Studies On Synergistic Mechanism Of Nonthermal Plasma Assisted Catalytic Oxidation Of Nitric Oxide

The conversion of NO to NO2 by nonthermal plasma assisted catalytic process is critical for removal of nitrogen oxides form exhaust gases,because NO2 is easier to neutralize or to reduce to nitrogen using catalysis.In spite of the growing interest in plasma catalysis,the underlying mechanisms of the synergistic effects between nonthermal plasma and catalysts are not yet fully understood.A series of experiments were carried out to understand the mechanisms of NTP assisted catalytic process.Firstly,combination of optical emission spectroscopy(OES)and Fourier-transform infrared spectroscopy(FTIR)were used to find out the feature of active species in NTP and the functions of different active species on NO oxidation,respectively.Secondly,characterization of MnOx catalysts with and without O2-NTP pretreatment was performed by XAFS,XRD,XPS and FTIR methods to achieve the effect of NTP on catalyst.Then,the NO oxidation performance of the combination of NTP with several different commercial or prepared catalysts were also investigated to find out the effect of catalysts on the NTP chemical reaction.The NO adsorption performance over Al2O3 and ZSM-5 were also carried out under NTP environment,achieving the intrinsic kinetics during NTP assisted catalytic oxidation of NO.Finally,the possible mechanisms of NO oxidation were carried out under NTP alone and NTP catalysis,respectively.The OES results showed that the N2 second positive system(SPS),NO ?system,Hermans's IR system and N2 first positive system were observed in the 99.999%N2 discharge,and N2(A3?u+)plays an important role in the formation of this spectrum band.The 0 spectrum at 777.28 and 844.47 nm,which are response to transition of(5S0,9.15eV)-(5P,10.74eV)and(3S0,9.52eV)-(3P,10.99eV),respectively,were observed in 99.999%O2 discharge.Hermans's IR system was quenched by O2,and the density of both N2 SPS and NO ? system were decreased with the increase of O2 concentration.The intensity of NO ? system was enhanced with NO concentration increasing.The intensity of spectrum increased linearly with the increase of discharge power.According the results of NO conversion performance using NTP alone,inter-reaction between NO and NO2 is vital for the NO conversion in the NTP.N2O as a byproduct of NTP reaction was observed in the discharge occurring in 02 and N2.NO oxidation is the optimal approach for NTP industrial application in NO removal.Improving the NO oxidation efi-iciency and reducing the formation of N2O are the two most important targets for NTP application in NO removal.Both NO oxidation efficiency and N2O concentration followed sublinear trend with the increase of SED,residence time and O2 concentration.XAFS and XPS results of MnOx pretreated by O2-NTP showed the valence state of partial Mn were increased form+3 to+4,indicating the increase of lattice oxygen(O?).XPS results also show that the ratio of adsorbed O(O?)and O?increased compared to that of raw MnOx.The catalysts can capture electrons from NTP environment,generating a negative charged surface.MnOx with the pretreatment of O2-NTP showed better NO oxidation performance than that of raw MnOx.According the performance of NO oxidation using NTP assisted catalytic process,both the NO oxidation and N2O formation concentration were enhanced compared with that of NTP alone.It seems that the catalytic enhancement of N2O formation is better than that of NO formation in NTP assisted catalytic process.The discharge in DBD reactor packing with catalysts was decreased with the increase of metal oxide load of catalysts,meanwhile,the NO oxidation eflficiency was also decreased.The catalyst packed in DBD reactor can change the discharge form from filamentary discharge to surface discharge.NTP can enhance the NO chemical adsorption over the surface of catalyst.The results of Bolsigplus simulation indicated that the concentration of O(3P)is 1.83 times of that of O(1D),and N2(A3?u+)concentration is 8.45 times of N concentration.Acrroding the caculation results of ELENDIF,the amount of O(3P),O(1D)or N2(A3?u+)is an order of magnitude higher than that of N for per 100 eV.In summry,the main reactive species attributed NO oxidation is O radicals and O3.The reaction between N2(A3?u+)and O2 is the main source for N2O production.0 and O3 can react with catalysts and increase the amount of lattice O and adsorbed 0 of catalysts,which may improve the the NO oxidation performance of catalyst.The catalyst packed in DBD reactor can not only change the discharge form but also improve the NTP chemical reaction such as NO oxidation and N2O formation.