- Email: [email protected]
Electrochemical reduction of carbon dioxide to higher hydrocarbons in a KHCO3 aqueous solution
299
J. Electroanal. Chem., 294 (1990) 299-303 Elsevier Sequoia S.A., Lausanne
Preliminary note
Electrochemical reduction of carbon dioxide to higher hydrocarbons in a KHCO, aqueous solution Masashi Azuma * Department
of Applied Chemistry,
Osaka Institute of Technology, Omiya, Asahi-ku,
Osaka 535 (Japan)
Kazuhito Hashimoto Department of Synthetic Tokyo 113 (Japan)
Chemistry
Faculty of Engineering,
The University
of Tokyo, Hongo, Bunkyo-ku,
Masahiro Watanabe Laboratory of Electrocatalysts Kofu 400 (Japan)
for Fuel Ceils, Faculty of Engineering
Yamanashi
University,
Takeda,
Tadayoshi Sakata Department of Electronic Chemistry, The Graduate School at Nagatsuta, Nagatsuta 4259, Midori-ky Yokohama 227 (Japan) (Received
12 March 1990; in revised form 11 September
Tokyo Institute
of Technology,
1990)
Electrochemical CO, reduction provides a possible means of converting useless CO* into useful fuels and/or organic compounds. We have been investigating the electrochemical reduction of CO, on several metal electrodes in aqueous solutions [1,2]. In these papers we reported that some hydrocarbons such as methane, ethane, ethylene, propane and propylene are produced on almost all metal electrodes. This result suggests to us the possibility of direct electrochemical production of longer hydrocarbons from CO,. In this note, we report that many kinds of hydrocarbons from methane to hexane are produced on a Pd electrode in aqueous solution. Pd wire (0.5 mm 0, 99.9% purity, purchased from The Japan Lamp Industry Co., Ltd.) was used as an electrode. The surface of the electrode was chemically etched in diluted HNO, solution for several seconds before use. The experimental procedures were described in detail in the previous paper [2]. Analysis of the reduction products in both gaseous and liquid phases was carried out using gas chromatography and liquid chromatography as described previously [2]. Analysis of small amounts of hydrocarbons produced by the electrochemical reduction of CO, was carried out using a gas chromatograph with a flame ionization detector (carrier gas: N,, column packing: Porapak Q and VZ-10).
*To
correspondence
0022-0728/90/$03.50
should be addressed.
0 1990 - Elsevier Sequoia S.A.
300
The gas c~omato~ap~c patterns of the sample gas showed twelve peaks after the electrolysis in a CO,-saturated KHCO, aqueous solution as shown in Fig. 1. The first peak in Fig. 1 is due to shock noise of the gas chromatograph and the other peaks from 2nd to 12th arise from various organic gases which are contained in the sample gas. These eleven peaks were identified as methane, ethylene, ethane, propylene, propane, n- and i-butane, n- and i-pentane, n- and i-hexane, respectively, by comparing with the patterns of standard gases. The figure shows clearly that the sample gas after electrolysis contains butane, pentane and hexane as well as methane or ethylene. Two kinds of blank experiments were conducted in order to confirm that the hydrocarbons detected after the electrolysis were produced by the electrochemical reduction of CO,. The first blank experiment showed that such hydr~~bons were not detected after electrolysis in a N,-saturated solution. This result shows that these hydrocarbons are not present in the original KHCO, solution itself. It is also shown that they are not produced by the electrochemical reduction of impurities such as epoxy resin or KHCO, itself. The second blank experiment showed that these hydrocarbons were not detected in the gas phase of the CO,-saturated cell stirred for more than 3 h without any current. These results
0
5
10 r. t. I min
15
21
Fig. 1. Gas chromatographic patterns of the sample gas after electrolysis at -2.0 V vs. SCE in a CO,-saturated KHCO, aqueous solution at 30°C. Conditions of the gas c~omato~aph analysis: Porapak-Q column (30 mm 0 X2 m). 180°C, N, carrier, FID.
301
show clearly that these hydrocarbons were not present in the CO2 gas before the electrolysis. It is also shown that such hydrocarbons were not produced by the chemical reaction of CO, with some impurities in the electrolyte solution or KHCO,. Therefore, the results of the two blank experiments demonstrate that CO, is reduced electrochemically to such hydrocarbons. To our knowledge, this is the first report on the direct electrochemical production of such long-chain hydrocarbons from CO,. Table 1 shows current efficiencies for some reduction products on a Pd electrode in a CO,-saturated 0.05 mol dmp3 KHCO, solution at - 2.0 V vs. SCE. Various kinds of reduction products were detected as shown in this table, even though the efficiencies are quite low. It should be noted that these products include hydrocarbons with a long carbon chain. The current efficiency of the total CO, reduction decreases with temperature as reported in our previous papers [1,2]. Figure 2 shows the current efficiencies for hydrocarbons from C, to C, plotted against the temperature of the electrolyte solutions. This figure shows that high temperature conditions are advantageous for longer hydrocarbon production. Hori et al. [3] investigated the temperature effects on the electrochemical production of CH, and C,H, on a Cu electrode from CO,. They showed that the current efficiency of CH, production on the Cu electrode decreases significantly with temperature, while that for C,H, increases with temperature. Although they did not discuss the mechanism of these temperature effects, this may be considered as follows. The large current efficiency of the CH, production on the Cu electrode suggests that the production rate of CH, on the Cu electrode is rather large and that the activation energy of CH, formation on Cu must be very small. Then the current TABLE 1 Typical current efficiencies (W) for various CO, reduction products on a Pd electrode at - 2.0 V vs. SCE in CO,-saturated 0.05 M IU-ICO, aqueous solution Products
Cc) HCOOH CH4 C2H4 C2H6 C3H6 C,H, n-C4%0
i-C,H,, n-C,H,, i-C,H,, n-C,H,, i-GH14 H2
a nd: not detected.
Temperature/ o C 0
20
30
40
11.6 16.1 0.083 0.011 0.014 0.0075 0.0069 0.0025 0.0031 0.0015 0.0021 nd a nd ’ 73.3
3.2 8.6 0.31 0.061 0.078 0.036 0.039 0.014 0.014 0.0052 0.0086 0.0018 0.0022 90.3
1.7 5.2 0.36 0.078 0.092 0.078 0.048 0.060 0.069 0.024 0.041 0.0086 0.0097 88.6
0.86 1.8 0.42 0.082 0.105 0.085 0.055 0.068 0.076 0.026 0.043 0.0107 0.0125 98.6
302
efficiency depends only on the ~n~ntra~on of dissolved CO, in the solution and intermediates on the Cu surface. This assumption explains very well the temperature effect on the CH4 formation on the Cu electrode because the solubility of CO, gas into water decreases with increasing temperature [1,2]. In contrast to the CH, formation, it is estimated that the CJ-I, formation reaction on a Cu electrode has a rather large activation energy, resulting in a large temperature effect, i.e. the formation rate of C,H, increases significantly with temperature. In sharp contrast to this result, the current efficiencies of CH,, C,H, and longer hydrocarbons on a Pd electrode increase with temperature as shown in Fig. 2. The current efficiencies for the electrochemical formation of hydrocarbons on a Pd electrode is quite low, suggesting that these reactions have rather large activation energies, causing a temperat~e effect similar to that for C,H, production on Cu. It is shown in this study that the Pd electrode is effective for the production of longer hydrocarbons from CO,. Several other metal electrodes were also used for the production of hydrocarbons, but most of them showed no reduction products longer than propane. Only Sn and Ag electrodes showed small amounts of C, and C, products, whereas the current efficiencies were much lower than in the case of a Pd electrode. In summary, we have demonstrated that CO* can be converted electrochemically into higher hydrocarbons, although the efficiencies are quite low. Such an approach
Fig. 2. Temperature dependence of the current efficiencies (9) for the electrochemical production of hydrocarbons from C, (methane) to C, species (n- and i-hexane) produced on a Pd electrode in a CO,-saturated 0.05 M KHCO, aqueous solution at -2.0 V vs. SCE. (0): C,, (+): C,, (A): C,, (0): C,, (0): c,, (A): C6.
303
suggests to us the possibility of electrochemical production of valuable organic compounds such as glucose and hydrocarbons from CO, molecules. REFERENCES 1 M. Azuma, K. Hashimoto, M. Hiramoto, M. Watanabe and T. Sakata, J. Electroanal. Chem., 260 (1989) 441. 2 M. Azuma, K. Hashimoto, M. Hiramoto, M. Watanabe and T. Sakata, J. Electrochem. Sot., 137 (1990) 1772. 3 Y. Hori, K. Kikuchi, A. Murata and S. Suzuki, Chem. L.&t., (1986) 897.
J. Electroanal. Chem., 294 (1990) 299-303 Elsevier Sequoia S.A., Lausanne
Preliminary note
Electrochemical reduction of carbon dioxide to higher hydrocarbons in a KHCO, aqueous solution Masashi Azuma * Department
of Applied Chemistry,
Osaka Institute of Technology, Omiya, Asahi-ku,
Osaka 535 (Japan)
Kazuhito Hashimoto Department of Synthetic Tokyo 113 (Japan)
Chemistry
Faculty of Engineering,
The University
of Tokyo, Hongo, Bunkyo-ku,
Masahiro Watanabe Laboratory of Electrocatalysts Kofu 400 (Japan)
for Fuel Ceils, Faculty of Engineering
Yamanashi
University,
Takeda,
Tadayoshi Sakata Department of Electronic Chemistry, The Graduate School at Nagatsuta, Nagatsuta 4259, Midori-ky Yokohama 227 (Japan) (Received
12 March 1990; in revised form 11 September
Tokyo Institute
of Technology,
1990)
Electrochemical CO, reduction provides a possible means of converting useless CO* into useful fuels and/or organic compounds. We have been investigating the electrochemical reduction of CO, on several metal electrodes in aqueous solutions [1,2]. In these papers we reported that some hydrocarbons such as methane, ethane, ethylene, propane and propylene are produced on almost all metal electrodes. This result suggests to us the possibility of direct electrochemical production of longer hydrocarbons from CO,. In this note, we report that many kinds of hydrocarbons from methane to hexane are produced on a Pd electrode in aqueous solution. Pd wire (0.5 mm 0, 99.9% purity, purchased from The Japan Lamp Industry Co., Ltd.) was used as an electrode. The surface of the electrode was chemically etched in diluted HNO, solution for several seconds before use. The experimental procedures were described in detail in the previous paper [2]. Analysis of the reduction products in both gaseous and liquid phases was carried out using gas chromatography and liquid chromatography as described previously [2]. Analysis of small amounts of hydrocarbons produced by the electrochemical reduction of CO, was carried out using a gas chromatograph with a flame ionization detector (carrier gas: N,, column packing: Porapak Q and VZ-10).
*To
correspondence
0022-0728/90/$03.50
should be addressed.
0 1990 - Elsevier Sequoia S.A.
300
The gas c~omato~ap~c patterns of the sample gas showed twelve peaks after the electrolysis in a CO,-saturated KHCO, aqueous solution as shown in Fig. 1. The first peak in Fig. 1 is due to shock noise of the gas chromatograph and the other peaks from 2nd to 12th arise from various organic gases which are contained in the sample gas. These eleven peaks were identified as methane, ethylene, ethane, propylene, propane, n- and i-butane, n- and i-pentane, n- and i-hexane, respectively, by comparing with the patterns of standard gases. The figure shows clearly that the sample gas after electrolysis contains butane, pentane and hexane as well as methane or ethylene. Two kinds of blank experiments were conducted in order to confirm that the hydrocarbons detected after the electrolysis were produced by the electrochemical reduction of CO,. The first blank experiment showed that such hydr~~bons were not detected after electrolysis in a N,-saturated solution. This result shows that these hydrocarbons are not present in the original KHCO, solution itself. It is also shown that they are not produced by the electrochemical reduction of impurities such as epoxy resin or KHCO, itself. The second blank experiment showed that these hydrocarbons were not detected in the gas phase of the CO,-saturated cell stirred for more than 3 h without any current. These results
0
5
10 r. t. I min
15
21
Fig. 1. Gas chromatographic patterns of the sample gas after electrolysis at -2.0 V vs. SCE in a CO,-saturated KHCO, aqueous solution at 30°C. Conditions of the gas c~omato~aph analysis: Porapak-Q column (30 mm 0 X2 m). 180°C, N, carrier, FID.
301
show clearly that these hydrocarbons were not present in the CO2 gas before the electrolysis. It is also shown that such hydrocarbons were not produced by the chemical reaction of CO, with some impurities in the electrolyte solution or KHCO,. Therefore, the results of the two blank experiments demonstrate that CO, is reduced electrochemically to such hydrocarbons. To our knowledge, this is the first report on the direct electrochemical production of such long-chain hydrocarbons from CO,. Table 1 shows current efficiencies for some reduction products on a Pd electrode in a CO,-saturated 0.05 mol dmp3 KHCO, solution at - 2.0 V vs. SCE. Various kinds of reduction products were detected as shown in this table, even though the efficiencies are quite low. It should be noted that these products include hydrocarbons with a long carbon chain. The current efficiency of the total CO, reduction decreases with temperature as reported in our previous papers [1,2]. Figure 2 shows the current efficiencies for hydrocarbons from C, to C, plotted against the temperature of the electrolyte solutions. This figure shows that high temperature conditions are advantageous for longer hydrocarbon production. Hori et al. [3] investigated the temperature effects on the electrochemical production of CH, and C,H, on a Cu electrode from CO,. They showed that the current efficiency of CH, production on the Cu electrode decreases significantly with temperature, while that for C,H, increases with temperature. Although they did not discuss the mechanism of these temperature effects, this may be considered as follows. The large current efficiency of the CH, production on the Cu electrode suggests that the production rate of CH, on the Cu electrode is rather large and that the activation energy of CH, formation on Cu must be very small. Then the current TABLE 1 Typical current efficiencies (W) for various CO, reduction products on a Pd electrode at - 2.0 V vs. SCE in CO,-saturated 0.05 M IU-ICO, aqueous solution Products
Cc) HCOOH CH4 C2H4 C2H6 C3H6 C,H, n-C4%0
i-C,H,, n-C,H,, i-C,H,, n-C,H,, i-GH14 H2
a nd: not detected.
Temperature/ o C 0
20
30
40
11.6 16.1 0.083 0.011 0.014 0.0075 0.0069 0.0025 0.0031 0.0015 0.0021 nd a nd ’ 73.3
3.2 8.6 0.31 0.061 0.078 0.036 0.039 0.014 0.014 0.0052 0.0086 0.0018 0.0022 90.3
1.7 5.2 0.36 0.078 0.092 0.078 0.048 0.060 0.069 0.024 0.041 0.0086 0.0097 88.6
0.86 1.8 0.42 0.082 0.105 0.085 0.055 0.068 0.076 0.026 0.043 0.0107 0.0125 98.6
302
efficiency depends only on the ~n~ntra~on of dissolved CO, in the solution and intermediates on the Cu surface. This assumption explains very well the temperature effect on the CH4 formation on the Cu electrode because the solubility of CO, gas into water decreases with increasing temperature [1,2]. In contrast to the CH, formation, it is estimated that the CJ-I, formation reaction on a Cu electrode has a rather large activation energy, resulting in a large temperature effect, i.e. the formation rate of C,H, increases significantly with temperature. In sharp contrast to this result, the current efficiencies of CH,, C,H, and longer hydrocarbons on a Pd electrode increase with temperature as shown in Fig. 2. The current efficiencies for the electrochemical formation of hydrocarbons on a Pd electrode is quite low, suggesting that these reactions have rather large activation energies, causing a temperat~e effect similar to that for C,H, production on Cu. It is shown in this study that the Pd electrode is effective for the production of longer hydrocarbons from CO,. Several other metal electrodes were also used for the production of hydrocarbons, but most of them showed no reduction products longer than propane. Only Sn and Ag electrodes showed small amounts of C, and C, products, whereas the current efficiencies were much lower than in the case of a Pd electrode. In summary, we have demonstrated that CO* can be converted electrochemically into higher hydrocarbons, although the efficiencies are quite low. Such an approach
Fig. 2. Temperature dependence of the current efficiencies (9) for the electrochemical production of hydrocarbons from C, (methane) to C, species (n- and i-hexane) produced on a Pd electrode in a CO,-saturated 0.05 M KHCO, aqueous solution at -2.0 V vs. SCE. (0): C,, (+): C,, (A): C,, (0): C,, (0): c,, (A): C6.
303
suggests to us the possibility of electrochemical production of valuable organic compounds such as glucose and hydrocarbons from CO, molecules. REFERENCES 1 M. Azuma, K. Hashimoto, M. Hiramoto, M. Watanabe and T. Sakata, J. Electroanal. Chem., 260 (1989) 441. 2 M. Azuma, K. Hashimoto, M. Hiramoto, M. Watanabe and T. Sakata, J. Electrochem. Sot., 137 (1990) 1772. 3 Y. Hori, K. Kikuchi, A. Murata and S. Suzuki, Chem. L.&t., (1986) 897.