Synthesis Of Zr-Mn Mixed Oxides And Their Catalytic Performance For Benzene Combustion

Most volatile organic compounds（VOCs）have irritating odors and biological toxicity.VOCs emitted into the atmosphere are not only harmful to animals,plants and human health,but also can damage the environment by participating in atmospheric photochemical reactions and generating secondary pollutants.Catalytic combustion,due to its high conversion rate,low energy consumption and low secondary pollution,has been considered as a very promising technology for VOCs elimination.In recent years,Mn-based catalysts have attracted a lot of attention because of their good redox performance and controllable structure.However,the structure of single manganese oxide is unstable at high temperature and easily deactivated..It has been reported that compounding Mnwith a second metal to form a mixed oxide is one of the effective approaches to improve the catalytic performance of manganese oxides.Herein,a series of Mn-based mixed oxide catalysts were synthesized by a redox-precipitation method,and the catalytic combustion of benzene was used as a model reaction to evaluate the activities of the as-prepared catalysts.By screening the activity of different components of the mixed oxide catalysts,it was found that the addition of Zr could significantly improve the benzene catalytic activity of MnO₂.After that,Zr-Mnmixed oxide catalysts were used to investigate the influence of Zr/Mnmolar ratio,calcination temperature and reaction conditions（including reactant concentration,air velocity and relative humidity）on the benzene combustion activity.It was found that the mixed oxide which was calcined at 300℃ with a Zr/Mnmolar ratio of 0.2 exhibited the highest catalytic activity with a T₉₀of 195℃(1,000 ppm benzene,WHSV=60,000 ml·g^-1·h^-1,20 vol.%O₂/N₂),which is 71℃ lower than pure MnO₂.According to the catalytic activity and a series of characterization results,it was found that the high-level benzene combustion activity of Zr-Mnmixed oxide was attributed to its high specific surface area,a large number of surface lattice oxygen species,better redox properties and abundant strong acid sites.In addition,the possible pathway of benzene combustion over Zr-Mnmixed oxide was explored by in-situ DRIFTS,and the results showed that the oxidation of benzene followed the Mars-van Krevelen mechanism.Benzene was firstly oxidized by the surface lattice oxygen species on the catalyst surface to form intermediate species such as phenol,benzoquinone and so on.Then the intermediate products were further oxidized to CO₂and H₂O by reactive oxygen species on the catalyst surface.