J . Phys. Chem. 1987, 91, 3199-3801
3799
Electrocatalytic Aspects of Iron Phthalocyanine and I t s pox0 Derivatives Dispersed on High Surface Area Carbon A. A. Tanaka; C. Fierro, D. Scherson,* and E. B. Yeager* Case Center for Electrochemical Sciences, The Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106 (Received: November 25, 1986; In Final Form: March 16, 1987)
The cyclic voltammetry of iron phthalocyanine (FePc) and its two @-oxoderivatives obtained by mixing the materials in dry form with high-area Vulcan XC-72 carbon yielded a common set of voltammetric peaks. This provides evidence for the presence of a single type of surface species for the macrocycle in its various forms. The electrochemical activity of these dispersed specimens for 0,reduction in alkaline media using thin porous coating-rotating disk techniques was found to be essentially the same for both p-oxo derivatives. Comparable activities were observed in the case of bulk monomeric FePc only after polarizing the electrodes at fairly negative potentials. Some differences in activity were observed, however, for the materials in gas-fed electrodes of the type used in fuel cells in 4 M NaOH at 60 OC. Based on the results obtained in this work and quantum mechanical considerations, it has been concluded that (i) the increase in conductivity induced by the exposure of FePc to dioxygen is most likely due to the formation of p-oxo-type derivatives, and (ii) it is not necessary to invoke a metal spin crossover mechanism as the key factor in explaining the high electrocatalytic activity of FePc for the reduction of 02,as has been proposed earlier in the literature.
Introduction Among the growing number of transition-metal macrocycles investigated in connection with electrocatalytic processes,'-21 iron phthalocyanine (FePc) appears to have received special attention. Such interest may have been motivated in part by the ease by which FePc can be prepared and purified and also by its high electrocatalytic activity for O2 reduction in aqueous media. Considerable effort has been devoted by many research groups toward gaining a deeper understanding of the detailed mechanism involved in this technologically important electrocatalytic reaction. Attention has been focused primarily on the nature of the FePc-02 interactions including the bonding geometry and energetics of the reaction pathway intermediates. Although important information has been derived from in situ spectroelectrochemical studies involving FePc and its water-soluble tetrasulfonated derivative adsorbed on a variety of electrode materials,22-29certain key aspects of the macrocyclic-mediated 0, reduction have not yet been clarified. This communication will present spectroscopic and electrochemical data involving two recently reported @-oxoforms of FePc from which new insight has been gained regarding the physical state of the macrocycles dispersed in high-area carbon supports and their electrocatalytic activity for O2 reduction. A critical review of the pertinent literature in the area has also been included.
Background The ability of FePc to promote the rates of O2 reduction in aqueous media appears to have been first reported by Jahnke and Schonborn' in 1968. Shortly thereafter, Kozawa et al.* and Brodd et aL3conducted a series of experiments aimed at providing a more detailed characterization of the factors involved in this macrocyclic-mediated redox process. The results obtained with FePc adsorbed on a rotating pyrolytic graphite disk electrode were found to be comparable to those with platinum in 1 M KOH and to a lesser extent in neutral media. The activity in acid solution was not as high as in alkaline electrolytes and decreased upon repeated cycling to negative potentials. This effect was not due to the loss of the catalyst layer as judged by the fact that electrodes that had undergone deactivation in acid media showed excellent performance when transferred to an alkaline solution. Additional experiments indicated that FePc is capable of reducing hydrogen peroxide in alkaline and neutral but not in acid media. In fact, no hydrogen peroxide was detected over a wide potential region by using rotating ring-disk techniques in neutral and alkaline 'On leave from Instituto de Fisica e Quimica de Sao Carlos-USP, Sao Carlos, (SP), Brasil.
0022-3654/87/209 1-3799$01.50/0
electrolytes. These studies were later extended to include the effects of anions and cations and the nature of the substrate on
(1) Jahnke, H.; Schonborn, M. Electrocatalysis on Organic Semiconductors; presented at the 19th CITCE Meeting, Detroit, 1968. (2) Kozawa, A.; Zilionis, V. E.; Brodd, R. J. J . Electrochem. SOC.1970, 117, 1470; 1474; 1971, 118, 1705. (3) Brodd, R. J.; Leger, V. Z.; Scarr, R. F.; Kozawa, A. NBS Spec. Publ. 1975, 455, 253. (4) Savy, M.; Andro, P.; Bernard, C.; Magner, G. Electrochim. Acta 1973, 18, 191. ( 5 ) Randin, J. P. Electrochim. Acta 1974, 19, 83. (6) Meier, H.; Albrecht, W.; Tschirwitz, U.; Zimmerhackl, E. Ber. Bunsen-Ges. Phys. Chem. 1973, 77, 843. (7) Savy, M.; Bernard, C.; Magner, G. Elecfrochim. Acta 1975, 20, 383. (8) Appleby, A. J.; Savy, M. NBS Spec. Publ. 1975, 455, 241. (9) Appleby, A. J.; Fleisch, J.; Savy, M. J . Catal. 1976, 44, 281. (10) Beck, F. J . Appl. Electrochem. 1977, 7, 239. (11) Tachikawa, H.; Faulkner, L. R. J . A m . Chem. SOC.1978,100,4379. (12) van Veen, J. A. R.; Visser, C. Electrochim. Acta 1979, 24, 921. (13) Appleby, A. J.; Savy, M.; Caro, P. J . Electroanal. Chem. 1980, 1 1 1 , 91, and references therein. (14) Maroie, S.; Savy, M.; Verbist, J. J. Inorg. Chem. 1979, 18, 2560. (15) Behret, H.; Binder, H.; Sandstede, G.; Scherer, G.G . J . Electroanal. Chem. 1981, 117, 29. (16) Melendres, C. A.; Feng, X . J . Elecrrochem. SOC.1983, 130, 81 1 . (17) van den Brink, F.; Visscher, W. Barendrecht, E. J . Electroanal. Chem. 1984, 172, 301; 1984, 175, 279. (18) Zagal, J.; Bindra, P.; Yeager, E. J . Elecrrochem. Sot. 1980, 127, 1506. (19) (a) Collman, J. P.; Denisevich, P.; Konai, Y.; Marrocco, M.; Koval, C.; Anson, F. C. J . A m . Chem. SOC.1980,102,6027. (b) Durand Jr., R. R.; Bencosme, C. S.; Collman, J. P. Anson, F. C. J . Am. Chem. Sot. 1983, 105, 2710. (20) Liu, H. Y.; Weaver, M. J.; Wang, C. B.; Chang, C. K. J . Electroanal. Chem. 1983, 145,439. (21) Liu, H. Y.; Abdalmuhdi, I.; Chang, C. K.; Anson, F. C. J . Phys. Chem. 1985, 89, 665. (22) Nikolic, B. Z.; Adzic, R. R.; Yeager, E. B. J . Electroanal. Chem. 1979, 103, 281. (23) Adzic, R. R.; Simic-Glavaski, B.; Yeager, E. B. J . Electroanal. Chem. 1985., 194.. 1 5 5 . (24) Scherson, D.; Yao, S. B.; Yeager, E. B.; Eldridge, J.; Kordesch, M. E.; Hoffman, R. W. J . Phys. Chem. 1983,87, 932. (25) Simic-Glavaski, B:; Zecevic, S.; Yeager, E. J . Electroanal. Chem. 1983, 150, 469. (26) Melendres, C. A,; Rios, C. B.; Feng, X.; McMasters, R. J . Phys. Chem. 1983, 87, 3526 and references therein. (27) Scherson, D. A.; Fierro, C.: Yeager, E. B.; Kordesch, M. E.; Eldridge, J.; Hoffman, R. W.; Barnes, A. J . Electroanal. Chem. 1984, 169, 287. (28) Scherson, D. A.; Fierro, C. A.; Tryk, D.: Gupta, S. L.; Yeager, E. B.; Eldridge, J.; Hoffman, J . J . Electroanal. Chem. 1985, 184, 419.
0 1987 American Chemical Society
3800 The Journal of Physical Chemistry, Vol. 91, No. 14, 1987 the overall electrocatalytic activity. The addition of cyanide and EDTA to sodium acetate solutions, for example, led to a substantial decrease in the currents for 0, reduction most certainly as a result of a strong complexation with the metal center, whereas chloride, sulfate, and several cationic species had essentially no effect. No electrocatalytic effects were observed in solutions of about pH 7 when substrates other than carbon were employed. In fact, the activity intrinsic to the support was, in many instances, found to decrease upon adsorption of the macrocycle on the surface. In 1973, Savy et aL4 investigated the electrochemical properties of several monomeric phthalocyanines vapor deposited on gold supports and their corresponding polymeric forms dispersed on high-area carbon. Among all the compounds examined and irrespective of their degree of polymerization, FePc was found to exhibit the highest activity for 0, reduction in solutions of pH 1.3 and 6.7 followed by Co-, Ni-, and CuPc. This ordering coincides with that of the first oxidation potentials, the magnetic moments, and the enhancements in rates of certain gas-phase reactions associated with the monomeric macro cycle^.^ Furthermore, a similar correlation was also found between the activities of the polymeric forms of most of these materials and their intrinsic electrical conductivities.6 The physical and electrochemical properties of two different kinds of FePc films vapor deposited on gold supports were examined by Savy and co-workers.’ These were prepared by electron bombardment (type A ) and by resistive heating (type B). Electrodes of the first type yielded consistently higher catalytic activities for O2 reduction than those of type B. The X-ray photoelectron spectra indicated a much higher concentration of unpaired electrons in films of type A than B. This was in agreement with the shifts observed in the UV-vis spectra, providing evidence for the presence of an electron acceptor-type ligand, most likely molecular oxygen, bound to the iron center in the bulk of the material. Based on quantum mechanical considerations to be discussed later in more detail it was proposed that the Fe(III)-O; species thus formed would have an optimum electronic configuration for the subsequent activation and reduction of dioxygen. Some indication that the bulk of the material was involved in the catalysis was provided by the fact that the overall activity was found to be higher for thicker films. The electrocatalytic properties of polymeric (p-) and monomeric FePc supported on carbon substrates were investigated by Appleby, Fleisch, and S a ~ y . The ~ , ~polymeric compounds were prepared by two different synthetic routes yielding predominantly what were described as a dimeric and a hexameric forms of the material. The activity of O2 reduction in alkaline solutions was reported to decrease in the sequence: dimer > polymer >> monomer. Mossbauer and optical data provided evidence that the bulk of polymeric and monomeric forms of FePc contained principally Fe(II1) in intermediate-spin (is) or high-spin (hS) states and Fe(I1)-is, respectively. These observations led the authors to suggest that Fe(II1) centers in either is or hS state were directly involved in the catalysis and that the activity of such sites was higher than that associated with Fe(I1)-is. The reflection spectra obtained for specimens in contact with 1 M KOH and oxygen, however, indicated that only Fe(I1)Pc-iS molecules were present at the solid-solution interface for both the monomeric and polymeric species. Based upon these observations, it was postulated that in the case of p-FePc the first step in the reduction process involved the chemisorption of O2 activated by Fe(II1)-iS or FePc(II1)-hS in the bulk material. This would generate oxygen radicals which would then undergo migration through the lattice and react with water at the solid-electrolyte interface. In 1977, BeckToproposed that the first step in the FePc-mediated electroreduction of dioxygen was the formation of an Fe(1I)Pc-0, adduct with a partial metal to O2 charge transfer. This species would become protonated and subsequently undergo a two-electron reduction to yield peroxide. In the last stage the (29) B. Simic-Glavaski, B.; S. Zecevic, S.; E. Yeager, E. J . A m . Chem SOC.1985, 107, 5625.
Tanaka et al.
H z 0 2would be catalytically decomposed by the macrocycle itself generating water and oxygen. If it is assumed that the two-electron reduction is the rate-determining step, the suggested mechanistic scheme appears to explain a number of experimental observations. In particular it accounts among others for the 60 mV decade-’ Tafel slopes, the pH dependence of the equilibrium potentials, and the fact that the steady-state reduction occurs at potentials which are much more positive than those corresponding to the redox processes of the macrocycle in the absence of O2in solution. A substantial increase in the conductivity of FePc films deposited on gold substrates upon exposure to oxygen was reported by Tachikawa and Faulkner.” This effect was associated with an electron acceptor species, into the lattice the incorporation of 02, conferring the material p-type semiconductor characteristics. Although a similar phenomenon was observed by placing the films in contact with water vapor, much higher conductivities than those obtained with each individual gas could be achieved by exposing the specimens to an 02-H20 mixture. The electrocatalytic activity of this type of FePc films for O2reduction in unbuffered 0.1 M KNO, was found to be very low with the onset occurring at very negative potentials. This was attributed to the inability of this p-type material to effect reductions at voltages more negative than the flat band potential (-0.0 V vs. SCE). At very negative potentials, however, the bands would be expected to bend downward leading to a depletion of holes in the interfacial region. The fact that the films showed no photoeffect was assumed to be due to the highly doped p-character of the material. In 1979, van Veen and VisserT2reported a rather comprehensive study of various transition-metal phthalocyanines in monomeric form dispersed on high-area carbons. The activity of FePc for O2reduction was found to be highest both in alkaline (6 N KOH) and acid (8 N H2S04)solutions in agreement with the claims of Savy and c o - ~ o r k e r s .Several ~ arguments were also presented casting doubts on the validity of some of the correlations suggested by Randid regarding the electrocatalytic activity and certain physical properties of transition-metal phthalocyanines referred to earlier. In a more recent communication, Appleby, Caro, and SavyI3 pointed out that optimum activity for the macrocyclic-mediated O2 reduction could be achieved provided two conditions were fulfilled: (i) the occurrence of active centers capable of adsorbing and desorbing oxygen and reaction intermediates and products in a reversible fashion, and (ii) a charge transfer with low activation energy. X-ray photoelectron spectroscopy datal4 obtained for various forms of FePc indicated that the Fe-0, bonding occurred only when the transition-metal center was in the is or hS states and not in the low-spin (1s) state. Detailed quantum mechanical calculations involving the Fe-d5 configuration indicated that a spin crossover could be effected either by a modification of the crystal field parameters or by a distortion of the molecular geometry, a process that would be enhanced in the spin borderline regions. Hence, if the ferric species observed primarily in the polymeric samples could undergo a facile is-IS or hS-1S transition, the first of the requirements specified above would be fulfilled. In order to satisfy the second condition for effective O2electrocatalysis, Appleby et a1.I3 proposed a model for conduction involving electron hopping through a chain of alternate O 2and FePc molecules, a process that would also be enhanced at the multispin border. According to these authors, such 02. species would serve as doping centers conferring the material with high enough conductivity to sustain high currents, and also migrate to the surface where they could further react. In I98 1, Behret et aLT5investigated the electrocatalytic reduction of oxygen in 1 M KOH using polymeric (mixture of oligomers ranging from dimer to pentamer) and monomeric FePc. The onset of O2 reduction as measured with a rotating FePc (ordinary pyrolytic graphite, OPG) disk-Pt ring electrode was 0.9 V vs. DHE and the amount of H,Oz detected at the ring was very small. In contrast to the results reported by Appleby, Fleisch, and Savy8s9the catalytic activity was found to decrease in the sequence “high” polymer > monomer > “low” polymer. This was
Electrocatalytic Aspects of Iron Phthalocyanine
The Journal of Physical Chemistry, Vol. 91, No. 14, 1987 3801
cluding UV-vis reflectance and surface-enhanced raman specattributed to differences in the synthetic procedures as well as troscopies (SERS). Striking similarities have been observed, for electrode preparation. instance, between the UV-vis spectra obtained for FeTsPc adMelendres and Fengi6 examined the electrochemical behavior sorbed on either Pt or the basal plane of highly oriented pyrolytic of FePc adsorbed on gold electrodes in 0.05 M H2S04. Three graphite and that in solution phase.22 This has indicated that the anodic and three cathodic peaks were found in the cyclic voltinteractions between the orbitals associated with the absorption ammogram in the range between -0.5 and -0.95 V vs. Hg/Hg2S04 bands are not involved to a large extent in the bonding of the only after polarizing the electrode in the hydrogen evolution region molecule to the surface and thus that the physicochemical (--0.9 V vs. Hg/Hg2S0,). This behavior led the authors to characteristics of such adsorbed layers, such as aggregation and conclude that the voltammetric features were associated with the may not differ much from those of the species reactivity toward 02, adsorption and desorption of hydrogen on different surface sites in solution. Support for this view has been provided by rather of the FePc. The activity of the electrodes for O2reduction was recent SERS experiments involving single-crystal silver surfaces.23 found to decrease with successive scans, an effect that was atIn particular, the polarization behavior characteristics were found tributed primarily to the adsorption of the hydrogen peroxide to he consistent with a geometrical configuration in which the intermediate and the degradation of the catalyst through the loss adsorbate would be bound edge-on to the surface. In addition, of the metal ion. interesting correlations have been found in the case of polycrysMore recently, van den Brink et aI.l7 studied the electrochemical talline silver surfaces between the relative intensities and frequency properties of FePc films (50-700 nm) deposited on gold and shits for some of the spectral features as a function of the applied pyrolytic graphite electrodes. In contrast to the earlier claims potential.29 As has been stressed in an earlier c o m m ~ n i c a t i o n , ~ ~ of Savy and c o - ~ o r k e r sno , ~ correlation was found between the a better understanding of these phenomena will most certainly thickness and resistance of the films. Rotating FePc (Au or OPG) require a more detailed theoretical treatment of adsorbate surface disk-Pt ring data in 1 M KOH showed that oxygen is reduced interactions including the effects of the electric field at the invia a four-electron process in the potential range 0.1-0.9 V vs. terface. RHE, but with a mechanistic transition between 0.3 and 0.5 V vs. RHE. Voltammetric studies, however, showed no redox Experimental Section processes in the potential range investigated for oxygen reduction. Synthesis. The FePc (Eastman Kodak, New York) was purified At very negative potentials ( E < 0.1 V vs. RHE) an electrochromic by repeated sublimation at a reduced pressure. The two p o x 0 effect was observed with the FePc film changing from bluish-green forms of FePc were synthesized according to the method described to red-brown. The film was found to degrade after multiple cycles. by Ercolani et aL30 Such effect was explained in terms of a ring reduction process The first compound, denoted as FePc-poxo( 1) is obtained by which would lead to a demetallation and further cleavage of the bubbling O2 into a suspension of FePc in dimethylformamide macrocycle. Impedance measurements conducted by the same (DMF) for 24-48 h to yield very small blue crystals of the desired research group provided evidence for the presence of surface states material. These are then washed with methanol and dried under at 0.82, 0.67, and -0.40 V vs. RHE, leading these authors to vacuum at 150-160 O C for 2-3 h to eliminate traces of D M F propose that FePc in these films behaves as p-type material with occluded in the lattice. The second species, FePc-poxo(Z), is slow charge transport at negative polarization ( E = 0.4-0.8V vs. prepared by bubbling O2 into an FePc solution in concentrated RHE). Based primarily on ellipsometric evidence the observed sulfuric acid. After 20 min, the solution is poured into cold, changes in the mechanism for O2reduction at potentials of about air-saturated water. The purple crystals obtained are then washed 0.5 V vs. R H E were attributed to modifications in the hydrorepeatedly with water to remove traces of acid. phobic/hydrophylic properties of the film. Specifically, for E > Iron tetrasulfonated phthalocyanine (FeTsPc) was prepared 0.5 V vs. RHE, the film would be highly hydrophobic whereas and purified according to the procedure reported by Weber and at potentials more negative than 0.4 V, Le., in the region where B~sch.~~ one of the surface states is observed, it would turn hydrophilic. Spectroscopic and Electrochemical Measurements. The FTIR In this state the film would undergo swelling leading to an increase of all compounds were recorded with a Digilab FTS-14 spectroin the number of active sites in contact with the electrolyte solution photometer in KBr pellets and the Mossbauer spectra collected and thus to the observed change in the mechanism. in a Ranger Scientific (Burleson, TX) spectrometer described in The electrochemical properties of the water-soluble tetradetail elsewhere.32 Scanning electron micrographs of the materials sulfonated derivative of iron phthalocyanine, FeTsPc, have been were obtained with a JEOL microscope Model 840 at a voltage studied extensively by Yeager and co-workers.I8 A mechanistic of 25 kV. The samples were prepared by depositing the powders investigation involving the use of a rotating OPG disk-Au ring on double-sided adhesive ta e onto an appropriate holder. A electrode indicated that, in the case of the material adsorbed at Pd-Au coating of about 100 thickness was then deposited onto presumably monolayer coverages on OPG, no peroxide could be the particles by conventional sputtering techniques. detected over a substantial range of potentials at lower polariThe electrochemical properties of the standard monomeric FePc zations. This provided evidence that the reaction proceeds via and its h-oxo derivatives were examined by mixing the materials a four-electron process. At much larger polarization, a small in dry form with high-area Vulcan XC-72 carbon of 250 m2 g-I amount of H 2 0 2was observed due most probably to the reduction (Cabot Corp., Billerica, MA) to a macrocycle/carbon weight by of O2 on the bare carbon surface exposed to the solution. Two weight ratio of 7%. The FeTsPc was preadsorbed on the same types of mechanisms were proposed depending on whether the high-area carbon by dissolving the compound in water to a conreaction was carried out in acid or in base to explain the different centration of lo-' M followed by the gradual addition of the carbon values of the Tafel slopes obtained. In both cases, the main under ultrasonic agitation in an amount sufficient to obtain a 5% controlling factor was the potential associated with the Fe(II)/ macrocycle/carbon w/w ratio. This corresponds to a coverage Fe(II1) redox couple. between and of a monolayer. The mixture was filtered It is interesting to note that a four-electron reduction of O2in through a 1-km Nucleopore membrane and the solid material acid media has also been observed for certain face-to-face Co washed several times with water and later dried in a oven at 120 porphyrins adsorbed on carbon s ~ r f a c e s , ' ~in- which ~ ~ the Co-Co O C for 3 days. distance can be adjusted by changing the length of the linkage Thin porous coating rotating ring-disk electrodes were used groups connecting the two rings. Most recently, however, it has for cyclic voltammetry and polarization measurements. These been found21 that for some of these cofacial porphyrins a single cobalt center is sufficient to promote the four-electron reduction (30) Ercolani, C.;Gardini, M.; Monacelli, F.; Pennesi, G.; rossi, G.Inorg. of dioxygen. Chem. 1983, 22, 2584 and references therein. As was mentioned briefly in the Introduction, several in situ (31) Weber, J. H.; Busch, D. H. Inorg. Chem. 1965, 4 , 469. spectroscopic methods have been employed to examine transi(32) Eldridge, J.; Kordesch, M . E.; Hoffman, R. W. J . Vac. Sci. Technol. tion-metal macrocycles adsorbed on electrodes s ~ r f a c e s , in~ ~ - ~ ~ 1982, 20, 934.
1
3802 The Journal of Physical Chemistry, Vol. 91, No. 14, 1987
Tanaka et al.
-A g plated NI screen rHydrophobic
\
. ,
backlng
-F bodj
Au r,ng
/
r u b b e r gasket
-k: I:zer
A c t ~ v elayer
Figure 1. Rotating ring-disk electrode system for use with Teflon-bonded actived layer (thin porous coating electrode).
-
gasket
Threaded Kel-F plug
Figure 2. Schematic diagram of the porous gas-fed electrode assembly.
were prepared by placing 10 mg of the catalyst/carbon mixture in a small beaker with distilled water under ultrasonic agitation for about 5 min. Subsequently, a dilute suspension of a Teflon emulsion (du Pont T30B, 60% Teflon in water plus 7% Triton X-100 as a stabilizer) was added to the dispersion to yield 10% w/w of Teflon in the final dry solid material. The mixture was filtered through a I-pm Nucleopore membrane, kneaded with a spatula onto a glass slide and about 0.5 f 0.1 mg, based on dry weight, was applied into the shallow disk cavity (-0.1 mm deep) of a rotating Au ring/pyrolytic graphite disk electrode (see Figure 1). The surface of this porous carbon coating was smoothened by using a spatula so as to create a common plane with the front of the electrode assembly. Any material which may have been accidentally spread onto the gap, ring, or remainder of the mounting was carefully removed with a soft lens paper. Electrochemical measurements involving Powercat 2000 (Stonehart Associates Inc., Madison, CT) were also performed with the material in the form of a thin porous coating to allow a direct comparison with the FePc compounds. Gas-fed oxygen electrodes were prepared by adding a dilute aqueous suspension of Teflon T-30B (-0.2 wt %) to about 50 mg of the carbon/catalyst mixture suspended in water under ultrasonic agitation. This mixture was then filtered through a 1-pm Nucleopore membrane. The resulting paste was shaped into a disk of 1.75 cm 0.d. in a circular stainless steel die first, using hand pressure and subsequently employing a commercial unit at about 130 kg cm-2. The disk was then attached to a 0.5-mm-thick disk of a Teflon-acetylene black hydrophobic material (Electromedia Corp., Englewood, NJ) with an embedded silver-plated Ni screen. This dual assembly was pressed at approximately 300 kg cm-2 at room temperature and heat treated at 280 "C for 2 h in flowing helium to remove remaining water and decomposition products of the emulsion stabilizer (Triton X-100). The diskshaped electrode was then installed into a special Kel-F holder. A schematic diagram of the final electrode assembly is given in Figure 2. Prototech 5% w/w Pt on deashed RB carbon of 1200 m2 g-' (Electromedia Corp., Englewood, NJ) and heat treated (800 "C, 2 h under Ar atmosphere) iron (p-oxo) tetrakis(meth0xyphenyl)porphyrin 10% w/w (FeTMPP),O/RB, gas-fed electrodes were prepared following the same procedure as that used in the case of the FePc specimens. The cyclic voltammetry and rotating ring-disk polarization measurements were performed with a Pine RDE-3 bipotentiostat (Pine Instrument Co., Grove City, PA). A gold foil ( A 2 cm2) and Hg/Hg,OH- ( 1 M) were used as counter and reference electrodes, respectively. All experiments were carried out in 1 M NaOH prepared from special low-carbonate pellets (J.T. Baker Chemical Co., Phillipsburg, NJ). The O2 reduction polarization
-
-
1600
1400
I200
Wavenumber f em+)
1000
E00
600
Figure 3. FTIR spectra of FePc (a), FePc-p-oxo(1) (b), and FePc-N(c), in crystalline form. The arrows indicate features characteristic of the p-oxo derivatives.
oxo(2)
curves involving thin porous coatings were recorded point-by-point to avoid the high capacitive currents associated with this type of electrode. The Au ring was polarized at +0.13 V, a potential positive enough for the oxidation of peroxide to occur under complete diffusion control. Prior to recording each polarization curve the ring was activated by scanning the potential between +0.5 and -0.8 V three times at 100 mV s-I. The O2 reduction measurements involving gas-fed electrodes were conducted in the galvanostatic mode using a Stonehart BC-1200 potentiostat. Results and Discussion Spectroscopic Characterization. The FTIR spectra of FePc and its two p-oxo forms are shown in Figure 3 and 4, respectively. The most striking difference between FePc and FePc-poxo(l), as pointed out by Ercolani et al.,30is the presence of two strong bands at 852 and 824 cm-'. These have been a t t r i b ~ t e d ) ~ to -~' the antisymmetric Fe-0-Fe stretching mode in analogy with similar features associated with well-characterized linear or quasi-linear M-0-M systems, where M represents a metal center. Additional evidence in support of this assignment was provided30 (33) Liston, D. J.; Kennedy, B. J.; Murray, K. S.; West, B. 0. Inorg. Chem. 1985, 24, 1561.
(34) Liston, D. J.; West, B. 0. Inorg. Chem. 1985, 24, 1568. (35) Shimomura, E. T.; Phillippi, M. A,; Goff, H. M.; Scholz, W. F.; Reed, C. A. J . Am. Chem. Soc. 1981, 103, 6778. (36) Fleischer, E. B.; Palmer, J. M.; Srivastava, T. S.; Chatterjee , A. J . Am. Chem. SOC.1971, 93, 3162. (37) Cohen, I. A. J Am. Chem. SOC.1969, 91, 1980.
Electrocatalytic Aspects of Iron Phthalocyanine
The Journal of Physical Chemistry, Vol, 91, No. 14, I987
3803
TABLE I: Mbsbauer Parameters for Iron Phthalocyanine and Iron Porphyrin and Their Corresponding p-Oxo Derivatives 6,' mm s-' A? specimen vs. a-Fe mm s-I
FePc
0.39 0.18 0.25 0.18 0.35 0.61 0.36 0.32 0.29 0.20 0.66 0.22 0.12
0.71 2.18 2.48 1.14 0.50 3.52 1.32 1.80 2.70
47%
0.38 0.25
2.63 0.42
16%
0.40 0.30 0.39 0.28
2.63 0.66 2.61 0.41
0.40 0.34 0.36 0.3 1 0.18
2.62 0.66 2.48 1.11 0.53
0.38 0.25 0.32 -0.03 0.37 0.09 0.32 0.27 0.42 0.17 0.19
2.54 1.19 0.54 1.54 0.63 0.82 0.59 0.62 1.52 1.13 0.27
FePc-p-oxo(I ) FePc-p-oxo(2) FePc (sheared) FePc (monomer)
990
850
Wovenumber (cm-"l 750 700
800
Figure 4. Expanded 650-950-cm-' region of the FTIR spectrum shown in Figure 3.
2.61 1.02 0.38
FePc(po1ymer)
1 .oo
r,c mm s-'
ref
0.39 0.54 0.69 0.53 42
8, 9
8, 9
FePc/Vulcan XC-72 41
(DMF)
44%
41
(Me2SO) 12% 45%
41
(HzS04) 15%
FeTsPc FeTsPc/Vulcan XC-72, 5% w / w FePc(C O ) (FeTMPP),O ( FeTPP)20 FeTPP FeTPP(C0) FeTPP(CO),
1.03 0.38 0.39
0.26
43 47 47 46 45 44
lsomer shift. *Quadrupole splitting. e Width.
Figure 5. Mossbauer spectra of FePc (a), FePc-p-oxo(l) (b), FePc-p(c) in crystalline form.
oxo(2)
by the disappearance of the 852- and 824-cm-I absorption bands accompanied by the appearance of a new strong feature centered at 806 cm-I when the p-oxo(1) compound was prepared with highly enriched lSO. This would rule out the possibility of a pperoxo structure being formed. It may be noted that experiments involving FePc wondensed with O2in the absence and presence of an inert gas have indicated3* that the stretching frequency of O2in the FePc-0, adduct in the end-on configuration occurs at about 1207 cm-I. The Mijssbauer spectra of the three species under investigation, reported in an earlier c o m m ~ n i c a t i o n are , ~ ~shown in Figure 5 and the associated parameters are given in Table I. These are in excellent agreement with those obtained most recently by Ercolani et al." According to these authors, both pox0 species exhibit strongly antiferromagnetically coupled high-spin ( S = 5 / 2 ) Fe(II1) centers but rather different X-ray powder patterns, magnetic susceptibilities, and IR spectra. These were attributed (38) Watanabe, T.; Ama, T.; Nakamoto, K. J . Phys. Chem. 1984,88,440. (39) Tanaka, A. A.; Fierro, C. A.; Gupta, S. L.; Yeager, E. B.; Scherson, D. Absrrucr of Papers, Electrochemical Society Meeting, Las Vegas, October 1985; Vol. 85-2, Extended Abstract No. 430. (40) Ercolani, C.; Gardini, M.; Murray, K. S.; Pennesi, G.; Rossi, G. Inorg. Chem. 1986, 25, 3972 and references therein.
principally to the lack of coplanarity between the two Pc rings in the FePc-poxo(1) as compared to the ~ - 0 x 0 ( 2 ) . A number of interesting observations can be made based upon a comparison of the values of the isomer shift, 6, and the quadrupole splitting, A, for the specimens listed in Table I: i. The iron impurity present in the commercially available FePc most certainly corresponds to FePc-p-oxo(2). ii. The small quadrupole splitting doublet for the high loading samples of FePc on Vulcan XC-72 prepared from either D M F or M e 2 S 0 may be attributed to FePc-p-oxo( 1). iii. The magnitude of A for doublet 2 in the case of carbon dispersions obtained from sulfuric acid is very similar to that of FePc-~-oxo(2).Although the reported values of 6 appear to be significantly different, it is very likely that the species observed on carbon is in fact the poxo(2) in light of the method of preparation of the specimens. iv. Savy et aL8v9have reported five quadrupole split doublets for monomeric FePc samples. The three features exhibiting the largest resonant absorption area correspond almost exactly to those reported later by Melendres4' for FePc precipitated from concentrated H2S04from which only one corresponds to the pure bulk FePc. The other two doublets which combined amount to about 6% of the total signal have not been observed by other workers and can most likely be attributed to impurities present in the material. v. Melendres4I has suggested that the doublet denoted as 2 in low-loading FePc/Vulcan XC-72 specimens prepared from DMF, Me2S0, and pyridine solutions (see Table I) may be associated (41) Melendres, C. A. J . Phys. Chem. 1980, 84, 1936.
3804
The Journal of Physical Chemistry, Vol. 91, No. 14, 1987
Tanaka et al.
\
I
-2.0
-1.0
0
Ve/oc/fyfmmlsl 1.0 20
Figure 6. Mijssbauer spectra of FeTsPc in crystal form (a) and adsorbed on Vulcan XC-72 (5% w/w) (b).
with FePc molecules interacting with the carbon substrate. This conclusion was based upon the fact that the spectral parameters did not appear to be a function of the solvent used and that the relative resonant absorption area of the doublet in question increased when a much higher area carbon was used. Grenoble and Drickamer,42however, have reported that the shearing of sublimed FePc crystals in a mortar gives rise to a material with a quadrupole splitting much smaller than that of the original FePc compound. This species was found to exhibit an X-ray powder diagram showing only a few weak diffuse lines. Although these authors attributed this phenomenon to an amorphous nature of the material, a similar spectral behavior would be observed for very small microcrystals of the original compound. Regardless of its actual nature, the values of 6 and A associated with this species are very similar to those of doublet 2 referred to above. It is thus conceivable that the dispersion of a small amount of the FePc on carbon may result in the formation of either amorphous or microcrystalline particles which as the loading is increased would serve as nucleation sites for FePc and/or its p-oxo derivatives. It is also possible that such small particles may be formed initially in the carbon substrate pores and thus their size would be governed by the size of such pores. This appears to be consistent with the fact that the resonance absorption of doublet 2 is found to increase upon dispersing a fixed amount of FePc in a higher area carbon for which the number of pores is larger but their size is smaller. vi. There seems to be no spectroscopic evidence for the presence of molecular oxygen bound to FePc in bulk form. Rather, the O2which penetrates the lattice at room temperature appears to undergo dissociation to yield p-oxo-type compounds. Additional evidence in support of this view was provided by experiments conducted by Melendres4' in which FePc was sealed in an ampule with O2 and heated ocassionally. The Mossbauer spectra after this treatment exhibited a much enhanced doublet with parameters in excellent agreement with those of a p-oxo species. The Mijssbauer spectra of FeTsPc bulk shown in curve a, Figure 6, yielded a more negative value for the isomer shift than that found for FePc (see Table I). It seems likely that this may be due to a strong axial interaction between the metal center and peripheral sulfonic groups in the crystal lattice involving charge transfer from the Fe to the -SO3 ?r* orbitals. A similar effect has been observed43for compounds of the form FePc(C0)X where X represents a sixth axially coordinated ligand and also for iron tetraphenylporphyrin40 adduct^^^^^ (see Table I). In both cases (42) Grenoble, D. C.; Drickamer, H. G . J . Chem.Shys. 1971, 55, 1624. (43) Calderazzo, F.; Frediani, S.; James, B. R.; Pampaloni, G.; Reimer, K. J.; S a m , J. R.; Serra, A. M.; Vitali, D. Inorg. Chem. 1982, 22, 2302. (44) Reimer, K . J.; Sibley, C. A,; Sams, J. R. J. Am. Chem. SOC.1983, 105, 5 147.
-1.2 &
I
-0.9 l
l
l
"
E(V)vs.Hg/HqO 0 0.3
-0.3
-0.6 "
l
l
'
l
Figure 7. Cyclic voltammograms of 7% w/w FePc-p-oxo(1) (a), 7% w/w FePc-p-oxo(2) (b), and 7% w/w FePc dispersed on Vulcan XC-72 carbon (c) in a N2-saturated1 M NaOH solution. u = 20 mV s-'. Temperature = 22 "C.
backbonding to the CO T * orbital of either one or two axially coordinated CO molecules results in a decrease in the d electron density around the metal and thus in a much more negative isomer shift than the axially unsubstituted compound.46 A very different Mossbauer spectrum was observed, however, upon adsorbing the FeTsPc on Vulcan XC-72 carbon. (see curve b in Figure 6) The associated parameters in this case were found to be very similar to those of a number of bulk (FeP)20 compounds4' where P represents a porphyrin-type ring providing some evidence that FeTsPc is present in the form of a p-oxo compound on the carbon support. Electrochemical Measurements. The cyclic voltammetry of thin porous coating electrodes containing the FePc-p-oxo species dispersed on high surface area Vulcan XC-72 carbon are given in curves a and b in Figure 7. The peaks observed at about -0.15 V and -0.55 V vs. Hg/HgO,OH- are in agreement with those reported earlier for FePc bulk.** The charge under each of the peaks was evaluated by a straightforward integration using as a base line the almost purely capacitive current associated with the carbon support, yielding values of 0.8 and 0.6 mC for the p-oxo( 1) and 42), respectively. The amount of macrocycle-carbon mixture involved in this type of thin porous coating electrodes has been estimated to be (0.5 f 0.1) mg, based on dry weight; hence, about 10% to 15% of all FePc molecules present in the electrode are electrochemically active. As a means of gaining insight into the morphology of the materials examined, a series of scanning electron micrographs were taken for the macrocycles in powder form. The results shown in Figure 8 indicate that the average particle size of the FePc-poxo( 1) is much smaller than that of the poxo(2) or the FePc. If it is assumed that the crystallites are spherical and smooth, the number of surface molecules would account only for a small fraction of the charge associated with the voltammetry peaks. It may be argued that bulk molecules are also involved in the electron-transfer process and thus would contribute to the total measured charge. This, however, would result in an asymmetry in the voltammogram due to the slow ion transport through the lattice, an effect that is not observed in the actual experiments. Alternatively, it is conceivable that during the preparation of the thin coating electrode the macrocycle may have undergone dissolution and subsequent adsorption on the (45) Plouzennec, M.L.; Bondon, A,; Sodano, P.; Simonneaux, G . Inorg. Chem. 1986, 25, 1254. (46) Collman, J. P.; Hoard, J. L.; Kim, N.; Lang, G.;Reed, C. A. J . Am. Chem. SOC.1915, 97, 2676. (47) Torrens, M.A.; Straub, D. K.; Epstein, L. M. J . A m . Chem. SOC.
1972, 94,4160.
The Journal of Physical Chemistry, Vol. 91. No. 14, 1987 3805
Electrocatalytic Aspects of Iron Phthalocyanine
1 -Lo
0.3
I!
1
P'
1
-'"$E Figure 8. !hnning electron micrographs of FePc (a), FePc-p-oxo( 1) (b), and FePc-p-oxo(2) (c), in crystalline form.
carbon substrate. This possibility seems highly improbable in view of the lack of solubility of FePc in water. It thus appears that the disparities between the charge under the peaks and the apparent surface area of the particles are most likely due to a high degree of surface microroughness particularly for the p-oxo compounds. In the case of monomeric FePc, for which the apparent crystallite size was the biggest among the compounds investigated, no clear voltammetric features could be distinguished for specimens that had been cycled in the same potential region as that for the p-oxo species (see curve c, Figure 7). The same peaks obtained for the oxygenated derivatives were observed, however, upon extending the sweep in the negative direction to -1.1 V vs.
-98 -0.6
0
En/)vs. &o 7 2
-o? -92
-0.3
-a6
p
-0.9 ElVl VSMW
Figure 10. Steady-state O2 reduction polarization curves for a thin porous coating rotating disk electrodes containing 7% w/w FePepoxo( I ) (a), 7% w/w FePc-p-oxo(2) (b), and 7% w/w FePc dispersed on Vulcan XC-72 carbon (c) in 1 M NaOH. Rotation rate: 2500 rpm. The electrodes used are the same as those in Figure 7 (see text for details). The dotted lines were obtained after the FePc sample was polarized at -1.1 V VS. Hg/HgO,OH-.
Hg/Hg0 (see curve c', Figure 7). This could be due to a potential-induced modification in the intrinsic conductivity of the material and/or to a lattice rearrangement which leads to the exposure of a much higher number of molecules to the electrolyte. Although this issue has not as yet been clarified, the results obtained provide strong evidence that only surface molecules in the lattice are electrochemically active and that these are the same for all species regardless of their bulk structure. This is consistent with the report of Appleby and Savy8s9who observed similar reflectance spectra upon placing in contact with 1 M KOH solutions three types of FePc (monomer, dimer, and polymer) which exhibited widely different Miissbauer spectra in their corresponding dry forms. Two voltammetric peaks occurring at potentials somewhat positive with respect to those observed for FePc were found in the case of FeTsPc adsorbed at monolayer level on Vulcan XC-72 carbon (see Figure 9). Zagal et a1.I8 and later Zecevic et aL4* have assigned these features to redox reactions involving the metal center, i.e., Fe( IIl)TsPc(-2)/Fe(II)TsPc(-2) and Fe( II)TsPc(2)/Fe(I)TsPc(-2). It seems, therefore, reasonable to assume that the peaks at -0.15 and -0.55 V vs. Hg/HgO in Figure 7 are associated with the same processes. These voltammetric features occur at the same potentials as those of the surface states reported by Barendrecht and co-workers'' providing a clear illustration that both techniques can be used as probes of the electrochemical properties of surface species. Rotating disk polarization curves for O2reduction with FePc and its p-oxo derivatives dispersed on XC-72 in 1 M NaOH at rmm temperature, were performed with the same thin porous coatings employed in the cyclic voltammetry measurements. As shown in Figure 10, the activity for the p-oxo(2) derivative was found to be essentially the same as that of the p-oxo( 1) derivative. In the case of FePc specimens which exhibited a featureless (48) Zecevic, S.; Simic-Glavaski, B.; Yeager, E.; Lever, A. B. P.; Minor, P. C. J . Electroanal. Chem. 1985, 196, 339.
3806
The Journal of Physical Chemistry, Vol. 91, No. 14, 1987 iimAl
I
1.0
Tanaka et al.
-
0,-0.3
r (r, r
ui -0.2
=o.e
-
? -
-0.6
.1, -0.1
t
-0.4
01 t
Q - 0
-0.2 -0
0.2
o -02
-0.4 -0.6 -0.8
-1.0
-0
Current Denslty (mA c v i 2 )
-10
I
^ ^
CU
MAf
Figure 11. Steady-state rotating ring-disk polarization curves for O2 reduction on a 7% w/w FePc-p-oxo(1) dispersed on Vulcan XC-72 carbon (a), Powercat 2000 (b), and Vulcan XC-72 (c) in a 1 M NaOH solution. Collection efficiency: 0.38. The ring was polarized at +0.13 V vs. Hg/HgO. Other conditions are specified in caption of Figure IO.
voltammogram such as that shown in curve c in Figure 7, the O2 reduction currents were found to be much lower than for the p-oxo compounds. A response very similar to that corresponding to the p-oxo(2) species was obtained, however, by polarizing the electrodes a t -1.1 V vs. Hg/HgO until the well-defined cyclic voltammetry curve shown in curve c', Figure 7 was obtained. The increase in the O2 reduction current found in all samples at potentials of about -0.3 to -0/.4 V correlates with the voltammetric region located precisely between the two peaks indicating that the reaction mechanism is sensitive to changes in the redox properties of the catalyst. These results clearly indicate that only one species, common to all specimens, is responsible for the enhancements in the electron-transfer rates. The onset for O2 reduction for all compounds examined was found to occur at potentials more positive than those associated with the peak attributed to the Fe(III)Pc/Fe(II)Pc transition, in agreement with the results of Beck,Ioreferred to in the Background section. The same phenomenon has been observed for a variety of iron porphyrins in Shigehara and A n ~ o who n ~ ~suggested that the effect may be due to a high electrocatalytic activity of the reduced form of the macrocycle. According to these authors, however, this explanation does not appear to be in accordance with the fact that the plots of the limiting current vs rotation rate showed deviations from linearity at high rotation rates indicating partial kinetic control. It was also cited as additional evidence against such interpretation that the equilibrium constant for the reaction Fe(1I)TMPyP
+ O2
Fe(II1)TMPyP
+ 02-
where TMPyP denotes meso-tetrakis(N-methylpyridyl)porphine, was too small and thus that the two reactants would have no tendency to remain associated. Such equilibrium constant, however, would correspond to an outer-sphere electron transfer and thus provides no direct information regarding the possible formation of a Fe(II)TMPyP-02 adduct. Bettelheim and Kuwanaso have argued that such a thermodynamically calculated value does not seem in agreement with the fact that the rate constants for the forward and reverse reactions are of the same order of magnitude. They proposed that an Fe(II)-02 intermediate could be formed with a much more positive redox potential than the free macrocycle couple. Such arguments, however, do not seem to explain all the experimental observations and thus additional work will be required to obtain a full account of the phenomenon. A selected number of rotating ring-disk measurements were performed with a different set of thin porous electrodes prepared
Figure 12. Polarization curves for 02 reduction with 02-fed (P = 1 atm) electrodes in 4 M NaOH at 60 O C (see text for details). Results are also shown for 5% w/w Pt/RB carbon (Prototech) and heat treated 10% (FeTMPP),O/RB carbon which had been previously deashed.
with the same dispersions used for the other experiments. No attempt was made in this case to record cyclic voltammetry curves prior to obtaining the polarization data. The ring currents for the activated FePc and the two p-oxo forms of the macrocycles were found to be very low, particularly for FePc-p-oxo(1) for which the results are shown in Figure 11. This could indicate that the overall reduction occurs via a direct 4-electron mechanism. This possibility, however, cannot be confirmed by employing such thin-coating electrodes since the peroxide may undergo decomposition within the porous matrix generating 02,which can then be further reduced to yield effectively four electrons per dioxygen molecule. It is interesting to note that the activity associated with the p-oxo( 1) specis was comparable to that obtained with Powercat 2000 (see Figure 1l), a commercially available Pt-based catalyst, and much higher than that of the Vulcan XC-72 without catalyst which has also been included in the same figure for comparison. Oxygen reduction measurements performed with gas-fed electrodes in 4 M NaOH at 60 OC,a technologically relevant electrolyte, shown in Figure 12, yielded the same relative activity observed for the thin porous coatings. In this case, however, the performance of Prototech platinum, a commercially available catalyst dispersed on RB carbon, in a 5% w/w ratio was found to be much higher than that of any of the macrocycles examined. The activity of the Pt-based catalyst was comparable to that of the p-oxo form of iron tetrakis(methoxypheny1)porphyrin catalyst which had been first adsorbed at a 10 % w/w ratio on a deashed form of RB carbon and later heat treated at 800 O C in an inert atmosphere. These results are also given in Figure 12 for comparison. Theoretical Considerations. Quantum mechanical calculations and symmetry and overlap arguments indicate that Fe(I1)Pc would have optimum electronic properties for coordinating dioxygen in the axial position.51 Such interaction would involve charge transfer from the electron-rich metal center to the O2 A* orbitals leading to an overall weakening of the 0-0 bond. As was mentioned earlier, an FePc-02 adduct in an end-on configuration has indeed been detected and characterized in gas matrices at very low temperatures@ with a much reduced 0-0stretching frequency than gas-phase dioxygen. It thus seems reasonable to assume that the formation of this adduct is the most likely step in the initial activation of dioxygen. The removal of an electron from the iron center, on the other hand, would be expected to increase the energy difference between the metal d-orbitals and the O2A* orbitals thus decreasing the ability of the metal to transfer charge to the axially coordinated ligand. In fact, the affinity of O2 for macrocycles of the form LFe(III)P, where P represents the ring and L an axially coordinated ligand, has not as yet been documented. Based on these arguments it seems unlikely that a ferric form of FePc may play an important role in the initial activation of O2
~~~
(49)R. Shigehara, R., F. C Anson, F C. J . Phys. Chem. 1982,86, 2776 (50) Bettelheim. A , Kuwana, T Anal Chem 1979, 51, 2257
(51) Newton, J E.: Hall, M. B. Inorg. Chem. 1984, 23, 4627 and references therein.
Electrocatalytic Aspects of Iron Phthalocyanine as proposed earlier in the literature.89 Rather, the Fe(II1) centers detected in the active specimens serve only to increase the conductivity of the material without participating directly in the electrocatalytic reaction. One of the requirements for efficient electrocatalysis cited by Savy and c o - ~ o r k e r s 'involves ~ the reversible adsorption and desorption of reactants and products. It is important to note in this regard that the strength of the axial interactions between the HOT, iron(I1) center in FePc and simple species such as O,,OH-, and their corresponding radical derivatives is expected to depend critically on the nature of the orbitals involved. For instance, dioxygen has both electron donor and electron acceptor orbitals at the appropriate energy to mix with the half-filled metal orbitals, whereas hydroxyl ion may be expected to act mostly as an electron donor since the lower unoccupied molecular orbital is at too high an energy to accept charge from the metal center. It is thus possible that the affinity of Fe(I1)Pc for dioxygen may be very high compared to that for the reaction intermediates, products, or other solution-phase components. This strong interaction with dioxygen compared to that with other species can be of critical importance to the electrocatalytic activity involving a single type of iron site. Such a model would make it unnecessary to invoke a metal spin crossover mechanism for the uptake of the reactant and the subsequent release of the reaction product as suggested by Savy and co-workers.13 From a more general viewpoint, the degree of back-bonding between the iron and dioxygen will depend on the electronic density available at the metal center which in turn will be governed by the nature of the metal-ligand interactions. Fierro et a1.52have pointed out on the basis of semiempirical quantum mechanical calculations that in the absence of axially coordinated ligands, the energy gap between the occupied d,,d, orbitals and the empty e.&**) ring orbitals plays a key role in controlling the extent of metal-to-ring back-bonding. This energy gap can be varied by introducing appropriate structural and chemical modifications to the ring structure. Within this framework, the much higher reactivity of Fe(I1) porphyrins toward dioxygen compared to that of Fe(I1) phthalocyanines has been attributed to their longer Me-N bond length. This would lead to a decrease in the metal-to-ligand back-bonding and consequently in an increase in the electronic charge at the metal center. Theoretical studies of this type can give considerable insight into the factors involved in the activation and reduction of dioxygen and thus provide a basis for a rational search for macrocyclic structures with optimized electronic characteristics to effect this redox process at high rates and low overpotentials.
Summary i. Mossbauer experiments conducted in this and other laboratories indicate that four distinct species can be found in FePc specimens either in bulk form or dispersed on carbon supports from a number of solvents. These have been assigned to the standard monomeric form of FePc (6 = 0.39 mm s-l vs. a-Fe, A = 2.61 mm s-l), two p-oxo compounds (I, 6 = 0.25,A = 0.38; (52) Fierro, C.; Scherson, D.; Anderson, A. B.; Yeager, E. B., to be submitted for publication.
The Journal of Physical Chemistry, Vol. 91, No. 14, 1987 3807
1 1 , 6 = 0 . 1 8 , A = l.OO),andafourthspecies(6=0.35,A=0.71) believed to correspond to very small amorphous particles or microcrystals of FePc. The information so far obtained does not appear to provide conclusive evidence for the presence of individual FePc molecules bound directly to the support. ii. Thin porous coating Teflon-bonded electrodes containing small crystals of either monomeric FePc or any of the two FePc-p-oxo species, mixed with high-area carbon, yielded a common set of welldefined voltammetric peaks in alkaline media. The potentials associated with these peaks ( E , = -0.15 V, E2 = -0.55 V vs. Hg/HgO, OH-) are in good agreement with those of two surface states reported by Barendrecht and co-workers17 in the same electrolyte for FePc films vapor deposited on smooth carbon surfaces. Rotating ring-disk measurements for 0, reduction involving such macrocyclic-carbon dispersions yielded very similar results for both pox0 species and for monomeric FePc. In the latter case, however, it was found necessary to polarize the electrode at fairly negative potentials in order to achieve high catalytic activity. These observations indicate that despite the major differences in their spectral properties, the electrocatalytic and redox characteristics of all forms of FePc examined are associated with a single FePc surface species. iii. Quantum mechanical calculations and symmetry and overlap arguments reported elsewheres' indicate that Fe(1I)Pc has an optimum electronic configuration for interacting with dioxygen. The bonding would involve charge transfer from the electron-rich metal center to the O2A* orbital leading to an overall weakening of the 0-0bond in an end-on configuration. Evidence in support of this view has been provided by matrix isolation techniques from which an 0,-FePc adduct with this geometry has been unambiguously c h a r a c t e r i ~ e d . ~ ~ iv. No spectroscopic evidence has been reported for the presence of molecular oxygen bound to the FePc lattice at room temperature as suggested by Savy and c o - w ~ r k e r s .The ~ ~ ~increase in conductivity of FePc upon exposure of bulk specimens to O2is most probably related to the formation of p-oxo derivatives such as those characterized by IR and Mossbauer spectroscopy. v. The strength of the axial interactions between the iron(I1) center in FePc and 0,and its reduction intermediates and products is expected to depend quite critically on the nature of the orbitals involved. It is thus likely that the affinity of Fe(I1)Pc for dioxygen may be very high compared to that for other species. This may be of critical importance to the electrocatalysis involving a single type of iron site. Such a model would make it unnecessary to invoke a metal spin crossover mechanism to explain the high activity of FePc for 0, reduction as suggested by Savy and coworkers.13
Acknowledgment. This work has been partially supported by the Department of Energy through a subcontract with Lawrence Berkeley Laboratory. Additional support has been provided by the IBM Corp. through a Faculty Development Award to one of the authors (D.S.). The authors would like to express their appreciation to Mr. L. Bodalbhai for the synthesis of the FePc compounds, Prof. R. W. Hoffman for the Mossbauer spectra, and Dr. D. Tryk and Mr. W. Aldred for the gas-fed electrode measurements. A.A.T. thanks CNPq-Brasil for a scholarship.
