3238
J. Phys. Chem. 1981, 85,3238-3244
The Effect of Electrolyte Anion Adsorption on Current Efficiencies for the Evolution of Ozone Peter C. Foller" and Charles W. Tobias Departments of Chemistry and Chemical Engineering, University of California, Berkeley, California 94720 (Received: May 4, 198 1; In Final Form: June 26, 198 1)
Ozone may be obtained in relatively high yield by the electrolysis of acidic aqueous solutions with @-leaddioxide anodes. The anionic coverage of the @-Pb02electrode surface has a strong influence on the ozone current efficiencies obtained;phosphorous-32 radiotracer measurements have demonstrated that, where anion coverage of the Pb02 anode surface is high, ozone current efficiencies are low. Anionic adsorption on PbOz at ozone evolution potentials is so persistent that the polarization history of the anode determines its subsequent behavior with respect to ozone formation. Such a conclusion lead to adjusting the adsorptivity of Pb02for electrolyte anions through reduction of surface lead to oxygen stoichiometry. By so adjusting the adsorptivity, it is shown that one may alter ozone current efficiencies substantially. It is speculated that anionic coverage acta to weaken the free energy of adsorption of oxygen,and it is through such an influence that the formation of ozone is affected. Introduction Increasing interest in the uses of ozone as an environmentally desirable alternative in water and waste treatment has motivated this laboratory to reopen its earlier investigations1into the kinetics of the anodic oxidation of water to ozone. Nonporous lead dioxide anodes have only recently been shown to be among the most effective for the electrolytic production of The synthesis of ozone requires an inert anode with a high oxygen overvoltage, and is most efficiently carried out at high current densities in concentrated acid solution a t reduced temperature. The literature of electrolytic ozone evolution reflects some confusion arising from a lack of understanding of the fundamental processes affecting current efficiency. It appears that the reaction is controlled by the chemical nature and degree of anionic coverage of the anode surface employed.M The complex phenomena so arising have heretofore defied a comprehensive analysis. In the following it will be shown that (1)there is apparently an ideal anionic coverage of the anode surface that produces maximum ozone current efficiency, (2) on the P-Pb02 surface during ozone evolution, anionic coverage is sufficiently persistent to influence measurements of ozone current efficiency at any subsequently applied potential or current density, (3) the adsorptivity of the P-Pb02 surface may be altered through a reduction of surface Pb to 0 stoichiometry as a technique to enhance ozone yields, and (4)the influence of anion adsorption is probably through the alteration of the free energy of adsorption of oxygen atoms which may be intermediate to ozone synthesis. Ozone current efficiencies are strongly dependent upon current density. Simply stated, one must increase the anode potential well beyond that of the oxygen evolution reaction for measurable ozone evolution to occur, and increasing current density is one of the most obvious means to accomplish this end. Thus, plots of ozone current efficiency vs. current density have been used frequently in the literature to characterize the reaction and the effects of other experimental parameters. A problem exists, however, in that differing polarization procedures develop
differing portrayals of what yields may be achieved. One of the most convenient methods of taking ozone current efficiency data is to increase current density step-by-step while simultaneously measuring the ozone concentration in the anodic gas stream. The lower curve of Figure 1 shows the result of a 15 min per point stepby-step polarization procedure in 0 "C 2 M H2S04with a freshly prepared P-Pb02anode (deposited according to the method of Stojkovic7). It is apparent that this procedure yields information different from that obtained in discrete experiments, in each of which a single constant current density is maintained for 90 min. The result of the latter procedure appears as the upper curve in Figure 1. It is important to note that the irregularity in the 15 min per point curve is greatly amplified in the results of the separate polarizations. The magnitude of the yields is also considerably higher. This curious phenomenon is seen in phosphoric acid as well (Figure 2, curves B and C) and was selected for more detailed investigation. The evolution of ozone may be carried out at highest current efficiency in acids of the fluoro anions. Tetrafluoboric acid and hydrogen hexafluorophosphate are particularly effective electrolyte^.^ The present study, however, deals with specific phenomena arising in the electrolysis of phosphoric and sulfuric acids with P-Pb02 anodes, the rationalization of which should aid in the understanding of the ozone evolution reaction as a whole. The experimental apparatus used to determine ozone current efficiency (by the UV photometric method) has been previously d e ~ c r i b e d . ~ Phenomena Leading to the Hypothesis of Anionic Coverage Control There are several trends in the behavior of ozone current efficiency data that have been described previously which lead one to suspect that anionic coverage of the anode surface may be of great influence. Among these are the (1)Seader, J.; Tobias, C. Ind. Eng. Chem. 1952,44,9,2207-11. (2) Semchenko, D.;Lybushkina, E.; Lybushkin, V. Izu. Seu.-Kauk.
Nuuchn. Tsentra Vyssh. Shk. Ser. Tekn. Nuuk. 1975,3(1), 98-100. (3) Senchemko, D.;Lybushkina, E.; Lybushkin, V. Elektrokhimiya 1973,9,11, 1744. (4)Fritz, H.; Thanos, J.; Wabner, D. Z . Nuturforsch. B 1979, 43, 161 - - 7-27. . - ..
*Address correspondence t o this author at T h e Continental Group, Inc., Energy Systems Laboratory, 10432 N. Tantau Ave., Cupertino, CA 95014.
(5) Foller, P.; Tobias, C. Submitted to the J. Electrochen. SOC. (6) Foller, P., Dissertation, University of California, Berkeley, 1979. (7)Stojkovic, D.;Jaksic, M.; Nikolic, B. Bull. SOC.Chirn. Beogrud 1969,34,211-6.
0022-365418112085-3238$01.25/00 1981 American Chemical Society
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The Journal of Physical Chemistry, Vol. 85, Iflo.
Electrolytic Evolution of Ozone from Solutions I
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concentrations can more than double the ozone current efficiency. Among the most interesting of these phenomena is the dependence of the ozone current efficiency on current density with respect to varying the polarization procedure.
I
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2 M H2S04
s 2
P - P b 0 2 ,O"C
i
< 0.1
0.3
0.5
0.7
0.9
A /cm2
Figure 1. The dependence of ozone current efficiency on polarization procedure in the electrolysis of 2 M H2S04with p-Pb02 anodes at 0 OC: upper curve, six discrete experiments; lower curve, one experiment, polarizing in ascending steps of current density.
"I M
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22, 1981 3239
I
I
0 X
(u
E
ecn
al
c
c 0
a c 0
a
2 M -90 min/pt.
2 M -15 rnin/pt.
Figure 2. The relationship between phosphorous-containing coverage of the p-Pb02anode surface and ozone current efficiency in 2 M H3P04 at 0 'C: curve A, phosphorous-containing coverage as a function of current density: curve B, seven discrete polarizations: curve C, one experiment, polarizing in ascending steps of current density.
f ~ l l o w i n g :Ozone ~ ~ ~ yields vary markedly between electrolytes without correlation to physical properties. In oxy anion electrolytes an optimum electrolyte concentration is found above and below which ozone yields decline. (Figure 8 illustrates the maximum that is observed in sulfuric acid electrolyte.) This maximum is located at different electrolyte concentrations depending upon anode material. As a constant current density is maintained, ozone yields of P-Pb02anodes slowly increase, indicating a change in their properties. The influences of addition agents, such as the fluoride ion, can be dramatic; small
Measurement of Anionic Adsorption The radiotracer technique for the measurement of adsorption was selected due to difficulties that would have been encountered with voltammetric methods at the high current densities of interest in the evolution of ozone (0.1-1.25 A/cm2). The possibility of neutral radical formation was regarded as likely. Also, in some circumstances, work was to be performed in the presence of a coadsorbed addition agent (which will not be reported upon at this time). Thus, voltammetric (or any chargesensitive) techniques were not desirable. Such considerations led to the selection of the radiotracer method of adsorption measurement. Phosphorous-32 labeled H3P04electrolyte was chosen as the subject of experimentation due to the high counting efficiency of this 1.7-MeV /3 emitter, and the apparently anomalous behavior of ozone current efficiency with respect to polarization procedure. As reported previously, particularly high ozone current efficiencies are not observed in the P-Pb02/.H,P04 system. Experimentally, P-Pb02microelectrodes of 0.19 cm2area were prepared by electrodeposition onto Pt wires force-fit into Teflon. These electrodes were then polarized for exactly 2 min in 2 mL of labeled 2 M H3P04electrolyte containing 0.5-mCi activity. The electrodes were immediately polarized upon insertion into the electrolyte, and immediately withdrawn at the termination of polarization (a 5-10 s procedure). The electrodes were rinsed by agitation in three successive 50-mL beakers of distilled water for 30 s apiece. The residual activities of the electrodes were counted by liquid scintillation. The P-Pb02 electrodes were dissolved into 2 mL of a hydrogen peroxide and acetic acid solution. After decomposition of the peroxide with Mn02, this solution was added to 15 mL of a dioxane-based cocktail for counting. This procedure avoided counting efficiency problems in heterogeneous measurement. The microelectrodes were polarized galvanostatically, as it was desired to compliment previously gathered ozone yield data. It was found that the adsorption coverage produced at the high current densities employed is so persistent that such ex-situ methods are well justified. Coverage of P-Pb02Anodes by Phosphorous-Containing Material as a Function of Current Density The current density dependence of phosphorous-containing coverage on P-Pb02developed through the radiotracer method is depicted in Figure 2, curve A. Dual maxima are apparent, with the measurements at higher current density reproducing better than those at low current density. The nonmonotonic behavior of P-containing adsorption with current density seen in Figure 2, curve A, is striking. However, similar behavior has been observed by Potapova et a1.8 in the case of the adsorption of sulfur-containing species from sulfuric acid on platinum anodes during polarization at high potential. The adsorption of phosphorous-containing material on p-PbO2 anodes as a function of current density may be (8)Potapova, N.;Rakov, A.; Veselovskii, V. Elektrokhiniya 1969,5, 11, 1418-20.
3240
The Journal bf Physical Chemistty, Vol. 85, No. 22, 1981
rationalized in the following manner. In the first ascending region, the adsorption of negatively charged species (predominately H2POJ increases with increasing anodic potential. The coverage may decrease as the rate of oxygen evolution increases. Adsorbed oxygen (0and OH from the discharge of water) may displace adsorbed phosphate. This type of competitive relationship between adsorbed anions and oxygen has been previously demonstrated in radi~tracer,~ voltammetric,1°and electron spectroscopyll studies of anodic polarization. In the second ascending region, the driving force of increased anodic potential may counterbalance the adsorption of neutral oxygen, once again Coulombically attracting negatively charged species. Coverage may again decline if many of the phosphate anions lose their charge due to radicalization. Having become neutral radicals, they cannot be held as strongly, and less are found on the surface. The experiments performed are insufficient to elucidate the nature of these species; however, analogously, C104-and HS04-are known to form in HCIOl and H2S04 through a single electron transfer under severe anodic polarization. That the second decline in coverage is associated with radical formation appears likely, as the participation of the oxygen of the phosphate anions in oxygen evolution increases in this region of current density. Additionally, limited experimentation with H3POL8 prepared by the reaction of 21% labeled water with the anhydride, P205,was performed. Anodic gas samples (0, and O3 mixtures) were collected for mass spectroscopic analysis during polarization of labeled 2.5 M H3P04at 0.5 and 1.25 A/cm2 at 0 "C with P-Pb02 anodes. No l80was detected in the anodic gases at 0.50 A/cm2; however, a participation of anion oxygen to the extent of 1% was detected at 1.25 A/cm2. This latter current density corresponds to the second decline in coverage in Figure 2, curve A. No anion oxygen appears until the structure of adsorbed phosphate is sufficiently distorted by increased potential (probably to the point of radicalization). If a Pt anode is employed under identical conditions, the participation of anion oxygen at 0.5 A/cm2 is found to be zero, and, at 1.25 A/cm2, it approaches 5.0% (with oxygen from water discharge comprising the balance). This higher participation may be due to higher levels of anion adsorption on the Pt surface. The fact that only traces of ozone are observed here may confirm a high coverage. The exchange rate between oxygens in water and in phosphate and other oxy anions is so slow as to be negligible for the purposes of measurement of anion participation in anodic p ~ o c e s s e s . ~ ~ - ~ ~ At first glance, the coverages of the P-Pb02 microelectrodes by phosphorous-containing material shown in Figure 2, curve A, appear to be rather high. It has been observed by electron microscopy, however, that due to the erosion of P-PbO, anodes under conditions of high potential and low pH (reported upon in ref 20) surfaces of P-Pb02anodes accumulate a powdery degradation product during polarization at high current density. A ratio of 1OO:lOOO real to apparent surface area is possible, which would then put the P-containing coverage (estimated as (9)Balashova, N.Electrochim. Acta 1962, 7, 559-66. (10)Lenza, R.; de Taconi, N.; Arvia, A. J.Electrochem.SOC.1979,126, 12, 2140-3. (11) Hammond, J.; Winograd, N.J. Electroanal. Chem. 1977, 78, 55-69. (12) Hoering, T.; Kennedy, J. J. Am. Chem. SOC.1957, 79,56-60. (13) Keish, B.; Kennedy, J.; Wahl, A. J. Am. Chem. SOC.1958, 80, 4178-82. (14) Gerovich, M.; Kaganovich, R.; Vergelesov, J.; Gorokhov, L. Dokl. Acad. Nauk SSSR 1957,114,1049.
Foiier and Tobias
2
M H3P04 Pt
ooc
* e
00
0.25
0.50
0.75
1.00
A/crn2, lrnin
Flgure 3. The coverage of platinum anodes by phosphorous-containing species as a function of current density in 2 M H3P0, at 0 O C .
HZPOd-) at less than a monolayer. The rationalization presented for the nonmonotonic behavior of the adsorption of phosphorous-containing species on P-PbO, did not involve any particular properties of this anode material beyond its overvoltage for the evolution of oxygen. The displacement of anions by oxygen atoms and radicalization, the processes assumed to produce the two maxima, should take place on any inert anode. Thus, to further test these arguments, the adsorption of phosphorous-32-labeledmaterial on platinum anodes was briefly studied at high current density. Platinum microelectrodes of 0.16 cm2 were polarized according to the procedures previously defined. After rinsing, the electrodes were placed in 15 mL of dioxane cocktail for heterogeneous liquid scintillation counting. Probably due to the heterogeneous counting method, accuracy and reproducibility were somewhat poorer than in the previous case in which the Pb02 electrodes were dissolved into solution. Figure 3 presents the adsorption of phosphorous-containing material on Pt as a function of current density. A similar pattern of dual maxima is apparent. In the case of the Pt surface, where the oxidation state changes from PtO to PtO, as a function of anodic potential, the differing adsorption tendencies of anions on the changed surface may have to be taken into account. If, at the current densities of Figure 3, the surface is homogeneously PtO,, the displacement/radicalization explanation may still be the most appropriate. The coverage detected on platinum is much greater than that on PbOzwhen suitable roughness factors are included. This may account for the greater anion oxygen content of the anodic gases, and the evolution of only traces of ozone at all phosphoric acid concentrations (and thus no correlation to ozone current efficiency is possible in this anode/electrolyte combination). Correlation of 32P-ContainingAdsorption to Ozone Yield It is clear from Figure 2 that the adsorption of P-containing material on B-PbO, anodes and current efficiency
The Journal of Physical Chemistty, Vol. 85, No. 22, 198 1
Electrolytic Evolution of Ozone from Solutions
I
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20 m3
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8
m
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Boelter (1952)
- 20
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3241
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Potopova I
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IO
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A / C d
Figure 5. Ozone yield as a function of current density in 5 M HCIO, at 0 OC with a Pt anode, as determined by Boelter.’’
A Pospelova
0
0
Jo
vs. S.H.E.
Flgure 4. The relationship between sulfur-containing coverage of the pt anode surface and ozone current efficiency in 7.5 M H2S04at -30 OC.
taken through discrete point-by-point polarization show a symmetrical relationship. Where anionic coverage is high, ozone current efficiency is low, and vice versa. This remarkable correlation confirms that adsorption arguments can, indeed, be used to rationalize many of the trends in ozone yields observed. As a cautionary note, it must be pointed out that data taken over differing periods of time are being compared. This was necessary due to the inability to achieve steady-state ozone current efficiencies over short periods of polarization combined with excessive electrolyte loss in the small-volume radiotracer cells over longer periods of polarization. The persistency of the anionic coverage observed throughout the series of ex-situ measurements helps to explain the variation observed in ozone yield between different methods of polarization. Just as residual adsorption from prior experiments would have to be eliminated to obtain true rates of count in radiotracer experiments (if reuse of electrodes had been contemplated), residual adsorption from previously applied current densities or potentials can influence subsequent ozone yields if not eliminated. Polarization at single current densities, as in the experiments presented in Figure 2, curve B, and as in the radiotracer measurements, ensures a coverage arising solely from the circumstances of interest. In the measurement of ozone current density, one affixes a high surface coverage at lower current density (see Figure 2, curve A) which inhibits higher ozone yields that otherwise would have been found at subsequent current densities. A non-steady-state coverage is thus produced. Many seeming irregularities in yield curves taken by continuously stepping current density, such as the shallow minima at 0.45 A/cm2 in Figure 1 and 0.65 A/cm2 in Figure 2, curve C, may be attributable to the subtleties of the anionic coverage as a function of current density. It is clear that much the same arguments may apply to the dependence of ozone current efficiency on polarization procedure found in sulfuric acid. In light of the conclusions we have drawn, the statement by Fritz and c o - ~ o r k e r s“if , ~ an already used anode is employed for ozone electrosynthesis,no definite conclusion about the yield of the product may be made”, is quite understandable. One may construct from data available in the literature a simultaneous plot of anionc coverage and ozone current efficiency vs. potential for the electrolysis of 7.5 M H2S04
at -30 “C with a platinum anode. The 35S radiotracer adsorption measurements of Potapova8 and Pospelova15 may be combined to illustrate in a general fashion that, where coverage of the Pt anode by sulfur-containing species is high, ozone current efficiency is low, and, where coverage is low, ozone current efficiency is high. Figure 4 presents the combination of the work of the aforementioned authors. The exact polarization procedures utilized in Potapova’s measurements of ozone current efficiency and in the measurements of adsorption are not specified. Thus, it is not possible to rationalize why, in the region of rapidly oscillating coverage about 4 V vs. SHE, the ozone current efficiency data are anomalously featureless. Polarization through ascending steps of potential would tend to mask any subtle phenomena, as previously discussed. Again, changes in oxidation state that do not come into consideration in the case of Pb02 are possible on Pt. As determined in Potapova and Pospelova’s experimentation, the coverage of the platinum surface by sulfur-containing species exhibits three distinct minima. The situation is more complex than in H2P04,due to the confirmed synthesis of S052- and S20a2-at potentials of ozone formation. The synthesis of per compounds in sulfuric acid has been fully characterized in relation to the prior method of HzOz manufacture. The differing adsorption characteristics of these anions, and their radical precursors, may give rise to the complexity of the adsorption of sulfur-containing species illustrated in Figure 4. Potapova was also unable to offer a comprehensive rationalization of the behavior observed. The dual maxima in the ozone current efficiencies determined in this laboratory and by Potapova are probably analogous to the dual maxima reported by BoelteP in the low-temperature electrolysis of HC104 with a platinum anode. This result is reproduced in Figure 5. Beck17 proposed separate mechanisms of ozone formation in the two regions, the first involving merely the discharge of water molecules, and the second, at higher current density, involved the discharge of perchlorate anions and the subsequent chemical reaction of the radicals formed. Though discharge of the anions surely occurred, as evidenced by studies of the synthesis of diperchlorate sulfate and diperchlorate persulfate through the quenching of Clod. and HS04e radicals at low temperatures in the electrolysis of mixed electrolytes at high potential,18J9 it (15) Pospelova, N.; Rakov, A.; Veselovskii, V. Elektrokhimiya 1969, 5, 11, 1318-20. (16) Boelter, E., Dissertation, University of Washington, 1952. (17) Beck, T., Dissertation, University of Washington, 1952. (18) Rakov, A,; Potapova, G.; Veselovskii, V. Elektrokhimiya 1970,6, 11, 1732-43.
3242
The Journal of Physical Chemistry, Voi. 85, No. 22, 1981
Folier and Tobias Lead 4f 7/2
is probable that variations in the free energy of adsorption of oxygen due to the radicalization of adsorbed anionic species enhanced the ozone yields arising from the mechanism of water discharge, as proposed in the cases of H3P04and HzS04 previously discussed. Alteration of the Adsorptivity of PbOa Surfaces As it became clear that the coverage of anode surfaces by anionic material, to a great extent, determines what ozone current efficiencies are obtained, it also became of interest to influence the electrode surface to retain an "ideal" coverage. Ozone current efficiency reaches its maximum at rather high concentrations in all the acid electrolytes we have had occasion to explore. The problem with P-Pb02electrodes, when anodically polarized to the region of ozone evolution in acid electrolyes, is that a slow erosion of the surface takes place. It has been established that the mechanism of this erosion involves a high interfacial hydrogen ion concentration.20 Thus, it would be desirable to find conditions where high ozone current efficiencies are available at much lower acid concentrations, if a commercially viable technology of ozone manufacture which uses PbOz anodes is to be developed. The obvious approach is to enhance the adsorptivity of P-Pb02 for electrolyte anions, so that a coverage found at, for example, 0.5 A/cm2 in 8.5 M H2SO4 can be obtained at, say, 0.5 A/cm2 in 2 M H2S04,where the anodic corrosion reaction takes place at a far slower rate. It was hypothesized that the adsorption of electrolyte anions onto P-Pb02 may preferentially take place at defect sites, as @-PbO2is known never to achieve full 1 to 2 stoichiometry (it is, in fact, Pb01.96-l.97 as electrodeposited).21 The deficiency of oxygen is thought to be responsible for the conductivity of PbOz. Kittel has shown that the bulk stoichiometry of PbOz samples can be reduced by vacuum baking.22 It was shown that the bulk stoichiometry can be reduced to 1.70 without change of crystalline form, but with, however, a loss of conductivity. To assess the possible effects of reduced surface stoichiometry on ozone yield, P-PbO2 electrodes were baked at 110 "C at torr for 24 h. No color change is observed in the electrodes after this "mild" treatment. Two techniques appeared promising, to determine just how the surface stoichiometry of the P-Pb02samples had changed. Auger electron spectroscopy yields information on the first few layers of the surface, but has a problem in that reduction of the surface by the incident electron beam tends to occur with oxides possessing fairly low thermal decomposition temperatures. X-ray photoelectron spectroscopy (ESCA or XPS) is a less disruptive technique for surface analysis. It develops information leading to stoichiometry, as well as to how a species is bound, as seen from its ''chemical shift". However, XPS is a high vacuum technique, and, thus, no in-situ work can be performed. Also the stoichiometry of PbOz may change if O2 partial pressure is reduced to zero. These problems can be minimized by taking XPS measurements as soon as is possible after polarization and introduction to high vacuum. Other workers have reported no particular diffi~ulties.'~ photoelectron spectra Figure 6 presents the lead of a P-Pb02sample before and after 100 "C vacuum bak(19) Potapova, G.; Rakov, A.; Veselovskii, V. Elektrokhimiya 1973,9, 1054-58. (20) Foller, P.; Tobias, C.Submitted to the J. Electrochem. SOC. (21) Mjndt, W. J. Electrochem. SOC.1969,116,8, 1076-80. (22) Kittel, A,, Dissertation, Prague, Czechoslovakia, 1944. (23) Kim, K.; O'Leary, T.; Winograd, N. Anal. Chem. 1973, 45, 13, 2214.
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Figure 7. The ozone current efficiencies of as-electrodeposited and of reduced surface stoichiometry @-Pb02anodes in 1 and 11 M H,SO, as a function of time.
ing. Before the baking procedure, most surface lead is in the Pb4+oxidation state, exhibiting a binding energy of 138.07 eV, calibrated on carbon. After vacuum baking, the predominant peak is that of the Pb2+state at 138.6 eV. (The binding energy of the Pb2+state is higher than that of the Pb4+oxidation state. This apparent anomaly has been previously observed in the case of highly conductive oxides and has been discussed by Kim and WinogradZ3as being due to relaxation effects after electron loss from the conduction band.) After the mild baking procedure, the bulk of the Pb02 is not converted to PbO or Pb304,as the electrode remains highly conductive. However, deconvolution of the 4f7 spectra into separate Lorentzian contributions for Ph2+and Pb4+ shows that the surface is apparently reduced to PbOl.z. The same deconvolution procedure applied to an as-electrodeposited sample shows in accordance with the literature values of bulk Pb01.96f0.01 stoichiometry. Apparently the surface of PbOz can be modified to exist in a much more defective state than the bulk. The results of ozone evolution experiments polarizing at a single current density of 1A/cm2 by using the reduced surface stoichiometry (vacuum baked) anodes in 0 "C 1 and 11M sulfuric acid appear in Figure 7. It can be seen that vacuum baking dramatically improves the yield in 1 M yet reduces the yield in 11M. These results had been predicted through analysis of Figure 8, the dependence of ozone current efficiency on sulfuric acid concentration.
The Journal of Physical Chemistry, Vol. 85, No. 22, 1981 3243
Electrolytic Evolution of Ozone from Solutions
2 M HZS04
(Pt, 2OoC, 200 rnV/s) 3-
2N
-
2 0
4
6 8 IO H2S04 Concentration, M/2
2
12
Flgure 8. The H2S04 concentration dependence of ozone current efficlency for P-PbO2 anodes at 0.75 A/cm2 and 0 O C .
When as-electrodeposited P-Pb02anodes are used, ozone is evolved at highest yield from sulfuric acid at a concentration of 8.5 M. It is thought that the efficiency decreases at higher concentrations due to increased anionic adsorption. So, in 11M H2S04,vacuum baking to produce more defect sites could only further increase the anionic coverage of the surface, and further reduce the ozone yield. In 1 M H2S04,where surface coverage is apparently below the optimum, reduced surface stoichiometry can perhaps promote an adsorption coverage more characteristic of that obtained in electrolyte concentrations closer to the ideal (8.5 M). Thus, the enchanced yields arise. The magnitude of the yield of the reduced surface stoichiometry anodes in 1 M H2S04 is somewhat greater than that of untreated P-Pb02 in 8.5 M H2S04. There are, thus, aspects of the problem that remain not fully understood. The dramatic improvements in ozone yield obtainable through the alteration of anionic coverage of the P-PbO2 surface by reduction of surface stoichiometry represent another decisive confirmation of the influence of adsorbed anionic species on ozone current efficiency. I t is interesting to note from Figure 7 that the risetime to steady-state yield, which is as much as 90 min with as-electrodeposited P-Pb02 anodes in 1M H2S04,is significantly shorter when the surface stoichiometry is reduced. It is possible that the slow rise of current efficiency (which is not seen with Pt anodes-they achieve steadystate yield within 20 min), is caused by a gradual incorporation of electrolyte anions into the surface. A steadystate anion population may be more readily achieved if additional adsorption sites are available through prior surface stoichiometry reduction. Mechanism of the Influence of Anion Adsorption in the Electrocatalysis of the Ozone Evolution Reaction It has been shown that an ideal coverage of the P-Pb02 anode is required for maximum ozone current efficiencies to be achieved. Too much, or too little coverage, and substantially smaller current efficiencies are observed. Anion coverage competes with oxygen atoms which are intermediate to the evolution of O2 (and perhaps 0,) for available surface area. The presence of oxygen atoms as intermediates in oxygen evolution on Pb02is proposed in all contemporary mechanism^.^^-^^ The more anionic charge on the surface, the less the free energy of adsorption of the intermediate, 0, may be. Thus, it is thought that (24) Shih, H.; Yung-Chao, C.; Chang-Wen, C.; Kuo-Tung, Y. Communist Chin.Sci. Abstr. Chem. 1967, 51, 5. (25)Issa, I.; Abdelaal. M.; El Miligy, A. J. Appl. Electrochem. 1975, 5,271-7. (26)Kabanov, B.;Weisberg, E.; Romanova, 11; Krivolapova, E. Electrochim. Acta 1964, 9, 1197-1201.
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I
1.6
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V vs. S.H.E.
Figure 9. The voltammetric definition of the “shift” parameter used to assess the free energy of adsorption of oxygen on Pt. Heavier trace represents descent from 3 min of gaivanostatic control at 0.20 A/cm2.
it is an ideal free energy of adsorption of intermediate oxygen atoms that is actually sought in the optimization of the ozone evolution reaction in electrolytes composed of the acids of the oxy anions. (A distinction is drawn here in that the acids of the highly electronegative fluoro anions have been found to produce ozone yields greatly superior to those of the oxy anions. It is thought that this ability may be due, in part, to a different mechanism involving the stabilization of intermediate hydronium-type cations by complexation and withdrawal of electron density).6 The need for an ideal free energy of adsorption of oxygen in the selectivity of the anodic oxidation of water to either oxygen or ozone may be rationalized as follows. Anionic species compete with intermediate oxygen atoms for available anode surface, reducing the free energy of adsorption of oxygen in relation to their number. It is possible that if anion coverage is too high and, therefore, the free energy of adsorption of oxygen too low, the activation energy for the reaction of oxygen atoms to oxygen molecules is so low that there is insufficient residence time for the assembly of three oxygens to ozone to possibly occur. Conversely, if anion coverage is too low, many oxygen atoms would, on the average, be held very tightly, possibly so tightly that the formation of ozone (a change of free energy of just 48 to 39 kcal/mol with respect to oxygen in the atomic state in the gas phase) would be thermodynamically inhibited. The thermodynamic prohibition would never be fully operative, as the gas-evolving surface is in such a state of flux, due to constant changes in local potential, that the free energy of adsorption of oxygen probably assumes a broad distribution of values. An intermediate free energy of adsorption, produced by an ideal anionic coverage, would then produce the maximum ozone current efficiencies of which a particular anode and electrolyte combinationis capable. The “ideal anionic coverage” is probably different for each anion, and depends on its size, electronegativity, and polarizability. The proposed “partial thermodynamic prohibition” of adsorbed oxygen atoms reacting to ozone (operative when they are too tightly bound) would be plausible if it could be demonstrated that considerable differences in the free energy with which oxygen is held can be obtained through differing polarization procedures or at differing electrolyte anion concentrations. Differences averaging somewhat below the 9 kcal/mol value of the 0 to O3free energy change (gas-phase reaction) are probably required. A shift
J. Phys. Chem. 1981, 85,3244-3247
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minutes at 0.2 A/cm2
Figure 10. The free energy of adsorption of oxygen on Pt as measured by the “shift” parameter as a function of the duration of polarization.
in the reduction potential of surface oxygen as a result of highly anodic polarization would demonstrate the required variability of the strength of oxygen binding. Voltammetry on Pb02 to determine an oxygen reduction potential is impossible due to the masking effect of the reduction of Pb02 itself. Therefore, the Pt surface was chosen for such an experiment. Cyclic voltammetry was performed from 1.8 to 0.0 V vs. SHE in 2 M H2SO4 at 200 mV/s. The steady-state potential of the “oxide” reduction peak of 0.65 V was then used as the point of reference for the subsequent excursions to the ozone-evolving region. Figure 9 illustrates the steady-state voltammogram obtained by repeated cycling and defines the shift parameter used to calculate a change in the free energy with which oxide oxygen and adsorbed oxygen is bound to Pt. The heavier trace in the particular instance of Figure 9 represents the descent from 3 min of galvanostatic control at 0.20 A/cm2 (2.2-2.45 V vs. SHE): The potential and peak current of the oxide reduction peak are sensitive to scan rate; however, the “shift” parameter, being a difference, was seen to be much less
sensitive. From this shift, a free energy differenceresulting from the intensity and duration of anodic polarization can be calculated by assuming a two-electron reduction. Figure 10 plots the free-energy differences obtained at a scan rate of 200 mV/s when a Pt electrode is polarized for increasing lengths of time in 2 M H2S04at 200 mA/cm2 (2.2-2.45 V vs. SHE) vs. cycling to 1.80 V. Though subject to some uncertainty, average values of up to 6 kcal/mol are found in the case of Figure 10. Such free-energy differences also increase with the current density of the hold. It is clear that the strength with which surface oxygen is held is, indeed, a polarization-dependent variable as our interpretations of the ozone evolution reaction require. Perhaps of significance, the rise to a steady-state value seen in Figure 10 is typical of that required for the ozone evolution reaction to reach a steady-state current efficiency in 2 M H2S04(here PbOzanodes exhibit 90-min risetimes). Thus, we believe it will be of interest to refine such studies, possibly extending them to more directly correlate oxygen binding, anion coverage, and ozone current efficiency. Conclusions The investigation of several puzzling phenomena observed in studies of ozone current efficiency in sulfuric and phosphoric acids has led to an apparently consistent hypothesis as to the fundamental factors which control the selectivity of the anodic process for either oxygen or ozone. It is hoped that the conclusions developed can lead to the optimization of the reaction in the fluoro anion electrolytes, tetrafluoboric acid, or hexafluorophosphoric acid, already known to be capable of producing substantially higher current efficiencies.6s6 Acknowledgment. The authors are grateful to the Board of Patents of the University of California, Ametek Corporation, and the Department of Chemical Engineering, University of California, Berkeley, for their partial support of this work.
Transitions in the Sign of the Diamagnetic Anisotropy of a Lyotropic Mesophase without a Phase Change. Type 0 Disk Micelle Systems Bruce J. Forrest, Leonard W. Reeves,’ and Carol J. Roblnson Chemistry Depaftment, University of Waterloo, Waterloo, Ontarlo, Canada N2L 301 (Received: May 4, 198 1; In Flnal Form: Ju& 6, 108 I )
The diamagnetic anisotropy of magnetically aligning disk micelle lyotropic liquid crystals has been reversed by the inclusion of aromatic amphiphiles. This reversal occurs without a phase change and at the point of transition a nonaligning type 0 disk micelle mesophase is formed. Different host mesophases have been taken through the change in sign of the diamagnetic anisotropy, and the effects of temperature variation investigated. The rate of alignment of the type I disk micelle mesophases is a linear function of aromatic amphiphile concentration. Introduction For a number of years we have studied a new series of lyotropic liquid cryst& which gain orientational order of the component disklike or rodlike micelles when placed into magnetic fields.12 These micelles, however, possess
little or no positional order of their centers of g r a ~ i t y . ~ The rod micelle mesophases have been named type I CM (cylindrical micelle), indicating positive diamagnetic SUSceptibility (Ax > 0))while the smmd has been m r ~ ~ tYPe ed I1 DM, indicating disk micelles with negative diamagnetic
(1) F. Y. Fujiwara, L. W. Reeves, M. Suzuki, and J. A. Vanin in “Solution Chemistry of Surfactants”,K. L. Mittal, Ed., Plenum Press, New York, Vol. 1, 1979, p 63.
(2) B. J. Forrest and L. W. Reeves, Chen. Reu., 81, 1 (1981). (3)L.Q.Amaral and M. R. Tavares, Mol. Cryst. Liq. Cryst. Lett., 56, 203 (1980).
0022-3654/81/2085-3244$01.25/0
0 1981 American Chemical Society
