Langmuir 1999, 15, 1141-1146
1141
Free-Radical Chain Reactions Involving Hydrogen and Bromine Atom Transfer Induced by TiO2-Mediated Photocatalysis Shlomo Gershuni, Norbert Itzhak, and Joseph Rabani* Department of Physical Chemistry and the Farkas Center, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Received July 6, 1998. In Final Form: November 18, 1998 Bromoform and cyclohexyl bromide are produced at high yields upon room-temperature UV photolysis of TiO2 powder suspended in deaerated tetrabromomethane dissolved in cyclohexane or cyclohexane-CCl4 mixtures. No reaction takes place in the absence of TiO2. Cyclohexyl chloride is produced in the presence of CCl4 when no CBr4 is added. The CBr4-toluene-TiO2 system behaves similarly, producing high yields of bromoform and benzyl bromide. UV photolysis of toluene solutions containing CBr4 in the absence of TiO2 also leads to efficient formation of the same products. The chain reaction mechanisms of the photocatalytic processes are discussed.
Introduction Poly(halomethanes) are known to halogenate saturated hydrocarbons and side chains of alkylaromatic compounds either under nonradical Gif conditions1 (FeCl3‚6H2O, picolinic acid, H2O2, in pyridine/acetic acid) or by a radical reaction where the radicals are initiated in various ways including dissociation of peroxides,2 photolysis,3 reactions of transition-metal complexes,4 or by heat.5 In the present manuscript we report for the first time TiO2-induced photochemical halogenation of hydrocarbons (cyclohexane and toluene) by poly(halomethane) in a radical-chain reaction. Unlike most other works concerning reactions sensitized by TiO2,6 our experiments were carried out in an organic medium in the absence of oxygen or any other substance which traps the photogenerated electron except the reactants. We observed high quantum yields of bromination and chlorination up to high conversions. The results provide information on the reaction mechanism. Experimental Section Materials. TiO2 was generously donated by Degussa (P25, average diameter 25 nm) and by Sachtleben Chemie GmbH (Hombikat UV100, average diameter 5 nm). Both materials are highly agglomerated (0.1 and 1.0 µm aggregates, respectively). Cyclohexane, toluene, tetrachloromethane, chloroform, and other solvents were commercial samples of analytical grade and were used without further purification. Tetrabromomethane was purchased from Aldrich and recrystallized from ethanol-water prior to use. The solvents have usually been dried over MgSO4, or sodium metal, although small amounts of dissolved water did not affect the results. Analyses. Analyses were performed on a Hewlett-Packard 5890 series ii gas chromatograph equipped with a TC detector, a HP-1 methyl silicone 5 m × 0.53 mm column, and a HP 3396 * To whom correspondence is addressed. Fax: 972 2 658 6925. Telephone: 972 2 658 5292. E-mail:
[email protected]. (1) Barton, D. H. R.; Csuhai, E.; Doller, D. Tetrahedron Lett. 1992, 33 (24), 3413. (2) Huyser, E. S. Free-Radical Chain Reaction; Wiley-Interscience: New York, 1970; pp 126-129. (3) Huyser, E. S. J. Am. Chem. Soc. 1960, 82, (2), 391. (4) Davis, R.; Durrant, J. L. A.; Rowland, C. C. J. Organomet. Chem. 1986, 316, 147. (5) Hunter, W. H.; Edgar, D. E. J. Am. Chem. Soc. 1932, 54, 2025. (6) Baciocchi, E.; Rol, C.; Sebastiani, G. V.; Taglieri, L. J. Org. Chem. 1994, 59, 5272 and references therein.
integrator. A sample of the crude reaction mixture was injected to the column, and after injection, the column was heated to 45° for 3 min and then heated to 190° at a rate of 10 °C/min. The products were identified by comparison to authentic samples. GC-MS analyses were performed using a Hewlett-Packard 5989A mass spectrometer. UV and visible spectra were recorded on a UVICON 860 spectrophotometer. Photolysis. All irradiations were carried out with either an Osram XBO or Hamamatsu HPK 150 W high-pressure xenon lamp. The light was filtered through a 0.5 cm quartz cell containing either H2O or a dilute colloidal solution of TiO2 in water. One or more cutoff filters were used in order to minimize visible light (Corning No. 5840 and/or No. 5970 filters were used). The combination of filters used produced an optical window in the range 325-375 nm. Reduction of the excitation light was achieved by a 1 cm cell containing 4:7 or 2:7 w/w Ni(ClO4)2 in water or by a colloidal TiO2 solution (2 g/L). Although visible light is not absorbed by TiO2, its elimination was necessary for meaningful actinometry. The entire irradiation system was covered with an aluminum foil in order to minimize energy losses because of light scattering by the TiO2 particles. The Al foil had a small opening, just enough for the entrance of the excitation light. It increased the rate of product buildup by 35% and protected the sample from illumination by the ordinary light in the room. Product analysis was carried out by gas chromatography. In a typical reaction 0.05 M CBr4 in cyclohexane or toluene, with or without added CCl4, or a CCl4-cyclohexane mixture (no CBr4) was used. The solution (4 mL) was placed in a cylindrical Pyrex vial (total volume ca. 10 mL) containing a magnetic stirring bar. A total of 40 mg of TiO2 powder was added, and the vial was sealed by a rubber stopper and an aluminum seal. The reaction mixture was then bubbled with argon gas for about 10 min. Corrections have been applied for loss of CCl4 and cyclohexane. The vial was irradiated for several hours while the TiO2 was suspended in the solution by vigorous magnetic stirring. The intensity of the exciting light was measured by potassium ferrioxalate actinometry.7
Results Typical results showing the accumulation of cyclohexyl bromide upon illumination of cyclohexane solutions containing CBr4 are presented in Figure 1. Accumulation of products is linear with illumination time up to at least (7) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London, Ser. A 1956, 235, 518.
10.1021/la980807t CCC: $18.00 © 1999 American Chemical Society Published on Web 01/27/1999
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Figure 1. Accumulation of cyclohexyl bromide. Initial conditions: 4 cm3 solution containing 40 mg of TiO2 (UV100) and 0.05 M CBr4 in air-free cyclohexane. Light intensity 7.1 × 10-8 einstein L-1 s-1.
3 h at all intensities used (1.3 × 10-8-3.3 × 10-6 einstein L-1 s-1). Deviations from linearity are observed above 33% conversion of the CBr4. Yields are about 50% higher with UV100 TiO2 than with the P25 product. This difference may be related to the smaller nanocrystallite’s size of the former type TiO2. The sole products which can be quantitatively determined during the first 24 h of illumination are bromoform and cyclohexyl bromide. Minor amounts of dibromoethane and tetrabromoethylene as well as traces of other (unidentified) products, the amounts of which were too low for quantitative analysis, have also been detected by GC-MS after prolonged illumination (>24 h). Thus, the predominant overall photolytic reaction is represented by eq 1. The concentrations of cyclohexyl hν,TiO2
C6H12 + CBr4 98 C6H11Br + CHBr3
I
L
Figure 2. Effect of light intensity on the buildup of cyclohexyl bromide. Initial conditions: 4 cm3 solution containing 40 mg of TiO2 (UV100) and 0.05 M CBr4 in air-free cyclohexane.
(1)
bromide and bromoform show a similar dependency on light intensity, although the quantum yields of bromoform are systematically 5-20% lower than the corresponding yields of cyclohexyl bromide. This difference between the yields of cyclohexyl bromide and bromoform may arise from destruction of the latter. Indeed, formation of cyclohexyl bromide is also observed when CBr4 is replaced by CHBr3. The results of Figure 1 represent a quantum yield of φ ) 24. Such a high yield is undoubtedly due to a chain reaction. This is further confirmed by the linear dependency of the concentration of cyclohexyl bromide on the square root of the light intensity, I1/2 (Figure 2). The effect of the concentration of cyclohexane on the quantum yield of cyclohexyl bromide is shown in Figure 3. The linear increase in yield indicates that the chain propagation ratedetermining step involves reaction of cyclohexane. Note that the variation of the cyclohexane concentration has been achieved by dilution with CCl4, keeping the total volume constant at 4 cm3. At the highest cyclohexane concentration used, no CCl4 was added. In the absence of CBr4, the cyclohexane-CCl4-TiO2 system produces cyclohexyl chloride instead of cyclohexyl bromide (see the next section). The yield of cyclohexyl chloride is less than 20% of the corresponding yield of cyclohexyl bromide which is measured under the conditions of Figure 3 in the absence of CCl4. When CBr4 is added to a solution containing both cyclohexane and CCl4, the formation of cyclohexyl chloride
Figure 3. Yield of cyclohexyl bromide as a function of cyclohexane concentration. Initial [CBr4] ) 0.2 M, 40 mg of TiO2. Total volume of the suspension 4 cm3 (Total volumes of cyclohexane and CCl4). Light intensity 7.5 × 10-8 einstein L-1 s-1. Time of illumination 0-4 h.
is completely suppressed (ratios [CCl4]/[CBr4] up to 20 have been tested). The yields of both cyclohexyl bromide and bromoform are independent of [CBr4] in the range 0.025-0.25 M. This observation confirms that the ratedetermining step in the propagation process involves cyclohexane and not CBr4. The amount of TiO2 (9-250 mg added to 4 cm3 liquid) has no effect on the quantum yields of the products. Cyclohexane-CCl4 System. The initial buildup of cyclohexyl chloride is linear with illumination time (Figure 4). Addition of either cyclohexyl chloride or chloroform (which are the expected initial stable products of reaction) has only a minor effect, if any, on the buildup. Unlike the CBr4 systems, where the rate of cyclohexyl bromide formation is directly proportional to I1/2, cyclohexyl chloride buildup is hardly affected by light intensity in the range
Free-Radical Chain Reactions Involving H and Br Transfer
Figure 4. Accumulation of cyclohexyl chloride as a function of illumination time. Initial conditions: 4 cm3 solution containing 40 mg of TiO2 (UV100) and 0.05 M CCl4 in air-free cyclohexane. Light intensity 1.2 × 10-7 einstein L-1 s-1.
Langmuir, Vol. 15, No. 4, 1999 1143
I
L
Figure 6. Effect of light intensity on the formation of benzyl bromide. Initial conditions: 4 cm3 solution containing 40 mg of TiO2 (UV100), 3.75 M toluene, and 0.05 M CBr4 in air-free CCl4. Light intensity 1.3 × 10-8 einstein L-1 s-1. Initial conditions: 4 cm3 solution containing 40 mg of TiO2 (UV100), 2.5 M toluene, and 0.05 M CBr4 in air-free CCl4.
products also in the absence of TiO2. This is attributed to direct excitation of toluene by the UV light. The thermal effect has been corrected by subtracting the results of control experiments. No corrections were required for the direct photolysis of toluene, although it has been found to proceed with a relatively high yield. This is because, under our conditions, almost all of the light which could excite toluene was absorbed by the inner filter effect of TiO2. As in the case of cyclohexane, the amount of TiO2 (9-250 mg added to 4 cm3 liquid) has no effect on the quantum yields of the products. Yields are about 50% higher with UV100 TiO2 than with the P25 product. This difference may be related to the nanocrystallite’s size, although other factors could induce the different behavior of these two different types of TiO2. Figure 5. Accumulation of benzyl bromide.
5 × 10-8-7 × 10-7 einstein L-1 s-1. This observation may be related to the relatively lower reactivity of CCl4 toward the reducing species which initiate the chain reaction and will be discussed later. The observation of a chain reaction involving CCl4, however, is in agreement with previous studies in homogeneous solutions2,3,8-10 and is evident from the high quantum yields (φ ) 8 is observed at the lowest intensity). Toluene-CBr4. Benzyl bromide is produced instead of cyclohexyl bromide upon replacement of the cyclohexane by toluene. The effects of illumination time and I1/2, respectively, on the accumulation of products are shown in Figures 5 and 6. Bromoform is obtained with a quantum yield comparable to that of benzyl bromide. In contrast to the cyclohexane systems, the toluene shows a significant dark reaction amounting to 2-7% of the product yields. The toluene systems produce the same (8) West, J. P.; Schmerling, L. J. Am. Chem. Soc. 1950, 72, 3525. (9) Katz, M. G.; Baruch, G. G.; Rajbenbach, L. Int. J. Chem. Kinet. 1976, 8, 599. (10) Alfassi, Z. B.; Feldman, L. Int. J. Chem. Kinet. 1980, 12, 379.
Discussion The primary step in the reaction mechanism is the photogeneration of pairs of electrons and holes in the TiO2 particles (reaction 2). Trapping of the electrons (reaction 3) and holes (reaction 4) occurs within less than 30 ps.11-16 hν
TiO2 98 hVB+ + eCB-
(2)
eCB- f etr-
(3)
hVB+ f htr+
(4)
Efficient second-order decay has been attributed to fast electron-hole recombination at high (pulsed laser) light (11) Howe, R. F.; Graetzel, M. J. Phys. Chem. 1985, 89, 4495. (12) Rothenberger, G.; Moser, J.; Graetzel, M.; Serpone, N.; Sharma, D. K. J. Am. Chem. Soc. 1985, 107, 8054 (13) Serpone, N.; Lawless, D.; Khairutdinov, R.; Pelizzetti, E. J. Phys. Chem. 1995, 99, 16655. (14) Nozik, A. J.; Memming, R. J. Phys. Chem. 1996, 100, 13061. (15) Skinner, D. E.; Colombo, D. P.; Cavaleri, J. J.; Bowman, R. M. J. Phys. Chem. 1995, 99, 7853 (16) Colombo, D. P.; Bowman, R. M. J. Phys. Chem. 1996, 100, 18445.
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intensities (reactions 5-7).12,13,17 Even under steady-state
eCB- + hVB+ f TiO2
(5)
etr-
f TiO2
(6)
etr- + htr+ f TiO2
(7)
+
hVB+
illumination, when an average of less than one electronhole pair is produced per nanocrystallite, electron-hole recombination is effective, although there is evidence that electron and hole transfer to scavengers may successfully compete with mobile electron-hole recombination.16,18,19 In water suspension, the trapped holes have been assumed to be a surface-bound OH• radical, produced by oxidation of the surface water or OH- ions (reaction 8a or 8b, respectively).20-36 The yield of adsorbed hydroxy
hVB+ + TiIV-O2--TiIVOH2 f {TiIV-O2--TiIV}OH• + H+ (8a) hVB+
IV
2-
IV
-
+ Ti -O -Ti OH f •
{Ti -O -Ti }OH (8b) IV
2-
IV
radicals has been determined for Aldrich TiO2 powder as 0.04.37 In view of the different behavior of different preparations of TiO2,13,38,39 it is questionable whether quantitative data obtained in water suspensions are relevant to the organic media used in the present work. However, we have found that small amounts of dissolved water had no effect on the results (compared to the use of dried organic solvents). It is conceivable that adsorbed water molecules which are not removed by the hydrophobic media are oxidized by the holes and produce adsorbed OH• radicals despite the organic environment. This conclusion is supported by the lack of heating effect on the photocatalytic activity of titania at 500 °C, which is in contrast to the largely modified catalytic properties observed upon heating to 1000 °C.40 The heating effect has been attributed to loss of surface hydroxyls, which (17) Colombo, D. P.; Bowman, R. M. J. Phys. Chem. 1995, 99, 11752. (18) Kamat, P. V. Langmuir 1985, 1, 608. (19) Bahnemann, D.; Henglein, A.; Spanhel, L. Faraday Discuss. Chem. Soc. 1984, 78, 151. (20) Fujihira, M.; Satoh, Y.; Osa, T. Nature 1981, 293, 206. (21) Ollis, D. F.; Hsiao, C.-Y.; Budiman, L.; Lee, C.-L. J. Catal. 1984, 88, 89. (22) Al-Ekabi, H.; Serpone, N.; Pelizzetti, E.; Minero, C.; Fox, M. A.; Draper, R. B. Langmuir 1989, 5, 250. (23) Turchi, C. S.; Ollis, D. F. J. Catal. 1990, 122, 178. (24) Okamoto, K.; Yamamoto, Y.; Tanaka, H.; Tanaka, M.; Itaya, A. Bull. Chem. Soc. Jpn. 1985, 58, 2015. (25) Minero, C.; Aliberti, C.; Pelizzetti, E.; Terzian, R.; Serpone, N. Langmuir 1991, 7, 928. (26) Milles, G.; Hoffman, M. R. Environ. Sci. Technol. 1993, 27, 1681. (27) Matthews, R. W. J. Chem. Soc., Chem. Commun. 1983, 177. (28) Matthews, R. W. J. Chem. Soc., Faraday Trans. 1 1984, 80, 457. (29) Wei, T.-Y.; Wan, C. J. Photochem. Photobiol. A 1992, 69, 241. (30) Mills, A.; Morris, S.; Davies, R. J. Photochem. Photobiol. A 1993, 70, 183. (31) Draper, R. B.; Fox, M. A. Langmuir 1990, 6, 1396. (32) Richard, C. J. Photochem. Photobiol. A 1993, 72, 179. (33) Cunningham, J.; Srijarana, S. J. Photochem. Photobiol. A 1988, 43, 329. (34) Jaeger, C. D.; Bard, A. J. J. Phys. Chem. 1979, 83, 3146. (35) Lawless, D.; Serpone, N.; Meisel, D. J. Phys. Chem. 1991, 95, 5166. (36) Goldstein, S.; Czapski, G.; Rabani, J. J. Phys. Chem. 1994, 98, 6586. (37) Sun, L.; Bolton, J. R. J. Phys. Chem. 1996, 100, 4127. (38) Martin, S. T.; Herrmann, H.; Hoffmann, M. R. J. Chem. Soc., Faraday Trans. 1994, 90, 3323. (39) Martin, S. T.; Herrmann, H.; Choi, W.; Hoffmann, M. R. J. Chem. Soc., Faraday Trans. 1994, 90, 3315. (40) Minero, C.; Maurino, V.; Pelizzetti, E. Langmuir 1995, 11, 4440.
takes place only at the higher temperature,41 implying strong bonding of the hydroxyl groups to the TiO2 surface. Under our conditions, the yields of scavengeable electrons and holes are probably of the same order as those observed in water suspensions, although in addition to the different TiO2, we have also used very high concentrations of the organic scavengers which may decrease the scavenging yields if the produced transients recombine with electrons very quickly.42 The chain reaction mechanism (in terms of the cyclohexane system) is proposed to include reactions 9-15.
{TiIV-O2--TiIV}OH• + C12H12 f TiIV-O2--TiIVOH2 + C12H11• (9) e-(TiO2) + CBr4 f CBr3• + Br-
(10)
C12H11• + CBr4 f CBr3• + C12H11Br
(11)
CBr3• + C12H12 f CHBr3 + C12H11•
(12)
2CBr3• f C2Br6
(13)
2C12H11•
f C24H22
CBr3• + C12H11• f C12H11CBr3
(14) (15)
Analogous reactions take place in the toluene system. Oxidation of Cl- and Br- by TiO2 holes, following reduction of CCl4 or CBr4, respectively, may also take place under our conditions.43 The Br•/Cl• radicals are expected to initiate the same chain process by abstraction of hydrogen from cyclohexane. This possibility does not affect our analysis of the results because in either case, whether cyclohexane reacts directly with OH• or indirectly via the halogen atom intermediate, the same C12H11• radical transient is obtained. The observation that cyclohexyl bromide and bromoform are the major products, with no observable amounts of the dimerization products of reactions 13-15 implies that the quantum yield of initiation (and, hence, the quantum yield of termination) is much less than unity. This is not surprising in view of the usually observed very small quantum yields in TiO2 photocatalysis. In fact, none of the products on the right-hand sides of eqs 13-15 have been identified by GC-MS, although tetrabromoethylene might have been produced (verified by control experiments) from decomposition of hexabromoethane during analysis. It is well-known that reducing additives in aqueous solutions compete more effectively with the TiO2 electronhole recombination compared to electron scavengers. This is related to the large positive redox potential of holes and OH• radicals as opposed to the mild driving force for reduction by TiO2 electrons. It is therefore reasonable to assume that reaction 9 is faster than reaction 10, and its competition with electron-hole recombination determines the rate of initiation. Although these features have been observed in water solutions, the reaction of adsorbed OH• radicals with cyclohexane (reaction 9) may still be relatively much faster than reaction 10: while hydrogen abstraction by OH• radicals produces uncharged radicals, (41) Sclafani, A.; Palmisano, L.; Schiavello, M. J. Phys. Chem. 1990, 94, 829. (42) Rabani, J.; Yamashita, K.; Ushida, K.; Stark, J.; Kira, A. J. Phys. Chem. 1998, 102, 1689. (43) Minero, C.; Maurino, V.; Calza, P.; Pelizzetti, E. New J. Chem. 1997, 21, 841.
Free-Radical Chain Reactions Involving H and Br Transfer
electron scavenging by CBr4 is expected to produce Brions. An activation barrier may be introduced by the organic environment. Note that the presence of the organic medium may inhibit the oxidation of water to the OH• radical, because this process is accompanied by formation of a proton. However, the driving force for oxidation is considerably larger than that for reduction. The excess proton which is produced by the oxidation of a water molecule at the TiO2 surface remains bound to a nearby (unreacted) water molecule and is neutralized by a bromide ion which is subsequently produced by reaction 10. In any case, material balance requires that both reactions 9 and 10 initiate the chain; otherwise, buildup of unreacted electrons would have been observed. (Such electrons are readily noticed by their blue color.) Of the two propagation reactions, reaction 12 must be rate-determining. This conclusion is based on the effect of cyclohexane concentration on the quantum yield of cyclohexyl bromide as opposed to the lack of any effect of CBr4 concentration. Reaction 12 is first-order in cyclohexane, and indeed Figure 3 shows that the chain length increases linearly with its concentration. Consequently, under our conditions k12[C12H12] , k11[CBr4]. The rate of propagation is therefore determined by eq 16.
d[C12H11Br]/dt ) d[CHBr3]/dt ) k12[CBr3•][C12H12] (16) Note that reactions 9 and 10 take place at the TiO2 surface. The propagation reactions 11 and 12 may take place at or near the TiO2 surface, or in the bulk, depending on the rates of reaction relative to the rates of desorption and diffusion of the reactants. However, since reaction 12 is rate-determining and there is always a high concentration of excess cyclohexane (or toluene), the half-life time of the propagation (and, hence, the determination of k12) is not expected to depend on the volume distribution of CBr3• radicals. In view of this it is useful to discuss the chain reactions in terms of homogeneous rate constants. Moreover, as will be discussed later, the termination reactions take place in the bulk. The steady-state concentration of the CBr3• radical must be much higher than that of the C12H11• radical. This conclusion is based on the assumption that reactions 1315 have comparable rate constants, with all three reactions being diffusion controlled. In this case, the more slowly reacting chain carrier radical, CBr3•, builds up to a higher steady-state concentration compared with the other chain carrier radical, namely, C12H11•. Thus, the predominant termination reaction is eq 13. On the basis of this it is possible to obtain the kinetic equations 17 and 18, and by their addition eq 19 is obtained.
d[CBr3•]/dt ) 0 ≈ I + k11[C12H11•][CBr4] k12[CBr3•][C12H12] - 2k13[CBr3•]2 k15[CBr3•][C12H11•] (17) d[C12H11•]/dt ) 0 ≈ I - k11[C12H11•][CBr4] + k12[CBr3•][C12H12] - k13[CBr3•][C12H11•] - k14[RH•]2 (18) I ) k13[CBr3•]2 + k14[C12H11•]2 + k15[CBr3•][C12H11•] (19) Neglecting the second and third terms on the righthand side of eq 19 and inserting the resulting expression for [CBr3•]s.s in eq 16 gives the rate equation 20.
Langmuir, Vol. 15, No. 4, 1999 1145
d[C12H11Br]/dt ) d[CHBr3]/dt ) {k12k13-1/2[C12H12]}xI (20) At constant light intensity, the accumulation of cyclohexyl bromide per unit time is expected to be linear with I1/2, as we indeed observed. The slopes of Figures 2 and 6 are (1.0 ( 0.2) × 10-2 and (9 ( 1.5) × 10-4 mol L-1/2 s-1/2 einstein-1/2, respectively. Under the conditions of Figure 2, the cyclohexane concentration was 9.3 M; hence, k12k13-1/2 ) (1.1 ( 0.2) × 10-3 mol-1/2 L1/2 s-1/2. The slope of Figure 3 is (3.2 ( 0.5) × 10-7 s-1, yielding k12k13-1/2 ) (1.2 ( 0.2) × 10-3 mol-1/2 L1/2 s-1/2, in good agreement with the results of Figure 2. The corresponding value for toluene is k12k13-1/2 ) (3.5 ( 0.5) × 10-4 mol-1/2 L1/2 s-1/2. Because the predominant termination reaction is presumably the same in both systems, the difference reflects a slower rate for hydrogen abstraction by the CBr3• radical from toluene, compared to cyclohexane. Assuming that reaction 13 is diffusion-controlled, namely, k13 ) 109 M-1 s-1 yields k12 ) 35 M-1 s-1 for cyclohexane and 11 M-1 s-1 for toluene. The diameter of the primary particle of P25 is approximately 20-30 nm; that of Hombikat is only 5 nm. Both materials are highly agglomerated (0.1 and 1.0 µm aggregates, respectively). Taking 0.4 nm for the TiO2 molecular diameter, one can determine that each nanocrystallite contains an average of ≈2000 molecules (Hombikat) or ≈250 000 (P25). This means that the average concentration of the Hombikat TiO2 nanocrystallites, with which almost all experiments were carried out, is ≈6 × 10-5 M. Because the light intensity ranged from 2 × 10-8 to 6 × 10-7 einstein L-1 s-1, this means that, even at the highest intensity employed, a nanocrystallite absorbed on the average 1 photon/min, and given the low quantum yield of initiation, it can be concluded that under most conditions a nanocrystallite could absorb only a small number of photons during the entire illumination period. It can be concluded that one pair of CBr3• radicals may be initially produced at the surface of a light-activated nanocrystallite. This conclusion is based on the slower rate of reaction 12 as compared to that of reaction 11. Upon propagation of the chain, CBr3• and C12H11• alternate. However, as discussed above, the contribution of C12H11• to the termination process can be neglected in view of its relatively fast reaction 11. It is reasonable to assume that a single pair of CBr3• radicals randomly produced at or near the TiO2 nanocrystallite surface has a considerably higher probability to escape to the bulk than to combine at the surface. Moreover, on the basis of the derived values of k12 and with use of typical diffusion coefficients, it may be concluded that diffusion to the bulk may successfully compete even with the rate of propagation, so that the major fraction of the chain reaction is in the bulk. Note that this model implies that electron and hole migration within the agglomerates is slow relative to the scavenging reactions. This is apparently the case in the CBr4 systems. The reaction mechanism in the presence of CCl4 and absence of CBr4 is expected to be similar, with the major products being cyclohexyl (or benzyl) chloride and chloroform instead of the bromine products. The observation that the quantum yield of cyclohexyl chloride is nearly inversely proportional to the light intensity (namely, the concentration produced per unit time is nearly independent of intensity) indicates that CCl4 reacts only slowly with e-(TiO2). Unreacted electrons accumulate in the same agglomerate to levels which open efficient channels for recombination with holes.
1146 Langmuir, Vol. 15, No. 4, 1999
When both CCl4 and CBr4 are present, only bromine products are observed. This is expected because CBr4 successfully competes with CCl4 for e-(TiO2), as suggested above. If the chain is long, the minor amounts of chloroform which may be produced at the expense of bromoform will not be noticeable.
Gershuni et al.
Acknowledgment. This work has been sponsored by MOS (Israel) and KFA-BEO (Germany). We are grateful to Dr. Detlef Bahnemann for useful discussions. LA980807T
