Construction Of Cu-Mn Catalyst And Its Performance For Toluene Catalytic Oxidation

Volatile organic compounds（VOCs）are one of the main precursors of haze formation and organic aerosol pollution,which is harmful to human health and the environment.Therefore,it is crucial and urgent to develop effective VOCs degradation technology.Catalytic oxidation has been commonly considered as a promising method of VOCs control because of its lower energy consumption,higher removal capacity and less secondary pollution.Compared with noble metal catalysts,transition-metal oxides are promising alternative catalysts due to their low cost and rich resource.However,it is still a difficult point to improve their catalytic oxidation performance at low temperature.Mn-based oxide catalysts show excellent activity for catalytic oxidation with a possible double-exchange behavior between Mn³⁺and Mn⁴⁺which can facilitate electron transfer.In addition,it is well known that the catalytic oxidation of VOCs over transition metal oxides follows the Mars-van-Krevelen mechanism,where the VOCs are first oxidized by surface lattice oxygen species of metal oxides,then the bulk lattice oxygen is transferred to the surface to participate in the reaction,and the metal oxides are re-oxidized when gas phase oxygen is introduced.And during this process,the oxygen vacancy is the main active center for oxygen cycle and catalytic reaction.It not only increases the adsorption of reactants and oxygen on the catalyst,but also promotes the migration and activation of oxygen species in the catalyst.In this paper,a series of copper-manganese spinel were prepared by co-precipitation method;a series of Cu-Mn composite oxides were prepared by hydrothermal method and urea solid phase reaction method,respectively.These catalysts were applied to the oxidation of toluene and the effect of oxygen vacancy on the catalytic activity of toluene was investigated.The specific research contents and results are as follows:1.A series of Cu_1.5Mn_1.5O₄ spinel（CM-T,T=300,400,500 and 600）were prepared by co-precipitation method by adjusting calcination temperature.Through characterization and activity test,it was found that the higher the calcination temperature,the higher the crystallinity of the catalyst was.However,the catalyst prepared by calcination at 300℃has the smallest grain size,the largest specific surface area,and the highest concentration of Cu²⁺,Mn³⁺and adsorbed oxygen species on the surface,indicating that the oxygen vacancy content of the catalyst is the highest.And the presence of oxygen vacancy leads to the weakening of metal oxygen bond,the enhancement of reducibility,the increase of reactive oxygen species,and the ability to react with toluene at a lower temperature of the catalyst.Thus,the CM-300 catalyst showed good catalytic activity for the oxidation of toluene,and the complete conversion of toluene was achieved at 240℃.2.Cu-Mn composite oxide catalyst was firstly synthesized by hydrothermal method and then modified by urea solid reaction treatment to obtain CM-X catalysts（X=2,4,6,where x represents the weight ratio of urea/CM catalyst）with different oxygen vacancy concentrations.It was found that the oxygen vacancy concentration in the compound oxides could be effectively tuned by adjusting the ratio of urea to CM.With the increase of urea content,the specific surface area and oxygen vacancy concentration of the catalyst changed in a volcanic shape and reached the maximum in the CM-4 sample.Moreover,the presence of abundant oxygen vacancies can lead to the amount increase of adsorbed oxygen species on the surface and high migration ability of lattice oxygen,which can improve the catalyst reducibility and oxygen species activity.Therefore,the CM-4 catalyst showed the excellent catalytic activity and stability for toluene oxidation,and toluene could be completely oxidized into CO₂ and H₂O at 215°C.And it was found by in situ DRIFT experiment that the oxidation of toluene occurs via the benzyl alcohol-benzoate-anhydride reaction pathway on CM-4 catalyst.