Study On Microstructure Regulation And Doping Modification Of LaMnO₃ Catalysts And Their Performance And Mechanism For Catalytic Oxidation Of Toluene In The Air

Catalytic combustion is one of the most effective methods for controlling VOCs emissions.Photothermal catalytic technology introduces light into the thermal catalytic system,which effectively promotes the synergistic photothermal effect,which leads to more electron excitation and enhances photothermal catalytic activity.The catalysts are mainly precious metals,metal oxides and composite oxides.Noble metal catalysts have high catalytic activity,but their high cost has limited their application.Perovskite oxides（ABO₃）have a high degree of catalytic oxidation performance,and their unique lattice structure makes the catalyst have good thermal stability,gradually becoming one of the most promising catalysts for catalytic combustion of VOCs.The catalytic performance of ABO₃ is mainly related to its specific surface area,defect nature and density,oxygen species,and reducibility.However,the LaMnO₃ catalyst prepared by the conventional method has a non-porous structure and a relatively small specific surface area.Therefore,a porous LaMnO₃catalyst with a novel material of ordered mesoporous carbon（OMC）as a template was prepared by the impregnation method in order to improve the specific surface area and catalytic performance of the catalyst.The catalytic performance of ABO₃ can also be improved by doping modification.Therefore,the Fe-doped LaMnO₃ catalysts were prepared by the traditional co-precipitation method,and the catalytic effects were explored.The main contents are as follows:A porous LaMnO₃ catalyst was prepared using ordered mesoporous carbon as a template.XRD,TG,SEM,TEM,BET,XPS and other analytical techniques were used to characterize the physical and chemical properties of porous LaMnO₃ samples.The catalytic performance of the samples was evaluated by catalytic combustion of toluene.It was found that an appropriate amount of OMC can increase the specific surface area of the catalyst and improve its catalytic performance.Among the prepared LaMnO₃ samples,the LaMnO₃-2 catalyst prepared with 20 wt.%OMC has the largest specific surface area（21.32 m²/g）,the highest O_latt/O_ads molar ratio（1.50）,and the Mn⁴⁺/Mn³⁺molar ratio（1.74）.Under conditions of gas flow rate=60 m L min^-1,relative humidity=0%,and toluene concentration=100 ppm,the toluene catalytic combustion performance of porous LaMnO₃ catalysts were better than LaMnO₃-bulk.Among them,LaMnO₃-2 catalyst showed the best catalytic activity （T10% = 161 °C,T50% = 198 °C,T90% = 218 °C）.By studying the kinetics of toluene catalytic oxidation,the results show that the Ea value（43 k J/mol）of LaMnO₃-2catalyst is much lower than the Ea value（166 k J/mol）of LaMnO₃-bulk burning toluene.Finally,the process of toluene oxidation catalyzed by porous LaMnO₃catalyst was analyzed according to Mars-van Krevelen mechanism.Iron-doped LaMnO₃(La Fe_xMn_1-xO₃)was prepared by traditional co-precipitation method.The composition,physical properties and photocatalytic activity of the prepared catalysts were characterized by XRD,DRS,PL and XPS technologies.Under the irradiation of xenon lamp,the activity of iron-doped LaMnO₃ catalysts to oxidize toluene under photothermal conditions was studied.It was found that when x=0.2,the activity of catalyst can be effectively improved with the toluene conversion of 78%and the CO₂ yield of 75%.La Fe_xMn_1-xO₃（x=0.2）owns a photothermal catalytic stability.The effective conversion of photothermolysis drives the catalytic reaction of toluene oxidation,and the light activates the active species on the catalyst,thereby improving the catalytic activity of the catalyst.The reaction of La Fe_xMn_1-xO₃catalysts to degrade toluene under photothermal conditions follows the Mars-van Krevelen redox cycle mechanism.In photothermal catalytic oxidation of toluene,lattice oxygen is the most important active substance.A small amount of iron doping can increase the lattice oxygen content of perovskite increasing the catalytic activity.In addition,a small amount of iron doping also helps to promote the separation efficiency of photogenerated electrons and holes,improving the photocatalytic activity of the catalyst.