence of precursor solution pH value on the final products was systematically studied.The photocatalytic performance of the products was evaluated through the removal efficiency of elemental mercury in flue gas.Moreover,the stability and the possible mechanism for the Hg0 oxidation removal of BiOIO3 were studied.The generated Hg2+after photocatalytic oxidation was also discussed.The results showed that the precursor solution pH value had great influence on composition and morphology of final products.The pure BiOIO3 products were obtained only under the acidic condition and the pH value of precursor solution did not exceed 3.80.The BiOIO3sample exhibited superior photocatalytic activity and nearly 100%of Hg0 were removed under UV light(25 min)and LED light(100 min)irradiation.The photogenerated holes were the main oxidative species responsible for the oxidation removal of Hg0 and almost all of the generated Hg2+was flowed into the subsequent liquid absorption system.This new BiOIO3 photocatalysts with superior photocatalytic activity and good stability have potential application for the Hg0oxidation removal.