J . Phys. Chem. 1993, 97, 12882-12887
12882
Semiconductor Photocata1ysis:l Effect of Light Intensity on Nanoscale CdS-Catalyzed Photolysis of Organic Substrates Tsutomu Shiragami,? Shinako Fukami, Yuji Wada, and Shozo Yanagida' Chemical Process Engineering, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan Received: July 7, 1993'
The relationship between light intensity and product distribution in semiconductor photocatalysis was investigated by using nanoscale CdS microcrystallites (CdS-0) as photocatalysts, triethylamine (TEA) as the electron donor, and either aromatic ketones, electron-deficient alkenes, or 1-benzylnicotinamide (BNA+) as substrates. In the case of the ketones and BNA+, the yield of their respective one-electron reduction products, pinacols and the dimer, (BNA)2, increases with decreasing light intensity. When alkenes are employed in the CdS-0 system, cis-trans photoisomerization always occur regardless of the light intensity. The kinetics for the photocatalysis of the alkenes and the measurement of the initial formation rate of active lattice Cd atoms (CdO) (which act as catalytic sites for two-electron-transfer reductions) reveal that CdO formation is proportional to the square of the relative light intensity, Zr2. The chemoselectivity in the photocatalysis using nanoscale CdS should be affected by the quantity of the CdO, whose formation strongly depends on the light intensity.
Introduction Photocatalytic reactions in semiconductor particulate systems have been studied not only for utilizing solar energy but also for developing organic photoreactions under mild conditions.24 With regard to organic photoreactions, it is very important to control the reaction selectivity. There are some reports that show the reaction selectivitycan be controlled by choosing the appropriate semiconductor photo catalyst^.^ In photolysis of a-hydroxycarboxylic acid, for example, platinized CdS (PtICdS) photocatalyst gives pyruvic acid through the oxidation of a hydroxyl group, while Pt/TiOz leads to decarboxylation, giving acetaldehyde. It was explained that the difference between the redox potential of the substrates and the potential of the valence and conduction bands of semiconductors should play an important role in determining the course of the ph~tocatalysis.~~ On the other hand, most processes in heterogeneous photocatalysis are known to be influenced by the photoformed catalyticsites on the irradiated semiconductor Understanding the energy relationships between photocatalysts and substrates, as well as discovering the photoproduced catalytic sites, should lead to improvements in the efficiency and selectivity of semiconductor photocatalysis. Recently, we reported that freshly prepared nanoscale CdS suspensions (CdS-0) catalyze the sequential two-electron photoreduction of aromatic ketones, electron-deficientalkenes, and 1-benzylnicotinamide(BNA+), to alcohols, alkanes,and 1-benzyl1,4-dihydronicotinamide,respectively, and that photogenerated CdO on the lattice acts as the catalytic site for this two-electron reduction.10.11 Furthermore, in the previous paper, we also found that the formation of CdO can be suppressed by the addition of the excess sulfide ion (S2-) and that the suppression of CdO formation leads to selective one-electron reduction of the organic substrates (instead of two-electron transfer). Accordingly, our photoreaction system is a clear example of how chemoselectivity can be achieved by controlling the photoformed catalytic site, Le., the lattice CdO. Henglein et al. discovered the mechanism for the formation of CdO on irradiated CdS surfaces and suggested that the formation of Cdo should depend on the irradiated light intensity.lk If CdO formationcan be controlled by light intensity,it should be possible f Present address: Department of Industrial Chemistry, Faculty of Technology,Tokyo Metropolitan University, 1- 1 Minami-Ohsawa,Hachioji, Tokyo, 192-03, Japan. Abstract published in Advance ACS Abstracts, November 1, 1993.
0022-365419312097-12882$04.00/0
to produce desirable reduction products by controlling the light intensity in the CdS-photocatalyzed system. The present report deals with this dependence of the CdS-0catalyzed photocatalysis on light intensity. The relationship between photoformed Cd atoms in the lattice and the mechanism is discussed in terms of the kinetics for the photolysis of alkenes.
Experimental Section Materials. Ketones were obtained from the following sources. Benzophenone(Guaranteed reagent (GR) grade) was purchased from Nacalai Tesque and 4,4'-dichlorobenzophenone (GR grade) from Wako Pure Chemical Industries. p-Cyanobenzophenone was prepared by a Friedel-Crafts reaction between benzene and p-cyanobenzoic acid. Dimethyl maleate, dimethyl fumarate, and dimethyl succinatewere GR grade, purchased from Tokyo Kasei. p-Cyanocinnamonitrile was prepared and purified according to the literature.l3 1-Benzylnicotinamide chloride (BNA+Cl-), 1-benzyl-1,4-dihydronicotinamide (BNAH), and the dimer of BNA+ ((BNA)z) were also prepared and purified according to the 1iterat~re.l~GR grade methyl viologen dichloride was purchased from Nacalai Tesque. GR grade triethylamine (TEA) and diethylamine (DEA) were purchased from Wako Pure Chemical Industries. Liquid amines were purified by fractional distillation before use. Sodium sulfide (GR grade from Nacalai Tesque) and cadmium perchlorate (GR grade from Mitsuwa) were used as delivered. Analysis. Ketones, alcohols, pinacols BNA+, (BNA)2, and BNAH were analyzed by liquid chromatography using a Cosmosil-ODS column and a UV detector (at 230 nm) (Tosoh; UV8000). As an eluent, a 6:4 mixture of methanol and buffered aqueous solution (KH2P04-NaOH; pH = 7) was employed with an eluent rate of 0.5 mL/min-'. Alkenes and diethylamine were analyzed by gas chromatography using a Shimadzu GC-12A apparatus equipped with a flame ionization detector. The chromatographycolumnwas a Shimadzu 25 m X 0.2 mm capillary column of OV-1 for alkenes and a 2m X 3mm column of ASC-L for diethylamine. Methyl benzoate, methyl myristate, and 2-propanol were used as internal standards. heparationof Cds-0 Photmtalyst. As reported in a previous paper,"JCdS-0 suspensionswere prepared in situ under an argon atmosphere by mixing q u a l quantities of methanolic solutions (2.5 X 10-2mol dm-3) eachof Cd(C104)2 andNa2S. This solution was magnetically stirred and cooled in an ice bath. 0 1993 American Chemical Society
Light Intensity and CdS-Catalyzed Photolysis
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12883
TABLE k Dependence of the Product Distribution on Relative Light Intensity (I,) by Photoreduction of Aromatic Ketones Catalyzed by CdS-0. product ratio [pinacol/(pinacol + alcoho1)l subst redb-Em Ir = 1 I, = 0.3 I, = 0.07 la lb IC
Id
1.32 1.35 1.55 1.56
0.14
0.W 0.W 0.ow
0.20 0.20
0.36 0.30
0.W
0.W 0.W
0.ow
Irradiated in MeOH for 6 h. b Polarographic half-wave reduction potential vs SCE in MeOH. C Only alcohols were formed.
"
0 10 20 30 40'50 60
a
SCHEME I
CdS-CatalyzedPhotoreductionof Ketones, ALkews, and BNA+. A stirred methanol suspensioncontainingCdS-0 (1 3 pmol), TEA(1 mol dm-3), and either a ketone (2 mmol dm-3), an alkene (10 mmol dm-') or BNA+, where [TEA] = 0.5 mol dm-3 for BNA+) was flushed with argon gas while cooling in an ice bath. The container was then closed with a rubber stopper and irradiated with a 300-Whigh-pressure mercury lamp through an aqueous copper dichloride and calcium dichloride solution filter (at 436 nm).l5 The light intensity at 436 nm was controlled by changing the concentration of the filter solution. Deuterium Incorporation Experiments. Deuterium incorporation experiments were performed during photoisomerization and photoreduction of alkenes in methanol-OD. When about 10-2095 of the substrate disappeared the irradiation was stopped, and deuterium isotopic distribution for the substrate and the products was determined by GC-mass spectroscopy (Shimadzu GCMS-QP1000). Determination of the Amount of Cdo. The formation of CdO was determined by the method of Henglein et al. as well as by the method outlined in previous papers.1°J2
Time / min Time / min Figure 1. Time-course plots for CdS-0-catalyzedphotoreduction of 4,4'dichlorobenzophenone by TEA in MeOH irradiated at various relative light intensities (Ir): (a) Ir= 1 and (b) Ir = 0.4;(0)ketone, ( 0 )alcohol, (A) pinacol, and (0) Cdo.
0
30
60
0
Time / min
60
120
Time / min
Y
0
60
120
0
60
I20
Results and Discussion
Time / min Time / min Figure 2. Time-course plots for CdS-0-catalyzed photoreduction and photoisomerization of dimethyl maleate by TEA in MeOH irradiated at various relative light intensities: (a) Ir = 1, (b) I, = 0.8, (c) 1, = 0.3, and (d) I, = 0.2; (0)dimethyl maleate, (A) dimethyl fumarate, and ( 0 ) dimethyl succinate.
Dependence of the Irradiated Light Intensity on Photoreduction of Aromatic Ketones and BNA+ Catplyzed by CdS-0. Table I shows the results for the CdS-0-catalyzed photoreduction of aromatic ketones irradiated at the various relative light intensities (I,) (Scheme I). The product ratio in Table I is the ratio of the yield of pinacols to the total yield of the reduction products (pinacol + alcohol). The value of 0.0 means that the corresponding alcohol is almost quantitatively produced in CdS-0 system. At Zr= 1, the alcohols were produced almost exclusively, whereas when the light intensity was decreased to Zr= 0.3 and 0.07,the pinacols were formed competitively, giving a product ratio dependent on the light intensity. Interestingly, the effect of light intensity on the product distribution was remarkable only for l a and l b but not for IC and Id. These results imply that the effect of light intensity should depend on reduction potentials of the ketones. Figure 1 shows time-course plots for the photoreduction of l b catalyzed by CdS-0 under the condition of 1, = 1 and 0.4. Irradiation at 1, = 1 led to a quantitative two-electron reduction of Ib to the alcohol, with the concurrent formation of lattice Cd atom (CdO). At I, = 0.4,the rate of the CdO formation was slower, and the pinacol was formed at an early stage of the photolysis. The formation of alcohol was observed with the
gradual formation of CdO. The formation of the pinacol stopped after the formation of CdO leveled off. As for the CdS-0-catalyzed photoreduction of BNA+ in the presence of TEA (as electron donor), the photolysis proceeded as shown in Scheme 11.11 Under the normal condition (Z, = l), BNAH was obtained as a two-electron reduction product with a yield of 10% after 2 h of irradiation. However, under irradiation at a lower light intensity (Ir= 0.3),BNAH was scarcely observed, but the dimer of BNA+ ((BNA)*) was effectively produced with a yield of 80% after 2 h of irradiation. In the latter case, no CdO was observed during the irradiation. These results indicate that the formation of CdO should be suppressed under low light intensity, thus enhancing the contribution of the one-electron reduction. Dependence of the Irradiated Light Intensity on Photolysis of Alkenes Catalyzed by CdS-0. Figure 2 shows time-course plots for the CdS-0-catalyzed photoisomerizationand photoreduction of dimethyl maleate in the presence of TEA in MeOH at the various light intensities (I, = 14.2). At Z, = 1 ,selective twoelectron reduction was observed with concomitant Cdoformation. Interestingly, the cis-trans photoisomerizationoccurred regardless of the light intensity, and the two-electron reduction was almost
Shiragami et al.
12884 The Journal of Physical Chemistry, Vol. 97,No. 49, 1993
SCHEME I1
'ONH2
CH2Ph
CH 2P h
CH ?Ph
CONH,
4,4'-(BNA)2
4,6'-(BNA)2
CdS, hv ( > 400 nm )
TEA In Y O bH2Ph
CH2Ph
BNA'
BNAH
Cds.0
subst RCH4HR' R
R'
isomerization reduction initial yieldo/ yieldo/ initial rate/pmol/h % I, rate/pmol/h %
p-CNPh CN
1.0
COzMc COzMe
0.80 0.30 0.20 1.0 0.80 0.30 0.20
32.4 22.2 8.7 7.1 11.1 9.0 4.1 2.4
47.0 76.8 90.6 94.7 44.3 34.9 57.2 75.2
23.6 2.0 0.6 0.3 16.8 0.9 0.2 0.1
30.3 12.2 7.9 5.2 44.3 11.7 4.8 2.1
TABLE IIk Deuterium Isotopic Distribution of Product Formed by CdS-0-Catalyzed Isomerization and Reduction of cis-pCyanocinnamoaiMlein MeOD Irradiated at the Various Relative Light Intensities (6) Yield/%" cis trans dihydro compound I, convn/% do dl dz do dl d2 do dl d2 dl 1.o 0.30 0.20
18 13 9
99 98 99
1 2 1
0 0
69 50 49
31 50 51
0 0 0
0 0
16 1
84 99
0 0
Determined by GC-MS analysis.
'Based on the substrate converted at maximum points for the isomerization to truns form. completely suppressed at I, = 0.05. A similar tendency was also observed in the photolysis of cis-p-cyanocinnamonitrile. In Table 11, the initial rate of the isomerizationof cis-alkenes to the rrans form and their reduction to the dihydro compound are summarized along with their respective yields. The chemical yields are based on the substrate converted (at the maximum points) for the formation to the trans form. At the lower I,, of course, no formation of CdO was observed, and the chemical yield of the rrans form was high. These results indicate that the CdS-0 suspensions should favor the cis-trans photoisomerizationat the lower I,, where CdO is suppressed. Only diethylamine (DEA) was obtainedas an oxidation product of TEA, during the photocatalysis at low I,. Neither the dimer of the TEA radical (derived from TEA radical cation) nor the adducts between TEA and alkenesweredetected.'JO As reported, the formation of DEA can be explained by hydrolysis of the iminium intermediate [(Et)*N=CHCHs]+ formed through the two-electron oxidation process.17 Figure 3 shows time-course plots of DEA formation from the photolysis of cis-p-cyanocinnamonitrile by CdS-0 irradiated at I, = 0.15. The formation of DEA was observed in parallel with that of dihydro compound, indicating an electron balance between the photoreductive formationof dihydrocompound and the photooxidative formation of DEA. These results suggestthat TEA should not be consumed in thecis-trans photoisomerization,althoughTEA is indispensable for effective photoisomerization. In order to elucidate the mechanism of the isomerization induced at the lower I , in detail, deuterium incorporation experimentsfor the photolysis of cis-p-cyanocinnamonitrilewere carried out in methanol-OD. Table I11 shows the isotopic distribution of deuterium incorporated into the recovered alkenes and the dihydro compound. These products were analyzed by GC-mass spectroscopy at an early stage of the photoreaction after about 15%conversion of the starting alkenes. Interestingly, the monodeuterated trans form was inevitably produced along with the nondeuterated trans form during the photolysis of the cis form in methanol-OD. A similar deuterium distribution was observed in the bulk-CdS-catalyzedphotolysis of the same alkene, indicating that the same mechanism is operating for the photoisomerization in both cases. (The oxidation of the intermediate alkyl radical formed by the protonation of the alkene
0
60
120
Time / min Figure 3. Time-course plot for CdS-0-catalyzed photoreduction and photoisomerization of cis-p-cyanocinnamonitrile by TEA in MeOH irradiated at I, = 0.15: (0) cis form, (A) truns form, (0) dihydro compound, and ( 0 )diethylamine.
radical anion (alkene'-) is the mechanism for the monodeuterated trans form, and the oxidation of alkene*-is the mechanism for the nondeuterated one.) Two possible oxidants of alkene*- and the alkyl radical are conceivable: oxidation (1) by TEA'+ and (2) by holes of CIS. The following facts suggest that TEA'+ should play a decisive role as an oxidant: (1) TEA was not consumed during the photoisomerization, although TEA is required for the isomerization (see Figure 3). (2) TEA was not consumed during the formation of coupling products with the radical of TEA, Le., (Et)*NCHCH3 with the alkyl radicals, alkene*-, or alkene itself. (3) Since it is expected that the oxidation of TEA and the reduction of TEA'+ should occur effectively on the irradiated CdS-0 surface, the steady-state concentration of TEA'+ could be very high. On the other hand, the dihydro compound consists almost entirely of the dideuterated one. These results strongly suggest that the alkyl radical should undergosequentialreductiveelectron transfer without disproportionation, leading to the formation of the dihydro compound. From these results, we propose that the photoisomerizationof CdS-0 irradiated at the low Zr proceeds spontaneously through two different mechanisms: (1) through the back electron-transfer process between the alkene*-and TEA'+ (eq 1) and (2) through thereoxidationofthealkene*byTEA'+(eq 2) asshowninscheme 111.
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12885
Light Intensity and CdS-Catalyzed Photolysis
m"
SCHEME IV O t -1 1
Id.
dllkd
-7'
I
\ TEA
1
'TEA!
Kinetic Analysis of the Pbotoisomerizatioaand Photoreduction of Alkenes by CdS-0. In order to confirm the proposed mechanism as shown in Scheme IV, the kinetics of the photolysis were investigated as a function of I,. Each rate constant was defined as follows (Scheme IV): a and kd are the proportional constants for the light absorption and the recombination rate constant, respectively. The rate constants of the electron transfer are kl, for CdS-0 to cis-alkene; k ~ for , the protonation of alkene*-; kz, for the electron transfer from CdS-0 to alkene'; kb, for the isomerization of cis-alkene*-;and k,for the reoxidation of alkene. k3 is the rate constant of the electron transfer from TEA to holes of CdS-0. The equations for the rate of formation of conduction band electrons and valence band holes are d[el/dt = al, - &,[e] [h] - k,[e] [Cd"] - kBT[e][TEA"] k,[cis-alkene] [e] - k,[e] [alkene'] (3) (4)
In these equations, k, is the rate constant for CdO formation, and km is the rate constant for electron transfer between the conduction band electron and TEA'+ formed by the oxidation of positive holes. Almost all of the photoformed positive holes may be transferred via TEA for three reasons: (1) CdS-0 is nonemissive, (2) the electron transfer between the holes and TEA is exothermic,l* and (3) the concentration of TEA is very high (1 mol dm-3). Furthermore, the quantum yield of CdO formation is relatively high.19 Assuming that k,, ~ B T and , k3 are large compared with k l , k2, and kd, eqs 3 and 4 can be approximated as follows:
+ k,#'EA'+])
(5)
Since the steady-state assumption is used for eqs 5 and 6,the equations [e] = c~Z,/(k,[Cd~+] + k&EA'+])
5
6
In Ir
Qi =
d[e]/dt = aI,- [e](k,[Cd2']
4
form formation (@i) is then expressed as follows:
H' 'R (Irma)
-k,[TEA][h]
3
Figure 4. Kinetic plots In Vi vs In I, for CdS-0-catalyzed photoreduction of dimethyl maleate: 6 is the initial rate of isomerization and I, is the relative light intensity.
cds
d[h]/dt=d,-kd[le][h]
2
RCH&H,R'
(7)
and [h] = aZr/k3[TEA] (8) can be derived from eqs 5 and 6. The quantum yield of trans
k,[cis-alkene] kd
X
+ k,[cis-alkene] + ~BT[TEA*+] kb[TEA'+]
kb[TEA'+]
+ kH[H+] k,[TEA'+] kz[alkene']
+ k,[TEA'+]
At an early stage in the photoisomerization, the rate of formation of the dihydro compound (k2)was much smaller than that of the isomer (k8). Furthermore, the chemical yield of the dihydro compound was very low compared with that of the trans form. If the assumption that k2
