Environ. Sci. Technol. 2009, 43, 2216–2227
Removal of Volatile Organic Compounds by Single-Stage and Two-Stage Plasma Catalysis Systems: A Review of the Performance Enhancement Mechanisms, Current Status, and Suitable Applications HSIN LIANG CHEN,† HOW MING LEE,‡ S H I A W H U E I C H E N , ‡ M O O B E E N C H A N G , * ,† SHENG JEN YU,§ AND SHOU NAN LI§ Graduate Institute of Environmental Engineering, National Central University, Chung-Li, Taoyuan County, 320, Taiwan (R.O.C.), Physics Division, Institute of Nuclear Energy Research (INER), Longtan, Taoyuan County, 325, Taiwan (R.O.C.), and Industrial Technology Research Institute (ITRI), Hsinchu 310, Taiwan (R.O.C.)
Received September 22, 2008. Revised manuscript received January 13, 2009. Accepted January 13, 2009.
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 combinestheadvantagesoffastignition/responsefromnonthermal 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 twostage 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 for the singlestage 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-
* Corresponding author phone: +886-3-4227151 ext. 34663; fax: +886-34221602; e-mail:
[email protected]. † National Central University. ‡ Institute of Nuclear Energy Research (INER). § Industrial Technology Research Institute (ITRI). 2216
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stage and two-stage configurations are suggested to be more suitable for industrial and indoor air applications, respectively.
1. Introduction Control of volatile organic compound (VOC) emission has become an important issue in that they are not only hazardous to human health but also harmful to the environment. Many VOCs have been proved to be carcinogenic and/ or mutagenic. Moreover, once emitted into the atmosphere, VOCs could be the precursors for the formation of photochemical smog, secondary aerosol, and ozone. Although process optimization is a feasible approach for VOC reduction, a more effective and economical abatement technique is still needed to meet the increasingly stringent emission regulations worldwide. Nonthermal plasma (NTP), such as dielectric barrier discharge (DBD), corona discharge, surface discharge, and packed-bed plasma reactor, featured by the nonequilibrium characteristic between electrons and heavy particles (including ions, radicals, and neutrals), has been investigated for VOC abatement over two decades. Compared with the hightemperature-based technologies such as combustion and catalytic oxidation, NTP can be ignited and works at room temperature. Hence, no warm-up time is required. In contrast to adsorption, NTP could directly decompose VOCs, rather than phase transfer. Nonthermal plasma can be applied to a wide range of VOC concentration (1-10,000 ppm), especially effective for concentrations lower than 100 ppm, in which the conventional technologies are not suitable for the treatment (1). However, the energy efficiency and CO2 selectivity (the ratio of the carbon amount from effluent CO2 to that from VOCs removed) still need to be improved for further industrial application (2, 3). To resolve these problems simultaneously, a novel technique termed as plasma catalysis has been developed accordingly. This novel technique combines the advantages of high selectivity from catalysis and the fast ignition/response from plasma technique. It has been experimentally demonstrated that plasma catalysis could enhance energy efficiency and improve CO2 selectivity in numerous studies, showing its promising potential for VOC removal. 10.1021/es802679b CCC: $40.75
2009 American Chemical Society
Published on Web 02/20/2009
FIGURE 1. Illustration of the performance enhancement mechanisms in SPC and corresponding effects on the performance. According to the location of catalyst, the plasma catalysis system can be divided into two categories, namely, singlestage and two-stage systems. Several different terms have been proposed to represent the single-stage plasma catalysis system, e.g., in-plasma catalysis reactor (IPCR) (2), plasmadriven catalysis (PDC) (4), combined plasma catalysis (CPC) (5), and in-plasma catalysis (IPC) (6), while postplasma catalysis reactor (PPCR) (2), plasma-enhanced catalysis (PEC) (4), and postplasma catalysis (PPC) (6) are used to denote the two-stage type. To avoid any misunderstanding, only single-stage plasma catalysis (SPC) and two-stage plasma catalysis (TPC) are adopted in the following. In a plasma catalysis system, nonthermal plasma and catalysis are not just working independently either in SPC or TPC. Some extra performance enhancement mechanisms would be induced. However, the mechanisms in these two configurations are quite different. In terms of gaseous pollutant removal via plasma catalysis, two excellent review works have been published (4, 6). Different from these studies, this review paper mainly focuses on VOC abatement, and more performance enhancement mechanisms possibly occurring in plasma catalysis are covered. It is noted that, to better interpret these mechanisms, the literature using plasma catalysis for NOx removal and hydrogen reforming to produce H2 are also cited. The current status of the applications of SPC and TPC on VOC removal and the suggestions on the suitable application for these different systems are also provided. In the following, the nonthermal plasma technologies are operated at atmospheric pressure unless specified otherwise.
2. Construction and Performance Enhancement Mechanisms of SPC 2.1. Construction of SPC. As can be literally understood, the SPC is constructed by integrating nonthermal plasma and catalysis in the same reactor. The catalyst can be introduced in the form of pellets, foam, honeycomb monolith, or coating on the electrode(s). The catalyst region can partially or completely occupy the discharge zone. In the case of pellets, the nonthermal plasma reactor can be packed with purely catalytic pellets or a mixture of noncatalytic and catalytic ones.
2.2. Performance Enhancement Mechanisms in SPC. In this configuration, the performance enhancement mechanisms are rather complicated because both plasma and catalysis take place simultaneously and interact with each other. The possible mechanisms and the corresponding effects on the performance are illustrated in Figure 1. These mechanisms can be elaborated from two aspects: (a) the influence of the presence of catalyst on the plasma treatment of pollutants and (b) the influence of plasma discharge on catalysis. The principles of how these mechanisms result in the positive effects on the performance are discussed in detail as follows. 2.2.1. Influence of the Presence of Catalyst on the Plasma Treatment of Pollutants. When the catalyst is packed within the plasma reactor, the discharge gap would be shortened, causing a change in the plasma characteristics and the discharge behavior. Moreover, the adsorption of pollutant on catalyst surface would change its retention time and concentration. The possible effects on plasma treatment of gaseous pollutants caused by these phenomena are individually described in the following. Packed-Bed Effect. Once the catalyst is introduced into a nonthermal plasma reactor as pellets, it would form the socalled packed-bed reactor (PBR), which is characterized by the numerous contact points between pellets and pellets/ electrodes. Compared with the nonpacked plasma reactor (e.g., DBD, corona discharge, and surface discharge), the average electric field in a PBR would be enhanced because of the short distance in the adjacency of contact points. When the electric field is enhanced, the plasma energy tends to be consumed by the electron-impact dissociation and ionization reactions, which are responsible for pollutant removal. In other words, the energy dissipated by the useless electronimpact reactions including rotation and vibration would be decreased. Therefore, PBRs could achieve a higher energy efficiency compared with nonpacked ones (7, 8). Enhancement in Plasma Generation. How the packing pellets in SPC affect the discharge behavior has been reviewed by Van Durme et al. (6). They indicated that the packing pellets are helpful for expanding the discharge region because the streamers (or microdischarges) are apt to propagate along the solid surface. More recently, Kim et al. (9) reported that the discharge mostly occurs around the contact points when VOL. 43, NO. 7, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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zeolite (MS-13X) pellets are packed inside a surface discharge reactor. On the other hand, the discharge would spread over the pellet surface when 10 wt% Ag/MS-13X catalyst is used instead. As the plasma discharge region expands, the concentrations of active species would increase due to the higher collision probability between electrons and gas molecules, leading to the acceleration of plasma chemistry reactions. Adsorption of Pollutant on Catalyst Surface. For SPC, if the catalyst has a significant adsorption capacity for pollutant molecules, it would bring about two functions, i.e., prolonging the pollutant retention time and increasing the pollutant concentration in the plasma discharge zone. The former could improve the removal efficiency and selectivity simultaneously because of the higher collision probability between pollutant molecules and the active species generated in plasma. Moreover, the latter could enhance the energy efficiency for pollutant abatement because the fraction of electrons or radicals reacting with pollutant molecules would be higher as the initial pollutant concentration increases. The above-mentioned merits have been experimentally demonstrated by Ogata et al. (10). They adopted two PBRs, packed with either BaTiO3 pellets or a mixture of BaTiO3/ zeolite (MS-13X) pellets, to remove benzene from gas streams. The zeolite pellets used have a significant adsorption capacity for benzene. while the BaTiO3 pellets show little adsorption ability. The energy efficiency of PBR with mixed pellets is 1.4 times higher. Moreover, the ratio of CO2 concentration to the summation of CO and CO2 concentrations for the PBR with mixed pellets (74%) is also higher than that with BaTiO3 pellets only (67%). As a result, the ability of catalyst to adsorb pollutant might play an important role in SPC. 2.2.2. Influence of Plasma Discharge on Catalysis. As far as SPC is concerned, the most significant differences from the conventional catalysis include the existence of abundant short-lived active species (i.e., excited species, radicals, and positive/negative ions), the occurrence of plasma discharge on catalyst surface, and a voltage potential and a current flow across the catalyst. The first phenomenon changes the status of gas-phase reactants for catalytic reactions, while the other two affect the physical and chemical properties of the catalyst. How these phenomena influence the catalytic performance are individually discussed as follows. Change of the Status of Gaseous Reactants. In conventional catalysis, most gaseous reactants are in their ground state. Nevertheless, for SPC, a certain portion of the gaseous reactants would be transformed into chemically active species including excited species, radicals, and/or ions. Because of the higher internal energy, these active species might show higher activity for catalytic reactions. Authors have reviewed the influences of these active species on the catalytic hydrocarbon reforming for H2 production (8) and reached the following conclusions based on the available data. • Ions and electronically excited species would have deexcited before they reach the catalyst surface. • The internal energy of the species in rotational state is not sufficient to induce further reactions. • Radicals generally show a much higher sticking coefficient for chemisorption, an essential step of catalytic reactions. • Although the internal energy of vibrationally excited species is not enough to induce plasma chemistry reactions, they are the active species produced in plasma with the minimum internal energy to improve catalytic reactions. Numerous studies have confirmed that elevating the vibrational energy of reactants could enhance the dissociative adsorption in catalytic reactions. It is also of much importance to understand whether the radicals and the vibrationally excited species could assist the catalytic reactions for pollutant removal. Especially for the 2218
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vibrationally excited species, which are considered useless for plasma chemistry reactions, the energy efficiency would be further enhanced if they could assist the catalysis. However, there is a significant difference between useful chemical synthesis and gaseous pollutant removal, i.e., the initial concentrations of the substance to be treated are typically >10% and TiO2 > 4.6 wt% V2O5/TiO2. All the above-mentioned comparisons are made based on SIE. The deterioration of the VOC removal efficiency caused by depositing active component on the catalytic support could be interpreted from two aspects: the change in the catalytic activity and plasma characteristics. When the active component is loaded, the catalytic activity might reduce due to the decrease in surface area. Kim et al. (3) and Harling et al. (14) both indicated that the surface area would decrease when Ag is loaded on TiO2. For instance, the surface area of TiO2 is decreased when 0.5 wt% Ag catalyst was coated (from 137.4 to 28.4 m2/g), and the benzene removal efficiency with catalysis at 267 °C was also decreased (from 42% to 5%) (14). On the other hand, the addition of active component on catalyst support could change the plasma characteristics of SPC as well. A DBD reactor filled with SiO2 pellets with or without 3wt% loading of Ni was experimentally tested for steam methane reforming to produce hydrogen by Nozaki et al. (49). The optical emission spectroscopy (OES) results show that the rotational temperature (generally deemed as gas temperature) stays the same, regardless of the presence of Ni; however, the deposition of Ni changes the vibrational temperature substantially. The vibrational temperature is sensitive to the electron energy distribution function (EEDF) (50), implying the active component might cause a significant influence on the plasma characteristics and the corresponding VOC removal efficiency. Nevertheless, to the authors’ knowledge, there is no study focusing on this topic. It is worth further investigation to get insight into the interaction between plasma and catalysis. Overall speaking, SPC has been proved to be an effective approach to improve the energy efficiency and CO2 selectivity for VOC abatement. However, it needs to be noticed that the carbon balance (the ratio of the removed carbon presented as gaseous substances to removed carbon) might decrease in SPC instead, as observed in ref 51. The lower the carbon balance, the more carbon-containing deposited would be generated on the catalyst surface, which would lead to the deactivation of the catalyst. In general, the carbon balance could be improved by increasing SIE. For the case that the SIE is constrained by the formation of arc discharge, a moderate heating of the catalyst might be needed. Up to date, it is still difficult to conclude how to select an appropriate catalyst based on the available data, since different discharge types, reactor geometry, catalysts, and target pollutants are investigated in the relevant studies. One should be careful in selecting the active component. Choosing
an inappropriate active component would decrease the VOC removal efficiency instead. 4.2. TPC. The literature adopting TPC for VOCs abatement is summarized in Table 3. Compared to plasma-alone, TPC could enhance VOC removal efficiency, CO2 selectivity, and O3 destruction. It is not surprising that the TPC with the catalyst heated to a high temperature could achieve a better performance than plasma-alone, since the catalyst is thermally activated and could remove the residual VOCs from the plasma reactor. However, the overall performance could be improved even at room temperature. More importantly, it has been indicated in various studies (32, 35, 37, 60) that a synergistic effect, i.e., the VOC removal efficiency obtained with TPC is higher than the summation of plasma-alone and catalysis-alone, could take place in TPC. As mentioned previously, ozone decomposition on catalyst surface plays a crucial part to induce the performance enhancement mechanism in TPC. It is certain that the suitable catalyst for TPC must possess two characteristics, i.e., the effectiveness in decomposing ozone and the generation of reactive oxygen species toward VOCs. Actually, ozone decomposition via catalysis has been extensively investigated because ozone is hazardous to human health. Under certain circumstances (e.g., in workplace with ozone generation source or in airplane cabin), it is necessary to reduce the ozone concentration as low as possible to protect people from ozone exposure. Dhandapani and Oyama (61) have comprehensively reviewed various patents and studies regarding catalytic ozone decomposition. They indicated that the catalysts are usually supported by γ-Al2O3, SiO2, TiO2, or activated carbon. The active components can be divided into two categories: noble metals (Pt, Pd, Rh, etc.) and metal oxides (MnO2, Co3O4, CuO, Fe2O3, NiO, and Ag2O). Due to the economic attraction, metal oxides are more commonly used than noble metals. Among the various metal oxides, MnO2 prevails in the relevant patents and studies possibly because of the highest activity for ozone decomposition, which has been further experimentally confirmed by Dhandapani and Oyama (61). In the case of pure catalysis, ten catalysts with different metal oxides deposited on γ-Al2O3 support were used to remove ozone from a gas stream at 40 °C. The loading amount of metal oxide is fixed at 10 wt%, while the initial O3 concentration is 2 ppm. The ozone removal efficiency is in the following order: MnO2 > Co3O4 > NiO > Fe2O3 > Ag2O > Cr2O3 > CeO2 > MgO > V2O5 > CuO. However, it is worth noting that ozone decomposition is just one of the essential characteristics for ozone-assisted catalysis. More importantly, the oxygen species formed on the catalyst surface after ozone decomposition has to be reactive for VOCs. The best catalyst for these two reactions might not be the same. For example, Einaga and Futamura (62) used six different metal oxides catalyst (Mn, Ag, Ni, Co, Fe, and Cu) supported on γ-Al2O3 to investigate the catalytic oxidation of benzene and cyclohexane with the addition of ozone. The experimental tests are carried out at 22 °C, and the initial concentration of ozone and benzene are 1000 and 100 ppm, respectively. In the absence of VOC, the activity for ozone decomposition is in the following order: Fe > Co ≈ Ni > Mn >Ag > Cu. The order is not exactly the same as that of Dhandapani and Oyama (61). The discrepancy might result from the difference in the catalyst preparation methods. However, when benzene or cyclohexane is present in the gas stream, the Mn oxide catalyst shows the best performance for either ozone decomposition or benzene removal, reinforcing that the suitable catalyst for TPC must meet the requirements of effective ozone decomposition and generating active oxygen species for VOCs at the same time. Moreover, for TPC, it needs to be emphasized that the catalyst temperature must be controlled at an appropriate range, in which the deterioration of catalytic activity caused VOL. 43, NO. 7, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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catalyst 66.2 26 18 23 19 11 44 52
36
20 170-245 170-245 150-245 25 16-300 ∼25 150-240
∼25
6
-----
-----
48 -------
---
60 750
50 35
----750 750
38.8
-----
-----
76 -------
100 68 43 66 78 >95 23 45 46 45 64 79 74 88 100
56 99 ------------------24 11 18 30
----0 0 38 6 --8 7 4 0 ---------
80 100 ------------------52.6 25.7 36.6 55
(66) (67)c,d,e, and f
(35) (60)c,d,e, and f (37)c,d,e, and f (37)c,d,e, and f
ref
172 (68)
--(36) 142 (37)c,d,e, and f
10 9.2
1260 330 142 142
removal carbon removal CO2 carbon catalyst temp efficiency CO2 selectivity O3 concn balance efficiency selectivity O3 concn balance SIE (°C) (%) (%) (ppm) (%) (%) (%) (ppm) (%) (J/L)
TPC
1. The symbol --- represents that the data are not given in the original paper. 2. The removal efficiency or CO2 selectivity in Table 3 is not always the best results obtained in the relevant papers. The purpose of the performance comparison in Table 3 is to demonstrate that TPC is capable of improving the performance. More information regarding the experimental details should be referred to the original paper. b APGD stands for atmospheric pressure glow discharge. c,d,e, and f Indicate that the removal efficiency, CO2 selectivity, and O3 concentration of TPC are achieved with the catalyst temperatures of 210, 170, 150, and 16 °C, respectively.
a
toluene
plasma reactor
DBD MnO2 corona discharge Pt/γ-Al2O3 corona discharge Pt/honeycomb corona discharge Pt/honeycomb TiO2 0.5 corona discharge CuO/MnO2/TiO2 (mass ratio 3:6.8:100) 45 APGDb Cu-Mn/Al2O3 Ni oxide/honeycomb V oxide/honeycomb 30 surface discharge Mn-Cu oxide/honeycomb 330 corona discharge Pt/honeycomb MnO2-FeO3 (mass ratio 40:60) γ-Al2O3 PBR packed with 9% MnO2/γ-Al2O3 3% MnO/activated carbon 240 glass beads
106 120 120 isopropyl alcohol 330
benzene butyl acetate
pollutant
conc. (ppm)
plasma-alone
TABLE 3. Literature Regarding TPC for VOCs Abatement and the Performance Comparison between Plasma-Alone and TPCa
FIGURE 2. Butyl acetate, isopropyl alcohol, and toluene decomposition by catalysis. ηwith and ηwithout represent the removal efficiency achieved with and without ozone addition, respectively. The figure is plotted based on the experimental results of ref 37. The catalyst used is Pt/honeycomb. The initial concentrations of butyl acetate, isopropyl alcohol, toluene, and ozone are 120, 330, 330, and 750 ppm, respectively. by the formation of carbon-containing deposits on catalyst surface can be avoided and the thermal ozone decomposition in gas phase is insignificant to make sure ozone can be used to assist heterogeneous catalysis. Although some catalysts are capable of decomposing ozone and removing VOCs simultaneously at room temperature, the catalysts would gradually deactivate because of the formation of carboncontaining intermediates, which has been reported in refs 63 and 64. To avoid such a problem, a moderate heating of the catalyst seems necessary. For example, Einaga and Futamura (65) investigated the catalytic oxidation of benzene assisted by ozone addition and indicated that the temperature must be maintained at a certain level. If the temperature is below 70 °C, the catalyst would gradually lose its activity due to the build-up of organic byproducts on catalyst surface. Nevertheless, it does not mean the catalyst can be heated without any limitation because ozone tends to decompose at high temperature. As shown in Figure 2, the higher the temperature, the lower the ratio of the VOC removal efficiency with ozone addition to that without. When the temperature is over a certain value, the ratio is approaching unity, suggesting that ozone could not effectively assist catalysis because it is mostly decomposed in the gas phase or on the catalyst surface without an active site.
5. Suggestion on the Suitable Applications for Different Plasma Catalysis Systems Although a systematic comparison between SPC and TPC is still lacking, it has been experimentally demonstrated in various studies that SPC could achieve a better performance for gaseous pollutant removals (13, 14, 32, 69-73), which are believed to be stemmed from the performance enhancement mechanisms described in section 2.2. However, it does not mean SPC is more suitable for all applications than TPC. In terms of VOC abatement, the byproducts generated after plasma treatment, such as CO, NOx and O3, is an important issue that should be taken into account. The typical VOC concentrations from industrial sources and in indoor air are 102-103 and
