The Role of Ozone in the Reaction Mechanism of a Bare Zeolite-Plasma Hybrid System

We investigated the reaction mechanism of a metal-unloaded zeolite-plasma hybrid system for decomposition of toluene at room temperature. Short-lived radicals and fast electrons did not contribute substantially to the reaction mechanism of toluene decomposition in the zeolite-plasma hybrid system. The main factor enhancing the reaction mechanism was gas-phase O3 directly reacting with toluene adsorbed onto the zeolite (the Eley-Rideal mechanism). CO2 selectivity was not improved by using H-Y zeolite due to its low ability to retain the active oxygen species formed by O3. The gas-phase O3 reacted with adsorbed toluene to form a ring cleavage intermediate that was slowly converted into formic acid. The decomposition rate of formic acid was much lower than that of toluene on the H-Y surface.


Introduction
Non-thermal plasma processes have attracted significant interest as promising technologies for decomposing volatile organic compounds (VOCs) [1].Over the last decade, research has focused on improving the process efficiency of decomposition of VOCs using various plasma reactors such as pulsed corona [2][3][4], ferroelectric pellet packed-bed discharge [5], silent (dielectric barrier) discharge [6], and surface discharge [7,8] reactors.The feasibility of plasma processes is limited by OPEN ACCESS energy efficiency and formation of toxic by-products.To improve energy efficiency and CO2 selectivity, catalyzed hybrid plasma reactors have been designed [9][10][11].For example, it has been reported that the combination of dielectric barrier discharge and a metal loaded catalyst enhances trichloroethylene decomposition and CO2 selectivity [12,13].Loading of active metals not only enhances CO2 selectivity for various VOCs [9,14,15], but also improves plasma coverage on the surface [16].We have also proposed a non-thermal plasma reactor combined with a catalyst [8,17] and cyclic plasma operation [18].Plasma induced surface oxygen species on metal nanoparticles play a significant role in improvement of the process [19,20].In particular, O3 enhances formation of active oxygen species on the catalyst surface.Manganese oxides (MnOx) are well known as promising catalysts for decomposition of VOCs by O3 at low temperatures through the Langmuir-Hinshelwood (L-H) mechanism, as follows [21]: *Oads + VOCads → COx + H2O + * (2) where * is an active sites on the catalyst and *Oads is the active oxygen species on the catalyst.Previous studies have reported that metal nanoparticles on zeolite strongly retain the active oxygen species formed by plasma compared with bare zeolite.We previously investigated active oxygen species formed by plasma on a silver nanoparticle-loaded zeolite [22].The amount of active oxygen fixed onto the catalyst surface by O2 plasma was approximately proportional to the square of the amount of supported silver.Thus, most of the active oxygen was present on the silver surface.Interestingly, the ozone demand factor (DFO3 = Δ[O3]/Δ[toluene]), i.e., the amount of ozone required for oxidation of toluene of bare zeolite (ZSM-5), was half that of silver-loaded zeolite [23].Smaller DFO3 values correlate with better performance during ozone-assisted catalysis of VOCs.These findings suggest that the supported metal does not necessarily improve the efficiency of the plasma hybrid system and that performance improvements in that system cannot be explained solely by surface reactions of active oxygen species formed by O3 (the L-H mechanism).
In this study, we focused on reaction mechanisms of a metal-unloaded zeolite in a plasma hybrid system for toluene decomposition.The role of metal nanoparticles has been studied by several research groups and has been found to be important in VOC removal, affecting decomposition efficiency, the carbon balance, and CO2 selectivity.However, the role of supports (e.g., without active metals) has not been well studied in terms of interaction with plasma.The role of zeolite in the toluene decomposition process was investigated by changing the position of the zeolite in the plasma reactor.The reaction mechanisms of toluene decomposition in the gas phase and on the zeolite surface were studied using gaseous and surface FT-IR analyses.

Role of Zeolites in the Plasma Hybrid System
Energy efficiency in the decomposition of VOCs was enhanced by using a zeolite-hybrid plasma reactor [18].Improvements in efficiency differed widely depending on the type of zeolite and its position in the reactor.Table 1 summarizes the performance of the zeolites in toluene oxidation, and their basic properties.Decomposition efficiency in the downstream position was 3.7 times higher than that in a conventional reactor, while that in the upstream position was 1.2-1.5 times higher.Additionally, decomposition efficiency in the downstream position was significantly affected by the type of zeolite, compared with upstream.The dependence of the decomposition efficiency on the position of the zeolite suggests that improvements in performance cannot be explained solely by activation of the zeolite surface by plasma.Figure 1 shows the relationship between the amount of toluene adsorbed and decomposition efficiency (normalized to that of a reactor without zeolites).In general, the surface area greatly affects the adsorption capacity and decomposition of VOCs.In a previously reported ozone-assisted catalyst system, the rate of benzene oxidation linearly increased with the specific surface area of the MnOx-loading catalyst, regardless of the type of catalyst support [24].In the present study, the amount of toluene (~6 Å) adsorbed by ferrierite (FER) was small and limited to the surface due to the micropore size (4.3 × 5.5 Å 2 ) of FER.The decomposition efficiency of the downstream configuration was linearly proportional to the adsorption capacity (Figure 1).On the other hand, that of the upstream configuration was near constant.If the toluene adsorbed in the internal areas of the zeolite was decomposed by short-lived radicals and fast electrons, the decomposition efficiency would not depend on the zeolite position.We conclude that reactions between toluene adsorbed onto the zeolite and short-lived radicals or fast electrons did not contribute to improvements in decomposition efficiency.One possible explanation is the limited area of plasma for bare zeolite compared to metal-loaded zeolites; plasma mainly formed at the points of contact with the zeolite pellets [16].This limited plasma area reduced effective interactions at the surface of the zeolite.These results confirm the important contribution of metal nanoparticles to both plasma properties and catalytic performance.
In addition, the diffusion lengths of short-lived radicals (O and OH) and fast electrons are <100 μm at atmospheric pressure.Thus, short-lived radicals and fast electrons had limited effect in this system.Ozone has a long diffusion length (>10 mm) due to its long lifetime, and can reach the inner area of micropores.Figure 2 shows the decomposition efficiency as a function of the outlet O3 concentration, normalized to that of a reactor without zeolites.In the downstream position, the O3 concentration was affected by the type of zeolite and decreased with increasing decomposition efficiency.Therefore, O3 was consumed approximately linearly with toluene oxidation.In the upstream position, there were no significant differences in O3 concentration or decomposition efficiency among the zeolite types.This can be explained by the O3 concentration at the entrance of the plasma reactor.The distribution of O3 concentrations in the reactor tube were measured using a dedicated reactor (shown in section 3.2.). Figure 3a,b shows the distribution of O3 in an unfilled reactor under dry air flow (without toluene or H2O) in the upstream and downstream configurations, respectively.The O3 concentration increased linearly with distance from the entrance of the reactor in both directions.When 1.0 g of H-Y was placed in the upstream or downstream position, the O3 concentration remained constant in the zeolite-filled region (Figure 4a,b)).These results were also observed for other zeolite types.Thus, formation of O3 was strongly suppressed in the zeolites-filled region.Based on these results, the low decomposition efficiency of the upstream configuration was likely due to a low O3 concentration, while the high efficiency of the downstream configuration was due to a high O3 concentration.Therefore, adsorbed toluene in the internal area of the zeolite was manly decomposed by O3.In addition to reactions (1) and ( 2 In reactions ( 1) and ( 2), O3 initially decomposes at the zeolite surface.Figure 5 shows the outlet concentration of O3 in the downstream reactor filled with 1.0-3.0g of H-Y.No decomposition of O3 was observed for any case.Thus, H-Y has little ability to decompose O3 and form active oxygen species on its surface.Therefore, the main process involved in toluene decomposition is likely direct reaction of gas phase O3 with toluene adsorbed onto H-Y, i.e., the Eley-Rideal (E-R) mechanism.This process is different from that for other types of zeolites and metal-loaded zeolites, i.e., the L-H mechanism.For example, active oxygen species formed by O3 on the MCM-41 and MS-13X surfaces significantly contributed to VOC decomposition [25,26].These active oxygen species on the catalyst surface are consumed in reactions with VOCs, O3, O, etc. [27][28][29].In other words, some of the active oxygen species (=O3) form unwanted byproducts in the L-H process, unlike in E-R.Therefore, in the E-R process, the H-Y zeolite has the potential for achieving low DFO3 in VOC decomposition.CO2 selectivity was similar between the H-Y filled and unfilled reactors (Table 1).On the other hand, production of formic acid was lower for the H-Y filled reactors.Considering the role of O3 in this system (reaction (3)), it may be difficult for gaseous O3 to oxidize the adsorbed CO to CO2, unlike for toluene and formic acid.Generally, in a plasma-catalyst hybrid system, bare zeolite contributes little to oxidization of CO.Metal-loaded zeolites significantly improves CO2 selectivity due to surface reaction with the active oxygen species formed by O3, most of which are present on the silver surface of the catalyst [22].Thus, active oxygen species on the metal surface contribute greatly to improvements in CO2 selectivity.The H-Y zeolite cannot easily retain active oxygen species on its surface through reactions with O3, leading to low CO2 selectivity.
Higher concentrations of VOCs result in higher energy efficiencies for removal [30].Kuroki et al. [31] reported that adsorbed toluene desorbed easily by DC pulsed plasma discharge, resulting in an increase in the concentration of VOCs.Although this experimental result using standard adsorbents cannot be directly compared with zeolites, if such an effect occurred in our system, the efficiency in the upstream reactor should be nearly the same as that in the downstream one.However, efficiency in the downstream configuration was much higher than that in the upstream reactor.This suggested that enhancement due to desorption of toluene from the zeolite was negligible.
The hydrophilicity of zeolite depends on the SiO2/Al2O3 ratio and affects the performance of thermal catalytic processes.Generally, hydrophilicity decreases with increasing SiO2/Al2O3 ratio.However, the difference in hydrophilicity between H-Y( 1) and H-Y(2) did not affect decomposition efficiency in our system (Table 1), unlike for thermal catalytic processes.

Decomposition of Toluene Adsorbed onto the Zeolites by O3
Figure 6a,b shows spectra for toluene adsorbed onto H-Y and FER, respectively, measured 10 min after introducing 200 ppm of toluene in N2. Figure 6c also shows the spectrum for toluene in the gas phase as a reference.IR peaks assigned to the aromatic ring were observed between 2000 and 1750 cm −1 in both Figure 6a,b.These peak positions were shifted compared with those in the gas phase.Table 2 summarizes the IR peaks for toluene in both the gas and liquid phases.The adsorption peaks near 3000 cm −1 were assigned to the CH3 group of toluene and those at 2000-1700 cm −1 were assigned to the C-H bonds in the aromatic ring.The shift in peaks for toluene adsorbed onto the zeolite is due to the perturbation of the π-electric system by adsorption onto the H-Y zeolite.These peaks were assigned to toluene physically adsorbed onto the solid surface, because they completely disappeared upon heating the sample to 200 °C.The peak signal for toluene on FER was much smaller than that on H-Y, because the adsorption capacity of FER for toluene is limited due to its small micropore size.The reactivity of toluene adsorbed onto H-Y was then investigated.Initially, toluene was adsorbed onto the H-Y sample for 10 min (Figure 7a).N2 gas was then supplied to remove the gas phase toluene from the reactor.A mixture of 400 ppm O3 in O2 was then introduced and the FT-IR spectrum was recorded (Figure 7b-f).The IR peaks for toluene disappeared immediately after O3 introduction, in less than 1 min.In contrast, IR peaks at 1390, 1630 and 1725 cm −1 appeared and their intensities increased for 40 min after O3 introduction.These peaks were assigned to formic acid (HCOOH).These results suggest that gas-phase O3 reacted with adsorbed toluene to form ring cleavage intermediate that was slowly converted to formic acid.We anticipate that the performance of the zeolite-plasma hybrid system could be improved by increasing the operating temperature, because the decomposition rate of formic acid is significantly affected by temperature [18].A distinctive peak for the active oxygen species formed by O3 on MCM-41 and MS-13X was observed at around 1380 cm −1 [25,26].In the present experiment, this peak was not observed.This result supports the observation that the H-Y zeolite does not retain active oxygen species on its surface through reaction with O3.
Figure 8 shows IR spectra for formic acid adsorbed to H-Y for 10 min under HCOOH(200 ppm)/N2 flow.The observed peaks were assigned to the carboxylic acid group (-COOH).The peak positions in the spectra and the profiles for formic acid adsorbed onto H-Y (Figure 8b) were in good agreement with those for toluene adsorbed onto H-Y treated with O3 (Figure 7c-f).Therefore, the peaks in Figure 7c-f can be assigned to formic acid.When a mixture of 400 ppm O3 in O2 was introduced for 10 min, the peak at 1725 cm −1 decreased by approximately half of its initial intensity, and the peak at 1630 cm −1 increased significantly in intensity.The decomposition rate of formic acid was much lower than that of toluene under the same conditions.Therefore, the benzene ring was relatively easily decomposed on the H-Y surface compared to carboxylic acid.With reference to the IR spectra of H2O (Figure 8d), the peak at 1630 cm −1 with increased intensity was assigned to H2O the final product of formic acid oxidation.

Decomposition of Toluene (C7H8)
The configuration of the surface discharge reactor is shown in Figure 9a.A quartz tube (i.d.: 10 mm, thickness: 1.5 mm) was used as the dielectric barrier.A spiral platinum wire coil (70 turns/100 mm) with a 0.3 mm diameter was placed in contact with the inner wall of the barrier tube.A piece of copper foil was wrapped around the outside of the barrier tube as the ground electrode; the outer electrode length was 100 mm.Zeolites (1 g) were packed into one end of the reactor.The reaction conditions were adjusted based on the flow direction and the position of the zeolites.The position of the zeolites was near the gas inlet in the upstream configuration and near the gas outlet in the downstream configuration.
The reactant gas consisted of 200 ppm toluene (C7H8), 0.5% H2O, and 20.0%O2, with N2 making up the remaining fraction.It was fed at a flow rate of 500 cm 3 /min into the reactor, where a high AC voltage was applied between the two electrodes.The plasma energy was supplied using an AC high-voltage neon transformer (24 kHz, NEON M-5; Lecip Co., Gifu, Japan) and the plug-in power was measured using a digital power meter (WT110; Yokogawa Electric Co., Tokyo, Japan).The input power was 5 W.
The plasma was cycled four times in 160 min (off for 30 min → on for 10 min → off for 30 min → etc.) (Figure 10), because cyclic operation improves energy efficiency by inducing periods of adsorption followed by plasma decomposition [8,13].The concentrations of products such as COx were consistent during each plasma period.The experiment was carried out at room temperature.Under plasma operation, the reactor temperature was increased to approximately 60 °C.The effluent gas was monitored continuously using an FT-IR spectrometer (FTS135; Bio-Rad Laboratories, Hercules, CA, USA) equipped with a gas cell (2.4 m path, 125 cm 3 volume; Infrared Analysis Inc., Anaheim, CA, USA) and the total amounts of the products were calculated from their

Conclusions
The reaction mechanism of the zeolite-plasma hybrid system for toluene decomposition was studied using gas and surface FT-IR analyses.The contributions of short-lived radicals and fast electrons to decomposition of adsorbed toluene were low.On the other hand, O3 directly decomposed toluene adsorbed in the inner areas of the zeolite micropore because it has a long life time.O3 provided the main mechanism of toluene decomposition in the zeolite-plasma hybrid system, unlike in metal-loaded zeolite-plasma systems.CO2 selectivity was not improved by use of the bare zeolite, because H-Y zeolite does not retain active oxygen species on its surface through surface reactions of O3.Gas-phase O3 reacted with adsorbed toluene to form a ring cleavage intermediate that was slowly converted to formic acid.The decomposition rate of formic acid was much slower than that of toluene on the H-Y surface.

Figure 1 .
Figure 1.Relationship between the decomposition efficiency and adsorption capacity for toluene.

Figure 2 .
Figure 2. Relationship between the relative decomposition efficiency and relative amount of ozone.

Figure 3 .
Figure 3. Ozone distribution in a conventional plasma reactor: (a) upstream configuration and (b) downstream configuration.

3 )Figure 4 .
Figure 4. Relationship between the decomposition efficiency and the adsorption of ozone: (a) upstream configuration and (b) downstream configuration.

Figure 5 .
Figure 5. Concentration of ozone in the downstream reactor with amount of H-Y(1).

Figure 9 .
Figure 9. Configurations of (a) the zeolite hybrid reactor and (b) the reactor for monitoring the ozone distribution in the plasma region.

Figure 11 .
Figure 11.Configurations of (a) the sample holder and (b) the IR cell equipped with a furnace.

Table 1 .
Decomposition efficiency in hybrid plasma reactors and the properties of zeolites.

Table 2 .
Wave number of IR peaks for toluene in gas phase, in the liquid film, and on H-Y.