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Electrochemical reduction of CO2 with high current density in a CO2 + methanol medium at various metal electrodes
J OIJnNAL OF
ELSEVIER
Journal of Electroanalytical Chemistry 404 (1996) 299-302
Short communication
Electrochemical reduction of CO 2 with high current density in a CO 2 + methanol medium at various metal electrodes Tomonori Saeki a,l, Kazuhito Hashimoto a, Naokazu Kimura b, Koji Omata b, Akira Fujishima a,* a Department of Applied Chemistry, Faculty of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan b Electric Power Development Corporation, 6-15-1 Ginza, Chuo-ku, Tokyo 104, Japan Received 11 September 1995; in revised form 22 September 1995
Abstract Various metals were used as working electrodes in the electrochemical reduction of highly concentrated CO 2 in a CO 2 + methanol medium. Reduction products were CO, CH4, C2H 4 and methyl formate (HCOOCH3). Methyl formate was formed by the reaction between solvent, methanol, and a CO 2 reduction product, formic acid, which corresponds to formic acid formation in aqueous systems. Basically, most electrodes gave the same principal product both in the present system and in the aqueous system. At W, Ti and Pt electrodes, CO 2 reduction was inefficient. Sn and Pb electrodes were active in formate production. However, CO was formed much more efficiently in the present system than in the aqueous system. It was indicated that Sn and Pb served as electrodes catalyzing formate production, while the supporting electrolyte, tetrabutylammonium cation, promoted CO formation. Electrolysis at Ag, Zn and Pd electrodes yielded CO mainly. Hydrocarbon formation at a Cu electrode was less efficient than in aqueous systems. However, hydrocarbon was formed efficiently at an Ni electrode. These differences in hydrocarbon formation in the present system, in comparison with aqueous systems, could be explained by the balance between hydrogen atom and CO 2 reduction intermediates on the electrode surface. Keyword~: Electrochemical reduction; CO2; Metal electrodes
1. Introduction High rate reduction of CO 2 into useful products with low input energy is one of the most important subjects in electrochemistry. Recently, the electrochemical reduction of CO 2 with high current density has been studied aggressively by many researchers using gas diffusion electrodes [1,2] and high pressure aqueous systems [3-5]. We are studying the high rate reduction of CO 2 in a CO 2 + methanol mixture [6-9]. Concentration of CO 2 in this medium can be controlled from 0 to 100% by changing the pressure of CO 2 from 0 atm. to liquefaction pressure. In this system, a high current density of CO 2 reduction .as
* Corresponding author. ~Present address: Production Engineering Research Laboratory, Hitachi Co. Ltd., 292 Yoshida-cho, Totsuka-ku, Yokohama 244, Japan. 0022-0728/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSDI 0 0 2 2 - 0 7 2 8 ( 9 5 ) 0 4 3 7 4 - 8
well as a high current efficiency is achieved. The total current density of CO 2 reduction reaches the value that is used in other industrial electrolyses. The concentration of CO 2 is sufficiently high that the reaction is not controlled by the mass transfer process of CO 2 in this system [6,7]. W e have also reported a significant effect of supporting salt on the product distribution [8,9]. In order to reduce CO 2 to useful products with low input energy, the design of electrocatalysts, both heterogeneous and homogeneous, is important. Many homogeneous catalysts have been tested in the electroreduction of CO 2 in aqueous systems [10-12]. It is also well known that metal electrodes themselves serve as catalysts; the product distribution strongly depends on the electrode material used [13-15]. We will report here the effect of such an electrode used on the high rate electrochemical reduction of CO 2 in the CO 2 + methanol system. The results will be discussed in relation to those in aqueous systems.
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2. Experimental The electrodes used were Ti, W, Ni, Pd, Pt, Cu, Ag, Zn, Sn and Pb in the form of wire. The purity of the metals was higher than 99.99%. Copper was donated by Sumitomo Electric Industry Co. and the other metals were purchased from Nilaco Co. The counter electrode used was a Pt wire. Both working and counter electrodes were sealed in a stainless steel tube by epoxy resin (Tort Seal ~, Varian Co.). The working electrode was etched chemically or electrochemically prior to use: Cu, Ag, Ni and Zn electrochemically in a concentrated H3PO 4 solution, Ti electrochemically in a n H 2 0 2 + HF solution, W chemically in an HF solution, Sn and Pb electrochemically in an HClO 4 + (CH3CO)20 and Pt chemically in aqua regia. Palladium was cleaned by cyclic negative-positive scans in a dilute H 2 S O 4 solution until residual current was minimized. Tetrabutylammonium perchlorate (TBAP, Tokyo Kasei) was used as a supporting electrolyte. This salt was electrochemically stable in our system. Only at an Ni electrode at 2000 mA cm -2 did the salt decompose slightly, forming 1-butene and tributylamine. The current efficiency of this decomposition was, however, negligible (less than 0.1%). A glass test tube containing 0.3 M (1 M = 1 mol dm -3) TBAP + methanol solution was placed in a high pressure cell. Carbon dioxide of controlled pressure was then introduced into this solution. The system was kept for about 1 h until equilibrium. Electrolyses were performed galvanostatically at 25 ( + 0.5)°C, 41 ( + 0 . 5 ) atm., mainly at 200 mA cm -2, using a potentio-galvanostat (Hokuto HA-501). After the electrolysis, high pressure gas was released into a storage tank to reduce its pressure. Reaction products were quantitatively analyzed by gas chromatography. Hydrogen was separated by a molecular sieve 13X column and detected by a thermal conductivity detector (TCD, Hitachi GC-163). Carbon monoxide was separated by Porapak Q and converted catalytically to methane over Ru + AI203 at 450°C, and was then detected by a flame ionization detector (FID, Ohkura GC-202). Hydrocarbons were separated by P0rapak R and detected by FID (Ohkura GC-202). Methyl formate (HCOOCH3) and dimethoxymethane (DMM, CH3OCH2OCH 3) were separated by PEG 1000 and detected by FID (Ohkura GC-103).
HCOOCH 3 CO C2H6 C2H4 CH 4
Sn
H2
1
Pb
Ag Zn Pd Cu W Ti Pt Ni 0
20
40
60
80
100
120
Current Efficiency / % Fig. 1. Electrochemical reduction of CO 2 at various metal electrodes in a CO 2 + methanol medium. Electrolyses were performed galvanostatically at 25°C and 41 atm., galvanostatically at 200 m A cm z.
was confirmed that methyl formate was formed from formic acid and methanol; the former was the CO 2 reduction product and the latter was a component of the solvent. Methyl formate in the present medium and formic acid in aqueous media are therefore produced in the same electrode reaction. Both reactions will be referred to hereafter as formate production. The current efficiencies of the reduction products are shown in Fig. 1. Formate production is fairly favorable at all electrodes in comparison with that in aqueous systems. For example, at Pt and Ni electrodes the current efficiencies of formic acid formation in aqueous systems are lower than 5% [15], while those of methyl formate production in the present system are higher than 20%. Nakagawa et al. [3] studied the electroreduction of high pressure CO 2 in an aqueous system and reported that the current efficiency of formic acid was higher than that under atmospheric pressure at most Group VIII metal electrodes. This report and our result indicate that formate production is favored by high CO 2 pressure. Each metal shows the characteristic activity on high rate reduction of CO 2. The electrode activity of metals can be classified into several groups as will be described below.
3. Results and discussion
3.1. W, Ti and Pt electrodes
The reduction product distribution depends strongly on the electrode material in the CO 2 + methanol system, in a way similar to that in aqueous systems. Detected reaction products are H e, CO, CH 4, C2H4, C2H 6, HCOOCH 3 and DMM. Experiments using labeled CO 2 a n d / o r methanol confirmed DMM to be an oxidation product of the solvent and the others to be the CO 2 reduction products [6,7]. It
These metals show poor activity in the electrochemical reduction of CO 2 in aqueous systems at room temperature. Total efficiencies of CO 2 reduction at W, Ti and Pt electrodes in an aqueous system are 2.0%, 1.6% and 0.0% respectively [14]. In the present system, although hydrogen production is predominant at these electrodes, the efficiency of CO 2 reduction is enhanced. Formate production
T. Saeki et al. / Journal of Electroanalytical Chemistry 404 (1996) 299-302
120
is particularly favored; 10.1% at W, 5.1% at Ti and 29.0% at Pt electrodes.
. . . . . . . .
301 ,
. . . . . . . .
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3.2. Pb and Sn electrodes
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In aqueous systems, formic acid is produced selectively at Pb and Sn electrodes [13], the current efficiency of CO production being less than 10%. In the present system, formate production is also the main reaction, but CO is produced simultaneously with an efficiency of more than 20%. This result can be explained by the effect of the supporting salt on the product distribution. We reported that the formation of CO is favored when tetrabutylammonium (TBA) salt is used as a supporting electrolyte, in comparison with lithium or tetraethylammonium salts [8,9]. In the present electrolyses, Pb and Sn electrodes may catalyze formate production, whereas the TBAP supporting electrolyte may promote CO formation. 3.3. Ag, Zn and Pd electrodes Carbon monoxide is the principal product at Ag, Zn, Pd and Cu electrodes in the present system. In aqueous systems, CO 2 is selectively reduced to CO at a Ag electrode [13-15]. At a Zn electrode, both CO and formate are produced in aqueous systems with comparable current efficiency [13-15], or CO is formed more selectively at a certain potential [14]. At a Pd electrode, although the efficiency of hydrogen production is rather high, CO is the main product from CO 2 in aqueous systems [14,15]. These electrodes show similar electrode activity on CO 2 reduction both in aqueous systems and in the present system. We would emphasize here that the total current efficiency of CO 2 reduction is higher in the CO 2 + methanol system than in aqueous systems. 3.4. Hydrocarbon formation at Cu and Ni electrodes Hydrocarbon formation is of great interest in electrochemistry, since more electrons transfer to one molecule of CO 2 than in CO and formate production. Copper has been known as the only metal at which CO 2 is reduced efficiently to hydrocarbons in aqueous systems [13-15]. Hydrocarbon formation at a Cu electrode in the present system is, however, less efficient than that of the other products when TBAP is used as a supporting salt. It is also less efficient than that in aqueous systems under both atmospheric pressure and high pressure [5]. At a Ni electrode, however, hydrocarbon formation is rather favored in the present system. The product distribution depends on the reduction current density, as is shown in Fig. 2. Hydrocarbon formation is favored in the range 20-2000 mA cm -2. The maximum efficiency of hydrocarbon formation is 23.5%, which corresponds to the partial current density of 200 mA cm -2. This electrode behavior is rather different from that in aqueous ambient
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j / mA.cm-2 Fig. 2. Electrochemical reduction of CO 2 at a Ni electrode in a CO 2 + methanol medium. Electrolyses were performed galvanostatically at 25°C and 41 atm. A: H 2 ( 0 ) and HCOOCH 3 (E]). B: CO (D), CH 4 ( O ) , C2H 4 (~x) and C2H 6 ( ~ ) .
systems; H 2 is the major product at room temperature and CO is efficiently formed at about 0°C [13]. The role of Cu and Ni electrodes in hydrocarbon formation can be understood qualitatively in terms of the balance of hydrogen atom and carbon atom concentrations on the electrode surface [5,7]. The CO 2 concentration is much higher in the CO 2 + methanol system than in the aqueous system. The supply of CO 2 to the electrode surface is, therefore, more sufficient in the CO 2 + methanol system. At a Cu electrode, the concentration of CO 2 reduction intermediates and hydrogen atoms on the surface may be in good balance at atmospheric pressure. However, CO 2 reduction proceeds so efficiently that concentration of its reaction intermediates may be excessive in the CO 2 + methanol system. Hydrogenation of CO z reduction intermediates therefore does not proceed efficiently, resulting in less hydrocarbon formation in the CO2-methanol system than in an aqueous system. However, at a Ni electrode, the concentration of hydrogen atoms at the electrode surface may be too large for efficient CO 2 hydrogenation, owing to the low hydrogen-overvoltage of Ni. Hydrogen production is the principal reaction at atmospheric pressure at room temperature [14]. Although CO 2 reduction efficiency is low, the main products from CO 2 are hydrocarbons. This indicates that Ni has a high activity of hydrogenation of CO z reduction intermediates. The efficiency of CO 2 reduction is enhanced in such systems of high CO 2 concentration as an aqueous high pressure system and the present CO 2 + methanol system. In these systems, since the concentration of CO 2 reduction intermediates on the
302
T. Saeki et al./ Journal of Electroanalytical Chemistry 404 (1996) 299-302
electrode is larger, hydrogenation of the intermediates proceeds more efficiently. Efficiency of hydrocarbon formation increases with increasing efficiency of CO 2 concentration, in the order: aqueous ambient system [14], aqueous high pressure system [4] and the present CO 2 + methanol system.
Acknowledgements The present work was partially supported by a Grant-inAid for Scientific Research from the Ministry of Education, Science and Culture of Japan, and by the New Energy Development Organization (NEDO).
References 4. Summary CO 2 reduction activity of several metal electrodes in a CO 2 + methanol system is discussed. Formate production in the present system is more efficient than that in aqueous system at most metals. At the metals which show poor catalytic activity on CO 2 reduction in aqueous systems (W, Ti and Pt), although hydrogen production proceeds mainly also in the present system, CO 2 reduction efficiency is enhanced in comparison with the aqueous systems. At the metals at which formate production proceeds exclusively in aqueous systems (Sn and Pb), formate production takes place mainly, but CO formation is also rather favored. The enhanced formation of CO is understood in terms of the effect of supporting electrolyte. The electrodes catalyze formate production effectively, while tetrabutylammonium cation, the supporting salt, promotes CO formation. Ag, Zn and Pd, at which CO 2 is reduced mainly to CO in aqueous systems, show similar electrode activities in the present system. The efficiency of hydrocarbon formation at a Cu electrode is lower, whereas that at a Ni electrode is much higher, than those in aqueous systems. This is explained qualitatively by the balance of hydrogen and carbon atom concentrations on the electrode surface.
[1] R.L. Cook, R.C. MacDuff and A.F. Sammels, J. Electrochem. Soc., 137 (1990) 607. [2] N. Furuya and K. Matsui, J. Electroanal. Chem., 271 (1989) 181. [3] S. Nakagawa, A. Kudo, M. Azuma and T. Sakata, J. Electroanal. Chem., 308 (1991) 339. [4] A. Kudo, S. Nakagawa, A. Tsuneto and T. Sakata, J. Electrochem. Soc., 140 (1993) 1541. [5] K. Hara, A. Tsuneto, A. Kudo and T. Sakata, J. Electrochem. Soc., 141 (1994) 2097. [6] T. Saeki, K. Hashimoto, Y. Noguchi, K. Omata and A. Fujishima, J. Electrochem. Soc., 141 (1994) L130. [7] T. Saeki, K. Hashimoto, N. Kimura, K. Omata and A. Fujishima, J. Phys. Chem,, 99 (1995) 8440. [8] T. Saeki, K. Hashimoto, N. Kimura, K. Omata and A. Fujishima, J. Electroanal. Chem., 390 (1995) 77. [9] T. Saeki, K. Hashimoto, N. Kimura, K. Omata and A. Fujishima, Chem. Lett., (1995) 361. [10] M. Beley, J.-P. Collin, R. Ruppert and J.-P. Sauvage, J. Chem. Soc., Chem. Commun., (1984) 1315. [11] B.R. Eggins, J.T.S. Irvine and J. Grimshaw, J. Electroanal. Chem., 266 (1989) 125. [12] H. Ishida, K. Tanaka and T. Tanaka, Chem. Lett., (1987) 1035. [13] M. Azuma, K. Hashimoto, M. Hiramoto, M. Watanabe and T. Sakata, J. Electrochem. Soc., 137 (1990) 1772. [14] H. Noda, S. Ikeda, Y. Oda, K. Imai, M. Maeda and K. Ito, Bull. Chem. Soc. Jpn., 63 (1990) 2459. [15] Y. Hori, H. Wakebe, T. Tsukamoto and O. Koga, Electrochim. Acta, 39 (1994) 1833.
ELSEVIER
Journal of Electroanalytical Chemistry 404 (1996) 299-302
Short communication
Electrochemical reduction of CO 2 with high current density in a CO 2 + methanol medium at various metal electrodes Tomonori Saeki a,l, Kazuhito Hashimoto a, Naokazu Kimura b, Koji Omata b, Akira Fujishima a,* a Department of Applied Chemistry, Faculty of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan b Electric Power Development Corporation, 6-15-1 Ginza, Chuo-ku, Tokyo 104, Japan Received 11 September 1995; in revised form 22 September 1995
Abstract Various metals were used as working electrodes in the electrochemical reduction of highly concentrated CO 2 in a CO 2 + methanol medium. Reduction products were CO, CH4, C2H 4 and methyl formate (HCOOCH3). Methyl formate was formed by the reaction between solvent, methanol, and a CO 2 reduction product, formic acid, which corresponds to formic acid formation in aqueous systems. Basically, most electrodes gave the same principal product both in the present system and in the aqueous system. At W, Ti and Pt electrodes, CO 2 reduction was inefficient. Sn and Pb electrodes were active in formate production. However, CO was formed much more efficiently in the present system than in the aqueous system. It was indicated that Sn and Pb served as electrodes catalyzing formate production, while the supporting electrolyte, tetrabutylammonium cation, promoted CO formation. Electrolysis at Ag, Zn and Pd electrodes yielded CO mainly. Hydrocarbon formation at a Cu electrode was less efficient than in aqueous systems. However, hydrocarbon was formed efficiently at an Ni electrode. These differences in hydrocarbon formation in the present system, in comparison with aqueous systems, could be explained by the balance between hydrogen atom and CO 2 reduction intermediates on the electrode surface. Keyword~: Electrochemical reduction; CO2; Metal electrodes
1. Introduction High rate reduction of CO 2 into useful products with low input energy is one of the most important subjects in electrochemistry. Recently, the electrochemical reduction of CO 2 with high current density has been studied aggressively by many researchers using gas diffusion electrodes [1,2] and high pressure aqueous systems [3-5]. We are studying the high rate reduction of CO 2 in a CO 2 + methanol mixture [6-9]. Concentration of CO 2 in this medium can be controlled from 0 to 100% by changing the pressure of CO 2 from 0 atm. to liquefaction pressure. In this system, a high current density of CO 2 reduction .as
* Corresponding author. ~Present address: Production Engineering Research Laboratory, Hitachi Co. Ltd., 292 Yoshida-cho, Totsuka-ku, Yokohama 244, Japan. 0022-0728/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSDI 0 0 2 2 - 0 7 2 8 ( 9 5 ) 0 4 3 7 4 - 8
well as a high current efficiency is achieved. The total current density of CO 2 reduction reaches the value that is used in other industrial electrolyses. The concentration of CO 2 is sufficiently high that the reaction is not controlled by the mass transfer process of CO 2 in this system [6,7]. W e have also reported a significant effect of supporting salt on the product distribution [8,9]. In order to reduce CO 2 to useful products with low input energy, the design of electrocatalysts, both heterogeneous and homogeneous, is important. Many homogeneous catalysts have been tested in the electroreduction of CO 2 in aqueous systems [10-12]. It is also well known that metal electrodes themselves serve as catalysts; the product distribution strongly depends on the electrode material used [13-15]. We will report here the effect of such an electrode used on the high rate electrochemical reduction of CO 2 in the CO 2 + methanol system. The results will be discussed in relation to those in aqueous systems.
300
T. Saeki et al. /Journal o f Electroanalytical Chemistry 404 (1996) 299-302
2. Experimental The electrodes used were Ti, W, Ni, Pd, Pt, Cu, Ag, Zn, Sn and Pb in the form of wire. The purity of the metals was higher than 99.99%. Copper was donated by Sumitomo Electric Industry Co. and the other metals were purchased from Nilaco Co. The counter electrode used was a Pt wire. Both working and counter electrodes were sealed in a stainless steel tube by epoxy resin (Tort Seal ~, Varian Co.). The working electrode was etched chemically or electrochemically prior to use: Cu, Ag, Ni and Zn electrochemically in a concentrated H3PO 4 solution, Ti electrochemically in a n H 2 0 2 + HF solution, W chemically in an HF solution, Sn and Pb electrochemically in an HClO 4 + (CH3CO)20 and Pt chemically in aqua regia. Palladium was cleaned by cyclic negative-positive scans in a dilute H 2 S O 4 solution until residual current was minimized. Tetrabutylammonium perchlorate (TBAP, Tokyo Kasei) was used as a supporting electrolyte. This salt was electrochemically stable in our system. Only at an Ni electrode at 2000 mA cm -2 did the salt decompose slightly, forming 1-butene and tributylamine. The current efficiency of this decomposition was, however, negligible (less than 0.1%). A glass test tube containing 0.3 M (1 M = 1 mol dm -3) TBAP + methanol solution was placed in a high pressure cell. Carbon dioxide of controlled pressure was then introduced into this solution. The system was kept for about 1 h until equilibrium. Electrolyses were performed galvanostatically at 25 ( + 0.5)°C, 41 ( + 0 . 5 ) atm., mainly at 200 mA cm -2, using a potentio-galvanostat (Hokuto HA-501). After the electrolysis, high pressure gas was released into a storage tank to reduce its pressure. Reaction products were quantitatively analyzed by gas chromatography. Hydrogen was separated by a molecular sieve 13X column and detected by a thermal conductivity detector (TCD, Hitachi GC-163). Carbon monoxide was separated by Porapak Q and converted catalytically to methane over Ru + AI203 at 450°C, and was then detected by a flame ionization detector (FID, Ohkura GC-202). Hydrocarbons were separated by P0rapak R and detected by FID (Ohkura GC-202). Methyl formate (HCOOCH3) and dimethoxymethane (DMM, CH3OCH2OCH 3) were separated by PEG 1000 and detected by FID (Ohkura GC-103).
HCOOCH 3 CO C2H6 C2H4 CH 4
Sn
H2
1
Pb
Ag Zn Pd Cu W Ti Pt Ni 0
20
40
60
80
100
120
Current Efficiency / % Fig. 1. Electrochemical reduction of CO 2 at various metal electrodes in a CO 2 + methanol medium. Electrolyses were performed galvanostatically at 25°C and 41 atm., galvanostatically at 200 m A cm z.
was confirmed that methyl formate was formed from formic acid and methanol; the former was the CO 2 reduction product and the latter was a component of the solvent. Methyl formate in the present medium and formic acid in aqueous media are therefore produced in the same electrode reaction. Both reactions will be referred to hereafter as formate production. The current efficiencies of the reduction products are shown in Fig. 1. Formate production is fairly favorable at all electrodes in comparison with that in aqueous systems. For example, at Pt and Ni electrodes the current efficiencies of formic acid formation in aqueous systems are lower than 5% [15], while those of methyl formate production in the present system are higher than 20%. Nakagawa et al. [3] studied the electroreduction of high pressure CO 2 in an aqueous system and reported that the current efficiency of formic acid was higher than that under atmospheric pressure at most Group VIII metal electrodes. This report and our result indicate that formate production is favored by high CO 2 pressure. Each metal shows the characteristic activity on high rate reduction of CO 2. The electrode activity of metals can be classified into several groups as will be described below.
3. Results and discussion
3.1. W, Ti and Pt electrodes
The reduction product distribution depends strongly on the electrode material in the CO 2 + methanol system, in a way similar to that in aqueous systems. Detected reaction products are H e, CO, CH 4, C2H4, C2H 6, HCOOCH 3 and DMM. Experiments using labeled CO 2 a n d / o r methanol confirmed DMM to be an oxidation product of the solvent and the others to be the CO 2 reduction products [6,7]. It
These metals show poor activity in the electrochemical reduction of CO 2 in aqueous systems at room temperature. Total efficiencies of CO 2 reduction at W, Ti and Pt electrodes in an aqueous system are 2.0%, 1.6% and 0.0% respectively [14]. In the present system, although hydrogen production is predominant at these electrodes, the efficiency of CO 2 reduction is enhanced. Formate production
T. Saeki et al. / Journal of Electroanalytical Chemistry 404 (1996) 299-302
120
is particularly favored; 10.1% at W, 5.1% at Ti and 29.0% at Pt electrodes.
. . . . . . . .
301 ,
. . . . . . . .
,
•
.
.
A
100 80
3.2. Pb and Sn electrodes
60 •
In aqueous systems, formic acid is produced selectively at Pb and Sn electrodes [13], the current efficiency of CO production being less than 10%. In the present system, formate production is also the main reaction, but CO is produced simultaneously with an efficiency of more than 20%. This result can be explained by the effect of the supporting salt on the product distribution. We reported that the formation of CO is favored when tetrabutylammonium (TBA) salt is used as a supporting electrolyte, in comparison with lithium or tetraethylammonium salts [8,9]. In the present electrolyses, Pb and Sn electrodes may catalyze formate production, whereas the TBAP supporting electrolyte may promote CO formation. 3.3. Ag, Zn and Pd electrodes Carbon monoxide is the principal product at Ag, Zn, Pd and Cu electrodes in the present system. In aqueous systems, CO 2 is selectively reduced to CO at a Ag electrode [13-15]. At a Zn electrode, both CO and formate are produced in aqueous systems with comparable current efficiency [13-15], or CO is formed more selectively at a certain potential [14]. At a Pd electrode, although the efficiency of hydrogen production is rather high, CO is the main product from CO 2 in aqueous systems [14,15]. These electrodes show similar electrode activity on CO 2 reduction both in aqueous systems and in the present system. We would emphasize here that the total current efficiency of CO 2 reduction is higher in the CO 2 + methanol system than in aqueous systems. 3.4. Hydrocarbon formation at Cu and Ni electrodes Hydrocarbon formation is of great interest in electrochemistry, since more electrons transfer to one molecule of CO 2 than in CO and formate production. Copper has been known as the only metal at which CO 2 is reduced efficiently to hydrocarbons in aqueous systems [13-15]. Hydrocarbon formation at a Cu electrode in the present system is, however, less efficient than that of the other products when TBAP is used as a supporting salt. It is also less efficient than that in aqueous systems under both atmospheric pressure and high pressure [5]. At a Ni electrode, however, hydrocarbon formation is rather favored in the present system. The product distribution depends on the reduction current density, as is shown in Fig. 2. Hydrocarbon formation is favored in the range 20-2000 mA cm -2. The maximum efficiency of hydrocarbon formation is 23.5%, which corresponds to the partial current density of 200 mA cm -2. This electrode behavior is rather different from that in aqueous ambient
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.
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.
.
.
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j / mA.cm-2 Fig. 2. Electrochemical reduction of CO 2 at a Ni electrode in a CO 2 + methanol medium. Electrolyses were performed galvanostatically at 25°C and 41 atm. A: H 2 ( 0 ) and HCOOCH 3 (E]). B: CO (D), CH 4 ( O ) , C2H 4 (~x) and C2H 6 ( ~ ) .
systems; H 2 is the major product at room temperature and CO is efficiently formed at about 0°C [13]. The role of Cu and Ni electrodes in hydrocarbon formation can be understood qualitatively in terms of the balance of hydrogen atom and carbon atom concentrations on the electrode surface [5,7]. The CO 2 concentration is much higher in the CO 2 + methanol system than in the aqueous system. The supply of CO 2 to the electrode surface is, therefore, more sufficient in the CO 2 + methanol system. At a Cu electrode, the concentration of CO 2 reduction intermediates and hydrogen atoms on the surface may be in good balance at atmospheric pressure. However, CO 2 reduction proceeds so efficiently that concentration of its reaction intermediates may be excessive in the CO 2 + methanol system. Hydrogenation of CO z reduction intermediates therefore does not proceed efficiently, resulting in less hydrocarbon formation in the CO2-methanol system than in an aqueous system. However, at a Ni electrode, the concentration of hydrogen atoms at the electrode surface may be too large for efficient CO 2 hydrogenation, owing to the low hydrogen-overvoltage of Ni. Hydrogen production is the principal reaction at atmospheric pressure at room temperature [14]. Although CO 2 reduction efficiency is low, the main products from CO 2 are hydrocarbons. This indicates that Ni has a high activity of hydrogenation of CO z reduction intermediates. The efficiency of CO 2 reduction is enhanced in such systems of high CO 2 concentration as an aqueous high pressure system and the present CO 2 + methanol system. In these systems, since the concentration of CO 2 reduction intermediates on the
302
T. Saeki et al./ Journal of Electroanalytical Chemistry 404 (1996) 299-302
electrode is larger, hydrogenation of the intermediates proceeds more efficiently. Efficiency of hydrocarbon formation increases with increasing efficiency of CO 2 concentration, in the order: aqueous ambient system [14], aqueous high pressure system [4] and the present CO 2 + methanol system.
Acknowledgements The present work was partially supported by a Grant-inAid for Scientific Research from the Ministry of Education, Science and Culture of Japan, and by the New Energy Development Organization (NEDO).
References 4. Summary CO 2 reduction activity of several metal electrodes in a CO 2 + methanol system is discussed. Formate production in the present system is more efficient than that in aqueous system at most metals. At the metals which show poor catalytic activity on CO 2 reduction in aqueous systems (W, Ti and Pt), although hydrogen production proceeds mainly also in the present system, CO 2 reduction efficiency is enhanced in comparison with the aqueous systems. At the metals at which formate production proceeds exclusively in aqueous systems (Sn and Pb), formate production takes place mainly, but CO formation is also rather favored. The enhanced formation of CO is understood in terms of the effect of supporting electrolyte. The electrodes catalyze formate production effectively, while tetrabutylammonium cation, the supporting salt, promotes CO formation. Ag, Zn and Pd, at which CO 2 is reduced mainly to CO in aqueous systems, show similar electrode activities in the present system. The efficiency of hydrocarbon formation at a Cu electrode is lower, whereas that at a Ni electrode is much higher, than those in aqueous systems. This is explained qualitatively by the balance of hydrogen and carbon atom concentrations on the electrode surface.
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