J. Phys. Chem. 1988, 92, 5773-5777 due to U,. This can be accomplished only by reducing the volume of the rim relative to the volume of the central core, which occurs when the micelles grow. The concentration of CsPFO within the rim (where nearly all CsPFO molecules reside) will thereby increase, as required by the initial assumption that Urn < UoD. When the micelles are sufficiently large and the composition of the rim is heavily CsPFO and the composition of the core CsPFD, further substitution of the shorter CsPFO molecule would result in a decrease in micelle size, as observed experimentally. Again it is desirable for the additional CsPFO to partition to the rim, both from a flexing standpoint and because of the pair interaction. For this to occur, however, the rim to core volume ratio must now increase to accommodate the additional CsPFO, which is accomplished by smaller micelles which are greater in number. Note, by the way, that the earlier developments for A Ax,, and Aq assumed a uniform distribution of CsPFO and CsPFD, which is slightly at variance with the conclusion drawn above. A partitioning of the surfactants would thus alter the quantitative results slightly, although not significantly. It should be kept in mind that the foregoing represents only one possible scenario (in which temperature and entropy are not even considered), and it is beyond the scope of this paper to develop a complete theoretical model. The nonmonotonic behavior can, in principle, be tested by using other methods as well. Hendrikx et al. have performed an elegant
5773
neutron scattering experiment using a contrast variation techn i q ~ e . They ~ ~ not only were able to determine the micellar dimensions but also observed the segregation of a nonionic alcohol and ionic surfactant within the micelle. If one could appropriately label the fluorocarbon surfactant (which may be quite difficult), such a measurement would be useful in testing the above hypothesis as well. In summary, although the magnetic birefringence technique can yield precise size information for nearly single component micelles, its interpretation is clouded for fully mixed systems. Nevertheless, qualitative results indicate that along the N I transition line a mixture of two different length surfactants results in substantially larger oblate micelles than occurs with only one species or the other present. A partitioning argument was presented to explain this behavior. Acknowledgment. This work was supported by the National Science Foundation Solid State Chemistry Program under grant DMR-8796354. The Francis Bitter National Magnet Laboratory is supported by the National Science Foundation under cooperative agreement DMR-8511789. (24) Hendrikx, Y.; Charvolin, J.; Rawiso, 1984, 100, 597.
M.J . Colloid Interface Sci.
Effect of Surface-Adsorbed 2-Propanol on the Photocatalytic Reduction of Silver and/or Nltrate Ions in Acidic Ti02 Suspension Bunsho Ohtani,* Masaya Kakimoto, Hiroshi Miyadzu, Sei-ichi Nishimoto, and Tsutomu Kagiya Department of Hydrocarbon Chemistry, Faculty of Engineering, Kyoto University, Sakyo- ku, Kyoto 606, Japan (Received: December 16, 1987; In Final Form: March 22, 1988)
The effect of 2-propanol addition on the photocatalytic reaction (Ae,, > 300 nm)of anatase TiOz (Merck) suspended in an acidified aqueous solution of silver nitrate or silver sulfate was investigated at room temperature under Ar. Deposition of Ag onto TiOz together with the formation of acetone, but not 02,occurred even under the acidic conditions. The amount of Ag+ adsorbed on the Ti02 surface in the dark, which was not affected by the addition of 2-propanol, had no influence on the rate of acetone formation. The initial rate of the acetone formation, as well as that of the Ag deposition, followed a formal Langmuir equation as a function of the 2-propanol concentration; the rate was proportional to the amount of surface-adsorbed 2-propanol as a hole scavenger. In the absence of Ag' in the suspension, nitrate anion was reduced to ammonia, together with acetone formation. The photodeposited Ag metal on TiOz enhanced the photocatalytic production of ammonia.
Introduction The mechanism of photocatalytic reaction by semiconductor particles suspended in aqueous or nonaqueous solutions has been described as the formation of a conduction-band electron and a valence-band positive hole, followed by the surface reaction of these active species with substrates to be reduced and oxidized, respectively.' (semiconductor) + hv e-(conduction band) h+(valence band) (1)
-
+
Intermediacy of hydrogen atoms adsorbed on the noble metal islands deposited on the semiconductor surface,z and hydroxyl radicals3 as well, is presumed in the photocatalytic reaction by ( 1 ) For a review see: Bard, A. J. J . Phys. Chem. 1982, 86, 172. (2) Baba, R.; Nakabayashi, S.;Fujishima, A.; Honda, K. J . Phys. Chem. 1985, 89, 1902. (3) (a) Jaeger, C. D.; Bard, A. J. J. Phys. Chem. 1979, 83, 3146. (b) Nishimoto, S.;Ohtani, B.; Kagiya, T. J. Chem. Soc., Faraduy Trans. 1 1985, 81,2467. (c) Nishimoto, S.;Ohtani, B.;Shirai, H.; Kagiya, T. J . Chem. Soc., Perkin Trans. 2 1986, 661.
0022-3654/88/2092-5773$01.50/0
-
suspended titanium dioxide (Ti02)
+ H+ h+ + OHe-
-
'H
(2)
*OH
(3)
Successive dark reactions induced by the photoirradiation have been investigated in many photocatalytic reaction^.^ The overall kinetics of these photocatalytic reactions seems to depend considerably on the primary step, especially on the charge separation of electron-hole pairs; prevention of recombination of the pairs is indispensable for the improvement of efficiency. In the photoelectrochemical cell, which consists of photoirradiated semiconductor and dark counter electrodes immersed in an aqueous electrolyte solution, the efficient charge separation is attributable to the space charge layer formed in the doped sem(4) (a) Nishimoto, S.; Ohtani, B.;Yoshikawa, T.; Kagiya, T. J. Am. Chem. SOC.1983, 105, 7180. (b) Ohtani, B.; Osaki, H.; Nishimoto, S.;Kagiya, T. J. Am. Chem. SOC.1986,108, 308. (c) Ohtani, B.; Osaki, H.; Nishimoto, S.; Kagiya, T. Tetrahedron Lett. 1986,27, 2019. (d) Nishimoto, S.;Ohtani, B.; Shirai, H.; Adzuma, S.; Kagiya, T. Polym. Commun. 1985, 26, 292.
0 1988 American Chemical Society
5774 The Journal of Physical Chemistry, Vol. 92, No. 20, 1988 iconductor being in contact with an electrolyte solution. In fact, quite high quantum efficiency could be obtained in such a photoelectrochemical cell.5 Although the depth of the space charge layer depends on the impurity density, it has been proposed to be in the range of 0.1-1000 pm for Ti02. In the particulate system, the effect of such a space charge layer on the separation of photogenerated charges seems to be minimal, because the size of each particle is less than that of the layer. Therefore, chemical reactions that prevent the recombination should be the determinants for the stimulation of photocatalytic reaction by semiconductor particles.' In recent publi~ations,6*~ we have described the pH-dependent photocatalytic activity of T i 0 2 suspended in aqueous silver nitrate solutions. The amount of silver ion (Ag') adsorbed on the TiOz surface predominantly determined the reaction rate; the pH dependence of reaction rate was interpreted in terms of the pHdependent Ag' adsorption. This means that the overall reaction rate depends only on the chemical step with photogenerated electrons, irrespective of the hole reaction to produce molecular oxygen (02).This paper describes the photocatalytic reaction of T i 0 2 suspended in acidified aqueous solution of silver salts in the presence of 2-propanol as a hole scavenger. In contrast to the previous ~ y s t e mthe , ~ rate mainly depended on the chemical step with photogenerated holes. Further, we have found that the nitrate anion can be reduced to ammonia (NH3) by the electrons, with the aid of hole scavenging by the surface-adsorbed 2-propanol. The mechanism of these photocatalytic reactions is discussed.
Experimental Section Anatase TiOz powder was supplied from Merck and used without further activation. The detailed characterization of this T i 0 2 has been described elsewhereS6 To a 5.00-cm3 portion of an aqueous solution of silver nitrate (AgNOJ or silver sulfate (Ag2S04)(Ag': 0.0100-0.0500 mol dm-3) was added the finely ground Ti02powder (50 or 250 mg). The pH of the suspension was adjusted by adding 0.100 mol dm-3 NaOH or H N 0 3 (in the case of AgZSO4solution, HzS04was used as acid). A HitachiHoriba M-8s pH meter and a double-junction-type reference electrode were used for the adjustment.' The suspension in a glass tube (18 mm in diameter and 180 mm in length, transparent to light of wavelength >300 nm) was purged by bubbling Ar for >30 min and sealed off with a rubber stopper. 2-Propanol was injected through the cap by a microsyringe. Photoirradiation was performed with a 400-W high-pressure mercury arc at 297 f 2 K under vigorous magnetic stirring. After the irradiation, the catalyst was centrifuged, washed repeatedly with distilled water, and dried overnight at 353 K. The photodeposited Ag was dissolved with concentrated H N 0 3 and measured by atomic absorption spectroscopy (Shimadzu AA 780). Acetone in the solution phase was analyzed with a Shimadzu GC 6A gas chromatograph equipped with a PEG 20M column (20% on Celite 545,60-80 mesh; 3 mm in diameter and 3 m in length) and an FID. Amounts of nitrite anion (NO;)8 and NH39were determined by spectroscopic measurements on a Shimadzu UV-ZOOS spectrophotometer. Results and Discussion Characteristics of Photocatalytic Reaction of Acidic Silver Salt Solution. As reported in the previous papers,6q7 the predominant reaction occurring in the silver salt solutions in the presence of suspended TiOz was deposition of Ag metal onto TiO, and concomitant liberation of O2 in the gas phase. 4Ag'
+ 2H20
-
4Ag
+ 0 2 + 4H'
(4)
(5) Wrighton, M. S.; Wolczanski, P. T.; Ellis, A. B. J. Solid State Chem. 1977, 22, 17. (6) Nishimoto, S.; Ohtani, B.; Kajiwara, H.; Kagiya, T. J . Chem. SOC., Faraday Trans. 1 1983, 79, 2685. (7) Ohtani, B.; Okugawa, Y . ;Nishimoto, S.; Kagiya, T. J . Phys. Chem. 1987, 91, 3550. ( 8 ) Weiss, K. G.; Boltz, D. F. Anal. Chim. Acta, 1971, 55, 77. (9) Morita, Y.;Kogure, Y . Nippon Kagaku Zasshi 1963, 84, 816.
Ohtani et al.
2oorF-l
0 '
10
20
30
50
40
Irradiation/h
50 -
$L 00
12.5
25
Acetone/ pmo I
Figure 2. Linear relation between the yields of acetone and Ag deposit. Anatase TiO, (250 mg) was suspended in an aquouas Ag2S04or AgN03 solution (Ag': 0.0100 mol dm-3, 5.00 cm3), which was acidified with concentrated sulfuric acid and nitric acid, respectively, and irradiated for 5-120 min.
Figure 1 is a representative time dependence of this photocatalytic reaction. The reaction rate at a given time of irradiation was almost proportional to the amount of Ag' adsorbed on the T i 0 2 surface.' As the photocatalytic reaction proceeds, the pH of the unbuffered reaction mixture (Le., in the absence of counter anion of weak acid, such as F)decreased according to eq 4 and therefore the amount of adsorbed Ag' d e c r e a ~ e d . ~This , ~ leads to the deactivation of Ti02 in the course of the photoirradiation, as depicted in Figure la. Since addition of an alkali solution activated the O2 evolution (Figure lb),6 the quenched photoreaction is due to the pH effect but not to the inner filter effect by the deposited Ag. Addition of 2-propanol to the deactivated reaction mixture led to the redeposition of Ag metal on T i 0 2 together with the formation of acetone but not 02.6 The photocatalysis of T i 0 2 was indicated from the facts that the same reaction could be observed
The Journal of Physical Chemistry, Vol. 92, No. 20, 1988 5115
Photocatalytic Reduction of Ag Ions
100
i-
Y 0 ' 1
;
5 PH
+
d Irradiation/min
Figure 3. Amount of Ag+ adsorbed on anatase Ti02 (5.00g) surface as a function of pH of the suspension (0.0100 mol dm-3 AgNO' with 0.09 mol dm-3 KNOB, 100 cm'), in the absence (0)and presence (0) of 2-propanol (0.25 mol dm-').
Figure 5. Time dependence of the photocatalyticacetone production by anatase Ti02 (250 mg) suspended in AgNO, (W) and Ag2S04(0)solutions (Ag+: 0.0100 mol dm-', 5.00 cm') in the presence of 2-propanol (6 mmol). Dashed line shows 100% depletion of Ag+. TABLE I: Constants for Langmuir Equations Obtained in the Ti02 Photocatalytic Reactions co- Ti02/ K/dm3 s/pmol adsorbate pH product mg mol-' m i d ref 2-propanol ca. 2 Ag 250 1.8 1.4 this work 440 46' 7 0 2 250 Ag+ 4 620 65" 7 0 2 250 Ag+ 5 7 250 1060 82' 0 2 Ag+ 6 87 0.06C*e 3b 2-propanol H2 SOb ca. 6 0.05c*c 3c tert-butyl SOb 44 ca. 14d H2
alcohol
'Ag deposition rate. bTi02-(5 wt %)Pt. Irradiation conditions NaOH. were different from the present experiments. d l mol H2 evolution rate.
5 1
,
OO
0.5
,
1
I
C / mol dm-3 Figure 4. (a) Yields of Ag deposit (0) and acetone (0)obtained by 10-minirradiation, and (b) ratio of 2-propanol concentration (C) with the rate of photocatalytic acetone production (racotono), as a function of C. The photocatalytic reaction was carried out in the aqueous Ag2S04 solution acidified with H2SO4.
in the preacidified aqueous solution of silver nitrate and 2-propanol and that negligible amounts of Ag and acetone were obtained by the photoirradiation in the absence of Ti02. As Figure 2 shows the molar ratio of Ag deposit to acetone satisfies the following stoichiometry 2Ag' + (CH3)2CHOH 2Ag (CH3)2C=O + 2H' (5)
-
+
Thus, Ag deposition linked with the acetone formation occurs in such an acidified suspension. The deactivation of T i 0 2 suspension without 2-propanol is attributed to the disappearance of adsorbed Ag'.' In a separate experiment (Figure 3), we have confirmed that the 2-propanol addition has a negligible effect on the Ag' adsorption; the amount of adsorbed Ag' was almost zero even in the presence of 2propanol under acidic conditions (pH ca.2). Consequently, under such conditions the reaction rate would be determined by the oxidation step to produce acetone from 2-propanol. Rate of Photocatalytic Reaction as a Function of 2-Propanol Concentration. Figure 4a shows the dependence of product yields on the concentration of 2-propanol (C) in the Ti02/AgzS04 (pH ca. 2)/2-propanol system. The stoichiometry of eq 5 was also confirmed in this experiment. Because the acetone yield was, as shown in Figure 5 , apparently proportional to the photoirradiation period during the initial 10-min irradiation for C = 1.2 mol drn-', the initial rate of the photocatalytic acetone production (ramtone) was calculated from the data shown in Figure 4a on the assumption that the yield in each suspension is proportional to the irradiation period. Figure 4b is a plot of C/ram,,,, as a function of C. From
the linear relation, a formal Langmuir equation between r,,,, (which should be half that of Ag deposition (rAg))and C could be derived as follows
where s is the limiting rate of photocatalytic acetone production, mol min-' and K is the adsorption constant, 1.8 mol-' 1.4 X dm3. These constants are collected in Table I together with the previously reported values for Ag+, 2-propan01,~~ and tert-butyl alcohol.k The value of s markedly depends on the photoirradiation conditions, especially on the photon flux, while K is independent of the photoirradiation conditions except the reaction temperature. In the rough estimation, the value of s for the present system was one order of magnitude smaller than that for the reaction in the silver nitrate solution in the absence of 2-propanol. The K value was considerably smaller than that for Ag+ adsorption3bon the same TiOz powder suspended in an acidic solution. This indicates the weak adsorption ability of 2-propanol. In fact, the amount of adsorbed 2-propanol under conditions similar to the photocatalytic reaction was within the experimental error limit (+a. 5 kmol g-'). Furthermore, the K value for 2-propanol under the present acidified conditions was smaller than that at pH ca. 6. Similar to the Ag+ adsorption (see Table I),' the adsorption of 2-propanol onto the protonated TiOz surface may be reduced. As described above, the time dependence of the photocatalytic reaction was linear during the initial 10-min irradiation (see Figure 5 ) . Noticeable is the fact that the linear increase of the product yield was observed as long as Ag+ remained in the Ag2S04system (further increase of the acetone yield in the A g N 0 3 system is discussed in the following section). It follows that the rate of photocatalytic reaction is independent of the concentration of Ag' in the aqueous solution and/or on the T i 0 2 surface. On the basis of these results, the mechanism of photocatalytic reaction is presumed as follows. (i) The photogenerated electron-hole pair cannot be utilized to produce Ag deposit and O2in the absence of 2-propanol under the conditions of the present reaction, in which the amount of
5776 The Journal of Physical Chemistry, Vol. 92, No. 20, I988
-
Ohtani et al.
surface-adsorbed Ag+ is negligible (e--h+)
TiO, (no chemical reaction)
(7)
At present, we have no evidence for the "electron-hole pair" as a close existence of the geminate negative and positive charges such as an "exciton". The term electron-hole pair is used as the tentative species having short lifetime. (ii) 2-Propanol adsorbed on the TiOz surface reacts with photogenerated electron-hole pairs to yield unpaired electrons, which have markedly longer lifetime than the electron paired with hole and react steadily during the lifetime.
-+
'/2((CH3)2CHOH)ads + (e--h+) !/z(CH3)2C=0
H+ + e-(unpaired) (8)
(iii) Reaction of the unpaired electrons with Ag' occurs to produce Ag deposit. Although the amount of Ag+ adsorbed on the surface is rather small under the acidic conditions, the overall rate is independent of the Ag+ amount. e-(unpaired)
+ Ag+
-
Ag
+ keCe + khch)
(10)
where 6 is quantum efficiency (
