Experimental Study On Regulating The Morphology And Interface Of Bismuth-based Catalyst And Its Application Of Removal For Power Plant Pollutants

With the rapid advancement of industrialization and the massive consumption of fossil energy,air pollution has become an inevitable problem in the lives of all human beings.Mercury,as a typical air pollutant,can cause serious damage to the ecological environment and human health due to its high toxicity,bioaccumulation,and long-distance mobility in the atmosphere.In August 2017,the"Minamata Convention on Mercury"came into effect,aiming to reduce and control global man-made mercury emissions.The latest global mercury emission assessment report in 2018 stated that the anthropogenic mercury emission source is about 2,220 tons,of which coal-fired power plants are identified as the main anthropogenic source of atmospheric mercury emissions.Both oxidized mercury(Hg²⁺)and particulate bound mercury（Hg^p）in coal-fired flue gas can be effectively reduced through existing air pollutant control equipment in coal-fired power plants.However,the gas phase elemental mercury（Hg⁰）has the characteristics of high volatility and insoluble in water,and effective control of its emissions is still a challenge.In recent years,photocatalytic oxidation technology has the advantages of direct use of sunlight,cost-effectiveness,and environmental friendliness,and has received extensive attention in environmental purification.Photocatalytic oxidation of mercury in coal-fired flue gas has become a research hotspot,and the core of using photocatalytic oxidation technology to remove Hg⁰ pollutants in flue gas is to develop efficient photocatalysts.The bismuth-based semiconductor catalyst has a special layered polar structure,which makes it have built-in electric fields（BEF）,and its valence band is composed of Bi6s orbitals and O 2p orbitals hybridization,which increases the dispersion of the valence band and reduces the band gap.This can promote the separation and transfer of photogenerated charge carriers,so that it has better photocatalytic activity.Therefore,the bismuth-based catalyst is an effective photocatalyst for removing Hg⁰ in coal-burning flue gas.However,the band gap of the bismuth-based catalyst is still large,and the photoresponse range（approximately 410 nm of the light absorption boundary）is still small,which makes the photo-generated charge carrier have a large recombination rate,which is not conducive to the improvement of photocatalytic efficiency.It hinders the practical application of photocatalytic purification of power plant flue gas pollutants.For the above issues,a series of bismuth-based catalysts have been constructed and synthesized via morphology and interface control.The effects of different morphologies and interface heterojunctions on the photocatalytic removal of gaseous Hg⁰ were studied.In addition,morphology,structure,chemical composition,element state,photoelectrochemical properties,and carrier fluorescence lifetime were analyzed by physical and chemical characterization methods.Furthermore,the reaction mechanism of the bismuth-based catalyst and gaseous Hg⁰ was deeply explored,and the effect of flue gas components on the performance of mercury removal was studied.Finally,quantum chemical density functional theory was used to calculate and analyze the microscopic characteristics of the energy band structure,density of states and surface work function,and reveal the structure-activity relationship of the catalyst.It provides a new route for the synthesis of new high-efficiency bismuth-based photocatalysts to control heavy metals such as mercury and arsenic in the coal-fired flue gas of power plants.The major research conclutions are summarized as follows:1.Morphology control is one of the most effective ways to improve the physical and chemical properties of the photocatalyst.The modified solvothermal method was used to prepare Fe³⁺doped Bi₇O₉I₃ with different morphologies,and the photocatalytic oxidation removal mechanism between gaseous Hg⁰ and Fe³⁺doped Bi₇O₉I₃ was studied.The results indicate that Bi₇O₉I₃with different Fe³⁺doping amounts exhibits different morphology and photocatalytic activity.When the Fe³⁺doping amount is 0.3%molar ratio,Bi₇O₉I₃ shows a rosette morphology and has the best catalytic activity for photocatalytic oxidation to remove gaseous Hg⁰.The optimized doping of Fe³⁺can not only tune the oxygen vacancy content on the Bi₇O₉I₃ surface to optimize exciton dissociation,but also promote charge carrier separation and molecular oxygen activation.Moreover,the oxygen vacancies and Fe²⁺/Fe³⁺on the surface of the catalyst can be acted as the active centers of the catalytic oxidation reaction,which significantly enhances the performance of photocatalytic oxidation to remove Hg⁰.2.It is a very feasible photocatalyst modification strategy to construct an interface heterojunction with interface regulation coupled with a semiconductor.The construction of an interface heterojunction can effectively improve the separation efficiency of the charge carrier of the catalyst and thus enhance the photocatalytic activity.The BiOIO₃ nanosheets were self-grown on BiOBr microspheres by solvothermal method to construct a three-dimensional hierarchical BiOBr/BiOIO₃ Z-scheme heterojunction photocatalyst with rich oxygen vacancies.Based on the experimental and characterization results,it is found that the synergistic effect of Z-scheme heterojunction and oxygen vacancies can not only promote the separation and transport efficiency of electron-hole pairs,but also maintain high redox performance,which is beneficial to photocatalytic removal of gaseous Hg⁰.Moreover,BiOBr/BiOIO₃Z-scheme heterojunction can effectively promote the activation of molecular oxygen in the simulated flue gas to generate more reactive oxygen species（·O₂^-and·OH）.This enables the catalyst to further oxidize and remove gaseous Hg⁰during the photocatalytic reaction process,thereby enhancing the photocatalytic performance of the catalyst for removing gaseous Hg⁰.3.The combination of morphology and interface tuning can synthesize a composite photocatalyst that can effectively remove gaseous elemental Hg⁰ by photocatalytic oxidation.CeO₂ nanoparticles were grown on BiOBr microspheres by solvothermal method to form CeO₂/BiOBr composites,and then reduced graphene oxide（r GO）was used as a supporter to synthesize a ternary composite CeO₂/BiOBr/r GO catalyst for photocatalytic removal Hg⁰,and further explored the catalytic relationship between the photocatalyst and gaseous Hg⁰.The experimental results of photocatalytic removal of gaseous Hg⁰ suggest that the CeO₂/BiOBr/r GO ternary composite catalyst exhibits the best photocatalytic mercury removal performance.The characterization results indicate that the interfacial Z-scheme heterojunction was formed after the r GO supporter loaded with CeO₂/BiOBr.Due to the excellent conductivity of r GO,the charge carrier transport performance of the ternary composite catalyst was significantly improved,which strengthened the oxidation process of elemental mercury in the photocatalytic reaction.Meanwhile,the oxygen vacancies on the surface of ternary composite catalyst can effectively promote the activation of molecular oxygen.The effect of actual flue gas components（SO₂,NO and HCl）on the photocatalytic oxidation of the catalyst to remove mercury was further studied.When the SO₂ was introduced,due to the competitive adsorption of SO₂ and Hg⁰on the catalyst surface,the mercury removal performance of the catalyst was significantly decreased.After the introducing of NO and HCl,active nitrate species and active chlorine species were generated during the catalytic reaction,which was beneficial to the further oxidation and removal of Hg⁰.Finally,combined with density functional theory calculations,the band structure,density of states and surface work function of CeO₂ and BiOBr were analyzed,and it is revealed that the existence of surface oxygen vacancies is beneficial to electron transfer and the photocatalytic mercury removal reaction of the interface Z-scheme heterojunction mechanism.