Study Of The Performance Of Toluene Catalytic Oxidation And The Cycle Of Oxygen Species Over Mn-based Catalysts

Volatile Organic Compounds(VOCs)are major atmospheric pollutants which are hazardous for human bodies and the environment.Catalytic oxidation is an efficient and environment-friendly technique,which is widely used for the removal of VOCs in industrial waste gases.Due to the high cost and deactivation of noble metal catalysts,it is vital for the technique to develop inexpensive non-noble-metal catalysts.Manganese oxide catalysts with good capability of oxygen storage/release were investigated as the active species in this thesis.Molecular sieves with structures of multi-pores and high surface area were used to enhance the dispersion of active species and adsorption and diffusion of pollutant molecules,and a second component(Ag)was added to facilitate the catalytic activity of toluene.The effect of supports on the reduction of manganese oxides and interaction between active species and supports were investigated and the structure-activity relationship was discussed;the migration,transformation and circulation of oxygen species in toluene catalytic oxidation reaction was deeply discussed combined with in-situ surface reaction MS techniques and isotopical experiments;a small amount of Ag was doped to adjust the structure of manganese oxides,facilitate the cycle of oxygen species and finally improve the catalytic activity.The detail results are as follows:(1)Toluene and acetone oxidation over the manganese oxide catalyst were investigated.It was shown that CO2 selectivity of the two VOCs differed greatly.The completed conversion temperature of toluene oxidation was 220?,and CO2 selectivity reached more than 80%in the whole reaction.Benzaldehyde and benzoic acid were produced as intermediate products.The 100%conversion of acetone oxidation was acquired when the temperature was just 140?,but the CO2 selectivity was only 3.6%,and it reached 83%and 100%at 180? and 220?,respectively.Large amount of acetate was found in the reaction.Compared with acetone oxidation,oxygen species of the catalyst was replenished more easily after toluene oxidation,and it facilitated the circulation of oxygen species in the oxidation reaction and was in favor of the removal of pollutants.(2)Rod-like(R),hexagonal(H)and fiber-like(F)SBA-15 were synthesized by hydrothermal method and manganese oxides were supported on the SBA-15 by precipitation noted as MnOx/SBA-15.It was found the morphology and properties of SBA-15 had great effect on the catalytic activity.MnOx/R-SBA-15 showed best catalytic activity among the catalysts,and the conversion reached more than 90%at 230?(78%for MnOx/H-SBA-15 and 61%for MnOx/F-SBA-15,respectively).The key factor for toluene catalytic activity on MnOx/R-SBA-15 was the good migration of oxygen species,more Mn2O3 and surface lattice oxygen,and the catalyst was in favor of the diffusion of toluene molecules.(3)Combined with in-situ surface reaction MS techniques and isotopical experiments,the migration,transformation and circulation of oxygen species in toluene catalytic oxidation reaction was deeply investigated to acquire the condition of oxygen species cycle in the reaction.Parameters(pre-exponential factors and rate constants)of the reaction were tested and calculated to confirm the reaction mechanism of toluene oxidation at different temperatures in theory.It was found lattice oxygen participated in the reation and adsorded oxygen decreased the reaction temperature by in-situ surface reaction MS techniques.In temperature programming surface reaction by 18O2 it was found C16O2,C16O18O and C18O2 were produced,and the formation of C16O2,C16018O followed Mars and van Krevelen mechanism.In multi-pulse experiments,the formation of C18O2 was different from the other two,and it followed Langmuir-Hinshelwood mechanism.With the increase of temperature,the amount of C18O2 showed the same rule of C16O18O and C16O2,and the ratio of C16O18O increased,which indicated that the lattice oxygen was the activity species and adsorbed oxygen didn't participated in the reaction directly.The whole process followed Mars and van Krevelen mechanism.In the reaction,surface lattice oxygen was consumed and replenished and bulk lattice oxygen moved to surface and participated in the reaction according to Raman results.The replenishment of bulk oxygen took place by gaseous oxygen.Therefore,the surface and bulk lattice oxygen,adsorbed oxygen and gaseous oxygen formed a cycle of oxygen species.(4)The effect of Ag on the structure and toluene catalytic oxidation capability for MnOx/SBA-15 was studied.It was investigated that 1 wt.%Ag obviously enhanced the activity at low temperature(increased by 22%at 220? and 13%at 225?).The addition of Ag increased more surface lattice oxygen and oxygen migration.In in situ surface reactions more lattice oxygen was activated by Ag,and the amount of C16O2 and C16O18O rised obviously.The ratio of C16018O on Ag-MnOx/SBA-15 was much higher than MnOx/SBA-15,which meant the rate of transformation increased.Compared with i-Ag-MnOx/SBA-15,interaction with defects between Ag and MnOx was formed on Ag-MnOx/SBA-15 and the structure directly promoted the rate of transformation and cycle.