504
J . Phys. Chem. 1990, 94, 504-506
Drifford,19 who considered the influence of the finite diffusivity Dionof the cc-ion and counterion on micellar diffusivity D,. @ion is computed by use of eq 9, for a given ion radius.) The corrected micellar diffusivity, DmC,is given by Dmc = Dm11 - DmS(0)z2/(DmS(O)z2 + Dim201
(18)
where I is the solution ionic strength. We calculated values for DmCusing a small-ion Rh of 1.5 8, for C1- and Na+, which yields the best fit to the data, Figure 3. The fit is not very sensitive to the choice of small-ion radius. The H N C approximation for g(r) gives good agreement with experiment for both the structure factor S(Q) and diffusivity D, for strongly repulsive ionic micelles in low salt. This work extends the region of effectiveness of this approach, as compared to earlier studies of Neal et a1.I6 for BSA and Cantu et aL2 for gangliosides. (19)Belloni, L.;Drifford, M.; Turq, P. J . Phys. Lett. 1985,46, L-207. Belloni, L.; Drifford, M. J . Phys., Lett. 1985,46, L-1183.
The main difference between these systems and CTACl micelles is that the latter exhibit stronger repulsive interactions because of larger volume fractions 4 and fractional ionization a. The outlined approach provides a simple method for obtaining reliable estimates of micellar fractional ionization over a wide concentration range. This method provides a powerful tool for exploring interparticle interactions in many colloidal systems. Previous studies of polyelectrolytes showed that electrostatic repulsions yield long-range correlations, leading to liquidlike structures. Self-assembling micelles are appealing candidates for these studies, in that the physical properties of these “model” particles (i.e., size, shape, and charge) can be modulated through changes in surfactant/ counterion structures.
Acknowledgment. We gratefully acknowledge the assistance of Dr. David Cannel1 in the implementation of H N C calculations, as well as financial support from the “Fundacion del Amo” and the National Science Foundation (Chemical Dynamics Program).
Visble-Light- Induced Two-Electron-Transfer Photoreductions on CdS: Effects of Morphology Tsutomu Shiragami, Chyongjin Pac, and Shozo Yanagida* Chemical Process Engineering, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan (Received: September 14, 1989)
Freshly prepared CdS suspensions (CdS-0) consisting of quantized particles and their l o w aggregation catalyze photoreductions of aromatic ketones and olefins in methanol under visible light irradiation using triethylamine as sacrificial electron donor, yielding alcohols and dihydro compounds, respectively,which are more selective than photocatalysis of commercially available crystalline CdS (Aldrich) (CdS-Ald). Deuterium incorporation experiments in photolysis of dimethyl maleate in methanol-OD revealed that CdS-0 catalyzes sequential two-electron-transfer photoreduction, affording dideuterated dimethyl succinate, while CdS-Ald induces both photoreduction and photoisomerization through disproportionation between one-electrontransfer-reduction intermediates, yielding much trideuterated dimethyl succinate and monodeuterated dimethyl fumarate and maleate.
Introduction Photochemical multielectron transfer in semiconductor particulate systems has been of much interest in view of solar energy conversion,] organic synthesis,” and artificial C 0 2 fmation.8 We (1)(a) Bard, A. J. J . Phys. Chem. 1982,86,172. (b) Gritzel, M. Energy Resources through Photochemistry and Catalysis; Academic Press: New York, 1983. (c) Kalyanasuadram, K.;Grgtzel, M. Photochem. Photobiol. 1984,40, 807. (2)(a) Fox, M. A. Arc. Chem. Res. 1983,16, 314. (b) Sakata, T. In Homogeneous ana‘ Heterogeneous Photocatalysis; Pelizzetti,E., Serpone, N., Eds.; Reidel: Dordrecht, 1986;p 397. (c) AI-Ekabi, H.; de Mayo, P. J. Org. Chem. 1987,52,4756.(d) Harada, H.; Sakata, T.; Ueda, T. J . Am. Chem. SOC.1985,107, 1773. (e) Harada, H.; Ueda, T.; Sakata, T. J. Phys. Chem. 1989,93, 1542. (3) (a) Ward, M. D.; White, J. R.; Bard, A. J. J . Am. Chem. SOC.1983, 105,27. (b) Brown, G.T.; Darwent, J. R. J . Phys. Chem. 1984,88,4955. (4)(a) Bahnemann, D.; Henglein, A.; Lilie, J.; Spanhel, L. J . Phys. Chem. 1984, 88, 709. (b) Bahnemann, D.; Henglein, A.; Spanhel, L. Furuduy Discuss. Chem. SOC.1984,78, 151. ( 5 ) (a) Nishimoto, S.; Ohotani, B.; Yoshikawa, T.; Kagiya, T. J . Am. Chem. Soc. 1983, 105, 7180. (b) Baba, R.;Nakayama, S.; Fujishima, A.; Honda, K. J . Am. Chem. Soc. 1987,109,2273. (c) Bahnemann, D.;Monig, J.; Chapman, R. J. Phys. Chem. 1988,92,3476. (6)Yanagida, S.; Ishimaru, Y.; Miyake, Y.; Shiragami, T.; Pac, C; Hashimoto, K.; Sakata, T. J . Phys. Chem. 1989,93, 2576. (7)Cuendet, P.;GrPtzel, M . J . Phys. Chem. 1988,92,3476. (8)(a) Halmann, M.Nature (London) 1978,275, 115-1 16. (b) Inoue, T.;Fujishima, A.; Koishi, S.; Honda, K. Nature (London) 1979,277,637.
have been interested in photocatalysis of freshly prepared semiconductors, since quantum size effect would be expected from their loose aggregations with colloidal morphology. In fact, it has recently been elucidated that freshly prepared ZnS7 and colloidal Ti02*can catalyze two-electron-transfer photoreductions of organic substrates. On the other hand, we recently reported that
0022-3654/90/2094-0504$02.50/00 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 2, I990 505
Letters
TABLE I: CdS-Catalvzed Photoreduction of Ketones and Olefins with TEA in MeOH” yield/%(
compd 1ad lb
-Ell,Id/Vb 1.17 1.32
IC
Id le If
4ae 4bc 4c
4d
1.55 1.56 1.56 1.60 1.56 1.75 1.73
convn/% CdS-0 CdS-Ald 100 98 98 100 97 52 100 100 70 68
3
2
100 95 98 100 98
IO
CdS-0
CdS-Ald
CdS-0
95 70 90 88 33 15
90 15 4 28 3 0
0 12 4
5
CdS-Ald
CdS-0
CdS-Ald
70 60 47 41
20 10 42 26
0 75 94 65 66
11
26 40
trace
100 100 98 95
“Irradiated in MeOH at 400 nm for 6 h. bPolarographichalf-wave reduction potentials vs SCE in MeOH. CYieldsare based on the substrate converted. Unknown products were detected. dIrradiated for 3 h. CIrradiatedfor 2 h. TABLE 11: Deuterium Isotopic Distribution in the Recovered Dimethyl Maleate and the Reduction from CdS-Catalyzed Photoreactionsof Dimethyl Maleate in MeOD
yield/%” CdS CdS-0 CdS-Aldb CdS-Ald‘
4a-do 98 24 44
4a-d, 2 27 50
4a-d2 49 6
4b-d0 2 6
4bdl 96 92
4b-d2
Sa-do 8
Sa-d, 20
2 2
5a-d2
5a-d3
72 20
71
9
“Determined by GC-MS analysis. bAnalyzedwhen the conversion of substrate is 50%. cAnalyzed when the conversion of 4a is 20%, where only the photoisomerization occurred (see Figure I). some commercially available CdS crystallites catalyze photoreduction of aromatic ketones to alcohols and/or pinacols by triethylamine (TEA) used as a sacrificial electron donor under visible light irradiation in a~etonitrile.~Here we report that a CdS suspension (CdS-0) prepared freshly from methanolic Cd(C104)2 and Na2S solutions efficiently photocatalyzes selective two-electron reductions of aromatic ketones and electron-deficient olefins to alcohols and dihydro compound by TEA, respectively (Scheme 1).
Experimental Section Highly pure (99.999%) CdS powder was obtained from Aldrich (CdS-Ald). The crystal form was hexagonal, and the average particle size was 1.5 pm. According to the method of Brus et a1.,I0 CdS-0 suspension was prepared in situ under an argon atmosphere by mixing equal amounts of methanolic solution (5 X 10” M) of Cd(C10& and NazS under magnetic stirring and cooling with ice and water. A stirred methanol suspension (2 mL) containing CdS powder (70 pmol) or CdS-0 (5 pmol), a ketone (2 mM) or an olefin ( 1 0 mM),and TEA (1 M) was flushed with argon gas and then irradiated with a 500-Wtungsten-halogen lamp through an aqueous sodium nitrite solution filter (>400nm). The reaction was followed by GLC and HPLC. Result and Discussion The UV absorption spectrum of CdS-0, which was measured after diluting to a tenth of the suspension, was similar to the reported one of a suspension with particle size of about 5.4 nm,Io suggesting that it should consist of very small particles (quantized CdS) and their loose aggregation. Table I shows the results for photoreductions of some aromatic ketones and electron-deficient olefins. Interestingly, compared with CdS-Ald, CdS-0 efficiently catalyzed photoreductions of ketones la-f to alcohols 2a-f and of electron-deficient olefins 4a-d to dihydro compounds 5a-d. Products other than pinacols and alcohols were not detected in their photoreductions. Pinacols 3a-f increased with an increase of reduction potential of the ketones. In the case of photoreductions of electron-deficient olefins, GC-mass spectroscopy demonstrated the formation of unidentified products. Hydrogen (9) Shiragami, f.; Pac, C.; Yanagida, S.J . Chem. Soc., Chem. Commun.
1989, 831. (10) Rossetti, R.; Hull, 82, 5 5 2 .
R.; Gibson, J. M.; Brus, L.E.J . Chem. Phys. 1985,
(*/a)
100
50
0
CdS-AId
I\ 30
60
90
I20
Figure 1. Time-conversion plots for photoreduction of dimethyl maleate catalyzed (a) by CdS-0; (b) by CdS-Ald. -0-, disappearance of dimethyl maleate; Q,isomerization to dimethyl fumarate;-A-, reduction to dimethyl succinate.
evolution was not observed during the photoreductions by both CdS catalysts because of the absence of HzO. It was also found that efficient photoreduction requires a higher concentration of TEA than 0.5 M, which might prevent photocorrosion and consumption of the catalyst. In addition, CdS-0 showed high activity even after repeated use (less than three times), and then the activity was lowered slightly possibly due to partial photocorrosion. However, no appreciable change in size of the cluster was observed as far as we know from the onset of the absorption spectrum of the photolysate. Figure 1 shows time-conversion plots for the photoreduction of dimethyl maleate (4a) catalyzed by CdS-Ald and by CdS-0. The CdS-Ald-catalyzed photoreduction was accompanied by the rapid cis-trans photoisomerization of 4a to dimethyl fumarate (&), with dimethyl succinate (5a). In contrast, CdS-0 catalyzed the more efficient and selective two-electron photoreduction to 5a with little photoisomerization to 4b. Similar
J . Phys. Chem. 1990, 94, 506-508
506 SCHEME 11
MeOOC,
,COOMe
MeOOC,
,COOMe
C=C H/
H ' 4a
6a MeOOC,
,COOMe
C - C H/A AH '
(2)
5a
-%
6a
5a
-
+
hvs -H'
6a
+ 4b
4a
4a
(3)
+ 4b
1
Id
I
H
2d
7d
+ 7d
7d
-
Ph Ph
I I
Ph-C-C--Ph
(6)
dH dH
Et3N
2hv~,-H*
3d
+
Et2N=CHMe
OH-
EtzNH
+
CHsCHO(7)
results were obtained in the photoreductions of 4b and p-cyanocinnamonitriles (4c-d). These notable findings for chemoselectivity suggest that CdS-0 and CdS-Ald should be different from mechanistic point of view of their photocatalyses. In order to elucidate mechanistic difference, deuterium incorporation experiments for the photolysis of 4a were carried out in methanol-OD. Table I1 shows each deuterium isotopic distribution incorporated in the recovered olefins, 4a and 4b, and the dihydro product 5a, which were analyzed by GC-mass spectroscopy at 20% and 50% conversions of 4a. In the CdS-0 photocatalysis, 5a-d, consists of 70%in the total 5a, whereas the deuterium incorporation in the recovered 4a is very low. On the other hand, in the case of CdS-Ald, most of the recovered 4a was a mixture of nondeuterated, monodeuterated, and dideuterated 4a, and the dihydro product (Sa) incorporated up to three deuterium atoms into the molecule. In addition, at 20% conversion
in the photocatalysis where only the photoisomerization occurred, both recovered 4a and 4b were found monodeuterated. These results suggest that CdS-0-catalyzed two-electron photoreduction should proceed sequentially (ECE process) as shown in Scheme I1 (eqs 2 and 5 ) and that CdS-Ald-catalyzed photoreduction should proceed through the disproportionation of one-electron-transfer reduction intermediate 6a (eq 3). The exclusive formation of pinacols in CdS-Ald-catalyzed photoreduction of some ketones suggests that the second electron transfer to 7d cannot be induced by CdS-Ald. Furthermore, in addition to the effective photoisomerization of olefins occurring via the disproportionation process, the isomerization at an early stage of the photolysis may be explained as due to a reoxidation process (eq 4). Diethylamine was found during the photoreduction and is explained as due to hydrolysis of the imminium intermediate produced by the oxidation of TEA (eq 9).9 It is important from the point of view of semiconductor photocatalysis that nonmetallized CdS shows chemoselectivity in photoreactions of organic substrates under visible light irradiation in methanol. Selective photocatalyses were reported to depend on the kind of semiconductor.2e Surface states of semiconductors were also found to affect courses of photoreactions as was observed for photocatalysis of freshly prepared ZnS suspensions.6 To the best of our knowledge, this is the first observation of chemoselectivity affected by morphology of semiconductor photocatalysts and may be suggesive of a quantum size effect: preferential and sequential electron transfer from discrete excited electronic states which are higher in energy than the conduction band of the corresponding bulk ~ o l i d s . ~ J ~ - ' ~
Acknowledgment. We thank Dr. T. Sakata and Dr. K. Hashimoto (Institute for Molecular Science) for helpful discussions. This work was supported by the Iwatani Naoji Foundation's Research Grant, and by a Grant-in-aid for Scientific Research from the Ministry of Education, Science and Culture, Japan (Nos. 61550617 and 62213021). (11) Nedeljkovc, J. M.; Nenadovi, M. T.; Micic, D. I.; Nozik, A. J. J . Phys. Chem. 1986, 90, 12. (12) Bahnemann, D. W.; Kormann, C.; Hoffmann, M. R.J . Phys. Chem. 1987, 91, 3189. (13) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. J . Am. Chem. SOC. 1987, 109, 5649. (14) Anpo, M.; Shima, T.; Kodama, S.; Kubokawa, Y. J. Phys. Chem. 1987, 91, 4305. (15) Yanagida, S.; Yoshiya, Y.; Shiragami, T.; Pac, C. J . Phys. Chem., submitted for publication.
An ESR Study of Surface Properties of MgO and Metal-Ion-Modified MgO Ceng Zhang,* Tsunehiro Tanaka, Tsutomu Yamaguchi, Hideshi Hattori, and Kozo Tanabe Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060, Japan (Received: September 25, 1989: In Final Form: November 29, 1989) Surface properties of MgO and metal-ion-modified MgO were studied by means of ESR with NO as a probe molecule. Addition of guest ions (Na', AI3+,and Zr4+)caused a great increase in the number of NO$- radicals formed on strongly basic sites. Splitting of T orbitals of NO adsorbed was greater on metal-ion-modifiedMgO than on pure MgO. Those results suggested that both the amount and the strength of basic sites on MgO surface were enhanced due to guest ion addition.
Modification of MgO with guest metal ions results in active catalysts for such reactions as oxidative coupling of methane,' synthesis of a,D-unsaturated nitriles from ketone and methanol,*
the aldol addition of acetone, and the hydrogenation of conjugated dienes3 We reported that an increase in the catalytic activity for aldol addition of acetone by modification of MgO with guest ions is due to enhancement of the base strength of the MgO matrix
(1) Ito, T.; Lunsford, J. H. Nafure (London) 1985, 214, 721. Ito, T.; Wang, J.-X.; Lin, C.-H.; Lunsford, J. H. J . Am. Chem. SOC.1985, 107, 5062.
(2) Ueda, W.; Yokohama, T.; Moro-oka, Y.; Ikawa, T. J . Chem. SOC., Chem. Commun. 1984, 39. (3) Tanabe, K.;Zhang, G.; Hattori, H. Appl. Cafal. 1989, 48, 63.
Introduction
0022-3654/90/2094-0506$02.50/0
0 1990 American Chemical Society
