Abatement of VOCs Using Packed Bed Non-Thermal Plasma Reactors : A Review

Non thermal plasma (NTP) reactors packed with non-catalytic or catalytic packing material have been widely used for the abatement of volatile organic compounds such as toluene, benzene, etc. Packed bed reactors are single stage reactors where the packing material is placed directly in the plasma discharge region. The presence of packing material can alter the physical (such as discharge characteristics, power consumption, etc.) and chemical characteristics (oxidation and destruction pathway, formation of by-products, etc.) of the reactor. Thus, packed bed reactors can overcome the disadvantages of NTP reactors for abatement of volatile organic compounds (VOCs) such as lower energy efficiency and formation of unwanted toxic by-products. This paper aims at reviewing the effect of different packing materials on the abatement of different aliphatic, aromatic and chlorinated volatile organic compounds.


Introduction
Abatement of low concentration of volatile organic compounds (VOCs) from air is of great interest in the past two decades for environmental remediation.VOCs are organic compounds with very high vapor pressure, which are found in trace amounts in the atmosphere (typically less than 1000 ppm).Emission of VOCs into the atmosphere is of great concern due to its adverse effect on humans (carcinogen, affects central nervous system (CNS), causes respiratory problem, etc.) and the environment (ozone depletion, photochemical smog, global warming, etc.).The sources and the adverse effects of different VOCs on human and environment are shown in Table 1.
Conventional techniques like adsorption [1], absorption [2,3], condensation [4,5], catalytic oxidation [6,7], photocatalysis [8][9][10], membrane purification [11] and biological treatment [12,13] which have been used for removal of VOCs have disadvantages such as low efficiency and high energy consumption.The merits and demerits of various technologies available for the abatement of VOCs are reviewed in the work by khan et al. [14], Parmar et al. [15] and Luengas et al. [16].For example, in catalytic oxidation, the catalyst has to be heated and maintained at higher temperature throughout the process [17].In thermal oxidation, large amount of energy is used to heat the large volume of gas containing very low concentration of VOCs [18].Thus, the above mentioned techniques are not cost effective to treat the waste gas when the concentration of VOC is less than 1000 ppm, as it is difficult to maintain the adiabatic conditions [19].Alternatively, non-thermal plasma (NTP) technology is cost effective in converting diluted VOCs in large volume of air into less toxic substances [20][21][22][23].NTP technology utilizes the supplied energy to create high energy electrons whose temperature is in the range of 10,000-250,000 K (1-25 eV) [24]; whereas the temperature of other particles remains close to room temperature [25,26].Thus, the electrons are not in thermal equilibrium with other particles and the overall temperature of the system is maintained close to room temperature.These energetic electrons collide with carrier gas molecules, which is predominantly air and generate highly reactive species such as ground and excited atomic oxygen and nitrogen, and vibrationally and electronically excited oxygen and nitrogen molecules.These reactive species in turn oxidize the diluted VOCs into less toxic substances such as H 2 O, CO 2 and other by-products.
Although different types of NTP discharge at atmospheric pressure and room temperature such as corona discharge [27][28][29][30], surface discharge (SD) [31], microwave discharge [32], dielectric barrier discharge (DBD) [33][34][35][36][37], packed bed DBD (PBDBD) [38][39][40] have been widely investigated as an alternative technology to remove VOCs from exhaust gas, formation of unwanted by-products and high energy consumption are the main bottlenecks for the commercialization of the technology.For the development of better NTP technology for abatement of VOCs, the two important factors to be considered in addition to decomposition efficiency are (i) reducing energy consumption and (ii) controlling the formation of unwanted and toxic by-products such as newly formed other VOCs, ozone, NO x , SO x [41].It has been widely reported in the literature that DBD reactors packed with non-catalytic or catalytic materials enhance the energy efficiency [20,42] and reduce the formation of unwanted by-products by total oxidation of aliphatic (methane, butane, ethylene, formaldehyde, acetaldehyde, acetone, bromomethane), aromatic (benzene, toluene, styrene, xylene, chlorobenzene) and halogenated (carbon tetrachloride, dichloromethane, perfluoroethane) VOCs.The effect of different packing materials on the discharge characteristics of the reactor, decomposition efficiency of different VOCs and formation of by-products are summarized in Table 2.
The presence of packing material in the discharge region of the reactor enhances the electric field near the contact points between the dielectric pellets and increases the residence time of VOCs in the discharge region resulting in enhanced removal efficiency [43] and deep oxidation [44] of VOCs respectively.The total oxidation of VOCs in packed bed reactors depends on the properties of packing materials (dielectric constant, size, shape, surface properties and catalytic activity), presence of humidity, energy density and nature of VOC itself.For example, aromatic compound with aside chain requires lower energy for complete oxidation when compared to the aromatic compound without sidechain like benzene [45].This is because the ionization potential of benzene is higher than aromatic compounds with sidechain like xylene or toluene [46].
In this review, the overview of the literature on the packed bed dielectric barrier discharge (PBDBD) reactors for the abatement of different volatile organic compounds is presented.In the first part, different types of packed bed reactors which have been used for VOC removal are reviewed.In the second part, the influence of different properties of the packing materials such as dielectric constant, packing material shape and size, surface properties (porosity and surface area) on the performance of packed bed reactors for the decomposition of different VOCs are reported.The effect of addition of catalysts or catalytic packing material on decomposition efficiency (η), carbon balance (the ratio between the carbon converted from VOC to gaseous by-products and the total carbon converted in the process), CO 2 selectivity (the ratio between the carbon converted from VOC to CO 2 and the total carbon converted in the process) and by-product formation is also reviewed.

Packed Bed Reactors
Packed bed reactors are single stage reactors in which a non-catalytic/catalytic packing material is located in the discharge region of the reactor.The schematic of the most commonly used configuration of packed bed reactors are shown in Figure 1.The PBDBD reactors used for the abatement of VOCs consist of a discharge (high voltage) electrode and a ground electrode separated by one or two dielectric layers and the space between the dielectric layer and one of the electrode is filled with the packing material.Quartz or glass tubes are widely used as dielectric barrier in the packed bed reactors used for the decomposition of VOCs.The high voltage (or discharge) electrode can be made of stainless steel rod [47][48][49][50][51], stainless steel wire [52,53], stainless steel spring [54,55], stainless steel bolt [56,57], aluminum rod [58], copper rod [59], molybdenum rod [60] or tungsten wire [43,[61][62][63].The surface area and geometry of the inner electrode plays an important role in the discharge characteristics of the plasma reactor.For example, increasing the diameter of the inner electrode, reduces the discharge gap and increases the superficial area resulting in a uniform distribution of microdischarges and a higher number of energetic secondary electrons [61] respectively.The geometry of the electrode affects the number of microdischarges per cycle of applied voltage [64].Using a bolt instead of a wire or rod as high voltage electrode enhances electric field at the sharp edges of the bolt and increases the number of microdischarges resulting in higher removal and power efficiency [37,56,65].In PBDBD reactors, the length of the ground electrode determines the effective length of the reactor.The ground electrode can be made of aluminum tape [47,52,55], copper mesh [52,58,63,66], stainless steel mesh [43,56,60], iron mesh [61], aluminum mesh [50], silver paste [51,52], stainless steel foil [53], brass wire [62] or aluminum foil [59,67,68].Using metal wire, tape or gauze as ground electrode can form void between ground electrode and dielectric barrier which results in a corona discharge in the void.The energy consumed by the corona discharge in the void does not contribute to the decomposition of VOC and results in increased energy consumption and decreased conversion efficiency.In literature, it has been mentioned that painting silver paste on the wall of the discharge tube [41,52,69] prevents the formation of voids and improves the conductivity between electrode.
In PBDBD reactors, when high voltage is applied across the dielectric barrier, the dielectric packing material is polarized which enhances the electric field around the contact points resulting in a partial discharge [71,72].As shown in Figure 2, the presence of packing materials in the discharge region maximizes the number of micro streamers and thus producing more high energy electrons [18].These energetic electrons produce reactive intermediates resulting in increased VOC removal efficiency when compared to the unpacked reactors [57,73,74].Surface discharges produced along the surface of the packing material may distribute the plasma more evenly and intensively throughout the reactor [75] and the catalytic reactions occur on the surface of the packing material.Thus, the use of packed bed reactor gives the advantage of uniform distribution of gas flow and discharge [76].In packed bed reactors, absorbents are also used as packing material which increase the residence time and minimize the volume of free gas in the discharge region [77] resulting in increased collisional probability between VOC and active species.the residence time and minimize the volume of free gas in the discharge region [77] resulting in increased collisional probability between VOC and active species.The presence of packing material enhances the energy efficiency of the NTP reactor.The presence of packing material in the discharge region reduces the breakdown voltage required for ignition of plasma by enhancing the electric field near the contact points.As shown in Figure 3, the power consumption of the packed reactors are smaller when compared to the unpacked reactor [78] under the same operational conditions.The presence of the packing material in the reactor can reduce the output power and output current [79] for the same applied voltage due to the higher space charge effect.However the average power density (=input power/gas space) of the packed reactor is higher than the unpacked reactor as the volume of the plasma zone in the packed reactor is smaller than the unpacked one.The performance of the non-packed and packed reactors can also be compared using the energy effectiveness (=g VOC removed/kWh).In [79], Lin et al. showed that the energy effectiveness of a glass beads packed bed reactor is 1.5 times higher when compared to the unpacked reactor.The specific energy density required for the non-packed reactor is quite higher than the packed reactor to obtain similar decomposition efficiency [80].The presence of packing material enhances the energy efficiency of the NTP reactor.The presence of packing material in the discharge region reduces the breakdown voltage required for ignition of plasma by enhancing the electric field near the contact points.As shown in Figure 3, the power consumption of the packed reactors are smaller when compared to the unpacked reactor [78] under the same operational conditions.The presence of the packing material in the reactor can reduce the output power and output current [79] for the same applied voltage due to the higher space charge effect.However the average power density (=input power/gas space) of the packed reactor is higher than the unpacked reactor as the volume of the plasma zone in the packed reactor is smaller than the unpacked one.The performance of the non-packed and packed reactors can also be compared using the energy effectiveness (=g VOC removed/kWh).In [79], Lin et al. showed that the energy effectiveness of a glass beads packed bed reactor is 1.5 times higher when compared to the unpacked reactor.The specific energy density required for the non-packed reactor is quite higher than the packed reactor to obtain similar decomposition efficiency [80].
The different kinds of power source used to provide energy for the packed bed plasma reactor are alternating current (ac) or bipolar pulsed power sources [54].Although a bipolar pulsed power source is expensive, it has advantages over ac power source such as instantaneous energy input [54] and can avoid the accumulation of net charges on the surface of dielectrics [54].In [81], Duan et al. compared the traditional ac power supply of 50 Hz and bipolar pulse power supply of 50 Hz (peak-to-peak voltage V pp = 0 to 30 kV; pulse width 100 ns and pulse rise rate 500 V•(ns) −1 ).In this work, it is shown that the discharge current produced by bipolar pulsed power supply is higher and longer when compared to ac power supply with same frequency and applied voltage [81] and the bipolar pulsed power can enhance the chlorobenzene removal efficiency by 1.6 times when compared to ac power supply.However, for economic reason, ac power supply is widely used.The different kinds of power source used to provide energy for the packed bed plasma reactor are alternating current (ac) or bipolar pulsed power sources [54].Although a bipolar pulsed power source is expensive, it has advantages over ac power source such as instantaneous energy input [54] and can avoid the accumulation of net charges on the surface of dielectrics [54].In [81], Duan et al. compared the traditional ac power supply of 50 Hz and bipolar pulse power supply of 50 Hz (peak-to-peak voltage Vpp = 0 to 30 kV; pulse width 100 ns and pulse rise rate 500 V•(ns) −1 ).In this work, it is shown that the discharge current produced by bipolar pulsed power supply is higher and longer when compared to ac power supply with same frequency and applied voltage [81] and the bipolar pulsed power can enhance the chlorobenzene removal efficiency by 1.6 times when compared to ac power supply.However, for economic reason, ac power supply is widely used.
The power consumed by packed bed reactors for the discharge can be calculated using Lissajous curve [25] or by integrating the current and voltage waveform over one period [82,83].In Lissajous method, the voltage applied on the high voltage electrode is measured using a high voltage probe and the dissipated charge is measured across a capacitor of capacitance , which is connected in series with the ground electrode.The energy deposited in the plasma reactor per cycle is calculated from the area of the V-Q Lissajous (Figure 2a,b) diagram which has been recorded using a digital oscilloscope.The average power during discharge is calculated by multiplying the energy deposited per cycle by ac power frequency [84] In the second method, the average power, P, of the discharge is calculated using following equation from the measurement of the current and the voltage over one cycle.
where and are instantaneous voltage and current.The discharge current, I, is measured through a resistor in series with the grounded electrode.The applied voltage V(t), and the current variations I(t) with time are visualized using a digital oscilloscope.The specific input energy (SIE) which is widely used in literature to study the energy efficiency of the process and reactor in abatement of VOC is defined as the energy deposited in the flue gas per unit volume and it is given by [85]: The power consumed by packed bed reactors for the discharge can be calculated using Lissajous curve [25] or by integrating the current and voltage waveform over one period [82,83].In Lissajous method, the voltage applied on the high voltage electrode is measured using a high voltage probe and the dissipated charge is measured across a capacitor of capacitance C m , which is connected in series with the ground electrode.The energy deposited in the plasma reactor per cycle is calculated from the area S of the V-Q Lissajous (Figure 2a,b) diagram which has been recorded using a digital oscilloscope.The average power P during discharge is calculated by multiplying the energy deposited per cycle by ac power frequency f [84], In the second method, the average power, P, of the discharge is calculated using following equation from the measurement of the current and the voltage over one cycle.
where V(t) and I(t) are instantaneous voltage and current.The discharge current, I, is measured through a resistor in series with the grounded electrode.The applied voltage V(t), and the current variations I(t) with time are visualized using a digital oscilloscope.The specific input energy (SIE) which is widely used in literature to study the energy efficiency of the process and reactor in abatement of VOC is defined as the energy deposited in the flue gas per unit volume and it is given by [85]: The frequency of the ac power supply also influences the energy efficiency of the DBD reactor.Ogata et al. [66,86] reported that the conversion of benzene decreases with increase in ac frequency applied to the DBD reactor.In [65], it has been reported that the energy efficiency for trichloroethylene (TCE) removal is higher with a 50 Hz ac power supply when compared to higher frequencies such as Catalysts 2017, 7, 113 6 of 33 500 and 2000 Hz.This decrease in energy efficiency at higher frequency is mainly due to the significant convection [87] (thermal transfer between plasma and dielectric surface) and displacement [87] (the current generated by charging the capacitance of the DBD reactor) current which are not useful for the decomposition of the chemical compounds.

Dielectric Constant
When an electric field is applied across a dielectric material, the factor by which the effective electric field is reduced by polarization of the dielectric material itself is the dielectric constant.The dielectric constant of a packing material determines the amount of energy the reactor can store during a single discharge [88].When the applied ac voltage is beyond the breakdown voltage of the interstitial space between packing material, a discharge plasma is formed.The electric field in these interstitial space is amplified by the presence of packing material near the contact points [22] of the packing material and the electric field is higher for the pellets of higher dielectric constant due to boundary conditions of electric flux [89].The applied voltage and discharge current waveforms for BaTiO 3 pellets of different dielectric constant (ε s = 10,000, 5000 and 660) are shown in Figure 4.The discharge current increases with increase in the dielectric constant of the pellet and it is maximum for the pellet of dielectric constant 10 4 .
applied to the DBD reactor.In [65], it has been reported that the energy efficiency for trichloroethylene (TCE) removal is higher with a 50 Hz ac power supply when compared to higher frequencies such as 500 and 2000 Hz.This decrease in energy efficiency at higher frequency is mainly due to the significant convection [87] (thermal transfer between plasma and dielectric surface) and displacement [87] (the current generated by charging the capacitance of the DBD reactor) current which are not useful for the decomposition of the chemical compounds.

Dielectric Constant
When an electric field is applied across a dielectric material, the factor by which the effective electric field is reduced by polarization of the dielectric material itself is the dielectric constant.The dielectric constant of a packing material determines the amount of energy the reactor can store during a single discharge [88].When the applied ac voltage is beyond the breakdown voltage of the interstitial space between packing material, a discharge plasma is formed.The electric field in these interstitial space is amplified by the presence of packing material near the contact points [22] of the packing material and the electric field is higher for the pellets of higher dielectric constant due to boundary conditions of electric flux [89].The applied voltage and discharge current waveforms for BaTiO3 pellets of different dielectric constant (εs = 10,000, 5000 and 660) are shown in Figure 4.The discharge current increases with increase in the dielectric constant of the pellet and it is maximum for the pellet of dielectric constant 10 4 .Packing material with a higher dielectric constant decreases the breakdown voltage [91] and enhances the electric field resulting in enhanced VOC removal efficiency [92].Zheng et al. [88] pointed out that the packing material with higher dielectric constant (γ-Al2O3) enhances the removal efficiency of acetone when compared to glass beads.When an external ac voltage beyond onset voltage is applied across a dielectric barrier with higher dielectric constant, an intense electric field is produced in the area near the contact point of the pellets and results in a combination of surface discharges on the packing pellets and filamentary discharges in the voids [43,93].As shown Reprinted from [90].Copyright (2016), with permission from IEEE.
Packing material with a higher dielectric constant decreases the breakdown voltage [91] and enhances the electric field resulting in enhanced VOC removal efficiency [92].Zheng et al. [88] pointed out that the packing material with higher dielectric constant (γ-Al 2 O 3 ) enhances the removal efficiency of acetone when compared to glass beads.When an external ac voltage beyond onset voltage is applied across a dielectric barrier with higher dielectric constant, an intense electric field is produced in the area near the contact point of the pellets and results in a combination of surface discharges on the packing pellets and filamentary discharges in the voids [43,93].As shown in Figure 5, ferroelectric packing materials with dielectric constant ε r ≥ 1100 are energy efficient in benzene removal (amount of benzene removed per watt) [86].In [61], Liang et al. reported that the BaTiO 3 packed bed reactor has the highest toluene removal efficiency when compared to a NaNO 2 PBDBD reactor, as the spontaneous polarization intensity of BaTiO 3 is higher than NaNO 2 .Thus, the usage of ferroelectric pellets enhances the electric field (by a factor of 10-250) by polarization [72], resulting in the formation of higher-energetic species in the discharge zone [94] and thus an enhanced VOC removal efficiency.
Apart from VOC removal efficiency, the dielectric constant of the packing material also plays an important role in the formation of by-products such as ozone, NO x .According to Futamura et al. [41], the nature of the catalyst or the packing material has less effect on the initial conversion of VOC.However, it has more influence on the reaction of intermediates formed leading to a difference in the carbon balance and CO 2 selectivity.Ogata et al. [86] reported that the ferroelectric packing material with lower dielectric constant suppresses the formation of NO x , but produces large amount of ozone.Coating dielectric material with metals of higher conductivity reduces the polarization of the pellets and thus the decomposition efficiency.In [71], it is reported that BaTiO 3 pellets showed better removal efficiency when compared to 0.2 wt % Pt coated BaTiO 3 due to the increased conductivity of the pellets by platinum coating.The dielectric constant of the material varies as a function of temperature.For example, the dielectric constant of the ferroelectric packing material Urashima et al. [92] reported that the temperature of the DBD reactor and thus the packing material increases during the operation, resulting in a reduced dielectric constant of the ferroelectric packing material.Thus, the electric field in the vicinity of the pellets is reduced resulting in reduced VOC removal efficiency.
Catalysts 2017, 7, 113 7 of 33 in Figure 5, ferroelectric packing materials with dielectric constant ≥ 1100 are energy efficient in benzene removal (amount of benzene removed per watt) [86].In [61], Liang et al. reported that the BaTiO3 packed bed reactor has the highest toluene removal efficiency when compared to a NaNO2 PBDBD reactor, as the spontaneous polarization intensity of BaTiO3 is higher than NaNO2.Thus, the usage of ferroelectric pellets enhances the electric field (by a factor of 10-250) by polarization [72], resulting in the formation of higher-energetic species in the discharge zone [94] and thus an enhanced VOC removal efficiency.
Apart from VOC removal efficiency, the dielectric constant of the packing material also plays an important role in the formation of by-products such as ozone, NOx.According to Futamura et al. [41], the nature of the catalyst or the packing material has less effect on the initial conversion of VOC.However, it has more influence on the reaction of intermediates formed leading to a difference in the carbon balance and CO2 selectivity.Ogata et al. [86] reported that the ferroelectric packing material with lower dielectric constant suppresses the formation of NOx, but produces large amount of ozone.Coating dielectric material with metals of higher conductivity reduces the polarization of the pellets and thus the decomposition efficiency.In [71], it is reported that BaTiO3 pellets showed better removal efficiency when compared to 0.2 wt % Pt coated BaTiO3 due to the increased conductivity of the pellets by platinum coating.The dielectric constant of the material varies as a function of temperature.For example, the dielectric constant of the ferroelectric packing material Urashima et al. [92] reported that the temperature of the DBD reactor and thus the packing material increases during the operation, resulting in a reduced dielectric constant of the ferroelectric packing material.Thus, the electric field in the vicinity of the pellets is reduced resulting in reduced VOC removal efficiency.

Packing Material Size
The size of the packing materials plays an important role in determining the characteristics of discharges in the reactor and the efficiency of VOC decomposition.The diameter of the packing material is chosen to maximize the contact points between the packing materials of same size without completely filling the discharge gap.Ogata et al. [86,95] mentioned that BaTiO3 pellets of larger diameter (3 mm) are less efficient in decomposing benzene when compared to the smaller pellets.Similarly, Yamamoto et al. [38] mentioned that much higher toluene decomposition efficiency is obtained by using BaTiO3 pellets of smaller diameter.In [71], it is reported that BaTiO3 pellets of smaller diameter (1 mm) are more efficient in removing CCl4 when compared to pellets of

Packing Material Size
The size of the packing materials plays an important role in determining the characteristics of discharges in the reactor and the efficiency of VOC decomposition.The diameter of the packing material is chosen to maximize the contact points between the packing materials of same size without completely filling the discharge gap.Ogata et al. [86,95] mentioned that BaTiO 3 pellets of larger diameter (3 mm) are less efficient in decomposing benzene when compared to the smaller pellets.Similarly, Yamamoto et al. [38] mentioned that much higher toluene decomposition efficiency is obtained by using BaTiO 3 pellets of smaller diameter.In [71], it is reported that BaTiO 3 pellets of smaller diameter (1 mm) are more efficient in removing CCl 4 when compared to pellets of diameter 3 mm.This is attributed to the enhanced electric field and more uniform plasma throughout the reactor volume when packed with smaller pellets.Contrarily, it has been reported in [96] that very fine titania pellets (~0.5 to 1 mm) reduced the removal efficiency of TCE.This is due to the insufficient generation of plasma in the reactive region as there is lack of void due to the compact packing of titania.

Packing Material Shape
Packing material made of the same material but with different shape can influence the capacitance of the reactor which changes the discharge characteristics [97] and thus VOC removal efficiency [79].The waveforms of applied voltage and discharge current for various pellet shapes such as (a) sphere, (b) solid cylinder and (c) hollow cylinder are shown in Figure 6.The discharge current waveforms of the three different packing shapes show that the formation of microdischarges follows the following sequence: hollow cylinder > cylinder > sphere.Takaki et al. [90] showed that for the same applied voltage, the quantity of charge accumulated with the microdischarge current in a hollow cylindrical pellet is higher when compared to spherical and cylindrical pellet.The micro-discharges start appearing at lower voltage for the hollow cylinder.This is because the enhancement of electric field is higher for the hollow cylinder due to the presence of sharp edges [98].The DBD reactor filled with hollow cylindrical pellet showed higher energy efficiency (g/kWh) in the abatement of C 2 F 6 when compared with spherical pellets.In [96], Oda et al. showed that the removal efficiency showed by disc shaped randomly crushed TiO 2 is higher when compared to spherical ones.This is due to the presence of sharp edges in the packing material which enhances the electric field.
Catalysts 2017, 7, 113 8 of 33 diameter 3 mm.This is attributed to the enhanced electric field and more uniform plasma throughout the reactor volume when packed with smaller pellets.Contrarily, it has been reported in [96] that very fine titania pellets (~0.5 to 1 mm) reduced the removal efficiency of TCE.This is due to the insufficient generation of plasma in the reactive region as there is lack of void due to the compact packing of titania.

Packing Material Shape
Packing material made of the same material but with different shape can influence the capacitance of the reactor which changes the discharge characteristics [97] and thus VOC removal efficiency [79].The waveforms of applied voltage and discharge current for various pellet shapes such as (a) sphere, (b) solid cylinder and (c) hollow cylinder are shown in Figure 6.The discharge current waveforms of the three different packing shapes show that the formation of microdischarges follows the following sequence: hollow cylinder > cylinder > sphere.Takaki et al. [90] showed that for the same applied voltage, the quantity of charge accumulated with the microdischarge current in a hollow cylindrical pellet is higher when compared to spherical and cylindrical pellet.The micro-discharges start appearing at lower voltage for the hollow cylinder.This is because the enhancement of electric field is higher for the hollow cylinder due to the presence of sharp edges [98].The DBD reactor filled with hollow cylindrical pellet showed higher energy efficiency (g/kWh) in the abatement of C2F6 when compared with spherical pellets.In [96], Oda et al. showed that the removal efficiency showed by disc shaped randomly crushed TiO2 is higher when compared to spherical ones.This is due to the presence of sharp edges in the packing material which enhances the electric field.

Surface Properties
The surface properties of the packing material such as surface area, pore volume and pore size play an important role in complete oxidation of VOCs.These properties of the packing material affect the residence time of the VOCs in the discharge region of the reactor by adsorption and increases the collision probability between adsorbed pollutant molecules and the plasma generated active species.As the adsorbed molecules can only vibrate at their fixed position, they can be more easily attacked by the reactive species when compared to the randomly moving gas phase

Surface Properties
The surface properties of the packing material such as surface area, pore volume and pore size play an important role in complete oxidation of VOCs.These properties of the packing material affect the residence time of the VOCs in the discharge region of the reactor by adsorption and increases the collision probability between adsorbed pollutant molecules and the plasma generated active species.
As the adsorbed molecules can only vibrate at their fixed position, they can be more easily attacked by the reactive species when compared to the randomly moving gas phase molecules.For example, adsorption packing materials such as Al 2 O 3 and zeolites concentrate VOCs on their surface resulting in enhanced collisional probability and deep oxidation of VOCs.Adsorption lowers the bond energy of the VOC molecule [88] and weakens the chemical bond which enhances the dissociation when these adsorbed molecules encounters the active species in the plasma [99].The most commonly used porous packing materials in DBD reactor for the abatement of VOCs are porous alumina, activated carbon and zeolites.
Lee et al. [100] reported that a reactor packed with porous γ-Al 2 O 3 results in complete oxidation of benzene and enhanced CO 2 selectivity due to an increase in residence time.Liang et al. [43] reported that Al 2 O 3 increases the residence time of toluene in the reactor resulting in enhanced toluene removal rate.This is because porous alumina possess electron-accepting acidic center which acts as an adsorption material for compound with π-electrons [101].He et al. [73] reported that the activated carbon exhibits higher removal efficiency of benzene due to its higher surface area (S BET = 609.3m 2 /g) when compared to other packing materials such as alumina (S BET = 230 m 2 /g) and molecular sieve (S BET = 45 m 2 /g).
In literature, it has been reported that packing the reactor with the same material but different surface properties like surface area and porosity influences the oxidation of VOCs and formation of by-products.Gandhi et al. [102], reported that the ethylene decomposition efficiency is enhanced with porous alumina (S BET = 348.3m 2 /g) when compared to non-porous alumina (S BET = 1.53 m 2 /g).Also the complete oxidation of ethylene to CO 2 and CO is obtained with porous alumina, whereas non-porous alumina leads to the formation of by-products such as acetaldehyde, acetylene and methane in addition to CO and CO 2 .Ogata et al. [95] found that CO 2 selectivity has been enhanced with a DBD reactor packed with porous Al 2 O 3 when compared to non-porous Al 2 O 3 due to complete oxidation of benzene in the adsorbed phase.Zheng et al. [88] reported that alumina with higher surface area (γ-Al 2 O 3 , S BET = 174 m 2 /g) shows better acetone removal due to adsorption when compared to the α-Al 2 O 3 (S BET = 53 m 2 /g).In [42], it has been reported that the removal efficiency of formaldehyde (HCHO) with γ-Al 2 O 3 (S BET ≈ 220 m 2 /g) is higher when compared to α-Al 2 O 3 (S BET ≈ 10 m 2 /g) due to the difference in the specific surface area.Adsorption of HCHO reduces the dissociation threshold energy.Ogata et al. [44] reported that the energy efficiency of a Al 2 O 3 /BaTiO 3 packed bed reactor is enhanced when compared to a BaTiO 3 packed reactor.This finding shows that in addition to decomposition of benzene in the gas phase on the surface of BaTiO 3 , there is also the disappearance of benzene due to adsorption on porous Al 2 O 3 .In [96], it has been reported that titania pellets with higher surface area showed an enhanced removal efficiency of TCE.Pangilinan et al. [49] showed that silver loaded TiO 2 with lower surface area (S BET = 49 m 2 /g) decomposes benzene up to 20%; whereas the Ag/TiO 2 with higher surface area (S BET ≈ 63 m 2 /g) completely oxidizes benzene.The increase in surface area enhances the adsorption of benzene on a catalytic surface results in enhanced decomposition.In [18], Cu-Ce catalyst showed both a better removal efficiency of HCHO and a better CO 2 selectivity when compared to CuO or CeO 2 .This is because the specific surface area and the pore volume of the Cu-Ce catalyst is larger when compared to the other catalysts which promotes the adsorption of VOCs and increases the retention time of the pollutant in the discharge region and enhances the deep oxidation [103].
Hu et al. [54] and Ogata et al. [104] reported that CO 2 selectivity is greatly enhanced by zeolite hybrid reactors due to the porous structure of zeolites which increases the residence time of the adsorbed species resulting in an increased collisional probability between VOC and active species.The external surface area of zeolites is much smaller when compared to the total surface area (which includes micro and macro-pores).However the results suggest that benzene adsorbed on the external pores are readily oxidized when compared to the ones adsorbed on the internal pores [104].The higher surface area and pore volume of HZSM-5 packing material enhance the adsorption of chlorobenzene and residence time and results in a higher removal efficiency and a good CO 2 selectivity [105].In [65], it has been reported that the presence of V 2 O 5 /TiO 2 or Cu-ZSM-5 enhances the decomposition of TCE partly due to the adsorption of TCE on the packing material.
The porous packing material absorbs/adsorbs molecules of sizes smaller than its pore size [106].When the pore size of a packing material is larger than the size of the VOC molecule, there is an increased probability of VOC adsorption in these micro-pores.In [41], benzene decomposition is studied using two types of zeolite, H-Y and ferrierrite, with pore diameter 7.4 Å and 4.3-5.3Å respectively.It is shown that benzene (molecular size = 5.9 Å) can assimilate well in the micro-pores of H-Y when compared to ferrierite.This is because the pore size of H-Y is bigger than the molecular size of benzene.
Packing the DBD reactor with porous material (Al 2 O 3 and SiO 2 ) enhances the discharge power when compared to non-porous (ZrO 2 and GW) packing material.This is due to the formation of micro-discharges in the micro-pores in addition to the micro-discharges in the gas phase [107].In addition to the enhanced discharge power, the adsorption of ethylene in these porous material enhances the decomposition of ethylene, CO x selectivity, carbon balance and reduces the formation of unwanted by-products such as N 2 O.
The effect of the presence of humidity in a gas stream on the removal of VOCs has to be considered for more realistic environmental and industrial applications.The presence of humidity in flue gasses can have both positive and negative effect on the decomposition of VOC and the product selectivity.The presence of humidity can modify the surface state of the packing material and quench the electrons which affects the discharge characteristics.The presence of water can greatly influence the chemical reactions by quenching electrons and other excited reactive species and by producing OH radicals.
Ogata et al. [46] reported that the presence of water (5%) reduces the decomposition efficiency of benzene, toluene and xylene in a BaTiO 3 packed bed DBD reactor.Urashima et al. [92] showed that the removal efficiency of C 2 F 6 is reduced in the presence of humidity (relative humidity = 70%) due to the extra electron energy loss for the dissociation of water molecules.In [47,108], it is reported that the presence of humidity changes the surface states of BaTiO 3 in a PBDBD reactor, resulting in a reduced benzene removal efficiency.Wu et al. [68] reported that the presence of humidity (4.43 vol %) reduces the toluene removal efficiency due to quenching of high energy electrons by water and reducing the formation of active oxygen.
Zhu et al. [48] reported that the presence of humidity reduces the benzene removal efficiency due to the competitive adsorption of water on the surface of TiO 2 .Wu at al [68] reported that the presence of humidity reduces the toluene decomposition efficiency due to competitive adsorption of water on active sites of the catalyst.On the other hand, Zhao et al. [109] reported that the presence of humidity has no significant effect on the removal of formaldehyde using Ag/Cu-Hz as packing material due to the selective absorption.Thus, the humidity tolerance of the PBDBD reactor depends on the ability of packing to selectively adsorb the pollutant.
The presence of humidity suppresses the formation of toxic by-products such as CO and NO x and enhances CO 2 selectivity.For the reactor packed with BaTiO 3 , the deactivation of lattice oxygen species in BaTiO 3 by H 2 O reduces CO [46] and NO x formation [108].
The lifetime of the packing material is very important for the commercialization of the PBDBD reactors for the abatement of VOC.The micro-pores of the porous material can get clogged by the accumulation of solid deposits resulting in a decrease in the decomposition efficiency of VOCs.In [107], it is reported that the accumulation of solid deposit on the surface of porous packing material (α-Al 2 O 3 ) blocks the pores leading to the deactivation of the packing material.The activity of the porous material can be regenerated by an oxygen plasma treatment of the packing material [107].On the other hand, the non-porous packing material does not show any time dependent deterioration in its performance [107].In [67], it has been reported that the presence of TiO 2 enhances the benzene oxidation by decomposing ozone to form active oxygen.However, a poor carbon balance × 100% is obtained due to deposition of intermediates on the surface of TiO 2 .

By-Product Formation
The production of toxic by-products during the abatement of VOC using NTP technology is one of the major concern for the commercialization of the discussed plasma technology for air purification.The most common by-products produced by an NTP reactor during the abatement of VOC in air are ozone, NO x , aerosols and other VOCs.It has been widely reported in literature that the presence of the packing material reduces the formation of toxic by-products such as ozone and NO x either by decomposition or by adsorption of the by-product and enhances CO 2 selectivity by complete oxidation of organic compounds.
The fact that the production of ozone is reduced in the presence of a packing material is mainly due to the decomposition of ozone on the surface of the packing material to form active oxygen species.The activity of active oxygen is higher than ozone and this results in an enhanced VOC removal efficiency.Chae et al. [45] mentioned that the production of ozone in a PBDBD reactor is 60% lower than in an unpacked reactor.Zheng et al. [88] reported that the formation of ozone is highly reduced in the DBD with a packing material when compared to an empty reactor.In [110], ozone formation is suppressed by the presence of TiO 2 /Al 2 O 3 /Ni foam when compared to unpacked reactor.Whereas Zhu et al. [48] showed that the presence of TiO 2 coated RR increases the ozone concentration enhancing benzene removal efficiency.
The CO 2 selectivity during the abatement of VOCs is enhanced by the complete oxidation of the organic compounds to form CO 2 and H 2 O.The formation of CO 2 is enhanced by the presence of a porous packing material due to the adsorption of VOC in the pores which increases the VOC retention time in the discharge region resulting in deep oxidation of VOC.Ogata et al. [44,111] reported that the decomposition of benzene and CO 2 selectivity are enhanced in an Al 2 O 3 /BaTiO 3 packed reactor.Enhancement of CO 2 selectivity has also been reported for reactors packed with photocatalysts such as TiO 2 /RR [48,53] and TiO 2 /glass beads [100].Kim et al. [47] reported that a BaTiO 3 packed bed reactor inhibits the formation of aerosol by enhancing the oxidation of benzene to CO and CO 2 .
Ogata et al. [44] found that the presence of porous Al 2 O 3 in Al 2 O 3 /BaTiO 3 in a hybrid PBDBD reactor suppresses the formation of N 2 O. Zheng et al. [88] reported that the formation of NO 2 is highly reduced to 0.5 ppm in the DBD reactor packed with γ-Al 2 O 3 when compared to an empty reactor.This is due to the adsorption of NO 2 on the surface of the packing material and only 0.5 ppm of NO 2 was produced.
The formation of yellow, brownish yellow or brownish solid deposit occurs on the wall of the DBD reactor which leads to a poor carbon balance during the abatement of VOC.The presence of packing material suppresses the formation of solid deposits by complete oxidation of the organic by-products.In [57], Gandhi et al. reported the formation of unwanted by-products such as acetylene and acetaldehyde from the decomposition of butane disappeared in the presence of glass wool loaded with TiO 2 or GO as packing material.In [112], Trinh et al. reported that the presence of MnO 2 coated monolith packing material inhibits the formation of polymer like deposit in the process of acetone abatement.
On the other hand, even in the presence of packing material, the formation of polymeric solid deposit is observed on the wall of the reactor and the surface of the packing material.During the decomposition of methane in the absence of O 2 , brown yellowish deposit was found in the inner electrode [113].Decomposition of xylene in N 2 , BaTiO 3 packed bed reactor results in poor carbon balance due to the formation of a carbon deposit, polymer, decomposed fragment, cluster, or particulate [46].During the process of decomposition of benzene in the absence of O 2 , carbonaceous deposit was found on the surface of the catalyst [114,115].Thus, it is evident that N 2 plays an important role in the polymerization process and this problem can be rectified by using sufficient amount of O 2 in the feed gas [57].

Catalyst Loading on Packing Materials
Catalysts in single stage reactors are in direct contact with plasma.The plasma assisted reactions on the surface of the catalyst play an important role in addition to the gas phase reaction in VOC removal [75].The pathway of destruction of one of the model VOC (i.e., Toluene) in FeO x /SBA-15 packed bed DBD reactor is shown in Figure 7.As shown in the mechanism pathway, the destruction of toluene takes place: (i) in gas phase (by direct collision of toluene and other intermediate by-products with electron and other reactive species such as O•, OH•, N 2 •, NO•, N 2 O•) and (ii) on the surface of the catalyst (by the reaction between absorbed toluene and other intermediate by-products with active species such as O• and OH•) [116].The combined effect of the excellent toluene absorption capability of SBA-15 and O 2 absorption capacity of Fe 2+ greatly increased the toluene removal efficiency, CO x selectivity and remarkably reduced the formation organic byproducts.Thus, the key points for screening the catalyst for the abatement of VOCs are removal efficiency and carbon balance.In literature, it is shown that CO 2 selectivity is independent of SIE whereas the removal efficiency increases with increase in SIE.The initial decomposition of VOC is determined by the SIE applied, whereas the catalyst is crucial in the decomposition of the intermediates formed during the decomposition of VOC resulting in a better carbon balance.Thus, the catalyst loading on the packing materials improves the VOC removal efficiency [73], suppresses the formation of unwanted by-products like CO, NO x [73], ozone [117] [116].The combined effect of the excellent toluene absorption capability of SBA-15 and O2 absorption capacity of Fe 2+ greatly increased the toluene removal efficiency, COx selectivity and remarkably reduced the formation organic byproducts.Thus, the key points for screening the catalyst for the abatement of VOCs are removal efficiency and carbon balance.In literature, it is shown that CO2 selectivity is independent of SIE whereas the removal efficiency increases with increase in SIE.The initial decomposition of VOC is determined by the SIE applied, whereas the catalyst is crucial in the decomposition of the intermediates formed during the decomposition of VOC resulting in a better carbon balance.Thus, the catalyst loading on the packing materials improves the VOC removal efficiency [73], suppresses the formation of unwanted by-products like CO, NOx [73], ozone [117] [117], NiOx [68,117], FeOx [116] and noble metal coated or impregnated porous or metal oxide catalysts.Reprinted from [116].Copyright (2017), with permission from Elsevier.

Removal Efficiency
The transition metal oxides have lattice oxygen, surface oxygen and adsorbed oxygen [123,124] which play an important role in VOC oxidation in addition to the gas phase oxygen present in the flue gas [125,126].Zhu et al. [118] reported that a better acetone removal efficiency was obtained for 5 wt % CuO/γ-Al2O3 due to its higher reducibility and abundance of highly mobile oxygen species on the surface of the catalyst.In [18], it has been reported that the combination of plasma and Cu-Ce binary metal catalyst enhanced the removal efficiency of formaldehyde.This is mainly attributed to the enhanced oxygen vacancy and adsorbed oxygen on the pores of the surface of the catalyst which

Removal Efficiency
The transition metal oxides have lattice oxygen, surface oxygen and adsorbed oxygen [123,124] which play an important role in VOC oxidation in addition to the gas phase oxygen present in the flue gas [125,126].Zhu et al. [118] reported that a better acetone removal efficiency was obtained for 5 wt % CuO/γ-Al 2 O 3 due to its higher reducibility and abundance of highly mobile oxygen species on the surface of the catalyst.In [18], it has been reported that the combination of plasma and Cu-Ce binary metal catalyst enhanced the removal efficiency of formaldehyde.This is mainly attributed to the enhanced oxygen vacancy and adsorbed oxygen on the pores of the surface of the catalyst which facilitate the redox reaction between Ce and Cu species in the catalyst.Zhu et al. [53] reported that the decomposition efficiency of benzene is improved by the presence of TiO 2 catalyst in a RR packed bed DBD reactor.In [121], it has been reported that WO 3 coated on TiO 2 has better removal efficiency of TCE when compared to V 2 O 5 coated TiO 2 ; however the reason for this behavior is not explained in the article.In [96], it has been reported that the removal efficiency of TCE is enhanced by coating TiO 2 with V 2 O 5 .
Uniform dispersion of a metal catalyst on the packing material is very important for enhanced VOC removal efficiency due to the increase in the active metal surface [49].Lu et al. [116] reported that loading SBA-15 with FeO x enhances the toluene removal efficiency and CO 2 selectivity by reducing the unwanted by-products like benzaldehyde, benzyl alcohol, formic acid, acetic acid, benzenemethanol, tetradecane, hexadecane and heptadecane.The excellent toluene absorbing property of SBA-15 and oxygen absorbing property of Fe 2+ ion generated by FeO x are decisive in the abatement of toluene.3% Fe loading is found to be optimal due to its high dispersion nature on the support material.In [68], it is shown that NiO loaded γ-Al 2 O 3 enhanced the removal of toluene as NiO decomposes ozone to atomic oxygen much more efficiently when compared to other catalysts.The metal loading up to 5 wt % increases the removal efficiency whereas further increase in Ni loading decreases the decomposition efficiency.This is probably due to the formation of bulk NiO which starts screening the active sites as 5 wt % of Ni loading is closer to the threshold of monolayer dispersion of NiO on Al 2 O 3 .Kang et al. [127] reported that TiO 2 supported by Al 2 O 3 shows better decomposition efficiency of toluene when compared to glass beads.This is because more TiO 2 is attached to Al 2 O 3 than to glass beads and Al 2 O 3 acts as an electron acceptor.
On the other hand, loading the porous support material with catalyst can affect the surface properties of the packing material by reducing the overall pore volume and surface area.Zhu et al. [117] reported that the removal efficiency of acetone is enhanced with MO x /γ-Al 2 O 3 when compared to pure γ-Al 2 O 3 even though the metal oxide supported alumina has smaller surface area (S BET = 148 ± 2 m 2 /g) when compared to pure alumina (S BET = 187 m 2 /g).This emphasizes the role of catalytic activity of the metal oxides in the decomposition of VOCs.The catalytic activity of metal oxides is determined by their reducibility and the presence of oxygen on the surface of the catalyst.The acetone removal efficiency of CuO x /γ-Al 2 O 3 is higher when compared to other metal oxides due to its higher reducibility and a carbon balance CB(%) = × 100% higher than 95% is obtained due to the presence of surface oxygen species.
The presence of an oxidation catalyst as packing material plays an important role in the formation of active oxygen on the surface of the catalyst which enhances VOC removal efficiency.The active oxygen is formed from ozone by two different pathways as follows: (i) direct collision between active species and air molecules (Equations ( 4) and ( 5)) [105] and (ii) decomposition of ozone on the catalyst surface (Equation ( 6)).The amount of active atomic oxygen produced by the second pathway is dependent on the nature of the catalyst.In [119], it has been reported that the presence of MnO 2 enhances the decomposition of TCE and reduced the formation of by-products such as dichloro-acetylchloride (DCAC), phosgene and trichloro-acetaldehyde (TCAA).In [67], it has been reported that the presence of MnO 2 packing material enhances the decomposition of benzene.In [128], it is shown that the MnO 2 loaded γ-Al 2 O 3 enhances the removal of toluene due to an enhancement of the redox reaction by MnO 2 .In [105], it has been shown that CuO supported on MnO 2 enhances the removal efficiency of chlorobenzene.Thus, MnO 2 accelerates the decomposition of ozone to form active O species resulting in an enhanced and complete oxidation of VOCs.e + O 2 → 2O + e (4) The removal efficiency of chlorobenzene and CO 2 selectivity are enhanced by a CeO 2 /HZSM-5 packed reactor when compared with an unpacked reactor due to the good catalytic activity, the presence of oxygen vacancy and the oxygen activity of CeO 2 [105] and due to the high surface area and Brønsted acid sites of HZSM-5 [122].The level of ozone in the exhaust is also reduced, probably due to the decomposition of ozone on the surface of the catalyst [129] and utilized for chlorobenzene conversion.In [65], it has been reported that the presence of V 2 O 5 /TiO 2 or Cu-ZSM-5 enhances the decomposition of TCE partly due to the adsorption of TCE on the packing material.
Kim et al. [41] studied the effect of Ag loading on TiO 2 on the decomposition efficiency of benzene and the carbon balance.The results showed that higher the silver content the better the carbon balance × 100% is as silver aids the decomposition of intermediates formed on the surface of TiO 2 .However the decomposition efficiency starts to decline when the Ag loading is more than 4 wt %, as the loaded silver decreases the surface area of TiO 2 which is responsible for the initial decomposition of benzene.Whereas in [120], it is mentioned that the TiO 2 is more effective in benzene decomposition when compared to 0.5 wt % Ag impregnated TiO 2 .In the above mentioned works, although the Ag loading on TiO 2 is done by the impregnation method, the difference in the results can be explained by the difference in calcination (thermal treatment of catalyst in oxidation environment) temperature and calcination time of the catalyst.Kim et al. [41] studied the loading of different metals such as Ag and Ni on the TiO 2 packing material for the removal efficiency of benzene and carbon balance.Although Ni loaded TiO 2 showed higher removal efficiencies when compared to Ag, there was a problem of a poor carbon balance.This shows that Ni has lower catalytic reactivity towards the intermediates produced during the process and these intermediates are deposited on the surface of Ni/TiO 2 .
Li et al. [50] studied the effect of Pt loading on γ-Al 2 O 3 on the decomposition efficiency of benzene, CO 2 selectivity and formation of by-products.Pt/γ-Al 2 O 3 showed higher decomposition efficiency when compared to unloaded γ-Al 2 O 3 .This is because the presence of Pt increases the number of active sites and reduces the activation energy of the decomposition reaction.Pt/γ-Al 2 O 3 also suppresses harmful NO x formation by (i) reducing the energy of electrons; thus reducing the number of excited N 2 , (ii) accelerate oxygen decomposition and oxidation.Also, the formation of CO is reduced in the presence of Pt/γ-Al 2 O 3 as Pt can promote O 3 decomposition resulting in enhanced CO oxidation.An et al. [62] studied the effect of metal (Cu) loading on molecular sieve and Cu, Co, Ag or Au loading on Al 2 O 3 on the removal efficiency of toluene and formation of by-products and the results show that the metal loading enhances the removal efficiency by decomposition of ozone on the catalyst surface.

By-Product Formation
The formation of unwanted by-products such as CO, ozone, NO x and other organic intermediates is one of the main disadvantages of NTP application for the abatement of VOCs.In the literature, it has been widely discussed that the metal catalysts loaded packing material plays an important role in the deep oxidation of VOCs resulting in increased CO 2 selectivity due to deep oxidation of the intermediate organic by-products.
The formation of CO is reduced by using Ni loaded BaTiO 3 as packing material when compared to a BaTiO 3 pellet packed bed reactor [71].Apart from the enhanced removal efficiency, Pt-Pd loaded Al 2 O 3 drive the oxidation of styrene towards CO 2 rather than the formation of CO [75].In [51], it is shown that loading HZSM-5 with Ag results in complete oxidation of benzene to CO 2 and only a very negligible amount of CO is produced.This is due to strong π-complexation of Ag with benzene [101].Kim et al. [55] reported that the catalyst loading (Ag/Pt) on TiO 2 enhances CO 2 selectivity during benzene decomposition.However Pt loaded TiO 2 has a negative effect on the decomposition efficiency when compared to TiO 2 .In [57], it is reported that glass wool loaded with TiO 2 or graphene oxide produced less CO and other organic by-products when compared to glass wool loaded with ZrO 2 or alumina.This is probably due to the photocatalytic properties of graphene oxide and TiO 2 which enhanced the formation of CO 2 .
The amount of catalyst loading on the packing material plays an important role in CO 2 selectivity.Kim et al. [52] reported that CO 2 selectivity is independent of SIE when Ag loading is more than 0.5 wt %; whereas for Ag loading less than 0.3 wt %, CO 2 selectivity decreases with increase in SIE due to the deposit of intermediates on the packing material.
Ogata et al.
[95] mentioned that the formation of N 2 O during decomposition of benzene is greatly reduced by packing the DBD reactor with metal loaded porous Al 2 O 3 /BaTiO 3 .Kim et al. [55] reported that the catalyst (Ag/Pt) loaded TiO 2 suppresses the formation of undesired by-product such as NO x .An et al. [62] reported that NO x produced by the plasma discharge during toluene decomposition is reduced by the presence of 1 wt % Au/Al 2 O 3 as packing material.Pangilinan et al. [49] reported that the formation of ozone is reduced with Ag/TiO 2 and decreases with the increase in metal loading on the catalyst.

Catalyst Poisoning
The activity of the catalyst used as packing material in PBDBD reactor shows time dependent deterioration due to the accumulation of organic intermediates on the surface of the catalyst or absorption of certain organic compounds.
During the process of decomposition of benzene, toluene and xylene, the activity of MnO x /Al 2 O 3 shows a decreased conversion efficiency after 150 h of usage [130].This is probably due to the formation of the organic intermediates which is mainly composed of saturated aldehydes and/or ketones, carboxylic acids, phenols, primary alcohols, acid amides and nitro-aromatic compounds on the surface of the catalysts [130].In [122], Jiang et al. reported the deactivation of catalyst in CeO 2 /HZSM-5 packing material which is used for decomposition of chlorobenzene after 75 h of usage.The deactivation is mainly due to the absorption and deposition of methyl and organic halide on the surface of the catalyst which covers the active sites of the catalyst.In [131], Ran et al. studied the catalytic activity of Ru/Al 2 O 3 packing material used for the abatement of dichloromethane in PBDBD reactor and observed the deactivation of catalysts which is mainly due to the chlorine deposition.Zhu et al. reported slight deactivation of Cu1Ce1 catalyst after 5 h of usage for the abatement of formaldehyde [18].This is due to the formation of coke on the surface of the catalyst which blocks the micropores [18].
As prolonging the service life of the catalyst is very important for practical application, a suitable method for the regeneration of deactivated catalyst is very important.In most of the cases, the activity of the catalyst is gained by calcination of the catalyst in the presence of O 2 or air flow.In [130], Fan et al. gained the activity of MnO x /Al 2 O 3 by calcination of catalyst in air stream at 773 K for 4 h and decomposition efficiency obtained by using the calcined catalyst showed that the catalyst was fully regenerated.

Conclusions
In this paper, an overview of the literature on packed bed dielectric barrier discharge (PBDBD) reactors for the abatement of volatile organic compounds is presented.The configuration of the packed bed DBD reactors such as the powered electrode, ground electrode and power supply play an important role in determining the energy efficiency of the reactor.Using bolt as high voltage electrode enhances the electric field due to the presence of sharp edges and increases the number of microdischarges.Silver paint as ground electrode is advantageous as it avoids the formation of corona discharge in the voids outside the reactor.Thus, bolt as high voltage electrode and silver paint as ground electrode enhances the energy efficiency of the PBDBD reactors.
From this review, it is evident that the VOC removal efficiency of packed bed reactors is higher than of unpacked reactors.The presence of packing material in the reactor enhances the electric field in the area near the contact points between the pellets and homogenously distributes the plasma in the reactor.The effect of various properties of packing materials such as dielectric constant, size, shape, surface properties (surface area, pore volume and pore size) on the discharge characteristics and abatement of different VOCs has been reviewed.The dielectric constant of the packing material is important in determining the discharge characteristics and decomposition efficiency of the PBDBD.The packing material with higher dielectric constant increases the decomposition efficiency of the reactor and the increase in decomposition efficiency saturates at particular dielectric constant of the packing material.The size of the packing material is chosen to maximize the contact points between the packing material without completely filling the reactor.Maximizing the contact points enhances the electric field and produces uniform plasma and the presence of gaps in the packing facilitates the generation of plasma.The hollow cylindrical shape of the packing material enhances the electric field due to the presence of sharp edges and thus increases the decomposition efficiency of the PBDBD reactor.The presence of a porous packing material increases the residence time of VOC in the discharge zone by adsorption and enhances the collisional probability between a VOC molecule and plasma generated active species.Porous material with higher surface area enhances the decomposition efficiency, increases CO 2 selectivity and results in complete oxidation of VOC.The pore size of the packing material is crucial for the adsorption of VOC as the porous material is efficient in adsorbing molecules smaller than its pore size.The porous packing material also enhances the discharge power due to the formation of additional microdischarges inside the pores.On the other hand, porous packing material shows deactivation with continuous usage due to the formation of coke deposit on the surface which blocks the pores.The activity of the packing material can be regenerated by oxygen plasma treatment.
Apart from enhancing the energy efficiency and decomposition efficiency, the presence of packing material significantly reduces the formation of unwanted by-products and enhances CO 2 selectivity.The presence of packing material reduces the formation of ozone due to decomposition of ozone on the surface of packing material.Packing the DBD reactor with porous packing material increases CO 2 selectivity and suppresses NO x formation.On the other hand, the problem of formation of polymeric deposit due to continuous usage is still prevalent in the packed bed DBD reactor which results in poor carbon balances.The formation of polymeric deposit can be significantly reduced by presence of at least 1% O 2 in the feed gas and the solid carbon deposit can be removed using oxygen plasma.
The presence of a catalytic packing material produces active oxygen on the surface of the catalyst by the decomposition of ozone and this enhances the deep oxidation of VOCs and reduces the ozone concentration in the exhaust.The amount of metal loading on the packing material plays an important role in the formation of CO 2 by deep oxidation of organic intermediate products.The presence of metal catalysts in the packing material suppresses the formation of oxides of nitrogen which is one of the important toxic by-products produced in the presence of air in the feed gas.
Thus, the presence of packing material in the discharge region of the dielectric barrier discharge reactor enhances the energy efficiency, increases VOC removal efficiency, suppresses the formation of unwanted toxic by-products such as NO x , ozone, other VOCs and significantly improves CO 2 selectivity when compared to the unpacked DBD reactor.On the other hand, for the commercialization of this technology, the lifetime of the porous packing material, deactivation of catalyst and the reaction kinetics for the different types of pollutants and catalysts in PBDBD reactors are important this subject of research.An extensive investigation on the modification of the packing material and catalyst used in packed bed DBD reactors by the non-thermal plasma will be an important subject for this field of research.

Figure 1 .
Figure 1.Schematic diagram of packed bed non-thermal plasma reactor and its cross-sectional view (a) Power source; (b) Packing material in discharge gap; (c) Dielectric barrier; (d) High voltage (or powered electrode) and (e) Ground electrode.

Figure 2 .
Figure 2. V-Q Lissajous figure in the (a) absence and (b) presence of ceramic Raschig rings (RR), voltage and current waveform in the (c) absence and (d) presence of ceramic Raschig rings (RR) at applied voltage of 12 kV (50 Hz and sine wave).Reprinted from [63].Copyright (2016), with permission from Elsevier.

Figure 2 .
Figure 2. V-Q Lissajous figure in the (a) absence and (b) presence of ceramic Raschig rings (RR), voltage and current waveform in the (c) absence and (d) presence of ceramic Raschig rings (RR) at applied voltage of 12 kV (50 Hz and sine wave).Reprinted from [63].Copyright (2016), with permission from Elsevier.

Figure 3 .
Figure 3.Effect of presence of packing material on the power to the plasma reactor (frequency of power supply = 60 Hz; packing material: BaTiO3 pellets).Reprinted from [78].Copyright (2016), with permission from IEEE.

Figure 3 .
Figure 3.Effect of presence of packing material on the power to the plasma reactor (frequency of power supply = 60 Hz; packing material: BaTiO 3 pellets).Reprinted from [78].Copyright (2016), with permission from IEEE.

Figure 5 .
Figure 5.Effect of dielectric constant on the removal efficiency of benzene (Initial benzene concentration: 200 ppm; carrier gas: dry air; flow rate: 0.2 L/min; shape and size of the packing material: sphere of φ2 mm, numbers in legend box refer to dielectric constant value, M = Mg2TiO4, C = CaTiO3, S = SrTiO3, B = BaTiO3).Reprinted from [86].Copyright (2016), with permission from IEEE.
by deep oxidation of the intermediate by-products.The most commonly used catalyst in the PBDBD reactors for the abatement of VOCs are metal oxides such as CuO x [18,68,73,117,118], MnO x [67,68,73,81,105,117,119-121], CeO x [18,68,74,105,117,122], CoO x [117], NiO x [68,117], FeO x [116] and noble metal coated or impregnated porous or metal oxide catalysts.Catalysts 2017, 7, 113 12 of 33 N2O•) and (ii) on the surface of the catalyst (by the reaction between absorbed toluene and other intermediate by-products with active species such as O• and OH•)

Figure 7 .
Figure 7. Pathways of toluene degradation in the gas phase and on the FeO x /SBA-15 surface.Reprinted from [116].Copyright (2017), with permission from Elsevier.

Table 1 .
Source of different VOCs and their effect on human and environment.

Table 2 .
Overview of published papers on abatement of VOCs using packed bed non-thermal plasma reactors.