Toluene Removal With Nickel-based Catalysts In Adsorptiondischarge Plasma Systems: Property And Products Study

Volatile Organic Compounds(VOCs)are regarded as one of the main precursors of ozone(O3)and secondary organic aerosols(SOA),and its control are one of the most crucial issues for air pollution.Non-thermal plasma(NTP)technique is a promising solution to remove VOCs due to its unique advantages of quick reaction at room temperature,low cost and convenient operation.However,high energy consumed,poor CO2 selectivity and the formation of toxic and hazardous by-products are the major problem for VOCs removal by using NTP technique alone.Recently,more works,mainly focused on optimization of experimental operating parameters in continuously introduced plasma system and introduction of various catalysts to maximum their synergetic effect,has been carried out in order to obtain more effective effect on VOCs removal.However,the research on the whole reaction process and novel plasma catalysis system are rarely reported.In this work,toluene is used as model VOC pollutant,the adsorption capacity and catalytic activity of the catalysts in different plasma systems(including continuously introduced and adsorption-discharge plasma system)were investigated.In addition,different detection techniques including in-situ FT-IR spectra,TOF-MS and GC-MS were used to explore the mechanisms of different plasma systems for toluene removal.Firstly,toluene degradation performance between two plasma systems,including continuously introduced and adsorption-discharge plasma system,were discussed.Silver and nickel-impregnated ?-Al2O3(recorded as AgOx/Al2O3 and NiO/Al2O3)were synthesized by an impregnation method,and the as-prepared catalysts were characterized by X-ray diffraction(XRD),H2-temperature-programmed reduction analyses(H2-TPR),transmission electron microscopy(TEM)and nitrogen adsorptiondesorption isotherms.The results showed that the size of NiO in NiO/Al2O3 catalyst was smaller than that of AgOx in AgOx/Al2O3 sample.The adsorption and plasma catalytic performance experiments displayed that NiO/Al2O3 catalyst exhibited better adsorption and catalytic activity than AgOx/Al2O3 and Al2O3 catalysts.This can be attributed to the high specific surface areas,remarkable ozone decomposition performance and well dispersion of NiO in the Al2O3 carrier.Meanwhile,different plasma catalysis systems showed that adsorption-discharge plasma system possessed good toluene conversion,carbon balance and CO2 selectivity.On one hand,new plasma system extended the residence time so that toluene could adequately be removed in the reactor.On the other hand,the surface active site of the catalysts increased with the degradation of the adsorbed toluene,and it was benefit for the collision between toluene molecule and active species,and at last improved toluene degradation performance.In this part,the destruction of low concentrations of toluene with nickel loaded SBA-15(Ni/SBA)catalyst in adsorption-discharge plasma system was developed,and the regenerative properties and stability of the catalyst were also investigated.The results showed that Ni/SBA sample exhibited better catalytic activity than SBA.This can be attributed to the fact that loading nickel into SBA could improve ozone decomposition,and more active oxygen atoms were formed in the surface of the catalyst and at last promoted toluene degradation.In-situ FT-IR spectra suggested that some chemical reactions occurred in the methyl group over Ni/SBA catalyst which indicated that the incorporation of Ni led to a partial oxidation of toluene after adsorption.Several adsorption/regeneration experiment showed that toluene adsorption amount and catalytic performance of Ni/SBA catalyst increased after plasma treatment due to the decrease of metal oxide particle size and enhancement of dispersion.In addition,Ni/SBA catalyst possessed good regenerability,its toluene mineralization rate kept above 70 % after five cycles of adsorption/regeneration process.However,the CO2 selectivity was not so high with Ni/SBA catalyst when toluene was degraded in adsorption-discharge plasma system.Therefore,catalysts surface and gas phase products during plasma catalysis process were analyzed by in-situ FTIR,GCMS and TOF-MS techniques.Water temperature programmed desorption(H2O-TPD)was achieved in order to explore the interaction between water and catalyst,and the effect of water vapor on plasma oxidation of toluene was also investigated.In-situ FTIR data showed that some aromatic compounds(aldehydes,carboxylic acids,esters,nitro organics,and so on)formed in the surface of Ni/SBA catalyst during 60 min plasma catalysis,and most of products in the surface of Ni/SBA sample was degraded while lots of products accumulating in the surface of pure SBA as the discharge time extended.GC-MS result showed that the accumulation of oxygen-containing organic compounds and alkanes products respectively affected the catalyst activity of Ni/SBA and SBA.Gas-phase products was detected by in-situ TOF-MS,and it revealed that benzoic acid was hard to be degraded in gas phase reaction and it was not the key substance for the ring-opening reactions of toluene.The presence of water vapor(relative humidity = 20 %,40 %,60 % and 80 %)had negative influence on the toluene mineralization rates of each catalyst,however,humidity led decrease the yield of CO and increase CO2 production that would be attributed to the enhanced oxidation of both CO and other intermediates during the reaction.Different Si/Al made different contribution to plasma catalysis activity,some researches had reported that different surface acidity and pore diameter of zeolite affected the partial discharge property of the catalysts,and at last influenced the catalytic performance.Therefore,this part based on the above study of toluene removal in adsorption-discharge plasma system by pure silicon molecular sieve,and aimed at exploring the toluene degradation performance and products of the catalysts.The result showed that the adsorption capacity and toluene mineralization rate were improved with the ratio of Si/Al increasing,and these were attributed to the increase in redox properties and the decrease in acid content.GC-MS result showed the presence of water vapor(relative humidity = 20 %)had negative influence on the toluene removal,and it prevented the formation of phenol,benzoic acid and nitrotoluene on catalyst surface after plasma catalysis.Water vapor impeded the generation of reactive species or quenched part of active species.In-situ FT-IR spectra suggested that some chemical reactions occurred in adsorption processes over Ni/ZSM-170 catalyst.Carboxylic acids,aldehydes,alcohol or esters was detected in the surface of Ni/ZSM-170 catalyst during 60 min plasma catalysis.Meanwhile,in-situ TOF-MS showed less gas-phase byproducts was found in Ni/ZSM-170 sample during catalysis process,most of the products was adsorbed in the catalyst and then degraded into CO2,H2 O and other small molecules.