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Demineralization of organic pollutants on the dye modified TiO2 semiconductor particulate system using visible light
Applied Catalysis B: Environmental 33 (2001) 119–125
Demineralization of organic pollutants on the dye modified TiO2 semiconductor particulate system using visible light Debabrata Chatterjee∗ , Anima Mahata Chemistry Section, Central Mechanical Engineering Research Institute, Durgapur 713209, India Received 7 December 2000; received in revised form 13 March 2001; accepted 14 March 2001
Abstract Photodegradation of organic pollutants, viz. phenol, chlorophenol, 1,2-dichloroethane and trichloroethylene in water has been achieved on the surface of TiO2 semiconductor modified with thionine and eosin Y by using visible light. After 5 h of irradiation with a 50 W tungsten lamp, over 55–72% degradation of pollutants is achieved. A working mechanism involving excitation of surface bound dye, followed by charge injection into the TiO2 conduction band is proposed. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Photodegradation; Dye sensitization; TiO2 photocatalyst; Organic pollutants; Visible light; Thionine; Eosin Y
1. Introduction Elimination of toxic and bioresistant of organic pollutants from wastewater by their transformation into non-hazardous species using semiconducting TiO2 particle and sunlight is a demanding area of research. Although the process of TiO2 -photocatalyzed degradation [1–6] studied exhaustively during last decade provides a reasonable solution to this problem. The vital snag of TiO2 semiconductor is that it absorbs a small portion of solar spectrum in the UV region (band gap energy of TiO2 is 3.2 eV). Hence, in order to reap maximum solar energy, it is necessary to shift the absorption threshold towards visible region. Conceptually, dye sensitization [7–12] seems to be a viable alternative method to deal with this issue. Dye (sensitizer) adsorbed on TiO2 surface is excited by absorbing visible light and effects charge injection ∗
Corresponding author. Tel.: +91-343-546818; fax: +91-343-546745. E-mail address: [email protected] (D. Chatterjee).
into the conduction band of TiO2 semiconductor at sub-band gap excitation and the catalytic processes is followed through interfacial electron transfer. Our recent research interest is focused on the exploration of visible light assisted catalytic transformation of organic molecules using TiO2 semiconductor particulate system [13,14]. In the present investigations, we explore the possibility of photodegradation of water-bound organic pollutants using TiO2 semiconductor particles modified with thionine and eosin Y (Fig. 1). The excited state redox properties of dyes thionine and eosin Y, selected for this work are different with regards to their electron transfer process. Thionine at the excited state is known to accept one electron and converts into semithionine [15], whereas, eosin Y gives up one electron upon excitation with visible light [16]. We wish to report herein, the results of our studies of visible light assisted decomposition of some common organic pollutants, viz. phenol, chlorophenol, 1,2-dichloroethane and trichloroethylene catalyzed by suspended dye modified TiO2 semiconductor particulate system.
0926-3373/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 1 ) 0 0 1 7 0 - 9
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D. Chatterjee, A. Mahata / Applied Catalysis B: Environmental 33 (2001) 119–125
Fig. 1. Pictorial representation of the various dye species.
2. Experimental Surface modification of semiconductor grade TiO2 (obtained from Fluka) particle and characterization of surface modified dyes were carried by adopting the published procedure [17]. In a typical experiment, a saturated solution of dye (pH ∼ 3) containing TiO2 (500 mg) was magnetically stirred for 4 h in dark. The uptake estimated spectrophotometrically by using a GBC Cintra 10 spectrophotometer by measuring of free dye and in the supernatant liquid obtained after filtration was found to be 543 eqv/g for thionine and 521 eqv/g for eosin Y. Spectra of the dyes adsorbed on TiO2 surface was taken in a qualitative manner by rubbing the solid sample on a piece of transparent paper and placing it into the optical path of built-in cell holder of the spectrophotometer (another piece of the same paper was placed in the reference cell-holder). All other chemicals used were of AR grade and doubly-distilled water was used throughout the experiment. In a typical photocatalytic experiment, aqueous suspension (50 ml) of pollutant (1 mmol) containing 100 mg of surface modified photocatalyst designated as TiO2 -D was taken in a flat-surfaced glass reactor. A 50 W tungsten lamp (Philips Medical Spot Lamp, covering the wavelength range 420–800 nm, inner diameter of the focus tube was 5.1 cm) was used for the irradiation. The distance between the lamp and the glass-reactor containing reaction mixture was fixed at 8 inch. The pre-aerated reaction mixture was
magnetically stirred during irradiation. The pH of reacting system containing dye fixed TiO2 and pollutant was fixed at 5.0 (NaOH). After irradiation (5 h), the reaction mixture were filtered and subjected to GC (CE 8000Top Series) analysis. A Tennex column operating at FID detector was used for this purpose. The depletion of dye was estimated spectrophotometrically. Fluorescence measurements were carried out in Chemistry Department, Burdwan University by using a Hitachi F-4010 spectrofluorometer. Production of CO2 was demonstrated by the precipitation of BaCO3 in the Ba(OH)2 solution. The evolved CO2 was flushed with oxygen through alkali (KOH) scrubber and estimated by titration against acid (HCl). Presence of Cl− (confirmed by AgNO3 test) at the end of the experiment indicated the dechlorination of halocarbons under investigation. The extent of chlorination was measured titrimetrically with AgNO3 solution using potassium chromate as an indicator. The photodegradation reaction was followed gas chromatographically by monitoring the disappearance of GC-peak corresponding to the reacting pollutant (i.e. phenol, chlorophenol, 1,2-dichloroethane and trichloroethylene).
3. Results and discussion Absorption spectra of the aqueous solutions of thionine and eosin Y at pH 3.0 revealed characteristic absorption maxima at 600 and 518 nm, respectively.
D. Chatterjee, A. Mahata / Applied Catalysis B: Environmental 33 (2001) 119–125
121
Fig. 2. Spectral changes that occurs during photolysis of aerated aqueous solution of (a) thionine (1 × 10−4 M) and phenol (2.7 × 10−3 M), and (b) eosin Y (1 × 10−4 M) and phenol (2.7 × 10−3 M) at pH = 5.0.
The emission spectra of thionine and eosin Y at pH 3.0 in water exhibited emission maxima at 622 and 546 nm, respectively. Spectra of the dyes adsorbed on to the surface of the TiO2 recorded in the solid state revealed similar features as reported earlier [17] and not much differed with those obtained in the aqueous solutions. A series of preliminary experiments (a) with TiO2 in dark; (b) with TiO2 and visible light; (c) with dye and visible light; and (d) TiO2 modified with dye and visible light, reveals the fact that at about 10–15% of the initial concentration each organic compound under investigation, viz. phenol, chlorophenol, 1,2-dichloroethane and trichloroethylene is adsorbed on the surface of TiO2 catalyst in the dark under spec-
ified conditions. No appreciable photodegradation of these compounds was observed by illumination of plain TiO2 with visible light. No change in the spectral pattern was observed after 5 h of stirring the aqueous solution of dyes with phenol in dark, however, an appreciable decrease in absorbance (Fig. 2) together with the decrease in concentration of phenol (Table 1) was noticed when air equilibrated aqueous dye solutions containing phenol were illuminated with visible light for 5 h. Formation of carbon dioxide, though in trace amount, confirms the occurrence of multi-electron oxidation of phenol under specified conditions. Considering phenol depletion (Table 1) which involves multi-electron oxidation process (for example four electrons oxidation of phenol results in
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Table 1 Results of photolysis of aerated aqueous solution of dye in presence of phenol at pH = 5.0 Dye (D) Thionine Eosin Y
[D]i (M) 10−4
1× 1 × 10−4
[Phenol]i (M) 10−3
2.7 × 2.7 × 10−3
benzoquinone formation) as compared to the one electron dye reduction (which contributes to the observed spectral changes as shown in Fig. 2), the results in Table 1 essentially suggest the participation of some other secondary dark reaction involving O2 •− /HO2 • radical (formed in the reaction of one electron reduced dye radicals, i.e. semithionine/eosin Y− with dissolved oxygen) in phenol degradation. Although in parallel to the phenol oxidation the possibility of dye degradation by superoxide/hydroperoxide radicals (O2 •− /HO2 • ) cannot be ruled out, the results in Table 1 suggests that phenol present in large excess is kinetically more vulnerable towards superoxide/hydroperoxide radicals (O2 •− /HO2 • ) mediated autocatalytic oxidation process. Although spectral bleaching of thionine is typical as thionine in its excited state is known to accept one electron to form semithionine, for eosin Y it appeared to be remarkable. However, a similar observation was also reported very recently by Arakawa et al. [18] in case of free eosin Y/TEOA system. It had been proposed that eosin Y in presence of light accept one electron from TEOA to produce trianion radical which sub-
[D]f (M) 10−4
0.7 × 0.3 × 10−4
[Phenol]f (M) 10−3
2.63 × 2.45 × 10−3
CO2 (mmol) Trace Trace
sequently undergoes hydrogenation to form a stable species spectrum of which is almost featureless in 400–800 nm region. However, the excited state redox behavior of the eosin Y dye is completely altered [18] when adsorbed at TiO2 as it reduces conduction band (CB) of TiO2 by giving up one electron. The one electron deficient eosin Y adsorbed at TiO2 subsequently gets its electron back from TEOA. A very similar thing is also happened in the our case when TEOA is replaced by phenol. It seems that the kinetic barrier for one electron transfer from eosin Y to the conduction band of TiO2 dropped substantially when eosin Y is adsorbed at TiO2 surface, whereas TEOA/phenol though appears to be a stronger reductant as it reduces free eosin Y in presence of light, could not transfer electron to the conduction band of TiO2 , but reduces one electron oxidized dye species. Results of photodegradation of water bound organics are summarized in Table 2. Time course of photodegradation of phenol is shown in Fig. 3. Although GC analysis of the sample solution taken at initial (after 30 min) stage of photolysis revealed a peak comparable to the that of authentic sample of quinone,
Table 2 Results of TiO2 -(D) catalyzed photodegradation of organic pollutants in water Pa
Pur b (mmol)
Degradation of P (%)c
Photocatalyst
CO2 (mmol)
Cl− (mmol)
Phenol Chlorophenol Trichloroethylene 1,2-Dichloroethane
0.36 0.45 0.39 0.36
64 55 61 64
TiO2 -thionine
3.5d 2.9d 0.98d 1.1d
– 0.47e 1.73e 1.19e
Phenol Chlorophenol Trichloroethylene 1,2-Dichloroethane
0.28 0.42 0.32 0.29
72 58 68 71
TiO2 -eosinY
4.1d 3.1d 0.98d 1.1d
– 0.50e 1.83e 1.1e
a
Initial concentration of pollutant is 1 mmol. Concentration of unreacted pollutant (Pur ) after 5 h of reaction. c Based on pollutant concentration taken, i.e. (1 − P ) × 100. ur d Mineralized CO present in the reacting system after 5 h of reaction. 2 e Organic chlorine recovered as Cl− after 5 h of reaction. b
D. Chatterjee, A. Mahata / Applied Catalysis B: Environmental 33 (2001) 119–125
123
Fig. 3. Time dependence of photocatalytic decomposition of phenol in dye fixed TiO2 system at pH = 5.0, phenol = 1 mmol, (䉱) eosin Y, (䊏) thionine.
formation of any stable intermediate(s) in the reaction mixture was not evidenced gas chromatographically after prolonged (4–5 h) photolysis. Based on the above experimental results and considering the earlier reports on the excited state redox properties of thionine and eosin Y adsorbed on the surface of TiO2 semiconductor [17], the following working mechanisms commensurate with one very recently reported dye sensitization reaction [18] are proposed for dye-sensitized photocatalytic oxidation of water bound organic pollutants in Eqs. (1)–(12) for thionine and eosin Y, respectively. hν
TiO2 -(D1 )s →TiO2 -(D1∗ )s
(1)
TiO2 -(D1∗ )s + P → TiO2 -(D1− )s + P•+
(2)
TiO2 -(D1− )s → TiO2 -(D1 + eCB − )s
(3)
TiO2 -(D1 +eCB − )s +O2 →TiO2 -(D1 )s + O2 •−
(4)
O2
•−
+
+ H → HO2
•
O2 •− /HO2 • + P/P•+ → Products
(5) (6)
where hν: visible light; D1 : thionine; P: pollutant. hν
TiO2 -(D2 )s →TiO2 -(D2∗ )s
(7)
TiO2 -(D2∗ )s → TiO2 -(D2+ + eCB − )s
(8)
TiO2 -(D2+ +eCB − )s +P→TiO2 -(D2 + eCB − )s + P•+ (9) TiO2 -(D2 + eCB − )s + O2 → TiO2 -(D2 )s + O2 •− (10) +
O2 •− + H → HO2 •
(11)
O2 •− /HO2 • + P/P•+ → Products
(12)
where hν: visible light; D2 : eosin Y; P: pollutant. As proposed in Eqs. (1)–(6), illumination of the catalytic system with visible light (hν) results in the excitation of the surface adsorbed thionine (Eq. (1)) which, analogous to visible light irradiated homogeneously thionine/phenol system, reacts with the reducing pollutant molecule to form semithionine (Eq. (2)). Semithionine then effects charge transition into the conduction band (Eq. (3)). Empty conduction band edge of TiO2 (E CB = 0.3 V versus NHE at pH = 5.0) [17] seems to be thermodynamically competent to oxidize one electron rich surface adsorbed semitionine (E ox = −0.064 V versus NHE) [17]. As the dye molecules are immobilized onto the surface of the TiO2 , disproportionation of semithionine to thionine and leucothionine is prevented in the present case. The essential role of oxygen as
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D. Chatterjee, A. Mahata / Applied Catalysis B: Environmental 33 (2001) 119–125
an typically conduction band electron receiver in photooxidation of organic molecules in TiO2 semiconductor particulate system is well documented in the literature [1,19]. Electron transfer from conduction band to dioxygen is reportedly very fast and kinetically favored as compared to the transfer of electron trapped in the TiO2 surface to dioxygen [19]. In the present Eqs. (1)–(6), it is suggested that dioxygen (O2 ) takes up the conduction band electron (eCB − ) leading to the formation of superoxide/hydroperoxide radicals (O2 •− /HO2 • ) in the reacting system (Eqs. (4) and (5)). It is presumed that the repeated attacks of O2 •− /HO2 • radical on the aromatic ring [1] cause the deep oxidation of aromatics involving radicaloid intermediates leading to the formation of carbon dioxide and chloride as end products (Eq. (6)). In order to examine that whether the dye adsorbed at TiO2 can survive the attack of O2 •− /HO2 • radicals, we had performed photolysis of aerated eosin Y modified TiO2 using visible light and noticed that the spectrum of the solid mass, obtained after filtration of the reaction mixture that undergone prolonged photolysis (10 h), revealed no characteristic spectral features of eosin Y. This suggests that the dyes fixed on TiO2 are vulnerable to O2 •− /HO2 • radicals attack and undergo gradual photodegradation in absence of pollutants (usually electron donors) under specified conditions. However, the case is different, as observed experimentally (Table 2) in presence of pollutants. Under specified reaction conditions, the concentration of pollutants is highly in excess over the concentration of adsorbed dyes. As a result free pollutant molecules are kinetically more susceptible to undergo oxidation by O2 •− /HO2 • radicals probably through a diffusion limited electron transfer process. Interaction of immobilized dye species (present in very low concentration) with O2 •− /HO2 • radicals in competition with free organic pollutant molecules present comparatively in large excess seems to be not propitious in the present case. For eosin Y adsorbed at the surface of TiO2 , it gives up one electron to the conduction band [18] upon irradiation as outlined in Eq. (8). Phenol and other halocarbons taken under investigation serve to reduce the oxidized form of surface adsorbed eosin Y (Eq. (9)), while electrons in conduction band (eCB − )
are consumed in generating superoxide/hydroperoxide (O2 •− /HO2 • ) radicals to initiate pollutant degradation (Eqs. (10)–(12)).
4. Conclusion In conclusion, the results of the present work clearly demonstrate that the surface adsorbed thionine or eosin Y dyes though differ in their excited state redox behaviors, can sensitize TiO2 semiconductor particulate system for degradation of phenol and other pollutants studied using visible light. No appreciable leaching of dyes was noticed spectrophotometrically after 5 h of photolysis. The results of the present studies explores the possibility of using dye fixed TiO2 semiconductor in visible light driven photodetoxification of contaminated water. Studies pertinent to this matter are in progress.
Acknowledgements We gratefully acknowledge the financial support (no.15/6/99-(ST)) obtained from MNES, Govt. of India. We are thankful to Dr. B.K. Sinha, Director of this institute for his encouragement. References [1] J. Sabate, M.A. Anderson, H. Kikkawa, M. Edwards, C.G. Hill Jr., J. Catal. 127 (1991) 167. [2] J.-C. D’Oliveira, C. Minero, E. Pelizzetti, P. Pichat, J. Photochem. Photobiol. A: Chem. 72 (1993) 61. [3] O. Legrini, E. Oliveros, A.M. Braun, Chem. Rev. 93 (1993) 671. [4] L. Muszkat, L. Bir, L. Feigelson, J. Photochem. Photobiol. A: Chem. 87 (1995) 85. [5] H.Y. Chen, O. Zahraa, M. Bouchy, F. Thomas, J.Y. Bottero, J. Photochem. Photobiol. A: Chem. 85 (1995) 179. [6] M.R. Hoffmann, S.C. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [7] E. Borgarello, J. Kiwi, E. Pelizzetti, M. Visca, M. Gratzel, Nature (London) 279 (1980) 158. [8] V.H. Houlding, M. Graetzel, J. Am. Chem. Soc. 105 (1983) 5695. [9] R. Memming, Prog. Surface Sci. 17 (1984) 7. [10] M.A. Ryan, E.C. Fitzegerald, M.T. Spitler, J. Phy. Chem. 93 (1989) 6150. [11] R. Amadelli, R. Argazzi, C.A. Bignozzi, F. Scandola, J. Am. Chem. Soc.112 (1990) 7099.
D. Chatterjee, A. Mahata / Applied Catalysis B: Environmental 33 (2001) 119–125 [12] G. Stewart, M.A. Fox, Res. Chem. Intermed. 21 (1995) 933. [13] D. Chatterjee, C. Bhattacharya, Indian J. Chem. 32A (1999) 1256. [14] D. Chatterjee, A. Mahata, Catal. Commun. 2 (2000) 1. [15] G.R. Brown, F.R. Darwent, P.D.I. Fletcher, J. Chem. Soc. 107 (1985) 6446.
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[16] P.V. Kamat, J. Photochem. 28 (1985) 513. [17] M.A. Fox, M.T. Dulay, J. Photochem. Photobiol. A: Chem. 98 (1996) 91. [18] R. Abe, K. Hara, K. Sayama, K. Domen, H. Arakawa, J. Photochem. Photobiol. A: Chem. 137 (2000) 63. [19] H. Gerischer, A. Heller, J. Phy. Chem. 95 (1991) 5261.
Demineralization of organic pollutants on the dye modified TiO2 semiconductor particulate system using visible light Debabrata Chatterjee∗ , Anima Mahata Chemistry Section, Central Mechanical Engineering Research Institute, Durgapur 713209, India Received 7 December 2000; received in revised form 13 March 2001; accepted 14 March 2001
Abstract Photodegradation of organic pollutants, viz. phenol, chlorophenol, 1,2-dichloroethane and trichloroethylene in water has been achieved on the surface of TiO2 semiconductor modified with thionine and eosin Y by using visible light. After 5 h of irradiation with a 50 W tungsten lamp, over 55–72% degradation of pollutants is achieved. A working mechanism involving excitation of surface bound dye, followed by charge injection into the TiO2 conduction band is proposed. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Photodegradation; Dye sensitization; TiO2 photocatalyst; Organic pollutants; Visible light; Thionine; Eosin Y
1. Introduction Elimination of toxic and bioresistant of organic pollutants from wastewater by their transformation into non-hazardous species using semiconducting TiO2 particle and sunlight is a demanding area of research. Although the process of TiO2 -photocatalyzed degradation [1–6] studied exhaustively during last decade provides a reasonable solution to this problem. The vital snag of TiO2 semiconductor is that it absorbs a small portion of solar spectrum in the UV region (band gap energy of TiO2 is 3.2 eV). Hence, in order to reap maximum solar energy, it is necessary to shift the absorption threshold towards visible region. Conceptually, dye sensitization [7–12] seems to be a viable alternative method to deal with this issue. Dye (sensitizer) adsorbed on TiO2 surface is excited by absorbing visible light and effects charge injection ∗
Corresponding author. Tel.: +91-343-546818; fax: +91-343-546745. E-mail address: [email protected] (D. Chatterjee).
into the conduction band of TiO2 semiconductor at sub-band gap excitation and the catalytic processes is followed through interfacial electron transfer. Our recent research interest is focused on the exploration of visible light assisted catalytic transformation of organic molecules using TiO2 semiconductor particulate system [13,14]. In the present investigations, we explore the possibility of photodegradation of water-bound organic pollutants using TiO2 semiconductor particles modified with thionine and eosin Y (Fig. 1). The excited state redox properties of dyes thionine and eosin Y, selected for this work are different with regards to their electron transfer process. Thionine at the excited state is known to accept one electron and converts into semithionine [15], whereas, eosin Y gives up one electron upon excitation with visible light [16]. We wish to report herein, the results of our studies of visible light assisted decomposition of some common organic pollutants, viz. phenol, chlorophenol, 1,2-dichloroethane and trichloroethylene catalyzed by suspended dye modified TiO2 semiconductor particulate system.
0926-3373/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 1 ) 0 0 1 7 0 - 9
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D. Chatterjee, A. Mahata / Applied Catalysis B: Environmental 33 (2001) 119–125
Fig. 1. Pictorial representation of the various dye species.
2. Experimental Surface modification of semiconductor grade TiO2 (obtained from Fluka) particle and characterization of surface modified dyes were carried by adopting the published procedure [17]. In a typical experiment, a saturated solution of dye (pH ∼ 3) containing TiO2 (500 mg) was magnetically stirred for 4 h in dark. The uptake estimated spectrophotometrically by using a GBC Cintra 10 spectrophotometer by measuring of free dye and in the supernatant liquid obtained after filtration was found to be 543 eqv/g for thionine and 521 eqv/g for eosin Y. Spectra of the dyes adsorbed on TiO2 surface was taken in a qualitative manner by rubbing the solid sample on a piece of transparent paper and placing it into the optical path of built-in cell holder of the spectrophotometer (another piece of the same paper was placed in the reference cell-holder). All other chemicals used were of AR grade and doubly-distilled water was used throughout the experiment. In a typical photocatalytic experiment, aqueous suspension (50 ml) of pollutant (1 mmol) containing 100 mg of surface modified photocatalyst designated as TiO2 -D was taken in a flat-surfaced glass reactor. A 50 W tungsten lamp (Philips Medical Spot Lamp, covering the wavelength range 420–800 nm, inner diameter of the focus tube was 5.1 cm) was used for the irradiation. The distance between the lamp and the glass-reactor containing reaction mixture was fixed at 8 inch. The pre-aerated reaction mixture was
magnetically stirred during irradiation. The pH of reacting system containing dye fixed TiO2 and pollutant was fixed at 5.0 (NaOH). After irradiation (5 h), the reaction mixture were filtered and subjected to GC (CE 8000Top Series) analysis. A Tennex column operating at FID detector was used for this purpose. The depletion of dye was estimated spectrophotometrically. Fluorescence measurements were carried out in Chemistry Department, Burdwan University by using a Hitachi F-4010 spectrofluorometer. Production of CO2 was demonstrated by the precipitation of BaCO3 in the Ba(OH)2 solution. The evolved CO2 was flushed with oxygen through alkali (KOH) scrubber and estimated by titration against acid (HCl). Presence of Cl− (confirmed by AgNO3 test) at the end of the experiment indicated the dechlorination of halocarbons under investigation. The extent of chlorination was measured titrimetrically with AgNO3 solution using potassium chromate as an indicator. The photodegradation reaction was followed gas chromatographically by monitoring the disappearance of GC-peak corresponding to the reacting pollutant (i.e. phenol, chlorophenol, 1,2-dichloroethane and trichloroethylene).
3. Results and discussion Absorption spectra of the aqueous solutions of thionine and eosin Y at pH 3.0 revealed characteristic absorption maxima at 600 and 518 nm, respectively.
D. Chatterjee, A. Mahata / Applied Catalysis B: Environmental 33 (2001) 119–125
121
Fig. 2. Spectral changes that occurs during photolysis of aerated aqueous solution of (a) thionine (1 × 10−4 M) and phenol (2.7 × 10−3 M), and (b) eosin Y (1 × 10−4 M) and phenol (2.7 × 10−3 M) at pH = 5.0.
The emission spectra of thionine and eosin Y at pH 3.0 in water exhibited emission maxima at 622 and 546 nm, respectively. Spectra of the dyes adsorbed on to the surface of the TiO2 recorded in the solid state revealed similar features as reported earlier [17] and not much differed with those obtained in the aqueous solutions. A series of preliminary experiments (a) with TiO2 in dark; (b) with TiO2 and visible light; (c) with dye and visible light; and (d) TiO2 modified with dye and visible light, reveals the fact that at about 10–15% of the initial concentration each organic compound under investigation, viz. phenol, chlorophenol, 1,2-dichloroethane and trichloroethylene is adsorbed on the surface of TiO2 catalyst in the dark under spec-
ified conditions. No appreciable photodegradation of these compounds was observed by illumination of plain TiO2 with visible light. No change in the spectral pattern was observed after 5 h of stirring the aqueous solution of dyes with phenol in dark, however, an appreciable decrease in absorbance (Fig. 2) together with the decrease in concentration of phenol (Table 1) was noticed when air equilibrated aqueous dye solutions containing phenol were illuminated with visible light for 5 h. Formation of carbon dioxide, though in trace amount, confirms the occurrence of multi-electron oxidation of phenol under specified conditions. Considering phenol depletion (Table 1) which involves multi-electron oxidation process (for example four electrons oxidation of phenol results in
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D. Chatterjee, A. Mahata / Applied Catalysis B: Environmental 33 (2001) 119–125
Table 1 Results of photolysis of aerated aqueous solution of dye in presence of phenol at pH = 5.0 Dye (D) Thionine Eosin Y
[D]i (M) 10−4
1× 1 × 10−4
[Phenol]i (M) 10−3
2.7 × 2.7 × 10−3
benzoquinone formation) as compared to the one electron dye reduction (which contributes to the observed spectral changes as shown in Fig. 2), the results in Table 1 essentially suggest the participation of some other secondary dark reaction involving O2 •− /HO2 • radical (formed in the reaction of one electron reduced dye radicals, i.e. semithionine/eosin Y− with dissolved oxygen) in phenol degradation. Although in parallel to the phenol oxidation the possibility of dye degradation by superoxide/hydroperoxide radicals (O2 •− /HO2 • ) cannot be ruled out, the results in Table 1 suggests that phenol present in large excess is kinetically more vulnerable towards superoxide/hydroperoxide radicals (O2 •− /HO2 • ) mediated autocatalytic oxidation process. Although spectral bleaching of thionine is typical as thionine in its excited state is known to accept one electron to form semithionine, for eosin Y it appeared to be remarkable. However, a similar observation was also reported very recently by Arakawa et al. [18] in case of free eosin Y/TEOA system. It had been proposed that eosin Y in presence of light accept one electron from TEOA to produce trianion radical which sub-
[D]f (M) 10−4
0.7 × 0.3 × 10−4
[Phenol]f (M) 10−3
2.63 × 2.45 × 10−3
CO2 (mmol) Trace Trace
sequently undergoes hydrogenation to form a stable species spectrum of which is almost featureless in 400–800 nm region. However, the excited state redox behavior of the eosin Y dye is completely altered [18] when adsorbed at TiO2 as it reduces conduction band (CB) of TiO2 by giving up one electron. The one electron deficient eosin Y adsorbed at TiO2 subsequently gets its electron back from TEOA. A very similar thing is also happened in the our case when TEOA is replaced by phenol. It seems that the kinetic barrier for one electron transfer from eosin Y to the conduction band of TiO2 dropped substantially when eosin Y is adsorbed at TiO2 surface, whereas TEOA/phenol though appears to be a stronger reductant as it reduces free eosin Y in presence of light, could not transfer electron to the conduction band of TiO2 , but reduces one electron oxidized dye species. Results of photodegradation of water bound organics are summarized in Table 2. Time course of photodegradation of phenol is shown in Fig. 3. Although GC analysis of the sample solution taken at initial (after 30 min) stage of photolysis revealed a peak comparable to the that of authentic sample of quinone,
Table 2 Results of TiO2 -(D) catalyzed photodegradation of organic pollutants in water Pa
Pur b (mmol)
Degradation of P (%)c
Photocatalyst
CO2 (mmol)
Cl− (mmol)
Phenol Chlorophenol Trichloroethylene 1,2-Dichloroethane
0.36 0.45 0.39 0.36
64 55 61 64
TiO2 -thionine
3.5d 2.9d 0.98d 1.1d
– 0.47e 1.73e 1.19e
Phenol Chlorophenol Trichloroethylene 1,2-Dichloroethane
0.28 0.42 0.32 0.29
72 58 68 71
TiO2 -eosinY
4.1d 3.1d 0.98d 1.1d
– 0.50e 1.83e 1.1e
a
Initial concentration of pollutant is 1 mmol. Concentration of unreacted pollutant (Pur ) after 5 h of reaction. c Based on pollutant concentration taken, i.e. (1 − P ) × 100. ur d Mineralized CO present in the reacting system after 5 h of reaction. 2 e Organic chlorine recovered as Cl− after 5 h of reaction. b
D. Chatterjee, A. Mahata / Applied Catalysis B: Environmental 33 (2001) 119–125
123
Fig. 3. Time dependence of photocatalytic decomposition of phenol in dye fixed TiO2 system at pH = 5.0, phenol = 1 mmol, (䉱) eosin Y, (䊏) thionine.
formation of any stable intermediate(s) in the reaction mixture was not evidenced gas chromatographically after prolonged (4–5 h) photolysis. Based on the above experimental results and considering the earlier reports on the excited state redox properties of thionine and eosin Y adsorbed on the surface of TiO2 semiconductor [17], the following working mechanisms commensurate with one very recently reported dye sensitization reaction [18] are proposed for dye-sensitized photocatalytic oxidation of water bound organic pollutants in Eqs. (1)–(12) for thionine and eosin Y, respectively. hν
TiO2 -(D1 )s →TiO2 -(D1∗ )s
(1)
TiO2 -(D1∗ )s + P → TiO2 -(D1− )s + P•+
(2)
TiO2 -(D1− )s → TiO2 -(D1 + eCB − )s
(3)
TiO2 -(D1 +eCB − )s +O2 →TiO2 -(D1 )s + O2 •−
(4)
O2
•−
+
+ H → HO2
•
O2 •− /HO2 • + P/P•+ → Products
(5) (6)
where hν: visible light; D1 : thionine; P: pollutant. hν
TiO2 -(D2 )s →TiO2 -(D2∗ )s
(7)
TiO2 -(D2∗ )s → TiO2 -(D2+ + eCB − )s
(8)
TiO2 -(D2+ +eCB − )s +P→TiO2 -(D2 + eCB − )s + P•+ (9) TiO2 -(D2 + eCB − )s + O2 → TiO2 -(D2 )s + O2 •− (10) +
O2 •− + H → HO2 •
(11)
O2 •− /HO2 • + P/P•+ → Products
(12)
where hν: visible light; D2 : eosin Y; P: pollutant. As proposed in Eqs. (1)–(6), illumination of the catalytic system with visible light (hν) results in the excitation of the surface adsorbed thionine (Eq. (1)) which, analogous to visible light irradiated homogeneously thionine/phenol system, reacts with the reducing pollutant molecule to form semithionine (Eq. (2)). Semithionine then effects charge transition into the conduction band (Eq. (3)). Empty conduction band edge of TiO2 (E CB = 0.3 V versus NHE at pH = 5.0) [17] seems to be thermodynamically competent to oxidize one electron rich surface adsorbed semitionine (E ox = −0.064 V versus NHE) [17]. As the dye molecules are immobilized onto the surface of the TiO2 , disproportionation of semithionine to thionine and leucothionine is prevented in the present case. The essential role of oxygen as
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an typically conduction band electron receiver in photooxidation of organic molecules in TiO2 semiconductor particulate system is well documented in the literature [1,19]. Electron transfer from conduction band to dioxygen is reportedly very fast and kinetically favored as compared to the transfer of electron trapped in the TiO2 surface to dioxygen [19]. In the present Eqs. (1)–(6), it is suggested that dioxygen (O2 ) takes up the conduction band electron (eCB − ) leading to the formation of superoxide/hydroperoxide radicals (O2 •− /HO2 • ) in the reacting system (Eqs. (4) and (5)). It is presumed that the repeated attacks of O2 •− /HO2 • radical on the aromatic ring [1] cause the deep oxidation of aromatics involving radicaloid intermediates leading to the formation of carbon dioxide and chloride as end products (Eq. (6)). In order to examine that whether the dye adsorbed at TiO2 can survive the attack of O2 •− /HO2 • radicals, we had performed photolysis of aerated eosin Y modified TiO2 using visible light and noticed that the spectrum of the solid mass, obtained after filtration of the reaction mixture that undergone prolonged photolysis (10 h), revealed no characteristic spectral features of eosin Y. This suggests that the dyes fixed on TiO2 are vulnerable to O2 •− /HO2 • radicals attack and undergo gradual photodegradation in absence of pollutants (usually electron donors) under specified conditions. However, the case is different, as observed experimentally (Table 2) in presence of pollutants. Under specified reaction conditions, the concentration of pollutants is highly in excess over the concentration of adsorbed dyes. As a result free pollutant molecules are kinetically more susceptible to undergo oxidation by O2 •− /HO2 • radicals probably through a diffusion limited electron transfer process. Interaction of immobilized dye species (present in very low concentration) with O2 •− /HO2 • radicals in competition with free organic pollutant molecules present comparatively in large excess seems to be not propitious in the present case. For eosin Y adsorbed at the surface of TiO2 , it gives up one electron to the conduction band [18] upon irradiation as outlined in Eq. (8). Phenol and other halocarbons taken under investigation serve to reduce the oxidized form of surface adsorbed eosin Y (Eq. (9)), while electrons in conduction band (eCB − )
are consumed in generating superoxide/hydroperoxide (O2 •− /HO2 • ) radicals to initiate pollutant degradation (Eqs. (10)–(12)).
4. Conclusion In conclusion, the results of the present work clearly demonstrate that the surface adsorbed thionine or eosin Y dyes though differ in their excited state redox behaviors, can sensitize TiO2 semiconductor particulate system for degradation of phenol and other pollutants studied using visible light. No appreciable leaching of dyes was noticed spectrophotometrically after 5 h of photolysis. The results of the present studies explores the possibility of using dye fixed TiO2 semiconductor in visible light driven photodetoxification of contaminated water. Studies pertinent to this matter are in progress.
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