J. Phys. Chem. B 2001, 105, 10659-10668
10659
Microelectrode Studies of the Reaction of Superoxide with Carbon Dioxide in Dimethyl Sulfoxide Jay D. Wadhawan, Peter J. Welford, Emmanuel Maisonhaute, Victor Climent, Nathan S. Lawrence, and Richard G. Compton* Physical and Theoretical Chemistry Laboratory, Oxford UniVersity, South Parks Road, Oxford OX1 3QZ, United Kingdom
Hanne B. McPeak and Clive E. W. Hahn Nuffield Department of Anaesthetics, Oxford UniVersity, Radcliffe Infirmary, Oxford OX2 6HE, United Kingdom ReceiVed: June 6, 2001; In Final Form: August 25, 2001
The reaction of superoxide with carbon dioxide is studied using voltammetry and potential step chronoamperometry at polycrystalline gold disk microelectrodes in a DMSO electrolyte. In agreement with prior work, it is found that a reaction occurs between the superoxide anion radical and carbon dioxide, effectively precluding their simultaneous detection at low levels of carbon dioxide. The reaction rate is found to be first-order with respect to both carbon dioxide and superoxide, consistent with an ECE or DISP1 type process. A rate constant is determined for this reaction based upon two independent methods: fast scan cyclic voltammetric measurements and steady-state voltammetric signals. These methods yield a consistent rate constant of 3.7 ( 1.6 × 105 M-1 s-1. Potential step chronoamperometric measurements reveal that oxygen adsorbs onto a gold electrode surface, to form a monolayer both in the presence and absence of carbon dioxide. A rate constant for the reduction of surface-bound oxygen to superoxide is reported.
to be8,22
Introduction Oxygen and carbon dioxide are important respiratory gases; the ability to detect them simultaneously is critical for the practising anaesthetist and intensivist. Several methods have been reported for their analytical detection.1 Electrochemical detection strategies have been employed for over 50 years, and have mainly been based upon the detection of a single species in an aqueous solution.2-6 In contrast, several workers have developed sensors that provide a means of deducing the concentrations of both gases in the presence of each other.7-16 These generally employ aprotic solvents. In particular, dimethyl sulfoxide (DMSO) as the solvent has proved a popular choice due to its use in medicine, in particular in providing symptomatic relief from interstitial cystitis,17,18 encouraging its use in a clinical measurement sensor. For electrochemical purposes, DMSO has the advantages of a wider cathodic potential window compared to water, so as avoiding solvent breakdown as a competing reaction, and having a significantly greater solubility of both gases. Problems arise for the simultaneous electrochemical detection of carbon dioxide and oxygen in nonaqueous solutions since oxygen is more easily reduced than carbon dioxide.7 The superoxide radical anion that is initially formed, as evidenced by the appearance of a yellow coloration in solution upon prolonged electrolysis of oxygen, and by using UV-vis spectroscopy (λmax ) 243-253 nm)8,19-21 and laser Raman spectroscopy,22 attacks carbon dioxide to form the peroxydicarbonate anion. The mechanistic pathway has been speculated * To whom all correspondence should be addressed, Email: compton@ ermine.ox.ac.uk, Tel.: +44 (0) 1865 275 413, FAX: +44 (0) 1865 275 410.
2O2 + 2e- f 2O2-•
(1)
O2-• + CO2 f CO4-•
(2)
CO4-• + CO2 f C2O6-•
(3)
C2O6-• + O2-• f C2O62- + O2
(4)
Overall: O2 + 2CO2 + 2e- f C2O62-
(5)
At large-sized electrodes, this attack effectively precludes the simultaneous electrochemical detection of both gases, as it has been reported that the CO2 reduction wave completely disappears in the presence of oxygen.2,9b Methods employed to circumvent these problems include the use of a metallised membrane to act as an oxygen filter,7 and the elegant, if elaborate, employment of a pulsed titration approach:8 a generation pulse reduces any oxygen present to superoxide, the latter rapidly titrates with any CO2 present. A recovery pulse then follows in which the amount of unreacted superoxide is determined. Evidently, all things being equal, the greater the CO2 concentration, the smaller the amount of superoxide is reoxidized in the recovery pulse. Very recently, the use of microelectrodes for simultaneous sensing purposes has been advocated10 because the kinetics of reactions 2 to 4 were thought to be “out run” by the much faster diffusional loss of species from the electrode | electrolyte interface, thereby giving rise to oxygen and carbon dioxide signals that are independent of each other.
10.1021/jp012160i CCC: $20.00 © 2001 American Chemical Society Published on Web 10/03/2001
10660 J. Phys. Chem. B, Vol. 105, No. 43, 2001 In this paper, we present a systematic study of the reaction of electrochemically generated superoxide with carbon dioxide at gold disk microelectrodes. We investigate the hypothesis10 that the kinetics of the superoxide/carbon dioxide reaction are out run by the very fast transport resulting from convergent diffusion at microelectrodes, deduce kinetic parameters for the rate of reaction between superoxide and carbon dioxide, and study the effect of adsorbed oxygen at a polycrystalline gold microdisk electrode both in the presence and absence of carbon dioxide. Experimental Section Chemical Reagents. Solvents used for electrochemical experiments were dimethyl sulfoxide (DMSO, Spectrosol grade, BDH Chemicals Ltd.), and acetonitrile (Fisons, dried and distilled). The former solvent was carefully treated by drying with Linde 5A molecular sieves (Aldrich) and with ICN alumina super grade I (ICN Biomedicals, Eschwege, Germany) as outlined previously.23 Supporting electrolytes were tetraethylammonium perchlorate (TEAP), in the case of DMSO, and tetrabutylammonium perchlorate (TBAP both Fluka), when acetonitrile was used as the solvent. Ferrocene was also purchased from Aldrich in the highest commercially available grade and used without further purification. Carbon dioxide (MG Gases, UK) and impurity-free oxygen and nitrogen (BOC, Guildford, Surrey, UK) were used for electrochemical experiments as outlined below. Instrumentation. For electrochemical experiments, a commercially available potentiostat (PGSTAT30, Eco Chemie, Netherlands) controlled by a Pentium III computer was employed. The small-volume electrochemical cell (ca. 10 cm3) consisted of a three-electrode arrangement with a gold wire counter electrode and unless otherwise stated, a silver wire pseudo-reference electrode (Goodfellow Cambridge Ltd., Cambridge, UK). For fast scan cyclic voltammetric experiments, a potentiostat (constructed in house) was utilized with ohmic drop compensation and a current amplification of 0.9 × 105. This potentiostat is analogous to that described by Amatore and coworkers,24 and is capable of sweeping the potential of the working electrode at rates up to 30 kV s-1. In these experiments, data were recorded using a THANDAR TG1304 function generator and a Tetronix TDS 3032 oscilloscope (300 MHz band-pass, 2.5 GS/s). The electrochemical cell was shielded from direct sunlight, to minimize light-accelerated DMSO disproportionation.25 The working electrode employed was a gold microdisk electrode of various diameters (nominally 10 µm, Microglass Instruments, Greensborough, Australia; 5 µm, 125 µm constructed in house). These electrodes comprise gold wires sealed in borosilicate glass,26 the ends of which were polished to reveal the microdisk surface. These microdisk electrodes were all carefully polished27 according to a recipe previously outlined.23 The electrodes were visually inspected regularly using an optical microscope, to ensure that they were not recessed from the glass surface. Diameters of the microdisks were calibrated electrochemically using 2 mM ferrocene in acetonitrile, 0.1 M TBAP (using a value for the diffusion coefficient as 2.3 × 10-5 cm2 s-1 as given by Sharp28) All experiments were undertaken in a thermostated water bath at 25 ( 2 °C. Carbon dioxide and oxygen were introduced to the electrochemical cell in selected volume ratios from a Wo¨sthoff (Bochum, Germany) triple gas-mixing pump (model 2 M 302/ a-F) which was accurate to ( 1% relative to the selected ratio. Nitrogen made up the rest of the gas mixture. Gas mixtures
Wadhawan et al. were bubbled through DMSO solutions for at least 10 min prior to voltammetric interrogation, to ensure that full equilibration of the gases and solvent could be obtained; the gas supply to the electrochemical cell was stopped for the duration of each voltammetric measurement so as to ensure that measurements were taken in stationary solutions, under diffusion-only conditions. Bubbling was recommenced immediately thereafter. Results and Discussion Electrochemical Reduction of Pure Oxygen or Carbon Dioxide on Gold Microdisk Electrodes in DMSO/TEAP Solutions. Figure 1a shows voltammograms for the reduction of carbon dioxide at a 9.0 µm (diameter) gold microdisk electrode immersed in 0.2 M TEAP/DMSO, consisting of a single reduction plateau occurring at E1/2 ) -2.0 ( 0.1 V vs Ag, in agreement with data previously reported.10,23,29 We have previously characterized the mechanism by which this reduction takes place, and concluded that is likely to involve the reduction of carbon dioxide to yield adsorbed CO2-•, which then slowly reacts with incoming material to form a dimerized species.23 A subsequent, fast transfer of a second electron to this anion radical dimer forms the reaction products kD
+CO2,k2,slow
k1
CO2(bulk) 98 CO2(surf) 98 CO2 - •(ads) 98 +e-,fast
(CO2)2-• 98 products Numerical simulation of this mechanism and its subsequent application to the experimental data indicated that the chargetransfer coefficient of this to be approximately 0.43 ( 0.05.23 In the Appendix of this paper, we report the results of the reduction of carbon dioxide at an Au(111) single crystal plane in 0.2 M TEAP/DMSO; further evidence for the presence of an adsorbed intermediate is inferred from hysteresis between the forward and the reverse scans in a cyclic voltammetric experiment and from the observation of a change in the voltammetry of Au(111) single cryastal electrode in aqueous sulfuric acid before and after CO2 experiments in 0.2 M TEAP/ DMSO. In Figure 1b, cyclic voltammograms corresponding to the reduction of oxygen at a 10.4 µm (diameter) gold microdisk electrode in a 0.2 M TEAP/DMSO solution are shown. Two well-defined signals are observed at half-wave potentials, E1/2 ) -0.37 ( 0.04 and -1.55 ( 0.04 V vsAg in a closely 1:2 height ratio, consistent with the following processes
O2 + e- f O2-•
(6)
O2 + 2e- f O22-
(7)
Tafel analyses of these data suggest that both processes are electrochemically reversible on the microelectrode time scale. From the inset figure, it can be seen that the first electrochemical signal is directly proportional to the oxygen concentration within the electrolyte solution, with a slope of -8.4 ( 1.5 nA vol %-1. The limiting current at a microdisk electrode (ilim) is given by the expression
ilim ) 4nFDrecbulk
(8)
in which n is the number of electrons transferred during the transport-limited process, F is the Faraday constant (96 485 C mol-1), D is the diffusion coefficient of the electroactive species, whereas cbulk is its concentration in bulk solution. The microdisk radius is re. The currents observed for the electroreduction of
Reaction of Superoxide with Carbon Dioxide
J. Phys. Chem. B, Vol. 105, No. 43, 2001 10661
Figure 1. (a) Cyclic voltammograms (100 mV s-1 scan rate) for the reduction of CO2 in 0.2 M TEAP/DMSO at a 9.0 µm (diameter) Au microdisk electrode. (A) 10, (B) 15, (C) 20, (D) 40, (E) 80 vol % CO2. (b) Cyclic voltammograms (100 mV s-1 scan rate) for the reduction of O2 in 0.2 M TEAP/DMSO at a 10.4 µm (diameter) Au microdisk electrode. (A) 5, (B) 10, (C) 15, (D) 20, (E) 40, (F) 80 vol % O2.
oxygen are significantly smaller than those for the same volume ratio of carbon dioxide, reflecting a lower solubility of oxygen in 0.2 M TEAP/DMSO30,31 as reported in Table 1. A previous paper23 reports the use of a dilatometric technique for the accurate measurement of the solubility of CO2 in 0.2 M TEAP/ DMSO. However, because literature values suggest that the solubility of O2 in DMSO solutions is approximately 2 orders of magnitude smaller than that of CO2, it effectively precludes this technique from being applied in this case; the data reported in Table 1 for oxygen have been taken from two sources, where we have assumed that the supporting electrolyte does not affect the solubility of the gas.23,32 For comparative purposes, also shown are the solubilities of carbon dioxide and oxygen in water.33 Using the pertinent solubility values, we may deduce diffusion coefficients of carbon dioxide and oxygen in 0.2 M TEAP/DMSO from the slope of the current | concentration graphs illustrated as insets in Figure 1. These are reported in Table 1. We next turn to the electrochemical reduction of both gases in the presence of each other. Simultaneous Electrochemical Reduction of Oxygen and Carbon Dioxide at Gold Microdisk Electrodes in TEAP/ DMSO Solutions. Figure 2 illustrates voltammograms for the reduction of oxygen and carbon dioxide in the presence of each
TABLE 1: Solubility Data and Diffusion Coefficients of the Species Involved in the Chemistry Presented in This Work at 25 °C species O2 O2-• CO2
solubility in DMSOa/mM 2.1d 125 ( 13e
solubility in H2Ob/mM 1.32 33.0
105 Dc/cm2 s-1 2.16 ( 0.4f 1.08g 1.02 ( 0.1h
a
Concentration when solution is saturated with the relevant gas. b Data taken from ref 33, and refers to the solubilites of the gases in pure water (no electrolyte). c Diffusion coefficients have been inferred from voltammetric data, and so refer to those values in the presence of at least 0.1 M TEAP. d Measured by Sawyer and co-worker30 for DMSO containing 0.1 M TEAP, using a modified Winkler titration method. The value quoted here is 4.8 times larger than that quoted by Fujinaga et al.31 for neat DMSO. e Value measured using a dilatometric method.23 Using the same method, a value of 113 ( 8 mM was reported as the saturation concentration of carbon dioxide in neat DMSO. f This value was determined from the voltammetric data presented in this work. g Taken from ref 30 for 0.1 M TEAP/DMSO. h Taken from ref 23 for 0.2 M TEAP/DMSO.
other at a 9.8 µm (diameter) gold disk microelectrode in 0.2 M TEAP/DMSO. In Figure 2a, the effect of varying the oxygen concentration at a fixed, nonzero carbon dioxide concentration
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Figure 2. Voltammograms for the simultaneous electrochemical reduction of oxygen and carbon dioxide at a 9.8 µm (diameter) gold disk microelectrode (scan rate 50 mV s-1) in 0.2 M TEAP/DMSO. (a) 3 vol % CO2 with (A) 10, (B) 20, (C) 30, and (D) 40 vol % O2. (b) 20 vol % O2 with (A) 1, (B) 3, and (C) 6 vol % CO2.
is shown, whereas Figure 2b depicts the converse. Only two signals are observed: the first corresponds to the reduction of dioxygen to superoxide, whereas the second is due to both further reduction of dioxygen and reduction of carbon dioxide, as from above, these two signals are essentially superimposed. In Figure 3, the reduction-limiting current of one gas has been plotted as a function of the concentration of the other gas. Thus, Figure 3a illustrates the variation of the carbon dioxide signal (obtained by subtracting the height of the first wave observed in the presence of CO2, from that of the second) with oxygen concentration, and shows that this signal is independent of the concentration of oxygen. In contrast, Figure 3b shows that this is not the case for the oxygen signal: while the oxygen signal appears to be independent of the carbon dioxide concentration at concentrations of the latter greater than 2 vol %, it is not independent below 2 vol %. Furthermore, it can be seen that at low concentrations of carbon dioxide, the current due to the reduction of oxygen is in fact one-half of that observed at high concentrations of carbon dioxide. This is indicative of a “switch” in the mechanistic pathway from the consumption of one over to two electrons. This is not surprising: at low CO2 levels, at the lower potentials, oxygen is reduced in a one electron process to superoxide, whereas at higher CO2 levels a “titration” occurs whereby superoxide reacts with carbon dioxide, in an overall two electron process 8,22
O2 + 2CO2 + 2e- f C2O62-
(9)
The apparent independence of the carbon dioxide signal with oxygen concentration arises as a consequence of the much greater solubility of carbon dioxide in 0.2 M TEAP/DMSO than
Figure 3. (a) Plot of the CO2 limiting reductive current (obtained after subtraction of the oxygen-only reductive current) in the presence of varying amounts of oxygen at a 9.8 µm (diameter) Au microdisk electrode (obtained from voltammograms swept at 50 mV s-1). × (1 vol % CO2), + (3 vol % CO2), and ( (6 vol % CO2). (b) Plot of the first O2 limiting reductive current in the presence of varying amounts of carbon dioxide at a 5.2 µm (diameter) Au microdisk electrode (obtained from voltammograms swept at 100 mV s-1). × (5 vol % O2), + (10 vol % O2), ˆI (15 vol % O2), ‘ (20 vol % O2), and ∼(40 vol % O2).
O2 (Table 1 indicates that CO2 is ca. 60 times more soluble in this medium than O2). Thus, at the larger concentrations of carbon dioxide, the titration of a comparatively small amount of carbon dioxide with superoxide makes little difference to the observed voltammetry.34 To detect electrochemically oxygen and carbon dioxide simultaneously, Hahn and co-workers10 hypothesized that if the electrode is small enough, titration of carbon dioxide by superoxide will be negligible. However, to obtain an estimate of the electrode size for this to be possible, the kinetics of the reaction need to be clearly understood. We thus next turn to the determination of a rate constant for this reaction. Determination of the Rate Constant for the Reaction of Superoxide with Carbon Dioxide: Fast Scan Cyclic Voltammetric Investigation into the Reaction of Superoxide with Carbon Dioxide and the Analysis of Steady-State Data at Gold Microdisk Electrodes. Fast scan cyclic voltammetry is a useful tool in the pursuit of kinetics in the nanosecond time scale.24,35 At small electrodes, this technology has the additional advantage of a reduced magnitude of ohmic drop and electrochemical cell time constants.35,36 Figure 4 shows fast scan cyclic voltammograms for oxygen reduction at a 125 µm (diameter) gold disk electrode to generate superoxide both in the presence and absence of carbon dioxide, and at two scan rates. Figure 4a shows the voltammetric response at a scan rate of 72 V s-1. In the absence of carbon dioxide, two peaks are observed, with Emid ) 1/2(Epred + Epox) ) -0.36 V vs Ag: the forward peak corresponds to one-electron formation of superoxide, whereas the reverse peak represents its reoxidation to dioxygen. The difference in height between the forward and reverse peaks is likely due to the diffusion coefficient of superoxide in DMSO
Reaction of Superoxide with Carbon Dioxide
J. Phys. Chem. B, Vol. 105, No. 43, 2001 10663 1-4 have been speculated upon as the pathway for this reaction, based upon the structure of the peroxydicarbonate dianion detected.8,22 However, experimental voltammograms obtained have not been examined in the light of these mechanistic pathways. The mechanism involves the heterogeneous transfer of an electron to dioxygen at the gold electrode surface (eq 1), followed by two homogeneous chemical steps (both first-order with respect to carbon dioxide, eqs 2 and 3), and ends with a disproportion step (eq 4). This nonsimple mechanism may be approximated by a mixture of the well-known ECE, DISP1 and DISP2 processes because, overall, either one or two electrons are transferred per mole of dioxygen, depending upon the extent of the follow-on reactions.38 Mixtures of ECE/DISP1 type processes are known.39 For the case in hand, the mechanism can be described as follows
ECE: O2 + e- f O2-• k,slow
O2-• + CO2 98 CO4-• fast
CO4-• + CO2 98 C2O6-• C2O6-• + e- f C2O62Figure 4. Fast scan cyclic voltammograms for the electroreduction of oxygen to superoxide at a 125 µm Au electrode immersed in 0.2 M TEAP/DMSO, employing scan rates of (a) 72 V s-1 and (b) 360 V s-1. In both cases, the solid line is the response in pure oxygen (10 vol %), whereas the dashed line is the response of when the electrode was immersed in a solution containing 10 vol % O2 and 1 vol % CO2.
electrolyte solutions being ca. two times smaller than that of dioxygen, as inferred via DIGISIM modeling of the voltammetry using the approximation of a hemispherical diffusion regime in which rehemisphere ) 2/π redisk.37 In the presence of CO2, although the formation of superoxide is observed at this scan rate, only a very small reverse peak is detected. Furthermore, the superoxide formation wave is enhanced by the presence of carbon dioxide. These results can be interpreted by a reaction of superoxide with carbon dioxide (vide supra), involving, inter alia, an additional electron-transfer process (eqs 1-4). These results are in agreement with the large electrode studies of this reaction, previously reported.8,22 Figure 4b shows fast scan cyclic voltammograms for the reduction of oxygen in the presence of carbon dioxide employing a potential sweep rate of 360 V s-1. Also shown, for clarity is the formation of superoxide in the absence of any carbon dioxide. The time scale of the experiment is shorter, resulting in a reduced reaction of superoxide with CO2; the presence of a peak corresponding to the reoxidation of superoxide in the presence of CO2 is evident. However, even at these scan rates, the reaction still occurs because the oxygen reduction signal is enhanced by the presence of CO2. The kinetics of the nucleophilic addition reaction are just about “out-run” in the time scale of this experiment (less than 10-3 s). DIGISIMTM 2.0 was used to simulate the cyclic voltammetric response at a 80 µm (diameter) hemispherical electrode, and thus deduce an estimate of the rate constant for the reaction of superoxide with CO2 for scan rates in the range from 72 to 1080 V s-1. After correction for capacitative charging, good agreement was observed between the experimentally observed and the simulated data using a rate constant of 5 × 105 M-1 s-1. We next turn to the mechanism by which the nucleophilic addition of superoxide to carbon dioxide takes place. Equations
DISP1: 2O2 + 2e- f 2O2-• k,slow
O2-• + CO2 98 CO4-• CO4-• + CO2 f C2O6-• C2O6-• + O2-• f C2O62- + O2 DISP2: 2O2 + 2e- f 2O2-• O2-• + CO2 h °CO4-• CO4-• + CO2 h °C2O6-• k,slow
C2O6-• + O2-• 98 C2O62- + O2 Alden and Compton37 have previously described the solutions of the mass-transport equations at a microdisk electrode under diffusion-only conditions and at steady-state, under the appropriate boundary conditions, and the assumption of equal diffusion coefficients using ILU preconditioned Krylov subspace methods in conjunction with the Amatore and Fosset method for the conformal mapping of the microdisk electrode geometry.40 These numerical simulations were used to generate working curves for ECE, DISP1 and DISP2 mechanisms at microdisk electrodes, permitting quantitative mechanistic analysis for these mechanisms without the need for further simulation. These working curves have been reproduced in Figure 5, and represent how the effective number of electrons transferred during the mechanism (Neff) varies as a function of a dimensionless, mechanism-dependent, rate constant, K
K)
k[O2] mbulkre2 D
(10)
where m is a mechanism-dependent parameter and is 0 for ECE or DISP1 processes, and is 1 for DISP2 processes, and k is a first-order rate constant (for ECE and DISP1 processes) or a second-order rate constant (for DISP2 processes).
10664 J. Phys. Chem. B, Vol. 105, No. 43, 2001
Wadhawan et al.
Figure 5. Working curves for ECE (×), DISP1 (+) and DISP2 (() processes at microdisk electrodes (see text).
Figure 6. Graph plotting Neff (obtained from experimental voltammograms at a 9.8 µm Au microdisk electrode immersed in 0.2 M TEAP/ DMSO containing various amounts of CO2 and O2) as a function of the carbon dioxide and oxygen concentrations (see text).
Experiments were undertaken at a both a 5 and 10 µm gold disk microelectrode for the reduction of varying amounts of both oxygen and carbon dioxide in 0.2 M TEAP/DMSO. From the experimentally determined data, an effective number of electrons transferred was deduced
Neff )
2 i1stO lim
4FDO2re[O2]bulk
(11)
from which, comparison with the working curve permits the 2 inference of a rate constant (k) for the reaction. In eq 15, i1stO lim is the limiting current for the reduction of dioxygen to superoxide. With both electrode sizes, it was found that although Neff depended very strongly upon the concentration of carbon dioxide present within the DMSO electrolyte, it varied negligibly with oxygen concentration, Figure 6. The rate constant, k, deduced in this manner will necessarily be dependent upon the carbon dioxide concentration
k ) kh [CO2]n
(12)
where kh is the true rate constant for the reaction, and the order of the rate-limiting step with respect to carbon dioxide is n.
Thus, if eq 2 is rate limiting, the reaction would exhibit firstorder kinetics with respect to [CO2], characteristic of an ECE or a DISP1 pathway; the observation of second-order kinetics is characteristic of a DISP2 mechanism and would suggest that either eq 3 or eq 4 is rate limiting. From eq 12, if the carbon dioxide concentration is excessive, the rate constant, k, will be a constant. This is likely to hold for the experiments considered below because the solubility of carbon dioxide is 60 times greater than that of oxygen (see Table 1). Figure 7 shows how log10(k) depends on log10[CO2] for the three mechanisms obtained from the 10 µm electrode voltammetric data. Because Neff is only weakly dependent upon [O2], an “oxygen-average” can be made for each CO2 concentration. It can be seen that the best fit occurs for an ECE or DISP1 mechanism, with a gradient close to 0.9, suggesting that the slow step of the reaction is that in eq 2: reaction of the initially generated superoxide with carbon dioxide. From the ordinate intercepts of the plots shown in Figure 7a and b, the rate constant for the slow reaction of superoxide with CO2 can be estimated to be 2.1 ( 0.5 × 105 M-1 s-1 (ECE) and 4.0 ( 1.3 × 105 M-1 s-1 (DISP1). From the approach adopted here, it is not possible a priori to distinguish between which of these two mechanisms approximates the reaction best. However, it is clear that the determined rate constants are essentially in agreement with that estimated by the fast scan cyclic voltammetric experiments. These estimations of the rate constant are 2 orders of magnitude greater than those estimated by Albery et al.8 and Sawyer and co-workers22 (ca. 1.3 × 103 M-1 s-1). In those reports, the kinetics of the superoxide reaction with carbon dioxide in DMSO were studied using a rotating ring-disk titration experiment in which a rotating gold disk is used to generate superoxide in the presence of a known amount of carbon dioxide, with which the former species reacts. At the platinum ring electrode, any unreacted superoxide is oxidized back into oxygen; increasing the mass transport of carbon dioxide to the double electrode, by, for example increasing the speed at which the dual electrode system is rotated, results in more carbon dioxide participating in reaction with superoxide, and thus a reduced collection efficiency. The disagreement between the previously reported rate constant for the superoxide-CO2 reaction and that reported in this work is likely to be due to the different time scales of the experiments used: in the experiments undertaken in this work, the experimental time scale is smaller permitting the deduction of primarily the loss of carbon dioxide via homogeneous reaction with superoxide; in
Reaction of Superoxide with Carbon Dioxide
Figure 7. Analysis of the rate constants determined from the comparison of the experimental data (at a 9.8 µm Au miocrdisk electrode) with the working curves in Figure 5 (see text). (a) ECE, (b) DISP1, and (c) DISP2. In all three cases, each point at individual carbon dioxide concentrations represents an average over the oxygen concentrations.
the rotating ring-disk experiments, the longer time scale allows a rate constant to be deduced that includes other steps in the mechanistic pathway because step 4 also involves the loss of superoxide. Taken together, the two experiments imply a DISP1 pathway is most likely. The fast reaction of superoxide with carbon dioxide effectively precludes the simultaneous detection of both species at low levels of CO2 using a steady-state method described above regardless of whether a macroscopic electrode or micron sized microelectrodes are used. However, a reduction in the time scale of the experiment so as to “out run” the kinetics of the superoxide/carbon dioxide reaction may be possible. To this end, chronoamperometric experiments were undertaken, the results of which are reported below. Potential Step Chronoamperometric Studies into the Electroreduction of Oxygen in the Presence of Carbon Dioxide at Gold Microdisk Electrodes in TEAP/DMSO Solutions. The fast kinetics of the nucleophilic attack of carbon dioxide by superoxide suggests that the kinetics of the reaction will be effectively “out-run” at fast time scales (ca. 1 ms or below). Consequently, chronoamperometric experiments were undertaken in 0.2 M TEAP/DMSO, by stepping the potential applied to the 10.2 µm (diameter) working electrode from 0.0 V vs Ag (corresponding to no Faradaic current) to -1.2 V vs Ag (corresponding to transport-limited oxygen reduction), at various solution oxygen concentrations. The results are reported
J. Phys. Chem. B, Vol. 105, No. 43, 2001 10665
Figure 8. Typical potential step chronoamperometic transients observed by stepping the potential of the 10.8 µm working electrode to -1.2 V vs Ag (see text). (a) Double layer effects: 0 vol % O2 and 0 vol % CO2. (b) (b) Transient observed in the presence of 5% O2 (×). Also shown (solid line) is the transient expected under diffusion-only conditions (see text), calculated using D ) 2.16 × 10-5 cm2 s-1, re ) 5.4 µm and [O2] ) 5 vol %.
in Figure 8. It can be seen (Figure 8a) that effects due to the charging of the double layer formed at the electrode | electrolyte interface are negligible (at least 1 order of magnitude smaller) in comparison with the transient observed in the presence of 5 vol % dioxygen (Figure 8b). DMSO is known to very slowly adsorb on to gold surfaces,41-43 but there is no evidence that this occurs under these conditions. Also shown in Figure 8b is the theoretical transient, assuming solely diffusive mass transport, calculated for the system using the expressions for chronoamperometric transients deduced by Aoki and Osteryoung44
{
i ) 4nFDrecbulk 1 +
i ) 4nFDrecbulk
}
0.718 35 0.056 26 0.006 46 + ... + xτ xτ3 xτ5 (for large values of τ) (13)
{x
}
π π + + 0.094xτ + ... 4τ 4 (for small values of τ) (14)
in which the dimensionless time variable, τ, is given by
τ)
4Dt re2
(15)
These two curves overlap in the domain 0.82 < τ
