Density Functional Theory Study On Mechanism Of Synergistic Purification Of SO₂,NO,Hg⁰ With Copper-loaded Graphene

Yunnan Province is a non-ferrous metal kingdom,and its lead and zinc ore reserves account for the largest in the country.Lead-zinc smelting,as one of the pillar industries in Yunnan Province,brings economic benefits as well as environmental pollution.Most lead-zinc smelting technologies use sulphide concentrates as raw materials and smelt under the conditions of high temperature and high oxygen.Therefore,a large amount of SO₂,NOx and Hg⁰ will be produced during the production process.After these gases undergo acidification process,most of SO₂,NOx are converted into sulfuric acid and nitrosylsulfuric acid and part of Hg⁰ also enters the sewage acid and sewage acid residue.A small amount of SO₂,NOx and Hg⁰ are discharged into the air along with the acidification tail gas.These low-concentration gases are usually directly discharged at high altitudes,and the concentration of pollutants is reduced by means of flue gas dilution to meet environmental protection requirements,but these measures have not reached the ultra-low-concentration emission standards.Therefore,the combined purification of sulfur,nitrate,mercury has become the focus of current research.Currently,carbon-based materials supported metal oxides are used as catalysts,and there have been many studies on the catalytic oxidation/reduction of SO₂,NO and Hg⁰,but the reaction mechanism is still in the exploration stage,and there are few related reports.Therefore,in this study,a copper-loaded graphene model was established.First,graphene was used as a carbon-based material to simplify the model.All possible adsorption structures of SO₂,NO and Hg⁰ on graphene were calculated by quantum chemical methods,and then,it was found that the catalytic oxidation reaction process of SO₂ and Hg⁰ and the catalytic reduction reaction process of NO on the surface of metal oxide Cu O.In this study,the density functional theory method was used to study the competitive adsorption of SO₂,NO and Hg⁰ on graphene and the changes in charge transfer.By analyzing the partial state density functions of the most stable structures of the three gas molecules,it can be found that there is no coincidence of peaks between the orbitals.The results show that all the adsorption of SO₂,NO and Hg⁰ is caused by physical interactions.Hg⁰ is at the top position,SO₂ is V-shaped at the top position,and the adsorption energy values of nitrogen atoms in NO at the bridge position are:SO₂（VT）-26.4 k J/mol>NO（NB）-15.0k J/mol>Hg⁰（T）-5.3 k J/mol,it is found that the adsorption energy of SO₂（VT）is the highest,and the adsorption energy of Hg⁰（T）is the lowest.Therefore,the adsorption of SO₂ on graphene is more stable than that of NO and Hg⁰,and SO₂ can be preferentially adsorbed and removed.According to Mulliken charge transfer,SO₂（VT）0.116e>NO（NB）0.030e>Hg⁰（T）0.008e,a strong net charge transfer occurs between SO₂ and the graphene surface,which also proves that SO₂ can be preferentially removed by adsorption.In order to show the relationship of adsorption competition between the three gases,a model of gas molecules coexisting on the surface of graphene was established.The results show that NO and Hg⁰,SO₂ and Hg⁰ mutually promote the simultaneous adsorption on the graphene surface,the simultaneous adsorption of SO₂ and NO on the surface of graphene can inhibit each other.Compared with single-molecule adsorption,the adsorption type of bi-molecular adsorption did not change.However,due to the interaction between Hg⁰,SO₂ and NO the adsorption type of Hg⁰,SO₂ and NO at the same time on the graphene surface becomes chemisorption.From the aspect of theoretical mechanism,the reaction mechanism of catalytic oxidation and reduction mechanism of Hg⁰,SO₂ and NO on the surface of Cu O was analyzed.In the catalytic oxidation or reduction of Hg⁰,SO₂ and NO,it was found that Cu O plays a vital role in catalytic oxidation and catalytic reduction reactions.First select the best reaction surface for different gases on Cu O.For the adsorption behavior of Hg⁰,SO₂,NO,O₂ and NH₃ on the surface of Cu O at the same time,it can be found that Hg⁰,NO,O₂ are more likely to form a stable state on the surface of Cu O（110）,and SO₂ and NH₃ are more likely to form a stable state on the surface of Cu O-O（100）.So the catalytic oxidation of Hg and the catalytic reduction of NO mainly occur on the surface of Cu O（110）,and the catalytic oxidation of SO₂ mainly occurs on the surface of Cu O-O（100）.Then search the reaction path of the three gases on the best reaction surface.The results show that:（1）Hg⁰ and O₂ form a stable adsorption structure on the surface of Cu O（110）,with an adsorption energy of-10.13 kcal/mol,which is a chemical adsorption process caused by chemical bond interactions.O₂ breaks the bond to form O atoms and combines with Hg on the surface of Cu O to form the intermediate Hg₂O₂.The final Hg O formed after breaking the bond forms a bond with Cu and O on the surface of Cu O.The maximum energy barrier to be overcome in this process is48.17kcal/mol;（2）SO₂ and O₂ form a stable adsorption structure on the surface of Cu O-O（100）,with an adsorption energy of-19.19 kcal/mol,which is also a chemical adsorption process caused by chemical bond interactions.When SO₂ and O₂ react on the surface of Cu O,the O atoms formed after the O₂ bond is broken are sequentially combined with SO₂ to form SO₃,and the maximum energy barrier to be overcome is96.86kcal/mol;（3）NO,NH₃,and O₂ form a stable adsorption structure on the surface of Cu O（110）,with an adsorption energy of-65.02 kcal/mol.It is also a chemical adsorption process caused by chemical bond interactions,mainly through Cu=O bond or N≡H bond is formed.There are many possibilities for the intermediate products of this reaction.Among them,NO-NH₂ is the easiest to form.H₂O in the product is close to Cu atoms on the surface of Cu O and adsorbed on the surface of Cu O（110）.The maximum energy barrier that this path needs to overcome is 119.54 kcal/mol.This study provided theoretical support for the development of a metal-modified carbon-based catalyst for simultaneous removal of SO₂,NO and Hg⁰.