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Heterogeneous photocatalytic decomposition of halosubstituted benzyl alcohols on semiconductor particles in aqueous media
Chemosphere 41 (2000) 1451±1455
Heterogeneous photocatalytic decomposition of halosubstituted benzyl alcohols on semiconductor particles in aqueous media Katarina S. Wissiak
a,*
b , Boris Sket , Margareta Vrtacnik
a
a
b
Faculty of Natural Sciences and Engineering, Department of Chemical Education and Informatics, Vegova 4, P.O. Box 18/1, 1001 Ljubljana, Slovenia Faculty of Chemistry and Chemical Technology, Department of Chemistry and Biochemistry, Asker ceva 5, 1000 Ljubljana, Slovenia Received 15 March 1999; accepted 17 November 1999
Abstract The photodegradation of 2-, 3- and 4-halosubstituted benzyl alcohols (HBAs) on semiconductive oxides (TiO2 , ZnO) was studied. It was found out that the photodegradation rate increases from the ¯uoro to bromo derivates in the case of 2- and 4-HBAs, whereas in the case of 3-HBAs the reverse trend was observed. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Halosubstituted benzyl alcohols; Heterogenous photocatalysis; Photodegradation; Oxidation potentials
1. Introduction Semiconductive oxides suspended in solution can be used to carry out heterogeneous photocatalytic and photosynthetic processes (Bard, 1980; Mills and Le Hunte, 1997). For example, TiO2 and ZnO are used in photocatalytic oxidation of cyanides (Frank and Bard, 1977a,b), sulphites (Frank and Bard, 1977a), acetates (Kraeutler and Bard, 1978a) and other substances (Miyake et al., 1977; Watanabe et al., 1977; Kraeutler and Bard, 1978b; Inoue et al., 1979). Irradiation of TiO2 powders suspended in an oxygensaturated acetonitrile solution leads to selective oxidation of the benzylic methylene group, and a carbonyl group is formed as a result. This method can be used for the preparation of esters from the corresponding ethers, or ketones from the corresponding hydrocarbons (Pin-
*
Corresponding author. Tel.: +386-6121-4326; fax: +3866112-58684. E-mail address: [email protected] (K.S. Wissiak).
cock et al., 1985). Izumi et al. (1981) have studied heterogeneous photocatalytic decomposition of benzoic and adipic acid in aqueous solutions on platinized TiO2 . They report CO2 and butane as the predominant reaction products and negligibly small quantities of oligomeric materials in case of the decomposition of adipic acid, while the photodecomposition of benzoic acid in oxygen-containing solution leads predominantly to the formation of CO2 , with intermediate production of salicylic acid and phenol. Halosubstituted aromatic chemicals have potentially adverse impacts on environmental systems (Hutzinger, 1982; Boh and Musar, 1996). Since their biological and chemical degradation is rather low, a photodegradation study was carried out using semiconductive oxides (Al-Ekabi and Serpone, 1988) to accelerate their degradation. To ascertain the impact of a halogen atom on the aromatic ring on the photodegradation pathways of halosubstituted benzyl alcohols (HBAs), 2-, 3- and 4HBA were selected as model substances, since the oxidation of the benzyl group is relatively simple.
0045-6535/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 5 - 6 5 3 5 ( 9 9 ) 0 0 5 5 4 - 8
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K.S. Wissiak et al. / Chemosphere 41 (2000) 1451±1455
Fig. 1. Kinetics of the photocatalytic degradation of ¯uoro benzyl alcohols in the presence of TiO2 powder.
Fig. 2. Kinetics of the photocatalytic degradation of chloro benzyl alcohols in the presence of TiO2 powder.
Fig. 3. Kinetics of the photocatalytic degradation of bromo benzyl alcohols in the presence of TiO2 powder.
2. Materials and methods 2-Methyl imidazol was obtained from Fluka Chemie AG, and H2 O2 was obtained from Belinka, Ljubljana. All other chemicals used in the experiment were obtained from Aldrich Chemical. A typical experiment for the photooxidation of HBAs was performed as follows: a suspension containing 20 mg of HBA and 100 mg of TiO2 (anatas) or ZnO
powder in 50 ml double distilled water was exposed to an ultrasonic bath for 5±10 min in order to obtain a ®ne suspension. The suspension was then irradiated with a Hg HPQ 125 W lamp. During irradiation the mixture was bubbled with oxygen and samples were removed every 10 min. The samples were further analysed by GC and GC/MS using 2-methyl imidazol as an internal standard.
K.S. Wissiak et al. / Chemosphere 41 (2000) 1451±1455
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Table 1 Rate constants of photodegradation k (10ÿ4 sÿ1 ) of selected HBA in the presence of TiO2 and ZnO powder HBA ÿ4
ÿ1
k (10 s ) TiO2 k (10ÿ4 sÿ1 ) ZnO
2-F
3-F
4-F
2-Br
3-Br
4-Br
4.8 4.2
12.6 17.3
4.2 5.8
8.3 13.2
3.1 10.8
12.0 14.7
Table 2 Rate constants of photodegradation (10ÿ4 sÿ1 ) of benzyl alcohol and selected HBA in the presence of TiO2 powder Halogen atom
Position 2
Position 3
Position 4
F Cl Br Benzyl alcohol
4.8 5.5 8.3
12.6 11.8 3.1 4.3
4.2 5.0 12.0
spectra were recorded using a Brucker ESP 300 spectrometer in (CD3 )2 CO solution with TMS as internal reference. Analytical gas chromatography (GC) was carried out on a Hewlett Packard 6890 Series Gas Chromatograph System using a 30 m Innowax column. Irradiation of solutions was performed in a quartz photoreactor equipped with a cooling system using Hg HPQ 125 W. Cyclic voltammetry was carried out in a 50 ml single compartment cell. The working disc electrode was a Pt disc electrode, and was ®xed to a cylindrical te¯on holder and mounted on the shaft of an EG&G PARC RDE Model 616. The reference electrode was Ag/AgCl (sat. KCl) to which all potentials are referred. The counter electrode was a Pt-wire. The instrumentation employed was an EG&G PARC Model 273 Potentiostat/Galvanostat with a model 270 Electrochemical Analysis Software System.
3. Results and discussion When the irradiation of the reaction mixture was ®nished and the TiO2 was removed from the remainder, the water was evaporated in vacuo. It was found that a few milligrams of polymeric product was obtained. It was insoluble in ether and chlorinated solvents but soluble in methanol and acetone. The product was analysed by IR spectroscopy. One broad signal at 3400 cmÿ1 and two peaks at 1725 and 1630 cmÿ1 were obtained. Regarding the signal at 1725 cmÿ1 in the IR spectrum it can be inferred that the polymeric product contains carbonyl groups. The same polymeric product was also analysed by 1 H NMR spectroscopy and several signals were
Scheme 1. Proposed reaction mechanism.
After 60 min of irradiation the TiO2 was removed and the reaction mixture was evaporated in vacuo. A small quantity of polymeric product was obtained and analysed by IR and 1 H NMR spectroscopy. IR spectra were recorded using a Perkin-Elmer 1310 spectrometer. 1 H nuclear magnetic resonance (NMR) Table 3 Oxidation potentials for benzyl alcohol at 1 mA and selected HBA HBA
2-F
2-Cl
2-Br
3-F
3-Cl
3-Br
4-F
4-Cl
4-Br
b.a.
E1 mA (V)
1.390
1.353
1.367
1.481
1.453
1.451
1.375
1.367
1.375
1.419
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K.S. Wissiak et al. / Chemosphere 41 (2000) 1451±1455
obtained in the region of aliphatic, aromatic and alkene protons. Results illustrating photodegradation of the ¯uoro, chloro and bromo benzyl alcohols in the presence of TiO2 powder are presented in Figs. 1±3. The dierence between the energy of the energetic positions of the valence and conduction band edges dictate the applicability of the speci®c semiconductor for the oxidation of organic compounds. Therefore experiments under the same conditions as above with ZnO as the semiconductor were carried out. The trend of photodegradation rates using ZnO as a semiconductor is similar as in the case of TiO2 . Nevertheless, the photodegradation rate constants are slightly diferent (Table 1). Based on the photodegradation results obtained it is possible to deduce that photodegradation rate depends on the type and position of the halogen atom on the aromatic ring of HBA. Comparing the photodegradation rate constants it can be concluded that the photodegradation rate of the substance in the case of 2-HBAs increases from the ¯uoro to bromo derivates. Similar results are obtained also for 4-HBAs, whereas in the case of 3-HBAs the rate of photooxidation exhibits exactly the reverse trend, increasing from the bromo to ¯uoro derivates. The results are presented in Table 2. To explain the eects described it is necessary to look into the proposed reaction mechanism (Pincock et al., 1985). The ®rst step is connected with absorption of light by TiO2 to generate an electron±hole pair, and electron transfer from the molecule of benzyl alcohol to the molecule of TiO2 -producing ion radicals. Further reaction may be connected with proton transfer and formation of benzyl and peroxyl radicals, which is followed by their linkage and formation of hydroperoxide, or with another electron and proton transfer and formation of a hydroxy benzyl cation. However, hydroperoxide can also be produced by the reaction of a benzyl alcohol radical and oxygen, in which a peroxyl radical is produced. After the peroxyl radical accepts an electron from TiO2 and is converted into the anion, it is further protonated to hydroperoxide. As a ®nal step, hydroperoxide is converted into substituted benzoic acid (Scheme 1). Taking into account the mechanism proposed, it seems that the crucial step in the reaction mechanism is the ion±radical formation, which is followed by its transformation into the hydroxy benzyl cation or radical. Since the ease of oxidation is certainly connected with oxidation potentials (Mayeda et al., 1972), cyclic voltammetry was used in order to ®nd out the impact of the type and position of the halogen atom on the aromatic ring on the change in oxidation potential. Table 3 shows the oxidation potentials at 1 mA (E1 mA ) for benzyl alcohol and selected HBAs.
Provided that the crucial step in the reaction mechanism is connected with the benzyl ion±radical, lower photodegradation rate would be expected in the case of 3-HBA than in the case of benzyl alcohol, and higher in the case of 2- and 4-HBAs. The photodegradation rate constants do not correspond with the dierences in oxidation potentials. This also indicates other possible effects which have an in¯uence on the photodegradation rate of HBAs. It has already been reported in previous studies (Izumi et al., 1981) of the oxidative decomposition of benzoic acid in oxygen containing solution on platinized TiO2 , that CO2 is the principal product, with intermediate production of a polymeric product with a carbonyl group. With reference to this, formation of benzoic acid as the primary photooxidation product can be predicted, followed by its oxidation to CO2 or by polymerisation after attack by OHá radicals on the aromatic ring. 4. Conclusion Considering the mechanism proposed, it can be concluded that photooxidation of selected benzyl alcohols is connected with the formation of a cation radical of the HBA which is converted into a benzyl radical or cation. Similar trends of photooxidation rates were obtained for 2- and 4-HBAs while exactly the reverse was detected in the case of 3-HBAs. The dierences in photodegradation rates cannot be explained by the dierences in the oxidation potential for 2-, 3-, and 4-HBAs alone. Nevertheless, the type and position of the halogen atom on the aromatic ring may also have an eect on photodegradation of the HBAs.
Acknowledgements We wish to thank the Ministry of Science and Technology, the Faculty of Chemistry and Chemical Technology and Faculty of Natural Sciences and Engineering for supporting the research. We are also grateful to Prof. B. Pihlar for essential assistance.
References Al-Ekabi, H., Serpone, N., 1988. Kinetic studies in heterogeneuos photocatalysis 1. Photocatalytic degradation of chlorinated phenols in aerated aqueous solutions over TiO2 supported on a glass matrix. J. Phys. Chem. 92, 5726±5731. Bard, A.J., 1980. Photoelectrochemistry. Science 207, 139±144. Boh, B., Musar, A., 1996. Toxic waste and Biodiversity. In: Cimerman, A., Gunde Cimerman, N. (Eds.), Biodiversity.
K.S. Wissiak et al. / Chemosphere 41 (2000) 1451±1455 Slovenian National Commission for UNESCO and National Institute of Chemistry. Frank, S.N., Bard, A.J., 1977a. Heterogeneous photocatalytic oxidation of cyanide and sul®te in aqueous solutions at semiconductor powders. J. Phys. Chem. 81, 1484±1488. Frank, S.N., Bard, A.J., 1977b. Semiconductor electrodes. 12. Photoassisted oxidations and photoelectrosynthesis at polycrystalline TiO2 electrodes. J. Am. Chem. Soc. 99, 4667±4675. Hutzinger, O., 1982. The Handbook of Environmental Chemistry. Springer, Berlin. Inoue, T., Fujishima, A., Konishi, S., Honda, K., 1979. Photoelectrocatalytic reduction of carbon dioxide in aqueous suspensions of semiconductor powders. Nature (London) 277, 637±638. Izumi, I., Fan, F.F., Bard, A.J., 1981. Heterogeneous photocatalytic decomposition of benzoic acid and adipic acid on platinized TiO2 powder. The photo-Kolbe decarboxylative route to the breakdown of the benzene ring and to the production of butane. J. Phys. Chem. 85, 218±223. Kraeutler, B., Bard, A.J., 1978a. Heterogeneous photocatalytic decomposition of saturated carboxylic acids on TiO2
1455
powder Decarboxylative route to alkanes. J. Am. Chem. Soc. 100, 5985±5992. Kraeutler, B., Bard, A.J., 1978b. Heterogeneous photocatalytic synthesis of methane from acetic acid-new Kolbe reaction pathway. J. Am. Chem. Soc. 100, 2239±2240. Mayeda, E.A., Miller, L.L., Wolf, J.F., 1972. Electrooxidation of benzylic ethers, esters, alcohols, and phenyl epoxides. J. Am. Chem. Soc. 94, 6812±6816. Mills, A., Le Hunte, S., 1997. An overview of semiconductor photocatalysis. J. Photochem. Photobiol. A: Chem. 108, 1±35. Miyake, M., Yoneyama, H., Tamura, H., 1977. The correlation between photoelectrochemical cell reactions and photocatalytic reactions on illuminated rutile. Bull. Chem. Soc. Jpn. 50, 1492±1496. Pincock, J.A., Pincock, A.L., Fox, M.A., 1985. Controlled oxidation of benzyl ethers on irradiated semiconductor powders. Tetrahedron 41, 4107±4117. Watanabe, T., Takizawa, T., Honda, K., 1977. Photocatalysis through excitation of adsorbates. 1. Highly ecient Ndeethylation of rhodamine B adsorbed to CdS. J. Phys. Chem. 81, 1845±1851.
Heterogeneous photocatalytic decomposition of halosubstituted benzyl alcohols on semiconductor particles in aqueous media Katarina S. Wissiak
a,*
b , Boris Sket , Margareta Vrtacnik
a
a
b
Faculty of Natural Sciences and Engineering, Department of Chemical Education and Informatics, Vegova 4, P.O. Box 18/1, 1001 Ljubljana, Slovenia Faculty of Chemistry and Chemical Technology, Department of Chemistry and Biochemistry, Asker ceva 5, 1000 Ljubljana, Slovenia Received 15 March 1999; accepted 17 November 1999
Abstract The photodegradation of 2-, 3- and 4-halosubstituted benzyl alcohols (HBAs) on semiconductive oxides (TiO2 , ZnO) was studied. It was found out that the photodegradation rate increases from the ¯uoro to bromo derivates in the case of 2- and 4-HBAs, whereas in the case of 3-HBAs the reverse trend was observed. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Halosubstituted benzyl alcohols; Heterogenous photocatalysis; Photodegradation; Oxidation potentials
1. Introduction Semiconductive oxides suspended in solution can be used to carry out heterogeneous photocatalytic and photosynthetic processes (Bard, 1980; Mills and Le Hunte, 1997). For example, TiO2 and ZnO are used in photocatalytic oxidation of cyanides (Frank and Bard, 1977a,b), sulphites (Frank and Bard, 1977a), acetates (Kraeutler and Bard, 1978a) and other substances (Miyake et al., 1977; Watanabe et al., 1977; Kraeutler and Bard, 1978b; Inoue et al., 1979). Irradiation of TiO2 powders suspended in an oxygensaturated acetonitrile solution leads to selective oxidation of the benzylic methylene group, and a carbonyl group is formed as a result. This method can be used for the preparation of esters from the corresponding ethers, or ketones from the corresponding hydrocarbons (Pin-
*
Corresponding author. Tel.: +386-6121-4326; fax: +3866112-58684. E-mail address: [email protected] (K.S. Wissiak).
cock et al., 1985). Izumi et al. (1981) have studied heterogeneous photocatalytic decomposition of benzoic and adipic acid in aqueous solutions on platinized TiO2 . They report CO2 and butane as the predominant reaction products and negligibly small quantities of oligomeric materials in case of the decomposition of adipic acid, while the photodecomposition of benzoic acid in oxygen-containing solution leads predominantly to the formation of CO2 , with intermediate production of salicylic acid and phenol. Halosubstituted aromatic chemicals have potentially adverse impacts on environmental systems (Hutzinger, 1982; Boh and Musar, 1996). Since their biological and chemical degradation is rather low, a photodegradation study was carried out using semiconductive oxides (Al-Ekabi and Serpone, 1988) to accelerate their degradation. To ascertain the impact of a halogen atom on the aromatic ring on the photodegradation pathways of halosubstituted benzyl alcohols (HBAs), 2-, 3- and 4HBA were selected as model substances, since the oxidation of the benzyl group is relatively simple.
0045-6535/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 5 - 6 5 3 5 ( 9 9 ) 0 0 5 5 4 - 8
1452
K.S. Wissiak et al. / Chemosphere 41 (2000) 1451±1455
Fig. 1. Kinetics of the photocatalytic degradation of ¯uoro benzyl alcohols in the presence of TiO2 powder.
Fig. 2. Kinetics of the photocatalytic degradation of chloro benzyl alcohols in the presence of TiO2 powder.
Fig. 3. Kinetics of the photocatalytic degradation of bromo benzyl alcohols in the presence of TiO2 powder.
2. Materials and methods 2-Methyl imidazol was obtained from Fluka Chemie AG, and H2 O2 was obtained from Belinka, Ljubljana. All other chemicals used in the experiment were obtained from Aldrich Chemical. A typical experiment for the photooxidation of HBAs was performed as follows: a suspension containing 20 mg of HBA and 100 mg of TiO2 (anatas) or ZnO
powder in 50 ml double distilled water was exposed to an ultrasonic bath for 5±10 min in order to obtain a ®ne suspension. The suspension was then irradiated with a Hg HPQ 125 W lamp. During irradiation the mixture was bubbled with oxygen and samples were removed every 10 min. The samples were further analysed by GC and GC/MS using 2-methyl imidazol as an internal standard.
K.S. Wissiak et al. / Chemosphere 41 (2000) 1451±1455
1453
Table 1 Rate constants of photodegradation k (10ÿ4 sÿ1 ) of selected HBA in the presence of TiO2 and ZnO powder HBA ÿ4
ÿ1
k (10 s ) TiO2 k (10ÿ4 sÿ1 ) ZnO
2-F
3-F
4-F
2-Br
3-Br
4-Br
4.8 4.2
12.6 17.3
4.2 5.8
8.3 13.2
3.1 10.8
12.0 14.7
Table 2 Rate constants of photodegradation (10ÿ4 sÿ1 ) of benzyl alcohol and selected HBA in the presence of TiO2 powder Halogen atom
Position 2
Position 3
Position 4
F Cl Br Benzyl alcohol
4.8 5.5 8.3
12.6 11.8 3.1 4.3
4.2 5.0 12.0
spectra were recorded using a Brucker ESP 300 spectrometer in (CD3 )2 CO solution with TMS as internal reference. Analytical gas chromatography (GC) was carried out on a Hewlett Packard 6890 Series Gas Chromatograph System using a 30 m Innowax column. Irradiation of solutions was performed in a quartz photoreactor equipped with a cooling system using Hg HPQ 125 W. Cyclic voltammetry was carried out in a 50 ml single compartment cell. The working disc electrode was a Pt disc electrode, and was ®xed to a cylindrical te¯on holder and mounted on the shaft of an EG&G PARC RDE Model 616. The reference electrode was Ag/AgCl (sat. KCl) to which all potentials are referred. The counter electrode was a Pt-wire. The instrumentation employed was an EG&G PARC Model 273 Potentiostat/Galvanostat with a model 270 Electrochemical Analysis Software System.
3. Results and discussion When the irradiation of the reaction mixture was ®nished and the TiO2 was removed from the remainder, the water was evaporated in vacuo. It was found that a few milligrams of polymeric product was obtained. It was insoluble in ether and chlorinated solvents but soluble in methanol and acetone. The product was analysed by IR spectroscopy. One broad signal at 3400 cmÿ1 and two peaks at 1725 and 1630 cmÿ1 were obtained. Regarding the signal at 1725 cmÿ1 in the IR spectrum it can be inferred that the polymeric product contains carbonyl groups. The same polymeric product was also analysed by 1 H NMR spectroscopy and several signals were
Scheme 1. Proposed reaction mechanism.
After 60 min of irradiation the TiO2 was removed and the reaction mixture was evaporated in vacuo. A small quantity of polymeric product was obtained and analysed by IR and 1 H NMR spectroscopy. IR spectra were recorded using a Perkin-Elmer 1310 spectrometer. 1 H nuclear magnetic resonance (NMR) Table 3 Oxidation potentials for benzyl alcohol at 1 mA and selected HBA HBA
2-F
2-Cl
2-Br
3-F
3-Cl
3-Br
4-F
4-Cl
4-Br
b.a.
E1 mA (V)
1.390
1.353
1.367
1.481
1.453
1.451
1.375
1.367
1.375
1.419
1454
K.S. Wissiak et al. / Chemosphere 41 (2000) 1451±1455
obtained in the region of aliphatic, aromatic and alkene protons. Results illustrating photodegradation of the ¯uoro, chloro and bromo benzyl alcohols in the presence of TiO2 powder are presented in Figs. 1±3. The dierence between the energy of the energetic positions of the valence and conduction band edges dictate the applicability of the speci®c semiconductor for the oxidation of organic compounds. Therefore experiments under the same conditions as above with ZnO as the semiconductor were carried out. The trend of photodegradation rates using ZnO as a semiconductor is similar as in the case of TiO2 . Nevertheless, the photodegradation rate constants are slightly diferent (Table 1). Based on the photodegradation results obtained it is possible to deduce that photodegradation rate depends on the type and position of the halogen atom on the aromatic ring of HBA. Comparing the photodegradation rate constants it can be concluded that the photodegradation rate of the substance in the case of 2-HBAs increases from the ¯uoro to bromo derivates. Similar results are obtained also for 4-HBAs, whereas in the case of 3-HBAs the rate of photooxidation exhibits exactly the reverse trend, increasing from the bromo to ¯uoro derivates. The results are presented in Table 2. To explain the eects described it is necessary to look into the proposed reaction mechanism (Pincock et al., 1985). The ®rst step is connected with absorption of light by TiO2 to generate an electron±hole pair, and electron transfer from the molecule of benzyl alcohol to the molecule of TiO2 -producing ion radicals. Further reaction may be connected with proton transfer and formation of benzyl and peroxyl radicals, which is followed by their linkage and formation of hydroperoxide, or with another electron and proton transfer and formation of a hydroxy benzyl cation. However, hydroperoxide can also be produced by the reaction of a benzyl alcohol radical and oxygen, in which a peroxyl radical is produced. After the peroxyl radical accepts an electron from TiO2 and is converted into the anion, it is further protonated to hydroperoxide. As a ®nal step, hydroperoxide is converted into substituted benzoic acid (Scheme 1). Taking into account the mechanism proposed, it seems that the crucial step in the reaction mechanism is the ion±radical formation, which is followed by its transformation into the hydroxy benzyl cation or radical. Since the ease of oxidation is certainly connected with oxidation potentials (Mayeda et al., 1972), cyclic voltammetry was used in order to ®nd out the impact of the type and position of the halogen atom on the aromatic ring on the change in oxidation potential. Table 3 shows the oxidation potentials at 1 mA (E1 mA ) for benzyl alcohol and selected HBAs.
Provided that the crucial step in the reaction mechanism is connected with the benzyl ion±radical, lower photodegradation rate would be expected in the case of 3-HBA than in the case of benzyl alcohol, and higher in the case of 2- and 4-HBAs. The photodegradation rate constants do not correspond with the dierences in oxidation potentials. This also indicates other possible effects which have an in¯uence on the photodegradation rate of HBAs. It has already been reported in previous studies (Izumi et al., 1981) of the oxidative decomposition of benzoic acid in oxygen containing solution on platinized TiO2 , that CO2 is the principal product, with intermediate production of a polymeric product with a carbonyl group. With reference to this, formation of benzoic acid as the primary photooxidation product can be predicted, followed by its oxidation to CO2 or by polymerisation after attack by OHá radicals on the aromatic ring. 4. Conclusion Considering the mechanism proposed, it can be concluded that photooxidation of selected benzyl alcohols is connected with the formation of a cation radical of the HBA which is converted into a benzyl radical or cation. Similar trends of photooxidation rates were obtained for 2- and 4-HBAs while exactly the reverse was detected in the case of 3-HBAs. The dierences in photodegradation rates cannot be explained by the dierences in the oxidation potential for 2-, 3-, and 4-HBAs alone. Nevertheless, the type and position of the halogen atom on the aromatic ring may also have an eect on photodegradation of the HBAs.
Acknowledgements We wish to thank the Ministry of Science and Technology, the Faculty of Chemistry and Chemical Technology and Faculty of Natural Sciences and Engineering for supporting the research. We are also grateful to Prof. B. Pihlar for essential assistance.
References Al-Ekabi, H., Serpone, N., 1988. Kinetic studies in heterogeneuos photocatalysis 1. Photocatalytic degradation of chlorinated phenols in aerated aqueous solutions over TiO2 supported on a glass matrix. J. Phys. Chem. 92, 5726±5731. Bard, A.J., 1980. Photoelectrochemistry. Science 207, 139±144. Boh, B., Musar, A., 1996. Toxic waste and Biodiversity. In: Cimerman, A., Gunde Cimerman, N. (Eds.), Biodiversity.
K.S. Wissiak et al. / Chemosphere 41 (2000) 1451±1455 Slovenian National Commission for UNESCO and National Institute of Chemistry. Frank, S.N., Bard, A.J., 1977a. Heterogeneous photocatalytic oxidation of cyanide and sul®te in aqueous solutions at semiconductor powders. J. Phys. Chem. 81, 1484±1488. Frank, S.N., Bard, A.J., 1977b. Semiconductor electrodes. 12. Photoassisted oxidations and photoelectrosynthesis at polycrystalline TiO2 electrodes. J. Am. Chem. Soc. 99, 4667±4675. Hutzinger, O., 1982. The Handbook of Environmental Chemistry. Springer, Berlin. Inoue, T., Fujishima, A., Konishi, S., Honda, K., 1979. Photoelectrocatalytic reduction of carbon dioxide in aqueous suspensions of semiconductor powders. Nature (London) 277, 637±638. Izumi, I., Fan, F.F., Bard, A.J., 1981. Heterogeneous photocatalytic decomposition of benzoic acid and adipic acid on platinized TiO2 powder. The photo-Kolbe decarboxylative route to the breakdown of the benzene ring and to the production of butane. J. Phys. Chem. 85, 218±223. Kraeutler, B., Bard, A.J., 1978a. Heterogeneous photocatalytic decomposition of saturated carboxylic acids on TiO2
1455
powder Decarboxylative route to alkanes. J. Am. Chem. Soc. 100, 5985±5992. Kraeutler, B., Bard, A.J., 1978b. Heterogeneous photocatalytic synthesis of methane from acetic acid-new Kolbe reaction pathway. J. Am. Chem. Soc. 100, 2239±2240. Mayeda, E.A., Miller, L.L., Wolf, J.F., 1972. Electrooxidation of benzylic ethers, esters, alcohols, and phenyl epoxides. J. Am. Chem. Soc. 94, 6812±6816. Mills, A., Le Hunte, S., 1997. An overview of semiconductor photocatalysis. J. Photochem. Photobiol. A: Chem. 108, 1±35. Miyake, M., Yoneyama, H., Tamura, H., 1977. The correlation between photoelectrochemical cell reactions and photocatalytic reactions on illuminated rutile. Bull. Chem. Soc. Jpn. 50, 1492±1496. Pincock, J.A., Pincock, A.L., Fox, M.A., 1985. Controlled oxidation of benzyl ethers on irradiated semiconductor powders. Tetrahedron 41, 4107±4117. Watanabe, T., Takizawa, T., Honda, K., 1977. Photocatalysis through excitation of adsorbates. 1. Highly ecient Ndeethylation of rhodamine B adsorbed to CdS. J. Phys. Chem. 81, 1845±1851.