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Effect of the oxygen layer at solid Ru electrodes on hydrogen oxidation in acid solution
547
J. Electroanal. Chem., 214 (1986) 547-554 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
EFFECT OF THE OXYGEN LAYER AT SOLID Ru ELECTRODES HYDROGEN OXIDATION IN ACID SOLUTION *
ON
M.W. BREITER lnstltut fdr Techmsche Elektrochemie, TU Wien, 9 Getreidemarkt, 1060 Wlen (Austrra) (Received 16th April 1986)
ABSTRACT
The inhibition of the anodic oxidation of Ha on a solid Ru electrode by the oxygen layer was investigated under voltammetric conditions in sulphuric acid solution at room temperature. Relatively small sweep rates had to be applied to be able to neglect the contribution of the partial currents of the formation or reduction of the oxygen layer with respect to the partial current of the Hz oxidation. It was demonstrated by voltammetry with a superimposed ac current at 1000 Hz that the process of Ha oxidation does not affect the formation or reduction of the oxygen layer. The inhibition occurs in a gradual fashion on the Ru electrode. Almost a monolayer coverage is necessary before the partial current of Hz oxidation reaches an almost constant value in the oxygen region. Ru behaves completely different from Pt, Rh and Pd in this respect.
INTRODUCTION
It was demonstrated in earlier papers [l-3] lar hydrogen in acid solutions l/2
H, ==H++e-
that the anodic oxidation of molecu-
(1)
is inhibited by the oxygen layer on smooth electrodes of platinum [1,2], iridium [l], rhodium [3] and palladium [3]. On platinum [2], a relatively small coverage with oxygen (about 10%) is sufficient to change the rate-determining step from convective diffusion of molecular hydrogen at potentials below the oxygen region to reaction control in the oxygen region. A similar result is observed in the anodic oxidation of methanol in acid solution under voltammetric conditions. The height of
* Dedicated to the memory of Professor H.W. Ntirnberg. 0022-0728/86/$03.50
0 1986 Elsevier Sequoia S.A.
548
the first wave during the positive sweep is determined [4] by the inhibiting effect of the oxygen coverage on the said platinum metals. The effect of the oxygen coverage of smooth ruthenium on the rate of reaction (I) has now been studied. Ruthenium differs in two aspects from the above platinum metals: (a) The formation of the oxygen layer occurs [5,6) on smooth ruthenium at less positive potentials. (b) The rate of reaction (1) is not controlled solely [7] by convective diffusion at potentials below the oxygen region as on the said platinum metals. Volt~et~ with and without a superimposed ac current was used as the technique. EXPERIMENTAL
The experiments were carried out in a Pyrex glass vessel of conventional design at room temperature (22 rt: l°C). The electrolyte was stirred either with purified Nz or with H,. The electrode potential was determined against a hydrogen electrode in the same solution (A4 H2S04). The electrolytic resistance R, between the tip of the Luggin capillary, leading to the compartment of the reference electrode, and the test electrode was obtained by cathodic pulses as described in ref. 7. Corrections, resulting from I&, were applied only in the impedance m~surements. The dc currents were small enough in the current-potential curves, measured at relatively low sweep rates, to neglect the influence of IR,. The set-up for the impedance measurements by voltammetry with a superimposed ac current was the same as that in previous work 14 . The test electr oJ e consisted of a rectangular piece cut from ruthenium foil, and had a geometric area of 1.64 cm2. Details of the construction and purity of the test electrode have been given before 171. The Ru foil in this study was of the same material as that in ref. 5. RESULTS
Voltammetric current-potential curves were recorded at 10 and 1 mV/s between about 0.03 and 1 V for H, stirring. The potential range was restricted to 0.1 V as the negative potential of reversal at N, stirring to eliminate the possible contribution of molecular hydrogen, formed at potentials below about 0.1 V during the negative sweep, on the subsequent positive sweep. The current-potential curves reveal the presence of strong current fluctuations in a wide potential range at H, stirring. This is clearly shown in Fig. 1 by the curve obtained at 10 mV/s. Vertical bars mark the extent of the current fluctuations at 1 mV/s in Fig. 2. The direction of the potential sweep is indicated by arrows. The effect of different stirring intensities is shown in Fig. 2. Curves a and b were taken at 10 or 1 mV/s and Nz stirring (1 cm3/s). The bars indicate the extent of the current fluctuations in Fig. 3.
549
t 0.6 -
as% 0 cl4 Q
E ._
QZ-
Fig. 1. Voltammetric current-potential curve at 10 mV/s and 1 cm3 Hz/s on smooth ruthenium in M H-/SO,.
The ohmic component R, and the capacitive component l/&s of the electrode impedance in an analogue series circuit were determined as a function of the potential at 1000 Hz by voltammetry with a superimposed ac current at sweep rates of 10 and 1 mV/s for H, or N, stirring. At a given sweep rate, the same R,E curves and l/wC,-E curves were obtained within the limits of reproducibility for both types of stirring. The results at 1 mV/s are given as an example in Fig. 4.
O[
O
,
92
0.4 E/V
03
aa
I-
1.0
Fig. 2. Current-potential curves at 1 mV/s and 1 cm3 Hz/s (curve a) or 0.2 cm3 Hz/s (curve b).
Fig. 3. Current-potential
curves at 10 mV/s
0
O
02
ah
(curve a) or 1 mV/s
E Iv
0.6
(curve b) at 1 cm3 Nz/s.
0.8
Fig. 4. Ohmic and capacitive component of the electrode impedance in an analogue Hz and 1 mV/s. The same curves were obtained at N, or H, stirring.
I-
1P
series circuit
at 1000
DISCUSSION
General considerations When the experimental results of different authors for solid Ru electrodes are compared, the following points have to be considered:
551
(a) The rne~lur~c~ procedures in the preparation of solid Ru are difficult. (b) The surface of Ru may be affected by the sealing into glass during assembly of the electrode. (c) Since Ru dissolves to a noticeable extent during positive sweeps [5], the roughness of the electrode will change with time. An aged electrode will differ from a fresh electrode in this respect. Another feature is the growth of a thick oxide film when the electrode is cycled between certain potential limits [5,6,8], leading to a change in the shape of the voltammetric current-potential curves. The foil electrode of the present inv~tigation was of the same material as that in ref. 5. The aging by prior use in other experiments was’responsible for the electrode roughness in this work being larger (three times) than that in ref. 5. Comparison of the current-potential curves of refs. 5 and 6 reveals a marked difference at the beginning of cycling. The initial curves recorded [6,8] during cycling between about 0 and 1.4 V in acid solution look somewhat similar to the respective curves on smooth Rh [9], with a well-defined reduction wave of the oxygen layer during the negative sweep and a wide hydrogen peak of the same shape for positive and negative sweeps. In contrast, the initial curves on the Ru foils do not display a hydrogen peak of nearly the same shape for positive and negative sweeps under any conditions. It is suggested that the described difference is due to reason (a} above. A voltammetric study of surfaces with different orientations of single crystals of RuO, was published [lo] recently. The reader is referred to this paper for additional references on work with RuO,. It is pointed out [lo] that “oxygen-free” surfaces exist for the following oscillographic orientations in the ideal case: (lOl), (111) and (100). Crystals with these orientations have only Ru atoms on the surface. Only the current-potential curve for the (100) face is given for a low sweep rate in 0.5 M H,SO,. This curve looks vastly different from curve b in Fig. 3 in a wide potential range. It is difficult to compare the results in ref. 10 with the results on solid Ru electrodes. There is a great difference in the behaviour of smooth electrodes of Pt, Rh and Pd, on the one hand, and Ru, on the other hand. While the study of the inhibition of reaction (1) may be carried out at sweep rates as large as 100 mV/s without noticeable interference by the partial currents due to the formation or reduction of the oxygen layer, the sweep rate cannot be larger than 10 mV/s on Ru otherwise the partial currents become too large. This was demonstrated experimentally by curves taken for H, stirring at 100 mV/s. These curves are not shown here because the ~nt~bution of the partial currents produces an effect which looks like a large hysteresis between the positive and negative sweeps, but really results from the overlapping of currents from different electrode reactions. Sweep rates of 10 and 1 mV/s were used predominantly for this reason. Large fluctuations of the anodic current can be observed in the curves of Figs. 1 and 2 in a wide potential range. They are attributed mainly to oscillations of the thickness of the diffusion layer on the rectangular electrodes immersed in a vertical position. This conclusion is supported by the experimental evidence in Fig. 2. The anodic limiting current density i,, observed between about 0.1 and 0.4 V, is
rn~k~ly smaller at 0.2 ems HJs than at 1 err? HJs because the diffusion layer thickness decreases. It was concluded in previous work [7] that the anodic limiting current density is given by:
Here, id,H2stands for the limiting current density due to convective diffusion and i,,T represents the exchange current density of the Tafel reaction occurring on a surface with a small coverage of H atoms. The inhibition of reaction (1) by the oxygen layer is caused by a transition from rate control by convective diffusion to control by a heterogen~us reaction on Pt, Rh and Pd. The exact nature of this transition and of the heterogeneous reaction is not yet fully understood. On Ru, the situation is more complicated. The inhibition involves a transition from mixed control by convective diffusion and the Tafel reaction to control by a heterogeneous reaction which need not be identical with the Tafel reaction. Formation and reduction of the oxygen layer The anodic formation and cathodic reduction of the oxygen layer on solid Ru electrodes has been studied and discussed previously [5,6,8]. There seems to be general agreement on several points: (d) Monolayer formation is followed by multilayer formation, probably RuOz, at larger positive potentials. The potential where this transition occurs is not quite certain Ill], but appears to be in the vicinity of 1 V. (e) The hydrogen layer and oxygen layer overlap, especially during the negative sweep. Separation is difficult. The general discussion in the preceding section makes it clear that information on the oxygen layer has to be obtained and correlated to information on the inhibition of reaction (1) on the same electrode under the same experimental conditions. In this context the question arises, as to whether the simultaneous occurrence of reaction (1) influences the formation and reduction of the oxygen layer under cycling between about 0.05 and 1 V. To answer this question, the electrode impedance was measured for both H, and N, stirring as a function of potential at 1000 Hz by voltammetry with a superimposed ac current. While the relatively large currents of reaction (1) mask the currents for the formation or reduction of the oxygen layer at small sweep rates in a wide potential range, the electrode impedance reflects [4] the influence of the oxygen layer on Pt, Rh and Pd. Thus information on the formation and reduction of the oxygen layer in the presence of reaction (1) is available. The concept was developed by the author in studies on Pt. It is now applied to Ru. The results of some of the impedance measurements are given in Fig. 4. Since the same R,-E curves and l/c&,-E curves were obtained at a given sweep rate (10 or 1 mV/s) for H, or N2 stirring, it is concluded that the formation or reduction of the oxygen layer on Ru is independent of the type of stirring. If there were an influence
553
of reaction (1) on the processes connected with the oxygen layer, the amount of oxygen at a given potential would be different for H, and N, stirring. This situation would be recognizable by different R,--E curves and l/&,-E curves for both types of stirring. Information on the formation and reduction of the oxygen layer is supplied for both types of stirring by the voltammetric current-potential curves taken at N, stirring. Although they are not shown here, such curves were also recorded at 100 mV/s. It was found, in agreement with earlier work [5,6,12], that the charge for the formation of the oxygen layer during the positive sweep was nearly the same (within 5%) as that for the reduction during the negative sweep at 100 mV/s. However, there is a systematic trend in the present study that the anodic charge becomes larger than the cathodic charge (ca. 15% at 10 mV/s and 100% at 1 mV/s). It is considered likely that Ru dissolution is responsible for this effect. Ru dissolution has to occur at lower potentials than 1.4 V, as was proposed in ref. 5. In the present study the charge for the formation of the oxygen layer at potential E has to be taken as the charge for the negative sweep starting at E. Additional information on the processes associated with the oxygen layer and on the overlapping of the oxygen and hydrogen region can be obtained from the impedance measurements. The capacity C, has a value of 706 pF/cm* at 0.1 V during the positive sweep. This value corresponds to the maximum in the l/&,-E curve (see Fig. 4). The magnitude of C, for 1000 Hz implies that there is a contribution from a pseudo-capacitance due to a faradaic reaction. The value of C, increases to about 1400 pF/cm* at 0.7 V and remains the same between 0.7 and 1 V. It is suggested that a partial step such as H,O=OH,+H++e-
(3)
is responsible for the pseudo-capacitance. A similar situation was found at more positive potentials for smooth electrodes of Ir [13]. The existence of a relatively rapid process was postulated for Ru in ref. 12. The increase of C, with potential between 0.1 and 0.7 V during the positive sweep implies that the coverage with adsorbed species in a reaction such as (3) becomes larger up to 0.7 V and remains practically the same between 0.7 and 1 V. The hysteresis of the l/&,-E curves of the positive and negative sweeps indicates that the coverage with the adsorbed species is not quite the same at a given potential. The maximum of the l/&,-E curve is interpreted as the result of the overlapping of the oxygen and hydrogen region. The slight increase of C, between 0.07 and 0.1 V during the positive sweep (see Fig. 4) reflects the influence of the hydrogen coverage. The latter conclusion is in agreement with the interpretation of the reflectance data in ref. 12. Inhibition
of the H, oxidation
by the oxygen layer
In contrast to Pt, Rh and Pd, there is a gradual decrease of the anodic current due to reaction (1) with potential above about 0.3 V under voltammetric conditions.
554
As discussed in the preceding section, the oxygen coverage begins to form at about 0.1 V during the positive sweep. The oxygen coverage present between 0.1 and about 0.3 V does not seem to affect reaction (1). On Pt, Rh and Pd, a relatively small amount of oxygen is sufficient to produce a large inhibiting effect, probably by blocking the most active sites for reaction (1). On Ru, an oxygen layer coverage of about 1 appears to be necessary to reach an almost constant current of reaction (1) in the oxygen region. The oxygen coverage increases on Ru in a linear fashion [11,12] with potential up to about 0.9 V. The shape of the i-E curves in Fig. 3 demonstrates that the same situation exists for the Ru electrodes of this work. The inhibition of reaction (1) appears to depend linearly on the oxygen coverage between about 0.4 and 0.7 V. Since two types of oxygen species (probably OH, and 0,) are present in the latter potential range, it is difficult to develop a detailed model of the mechanism of inhibition. In a simple model, the inhibition of reaction (1) between 0.3 and 0.7 V may be considered to be caused by blocking of the free surface by oxygen species. The oxygen coverage from the i-E curves represents a measure of the coverage by both oxygen species. Additional inhibiting effects are present between 0.7 and 1 V. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13
K. Franke, C.A. Knorr and M. Breiter, 2. Eiektrochem., 63 (1959) 226. M.W. Breiter, Electrochim. Acta, 7 (1962) 601. M. Breiter and F.G. Will, 2. Elektrochem., 61 (1957) 1177. M.W. Breiter, Electrochim. Acta, 8 (1963) 973. D. Mitchell, D.A.J. Rand and R. Woods, J. Electroanal. Chem., 89 (1978) 11. S. Hadzi-Jordanov, H. Angerstein-Kozlowska, M. Vukovic and B.E. Conway, J. Electrochem. Sot., 125 (1978) 2471. M.W. Breiter, J. Electroanal. Chem., 178 (1984) 53. V. Birss, R. Myers, H. Angerstein-Kozlowska and B.E. Conway, J. Electrochem. Sot., 131 (1986) 1502. For a recent review, see B.E. Conway in S. Trasatti (Ed.), Electrochemistry of Conductive Metal Oxides, Vol. 13, Elsevier, Amsterdam, 1980. T. Hepel, F.H. Pollak and W.E.O’Grady, J. Electrochem. Sot., 131 (1984) 2094. M.A. Quiroz, Y. Maas, E. Lamy-Pitara and J. Barbier, J. Electroanal. Chem., 157 (1983) 165. S. Hadzi-Jordanov, H. Angerstein-Kozlowska, M. Vukovic and B.E. Conway, J. Phys. Chem., 81 (1977) 227. M.W. Breiter, Z. Phys. Chem., N.F., 52 (1967) 73.
J. Electroanal. Chem., 214 (1986) 547-554 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
EFFECT OF THE OXYGEN LAYER AT SOLID Ru ELECTRODES HYDROGEN OXIDATION IN ACID SOLUTION *
ON
M.W. BREITER lnstltut fdr Techmsche Elektrochemie, TU Wien, 9 Getreidemarkt, 1060 Wlen (Austrra) (Received 16th April 1986)
ABSTRACT
The inhibition of the anodic oxidation of Ha on a solid Ru electrode by the oxygen layer was investigated under voltammetric conditions in sulphuric acid solution at room temperature. Relatively small sweep rates had to be applied to be able to neglect the contribution of the partial currents of the formation or reduction of the oxygen layer with respect to the partial current of the Hz oxidation. It was demonstrated by voltammetry with a superimposed ac current at 1000 Hz that the process of Ha oxidation does not affect the formation or reduction of the oxygen layer. The inhibition occurs in a gradual fashion on the Ru electrode. Almost a monolayer coverage is necessary before the partial current of Hz oxidation reaches an almost constant value in the oxygen region. Ru behaves completely different from Pt, Rh and Pd in this respect.
INTRODUCTION
It was demonstrated in earlier papers [l-3] lar hydrogen in acid solutions l/2
H, ==H++e-
that the anodic oxidation of molecu-
(1)
is inhibited by the oxygen layer on smooth electrodes of platinum [1,2], iridium [l], rhodium [3] and palladium [3]. On platinum [2], a relatively small coverage with oxygen (about 10%) is sufficient to change the rate-determining step from convective diffusion of molecular hydrogen at potentials below the oxygen region to reaction control in the oxygen region. A similar result is observed in the anodic oxidation of methanol in acid solution under voltammetric conditions. The height of
* Dedicated to the memory of Professor H.W. Ntirnberg. 0022-0728/86/$03.50
0 1986 Elsevier Sequoia S.A.
548
the first wave during the positive sweep is determined [4] by the inhibiting effect of the oxygen coverage on the said platinum metals. The effect of the oxygen coverage of smooth ruthenium on the rate of reaction (I) has now been studied. Ruthenium differs in two aspects from the above platinum metals: (a) The formation of the oxygen layer occurs [5,6) on smooth ruthenium at less positive potentials. (b) The rate of reaction (1) is not controlled solely [7] by convective diffusion at potentials below the oxygen region as on the said platinum metals. Volt~et~ with and without a superimposed ac current was used as the technique. EXPERIMENTAL
The experiments were carried out in a Pyrex glass vessel of conventional design at room temperature (22 rt: l°C). The electrolyte was stirred either with purified Nz or with H,. The electrode potential was determined against a hydrogen electrode in the same solution (A4 H2S04). The electrolytic resistance R, between the tip of the Luggin capillary, leading to the compartment of the reference electrode, and the test electrode was obtained by cathodic pulses as described in ref. 7. Corrections, resulting from I&, were applied only in the impedance m~surements. The dc currents were small enough in the current-potential curves, measured at relatively low sweep rates, to neglect the influence of IR,. The set-up for the impedance measurements by voltammetry with a superimposed ac current was the same as that in previous work 14 . The test electr oJ e consisted of a rectangular piece cut from ruthenium foil, and had a geometric area of 1.64 cm2. Details of the construction and purity of the test electrode have been given before 171. The Ru foil in this study was of the same material as that in ref. 5. RESULTS
Voltammetric current-potential curves were recorded at 10 and 1 mV/s between about 0.03 and 1 V for H, stirring. The potential range was restricted to 0.1 V as the negative potential of reversal at N, stirring to eliminate the possible contribution of molecular hydrogen, formed at potentials below about 0.1 V during the negative sweep, on the subsequent positive sweep. The current-potential curves reveal the presence of strong current fluctuations in a wide potential range at H, stirring. This is clearly shown in Fig. 1 by the curve obtained at 10 mV/s. Vertical bars mark the extent of the current fluctuations at 1 mV/s in Fig. 2. The direction of the potential sweep is indicated by arrows. The effect of different stirring intensities is shown in Fig. 2. Curves a and b were taken at 10 or 1 mV/s and Nz stirring (1 cm3/s). The bars indicate the extent of the current fluctuations in Fig. 3.
549
t 0.6 -
as% 0 cl4 Q
E ._
QZ-
Fig. 1. Voltammetric current-potential curve at 10 mV/s and 1 cm3 Hz/s on smooth ruthenium in M H-/SO,.
The ohmic component R, and the capacitive component l/&s of the electrode impedance in an analogue series circuit were determined as a function of the potential at 1000 Hz by voltammetry with a superimposed ac current at sweep rates of 10 and 1 mV/s for H, or N, stirring. At a given sweep rate, the same R,E curves and l/wC,-E curves were obtained within the limits of reproducibility for both types of stirring. The results at 1 mV/s are given as an example in Fig. 4.
O[
O
,
92
0.4 E/V
03
aa
I-
1.0
Fig. 2. Current-potential curves at 1 mV/s and 1 cm3 Hz/s (curve a) or 0.2 cm3 Hz/s (curve b).
Fig. 3. Current-potential
curves at 10 mV/s
0
O
02
ah
(curve a) or 1 mV/s
E Iv
0.6
(curve b) at 1 cm3 Nz/s.
0.8
Fig. 4. Ohmic and capacitive component of the electrode impedance in an analogue Hz and 1 mV/s. The same curves were obtained at N, or H, stirring.
I-
1P
series circuit
at 1000
DISCUSSION
General considerations When the experimental results of different authors for solid Ru electrodes are compared, the following points have to be considered:
551
(a) The rne~lur~c~ procedures in the preparation of solid Ru are difficult. (b) The surface of Ru may be affected by the sealing into glass during assembly of the electrode. (c) Since Ru dissolves to a noticeable extent during positive sweeps [5], the roughness of the electrode will change with time. An aged electrode will differ from a fresh electrode in this respect. Another feature is the growth of a thick oxide film when the electrode is cycled between certain potential limits [5,6,8], leading to a change in the shape of the voltammetric current-potential curves. The foil electrode of the present inv~tigation was of the same material as that in ref. 5. The aging by prior use in other experiments was’responsible for the electrode roughness in this work being larger (three times) than that in ref. 5. Comparison of the current-potential curves of refs. 5 and 6 reveals a marked difference at the beginning of cycling. The initial curves recorded [6,8] during cycling between about 0 and 1.4 V in acid solution look somewhat similar to the respective curves on smooth Rh [9], with a well-defined reduction wave of the oxygen layer during the negative sweep and a wide hydrogen peak of the same shape for positive and negative sweeps. In contrast, the initial curves on the Ru foils do not display a hydrogen peak of nearly the same shape for positive and negative sweeps under any conditions. It is suggested that the described difference is due to reason (a} above. A voltammetric study of surfaces with different orientations of single crystals of RuO, was published [lo] recently. The reader is referred to this paper for additional references on work with RuO,. It is pointed out [lo] that “oxygen-free” surfaces exist for the following oscillographic orientations in the ideal case: (lOl), (111) and (100). Crystals with these orientations have only Ru atoms on the surface. Only the current-potential curve for the (100) face is given for a low sweep rate in 0.5 M H,SO,. This curve looks vastly different from curve b in Fig. 3 in a wide potential range. It is difficult to compare the results in ref. 10 with the results on solid Ru electrodes. There is a great difference in the behaviour of smooth electrodes of Pt, Rh and Pd, on the one hand, and Ru, on the other hand. While the study of the inhibition of reaction (1) may be carried out at sweep rates as large as 100 mV/s without noticeable interference by the partial currents due to the formation or reduction of the oxygen layer, the sweep rate cannot be larger than 10 mV/s on Ru otherwise the partial currents become too large. This was demonstrated experimentally by curves taken for H, stirring at 100 mV/s. These curves are not shown here because the ~nt~bution of the partial currents produces an effect which looks like a large hysteresis between the positive and negative sweeps, but really results from the overlapping of currents from different electrode reactions. Sweep rates of 10 and 1 mV/s were used predominantly for this reason. Large fluctuations of the anodic current can be observed in the curves of Figs. 1 and 2 in a wide potential range. They are attributed mainly to oscillations of the thickness of the diffusion layer on the rectangular electrodes immersed in a vertical position. This conclusion is supported by the experimental evidence in Fig. 2. The anodic limiting current density i,, observed between about 0.1 and 0.4 V, is
rn~k~ly smaller at 0.2 ems HJs than at 1 err? HJs because the diffusion layer thickness decreases. It was concluded in previous work [7] that the anodic limiting current density is given by:
Here, id,H2stands for the limiting current density due to convective diffusion and i,,T represents the exchange current density of the Tafel reaction occurring on a surface with a small coverage of H atoms. The inhibition of reaction (1) by the oxygen layer is caused by a transition from rate control by convective diffusion to control by a heterogen~us reaction on Pt, Rh and Pd. The exact nature of this transition and of the heterogeneous reaction is not yet fully understood. On Ru, the situation is more complicated. The inhibition involves a transition from mixed control by convective diffusion and the Tafel reaction to control by a heterogeneous reaction which need not be identical with the Tafel reaction. Formation and reduction of the oxygen layer The anodic formation and cathodic reduction of the oxygen layer on solid Ru electrodes has been studied and discussed previously [5,6,8]. There seems to be general agreement on several points: (d) Monolayer formation is followed by multilayer formation, probably RuOz, at larger positive potentials. The potential where this transition occurs is not quite certain Ill], but appears to be in the vicinity of 1 V. (e) The hydrogen layer and oxygen layer overlap, especially during the negative sweep. Separation is difficult. The general discussion in the preceding section makes it clear that information on the oxygen layer has to be obtained and correlated to information on the inhibition of reaction (1) on the same electrode under the same experimental conditions. In this context the question arises, as to whether the simultaneous occurrence of reaction (1) influences the formation and reduction of the oxygen layer under cycling between about 0.05 and 1 V. To answer this question, the electrode impedance was measured for both H, and N, stirring as a function of potential at 1000 Hz by voltammetry with a superimposed ac current. While the relatively large currents of reaction (1) mask the currents for the formation or reduction of the oxygen layer at small sweep rates in a wide potential range, the electrode impedance reflects [4] the influence of the oxygen layer on Pt, Rh and Pd. Thus information on the formation and reduction of the oxygen layer in the presence of reaction (1) is available. The concept was developed by the author in studies on Pt. It is now applied to Ru. The results of some of the impedance measurements are given in Fig. 4. Since the same R,-E curves and l/c&,-E curves were obtained at a given sweep rate (10 or 1 mV/s) for H, or N2 stirring, it is concluded that the formation or reduction of the oxygen layer on Ru is independent of the type of stirring. If there were an influence
553
of reaction (1) on the processes connected with the oxygen layer, the amount of oxygen at a given potential would be different for H, and N, stirring. This situation would be recognizable by different R,--E curves and l/&,-E curves for both types of stirring. Information on the formation and reduction of the oxygen layer is supplied for both types of stirring by the voltammetric current-potential curves taken at N, stirring. Although they are not shown here, such curves were also recorded at 100 mV/s. It was found, in agreement with earlier work [5,6,12], that the charge for the formation of the oxygen layer during the positive sweep was nearly the same (within 5%) as that for the reduction during the negative sweep at 100 mV/s. However, there is a systematic trend in the present study that the anodic charge becomes larger than the cathodic charge (ca. 15% at 10 mV/s and 100% at 1 mV/s). It is considered likely that Ru dissolution is responsible for this effect. Ru dissolution has to occur at lower potentials than 1.4 V, as was proposed in ref. 5. In the present study the charge for the formation of the oxygen layer at potential E has to be taken as the charge for the negative sweep starting at E. Additional information on the processes associated with the oxygen layer and on the overlapping of the oxygen and hydrogen region can be obtained from the impedance measurements. The capacity C, has a value of 706 pF/cm* at 0.1 V during the positive sweep. This value corresponds to the maximum in the l/&,-E curve (see Fig. 4). The magnitude of C, for 1000 Hz implies that there is a contribution from a pseudo-capacitance due to a faradaic reaction. The value of C, increases to about 1400 pF/cm* at 0.7 V and remains the same between 0.7 and 1 V. It is suggested that a partial step such as H,O=OH,+H++e-
(3)
is responsible for the pseudo-capacitance. A similar situation was found at more positive potentials for smooth electrodes of Ir [13]. The existence of a relatively rapid process was postulated for Ru in ref. 12. The increase of C, with potential between 0.1 and 0.7 V during the positive sweep implies that the coverage with adsorbed species in a reaction such as (3) becomes larger up to 0.7 V and remains practically the same between 0.7 and 1 V. The hysteresis of the l/&,-E curves of the positive and negative sweeps indicates that the coverage with the adsorbed species is not quite the same at a given potential. The maximum of the l/&,-E curve is interpreted as the result of the overlapping of the oxygen and hydrogen region. The slight increase of C, between 0.07 and 0.1 V during the positive sweep (see Fig. 4) reflects the influence of the hydrogen coverage. The latter conclusion is in agreement with the interpretation of the reflectance data in ref. 12. Inhibition
of the H, oxidation
by the oxygen layer
In contrast to Pt, Rh and Pd, there is a gradual decrease of the anodic current due to reaction (1) with potential above about 0.3 V under voltammetric conditions.
554
As discussed in the preceding section, the oxygen coverage begins to form at about 0.1 V during the positive sweep. The oxygen coverage present between 0.1 and about 0.3 V does not seem to affect reaction (1). On Pt, Rh and Pd, a relatively small amount of oxygen is sufficient to produce a large inhibiting effect, probably by blocking the most active sites for reaction (1). On Ru, an oxygen layer coverage of about 1 appears to be necessary to reach an almost constant current of reaction (1) in the oxygen region. The oxygen coverage increases on Ru in a linear fashion [11,12] with potential up to about 0.9 V. The shape of the i-E curves in Fig. 3 demonstrates that the same situation exists for the Ru electrodes of this work. The inhibition of reaction (1) appears to depend linearly on the oxygen coverage between about 0.4 and 0.7 V. Since two types of oxygen species (probably OH, and 0,) are present in the latter potential range, it is difficult to develop a detailed model of the mechanism of inhibition. In a simple model, the inhibition of reaction (1) between 0.3 and 0.7 V may be considered to be caused by blocking of the free surface by oxygen species. The oxygen coverage from the i-E curves represents a measure of the coverage by both oxygen species. Additional inhibiting effects are present between 0.7 and 1 V. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13
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