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Evidence for the Formation of Cobalt Porphyrin-Quinone Complexes Stabilized at Carbon-Based Surfaces Toward the Design of Efficient Non-Noble-Metal Oxygen Reduction Catalysts L. Elbaz, E. Korin, L. Soifer, and A. Bettelheim* Chemical Engineering Department, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 84105, Israel
ABSTRACT The interaction of electropolymerized Co(III)TAPP (polyCoTAPP) films with adsorbed resorcinol on glassy carbon (GC) and with surface quinone functionalities on aerogel carbon (AEC) were studied using reflection UV-visible spectroscopy and X-ray photoelectron spectroscopy. A red shift of the Soret band and the appearance of new Q bands appearing after adsorption of resorcinol on a GC/polyCoTAPP film was interpreted as being due to change of the metalloporphyrin electronic structure. The photoelectron depth profiles for an AEC/polyCoTAPP film showed that the cobalt ion is mostly in the Co(III) state at the outer layers of the film, while the amount of cobalt ion in the formal þ2 state gradually increases in the inner film layers. This seems to indicate the formation of chargetransfer complexes between the metalloporphyrin and reduced quinone functionalities on the AEC surface. Understanding the nature of metalloporphyrin/porous carbon structures is an important step toward the design of reliable and low-cost non-noble-metal oxygen reduction catalytic electrodes and their application in fuel cells and batteries. SECTION Surfaces, Interfaces, Catalysis
surface area (up to 1100 m2/g).11 We have recently shown that Co(III) tetra(oaminophenyl)porphyrin (Co(III)TAPP; structure in Scheme 1) and Co(III) tetra(psulfonatophenyl)porphyrin (Co(III)TPPS) incorporated through adsorption (both metalloporphyrins) or electropolymerization (Co(III)TAPP) in AEC electrodes show catalytic behavior toward oxygen reduction at more positive potentials as compared to these metalloporphyrins at other carbon-based electrodes.12 Moreover, controlled quantities of reduced o-, m-, and p-benzoquinones (o-, m-, p-BQ): catechol (Cat), resorcinol (Res), and hydroquinone (HQ) were adsorbed in AEC electrodes, followed by Co(III)TPPS adsorption.13 These systems were found to be efficient catalysts for the reduction of oxygen to water. On the basis of indirect electrochemical evidence, it has been assumed that the active sites are complexes formed between the metalloporphyrin and quinone system and stabilized within the AEC porous structure.12,13 In the present paper, direct evidence is presented for the formation of surface charge-transfer complexes between electropolymerized Co(III)TAPP and quinone/hydroquinone systems existing on the surface of AEC (mostly resorcinol/m-quinones in nature13) or introduced through adsorption on glassy carbon (GC) electrodes.
P
orphyrins and quinones are classes of compounds which play important roles in biological redox systems. Porphyrins and particularly porphyrin-quinone compounds are important model systems for photoinduced electron-transfer studies.1 However, these studies in this area have been performed with quinones covalently2 or noncovalently3 bound to the macrocycle periphery. Very few and nonrecent studies concerning metal-centered interactions of some metalloporphyrins with quinones have been reported,4,5 including the use of cobalt porphyrin/quinone/Pd systems for the aerobic catalytic oxidation of organic compounds.6 It has been established that high-spin Fe(II) porphyrins are oxidized by quinones, while low-spin Fe(III) porphyrins are reduced by hydroquinone.4 Weak 1:1 molecular complexes between Co(II) mesoporphyrin IX dimethyl ester and 2-methylnaphthaquinone (vitamin K3) have been characterized by UV-visible and NMR spectroscopy.7-9 With the exemption of a study of hemoproteins,4 these studies,5,7-9 including the reduction of Mn(III) porphyrins by semiquinone radical anion derivatives,10 were conducted in organic media, where the reversible one 2e- reduction process of the quinone/hydroquinone couple does not occur due to a lack of a proton source. Aerogel carbons (AECs) are porous carbon materials commonly prepared using resorcinol and formaldehyde as chemical precursors. After carbonization at 700-1000 °C, they consist of interconnected nanosized particles with interstitial pores with diameters of less than 50 nm and have a high
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Received Date: November 24, 2009 Accepted Date: December 7, 2009 Published on Web Date: December 11, 2009
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DOI: 10.1021/jz900310c |J. Phys. Chem. Lett. 2010, 1, 398–401
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on GC/polyCoTAPP causes an 8 nm red shift for the Soret band, and Q bands appear as shoulders at ∼485 and ∼550 nm (indicated by arrows in curve b, Figure 1). These spectral changes are attributed to the interaction of the cobalt porphyrin and resorcinol. Such a red shift of the Soret band and reflectance spectral changes for a SiO2-attached cobalt porphyrin after exposure to imidazole were attributed to changes of the electronic structure of the metalloporphyrin due to a complex formed with imidazole.15 Further characterization of the GC/polyCoTAPP and GC/ polyCoTAPP/Res films was achieved using X-ray photoelectron (XP) spectroscopy. Figure 2A displays the Co2p3/2 signal of polyCoTAPP on the GC surface. The main peak at 782.0 eV is located at a position typical of cobalt in the formal oxidation of þ3.16 However, after adsorption of resorcinol, the Co2p3/2 XP spectrum shows a broad band, and its deconvolution indicates the presence of two peaks with almost identical intensity at 782.0 and 780.0 eV (Figure 2B). The increase of the contribution of the peak with the smaller binding energy, typical to cobalt in the formal oxidation state of þ2,17 is attributed to the formation of a CoTAPP-Res complex in which a partial charge has been transferred from the Res to Co(III)TAPP. Figure 3A shows the Co2p3/2 signal obtained for polyCoTAPP on AEC. The spectrum consists of a main peak at 782.0 eV with a small contribution at 779.5. This is consistent with the cobalt being mostly in the Co(III) state. The O1s spectrum showed two peaks at 531.8 and 533.1 eV as compared to 531.7 and 532.4 eV for bare AEC. The first peak is attributed to surface organic carbonyl groups,18 while the second is due to reduced quinones, as evidenced by its increased intensity after adsorption of resorcinol in AEC. The fact that only the second peak is significantly shifted (by 0.7 eV) seems to imply the existence of a complex formed between Co(III)TAPP and ligated surface-reduced quinones. Although, to the best of our knowledge, ligation of Co(III)/Co(II) porphyrins by quinones has not been reported, it has been shown that Co(II) and Co(III) tetrakis(N-methyl-4-pyridyl) porphyrins ligate methanol and ethanol through oxygen atoms.19 This and our own unsuccessful attempts to determine such a ligation for dissolved Co(III)TAPP and Co(III)TPPS by various quinone/
Curve a in Figure 1 shows a reflection UV-visible spectrum obtained for an electropolymerized film of Co(III)TAPP (polyCoTAPP) on a GC electrode. Soret and Q bands with λmax = 431 and 544 nm, respectively, are observed. The reflection spectrum obtained for the polyCoTAPP film is similar to that of “monomeric” Co(III)TAPP dissolved in Me2SO (λmax = 430 and 546 nm).14 Adsorption of resorcinol
Figure 1. Reflection UV-visible spectra obtained for a film polyCo(III)TAPP on GC (a) and for the same film after adsorption of resorcinol (b). Scheme 1. CoTAPP Structure and Its Interaction with Resorcinol on the Surface and with Oxygen
Figure 2. Co2p3/2 XP spectra (black: original; red: deconvoluted) for a film of polyCo(III)TAPP on GC (A) and for the same film after adsorption of resorcinol (B).
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DOI: 10.1021/jz900310c |J. Phys. Chem. Lett. 2010, 1, 398–401
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Figure 3. Co2p3/2 XP spectra (black: original; red: deconvoluted) for a film of polyCo(III)TAPP in AEC before (A) and after sputtering periods of 1, 2, and 4 min. (B, C, and D, respectively).
prepared cofacial or multinuclear cobalt porphyrins.20 In accordance with the present and previous studies, surface quinones serve as electron-transfer agents for the electroreduction of Co(III) porphyrin, which in turn reduces oxygen to water (Scheme 1). The possible role of the AEC porous structure in reducing the yields of H2O2 by its dismutation, due to the high local density of the catalytic sites, cannot be ruled out. Aiming to control the nature and activity of these sites and in view of the importance of developing low-cost non-noble-metal catalysts, we are engaged in a more thorough study of various metalloporphyrin/quinone interactions in porous carbon-based substrates. Organic aerogels are being prepared using different reduced quinone derivatives as starting materials, and metalloporphyrins are introduced in various stages of the areogel carbon electrode preparation, toward the design of effective catalytic electrodes and their possible use in electrochemical energy conversion devices (fuel cells and batteries).
hydroquinone systems led to the conclusion that these complexes are stabilized only when attached to the surface. Depth Co2p3/2 XPS profiles for a film of polyCoTAPP on AEC were obtained by argon sputtering (rate of about 20 Å/min). Figure 3B, C, and D was obtained after sputtering periods of 1, 2, and 4 min (depth of ∼20, 40, and 80 Å, respectively). It can be clearly seen that the intensity of the 779.5 eV peak consistently increases, while that of the 782 eV peak decreases as a function of depth (intensity ratio of the first peak relative to that of the second one of 0.3, 1.3, 1.7, and 3.0 for depths of 0, ∼20, 40, and 80 Å, respectively). This seems to indicate that the amount of cobalt ion in the formal þ2 state increases in inner layers of the film due to the closer vicinity to the AEC surface and that charge-transfer complexes are formed between the metalloporphyrin and reduced quinones. This is illustrated in Scheme 1 for the interaction of Co(III)TAPP and surface resorcinol-like functionalities. It has been shown that the catalytic activity of Co(III)/Co(II) porphyrin in AEC electrodes is affected not only by the porous structure of the substrate but also most importantly by its chemical nature.12,13 These electrodes were shown to mediate O2 reduction almost completely to water at high potentials in aqueous acidic solutions (∼þ0.5 V versus Ag/AgCl), similar to the catalytic activity reported for the difficultly
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EXPERIMENTAL METHODS Surface coverage of polyCoTAPP was determined by following the absorbance of the dissolved metalloporphyrin in 1 M H2SO4 at 536 nm (ε = 1.2 104 M-1 cm-1) with an Ocean Optics USB4000 spectrophotometer. Reflection UV-visible
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spectra were obtained using the above spectrophotometer coupled with an Ocean Optics QR400-7 reflection probe, which was positioned within 2 mm of the substrate surface. Escalab 250 with an Al X-ray source and monochromator were used for the XP spectroscopy measurements. All of the electrochemical experiments were conducted at 25 °C using a conventional three-electrode cell and were performed with a Pine Instruments bipotentiostat. Electropolymerization of Co(III)TAPP was achieved from a 1 mM Co(III)TAPP (Cl-) solution in 1 M H2SO4 by cyclic voltammetry in the 0 to þ1.2 V (versus Ag/AgCl/KClsatd.) range.14 Twenty cycles were applied for GC and AEC working electrodes at scan rates of 50 and 1 mV/s, yielding metalloporphyrin approximate surface coverages of 2.0 10-9 and 1.5 10-7 mol/cm2, respectively. Adsorption of resorcinol was achieved by immersing overnight GC/polyCoTAPP in 1 M resorcinol aqueous solution.
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AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: armandb@ bgu.ac.il.
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ACKNOWLEDGMENT We thank the Israel Ministry of Infrastructures for partial financial support of this research. One of the authors (L.E) would like to thank the Kreitman foundation of Ben-Gurion University of the Negev, Beer-Sheva, Israel, for a Ph.D. fellowship.
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