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Electrocatalytic reduction of carbon dioxide to methanol
Journal of Molecular
Catalysis,
41 (1987) 303 - 311
303
ELECTROCATALYTIC REDUCTION OF CARBON DIOXIDE TO METHANOL PART 7. WITH QUINONE DERIVATIVES IMMOBILIZED ON PLATINUM AND STAINLESS STEEL KOTARO OGURA and MINORU FUJITA Department
of Applied
Chemistry,
(Received August 5,1986;
Yamaguchi
University,
Ube 755 (Japan)
accepted November 21,1986)
Catalytic reduction of carbon dioxide has been performed at quinone derivative-confined electrodes such as p-benzoquinone, 2-aminoanthraquinone, indigo and alizarin. These compounds worked as an electrode mediator in the catalytic reduction of CO?, although they tended to shed from the substrate at a large negative potential. The redox reaction of a favorable mediator must be reversible, and in this context the electrode modified with phthalocyanine cobalt did not operate as a mediator since the redox reaction of this compound was completely irreversible. The reduction of COZ to methanol was induced via a homogeneous catalysis by oxidation of the leuco-type of quinone, and regeneration of the active mediator was electrochemically achieved.
Introduction The reduction of carbon dioxide to an organic substance is an attractive subject, since the cyclic utilization of a carbon resource is possible and we can go beyond the problems of depletion of petroleum. Among many proposed methods for the COZ conversion, the electrochemical one is most promising [ 11. However, the weakness of this method is that it requires a large over-potential, and formic acid is the final product whose further reduction is very difficult in aqueous solution. In our previous works [2 - 41, an indirect electrochemical reduction was developed in which COZ is reduced at a mediated electrode. This process involves the electron transfer reaction at the mediated electrode/solution interface via homogeneous catalysis. The scheme of this process is shown in Fig. 1, where Ox represents COZ,* M, homogeneous catalyst; M-Ox and M-Red, intermediates; Red, reduction product, Y and X, reduced and oxidized forms, respectively, of the mediator immobilized on the electrode. 0304-5102/87/$3.50
0 Elsevier Sequoia/Printed in The Netherlands
304 Mediator Red
Fig. 1. Schematic representation reduction of CO, (Ox).
of the electrode/mediator/homogeneous
catalysis in the
Aqueous CO* is first captured by the homogeneous catalyst to form M-Ox, The intermediate is reduced to M-Red by the reaction with the reduced form of the mediator, and regeneration of the active mediator is electrochemically achieved. The homogeneous catalyst consists of a metal complex and a primary alcohol, and various effective systems have been investigated [ 31. The requirements for a substance capable of operating as an electrode mediator are to be stable in aqueous solution, to be easily immobilized on the substrate, and to have a suitable redox property. Previously [Z, 31, we have used Everitt’s salt (ES, KZFen[Feu(CN),]) for this purpose, which is the reduced form of Prussian blue (PB, KFem[Fen(CN)6]). In the present work, further studies on the electrode mediator were performed, and quinone derivatives immobilized on platinum and stainless steel were found to act satisfactorily for the reduction of CO, to meth~ol. Experimental Q&none derivatives used as the electrode mediator were p-benzoquinone (C6H402), 2aminoanthraquinone (&H,NO,), indigo (C16H10N~02) and alizarin (C14Hs04). Everitt’s salt and phthalocyanine cobalt (Cs2H16CON,) were also employed for comparison. Fixing of quinone derivatives and phthalocyanine cobalt to the electrode was carried out by coating a paste film [ 5 1. The paste film was prepared by mixing these compounds and graphite powder (1:lO wt.%), and adding a small amount of ethyl cellosolve solution (2%) containing copolymer of methyl metha~~la~-methac~lic acid [6]. The film of ES was formed by the elec~ochemic~ reduction of the PB film coated on the electrode [7,8]. The substrates were platinum (4.2 or 15 cm2) and stainless steel (15 cm’). The catalyst solution was 0.1 M KC1 (pH 3.5) containing 10 mM (mmol dmm3) aquapentacyanoferrate(I1) (Na:,[Fe(CN),(H,O)]) and 15 mM methanol. The solution was kept in a reservoir of 1 dm3 through which CO* bubbled at a flow rate of 200 cm3 mm-’ for 1 h. 40 cm3 of this solution was transferred respectively to the cathode and anode compartments in the electrolytic cell, which were separated by a fine porosity glass frit. The modified electrode and the bright platinum plate were set in the main and counter
305
electrode compartments, respectively. The electrode potentials measured are all referred to the saturated calomel electrode, and all experiments were carried out at 40 f 1 “c. Electrochemical measurements were carried out with a potentiostat (Nichia, NP-IR 1000) and a potential programmer (Nikko Keisoku Model NPS-2). The quantitative analysis of methanol added initially and produced finally was performed by a gas chromatograph (JGC-1100) and a steam chromatograph (Model SSC-1, Ohkura). The sampling procedure for the gas chromatograph was described previously [9], and the aqueous sample was used without any p~~eatment for the steam c~omato~ph.
Results and discussion Quinone derivatives used are typical dyes for yellow, deep blue or deep reddish-purple. In a general dyeing process, the dye is reduced to a leuco compound by a reducing agent such as sodium hydrosulfite. The leuco type is soluble in aqueous solution and has adequate affinity for a fiber through hydrogen bonding. The leuco compound combined to the fiber is re-oxidized to the quinone by air. Hence it may be expected that the redox reaction of the quinone derivatives proceeds under ordinary electrochemical conditions.
0
0.2
0.4 E/
0.6
0.8
VVSSCE
Fig. 2. i-E curves of quinone derivative-confined electrodes in 0.1 M KC1 (pH 3.5) at the sweep rate of 5 mV s-l. (), p-benzoquinone; (.. - ..), 2-aminoanthraquinone; (. - .), indigo; (__ ), alizarin; platinum surface area, 4.2 cm2.
The cyclic voltammograms of quinone derivatives immobilized on the platinum electrode are shown in Fig. 2. The reduction of these derivatives is observed at +0.07 V us. SCE ~-benzoqu~one), +0.27 V (&aminoanthraquinone), +0,40 V (~digo) and +0.52 V (size). The redox reaction of the quinone is represented as:
306
0 +
OH
0
-Q
2ri+ + 2e- -
d (Q)
B
OQQ)
where H2Q is the leuco type of quinone. The indigo- and ES-confined electrodes were applied to the reduction of aqueous Cc&, and the concentration of methanol obtained is shown versus the electrolysis time in Fig. 3. The reduction was performed in CO2 as well as Nz atmosphere. In both conditions, CO2 is reduced to methanol at the ~digo~~~f~ed e&&rode, although the amount of methanol is about half of that obtained with the ES-confined electrode at the electrolysis time of 9 h. In the prolonged electrolysis, the methanol formation is inclined to approach a certain value, which is determined by the depletion of aqueous COz. The same reason may be invoked to explain the smaller amount of methanol acquired in the N, atmosphere,
0
2
4
6
3
t/h
Fig. 3. Methanol formation as a function of electrolysis time with the mediated electrode: (a, O), ES; (A, A), indigo. The catholyte was the COa-saturated KC1 solution containing 10 mM Na~[Fe(CN)#-I~O)f and 15 mM C&OH, The electrolysis was performed in an atmosphere of CO, (a, A) and N2 (0, A).
307
The overall reaction for the reduction of CO2 at the ES-confined electrode has been proposed as shown in reaction (2) f2], co2 + 6ES + 6H+ 5
CH,OH + 6PB + 6K+ + Hz0
(2)
6fF
I
The total reaction on the quinone-confined represented as: COz + 3HzQ e
electrode can be similarly
CH,UH + 3Q C f-I20
(3)
t In both reactions, the conversion of CO2 to methanol is the redox reaction being induced by the oxidation of Everitt’s salt or leuco-type quinone. The m-reduction of the oxidized form to the active mediator must be achieved by the introduction of electrons from an external source of energy. The protons in solution are consumed in reaction (2), and the pH increases during the reduction of COz. In reaction (3), however, the protons consumed in the reduction are supplied by the re-reduction of the quinone, and the pH of the catholyte does not change as the reaction advances. Table 1 shows the current efficiency of the methanol formation obtained with the indigo- and ES-confined electrodes. At the former electrode, TABLE
1
Results of the concentration the current efficiency (r7i)a
Mediator ES
Electrolysis time (h)
[Methanol] (mm01 dmm3)
9
1.72 3-46 5.37 6.43 7.06 10.04 11.50
0.035 0.082 0.139 0,144 0.172 0.207 0.225
43.1 56.3 56.1 51.9 54.3 47.3 45.3
1 2 3 4 5 7 9
0.97 1.65 2.60 3.27 3.92 5.57 8.47
0.017 0.050 0.074 0.081 0.104 0.125 0.136
40.6 70.2 59.1 57.4 61.4 52.0 37.2
1
2 3 4 ;
indigo
of methanol produced, the electric charge passed (Q), and
%atalyst solution (40 cm3, pH 3.5): CC&saturated 0.1 M KCI containing 10 mM NasfFe(CN)s(H~O) f and 15 mM CHaUH; eleetx~?iysis potential, -0.7 V US,SCE; surface area, 4.2 cm2; in CC& atmosphere.
308 the electriccharge and the amount of methanol are both smaller than those obtained at the latter, but there is no big difference in the current efficiencies. The smaller amount of methanol at the indigo-confined electrode is ascribed to a smaller quantity of active mediator per unit area. A cyclic voltammogram of the ES-modified platinum electrode in 0.1 M KC1 showed a sharp peak (about 0.2 mM cme2) at +0.15 V (us. SCE) with a half-width of -15 mV and a peak separation of 10 mV. The amount of ES on the substrate was evaluated coulometricahy in the KC1 solution. The averaged value was -2.8 X lo-’ mol cmm2 in the concentration of high-spin iron ions. On the other hand, the current peak (about 0.03 mA cmm2) in the voltammogram of the indigo-modified electrode is not sharp, as shown in Fig. 2. In this system, graphite powder needed to be added to the paste containing the quinone in order to elevate its conductivity. Consequently, the amount of the quinone per unit area was to be kept within a limit, and the concentration of the quinone being exposed at the solution/paste electrode interface must be far lower than that of ES. In Fig. 4, the methanol formation is plotted uersus the cathodizing potential. Methanol is produced at negative potentials uersus SCE, but the plots show maxima at about -0.5 V (2aminoanthraquinone), -0.7 V (p-benzoquinone) , -0.7 V (alizarin) and -0.9 V (indigo). At potentials more negative than about -0.9 V (2aminoanthraquinone, p-benzoquinone and alizarin) or -1.3 V (indigo), there is no reduction of C02. The current
E/V
vs SCE
Fig. 4. The concentration of methanol produced versus anodizing potential at the quinone-confined electrodes. (O), indigo; (A), alizarin; (a), p-benzoquinone; (a), a-aminoanthraquinone. The catalyst solution was COz-saturated KC1 solution containing 10 mM Nas[Fe(CN),(HzO)] and 15 mM CHsOH.
309 TABLE 2 Results of the concentration current efficiency*
of methanol produced, the electric charge passed, and the
Mediator
Potential (V vs. SCE)
indigo
+0.1 -0.1 -0.3 -0.5 -0.7 -0.9 -1.1 -1.3
1.29 2.38 2.59 2.39 2.85 7.86 14.30
(-0.023) 0.019 0.041 0.943 0.070 0.018 0.062 (-0.037)
+1 .I -0.1 -0.3 -0.5 -0.7 -0.8 -0.9
0.62 1.64 2.03 2.27 4.08 3.98 6.40
(-0.003) 0.036 0.048 0.068 0.078 0.066 (-9,002)
alizarin
[Methanol] (mmol dm-““) 0.48
34.1 40.0 38.5 67.6 63.4 18.3 -
49.4 54.6 69.4 44.3 38.4 -
a Catalyst solution (40 cmas pH 3.5): COa-saturated 0.1 M KC1 containing IO mM Nas[Fe(CN)s(HaO)f end 15 mM CHaOH; electrolysis time, 3 h; surface area, 4.2 cm’; in Na atmosphere.
efficiencies obtained with the indigo- and a&u&confined electrodes are 8sbown in Table 2. Quinone derivatives tended to shed from the substrate at such a negative potential, which leads to difficulty in the mediated electron transfer. Hence, a large negative potential cannot be applied in the electrolysis with the quinone-confined electrodes. This is a shortcoming in using these electrodes as a mediator, but there was no such restriction with the ES-confined electrode. The concentrations of methanol and the current efficiencies obtained withvarious modified electrodes are shown in Table 3. In this experiment, platinum and stainless steel are both used as the substrate, The results show that the quinone derivatives as well as ES work as mediators, but phth~ocy~~e cobalt shows no mediatory effect. This is caused by the lack of the redox property required for a compound to operate as a mediator. The redox reaction of the favorable mediator is always reversible, as shown in Fig. 2 for the quinones and in [6] and [7] for Everitt’s salt, but the i-E curve of the cobalt complex with phthalocyanine is completely irreversible [lo]. The effect of the substrate is obvious in Table 3, and the amount of methanol obtained with stainless steel is about half of that with platinum. This result, attributed to the slight dissolution of stainless steel during the course of electrolysis, is supported by the results shown in the same table. The current efficiency in the case of platinum is generaBy higher at -0.3 V than at -0.7 V, but the relationship is reversed in the case of stainless steel.
310 TABLE 3 Results of the concentration current efficiencya
of methanol produced, the electric charge passed, and the
[Methanol] (mmol dme3)
Mediator
Substrate
Potential (V vs. SCE)
A B C D E F
platinum
-0.3
9.25 13.05 7.40 6.82 12.16 10.21
0.241 0.122 0.105 0.089 0.140 (-0.005)
60.3 21.7 32.9 30.2 26.7 -
A B C D E F
platinum
-0.7
24.91 15.38 17.10 20.82 30.58 20.99
0.449 0.256 0,155 0.151 0.080 (-0.020)
41.7 38.5 21.0 16.8 6.1 -
A B C D E F
stainless steel
-0.3
8.66 8.59 6.87 6.39 9.23 6.50
0.171 0.099 0.076 0.068 0.119 (-0.008)
45.7 26.7 25.6 24.6 29.9 -
A B C D E F
stainless steel
-0.7
9.00 10.55 8.00 8.36 9.01 9.30
0.177 0.195 0.107 0.111 0.140 (-0.013)
45.5 42.8 31.0 30.8 36.0 -
Watalyst solution (40 cm3, pH 3.5): COzsaturated 0.1 M KC1 containing 10 mM Nas[Fe(CN)s(HzO)] and 15 mM CHsOH; electrolysis time, 5 h; surface area of the substrate, 15 cm2; in CO2 atmosphere. A, Everitt’s salt; B, indigo; C, 2-aminoanthraquinone; D, alizarin; E,p-benzoquinone; F, phthalocyanine cobalt.
This is responsible for the tendency of stainless steel to dissolve at a potential more noble than the rest potential (i.e., anodic dissolution). The existence of a homogeneous catalyst consisting of a metal complex and a primary alcohol is prerequisite to the reduction of COz. As discussed previously [ 41, the homogeneous catalyst first captures CO2 and converts it to an electroactive species. The intermediate is assumed to be a type of formate, since IR spectra showed two bands at 1090 and 1180 cm-’ due to C-O-C in the CO,-saturated solution containing the homogeneous catalyst. The detailed mechanism of reaction (3) can be represented as in the reaction with ES [2] (see Fig. 5). In this scheme, the electroactive species is reduced by reaction with the leuco type of quinone to produce finally methanol and- the initial catalyst. Quinone is re-reduced electrochemically to the leuco type, which is why the
311
'20
_:H,Q
Fig. 5. Reaction scheme.
redox reaction of the favorable mediator must be reversible. The coordination of the cobalt atom to phthalocyanine gives an increase in the electron density on the dt2 orbital, and hence reduction of the electroactive species might be more advantageous at the electrode modified with this compound. As discussed above, however, the irreversibility of the electron-transfer reaction of phthalocyanine cobalt does not lead to the reduction of COz. The catalytic reduction of CO2 to methanol is feasible with the quinone derivative-confined electrodes, although the yield of methanol and the current efficiency are lower than those obtained with the ES-confined electrode. Acknowledgement Financial support from the Iwatani Naoji Foundation’s Research Grant is gratefully acknowledged.
References 1 2 3 4 5 6 7 8 9 10
M. Uknan, A. Aurian-BIajeni and M. Halmann, Chemtech, (1984) 235. K. Ogura and K. Takamagari, J. Chem. Sot., Dalton Trans., (1986) 1519. K. Ogura and M. Takagi, J. Electroanal. Chem., 201 (1986) 359. K. Ogura and I. Yoshida, J. Mol. Catal., 34 (1986) 67,309. M. Lamache, Electrochim. Acta, 24 (1979) 78. I. I-IaBer, R. Feder, M. Hatzakis and E. Spiller, J. Ekctrochem. Sot., 126 (1979) V. D. Neff, J. Electrochem. Sot., 125 (1978) 886. K. Itaya, H. Akahoshi and S. To&ma, J. Electrochem. Sot., 129 (1982) 1498. K. Ogura and S. Yamasaki, J. Chem. Sot., Faraday Trans. 1, 81 (1985) 267. K. Ogura and S. Yamasaki, J. Appl. Ekctrochem., 15 (1985) 279.
154.
Catalysis,
41 (1987) 303 - 311
303
ELECTROCATALYTIC REDUCTION OF CARBON DIOXIDE TO METHANOL PART 7. WITH QUINONE DERIVATIVES IMMOBILIZED ON PLATINUM AND STAINLESS STEEL KOTARO OGURA and MINORU FUJITA Department
of Applied
Chemistry,
(Received August 5,1986;
Yamaguchi
University,
Ube 755 (Japan)
accepted November 21,1986)
Catalytic reduction of carbon dioxide has been performed at quinone derivative-confined electrodes such as p-benzoquinone, 2-aminoanthraquinone, indigo and alizarin. These compounds worked as an electrode mediator in the catalytic reduction of CO?, although they tended to shed from the substrate at a large negative potential. The redox reaction of a favorable mediator must be reversible, and in this context the electrode modified with phthalocyanine cobalt did not operate as a mediator since the redox reaction of this compound was completely irreversible. The reduction of COZ to methanol was induced via a homogeneous catalysis by oxidation of the leuco-type of quinone, and regeneration of the active mediator was electrochemically achieved.
Introduction The reduction of carbon dioxide to an organic substance is an attractive subject, since the cyclic utilization of a carbon resource is possible and we can go beyond the problems of depletion of petroleum. Among many proposed methods for the COZ conversion, the electrochemical one is most promising [ 11. However, the weakness of this method is that it requires a large over-potential, and formic acid is the final product whose further reduction is very difficult in aqueous solution. In our previous works [2 - 41, an indirect electrochemical reduction was developed in which COZ is reduced at a mediated electrode. This process involves the electron transfer reaction at the mediated electrode/solution interface via homogeneous catalysis. The scheme of this process is shown in Fig. 1, where Ox represents COZ,* M, homogeneous catalyst; M-Ox and M-Red, intermediates; Red, reduction product, Y and X, reduced and oxidized forms, respectively, of the mediator immobilized on the electrode. 0304-5102/87/$3.50
0 Elsevier Sequoia/Printed in The Netherlands
304 Mediator Red
Fig. 1. Schematic representation reduction of CO, (Ox).
of the electrode/mediator/homogeneous
catalysis in the
Aqueous CO* is first captured by the homogeneous catalyst to form M-Ox, The intermediate is reduced to M-Red by the reaction with the reduced form of the mediator, and regeneration of the active mediator is electrochemically achieved. The homogeneous catalyst consists of a metal complex and a primary alcohol, and various effective systems have been investigated [ 31. The requirements for a substance capable of operating as an electrode mediator are to be stable in aqueous solution, to be easily immobilized on the substrate, and to have a suitable redox property. Previously [Z, 31, we have used Everitt’s salt (ES, KZFen[Feu(CN),]) for this purpose, which is the reduced form of Prussian blue (PB, KFem[Fen(CN)6]). In the present work, further studies on the electrode mediator were performed, and quinone derivatives immobilized on platinum and stainless steel were found to act satisfactorily for the reduction of CO, to meth~ol. Experimental Q&none derivatives used as the electrode mediator were p-benzoquinone (C6H402), 2aminoanthraquinone (&H,NO,), indigo (C16H10N~02) and alizarin (C14Hs04). Everitt’s salt and phthalocyanine cobalt (Cs2H16CON,) were also employed for comparison. Fixing of quinone derivatives and phthalocyanine cobalt to the electrode was carried out by coating a paste film [ 5 1. The paste film was prepared by mixing these compounds and graphite powder (1:lO wt.%), and adding a small amount of ethyl cellosolve solution (2%) containing copolymer of methyl metha~~la~-methac~lic acid [6]. The film of ES was formed by the elec~ochemic~ reduction of the PB film coated on the electrode [7,8]. The substrates were platinum (4.2 or 15 cm2) and stainless steel (15 cm’). The catalyst solution was 0.1 M KC1 (pH 3.5) containing 10 mM (mmol dmm3) aquapentacyanoferrate(I1) (Na:,[Fe(CN),(H,O)]) and 15 mM methanol. The solution was kept in a reservoir of 1 dm3 through which CO* bubbled at a flow rate of 200 cm3 mm-’ for 1 h. 40 cm3 of this solution was transferred respectively to the cathode and anode compartments in the electrolytic cell, which were separated by a fine porosity glass frit. The modified electrode and the bright platinum plate were set in the main and counter
305
electrode compartments, respectively. The electrode potentials measured are all referred to the saturated calomel electrode, and all experiments were carried out at 40 f 1 “c. Electrochemical measurements were carried out with a potentiostat (Nichia, NP-IR 1000) and a potential programmer (Nikko Keisoku Model NPS-2). The quantitative analysis of methanol added initially and produced finally was performed by a gas chromatograph (JGC-1100) and a steam chromatograph (Model SSC-1, Ohkura). The sampling procedure for the gas chromatograph was described previously [9], and the aqueous sample was used without any p~~eatment for the steam c~omato~ph.
Results and discussion Quinone derivatives used are typical dyes for yellow, deep blue or deep reddish-purple. In a general dyeing process, the dye is reduced to a leuco compound by a reducing agent such as sodium hydrosulfite. The leuco type is soluble in aqueous solution and has adequate affinity for a fiber through hydrogen bonding. The leuco compound combined to the fiber is re-oxidized to the quinone by air. Hence it may be expected that the redox reaction of the quinone derivatives proceeds under ordinary electrochemical conditions.
0
0.2
0.4 E/
0.6
0.8
VVSSCE
Fig. 2. i-E curves of quinone derivative-confined electrodes in 0.1 M KC1 (pH 3.5) at the sweep rate of 5 mV s-l. (), p-benzoquinone; (.. - ..), 2-aminoanthraquinone; (. - .), indigo; (__ ), alizarin; platinum surface area, 4.2 cm2.
The cyclic voltammograms of quinone derivatives immobilized on the platinum electrode are shown in Fig. 2. The reduction of these derivatives is observed at +0.07 V us. SCE ~-benzoqu~one), +0.27 V (&aminoanthraquinone), +0,40 V (~digo) and +0.52 V (size). The redox reaction of the quinone is represented as:
306
0 +
OH
0
-Q
2ri+ + 2e- -
d (Q)
B
OQQ)
where H2Q is the leuco type of quinone. The indigo- and ES-confined electrodes were applied to the reduction of aqueous Cc&, and the concentration of methanol obtained is shown versus the electrolysis time in Fig. 3. The reduction was performed in CO2 as well as Nz atmosphere. In both conditions, CO2 is reduced to methanol at the ~digo~~~f~ed e&&rode, although the amount of methanol is about half of that obtained with the ES-confined electrode at the electrolysis time of 9 h. In the prolonged electrolysis, the methanol formation is inclined to approach a certain value, which is determined by the depletion of aqueous COz. The same reason may be invoked to explain the smaller amount of methanol acquired in the N, atmosphere,
0
2
4
6
3
t/h
Fig. 3. Methanol formation as a function of electrolysis time with the mediated electrode: (a, O), ES; (A, A), indigo. The catholyte was the COa-saturated KC1 solution containing 10 mM Na~[Fe(CN)#-I~O)f and 15 mM C&OH, The electrolysis was performed in an atmosphere of CO, (a, A) and N2 (0, A).
307
The overall reaction for the reduction of CO2 at the ES-confined electrode has been proposed as shown in reaction (2) f2], co2 + 6ES + 6H+ 5
CH,OH + 6PB + 6K+ + Hz0
(2)
6fF
I
The total reaction on the quinone-confined represented as: COz + 3HzQ e
electrode can be similarly
CH,UH + 3Q C f-I20
(3)
t In both reactions, the conversion of CO2 to methanol is the redox reaction being induced by the oxidation of Everitt’s salt or leuco-type quinone. The m-reduction of the oxidized form to the active mediator must be achieved by the introduction of electrons from an external source of energy. The protons in solution are consumed in reaction (2), and the pH increases during the reduction of COz. In reaction (3), however, the protons consumed in the reduction are supplied by the re-reduction of the quinone, and the pH of the catholyte does not change as the reaction advances. Table 1 shows the current efficiency of the methanol formation obtained with the indigo- and ES-confined electrodes. At the former electrode, TABLE
1
Results of the concentration the current efficiency (r7i)a
Mediator ES
Electrolysis time (h)
[Methanol] (mm01 dmm3)
9
1.72 3-46 5.37 6.43 7.06 10.04 11.50
0.035 0.082 0.139 0,144 0.172 0.207 0.225
43.1 56.3 56.1 51.9 54.3 47.3 45.3
1 2 3 4 5 7 9
0.97 1.65 2.60 3.27 3.92 5.57 8.47
0.017 0.050 0.074 0.081 0.104 0.125 0.136
40.6 70.2 59.1 57.4 61.4 52.0 37.2
1
2 3 4 ;
indigo
of methanol produced, the electric charge passed (Q), and
%atalyst solution (40 cm3, pH 3.5): CC&saturated 0.1 M KCI containing 10 mM NasfFe(CN)s(H~O) f and 15 mM CHaUH; eleetx~?iysis potential, -0.7 V US,SCE; surface area, 4.2 cm2; in CC& atmosphere.
308 the electriccharge and the amount of methanol are both smaller than those obtained at the latter, but there is no big difference in the current efficiencies. The smaller amount of methanol at the indigo-confined electrode is ascribed to a smaller quantity of active mediator per unit area. A cyclic voltammogram of the ES-modified platinum electrode in 0.1 M KC1 showed a sharp peak (about 0.2 mM cme2) at +0.15 V (us. SCE) with a half-width of -15 mV and a peak separation of 10 mV. The amount of ES on the substrate was evaluated coulometricahy in the KC1 solution. The averaged value was -2.8 X lo-’ mol cmm2 in the concentration of high-spin iron ions. On the other hand, the current peak (about 0.03 mA cmm2) in the voltammogram of the indigo-modified electrode is not sharp, as shown in Fig. 2. In this system, graphite powder needed to be added to the paste containing the quinone in order to elevate its conductivity. Consequently, the amount of the quinone per unit area was to be kept within a limit, and the concentration of the quinone being exposed at the solution/paste electrode interface must be far lower than that of ES. In Fig. 4, the methanol formation is plotted uersus the cathodizing potential. Methanol is produced at negative potentials uersus SCE, but the plots show maxima at about -0.5 V (2aminoanthraquinone), -0.7 V (p-benzoquinone) , -0.7 V (alizarin) and -0.9 V (indigo). At potentials more negative than about -0.9 V (2aminoanthraquinone, p-benzoquinone and alizarin) or -1.3 V (indigo), there is no reduction of C02. The current
E/V
vs SCE
Fig. 4. The concentration of methanol produced versus anodizing potential at the quinone-confined electrodes. (O), indigo; (A), alizarin; (a), p-benzoquinone; (a), a-aminoanthraquinone. The catalyst solution was COz-saturated KC1 solution containing 10 mM Nas[Fe(CN),(HzO)] and 15 mM CHsOH.
309 TABLE 2 Results of the concentration current efficiency*
of methanol produced, the electric charge passed, and the
Mediator
Potential (V vs. SCE)
indigo
+0.1 -0.1 -0.3 -0.5 -0.7 -0.9 -1.1 -1.3
1.29 2.38 2.59 2.39 2.85 7.86 14.30
(-0.023) 0.019 0.041 0.943 0.070 0.018 0.062 (-0.037)
+1 .I -0.1 -0.3 -0.5 -0.7 -0.8 -0.9
0.62 1.64 2.03 2.27 4.08 3.98 6.40
(-0.003) 0.036 0.048 0.068 0.078 0.066 (-9,002)
alizarin
[Methanol] (mmol dm-““) 0.48
34.1 40.0 38.5 67.6 63.4 18.3 -
49.4 54.6 69.4 44.3 38.4 -
a Catalyst solution (40 cmas pH 3.5): COa-saturated 0.1 M KC1 containing IO mM Nas[Fe(CN)s(HaO)f end 15 mM CHaOH; electrolysis time, 3 h; surface area, 4.2 cm’; in Na atmosphere.
efficiencies obtained with the indigo- and a&u&confined electrodes are 8sbown in Table 2. Quinone derivatives tended to shed from the substrate at such a negative potential, which leads to difficulty in the mediated electron transfer. Hence, a large negative potential cannot be applied in the electrolysis with the quinone-confined electrodes. This is a shortcoming in using these electrodes as a mediator, but there was no such restriction with the ES-confined electrode. The concentrations of methanol and the current efficiencies obtained withvarious modified electrodes are shown in Table 3. In this experiment, platinum and stainless steel are both used as the substrate, The results show that the quinone derivatives as well as ES work as mediators, but phth~ocy~~e cobalt shows no mediatory effect. This is caused by the lack of the redox property required for a compound to operate as a mediator. The redox reaction of the favorable mediator is always reversible, as shown in Fig. 2 for the quinones and in [6] and [7] for Everitt’s salt, but the i-E curve of the cobalt complex with phthalocyanine is completely irreversible [lo]. The effect of the substrate is obvious in Table 3, and the amount of methanol obtained with stainless steel is about half of that with platinum. This result, attributed to the slight dissolution of stainless steel during the course of electrolysis, is supported by the results shown in the same table. The current efficiency in the case of platinum is generaBy higher at -0.3 V than at -0.7 V, but the relationship is reversed in the case of stainless steel.
310 TABLE 3 Results of the concentration current efficiencya
of methanol produced, the electric charge passed, and the
[Methanol] (mmol dme3)
Mediator
Substrate
Potential (V vs. SCE)
A B C D E F
platinum
-0.3
9.25 13.05 7.40 6.82 12.16 10.21
0.241 0.122 0.105 0.089 0.140 (-0.005)
60.3 21.7 32.9 30.2 26.7 -
A B C D E F
platinum
-0.7
24.91 15.38 17.10 20.82 30.58 20.99
0.449 0.256 0,155 0.151 0.080 (-0.020)
41.7 38.5 21.0 16.8 6.1 -
A B C D E F
stainless steel
-0.3
8.66 8.59 6.87 6.39 9.23 6.50
0.171 0.099 0.076 0.068 0.119 (-0.008)
45.7 26.7 25.6 24.6 29.9 -
A B C D E F
stainless steel
-0.7
9.00 10.55 8.00 8.36 9.01 9.30
0.177 0.195 0.107 0.111 0.140 (-0.013)
45.5 42.8 31.0 30.8 36.0 -
Watalyst solution (40 cm3, pH 3.5): COzsaturated 0.1 M KC1 containing 10 mM Nas[Fe(CN)s(HzO)] and 15 mM CHsOH; electrolysis time, 5 h; surface area of the substrate, 15 cm2; in CO2 atmosphere. A, Everitt’s salt; B, indigo; C, 2-aminoanthraquinone; D, alizarin; E,p-benzoquinone; F, phthalocyanine cobalt.
This is responsible for the tendency of stainless steel to dissolve at a potential more noble than the rest potential (i.e., anodic dissolution). The existence of a homogeneous catalyst consisting of a metal complex and a primary alcohol is prerequisite to the reduction of COz. As discussed previously [ 41, the homogeneous catalyst first captures CO2 and converts it to an electroactive species. The intermediate is assumed to be a type of formate, since IR spectra showed two bands at 1090 and 1180 cm-’ due to C-O-C in the CO,-saturated solution containing the homogeneous catalyst. The detailed mechanism of reaction (3) can be represented as in the reaction with ES [2] (see Fig. 5). In this scheme, the electroactive species is reduced by reaction with the leuco type of quinone to produce finally methanol and- the initial catalyst. Quinone is re-reduced electrochemically to the leuco type, which is why the
311
'20
_:H,Q
Fig. 5. Reaction scheme.
redox reaction of the favorable mediator must be reversible. The coordination of the cobalt atom to phthalocyanine gives an increase in the electron density on the dt2 orbital, and hence reduction of the electroactive species might be more advantageous at the electrode modified with this compound. As discussed above, however, the irreversibility of the electron-transfer reaction of phthalocyanine cobalt does not lead to the reduction of COz. The catalytic reduction of CO2 to methanol is feasible with the quinone derivative-confined electrodes, although the yield of methanol and the current efficiency are lower than those obtained with the ES-confined electrode. Acknowledgement Financial support from the Iwatani Naoji Foundation’s Research Grant is gratefully acknowledged.
References 1 2 3 4 5 6 7 8 9 10
M. Uknan, A. Aurian-BIajeni and M. Halmann, Chemtech, (1984) 235. K. Ogura and K. Takamagari, J. Chem. Sot., Dalton Trans., (1986) 1519. K. Ogura and M. Takagi, J. Electroanal. Chem., 201 (1986) 359. K. Ogura and I. Yoshida, J. Mol. Catal., 34 (1986) 67,309. M. Lamache, Electrochim. Acta, 24 (1979) 78. I. I-IaBer, R. Feder, M. Hatzakis and E. Spiller, J. Ekctrochem. Sot., 126 (1979) V. D. Neff, J. Electrochem. Sot., 125 (1978) 886. K. Itaya, H. Akahoshi and S. To&ma, J. Electrochem. Sot., 129 (1982) 1498. K. Ogura and S. Yamasaki, J. Chem. Sot., Faraday Trans. 1, 81 (1985) 267. K. Ogura and S. Yamasaki, J. Appl. Ekctrochem., 15 (1985) 279.
154.