Study On Surface/interface Modification Over Manganesebased Oxides And Their Catalytic Performances For VOCs Purification

Volatile organic compounds（VOCs）are a series of low-boiling organic substances that pose a tremendous threat to the ecological environment and human health.Among the various control technologies of VOCs,the catalytic oxidation method stands out because of its ability to completely convert VOCs into CO₂ and H₂O,no secondary pollution,low energy requirements,and high efficiency.As for the selection of catalysts,manganese-based oxide catalysts have received extensive attention due to their low price,environmental friendliness and special physical and chemical properties.However,the pristine manganese-based oxide catalysts usually exhibit insufficient defect sites and few active oxygen species,resulting in low catalytic activity.During the high temperature reaction,it is easy to change phase and deactivate,resulting in poor stability.Therefore,finding suitable surface/interface modification strategies to improve the catalytic activity and thermal stability of manganese-based catalysts would have important research significance.In this thesis,we mainly study the surface/interface regulation of manganese-based oxide catalysts by different modification methods（surface detoxification,surface acid treatment,two-phase composite by topotactic transformation and quenching modification）to improve the catalytic performance of VOCs degradation.The specific research contents and experimental results are as follows:（1）The significant poisoning effect of adsorbed sulfate on MnO₂ catalyst in the catalytic oxidation of VOCs was revealed.The manganese sulfate-derived MnO₂ catalyst exhibits significantly enhanced performance after multiple water washing.The blockage of surface oxygen species and active sites is considered to be the decisive reason of sulfate poisoning.Additionally,the quenching strategy can be used to significantly improve the removal efficiency of adsorbed sulfate.The results show that the toluene conversion of MnO₂ without absorbed sulfate can reach 100%at 230°C under the gas hourly space velocity（GHSV）of72000 m L g^-1 h^-1.（2）The surface/interface modification ofδ-MnO₂ by acid treatment was studied,and its catalytic performance for VOCs oxidation was also investigated.This results show that the catalytic activity of the acid-treatedδ-MnO₂ catalyst for VOCs degradation is significantly improved.Different concentrations of hydrochloric acid have different effects onδ-MnO₂.When the concentration of hydrochloric acid was increased to 1 M,the improvement of catalytic performance is negligible.The excellent catalytic performance of VOCs is mainly attributed to the increase of the specific surface area and the formation of a large number of active surface lattice oxygen species.The acid-treated catalyst can completely convert toluene into CO₂ and H₂O at 280 ^oC under the GHSV of 72000 m L g^-1 h^-1,and it can maintain good catalytic activity for more than 50 h without obvious potential decay.In addition,acid treatment experiments were also extended toα-MnO₂,β-MnO₂ andγ-MnO₂.It was found that the performance was improved,but the degree of improvement was inferior to that of layeredδ-MnO₂.（3）A hierarchical porousα-MnO₂/Mn₃O₄ heterostructure with rich interfaces was designed and synthesized by using topotactic transformation strategy.The growth kinetics process fromδ-MnO₂ toα-MnO₂/Mn₃O₄ was observed in real-time by in-situ heating transmission electron microscopy.The results show that theα-MnO₂/Mn₃O₄ catalyst not only provides a large number of intimate interfaces,but also forms a Z-type heterojunction,which greatly promotes the generation of superoxide radicals.The catalyst also has numerous oxygen vacancies and can adsorb more oxygen molecules on the surface,thereby forming more superoxide radicals per unit reaction time.Meanwhile,the catalyst can absorb light in the entire spectrum of sunlight to achieve efficient light-to-heat conversion.Compared with pure phaseα-MnO₂ and Mn₃O₄,the catalyst exhibits better catalytic activity for VOCs degradation under the radiation of full sunlight.Under the light intensity of 764 m W cm^-2,the equilibrium temperature of theα-MnO₂/Mn₃O₄ catalyst is up to 135 ^oC,and the toluene conversion reaches more than 95%.（4）A new catalyst Co₃O₄/La Co_xMn_1-xO₃ for catalytic purification of VOCs was synthesized by quenching technology.The study shows that quenching can cause partial Mn on LaMnO₃ by Co substitution.Meanwhile,electrostatic adsorption can promote the formation of Co OOH on the surface of LaMnO₃.Compared with pure LaMnO₃ and Co₃O₄/LaMnO₃,the Co₃O₄/La Co_xMn_1-xO₃ catalyst formed after annealing exhibits better catalytic performance for toluene oxidation,which is mainly attributed to a larger specific surface area,more oxygen vacancies and better low temperature reducibility.Additionally,the catalyst has good catalytic stability.Under the GHSV of 72000 m L g^-1 h^-1,the toluene conversion reaches more than 95%at 280 ^oC,and can be maintained for 60 h.Furthermore,it is confirmed by DFT calculation that the Co₃O₄/La Co_xMn_1-xO₃ catalyst has lowest C-H bond activation energy,which is attributed to Co doping and interfacial effects.