Discharge photoelectrocatalytic system for the degradation of aromatics

We have introduced the discharge photoelectrocatalytic system, in which TiO2 thin film coating on aluminum plate is subjected simultaneously to both UV irradiation and high voltages in excess of 3000 volts. Due to high voltages O3 is generated; however, efficient removal of O3 is observed in this photoelectrocatalytic system. In terms of the removal of volatile organic compounds (VOCs), the discharge photoelectrocatalytic system has been applied to the removal of aromatic compounds such as benzene and toluene. Based on the experimental data, the rates of the removal of these compounds in this system are higher compared to either the discharge—only system (without the photocatalyst) or the photocatalyst— only system (without high voltage discharge), and the higher rates of degradation of these compounds in the photoelectrocatalytic system are attributed to the reduced rate of electron-hole recombination in TiO2.


INTRODUCTION
Much attention has been given to titanium dioxide (TiO 2 ) due to its outstanding photocatalytic properties such as photochemical stability and high oxidation rates.Nevertheless, its industrial applications such as removal of pollutants [1,2] in the industrial sites and wastes have been slow due to the limitations associated with the requirement of UV light for the generation of excited electrons and holes and their subsequent rapid recombinations [3,4].The discharge photoelectrocatalytic system was recently designed in this group for the purpose of increasing the photocatalytic activity by trapping electrons generated in TiO 2 upon UV irradiation, and this system was used for the removal of aromatic compounds such as benzene and toluene.The discharge photoelectrocatalytic system maximizes the photocatalytic activity by retarding electron-hole recombination, thereby increasing the lifetime of holes responsible for the mineralization of these compounds.In the discharge photoelectrocatalytic system, the degradation rates of these compounds increased with the increase in the applied voltage on the aluminum plate.Overall, the degradation rates of these compounds were higher under the condition of simultaneous UV irradiation and applied voltage in comparison to the conditions of either UV irradiation only or the applied voltage only.Our discharge photoelectrocatalytic system also demonstrates generation and consumption of ozone (O 3 ), which are due to the discharge effect and the photocatalytic effect, respectively [5].† E-mail: hskim@sunmoon.ac.kr

EXPERIMENTAL
The photocatalyst used in this study was sol type titanium dioxide (Enpion, Korea; 15% TiO 2 ) suspended in alcoholic solvent, which will be referred to as Enpion TiO 2 .Enpion TiO 2 was mixed with silicon binder solution (silicon, alcohol, water, acid) of pH 3.5 to increase the coherence of titanium dioxide on the substrate, aluminum plate.Evaporation of the solvent followed by drying at 80 • C for 4 hours and shredding turned the Enpion TiO 2 into powder for the BET and XRD measurements.Surface area analysis was done using ASAP 2010 (Micrometric Science) and structural measurement by X ray Diffractometer (Rigaku D-Max 2200).For the XRD measurement Cu Kα radiation was used, and the sample scans were collected between 8 • and 20 • 2θ.The Enpion TiO 2 powder sample was viewed by Jeol JSM-6400 scanning electron microscope (SEM).
Aluminum plate was coated with Enpion TiO 2 using a bar-coater, which was dried at 120 • C for 30 mins in the air without calcinations.TiO 2 coated aluminum plate was then used as a cathode for the discharge photoelectrocatalytic system and an array of Cu strips in the shape of saw tooth was used as an anode (Figure 1).High Voltage DC bias was used for the discharge in the system, and voltages of 3000 volts and 5000 volts were applied to observe the effect of the electric field on the photocatalytic activity.Three 8 W UV lamps (GLK8CQ, Sankyo Denki) were used for the irradiation of TiO 2 coated aluminum plate.
Air flowed through a mass flow controller into the saturator placed in a constant temperature bath at 20 • C and 1 atm, and was mixed with 90% air coming from another mass flow controller in order to obtain 1 vol.%aromatic compound in air.This aromatic mixture flowed into and out of a 10l batch reactor initially, to allow adsorption to take place in the reactor, and samples of outlet mixture were periodically analyzed with a gas chromatoghaph (HP6890) equipped with a flame ionization detector and a capillary column (HP5) to be compared with the target composition.When the target composition of 1 vol.% was achieved, the inlet and outlet of the reactor were closed, and photoelectrocatalytic degradation of the aromatic compound was carried out in this batch reactor at 20 • C and 1 atm.During the experiment gas samples were taken at 30 min.interval for GC analysis to monitor variations in the concentration of the aromatic compound.

RESULTS AND DISCUSSION
Surface area analysis results for Enpion TiO 2 are presented in Table 1 along with the results for the reference compound (P-25 TiO 2 ).BET measurements in Table 1 reveal that the specific surface area of Enpion TiO 2 is ∼ 5 times larger than P-25.
The XRD patterns for the photocatalysts in Figure 2. showed that only anatase existed in Enpion TiO 2 .On the other hand, the reference compound (P-25 TiO 2 ) showed both anatase and rutile crystallinity.  in Figure 3(a), showing signs of aggregation.Significant increase in the size of the particles is seen in Figure 3(c) due to clustering, and this clustering might have been accelerated by the silicon binder.These clusters are expected to provide a highly porous morphology [2,3], and this agrees with the large BET values for Enpion TiO 2 .
Figures 4 and 5 show the degradation of the aromatic compounds as a function of UV irradiation time for the three cases of operating conditions: (a) discharge only (plasma effect), (b) UV irradiation only (conventional photocatalytic effect) and (c) discharge with UV irradiation (photoelectrocatalytic effect).In the figures, the dot symbol corresponds to the condition (a), the circle to the condition (b), and the reverse triangle to the condition (c).For both benzene (Figure 4) and toluene (Figure 5), the degradation rate of case (c) was significantly higher than either case (a) or case (b), showing the highest catalytic activity.Between the case (a) and the case (b), the degradation rate of case (b) was slightly higher than that of case (a).under the plasma effect only and ∼ 2.2 times faster than under the photocatalytic effect only.
An explanation for the highest degradation rate of the case (c) can be found in the trapping of the photogenerated electrons, and such an effect would retard electron-hole recombination, thereby increasing the lifetime of the holes capable of mineralizing benzene into carbon dioxide (CO 2 ) and water (H 2 O).
As to the difference in the degradation rates between the case (a) and the case (b), it is more pronounced for toluene than for benzene, and this obervation may have to do with the fact that toluene has nonzero dipole moment (∼ 0.33 debye).In the presence of the electric field, toluene molecules are likely to align with methyl groups pointing toward the Cu strips; however, such an orientation might have resulted in adverse effect in our experiment.
The effect of voltage on the degradation of the aromatic compounds in the discharge photoelectrocatalytic system is shown for benzene in Figure 6 concentration under the discharge of 3000 volts, and the dot symbol corresponds to that of 5000 volts, and the curves represent a single exponential decay fit to the data.As shown in the figure, the benzene degradation rates were higher with higher applied voltage on the aluminum plate.In terms of the pseudo first order rate constant, it is 12.41 × 10 −4 min −1 at 5000 volts and 7.2 × 10 −4 min −1 at 3000 volts.Based on this result we suggest that higher electric field induces larger number of trapped electrons on the aluminum plate and the stronger plasma effect, thus leading to higher rate of degradation of benzene.
The generation and the removal of ozone (O 3 ) in the discharge photoelectrocatalytic system are shown in Figure 7.As shown in the figure, ozone (O 3 ) produced in the discharge system could be removed by UV irradiation onto TiO 2 , which was demonstrated by a dramatic decrease in ozone concentration immediately following irradiation of UV light.Recently, the plasma system as well as the electrostatic precipitator based on the discharge system are being launched as advanced air cleaning system.However, in both cases, the amount of ozone drawn out of the air cleaner is not negligible, and it can be potentially harmful for people in the confined space over extended period of time.The discharge photoelectrocatalytic system will be an answer to such health hazardous problems.

CONCLUSIONS
The degradation of benzene and toluene was carried out in the discharge photoelectrocatalytic system consisting of TiO 2 thin film, its aluminum substrate as cathode, anode of Cu strips, high voltage power supply and UV lamps.Compared to either the plasma effect only or the photocatalytic effect only, higher degradation rates were observed for both benzene and toluene in the discharge photoelectrocatalytic system.In terms of the effect of the voltage on the degradation of the aromatic compounds, higher degradation rate at higher applied voltage was observed for benzene.Between benzene and toluene, degradation of benzene was faster than toluene in the discharge photoelectrocatalytic system, and benzene decomposed ∼ 1.4 times faster than toluene based on the pseudo first order rate constants.The higher rates of degradation for these aromatic compounds are attributed to the longer lifetime of holes in TiO 2 due to reduced electron-hole recombination.
The discharge photoelectrocatalytic system produces ozone by the plasma effect due to the high voltage discharge.However, ozone is consumed photocatalytically upon production within the system, and therefore, the amount of O 3 coming out of the discharge photoelectrocatalytic system is quite low.Compared to the UV lamps, the discharge photoelectrocatalytic system consumes little amount of electricity (2-4W), and thus has the advantage of being energy efficient.

Figure 1 .
Figure 1.Schematic diagram of the discharge Photoelectrocatalytic System.

Figure 4 .Figure 5 .
Figure 4. Photodegradation of benzene as a function of UV irradiation time.

Figure 6 .
Figure 6.The effect of applied voltage on the photodegradation of benzene.

Figure 7 .
Figure 7. Generation of O 3 by the plasma effect and its removal by the photocatalytic effect in the discharge photoelectrocatalytic system.

Table 1 .
Specific surface areas and pore volumes of P-25 and Enpion TiO 2 .