Non-thermal Plasma Catalytic Oxidation Of Toluene By Cobalt Incorporated MCM-41: A Process Study

Non-thermal plasma(NTP)catalysis has attracted widespread attention in VOCs abatement due to its unique advantages of quick reaction at room temperature and atmospheric pressure.However,non-thermal plasma catalysis still suffers from the problems of high energy consumed,poor products selectivity as well as the decreasing of catalytic performance in the presence of water vapor.Moreover,the reaction process of VOCs oxidation in plasma catalytic system is not clear,and thus impedes the application of the plasma catalysis technique.In this stduy,a series of highly active cobalt modified MCM-41 catalysts were synthesized by hydrothermal method and impregnation method.The destruction of low concentrations of toluene in air streams using a hybrid plasma-catalysis system was investigated.Both the effect of cobalt incorporation amount and the effect of synthesized methods were studied and some experiments were carried out on comparing the difference between two types of catalysts.Then,in order to solve the problems of high energy consumed and the adverse impact of water vapor,an adsorption-discharge plasma system was proposed and studied.Both the adsorption capacity and catalytic activity of the catalysts were investigated in this catalytic system.The difference between two systems,including continuously introduced and adsorption-discharge plasma system,were disscussed.In addition,the reaction intermediates and products were also carried out by in-situ FTIR spectra,GC-MS and TOF-MS.Moreover,the plausible reaction mechanisms were discussed based on the organic intermediates generated in the reaction.The detailed work and summarized results were drawn as follows:1.A series of highly active cobalt incorporated MCM-41(Co-MCM-41)catalysts were synthesized by a hydrothermal method.For comparison,cobalt-impregnated MCM-41(Co-MCM-41_imp)catalysts were prepared via an impregnation method.The synthesized samples were characterized by X-ray diffraction(XRD),nitrogen adsorption-desorption isotherms,ultraviolet–visible diffuse reflection spectrum(UV–Vis DRS),H2-temperature-programmed reduction analyses(H2-TPR),scanning electron microscope(SEM)and transmission electron microscopy(TEM).The results showed that the cobalt modified samples retained the highly ordered mesopore structure of MCM-41.In addition,the cobalt species on Co-MCM-41 was highly dispersed in the silica framework of MCM-41 molecular sieves,while the cobalt on Co-MCM-41_imp was present as cobalt oxide clusters or Co3O4 nano-particles on the external surface of MCM-41.The non-thermal plasma catalytic performance of the catalysts was evaluated by toluene oxidation using a dielectric barrier discharge(DBD)reactor.The results showed that the Co-MCM-41 exhibited better catalytic activity and stability than Co-MCM-41_imp.This can be attributed to the well dispersion of cobalt species,good ozone decomposition activities and the high ability to degrade organic intermediates on catalyst surface.2.In the non-thermal plasma system without catalysts,the toluene decomposition efficiency increased with the water wapor in the gas stream.It maybe attributed to the reason that H2 O molecules collide with high-energy electrons and form OH radicals,which have higher reaction activity than ozone and active oxygen atoms.However,when the catalyst was introduced into the plasma system,the results showed that toluene removal decreased significantly.This is because that water has the electronegative characteristics,which could limits the electron density in the system and quench the activated chemical species.On the other hand,it has the competitive adsorption of water,toluene and the active species on the catalyst surfaces,thus decreased the toluene decomposition efficiency.3.Toluene adsorption capacity and plasma catalytic performance in the adsorptiondischarge plasma system were studied and the results suggested that the as-prepared Co-MCM-41 catalyst possesses not only large VOCs adsorption capacity,but also good plasma catalytic performance.Compared to traditional continuously introduced plasma system,the adsorption-discharge plasma system significantly increased toluene conversion,carbon balance and CO2 selectivity.In addition,the catalytic oxidation of toluene in adsorption-discharge plasma system lowered the energy consumption and improved the moisture resistant.4.The reaction intermediates and products were also carried out by GC-MS,in-situ FTIR spectra,and TOF-MS.Several types of organic byproducts were identified on the gas phase and catalyst surface.The main byproducts during toluene oxidation were benzaldehyde,phenylcarbinol,benzoic acid,phonel and nitrobenzene.The detected byproducts provided the direct evidence for plasma catalytic oxidation of toluene.Moreover,based on the organic intermediates generated in the reaction.the plausible reaction mechanisms were discussed and the possible reaction pathways were proposed.