Effect Of Precursor Anion On The Structure Of Copper Manganese Oxide Catalysts And Catalytic Combustion Performance Of Toluene

In recent years,a large number of volatile organic compounds（VOCs）emitted from industrial production have seriously affected the atmospheric environment.The solution of air pollution problems such as fine particulate matter(PM_2.5)and ozone（O₃）depends on the effective control of VOCs emission.Among many VOCs treatment technologies,catalytic combustion is considered to be one of the most promising technologies because it can achieve complete conversion of VOCs at low temperature,but the high-activity and low-cost catalysts are the key to large-scale application of this technology.Compared with the commercial noble metal catalysts,manganese oxide catalysts have the advantages of low cost,high catalytic activity and good stability,and are expected to obtain the same catalytic activity as noble metal catalysts.There are many factors that affect the activity of manganese oxide catalysts,but the influence of anions inevitably introduced in the preparation of manganese oxide on the activity of the catalysts has rarely been reported in-depth.In order to further understand the structural characteristics of manganese oxide catalysts and improve their VOCs catalytic activity,two kinds of Cu-Mn oxide catalysts with different structures were prepared by selecting manganese precursors and copper precursors containing different anions.The effects of different anionic precursors on the structure of Cu-Mn oxide catalysts and their performance for the catalytic oxidation of toluene,a representative VOCs pollutant,were studied.The reasons for the effects of different anions on the catalytic performance and the internal relationship between the structure and performance of the catalysts were revealed.In this paper,a series of copper manganese oxide catalysts with tunnel structure and layered structure were successfully prepared by the same hydrothermal method with manganese precursors and copper precursors containingNO₃^-,SO₄^2-andCl^-by adjusting the p H of the solution.And the catalytic combustion activity for toluene was investigated in the presence of water vapor.The results showed that under the same preparation conditions with only anion difference,the catalytic performance of the catalysts showed obvious differences.Among them,theα-Cu Mn-N andδ-Cu Mn-N catalysts withNO₃^-showed the best catalytic activity(α-Cu Mn-N:T₅₀=174℃,T₉₀=187℃;δ-Cu Mn-N:T₅₀=184℃,T₉₀=205℃),CO₂ selectivity,normalized reaction rate and the lowest apparent activation energy in their respective systems.The structure and physicochemical properties of the catalysts were analyzed by XRD,SEM,TEM,BET,XPS,H₂-TPR,O₂-TPD and so on.The results show thatα-Cu Mn-N andδ-Cu Mn-N catalysts have the highest contents of Mn³⁺,Cu⁺and O_latt in their respective systems.Higher content of Mn³⁺and Cu⁺will lead to more oxygen vacancies on the surface of the catalyst and promote the redox cycle.O_latt is directly involved in the catalytic reaction of toluene,and high content of O_latt can directly promote the catalytic reaction.At the same time,α-Cu Mn-N andδ-Cu Mn-N catalysts have the best low-temperature reducibility and oxygen migration in their respective systems,which will be conducive to electron transfer and improve the redox cycle.The above results show that the introduction ofNO₃^-increases the content of Mn³⁺and Cu⁺in the process of catalyst synthesis,promotes the formation of more oxygen vacancies,improves the content and mobility of lattice oxygen,and directly promotes the catalytic reaction.In the third part of this paper,the catalytic performance of the catalyst for toluene was investigated under different conditions（space velocity,toluene concentration,moisture content,catalytic reaction time）.The results show that,compared with other manganese oxide catalysts,α-Cu Mn-N catalyst can exhibit excellent catalytic activity at low,medium,and high space velocity and toluene concentration,and it has good long-term stability.Under different water-containing conditions,the activity ofα-Cu Mn-N catalyst is different.The results show that the catalyst can completely convert toluene in 12 hours when the water vapor content is 15%or less;However,when thewater vapor content increases to 25%,the competitive adsorption between water and toluene may occur,which affects the catalytic activity of toluene.