Structure Regulation Of Lanthanum Cobaltate Perovskite And Its Effect Mechanism On Simultaneous Catalytic Oxidation Of NO And Hg~0

The existence of nitrogen oxides（NO_x）and heavy metal mercury（Hg⁰）in the atmosphere can cause a series of human health and environmental problems,and thus has received close attention from countries around the world.Coal combustion is one of the main sources of nitrogen oxides and mercury in the atmosphere.According to the status quo of Chinese country’s energy system,coal will still be the most widely used energy source in China for a long time to come.Therefore,how to reduce the nitrogen oxides and heavy metal mercury produced by coal combustion has become a major concern.Based on the current denitration and mercury removal technology,catalytic oxidation is one of the efficient and promising technologies.The simultaneous catalytic oxidation technology is to oxidize NO and Hg⁰ in coal-fired flue gas into nitrogen dioxide（NO₂）and Hg²⁺which are soluble in water under the action of catalyst.The oxidation products can be absorbed and processed by wet flue gas desulfurization system.The key about catalytic oxidation is the development of efficient and active catalysts.At present,the most common catalysts with catalytic oxidation activity are transition metal oxide catalysts,but these catalysts are often limited in use due to cost and activity issues.The perovskite catalyst（structural formula is ABO₃）has excellent oxidation ability and high stability,and has become an important field of catalyst development in recent years.In this study,perovskite-type oxides were used as catalysts for the simultaneous catalytic oxidation of NO and Hg⁰.La BO₃（B=Co,Mn,Ni and Cu）perovskites with different B sites were prepared on the basis of lanthanide perovskites,and La Co O₃perovskites with better performance were screened and modified and heterogeneous element doping treatment（preparation method,acetic acid etching,A-site Sr doping and B-site Mn doping）to improve its catalytic activity,and analyze its influence on catalytic activity from the structural change of lanthanum cobaltate mechanism,and the following main conclusions are drawn:（1）Using La as the A-site ion of perovskite,and four transition metals Co,Mn,Ni and Cu were used to prepare perovskite catalysts as the B site.The activity of La BO₃（B=Co,Mn,Ni and Cu）for the simultaneous oxidation of NO and Hg⁰ was studied,and the effects of oxygen content and the presence of NO and Hg⁰ on the oxidation activity of La Mn O₃ and La Co O₃ were deeply analyzed.The results show that the oxidation activity of B-site ions can be summarized as:Mn⁴⁺>Co³⁺>Ni²⁺>Cu²⁺,the oxygen content of 10%can fully meet the oxidation demand,and there is a competitive adsorption relationship between NO and Hg⁰,which affects the catalytic activity.（2）La Co O₃ perovskite was prepared by molten salt method,sol-gel method and co-precipitation method,and the influence of preparation method on the structure of La Co O₃ was investigated.The results show that the three methods can successfully prepare the perovskite structure of ABO₃,but the K and Na in the molten salt method and the co-precipitation method can promote the oxidation activity to a certain extent.At the same time,in different preparation methods,in order to synthesize stable perovskite structure,La Co O₃ makes the samples show different physicochemical properties,which makes the activity of La Co O₃ to oxidize NO and Hg⁰ different.（3）The 1 M,2 M and 3 M acetic acid solutions were used to etch the La Co O₃perovskite,and the effect of different acetic acid concentrations on the structure of La Co O₃ was studied,and the activity changes of its simultaneous oxidation of NO and Hg⁰ were analyzed..The results show that the etching treatment of acetic acid can improve the oxidation activity of lanthanum cobaltate,because under the etching treatment of acetic acid,the La element on the surface of the perovskite gradually dissolves,so that the active component Co is exposed,but if the concentration of acetic acid is too high,it will further dissolve the active component Co,thereby reducing the activity.（4）The effect of A-site heteroelement Sr doping on the structure of lanthanum cobaltate perovskite was investigated,and the mechanism of Sr on the oxidation activity of lanthanum cobaltate perovskite was summarized.The results show that Sr doping improves the activity of La_1-xSr_xCo O₃（x=0.1–0.5）perovskites for simultaneous oxidation of NO and Hg⁰,but Sr has a solubility limitation in lanthanum cobaltate perovskites,when the doping amount exceeds 0.4,Sr can only exist on the surface of lanthanum cobaltate in the form of inert substance Sr CO₃,and the content of active components in the structure decreases,which leads to a decrease in oxidation activity.（5）The effect of B-site heteroelement Mn doping on the structure of La Co O₃perovskite was investigated,and the mechanism of Mn on the oxidation activity of La Co_1-xMn_xO₃（x=0.1–0.5）perovskite was summarized.The results show that Mn has a strong affinity for Hg⁰,so the doping of Mn makes the competitive adsorption capacity of Hg⁰ stronger than that of NO,and the oxidation activity of Hg⁰ gradually increases with the increase of Mn doping amount,while that of NO decreases.In conclusion,the La Co O₃ perovskite prepared in this study has a good ability to simultaneously catalyze the oxidation of NO and Hg⁰,which provides a solution for the selection and application of catalytic materials in the future.At the same time,the treatment methods on surface modification of the perovskite in this study（different preparation methods,acetic acid etching and doping with hetero-elements Sr and Mn）also provide new ideas for improving the activity of perovskite catalysts.