Toluene Degradation Performance And Mechanism Over Nb-Doped MnO_x/Zeolite Catalyst Coupled With Non-thermal Plasma

The emission of low concentration of volatile organic compounds(VOCs)from many industry processes has caused great harm to environment and human health.The non-thermal plasma(NTP)coupled with catalyst is one of the most effective technologies for treating VOCs.The catalyst is key factor for mineralizing VOCs completely,among which MnOx supported on zeolites(Mn/zeolites)is widely used as catalyst in NTP-catalysis.In this paper,the degradation performance of toluene on Mn/zeolites in NTP-catalysis was studied.The influence of the skeleton structure,pore size,Si/Al ratio of zeolite and doping of the second metal(Nb,Cu,Ce,Co)on toluene degradation were investigated;moreover,the degradation mechanism of toluene was also discussed.The main research contents and results are as follows:(1)The supported catalysts were prepared by loading 3 wt%of manganese on different zeolites(MCM-41,ZSM-5 and 13X).The influence of the skeleton structure of zeolites on toluene degradation in NTP-catalysis was studied.The results showed that:?The mesoporous MCM-41 has larger pore diameter higher specific surface area and electronegative surface than microporous zeolites ZSM-5 and 13X,so MnOx was dispersed more uniformly on MCM-41;Moreover,Mn/MCM-41 had the highest oxidation reduction activity and oxygen storage capacity.?Compared to NTP alone or NTP-zeolites system,the toluene degradation was improved significantly in NTP-Mn/zeolites system.The toluene removal efficiency,CO2 selectivity and carbon balance could up to 99.4%,27.7%and 56.5%in DBD reactor packed with Mn/MCM-41,which has the highest catalytic activity.(2)The catalysts were prepared by loading 3 wt%of X(Nb,Cu,Ce,Co)-Mn on MCM-41(X:Mn=1:1)by impregnation method.The influence of doping X(Nb,Cu,Ce,Co)on toluene degradation was studied in the NTP-catalysis system.The results showed that:? After doping Nb,Cu,Ce and Co,respectively,the metal species were highly dispersed on the MCM-41 with a 1.5nm average particle size,and the redox activity and catalytic activity of catalysts were improved.Among the bimetallic catalysts,the H2 consumption and the lattice oxygen content of Nb-Mn/MCM-41 are the largest.In addition,Nb doping could increace the acidity of catalyst,which is benefit for the decomposition of ozone.?When the initial concentration of toluene increased to 220ppm,the toluene removal efficiency,CO2 selectivity and carbon balance of Mn/MCM-41 could up to 78.4%,32.2% and 61.1%.Compared to Mn/MCM-41,the toluene degradation was improved significantly by doping the second metal.Nb-Mn/MCM-41 had the best catalytic performance,and the toluene removal efficiency,CO2 selectivity and carbon balance were 17.4%,20.7%and 24.4%higher than Mn/MCM-41 respectively..(3)Nb-Mn/MCM-41(Nb:Mn=1:1)catalysts with different Mn loading amount(3%,5%and 7%)were prepared by direct synthesis method and impregnation method,respectively,which was named as Nb-Mn/M ? and Nb-Mn/M ?.The influence of preparation method on the degradation of toluene was studied in the NTP-catalysis system.The results showed that the metal atoms could mainly enter into the skeleton of zeolite by direct synthesis method,while Nb and Mn mainly exist on the surface or in the pore structure in the form of oxidation state by impregnation method.The total oxygen consumption and lattice oxygen content of Nb-Mn/M ? were slightly higher than Nb-Mn/M ?,resulting in that Nb-Mn/M ? had better catalytic performance than Nb-Mn/M ?,and the toluene removal efficiency,CO2 selectivity and carbon balance were 96.4%,55.0%and 85.4% respectively.(4)The bimetal Nb-Mn supported on different pore size of MCM-41 were prepared by impregnation method.The influence of pore size of MCM-41 on the toluene degradation was studied in the NTP-catalysis system.Besides the specific surface areas,the pore size played a significant role in the dispersion of metal oxides.The proper specific surface area and pore size are beneficial to the dispersion and oxygen storage capacity of metal oxides.The larger the pore size of MCM-41,the higher oxidation reduction activity and oxygen storage capacity the Nb-Mn/MCM-41 catalyst had.Among the four catalysts,when the pore size increased,the toluene removal efficiency,CO2 selectivity and carbon balance increased by 3.3%,26.0%and 39.1%respectively.(5)The bimetal Nb-Mn supported on different Si/Al ratio of MCM-41 were prepared by impregnation method.The influence of Si/Al ratio on the toluene degradation was studied in the NTP-catalysis system.The results showed that with increasing the Si/Al ratio of MCM-41,the dispersion,oxygen storage capacity,acidity and hydrophilicity of Nb-Mn/MCM-41 were all increased.Under the condition of dry air,the lower Si/Al was more beneficial for the degradation of toluene,and Nb-Mn/MCM-41 with the lowest Si/Al of 25 had a best toluene removal efficiency of 98.2%.Under the condition of 3 vol%wet air,the catalytic performance of all catalysts decreased.The lower Si/Al ratio of MCM-41,the more unfavorable for the degradation of toluene,the toluene removal efficiency of Nb-Mn/MCM-41 with Si/Al of 25 decreased by 15.9%,while Nb-Mn/MCM-41 with Si/Al of 200 just decreased by 3%.