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Effect of pressure on the electrochemical reduction of CO2 on Group VIII metal electrodes
339
J. Electroanal. Chem., 308 (1991) 339-343 Elsevier Sequoia S.A., Lausanne
Preliminary
note
Effect of pressure on the electrochemical on Group VIII metal electrodes Shinji Nakagawa,
Akihiko
The Graduate School at Nagatsuta, Yokohama 227 (Japan) (Received
Kudo,
Masashi
Azuma
reduction
*, and Tadayoshi
Tokyo Institute of Technology. 4259 Nagatsuta,
of CO,
Sakata
**
Midori-ku,
22 April 1991)
Recently, electrochemical reduction of CO,, especially on metal electrodes, has been studied extensively. This work makes clear that the reduction products of CO, depend strongly on the electrode materials [l-3]. In aqueous solution, however, the concentration of CO, is small (0.033 M in water at 25’C) under ambient pressure owing to its low solubility. Because of the increase of CO, concentration, electrolysis at low temperature or under high pressure seems to improve the reduction efficiency. Actually, it has been reported that at low temperature (ca. 0 o C) the faradaic efficiency for the electrochemical reduction of CO, was increased and the selectivity of the reduction products was changed, compared with those at room temperature [2,4]. On the other hand, some workers have reported the electrochemical reduction of CO, under high pressure [5,6]. Ito et al. have examined the effect of pressures up to 20 atm on the reaction at Zn, In, Pb, and Sn electrodes whose overpotential for hydrogen evolution is large [5]. They found that current density and yield of formic acid (the main product in the liquid phase) were increased by increasing pressure. Since the concentration of CO, in the aqueous electrolyte is increased by increasing the CO, pressure, the electrochemical reduction of carbon dioxide would be enhanced and the selectivity of the reduction products could be changed under high pressure, even for metal electrodes with a small overpotential for hydrogen evolution. Moreover, hydrocarbon formation, i.e. products reduced further than carbon monoxide or formic acid, might be expected since the reduction of CO, competes with hydrogen evolution from water. In the present study, the electrochemical reduction of CO, under high pressure (up to 60 atm) on Group VIII metal electrodes (Fe, Co, Ni, Pd, and Pt), which
l Department of Applied Chemistry, Osaka Institute Japan. l * To whom correspondence should be addressed.
OC22-0728/91/$03.50
0 1991 - Elsevier Sequoia
S.A.
of Technology,
Omiya,
Asahi-ku,
Osaka
535,
1
lC 50 1= 60 lC 60 1= 50 1’ 60
Fe Fe co co Ni Ni Pd Pd Pt Pt
0 6.7 1.2 14.4 0 10.4 5.3 51.9 0 9.3
Faradaic Co
4.1 0.1 23.2 4.4 4.4 0 24.1
t
0 3.1
HCOOH 0 1.61 0.31 0.93 0.62 1.78 0 0 0.02 0.31
CH, 0 1.32 0.2 0.45 0 0.88 0 0 t 0.08
C2I-b
of CO* reduction
VIII metal electrodes
efficiency
of CO, on Group
C2H4
0 1.53 0.19 0.48 0.06 0.41 0 0 0 0.08
C3H,
0 0 0 t 0 0.06 0 0 0 t
C3H,
0 0.19 t t 0 t 0 0 0 0
i-C,H,,
0 1.01 0.09 0.33 0 0.3 0 0 0 t
n-C,H,,
0 16.0 2.0 20.1 1.4 31.5 9.7 62.3 0.02 33.9
30 o C,
H.C. iO, b 0 5.7 0.8 2.3 1.3 3.9 0 0 0.02 0.47
temperature:
COztO, a
of 50 or 60 atm. t: trace, reaction
and high pressure
to give a total pressure
0 0.05 t 0.13 0.63 0.44 0 0 0 t
(W)
ambient
product
under
a Total faradaic efficiency for CO, reduction. b Total faradaic efficiency for hydrocarbon formation. ’ For experiments under 1 atm CO, partial pressure, Ar was introduced potential: - 1.8 V vs. Ag/AgCl, electrolyte: 0.1 M KHCO,.
Pressure of CO,/atm
reduction
Electrode
Electrochemical
TABLE
341
hardly reduce COZ in aqueous solution under 1 atm of CO, (except for Pd [7]), is reported. The electrochemical reduction of CO, was carried out in aqueous KHCO, solution (0.1 M), in an autoclave (limit: 100 atm) equipped with three electrode ports, a pressure gauge, and a gas inlet and outlet, as shown in Fig. 1. The electrodes were insulated from the stainless steel body with Teflon sleeves. In the autoclave used here, the influence (if any) of oxygen evolved on the Pt counter electrode cannot be excluded, because the working electrode compartment is not separated from that of the counter electrode. Wire electrodes (Nilaco, surface area ca. 3 cm2, purity: Fe 99.9%, Co 99.998, Ni 99.9%, Pd 99.958, Pt 99.98%) were chemically etched with (ca. 1 M) nitric acid before use. After the aqueous KHC03 solution was deoxygenated by bubbling CO,, pre-electrolysis was performed at -2.5 V vs. a Ag/AgCl/KCl (sat) reference electrode with bubbling CO, to stabilize the electrode surface. Electrolysis for CO, reduction was carried out at -1.8 V vs. Ag/AgCl using a potentiostat (Hokuto HA-501). For experiments under 1 atm of CO, partial pressure, argon was introduced as a balance to make up a total pressure of 50 or 60 atm. The charge passed (1000-2000 C in the present study) was monitored by a coulometer (Hokuto HF-201). Carbon monoxide, hydrogen and hydrocarbons were determined using gas chromatography (Ohkura Model-802 (TCD detector, MS-13X column for CO and Active Carbon for HZ) and GC202 (FID detector, VZ-10 for hydrocarbons)). The lowest detectable limits of the faradaic efficiencies for carbon monoxide and hydrocarbons were 0.1% and 0.01% respectively, because the dead volume of the gas phase was large. Formic acid was determined using liquid chromatography (Shimadzu LC-4A, Shodex Ionpack KC-811 column). Table 1 shows faradaic efficiencies of CO, reduction products on various electrodes of Group VIII metals under high pressures of CO, (at 50 or 60 atm) as
I
+gkl=F 2
/L
.
I I
CO2
6
Fig. 1. Cell for electrochemical reduction of CO* under high pressure. (1) inlet, (2) outlet, gauge, (4) working electrode, (5) counter electrode, (6) reference electrode (Ag/AgCl/KCl trode), (7) glass filter, (8) stirrer bar, (9) glass support.
(3) pressure (sat) elec-
342
well as at 1 atm. Hydrogen was also evolved in all cases *. It is well known that CO, is hardly reduced on these Group VIII metal electrodes in aqueous solutions, because of the small overvoltages for hydrogen evolution [l-3,8]. As shown in Table 1, it was found that CO, is reduced with considerable current efficiencies of 16-62s on these metal electrodes at high CO, pressures, producing carbon monoxide and formic acid as the main products. Corrosion was observed during electrolysis for the Fe and Co electrodes; the other metals were stable. Even on a Pt electrode, on which CO, cannot be reduced but where water is preferentially reduced to hydrogen at 1 atm of CO,, CO, can be reduced when the CO, pressure is increased to 60 atm with a total CO, reduction efficiency of 34%. For a Pd electrode, the current efficiency of CO production increased ten-fold compared with that at a partial pressure of 1 atm, when the CO, pressure was increased to 50 atm. It should be noted that C, and C, hydrocarbon chains, such as are produced on Fe, Co and Ni electrodes, at C,H,, C,H,, i-C,H,, and n-C,H,,, elevated CO, pressure, while at 1 atm of CO, only a small amount of CH, and C,H, are produced on these electrodes [l-3, 81. Azuma et al., have reported the formation of higher hydrocarbons, up to C,, on a Pd electrode at 1 atm CO, [7]. However, in the present study, hydrocarbons were not detected on the Pd electrode **. In the case of the Fe electrode, current efficiencies for hydrocarbon production were the largest of the Group VIII metals, while the effect of CO, pressure on the current efficiency was not as marked as with the other metals. Control experiments, in which Ar gas was used instead of CO,, did not produce hydrocarbons, demonstrating clearly that the hydrocarbon formation is due to the reduction of CO,. It is thought that concentrations of reduction intermediates of CO, are increased by raising the CO, pressure, and these react with the hydrogen atoms which are produced simultaneously by reduction of water and adsorbed on the metal electrode. All the metals in Table 1 are well known as active catalysts for hydrocarbon synthesis from CO, and H, [9,10], which suggests a strong relationship between the electrocatalysis and the thermal catalytic reaction.
l In Table 1, the current efficiency of H, evolution is excluded. The sum of faradaic efficiencies in the present work was less than 100% (ca. 70%) even when the current efficiency of H, production was included; although it should be 100% theoretically. It was found that the current efficiency of H, evolution includes fairly large experimental errors, probably because the hydrogen evolved was partly consumed by the back reaction with oxygen to form water on the working electrode and/or on the counter electrode in the autoclave. ** As reasons for the discrepancy from the result in ref. 7, the following are considered. (1) Different detection limit. For the experiment at 1 atm partial pressure CO, in the present experiment argon was introduced as a balance to give a total pressure of 50 or 60 atm. (2) Since the compartment of the working electrode was not separated from that of the counter electrode, it is possible that the oxygen evolved at the counter electrode moves to the working electrode to interfere with the cathodic reactions, as mentioned in the experimental section. (3) The electrode potential was different from that in the experiment reported previously.
343 REFERENCES 1 Y. Hori, K. Kikuchi and S. Suzuki, Chem. Lett., (1985) 1695. 2 M. Azuma, K. Hashimoto, M. Hiramoto, M. Watanabe and T. Sakata, J. Electroanal. Chem., 260 (1989) 441; J. Electrochem. Sot., 137 (1990) 1772. 3 H. Noda, S. Ikeda, Y. Oda, K. Imai, M. Maeda and K. Ito, Bull. Chem. Sot. Jpn., 63 (1990) 2459. 4 Y. Hori, K. Kikuchi, A. Murata and S. Suzuki, Chem. Lett., (1986) 897. 5 K. Ito, S. Ikeda and M. Okabe, Denki Kagaku, 48 (1980) 247; K. Ito, S. Ikeda, T. Iida and Y. Niwa, De&i Kagaku, 49 (1981) 106; K. Ito, S. Ikeda, T. Iida and A. Nomura, Denki Kagaku, 50 (1982) 463. 6 B. Aurian-Blajeni, M. Halmann and J. Manassen, Solar Energy Mat., 8 (1983) 425. 7 M. Azuma, K. Hashimoto, M. Watanabe and T. Sakata, J. Electroanal. Chem., 294 (1990) 299. 8 Y. Hori and A. Murata, Electrochim. Acta, 35 (1990) 299. 9 G.D. Weatherbee and C.H. Bartholomew, J. Catal., 68 (1981) 67; 87 (1984) 352. 10 Y. Arakawa, Shokubai (Catalyst), 31 (1989) 558.
J. Electroanal. Chem., 308 (1991) 339-343 Elsevier Sequoia S.A., Lausanne
Preliminary
note
Effect of pressure on the electrochemical on Group VIII metal electrodes Shinji Nakagawa,
Akihiko
The Graduate School at Nagatsuta, Yokohama 227 (Japan) (Received
Kudo,
Masashi
Azuma
reduction
*, and Tadayoshi
Tokyo Institute of Technology. 4259 Nagatsuta,
of CO,
Sakata
**
Midori-ku,
22 April 1991)
Recently, electrochemical reduction of CO,, especially on metal electrodes, has been studied extensively. This work makes clear that the reduction products of CO, depend strongly on the electrode materials [l-3]. In aqueous solution, however, the concentration of CO, is small (0.033 M in water at 25’C) under ambient pressure owing to its low solubility. Because of the increase of CO, concentration, electrolysis at low temperature or under high pressure seems to improve the reduction efficiency. Actually, it has been reported that at low temperature (ca. 0 o C) the faradaic efficiency for the electrochemical reduction of CO, was increased and the selectivity of the reduction products was changed, compared with those at room temperature [2,4]. On the other hand, some workers have reported the electrochemical reduction of CO, under high pressure [5,6]. Ito et al. have examined the effect of pressures up to 20 atm on the reaction at Zn, In, Pb, and Sn electrodes whose overpotential for hydrogen evolution is large [5]. They found that current density and yield of formic acid (the main product in the liquid phase) were increased by increasing pressure. Since the concentration of CO, in the aqueous electrolyte is increased by increasing the CO, pressure, the electrochemical reduction of carbon dioxide would be enhanced and the selectivity of the reduction products could be changed under high pressure, even for metal electrodes with a small overpotential for hydrogen evolution. Moreover, hydrocarbon formation, i.e. products reduced further than carbon monoxide or formic acid, might be expected since the reduction of CO, competes with hydrogen evolution from water. In the present study, the electrochemical reduction of CO, under high pressure (up to 60 atm) on Group VIII metal electrodes (Fe, Co, Ni, Pd, and Pt), which
l Department of Applied Chemistry, Osaka Institute Japan. l * To whom correspondence should be addressed.
OC22-0728/91/$03.50
0 1991 - Elsevier Sequoia
S.A.
of Technology,
Omiya,
Asahi-ku,
Osaka
535,
1
lC 50 1= 60 lC 60 1= 50 1’ 60
Fe Fe co co Ni Ni Pd Pd Pt Pt
0 6.7 1.2 14.4 0 10.4 5.3 51.9 0 9.3
Faradaic Co
4.1 0.1 23.2 4.4 4.4 0 24.1
t
0 3.1
HCOOH 0 1.61 0.31 0.93 0.62 1.78 0 0 0.02 0.31
CH, 0 1.32 0.2 0.45 0 0.88 0 0 t 0.08
C2I-b
of CO* reduction
VIII metal electrodes
efficiency
of CO, on Group
C2H4
0 1.53 0.19 0.48 0.06 0.41 0 0 0 0.08
C3H,
0 0 0 t 0 0.06 0 0 0 t
C3H,
0 0.19 t t 0 t 0 0 0 0
i-C,H,,
0 1.01 0.09 0.33 0 0.3 0 0 0 t
n-C,H,,
0 16.0 2.0 20.1 1.4 31.5 9.7 62.3 0.02 33.9
30 o C,
H.C. iO, b 0 5.7 0.8 2.3 1.3 3.9 0 0 0.02 0.47
temperature:
COztO, a
of 50 or 60 atm. t: trace, reaction
and high pressure
to give a total pressure
0 0.05 t 0.13 0.63 0.44 0 0 0 t
(W)
ambient
product
under
a Total faradaic efficiency for CO, reduction. b Total faradaic efficiency for hydrocarbon formation. ’ For experiments under 1 atm CO, partial pressure, Ar was introduced potential: - 1.8 V vs. Ag/AgCl, electrolyte: 0.1 M KHCO,.
Pressure of CO,/atm
reduction
Electrode
Electrochemical
TABLE
341
hardly reduce COZ in aqueous solution under 1 atm of CO, (except for Pd [7]), is reported. The electrochemical reduction of CO, was carried out in aqueous KHCO, solution (0.1 M), in an autoclave (limit: 100 atm) equipped with three electrode ports, a pressure gauge, and a gas inlet and outlet, as shown in Fig. 1. The electrodes were insulated from the stainless steel body with Teflon sleeves. In the autoclave used here, the influence (if any) of oxygen evolved on the Pt counter electrode cannot be excluded, because the working electrode compartment is not separated from that of the counter electrode. Wire electrodes (Nilaco, surface area ca. 3 cm2, purity: Fe 99.9%, Co 99.998, Ni 99.9%, Pd 99.958, Pt 99.98%) were chemically etched with (ca. 1 M) nitric acid before use. After the aqueous KHC03 solution was deoxygenated by bubbling CO,, pre-electrolysis was performed at -2.5 V vs. a Ag/AgCl/KCl (sat) reference electrode with bubbling CO, to stabilize the electrode surface. Electrolysis for CO, reduction was carried out at -1.8 V vs. Ag/AgCl using a potentiostat (Hokuto HA-501). For experiments under 1 atm of CO, partial pressure, argon was introduced as a balance to make up a total pressure of 50 or 60 atm. The charge passed (1000-2000 C in the present study) was monitored by a coulometer (Hokuto HF-201). Carbon monoxide, hydrogen and hydrocarbons were determined using gas chromatography (Ohkura Model-802 (TCD detector, MS-13X column for CO and Active Carbon for HZ) and GC202 (FID detector, VZ-10 for hydrocarbons)). The lowest detectable limits of the faradaic efficiencies for carbon monoxide and hydrocarbons were 0.1% and 0.01% respectively, because the dead volume of the gas phase was large. Formic acid was determined using liquid chromatography (Shimadzu LC-4A, Shodex Ionpack KC-811 column). Table 1 shows faradaic efficiencies of CO, reduction products on various electrodes of Group VIII metals under high pressures of CO, (at 50 or 60 atm) as
I
+gkl=F 2
/L
.
I I
CO2
6
Fig. 1. Cell for electrochemical reduction of CO* under high pressure. (1) inlet, (2) outlet, gauge, (4) working electrode, (5) counter electrode, (6) reference electrode (Ag/AgCl/KCl trode), (7) glass filter, (8) stirrer bar, (9) glass support.
(3) pressure (sat) elec-
342
well as at 1 atm. Hydrogen was also evolved in all cases *. It is well known that CO, is hardly reduced on these Group VIII metal electrodes in aqueous solutions, because of the small overvoltages for hydrogen evolution [l-3,8]. As shown in Table 1, it was found that CO, is reduced with considerable current efficiencies of 16-62s on these metal electrodes at high CO, pressures, producing carbon monoxide and formic acid as the main products. Corrosion was observed during electrolysis for the Fe and Co electrodes; the other metals were stable. Even on a Pt electrode, on which CO, cannot be reduced but where water is preferentially reduced to hydrogen at 1 atm of CO,, CO, can be reduced when the CO, pressure is increased to 60 atm with a total CO, reduction efficiency of 34%. For a Pd electrode, the current efficiency of CO production increased ten-fold compared with that at a partial pressure of 1 atm, when the CO, pressure was increased to 50 atm. It should be noted that C, and C, hydrocarbon chains, such as are produced on Fe, Co and Ni electrodes, at C,H,, C,H,, i-C,H,, and n-C,H,,, elevated CO, pressure, while at 1 atm of CO, only a small amount of CH, and C,H, are produced on these electrodes [l-3, 81. Azuma et al., have reported the formation of higher hydrocarbons, up to C,, on a Pd electrode at 1 atm CO, [7]. However, in the present study, hydrocarbons were not detected on the Pd electrode **. In the case of the Fe electrode, current efficiencies for hydrocarbon production were the largest of the Group VIII metals, while the effect of CO, pressure on the current efficiency was not as marked as with the other metals. Control experiments, in which Ar gas was used instead of CO,, did not produce hydrocarbons, demonstrating clearly that the hydrocarbon formation is due to the reduction of CO,. It is thought that concentrations of reduction intermediates of CO, are increased by raising the CO, pressure, and these react with the hydrogen atoms which are produced simultaneously by reduction of water and adsorbed on the metal electrode. All the metals in Table 1 are well known as active catalysts for hydrocarbon synthesis from CO, and H, [9,10], which suggests a strong relationship between the electrocatalysis and the thermal catalytic reaction.
l In Table 1, the current efficiency of H, evolution is excluded. The sum of faradaic efficiencies in the present work was less than 100% (ca. 70%) even when the current efficiency of H, production was included; although it should be 100% theoretically. It was found that the current efficiency of H, evolution includes fairly large experimental errors, probably because the hydrogen evolved was partly consumed by the back reaction with oxygen to form water on the working electrode and/or on the counter electrode in the autoclave. ** As reasons for the discrepancy from the result in ref. 7, the following are considered. (1) Different detection limit. For the experiment at 1 atm partial pressure CO, in the present experiment argon was introduced as a balance to give a total pressure of 50 or 60 atm. (2) Since the compartment of the working electrode was not separated from that of the counter electrode, it is possible that the oxygen evolved at the counter electrode moves to the working electrode to interfere with the cathodic reactions, as mentioned in the experimental section. (3) The electrode potential was different from that in the experiment reported previously.
343 REFERENCES 1 Y. Hori, K. Kikuchi and S. Suzuki, Chem. Lett., (1985) 1695. 2 M. Azuma, K. Hashimoto, M. Hiramoto, M. Watanabe and T. Sakata, J. Electroanal. Chem., 260 (1989) 441; J. Electrochem. Sot., 137 (1990) 1772. 3 H. Noda, S. Ikeda, Y. Oda, K. Imai, M. Maeda and K. Ito, Bull. Chem. Sot. Jpn., 63 (1990) 2459. 4 Y. Hori, K. Kikuchi, A. Murata and S. Suzuki, Chem. Lett., (1986) 897. 5 K. Ito, S. Ikeda and M. Okabe, Denki Kagaku, 48 (1980) 247; K. Ito, S. Ikeda, T. Iida and Y. Niwa, De&i Kagaku, 49 (1981) 106; K. Ito, S. Ikeda, T. Iida and A. Nomura, Denki Kagaku, 50 (1982) 463. 6 B. Aurian-Blajeni, M. Halmann and J. Manassen, Solar Energy Mat., 8 (1983) 425. 7 M. Azuma, K. Hashimoto, M. Watanabe and T. Sakata, J. Electroanal. Chem., 294 (1990) 299. 8 Y. Hori and A. Murata, Electrochim. Acta, 35 (1990) 299. 9 G.D. Weatherbee and C.H. Bartholomew, J. Catal., 68 (1981) 67; 87 (1984) 352. 10 Y. Arakawa, Shokubai (Catalyst), 31 (1989) 558.