Synthesis Of Mesoporous Manganese Oxide Catalysts And Their Excellent Performance For Toluene Catalytic Oxidation

With the development of industry in China,the environment quality is becoming serious day by day.Volatile organic compounds（VOCs）are one of major atmospheric pollutants with a large quantities.Hence,it is imperative to develop efficient approaches for VOCs abatement.Catalytic combustion is a common solution for VOCs treatment and satisfactory catalyst is the premise for this technology.Transition mental oxides are becoming increasingly popular in view of advantages such as low cost,poison resistance,and thermal stability,MnO_x is considered to be one of the most potential catalyst for VOCs treatment.However,most currently reported MnO_x catalysts show poor activity for VOCs catalytic combustion.In order to improve the catalytic activity of MnO_x catalysts,this paper major in the design and synthesis of Mn₃O₄ with abundant oxygen defects,acid-treatment and influence of different synthesis condition.The catalyst activity was evaluated by toluene catalytic combustion reaction.The main research work is as follows:（1）Mesoporous Mn₃O₄（m-Mn₃O₄）was prepared through using by-product KCl as template.KCl protected the oxygen defects on the surface of Mn₃O₄,thus the as-prepared catalyst exhibited large specific surface area and rich oxygen defects which were the reason for its abundant surface active sites.Of the Mn₃O₄ samples tested,m-Mn₃O₄presented superior catalytic activity than b-Mn₃O₄ and c-Mn₃O₄,in which 90%conversion of toluene was achieved at 230 ^oC at a WHSV of 40,000 ml·g^-1·h^-1.We propose a facile and green method of fabricating m-Mn₃O₄ with rich oxygen defects at room temperature.（2）γ-MnO₂ with different acid-treatment was obtained by treating the m-Mn₃O₄ using different acid（HNO₃,HCl,HAc and H₂SO₄）.A series of characterizations showed that the order of specific surface area of the samples after acid-treatment was as follows:MnO₂-HN≈MnO₂-HC>MnO₂-HA>MnO₂-HS;the order of surface oxygen vacancies follows:MnO₂-HN>MnO₂-HS>MnO₂-HC>MnO₂-HA.Therefore,the sample treated with nitric acid showed best activity for toluene catalytic combustion,while the sample treated with sulfuric acid showed better activity because of its superior concentration of surface oxygen although it shows the lowest specific surface area.This result indicated that the surface chemical properties are main factors for catalytic activity.The sample treated with acetic acid showed the worst catalytic performance.In this part,the differences in physicochemical properties and catalytic performance ofγ-MnO₂ prepared by different acid-treatments were investigated.（3）Theδ-MnO₂ with different interlayer ions（Na⁺,K⁺）were synthesized by solvothermal method.The effects of the amount of H₂O₂,reaction temperature and time on the structure and catalytic activity were investigated.It can be seen that higher temperature was conductive to the transformation from Mn₃O₄ toδ-MnO₂ and the formation of lamellar morphology for Na-δ-MnO₂.Longer reaction time was beneficial to the crystal transformation,and excessive oxidant could induce the formation of nanosheets morphology.For K-δ-MnO₂,higher temperature and longer reaction time was favorable for the crystal transformation and the formation of the nanosheets morphology.As for catalytic performance,NM-80 was better than KM-80 in terms of reducibility,ability of oxygen adsorption,fluidity of surface lattice oxygen and concentration of oxygen vacancy,and this why NM-80 showed better catalytic activity.However,both of the AOS of surface Mn elements were 3.4,indicating that interlayer ions have a great influence on the inherent chemical properties ofδ-MnO₂.