1057
Anal. Chem. 1986, 58, 1057-1062
LITERATURE CITED
APPENDIX The following.~ equations were taken from ref 13
1
VMOHMMOH - VAHMAH (H+)- K , = 0 VAH+ VMOH
(H+)2+
1
(7)
VMOHMMOH + KBH+ (H+)' + VBH + VMOH KBH+MMOH VMOH- KBH+MBH+VBH+ VBH + VMOH K w K ~=~0+(8)
(H+)3+
VMOH= -VBH+((H+)~ + KBH+(H')' - {Kw+ KBH+MBH+](H+) - KwKBH+/ (H+)3+ (KBH+ + MMoHKH+)~+ (KBH+MMoH- K,l(H+) - KwKBH+J(9) In eq 8 and 9 V B ~refers + to the volume of the solution under titration after having finished the first phase, and MBH+refers to'the corresponding concentration of B. Registry No. Sulfanilamide,63-74-1;sodium acetate, 127-09-3.
Watters, J. I.I n "Treatise on Analytical Chemistry", 1st ed.; Kolthoff, I . M., Elving, P. J., Eds.; Wiley: New York, 1975; Part I,Vol. 11, Chapter 114. Cox, D. C. J . A m . Chem. SOC.1925, 4 7 , 2138. Furman, N. H. Ind..€ng. Chem. Anal. Ed. 1930, 2 , 213. Luzzana, M.; Perreila, M.; Rossi-Bernardi, L. Anal. Biochem. 1971, 4 3 , 556. Busch, N.; Freyer, P. Anal. Biochem. 1977, 79, 212. Busch, N.; Freyer, P.; Szameit, H. Anal. Chem. 1978, 50(14), 2166. Fleck, G. M. "Equilibrios en disoiucibn", I r a ed.; Editorial Alhambra S. A,: Madrid, Espafia, 1967; p 99. Matsushita, H.; Ishikawa, N. Nippon Kagaku Kaishi 1976, ( l l ) , 1710. "Vogel'sTextbook of Practical Organic Chemistry", 4th ed.; Longman: New York, 1978; p 651. Martin, N. A,; Swarbrick, J.; Cammarata, A. "Physical Pharmacy", 2nd ed.; Lea & Febiger: Philadelphia, PA, 1969; p 194. "The Merck Index", 10th ed.; Merck & Co., Inc., 1983; p 1280. Westcott, C. Clarck "pH Measurements"; Academic Press: New York, 1978; p 158. Fleck, G. M. "Equilibrios en disolucibn", I r a ed.; Editorial Alhambra S. A,: Madrid, Espafia, 1967; p 71.
RECEIVED for review May 13,1985. Accepted November 11, 1985. Support of this work was from CONICET and CONICOR (Grants 560/84 and 340/84, respectively).
Electrochemical Reduction of Dioxygen to Perhydroxyl (HO,.) Aprotic Solvents That Contain Brmsted Acids
in
Pablo Cofr6 and Donald T. Sawyer*
Department of Chemistry, Texas A&M University, College Station, Texas 77843
I n acetonltrlle (MeCN) and dlmethylformamlde the effect of proton sources (HC104 and PhOH) on the electrochemistry of 0, at platlnum (PI) and glassy carbon (GC) electrodes has been studied by rotated ring-disk and cyclic voltammetry. With weak Brernsted acids (H20 and PhOH) the reverslble reduction of 0, to O,-. Is followed by protonation to form HO,. A-). For stronger acids perhydroxyl (O,-. 4-HA at GC there Is dlrect formation of HO,. (0, HA eH02'(ads) A-), which chemlsorbs to the electrode surface and dlsproportlonates to 0, and H202;wlth excess protons a HA eH202 second electron transfer occurs (HO,.,,,,, 4- A'-). For strong acids In MeCN direct reduction of the proton occurs at PI (H+ eprlor to electron transfer to 0,; the chemisorbed hydrogen atoms react wlth O2 ("(ads) + O2 H02'(ads) '/2H202 + '/202)*
-
+
+
+
+
+
+
+
+
-
-
-+
+
The electrochemistry of dioxygen (0,) is one of the most extensively studied processes (1-4), but a reasonable understanding of the electron-transfer mechanism for its reduction has been gained only during the past 2 decades through the use of dipolar aprotic solvents (5-14). Thus, in acetonitrile (MeCN) and dimethylformamide (DMF) the reduction of O2 is a reversible one-electron process (15) 0 2
+ e- + 0 2 - - EMecN0' =
-0.90 V vs. SCE
(1)
The resulting superoxide ion is stable in the absence of proton sources but rapidly disproportionates upon protonation (16)
HOy
+ HO2.
-
HzOz + 0
2
(2)
Although the effect of protons on the electrochemistry of O2 has been noted in several previous studies (15, 16),there has not been a systematic characterization of the electrontransfer reduction for O2 in the presence of Bronsted acids. Superoxide is a natural intermediate that is produced in biological respiration and metabolism; its protonated form, H02., can initiate lipid peroxidation and autoxidation ( 17). This occurs because H02. can oxidize substrates with allylic functions via hydrogen-atom abstraction (e.g., 1,4-cyclohexadiene (1,6CHD)). The present study has been directed, in part, to ascertain whether direct production of H02. by electron transfer to O2 in biomembranes is feasible and a potential biohazard. The effect of solvent and electrode material on the electrochemical reduction of oxygen in the presence of Bronsted acids is the primary focus of the study. Rotated ring-disk voltammetry has been the primary technique for the investigation, because the product species that are generated via reduction of O2 at the disk can be characterized a t the ring electrode within a few milliseconds.
EXPERIMENTAL SECTION Instrumentation. The rotated ring-disk measurementswere made with a Pine Instruments Co. Model PIR rotator with either Pt-Pt or GC-GC ring-disk electrodes. The parameters of the electrodes were as follows: Pt-Pt electrode, rI = 0.382 cm, r2 = 0.399 cm, r3 = 0.422 cm, N = 0.178; GC-GC electrode, r1 = 0.382 cm, r2 = 0.416 cm, r3 = 0.556 cm, N = 0.418. Potential control was provided by a Pine Instruments Co. Model RDE 3 dual potentiostat. The sample solutions and electrode assembly (including a Pt auxiliary electrode in a separate tube with a medium-porosity fritted-glass disk at the end and a Ag/AgCl reference electrode in a luggin capillary (15))were contained in a 150-mL beaker with a Leeds and Northrup plastic cell top. 1986 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
The ring currents and disk currents vs. disk potential in the collection-efficiency experiments were recorded simultaneously on two Houston Model 2000 X-Y recorders. The linear-sweep voltammetric experiments were accomplished with the same instruments and electrodes. Reagents and Solvents. Acetonitrile (MeCN) and dimethylformamide (DMF) were supplied by Burdick and Jackson (“distilled in glass grade”) and were used without further treatment. All of the sample solutions contained 0.1 M tetraethylammonium perchlorate (TEAP) (reagent grade, G. F. Smith Chemical Co.) as the supporting electrolyte. Stock solutions of 0.1 M HCIOl (reagent grade) in water and 0.1 M PhOH in MeCN and in DMF were used as proton sources. The concentration of O2in the sample solutions for each experiment was adjusted by saturating them with 02/Ar gas mixtures of variable composition. The resulting equilibrium 0, concentration was determined by measuring its limiting reduction current at a rotated-disk electrode. The different 02/Ar gas mixtures were obtained by adjusting their relative gas flows as described elsehwere (17). The solubility of O2in the solvents has been reported previously (15). Methodology. After the electrode surface was polished with Buehler no. 3 (0.05-pm) alumina, its potential was scanned within the voltage limits of the solvent. The oxygen-reduction limiting current was used to determine the oxygen concentration and the electron stoichiometry. The proper potential setting of the ring electrode for diffusion-controlled currents of product species was determined by setting the disk-electrodepotential at the desired negative potential and scanning the ring-electrode potential. Collection-efficiency experiments were done by setting the ring electrode at the desired constant potential, while scanning the disk-electrode potential in a negative direction at a scan rate of 10 mV/s. The simultaneous recording of ring and disk currents was accomplished with separate recorders.
RESULTS Strong Acid. Figure 1 illustrates the effects of protons, solvent (MeCN and DMF), and electrode material (platinum, section A and B, and glassy carbon, section C and D) on the electrochemical reduction of dioxygen. The curves (a-c or f) in the upper half of each section (A-D) represent the reduction current at the disk electrode as a function of its potential for proton/02 mole ratios that range from 0 to 30. The primed curves (a’-c’ or f’) of the lower half of each section (A-D) represent the current at the ring electrode (with its potential held at +0.3 V vs. SCE for Pt and 0.0 V vs. SCE for GC) that results from the products produced by the reductions a t the disk electrode (as a function of Edlsk).In the absence of protons oxygen is reduced to 0,- (a); the product is reoxidized a t the ring electrode (a’). The addition of HC104to the O,/Pt/MeCN system causes a new reduction wave to appear at -0.2 V vs. SCE (curve b, Figure 1A) and that due to O2to disappear. The limiting current for this new wave is directly proportional to the proton concentration (c). The ring-current trace also is affected by the addition of HCIO1. The initial oxidation-current trace for the reduction product of O2 (a’) is eliminated (b’), but a small reduction current is obtained instead for disk-electrode POtentials more negative than -1.0 V. With higher proton concentrations the reduction current of curve b’ is not observed, but an oxidation peak appears when ED is -0.4 V (c’). This peak current is proportional to proton concentration. For the 02/Pt/DMF system the addition of protons causes the O2 reduction wave (curve a, Figure 1B; Ell2 = -0.9 V VS. SCE) to decrease and a new reduction wave to develop at more positive potentials (b-d; Elj2= -0.3 V). The current for this new wave increases with proton concentration until the initial oxygen reduction wave is totally replaced. With a large excess of protons, a new reduction occurs at -0.15 V. The oxidation current a t the ring electrode for the product of O2reduction a t the disk electrode decreases as the proton concentration is increased (a’+; a small reduction current is observed when
A C
\
L/
IZoopA 1
0
1
1
-0.8
1
I
I
0 lA: * l
-16
-08
Ll -1.6
E, vs S C E / V
Figure 1. Effect of protons, solvent, and electrode material on the voltammetric reduction of oxygen at a disk electrode and the electrochemistry of its reduction products at a ring electrode with a fixed potential: (A) 0.22 mM O2in MeCN (0.1 M TEAP) at Pt/Pt (1600 rpm) with (a) 0, (b) 0.27, and (c) 1.62 mM HC104 ( E R= +0.3 V vs. SCE); (B) 0.54 mM O2in DMF (0.1 M TEAP) at Pt/Pt (1600 rpm) with (a) 0, (b) 0.14, (c)0.54, (d) 3.24, (e) 13.0,and (f) 17.8 mM HC104 (ER= +0.3 V vs. SCE);(C) 2.97 mM O2in MeCN (0.1 M TEAP) at GC/GC (1600 rpm) with (a)0, (b) 3, and (c) 10 mM HCIO, ( E R = 0.0 V vs. SCE);(D) 4.8 mM O2in DMF (0.1 M TEAP) at GC/GC (1600 rpm) with (a)0, (b) 6, and (c) 48 mM HCIO4 ( E R= 0.0 V vs. SCE). The primed letters (af-ff)are for the currents that result at the ring-electrode potentials (ER)during the course of the voltammetric scan of the disk electrode.
EDis more negative than -0.9 V if the proton concentration is carefully adjusted (d’). A new oxidation wave appears when E D is at -0.5 V (d’), which increases in height as the proton concentration is increased (e’ and f’; the small reduction current of curve d’ disappears). The addition of HC104to the 02/GC/MeCN system causes disk-ring traces for oxygen (curves and a’, Figure 1C) to change. For a 1:l ratio of HC104:02(b) a prewave at -0.6 V appears, which increases in height with further increases in HC104concentration. The ring trace for the reduction product of O2disappears (b’), and a small reduction peak appears when ED is more negative than -1.0 V. Higher proton concentrations result in a single reduction wave (Ell2= -0.9 V) with a small prewave at -0.2 V that is independent of HC104concentration (c). A small reduction current occurs at the ring electrode when E D is more negative than -1.1 V (c’). When HC104 is added to the 02/GC/DMF system the initial O2 reduction wave (curve a, Figure 1D; Ellz = -1.0 V) is replaced by a wave with a E l p of -0.65 V (b),which increases in height with HC104 concentration and becomes the only reduction (c). The limiting current for this wave becomes independent of HC104 concentration, and its height can be up to 80% higher than the initial wave for O2 in a proton-free system. Addition of HC104causes the ring current to decrease (curve b’, Figure lD), and with excess HC104the presence of oxidizable products is completely suppressed and a small
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986 A
1059
C
GC-GC ID
C
I,
I
0
1
I
-0.4 -0.8 -1.2
I -1.6
I -2.0
I
1
-0.4 -0.8
I
I
-1.6
-2.0
I
-1.2
I
-0.4
I ’
-0.8
1 -1.2
I -1.6
I
-2.0
E, vs S C E / V Flgure 2. Effect of acid strength and electrode material on the voltammetric reduction of oxygen at a disk electrode and the et-ctrochemistry of its products at a ring electrode with a fixed potential: (A) 0.4 mM O2 in DMF (0.1 M TEAP) at GC/GC (3600 rpm) with (a) 0, 0)0.4, (c) 0.8, and (d) 0.8 mM HCIOl plus 0.1 M 1,Ccyclohexadiene (ER= 0.0 V vs. SCE); (B) 0.49 mM O2in MeCN (0.1 M TEAP) at Pt/Pt (1600 rpm) with (a) 0,(b) 0.27, (c) 0.54, (d) 1.1, and (e) 2.7 mM PhOH (ER= 4-0.2V vs. SCE); (C) 0.49 mM O2 in DMF (0.1 M TEAP) at GC/GC (1600 rpm) with (a) 0 and (b) 9 mM PhOH (ER = 0.0V vs. SCE). The primed letters (a’-e‘) are for the currents that result at the ring-electrode potentials during the course of the voltammetric scan of the disk electrode.
reduction limiting current occurs for disk potentials that are more negative than -0.9 V. When the previous experiment is repeated at lower oxygen (0.4 mM) and HCIOl concentrations (Figure 2A), an unexpected disk trace is obtained. Curves a and af represent the reduction of O2 and the oxidation of 02-., respectively. The addition of controlled amounts of HC104 causes two new reduction waves to develop at less negative potentials (b and c). An increase in proton concentrationcauses the first to grow proportionately, while the height of the second remains constant (this is characteristic of an adsorbed species at the electrode surface). The corresponding ring traces (b’ and c’) show the expected decrease in oxidation current; oxidation current does not result from the disk currents through the first two reduction waves. Weak Acid. The curves of Figure 2B (Pt/MeCN) illustrate the effect on the electrochemistry for O2 and its reduction products from the addition of a weak acid (phenol). The addition of phenol causes the reduction wave for O2to increase in height and its half-wave potential to shift to less negative potentials (a-c); the limiting current grows with PhOH concentration to more than twice the initial value (e vs. a). The traces for ring currents indicate a decrease in oxidation current together with an initial wave shift toward less negative potentials (b’-df). The gradual increase in PhOH concentration causes the development of some peaks in ring current (b’-d’) that vanish at higher concentrations. A large excess of PhOH does not completely suppress the ring oxidation current, but results in a further increase in oxidation current at potentials close to the negative end of the disk-potential scan. Addition of PhOH to a 02/GC/DMF system causes the disk current for oxygen reduction to increase and the Ellz to shift to less negative potentials. The potential shift is less than in MeCN, and with a large excess of PhOH the limiting current becomes constant and about 80% larger (curve b, Figure 2C) than the initial value (a). The ring currents for the product species decrease, but are not completely suppressed by the addition of PhOH (bf). Surface Phenomena. Rotated ring-disk experiments with low oxygen and HCIOl concentrations (Figure 2A) yield a reduction wave (b-d) that is characteristic of surface phe-
nomena. Figure 3A illustrates linear-sweep voltammograms for the reduction of O2 in DMF at a GC electrode (a) and with increasing amounts of HC104 (b-e). With small amounts of acid the height of the initial O2 reduction peak (Epc= -0.9 V, curve a) decreases, its peak potential shifts toward less negative values, and a peak at -0.6 V develops that is preceded by a small shoulder at -0.45 V (b). The effect of increased amounts of HC104is more pronounced for the -0.45-V peak (c). A small excess of HC104 (d) causes this peak to become larger than the -0.6-V peak, which essentially remains constant in height. Both of these shift to more negative potentials with increased HC104,while the O2 reduction peak decreases in height and shifts toward less negative potentials. With a large excess of HC104 only a single reduction peak at -0.65 V is observed (e). The height of this peak becomes independent of HC104 concentration with further increases. The peak at -0.45 V (Figure 3A) increases linearly with the square root of voltage scan rate, but that at -0.6 V increases linearly with scan rate (characteristic of an adsorbed species). The O2 reduction peak does not shift with scan rate, but the two prepeaks shift toward more negative potentials. Integration of the peak current for the peak at -0.6 V gives a surface charge of 20 KC cm-2. When the solvent is changed from DMF to MeCN (Figure 3B) the same pattern results from the addition of HC104 (b-e). The peak potentials are shifted more in MeCN with the addition of protons and are better resolved. The second prepeak, which increases linearly with scan rate, splits into two adjacent peaks (c and d). Integration of the peak current gives a surface charge of 19.6 KC cm-2. When the electrode material is changed to Pt, the addition of HCIOl causes a new peak at 0.0 V to develop that is followed by one or two small peaks (curves c and d, Figure 3C). The heights of the latter are independent of proton concentration and increase linearly with scan rate. The peak at 0.0 V increases directly with HC104concentration (b-e) at the expense of the initial oxygen reduction peak (a). Gold Electrode. When a gold electrode is used the reduction of O2and the effect of added protons are a compromise between those obtained at platinum and GC electrodes. Oxygen reduction occurs at a slightly more negative potential
1060
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986 A
A
I'
DMF
DISK a
I
0
I
0
I
I
I
I
-0.4
-1.2
-0.8
I
-1.6
- 3
ED v s SCE/V
-0.2
-0.4
-0.6
-0.6
-1.0
-1.2
Figure 4. Effect of 0.1 M 1,4-cyclohexadiene (b) on the electrochemical reduction of 1.4 mM O2plus 1.4 mM HCIO, in MeCN (0.1 M TEAP) at a Pt disk electrode (1600 rpm) and on the current at its Pt-ring electrode (b'; E, = +0.3 V vs. SCE). Curves a and a' are for the system without any 1,CCHD.
MeCN GC-GC
tu.*
0
-0.4
-0.6
-1.2
I
I
-1.6
-2.0
E vs S C E I V
Figure 3. Linear sweep voitammograms (1 V min-') of dissolved oxygen in the presence of increasing concentrations of HCIO,: (A) 0.37 mM O2in DMF (0.1 M TEAP) at GC with (a) 0, (b) 0.11, (c) 0.37, (d) 1.11, and (e) 1.85 mM HCIO,; (B) 0.22 mM O2in MeCN (0.1 M TEAP) at GC with (a) 0, (b) 0.14, (c) 0.24, (d) 0.79, and (e) 1.23 mM HCIO,; (C) 0.61 mM 0, in MeCN (0.1 M TEAP) at Pt with (a) 0, (b) 0.27, (c) 1.08, (d) 2.16, and (e) 3.24 mM HCIO,.
than at GC (in DMF; Elj2(GC) = -0.96 V, Elj2(Pt)= -1.04 V, Elj2(Au)= -1.02 V). As with GC, the addition of HC104 causes the oxygen reduction wave to be replaced by a wave at -0.6 V. With MeCN even small relative concentrations of HC104 make possible the detection of a reduction-current peak at an Au-ring electrode ( E R = +0.3 V) once the disk potential is more negative than the first reduction wave. This reduction peak grows with the HC104concentration into a well-defined wave. The height of this reduction wave is decreased by the presence of 1,4-cyclohexadiene. Linear sweep voltammetry of O2 with HC104 in MeCN yields results that are similar to those for GC. The main difference is that an excess of HC104causes not only the initial O2reduction peak (EP= -1.0 V) to be replaced by peak at a less negative potential (Ep= -0.4 V) but also the development of new reduction peaks at potentials more negative than -1.0 V. The peak at -0.4 V increases proportionately to HC104 concentration without apparent limit. The adsorption peaks that are present with Pt and GC also appear at an Au electrode. Electrochemistry of H O p Attempts to detect the formation of H02. at the disk electrode from the reduction O2 in the presence of protons failed for a variety of conditions with ambient concentrations of O P However, if the concen-
+0.8
+0.4
0
E,
-0.4
-0.8
-1.2
vs S C E I V
Figure 5. Effect of 1,4-~yciohexadiene on the current-voltage response curve for the products that result from the electrolysis at a GC disk electrode of 2.25 mM O2 plus 4.5 mM HC10, in MeCN (0.1 M TEAP). Curve a is the response for E , = 0.0 V vs. SCE and without 1,4-CHD, curve b for Eo = -1.2 V vs. SCE and without 1,4-CHD, and curve c for E , = -1.2 V vs. SCE and with 0.1 M 1,4-CHD; curve b' = (curve b - curve a).
trations are lowered by an order of magnitude and the proton-to-02 ratio also is decreased then two separate O2 reduction waves result. The first corresponds to the reduction of O2 in the presence of protons to produce H02. at the electrode surface, while the second is due to the reduction of O2 to 02-..Because the HOz. that is formed readily disproportionates to H,02 and 02,the second reduction wave height is enhanced by this newly formed 02. The addition of a large excess of 1,4-cyclohexadiene (1,4-CHD) should react with H02- before it can disproportionate. This would decrease the oxygen contribution to the second reduction wave and lower its limiting current. Figure 4 illustrates the results for such an experiment with a Pt-Pt ring-disk electrode in MeCN and a proton:02 mole ratio of 0.6:l. The reduction current at the ring electrode (ER= +0.3 V, curve a') is due to a product that is formed when the disk electrode is scanned past the first reduction wave for O2 (ED more negative than -1.2 V) to give 02-.(a). The addition of 1,CCHD causes the disk current to decrease (b) as well as the ring reduction current (b').
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
1061
Figure 5 illustrates some ring electrode scans at GC in MeCN for an optimal proton-02 concentration ratio for HOzgeneration. Curve a represents the ring trace when there is no disk reaction (E, = 0.0 V). Curve b corresponds to a disk potential (ED= -1.2 V) that is negative enough to cause oxygen to be reduced to HOz. and O p . Subtraction of curve a from curve b gives curve b’, which represents the net reduction peak for a species formed a t the disk (apparent E,, = -0.05 V). The addition of l,$-CHD hinders that reduction peak as shown by curve c.
concentration when it is added in excess (curves b and c, Figure 1D). Because the ring-electrode trace does not exhibit an oxidation current when the disk-electrode potential is at the limiting current for this new wave (curves b’ and c‘, Figure lC,D), O p is not a reduction product. The limiting current for the 02/02-. process also decreases with increasing HC104 concentration (curves b and c, Figure lC,D), which means that 0, is consumed by this new reduction process. These observations are consistent with a concerted HC/02reduction process
DISCUSSION AND CONCLUSIONS The electrochemical reduction of 0, in an aprotic matrix is essentially independent of solvent and electrode material and is a reversible process (curves a and a’ of Figures 1-3)
02
O2
+ e- + 02-. E I j z= -0.9
f 0.2
V
+ e-
-
H.(Pt) El12= -0.4 V vs. SCE
(4)
-
2
-
HOY(ads)
(5)
This process consumes 0, and does not produce 02-., which accounts for the decrease in the anodic current at the ring electrode and for the absence of a ring-electrode current when the disk electrode potential reduces protons (curves b’-d‘, Figure 1B). With excess protons their reduction produces H,, which is oxidized by the ring electrode (curve e’, Figure 1B). The results are similar for the 02/HC104/Pt/MeCNsystem (Figure lA), but the O2reduction wave o c c w at more negative potentials, and the H+ and (H+ 0,)reduction occur at more positive potentials. This is the consequence of MeCN being a much poorer solvating agent for protons and anions than is DMF. Another important difference is that a smaller amount of HC104 will completely suppress the 02/02-. reduction peak (curves b and b‘, Figure 1A). This indicates that reaction 4 is so efficient that it results in the complete coverage of the Pt electrode such that all 0, molecules which contact the surface are converted to H02.(ads). The same overall mechanism prevails when PhOH rather than HClO, is the proton source (Figure 2B). However, proton reduction occurs a t a much more negative potential and the product (PhO-) is oxidized at the ring electrode (curves c’-e’, Figure 2B). In general the same mechanism holds for reduction of 02/HA systems at gold as for platinum electrodes. Glassy Carbon. Although protons can be reduced at GC electrodes, much more negative potentials are required than for Pt or Au. Furthermore, oxidation of H, is not observed at GC electrodes (18),which indicates that the species is not stabilized by this surface. With a GC electrode the protons of HC104 are reduced at a more negative potential (-1.4 V vs. SCE) than is 0,. Furthermore, the limiting current for the new wave that develops upon the addition of HC104 becomes independent of acid
+
HOy(ads) E l p = -0.7 V
VS.
SCE
(6)
With excess acid the latter species is more easily reduced than the Oz/H+ system
+ H+ + e-
H02*,,ds)
This represents the initial step in Hz generation at a Pt electrode (2H.(Pt) H2) (18). When excess 0, is present the reduction process of eq 4 occurs at a more positive potential (for HClO, in DMF, E , / , = -0.16 V). Thus, the presence of O2 facilitates the removal of H-(Pt) from the electrode surface by the chemical reaction H*(Pt) + 0
-
(3)
However, the presence of protons causes the electrochemistry to become much more complex and dependent upon electrode material and solvent (curves b and c and b’ and c’ of Figures 1 and 2). Platinum. With a Pt electrode and a strong Brransted acid (HClO,) proton reduction to H.(Pt) precedes electron transfer to O2(curves b and c, Figure 1A; curves b-f, Figure 1B). This proton reduction wave shifts to more negative potentials for weak acids (PhOH, curves b e , Figure 2B) and for more basic solvents (HClO, in DMF; curves b-f, Figure 1B) HA(so1v)
+ H+ + e-
--*
&Oz
(7)
which accounts for the apparent two-electron per 02 stoichiometry of curves c in parts C and D of Figure 1 and the absence of any anodic current at the ring electrode. The perhydroxyl radical (HO,.) is unstable and rapidly disproportionates, both in the solution phase (18) and at the electrode surface 2H02. HzOz + 0 2 (8)
-
Curve b’ of Figure 1C indicates the development of a reduction wave at the ring electrode (ER= 0.0 V vs. SCE) when the HC104concentration is carefully adjusted and the diskelectrode potential is scanned negatively (curve b, Figure 1C). This reduction wave at the ring must be due to HOz. that is formed in the solution phase
02-. + H+
H02. + e-
-
HOz.
- HOC
H+
H,Oz
(9) (10)
because reduction current is observed at the ring electrode only after the disk potential is sufficiently negative to reduce 0,to 0;. The rapid disproportionation of HO,. via reaction 8 limits the amount of HO,. that reaches the ring electrode, which results in extremely low collection efficiencies (curve b, Figure 1D). High proton concentrations shift the 02-./H+ reaction zone toward the disk, which facilitates reaction 8 and eliminates the HO,. reduction at the ring electrode (curve c’, Figure IC). The appearance of a reduction current at the ring electrode (curve a’, Figure 4; ER = +0.3 V vs. SCE) only after the disk-electrode potential is more negative than -1.2 V (0, + e- O,-.; curve a, Figure 4).is consistent with the formation of HO,. via reactions 3 and 9. This is a system with limiting amounts of protons relative to the 0, flux to the electrode. The complete suppression of reduction current at the ring electrode by the addition of excess 1,4-cyclohexadieneis independent proof that it is due to HO,. (curve b’, Figure 4). Previous work (19)has demonstrated that HO,. is consumed by 1,4-CHD HOz. 1,4-CHD HzO2 + 1,3-CHD. (11)
-
+
1,3-CHD.
-
-
‘/,1,3-CHD
+ 1/2PhH
(lla)
By subtracting the current-voltage curve for the GC ring electrode (with the GC disk electrode at 0.0 V vs. SCE) for a 2:l mole ratio of HC104to 0, in MeCN from that with the GC disk electrode at -1.2 V vs. SCE (curves a and b, Figure 5), a net reduction wave (b’) is obtained that can be attributed to electron transfer to HO,.
+ H+ + e-
-
H 2 0 2 E l l , = +0.05 V vs. SCE (12) The presence of 1,4-CHD substantially suppresses the ring current and completely eliminates this differential current
HO,.
1062
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
(curve c, Figure 5). The enhanced anodic current of the latter curve is probably due to the oxidation of the intermediate products of reaction 11. Because reaction 6 requires free protons, the reduction of O2 at GC in the presence of a weak acid (PhOH) is a simple electron-transfer process (eq 3); the ring-current trace indicates the formation of 0,- for all disk potentials where O2is reduced (curves b' and b, Figure 2C). Phenol is not reduced at GC electrodes, which means that the positive shift for O2reduction in its presence is due to a chemical reaction after electron transfer (eq 3) (18)
02-.+ PhOH -!., H02. + PhOThe increase in the O2 limiting current with increased concentrations of PhOH is due to the disproportionation of H02. to O2and H202(eq 8). The limiting current increases by up to a factor of 1.8; a larger number of turnovers (eq 3, 9, 13, and 8) would be required to achieve a limiting current that is consistent with an overall two-electron reduction of 02. Adsorption of €IO2.. For conditions of low Oz and HC104 concentrations the current-voltage curves for a GC electrode exhibit two prewaves (curves b-d, Figures 2A and 3A). The height of the second of these does not increase with an increase in HC104concentration and can be attributed to the reduction of HOz.(ads)that is formed by reaction 6. This process occurs at a more negative potential than that for H02. formed in solution because the H+ concentration at the electrode surface is much lower than in bulk solution H02*(ads)
+ e-
GC, DMF
HOz
= -0.6 V
VS.
SCE (14)
Thus, the H02. that is produced at the electrode surface is an adsorbed species which is either reduced by reaction 7 or 14 or disproportionates at the electrode surface via reaction 8 (Figures 1 and 2). The integrated current-voltage curves for the adsorption peaks (peak current directly proportional to voltage scan rate) under limiting proton fluxes (curves b and c, Figure 3A-C) are consistent with 9% coverage of an electroactive species (H02yads)) from the (0,+ H') reduction of reaction 6. This assumes a cross sectional area for H o p of 6.7 X cm2 and close packing on a flat surface. Hence, electrogenerated H02-does not escape the electrode surface
until reduced to H0f/H202or disproportionated to H202and 02.
The only way to produce H02-in the solution phase from electrochemicalreduction of O2is to adjust the proton-02 ratio such that some 02-.is produced at the disk electrode, which diffuses away from the surface and reacts with a proton in the bulk solution (reactions 3 and 9).
ACKNOWLEDGMENT Most of the experiments were performed at the University of California, Riverside. We are especially grateful to Rafael Gana of the Catholic University, Santiago,Chile, for the award of a sabbatical leave to Pablo Cofr6. Registry No. Oz, 7782-44-7;H02., 3170-83-0;Pt, 7440-06-4; HC104,7601-90-3;PhOH, 10895-2;Au, 7440-57-5;HzOz, 7722-84-1; carbon, 7440-44-0; 1,4-cyclohexadiene,628-41-1. LITERATURE CITED (1) Bagotskii, V. S.; Nekrasov, L. N.; Shumiiova, N. A. Usp. Khim. 1965, 34, 1697 [Russ. Chem. Rev. (Engl. Transl.) 1985, 3 4 , 7171. (2) Damjanovic, A. Mod. Aspects Nectrochem. 1969, 5 , 369. (3) Hoare, J. P. Encycl. Electrochem. Elem. 1974, 2 , 191. (4) Appieby, A. J. Mod. Aspects Electrochem. 1974, 9 , 369. (5) Maricie, D. L.; Hodgson, W. G. Anal. Chem. 1965, 3 7 , 1562. (6) Sawyer, D. T.; Roberts, J. L., Jr. J . Electroanal. Chem. 1966, 12, 90. (7) Peover, M. E.; Whlte, B. S. Electrochim. Acta 1966, 1 1 , 1081. (8) Johnson, E. L.; Pool, K. H.; Hamm, R. E. Anal. Chem. 1967, 3 9 , 88. (9) Dubrovina, H. I.; Nekrasov, L. N. Electrokhimiya 1972, 8, 1503 [Sov. Nectrochem. (Engl. Transl.) 1972, 8 , 14661. (10) Bauer, D.; Beck, J. P. J. Electroanal. Chem. Interfacial Electrochem. 1972, 4 0 , 233. (11) Chevaiet, J.; Rouile, L.; Gierst, L.; Lambert, J. P. J . Electroanal. Chem. Interfacial Electrochem. 1972, 3 9 , 201. (12) Dlvesek, J.; Kastening, B. J . Nectroanal. Chem. Interfacial Nectrochem. 1975, 6 5 , 603. (13) Sawyer, D. T.; Seo, E. T. Inorg. Chem. 1977, 16, 499. (14) Wilshire, J.; Sawyer, D. T. Acc. Chem. Res. 1979, 72, 105. (15) Sawyer, D. T.; Chlericato, G., Jr.; Angelis, C. T.; Nanni, E. J.; Tsuchiya, T. Anal. Chem. 1982, 54, 1720. (16) Chin, D. H.; Chiercato, G., Jr.; Nanni, E. J., Jr.; Sawyer, D. T. J. Am. Chem. SOC. 1982, 104, 1298. (17) Roberts, J. L., Jr.; Sawyer, D. T. I s r . J . Chem. 1983, 23, 430. (18) Barrette, W. C., Jr.; Johnson, H. W., Jr.; Sawyer, D. T. Anal. Chem. 1984. 56, 1890. (19) Sawyer, D. T.; Roberts, J. L., Jr.; Caiderwood, T. S.; Sugimoto, H.; McDoweil, M. S. Philos. Trans. R . SOC.London 6 1985, 371, 483.
RECEIVED for review October 28, 1985. Accepted January 9, 1986. This work was supported by the National Science Foundation under Grant CHE-8212299 and the National Institutes of Health under Grant GM-36289.
