59 1
Anal. Chem. 1985, 57,591-595 (21) Galvez, J.; Moiina. A.; Serna, C. J . Electfoanal. Chem. 1981, 121, 85. (22) Galvez, J.; Molina, A.; Serna, C. J. Nectfoanal. Chem. 1881, 124, 201. (23) Ferrier, R. D.; Schroeder, R. R. J. Electroanal. Chem. 1973, 45, 343. (24) Heyrovsky, J.; Kuta, J. “Principles of Polarography”; Academic Press: New York, 1966; p 542. (25) Kouteckv, J. Czech. J. Phys. 1953, 2, 50. (26) Heyrovsky, J.; Kuta, J. “Principles of Polarography”; Academlc Press: New York, 1966; p 137.
(27) Salto, A.; Himeno, S. J. Electroanal. Chem. 1979, 101, 257. (28) Heyrovsky, J.; Kuta, J. “Principles of Polarography”; Academic Press: New York, 1966; pp 106-107. (29) Galvez, J. Doctoral Dissertation, Universidad de Murcia, 1973.
RECEIVED for review July 18, 1984. Accepted November 5, 1984* We thank the Cornision Asesora de InvestigaciBn Cientifica y TBcnica for supporting this study.
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Electrocatalytic and Analytical Response of Cobalt Phthalocyanine Containing Carbon Paste Electrodes toward Sulfhydryl Compounds Mary Kim Halbert and Richard P. Baldwin* Department of Chemistry, University of Louisville, Louisville, Kentucky 40292
Chemlcally modified electrodes, constructed by incorporating cobalt phthaiocyanlne (CoPC) Into the graphlte powder/Nujol oil matrix used to fabrlcate conventional carbon paste electrodes, were shown to catalyze the electrooxldatlon of sulfhydryl-contalning compounds inciudlng cystelne, homocysteine, N-acetylcysteine, and glutathione. The sulfhydryl oxidation occurred at 0.75-0.85 V vs. Ag/AgCI, the same potentlai observed for the CoPC oxldatlon both in solutlotl and after addltion to the carbon paste. When used as the sensing electrodes in amperometric detection following iiquld chromatography, the CoPC electrodes permltted detection of the compounds at similar values of applied potentlai. Detection iimlts of 2.7 pmol were obtained for cysteine, N-acetylcystelne, and homocystelne; and electrode response was linear up to 270 pmoi injected. Different carbon paste surfaces renewed in the standard manner yielded chromatographic peak currents with a reproducibility of 7 %.
In recent years, electrodes possessing specific chemical functionalities intentionally linked to their surface have been demonstrated to possess distinct advantages over conventional electrode substrates in numerous application areas including electrosynthesis, electrocatalysis, and photoelectrochemistry ( I , 2). In principle, such chemically modified electrodes (CMEs) should also be able to provide enhanced performance in the area of electroanalysis as well. However, to date, instances involving the utilization of CMEs for such purposes have not been extensively documented despite the potential advantages which might be expected to accrue. One of the important properties of CMEs which has been the object of considerable study has been their ability to catalyze the oxidation or reduction of solute species which exhibit high overvoltages at unmodified surfaces and consequently are not ideally suited to quantitative determination via conventional electrochemical approaches. Since the major effect of such CME electrocatalysis consists of the lowering of the potential required for the electrolysis of the catalyzed redox system, these electrodes should possess direct application in a variety of analytical situations. In particular, their use should greatly improve the capabilities of electrochemical detection techniques in liquid chromatography (LCEC) where
sensitivity and selectivity are directly dependent on the magnitude of the electrode potential required to produce the desired electrode reaction (3). In previous work in our laboratory, CMEs formed both by electrochemicalpretreatment of glassy carbon ( 4 , 5 )and by the addition of cobalt phthalocyanines to ordinary carbon paste (6) have been shown to enhance significantly the LCEC determination of hydrazine compounds-a family of compounds which can generally be oxidized only at relatively high, positive potentials greater than +1 V vs. Ag/AgCl. However, when either of the abovementioned CMEs was employed, the potential required for the oxidation was reduced by 0.5 to 1V, and detection limits obtained were some 2 to 3 orders of magnitude below those attainable a t unmodified electrodes even at considerably higher values of applied potential. In view of the well-known catalytic activity of metallophthalocyanines toward a wide variety of redox systems (7), we have continued to investigate the use of phthalocyaninecontaining carbon paste CMEs as analytical sensors in voltammetry and LCEC. In particular, we have considered the application of cobalt phthalocyanine (CoPC) electrodes for the electrocatalytic determination of sulfhydryl compounds. At most conventional electrodes, sulfhydryls exhibit irreversible oxidations requiring extreme positive potentials. Consequently, electrochemical detection of these species is usually performed at mercury or mercury amalgam electrodes at which the mercury sulfide species formed can be oxidized at comparatively modest values of applied potential (8-10). In this work, we report on an alternate approach for the detection of sulfhydryls at lowered potentials at CoPC-CMEs. The response of these electrodes toward a variety of sulfurcontaining species and their utility for the LCEC detection of cysteine and several related compounds are described.
EXPERIMENTAL SECTION Reagents. Cobalt phthalocyanine was obtained from Eastman Kodak Co. Cobalt phthalocyaninetetrasulfonate was prepared and purified according to the procedure of Weber and Busch (11).
Because the monosodium salt of 4-sulfophthalic acid required for the procedure could not be obtained commercially, it was prepared in our laboratory according to ref 12. With the exception of Captopril (provided by E. R. Squibb and Son, Inc.), all thiol compounds used in this work were obtained from Sigma Chemical Co. The sodium salt of 1-octanesulfonic acid, used as an ionpairing agent, was also obtained from Sigma. All reagents were
0003-2700/85/0357-0591$01.50/00 1985 American Chemlcai Soclety
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do
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1
Ole
0!4
0.6 Volts
vS
012
0
lo
Ag/AgCI
Flgure 1. Cyclic voltammograms of 1.0 X M cobalt phthalocyaninetetrasulfonate at unmodified carbon paste electrodes: (a) in 0.05 M H2S0,; (b) same as (a) but with 1.0 mM cysteine added; (c) same as (a) but with 2.0 mM cysteine added. Scan rate was 100
mVls.
used as received without further purification. Working Electrodes. Carbon paste for unmodified electrodes was made by thoroughly hand mixing 5 g of graphite powder (Fisher ScientificCo.) with 3 mL of Nujol oil (McCarthyScientific Co., Fullerton, CA) in a mortar and pestle. Modified carbon pastes were prepared similarly except that the appropriate weight of CoPC was first mixed with the graphite powder. The Nujol oil was then added and ground with the graphite mixture for 30 min. The concentration of the phthalocyanine in the CME was described on a percent basis as the weight of the CoPC added to the 5 g of graphite powder. Apparatus. Cyclic voltammetry was performed with a Bioanalytical Systems (W. Lafayette, IN) Model CV-1B potentiostat. A carbon paste working electrode (with or without CoPC),a saturated Ag/AgCl reference electrode,and a platinum wire counterelectrodewere used for all experiments. Flow injection analysis and liquid chromatography were performed with a Perkin-Elmer Series 10 pump, a Rheodyne (Berkeley,CA) Model 7125 injector with a 20-wL loop, and an IBM Model EC/230 amperometric detector. Chromatography was performed with an IBM 15-cm octadecylsilane column. RESULTS AND DISCUSSION Solution-Phase Catalysis. Among the compounds whose redox reactions have been shown to be catalyzed by metallophthalocyanines are cysteine and several other sulfhydryl-containing species (13-18). In view of the importance of these compounds in a number of biological, pharmaceutical, and environmental areas, we chose to investigate the application of CoPC-CMEs for their determination. We began by examining the electrocatalysis of cysteine by solution-phase CoPC. In order to obtain a phthalocyanine possessing sufficient solubility in aqueous solution to be of use, the tetrasulfonate derivative of CoPC was synthesized as described in the Experimental Section. All subsequently described solution-phase studies were conducted with this CoPC derivative. Previous voltammetric studies of the tetrasulfonate derivative of CoPC at platinum electrodes have reported the occurrence of two anodic processes for the compound (12,191. The first, a t lower potentials, was assigned to the Co(I1)Co(II1) oxidation while the higher potential process consisted of a degradative ring oxidation. A cyclic voltammogram (CV) obtained in our laboratory at a conventional carbon paste electrode for an aqueous solution of the same compound is shown for comparison in Figure 1 (curve a). Under the conditions employed, two anodic waves were observed. Because the CV remained virtually unchanged on subsequent scans, neither of the oxidations apparently corresponded to the CoPC-degrading process. Thus, the reversible, higher potential wave at +0.93 V vs. Ag/AgCl presumably involved the Co(I1) oxidation. Since the oxidation a t +0.77 V continued to be observed even after removal of the electrode from the
phthalocyanine solution, thorough rinsing of the surface, and reimmersion into a blank solution containing no CoPC, it most likely corresponded to the same redox process for adsorbed CoPC. In addition, several small cathodic waves were observed a t less positive potentials on the reverse scan; although always comparatively small in amplitude, the specific occurrence of these reductions was a complicated function of the potential scan rate, the solution pH, and the past history of the electrode surface. When cysteine was added to the tetrasulfonated CoPC solution, the CVs shown in curves b and c of Figure 1 were obtained. The principal changes which resulted consisted of a marked increase in the current level associated with the CoPC oxidation waves and a complete absence of reduction current on the reverse scan. On subsequent cycles, the anodic currents between +0.7 V and + L O decreased in magnitude as the cysteine concentration became depleted near the electrode surface; and the voltammogram gradually started to resemble that seen above for the CoPC compound in the absence of cysteine. The initial increase in current was proportional to the concentration of cysteine added. In the absence of the phthalocyanine compound, no current for the direct electrooxidation of cysteine was observed in the potential range examined. The observations described above are essentially those which would be expected for a two-step electrocatalytic process initiated by the electrochemical oxidation of the cobalt(I1) phthalocyanine compound to a Co(II1) species and completed by the chemical oxidation of the cysteine in a redox reaction which also serves to regenerate the Co(I1) form of the phthalocyanine. Previous studies (13-18) of cysteine oxidation by CoPC compounds have indicated that, depending on solution pH, both the +2 and +3 states are capable of carrying out the sulfhydryl oxidation. However, under the acidic conditions employed in this work and necessary for the optimum chromatographic retention and separation of these zwitterionic species, the strongest and most consistently observed cysteine oxidation occurred in the potential region assigned to Co(II1)formation. When neutral or basic solutions were employed, increases in anodic current were also noted at lower potentials. This could be indicative of cysteine oxidation by CO" PC species in solution of higher pH. But it appeared in general that the higher oxidation state of the phthalocyanine, once produced electrochemically, was a much more effective electrocatalyst under the conditions employed. CME Electrocatalysis. In several previous studies, a variety of metallophthalocyanines have been attached both to graphite and to metal oxide surfaces by silane formation (19),vapor deposition (20),and direct adsorption (21). In our laboratory, we have recently shown that stable phthalocyanine CMEs can also be constructed simply by addition of the desired compound to the graphite powder/Nujol oil matrix used to fabricate conventional carbon paste electrodes (6). Because of their ease of construction and the capability for rapid, quantitatively reproducible surface renewal, the latter approach involving chemically modified carbon paste was chosen for use here. The CVs obtained for a CoPC-containing carbon paste CME immersed both in 0.05 M H2S04blank solution and in M cysteine are shown the same solution doped with 1 X in Figure 2. The CoPC-CME employed here and subsequently in this work represented a phthalocyanine doping level of 2% by weight; CMEs containing higher modifier concentrations were not found to yield significantly higher current levels and thus were not investigated further. By itself, the CoPC-CME exhibited only a single anodic wave when cycled in blank over positive potentials not exceeding + L O V vs. Ag/AgCl. This wave occurred at +.077 V vs. SCE and closely
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
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V \
/
1
V o l t s v s . Ag/AgCI
Flgure 2.
Cyclic voltammograms of (a) unmodified carbon paste electrode, (b) 2.0% CoPC carbon paste electrode, and (c) 1.0 mM cysteine at 2.0% CoPC carbon paste electrode. Dashed line (- - -) shows the voltammogram obtained for 1.0 mM cysteine at an unmodified electrode. Electrolyte was 0.05 M H,SO,; scan rate was 100 mV/s.
matched the adsorption wave observed for a CoPC solution in Figure 1. Several relatively small reductions were seen on the reverse scan, their relative intensity dependent on the potential scan rate employed and the positive potential limit of the scan. As with the solution-phaseCoPC tetrasulfonate system, the introduction of a sulfhydryl compound such as cysteine to this CME/solution system resulted in a marked increase in current at the Co(I1)-Co(II1) oxidation wave, greatly exceeding that observed either for the CoPC-CME in the absence of the sulfhydryl or for a plain carbon paste electrode exposed to the same sulfhydryl concentration. As before, the catalytic currents observed were proportional to the cysteine concentration; and no cathodic currents were seen on the reverse scan. Further, the currents due to the sulfhydryl oxidation were diffusion controlled, exhibiting a linear dependence on the square root of the scan rate for the scan rates as high as 600 mV/s. These observations are roughly the same as those of Zagal et al. (21)who reported a catalytic oxidation of cysteine via cobalt phthalocyaninetetrasulfonate adsorbed onto pyrolytic graphite. Analogous CV results were obtained at 2.0% CoPC electrodes for a series of sulfhydryl species including 2mercaptoethanol, 3-mercaptopropionic acid, the antitumor drug 6-mercaptopurine,the antihypertensive agent Captopril, penacillamine, homocysteine, N-acetylcysteine, and the cysteine-containing tripeptide glutathione. This is illustrated in Figure 3 for the last three compounds. Note that, unlike the voltammograms shown previously, these were run in pH 2.4 phosphate buffer. Because our eventual objective entailed the use of these CME systems in LCEC detection, it was appropriate to examine the electrocatalytic processes under less acidic solution conditions more directly compatible with liquid chromatographic instrumentation. In general, the affect of increasing the solution pH was a modest shift of the catalytic waves to lower potentials. For all the above compounds similar but not identical increases in anodic current were observed at the CME in the potential region of the CoPC oxidation. As with cysteine, the catalytic waves were irreversible with no cathodic current seen on the reverse scan. Further, although the anodic peak currents showed a linear dependence on the square root of the potential scan rate, the oxidation waves failed to exhibit the usual broad tailing characteristic of simple diffusion-limited processes; and the
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0,’8
0!6
014
0’2
O!O
Volts v a Ag/AgCI
Flgure 3. Cyclic voltammograms of (a) N-acetylcysteine, (b) homocysteine, and (c) glutathione at unmodified carbon paste (-- -) and 2.0% CoPC carbon paste (-) electrodes. All concentrations are 1.0 mM. Electrolyte was 0.05 M phosphate buffer (pH 2.43); scan rate was 30 mVls.
reverse scans sometimes became irregular in shape, showing ,unexpected anodic dips in current as the potential was cycled back in the cathodic direction. The reason for this nonideal behavior is not yet known and requires continued investigation. However, ongoing work in our laboratory involving the phthalocyanine-mediated oxidation of several organic compounds in addition to sulfhydryls indicates that such CV behavior is a generally observed phenomenon for CoPC electrocatalysis. A possible explanation for the adsorption-like CV behavior is that one step of the oxidation process consists of the slow or irreversible chelation of the thiol group to the phthalocyanine metal center. Some evidence for the involvement of such a complexation step in the homogeneous electrocatalytic process has been demonstrated in previous electrochemical and spectroscopic investigations (13-18). Other sulfur-containing compounds examined included cystine, methionine, and malathion. Of these, only cysteine showed enhanced anodic current at the CoPC-CMEs. For this compound, no oxidation was seen for the initial scan in the positive direction. However, when the potential was first scanned sufficiently far in the negative direction to reduce cysteine (presumably to the sulfhydryl form), subsequent scans to positive potentials exhibited current-voltage behavior similar to that observed for cysteine itself. LCEC. Despite some uncertainty concerning the catalysis mechanism, it was apparent that the response enhancement produced by the use of CoPC-containingCMEs should make their usage attractive in a number of analytical applications. One area where their use could be of particular advantage is as the sensing electrode for the detection and determination of sulfhydryl compounds following LCEC. For such applications, it is necessary that the analyte of interest be oxidized or reduced at a relatively low potential. The lower the potential required, the better is the sensitivity and selectivity that is expected to result (3). Hydrodynamic voltammograms (HDVs) obtained for cysteine, homocysteine, and N-acetylcysteine via flow injection are shown in Figure 4. They were obtained by recording the response of the CoPC-CME placed in the conventional thinlayer cell configuration ordinarily employed in LCEC but with a short length of narrow stainless steel tubing inserted in place of the column between the injector and detector. The HDVs
594
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3,MARCH 1985 40(
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300
I
250
t nA 200
150
a
100
L Volts
VI.
Ag/AgCI
Flgure 4. Hydrodynamic voltammograms of cysteine (0),homocysteine (O), and N-acetylcysteine (A)at a 2.0% CoPC electrode. Mobile phase was 5% CH,OH/95% 0.05 M phosphate buffer (pH 2.43) with 1.0 mM octanesulfonate. Flow rate was 1.0 mL/min.
possessed virtually the same shape for all three compounds; as in their CVs, only a single region of electrochemical activity was observed: an anodic wave which reached a maximum at approximately +0.8 V vs. Ag/AgCl and rapidly decreased at higher potentials. Also provided for comparison are the HDVs obtained at unmodified carbon paste which, as in CV, showed no response at potentials lower than +LO The mobile phase employed in these flow injection experimentsand in the LCEC experiments described below consisted of 5% CH30H/95% 0.05 M phosphate buffer (pH 2.4) containing M sodium octanesulfonate. As was observed earlier for the CVs of the sulfhydryl compounds at the CoPC-CMEs,the shapes of the HDVs obtained for the same analytes also were not consistent with that ordinarily observed for a simple diffusion-limited oxidation. Rather, the decreases in chromatographic peak currents that occurred at higher potentials were indicative of the occurrence of a potential-dependent electrode deactivationprocess which, as will be discussed below, severely limited the high potential operation of the electrode as an LCEC detector. In fact, operation at potentials greater than +0.85 V vs. Ag/AgCl for even a few chromatograms resulted in a marked decrease in the low potential electrode response compared to that observed for a fresh electrode surface. Furthermore, after HDVs were obtained in the usual manner by starting at low potentials and then proceeding to more positive values, subsequent attempts to reproduce the HDVs by changing the applied potential in the reverse direction yielded i-E profiles of the same shape but much reduced in magnitude. On the basis of the HDVs, it was apparent that the optimum potential for LCEC detection of these thiols at CoPC CMEs should fall between +0.70 and +0.80 V vs. AgIAgC1. Accordingly,the chromatogram shown in Figure 5 was subsequently generated with an octadecylsilane column and a detector potential of +0.75 V. As expected, well-formed anodic peaks were obtained at this potential only when the CoPC electrode was employed and not for unmodified carbon paste or glassy carbon electrodes. Under these conditions, calibration curves for each of the sulfhydryl compounds were linear for injected amounts of analyte from 270 to 2.7 pmol.
L
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1
2
i
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i
3
f
4
i
5
1
6
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7
,
6
,
9
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cysteine at (a) 2.0% CoPC carbon paste electrode and (b) unmodified carbon paste electrode. Concentrations were 1.0 X low5M; E = +0.75 V vs. Ag/AgCI; same mobile phase and flow rate as in Figure 4.
In all cases, the latter amount yielded a SIN of approximately 2 and thus represented the detection limit for all three species. The linearity was evaluated by least-squares analysis of data taken for seven concentrations over this range. The compounds yielded correlation coefficients ranging from 0.997 to 0.999; the slopes of the calibration curves varied from 1.88 nA/pmol for homocysteine to 1.50 nA/pmol for N-acetylcysteine. One of the principal advantages of carbon paste electrodes over solid electrodes is their capacity for easy and relatively reproducible generation of new surfaces. Likewise, one of the primary strengths of chemically modified carbon paste electrodes compared to CMEs formed by the usual methods involving adsorption, covalent attachment, or polymer coating onto solid substrates lies in their similar capacity for rapid generation of quantitatively reproducible modified surfaces (6). Accordingly, evaluations were made of the reproducibility of repeated injections of the identical sulfhydryl solutions when the carbon paste CME surface was replaced between each injection. Typical peak currents obtained in this manner for the injection of 1.0 X M cysteine solution yielded a relative standard deviation of 6.98% for nine successive trials. This level of reproducibility is not appreciably less than that expected for the repeated renewal of ordinary carbon paste surfaces (22). Finally, the stability of the CME response in LCEC was evaluated by placing the CoPC-containing electrode in the flowing mobile phase stream and following its current response toward the sulfhydryl compounds over an extended length of time. Typical results obtained for repeated injections of 1.0 X M homocysteine are shown in Figure 6. Similar stability was observed for cysteine and N-acetylcysteine as well. When the electrode was maintained at +0.75 V vs. SCE (curve a), peak heights decreased by 1-2%/h; as a result, CMEs ordinarily retained 85-90% of their original activity following 8 h of continuous exposure to the methanol/H,O
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All -SH compounds examined in our laboratory have shown enhanced response with CoPC CMEs exceeding that obtained with conventional carbon paste and glassy carbon electrodes and approaching that for Hg/Au amalgam electrodes. Extension of this work to include applications involving the practical determination of specific sulfhydryl compounds is continuing. In addition, it is apparent that electrocatalytic CMEs, whether constructed by the addition of the modifying species to carbon paste or by the other electrode modification methods, offer the possiblity of extending voltammetric and LCEC techniques to include several classes of compounds otherwise poorly suited to electroanalysis.
2000
1500
i
1000
ACKNOWLEDGMENT 50 0
J
0
I
100
I
200
1
I
300
400
500
time ( m i n )
Flgure 6. Electrode response for 1.O X lo4 M homocysteine vs. time immersed In flow injection system: (a) E = 4-0.75 V vs. Ag/AgCI; (b) E = +0.65 V; (c) E = 4-0.85V. Same conditions as in Figure 4.
stream. As seen in curve c, operation at'higher potentials resulted in a much more dramtic decrease in electrode response that was sufficiently rapid that, after 7 h (or more than 100 injections), measured currents were reduced to only 16% of those initially observed a t the fresh electrode surface. However, operation at +0.65 V (curve b) caused practically no electrode deactivation even after a full day of continuous operation. Most likely, most of the instability at the more positive LCEC potentials was related to the nondiffusion phenomena observed at similar potentials in both CVs and HDVs. In addition, it is possible that some of the decrease in electrode response was due to the gradual leaching of CoPC from the CME surface. In fact, some decreases in response have been observed previously even for conventional carbon paste electrodes in LCEC upon long-term exposure to a binary mobile phase containing a small fraction of organic components such as methanol or acetonitrile. In any case, however, the CoPC/carbon paste surface could be rapidly and reproducibly renewed if desired. Further, after surface regeneration, a period of only 5-10 min was required for the electrode to reach the optimum level at +0.75 V vs. Ag/AgCl, the potential required for this determination. It is clear that, as for hydrazine, phthalocyanine-containing CMEs possess considerable potential for use in the voltammetric and LCEC determination of sulfhydryl compounds.
The authors wish to thank E. R. Squibb & Sons, Inc., for their generous donation of the Captopril sample used in this work. Registry No. CoPC, 3317-67-7;carbon, 7440-44-0;cysteine, 52-90-4; homocysteine, 6027-13-0; N-acetylcysteine, 616-91-1; glutathione, 70-18-8.
LITERATURE CITED (1) Snell, K. D.; Keenan, A. G. J . Chem. SOC.,Chem. SOC.Rev. 1979, 8, 259-282. (2) Murray, R. W. Acc. Chem. Res. 1980, 13, 135-141. (3) Kissinger, P. T. Anal. Chem. 1977, 4 9 , 447A-456A. (4) Ravichandran, K.; Baldwin, R. P. Anal. Chem. 1983, 55, 1782-1786. (5) Ravichandran, K.; Baldwin, R. P. J . Liq. Chromatogr., in press. (6) Korfhage, K. M.; Ravlchandran, K.; Baldwin, R. P. Anal. Chem. 1984, 56, 1514-1517. (7) Moser, F. H.; Thomas, A. L. "The Phthalocyanlnes";CRC Press: Boca Raton, FL, 1983; Vol. 1. (8) Rabensteln, D. L.; Saetre, R. Anal. Chem. 1977, 4 9 , 1036-1039. (9) Saetre, R.; Rabenstein, D. L. Anal. Chem. 1978, 50, 276-280. (10) Allison, L. A.; Shoup, R. E. Anal. Chem. 1983, 55, 8-12. (11) Weber, J. H.; Busch, D. H. Inorg. Chem. 1965, 4 , 469-471. (12) Rollman, L. D.: Iwamoto, R. T. J . Am. Chem. SOC. 1988, 90, 1455-1 463. (13) Kundo, N. N.; Keier, N. P.; Glazneva, G. V.; Mamaeva, E. K. Klnetlka Kafallz 1987, 8, 1325-1330. (14) Kundo, N. N.; Keier, N. P. Russ. J . Phys. Chem. 1968, 42, 707-711. (15) Kundo, N. N.; Keier, N. P. Klnetlka Kataliz 1970, 1 1 , 91-99. (16) Dolansky, J.; Wagnerova, D. M.; Verprek-Siska, J. Collect. Czech. Chem. Commun. 1976, 4 1 , 2326-2332. (17) Skorobogaty, A.; Smith, T. D. J . Mol. Catal. 1982, 76, 131-147. (18) Brouner, W. M.; Piet, P.; German, A. L. folym. Bull. 1982, 8, 245-251. (19) Shepard, V. R.; Armstrong, N. R. J . Phys. Chem. 1979, 83, 1268-1276. (20) Green, J. M.; Fauikner, L. R. J . Am. Chem. SOC. 1983, 105, 2950-2955. (21) Zagai, J.; Fierro, C.; Rozas, R. J . Necfroanal. Chem. 1981, 119, 403-408. (22) Adams, R. N. "Electrochemistry at Solld Electrodes"; Marcel Dekker: New York, 1969; p 283.
RECEIVED for review August 3, 1984. Accepted December 5, 1984. This work was supported by the Graduate School of the University of Louisville. It was presented in part at the 187th National Meeting of the American Chemical Society.
