Density Functional Theory Study On The Mercury Removal Mechanism Of Cu S-based Adsorbent

Mercury is a toxic trace element,and mercury pollution as a global pollution poses great threat on human being and the environment.Coal-fired boilers are the main anthropogenic source of mercury pollution,and adsorbent injection technology is the most effective mercury pollution control technology for coal-fired power plants,so the development of adsorbents is important for the treatment of mercury pollution.In recent years,Cu S material has been proven to have excellent mercury removal performance,it not only has high mercury capacity and high mercury removal rate,the mercury removal performance is also very stable,almost unaffected by other atmospheres in the flue gas（such as O₂,H₂O and SO₂）.Besides,the preparation process of Cu S-based mercury adsorbents is relatively simple and the cost is relatively low.A lot of studies on Cu S-based mercury adsorbents have been reported,and Cu S materials are regarded as a new generation of mercury adsorbents that are expected to be commercialized.Experimental studies have confirmed that Hg is mainly adsorbed on Cu S-based sorbents in the form of Hg S,and Cu S has the best mercury removal performance among many metal sulfides.Although researchers have proposed some mercury removal mechanisms of Cu S materials based on experimental phenomena,there is still no unified understanding of the active components responsible for mercury removal on Cu S-based adsorbents.In addition,the reason for the unique mercury removal performance of Cu S materials in many metal sulfides is also unclear.Density functional theory is a mature computational chemistry method that can assist experimental studies to better explore reaction mechanisms.This thesis systematically studied the mercury removal mechanism of Cu S materials based on density functional theory,and the main contents are as follows:First,from the perspective of surface adsorption,the adsorption of Hg⁰,Hg Cl and Hg Cl₂ on the two stable Cu S（001）surfaces（a pair of asymmetric surfaces formed by breaking the Cu（2）-S（1）bond,named as slab1 and slab2）was explored,and the adsorption of Hg⁰ on the vacancy defect Cu S（001）surfaces is also investigated.The results show that Hg⁰ is physically adsorbed on the clean Cu S（001）surface,and the adsorption energy is-26.0～-46.0 k J/mol.While Hg Cl and Hg Cl₂ can be strongly adsorbed on the Cu S（001）surface,and their molecular adsorption and dissociative adsorption tends to occur on slab1 and slab2,respectively.The vacancy defects can strengthen the adsorption of Hg⁰ on the Cu S surface,and the adsorption energy can be increased to-65.1～-178.8 k J/mol.The promotion effect of vacancy defects on Hg⁰ adsorption is more prominent on slab1.In general,the adsorption of mercury species（including Hg⁰,Hg Cl and Hg Cl₂）on slab2 is stronger than their adsorption on slab1.Secondly,the effect of flue atmospheres（including chlorine-containing atmospheres:HCl and Cl₂,oxygen-containing atmospheres:O₂,H₂O and SO₂）on the adsorption of Hg⁰ on the Cu S（001）surface was explored.The results show that the adsorption of all atmospheres except HCl on slab2 is stronger than that on slab1.The physical adsorption of HCl on the surface of Cu S（001）has no obvious effect on the adsorption of Hg⁰.Cl₂ can dissociative adsorb on the two surfaces to form Cu S（001）surfaces containing Cl ions.The adsorption of Hg⁰ on the Cl-modified surfaces is significantly stronger than that on clean Cu S（001）surfaces.On slab1,Hg⁰ and the adsorbed Cl will generate Hg Cl₂.On the Cl modified slab2,the interaction between Hg⁰ and the surface is strengthened,and it is converted from physical adsorption to chemical adsorption.By discussing the adsorption behavior of oxygen-containing atmosphere on the clean Cu S（001）surfaces and the vancancy defect slab1,combined with the experimental phenomenon,it is deduced that the main active sites for mercury removal on the Cu S sorbent are Cu vacancy defects on slab1.Then,we found that the relaxation of the Cu S surface containing Cu vacancy defects can trigger the formation of polysulfides,and the formation of polysulfides and vacancy defects are complementary.Polysulfides are likely to exist on slab1,because the formation of polysulfides on slab1 containing Cu vacancy defects is a favorable reaction both on thermodynamic and kinetic,and polysulfides can stabilize slab1 containing Cu vacancy defects.But for slab2,only when the Cu defect concentration is quite high,the S atoms on the surface can couple each other to form polysulfides.The content of polysulfides on the surface of Cu S（mainly slab1）changes dynamically.When Cu defects are generated,polysulfides will appear on the surface of slab1,and when Hg moves to the surface of Cu S,polysulfides will decompose,causing Hg firmly adsorbed on Cu vacancy defects.Although polysulfides will not directly adsorb Hg⁰,and perhaps they can promot the adsorption of Hg⁰ on the surface of Cu S materials by increasing the adsorption sites.Last,we used the cluster model to simulate Cu S nanoparticles and AC supported（Cu S）_nclusters to simulate Cu S modified carbon materials,and then investigated the mercury removal mechanism of Cu S nanomaterials,explored the effect of supporter on the mercury removal ability of Cu S nanoparticles and revealed the fundamental reason why Cu S has the best mercury removal performance among various metal sulfides.The calculation results show that the interaction between（Cu S）_n（n=1-10）clusters and Hg⁰ belongs to chemical adsorption,and the adsorption energy is-57.4～-140.1 k J/mol.When Hg⁰ interacts with the（Cu S）_n cluster,Hg⁰ will form bonds with Cu first,expanding its Cu core into the structure of Cu core of the（Cu S）_n+1 cluster,and then Hg⁰ is further immobilized by external S ions.Besides,by comparing its Hg⁰ adsorption behavior with those of other metal sulfide clusters,it is revealed that the reason for the excellent mercury removal performance of Cu S material is the mixed valence state of Cu and S(Cu¹⁺/Cu²⁺and S^1-/S^2-)can both oxidize mercury.Through the above discussion,this thesis preliminarily solved the two problems mentioned above.The active sites responsible for mercury removal on the Cu S-based adsorbent are mainly Cu vacancy defects on slab1,and Hg is chemisorbed on the active sites with an adsorption energy of-178.8 k J/mol,and Hg-S bonds are formed,which is consistent with the experimental results.The Cu defects on the surface of Cu S will trigger the generation of polysulfides,and the formation of polysulfides and Cu defects promote each other.The polysulfides change dynamically.When Hg moves to the Cu S surface,the polysulfides will decompose and expose the original Cu defect sites,although polysulfides cannot directly adsorb Hg⁰,it can improve the mercury removal performance of Cu S materials by increasing the Hg adsorption sites.The reason why Cu S material is superior to other metal sulfides in mercury removal performance is that Cu S is formed by the incomplete redox reaction of Cu and S,thus both Cu and S have certain oxidizing ability to Hg⁰.