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J . A m . Chem. SOC.1988, 110, 4905-4913
Grants C H E 851 128 and C H E 8604007. N M R spectra were recorded at the N M R Facility for Biomedical supported by N I H Grant RR00292. Our cooperative program in this general area with Professor C. MacLean at the Free University, Amsterdam, has been aided by a NATO travel grant. We are grateful to Drs. Dolf Huis and Gerd van der Zwan of the Free University, Amsterdam, for helpful comments and to Scott Cramer for performing several calculations.
born in mind that the averaging applied in the two methods is quite different, ours yielding directly (sin2 a),whereas the NOE method yields (r3HH)-' which is related to a by complex trigonometric relations. The NOE method weights small HH distances (in this case the planar arrangement) strongly, so that a somewhat smaller angle should be predicted.
Acknowledgment. This work was supported in part by N S F
Electrocatalysis at a Novel Electrode Coating of Nonstoichiometric Tungsten(V1,V) Oxide Aggregates Pawel J. Kuleszat and Larry R. Faulkner*-* Contribution from the Department of Chemistry, University of Warsaw, Pasteura 1 , 02-093 Warsaw, Poland, and Department of Chemistry and Materials Research Laboratory, University of Illinois, 1209 West California Street, Urbana, Illinois 61 801, Received September 14, 1987. Revised Manuscript Received March 4, 1988
Abstract: Cycling the potential between 0.8 and -0.4 V vs. SCE in the colloidal mixture of W03.2H,0/W03.H20 existing in 2 M H2S04at 35 OC causes the electrodeposition of stable, mixed-valent W(V1,V) oxide aggregates on common electrode substrates. Infrared spectroscopy confirmed the presence of H 2 0 in the deposit and showed that the extent of hydration is lower than in W03.2H20. Well-defined redox transitions have been attributed to the reductive formation and oxidative elimination of hydrogen W oxide bronzes in dihydrate portions of the film. Reduction of monohydrate semirigid microstructures in the film is more irreversible and apparently leads to lower substoichiometric W oxides. Depth profiles were determined by Auger electron spectrometry and secondary ion mass spectrometry. The coating is easily permeable to ions, and the dynamics of charge propagation and electrochromism are comparable to those of dihydrate films. The W(V1,V) oxide film catalyzes the electroreduction of bromate to bromide in H2S04. At bare carbon, BrOC is not reduced prior to the onset of hydrogen evolution at about -0.7 V. At the modified surface, a bromate reduction peak appears at about -0.1 V vs SCE. The peak is linear with concentration from to M and is nearly diffusion-controlled. Cyclic and rotating disk voltammetry have been employed to characterize the catalytic reaction between Br0,- and reduced centers in the film. For typical quantities of the mol cm-*), a moderate apparent rate constant of the heterogeneous reaction (2 X lo4 cm W(V1,V) oxide film ((1-4) X s-l) was found.
Considerable recent effort has been devoted to the preparation and characterization of organic polymers as materials for coating electrode ~urfaces.l-~Potential applications based on the catalysis of electrochemical reactions have provided much of the incentive for this The attractiveness of such systems is in the combination of advantages from heterogeneous catalysis (particularly those of a catalyst attached to insoluble matrix) together with benefits of a three-dimensional distribution of catalytic centers, normally characteristic of homogeneous cata-
Other problems with modified electrodes arise from the lack of chemical stability among the parts of the microstructure. A reasonable approach is devising novel electrocatalytic surfaces is to consider inorganic mat rice^,^-'^ through which one could exploit the available information on heterogeneous catalysts and the electronic concepts of solid^."-^^ Inorganic polymeric oxides ~~
One of the important conflicts in these systems concerns the need to distribute electrons rapidly in the microstructure vs the need for the structure to engage in fast redox chemistry with substrate molecules in solution. Almost all catalytic modified electrodes have been developed by immobilizing good redox mediators into the microstructures.2 Normally, these are simple, coordinatively saturated, and substitutionally inert redox couples, e.g., Fe(CN):-I4-, MO(CN)~~/',and R u ( N H ~ ) ~ ~ + /They ' + . are effective as distributors of charge, because they interchange electrons readily among themselves, but they are unpromising in practical terms, because they are not potent catalysts.* In one of the most advanced illustrations of electrocatalysis at chemically modified electrodes, Buttry and Ansons separated the mediator and catalyst roles in a three-component system, wherein a redox mediator carried electrons rapidly to and from the immobilized, efficient catalyst sites. *To whom correspondence should be addressed. +Universityof Warsaw. *Universityof Illinois.
0002-7863/88/1510-4905$01.50/0
~
(1) (a) Chidsey, C. E. D.; Murray, R. W. Science 1986, 231, 25. (b) Wrighton, M. S. Science 1986, 231, 32. (2) Murray, R. W. Elecfroanal. Chem. 1984, 13, 191-369. (3) Faulher, L. R. Chem. Eng. News 1984, 52 (9), 28. (4) Zak, J.; Kuwana, T. J . Electroanal. Chem. 1983, 150, 645. (5) Buttry, D. A.; Anson, F. C. J. Am. Chem. Soc. 1983, 105, 685; 1984, 106, 59 and ref 1-6 therein. (6) Ikeda, T.; Leidner, C. R.; Murray, R. W. J . Electroanab Chem. 1982, 138, 343. (7) Oyama, N.; Anson, F. C. Anal. Chem. 1980, 52, 1192. (8) Anson, F. C.; Ohsaka, T.; Saveant, J.-M. J . Am. Chem. Sot. 1983,105, 4883. (9) Humphrey, B. D.; Sinha, S.; Bocarsly, A. B. J . Phys. Chem. 1987, 91, 586. (10) Cox, J. A.; Kulesza, P. J. Anal. Chem. 1984, 56, 1021. (1 1) Kulesza, P. J. J . Elecfrounal. Chem. 1987, 220, 295. (1 2) (a) Ellis, D.; Eckhoff, M.; Neff, V.D. J . Phys. Chem. 1981,85, 1225. (b) Itaya, K.; Ataka, T.; Toshima, S. J . Am. Chem. Soc. 1982, 104, 4767. (13) (a) Keita, B.; Nadjo, L.; Krier, G.; Muller, J. F. J. Electroanal. Cbem. 1987, 223, 287. (b) Keita, B.; Nadjo, L. J . Elecrroanal. Chem. 1987, 227, 265. (14) Ghosh, P. K.; Bard, A. J. J . A m . Chem. Soc. 1983, 105, 5691. ( 1 5 ) Liu. H.-Y.: Anson. F. C. J . Electroanal. Chem. 1985. 184. 411. i16) Murray, C. G.; Nowak, R. J.; Rolison, D. R. J . Elecfroanal. Chem. 1984, 164, 205. G
1988 American Chemical Society
4906 J . Am. Chem. SOC.,Vol. 110, No. 15, 1988 that contain chains of metal centers in different oxidation states are of interest, not only for their significant electrical conductivity but also as materials with defined acid-base properties and oxygen-transfer capabilities that can lead to new patterns of electrocatalytic reactivity. For example, mixed oxides with the perovskite structure have attracted attention as heterogeneous catalysts for different processes, including reduction of nitrous oxide,21CO oxidation,22or ethylene h y d r ~ g e n a t i o n . ~ ~ We recently described a general method for the electrochemical preparation of electrodes modified with mixed-valent tungsten(V1,V) oxides.24 Our work was stimulated by useful characteristics aside from catalysis, particularly the ease of preparation, chemical inertness, stability in strong acids, high porosity, electronic conductivity, and well-defined surface electrochemistry. The films underwent redox processes involving the generation of nonstoichiometric centers, such as hydrogen tungsten oxide bronzes (H,W03, 0 < x < 1) or lower tungsten oxides (W03,, 0 < y < I). The extent of hydration and the thickness of the film could be controlled by varying experimental parameters such as temperature and the length of potential cycling during the modification step. Redox transitions appeared to be faster in more hydrated films, but the durability was poorer. Optimum W oxide microstructures might be heterodispersed or unevenly hydrated, Le., in having both rigid, less hydrated portions and freer, more aquated ones. In this study, we describe the preparation and behavior of such compromise aggregate phases on conducting substrates. The promise of these films for electrocatalytic activity rests on the oxygen nonstoichiometry (oxygen vacancies) and on the electronic c o n d u c t i ~ i t y . ~ Thus, ~ - ~ ~ the system can exhibit both high reactivity toward the reduction of a substrate as well as high rates for charge propagation and redox transition. There are parallels between our work and that of Nadjo and co-worker~,'~ who have described electrodes modified by polyoxometalates. Our systems differ in physical extent and in chemical character, but there are similarities in the catalytic behavior. As substrates for catalytic electroreductions at the W(V1,V) oxide modified electrode, oxoanions that require removal of oxide are of interest. We have investigated the reduction of bromate as an example. This reaction is highly irreversible at bare elect r o d e ~but , ~it~can ~ ~readily ~ be performed by homogeneous reductants in the presence of catalyst^.^^,^^ The catalytic effect of W(V1) on the homogeneous reduction of bromate by iodide has been observed.32 Thermodynamically, Br03- can be reduced in strong acids prior to the redox transitions in W(V1,V) oxides; hence the electrochemically generated hydrogen tungsten bronzes or lower tungsten oxides may catalyze the electrochemical reduction of bromate. Translation of the homogeneous EC cata-
(1 7) Cousidine, D. M. Ed. Van Nostrands Scientific Encyclopedia; Van Nostrand Reinhold: New York, 1976; p 447. (1 8) Tamura, H.; Yoneyama, H.; Matsumoto, Y. Electrodes ojConductiue Metallic Oxides; Trasatti, S., Ed.; Elsevier: Amsterdam, 1980; Chapter 6. (19) Kazanski, V. B. Kinet. Katal. 1977, 18, 43. (20) Calvo, E. J.; Drennan, J.; Kilner, J. A,; Albery, W. J.; Steele, B. C. H. Proceedings of the Symposium on the Chemistry and Physics of Electrocatalysis; McIntyre, J. D. E., Weaver, M. J., Yeager, E. B., Eds.; The Electrochemical Society: Pennington, NJ, 1984; Vol. 84-12, pp 489-51 1. (21) Voorhoeve, R. J. H.; Remeika, J. P.; Johnson, D. W., Jr. Science 1972, 177, 353; 1973, 180, 62. (22) Tascon, J. M. D.; Tejuca, L. G. J . Chem. Soc., Faraday Trans. I 1981, 77, 591. (23) Petunchi, J. 0.;Nicastro, J. L.; Lombardo, E. A. J . Catal. 1981, 70, 356. (24) Kulesza, P. J.; Faulkner, L. R. J . Electroanal, Chem., in press. (25) Cotton, F. A.; Wilkinson, G. Adoanced Inorganic Chemistry, A Comprehensiue Text; Wiley: New York, 1980 p 850. (26) Weisman, P. J.; Dickens, P. G. J . Solid State Chem. 1973, 6, 314. (27) Crandall, R. S.; Faughnan, B, W. Appl. Phys. Lett. 1976, 28, 95. (28) Faughnan, B. W.; Crandall, R. S. Appl. Phys. Lett. 1975, 27 275. (29) Rozhdestvenskaya, Z. B.; Songina, 0.A. J. Anal. Chem. USSR 1960, 15, 155. (30) Toporova, V. F.; Vekslina, V. A. J . Anal. Chem. USSR 1975,30, 271. (31) Toporova, V. F.; Vekslina, V . A.; Chovnyk, N. G. J . Anal. Chem. USSR 1973, 28, 856. (32) Wolff, C. M.; Schwing, J. P. Bull. Soc. Chim. Fr. 1976, 619, 615.
Kulesza and Faulkner lysis33to the heterogeneous case is therefore possible. In this paper, we will show that very effective catalysis is indeed observed. Carbon substrates were used as the base electrodes because bromate reduction does not occur at the bare substrate, at least not before the onset of hydrogen evolution at about -0.7 V. Hence, the role of the modifying film in the electrocatalysis becomes apparent readily. The results presented here are of interest not only for their relevance of the analytical prospects for monitoring bromate but also because they concern the general promise of the nonstoichiometric mixed oxide films for catalyzing the electroreductions of oxo-substrates, such as inorganic anions or certain organic molecules.
Experimental Section The electrochemical experiments were performed with an IBM Instruments EC-225 or a Bioanalytical Systems BAS- 100 potentiostat. Rotating disk voltammetry was done with an ASR Rotator (Pine Instrument Co.) fitted with a glassy carbon disk having a geometric area of 0.50 cm2. All experiments were carried out in a three-electrode configuration. Potentials were measured and reported vs the saturated calomel electrode (SCE). All chemicals were commercial materials of the highest available purity (ACS Reagent Grade or Puratronic) and were used as received. The house distilled water was further treated with a Milli-Q water purification system. Solutions were deaerated for at least 5 min prior to the electrochemical experiments with use of prepurified N 2 saturated with water. A nitrogen atmosphere was maintained over the solution at all times. Unless otherwise stated, experiments were conducted at room temperature, 22 f 2 oc. A Kel-F sealed glassy carbon substrate from Bioanalytical Systems, with a geometric area of 0.071 cm2, was used for most voltammetric experiments. Prior to modification, such electrodes were cleaned and polished as described elsewhere.24 The electrode modification did not require very careful pretreatment of the glassy carbon.24 Unless otherwise stated, the modification of a clean substrate was accomplished by a 4-h cycling of the potential at 50 mV s-' between -0.4 and 0.8 V in the pale yellow colloidal mixture of W 0 3 . 2 H 2 0 and W03.H20. The mixture was freshly prepared by dissolution of 5 mL of 0.2 M Na2W04.2H20in 45 mL of 2.2 M H 2 S 0 4 at 35 f 2 OC. Film thicknesses were measured with an Alpha-step profilometer (Tencor Corp.) as described earlier.24 In order to prepare samples for Auger electron spectrometry (AES) or secondary ion mass spectrometry (SIMS), the electrodeposition was performed on pyrolytic graphite disks from Union Carbide (Chicago, IL) as detailed elsewhere.24 Auger electron spectra were recorded on a Physical Electronics Model 595 Auger electron spectrometer and secondary ion mass spectrometry data were obtained on a Cameca Instruments I M S 3F ion microanalyzer. These measurements were made with previously reported beam characteristic~.~~ Visible absorption spectra were recorded for films deposited on Sn02-coated glass slides (PPG Industries, "Nesa", 10-20 O/square). A Hewlett-Packard Model 8450A diode array spectrophotometer was used in the manner previously reported.24 Infrared spectra were taken with a Zeiss Model UR-20 spectrophotometer. For these measurements, the tungsten oxide was scraped from the graphite electrodes as reported by Reichman and Bard.34 To make pellets with KBr, a milligram of a finely ground sample was intimately mixed with about 100 mg of dried KBr powder. Special attention was paid to prepare homogeneous samples of similar weight. For quantitative measurements, known and controlled quantities of KBr and the test sample were taken. Additional IR measurements of Na2W04.2H20,and of this salt with graphite powder, suggested that there was no interference from the graphite residues in the region of 1300 to 4000 cm-' that was of our concern.
Results and Discussion Designing a successful heterogeneous electrocatalyst required careful consideration of the physical and chemical properties of W oxides. Since only in strong acids do W(V1,V) oxide coatings remain stable and provide high rates of charge p r o p a g a t i ~ n , ~ ~ - ~ * J ~ our experiments were generally done in 2 M H2S04. The next important factor was the degree of hydration and the porosity of (33) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980. (34) Reichman, B.; Bard, A. J. J . Electrochem. Soc. 1979, 126, 583, 2133.
J. Am.
Tungsten(VI,V) Oxide Aggregates
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4907
/
08
04
0
E /
-0h V
.i
SCE
Figure 1. Steady-state cyclic voltammogram of the W(V1,V) oxide deposit on glassy carbon in 2 M H2S04. Modification was performed from the sodium tungstate dihydrate solution as described in the Experimental Section. Scan rate, 50 mV s-'; geometric area of the electrode, 7.1 mm2.
the film. With a gradual increase of temperature from 15 to 90 OC the film converted, in practice irreversibly, from dihyrate to m o n ~ h y d r a t e .In ~ the ~ ~ latter ~ ~ ~ form, ~ ~ redox transitions appeared to be slower, but the stability was better.24.34Thus, our optimum modification procedure involved electrodeposition coupled with sol-gel aggregation upon reduction.24 We worked from the pale-yellow colloidal mixture of W0,.2H20, with some W03.H20, existing in 2 M H 2 S 0 4at 35 0C.36 The films were grown slowly by potential cycling as described in the Experimental Section. This approach is known to favor formation of porous, hydrated metal oxide coating^.^' When thickness measurements were made by profilometer, a mean value of 2400 f 300 .& was obtained. Physicochemical Identity and Voltammetric Behavior of the Films, Figure 1 shows the cyclic voltammogram obtained in 2 M H 2 S 0 4with a glassy carbon electrode coated with the oxide deposit. The film underwent a well-defined and well-separated double reduction in the range from +0.8 to -0.7 V, and the reduced film showed a corresponding double oxidation. The reduction peaks are observed at E,, = -0.090 V and EF2 = -0.580 V, whereas the oxidation peaks appear at Epal= -0.045 and Epa2 = -0.500 V. Comparison of these results with those previously reported by us,24 dealing with the electrochemical behavior of dihydrate, monohydrate, and anhydrous W(V1,V) oxide coatings, suggests that the pattern in Figure 1 is characteristic for a coating somewhat less hydrated than the dihydrate. With films of lower water content, the E,, and EF2reduction peaks are shifted toward more negative value^.^^^^^ Further, IR measurements of pellets prepared with the W(V1,V) oxides (in air, practically WO,) scraped from the electrode show that the amount of water in the films is lower than that in the dihydrate ~ t a n d a r d , W03-2H20. ,~ A typical infrared spectrum of the WO, films (Figure 2, curve A) clearly indicates the presence of water via its characteristic peaks near 1650 and 3500 cm-1.34938The heights of these peaks in various samples should express differences in water content. Indeed, when the W(V1,V) oxide film was prepared by electrodeposition at lower temperature, Le., at 17 "C, the heights increased virtually to levels characteristic for authentic dihydrate W03.2H20 (Figure 2, curve B), which was prepared according to Freedman,36 except for being precipitated from 2 M H2S04 instead of 3 M HCI. The extent of hyration of the W(V1,V) oxide coatings prepared at 35 OC should be related to the water content in the pale-yellow stable phase of WO, that exists as colloidal suspension in H 2 S 0 4 (Le., in the mixture for modification). In order to test this hypothesis, an additional voltammetric experiment was performed with a glassy carbon electrode that had been modified by me(35) DiPaola, A,; DiQuarto, F.: Sunseri, C. Corrosion Sci. 1980, 20, 1069, 1079. (36) Freedman, M . L.J . A m . Chem. SOC.1959, 81. 3834. (37) (a) Burke, L. D.; O'Sullivan, E. J . M. J . E/ectroana/. Chem. 1981, 117, 155. (b) Burke, L. D.; Twomey, T. A. M.: Whelan, D.P. J . Electroanal. Chem. 1980, 107, 201. (38) Hurditch, R. .I.EIectron Lett. 1975. 11 (7). 142.
36@l
3200
2800
2600
2000
1600 Cm
~
Figure 2. IR spectra of (A) the W(V1,V) oxide coatings prepared as described in the Experimental Section and scraped from the electrode substrate, (B) of W03.2H20.
chanical dispersion, Le., by polishing the electrode surface with the WO, filtrate (from the colloidal precipitate obtained at 35 "C in 2 M H2S04), in the manner proposed by Kuwana et al. for various metal oxide^.^^^^^ Indeed, such glassy carbon modified electrodes exhibited voltammetric behavior virtually identical (particularly in terms of E,,, Epal,E F 2 , and E ) with the one shown in Figure 1. This behavior contrasts signi/?lantlp with that of both the authentic monohydrate, which does not show these peaks, and the authentic dihydrate, which has a markedly narrower separation between the two sets of peaks.24 An estimation of the water content in the pale-yellow WO, phase was attempted via IR spectroscopy. In order to avoid dissolution or changes in the extent of hydration of WO,, these filtrates were rinsed with diluted 0.1 M H2S04,rather than with distilled water. To perform comparative IR measurements and to estimate the water content in the pale-yellow WO, phase, two colloidal suspensions, both the white dihydrate and the pale-yellow, were prepared at 16 and 35 "C, respectively, and subsequently filtered, rinsed, and dried. The peak at ca. 1650 cm-' was used for the evaluation, and it was assumed that Beer's law was obeyed. A base-line method for determination of absorbance, in which the contribution from aquated H2S04 was assumed to be constant, was employed. The extent of hydration was found to be roughly 1.8, which may indicate structural heterogeneity of the phase. When W 0 3 . H 2 0is brought into contact with 2 M H2S04, the solid does not disperse readily to form a colloid system, and therefore is lyophobic.40 In contrast, the lyophilic W03.2H20 produces a colloidal dispersion spontaneously. At present, there is not enough evidence to assume the formation of micelle-like polydispersions in the pale-yellow WO, phase, but the existence of two portions, the rigid lyophobic monohydrate and the redox-facile dihydrate, can be expected. The formation of the pale-yellow, incompletely hydrated WO, colloid takes place not only at 35 OC but also as a result of aging, e.g., for 48 h in 2 M H2SO4, of the white dihydrate suspension, even at 16 "C. The effect was confirmed both visually and with the use of IR spectroscopy. When the time of the aging was increased to 96 h, no further changes in the level of hydration were observed, implying that a metastable phase was formed. As before for filtrates, we estimated a number of water molecules associated with W03, z, and found that in all cases z = 1.8 f 0.1. In contrast, the aging of the dihydrate aggregates formed as W(V1,V) oxide coatings on carbon electrodes was much less pronounced. There were no changes in the positions of voltammetric peaks during 48 h of repetitive cycling of the electrode potential from 1.O to 4 . 5 V in 2 M H2SO4 at 22 "C. This improved stability of surface films is not surprising, since the solution phases are metastable sols, whereas the deposited films are probably semirigid inorganic gel aggregate^,^^ typically consisting of water trapped within a (39) Dong, S.; Kuwana, T. J . Electrochem. Soc. 1984, 131, 813. (40) (a) Levine, I . N. Physical Chemistry; McGraw-Hill: New York, 1978; pp 341-345. (b) Atkins, P. W. Physical Chemistry, 2nd ed.; W . H. Freeman: New York, 1982: pp 842-847
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J . A m . Chem. SOC.,Vol. 110, No. 15, 1988
I Table I. Voltammetric Parameters for Reduction Peaks at E,,“ u/mV s-’ i,llPA E,,jV i , , r - I / p A mV s-’ 500 200 100 50 20 10 5
1120 496 257 131 53.1 26.9 13.6
-0.110 -0.100 -0.095 -0.090 -0.087 -0.085 -0.085
2.24 2.48 2.57 2.62 2.66 2.69 2.72
Experimental conditions as for Figure 1, except that the scan rate, is varied.
a.
L‘,
Figure 3. Cyclic voltammograms recorded in 2 M H2S04with a glassy carbon electrode coated with the W(V1,V) oxide deposit of Figure 1 in the presence (A) and absence (B) of 2 mM K3Fe(CN)6. Scan rate, 100 mV SKI; geometric area of the electrode, 7.1 mm2.
three-dimensional network of tiny crystals held together by van der Waals forces.40 Wez4have interpreted voltammetry like that in Figure 1 in terms of two distinct, parallel electrode processes: (a) hydrogen uptake to give the hydrogen tungsten oxide b r o n z e ~ , ~ which ’ - ~ ~ may be written as W 0 3 x H + + xe- F? H,W03 (0 < x < l ) , and (b) the reduction of W(V1) to lower tungsten oxides,44which can be described as W 0 3 2ye- 2 yH+ zW03, + yH,O (0 < y < 1). In the case of dihydrate coatings, the insertion of a proton upon a valence adjustment of W (Le. bronze formation) and proton transport via H 3 0 +exchange between adjacent water molecules are relatively ~ n h i n d e r e d . ’ ~For . ~ ~monohydrate ~ or anhydrous W oxides, the mobility of H+ is significantly lowered, and the breaking of tungsten-xygen bonds to form oxygen ion vacancies, and thus substoichiometric W oxides, could start to predominate.41-44 For our unevenly hydrated coatings (prepared by electrodeposition at 35 “C), both types of redox process are to be expected. The appearance of two sets of well-developed and well-separated peaks, E,,/E,, and EF2fEpaZ,on an underlying broad background supports this hypothesis. Since the reduction to hydrogen tungsten bronzes takes place at generally more positive potentials than the formation of substoichiometric lower W oxi d e ~ $ one ~ - ~can ~ understand the peaks at E,, and E,, in terms of the reductive absorption of hydrogen into the oxide. The literature indicates the existence of two stable phases of the hydrogen W bronzes, H0,18W03and H O , ~ ~ W OThus, ~ . ~ ’the E,, peak should be, as before in the case of dihydrate coatings,24 attributed to formation of Ho,,gW03. The E,, current at potentials about 500 mV more negative than E,,, cannot be wholly attributed to the formation of Ho.35WO3. Judging from the appearance of significant background currents at potentials more negative than -0.2 V and referring to our previous report,24we believe that reduction to the substoichiometric (“blue”) W oxides (WO,,) also contributes substantially to the overall process. Integration of the voltammetric peak at E,,, above an estimated base line, yielded a charge of 4.1 mC cm-,, which approximately corresponds to 2.4 X lo-’ mol cm-2 of catalytic W03/H,W03 centers, or about 2400 catalyst monolayers. In this calculation, it is assumed that for charge balance to be maintained, the creation of 1 mol of Ho,18W03phase requires injection of 0.18 mol of electrons. Permeability, Stability, and Reversibility of Redox Transitions. Efficient electrocatalysis requires ready access of the reactant to catalytic sites within the film. Physical permeability was probed via the voltammetric behavior of the Fe(CN):-l4- couple. The cyclic voltammogram (Figure 3) recorded in 2 M H2S04at 200 mV s-’ reveals a measured Eo’ = 0.435 V, which is the value observed in the same solution at platinum. The peak current scales with the square root of scan rate in the range from 10 to 300 mV s-I, and the waveforms are characteristic for a diffusion-limited
process. The differences between the peak potentials were 64, 72, and 86 mV at 50, 100, and 200 mV s-’. Further, the peak current ratios remained equal to unity. Thus, the electrode reaction is almost electrochemically reversible. At potentials more positive than 0.3 V, the film behaves like a s e m i c o n d ~ c t o rand , ~ ~it~is~ ~ unlikely that there would be such reversibility if the electrode process were confined to the oxide/solution interface. Thus we conclude that the film is readily permeable to ferri/ferrocyanide. The characteristics of the cyclic voltammogram in Figure 3 do not change qualitatively even when the film is ca. 6 times thicker, Le., with a charge at E,, of 24 mC cm-*. However, the thicker film causes a 20% decrease in the voltammetric peak currents and an increase in the peak separation from 64 to 105 mV at 50 mV s-,. The virtually unchanged Eo‘ value at the modified electrode vs bare carbon suggests that there is no specific interaction between the coating and ferro- or ferricyanide. The stability of the coating was tested by measuring a decrease of the voltammetric peak current at E,, during various experiments in 2 M H2S04. Each measurement was performed at least three times with a freshly prepared W(V1,V) oxide film on a selected glassy carbon substrate. Solutions were stirred by bubbling nitrogen through them. After simply soaking the electrode in the electrolyte for 4 h, a decrease of 22 f 4% was observed in the current. In the case of potential cycling from -0.45 to 0.8 V at 50 mV s-l for 4 h, a decrease of 12 f 3% took place. No changes in the positions of peaks were observed. The error bars given above represent one standard deviation about the mean. The increased stability of the W(V1,V) oxide coatings described here, when compared to dihydrate films,24apparently results from the structural heterogeneity of the assemblies. The presence of the already postulated, less hydrated, lyophobic portions in the W oxide aggregates seems to decrease solubility and increase the overall durability of the coating. At scan rates up to 500 mV s-l, the voltammetric currents measured in the potential range from 0.8 to -0.7 V are proportional to scan rate, as expected for surface processes. The effect of L: on the peak current at E,, is summarized in Table I for a glassy carbon modified electrode in 2 M H2S04. In the range from 5 to 500 mV s-I, the function i,,u-I is nearly independent of L’, changing by only about 18%. The decrease at faster scan rates may reflect limitations associated with charge propagation in the film. For L? = 5-100 mV s-l, the peak potentials Epcland .Epal remain constant, and the difference between them is 40-45 mV. At higher L:,a wider splitting is observed. At all potentials where redox transitions are observed (
