No abstract

This paper provides a comprehensive review regarding the application of plasma catalysis, the integration of nonthermal plasma and catalysis, on VOC removal. This novel technique combinesthe advantages of fast ignition/response from nonthermal plasma and high selectivity from catalysis. It has been successfully demonstrated that plasma catalysis could serve as an effective solution to the major bottlenecks encountered by nonthermal plasma, i.e., the reduction of energy consumption and unwanted/hazardous byproducts. Instead of working independently, the combination could induce extra performance enhancement mechanisms either in a single-stage or a two-stage configuration, in which the catalyst is located inside and downstream from the nonthermal plasma reactor, respectively. These mechanisms are believed to be responsible for the higher energy efficiency and better CO2 selectivity achieved with plasma catalysis. A comprehensive discussion on the performance enhancement mechanisms is provided in this review paper. Moreover, the current status of the applications of two different plasma catalysis systems on VOC abatement are also given and compared. The catalyst plays an important role in both configurations. Especially for the single-stage type, depositing an inappropriate active component on catalytic support would decrease the VOC removal efficiency instead. To date, no definite conclusion on catalyst selection forthe single-stage plasma catalysis is available. However, MnO2 seems to be the best catalyst for two-stage configuration because it could effectively decompose ozone and generate active species toward VOC destruction. On the other hand, although the single-stage plasma catalysis has been proved to be superior to the two-stage configuration, it does not mean that the former is always the best choice. Considering the typical VOC concentrations from different sources and the characteristics of different plasma catalysis systems, the single-stage and two-stage configurations are suggested to be more suitable for industrial and indoor air applications, respectively.

Packed-bed plasma reactors, constructed by packing noncatalytic dielectric pellets inside nonthermal plasma reactors, have been demonstrated to effectively alleviate the major bottleneck encountered by nonthermal plasma, i.e., the energy efficiency needs to be further improved. As far as the environmental issues are concerned, packed-bed plasma reactors are mainly applied to ozone generation and gaseous pollutant removal. According to the available experimental data, for a given specific energy density, the energy efficiency for ozone generation and gaseous pollutant abatement obtained with packed-bed reactors, if compared to that of nonpacked reactors, could be 1.1-4.3 and 1.1-12 times higher, depending on the type of pollutant, the reactor geometry, and the packing pellets used. Nevertheless, it is worth noticing that the packing pellets suitable for ozone generation and pollutant removal are quite different. The influences of material, dielectric constant, size, and shape of the packing pellets on the performance for ozone generation and gaseous pollutant removal are comprehensively reviewed in this paper and guidelines for pellet selection are provided as well. For the single-stage plasma catalysis system, in which catalyst pellets are directly packed inside the plasma reactor, the physical parameters of catalyst would also have significant influence on the plasma characteristics and the performance. Therefore, the content of this review paper could provide useful information for singlestage plasma catalysis system from the viewpoint of plasma characteristics.