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Journal of the Chilean Chemical Society

On-line version ISSN 0717-9707

J. Chil. Chem. Soc. vol.51 no.2 Concepción June 2006

http://dx.doi.org/10.4067/S0717-97072006000200017 

 

J. Chil. Chem. Soc., 51, Nº 2 (2006) , pags: 923-926

 

"SPECTROELECTROCHEMICAL STUDIES ON ITO MODIFIED ELECTRODES WITH A CONDUCTING COBALT (II) MACROCYCLE FILM IN THE ELECTROCHEMICAL REDUCTION OF CO2"

 

PAULINA DREYSE1, GALO RAMIREZ2, ANDREA RIQUELME2, MAURICIO ISAACS1*

1Departamento de Química, Facultad de Ciencias, Universidad de Chile. Santiago, Chile.
2Departamento de Química de los Materiales, Facultad de Química y Biología, Universidad de Santiago de Chile, Santiago, Chile.


ABSTRACT

The spectroelectrochemical properties of a conducting polymer derived from Co(II) tetra-3-amino-phenyl-porphyrin,(poly-Co(II)-TAPP) were studied towards the electrochemical reduction of CO2 on ITO surface. Under inert atmosphere, the results show that the polymer present a stable Co(I) oxidation state only in basic pH while in more acidic solution this Co(I) does not stabilize. Under CO2 atmosphere the reduced polymer forms a stable adduct with no clear electronic localization. The formation and the stability of this adduct could explain the wide distribution of reaction products.

Key words: electrochemical reduction of carbon dioxide, porphyrins, conducting polymers.


INTRODUCTION

The electrochemical reduction of CO2 is an important topic in electrochemistry, because the green house effect-global warming is now more evident and it is a matter of public discussion and world concern [1,2]. For that reason, many efforts has been focused to find electrodic surfaces that present the possibility of recycling this raw material in fuels or starting materials for organic products [1,2]. The electrochemical reduction of carbon dioxide, in aqueous solution, has been studied in metallic, semiconductors or carbon electrodes [1,2]. Where each one of them, have presented their own drawbacks [1,2]. The carbon electrodes are not expensive, but they present a high overpotential and are not selective being the H2 production the principal competitive reaction. One way to avoid these problems is the use of coordination compounds as electro-catalysts. Among them aza-macrocyclic compounds containing transition metals seems to be an alternative to eliminate the problems described above. The most studied aza-macrocyclic complex is [Ni(cyclam)]2+ [3].This electro-catalyst present a low overpotential for the electrochemical reduction of carbon dioxide on Hg electrodes and is selective to the CO formation [3]. Other aza-macrocyclic compounds with electrocatalytic activity towards the electrochemical reduction of carbon dioxide are porphyrins and phthalocyanines these macrocycles present electrocatalytic activity either in organic or aqueous solution [4,5].

Different modifications of carbon surfaces with these macrocycles have shown promissory results and among them the electro-polymerization of porphyrins and phthalocyanines, on carbon surfaces, it seems to be one of the best approach since it is possible to obtain stable and robust electrodes considering the extreme potential where generally the reduction of carbon dioxide takes place (ca.-1.0 V vs Ag/AgCl)

In previous works we have studied the electrochemical reduction of CO2 on glassy carbon modified-electrodes with poly-tetra-amino-phthalocyanine, M-poly-tetra-amino-phthalocyanines where M = Co, Ni, Cu, Fe, (poly-M(II)TAPC) [6] and poly-Co(II)-tetra-3-amino-phenyl-porphyrin (poly-Co(II)-TAPP) [7]. Results have shown that Co(II) macrocycles are good electro-catalysts for the reduction of carbon dioxide. The conducting polymer poly-Co(II)TAPC is selective to formic acid formation. However, poly-Co(II)-TAPP is not selective giving CO, H2 and formic acid as products. In this communication we explain the low selectivity properties of poly-Co(II)-TAPP, in terms of UV-Visible spectroelectrochemical measurements to obtain information about its redox properties, either metal or ligand centered. As far as we know this is the first time that this kind of results related to the electrochemical reduction of CO2 are presented for poly-Co(II)-TAPP.

RESULTS AND DISCUSSION

Porphyrins and phthalocyanines containing Co (II) as metallic center have a well documented spectroelectrochemical characterization in aqueous and organic solution [8-15]. In both cases, Co(I) —L (metal-ligand) charge transfer band appears at 450 nm for phthalocyanines and at 540 nm for porphyrins compounds when the complexes are exposed to negative potentials. On transparent electrodes modified with conducting polymers derived from these macrocycles the spectra are similar, compared with the complexes in solution, when they are electrochemically reduced [16-18]. Figure 1 shows the spectra of poly-TAPP-Co (II) modified electrode, recorded at different potentials in 0.2 M KOH solution. This Figure depicts the enhancement of the Q bands with a small blue shift from 546 to 538 nm corresponding to the high energy Q band. The Soret band undergoes a shift from 445 to 425 nm when the potential is changed from open circuit potential (OCP) to -0.9V and also shows a continuous increasing in this intensity from OCP to this negative potential. This behavior at such extreme pH is typical for Co (I) species. Fujita have demonstrated for a series of Co(II) porphyrins complexes that when chemical, photochemical, radiolytic or electrochemical reductions are performed in basic solution the spectra obtained are the same in all cases. The explanation for that behavior is the OH- could coordinate as a fifth or even a sixth position stabilizing the Co (I) oxidation state [19-22].

In our case the spectra obtained for poly-TAPP-Co (II) modified electrode fit with the results reported before for Fujita, indicating the same kind of stabilization of the Co(I) intermediary.


  Figure 1: Spectroelectrochemical reduction of poly- Co(II)-TAPP/ITO surface in 0.2 M KOH,under N2 Atmosphere. Solid line: OCP . Dotted line: -0.5V. Dashed line: -0.9 V

When poly-Co(II)TAPP modified electrode is in NaClO4 0.1 M under N2 (see Figure 2) atmosphere, the Q bands decrease its intensity but remains at the same wavelength, 550 nm. The Soret band shifts from 442 to 434 and its absorbance also decreases. This behavior is completely different to that shown in Figure 1. Then, a possible explanation of this "bleaching" is that the applied negative potential does not reduce the Co(II) species. Instead, a reduction of the whole macrocycle, with no electronic localization, takes place. Other authors had informed that, in neutral or acidic solution of Co porphyrins complexes, the Co(I) oxidation state have a short life time decaying rapidly stabilizing the radical species or regenerating the Co(II) complex [19].


  Figure 2: Spectroelectrochemical reduction of poly- Co(II)-TAPP/ITO surface in 0.1 M NaClO4 ,under N2 Atmosphere. Solid line: OCP. Dotted line: -0.5V. Dashed line: -0.9 V

However, when poly-Co(II)TAP modified electrode is in NaClO4 0.1 M under CO2 atmosphere (see Figure 3), Q bands increase its intensity but remains at the same wavelength 545 nm. The Soret band shifts from 442 to 428 nm and its absorbance decreases. Worth noting is the appearing of a signal as a wide band between 600-650 nm indicating the possible presence of chlorins or phlorins species [19].


  Figure 3: Spectroelectrochemical reduction of poly- Co(II)-TAPP/ITO surface in 0.1 M NaClO4 ,under CO2 Atmosphere. Solid line: OCP. Dotted line: -0.5V. Dashed line: -0.9 V

In this case the spectra obtained resemble the spectra recorded in basic solution. The increment in the Q bands intensities could be related to an adduct formation between de CO2 and the reduced poly-TAPP-Co modified electrode [23,24] since the coordination of CO2 or most probably CO stabilizes the Co (I) oxidation state as in the case of OH- ions in basic solution [19].

Most of the Co (II) related macrocycles have an opposite behavior. For example, when the spectra of Co(II) hexa-aza-macrocycles, in organic solution, are registered in N2 atmosphere, a Co(I)—L (metal to ligand) charge transfer band arises at negative potentials and when the experiment is carried out in CO2 atmosphere the band disappears indicating an electron transfer from the metal to the coordinate molecule instead of the macrocyclic ring [6,23,24]. In other cases, some macrocycles present a similar behavior of the poly- Co(II)-TAPP shown here [25-27]. For instance, the electronic spectra of the complex [Ni(tmdnTAA)]2+ present a band centered at 358 nm when is reduced at -1.8 V in CO2 atmosphere. In this case, the absorption band was attributed to a stable intermediary [Ni(tmdnTAA)CO]2+. This intermediary is a stable adduct that interrupts the electrocatalysis [27]. This fact was corroborated by controlled potential experiments, where H2 was the unique reaction product [27].

In our case the adduct does not disappear when the potential is applied, therefore the electron transfer does not occur directly from the metallic center to the CO2 as in the case of Co (II) phthalocyanine polymer reported before [6]. Where the reduced ligand and Co(I) are the species responsible of the electrocatalysis [6] .

In the case of the poly- Co(II)-TAPP, the adduct formed between the polymeric complex and the CO2 molecule is stable, but not defined, because is not well known the form that CO2 is bind to the poly- Co(II)-TAPP.

However several theoretical works have been focused to this theme [23, 28-30] they have explained that different coordination modes could drive the distribution of reaction products. Probably, in our case, the carbon dioxide has several ways to bind the complex. Then, the formation of the adduct and the appearance of the wide bands over 600 nm could indicate the presence of species of different energies, resembling phlorins radicals for porphyrins in solution [19]. For this reason the electron transfer mechanism should be more complicated that the case of the poly-Co(II)TAPC. These facts should explain the differences in the selectivity towards the reaction products between poly-Co(II)TAPC and poly-Co(II)TAPP.

ACKNOWLEDGEMENTS

This work was supported by Fundación Andes "Programa de Inicio de Carrera Para Jóvenes Investigadores" C-14060/40 and U. de Chile Proyecto DI I2-04/09-2. G.R and A.R are grateful to CONICYT doctoral scholarship.

The authors acknowledge Dr. María Jesús Aguirre for helpful discussions and laboratory facilities.

 

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e-mail: misaacs@uchile.cl

 

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