TiO2 Photocatalytic Mechanisms in Water Purification
J. Phys. Chem. B, Vol. 101, No. 14, 1997 2657
References and Notes
2-aminobenzaldehyde. Formation of the former product by this
pathway does not involve superoxide; therefore it is not
decreased by SOD. On the other hand, it involves Q•+ and
accordingly it should be decreased at pH 3 because quinoline
is then mostly present as quinolinium ion which is a poorer
electron donor than quinoline; Table 2 shows that this expected
decrease was indeed observed. Also, comparison of the amounts
of 2-aminobenzaldehyde and its N-formyl derivative at pH 6
with and without SOD suggests that the addition of superoxide
to Q•+ (Scheme 1B) prevails over the addition of neutral
dioxygen to Q•+ (Scheme 1C) in the absence of SOD, as is
expected from the nucleophilic character of superoxide.
Finally, the hydroxylated dioxetane neutral radical shown in
Scheme 1C can also be formed in the hydroxyl radical-induced
degradation of quinoline, which might explain the presence of
2-aminobenzaldehyde among the intermediates obtained through
the photo-Fenton reaction (Figure 3). However, as already
emphasized, the hydroxyl radical preferentially adds to the
benzene ring of quinoline to yield 5- and 8-hydroxyquinoline,
as well as quinoline-5,8-dione (Table 2, Scheme 1A).
(1) Bahnemann, D.; Cunningham, J.; Fox, M. A.; Pelizzetti, E.; Pichat,
P.; Serpone, N. In Aquatic and Surface Photochemistry; Helz, G. R., Zepp,
R. G., Crosby, D. G., Eds.; Lewis: Boca Raton, FL, 1994; p 261.
(2) Jaeger, C. D.; Bard, A. J. J. Phys. Chem. 1979, 83 , 3146.
(3) Turchi, C. S.; Ollis, D. F. J. Catal. 1990, 122, 178.
(4) Cunningham, J.; Srijaranai, S. J. Photochem. Photobiol. A: Chem.
1991, 58, 361.
(5) Nada, H.; Oikawa, K.; Ohya-Nishiguchi, H.; Kamada, H. Bull.
Chem. Soc. Jpn. 1994, 67, 2031.
(6) Peterson, M. W.; Turner, J. A.; Nozik, A. J. J. Phys. Chem. 1991,
98, 6586.
(7) Brezova, V.; Stasko, A.; Biskupic, S.; Blazkowa, A.; Havlinova,
B. J. Phys. Chem. 1994, 98, 8977.
(8) Brezova, V.; Stasko, A. J. Catal. 1994, 147, 156.
(9) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341.
(10) Draper, R. B.; Fox, M. A. J. Phys. Chem. 1990, 94, 4628.
(11) Draper, R. B.; Fox, M. A. Langmuir 1990, 6, 1396.
(12) Fox, M. A.; Draper, R. B.; Dulay, M. T.; O’Shea, K. In
Photochemical ConVersion and Storage of Solar Energy; Pelizzetti, E.,
Serpone, N., Eds.; Kluwer: Dordrecht, 1991; p 323.
(13) Fox, M. A. In Photocatalytic Purification and Treatment of Water
and Air; Ollis, D. F., Al-Ekabi, H., Eds.; Elsevier: Amsterdam, 1993; p
163.
(14) Harbour, J. R.; Hair, M. L. J. Phys. Chem. 1979, 83, 652.
(15) Ceresa, E. M.; Burlamacchi, L.; Visca, H. J. Mater. Sci. 1983, 18.
(16) Lawless, D.; Serpone, N.; Meisel, D. J. Phys. Chem. 1991, 95, 5166.
(17) Mao, Y.; Scho¨neich, C.; Asmus, K.-D. J. Phys. Chem. 1991, 95,
10080.
Conclusions
(18) Mao, Y.; Scho¨neich, C.; Asmus, K.-D. J. Phys. Chem. 1992, 96,
8522.
(19) Gray, K. A.; Stafford, U. Res. Chem. Intermed. 1994, 20 (8), 835.
(20) Stafford, U.; Gray, K. A.; Kamat, P. V. J. Phys.Chem. 1994, 98,
6343.
(21) Vinodgopal, K.; Stafford, U.; Gray, K. A.; Kamat, V. J. Phys. Chem.
1994, 98, 6797.
(22) Goldstein, S.; Czapski, G.; Rabani, J. J. Phys. Chem. 1994, 98,
6586.
(23) Richard, C.; Boule, P. New J. Chem. 1994, 18, 547.
(24) Richard, C.; Boule, P. Sol. Energy Mater. Sol. Cells 1995, 38, 431.
(25) Lepore, G. P.; Langford, C. H. J. Photochem. Photobiol. A: Chem.
1993, 75, 67.
First, the present results demonstrate that quinoline is an
appropriate molecular probe to reveal the differences between
oxidation processes based on OH• radical production and the
TiO2 photocatalytic oxidative steps. Aromatic compounds with
only one ring or two identical rings cannot easily be used for
that purpose because the same products can be obtained through
different pathways. In other words, quantitative analyses of the
primary products formed by several degradation techniques are
informative only if the studied compound has a suitable
molecular structure.
(26) Howe, R. F.; Gra¨tzel, M. J. Phys. Chem. 1987, 91, 3906.
(27) Micic, O. I.; Zhang, Y.; Cromack, K. R.; Trifunac, A. D.; Thurnauer,
M. C. J. Phys. Chem. 1993, 97, 7277.
(28) Lepore, G.; Langford, C. H. Water Pollut. Res. J. Can. 1989, 24,
537.
(29) Amalric, L.; Guillard, C.; Pichat, P. Res. Chem. Intermed. 1994,
20, 579.
(30) Pichat, P.; Guillard, C.; Amalric, L.; Renard, A.-C.; Plaidy, O. Sol.
Energy Mater. Sol. Cells 1995, 38, 391.
(31) Gerischer, H.; Heller, A. J. Phys. Chem. 1991, 95, 5261.
(32) Schwitzgebel, J.; Ekerdt, J. G.; Gerischer, H.; Heller, A. J. Phys.
Chem. 1995, 99, 5633.
(33) Peral, J.; Casado, J.; Domenech, X. Electrochim. Acta 1989, 34,
1335.
(34) Tunesi, S.; Anderson, M. J. Phys. Chem. 1991, 95, 3399.
(35) Sun, Y.; Pignatello, J. J. EnViron. Sci. Technol. 1995, 29, 2065.
(36) Pelmont, J. Enzymes; Presses Universitaires de Grenoble: Grenoble,
France, 1995.
(37) Walling, C. Acc. Chem. Res. 1975, 8, 125.
(38) Eberhardt, M. K.; Raminez, G.; Ayala, E. J. Org. Chem. 1989, 54,
5922.
Second, considering that the hypothesis according to which
surface OH• radicals have not the electrophilic character
exhibited by free OH• radicals can reasonably be ruled out, the
quinoline products indicate that species other than the OH•
radicals are involved in TiO2 photocatalytic oxidative degrada-
tions in water as was initially proposed for reactions carried
out in acetonitrile.75 Together with the increasing evidence
regarding the role of reduction steps under TiO2 photosensiti-
zation, that illustrates the variety of mechanisms involved and
further enhances the interest for the use of this technique in
pollutant control.
Third, considering that several experimental facts denote that
superoxide dismutase is capable of exhibiting its expected
•-
enzymatic activity with respect to O2 anion radicals in UV-
irradiated TiO2 aqueous suspensions, it is inferred that super-
oxide is chemically involved in photocatalytic oxidative steps.
(39) Pereira, W. E.; Rostad, C. E.; Leiker, T. J.; Updegraff, D. M.;
Bennett, J. L. Appl. EnViron. Microbiol. 1988, 54, 827, and references cited
therein.
(40) Shukla, O. P. Appl. EnViron. Microbiol. 1986, 51, 1132.
(41) Aislabie, J.; Bej, A.-K.; Hurst, H.; Rothenburger, S.; Atlas, R. M.
Appl. EnViron. Microbiol. 1990, 56, 345.
(42) Miethling, R.; Hecht, V.; Deckwer, W.-D. Biotechnol. Bioeng. 1993,
42, 589.
(43) Kochany, J.; Maguire, R. J. Chemosphere 1994, 28, 1097.
(44) Faust, B. C.; Hoigne´, J. Atmos. EnViron. 1990, 24A, 79.
(45) Pignatello, J. J. EnViron. Sci. Technol. 1992, 26, 944.
(46) Haag, W. R.; Yao, C. C. D. EnViron Sci. Technol. 1922, 26, 1005.
(47) Zepp, R. G.; Faust, B. C.; Hoigne´, J. EnViron. Sci. Technol. 1992,
26, 313.
(48) Sun, Y.; Pignatello, J. J. EnViron. Sci. Technol. 1993, 27, 304.
(49) Benhelberg, J. J.; Warneck, P. J. Phys. Chem. 1995, 99, 5214.
(50) Cragoe, E. J.; Robb, C. M. Org. Synth. 1969, 40, 54.
(51) Skraup, Z.-M. Monatshefte 1882, 3, 535.
Fourth, mechanisms consistent with the pH and SOD effects
and based on the addition of either O2 or, to a lesser extent,
O2 to the quinoline radical cation, have been suggested.
•-
Fifth, inasmuch as the ionizing potential of quinoline is not
dissimilar from that of many aromatic pollutants, the occurrence
of several oxidative pathways in TiO2 photocatalysis is of
general significance. The relative importances of these path-
ways is influenced, inter alia, by the pH, in particular with
respect to the pKa of both the pollutant and superoxide, and by
the surface coverage in pollutant because of competitive hole
transfer to the pollutant or to H2O/OH- ions.
Acknowledgment. The authors gratefully acknowledge the
Galileo program (Ref. 94080) and the Universita` of Torino for
a scholarship to L.C., both of which made possible the
cooperation between the groups in France and Italy.
(52) Huntress, E. H.; Bornstein, J.; Hearon, W. M. J. Am. Chem. Soc.
1956, 78, 2225.
(53) Copini, D. Gazz. Chim. It., 1950, 80, 36.