2776
J. Phys. Chem. 1902, 86, 2776-2783
Electrocatalytlc Actlvlty of Three Iron Porphyrins in the Reductions of Dioxygen and Hydrogen Peroxide at Graphite Electrodes Klyotaka Shlgehara and Fred C. Anson' Arthur Amos Noyes Laboratories, Division of Chemistry and Chemhl Engineering, California Institute of Technology, Pasadena, Callforn& 9 1125 (Received:January 29, 1982; In Final Form: March 19. 1982)
Catalysis of the reduction of O2 at graphite electrodes by iron(II1) protoporphyrin IX, iron(II1) meso-tetraphenylporphine, and iron(II1) meso-tetra(3-pyridy1)porphine adsorbed on the electrode surface is described. Rotating ring-disk voltammetry was used to detect HzO2 as an intermediate and to demonstrate its further reduction at more negative potentials. Possible catalytic mechanisms are proposed, and some behavioral differences likely to be encountered in comparisons of dissolved and adsorbed catalysts are discussed.
As part of a continuing series of studies of the catalysis of dioxygen reduction by molecules adsorbed or otherwise attached to electrode surfa~es,l-~ we have conducted experiments with three iron porphyrin catalysts. Recent studies of iron porphyrins as dioxygen reduction catalysts*-" have reported that the catalyzed reaction can proceed by a two-electron pathway to produce hydrogen peroxide or by a net four-electron pathway to yield water. It is of interest to learn what chemical and structural features of these catalysts determine the pathways taken by the reactions that they catalyze. With a variety of adsorbed cobalt porphyrin catalysts, we have found the two-electron reduction pathway to be predominant1v3J2 In only one instance was a quantitative four-electron reduction achieved, and this required a dimeric cofacial cobalt porphyrin with a carefully fashioned structure.' Even small alterations in the structure caused the reduction catalyzed by the dimeric catalyst to revert to a two-electron pathway. Thus, certain monomeric iron porphyrins appear to provide catalytic pathways for dioxygen reduction that are unavailable with their cobalt analogues, and we sought to understand the reasons for the differences. The electrocatalytic behavior of three iron porphyrins was investigated by rotating disk and ring-disk proced u r e ~ . ~The ~ three were iron(II1) protoporphyrin IX (FePPIX), iron(II1) meso-tetraphenylporphine(FeTPP), and iron(II1) meso-tetra(3-pyridy1)porphine (FeTPyP). The first two porphyrins yielded four-electron reductions of dioxygen, but the catalytic activity of all three was relatively short-lived. The rate of reduction of hydrogen peroxide as catalyzed by the iron(II1) porphyrins was also measured as well as the rate of its catalyzed dispropor(1)J. P.Collman, P. Deniaevich, Y. Konai, M. Marrwco, C. Koval, and F. C. Anson, J.Am. Chem. SOC.,102,6027 (1980). (2)K. Shigehara and F. C. Anson, J. Electroanal. Chem., 132,107 (1982). (3)R.Durand and F. C. Anson, J.Electroanul. Chem., 134,273(1982). (4)F. van den Brink, E. Barendrecht, and W. V i h e r , J.Royal Neth. Chem. Soc. 99,253 (1980),and references therein. (5)H. Behret, W.Clauberg, and G. Sandstede, Ber. Bunsenges. Phys. Chem., 83, 139 (1979),and references therein. (6)T. Kuwana, M. Fugihlra, K. Sunakawa, and T. Osa, J.Electrocanal. Chem., 88, 299 (1979). (7)A. Bettelheim and T. Kuana, Anal. Chem., 51, 2257 (1979). (8)N. Kobayashi, M. Fujihira, K. Sunakawa, and T. Osa, J. Electroanal. Chem., 101,269 (1979). (9)N.Kobayashi, T.Mataue, M. Fujihira, and T. Osa, J.Electroanal. Chem., 103,427 (1979). (10)N.Kobayashi, M. Fugihira, T. Osa, and T. Kuwana, Bull. Chem. Soc. Jpn., 63,2195 (1980). (11)A. Bettelheim, R. J. H. Chan, and T. Kuwana, J. Electroanal. Chem., 110,93 (1980). (12)R. Durand and F. C. Anson, work in progress. (13) W. J. Albery and M. H. Hitchman, "Ring-Disk Electrodes", Oxford University Press, Oxford, United Kingdom, 1965. 0022-3654182/2086-2776$0 _1.2510 .-. #.
tionation. The results show that the catalyzed four-electron reductions of dioxygen generate hydrogen peroxide as an intermediate that is further reduced at more negative potentials.
Experimental Section Materials. Iron(II1) protoporphyrin M chloride (Strem Chemical Company) was recrystallized twice from pyridine-acetic acid. Iron(II1) meso-tetraphenylporphine bromide was synthesized according to a standard procedure.14 Cobalt(I1) mesoporphyrin diethyl ester3 was a gift from Professor J. P. Collman. Iron(II1) meso-tetra(3pyridy1)porphine was prepared as what we believe to be the hydroxide as follows. A mixture of 6.71 g (0.1 mol) of freshly distilled pyrrole and 10.7 g (0.1 mol) of 3pyridinecarboxaldehyde was added rapidly to 250 mL of propionic acid under reflux in an inert atmosphere. Refluxing was continued for 20 min. The solution was then cooled to room temperature and the solvent was removed by evaporation at reduced pressure at temperatures below 40 "C. The resulting solid was extracted with chloroform saturated with ammonia. The extract was applied to a silica gel column (ca. 350 mesh) and eluted with a 1:20 acetone-chloroform mixture. A red-purple band was collected and the solution was evaporated to obtain a solid that was recrystallized twice from a mixture of methylene chloride and heptane; 1.6 g of meso-tetra(3-pyridy1)porphine was obtained: ,A, (in CHC1,) 416, 514, 550, 590, 656 nm; A- (in 1N HC1) 441,584,639 nm. The porphine (500 mg) and 3 mL of pyridine were dissolved in 30 mL of deaerated THF. While the solution was refluxed, 2.5 g of Fe"Br2-2H20was added, and refluxing continued for 7 h. After evaporation,the solid was dissolved in a minimal amount of CHC1, and chromatographed on a neutral alumina column to exclude excess iron. The eluent was evaporated and applied to a silica gel column (70-230 mesh) and eluted with 1N HC1. The first, green band was protonated free base and the second, red-brown band was collected. After the volume was reduced by evaporation, the Fe"'TpyP complex was precipitated by adding a solution of NaOH. The solid was recrystallized twice from a chloroform-heptane-methanol mixture to obtain 320 mg of product. The absence of free base was confirmed from spectroscopy in acidic aqueous solution: A, (in 1M HCI) 398, 516, 580 nm. Dimethyl formamide and dichloroethane were purified by distillation and stored over 4A molecular sieves. Other chemicals were reagent grade and were used as received. (14)J. P.Collman, R. R. Gagiie, C. A. Reed, T. R. Halbert, G. Lang, and W. T. Robinson, J. Am. Chem. Soc., 97,1427 (1975).
0 1982 American Chemical Society
Electrocatalytic Activity of Three Iron Porphyrins
The Journal of Physical Chemistty, Vol, 86, No. 14, 7982 2777 r.
TABLE I: Electrolyte-Buffer Solutions pH range 0-2.5 2.5-4 4-6
6-9 9-11 11-14
electrolyte 0 . 2 M NaC10, t HC10, 0.1 M NaClO, t 0.1 M potassium citrate 0.1 M NaClO, + 0.1 M acetate buffer 0.1 M NaClO, t 0.1 M phosphate buffer 0.1 M NaClO, t 0.1 M carbonate buffer 0.2 M NaClO, t NaOH
[O,I, mM 1.3
1.0 1.1
1.1 1.3
1 . 3 ( p H 12)
Laboratory distilled water was further purified by passage through a purification train (Barnstead Nanopure). Pyrolytic graphite electrodes (Union Carbide Co., Chicago, IL) were prepared and mounted as previously described.15 Apparatus and Procedures. Cyclic and rotating disk voltammetry employed previously described procedures and instrumentation.16 For rotating ring-disk measurementa a recently described electrode with a demountable disk17 was utilized. For detection of hydrogen peroxide, the ring consisted of a platinum electrode maintained at a potential where hydrogen peroxide was oxidized to dioxygen. The platinum ring electrode was freshly cleaned and pretreated before each run to maintain its efficiency in the detection of hydrogen peroxide.18 The collection efficiency of the electrode (which is dependent on rotation ratel') was 0.27 at 100 rpm. For detection of dioxygen formed at the disk (for example, by the disproportionation of hydrogen peroxide) a carbon-paste ring electrode was employed. The electrode was coated with the cobalt(I1) porphyrin listed above which has been shown3to catalyze the reduction of dioxygen to hydrogen peroxide quantitatively. The coating was applied by dipping the electrode for 5 min in a 0.5 mM solution of the porphyrin in dichloromethane. The supporting electrolyte-buffer solutions that were employed are listed in Table I. The O2 concentration in each of the dioxygen-saturated buffers was measured with a Yellow Springs Instrument Model 5331 dioxygen probe and they are also listed in Table I. No significant effects of the buffer anions on the catalyst electrochemistry were noticed. Electrodes were coated with the porphyrin catalysts by pipetting aliquots of solutions of the metalloporphyrin in dichloroethane (FeTPP and FeTPyP) or DMF (FePPIX) onto a freshly cleaved electrode and allowing the solvent to evaporate slowly. (Solutions of the metalloporphyrins were utilized within a few hours of their preparation. Electrodes coated with older solutions exhibited less reproducible behavior.) After being coated, the electrodes were soaked in 0.1 M NaOH for a few minutes to replace any axial ligands with hydroxide, washed thoroughly, and transferred to the test solution. Measurements were conducted at 22 f 2 "C. Prepurified argon was used to deaerate solutions when necessary. Potentials are reported with respect to a sodium chloride saturated calomel electrode (SSCE). Results Catalyzed Dioxygen Reduction in Acidic Solutions. Figure 1 (solid surves) shows cyclic voltammograms for the three iron porphyrins adsorbed on graphite electrodes in an acidic electrolyte in the absence of dioxygen. The re(15)N.Oyama and F. C. Anson, J. Am. Chem. Soc., 101,3450(1979). (16)N.Oyama and F. C. Anson, Anal. Chem., 52, 1192 (1980). (17)T.Geiger and F. C. Anson, Anal. Chem., 52, 2448 (1980). (18)T.Geiger and F. C. Anson, J. Am. Chem. SOC.,103,7489(1981).
i
0.6
I
1
1
I
0.4
0.2
0
-0.2
I
-0.4
J
I
-0.6
-0.8
E vs SSCE, volt Figure 1. Cyclic voltammograms for adsorbed iron porphyrins in the absence (soHd curves, S = 5 MA) and in the presence (dashed curves, S = 25 MA) of O2 (satwated): (A) FeTPP, (6)FePPIX, (C) FeTPyP. The pyrolytic graphite electrode (0.17 cm2) was coated with 6 X lo4 mol cm-* of porphyrin. Supporting electrolyte: 0.1 M HCIO, -t 0.1 M NaCIO,. Scan rate: 20 mV s-'.
versible voltammetric responses do not diminish upon repeated cycling in the absence of dioxygen even for FeTPyP which is protonated and soluble in many electrolytes. If the FeTPyP coating is washed with 0.1 M HCl, all but about one monolayer of the porphyrin is removed as judged from the greatly diminished voltammetric response. However, the FeTPP and FePPIX coatings of Figure 1 survive such washings intact. The greater stability of the FeTPyP coating in Figure 1originates in the much lower solubility of the protonated porphyrin in perchlorate electrolytes. By systematically increasing the quantity of iron porphyrin deposited on the electrode surface and measuring the areas under the corresponding voltammograms, we established that virtually all of the porphyrin deposited remains on the surface when less than ca. mol cm-2 is adsorbed. With thicker coatings proportionately smaller responses result that could arise either from physical loss of a portion of the porphyrin or because not all of the porphyrin molecules in the thicker coatings retain their electroactivity. The peak currents of the voltammograms increase linearily with the potential scan rate, as expected for adsorbed reactant^.'^ The breadth of the voltammetric waves indicates the absence of fully Nernstian response as does the inequality of the peak potentials, especially for FePPIX. When the coated electrodes were transferred to dioxygen-saturated solutions, the dashed curves in Figure 1 resulted. At bare graphite electrodes dioxygen reduction does not commence before -0.5 V, so it is evident that all three iron porphyrins are active catalysts. The occurrence of the catalyzed reduction at potentials well ahead of those ~~~
(19)E.Laviron, J. Electroanal. Chem., 39, 1 (1972).
Shigehara and Anson
The Journal of Physical Chemistry, Vol. 86, No. 14, 1982
2778
a
I
I
4 0
1
~3
c 4
r z
3
,
c
o
EDISKd s
r z
,
c3
1
c4
c5
OE
c7
SSCE volt
Flgure 2. Current-potential curves during the reduction of O2 at a rotated graphlte disk-platinum ring electrode. The graphite disk (0.34 cm2) was coated with 6 X lo-' mol cm-* of (A) FeTPP, (B) FePPIX, and (C) FeTqrP. Ring electrode potential: 1.2 V. Rotation rate: 100 rpm. Supporting electrolyte: 0.1 M HCIO, -t 0.1 M NaCI, saturated with 02. Dashed cwves: recorded as the dlsk potential was scanned at 10 mV s-'. %id curves: formed by connecting the points for initiii steady-state currents measured at the corresponding potential with a freshly coated electrode.
where the catalysts are reduced in the absence of dioxygen speaks for an extremely rapid consumption of the reduced catalyst by its reaction with dioxygen and/or by subsequent reactions.20 In contrast with the stable responses observed in the absence of dioxygen, the peak currents for the catalyzed dioxygen reduction decreased upon repeated scanning. After 50 scans at 20 mV s-l the peak current was approximately one-half as great as the initial peak current for FePPIX and the decay was even faster with the other two catalysts. Visual inspection of the less active electrodes revealed a detectable color change of the catalyst coating. If the electrode was transferred to pure supporting electrolyte and rotated rapidly (8000 rpm) for a few minutes, the outermost layer of the less active catalyst appeared to dissolve and the electrode was temporarily restored to its full activity toward dioxygen reduction. The deactivation seems to result from the formation of a slowly soluble film of degraded catalyst of unknown composition. The time dependence of the catalytic responses produced by this catalyst degradation forced us to prepare fresh electrode coatings for each experiment and to discard the electrodes as soon as they showed evidence of degradation. Rotating Ring-Disk Experiments. The course followed by the dioxygen reduction reaction at the graphite disk electrode coated with catalyst becomes clear when a platinum ring electrode is used to detect any hydrogen peroxide formed a t the disk.lJ3 Figure 2 shows a set of current potential curves recorded with a rotating graphite disk-platinum ring electrode. The large anodic ring cur(20)
C.P.Andrieux and J. M. Saveant, J. Electroanol. Chem., 93,163
(1978).
20
40
80
60
I '0
Flgure 3. Levich plots of the limiting current for reduction of O2at rotating graphite disk electrodes coated with the iron porphyrin catalysts. Supporting electrolyte: 0.1 M HClO, 0.1 MNaCO, saturated with OF The dashed line glves the calculated response for the fourelectron, mass-transfer-ilmited reduction.
+
rents that flow as the reduction of O2 commences demonstrate the formation of HzOz. However, with FeTPP and FePPIX the ring currents pass through a maximum and decrease again to negligible values as the disk potential becomes more negative, so the production of H,02 is clearly potential dependent. The slow degradation of the catalyst coatings mentioned above is also evident in the ring-disk responses in the region of the plateau currents. The dashed lines represent the ring and disk currents obtained when the disk potential was scanned continously at 10 mV s-l. The plotted points (connected by the solid line) represent the currents measured when a freshly coated disk electrode was stepped to each potential and the current recorded as soon as it became steady (a few seconds). All quantitative comparisons and calculations in what follows are based on such initial, steady-state currents with freshly coated electrodes. The ring-disk response obtained when the graphite disk is coated with FemTPP (Figure 2A) shows two steps in the catalyzed reduction of dioxygen. The first step produces hydrogen peroxide, as is evident from the ring current response, but the reduction proceeds all the way to water when the disk potential reaches -0.1 V. At low electrode rotation rates the magnitude of the disk current on the second plateau is fairly close to that expected for the four-electron, mass-transfer-limited reduction of dioxygen. However, the currents fall below the expected values at higher rotation rates, as is shown in the Levich plots in Figure 3. Ring-disk measurements at the same rotation rates showed that no hydrogen peroxide formation accompanied the increasing deviation from the calculated linear Levich behavior so that the cause of the smaller currents was not the result of a change in the net reaction stoichiometry. A slow chemical reaction preceding the electron-transfer step is implicated by these results. The behavior of the FemPPIX catalyst (Figure 2B) is similar to that of FeInTPP except that the two reduction waves fall at potentials too close together for separate steps to appear in the disk current-potential curves. However, the ring response shows clearly that hydrogen peroxide is
Electrocatalytlc Activlty of Three Iron Porphyrins
The Journal of Physical Chemistry, Vol. 86, No. 14, 1982 2779
NE
a
E -
3
rFePPIX, mole cm.' Flgure 4. Variation in limiting disk currents for the reduction of O2 at a rotated graphite disk electrode coated with increasing quantities of FePPIX. Supporting electrolyte: 0.1 M HCIO, 0.1 M NaCIO, saturated with 02.
+
formed on the rising part of the wave but not on the plateau which commences at about the same potential for both FemPPIX and FemTPP. Figure 3 shows that limiting currents that deviate from the Levich line are also obtained with FePPIX. Figure 2C shows the contrasting behavior exhibited by the FeInTPyP catalyst: Hydrogen peroxide is detected at the ring at all potentials where dioxygen is reduced. However, the magnitude of the limiting disk current is somewhat larger than what would be expected if the formation of hydrogen peroxide were quantitative. Thus, this catalyst appears to yield mixed reaction pathways. Assuming that hydrogen peroxide and water are the only possible reduction products, the fraction of hydrogen peroxide in the products is given by 2iR/(NiD+ iR)where iR and iD are the ring and disk currents and N is the collection efficiency of the ring-disk e1e~trode.l~At a rotation rate, w, of 100 rpm the measured value of N was 0.27,l' from which the fraction of the dioxygen that was reduced to hydrogen peroxide was calculated to be 0.41. The limiting disk currents for coated electrodes depend on the quantity of catalyst present in the coating as shown in Figure 4 for FeII'PPIX. The current is maximal when rFereaches 8 X mol cm-2 based on the geometric electrode area and becomes independent of the catalyst loading a t higher values. The surface roughness of the basal plane pyrolytic graphite is unlikely to exceed 2-3 so that 8 X mol cm-2 would correspond to several equivalent monolayers of catalyst (a monolayer of porphyrin adsorbed in a flat configuration requires ca. 1 X 10-lo mol cm-2). In solution, the porphyrins are known to associate into oligomers,21and, if the same were true on the surface, this could account for the behavior shown in Figure 4. The small maxima may mark the point where the entire area of the electrode becomes coated with catalyst. At higher loadings the "hills and valleys" of the surface may become leveled by the deposition of multiple layers of catalyst so that the catalytically effective area is decreased and lower currents result. Once about 8 X mol of porphyrin is on the electrode surface, additional catalyst produces no further increase in the current even at electrode rotation rates (e.g., 900 rpm) where the current is well below the mass-transferlimited value. This result makes it clear that only the (21) W. I. White in "The Porphyrins", Vol. V, D. Dolphin, Ed., Academic Press, New York, 1978.
3. IO
0.05
w-112,
(rpmjl/2
Flgure 5. Koutecky-Levich plots for the reduction of O2at rotating graphite disk electrodes coated with iron porphyrin catalysts. Experimental conditions and symbols as in Figure 3. The dashed line is calculated for a four-electron, mass-transfer-limitedreduction.
outermost portions of the catalyst coatings participate in the catalysis and that electron transfer from the underlying electrode to the active portion of the catalyst coating proceeds much more rapidly than the catalyzed reaction (otherwise smaller currents would result with the thicker coatings). Analysis of the Kinetic Current. Diagnosis of the kinetics of the chemical step responsible for the nonlinearity of the plots in Figure 3 is facilitated by means of Koutecky-Levich plots of (limiting current)-l vs. (rotation (ref 16 and 22) as shown in Figure 5. The straight lines obey the equation (1) l / i l h = l / i l e v+ l / i k where ilev is the Levich current16s22and ik is the kinetically limited current. ilev= n F A C o ~ o , 2 / 3 v - 1 / 6 ~ 1 / 2
(2)
where Co and Do2are respectively the bulk concentration and the diffusion coefficient of dioxygen, n is the number of electron involved in the reduction of dioxygen, and the other terms have their usual significance.22 For FeTPP and FePPIX the slopes of the lines in Figure 5 match that of the dashed line corresponding to n = 4 as would be expected from the absence of ring current in the experiments shown in Figure 2, A and B. The slope of the line for FeTPyP corresponds to a reduction involving 3.2 electrons in accord with the results quoted earlier on the basis of the ratio of the ring and disk currents at w = 100 rpm. This result indicates that H202and H20 are produced at electrodes coated with FeTPyP in about the same proportion at all of the electrode rotation rates investigated. The intercepts of the Koutecky-Levich lines in Figure 5 provide the values of ik for each catalyst. ik is the steady current that would flow if the concentration of O2 at the electrode surface could be maintained equal to its value in the bulk of the solution during its reduction. The values of ik are, therefore, direct measures of the maximum rates at which the O2 present can be reduced at the catalystcoated electrodes. Catalyzed Reduction of Hydrogen Peroxide. The absence of anodic ring current during the reduction of O2at the disk electrode on the plateau of two of the curves in (22) V. G . Levich, 'Physicochemical Hydrodynamics",Prentice-Hd, Englewood Cliffs,NJ, 1972;J. Koutecky and V. G. Levich, Zh. Fiz. Khim., 32, 1565 (1956).
2780
The Journal of phvsical Chemistry, Vol. 86, No. 14, 1982
Shigehara and Anson
TABLE 11: Kinetic Data for Catalyzed Reduction of Dioxygen and Hydrogen Peroxide at Porphyrin-Coated Graphite Rotating Disk Electrodes in Acidic ElectrolytesQ porphyrinC
*
,3ISK .
ik,f mA
Ef,d V
E,/,,e v
FePPIX FeTPP FeTPyP
-0 12 -0.1 +0.04
FePPIX FeTPP FeTPyP
-0.12 -0.1 -0.04
cm-,
Reduction of 0, 0.02 46(9.7h) 0.13,-0.04' 10 0.11 -2.8
Reduction of H,O,J -0.03 16.7 -0.04 9.3 0.11 0.48
nappg
R I W
0
o-.
+
-.
d
* IRING!
3.8 3.7 -3
0.4
02
0
-0.2
0 P
-06
- 0.4
EDISKv s SSCE, voll
Figure 6. Current-potential curves during the reduction of 1 mM H202 at a rotated graphlte disk-carbon-paste ring electrode. The disk (0.34 cm? was coated with 6 X 10" mol cm-* of FePPIX and the ring was dipcoated with a cobalt porphyrin catalyst (see text). Ring electrode potential: -0.3 V. Rotation rate: 100 rpm. Other condftions as in Figure 2.
2.0 1.9 1.9
Supporting electrolyte: 0.1 M HC10, + 0 . 1 M NaC10,. ElecDioxygen-saturated solutions; CO, = 1 . 3 mM. mol cm-, of porphyrin. trodes were coated with 6 x Formal potential of the adsorbed porphyrin taken as the average of the anodic and cathodic peak potentials in the absence of 0, (Figure 1). e Half-wave potential of the catalyzed reduction wave at w = 100 rpm. f Kinetic current evaluated from the intercepts of Koutecky-Levich plots. R Apparent number of electrons consumed in the reduction as determined from the slope of the KouteckyLevich plot. I/ Kinetic current in air-saturated solution; CO, = 0.26 mM. Two waves present. Concentration of H,O, = 1 . 0 mM.
Figure 2 could signal that H202is not an intermediate in the reduction at these disk potentials, or it could be the result of the rapid, further reduction or disproportionation of H202that is formed as an intermediate. In an attempt to remove this ambiguity, the behavior of H202 at rotating ring-disk electrodes was examined in solutions free of 02. The ring electrode for these experiments was carbon paste that had been coated with a cobalt porphyrin (see Experimental Section) that is a good catalyst for the reduction of dioxygen to hydrogen peroxide at the potential (-0.3 V) where the ring electrode was held. (A platinum ring was less satisfactory because it tended to catalyze the disproportionation of hydrogen peroxide.) The three iron porphyrin catalysts were adsorbed on the disk electrode where they exhibited similar behavior. Figure 6, recorded with a FePPIX coating, is also representative of the behavior of FeTPP. Considerably smaller limiting disk currents were obtained with FeTPyP coatings. Uncoated electrodes were essentially inert toward the reduction of peroxide so that all three porphyrins are clearly catalytically active for this reaction. The absence of cathodic ring current at disk potentials ahead of the H202 reduction wave in Figure 6 shows that Fe"'PP1X (the other porphyrins behave similarly) does not catalyze the disproportionation of H202at significant rates. The potentials where the catalyzed reduction of H202 commenced matched those where the anodic ring currents in Figure 2, A and B, began to decrease. Thus, the potential dependence of the ring currents in Figure 2, A and B, can
Figure 7. Formal potentials for adsorbed FePPIX as a function of pH. The potentials are the average of the cathodic and anodic peak potentials measured in dioxygen-free solutions at a scan rate of 20 mV S-1.
be explained by the reduction of O2commencing at disk potentials where H202is not reducible. A t more negative potentials H202 produced at the disk can be rapidly further reduced there so that it never reaches the ring electrode. Analysis of the kinetics of the catalyzed reduction of Hz02was also performed by means of Koutecky-Levich plots similar to Figure 5. For all three catalysts straight lines resulted with slopes corresponding to a two-electron reduction and intercepts attributable to a rate-limiting chemical step preceding electron transfer. The data obtained are summarized in Tables I1 and 111. p H Dependence of the Catalysis. The peak potentials of the adsorbed catalysts move to more negative values with increasing pH. For example, Figure 7 shows the dependence of the formal potential of adsorbed FePPIX
TABLE 111:" pH Dependenceb of the Catalyzed Reduction of Dioxygen and Hydrogen Peroxide reduction of 0, porphyrin
PH
Ef,V
E,,,, V
ik, mA cm-,
reduction of H,O,
napp
12 -0.68 -0.48 C 4 9.8 -0.54 -0.38 C 4 C 3.9 7.0 -0.37 -0.29 5.0 -0.32 -0.25 39 3.8 3.2 -0.17 -0.05 39 3.8 FeTPP 12 -0.65 - 0.37 27 4 FeTPyP 12 -0.65 -0.37 9 4 See Table I1 for definitions and experimental conditions. Supporting electrolyte Experimental Section. Intercept of Koutecky-Levich plot too small to measure.
FePPIX
E,,,, V
ik, mA cm-,
napp
-0.38 C 2 -0.36 C 2 - 0.32 C 1.9 2 -0.26 17 -0.11 17 1.9 - 0.39 12 1.9 -0.39 7 2 buffer compositions are given in the
The Journal of Physical Chemistry, Vol. 86, No. 14, 1982 2781
Electrocatalytic Activity of Three Iron Porphyrins
+
- _ ..=__ -....-.--..
Jf-7 IDISK
~
,:-j/ _ _ e - -
A -
'DiSK
X I
=o+
8
c
'RING
IRING
125P*
'0
'2.5~'
I2
+
t
. I
0
-0.2
-0.4
E,,S,
-0.6
v s SSCE, bolt
Flgure 0, Repetition of the experiment of Figure 6 at pH 8 (outer curves and pH 12 (inner curves). Rotation rate: 100 rpm. Ring electrode potential: -0.5 V. Other conditions as in Figure 6. 1
,
1
,
36 C 4 C 2
,
0 - C 2 0 4 - 0 6 - 0 8C 4 0 2
E vs SSCE volt
0 -02-04-060 8 - I 3
E vs SSCE, volt
Flgure 8. Cyclic voltammograms for adsorbed FePPIX in the absence (solid curves) and in the presence (dashed curves) of O2 (saturated) in NaCIO, solutions buffered at various pH values. The graphite electrode (0.17 cm2) was coated with 6 X 10" mol cm-' of FePPIX. Scan rate: 20 mV s-I.
as a function of pH. In the two pH ranges where the formal potential changes the most, the slope is ca. 60 mV per pH unit, indicating that one proton is consumed when the Fe(II1) center is reduced to Fe(I1) as would be true if the electrode reaction were Fe"'PPIX(0H)
+ e + H+ +=FeIIPPIX + H 2 0
(3)
between ca. pH 2.5 and 5.5 and Fe111PPIX(OH)2+ e
+ H+
Fe"PPIX(0H)
+ H20 (4)
a t pH values above ca. 8.5. The potential at which the catalyzed dioxygen reduction proceeds also moves to more negative values as the pH increases (Figure €9, but the reaction occurs a t more positive potentials than those where the catalyst is reduced at all pH values. Rotating ring-disk measurements were less useful at pH values above 5 because of problems with the detection of H202at the platinum ring in neutral and alkaline solutions. However, Koutecky-Levich plots of the disk limiting currents were linear with slopes indicating that the reduction of dioxygen was consuming four electrons. Kinetic currents for the catalyzed reduction were also evaluated from the intercepts of the plots except at pH values where the intercepts became too small to measure. The data are summarized in Table I11 along with analagous data for the catalyzed reduction of H202at several pH values. The disproportionation of H202is known to be catalyzed by iron porphyrin^,^^ and this has been proposed as a possible step in the mechanism by which they catalyze the reduction of dioxygen." At pH 1we have already shown that the disproportionation of H202a t a rotating disk coated with FePPIX is too slow for O2 to be detected at the ring electrode (Figure 6), but, since the maximum rate of disproportionation occurs near pH 8,% we repeated the experiment of Figure 6 at this pH as well as at pH 12. The resulh are shown in Figure 9. Dioxygen is detected at the ring electrode at potentials before the H202is reduced at both pH values, but the ring currents are much smaller than the value calculated from the known collection ef-
ficiency and the limiting disk current for the reduction of H20z. Thus, the catalyzed reduction of H202proceeds much more rapidly at potentials on the plateau of the wave than does its catalyzed disproportionation at potentials ahead of the wave. However, this observation does not rule out disproportionation (catalyzed by the reduced porphyrin) as a step in the catalyzed reduction of H202because, at potentials where the porphyrin is in its reduced form, the catalyzed reduction of O2 proceeds rapidly (Tables I1 and 111).
Discussion Kinetics and Mechanism of the Catalyzed Reductions. The voltammetric data provide two features that must be accommodated by any mechanism proposed for the catalytic mechanisms: (i) At all pH values dioxygen is reduced at potentials considerably more positive than those where the adsorbed iron porphyrins are reduced in the absence of dioxygen. (ii) The positive intercepts of the Koutecky-Levich ploh point to a chemical rate-determining step at high rotation rates. One possible scheme that would apply at potentials where H202is the reduction product (i.e., on the initial, rising portion of the reduction waves) is given in Scheme I, where PFe represents the iron Scheme I PFe"'
+ O2 PFe"'(02-) + 2H+ + e- PFe"' + H202
PFe" PFe"'(O2-)
-
(5) (6)
(7)
porphyrin catalyst. Half-reaction 5 cannot proceed extensively to the right at the potentials where the catalyzed reduction is observed so that writing it as the first step implies that the reduced porphyrin is an extremely potent catalyst that reacts with O2with a very high specific rate. Scheme I proposes the superoxide complex of the oxidized porphyrin, PFe1"(02-), as the product of the reaction. In nonaqueous solvents such complexes are formed at low temperature by both FemPPIX25and FemTPP,26but the complexes decompose into Fe"P and O2 at room temperature. In aqueous solution at pH 10 iron(II1) meso-tetra(N-methylpyridyl)porphine,FemTMPyP, reacts rapidly with 02-to yield O2and F~"TMPYP,~' which is in accord with the relative redox potentials of the 02/02-and Fem/"TMPyP couples at this pH (-0.5828and -0.20 V6 vs. (25)H. A. 0.Hill and D. R. Turner, Biochem. Biophys. . . Res. Commun., 66,739 (1974). (26)E.McCandlish, A. R. Miksztal, M. Nappa, A. Q. Sprenger, J. S. Valentine. J. D. Strone. and T. G. SDiro. * . J . Am. Chem. Soc.. 102,4268 (1980). (27)R.F.Pasternack and B. Halliwell, J.Am. Chem. Soc., 101,1026 (1979). I .
(23)M. L.Kremer,Nature (London),206,384(1965);Trans. Faraday Soc., 61, 1453 (1965);R.Gatt and M. L. Kremer, ibid. 64,721 (1968). (24)P. Waldmeier and H. Siegel, Inorg. Chim. Acta, 6 , 659 (1971).
+ e + PFe"
2782
Shigehara and Anson
The Journal of Physical Chemistry, Vol. 86, No. 14, 1982
SCE,respectively). Thus, for this porphyrin, the equiand the aflibrium constant of reaction 8 is ca. 2 X Fe"TMPyP
+ O2 t FeII'TMPyP + 02-
(8)
finity of FemTMPyP for Of is apparently not great enough to cause the two reactants to remain associated as proposed in reaction 6. However, with porphyrins that are adsorbed on the electrode surface, Scheme I could apply even when the equilibrium constant for reaction 6 was much smaller than unity so long as the combination of steps depicted as reaction 7 rapidly consumed the small amount of Fen1P(02-)that was formed. If reaction 7 were rapid ennough, it would be possible for O2reduction to proceed at potentials considerably more positive than the formal potential of reduction 5, as observed. Under these conditions the kinetic current, ik, obtained from the intercepts of the Koutecky-Levich plots for solutions of O2would be determined by the forward rate of the chemical step in Scheme I, reaction 6 . One difficulty with this interpretation is that the rate constant for reaction 6 in the forward direction would have to be close to the diffusion-limited value in order to account for the relatively large difference between the potential where the catalyzed reduction proceeds and the formal potential of the catalyst. This is incompatible with an equilibrium constant for reaction 6 as small as 2 X as well as with the magnitudes of the measured kinetic currents (Table II).= An equilibrium constant for reaction 6 that is larger for adsorbed FePPIX than for the soluble porphyrin of ref 27 might account for part of the apparent discrepancy, but the origin of the mismatch between the observed values of AB for these catalysts and those calculated from the measured values of i k remains to be established. The rate of the catalyzed reduction of O2 is greater at pH 12 than pH 1 (Tables 11and 111),but the available data at intermediate pH values are inadequate to permit the origin of the pH dependence to be identified unambiguously. At potentials on the plateau of current-potential curves such as those in Figure 2, the catalyzed reduction of O2 proceeds all the way to H 2 0 because H202is also catalytically reduced at these potentials (Figures 6 and 9). As with O2 reduction, the half-wave potentials of the catalyzed reduction waves for H202are much more positive than the formal potential of the catalyst couple so that very rapid reactions between the catalysts and H202are implicated. The difference in half-wave potentials persists in acidic electrolytes where the concentration of HOC, the species most likely to coordinate to the metal center in the porphyrins, is extremely low (pKAfor H202is 11.6). This suggests that the catalysis may be initiated by a rapid, outer-sphere reaction between the reduced porphyrin and H202 An alternative first step in which H202coordinates to the unreduced porphyrin5 seems less likely in view of the pH dependence of the half-wave potential for the H202 reduction wave (Table 111). El12becomes more negative within the range of pH values where the concentration of H202is not changing with pH. (Note that the pH dependence of the formal potential of FePPIX would not be expected to affect El12for the reduction of H202coordi(28) Y . A. Ilan, G. Zapski, and D. Musel, Biochem. Biophys. Acta, 420, 209 (1976). (29) The simplest kinetic models predict that the difference, AE,between the half-wave potentials of the catalyzed reaction and the formal potential of the catalyst will be given by (30) AE = (RT/F)In (1 ik/ih). For FePPIX at pH 1, AI3 = 140 m V while (RT/F)In (1 + ik/il)lsv) is 91 mV. (30) R. D. Rocklin and R. W. Murray, J.Phys. Chem., 85,2104 (1981).
+
nated to the Fe"' center at potentials where the latter is not reduced.) In neutral and acidic solutions all of the iron porphyrins studied catalyze the reduction of H202only at potentials where O2 is reduced. This suggests that the H202reduction results from its porphyrin-catalyzed disproportionation via a mechanism that commences with reaction 5 and is followed by reactions 9-11. The resulting PFe" H202 H+ PFe"' OH- H20 (9)
+
+
--
OH.+ H202
+ + + H2O
HOP H202 + 0
(10) 2H02. 2 (11) O2 could then be reduced via Scheme I. Reactions 9-11 are all highly favored thermodynamically with equilibrium and respecconstants of ca. 1O'O (for FePPIX), tively. The pH dependence of El12for the H202reduction wave could then arise from a combination of the effect of pH on Ef for reaction 5 and the rate of reaction 9. At pH values above 10 where the reduction of H202 proceeds at potentials somewhat more positive than those where O2 is reduced, the catalytic mechanism may involve reaction between the porphyrins and HOz- whose concentration becomes more significant at these higher pH values. Relative Catalytic Potency. To proceed from the experimental values of ik to rate constants for the currentlimiting reaction, a kinetic model must be selected from among a large number of po~sibilities.~'For our present purposes we have chosen to avoid this source of uncertainty by comparing the catalysts in terms of the kinetic currents which they can sustain without attempting to derive rate constants from the values of ik. On this basis, the order of catalytic potency of the three iron porphyrins studied is FePPIX > FeTPP > FeTPyP. This is also the order of the reducing strength of the adsorbed Fe" porphyrins (see E f values in Table 11). If reaction 6 , which involves the reduction of O2 by the Fe" center, is indeed the rate-limiting step, it is not surprising to find a correlation between the potency of the catalysts and their reducing strengths. Although two of the iron porphyrin catalysts studied here produce an overall four-electron reduction of 02,the rotating ring-disk electrode data make it clear that the reduction proceeds via H202as an intermediate at potentials on the rising part of the current-potential curves. This contrasts with the behavior of the most potent of the dicobalt cofacial porphyrin catalysts which achieves the four-electron reduction without the formation of H202.' This catalyst is also active at much more positive potentials at all pH values.'J2 Mononuclear cobalt porphyrin catalysts usually lead to H202as the O2reduction product, often quantitatively.lp3J2 The ability of the iron porphyrins to effect the further electroreduction of H202while mononuclear cobalt porphyrins apparently do not seem likely to be the result of the significantly more negative formal potentials of the iron porphyrins. Reaction 9 has a large favorable driving force in the case of iron porphyrins, but the equilibrium constant is less than unity for most cobalt porphyrins. There is little doubt that a number of other factors may also be involved in controlling the kinetics and mechanism, but the difference in formal potentials seems too large not to be important. Comparison with Previous Studies. Behret et al.5 have studied dioxygen reduction as catalyzed by FeTPP adsorbed on pyrolytic graphite electrodes in both H2S04and (31) C. P. Andrieux, J. M. Dumas-Bouchiat,and J. M. Saveant, J. ElectroanaL Chem., 131, 1 (1982).
Electrocatalytic Activity of Three Iron Porphyrins
KOH solutions. Their rotating ring-disk results are similar in some respects to those reported here including the maximum in the ring current response at potentials on the rising portion of the disk response (Figure 2). However, in contrast with the present results, quantitative reduction of dioxygen to water was not achieved at any potential and no deviations from linear Levich plots were observed at rotation rates as high as 4000 rpm. The reasons for these differences may be that we employed rather thick coatings of FeTPP (the quantities of catalyst employed and the details of the coating procedures used by Behret et al.S are not reported by them) that assured complete coverage of the pyrolytic graphite. With imperfect catalyst coatings the uncatalyzed reduction of O2 to H20 can occur on the uncovered portion of the pyrolytic graphite at potentials on the plateau of the wave for the catalyzed reduction. Kuwana, Osa, and co-workers6" have investigated the catalysis of the electroreduction of O2 by several watersoluble iron porphyrins. Hydrogen peroxide was initially reported to be the reduction product,610 but more re~ e n t l y ' ' *conditions ~~ were described under which both H202and H20 are produced. When iron porphyrins were attached to the surface of glassy carbon electrodes coated with poly(methacry1 chloride), the quantitative reduction of O2to HzO was achieved with high concentrations of one attached catalyst, but with lower quantities of a similar catalyst only Hz02was produced.l' The potentials where the catalyzed reaction proceeded at the coated electrode were significantly more positive than the potentials where the catalysts were reduced in the absence of OF The latter feature matches the behavior observed in our experiments, but it differs from that reported for the water-soluble iron We believe that this difference is significant and that it emphasizes one essential difference between soluble and adsorbed catalysts for the electroreduction of dioxygen: With soluble catalysts partially reduced intermediates formed at low concentrations by an encounter with a molecule of reduced catalyst (e.g., reaction 6) can be reduced further only in one-electron steps upon successive encounters. However, when the catalyst is present only on the electrode surface, electrons are immediately available for delivery (at appropriate potentials) to an initially formed intermediate no matter how low its concentration. If the electrode potential is such that reduction by three additional electrons is energetically favorable, this process can occur so rapidly that the metastable product, H202,is reduced before it escapes to the bulk solution. With soluble catalysts peroxide can escape from the coordination spheres of the iron porphyrin where it was formed before it can be further reduced and H202can thereby become the primary reduction product. This difference may well be the reason that H202is the product obtained with most soluble catalysts while some attached catalysts yield HzO with little or no Hz02. It may be worth emphasizing an inherent difficulty in comparisons of the kinetic responses of attached and dissolved porphyrins: Demonstrating the absence of adsorbed po;ph;?in in experiments with solutions of water soluble porphyrins is often not straightforward. Thus, the absence of adsorption is argued in ref 10 to be established (32) P. A. Forshey and T. Kuwana,preprint.
The Journal of Physical Chemistry, Vol. 86, No. 14, 1982 2703
by the linearity of plots of peak currents vs. (scan rate)'j2 for solutions of the porphyrin in the absence of 02. However, at the highest scan rate employed in this test (250 mV s-l) the measured peak current due to reduction of the dissolved, diffusing porphyrin is over 50 times greater than the maximum current that would result from e.g., 2 X lo-" mol cm-2 of adsorbed porphyrin-more than enough to make significant contributions to the catalytic current in the presence of 02.Under first-order limiting conditions such as those employed in ref 10 the ratio of the kinetic current contributed by adsorbed catalyst, iad, to that contributed by the soluble catalyst, is, can be shown to be given by eq 12, where k is the rate constant (cm3
iad/is = k1/2rcat/(Do11/2Cca~/2) (12) mol-' s-l) of the catalytic reaction, rat is the quantity of adsorbed catalyst, Do is the diffusion coefficient of dioxygen (2.6 X 10" cmj s-'), and Ccatis the concentration of soluble catalyst (mol cm-9. Thus, the larger the rate constant, k , the less negligible will be contributions from any adsorbed catalyst. For example, with the value of k = 4 X 1O'O cm3 mol s-' estimated in ref 6 and 10, almost 30% of the catalytic current is calculated to result from the reaction of O2 with the adsorbed catalyst even if Ceat were as large as lo+ mol cm-3 and rcat were as small as 1 X lo-'' mol cm-2 (i.e., near the limit of detectability). This calculation makes clear how difficult it is to eliminate heterogeneous catalytic pathways in solutions of soluble catalysts that may nevertheless be weakly adsorbed. One advantage of using insoluble catalysts that are present only on the electrode surface is the unambiguous elimination of kinetic pathways involving dissolved catalyst although other interpretive difficulties arise if more than one effective monolayer of catalyst is p r e ~ e n t . ~ '
Conclusions Thick coatings of FePPIX and FeTPP on graphite electrodes provide effective catalysis of the reductions of O2 and H202to H20 over a wide pH range. FeTPyP coatings are somewhat less active. Only the outer portions of the catalyst coatings participate in the catalysis. Possible catalytic mechanisms for the reduction of O2 and H202both involve an initial, rate-limiting reaction between the substrate and the reduced form of the adsorbed porphyrin. In the case of O2 reduction, the product of the initial reaction undergoes further rapid reduction by electrons from the electrode. High specific rates are required to account for the catalyzed reaction's proceeding at potentials that are considerably more positive than those where the catalysts are reduced in the absence of O2or H202and some interpretive ambiguities remain. Differences in the products resulting from the catalysis of the reduction of O2by adsorbed and dissolved iron porphyrins may result from the more rapid electroreduction of intermediates generated on the electrode surface when adsorbed catalysts are utilized. Acknowledgment. Helpful discussions with Richard Durand are a pleasure to acknowledge. We appreciated receiving several manuscripts from Professor Theodore Kuwana in advance of their publication. This work was supported by the National Science Foundation.
