Anal. Chem. 1998, 70, 468-472
A Ring-Disk Study of the Competition between Anodic Oxygen-Transfer and Dioxygen-Evolution Reactions Natasˇa D. Popovic´ and Dennis C. Johnson*
Department of Chemistry and Ames Laboratory, Iowa State University, Ames, Iowa 50011
Voltammetric data obtained at a rotated ring-disk electrode for oxidation of dimethyl sulfoxide (DMSO) to dimethyl sulfone support the conclusion that anodic O-transfer and O2(aq)-evolution reactions are in competition for a common reaction precursor. That species is concluded to be the adsorbed OH radical generated by anodic discharge of H2O. Because of this competition, an increase in the flux of the reactant in the O-transfer reaction causes a decrease in the rate of O2(aq) evolution. Therefore, difference voltammetry and hydrodynamically modulated voltammetry cannot be considered reliable for accurate deconvolution of the total electrode current resulting from these concomitant processes. A simple mathematical model is developed to describe the total current resulting from these simultaneous processes, and results of preliminary tests of this model are in good agreement with experimental data for DMSO oxidation.
electrode-solution interface, has been reported to increase substantially the rate of oxidation of dimethyl sulfoxide (DMSO) and tetramethylene sulfoxide (TMSO) to their corresponding sulfones.3 Originally, the Bi(V) sites in Bi(V)-doped β-PbO2 film electrodes were suspected to function by increasing the surface activity of adsorbed OH radicals generated by discharge of H2O; however, no experimental confirmation of this speculation has been obtained. It is speculated that the prerequisite for anodic O-transfer reactions is the anodic discharge of H2O at PbO2 sites to produce adsorbed OH radicals, as represented by
Anodic O-transfer reactions are electrochemical oxidations that involve the transfer of O atoms from H2O in the solvent phase to the oxidation products. On the basis of examination of thermodynamic tables, virtually all organic compounds are predicted to be oxidized to CO2 at potentials easily accessible at conventional anode materials (Au, Pt, GC) in aqueous solvents. Therefore, observations that the majority of possible anodic O-transfer reactions are slow or nonexistent at these anode materials is explained on the basis of kinetic rather than thermodynamic limitations. Furthermore, on the basis of these observations, it is concluded that hydrolysis cannot be considered as an efficient mechanism for the O-transfer step in these anodic processes. Research in this laboratory continues to be focused on the development and characterization of new electrode materials that can support anodic O-transfer reactions for a variety of organic and inorganic species. A common approach has been the incorporation of altervalent metallic species within thin films of β-PbO2 electrodeposited onto inert substrates.1-3 From its inception, this approach has been based on the premise that defects in the β-PbO2 surface lattice generated by the altervalent dopants can function catalytically within various O-transfer mechanisms. One dopant, cationic bismuth believed present as Bi(V) at the
S[OH] + R f S[ ] + RO + H+ + e-
(1) Yeo, I.-H.; Johnson, D. C. J. Electrochem. Soc. 1987, 134, 1973-1977. (2) Yeo, I.-H.; Kim, S.; Jacobson, R.; Johnson, D. C. J. Electrochem. Soc. 1989, 136, 1395-1401. (3) Chang, H.; Johnson, D. C. J. Electrochem. Soc. 1990, 137, 2452-2456.
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S[ ] + H2O f S[OH] + H+ + e-
(1)
where S[OH] represents adsorbed OH radicals and S[ ] corresponds to unoccupied surface sites.4 The O-transfer step, then, can be represented by
(2)
where R is the reactant and RO is the product of the O-transfer reaction.5,6 Evolution of O2(aq) can diminish the current efficiency for O-transfer reactions by consumption of adsorbed OH radicals, as represented by
S[OH] + H2O f S[ ] + O2(aq) + 3 H+ + 3 e-
(3)
Because of the competition of the O2(aq)-evolution and Otransfer reactions for the adsorbed OH species, any increase in the rate of the O-transfer reaction given by eq 2, e.g., an increase in concentration of R, will result in a decrease in the rate of O2(aq) evolution given by eq 3. Evidence for competition between O2(aq)-evolution and Otransfer reactions was first observed by Vitt and Johnson.5 They applied difference voltammetry at rotated disk electrodes (RDEs) in a study of several O-transfer reactions. In their procedure, current-potential (i-E) curves obtained for two rotation velocities were subtracted to yield a difference response (∆i-E) in hopes of extracting the rotation-dependent component of the total (4) Pavlov, D.; Monahov, B. J. Electrochem. Soc. 1996, 143, 3616-3629. (5) Vitt, J. E.; Johnson, D. C. J. Electrochem. Soc. 1992, 139, 774-778. (6) Janssen, L. J. J.; Van der Heyden, P. D. L. J. Appl. Electrochem. 1995, 25, 126-136. S0003-2700(97)00780-4 CCC: $15.00
© 1998 American Chemical Society Published on Web 02/01/1998
current. However, the ∆i-E curves consistently were observed to have a peaked shape with ∆i values decreasing substantially from their maximum value as E was increased within the potential region of O2 evolution. In contrast to that finding, the use of a rotated ring-disk electrode (RRDE) for detection of IO3- at the ring electrode during oxidation of I- at the disk electrode indicated I- oxidation persisted at a transport-limited rate throughout the potential region for which ∆i values decreased below their maximum. The ∆i-E curves for I- at a Pt RDE obtained by Austin et al. using square-wave hydrodynamically modulated voltammetry also had a peaked shape in the potential region where the O-transfer reaction occurred simultaneously with the O2(aq)evolution reaction.7 An equation can be derived from eqs 1-3 to quantitatively describe the competition between the O-transfer and O2(aq)evolution reactions. Accordingly, the total current (itot) is given by s itot ) FAΓ0{kWDRθS[ ] - k-WDRθS[OH]CH + +
3kOERθS[OH] + kOTRθS[OH]CsR} (4)
as a result of increases in the flux of reactant. This is because an increase in electrode potential is necessary to generate the increased flux of OH radicals required to achieve the half-wave current (i1/2) defined by
i1/2 ) 0.5ilim ) 0.5
{
}
nFADCb δ
(7)
where ilim is the convective-diffusional transport-limited current. For a rotated disk electrode, δ in eq 7 is the diffusion layer thickness given by
δ ) 1.61D1/3ν1/6ω-1/2
(8)
where D is the diffusion coefficient (cm2/s), ν is the kinematic viscosity (cm2/s), and ω is the rotational velocity (rad/s). At E ) E1/2, kWDR is given by
kWDR ) k°′ WDR{(RF/RT)(E1/2 - E°′ WDR)}
(9)
where R is the transfer coefficient. Solving for E1/2 gives In eq 4, kWDR and k-WDR are the forward and reverse rate constants, respectively, for the water discharge reaction in eq 1; kOTR is the rate constant for the anodic O-transfer reaction in eq 2; kOER is the rate constant of the O2-evolution reaction in eq 3; Γo is the density (mol/cm2) of surface sites available for adsorption of OH radicals; θS[OH] is the fractional surface coverage by adsorbed OH s radicals and θS[ ] ) 1 - θS[OH]; CH + is the surface concentration of s + H ; and CR is the surface concentration of the reactant. A steady-state assumption is applied for θS[OH], and eq 4 is rewritten as
{
itot ) 2FAkWDRΓo
2kOER + kOTRCsR
s s kWDR + k-WDRCH + + kOER + kOTRCR
}
(5) It becomes evident from an examination of eq 5 that a distinct wave for the anodic O-transfer reaction, i.e., no substantial contribution from coevolution of O2, is observed only when kOTR CsR >> kOER. Based on this approximation, and the assumptions that the back reaction for water discharge can be ignored, i.e., k-WDRCH+s ) 0, and kWDR E°OTR. Results consistent with this expectation have been given previously.3 Based on eq 6, it can be shown that small increases in E1/2 values are to be expected (7) Austin, D. S.; Johnson, D. C.; Hines, T. G.; Berti, E. T. Anal. Chem. 1983, 55, 2222-2226.
E1/2 ) E°′ WDR +
{
b
DCR 2.3RT log RF 2Γok°′ WDRδ
}
(10)
Based on eq 10, plots of E1/2 vs log{ω1/2} and vs log{CbR} are predicted to be linear with slopes of 2.3RT/RF. EXPERIMENTAL SECTION Instrumentation and Data Collection. Electrochemical data were obtained using an RDE4 bipotentiostat, AFMSR rotator, and AFMR28 Au-Au ring-disk electrode (r1 ) 2.29 mm, r2 ) 2.46 mm, r3 ) 2.69 mm, Adisk ) 0.165 cm2, Ntheory ) 0.22; Pine Instrument Co.). A three-compartment electrochemical cell was used to separate the working, reference, and counter electrodes. A saturated calomel electrode (SCE, Fisher Scientific) provided the reference potential for all experiments. The counter electrode was a coiled Pt wire (∼5 cm2). Data were collected using IBM-compatible computers equipped with DT2820 (Data Translations) or AT-MIO-16XE-50 data acquisition boards (National Instruments). Programs for data collection were written locally using Asyst (Asyst Software Tech., Inc.) or LabView (National Instruments) software. Calculations were performed using SigmaPlot 3.0 (Jandel Scientific) software. Chemicals. All chemicals were analytical reagent grade and were used as received from Aldrich Chemicals or Sigma. Solutions were prepared with water that had been doubly distilled and deionized (Nanopure, Barnstead). Procedures. Bismuth(V)-doped lead dioxide films were electrodeposited on a Au substrate at 1.60 V for 15 min under quiescent conditions from a solution containing 5 mM Bi(NO)3 and 10 mM Pb(NO)2 in 1 M HClO4. Films deposited in this manner contain a Bi/Pb ratio equal to ∼1:2.8 The films were removed from the Au substrate by electrochemical stripping at 0.2 V. The Au RRDE was polished after removal of the films with alumina slurry (0.3 µm, Buehler) on Buehler Microcloth. All Analytical Chemistry, Vol. 70, No. 3, February 1, 1998
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solutions were thoroughly deaerated with N2(g). The supporting electrolyte was 1.0 M HClO4. Caution: bismuth and lead salts are toxic substances. The experimental collection efficiency (Nexp) for O2(aq) at the RRDE was determined by scanning the potential of a 1:2 Bi-PbO2 disk and simultaneously detecting O2(aq) at the Au ring at -0.3 V. Values of iring were plotted vs idisk for seven values of disk potential. The plot was linear (r2 ) 0.9987) with a slope corresponding to Nexp ) 0.050. Difference voltammetric curves (∆i-E) were calculated by subtraction of the i-E curves obtained at two rotation rates at the disk of the RRDE. RESULTS AND DISCUSSION The rotated ring-disk electrode offers a powerful means to determine the contributions to total current from O-transfer and O2-evolution reactions that occur concomitantly at the disk electrode. To demonstrate this, a Bi-PbO2 film was deposited on the disk electrode of the Au-Au RRDE, whereas the ring electrode remained uncoated. DMSO was chosen as the model reactant for a variety of reasons: (i) it does not produce welldefined anodic waves at Au or at undoped β-PbO2 electrodes, (ii) it produces mass transport-limited response at Bi-PbO2 film electrodes, (iii) its oxidation to dimethyl sulfone (DMSO2) is believed to occur by the O-transfer mechanism depicted by eqs 1-3,8,9 and (iv) neither DMSO nor DMSO2 interfere with cathodic detection of O2(aq) at the Au ring electrode for Ering ) -0.3 V. For this RRDE system, the contribution to total disk current (idisk,tot) from the O2-evolution reaction (idisk,OER) can be calculated from the ring current (iring) for O2 reduction as indicated by
idisk,OER ) -iring/Nexp
(11)
where Nexp is the efficiency determined experimentally for collection of O2(aq) generated at the disk electrode. Typical voltammetric plots of idisk vs Edisk and iring vs Edisk obtained in this experiment are shown in Figure 1 as a function of increasing DMSO concentration. It is readily apparent that iring for Ering ) -0.30 V, corresponding to reduction of O2(aq) generated at the disk electrode at Ed > 1.75 V, decreases with increased DMSO concentration. This finding supports the premise that the anodic discharge of H2O to produce adsorbed OH radicals is a prerequisite of both the O2(aq)-evolution and O-transfer reactions. Accordingly, for generation of adsorbed OH radicals at a constant rate, i.e., constant Edisk, the fraction of the OH radicals used in generation of O2(aq) is diminished as the DMSO flux is increased. The current for oxidation of DMSO at the disk electrode (idisk,OTR) was calculated by subtraction from total disk current (idisk,tot) the current for the O2(aq)-evolution reaction (idisk,OER) estimated on the basis of the ring current (iring), as shown by
idisk,OTR ) idisk,tot - idisk,OER ) idisk,tot + iring/Nexp (12) Calculated values of idisk,OTR are plotted vs Edisk in Figure 2A for two rotational velocities. For comparison, Figure 2B shows a (8) Chang, H.; Johnson, D. C. Anal. Chim. Acta 1991, 248, 85-93. (9) Kawagoe, K. T.; Johnson, D. C. J. Electrochem. Soc. 1994, 141, 3404-3409.
470 Analytical Chemistry, Vol. 70, No. 3, February 1, 1998
Figure 1. Voltammetric response for DMSO in 1.0 M HClO4 at the Au disk electrode coated by 1:2 Bi-PbO2 film with detection of O2(aq) at the uncoated Au ring electrode. Edisk, 1.52-1.90 V at 40 mV/s. Ering, -0.30 V. DMSO concentration (mM): (a) 0, (b) 5.0, (c) 10.0, (d) 15.0, (e) 20.0. Note: ring current is divided by the experimental collection efficiency (Nexp ) 0.050) to aid comparison with the disk current. Rotational velocity, 168 rad/s.
difference voltammetric response (∆idisk - Edisk) for DMSO at the Bi-PbO2 disk electrode calculated by subtraction of the idisk,tot as a function of Ed at the two rotational velocities. It is apparent that the calculated currents for DMSO increase to a plateau value for Edisk f 1.9 V. In contrast, the experimental value of ∆idisk reaches a maximum at Edisk ) 1.84 V and then decreases for Edisk > 1.84 V. This decrease in ∆idisk for Edisk > 1.84 V is a consequence of a decrease in the value of idisk,OER at the higher DMSO flux, i.e., the larger rotational velocity. The results for collection of O2(aq) shown in Figures 1 and 2 are indirect evidence that oxidation of DMSO persists at a transport-limited rate for Edisk > ∼1.8 V. Direct evidence supporting this conclusion is presented by so-called shielding data shown in Figure 3 obtained at 94 and 168 rad/s for the presence and absence of 10.0 mM DMSO. In this experiment, both the ring and disk electrodes were coated with Bi-PbO2 films and Ering was set at 1.75 V for direct detection of DMSO. Both the iring Edisk and idisk - Edisk curves in Figure 3 are independent of rotational velocity in the absence of DMSO (curve a). In the presence of DMSO (curves b and c), the iring response is attenuated from the maximum values observed for Edisk < 1.6 V as Edisk is increased to values of >1.8 V. Of greatest significance is the observation that, for Edisk > 1.8 V, iring values remain virtually constant; that is, the rate of DMSO oxidation at the disk does not diminish as the rate of O2(aq) evolution increases. Hence, we can conclude with certainty that the decrease in the current in
Figure 2. (A) Voltammetric response for 5.0 mM DMSO in 1.0 M HClO4 calculated by subtracting the current for O2 evolution from the total disk current. Electrode, Au disk coated with 1:2 Bi-PbO2 film. Edisk, 1.50-1.90 V at 40 mV/s. Rotational velocity (rad/s): (a) 94 and (b) 262. (B) Difference voltammetric response for 5.0 mM DMSO at the same electrode obtained by subtracting the voltammogram at 94 rad/s from the voltammogram at 262 rad/s.
∆idisk for Edisk > 1.8 V (see Figure 2B) is a result of a decrease in the component of idisk,tot coming from the O2(aq)-evolution reaction as the DMSO flux is increased. The flux of DMSO at a constant concentration can be increased by increasing the rotational velocity of rotated electrodes. This fact is illustrated by data in Figure 4 obtained using the same ring-disk system used to obtain data in Figure 1. The disk electrode coated with the Bi-PbO2 film was controlled at Edisk ) 1.70 V, a potential for which some O2(aq) is generated, and the uncoated Au ring electrode was controlled at Ering ) -0.30 V to detect O2(aq) generated at the disk electrode. Figure 4 was produced by recording iring as a function of time while the rotation speed of the RRDE was toggled between 94 and 408 rad/s. Curve a demonstrates that the rate of O2(aq) generation is independent of rotational velocity in the absence of DMSO. However, curve b, obtained for 5.0 mM DMSO, demonstrates that the rate of O2(aq) generation is attenuated by the presence of DMSO and, furthermore, that the rate of O2(aq) is decreased further by increasing the flux of DMSO. The prediction in eq 10 of a linear dependence of E1/2 on ln{ω1/2} and ln{CbR} was tested. Shown in Figure 5 are idisk Edisk curves for DMSO as a function of rotational velocity. Values of E1/2 taken from Figure 5 are plotted vs ln{ω1/2} as shown in the inset of Figure 5. This plot is linear (r2 ) 0.9985) with a slope of 0.0385 corresponding to R ) 0.67. The plot of E1/2 vs log{CbDMSO} (not shown) also was linear (r2 ) 0.9990) with a slope corresponding to R ) 0.55. These results are concluded to support the validity of the model based on eqs 1 and 2.
Figure 3. Voltammetric response for DMSO in 1.0 M HClO4. Electrodes, Au disk and ring coated with 1:2 Bi-PbO2 films. Edisk, 1.50-1.90 V at 40 mV/s. Ering, 1.75 V. DMSO concentration (mM): (a) 0 and (b, c) 10.0. Rotational velocity (rad/s): (a) 94 and 168, (b) 94, and (c) 168.
Figure 4. Cathodic detection of O2 at the ring electrode as a function of modulated rotational velocity. Potential, Ering ) -0.30 V. DMSO concentration (mM): (a) 0; (b) 5.0. Rotational velocity, toggled between 94 and 408 rad/s.
Equation 5 was used to calculate the idisk,tot as a function of Edisk. The surface concentration of the reactant in this equation was expressed as CsR ) CbR - idisk,tot/nFAm0, where m0 ) D/δ is the mass-transfer coefficient.10 Constants used in this calculation were as follows: k°WDR ) 0.1 cm/s, k°-WDR ) 1 × 10-5 cm/s, s k°OER ) 1 × 10-3 cm/s, k°OTR ) 0.1 cm/s, CH + ) 1 M, Γo ) 1 × -10 2 4 10 mol/cm , E°WDR ) 1.40 V vs SCE, E°OER ) 0.988 V vs SCE,11 E°RO,R ) 0.0 V vs SCE for the RO-R redox couple, and R ) 0.5. Voltammograms calculated as a function of rotational velocity are (10) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley and Sons: New York, 1980; Chapter 1.
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R ) 0.57 derived from these calculations. Perhaps this is an artifact of the onset of O2(aq) generation for small values of θS[OH] in the region of E1/2 even though idisk,OER > kOER; that is, the O2(aq)-evolution reaction is ignored. Of greatest significance is confirmation that the E1/2 value for O-transfer reactions is shifted to more positive values as a consequence of increased flux of the reactant because of the need to increase the flux of OH radicals with that of the reactant. This fact also explains the reason that plots of idisk,tot vs Edisk exhibit distinct current plateaus only at low values of reactant flux, i.e., when potentials are sufficiently low so that idisk,OER
