3782
J . Phys. Chem. 1987, 91, 3782-3788
we must be on the resonance frequency. Were we not on the resonance frequency, then the approximation used in deriving eq 19 would not be valid. Secondly, our approximation still is a function of grating height. Note that we have used eq 15, which means that we are discarding higher orders of E for nonresonant plasmons. As these become large, then our approximation becomes less accurate. Nevertheless, we are happy to report that this method preserves the qualitative features of the full calculation. Figure 1 illustrates this very clearly. Here we have calculated the plasmon field strength (lA1l)for various grating heights using both our new method and the full calculation. It is clear that, for gratins up to 5 N 25 nm, the simple calculation preserves the qualitative features of the exact method. This corresponds to a value of kgE N 0.7, which is a very deep grating. The quantitative picture may also be better than appears in the graph. The calculation was done for k , = 2.95 X IO7 m-]. This is the resonance frequency for shallow gratings, but it is off resonance for deeper gratings. At 25 nm, we have observed that the resonance frequency is closer to 3.1 X IOw7 m-I. We have ignored this shift in frequency in our calculations. As mentioned previously, our result compares with the more general result of Weber.12 His is more general in that is explicitly accounts for three terms in the Rayleigh expansion rather than just one. However, at resonance, the other two terms can be
expected to contribute negligibly and hence our result should be nearly as accurate. Off resonance., of course, the other terms are significant and Weber's method is clearly preferable. Strict numerical comparison is impassible since he did his calculation using silver, with a small t2, whereas ours is for cadmium. Furthermore, his example is for a sawtooth grating while ours is for a sinusoidal grating. But it is very clear from a comparison of Figure 1 with his Table I that the same qualitative features hold, namely that the plasmon resonance intensity is underestimated. Our formalism, while not as general, is derived much more simply and is easier to use and is probably just as accurate where applicable. In a later paperls we shall apply this formalism to the problem of periodic, laser-induced, chemical vapor deposition. We should be able to predict the growth rate and maximum peak height using the above method.
Acknowledgment. D.A.J. gratefully acknowledges the invaluable assistance of Dr. Pui-Tak leung and also appreciates the help of other members of our research group. We thank a referee for bringing ref 11 and 12 to our attention. This research was supported by the Office of Naval Research and the Air Force Office of Scientific Research (AFSC), United States Air Force, under Contract F49620-86-C-0009. (15) Jelski, D. A.; George, T. F. J. Appl. Phys. 1987, 61, 2353.
Efflcient Photocatalysis of the I rreverslble One-Electron and Two-Electron Reduction of Halothane on Platinized Colloidal Titanium Dioxide in Aqueous Suspension Detlef W. Bahnemann,* Jorg Monig,*t and Rita Chapman Bereich Strahlenchemie, Hahn-Meitner Institut GmbH, DlOOO Berlin 39, West Germany (Received: October 1.5, 1986; In Final Form: February 2.5, 1987)
The irreversible one- and two-electron reductions of halothane (2-bromo-2-chloro-l,l,l-trifluoroethane) by conduction band electrons, ecB-, photogenerated in aqueous colloidal suspensions of platinized titanium dioxide (Ti02/Pt) have been investigated. Both bromide and fluoride ions, Le., the respective products of the one-electron and two-electron transfer to halothane, are formed with high quantum yields (aT = aB,-= 0.43 for photoplatinized TiO,) provided efficient hole scavengers, e.g., methanol, are present. The intermediacy of a carbon-centered radical, CF,CHCI, resulting from the initial reaction of halothane with eCB-,is strongly indicated. However, this radical is apparently well stabilized on the oxide surface and does not diffuse into the aqueous phase to an appreciable extent since typical product distributions for homogeneous free-radical reactions are not observed. Adsorption of the parent molecule halothane onto Ti02/R also appears to be an efficient process as deduced from several experimental results. Induction periods between 15 and 35 min, apparent in the concentration vs. illumination time profiles, are explained by photoinduced rearrangements of the Pt deposits resulting in better electron relay properties of the metal. Implications of the observed results on the design of suitable catalysts for synthetic organic photochemistry are discussed.
Introduction The photoelectrochemistry of small semiconductor particles, which are dispersed in solution, has been an area of very active research in the past 15 years.'-I2 The materials most thoroughly studied entail the oxides and sulfides2c.f~3c~d~1G12 of transition metals. Among them, Ti02 has received particular attention due to its notable stability against photocorrosion.1-5~9Furthermore, transparent colloidal suspensions of TiO, can be readily prepared2ad,9 and offer several advantages: (i) A high specific surface area leads to enhanced catalytic activity.I3 (ii) Both conduction band electrons (ecB-) and valence band holes (hve+), which are generated by absorption of photons with hue, 1 E, ( E g = 3.2 eV 2 band gap energy), can be used for chemical redox p r o c e s s e ~ . ~ ~ - ~ ~ 'Present address: GSF, Institut fiir Tieflagerung, D3300 Braunschweig, West Germany.
0022-3654/87/2091-3782$01.50/0
(iii) Due to negligible light scattering by the small particles ( d 5 X,bs/20),16 quantum efficiencies can be reliably measured. (1) (a) Furlong, D. N.; Wells, D.; Sasse, W. H. F. J . Phys. Chem. 1985, 89, 626. (b) Furlong, D. N.; Wells, D.; Sasse, W. H. F. J.Phys. Chem. 1985, 89,1922. (c) Furlong, D. N.; Wells, D.; Sasse, W. H. F. J.Phys. Chem. 1986, 90, 1107. (2) (a) Moser, J.; Gratzel, M. Helu. Chim. Acta 1982, 65, 1436. (b) Duonghong, D.; Borgarello, E.; Gratzel, M. J . Am. Chem. SOC.1981, 103, 4685. (c) Duonghong, D.; Ramsden, J.; Gratzel, M. J. Am. Chem. SOC.1982, 104, 2971. (d) Moser, J.; Gratzel, M. J. Am. Chem. Soc. 1983, 105, 6541. (e) Moser, J.; Gratzel, M. J . Am. Chem. SOC.1984, 106, 6557. (f) Serpone, N.; Sharma, D. K.; Jamieson, M. A.; Gratzel, M.; Ramsden, J. Chem. Phys. Lett. 1985, 115, 473. (8) Moser, J.; Gratzel, M.; Sharma, D. K.; Serpone, N. Helu. Chim. Acta 1985, 68, 1686. (h) Rothenberger, G.; Moser, J.; Gratzel, M.; Serpone, N.; Sharma, D. K. J. Am. Chem. SOC.1985,107,8054. (i) Desilvestro, J.; Gratzel, M.; Kevan, L.; Moser, J.; Augustynski, J. J.Am. Chem. SOC.1985, 107, 2988.
0 1987 American Chemical Society
Reduction of Halothane on Ti02/Pt Conventional spectroscopic techniques can be employed for mechanistic studies. Starting from the original concept to use these materials for the photosynthetic conversion of water into dihydrogen and dio~ygen,'~*''-'~ many additional chemical systems have meanwhile b x n investigated.m22 In most of these studies the strong oxidative power of hVB+ (E: = +2.8 V vs. N H E ) initialized the chemical reactions. For example, Fox and co-workers investigated the photocatalytic oxidation of alkenes and arenes,21while Ollis et ai. studied the degradation of various halogenated hydrocarbons.22 Apart from H 2 formation in aqueous solution, the chemistry which can be initiated by the reducing conduction band electron has received only limited It was shown that hole scavengers adsorbed on the semiconductor's surface are required to suppress electron-hole recombinati~n,'~+~ which is a particularly fast process.2h Also special relay compounds attached to the surface of the particles, e.g., Pt, are necessary for the dihydrogen f o r m a t i ~ n . The ~ ~ -mode ~ ~ of action of these metal deposits involves the stabilization of intermediate H' atoms as well as the catalysis of their bimolecular combination reaction forming H2.23 We wish to report here investigations of a model system which allows us to distinguish between one-electron and two-electron transfer steps in semiconductor dispersions. A particularly useful compound in this respect is 2-bromo-2-chloro-1,l ,I-trifluoroethane (CF3CH(Br)C1, halothane). Its free-radical-induced reduction has recently been investigated in homogeneous aqueous solut i ~ n . ~ * - ~Radiation ' chemical studies showed that the initial
(3) (a) Frank, S. N.; Bard, A. J. J. Phys. Chem 1977,81, 1484. (b) Ward, M. D.: White. J. R.: Bard. A. J. J. Am. Chem. Soc. 1983.105.27. (c) Becker. W. G.'; Bard,'A. J.'J.Phys. Chem. 1983, 87, 4888. (d) White, J:R.; Bard; A. J. J. Phys. Chem. 1985, 89, 1947. (4) (a) Kamat, P. V. J . Photochem. 1985, 28, 513. (b) Kamat, P. V. Langmuir, 1985, 1, 608. ( 5 ) (a) Brown, G. T.; Darwent, J. R.J . Chem. SOC.,Faraday Trans. 1 1984,80, 1631. (b) Brown, G. T.; Darwent, J. R. J . Phys. Chem. 1984,88, 4955. (c) Brown, G. T.; Darwent, J. R. J. Chem. Soc., Chem. Commun. 1985, 98. (d) Brown, G. T.; Darwent, J. R.; Fletcher, P. D. I. J . Am. Chem. SOC. 1985, 107, 6446. (6) Stramel, R. D.; Thomas, J. K. J . Colloid Interface Sci. 1986,110, 621. (7) Haupt, J.; Peretti, J.; VanSteenwinkel,R.Nouu.J . Chim. 1984,8,633. (8) Koch, U.; Fojtik, A.; Weller, H.; Henglein, A. Chem. Phys. Lett. 1985, 122, 507. (9) (a) Bahnemann, D.; Henglein, A,; Lilie, J.; Spanhel, L. J . Phys. Chem. 1984, 88, 709. (b) Bahnemann, D.; Henglein, A.; Spanhel, L. Faraday Discuss. Chem. SOC.1984, 78, 151. (10) (a) Kuczynski, J.; Thomas, J. K. Chem. Phys. Lett. 1982, 88, 445. (b) Kuczynski, J.; Thomas, J. K. J . Phys. Chem. 1983,87, 5498. ( 1 1) Albery, W. J.; Brown, G. T.; Darwent, J. R.; Saievar-Iranizad, E. J . Chem. SOC.,Faraday Trans 1 1985, 81, 1999. (12) (a) Henglein, A. Ber. Bunsenges. Phys. Chem. 1982, 86, 301. (b) Henglein, A. J. Phys. Chem. 1982, 86, 2291. (c) Alfassi, Z.; Bahnemann, D.; Henglein, A. J. Phys. Chem, 1982,86,4656. (d) Gutiirrez, M.; Henglein, A. Be?. Bunsenges. Phys. Chem. 1983,87, 474. (e) Henglein, A.; Gutitrrez, M. Ber. Bunsenges. Phys. Chem. 1983, 87, 852. (f) Henglein, A,; Gutierrez, M.; Fischer, Ch.-H. Ber. Bunsenges. Phys. Chem. 1984,88, 170. (13) Fendler, J. H. J. Phys. Chem. 1985.89, 2730. (14) Gerischer, H. J. Phys. Chem. 1984, 88, 6096. (15) Albery, W. J.; Bartlett, P. N. J . Electrochem. SOC.1984, 131, 315. (16) Ottewill, R. H.; Rastogi, M. C. Trans. Faraday SOC.1960,56, 866. (17) Gratzel, M. Acc. Chem. Res. 1981, 24, 376. (18) Somorjai, G. A,; Hendewerk, M.; Turner, J. E. Catal. Reu.-Sci. Eng. 1984, 26, 683. (19) Bockris, J. O'M.; Dandapani, B.; Cocke, D.; Ghoroghchian, J. Inr. J . Hydrogen Energy 1985, 10, 179. (20) (a) Kraeutler, B.; Bard, A. J. J. Am. Chem. SOC.1978, 100, 5985.
(b) Izumi, I.; Dunn, W. W.; Wilbourn, K. 0.;Fan, F.-R. F.; Bard, A. J. J . Phys. Chem. 1980.84, 3207. (c) Dunn, W. W.; Aikawa, Y.; Bard, A. J. J . Am. Chem. Soc. 1981, 103, 6893. (d) Izumi, I.; Fan, F.-R. F.; Bard, A. J. J. Phys. Chem. 1981, 85, 218. (21) Fox, M. A. Acc. Chem. Res. 1983, 16, 314 and references cited therein. (22) Ollis, D. F. Enuiron. Sci. Technol. 1985, 19,480 and references cited therein. (23) Henglein, A,; Lindig, B.; Westerhausen, J. J . Phys. Chem. 1981,85, 1627.
The Journal of Physical Chemistry, Vol. 91, No. 14, 1987 3783 electron transfer from a suitable one-electron reductant, e.g., the hydrated electron (ea;), leads to the formation of bromide ions and a carbon-centered radical.24 F
ens
t
H
I 1 F-C-C-Br
I I F CI
-
Br-
ii
F-C-C.
t
I
(2)
I
F
CI
In the presence of molecular oxygen this radical readily forms a peroxy radical ( k , = 1.3 X lo9 mol-' dm3 s-1)26327
F
I
H
I
F-C-C*
I I F CI
t 02
-
F
H
I
1
I
.
F-C-C-00.
(3)
I I
F
CI
which is converted into trifluoroacetate (TFA-) and other products in a complex series of subsequent reactions. Further one-electron reduction of the carbon-centered radical by appropriate compounds, e.g., the ascorbate ion at pH 2 7 , leads to the formation of fluoride ions.24 F
H
The other reaction product, 2-chloro-l,l-difluoroethane(CDFE), has also been detected and identified by GC-MS2*Even though a quantitative analysis of this compound has not been achieved, the absence of other nonionic products and results from similar two-electron reductions was taken as sufficient evidence for reaction 4.29 Thus, the detection of fluoride ions during the reduction of halothane can be conveniently used as a quantitative measure for the simultaneous formation of CDFE. Since reaction 4 is very slow (k4 5 lo5 mol-' dm3 s-'),~~it does not normally Hence, in homogeneous solution compete with the addition of 02. fluoride ions are only obtained when dioxygen is excluded.24 Bromide ion formation indicates the overall yield of halothane reduction while fluoride ions permit the quantification of twoelectron transfer processes.29 Both ionic products are stable, Le., they do not undergo further conversion, and can easily be determined by using modern analytical techniques. Experimental Section
The preparation of colloidal solutions of T i 0 2 and platinized T i 0 2 (Ti02/Pt) is described in detail e l ~ e w h e r e .In ~ principle, the following procedure was used. A clear colloidal suspension of T i 0 2 results when titanium tetraisopropoxide dissolved in 2propanol is added dropwise to a vigorously stirred 0.03 M aqueous hydrochloride solution followed by continued stirring in the dark for about 24 h. White, water-soluble powders of the colloid are obtained by rotary evaporation of the solvent. However, it should be noted that in order to obtain powders with reproducible activity, it is necessary to evaporate the solvent at temperatures not exceeding 300 K. Colloidal platinum results from reduction of 3 X lo4 mol dm-3 H2PtC1, by 1.7 X mol dm-3 sodium citrate in aqueous solution at 373 K for 1 h and subsequent removal of excess ions with an ion-exchange resin. Brown powders of Ti02/R are obtained by solvent evaporation from mixtures of the two ~
~~~
(24) Monig, J.; Krischer, K.; Asmus, K.-D. Chem.-Biol. Interacr. 1983, 45, 43. (25) Monig, J.; Asmus, K.-D. J . Chem. SOC.,Perkin Trans. 2 1984, 2057. (26) Monig, J.; Asmus, K.-D.; Schaeffer, M.; Slater, T. F.; Willson, R. L. J. Chem. SOC.,Perkin Trans 2 1983, 1133. (27) Monig, J.; Asmus, K.-D. Oxygen Radicals in Chemistry and Biology; Bors, W., Saran, M., Tait, D., Eds.; Walter de Gruyter: Berlin, FRG, 1984; p 57. (28) Asmus, K.-D.; Bahnemann, D.; Krischer, K.; Lal, M.; Monig, J. Lye Chem. Rep. 1985, 3, 1. (29) Monig, J. Ph.D. Thesis, TU Berlin (FB6) D 83, FRG, 1983, p 83.
3784 The Journal of Physical Chemistry, Vol. 91, No. 14, 1987 BEFORE IRRADIATION
ONE HOUR h v l X e , = 3 6 6 n m ) 1
(b)
Br-
I
VI
I
k
z
[
I
I
-
‘ li
.
W
a
+ I
F-
0. W
I Y
a W D.
f . ‘L u 1
2
L[
Chalolhanc?
Aex,
mol dm-3 nm 10 366 10 366 10 366 10 400 0 366 10 366
pH aBr- aF 11.5 0.068 0.008 11.5
0.054b 0.O2lb
1-3 11.5
0.002 a
11.5 a 11.5 a
a a a a
aTFA0.001 O.O0lb a
a a a
“No product formation observed, i.e., GBr- < 0.0002, 0, < 0.0010, < 0.0002. Mean values from at least five independent experi-
@TFA-
ments with three different batches of catalyst (standard deviation
increosed Y sens
4 6 8 0 2 4 CHROMATOGRAM RUN TIME/ MIN.
TABLE I: Product Yields Observed upon Illumination of Air-Saturated, Aqueous Colloidal Solutions in the Presence of 1 mol dm-3 Methanol and 0.5 g dm-’ of the Respective Catalyst
type Of colloid TiOt Ti02/Pt TiO,/Pt TiOz/Pt TiO,/Pt none
I
I
-7
i
Bahnemann et al.
*IS%). 6
@
Figure 1. Ion chromatograms from the aqueous supernatant (obtained after precipitation of the catalyst with H2S04)of an aerated solution containing 0.5 g dm-3 TiO,/Pt, 1 mol dm-’ methanol, and lo-* moles dm-’ halothane at pH 11.5; (a) before illumination; (b) after 1-h illumination (Aex = 366 nm).
colloidal solutions and contain a final concentration of 3 X mol dm-3 PtO. These powders are stable for months without changing their catalytic activity. Dissolution in water yields clear colloidal solutions with a pH of 3. These solutions contain positively charged particles and are stable without coagulation between pH 1 and pH 3 (adjusted with HCl). Fast addition of an appropriate amount of aqueous N a O H yields suspensions of negatively charged particles which do not flocculate between pH 10.2 and pH 12.0. All experiments were carried out in the absence of colloid stabilizers. Quartz cells containing 15 cm3 solution were illuminated with monochromatic light of a mercury-doped 450-W Xe lamp employing a Bausch & Lomb monochromator (bandwidth f5 nm). Infrared light was minimized by a IO-cm water filter. Actinometry mol dm-3 was carried out regularly with a solution of 5 X Aberchrome 540 in toluene,30which absorbs all incident photons. einstein The lamp output varied between 6 X 10” and 1 X L-I s-l Fluoride ion formation was determined with an ion-selective electrode. It was ascertained that the colloid does not interfere with the analysis. The minimal concentration which could reliably mol dm”. Bromide ions and tribe measured was 1 X fluoroacetate (TFA-) were quantified by means of high-performance ion chromatography using a Dionex 2010i equipped with a separation column AS4 and a conductivity d e t e ~ t o r . ~ Prior ’ to analysis the colloidal particles were precipitated from the solution by neutralization using appropriate amounts of H2S04or NaOH, respectively. The supernatant of a subsequent centrifugation at 3000 G for 10 min was taken for the analysis. The minimum concentrations thus measurable were 2 X 10” mol dm-3 for each ion. Calibrations were carried out daily using the experimental matrix containing known amounts of the ions.
Results Example chromatograms of an air-saturated solution containing 0.5 g dm-3 Ti02/Pt, 1.0 mol dm-3 methanol, and 1 X lo-* mol dm-’ halothane at pH 11.5 before and after 1-h illumination are shown in Figure 1 , a and b, respectively. The two peaks in chromatogram l a are identified as chloride, which stems from the preparation of the colloid, and sulfate, which is introduced during the analytical workup. As can be clearly seen, illumination of such solution with A, = 366 nm results in the formation of fluoride ions, trifluoroacetate ions (TFA-), and bromide ions. (30) Heller, H. G.; Langan, J. R. J . Chem. Soc., Perkin Trans. 2 1981, 341. (3 1 ) Weiss, J . Handbuch der Ionenchromatographie; VCH Verlagsgesellschaft: Basel, 1985.
I u
‘0
40 80 120 160 200 240 irrodiolion time/min
Figure 2. Fluoride concentration (measured with an ion-sensitive electrode) vs. irradiation time observed upon illumination of an aerated aqueous suspension of 0.5 g dm-’ TiOJPt, 1 mol dm-3 methanol, and lo-, mol dm-’ halothane at pH 11.5.
Photons with energies corresponding to wavelengths above 400 nm, Le., below the Ti02 band gap energy, do not induce ionic product formation (see Table I, entry 4). When either halothane or Ti02/Pt was omitted (entries 5 and 6), UV irradiation did not result in any product formation. The photolytic production of fluoride in the presence of oxygen represents a remarkable result, since F is not formed at all when reductions were carried out in air-saturated homogeneous solut i o n ~ All . ~ ~quantitative data on the F- formation incidentally stem from measurements with an ion-sensitive electrode although Figure l b illustrates that fluoride can principally be detected by ion chromatography. This seemed necessary because F- elutes within the dead time of the column.31 The peak is further perturbed by solvent ions eluting from the column due to the high ionic strength of our solutions. A detailed time dependence for the fluoride ion formation is shown in Figure 2. From the linear part of the curve, following the apparent induction period, a quantum yield of CP = 0.018 is calculated. (Experimental conditions were chosen so that 50% of the incident photons were absorbed.) Since the objective of this study was to elucidate the reductive site of the photocatalytic cycle on Ti02, efficient hole scavenging was a prereq~isite.’~ Therefore, methanol was added to the above solutions. Two other alcohols were also tested as hole scavengers (Figure 3), but in comparison methanol seems to be by far the most efficient one. Halothane is also reduced in the absence of any hVB+scavenger, indicating that electron transfer to halothane and/or hole transfer to water can prevent at least some electron-hole recombinations. All subsequent experiments were nevertheless performed in the presence of methanol to ensure highest possible reduction yields. As can be seen from Figure 4,
The Journal of Physical Chemistry, Vol. 91, No. 14, 1987
Reduction of Halothane on TiO,/Pt
8
X
-
N
2
3785
3l
2 - i
a I
l
a
&iLk
1-
0
n n
0 None
t-Butanol
2- Propanol Methanol
Figure 3. Quantum yields of ionic products observed upon illumination (Acx = 366 nm) of aerated aqueous suspensions of 1.O g dm-’ TiO,/Pt and 10-2 mol dm-’ halothane at pH 1 1.5 in the presence of various hole scavengers (calcohol= 1 mol dm-’). I
4
8
12
16
lo3 [CF3 CH Br CI ] / M
20
Figure 5. Quantum yields of ionic products observed upon illumination (Aex = 366 nm) of aerated aqueous suspensions of 0.5 g dm-3 TiO,/Pt and 1 mol dm-3 methanol at pH 11.5 as a function of the halothane concentration.
I
2.5
2.0 01
I
I
I.o
0.5
[TiO,/Pt] 05-
/ 1
I 1.5
1
/ g dm3
Figure 6. Quantum yields of ionic products observed upon illumination (Aex = 366 nm) of aerated aqueous suspensions containing 1 mol dm-’ I
TFA‘--
halothane at pH 11.5 and various catalyst concentrations. (TFA- yields are well below 0.01 and are thus omitted for graphic clarity.)
[CH3 OH] l M
Figure 4. Quantum yields of ionic products observed upon illumination (Aex = 366 nm) of aerated aqueous suspensions of 0.5 g dm-’ TiOJPt and mol dm-’ halothane at pH 11.5 as a function of the methanol concentration.
the concentration of this hole scavenger has a pronounced effect on both the total yield of halothane reduction and the relative distribution of the products. Thus, increasing the methanol concentration leads to an increase in the overall reduction yield (aBr-) and favors the two-electron transfer (aF/@Br-) at the expense of the one-electron transfer (@‘TFA-/@Br-). The dependence of the product yields on the halothane concentration is illustrated in Figure 5. An increased substrate concentration results in enhanced yields of all products, and the observed dependence resembles an adsorption isotherm. Figure 6 displays the decrease of product yields with increasing catalyst concentration. The influence of the gas atmosphere on the product distribution is shown in Figure 7. When solutions containing 0.5 g dm-3 Ti02/Pt, 1 mol dm-3 methanol, and 1 X lo-* mol dm-3 halothane at pH 1 1.5 were illuminated under nitrogen, the majority of the reductions are actually two-electron transfer steps, as is portended = 0.95. This ratio decreases by the relative product yield OF-/@.,,with increasing oxygen concentration; Le., one-electron transfer processes become relatively more important. Furthermore, the absolute reduction yield also drops with higher oxygen content = 0.081, 0.055, and as indicated by quantum efficiencies of aBr0.027 for N2-saturated, air-saturated, and oxygen-saturated solutions, respectively. As expected, hardly any TFA- is formed
-0 .?? 0.8
a > .+
2 02
n Air
02 Figure 7. Ratios of ionic products observed upon illumination (Aex = 366 nm) of aqueous suspensions containing 0.5 g dm-’ TiO,/Pt, 1 mol dm-’ methanol, and lo-* mol dm-’ halothane at pH 11.5 as a function of dioxygen content.
under a nitrogen atmosphere whereas in aerated solutions TFAgains in prominence, since more carbon-centered radicals formed in the first reduction step are intercepted by 02.The formation of small quantities of TFA- in N,-saturated solutions is attributed to surface-adsorbed molecular oxygen, which cannot be removed by purging with inert gases.32 It is surprising, however, to see that no TFA- at all was detected in oxygen-saturated solution. This result was checked several times with different batches of colloid preparations. (32) Many, A. CRC Crit. Rev. Solid State Sci. 1974, 5 1 5 .
3786 The Journal of Physical Chemistry, Vol. 91, No. 14, 1987
Bahnemann et al.
ecE- t
ii I I
F-C-c-Br F
I CII F
'-
t
The pH dependence of the product yields was also investigated. The results shown in Figures 1-7 all refer to solutions with pH 11.5. There are no pronounced differences in the pH range from 10.3 to 12.0. In acidic solutions, on the other hand, the overall reduction yields are much lower (see entry 3 in Table I). However, a more extensive study of this pH dependence is not feasible because of the rather limited stability region of colloidal TiOz (see Experimental Section). We are therefore unable to decide whether the well-known "Nernstian" behavior2' of T i 0 2 (Le., the shift of the redox potential of eCB-to more negative values by 59 mV with ApH = 1) or a competition between halothane and Hag+for the conduction band electrons is responsible for the decreased product yield. Both the presence of Pt deposits on the Ti02 colloid and their individual size affect the product yields markedly. Bare TiO, colloid is a fairly inefficient photocatalyst for fluoride formation whereas the Br- yield indicating the one-electron transfer hardly depends on the presence of the metal relay (entries 1 and 2 in Table I). Since it is known that photoplatinized Ti02 colloids (Ti02/PtPh)bear much smaller platinum deposits, such systems were prepared by UV irradiation of a solution containing 0.5 g dm-3 TiO, at pH 11.5 in the presence of I mol dm-3 methanol mol dm-3 H2PtC16for a duration of 40 min. Figure and 1 X 8 shows the time dependence of ionic product formation for such a colloid. Although the qualitative characteristics are the same as in Figure 2, it should be noted that the efficiency of Ti02/Ptp, is considerably higher, i.e., @F(Ti02/PtPh)= 0.43 as compared with @&TiO,/Pt) = 0.018, and that the induction period is much = 0.43 is calculated from shorter. Incidently, @Br-(TiOZ/PtPh) the results in Figure 8, which illustrates the remarkable efficiency of photoplatinized TiOz for the two-electron transfer to halothane even in the presence of air with @ F / @ B r - = 1 for Ti02/PtPhas compared with @ P F / @ ~ r -= 0.32 for TiO,/Pt (Figure 5) and only a very small yield of TFA-. Discussion The present study shows that halothane can be conveniently used to monitor one-electron and two-electron transfer processes occurring in colloidal aqueous suspensions of semiconductor particles. Since both pathways are irreversible and lead to the formation of different products, they can be easily quantified by measuring the respective halide ion formation. Hence, the extent of two-electron transfer is reflected by the ratio of [F-]/[Br-].
Br-
t
t F-
+
F-C-C.
(5)
I CII F
F Br-
H
I 1
CI
ii -
2eCE- t F-C-c-Br
F
H
\ C=C
F'
'
(6)
\CI
In homogeneous solution the latter reduction does not occur in the presence of molecular oxygen since. the carbon-centered radical is efficiently intercepted, yielding trifluoroacetate as one of the final products.27 Two different pathways can be envisaged for reaction 6. Either both electrons stem from the same colloidal particle, which implies a finite residence time of the intermediate radical on the surface, or, in contrast, the CF3CHC1 radical can diffuse into the bulk solution and accept the second electron from a different Ti02particle. The latter process is governed by typical homogeneous kinetics.12' High yields of bromide and fluoride are found upon illumination of aerated aqueous suspensions of Ti02/Pt with photons of an energy exceeding the band gap energy of Ti02 of 3.2 eV. The formation of fluoride in the presence of 0, clearly demonstrates that two electrons can be efficiently passed from the conduction band of one colloidal particle through the platinum relay onto a halothane molecule. Thus, the transient carboncentered radicals seemingly do not diffuse into the bulk of the solution but remain at the colloid surface. It has been shown9 that platinum deposits on semiconductor materials readily accept and store electrons, leading to a shift of the Fermi level to a more negative potential.23 In the case of colloidal Ti02 one electron-hole pair is, on average, generated in each particle every 1.6 s as calculated from the absorbed photon flux and particle c ~ n c e n t r a t i o n . Thus, ~ ~ it can be assumed that within seconds of illumination a stationary concentration of electrons on the Pt deposits is reached. Our results suggest that it is not necessary to invoke such a shift of the Fermi level in order to account for the reduction of halothane. It is known that the first one:electron reduction step of CF3CHBrCI leading to Brand CF3CHCI occurs at a more negative potential than the second step.24 However, considerable amounts of bromide ions are already formed when colloidal suspensions of bare TiO, are illuminated (see Table I, entry 1). On the other hand, the accumulation of electrons on the Pt deposits appears to be necessary for an efficient fluoride formation from halothane (cf. entries 1 and 2 in Table I). This is the expected behavior since, as will be shown below, othe: species besides molecular oxygen compete with eCB-for the CF,CHCI radical not yielding F-. The intermediacy of an interceptable carbon-centered radical is furthermore indicated by the formation of TFA- and by the decreasing [F]/[Br-] ratio with increasing oxygen concentration. The associated decrease in the total reduction yield is explained by the competition between O2and halotane for conduction band electrons34and by the known fact that superoxide, formed by the former reaction, is not able to reduce halothane in aqueous sol u t i o n ~ . ~It~ has been shown that the intermediacy of a CF3CHC102' radical quantitatively leads to the formation of chloride ions as final products.27 However, due to the high C1concentration in the colloidal matrix, analysis of this species is not possible in our system (cf. Figure la). While TFA- is the only other compound which has been positively identified when halothane was reduced in homogeneous aerated solutions, quantitative studies revealed the presence of other product^.^' As is obvious from Figure 7 , a balanced product spectrum is also not obtained when halothane is reduced in the presence of O2and the photocatalyst. We have so far not been able to identify any other reaction product, nor do we have an explanation for the unexpected (33) Spanhel, L. Diplomarbeit, TU Berlin (FB6), FRG, 1985, p 23. (34) Bahnemann, D. W.; Hoffmann, M. R.; Hong, A. P.; Kormann, C. In Chemistry of Acid Rain; Johnson, R. W., Elzerman, A. W., Gordon, G. E , Calkins, W., Eds.; American Chemical Society: Washington, D.C.; ACS Symp. Ser., in press.
The Journal of Physical Chemistry, Vol. 91, No. 14, 1987 3787
Reduction of Halothane on Ti02/Pt absence of detectable amounts of TFA- in oxygen-saturated suspensions. Further experiments, well beyond the scope of this paper, are currently under way to elucidate this intriguing result. A ratio of almost unity for the quantum yields of fluoride and bromide is observed when photoplatinized TiO, is the catalyst. Under such experimental conditions the reaction of molecular oxygen with freely diffusable CF3CHC1 radicals would prevent the fluoride ion formation e f f e ~ t i v e l y .Hence, ~~ the TFA- found in aerated solution should derive from a surface reaction, namely, between CF3CHC1 and dioxygen. Adsorption of the parent halothane molecule onto the surface of the oxide support explains two other dependencies observed in this study. First of all, the yields of all ionic products which rise steeply as the halothane concentration is increased from 1 to 5 X mol dm-3 and level off thereafter resemble a typical adsorption isotherm (Figure 5 ) . Secondly, Figure 6, which displays the significant decrease of the product yields with increasing catalyst concentration, can also be explained by adsorption phenomena, since the number of halothane molecules per catalyst particle (or per unit surface area) decreases with increasing Ti02/Pt concentration. The quantum yield calculation takes into account an augmented light absorption by a higher catalyst concentration. Therefore, the surface area is the only variable in these experiments. Quantum yields independent of the photocatalyst's concentration would be expected if adsorption of halothane was not a limiting parameter. The above discussion implies that, even at the highest halothane concentrations used in this study, a submonolayer coverage of the Ti02/Pt surface is achieved. This assumption is reasonable since the hydrophilic oxide surface should not easily accommodate nonpolar molecules. An efficient hole scavenger is necessary to obtain sizable quantum yields, since electron-hole recombination is a very fast process,2h which can even be catalyzed by the Pt deposits.9b Methanol, which can readily be oxidized, proved to be the most efficient hole scavenger tested (Figure 3). hVB++ C H 3 0 H
-
C H 2 0 H + Haq+
-
(CH3)2COH+ Hagf
(8)
but more strikingly the second electron transfer step hardly occurs at all, as is evident from the [F]/[Br-] ratio (see Figure 3). This finding can be rationalized by. an H' atom abstraction reaction from the alcohol by the CF3CHCI radical formed in the first reduction step. CF3CHC1 + C H 3 0 H CF3CHC1 + (CH3),CHOH
--
+
CF3CH2C1 C H 2 0 H
CF3CHCl
CF3CHC1 + 0,
TFA-
+ OH-surface
~ V B +
F
t
(10)
(35) Lilie, J.; Beck, G.;Henglein, A. Ber. Bunsenges. Phys. Chem. 1971,
-
+ other products
(1 2)
Appreciable bromide formation is also encountered in the absence of alcohol as a hole scavenger. Under these conditions hvB+should oxidize either halothane or OH-.9b Since direct oxidation of halothane appears to be unlikely, we propose the following reaction sequence.
CF3CH2C1+ (CH3),COH
+
-
+ CH,OH/(CH3),COH F + CF2CHCl + H" + CH20/(CH3)2CO (1 1)
seems unlikely, since it would not account for the observed dependence of the fluoride yield on the alcoholic hole scavenger. Based on the one-electron oxidation potentials of the a-hydroxy radicals, the bimolecular radical-radical reaction 11 should yield at least similar [F-]/[Br-] ratios when 2-propanol is used. Therefore, we conclude that the a-hydroxy radicals remain on the particle surface and suffer the loss of a second electron to the platinum deposit. A marked dependence of the product yields on the methanol concentration is observed (Figure 4) and can be accounted for by a different competition scheme. Photogenerated hvB+are more efficiently scavenged at higher methanol concentration, thus surpressing electron-hole recombination, which results in an increased Br- yield. Concomitantly, the [F]/[Br-] ratio rises while [TFA-]/[Br-] decreases. This suggests that the reaction of CF3CHCl with dioxygen competes with the uptake of the second electron.
(9)
It is known that in homogeneous solution containing 1 mol dm-3 of the respective alcohol reaction 10 has a half-life of 1 ms and is about 25 times faster than reaction 9.25 This H' atom abstraction competes with the uptake of a second electron by the intermediate radical. The a-hydroxyalkyl radicals formed in reactions 9 and 10 as well as by the oxidation of the alcohol by a valence band hole (reactions 7 and 8) are strong one-electron reductants. ,(E:(CH20H/CH20 + H+) = -0.98 V vs. N H E and E$((CH3)2COH/(CH3)2C0+ H+) = -1.30 V V S . N H E ) . ~In~ homogeneous solution their reaction with halothane initiated a chain reaction in the absence of 02,leading to very high yields of bromide.25 Such a process is not indicated under our experimental conditions. Taking the employed concentrations and the published rate constant; (Le., k ( C H 2 0 H 0,) = 5 X lo9 mol-' dm3 s-' and k((CH3),COH + 0,) = 4 X lo9 mol-] dm3 s-'),~~ 75, 458.
+
(7)
A corresponding result was found in previous studies on the formation of dihydrogen and explained by the hydrophilic character of methanol leading to strong adsorption on the surface of the colloidal particles.9b The overall reduction yield is diminished by a factor of 2 in the presence of 2-propanol hVB++ (CH3),CHOH
one calculates that the reactions of halothane and 0, with the a-hydroxyalkyl radicals should occur with nearly equal probability (43% and 57%, respectively, in the presence of 2-propanol) in CF3CHBrCI) = 7.6 air-saturated solution (Le., k((CH3)$OH X lo7 mol-' dm3 s-I).,~ However, the other chain-propagating reaction of the carbon-centered CF,CHCI radical with the alcohol is inhibited by dioxygen (reaction 3).25 With k9 = 27 mol-' dm3 s-l and klo = 670 mol-' dm3 s - I , ~ ~ it is evident that under typical experimental conditions reactions 9 and 10 have a probability of occurrence smaller than 0.01% and 0.2%, respectively, if compared with reaction 3. The product ratio [F]/[Br-] E 1 observed in the absence of O2 evinces the absence of. free a-hydroxyalkyl radicals in the bulk solution. A reaction between the CF3CHCl radical and an a-hydroxy radical
H
I 1 F-C-C-Br I I F CI
F
I I
F-C-6-Br
F
I
t 0 2 t H20
-+
-
OH*surface
(14) F
I . I I
H20 t F-C-C-Br
F
-
TFA-
+
CI-
(15)
CI
t Br-
+
CI other products
(16)
It is known that radicals bearing two halogen substituents at the tervalent carbon are quantitatively converted into their corresponding acids in aerated aqueous s o l ~ t i o n ; ~therefore, ' the low TFA- yield is at first sight surprising. Since half of the Br- yield results from reduction of halothane by eCB-,the [TFA-]/[Br-] ratio should exceed 0.5. Further oxidation of TFA- by highly oxidizing hvB+via a p h o t o d e c a r b o ~ y l a t i o nis~envisaged ~ ~ ~ ~ ~ ~ ~to ( 3 6 ) Adams, G.E.;Willson, R. L. Trans. Faraday SOC.1969, 65, 2981. (37) Lal, M.; MGnig, J.; Asmus, K.-D. J . Chem. SOC.,Perkin Trans. 2, submitted for publication.
3788 The Journal of Physical Chemistry, Vol. 91, No. 14, 1987 account for this loss of trifluoroacetate. A close inspection of the concentration/time dependencies (Figures 8 and 2) reveals two different induction periods of 15 and 30 min for Ti02/Ptphand Ti02/Pt, respectively. The length of these periods significantly exceeds the time necessary to attain the photostationary state for electrons on the Pt deposits. Hence, we propose a photoinduced change in the physical appearance of the metal deposit to account for this observation. In this respect it is interesting to note that the UV/vis absorption spectrum of the colloidal TiOZ/PtPhsuspensions remains unchanged during the first 15 min of illumination, while a marked absorption increase is observed in the visible part of the spectrum (1> 380 nm) a t longer irradiation times, in excellent agreement with the length of the induction period. The visible component of the absorption spectrum of platinized titanium dioxide is caused by the Pt deposit.9b (No particle coagulation which could lead to additional absorption incrementsI6 in this wavelength region is observed during these experiments.) We envisage a light-induced rearrangement in more finely dispersed Pt aggregates on the Ti02 surface to explain these apparent changes. The active reduction catalyst is thus formed during the initial stages of UV irradiation. An induction period of 35 min is observed in our third system when an aerated aqueous suspension of 0.5 g dm-3 Ti02 is illuminated in the presence of mol dm-3 H2PtCI, (other conditions are 1 mol dm-3 methanol, IOw2 mol dm-3 halothane, pH 11.5) before fluoride and bromide are formed with quantum yields of 0.2 and 0.4, respectively. While this result supports the proposed mechanism, its detailed analysis is rather obscured by the initial occurrence of several parallel reactions competing for the eCB-,e.g., the stepwise reduction of PtC12- leading to Pto, the reduction of halothane (reactions 5 and 6), and the dioxygen reduction. Since it seems highly unlikely that the proposed surface rearrangement of the platinum relay should be a unique property of the investigated system, it is suggested to inspect data for the photocatalytic formation of dihydrogen (and possibly other reduction processes) on TiO,/Pt in search of similar effects. Conclusion
We report here the Occurrence of an irreversible two-electron (38) Yoneyama, H.; Takao, Y.; Tamura, H.; Bard, A. J. J . Phys. Chem. 1983, 87, 1417.
Bahnemann et al. transfer from the conduction band of Ti02/Pt particles onto an organic molecule. Moser and Gratze12d studied the reversible two-electron reduction of various substituted methylviologens. Using laser flash photolysis, they were able to demonstrate that, depending on experimental conditions, both the initial and the second electron transfer step can be rate limiting. Even though we could not differentiate between the two possibilities, we unambigously proved that in our system an intermediate radical is formed which does not diffuse into the bulk solution to any appreciable extent. Since relatively mild conditions are sufficient to catalyze the overall process F
H
I I F-C-C-Er I
I
I
F
CI
t CH30H
TiOZlPt 7 F-
t ET t
t
we envisage that such systems could be employed to obtain substituted alkenes. The general applicability of the catalyst is suggested by studies using other halogenated ethanes, e.g., F,CCC13and F2C(Cl)C(F)C12. This novel procedure may sometimes be superior to existing synthetic pathways since catalysts with very high quantum efficiencies can be designed. In addition, a high specificity was observed in the present study especially when photoplatinized T i 0 2 was employed as the catalyst and when oxygen was excluded. This indicates that the formation of undesired side products can be minimized by chosing appropriate experimental conditions.
Acknowledgment. The authors express their sincere gratitude to Prof. Klaus-Dieter Asmus (Hahn-Meitner Institut, Berlin), who initiated this project and supported it throughout its entire duration. We greatly appreciated the continuous encouragement and scientific criticism by him and by Prof. Arnim Henglein (Hahn-Meitner Institut). The skillful assistance of Mrs. Martha Weller during part of the laboratory studies is gratefuly acknowledged. We also thank Dr. Andri M. Braun (EPFL Lausaune) for stimulating discussions.
