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Heterogeneous photocatalytic reactions on semiconductor materials
J. Electroanal. Chem., 126 (1981) 277--281
277
Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
Preliminary note HETEROGENEOUS PHOTOCATALYTIC REACTIONS ON SEMICONDUCTOR MATERIALS PART II. PHOTOELECTROCHEMISTRY AT SEMICONDUCTOR TiO2/ INSULATING AROMATIC HYDROCARBON LIQUID INTERFACE MASAMICHI FUJIHIRA*, YOSHIHARU SATOH and TETSUO OSA
Pharmaceutical Institute, Tohoku University, Aobayama, Sendai 980 (Japan)
(Received 15th July 1981)
Previously we reported the heterogeneous photocatalytic oxidation of aromatic compounds in the presence of illuminated semiconductor powders in the aqueous solutions of oxygen containing aromatics [1]. All the products expected from the Fenton reaction [2] were obtained. For example, benzaldehyde, cresols, and bibenzyl were obtained from toluene. F r o m these results and other observations, it was concluded that hydroxyl radical, • OH, which is formed either from the cathodic reaction product, H:O2, as in the Fenton reaction or from the anodic oxidation of O H - or H20, plays an important role in the above reaction [1]. Hydroxyl radical was also supposed to be the veritable oxidizing agent in the photodecomposition of hydrocarbons in oxygen-containing aqueous solutions at platinized TiO2 [3,4]. On the other hand, irradiation of TiO2 (or CdS) powders suspended in organic solvents containing 1,1-diphenylethylene and its derivatives under oxygen gave corresponding epoxides and benzophenone, and a mechanism via superoxide anion was proposed [5]. Oxidation of alkyltoluenes vapor by oxygen in contact with UV irradiated TiO2 has been reported to yield alkylbenzaldehydes selectively [6]. In this note, the heterogeneous photocatalytic oxidation of benzene and toluene by illuminated TiO2 powders, which are suspended in the insulating aromatics themselves under nitrogen and air atmosphere, will be compared with previous results of the aqueous solution to clarify the difference in product distribution caused by the absence and presence of water. A possible mechanism for the reactions will be also proposed. One gram of TiO2 (99.99% anatase, Rare Metallic Co.) was suspended in an organic substrate (100 cm a) by a magnetic stirrer and was irradiated for 2 h by a 500 W high pressure mercury arc lamp with a Toshiba UV filter UV-D35 at room temperature. Platinized anatase was prepared by the method described previously [1]. TiO~ powders were dried by heating at 100°C for 30 min before each experiment. Water in benzene and toluene was removed by contact with molecular sieves 4A for 24 h. The products were analyzed as *To whom correspondence should be addressed.
0022-0728/81/0000--0000/$ 02.50, © 1981, Elsevier Sequoia S,A.
278
described previously [1] by a Shimadzu GC-4CM gas chromatograph. The representative resl~_lts are summarized in Table 1. In the presence of oxygen, benzaldehyde was formed from toluene selectively by illuminated undoped anatase (no. 1). Other products, such as cresols and bibenzyl, which were obtained in the presence of water [1], were n o t obtained. Such a selective oxidation was also observed on platinized anatase (no. 3). On the other hand, bibenzyl was formed with a trace amount of benzaldehyde on either catalyst when nitrogen was bubbled for deaeration (nos. 2 and 4), although the a m o u n t of bibenzyl formed was much bigger on the platinized catalyst than on the u n d o p e d catalyst. Benzene was n o t so reactive as toluene (nos. 5 and 6) and a trace a m o u n t of phenol was formed only in the presence of oxygen (no. 5). In the absence of water, the main anodic reaction on the illuminated TiO2 powders will be the one-electron oxidation of aromatic hydrocarbons to the corresponding cation radicals by positive holes created in the valence band. This is rationalized by the consideration of energy levels of the valence-band edge of TiO2 and the redox potentials for the cation radical formation as shown in Fig. 1A. To comPensate for the lack of data, the energy levels of TiO2 [7] and the redox potential of benzene, toluene, hydrogen, and oxygen measured in acetonitrile [8--10] are adopted instead of those in aromatic hydrocarbons. The diagram for the aqueous system is shown in Fig. 1B (pH = 7). If the reaction is carried out in water, oxidation of O H - or H20 into -OH will be the main anodic reaction (Fig. 1B). In the case of toluene, the electrochemically generated cation radical in acetonitrile has been known to lose a proton irreversibly to give a benzyl radical, which undergoes dimerization to bibenzyl or is further oxidized to a benzyl cation [9,11]. The resultant benzyl cation reacts with CH3CN giving the acetamidation product. Consequently the selective formation of bibenzyl under nitrogen (nos. 2 and 4) TABLE 1 Heterogeneous photocatalytic oxidation of aromatic hydrocarbon--TiO 2 system Run no.
1 2 3 4 5 6
Type of catalyst a
a,u a,u a,d,Pt a,d,Pt a,u a,u
Substrate
Toluene Toluene Toluene Toluene Benzene Benzene
Conditions
under under under under under under
air N~ air N2 air N2
Product b/#mol
PhCHO
BiBz P h O H
175.9
n d
t d
69.5 t
Total ~mol
yield c/%
BiPh
6.1 n 55.4 t n
c/ Q u a n t u m
n n
175.9 12.2 69.5 110.8 t n
0.81 0.06 0.32 0.51 t n
a a = anatase, u = u n d o p e d , d = d o p e d b y h e a t i n g at 600°C for 3 h in vacuo, P t = p l a t i n i z e d p h o t o e l e c t r o c h e m i c a l l y by t h e m e t h o d in t h e literature [ 1 2 ] . b p h C H O = b e n z a l d e h y d e , BiBz = b i b e n z y l , P h O H = p h e n o l , BiPh = b i p h e n y l . c Total a m o u n t o f p r o d u c t s and t h e q u a n t u m yield are calculated o n t h e basis o f t h e p r o d u c t s f o r m e d b y o n e p h o t o n p r o c e s s such as b e n z a l d e h y d e ( m e c h a n i s m 1) and b e n z y l radical ( m e c h a n i s m 2). T h e r e f o r e o n e m o l e o f b i b e n z y l c o u n t s as t w o m o l e s o f p r o d u c t . d t = trace, n = n o t d e t e c t a b l e .
279
l - -
oo~ + e
i
= 02TuOjn1
2~:+ + 2e : H2 ~ -
2H + + 2e = H2 ~
tO U if)
0
14
-
-
02
+
e
04]
o2
=
--
02 + 2H + + 2e = H202 ~
--
02 + 4H + + 4e = 2H20 ~
>
+l
+l hl
+2 it,lit
ES]
--
T o l u e n e t + e = Toluene
--
Benzene" + e = Benzene E8 ]
TolueneT + e = Toluene [8] + 2 - -
+ ....
OH +
H+
+
e
=
H20
Benzene t + e = Benzene ~ ]
Fig. 1. Position of conduction-band and valence-band edge of TiO 2 [7,16 ] and redox potentials of solution species vs. SCE in acetonitrile (A) and in aqueous solution at pH 7 (B).
can be interpreted by reaction mechanism 1 (in the absence of oxygen): ~
e- + h +
(i)
-*
C6HsCH3 +
(2)
C6HsCH3 +
~
C6HsCH2" + H +
C6HsCH2"
-*
1/2 (C6HsCH2)2
(3) (4)
H+ + e -
-~
1/2 H2
(5)
hv C6HsCH3
+
h+
Overall reaction [(1)+(2)+(3)+(4)+(5)]: C6HsCH3 + hv
-+
1/2 (C6HsCH2)2 + 1/2 H2
(6)
In the absence of oxygen, the only possible cathodic reaction of the illuminated TiO2 powders will be the reduction of proton to hydrogen as shown in eqn. (5). The tremendous increase in the yield on the platinized catalyst can be explained by the lowering of the overpotential for the reaction of eqn. (5) on platinum. In the presence of oxygen, the benzyl radical reacts with oxygen and the resultant peroxy radical may be reduced to benzaldehyde [13] by an electron from the conduction band or a superoxide anion. The possibility of the direct reaction between a benzyl radical and 0 2 - as the reaction between a diphenylethylene cation radical and 0 2 - proposed by others [5] cannot be excluded. If hydrogen peroxide is formed by the cathodic reaction as in the aqueous system, the h y d r o x y l a t e d products such as cresols will also be formed. But the result appears to be opposite to what one would expect from this hypothesis
280 and the formation of H202 can be excluded. Consequently the reaction in the presence of oxygen can be summarized by mechanism 2: hv
~
e- + h ÷
(1)
C6HsCH3 + h +
-+
C6HsCH3 +
(2)
C6HsCH3 +
-+
C6HsCH2 " + H +
(3)
C6HsCH2" + 02
-+
C6HsCH20:'
(7)
C6HsCH202" + e-
~
C6HsCHO + O H -
(8)
02 + e-
~
O27
(9)
C6HsCH202" + 02 = ~
C6HsCHO + O H - + 02
(10)
C6HsCH2" + 02 =
~
C6HsCHO + O H -
(ii)
H++ OH-
~
H20
(12)
Overallreaction [(1)+(2)+(3)+(7)+(8)+(12)]: C6HsCH3 + 02 + hv ~ C6HsCHO + H20
(13)
As water is formed as one of the products, h y d r o x y l a t e d products such as cresols and phenol will be formed as in the aqueous system [1] by prolonged irradiation [17]. Figure 1A also explains well the reason why the oxidation of toluene proceeded but that of benzene did not. Very little is known about the electrochemistry of TiO2 in aromatic hydrocarbons because of the low dielectric constant of these solvents [18]. However, the present work suggests that the valence-band edge of TiO2 in contact with aromatic hydrocarbons lies in the potential range between +2.0 and +2.3 V vs. SCE. It is also interesting to point out that there is an excess charge in the particle due to a preferential reaction which is either cathodic (due to electrons) or anodic (due to holes). This excess charge leads to the formation of an electrical double layer with charged reaction products, i.e. the particle and the products held together by electrostatic attraction, as there is no other electrolyte in the solvent. In conclusion, the study of the heterogeneous photocatalytic reactions on semiconductor particle microelectrodes in contact with an insulating liquid can be a novel m e t h o d for elucidation of electrochemical processes at the semiconductor/insulating liquid interface.
REFERENCES 1 2 3 4 5
M. F u j i h i r a , Y. S a t o h a n d T. Osa, N a t u r e , in press. C. Walling, A c c . C h e m . Res., 8 ( 1 9 7 5 ) 1 2 5 . I. I z u m i , W.W. D u n n , K . O . W i l b o u r n , F . F . F a n a n d A . J . B a r d , J. P h y s . C h e m . , 8 4 ( 1 9 8 0 ) 3 2 0 7 . I. I z u m i , F . F . F a n a n d A . J . B a r d , J. P h y s . C h e m . , 8 5 ( 1 9 8 1 ) 2 1 8 . T. K a n n o , T. O g u c h i , H. S a k u r a g i a n d K. T o k u m a r u , T e t r a h e d r o n L e t t . , 21 ( 1 9 8 0 ) 4 6 7 .
281
6 M.N. M o z z a n e g a , J.M. H e r r m a n n a n d P. P i c h a t , T e t r a h e d r o n L e t t . , ( 1 9 7 7 ) 2 9 6 5 . 7 S.N. F r a n k a n d A . J . B a r d , J. A m . C h e m . S o c . , 9 7 ( 1 9 7 5 ) 7 4 2 7 . 8 E.S. P y s h a n d N.C. Y a n g , J. A m . C h e m . S o c . , 8 5 ( 1 9 6 3 ) 2 1 2 4 . 9 V.D. P a r k e r a n d R . N . A d a m s , T e t r a h e d r o n L e t t . , ( 1 9 6 9 ) 1 7 2 1 . 1 0 M.E. P e o v e r a n d B.S. W h i t e , E l e c t r o c h i m . A c t a , 1 1 ( 1 9 6 6 ) 1 0 6 1 . 11 V.D. P a r k e r a n d B.E. B u r g e r t , T e t r a h e d r o n L e t t . , ( 1 9 6 8 ) 2 4 1 1 . 1 2 B. K r a e u t l e r a n d A . J . B a r d , J. A m . C h e m . S o c . , 1 0 0 ( 1 9 7 8 ) 4 3 1 7 . 1 3 R. T o m a t a n d A. R i g o , J. A p p L E l e c t r o e h e m . , 9 ( 1 9 7 9 ) 3 0 1 . 1 4 D.T. S a w y e r a n d M.J. G i b i a n , T e t r a h e d r o n , 3 5 ( 1 9 7 9 ) 1 4 7 1 . 1 5 D. D o b o s , E l e c t r o c h e m i c a l D a t a , Elsevier, A m s t e r d a m , 1 9 7 5 . 1 6 E.C. D u t o i t , R. C a r d o n a n d W.P. G o m e s , Bet. B u n s e n g e s . P h y s . C h e m . , 8 0 ( 1 9 7 6 ) 4 7 5 . 1 7 R. O g u c h i , T. K a n n o , H. S a k u r a g i a n d K. T o k u m a r u , A b s t r a c t s o f A n n u a l M e e t i n g o f t h e C h e m i c a l S o c i e t y o f J a p a n , I, 1 9 8 1 , p. 2 5 6 . 1 8 S. N a k a b a y a s h i , A. F u j i s h i m a a n d K. H o n d a , J. E l e c t r o a n a l . C h e m . , 1 1 1 ( 1 9 8 0 ) 3 9 1 .
277
Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
Preliminary note HETEROGENEOUS PHOTOCATALYTIC REACTIONS ON SEMICONDUCTOR MATERIALS PART II. PHOTOELECTROCHEMISTRY AT SEMICONDUCTOR TiO2/ INSULATING AROMATIC HYDROCARBON LIQUID INTERFACE MASAMICHI FUJIHIRA*, YOSHIHARU SATOH and TETSUO OSA
Pharmaceutical Institute, Tohoku University, Aobayama, Sendai 980 (Japan)
(Received 15th July 1981)
Previously we reported the heterogeneous photocatalytic oxidation of aromatic compounds in the presence of illuminated semiconductor powders in the aqueous solutions of oxygen containing aromatics [1]. All the products expected from the Fenton reaction [2] were obtained. For example, benzaldehyde, cresols, and bibenzyl were obtained from toluene. F r o m these results and other observations, it was concluded that hydroxyl radical, • OH, which is formed either from the cathodic reaction product, H:O2, as in the Fenton reaction or from the anodic oxidation of O H - or H20, plays an important role in the above reaction [1]. Hydroxyl radical was also supposed to be the veritable oxidizing agent in the photodecomposition of hydrocarbons in oxygen-containing aqueous solutions at platinized TiO2 [3,4]. On the other hand, irradiation of TiO2 (or CdS) powders suspended in organic solvents containing 1,1-diphenylethylene and its derivatives under oxygen gave corresponding epoxides and benzophenone, and a mechanism via superoxide anion was proposed [5]. Oxidation of alkyltoluenes vapor by oxygen in contact with UV irradiated TiO2 has been reported to yield alkylbenzaldehydes selectively [6]. In this note, the heterogeneous photocatalytic oxidation of benzene and toluene by illuminated TiO2 powders, which are suspended in the insulating aromatics themselves under nitrogen and air atmosphere, will be compared with previous results of the aqueous solution to clarify the difference in product distribution caused by the absence and presence of water. A possible mechanism for the reactions will be also proposed. One gram of TiO2 (99.99% anatase, Rare Metallic Co.) was suspended in an organic substrate (100 cm a) by a magnetic stirrer and was irradiated for 2 h by a 500 W high pressure mercury arc lamp with a Toshiba UV filter UV-D35 at room temperature. Platinized anatase was prepared by the method described previously [1]. TiO~ powders were dried by heating at 100°C for 30 min before each experiment. Water in benzene and toluene was removed by contact with molecular sieves 4A for 24 h. The products were analyzed as *To whom correspondence should be addressed.
0022-0728/81/0000--0000/$ 02.50, © 1981, Elsevier Sequoia S,A.
278
described previously [1] by a Shimadzu GC-4CM gas chromatograph. The representative resl~_lts are summarized in Table 1. In the presence of oxygen, benzaldehyde was formed from toluene selectively by illuminated undoped anatase (no. 1). Other products, such as cresols and bibenzyl, which were obtained in the presence of water [1], were n o t obtained. Such a selective oxidation was also observed on platinized anatase (no. 3). On the other hand, bibenzyl was formed with a trace amount of benzaldehyde on either catalyst when nitrogen was bubbled for deaeration (nos. 2 and 4), although the a m o u n t of bibenzyl formed was much bigger on the platinized catalyst than on the u n d o p e d catalyst. Benzene was n o t so reactive as toluene (nos. 5 and 6) and a trace a m o u n t of phenol was formed only in the presence of oxygen (no. 5). In the absence of water, the main anodic reaction on the illuminated TiO2 powders will be the one-electron oxidation of aromatic hydrocarbons to the corresponding cation radicals by positive holes created in the valence band. This is rationalized by the consideration of energy levels of the valence-band edge of TiO2 and the redox potentials for the cation radical formation as shown in Fig. 1A. To comPensate for the lack of data, the energy levels of TiO2 [7] and the redox potential of benzene, toluene, hydrogen, and oxygen measured in acetonitrile [8--10] are adopted instead of those in aromatic hydrocarbons. The diagram for the aqueous system is shown in Fig. 1B (pH = 7). If the reaction is carried out in water, oxidation of O H - or H20 into -OH will be the main anodic reaction (Fig. 1B). In the case of toluene, the electrochemically generated cation radical in acetonitrile has been known to lose a proton irreversibly to give a benzyl radical, which undergoes dimerization to bibenzyl or is further oxidized to a benzyl cation [9,11]. The resultant benzyl cation reacts with CH3CN giving the acetamidation product. Consequently the selective formation of bibenzyl under nitrogen (nos. 2 and 4) TABLE 1 Heterogeneous photocatalytic oxidation of aromatic hydrocarbon--TiO 2 system Run no.
1 2 3 4 5 6
Type of catalyst a
a,u a,u a,d,Pt a,d,Pt a,u a,u
Substrate
Toluene Toluene Toluene Toluene Benzene Benzene
Conditions
under under under under under under
air N~ air N2 air N2
Product b/#mol
PhCHO
BiBz P h O H
175.9
n d
t d
69.5 t
Total ~mol
yield c/%
BiPh
6.1 n 55.4 t n
c/ Q u a n t u m
n n
175.9 12.2 69.5 110.8 t n
0.81 0.06 0.32 0.51 t n
a a = anatase, u = u n d o p e d , d = d o p e d b y h e a t i n g at 600°C for 3 h in vacuo, P t = p l a t i n i z e d p h o t o e l e c t r o c h e m i c a l l y by t h e m e t h o d in t h e literature [ 1 2 ] . b p h C H O = b e n z a l d e h y d e , BiBz = b i b e n z y l , P h O H = p h e n o l , BiPh = b i p h e n y l . c Total a m o u n t o f p r o d u c t s and t h e q u a n t u m yield are calculated o n t h e basis o f t h e p r o d u c t s f o r m e d b y o n e p h o t o n p r o c e s s such as b e n z a l d e h y d e ( m e c h a n i s m 1) and b e n z y l radical ( m e c h a n i s m 2). T h e r e f o r e o n e m o l e o f b i b e n z y l c o u n t s as t w o m o l e s o f p r o d u c t . d t = trace, n = n o t d e t e c t a b l e .
279
l - -
oo~ + e
i
= 02TuOjn1
2~:+ + 2e : H2 ~ -
2H + + 2e = H2 ~
tO U if)
0
14
-
-
02
+
e
04]
o2
=
--
02 + 2H + + 2e = H202 ~
--
02 + 4H + + 4e = 2H20 ~
>
+l
+l hl
+2 it,lit
ES]
--
T o l u e n e t + e = Toluene
--
Benzene" + e = Benzene E8 ]
TolueneT + e = Toluene [8] + 2 - -
+ ....
OH +
H+
+
e
=
H20
Benzene t + e = Benzene ~ ]
Fig. 1. Position of conduction-band and valence-band edge of TiO 2 [7,16 ] and redox potentials of solution species vs. SCE in acetonitrile (A) and in aqueous solution at pH 7 (B).
can be interpreted by reaction mechanism 1 (in the absence of oxygen): ~
e- + h +
(i)
-*
C6HsCH3 +
(2)
C6HsCH3 +
~
C6HsCH2" + H +
C6HsCH2"
-*
1/2 (C6HsCH2)2
(3) (4)
H+ + e -
-~
1/2 H2
(5)
hv C6HsCH3
+
h+
Overall reaction [(1)+(2)+(3)+(4)+(5)]: C6HsCH3 + hv
-+
1/2 (C6HsCH2)2 + 1/2 H2
(6)
In the absence of oxygen, the only possible cathodic reaction of the illuminated TiO2 powders will be the reduction of proton to hydrogen as shown in eqn. (5). The tremendous increase in the yield on the platinized catalyst can be explained by the lowering of the overpotential for the reaction of eqn. (5) on platinum. In the presence of oxygen, the benzyl radical reacts with oxygen and the resultant peroxy radical may be reduced to benzaldehyde [13] by an electron from the conduction band or a superoxide anion. The possibility of the direct reaction between a benzyl radical and 0 2 - as the reaction between a diphenylethylene cation radical and 0 2 - proposed by others [5] cannot be excluded. If hydrogen peroxide is formed by the cathodic reaction as in the aqueous system, the h y d r o x y l a t e d products such as cresols will also be formed. But the result appears to be opposite to what one would expect from this hypothesis
280 and the formation of H202 can be excluded. Consequently the reaction in the presence of oxygen can be summarized by mechanism 2: hv
~
e- + h ÷
(1)
C6HsCH3 + h +
-+
C6HsCH3 +
(2)
C6HsCH3 +
-+
C6HsCH2 " + H +
(3)
C6HsCH2" + 02
-+
C6HsCH20:'
(7)
C6HsCH202" + e-
~
C6HsCHO + O H -
(8)
02 + e-
~
O27
(9)
C6HsCH202" + 02 = ~
C6HsCHO + O H - + 02
(10)
C6HsCH2" + 02 =
~
C6HsCHO + O H -
(ii)
H++ OH-
~
H20
(12)
Overallreaction [(1)+(2)+(3)+(7)+(8)+(12)]: C6HsCH3 + 02 + hv ~ C6HsCHO + H20
(13)
As water is formed as one of the products, h y d r o x y l a t e d products such as cresols and phenol will be formed as in the aqueous system [1] by prolonged irradiation [17]. Figure 1A also explains well the reason why the oxidation of toluene proceeded but that of benzene did not. Very little is known about the electrochemistry of TiO2 in aromatic hydrocarbons because of the low dielectric constant of these solvents [18]. However, the present work suggests that the valence-band edge of TiO2 in contact with aromatic hydrocarbons lies in the potential range between +2.0 and +2.3 V vs. SCE. It is also interesting to point out that there is an excess charge in the particle due to a preferential reaction which is either cathodic (due to electrons) or anodic (due to holes). This excess charge leads to the formation of an electrical double layer with charged reaction products, i.e. the particle and the products held together by electrostatic attraction, as there is no other electrolyte in the solvent. In conclusion, the study of the heterogeneous photocatalytic reactions on semiconductor particle microelectrodes in contact with an insulating liquid can be a novel m e t h o d for elucidation of electrochemical processes at the semiconductor/insulating liquid interface.
REFERENCES 1 2 3 4 5
M. F u j i h i r a , Y. S a t o h a n d T. Osa, N a t u r e , in press. C. Walling, A c c . C h e m . Res., 8 ( 1 9 7 5 ) 1 2 5 . I. I z u m i , W.W. D u n n , K . O . W i l b o u r n , F . F . F a n a n d A . J . B a r d , J. P h y s . C h e m . , 8 4 ( 1 9 8 0 ) 3 2 0 7 . I. I z u m i , F . F . F a n a n d A . J . B a r d , J. P h y s . C h e m . , 8 5 ( 1 9 8 1 ) 2 1 8 . T. K a n n o , T. O g u c h i , H. S a k u r a g i a n d K. T o k u m a r u , T e t r a h e d r o n L e t t . , 21 ( 1 9 8 0 ) 4 6 7 .
281
6 M.N. M o z z a n e g a , J.M. H e r r m a n n a n d P. P i c h a t , T e t r a h e d r o n L e t t . , ( 1 9 7 7 ) 2 9 6 5 . 7 S.N. F r a n k a n d A . J . B a r d , J. A m . C h e m . S o c . , 9 7 ( 1 9 7 5 ) 7 4 2 7 . 8 E.S. P y s h a n d N.C. Y a n g , J. A m . C h e m . S o c . , 8 5 ( 1 9 6 3 ) 2 1 2 4 . 9 V.D. P a r k e r a n d R . N . A d a m s , T e t r a h e d r o n L e t t . , ( 1 9 6 9 ) 1 7 2 1 . 1 0 M.E. P e o v e r a n d B.S. W h i t e , E l e c t r o c h i m . A c t a , 1 1 ( 1 9 6 6 ) 1 0 6 1 . 11 V.D. P a r k e r a n d B.E. B u r g e r t , T e t r a h e d r o n L e t t . , ( 1 9 6 8 ) 2 4 1 1 . 1 2 B. K r a e u t l e r a n d A . J . B a r d , J. A m . C h e m . S o c . , 1 0 0 ( 1 9 7 8 ) 4 3 1 7 . 1 3 R. T o m a t a n d A. R i g o , J. A p p L E l e c t r o e h e m . , 9 ( 1 9 7 9 ) 3 0 1 . 1 4 D.T. S a w y e r a n d M.J. G i b i a n , T e t r a h e d r o n , 3 5 ( 1 9 7 9 ) 1 4 7 1 . 1 5 D. D o b o s , E l e c t r o c h e m i c a l D a t a , Elsevier, A m s t e r d a m , 1 9 7 5 . 1 6 E.C. D u t o i t , R. C a r d o n a n d W.P. G o m e s , Bet. B u n s e n g e s . P h y s . C h e m . , 8 0 ( 1 9 7 6 ) 4 7 5 . 1 7 R. O g u c h i , T. K a n n o , H. S a k u r a g i a n d K. T o k u m a r u , A b s t r a c t s o f A n n u a l M e e t i n g o f t h e C h e m i c a l S o c i e t y o f J a p a n , I, 1 9 8 1 , p. 2 5 6 . 1 8 S. N a k a b a y a s h i , A. F u j i s h i m a a n d K. H o n d a , J. E l e c t r o a n a l . C h e m . , 1 1 1 ( 1 9 8 0 ) 3 9 1 .