Experimental And Theoretical Study On Mercury Removal Via BiOIO₃ Composite Photocatalyst

The rapid development of national economy is accompanied by the development and utilization of fossil fuels.The burning of a large number of coal produces kinds of air pollution,which brings a bad impact on the living environment of the human.Among the many air pollutants,the heavy metal mercury pollution in coal-fired flue gas has been widely concerned by scholars due to its serious toxicity and bioaccumulation.The gaseous elemental mercury with the highest content in coal-fired flue gas is stable and insoluble in water,that it's the hardest one to deal with.Compared with adsorption and thermal catalysis,photocatalysis technology is highly efficient,stable and free of secondary pollution.In this paper,the BiOIO₃ semiconductor photocatalyst with unique lamellar structure was selected as the active component to remove the elemental mercury from flue gas.While the graphene and carbon nanotube which with high specific surface area and unique microstructure were used as the catalyst carrier to improve the defects of pure BiOIO₃.In this paper,the BiOIO₃/graphene nano-composite photocatalyst was prepared via hydrothermal method.The high specific surface area of graphene was conducive to the adsorption of reactants in flue gas.While its large layer of graphene nanosheets was conducive to the adhesion of the active component BiOIO₃ crystal.The composite samples exhibited excellent photocatalytic oxidation performance of elemental mercury,and the highest efficiency was 88.5 % under visible light.The introduction of graphene would lead to the reduction of band gap width in the composite system.And the free radical trapping experiment revealed the key role of photogenic holes in the catalytic oxidation of elemental mercury.The BiOIO₃/CNT nano-composite photocatalyst was successful synthesised by in situ crystallization of BiOIO₃ crystal on carbon nanotube via one-step hydrothermal method.The characterization of the samples and the removal of elemental mercury from flue gas were studied.The results showed that carbon nanotube was an excellent carrier for preparation of bismuth-based composite catalyst,and the presence of carbon nanotube would not affect the crystallization of BiOIO₃.The catalytic oxidation efficiency of elemental mercury was up to 83.6 %.The abundant pore structure provided by the rich carbon nanotube is conducive to the adsorption of reactants and the migration of photogenerated electrons to carbon nanotube.Finally,the mercury removal mechanism in flue gas from coal-fired power plants was studied via density functional theory calculation.The heterojunction model of BiOIO₃/graphene was constructed and the theoretical feasibility of forming heterojunction between BiOIO₃ and graphene was verified.The contribution of carbon doping level at the bottom of the conduction band position was the main reason for the reduction of band gap width of the composite system.The enhanced local electric field in the heterojunction interface attachment was beneficial to the migration of photogenerated electrons to graphene,while the photogenerated holes enriched in the valence band position were beneficial to the catalytic oxidation of elemental mercury.The theoretical calculation results were consistent with the experimental results.By the way,based on the transition state search theory,the feasibility of the traditional SCR catalyst active component for the catalytic oxidation of elemental mercury,and the mechanism that the removal of elemental mercury enhanced by surface activity chlorine on unburned carbon of fly ash were also studied.This paper provides some basic data and reference for the control of heavy metal mercury pollutant in coal-fired power plants and the development of highly efficient bismuth-based composite photocatalyst.