Combination Of Non-thermal Plasma With Catalysis For Degradating Volatile Organic Compounds

With the development of modern industry, the problems of air pollution caused by emissions of VOCs become more and more serious. Common techniques for VOCs treatment including: condensation; solution absorption; adsorption; membrane separation; biological treatment; combustion; ozonation and photocatalysis are difficult to eliminate multicomponent VOCs simultaneously. In contrast, non-thermal plasma technology can degrade different types of VOCs.at even ambient temperature. This technology also shows good VOCs removal efficiency and high energy utilization efficiency. Nevertheless, it has been found that, non-thermal plasma always produce many byproducts for VOCs degradation, and present low CO₂ selectivity.The performance of plasma alone for VOCs removal is studied at first in the paper. We intend to give a balance of VOCs removal efficiency and byproduct emissions simultaneously by optimizing the physical parameters of plasma reactor. The work concerns about developping more efficient catalysts to combine with plasma. Therefore, we synthesized different phase structure of MnO₂ catalysts by hydrothermal method and M’-OMS-2 catalysts by reflux-ion exchange method, respectively. The activity of each catalyst in plasma-catalysis reaction was investigated. On the other hand, XRD, TEM, BET, FTIR, TPR, TPD methods were applied to characterized those catalysts, and the mechanism of plasma-catalysis was also discussed. In the end, we designed a system which was consisted of a high-voltage electrostatic field and cobalt manganese composite metal oxide catalysts Co-Mn Ox. The system was applied in a simulated indoor environment for the effective removal of particulate matter. Simultaneously, formaldehyde could also be oxidized by ozone which was acted as byproduct of high-voltage electrostatic field.The results in this work showed that: for the removal of VOCs by plasma alone, VOCs removal efficiency was closely related to the power supply. Higher power of plasma often gave higher VOCs removal efficiency. However, more by-products can be formed. Lower plasma power may reduce the by-products, but led to less removal efficiency. Therefore, the reasonable power parameters for plasma should be chosen.Theoretically, the energy distribution of electrons in plasma satisfied Maxwell’s equations. Therefore, some electrons could have enough energy to break nitrogen triple bond to form NOx in the presence of oxygen. The experimental results showed that, NOx was primarily generated by the oxygen and reactive nitrogen species in the discharge process. But some oxygen-containing VOCs could also contribute their oxygen to the formation of NOx. Therefore, it is difficult to avoid NOx producing in plasma alone. The generation of NOx mainly belongs to excitation type when low voltage is used for plasma, however, it will translate to thermal type with voltage increasment. Low CO₂ selectivity was observed during acetaldehyde degradation by plasma, which was due to the formation of some intermediate products such as nitromethane, amines, acetic acid, and carbon monoxide.Three types of MnO₂ catalysts with different phase structure were synthesized and combined with plasma. All of them are active in plasma-caalysis. The experimental results showed that, the introduction of MnO₂ catalysts could improve the VOCs removal efficiency and CO₂ selective, while the byproducts were also reduced. α-MnO₂/Al₂O₃ was superior to other MnO₂ catalysts. The excellent activity may be ascribed to OH groups, VOCs adsorption capacity and mobility of oxygen of the material.For plasma-catalytsis reaction, the influence factors such as manner of combination, VOCs initial concentration, flow rate, volume of catalysts always played an important role on the VOCs removal efficiency and byproduct emissions. Our experiment showed that post plasma-catalytsis had a better permance than in plasma-catalytsis; higher VOCs initial concentration was not beneficial to VOCs degradation, but also produced more NOx. According to analysis of kinetics, the introduction of catalysts can not change plasma reaction order but may increase the reaction rate. Meanwhile, a moderate flow rate benefited both VOCs removal and byproduct suppression; and larger amount of catalysts could improve the performance of plasma-catalytsis.M’-OMS-2 catalysts synthesized by reflux-ion exchange method were more active than MnO₂ catalysts in plasma-catalytsis reaction. Meanwhile, these materials presented good stability. Co-OMS-2 showed highest activity among all the M’-OMS-2 samples. According to catalysts characterization, it was easier for cobalt to substitute manganese than other metal ions in M’-OMS-2, therefore, more surface oxygen vacancy could be formed for Co-OMS-2. Those oxygen vacancy were beneficial to trapping of reactive oxygen species in plasma-catalytsis reaction. In addition, high oxygen mobility and large surface area can also enhance catalytic activity for Co-OMS-2. Compare to plasma alone, the enhancement of CO₂ selectivity in acetaldehyde degradation by introducing Co-OMS-2 catalysts was unsignificant, becasue Co-OMS-2 catalysts could convert many plasma intermediates to CO rather than CO₂Drawing lessons from the plasma-catalysis technology, a high-voltage electrostatic field was combined with Co-MnOx catalysts to remove PM in an indoor environment, and formaldehyde could also be catalytically oxidized by ozone which was produced in the high-voltage electrostatic field. The synthesis temperature, cobalt and manganese mole ratio of catalysts had a significant impact on the system performance. Experimental results showed that the Co-Mn Ox catalysts with cobalt-manganese molar ratio of 1: 1, and 773 K calcination temperature presented highest catalytic activity. Catalytic activity of Co-Mn Ox was related to the formation of Co Mn O3, mixed valence state of metal ions and surface oxygen.