The Preparation And Ozone Catalytic Decomposition Performance Of The ?-MnO₂ Catalysts

In recent years,photochemical smog occurs frequently in summer in China,the situation of ozone pollution in the atmosphere is increasingly serious.As a result of air exchange,elevated outdoor ozone concentrations increase indoor ozone concentrations.The air purifier with ozone oxidation technology,the fresh air equipment with electric discharge and dust removal,and printers involving ultraviolet radiation are also important sources of indoor ozone.Excessive ozone concentration will cause cardiovascular and respiratory disorders and seriously harm human health;Ozone can even slow plant growth and reduce crop yields.Because of the strong oxidation ability of ozone,even low concentration of ozone will react with organic matter in the air,and produce a variety of chemical oxidation products,which will have greater harm to the human body.Therefore,the elimination of ozone has important significance for human health and ecological environment.The catalytic method is safe,economical and efficient for ozone decomposition.Among various catalysts,the manganese oxide catalyst has attracted extensive attention from researchers because of its low price and excellent performance.?-MnO2 is a kind of manganese dioxide that has one-dimensional channel structure with disordered 1 × 1 and 1× 2 tunnels,which has widely applied in catalytic decomposition of formaldehyde,toluene and NOx.In addition,?-MnO2 has the advantages of excellent performance,low price and low toxicity.?-MnO2 can effectively decompose ozone in dry conditions,but its activity is significantly inhibited in the presence of water vapor.The research mainly focused on the study of ?-MnO2 catalysts.The ?-MnO2,Ce-?-MnO2 and Co-?-MnO2 catalysts were prepared by doping transition metals(Ce and Co)in the hydrothermal synthesis process,and pH=1,2,4 and 7 catalysts were prepared by changing the post-treatment process(washing process)of the Ce-?-MnO2 catalyst.By optimizing the doping ratio,we found that the Ce-?-MnO2 catalyst with Ce/Mn of 1/8 had the best activity.The Ce-?-MnO2 catalyst showed ozone conversion of 96%for 40 ppm of O3 under relative humidity of 65%and space velocity of 840 L/(g·h)after 6 h at room temperature,which is far superior to the ?-MnO2 catalyst.Among the M-?-MnO2 catalysts doped with different transition metals(Ce or Co)with the same ratio(1/8),Ce-?-MnO2 catalyst(96%)had better performance than Co-?-MnO2(55%)and ?-MnO2(38%).Doping Ce leads to the formation of mixed crystals of ?-MnO2 and ?-MnO2 in the hydrothermal process of the Ce-?-MnO2 catalysts.With the increase of specific surface area(74 m2/g?120 m2/g),more surface defects and oxygen vacancies formed,which can provide more active sites and be favorable for the adsorption and decomposition of ozone.The Ce-?-MnO2 catalyst had different deactivation mechanisms under the dry and wet conditions.The deactivation in the wet airflow was due to the competitive adsorption of water molecules and ozone molecules,and the deactivation in the dry airflow was due to the toxicity of oxygen vacancy.Among the catalysts with different washing degrees,the pH=7 catalyst has better resistance to water and high space velocity.The residual acid ions on the surface of the catalyst will reduce its hydrophobicity,so the activity of other catalysts(pH=1,2 and 4)which are not washed thoroughly will decrease in the humid atmosphere.In addition,the residual acid ions will adsorb on the oxygen vacancy and reduce the content of oxygen vacancy,which will reduce the activity of the catalyst at dry atmosphere and high space velocity.Kinetic experiments showed that the pH=7 catalyst had the lowest apparent activation energy,so ozone molecules were more easily activated on the surface of the pH=7 catalyst.The DFT calculation revealed the deactivation mechanism about different kinds of oxygen vacancy on the catalyst.Under room temperature and the space velocity of 840 L/(g·h),the ozone decomposition activity of Ce-?-MnO2 catalyst washed to pH=7 at the relative humidity of 65%and dry atmosphere was 60%and 100%after 96 h,respectively,which indicated that the catalyst has a great practical application prospect.