Langmuir 1991, 7, 503-507
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Light-Induced Decomposition of Saturated Carboxylic Acids on Iron Oxide Incorporated Clay Suspended in Aqueous Solutions Hirokazu Miyoshi,? Hirotaro Mori,t and Hiroshi Yoneyama*p+ Department of Applied Chemistry, Faculty of Engineering, and Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, Yamada-oka 2-1,Suita, Osaka 565,J a p a n Received November 27, 1989.I n Final Form: August 30, 1990 Iron oxide prepared in sodium montmorillonite clay supports exhibited remarkable activities as photocatalysts for the decomposition of acetic acid, propionic acid, and n-butyric acid, being in marked contrast to cu-ferric oxide bulk powder. The activities were greatly dependent on solution pH, and the highest activity was observed at a specified pH. The charged conditions of the photocatalysts were analyzed as a function of pH, based on potentiometric titration data of photocatalyst suspensions, and adsorption of acetate was evaluated to discuss the observed pH dependence. Slight reduction of the photocatalyst surfaces was noticed during the course of the decarboxylation reactions as in the cases of other iron oxide photocatalysts.
Introduction Fairly intensive studies have been conducted on the photoelectrochemical properties of cu-Fe203'-'' as a stable photoanode. In contrast little work has been done with the use of this material as photocatalysts. The main reason for this is that most of the photocatalytic reactions studied so far concern those involving hydrogen evolution, but the flat band potential of a-Fe203 is too positive to cause hydrogen evolution from aqueous solutions,1'2a although Kiwi and GratzeP reported that colloidal a-FezO3 served as photocatalysts for evolving hydrogen from aqueous solutions. Oxygen e v ~ l u t i o n and ' ~ reduction of nitrogen to ammonial4-I6 were also reported a t a-Fe203 photocatalysts suspended in aqueous solutions. The present authors investigated previ~usly'~ the photoelectrochem-
* To whom correspondence should be addressed. Department of Applied Chemistry, Faculty of Engineering. * Research Center for Ultra-High Voltage Electron Microscopy.
f
(1)Quinn, R. K.; Nasby, R. D.; Baughman, R. J. Mater. Res. Bull. 1976,21, 1011. (2)(a) Kennedy, J. H.; Frese, W., Jr. J. Electrochem. SOC.1978,125, 723. (b) Kennedy, J. H.; Frese, W., Jr. J. Electrochem. Soc. 1978,125, 709. (3)(a) Wilhelm, S.M.; Yun, K. S.; Ballenger, L. W.; Hackerman, N. J . Electrochem. SOC.1979,126,419.(b) Yeh, Lun-Shu R.; Hackerman, N. J . Electrochem. SOC.1977,124,833.(c) Delnick, F. M.; Hackerman, N. J. Electrochem. Soc. 1979,126,732.(d) Fredlein, R. A.; Bard, A. J. J . Electrochem. SOC.1979,126,1982. (4)Gori, M.; Gruniger, H.-R.; Calzaffrri, G. J. Appl. Electrochem. 1980, 10, 345. (5)Iwanski, P.; Curran, J. S.; Gissler, W.; Memming, R. J. Electrochem. SOC.1981,128,2128. (6)Kennedy, J. H.; Anderman, M. J . Electrochem. SOC.1983,130,848. (7)Dare-Edwards,M. P.; Goodenough, J. B.; Hammett, A.; Trevellick, P. R. J . Chem. SOC.,Faraday Trans. 1 1983,79,2027. (8)Leygruf, Ch.; Hendewerk, M.; Somorjai, G. A. J . Phys. Chem. 1982, 86,4484. (9)Itoh, K.; Bockris, J. O'M. J. Electrochem. SOC. 1984,131,1266. (10)Anderman, M.; Kennedy, J. H. J . Electrochem. SOC.1984,131, 1565. (11)(a) Itoh, K.; Nakao, M.; Honda, K. J.Appl. Phys. 1985,57,5493. (b) Itoh, K.; Nakajima, Y.; Fujishima, A. Chem. Lett. 1987,2125. (12)Kiwi, J.; GrBtzel, M. J. Chem. SOC.,Faraday Trans. 1 1987,83, 1101. (13)Haupt, J.; Peretti, J.; Van Steenwinkel, R. Nouu. J . Chim. 1984, 8,633. (14)Tennakone, K.; Wickramanayake, S.; Fernando, C. A. N.; Ileperuma, 0. A.; Panchihewa, S. J . Chem. SOC.,Chem. Commun. 1987,1079. (15)Khader, M. M.; Somorjai, G. A. Langmuir 1987,3,303. (16)Khader, M. M.; Yurens, G. H.; Kim, I.-K.; Salmeron, M.; Somorjai, G. A. J . Am. Chem. SOC.1987,109,3581.
0743-7463/91/2407-0503$02.50/0
ical properties of size quantized FepO3 microcrystallites prepared on clay supports, which will be denoted in this paper as FezO3/clay. This material had a bandgap of 2.48 eV, being 0.28 eV greater than Fez03 bulk material, and possessed higher photocatalytic activity for the decomposition of acetic acid and propionic acid. In the present study, physical characterization of this material is reported employing transmission electron microscopy, and the effect of the solution pH on apparent activities for the photodecomposition of acetic acid, propionic acid, and n-butyric acid is investigated as an extension of our previous studies on the photocatalytic activity of size quantized Fe203. I t will be shown that the photocatalytic activity is greatly influenced by solution pH, and this finding will be discussed in terms of adsorption of acetate ions on surface hydroxyls of the photocatalysts.
Experimental Section Preparation of Iron Oxide Microcrystallites in Clay Support and Observation by Transmission Electron Microscope. The preparation of the FezOs/clay was accom-
plished following the published procedurela that was reported in our previous paper in detail.17 As reference to the Fen03/clay, cu-Fe203 powder was prepared by heating trinuclear iron(II1) acetate at 400 " C for 20 h in air. As alreadyreported," the specific surface area of the Fe~O3/claywas 290 m2 g-l and that of the &-Fez03was 75 m2g-l, respectively, and the basal spacing of the clay interlayers of the FezO3/clay was 0.66 nm. The FezO3/clay contained about 36 wt % FezO3. The size and form of Fez03 microcrystallites in the Fe203/claywere observed with a transmission electron microscope, TEM (Hitachi Model H-9000; acceleration 300 kV,point resolution 0.19 nm). The preparation of the samples in those cases was essentially the same as that reported previously. Sodium montmorillonite was suspended in a trinuclear iron(II1) acetate solution to exchange sodium ions with the iron salt, and the resulting colloids of 2.2 wt % were dropped on a glass plate, followed by heating at 400 O C for 20 h in air to give a Fez03/clay film of ca. 17 pm thickness. This film was stripped off and freeze-dried with liquid nitrogen, and then a cross section perpendicular to the basal plane of the clay was prepared by cutting with an ultramicrotome. Since electron bombardment of clay causes ionization damage in its silicate (17)Miyoshi, H.; Yoneyama, H. J. Chem. SOC.,Faraday Trans. 1 1989, 85,1873. (18)Yamanaka, S.;Doi, T.; Sako, S.; Hattori, M. Mater. Res. Bull. 1984,19,161.
0 1991 American Chemical Society
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Miyoshi et at.
.
Figure 1. Photograph (A) taken by TEM of a cross section perpendicular to the basal plane of clay of FezOs/clay. layersto make it amorphous,observationswere completedwithin the bombardment for not longer than ca. 60 s. Photocatalytic Experiments. A Pyrex tube of about 180 mm length and 18 mm diameter was used as a reaction cell for photocatalytic experiments. Usually 10 mg of the Fe203/clay or 3.6 mg of the a-Fe2O3powder was suspended in 10 mL of aqueous solutions containing reactants. After Ar gas was bubbled in photocatalyst suspensions for more than 2 h, the cell was closed with a septum and the suspension was illuminated with a 500-Whighpressure mercury arc lamp under magnetic stirring. Methane, ethane, propane, and carbon dioxide were determined using a gas chromatograph (Yanaco, C800-T). A molecular sieves 5A column was used for methane and a Porapack Q column for ethane, propane, and carbon dioxide. For cases of microanalysis of methane and ethane, the gas chromatograph was operated in a FID mode using the Potapack Q column. The temperature of the columns was 100 "C for all cases and Ar was used as a carrier gas. In some experiments, Ag+ was used as an electron scavenger. The amaunt of Ag deposited on the a-Fe2O3 powder and the Fe*O~/claywas determined by atomic absorption spectrometry after dissolvingthe Ag with 1mol dm+ HN03,followed by dilution with twice distilled water so as to give 3-30 ppm of Ag+. The amount of Fe*+dissolved from the photocatalysts in the course of the photocatalytic experiments was determined by absorption spectroscopy using the o-phenanthroline method.Ig Determination of Point of Zero Charge. The point of zero charge of the catalysts was determined by potentiometric titration." Different amounts, 0.1 g, 0.5 g, or 1.0 g, of oxides (Fe203/ clay or cu-Fe203) were suspended in 100 mL of a 0.01 mol dm-3 NaOH solution and then titrated with 0.013 mol dm-3 HN03 under magnetic stirring and N2 atmosphere. Changes in pH of the suspensions during the titration were measured with a TOA HM-1BA pH meter.
Results and Discussion Observation by TEM of Fe203/Clay. A photograph of Fe203/clay, taken by TEM, is shown in Figure 1. The width (a) given in the bottom of the left hand side of the picture gives the silicate layer thickness of clay, while a broad black stripe in the circle (b) shows the iron oxide layer incorporated in the interlayer spacings of clay, which is estimated to be ca. 0.66 nm, in agreement with X-ray diffraction analysisof the Fe203/clay reported previously.17 In the center of the picture, a black ellipse of 3 nm width and 12 nm length shown by an arrow is seen which is an iron oxide particle. Also seen in this picture are several large iron oxide particles having regular crystallographic (19) Snell, F. D.; Snell, C. T. Colorimetric Methods of Analysis,3rd ed.; D. Van Nostrand: New York, 1957; Vol. 2, p 316. (20) Park, C. A.; de Bruyn, P. L. J. Phys. Chem. 1962,66,967.
Concentration of acetate I mol dnii3
0.007
tr
0.07
2.05
5
0-
$ 1 =I
1
6
7
-
..:~
8
0
2ot "
4.m
10,
n.
E Xt E
3.88
2
3
4
5
PH
Figure 2. Effect of pH on the production rate of methane in photodecompositionof acetic acid. Photocatalyst: Fe203/clay (a),cu-FepO3 (A). The concentration of acetate was varied by changing pH of mixed solutions of acetate and acetic acid with fixing the total concentration to 4.1 mol dm-3. A 3.6-mgportion of cu-Fe203 (10 mg in Fe203/clay) was added to 10 mL of the solutions. Light source was a 500-Whigh-pressuremercury lamp. arrays that can be recognized by parallel line shapes in the particles. If the width covered by the parallel lines is divided by the number of the lines for the particle in the center of the picture given by an arrow, 3.62 A is obtained as a line width. This value corresponds to the face spacings of a (012) plane of a-Fe203 (3.68 A). Photodecomposition of Acetic Acid, Propionic Acid, a n d n-Butyric Acid at FepOa/Clay Catalysts. It is that the photocatalytic decomposition of carboxylic acids on Ti02 results in the corresponding alkane as the major reaction product. This was true also a t both FepOs/clay and &-Fez03catalysts, though the activity of the latter were quite low. The rate of decomposition of acetic acid to yield methane is given in Figure 2 as a function of pH. In the upper abscissa of this figure, the concentration of acetate calculated from its pKa is given. The photocatalytic activity of the Fe203/clay was greatly influenced by solution pH, and the highest activity appeared a t pH 4.8. The same was true for the photodecomposition of propionic acid and n-butyric acid, as Figures 3 and 4 show. The photodecomposition of acetic acid on Ti02 (anatase) photocatalysts in the presence of dissolved oxygen gave similar pH dependency, though the (21) (a) Kraeutler, B.; Bard, A. J. J. Am. Chem. SOC.1978, ZOO, 5985. (b) Yoneyama, H.; Takao, Y.; Tamura, H.; Bard, A. J. J. Phys. Chem. 1983,87,1417.
(22)Sakata, T.; Kawai, T.; Hashimoto, K. J. Phys. Chem. 1984,88, 2344.
Iron Oxide as Photocatalyst
Langmuir, Vol. 7,No. 3, 1991 505
Cwcentration of propiomte / moldni3
00048 0047
i2 30t
.. ... . 19
377
352
5
ci
;^ 4
01
"4"
5
.
7
/
"5'7
:A
PH
Figure 3. As in Figure 2, but for photodecomposition of propionic acid. The total concentration of propionate and propionic acid was fixed to 3.8 mol dm-3.
-'2
5
0
10
12
0 01 mol dni3 HNO3 / cm3
Figure 5. Potentiometric titration curves of the FepOs/clay suspended in 0.1 mol dm-3 NaOH solution with 0.01 mol dm-3 HN03solution. The amount of FepOa/clay was as follows: 0.1 g (A);0.5 g (v);1 g (0); none ( 0 ) .
Concentration of butyrate / mol de3 40 I
$ 30
g 5
7
I
b
3
10
3.8
20
0.006 0.06
195
367
I 4
3.88
I
...
*.
c
*. -5 3
4
5
6
7
8
PH PH
Figure 4. As in Figure 2, but for photodecomposition of n-butyric acid. The total concentration of n-butyrate and n-butyric acid was fixed to 3.9 mol dm-3. Table I. Major Reaction Products in Photo-Kolbe Reaction of 4.1 mol dm-* Acetate at pH 8.46. amt produced/pmol photocatalyst 7h 22 h FezO3/clay CHI 0.093 0.25 CZHR 0.003 u-FezO3 CH40.015 0.036 C2H6 0.019 0.037 10 mg of FenOa/clay or 3.6 mg of a-Fep03powder was suspended in 10 mL of 4.1 mol dm-3sodium acetate. (I
pH value giving the highest decomposition rate was not pH 4.8 but pH 3.4.21a The higher activity at the Fez03/ clay than a t the a-FezO3 powder was recognized even in high pH such as a t pH 8.46, as shown in Table I. According to the previous studies,21 not acetic acid molecules but acetate ions serve as the reactant. The high activity of the FezOa/clay seems to be related a t least in part to a large surface area of the photocatalyst. However, as easily recognized from the results shown in Figures 2-4, the observed activity differences cannot be explained from the surface area differences alone, because the highest activity of the Fe~O3/claywas more than 40 times greater than that of the a-FezO3. This subject will be discussed further in a later section. Titration curves obtained with different amounts of suspended oxide crossed a t about pH 4.8as shown in Figure 5; thus the point of zero charge (pzc) of the FezOs/clay was judged to be ca. pH 4.8. Similarly the pzc of the a-Fe2O3 powder was determined to be pH 7.5 from titration curves of its aqueous suspensions (not shown), the value being in good accord with literature values.23 The (23) Hesleitner, P.; Babit., D.; Kallay, N.; MatijeviC,E. Langmuir 1987, 3, 815.
Figure 6. Surface charge density of the FezOs/clay determined from Figure 5: 0.1 g (A);0.5 g (V);1 g (0). pzc of sodium montmorillonite used in the present study was determined to be pH 9.3 in our previous Consequently, the pzc of the FezOs/clay was much lower than that of the a-FezO3 powder and the clay alone. The clay usually has Na+ ions in its interlayers to compensate negative charges of silicate layers. In the preparation of FezOa/clay, the Na+ ions were replaced by positively charged trinuclear iron(II1) acetate ions, which was then decomposed to give iron oxide incorporated clay. Since the resulting iron oxide must be electrically neutral, adsorption of protons must occur on the oxide surfaces on the conversion of the iron acetate to iron oxide to keep electrical neutrality of the clay. The results obtained by the titration indicated that the protonated FezOa/clay keeps its electrical neutrality a t pH 4.8. The low pzc of the FezOs/clay compared to that of a-FezO3 seems to suggest that a high concentration of protons is needed to hold these adsorbed protons on the oxide. The surface charge density (a) of the FezOs/clay was estimated from differences in the amount of the titrant in the presence and absence of suspended oxide by using the following equationz5
where Vis the volume of suspension, m the weight of oxide, A its surface area, and (CH- COH)and (CH- CoH)init the difference in the concentration of protons and hydroxide ions in the presence and absence of oxide, respectively. Results obtained are given in Figure 6. Usually, the surface charge density of the FezOa/clay should not vary with differences in the amount of oxides titrated, but the titration of the smaller amount gave a little greater surface (24) Inoue, H.; Haga, S.; Iwakura, C.; Yoneyama, H. J. ElectroanaL Chem. Interfacial Electrochem. 1988, 249, 133. (25) Ohtani,B.; Okugawa, Y.; Nishimoto, S.;Kagiya, T. J.Phys. Chem.
1987,91, 3550.
506 Langmuir, Vol. 7, No. 3, 1991
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Miyoshi et al.
500
*
4 6
1 0
2
1
0
400
4
A
\
!
300
r
0
200
E
2
4
100
PH n
3
4
5
6
Figure 8. Products of the acetate concentration and the amount and neutral ( 0 )surface hydroxyls, and of positively charged (0) the sum of these products (A)for 1 g of FezO3/clay.
7
PH
Figure 7. Amount of negatively charged (A); neutral ( O ) ,and positively charged ( 0 )surface hydroxyls as a function of pH. The total amount of the surface hydroxyls was determined to be 505 pmol g-l. charge. This finding may have resulted from a buffer capacity of the clay which is indebted to cation-exchange capacity of the clay interlayers. It may be assumed that all the surface hydroxyls are neutral a t the pzc and that with decreasing (or increasing) pH they are charged a t the rate of 10-fold increase with a decrease (or increase) of unit pH. -H+
M-OH2+
+
-H+
MOH + M-0-
+H+
+H+
Then one can evaluate ionized and neutral surface hydroxyls as a function of pH,25as shown in Figure 7. In the construction of this figure, the amount of the total surface hydroxyls of 505 pmol g-' was used which was obtained by the extrapolation of l/r- vs plots that were made with the use of the data given in Figure 6, where ris the amount of the negatively charged hydroxyls. It seems reasonable to assume that acetate ions adsorb on both positively charged surfaces and neutral surfaces, but not on negatively charged surfaces. The amount of adsorption must be proportional to the quantity of these charges and the acetate concentration, and then the products of the concentration of acetate and the amount of positive and neutral surface hydroxyls are evaluated as shown in Figure 8. It is seen that the sum of these products shows its maximum at pH 5, being in close to the pH value where the maximum rate of decomposition of acetic acid is achieved (see Figure 2). Reaction Mechanisms of the Photodecomposition of Acetic Acid. Major reaction mechanisms for the production of methane in the photodecomposition of acetic acid are given by eqs 1-3,21,22where ht,+ and et,- are a CH,COO-
+ ht:
-
CH,
+ CO,
I J
E V
"
0
-
+ Hads
CH,
2
4
nitely produced. Two plausible explanations would be made for this. One concerns the contribution of iron oxide photocatalysts of very large quantization. According to our results on Fez03 microcrystallites of less than ca. 5 nm diameter prepared in Nafion,26a blue shift of more than 1.5 eV was observed in their absorption spectra due to the size quantization. If such small Fez03 particles were contained in the FezOs/clay, then the energy of the bottom of the conduction band edge would be high enough to allow reaction 2. The flat-band potential of the Fez03/ clay previously reported must be indebted to the greater Fez03 microcrystals whose size is the most predominant among Fez03 particles prepared on the clay support (see Figure 1). Another possibility would be the involvement of surface hydroxyls of the iron oxide microcrystallites, as given by eqs 4 and 5. Positively charged (-M-OHz+) and/or neutral M-OH2+ + CH,
+ etr-
(1)
-
+ CH, + etr--
M-OH (3)
surface-trapped hole and electron, respectively. According to the previous study,I7 the conduction band edge a t pH 0 is -0.01 V vs SCE for thk Fe~Os/clayand +0.22 V vs SCE for the a-Fe203; the valence band edge of the FezOs/clay is 2.47 V vs SCE and that of a-Fe203 2.42 V vs SCE, respectively. The potentials of the valence band edge of these materials are positive enough to oxidize acetate, while those of the conduction band edge may be insufficient to proceed to reaction 2. Nevertheless, methane was defi-
3
Figure 9. Effect of the concentration of acetic acid on the production rate of methane in the photo-Kolbe reaction. The (A). Solution pH photocatalyst was FezO3/clay ( 0 )and a-Fe~03 changed from pH 2.6 to pH 1.8 with increase in the amount of acetic acid added. A 3.6-mg portion of a-FezOa (10 mg in Fez03/ clay) was added to 10 mL of acetic acid solution.
(M-OH
CH,
1
Concentration of acetic acld /mol dd
+ H+
-
M-OH
+ CH,
M-0-+ CH,) M-OH2+
(4) (4')
(5)
surface hydroxyls (-M-OH) would be utilized for the hydrogen sources in the reaction with methyl radicals to give methane, but negatively charged surfaces (-M-0-) could not supply hydrogen atoms for the promotion of the (26) Miyoshi, H.; Tanaka, K.; Uchida, H.; Mori, H.; Sakata, T.; Yoneyama, H. J.EIectroanaL Chem. InterjacialElectrochem. 1990,295, 71.
Iron Oxide as Photocatalyst
Langmuir, Vol. 7, No. 3, 1991 507
+ A’
E
0
1 ,
0
1
2
3
4
5
Illumination time I h
Figure 10. Time course of the amount of CH4 produced and that of Fe(I1) dissolved from 10 mg of the FepOa/clay suspended Fe(I1) (0). in 4.1 mol dm-3 acetic acid at pH 4.1: CHI (0);
methane production. According to Figure 7, a large amount of these are available at pH below 5, being in agreement with results shown in Figure 2. The adsorption of acetate becomes scarce in pH below 3 because its concentration becomes low in this pH region. Nevertheless the methane production rate increased with decreasing pH from 2.5 as shown in Figure 2, suggesting that the involvement of acetic acid molecules seems likely under such high acidic conditions. Truely, it was observed that the methane production rate increased with an increase in the concentration of acetic acid, as shown in Figure 9. The carboxyl groups of acetic acid must interact with positively charged surface hydroxyls of the photocatalysts through hydrogen bonding to make the acetic acid molecules reactive. The discussion made here should be applicable to the cu-Fe203 photocatalyst. However, the activities of this material were very low compared to those of the Fe203/ clay. To account for such discrepancy, the contribution of the size quantization to raise the conduction band edge might be speculated, as already discussed above, but there seems to be another source. It is ~ell-established~7~~8 that (27) Henglein, A.; Kumar, A.; Janata, E.;Weller, H. Chem. Phys. Lett. 1986, 132, 133.
the size quantized semiconductor microcrystallites are rich in nonstoichiometry and exhibit high affinities for adsorption of foreign species. If it were the case in the FezO3/ clay too, high adsorbabilities of acetate ions and/or acetic acid molecules would be expected a t this material. Thermodynamic~~g predicts that photogenerated electrons reduce Fez03 of the FenOs/clay to Fe30r. If the reduction occurs, the resulting Fe(I1) will be dissolved in solution due to its high solubilities than Fe(III), as already observed for photocatalytic oxidation of oxalic acid at Fe00H,30 that of sulfur dioxide a t ( ~ - F e 2 0 3and , ~ ~y-ray radiolysis of a-FezO3-suspended water.32 The dissolution of Fe(I1) was truely observed in the present experiment, too, as shown in Figure 10 where the time course of the dissolution of Fe(I1) and the production of methane, obtainedat pH 4.71,are given. However, since the amount of dissolution was not great, the FezOs/clay behaved like stable photocatalysts. Illumination for 5 h did not result in any marked changes in absorption and fluorescence spectra. The dissolution of Fe(I1) was suppressed by adding 25 pmol of AgN03 to 10 mL of 4.1 mol dm-3 acetic acid solutions of pH 4.8. Ag+ ions worked as a very effective electron scavenger, as rationalized by very positive redox potential of Ag+/Ag. The methane production rate at pH 4.8 was 36 pmol h-’ in the presence of Ag+, being 2 pmol h-1 higher than that in the absence of Ag+. The increased rate may have resulted from the deposited Ag catalyzing the slowest step of decomposition of acetic acid.
Acknowledgment. TEM observation by Y. Murata a t TORAY is gratefully acknowledged. This work was supported by Grant-in-aid for Scientific Research on Priority Area No. 01603023 from the Ministry of Education, Science and Culture. (28) Kormann,C.;Bahnemann, D. W.; Hoffmann, M. R.J.Phys. Chem. 1988,92,5196. (29) Pourbaix, M. Atlas of Electrochemical Equilibra in Aqueous Solutions; Academic Press: New York, 1966; p 307. (30) Leland, J. K.; Bard, A. J. J. Phys. Chem. 1987, 91, 5076. (31) Faust, B. C.; Hoffmann, M. R.; Bahnemann, D. W. J . Phys. Chem. 1989, 93,6371. (32) Mulvaney, P.; Cooper, R.; Grieser, F.; Meisel, D. Langmuir 1988, 4,1206.
