Photocatalytic degradation of chlorinated ethenes

Degradation of three chlorinated ethenes, trichloroethylene, trans-1,2-dichloroethylene and cis1,2-dichloroethylene, by UV-light irradiated TiO2 catalyst prepared by the sol-gel method in dry air at ambient temperature have been examined by using FTIR measurement. The chlorinated ethenes rapidly decomposed to produce dichloroacethyl chloride, CO, HCl, and COCl2. For transand cis-1,2-DCE systems isomerization to each other is found to be the first step of the degradation. The C = C bond of the chlorinated ethenes interacts directly with TiO2 site and, consequently, the degradation results in several products on the catalyst surface in these systems.


INTRODUCTION
Large amount of volatile organic compounds (VOCs) such as trichloroethylene (TCE) have been widely used for a kind of metal cleaning and dry cleaning solvents.Owing to careless treatment, these chemical materials have contaminated air, soil, river and groundwater all over the world.Photocatalytic methods have been employed for VOCs degradation in gas or liquid phase nowadays.Some researchers have reported that the degradation of TCE by the photocatalytic reaction in air produces intermediates including poison gases [1][2][3][4].Furthermore, various researchers have proposed various mechanisms of photocatalytic degradation in the VOCs system.
Dibble and Raupp indicated that the rate of the degradation of TCE was fast and there were no byproducts [5,6].Anderson et al. proposed a mechanism of the TCE photocatalytic degradation in gas phase involving hydroxyl radical and observed intermediates such as monochloroacetate [7,8].Fan and Yates, in contrast, proposed that TCE was photooxidized on TiO 2 by electronically activated chemisorbed O 2 [3].Hwang et al. suggested that oxygen radical species involved in the system attacks the double bond of TCE in the initiation step [4].It is important, therefore, to elucidate detailed reaction products of the VOCs by photocatalyst from a practical point of view.From this clue, development of new materials would be expected greatly to improve photocatalytic degradation in the VOC systems.
In this paper, we will report on the detailed photocatalytic degradation products of TCE, cis-and trans-1,2-dichloroethylene (cis-and trans-1,2-DCE) by UV light irradiated titanium dioxide (TiO 2 ) catalyst in dry air at ambient temperature.Monitoring the FTIR spectra of the chemical species produced by the reaction were observed.We try to make clear the degradation scheme of chlorinated ethenes by UV irradiated TiO 2 .Based on the observation of the reaction products, we will also discuss the roll of TiO 2 catalyst existed on the surface by comparing the reaction products among the three ethene compounds, which have the C = C double bond and Cl atoms.

Preparation of TiO 2 catalyst. Glass wool
was washed with ethanol and then dried.The solution of the TiO 2 sol-gel system contained 15.0 mL of TPOT, 15.0 mL of ethanol and 8.0 × 10 −2 mL of 1.0 × 10 −3 M aqueous HCl solution as a catalysis.The solution was stirred during the addition and thoroughly for additional 10 min.The dipping wool was withdrawn vertically from the sol-gel solution in a vial at a speed of 10 mm min −1 under nitrogen gas at ambient temperature.Finally, the wool was kept at ambient temperature for 1 day, heated at 773 K for 1 hr in air and then cut 3 cm in length.Crystal structure of the TiO 2 film on glass is mainly anatase form under the condition of heating treatment at 773 K [9,10].

Photocatalytic degradation of the chlorinated ethenes.
The glass wool was placed in an in- frared cell, which was made from a Pyrex glass.KBr single crystals were used as two infrared windows and were sealed by two Teflon O-rings.Each chlorinated ethene gas for the degradation experiment was generated from a Permeater (GASTEC Co.; PD-1B) by using the dry air.The gases passed through the cell for 1 hr at 12 L/hr, and then valves of the cell were closed.Degradation reactions for the chlorinated ethenes were carried out in the cell by irradiation near-UV light from a 4 W black light bulb (Toshiba FL4BLB) at ambient temperature.Gas phase IR spectra were observed using a Shimadzu FTIR-8200A as a function of time.We can assume that the chemical species existed in the reaction cell are in equilibrium between the gas phase and on the TiO 2 surface.The decomposed species on the TiO 2 surface, therefore, appears in the FTIR spectra.
The obtained FTIR spectra were assigned by referring the vibrational and/or electronic spectrum of the National Institute  In Figure 1(a), there are three typical characteristic bands at 945 cm −1 (C − Cl stretching), 848 cm −1 (C − H stretching), and 783 cm −1 (C − H bending).After 40 min irradiation of the UV light 1(b), the spectrum indicated the decrease in the intensity of the TCE absorption bands and appearance of other bands originating from DCAC at 800 (C − Cl 2 stretching) and 741 cm −1 , and from COCl 2 at 850 cm −1 .After 100 min irradiation of the UV light 1(c), the intensity of the COCl 2 bands increased, while the intensities of the other bands decreased in the spectra.The results reveal that DCAC and COCl 2 are the main reaction products of the photocatalytic oxidation of TCE.These reaction products agree with the results of the previous works [1,3], in which passing through a maximum intensity at 100 min, and degreases in increasing the UV irradiation time.These results indicate that the degradation of TCE produces DCAC, CO, HCl, and COCl 2 .In addition to this, further irradiation would cause the decomposition of DCAC into CO, HCl, and COCl 2 .

Degradation paths of the chlorinated ethenes.
Though the above experiments are qualita- tive results, they indicated the paths of the degradation.In Figure 3, the maximum time of DCAC, CO, HCl, and COCl 2 were ca.40, 100, 60, and 100 min, respectively.It is indicated that a degradation path of TCE involves the conversion of TCE to CO, HCl, and COCl 2 via DCAC.
From Figure 4, the maximum time of the products converted from trans-1,2-DCE into cis-1,2-DCE, DCAC, CO, HCl, and COCl 2 were ca.70, 140, 150, 150, and 200 min, respectively.From Figure 5, the maximum time of the products converted from cis-1,2-DCE into trans-1,2-DCE, DCAC, CO, HCl, and COCl 2 were ca.100, 160, 170, 160, and 180 min, respectively.These results suggest that isomerization of 1,2-DCEs occurred in the first step of the degradation and 1,2-DCEs decomposed to DCAC, HCl, CO, and COCl 2 at almost same time.These results also suggest that degradation paths of 1,2-DCEs are a direct conversion to HCl, CO, and COCl 2 , or indirect conversion to them via DCAC.Furthermore, it is likely that the degradation of TCE convert to HCl, CO, and COCl 2 as well as the degradation of 1,2-DCEs.Based on the above discussions, we propose a degradation scheme of the chlorinated ethenes on the TiO 2 photocatalytic surface as shown in Figure 6.Path (A) is adsorption of the chlorinated ethenes, converting them to DCAC and then producing CO, HCl, and COCl 2 .Second path (B) is isomerization of themselves followed by the decomposition to CO, HCl, and COCl 2 as same as path (A).It is also possible to take the third path (C), direct conversion of the chlorinated ethenes to CO, HCl, and COCl 2 .
On the other hand, it is possible to occur the degradation reaction of the chlorinated ethenes in gas phase.Nimlos et al. [2] observed degradation products including Cl 2 molecule for photocatalytic degradation of TCE.It is noted that some intermediates are possible to be generated from the reaction of chlorinated ethenes with Cl radical in gas phase under UV irradiation (longer than 300 nm) [11].Therefore, a similar reaction among TCE, Cl 2 , and O 2 in gas phase would occur in our system and also generates DCAC, though Cl 2 was not detected in this work by FTIR measurements.In our reaction systems, since participated species in an initial reaction are chlorinated ethenes, O 2 , and photo-irradiated TiO 2 surface, it is concluded that the initial reaction occurs on the catalyst surface.

Degradation of the chlorinated ethenes on
the photocatalyst surface.Anpo et al. [12] re- ported that photocatalyzed isomerization of 2-butene on MgO powders without oxygen molecule (O 2 ).They suggested that the charge-transfer excited complex [M (n−1)+ − O − ] * on the catalyst surface plays a significant role in weaking the C = C double bond of 2butene and the interaction between the charge-transfer excited complex and the C = C double bond of 2-butene resulted in its geometrical isomerization.The degradation of trans-and cis-1,2-DCE produces DCAC.This result indicates that Cl radical exist in the neighborhood of the 1,2-DCE on the TiO 2 surface.The experimental results suggest that trans-cis isomerization between two 1,2-DCEs on the surface occur through the following ways.One would be based on the interaction between the [Ti 3 − O − ] * and the C = C bond resulting in the isomerization.The interaction weakens the C = C bond and a single bond character increases.Under the circumstances, it is possible to occur the rotation around C − C bond and cis-and trans-1,2-DCE isomerize to trans-and cis-1,2-DCEs, respectively.Another possible path is a Cl radical migration and ejection to the adsorbed 1,2-DCEs resulting in another 1,2-DCEs.
As we showed in the preceding sections, main part of the degradation for the chlorinated ethenes occurred on the TiO 2 surface.Figure 7  Lu et al. suggested that the substrate-mediated excitation of oxygen played an important role in the photocatalytic degradation on the TiO 2 surface in methyl chloride system [14].Hwang et al. also suggested that oxygen radical species created via UV activation of added O 2 were likely candidates to be primarily involved in attacking the TCE double bond in the initiation step [4].Kutsuna et al. reported that 1,1,1trichloroethane is inactive by UV irradiated TiO 2 photocatalyst in dry air [15].It is noted that our results indicate that chlorinated ehenes are reactive through the interaction between the photoactivated [Ti 3 − O − ] * and the C = C bond.

CONCLUSIONS
The degradation reactions of TCE, trans-1,2-DCE, and cis-1,2-DCE by UV irradiated TiO 2 without H 2 O have been investigated.The isomerization of the 1,2-DCEs occurred on the catalyst surface.It was observed that not only the rapid decomposition of the chlorinated ethenes, but also slow decomposition of intermediate such as DCAC were detected.There are mainly three paths for the degradation of the chlorinated ethenes : the chlorinated ethenes decomposed 1 : via DCAC, 2 : via isomerization and 3 : directly to CO, HCl and COCl 2 .It is concluded that interaction of C = C double bond in the ethenes with photoinduced charge transfer site [Ti 3 − O − ] * on the TiO 2 surface is essential to the degradation of the chlorinated ethenes.
of Standards and Technology (NIST).The wavenumbers for the intensity measurements of the FTIR bands originated from TCE, cis-1,2-DCE, trans-1,2-DCE, dichloroacethyl chloride (DCAC), CO, HCl, and phosgene (COCl 2 ) are at 945 (C − Cl stretching), 695 (C − H bending), 831 (C − Cl stretching), 741 (C − Cl 2 stretching) [3], 2175, 2944, and 850 (C − Cl 2 stretching) cm −1 , respectively.Since the absorption band of TCE (848 cm −1 : C − Cl stretching) and cis-DCE (857 cm −1 : C − Cl stretching) overlap with the absorption band of COCl 2 (850 cm −1 : C − Cl 2 stretching), the intensity change of COCl 2 was calculated by referring the absorption bands of TCE (945 cm −1 : C − Cl stretching) and cis-DCE (695 cm −1 : C − H bending).It was difficult to quantify of CO 2 in spite of observation of its absorption bands at 668 and 2362 cm −1 (total symmetric vibration) for the individual systems.However, their intensities for the bands increased quantitatively as a function of UV irradiation time for whole systems.We confirmed that there occurred no degradation reaction in the reaction cell under irradiation of the black light and without the photocatalyst.

Figure 1
shows FTIR spectra recorded (a) before, (b) 40 and (c) 100 min UV irradiation of TCE in a range from 700 to 1000 cm −1 .

− 1 Figure 3 .Figure 4 .Figure 5 .
Photocatalytic degradation reactions for trans-and cis-1,2-DCE were carried out under the same conditions.Figure3shows FTIR spectra recorded (a) before, (b) 80 min UV irradiation of trans-1,2-DCE, and (c) the difference spectra between (a) and (b) in a range from 680 to 880 cm −1 .FTIR spectrum for trans-1,2-DCE 3(a) shows a typical characteristic band at 831 cm −1 (C − Cl stretching).After 80 min irradiation of the UV light 3(b), the spectrum indicated the decrease in the intensity of the trans-1,2-DCE absorption band and appearance of other bands originating from cis-1,2-DCE at 695, 857 cm −1 , COCl 2 at 850 cm −1 and DCAC at 741 cm −1 .Figure 4 shows absorbance changes of the products and trans-1,2-DCE upon UV irradiation.The degradation reaction of trans-1,2-DCE occurred rapidly.The species of the photocatalytic degradation for trans-1,2-DCE are cis-1,2-DCE, DCAC, CO, HCl, and COCl 2 .The band intensities for DCAC, CO, and HCl increased simultaneously with UV irradiation time, and intensity of the each band reached their maximum at around 160 min.Intensity of COCl 2 band increased until around 200 min with UV irradiation time.These results show that a part of trans-1,2-DCE isomerizes to

Figure 7 .
Figure 7.A proposed structural model of a bridge type πcomplex interacting with the neighboring O − O complex on the TiO 2 surface.