Fundamental Research On Post Nonthermal Plasma-Nanocatalysis For The Synergistic Decomposition Of Volatile Organic Compounds

Volatile organic compounds?VOCs?are one of the main sources of air pollutants.The emerging plasma catalytic technology can take advantage of the high reactive activity of plasma,the high selectivity of catalyst and the synergistic effect of both,thereby effectively degrading VOCs.In post-plasma catalytic?PPC?process,the catalyst is placed downstream of the plasma region,which can catalytically convert the long-lived active species.Meanwhile,the plasma-induced reaction in the discharge region and the catalytic reaction on the catalyst surface are relatively independent,which is favorable for the extension and optimization of the reaction pathways,promoting commercial applications.The treatment of VOCs with low concentration and high flow rate at room temperature in PPC process usually faces with several challenges,such as the difficulty in balancing the decomposition efficiency and energy efficiency,the relative poor product selectivity and catalytic stability,which limits its large-scale industrial application.For the plasma reaction,the short-lived active species produced by plasma discharge will react directly with VOCs.Meanwhile,the discharge continuously consumes energy,directly determining the energy consumption of PPC process.For the catalytic reaction,the catalyst will convert the long-lived active species into active oxygen,further oxidizing the adsorbed VOCs and reaction intermediates.The aforementioned microscopic processes will directly affect the macroscopic performance of PPC process.However,systematic and comprehensive description and regulation are rarely reported.In this dissertation,the fundamental research on post nonthermal plasma-nanocatalysis for the synergistic decomposition of VOCs is carried out.Firstly,the correlation mechanism among the micro-discharge behavior of plasma,the distribution of plasma generated long-lived,short-lived active species and reaction intermediates as well as the macroscopic performance are studied.Secondly,the micro-morphology of the catalyst,the nanostructure of the support and the reaction condition are rationally designed and controlled according to the characteristics of PPC process.The improvement of the morphology and structure of the catalyst will enhance the catalytic reaction at room temperature and the gas diffusion and catalytic reaction at high gas flow rate,promoting the decomposition efficiency,energy efficiency,product selectivity and reaction stability.The research contents and main conclusions of this paper are as follows:Firstly,the relationship between the micro-discharge behavior of plasma,the distribution of plasma generated species and macroscopic performance are investigated by adjusting the discharge and gas parameters to change specific input energy?SIE?.The results show that the SIE increases with the increment of discharge voltage,enhancing the amount and intensity of micro-discharge as well as the generation of reactive species.As a result,the decomposition efficiency,the selectivity of carbon dioxide and the carbon balance can be improved.However,the energy efficiency decreases due to the increase of heat dissipation.While the increase of gas flow rate will reduce the SIE,thereby weakening the discharge intensity and reducing the amount of reactive species.Meanwhile,the collision possibility between gaseous pollutant molecules and plasma-active species is reduced,resulting in a significant decrease in toluene degradation efficiency,carbon dioxide selectivity and carbon balance.On the contrast,the plasma-generated active species can be more fully utilized,resulting in a significant increase in energy efficiency.Secondly,the transformation of the catalyst micro-morphology between solid urchin and hollow urchin is controlled to investigate the optimization strategy of the gas adsorption and catalytic processes on the catalyst surface.The results indicate that compared with the solid urchin,hollow urchin can enlarge the contact surface area of catalyst exposed to gas and prolong the residence time of gas on the catalyst surface.In addition,the open macroporous structure can facilitate the gas diffusion and adsorption on the catalyst surface at high flow rate.At the same time,the structure of hollow urchin can improve the amount of surface oxygen vacancy and low-temperature reducibility,thus promoting the catalytic conversion of ozone into active oxygen.Hence,the catalytic performance of hollow urchin is better than that of solid urchin.Compared with solid urchin MnO₂,hollow urchin MnO₂ exhibits better toluene decomposition efficiency,energy efficiency,carbon dioxide selectivity and carbon balance,with increases of 16%,18%,25%,and 16%,respectively.Therefore,the structure of hollow urchin can enhance the decomposition of VOCs.Thirdly,simultaneously realizing high catalytic performance and ultra-low pressure drop at high gas flow rate are studied by fabricating catalyst support with three-dimensional hierarchical nanostructure.Three-dimensional vertical graphene foam?VG foam?is prepared by using macroporous nickel foam as the template.Reduced graphene oxide?rGO?and activated carbon?AC?serve as a comparison.On one hand,the pressure drop of VG foam is 2-3 orders of magnitude lower than that of rGO and AC powders when the gas velocity and bed weight are the same.On the other hand,the MnO₂ nanopetals are highly dispersed on the surface of VG foam without obvious aggregation,forming a hierarchical petal-on-petal structure,which can avoid the appearance of dense channels,stacking layers and agglomeration of catalyst.Compared with MnO₂/rGO and MnO₂/AC,MnO₂/VG foam has lower Mnoxidation state and higher surface oxygen vacancy content,which can catalytically convert more ozone into reactive oxygen species.The toluene decomposition efficiency,carbon dioxide selectivity,carbon balance and ozone conversion efficiency of MnO₂/VG foam are significantly higher than those of MnO₂/rGO and MnO₂/AC,reaching 93%,60%,78%and 100%,respectively.Therefore,VG foam has obvious advantages over rGO and AC powder in structure,which can significantly improve the catalytic performance.Fourthly,the combination of solar thermal conversion and post-plasma catalysis is proposed for the first time by fabricating solar thermal nanocatalyst.The solar thermal effect can elevate the temperature of the catalyst to enhance its catalytic activity.The key to solar-enhanced post-plasma catalysis?SEPPC?is the catalyst support and light absorber?i.e.graphene fin foam,GFF?.MnO₂ nanofins are loaded on the surface of GFF by oxidation-reduction deposition to obtain hierarchical MnO₂/GFF catalyst.The results show that the spectral response range is greatly broadened due to the graphene fin structure in MnO₂/GFF.The light absorption over MnO₂/GFF is>95%across the whole solar spectrum.Moreover,the chemical connection between MnO₂and GFF is favorable for heat transfer.The steady-state surface temperature of the catalyst can reach up to 72.6°C under solar illumination,corresponding to a high solar thermal conversion efficiency of 62.2%.The obvious solar thermal effect enhances the toluene decomposition efficiency,energy efficiency,carbon dioxide selectivity and ozone conversion,which are 63%,57%,36%and 1.89 times higher than those of the conventional PPC process,respectively,reaching 93%,12.7 g k W h^-1,83%and 0.52 test under solar illumination.The solar-enhanced performance is mainly due to the solar-enhanced catalytic ozone conversion,solar thermal catalytic oxidation of toluene and their strong synergistic effect?42%?.