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Effect of solvents on photocatalytic reduction of carbon dioxide using semiconductor photocatalysts
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
553
Effect of solvents on photocatalytic r e d u c t i o n of c a r b o n dioxide using s e m i c o n d u c t o r photocatalysts
Tsukasa Torimoto, Bi-Jin Liu, and Hiroshi Yoneyama* Department of Applied Chemistry, Faculty of Engineering, Osaka University Yamada-oka 2-1, Suita, Osaka 565, Japan
Photocatalytic reduction of carbon dioxide on TiO 2 nanocrystals embedded in SiO 2 matrices (Q-TiO2/SiO2) and bulk CdS particles with and without surface modification by several thiol compounds was investigated in various kinds of solvents. Formate and carbon monoxide were obtained as the major reduction products and the ratio of the former to the latter was increased with an increase in the dielectric constant of the solvents used for the use of Q-TiO2/SiO 2 and bare CdS particles as photocatalysts. The surface modification of CdS particles with thiol compounds was effective in enhancing the ratio of formate to carbon monoxide. The observed selectivity of CO 2 reduction products was explained well in terms of the stabilization of reaction intermediates on the photocatalyst surface.
INTRODUCTION Artificial photosynthesis has been intensively studied in view of the chemical storage of light energy. One of the promising approaches is to utilize photoinduced reductions of carbon dioxide using semiconductor photocatalysts [1-7], and so far, several compounds such as formate, carbon monoxide, methanol and methane have been reported. What kind of reduction products are obtained may depend on preparation conditions of the photocatalysts and the environments under which the photocatalytic experiments are carried out. The importance of the latter factor has been suggested from the results that changes in the charged conditions of the CdS photocatalyst surface [3], adsorption of the In3+ metal ion on CdS photocatalyst surface [4], metal deposition on TiO 2 photocatalysts [5], and changes in the dispersion of photocatalysts in zeolite matrices [6] changed selectivities of CO 2 reduction. In the case of photocatalytic reaction using solution systems, environments under which the CO 2 reduction occurs are changed depending on the kinds of solvents used, so that the reduction products seem to be varied. However, little is known about the role of solvents in the reduction behavior of CO 2. In this paper, photocatalytic reduction of CO 2 in various kinds of solvents with use of TiO 2 and CdS as photocatalysts and the role of the solvents in CO 2 reduction behavior are reported.
554
EXPERIMENTAL
TiO 2 nanocrystals embedded in SiO 2 matrices (Q-TiO2/SiO2) and surface-modified bulk CdS particles were used as the photocatalysts. Q-TiO2/SiO 2 was prepared by hydrolysis of Ti(OEt)4 in the presence of Si(OEt)4 [7]. 25 cm 3 ethanol solution containing 80 mM HC1 and 4.4 M H20 was added to an ethanol solution of the same volume which contained 0.10 M Ti(OEt)~ and 1.0 M Si(OEt)4, resulting in a sol containing TiO 2 and SiO 2. After stirring for 2 h, 0.14 cm ~ of sol were cast on a quartz plate (2.0 cm 2) to give a transparent gel film of Q-TiO2/SiO 2 containing 7 ~tmol of TiO 2. In order to modify the surface of CdS particles, five kinds of thiol compounds were dissolved in appropriate solvents, that is, 0.1 M 2-aminoethanethiol in 2-propanol, 0.1 and 0.5 M 1-dodecanethiol in 2-propanol, 0.1 M sodium 2-mercaptoethane sulfonate in water, and 0.1 M sodium sulfide in water. CdS particles (0.72g) were suspended in these solutions and the suspensions were agitated overnight. By repeating centrifugation and washing with water, followed by drying in vacuum, surface-modified CdS particles were prepared. The photoreduction experiments were carried out using CO2-saturated solutions containing the photocatalyst and 1 M 2-propanol as a hole scavenger. 5 cm 3 of the solution was put in a quartz cell whose top was sealed with a rubber septum and irradiated with light of wavelengths longer than 300 nm, which was obtained by passing light from a 500 W high pressure Hg lamp through a colored filter. R E S U L T S AND D I S C U S S I O N
The photocatalytic reduction of CO 2 on the bare CdS particles suspended in acetonitrile in the presence of 2-propanol resulted in the formation of formate and carbon monoxide as the reduction products with a simultaneous production of H 2 (Figure 1). As an oxidation product of 2-propanol, acetone was produced, and no other oxidation products were obtained. As shown in 100 <>
o
E r o ...I -o o EL
0 0 o ro r
1
75
50
e
L
LL
<>
"o" "o. ~
25
f
"'-
h m
0
2
4
6
Time / h
Figure 1 Time course of the production of formate ( II ), carbon monoxide ( 9 ) hydrogen ( V ) and acetone (<~) in acetonitrile solution containing 1.0 M 2propanol. The photocatalyst used was naked CdS.
20
40
60
80
Dielectric constant
Figure 2 The fraction of carbon monoxide as a function of the dielectric constant of the solvent used. The results were obtained by irradiation of the naked CdS (11) and Q-TiO2/SiO 2 photocatalyst ( I--I). Solvents used were (a) carbon tetrachloride, (b)dichloromethane, (c) 2-propanol, (d) propionitrile, (e) ethylene glycol monoethyl ether, (f) acetonitrile, (g) sulfolane, (h) propylene carbonate, and (i) water.
555 Figure 1, the amount of the products increased linearly with time, indicating that the activity of photocatalyst did not deteriorate during the photoreduction experiments. By comparing the sum of the amount of reduction products with that of the oxidation product, it was found that the chemical stoichiometry of the reduction and oxidation products were maintained. In order to investigate the effect of solvents on the reduction behaviors of CO 2 in more details, photoreduction experiments were carried out using the naked CdS and Q-TiO2/SiO 2 in various solutions. It was found that in all cases, formate and/or carbon monoxide were produced and no other CO 2 reduction products were detected. Figure 2 shows the fraction of carbon monoxide in CO 2 reduction products as a function of the dielectric constant of the solvent used. In both cases, the fraction of carbon monoxide decreased with increase of the dielectric constant, though the selectivity of the reduction reactions was a little different, that is, CdS has a tendency to produce more CO than Q-TiO2/SiO 2. These results are explained well in terms of stabilization of reaction intermediate. The CO 2o- anion radicals formed by reduction of CO 2 with photogenerated electrons may be adsorbed on the photocatalyst surface, the degree being dependent on the solvation of CO2~ In high polar solvent, CO 2~ is greatly stabilized with solvent molecules, resulting in the weak interaction between CO2~ photocatalyst surface, as shown in Scheme 1. Then carbon atom of CO2otends to react with a proton to give formate. On the other hand, CO2~ in low polar solvent is strongly adsorbed on the photocatalyst surface, and then an attack of oxygen atom of CO2owith a proton to yield CO becomes feasible. If this view is valid, the results shown in Figure 2 suggest that the CdS surface has a higher affinity for CO2o- than TiO 2.
O'~'.;;O H+ O'~c/OH/ --x-M-x-> \--x-I~1-x-- ~ CO+OH"
HCOOH
1 O0
-~ 0 "10 0 i,..
(A)
fl co 80
',0 tO
60
9-
20
40
a) l o w - polar solvents a
Solvation
I
H
--~ HCO0
b) high - polar solvents
r
1 O0
0 L
80
'*-0
60
c
d
e
(B)
g 40
.~
~U_
20
a
Scheme 1 Illustration of hypothetical reduction pathway of CO 2 on photocatalyst surface. M: metal ion site on the surface.
b
b
c
d
e
Photocatalyst
Figure 3 The fraction of C O 2 reduction products obtained by irradiation of various surface-modified CdS photocatalysts in acetonitrile (A), and dichloromethane (B). Photocatalysts of (a) to (e) were the same as those given in Table 1.
556 Table 1 The molar ratio of adsorbed thiol to CdS determined by elemental analyses.
Catalyst
Surface-modification condition
a
none
b
Ratio of adsorbed thiol to CdS (mol%)
surface coverage (%)
0
0
0.1M NH2(CH2)2SH
0.60
29
c
0.1M n-C 12H25SH
0.65
30
d
0.1M NaSO3(CH2)2SH
0.90
45
e
0.5M n-C lzH25SH
1.30
65
In order to confirm the above hypothesis, photoreduction experiments of C O 2 w e r e performed with use of the surface-modified CdS particles. Figure 3 shows the fraction of formate and carbon monoxide in CO 2 reduction products obtained for use of various kinds of CdS photocatalysts in acetonitnle and dichloromethane. It is clearly seen that the fraction of formate was varied greatly depending on the kind of CdS photocatalyst used, and in both solutions, it increased in the order of photocatalyst (a) to (e). Though the same modifier of 1-dodecanethiol was used in the photocatalysts (c) and (e), the fraction of CO 2 reduction products was greatly different. Table 1 shows the molar ratio of the adsorbed thiol compounds to CdS determined by elemental analyses, and also listed is the surface coverage with thiol compounds which were obtained by using the molar ratio of thiols to CdS and the average diameter of 50 nm of CdS particles and by assuming that the surface density of Cd 2+ sites was the same as that of (0001) facet of hexagonal CdS. Comparing the results in Figure 3 with the surface coverage by thiol compounds on the CdS surface shown in Table 1, the fraction of formate production seems to be related to the degree of the surface coverage, and the presence of the functional groups o f - N H 2 a n d - S O 3- in thiol compounds did not greatly alter this tendency. It seems likely that CO2o-anion radicals in low polar solvents tend to predominantly adsorb on the positively charged Cd 2+ sites, resulting in carbon monoxide formation. Since the thiol compounds should be bound to the surface Cd 2+ sites, the amount of the adsorbed CO,~oon Cd 2+ sites must decrease with increase of the degree of surface modification, and then formate production becomes predominant with increasing the surface coverage. REFERENCES
1. 2. 3. 4. 5. 6
T. Inoue, A. Fujishima, S. Konishi, and K. Honda, Nature 277 (1979) 637. A. Henglein, M. Guttieretz, and C. H. Fischer, Ber. Bunsenges. Phys. Chem. 88 (1984) 170. H. Inoue, R. Nakamura, and H. Yoneyama, Chem. Lett. (1994) 1227. M. Kanemoto, M. Nomura, Y. Wada, T. Akano, and S. Yanagida, Chem. Lett. (1993)1687. Z. Goren, I. Willner, A. J. Nelson, and A. J. Frank, J. Phys. Chem. 94 (1990) 3784. M. Anpo, H. Yamashita, Y. Ichihashi, Y. Fujii, and M. Honda, J. Phys. Chem. B. 101 (1997) 2632. 7. H. Inoue, T. Matsuyama, B. -J. Liu, T. Sakata, H. Mori, and H. Yoneyama, Chem. Lett. 653 (1994).
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
553
Effect of solvents on photocatalytic r e d u c t i o n of c a r b o n dioxide using s e m i c o n d u c t o r photocatalysts
Tsukasa Torimoto, Bi-Jin Liu, and Hiroshi Yoneyama* Department of Applied Chemistry, Faculty of Engineering, Osaka University Yamada-oka 2-1, Suita, Osaka 565, Japan
Photocatalytic reduction of carbon dioxide on TiO 2 nanocrystals embedded in SiO 2 matrices (Q-TiO2/SiO2) and bulk CdS particles with and without surface modification by several thiol compounds was investigated in various kinds of solvents. Formate and carbon monoxide were obtained as the major reduction products and the ratio of the former to the latter was increased with an increase in the dielectric constant of the solvents used for the use of Q-TiO2/SiO 2 and bare CdS particles as photocatalysts. The surface modification of CdS particles with thiol compounds was effective in enhancing the ratio of formate to carbon monoxide. The observed selectivity of CO 2 reduction products was explained well in terms of the stabilization of reaction intermediates on the photocatalyst surface.
INTRODUCTION Artificial photosynthesis has been intensively studied in view of the chemical storage of light energy. One of the promising approaches is to utilize photoinduced reductions of carbon dioxide using semiconductor photocatalysts [1-7], and so far, several compounds such as formate, carbon monoxide, methanol and methane have been reported. What kind of reduction products are obtained may depend on preparation conditions of the photocatalysts and the environments under which the photocatalytic experiments are carried out. The importance of the latter factor has been suggested from the results that changes in the charged conditions of the CdS photocatalyst surface [3], adsorption of the In3+ metal ion on CdS photocatalyst surface [4], metal deposition on TiO 2 photocatalysts [5], and changes in the dispersion of photocatalysts in zeolite matrices [6] changed selectivities of CO 2 reduction. In the case of photocatalytic reaction using solution systems, environments under which the CO 2 reduction occurs are changed depending on the kinds of solvents used, so that the reduction products seem to be varied. However, little is known about the role of solvents in the reduction behavior of CO 2. In this paper, photocatalytic reduction of CO 2 in various kinds of solvents with use of TiO 2 and CdS as photocatalysts and the role of the solvents in CO 2 reduction behavior are reported.
554
EXPERIMENTAL
TiO 2 nanocrystals embedded in SiO 2 matrices (Q-TiO2/SiO2) and surface-modified bulk CdS particles were used as the photocatalysts. Q-TiO2/SiO 2 was prepared by hydrolysis of Ti(OEt)4 in the presence of Si(OEt)4 [7]. 25 cm 3 ethanol solution containing 80 mM HC1 and 4.4 M H20 was added to an ethanol solution of the same volume which contained 0.10 M Ti(OEt)~ and 1.0 M Si(OEt)4, resulting in a sol containing TiO 2 and SiO 2. After stirring for 2 h, 0.14 cm ~ of sol were cast on a quartz plate (2.0 cm 2) to give a transparent gel film of Q-TiO2/SiO 2 containing 7 ~tmol of TiO 2. In order to modify the surface of CdS particles, five kinds of thiol compounds were dissolved in appropriate solvents, that is, 0.1 M 2-aminoethanethiol in 2-propanol, 0.1 and 0.5 M 1-dodecanethiol in 2-propanol, 0.1 M sodium 2-mercaptoethane sulfonate in water, and 0.1 M sodium sulfide in water. CdS particles (0.72g) were suspended in these solutions and the suspensions were agitated overnight. By repeating centrifugation and washing with water, followed by drying in vacuum, surface-modified CdS particles were prepared. The photoreduction experiments were carried out using CO2-saturated solutions containing the photocatalyst and 1 M 2-propanol as a hole scavenger. 5 cm 3 of the solution was put in a quartz cell whose top was sealed with a rubber septum and irradiated with light of wavelengths longer than 300 nm, which was obtained by passing light from a 500 W high pressure Hg lamp through a colored filter. R E S U L T S AND D I S C U S S I O N
The photocatalytic reduction of CO 2 on the bare CdS particles suspended in acetonitrile in the presence of 2-propanol resulted in the formation of formate and carbon monoxide as the reduction products with a simultaneous production of H 2 (Figure 1). As an oxidation product of 2-propanol, acetone was produced, and no other oxidation products were obtained. As shown in 100 <>
o
E r o ...I -o o EL
0 0 o ro r
1
75
50
e
L
LL
<>
"o" "o. ~
25
f
"'-
h m
0
2
4
6
Time / h
Figure 1 Time course of the production of formate ( II ), carbon monoxide ( 9 ) hydrogen ( V ) and acetone (<~) in acetonitrile solution containing 1.0 M 2propanol. The photocatalyst used was naked CdS.
20
40
60
80
Dielectric constant
Figure 2 The fraction of carbon monoxide as a function of the dielectric constant of the solvent used. The results were obtained by irradiation of the naked CdS (11) and Q-TiO2/SiO 2 photocatalyst ( I--I). Solvents used were (a) carbon tetrachloride, (b)dichloromethane, (c) 2-propanol, (d) propionitrile, (e) ethylene glycol monoethyl ether, (f) acetonitrile, (g) sulfolane, (h) propylene carbonate, and (i) water.
555 Figure 1, the amount of the products increased linearly with time, indicating that the activity of photocatalyst did not deteriorate during the photoreduction experiments. By comparing the sum of the amount of reduction products with that of the oxidation product, it was found that the chemical stoichiometry of the reduction and oxidation products were maintained. In order to investigate the effect of solvents on the reduction behaviors of CO 2 in more details, photoreduction experiments were carried out using the naked CdS and Q-TiO2/SiO 2 in various solutions. It was found that in all cases, formate and/or carbon monoxide were produced and no other CO 2 reduction products were detected. Figure 2 shows the fraction of carbon monoxide in CO 2 reduction products as a function of the dielectric constant of the solvent used. In both cases, the fraction of carbon monoxide decreased with increase of the dielectric constant, though the selectivity of the reduction reactions was a little different, that is, CdS has a tendency to produce more CO than Q-TiO2/SiO 2. These results are explained well in terms of stabilization of reaction intermediate. The CO 2o- anion radicals formed by reduction of CO 2 with photogenerated electrons may be adsorbed on the photocatalyst surface, the degree being dependent on the solvation of CO2~ In high polar solvent, CO 2~ is greatly stabilized with solvent molecules, resulting in the weak interaction between CO2~ photocatalyst surface, as shown in Scheme 1. Then carbon atom of CO2otends to react with a proton to give formate. On the other hand, CO2~ in low polar solvent is strongly adsorbed on the photocatalyst surface, and then an attack of oxygen atom of CO2owith a proton to yield CO becomes feasible. If this view is valid, the results shown in Figure 2 suggest that the CdS surface has a higher affinity for CO2o- than TiO 2.
O'~'.;;O H+ O'~c/OH/ --x-M-x-> \--x-I~1-x-- ~ CO+OH"
HCOOH
1 O0
-~ 0 "10 0 i,..
(A)
fl co 80
',0 tO
60
9-
20
40
a) l o w - polar solvents a
Solvation
I
H
--~ HCO0
b) high - polar solvents
r
1 O0
0 L
80
'*-0
60
c
d
e
(B)
g 40
.~
~U_
20
a
Scheme 1 Illustration of hypothetical reduction pathway of CO 2 on photocatalyst surface. M: metal ion site on the surface.
b
b
c
d
e
Photocatalyst
Figure 3 The fraction of C O 2 reduction products obtained by irradiation of various surface-modified CdS photocatalysts in acetonitrile (A), and dichloromethane (B). Photocatalysts of (a) to (e) were the same as those given in Table 1.
556 Table 1 The molar ratio of adsorbed thiol to CdS determined by elemental analyses.
Catalyst
Surface-modification condition
a
none
b
Ratio of adsorbed thiol to CdS (mol%)
surface coverage (%)
0
0
0.1M NH2(CH2)2SH
0.60
29
c
0.1M n-C 12H25SH
0.65
30
d
0.1M NaSO3(CH2)2SH
0.90
45
e
0.5M n-C lzH25SH
1.30
65
In order to confirm the above hypothesis, photoreduction experiments of C O 2 w e r e performed with use of the surface-modified CdS particles. Figure 3 shows the fraction of formate and carbon monoxide in CO 2 reduction products obtained for use of various kinds of CdS photocatalysts in acetonitnle and dichloromethane. It is clearly seen that the fraction of formate was varied greatly depending on the kind of CdS photocatalyst used, and in both solutions, it increased in the order of photocatalyst (a) to (e). Though the same modifier of 1-dodecanethiol was used in the photocatalysts (c) and (e), the fraction of CO 2 reduction products was greatly different. Table 1 shows the molar ratio of the adsorbed thiol compounds to CdS determined by elemental analyses, and also listed is the surface coverage with thiol compounds which were obtained by using the molar ratio of thiols to CdS and the average diameter of 50 nm of CdS particles and by assuming that the surface density of Cd 2+ sites was the same as that of (0001) facet of hexagonal CdS. Comparing the results in Figure 3 with the surface coverage by thiol compounds on the CdS surface shown in Table 1, the fraction of formate production seems to be related to the degree of the surface coverage, and the presence of the functional groups o f - N H 2 a n d - S O 3- in thiol compounds did not greatly alter this tendency. It seems likely that CO2o-anion radicals in low polar solvents tend to predominantly adsorb on the positively charged Cd 2+ sites, resulting in carbon monoxide formation. Since the thiol compounds should be bound to the surface Cd 2+ sites, the amount of the adsorbed CO,~oon Cd 2+ sites must decrease with increase of the degree of surface modification, and then formate production becomes predominant with increasing the surface coverage. REFERENCES
1. 2. 3. 4. 5. 6
T. Inoue, A. Fujishima, S. Konishi, and K. Honda, Nature 277 (1979) 637. A. Henglein, M. Guttieretz, and C. H. Fischer, Ber. Bunsenges. Phys. Chem. 88 (1984) 170. H. Inoue, R. Nakamura, and H. Yoneyama, Chem. Lett. (1994) 1227. M. Kanemoto, M. Nomura, Y. Wada, T. Akano, and S. Yanagida, Chem. Lett. (1993)1687. Z. Goren, I. Willner, A. J. Nelson, and A. J. Frank, J. Phys. Chem. 94 (1990) 3784. M. Anpo, H. Yamashita, Y. Ichihashi, Y. Fujii, and M. Honda, J. Phys. Chem. B. 101 (1997) 2632. 7. H. Inoue, T. Matsuyama, B. -J. Liu, T. Sakata, H. Mori, and H. Yoneyama, Chem. Lett. 653 (1994).