**3. VOC removal by means of corona discharges**

Air plasma produced by corona discharges and its performance in the oxidation of VOCs are being investigated in Padova using a prototypal large corona reactor [18–20]. The reactor and the auxiliary apparatus were designed in order to achieve stable and reproducible plasma regimes and experimental conditions, which are necessary for quantitative kinetic and product studies. Reproducibility and stability of experimental conditions in our set-up allow to test and compare the performance of different corona regimes, notably dc+, dc– and pulsed+ within the same apparatus and under otherwise identical experimental conditions. The experimental set-up, comprising the corona reactor, the gas flow line and instrumentation for *in line* and *off line* analysis of the treated gas, is schematically reproduced in Figure 2.

**Figure 2.** Schematics of corona reactor, gas flow line and instrumentation for *in line* and *off line* analysis of the treated gas.

The corona reactor has a wire/cylinder electrode configuration. The wire electrode (stainless steel, outer diameter 1 mm) is electrically connected to the high voltage supply and fixed along the axis of a stainless steel cylinder (38.5 mm i.d. x 600 mm) which is electrically grounded.

The reactor can be energized by dc or pulsed high-voltage power. The dc power supply has an output voltage of ± 25 kV and an output current of 0 – 5 mA. For generating pulsed corona, a pulsed high voltage with dc bias (PHVDC) was used, based on a spark gap switch with air blowing, with the following specifications: dc bias of 0 – 14 kV, peak voltage of 25 – 35 kV (with dc bias), peak current up to 100 A, maximum frequency 300 Hz, rise-time of the pulses less than 50 ns. To measure the power input two homemade current probes (*shunt*), one of 1.1 Ω for pulsed current, the other of 52 Ω for dc current, were used. The experimental apparatus was described in detail previously [18].

The reactor is connected to a gas flow line made of Teflon tubing (inner diameter 4 mm). The air/VOC mixture is prepared by bubbling synthetic air (80% nitrogen: 20% oxygen from AirLiquide) through a sample of liquid VOC and by diluting the outcoming flow with a second flow of synthetic air to achieve the desired gas composition (VOC mixing ratio in the 100 - 1000 ppm range) and flow rate (usually kept constant at 450 mLN⋅min-1). The gas flow line is equipped with a loop for humidification and with a probe to measure the humidity. The treated gas exiting the reactor goes through a small glass reservoir equipped with a sampling port from which aliquots are withdrawn with a gastight syringe for off-line chemical analysis by GC-MS (Agilent Technologies 5973) and GC-TCD/FID (Agilent Technologies 7890). *In line* IR analysis is performed with an FTIR Nicolet 5700 spectrophotometer using a 10 cm long gas cell with windows of NaCl (for experiments with dry air) or of CaF2 (for experiments with humidified air). The determination of ozone, CO, and CO2 were performed by integration of characteristic IR bands as described previously [18]. The determined conversion data, i.e. [VOC]/[VOC]0 as a function of the specific input energy (SIE, also referred to as specific energy density SED) usually follow a first order exponential decay profile. The SIE was determined as described previously for dc [18, 21] and pulsed [20, 21] corona, respectively. The data are thus interpolated with the equation (1) to obtain the energy constant kE, which is a measure of the process energy efficiency.

**3. VOC removal by means of corona discharges**

*line* analysis of the treated gas, is schematically reproduced in Figure 2.

gas.

8 Current Air Quality Issues

was described in detail previously [18].

Air plasma produced by corona discharges and its performance in the oxidation of VOCs are being investigated in Padova using a prototypal large corona reactor [18–20]. The reactor and the auxiliary apparatus were designed in order to achieve stable and reproducible plasma regimes and experimental conditions, which are necessary for quantitative kinetic and product studies. Reproducibility and stability of experimental conditions in our set-up allow to test and compare the performance of different corona regimes, notably dc+, dc– and pulsed+ within the same apparatus and under otherwise identical experimental conditions. The experimental set-up, comprising the corona reactor, the gas flow line and instrumentation for *in line* and *off*

**Figure 2.** Schematics of corona reactor, gas flow line and instrumentation for *in line* and *off line* analysis of the treated

The corona reactor has a wire/cylinder electrode configuration. The wire electrode (stainless steel, outer diameter 1 mm) is electrically connected to the high voltage supply and fixed along the axis of a stainless steel cylinder (38.5 mm i.d. x 600 mm) which is electrically grounded.

The reactor can be energized by dc or pulsed high-voltage power. The dc power supply has an output voltage of ± 25 kV and an output current of 0 – 5 mA. For generating pulsed corona, a pulsed high voltage with dc bias (PHVDC) was used, based on a spark gap switch with air blowing, with the following specifications: dc bias of 0 – 14 kV, peak voltage of 25 – 35 kV (with dc bias), peak current up to 100 A, maximum frequency 300 Hz, rise-time of the pulses less than 50 ns. To measure the power input two homemade current probes (*shunt*), one of 1.1 Ω for pulsed current, the other of 52 Ω for dc current, were used. The experimental apparatus

The reactor is connected to a gas flow line made of Teflon tubing (inner diameter 4 mm). The air/VOC mixture is prepared by bubbling synthetic air (80% nitrogen: 20% oxygen from AirLiquide) through a sample of liquid VOC and by diluting the outcoming flow with a second flow of synthetic air to achieve the desired gas composition (VOC mixing ratio in the 100 - 1000 ppm range) and flow rate (usually kept constant at 450 mLN⋅min-1). The gas flow line is equipped with a loop for humidification and with a probe to measure the humidity. The treated gas exiting the reactor goes through a small glass reservoir equipped with a sampling port from which aliquots are withdrawn with a gastight syringe for off-line chemical analysis by

$$\left[VOC\right] = \left[VOC\right]\_0 \cdot e^{\left(^{-k\_x}\cdot S\bar{E}\right)}\tag{1}$$

Current/voltage characteristics of dc corona, both of positive and negative polarity, were monitored in synthetic air with and without VOC admixture (500 ppm VOC concentration). For each applied voltage, the mean current intensity was measured, after a stabilization time of 5 minutes, using a multimeter. The ions present in the air plasma produced by +dc and –dc corona were investigated using an APCI (*Atmospheric Pressure Chemical Ionization*) interfaced to a quadrupole mass analyzer (TRIO 1000 II, Fisons Instruments) [22, 23]. A schematic drawing of the arrangement and the gas inlet systems is shown in Figure 3.

**Figure 3.** Schematics of APCI ion source and gas inlet system (1) quadrupole analyzer, (2) rotary pump, (3) diaphragm pump.

The corona discharge is kept at atmospheric pressure by a flow of synthetic air (4–5 L⋅min-1) introduced through the nebulizer line, a capillary of ca. 2 mm (inner diameter). Vapors of the desired VOC, stripped by an auxiliary flow of synthetic air (typically 5–50 mL⋅min-1) from a liquid sample contained in a reservoir, enter the APCI source through another capillary (inner diameter 0.3 mm) placed coaxially inside the nebulizer line. A second line allows for the introduction of water vapors as desired. The needle electrode for corona discharge was kept at 3 kV. Ions leave the source through an orifice (50 µm in diameter) in the counter electrode, called the "sampling cone" and held at 0–150 V relative to ground. The ions then cross a low pressure region (down to ca. 10-2 Torr) and, through the orifice in a second conical electrode, called the "skimmer cone" and kept at ground potential, reach the low pressure region hosting the focusing lenses and the quadrupole analyser. Prior to running the experiments with the VOC, a preliminary analysis is routinely conducted to acquire the "background" spectra with only synthetic air and humidified synthetic air a).

The efficiency, products and mechanisms of VOC oxidation were studied systematically under variation of the corona type (dc or pulsed), the corona polarity (negative or positive), the VOC (a few hydrocarbons, halogenated and oxygenated organic compounds have been investigat‐ ed), the VOC inlet concentration and the level of humidity. These studies have provided a large body of experimental results which give insights into corona induced chemical oxidation and useful hints for its application.

The type of corona has major impact on the process efficiency. In Figure 4 an example is shown, which is reporting a comparison of the decay profile of toluene concentration as a function of SIE under three different corona regimes: dc+, dc– and pulsed+ [21]. The much better efficiency of pulsed+ corona with respect to dc corona of either polarity is evident. Also evident is the better performance of dc– with respect to dc+ corona. Analogous results were obtained in similar experiments with other VOCs, including *n*-hexane [18, 20] and dibromomethane [24].

**Figure 4.** Decay profile of toluene (500 ppm in synthetic air) as a function of SIE in corona induced oxidation under the following regimes: pulsed+, dc– and dc+ [21].

The better efficiency of pulsed+ corona with respect to dc corona of either polarity is consistent with the results of an emission spectroscopy study which showed that at any specific input energy significantly greater average electron energy is obtained with pulsed corona than with dc coronas [25]. Correspondingly, a higher density of reactive O atoms is observed in pulsed + corona than with dc coronas [25]. Due to the filamentary nature of the plasma, not only the energy but also the spatial distribution of electrons and other short-lived reactive species is very different from that in glow dc coronas: the plasma is affecting a relatively larger volume thus accounting for a more efficient process.

A second important variable is the VOC initial concentration. Usually, the corona induced oxidation efficiency decreases as the VOC initial concentration [X]0 is increased and often a linear correlation is observed between kE and 1/[X]0 within a significant range of concentra‐ tions. An example is shown in Figure 5.

pressure region (down to ca. 10-2 Torr) and, through the orifice in a second conical electrode, called the "skimmer cone" and kept at ground potential, reach the low pressure region hosting the focusing lenses and the quadrupole analyser. Prior to running the experiments with the VOC, a preliminary analysis is routinely conducted to acquire the "background" spectra with

The efficiency, products and mechanisms of VOC oxidation were studied systematically under variation of the corona type (dc or pulsed), the corona polarity (negative or positive), the VOC (a few hydrocarbons, halogenated and oxygenated organic compounds have been investigat‐ ed), the VOC inlet concentration and the level of humidity. These studies have provided a large body of experimental results which give insights into corona induced chemical oxidation and

The type of corona has major impact on the process efficiency. In Figure 4 an example is shown, which is reporting a comparison of the decay profile of toluene concentration as a function of SIE under three different corona regimes: dc+, dc– and pulsed+ [21]. The much better efficiency of pulsed+ corona with respect to dc corona of either polarity is evident. Also evident is the better performance of dc– with respect to dc+ corona. Analogous results were obtained in similar experiments with other VOCs, including *n*-hexane [18, 20] and dibromomethane [24].

**Figure 4.** Decay profile of toluene (500 ppm in synthetic air) as a function of SIE in corona induced oxidation under the

The better efficiency of pulsed+ corona with respect to dc corona of either polarity is consistent with the results of an emission spectroscopy study which showed that at any specific input energy significantly greater average electron energy is obtained with pulsed corona than with dc coronas [25]. Correspondingly, a higher density of reactive O atoms is observed in pulsed + corona than with dc coronas [25]. Due to the filamentary nature of the plasma, not only the energy but also the spatial distribution of electrons and other short-lived reactive species is very different from that in glow dc coronas: the plasma is affecting a relatively larger volume

A second important variable is the VOC initial concentration. Usually, the corona induced oxidation efficiency decreases as the VOC initial concentration [X]0 is increased and often a

only synthetic air and humidified synthetic air a).

useful hints for its application.

10 Current Air Quality Issues

following regimes: pulsed+, dc– and dc+ [21].

thus accounting for a more efficient process.

**Figure 5.** Dependence of process efficiency (kE) on the reciprocal of VOC initial concentration for dc+ corona induced oxidation of acetone in dry synthetic air.

Other similar cases are reported in the literature [14, 24, 26–29] and have been interpreted based on a simple scheme of inhibition by the intermediates formed in the VOC reaction [30]. Finally, the VOC chemical composition and structure also matters and different VOCs are oxidized with different efficiencies under the same experimental conditions. A few representative data are reported in Table 1.


a) Data are from ref. [27] unless otherwise specified. b) VOC initial concentration was 500 ppm. c) Data from ref. [25].

**Table 1.** Reaction efficiency data, expressed as kE in L kJ-1 units, for corona processing of different VOCs in dry and in humid (40% RH) synthetic air.

This outcome might not have been anticipated *a priori* since the generally accepted notion is that plasma chemical processes proceed via radical reactions which are usually very fast and poorly selective. This is the case, for example, for the reaction of OH radicals with organic compounds which is viewed as a major contributor to VOC oxidation in humid air plasmas.

The data in Table 1 show instead that air plasmas are somewhat selective. This selectivity might originate from either of two circumstances (or possibly a combination of the two): within a given type of plasma, say that produced by dc–, different VOCs either react along different paths or react with the same species but at different rates. The vast available bibliography on rate constants for reactions of many VOCs with air plasma reactive species (atoms, radicals, ions) and on ionization energies and electron affinities provides tools to exclude some possibilities and sort out which reactions are most likely paths. Chemical knowledge and intuition help in providing model VOCs to be used as reactivity probes.

A most intriguing and informative response is found in studying the effect of humidity on the efficiency of VOCs oxidation. The data in Table 1 show that for all VOCs considered, except CF2Br2, the presence of humidity in the air produces an increase in efficiency with dc– corona and no effect or a slight decrease in efficiency with dc+ corona. The increase in efficiency observed with dc– is rather straightforwardly attributed to the OH radicals formed by corona discharges in humid air. OH radicals are among the strongest known oxidants of VOCs. Compare for example the rate constants for reaction of toluene with atomic oxygen (2) [31] and with OH radical (3) [32], a channel becoming more available in humid air plasma:

$$\text{C}\_7\text{H}\_8 + \text{O} \rightarrow \text{Products} \qquad \qquad \text{k}\_{298} = 7.6 \cdot 10^{-14} \text{ cm}^3 \cdot \text{molecule}^{-1} \cdot \text{s}^{-1} \tag{2}$$

$$\text{C}\_7\text{H}\_8 + \text{OH} \rightarrow \text{Products} \qquad \qquad \text{k}\_{298} = \text{S}.7 \cdot 10^{-12} \text{ cm}^3 \cdot \text{molecule}^{-1} \cdot \text{s}^{-1} \tag{3}$$

Support for the conclusion that reaction with OH radicals is important in dc– corona induced oxidation of hydrocarbons and of CH2Br2 (Table 1) came from experiments with CF2Br2 (halon 1020) [27]. Like other perhalogenated saturated hydrocarbon, CF2Br2 is not attacked by OH and other atmospheric radicals: the reaction of CF2Br2 with OH radicals is more than 220 times slower [33] than that of CH2Br2. And indeed there was no increase in efficiency for dc– processing of CF2Br2 in humid air, but rather a slight decrease with respect to dry air. This slight decrease in efficiency was attributed to reaction (4) which contributes to reduce the average electron energy while producing OH radical, which is unable to attack this specific VOC.

$$H\_2O + e^- \rightarrow OH + H + e^- \tag{4}$$

Less straightforward was to explain the decrease in efficiency observed with dc+. To make sure that OH radicals also form in dc+ corona regime and to compare their relative densities in dc + and dc– air plasmas the well known reaction of OH with CO to form CO2 (eq. (5)) was used [34].

$$\text{CO} + \text{OH} \rightarrow \text{CO}\_2 + \text{H} \qquad \qquad \qquad \quad \text{k}\_{298} = 2.41 \cdot 10^{-13} \text{ cm}^3 \cdot \text{molecule}^{-1} \cdot \text{s}^{-1} \tag{5}$$

Indeed, in a control experiment CO did not react at all in dry air under the effect of either dc + or dc– corona. In contrast, in humid air (40% RH) reaction (5) occurs both with dc–, more efficiently, but also with dc+ corona, thus proving the presence of OH radicals in such plasmas. Since with dc+ oxidation of hydrocarbons is less efficient in the presence of OH radicals than it is in dry air (Table 1), it was concluded that reaction with OH radicals is not the dominant initiation channel for their oxidation in dc+ corona. Thus, it appears clearly that VOC oxidation induced by dc+ and dc– corona in air occurs by different mechanisms. For the investigated hydrocarbons (see Table 1) oxidation induced by dc+ corona is believed to be initiated by ionmolecule reactions. Support for this hypothesis comes from direct observation of the ions within the plasma achieved by APCI-mass spectrometry analysis and from comparison of current/voltage (I/V) profiles measured with only synthetic air and with VOC-containing synthetic air.

paths or react with the same species but at different rates. The vast available bibliography on rate constants for reactions of many VOCs with air plasma reactive species (atoms, radicals, ions) and on ionization energies and electron affinities provides tools to exclude some possibilities and sort out which reactions are most likely paths. Chemical knowledge and

A most intriguing and informative response is found in studying the effect of humidity on the efficiency of VOCs oxidation. The data in Table 1 show that for all VOCs considered, except CF2Br2, the presence of humidity in the air produces an increase in efficiency with dc– corona and no effect or a slight decrease in efficiency with dc+ corona. The increase in efficiency observed with dc– is rather straightforwardly attributed to the OH radicals formed by corona discharges in humid air. OH radicals are among the strongest known oxidants of VOCs. Compare for example the rate constants for reaction of toluene with atomic oxygen (2) [31] and with OH radical (3) [32], a channel becoming more available in humid air plasma:

7 8 <sup>298</sup> *C H O Products* k 7.6 10 cm molecule s - - - + ® =× × × (2)

7 8 <sup>298</sup> *C H OH Products* k 5.7 10 cm molecule s - - - + ® =× × × (3)

Support for the conclusion that reaction with OH radicals is important in dc– corona induced oxidation of hydrocarbons and of CH2Br2 (Table 1) came from experiments with CF2Br2 (halon 1020) [27]. Like other perhalogenated saturated hydrocarbon, CF2Br2 is not attacked by OH and other atmospheric radicals: the reaction of CF2Br2 with OH radicals is more than 220 times slower [33] than that of CH2Br2. And indeed there was no increase in efficiency for dc– processing of CF2Br2 in humid air, but rather a slight decrease with respect to dry air. This slight decrease in efficiency was attributed to reaction (4) which contributes to reduce the average electron energy while producing OH radical, which is unable to attack this specific

Less straightforward was to explain the decrease in efficiency observed with dc+. To make sure that OH radicals also form in dc+ corona regime and to compare their relative densities in dc + and dc– air plasmas the well known reaction of OH with CO to form CO2 (eq. (5)) was used

Indeed, in a control experiment CO did not react at all in dry air under the effect of either dc + or dc– corona. In contrast, in humid air (40% RH) reaction (5) occurs both with dc–, more efficiently, but also with dc+ corona, thus proving the presence of OH radicals in such plasmas. Since with dc+ oxidation of hydrocarbons is less efficient in the presence of OH radicals than it is in dry air (Table 1), it was concluded that reaction with OH radicals is not the dominant initiation channel for their oxidation in dc+ corona. Thus, it appears clearly that VOC oxidation

<sup>2</sup> <sup>298</sup> *CO OH CO H* k 2.41 10 cm molecule s - - - +® + =× × × (5)

14 3 1 1

12 3 1 1

<sup>2</sup> *H O e OH H e* - - +® ++ (4)

13 3 1 1

intuition help in providing model VOCs to be used as reactivity probes.

VOC.

12 Current Air Quality Issues

[34].

Figure 6 (a) reports *I*/*V* data monitored in experiments with toluene (500 ppm initial concen‐ tration). It is seen that for dc– the profiles determined with and without toluene are nearly superimposed, whereas in the presence of toluene for dc+ the current intensity measured is significantly lower, at any applied voltage, than found in pure synthetic air. Since corona current is due to ion transport across the drift region of the interelectrode gap, the results suggest that with dc+ corona different ions are present in pure air and in toluene containing air. Accordingly, different average ion mobilities are derived from the current/voltage characteristics of Figure 6 (a) [35–37]: 2.35 cm2 ⋅V-1⋅s-1 for pure air and 1.79 cm2 ⋅V-1⋅s-1 for toluene-containing air, respectively.

APCI-mass spectrometry is a powerful tool for monitoring and characterizing the ions formed by corona discharges and their reactions. The APCI-mass spectra reported in Figure 6 (b) show the ions present in the plasma produced in synthetic air by dc– and dc+ corona discharge, respectively. These ions are water clustered O2 – and O3 – ions (O2 – (H2O)n (n = 0-2: m/z 32, 50, 68), O3 – (H2O)n (n = 0-1: m/z 48, 66)) as well as O2 – (O2) (m/z 64) for dc– corona and H3O+ (H2O)n (n = 2 - 3: m/z 55, 73) and NO+ (H2O)n (n = 1 - 2; m/z 48, 66) for dc+ corona, respectively.

**Figure 6.** (a) Current/voltage profiles measured with dc– and with dc+ corona in pure air (open symbols) and in tol‐ uene (500 ppm) containing air (closed symbols). (b) APCI mass spectra recorded with dc– and dc+ corona in pure air. (c) APCI mass spectra recorded with dc– and dc+ corona in air containing toluene (500 ppm).

The APCI mass spectra recorded under the same experimental conditions except for the presence of a small amount of toluene (500 ppm) in the air are shown in panel (c) of Figure 6. The effects are significantly different for dc– and dc+ corona. Thus, with dc– corona the major ions observed in the plasma are the same regardless of whether toluene is present or not in the gas (compare the mass spectra on the left-hand side in panels (b) and (c) of Figure 6). These observations are fully consistent with and provide a rationale for the nearly identical *I*/*V* curves determined for dc– in pure air and in toluene containing air (panel (a) of Figure 6).

In contrast, in the case of dc+ the mass spectrum of toluene containing air is completely different from that of pure air (compare the mass spectra on the right-hand side in panels (b) and (c) of Figure 6). Thus, in air contaminated with toluene (500 ppm) the prevailing charged species are T+ (m/z 92) and [T+H]+ (m/z 93) (T stands for the toluene molecule), along with their ionmolecule complexes T+ (T) (m/z 184) and [T+H]+ (T) (m/z 185). These ions form [38-40] via exothermic charge- and proton transfer ion-molecule reactions (eq. 6 and 7), characterized by rate constants of 1.8x10-9 and 2.2x10-9 cm3 molecule-1 s-1, respectively [38], followed by ionmolecule complex formation (eq. 6a and 7a).

$$\begin{aligned} T + O\_2^\* &\to T^\* + O\_2\\ T^\* + T + M &\to T^\* \left(T\right) + M \quad (a) \end{aligned} \tag{6}$$

$$T + H\_3O^+ \rightarrow \left[T + H\right]^+ + H\_2O$$

$$\left[T + H\right]^+ + T + M \rightarrow \left[T + H\right]^\ast \left(T\right) + M \quad (a)\tag{7}$$

Finally, the NO+ (T) (m/z 122) ion-molecule complex is also observed. Thus, mass spectroscopic ion analysis provides a rationale, at the molecular level, for interpreting *I*/*V* curves observed with dc+ (Figure 6 panel (a) right hand side). In addition, these results suggest that ionic reactions might be responsible for the initial stages of toluene decomposition induced by dc+ corona. This hypothesis is consistent with the observed insensitivity of the dc+ process efficiency to the presence of humidity, which rules out a significant role of the OH radical.

Analogous results were obtained with other hydrocarbons leading to the conclusion that the initial step of oxidation depends on the plasma regime applied: ion-molecule reactions are favored with dc+ whereas reactions with O atoms and OH radicals prevail in the case of dc– corona discharges.

The yield of the final oxidation product, CO2, as a function of SIE has been determined and compared with the profile of VOC conversion (Figure 7). CO2 production is clearly less energy efficient than VOC conversion as is reasonable to expect for a process which involves many steps and oxidation intermediates.

In comparing pulsed and dc coronas, at any given value of VOC conversion the yield of CO2 increases in the order pulsed+ < dc– < dc+. This is evident from the data shown in Figure 8a concerning experiments with *n*-hexane. Comparing the results corresponding to a given

The APCI mass spectra recorded under the same experimental conditions except for the presence of a small amount of toluene (500 ppm) in the air are shown in panel (c) of Figure 6. The effects are significantly different for dc– and dc+ corona. Thus, with dc– corona the major ions observed in the plasma are the same regardless of whether toluene is present or not in the gas (compare the mass spectra on the left-hand side in panels (b) and (c) of Figure 6). These observations are fully consistent with and provide a rationale for the nearly identical *I*/*V* curves

In contrast, in the case of dc+ the mass spectrum of toluene containing air is completely different from that of pure air (compare the mass spectra on the right-hand side in panels (b) and (c) of Figure 6). Thus, in air contaminated with toluene (500 ppm) the prevailing charged species are

exothermic charge- and proton transfer ion-molecule reactions (eq. 6 and 7), characterized by rate constants of 1.8x10-9 and 2.2x10-9 cm3 molecule-1 s-1, respectively [38], followed by ion-

( )

[ ] ( )

ion analysis provides a rationale, at the molecular level, for interpreting *I*/*V* curves observed with dc+ (Figure 6 panel (a) right hand side). In addition, these results suggest that ionic reactions might be responsible for the initial stages of toluene decomposition induced by dc+ corona. This hypothesis is consistent with the observed insensitivity of the dc+ process efficiency to the presence of humidity, which rules out a significant role of the OH radical.

Analogous results were obtained with other hydrocarbons leading to the conclusion that the initial step of oxidation depends on the plasma regime applied: ion-molecule reactions are favored with dc+ whereas reactions with O atoms and OH radicals prevail in the case of dc–

The yield of the final oxidation product, CO2, as a function of SIE has been determined and compared with the profile of VOC conversion (Figure 7). CO2 production is clearly less energy efficient than VOC conversion as is reasonable to expect for a process which involves many

In comparing pulsed and dc coronas, at any given value of VOC conversion the yield of CO2 increases in the order pulsed+ < dc– < dc+. This is evident from the data shown in Figure 8a concerning experiments with *n*-hexane. Comparing the results corresponding to a given

(T) (m/z 122) ion-molecule complex is also observed. Thus, mass spectroscopic

2 2 ( ) *TO T O T TM TT Ma* + +

+®+

3 2 [ ] [ ] ( ) *T HO T H HO TH TM TH T M a*

+ + + ®+ + + ++ ® + +

+ +

(m/z 93) (T stands for the toluene molecule), along with their ion-

++ ® + (6)

(T) (m/z 185). These ions form [38-40] via

(7)

determined for dc– in pure air and in toluene containing air (panel (a) of Figure 6).

(T) (m/z 184) and [T+H]+

+ +

T+

(m/z 92) and [T+H]+

molecule complex formation (eq. 6a and 7a).

molecule complexes T+

14 Current Air Quality Issues

Finally, the NO+

corona discharges.

steps and oxidation intermediates.

**Figure 7.** Profiles of VOC decay and CO2 production as a function of SIE for treatment of toluene (500 ppm) with dc– corona in dry synthetic air.

decomposition fraction of the VOC, for example 0.7 (70% conversion), the corresponding amount of CO2 released with dc+ is about 4 times larger than with pulsed+ and about 1.6 times larger than with dc–. Thus, among the different types of corona tested, dc+ has the poorest efficiency for VOC conversion but the best selectivity for CO2 production. On the other hand, a consistently lower CO2/CO ratio is found with dc+ than with dc– and pulsed+ (Figure 8b).

**Figure 8.** CO2 production and CO2/CO ratio for treatment of *n*-hexane (500 ppm) with dc+, dc– and pulsed+ corona in dry synthetic air. The data are displayed as a function of the fraction of decomposed *n*-hexane.

In search for the missing fraction of organic carbon several oxidation intermediates were identified and quantified by means of GC/MS and GC/FID analysis and proper standards. In the case of *n*-hexane the major detected intermediates were a few aldehydes and ketones, as shown in Figure 9. It is seen that the concentration of most of these intermediates reaches a maximum and then decays, showing that they are in turn oxidized in air non-thermal plasmas. It is also seen that, with the exception of acetaldehyde, the concentrations of these organic intermediates are very small under any of the applied conditions. The experimental data have been fitted according to a simple kinetic model for consecutive reactions [41] to obtain relative reactivity data of the intermediates with respect to that of the precursor, *n*-hexane [18].

**Figure 9.** Aldehydes and ketones detected as intermediates in the oxidation of *n*-hexane (500 ppm in dry synthetic air) induced by pulsed+, dc– and dc+. The data are displayed as a function of the fraction of decomposed *n*-hexane [18].

**Figure 10.** FT-IR spectra recorded in experiments with dc+ corona (+19 kV) in (a) pure synthetic air, used as reference spectrum, and in synthetic air containing 500 ppm of the following VOCs: (b) *n*-hexane; (c) toluene; (d) CH2Br2; (e) CF2Br2.

In line FT-IR spectroscopy gives a comprehensive overview of the composition of the treated gas analyzed at the outlet of the corona reactor. Besides CO2 and CO, other species can be conveniently determined, including ozone, various nitrogen oxides and derivatives (N2O, HNO3, etc), and, depending on the specific VOC, also other volatile organic oxidation inter‐ mediates. A few examples, reported in Figure 10, show characteristic bands for VOC specific products such as formic acid in the case of toluene and CF2O in the case of CF2Br2.

The production of HNO3 is high and ozone is almost completely absent in corona discharge treatment of air containing bromo derivatives CH2Br2 and CF2Br2, (Figure 10 d and e). These observations have been explained considering reactions and catalytic cycles involving BrOx (x = 0, 1) and NOx (x = 1, 2) species [24], which have been extensively investigated as major contributors to the depletion of stratospheric ozone [42].
