Electrocatalytic activity of CoII TPP-pyridine complex modified carbon electrode for CO2 reduction

309

J. Electrounal. Chem., 318 (1991) 309-320

Elsevier Sequoia S.A., Lausanne

JEC 01739

Electrocatalytic activity of Co” TPP-pyridine complex modified carbon electrode for CO, reduction Takashi Atoguchi a, Akiko Aramata a**,Akio Kazusaka b and Michio Enyo a a Catalysis Research Center, Hokkaido University, Sapporo 060 (Japan) b Faculty of Veterinary Medicine, Hokkaido University, Sapporo 060 (Japan)

(Received 2 May 1991; in revised form 15 July 19911

Abstract A cobalt(IIltetraphenylporphyrin (Co”TPP)-pyridine complex modified glassy carbon electrode (CoTPP/py/GC) was prepared and characterized in the form of Co”TPP-py-NHCO-GC. The catalytic activity for the electrochemical reduction of CO, was studied in phosphate buffer; the ,electrode showed a high catalytic activity for CO, reduction to CO at potentials more negative than - 1.0 V vs. SCE (-0.4 V vs. RHE), with a current efficiency of 92% for CO production at - 1.1 V vs. SCE. CoTPP/py/GC also showed a high durability of the catalytic activity and the overall turnover number (mol of CO produced/m01 of Co”TPP on GC) exceeded 10’. The role of the pyridine in CoTPP/py/GC is discussed in connection with the activity and stability as an electrode.

1. INTRODUCTION

The fixation of CO, by homogeneous [1,21 and heterogeneous [31 catalysts has been studied extensively, where CO, was coupled with organic compounds such as alkynes, alkenes, conjugated dienes, aromatics, and hydrogen gas. In the above catalytic processes, a high temperature and/or a high pressure were applied, since CO, is chemically very stable. Electrochemical reduction of CO, [4] has also attracted our attention because the reduction proceeds at a moderate temperature and atmospheric pressure, giving a variety of reduction products such as formate, CO, methane, ethene, and ethanol in aqueous solutions [5,61 or for-mate, oxalate, and CO in aprotic media [7,8]. In particular, the electrolysis in aqueous solution is interesting, since the hydrogen source, H,O, is abundant. In spite of these advantages, electrochemical

l

To whom correspondence

0022-0728/92/$05.00

should be addressed.

0 1992 - Elsevier Sequoia S.A. All rights reserved

310

reductions of CO, at bare metal electrodes have suffered from large overvoltages of about l-2 V and competing hydrogen evolution [4]. Metal complexes have been applied successfully to reduce the large overvoltages by their mediator process. Tetraazamacrocyclic complexes of Con or Ni” catalyze the electrochemical reduction of CO, at a mercury electrode in aqueous solution [9]. Especially, the Ni”-1,4,8,11-tetraazatetradecane (cyclam) complex showed a high catalytic activity, with a high selectivity of CO, to CO [lo], i.e., the current efficiency of the reduction (nco) was 99% at - 1.0 V vs. SHE (pH 4.1). Unfortunately, some of these catalysts were usually rather unstable and degraded during electrolysis. Con phthalocyanine (PC) deposited on a carbon electrode showed a high catalytic activity and a noticeable stability for the electrochemical reduction of CO, [11,12]. The catalytic reduction current at this electrode sustained a steadystate value of the overall turnover number (mol of produced CO/mol of deposited Co”Pc) of over 105, but the selectivity to CO was less than 65% at - 1.15 V vs. SSCE (pH 5) [12], of which potential 0.53 V was overvoltage on the basis of the thermodynamic standard potential of CO, + 2 Hf+ 2 e-e CO + H,O at -0.1 V vs. RHE [4]. These previous studies led us to investigate the electrochemical reduction of CO, at a modified electrode, on which the complex is fixed as a mediator in an active and stable form. In order to construct an active and stable electrocatalyst, we attempted Co” tetraphenylporphyrin (TPP) fixation on glassy carbon (GC) through coordination of pyridine, which is denoted as CoTPP/py/GC, in which the pyridine is anchored chemically onto the surface of the GC electrode through an amido bond at CY of the pyridine ring (py/GC). According to the literature [13,14], basic ligands such as pyridine and imidazole are expected to improve the catalytic activity of Co”TPP for CO, reduction by coordination to the axial position of this complex. The surface states of the GC electrode at each preparation step of CoTPP/py/GC were characterized by means of FT-IR and UV-Vis, and XPS. The catalytic activity of the CoTPP/py/GC electrode for CO, reduction was examined by cyclic voltammetry and controlled potential electrolysis. The dependence of the current efficiency of CO, reduction on potential was also examined. EXPERIMENTAL

Co”TPP (Wake Pure Chemical Industries, Ltd.) and reagent grade 4-aminopyridine, NaH,PO,, Na,HPO, and SOCI, (Kant0 Chemical Co., Inc.) were used as received. Reagent grade benzene and CH,Cl, were refluxed over CaH, for 30 min, then distilled, and stored over 3A activated molecular sieves. Ar (99.9999%) and CO, (99.999%) were used. An aqueous phosphate buffer solution (a mixture of l/15 M NaH,PO, and l/15 M Na,HPO,) was prepared using Millipore pure water. The pH values of the buffer were 6.7 and 6.0 after He (or Ar) and CO, saturation, respectively. Cyclic voltammetry and controlled potential electrolysis were performed in a three-compartment Pyrex cell after He (or Ar) or CO, saturation of the solution in

311

//5 d (I)

ox/GC preparation

(2) COCVGC

(3)

prepamtion

rl

9 C

(GC)

+

‘CL

(4)

prepamtion

4-

CoTPP/py/GC prepamtion

(4) COTPP

(3) Amidization

(2) Chlorination

(SOCLZ)

py/GC

9

(4aminopyridne): “p@

=O 0 C” ‘CL CL

/

(plain GC)

(ox /GC 1

KOCI /GC)

KoTPP/py/GC

(py/GC)

Fig. 1. Scheme of the preparation procedure of the CoTPP/py/GC

1

electrode.

the working electrode compartment at room temperature. An 80 mesh 1 cm* Pt net was used as the counter electrode and a saturated calomel electrode (SCE) as reference electrode. In the following, potentials are given against the SCE. Electrochemical measurements were carried out potentiostatically using a potent\ostat (Toho 2OOOB),function generator (Toho 22301, and X-Y recorder (Rika Denki WE-11A). The Co”TPP-pyridine complex modified electrode (CoTPP/py/GC) was prepared in four steps, as shown in Fig. 1. Ox / GC preparation

The ox/GC surface in Fig. 1 was prepared by anodic oxidation of GC (3.5 cm*, GC 30s of Tokai Carbon) at 2.5 V for 10 min in an Ar saturated phosphate buffer. COCI / GC preparation After washing with water, the ox/GC was refluxed in SOCI, for 1 h. Py / GC preparation Amidization of the -COCl groups on the GC surface was carried out in a 4-aminopyridine saturated benzene solution for 2 h at room temperature, and then the electrode was washed with benzene. CoTPP/py

/ GC preparation

Finally, Co”TPP was fixed on the amidized surface by reflex of 0.3 mM Co”TPP in a 4: 1 mixture of benzene and CH,Cl, with py/GC for 1 h. Before use, the CoTPP/py/GC electrode was washed thoroughly with water.

312

The above ox/GC, COCl/GC and py/GC surfaces were characterized by diffuse reflectance FT-IR spectra which were observed in air by means of a Perkin-Elmer 1720X FT-IR spectrometer with diffuse reflectance accessories (Perkin-Elmer). Diffuse reflectance UV-Vis spectra of CoTPP/py/GC were observed in air by means of a Hitachi 330 spectrophotometer with an integrating sphere. XPS spectra of py/GC and CoTPP/py/GC were observed with a VG ESCA MK III apparatus. CO was analyzed by means of a gas chromatograph (Shimazu GC 8A) with a TCD detector, a 3 mm 0 x 2 m column of Unibeads C (60/80 mesh, Gasukuro, Kogyo Inc.), and He as carrier gas. For the observation of hydrogen, after removal of CO, and moisture by a pre-column of activated alumina, a Shimazu gas chromatograph GC 6AM with TCD, a 3 mm 0 X 2 m column of molecular sieve 13X-S (60/80 mesh, Gasukuro Kogyo Inc.), and N, carrier gas were used. Products in the solution in the working electrode compartment were observed by FT-IR ATR (attenuated total reflectance 1 spectroscopy. RESULTS

AND DISCUSSION

Characterization of the CoTPP/py

/ GC electrode

Glassy carbon was first oxidized to convert surface carbon into the -COOH group. A few methods for the introduction of -COOH functional groups onto a carbon electrode have been reported, such as (1) oxidation by heating at 300-400°C in air [15], (2) oxidation by oxidizing agents [16], and (3) anodization in aqueous solution [17]. Jannakoudakis et al. [17] reported that anodic oxidation of a carbon fiber at 2.3 V in aqueous 0.5 M Na,SO, solution gave the -COOH group selectivity. We examined the above three methods and found that the anodic oxidation technique was the best to introduce the -COOH group; this method, therefore, was employed. After ox/GC preparation by anodic oxidation at 2.5 V for 10 min, its diffuse reflectance FT-IR spectrum was observed as shown in Fig. 2a, where the absorption peak at 1711 cm-’ was assigned to C=O stretching of the -COOH group, and the peak at 1630 cm-’ was assigned to the C=O of the quinone group (>C=O) [18]. The formation of these functional groups was supported by observation of the cyclic voltammogram in Ar saturated phosphate buffer (Fig. 3). In the negative sweep, reduction peaks at 0.3 (on curve 31, -0.1 (on curves 4 and 5) and -0.8 V (on curve 1) were observed, which were assigned to the reduction currents of -OH, quinone, and the -COOH group, respectively [17,18]. In comparison with the CV of plain GC (Fig. 3, curve 5), the peak current on ox/GC at -0.8 V was enhanced significantly. The ox/GC surface is shown schematically in Fig. 1 (ox/G(Z). After treatment of the ox/GC with SOCl,, the peak at 1711 cm-’ disappeared and a new peak appeared at 1828 cm-’ (Fig. 2b), which is assigned to C=O stretching of the -COCl group [19]. Further, the decrease of the peak intensity at 1630 cm-’ is due to a decrease of the amount of quinone by the SOCI, treatment.

313

i

4000

3200

24002000

1600

Wovenumber Fig. 2. Diffuse

reflectance

IT-IR

1200

800

400

/cm-’ spectra

of ox/GC

(a), CoCl/GC

(b), and PY/GC

cc).

The diffuse reflectance FT-IR spectrum of py/GC is shown in Fig. 2c, where the absorption peak of C=O stretching of the amido (-CONH-) group can be observed at 1671 cm-’ together with the characteristic peaks of the pyridine ring at 1605 and 1541 cm-‘. These spectral features show that the GC surface was functionalized with a -CONH-py group, as shown in Fig. 1 (py/GC). The CV of the py/GC electrode at a sweep rate of 50 mV/s in phosphate buffer under an Ar atmosphere is shown in Fig. 4. Although the peaks at 0.3, -0.1 and -0.8 V were observed on ox/GC in the negative sweep, no such cathodic peak appeared on the CV of py/GC, implying that no reducible oxygenated groups such as -COOH or -OH remained on the py/GC surface. After the fixation of Co”TPP as shown in Fig. 1 (CoTPP/py/GC), its diffuse reflectance UV-Vis spectrum was observed at room temperature in air. This is shown in Fig. 5a, the UV-Vis spectra of Co”TPP in CH,Cl, and in 16% pyridine + CH,Cl, are given in Figs. 5b and c, respectively. In the spectrum of CoTPP/py/GC, an absorption peak was observed at 430 nm, the wavelength of

314

I (0)

I

I

I

-1.0

0

I.0

I

E /V

I

2.0

(SCE)

Fig. 3. Cyclic voltammograms of a GC electrode at a sweep rate of 50 mV/s with various upper limits: (1) 2.5; (2) 2,.0; (3) 1.5; (4) 1.0, and (5) 0.5 V; in phosphate buffer under an Ar atmosphere (pH 6.7).

which is nearly identical to the Soret band of Co”TPP in pyridine + CH,Cl, in air. Stynes et al. [201 reported that the shift of the Soret band of Con porphyrins to 430 nm was due to coordination of the pyridine and then successive adsorption of

(+I

Ito; ‘5m>-y i-1 I

-1.5

I

I

-1.0

I

- 0.5

0

E/V(SCE) Fig. 4. Cyclic voltammogram atmosphere. pH 6.7.

of py/GC

at a sweep rate of 50 mV/s

in phosphate

buffer

under

an Ar

31.5

al CoTPP/py/GC

400

b) Co’TPP

in CH&

cl Co’TPP

in 16% py/CHnCIP

500 Wavenumber

600

700

(nm 1

Fig. 5. Diffuse reflectance UV-Vis spectrum of CoTPP/py/GC (a), and W-Vis in CH,Cl, (b) and in 16% pyridine+CH,CI, (c) at room temperature in air.

spectra of Co”TPP

an 0, molecule. Therefore, the absorption peak at 430 nm in Fig. 5a was assigned to the Soret band of a Co”TPP-pyridine complex of 0, adducts on CoTPP/py/GC. This suggested that Co”TPP on the CoTPP/py/GC electrode is fixed as a pyridine complex with coordinated 0, in air. The adsorption of 0, by the Co”TPP-pyridine complex has been studied as an 0, carrier and this complex releases an 0, molecule reversibly under an inert atmosphere or in vacua at room temperature [21]. In the present case, the 0, coordinated to CoTPP/py/GC was removed within a few cycles of CV in phosphate buffer under an Ar atmosphere, during which a decrease of the reduction current was observed at potentials more negative than 0 V. Thus no cathodic current was observed except for the hydrogen evolution reaction. From the above CV observation and spectra shown in Fig. 5, we presumed that Co”TPP became a five-coordinate complex with a pyridine

316

a: co c

0: l

Fig. 6. Structure of the CoTPP/py/GC

:

N

electrode.

ligand of py/GC in the form of Co”TPP/py-NH-CO-GC. CoTPP/py/GC electrode is shown schematically in Fig. 6. Electrochemical reductioiz of CO, at the CoTPP/py

The structure of the

/ GC electrode

A typical CV of the CoTPP/py/GC electrode at a sweep rate of 1 mV/s in He saturated phosphate buffer (pH 6.7) is shown in Fig. 7, where an increase of the cathodic current can be observed at potentials more negative than - 1.1 V. In this potential region, hydrogen was detected in the working electrode compartment by gas chromatography. As no cathodic current was observed on py/GC in the same potential region, the pyridine modified surface itself is not active for hydrogen evolution at this potential region and, thus, Co”TPP, in the form as shown in Fig. 6, is catalytically active for the hydrogen evolution reaction. In CO, saturated phosphate buffer (pH 6.01, the cathodic current on CoTPP/py/GC was enhanced at potentials more negative than - 1.0 V as shown in Fig. 7, but no increase was observed on py/GC in the same potential region, as for the hydrogen evolution reaction. This indicates clearly that Co”TPP fixed on this electrode, CoTPP/py/GC, is catalytically active for the electrochemical reduction of CO, at potentials more negative than - 1.0 V. As this potential is more negative than the E,,2 of Co”TPP/Co’TPP at -1.03 V in pyridine [22], Co’TPP is suggested to be the active species in the electrochemical reduction of CO,, as was the case with Co”mesotetracarboxyphenylporphyrin (TCPP) and Corrtetraphenylporphyrinsulphonate (TPPS,) [23]. CO was the main reduction product of the controlled potential electrolysis at - 1.0 to - 1.3 V. Neither for-mate nor oxalate were detected. Figure 8 shows the current efficiency of CO production (rlco) against potential, where qco reaches a

317

0 Vvs RHE -1.2

-1.0

-0.8

-0.6

-0.4

-0.2

I

E / V (SCE) Fig. 7. Cyclic voltammograms of CoTPP/py/GC He (pH 6.7) and CO, (pH 6.0) atmospheres.

100 %I

I%)

50 .

0’

I

-1.0

I

I

-1.1

-1.2 E / V (SCE)

Fig. 8. Plots of TV- against potential.

at a sweep rate of 1 mV/s in phosphate buffer

318

maximum at - 1.1 V. At potentials more negative than - 1.2 V, it is reduced as a result of the competing hydrogen evolution. Trace amounts of CO were detected after the electrolysis at - 1.0 V (-0.4 V vs. RHE). Therefore, from Fig. 8, the electrochemical reduction of CO, on CoTPP/py/GC is found to occur at a potential of 0.3 V negative to the thermodynamic standard potential of CO, to CO reduction of -0.1 V vs. RHE [4]. Effect of the fixation of Co”TPP on the catalytic stability of CoTPP/py

/GC

In order to calculate the overall turnover number of Co”TPP for CO, reduction, we estimated the amount of Co”TPP on the CoTPP/py/GC electrode from XPS observations. An amount of Co”TPP of lo-‘* mol/cm* on the electrode surface was estimated from the XPS peak intensity and empirically derived atomic sensitivity factors of 01s and Co2p 1241,on the assumption that a monolayer of oxygenated functional groups of about lop9 mol/cm* is produced by the anodic oxidation, as a rough approximation. In the case of the electrolysis at - 1.1 V, where vco was 92%, the reduction current was sustained at a steady state value when a charge of 5.8 C was passed. This charge corresponds to 3 X 10e5 mol of two-electron reduction products such as CO and H,. Therefore, the overall turnover number (mol of CO produced/mol of Co”TPP on GC) was about 10’. In all electrolytes in the present work, the overall turnover numbers were well beyond 10’ without loss of the catalytic activity. The catalytic activity was maintained for at least 3 days when the electrode was stored in an Ar or CO, saturated phosphate buffer solution. In order to examine the effect of the fixation on the stability of Co”TPP, we carried out the electrochemical reduction of CO, at a GC electrode in a 0.1 M tetra-n-butylammoniumfluoroborate (TBAF) + DMF solution with homogeneously dissolved 7.5 x lo-’ mol Co”TPP 1251.In the electrolyses at potentials of - 1.3 to - 1.5 V, the main reduction product was formate ion with 77ncoo = 10%. After passing a charge of 5-8 C, the red colour of Co”TPP disappeared under black precipitates, and subsequently the catalytic activity was lost. This suggested that Co’TPP was not stable even in DMF and was degraded while the overall turnover number of Co”TPP was below 50, although the overall turnover number of Co”TPP on CoTPP/py/GC for CO, reduction was to be 10’. These results indicate that the catalytic activity and stability of Co’TPP, which was formed by reduction of Co”TPP, were favoured by the fixation on the surface of CoTPP/py/GC. This high durability of the catalytic activity of CoTPP/py/GC is due to the isolation of Co”TPP centers from each other by coordination of pyridine, of which the concentration was also estimated to be about lo-” mol/cm* and 10 times as much as that of Co”TPP by XPS peak intensity. Therefore, the mutual interaction between Co’TPP, which is isolated by pyridine coordination, is l

l

Upon addition of pyridine to the DMF solution, a shift of the redox peak of Co”TPP/Co’TPP

more negative

potentials

was observed

but the catalytic

activity was not enhanced.

to

319 TABLE 1 Overall turnover number and nco for different catalysts Catalyst CoTPP/py/GC b CoPc/C c Ni cyclam (on Hg) d

E/V

E/V vs. RHE

%Cl/%

TON a

- 1.1 (vs. SCE, pH 6) - 1.15 (vs. SSCE, pH 5) - 1.0 (vs. SHE, pH 4.1)

-0.5 -0.61 - 0.76

92 52.2 99

> 10’ > 3.1 x 105 >8~10~~

a Overall turnover number. b This work. ’ Cobalt phthalocyanine on carbon cloth [12]. d Ni cyclam adsorbed on a Hg electrode [lo]. e Since this value was calculated on the basis of the concentration of Ni cyclam dissolved in the cathodic cell solution (see ref. lo), it is difficult to compare it directly with the results for the modified electrodes.

suggested to be suppressed [121, and thus CoTPP/py/GC is stable, giving a high overall turnover number. The qco and overall.. turnover number are.- compared with the previously reported results for Ni”cyclam on Hg and Co”Pc on carbon cloth in Table 1, where Co”TPP on the CoTPP/py/GC electrode is found to be an effective and stable catalyst for the electrochemical reduction of CO,. The role of the pyridine ligand in the catalytic activity of the CoTPP/py electrode

/GC

As mentioned above, Co”TPP on the CoTPP/py/GC electrode is in the form of a five-coordinate complex with a pyridine ligand, with a vacant site at the trans-position to pyridine, as in the case of the polymer bonded imidazole-Co”TPP complex [26]. It is expected that CO, reduction occurs at a vacant site to which pyridine gives specific reactivity through the so-called tram effect [27]. The key step of the electrochemical reduction of CO, ‘has been proposed to be an electron transfer to CO, [4]. It is expected that the axial pyridine ligand favours an electron transfer from Co’ of the complex to CO,, since it increases the electron donating ability (or basicity) of Co, as in the case of an 0, carrier [20,21,26], and facilitates the reduction of CO,. In order to elucidate the effect of the axial ligand, a study with different bases as bridges between Co”TPP and GC is under progress.

ACKNOWLEDGEMENTS

The authors wish to acknowledge Dr. H. Konno of the Faculty of Engineering, Hokkaido University, for the XPS observations. We also thank Prof. T. Hayashi and Dr. R. Ohnishi of the Catalysis Research Center, Hokkaido University, for their helpful discussions.

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