| In recent years,with the acceleration of industrialization in China,the emission of volatile organic pollutants(VOCs)in the atmosphere is increasing.Among them,the emission of industrial source VOCs involves a large number of industries,high emission intensity and serious pollution.Non-thermal plasma has the advantages of high degradation efficiency at room temperature and non-selectivity to pollutants,so it is considered as one of the promising technologies for the treatment of VOCs and has been widely concerned by researchers.The research on atmospheric pressure discharge plasma treatment of VOCs is mainly focused on the generation method of atmospheric pressure discharge plasma,in order to produce a large number of high-energy electrons and active materials,thus to improve the energy efficiency of discharge plasma applied for VOCs degradation.There are many ways to generate discharge plasma,including dielectric barrier discharge,corona discharge and so on.It is difficult to satisfy both high energy efficiency and economically feasible excitation power supply with the current plasma generation methods.According to the research results on field emission and positive DC corona discharge plasma,this paper proposes to use oxide cathode as discharge cathode to enhance the physical and chemical activity of discharge plasma for VOCs removal.Toluene is used as the target,mainly focusing on the preparation and characterization of oxide cathode,characteristics of discharge plasma with oxide cathode,degradation performance and mineralization of toluene,optimization of operating parameters in the discharge system and toluene degradation mechanism,etc.Main research contents and relevant results are as follows:(1)The material selection and preparation methods of oxide cathodes were determined,and the enhancement of plasma current intensity by oxide cathodes under positive DC discharge was studied.The results show that compared with Ni cathode discharge,the MgO/Ni cathode,NiO/Ni cathode and sandwiched MgO/NiO/Ni cathode prepared by electrophoretic deposition can enhance the discharge current intensity of DC discharge under different gas pressure conditions(400-700 torr),and the sandwiched MgO/NiO/Ni cathode shows the best enhancement effect.The surface potential of the oxide cathode after DC discharge is measured.The results show that the surface potential produced by the positive charges accumulated on the surface of the oxide cathode is sufficient to cause the field-induced secondary electron emission,which is the main reason for the increase of discharge current.The influence of oxide cathode structure parameters on discharge was investigated.The optimal thickness of the two-layer MgO/NiO is 89.2 μm(MgO layer of 80.6μm,NiO layer of 8.6 μm).The oxide cathode prepared by relatively rough metal substrate is beneficial to the enhancement of DC discharge current.(2)The physical and chemical characteristics of atmospheric pressure air discharge plasma with MgO/NiO/Ni cathode were studied,including voltage-current characteristics,discharge morphology,spectral detection and so on.Compared with discharge with Ni cathode,the discharge current at the same voltage can be increased up to 3.3 times in the needle-plate reactor;while,the discharge current can be increased up to 11.8 times in the wire-plate reactor.Except for the discharge around the discharge electrode(needle and wire electrodes),the discharge also occurs on the surface of MgO/NiO/Ni cathode,which is mainly concentrated in the vertical position of the discharge electrode.The spectral intensity around the discharge electrode increases as well as the active substances,and the discharge products such as O3 increase significantly.(3)The treatment of toluene waste gas by atmospheric pressure discharge plasma with MgO/NiO/Ni cathode was studied.Compared with discharge with Ni metal cathode,discharge plasma with MgO/NiO/Ni cathode can significantly improve toluene degradation efficiency,energy utilization and mineralization efficiency.At almost the same discharge power,the toluene degradation efficiency,energy utilization efficiency and mineralization efficiency of discharge with MgO/NiO/Ni cathode(P=11.20 W)increased by 0.93,0.94 and 1.26 times than those of discharge with Ni cathode(P=11.28 W),respectively.On the one hand,the field-induced secondary electron emission of the oxide cathode can significantly increase the number of seed electrons,and then enhance the number of high-energy electrons and active substances in the whole discharge system.On the other hand,the discharge on the surface of the MgO/NiO/Ni cathode expands the spatial distribution of high-energy electrons and active materials.These two aspects can increase the collision probability of high-energy electrons and active substances with toluene molecules and intermediates,so as to obtain higher toluene degradation efficiency,utilization and mineralization efficiency.(4)The mechanism of toluene degradation by discharge plasma with MgO/NiO/Ni cathode was discussed.In wire-plate reactors,the possibility of catalytic decomposition of O3 and further for toluene degradation by MgO/NiO/Ni cathode is relatively small.The degradation products of toluene were analyzed by Fourier Transform Infrared Spectroscopy(FT-IR)and Gas Chromatography-Mass Spectrometry(GC-MS).The results show that a large number of high-energy electron collisions produced by oxide cathodes are the main pathway of methyl dehydrogenation on toluene,and a large amount of ·O and ·OH produced by oxide cathode discharge are the main active substances that cause ring-opening reactions to produce oxygen-containing products.The intermediates will be further oxidized and decomposed into small organic acids by high-energy electrons and oxygen-containing active groups,and eventually oxidized to CO2 and H2O. |
