Experimental And Mechanism Study On Catalytic Degradation Of Volatile Organic Compounds Via Microwave-Induced Metal Discharge

With the rapid industrial development in our country,the output and consumption of organic chemicals are gradually increased,and volatile organic compounds(VOCs)are inevitably produced as remarkable types of environmental pollutants during their manufacture and usage.In recent years,such typical environmental pollutants are becoming more and more serious,so the source control and emission reduction of VOCs have become an essential way to prevent and control air pollution in our country.However,due to the deficiencies in current treatment technologies,there is an urgent need to develop new technologies to efficiently control VOCs pollution.Microwave-induced metal discharge is a process that can produce significant photo and plasma effects,which has unique effects on promoting microwave heating,pyrolysis,and intensifying chemical reactions.However,there are few reports on the use of microwave-induced metal discharge to degrade VOCs.Therefore,this paper selected toluene/acetone as the substrates,to carry out the experimental research and theoretical calculation of the microwave-induced metal discharge degradation of VOCs and to reveal the reaction law and the reaction mechanism,finally to provide a theoretical basis for related applications.Firstly,the metal electrode materials were screened with excellent performances from factors such as the melting point and the electron overflow work,and their discharge performance was also explored.The primary selection of metal strip electrodes included such as aluminum,copper,iron,nickel,etc.,and then the discharge effects of nickel strips,paper clips,and metal tungsten needles were compared.The metal tungsten was determined as the preferred electrode material,and then improved the discharge effect from the shape,the number of electrodes,and placement method.At the same time,the physical characteristics of microwave-induced metal discharge such as thermal effect and plasma effect were also investigated in detail.The changes of reactor bed temperature and reactor outlet gas temperature,and the exploration of the characteristics of thermal effect under discharge conditions were studied,the results demonstrated that metal discharge strengthens the thermal effect of microwave selective heating,resulting in the significant increase of the temperature rise rate and the temperature range of the reactor bed and gas.Based on emission spectrum analysis,a set of photovoltage analysis system was established to study the luminescence effect of microwave-induced metal discharge.The analysis of the change process of metal discharge under different working conditions revealed that the pulse waveform of microwave-induced metal discharge photovoltage presents periodic sinusoidal changes.And the influence mechanism of the metal discharge plasma characteristics was clearly proved that microwave power and oxygen content have positive effects on the intensity of the emitted light voltage,and the gas flow rate has a negative effect on the emitted light voltage.Secondly,using toluene and acetone as the VOCs,a microwave-induced metal discharge degradation of toluene and acetone was carried out.The influence rules of microwave power,gas flow rate,initial concentration of toluene/acetone and oxygen content on the degradation rate of toluene/acetone were investigated,and GC-MS,FTIR,O3 detectors,and other methods were used to analyze gas product composition,mineralization rate,and O3 production,and to explore the microwave-induced metal discharge reaction mechanism of VOCs degradation.The results showed that the degradation rate of toluene/acetone using the nickel strip or the paper clip electrode only reached up to 65-70%,while the tungsten electrode had a better effect on degradation of toluene/acetone.When the microwave power was 900W and the discharge background atmosphere was air,the degradation rate of toluene and acetone reached up to 93%and 90%,respectively.The higher the initial concentration of VOCs would cause a decrease in the degradation rate,but the amount of degradation would continue to increase.Specifically,when the initial concentration of toluene/acetone increased up to 5 times,the decrease rate was reduced by 2%and 4%,respectively.When the flow rate increased up to 5 times,the degradation rate of toluene and acetone decreased by 3%,and the overall degradation rate was still above 90%,showing a high level of degradation.Using the metal tungsten electrodes caused the significant increase of degradation rate of VOCs treated by microwave-induced metal discharge,moreover,the concentration range of VOCs treatment was also increased by orders of magnitude,and its corresponding application prospects were broad.Then,in order to achieve a higher degradation rate and to effectively reduce the formation of harmful by-products,the catalysts were introduced to study microwave-induced metal discharge catalytic degradation of toluene/acetone.Selecting MnO2 as the catalyst,its crystal form did not change before and after the microwave-induced metal discharge degradation of VOCs,proving that MnO2 can be employed as a catalyst for the system.The introduction of catalyst MnO2 increased the degradation rate of toluene/acetone,and reduced the leaching amount of O3.When the catalyst was placed in another external reactor,the degradation rate of toluene/acetone reached up to 98%and 97%,and the removal rate of O3 reached up to 52%and 48%,respectively.These achieved a high degradation rate of toluene/acetone and a significant reduction of O3 emission.Meanwhile,the mineralization rate of toluene/acetone and the selectivity of CO2 were enhanced,showing that there is a synergetic coupling effect between the catalyst and the microwave-induced metal discharge,which significantly improves the VOCs treatment effect.After the reaction,the content of Mn3+ on the surface of the catalyst increased,and the relative content of Oads adsorbed on the surface increased from 25.5%before the reaction to 45.8%(toluene)and 48.4%(acetone),respectively.As combined with the product analysis,the possible degradation reaction path of toluene/acetone in the microwave-induced metal discharge catalytic system was speculated:In one part,the inelastic collision occurred between the VOCs gas molecules and the high-energy electrons generated by the discharge caused the VOCs gas molecules to break the bond,which was accompanied by the oxidation reaction of active radicals(O,OH,etc.)with VOCs gas molecules.In the other part,the reaction occurred on the surface of the catalyst,the great adsorption capacity of the MnO2 catalyst tunnel-type crystal structure,which can greatly absorb a large amount of O3 and H2O generated by the discharge process,resulting in the changes in the valence state of Mn(Mn4+,Mn3+,Mn2+).The O3 adsorbed on the surface of MnO2 can generate O radicals,or react with H2O to form OH radicals.These radicals underwent oxidation reactions with VOCs and their intermediate products to give the CO2 and H2O as final products.Finally,based on the experimental results,the density functional theory(DFT)calculation was carried out to explore the toluene/acetone degradation process by theoretical simulation calculations at the micro-atom level,to explore the reaction path of microwave-induced metal discharge degradation toluene/acetone,and to analyze the transition state structure,reaction potential energy barrier,and atomic bond energy,revealing the mechanism of the degradation of toluene/acetone by high-energy electrons and free radicals from the micro-level and explaining the theoretical possibility of its degradation products.The results confirmed that in the plasma field formed by microwave-induced metal discharge,toluene/acetone can be directly degraded through the cleavage of the covalent bond,or it can undergo an oxidation reaction with active free radicals to achieve indirect degradation,that is,oxidation reaction.The degradation products of toluene/acetone not only had CO2 and H2O,but also included other organic products.The theoretical calculation results were consistent with the experimental results,and the reaction path was further confirmed.The research in this thesis confirms that the microwave-induced metal discharge catalysis technology can achieve high-efficiency degradation of VOCs,and this technical route has the potential for large-scale application.