| Volatile organic compounds（VOCs）are the key precursors of fine particulate matter and ozone in atmospheric environment,and the effective control of VOCs is significant for the continuous improvement of Chinese atmospheric environmental quality.Among various end-to-pipe techniques,non-thermal plasma（NTP）exhibits great potential for low concentration and multi-component industrial VOCs control because it can be quickly turned on/off,operated at room temperature and atmospheric pressure,and has high economical efficiency,low operating resistance and simple processes.However,the present NTP technology has troubles such as low mineralization rate,various by-products generation and low energy efficiency,which make it is hard to reach the increasingly strict control requirements of industrial VOCs exhaust gas.Herein,a novel double dielectric barrier discharge reactor with two discharge zone was constructed,and the relationship between its discharge properties and toluene degradation performance was explored.The role of the inner and outer discharge zone and the reactive species were also investigated.Next,the double discharge barrier reactor was packed withγ-Al2O3 and ZSM-5 zeolite optimized in configuration.Ce-Co bimetallic catalyst was coupled with the reactor to deeply oxidize toluene,the impacts of catalysts properties on toluene degradation performance and the toluene degradation mechanism in plasma catalytic system were investigated.The finding in this work can provide design and theoretical guidance for the practical application of NTP technology.The main conclusions of this work are as follow:Firstly,a novel double dielectric barrier discharge reactor with two discharge zone was constructed and its inner barrier was located between the high voltage electrode and the outer barrier.Its discharge properties were analyzed based on discharge images,voltage-current waveforms,input power and the temperature of the reactor and the toluene degradation performance was evaluated by removal efficiency,mineralization rate and energy efficiency.Compared with single barrier reactor,the novel double barrier reactor exhibited a more uniform and stable discharge,lower input power and gas temperature.Under 18～24k V,the energy efficiency of the novel double barrier reactor was 2.5～3 times that of the single barrier reactor.The novel double barrier reactor had a stronger discharge intensity,and its removal efficiency and mineralization rate（86.4%～100%and 22.7～54.2%）were much higher than that of the traditional double barrier reactor（28.2%～80.5%and 8.9%～20.6%）.Next,in order to explore the role of the inner and outer discharge zones and different reactive species in toluene degradation,four reactors with different flow directions were designed based on the novel double barrier structure.Based on the characteristics of the inner and outer discharge zones and the toluene degradation performance under different voltage of the four reactors,it can be concluded that:the inner discharge zone was mainly contributed to the initial degradation of toluene due to its stronger discharge intensity,the function of the outer discharge zone was mainly the further oxidation of the organic intermediates since it has a longer gas residence time,and the grading degradation and oxidation of toluene can be achieved when the gas flowed from the inner to the outer discharge zone consecutively.For different plasma reactive species,excited nitrogen molecules,metastable nitrogen/oxygen species and oxygen atoms played acritical roles in toluene degradation and oxidation.The toluene degradation mechanism in the novel double barrier reactor was investigated based on the properties of the inner and outer discharge zones,the role of different reactive species and the detected organic intermediates.The degradation mechanism analysis suggested that toluene react with reactive nitrogen/oxygen species in the inner discharge zone to form small organic compounds such as acetic acid and 3-hexanol,which were further degraded in the outer discharge zone and oxidized by reactive oxygen species such as atomic O and 1O2 to COxand H2O.Subsequently,γ-Al2O3 and ZSM-5 were packed in the novel double barrier reactor to degrade toluene,and its configuration was optimized in terms of packing material type,packing methods,flow modes and electrode structure.The reactor packed with a mixed packing materials provided a mineralization rate of 6.1%and 14.1%greater than packing singleγ-Al2O3 and ZSM-5,respectively.For different packing methods,the reactor withγ-Al2O3 packing in the inner tube and ZSM-5 packing in the outer tube had the highest mineralization rate（63.3%）.For the four different gas flow modes,the mineralization rate was the highest when the gas flowed from the inner tube into the outer tube during both adsorption and discharge phases.Among different high voltage electrodes,the mineralization rate of the reactor equipped with a rod electrode was higher than that with a coil electrode.Finally,based on the optimal packed bed reactor configuration,Ce O2-Co3O4/γ-Al2O3 catalysts with different Ce/Co ratios were prepared to deeply oxidize toluene,and the structure and chemical properties of the catalysts were characterized by BET,XRD,TEM,XPS and H2-TPR.The results showed that the bimetallic catalyst had high toluene adsorption capacity,surface adsorbed oxygen content and low reduction temperature.Ce O2-Co3O4 solid solutions and surface defects were generated due to the interaction between the Ce and Co species,thereby facilitating the utilization of reactive oxygen species.The highest mineralization rate was obtained when the Ce/Co ratio was 1:3（89.7%）,followed by 1:1（84.1%）,3:1（81.8%）and single Ce O2（79.4%）,Co3O4（77.5%）and unloadedγ-Al2O3（69.2%）.The investigation of the toluene degradation mechanism in the NTP catalytic system indicates that the catalyst can reinforced discharge intensity and adsorb plasma reactive species and toluene of the catalyst,and the NTP can provide abundant reactive particles such as atomic O,1O2 and O3 for the catalytic reaction,which jointly promoted the deep oxidation of toluene. |
