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Investigation of ruthenium deposition onto a platinized platinum electrode in sulfuric acid media
233
J. Electroanal. Chem., 230 (1987) 233-240 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
INVESTIGATION OF RtJTJdENIUM DEPOSITION ONTO A PLATINIZED PLATINUM ELECIRODE IN SULFURIC ACID MEDIA
S. SZABd and I. BAKOS Central Research Institute for Chemistry, Hungarian Academy P. 0. Box I7 (Hungary)
of Sciences, H-1525 Budapest,
(Received 22nd July 1986; in revised form 15th January 1987)
ABSTRACT
Ruthenium deposition onto platinized F’t electrode in 0.5 M H,SO, solution is investigated, The Z-V profiles of the Pt electrode covered with Ru depend on the potential of Ru deposition. This phenomenon is explained by the increasing degree of oxidation of the Ru layers deposited at higher potentials. Oxidation of Ru deposited at low potentials begins with comparatively slow processes. A mechanism for Ru deposition via ionization of hydrogen adsorbed on platinked Pt electrode is proposed.
INTRODUCTION
In the past, several papers have been devoted to the investigation of underpotential deposition of noble metals onto noble metal substrates [l-4], but until now no paper has been published on the underpotential deposition of Ru onto a Pt surface. Several papers deal with the electrochemical and electrocatalytic properties of ruthenized Pt electrodes [5-81. These electrodes can be regarded, however, as Ru electrodes since the electrodeposited Ru layer is thick enough to cover the Pt. surface completely. The only exception is an observation of Watanabe and Motoo [5], who used a Pt electrode covered with Ru adatoms as a Ru/Pt electrocatalyst but failed to study the adsorption of ruthenium onto the Pt surface. Their attempt to calculate ruthenium coverage based on OH- adsorption is questionable in the light of later results [6-81. The purpose of our paper is to give an account of the work carried out to describe the spontaneous formation of a ruthenium layer on the surface of a platinized Pt electrode covered with preadsorbed hydrogen. The characteristics of the platinized Pt electrode covered with electrodeposited Ru are also dealt with. 0022-0728/87/$03.50
0 1987 Elsevier Sequoia S.A.
234 EXPERIMENTAL
All experiments were carried out in the same three-compartment cell described earlier [9]. A characteristic feature of our cell is that cell solutions can be replaced with the exclusion of air. It allows the study of the properties of a Pt electrode covered with deposited metal in the absence of depositing ions. The 0.5 M H,SO, supporting electrolyte was prepared from Merck reagents with triply distilled water as described earlier [9]. Purified nitrogen was bubbled through the cell to deoxygenate the solutions. 1 g RuCl, - l-3 H,O (Ventron GmbH) was dissolved in 100 cm3 1 M HCl and this solution was used as a source of Ru ions. The concentration at the beginning of Ru deposition was 0.5-l mM which, however, never dropped below 0.4 mM in the course of experiments. It must be taken into consideration, however, in the discussion of the results that the average valence state of ruthenium in this compound is near +4 [lO,ll]. The polycrystalline platinum electrode was platinized as described previously [9]. Its apparent surface area was ca. 2 cm’ and the roughness factor was ca. 250. All potentials are referred to the hydrogen electrode immersed into the same supporting electrolyte in the main compartment of the cell. RESULTS AND DISCUSSION
Ruthenium deposition via ionization of hydrogen a&orbed on platinized platinum First the cyclic voltammogram of the Pt electrode was determined in supporting electrolyte (curve 1 in Fig. 1) then the electrode was saturated with hydrogen by
Fig. 1. (1) Voltammogram of Pt electrode in 0.5 M H,S04 solution; (2) voltammogram of Pt electrode covered with ruthenium deposited via the ionization of preadsorbed hydrogen in 0.5 M H,SO, free of ruthenium ions. Sweep rate: 37 mV/s.
235
I
20
LO
t/mm
6o
Fig. 2. Variation of electrode potential during the deposition of ruthenium adsorbed on platinized platinum under open-circuit conditions.
via ionization
of hydrogen
strong cathodic polarization. When, under open-circuit circumstances, the potential reached 0.05 V owing to hydrogen desorption, 0.5 cm3 of Ru solution was introduced into the main compartment of the cell. It caused a potential shift in the positive direction (Fig. 2). (The amount of charge corresponding to this potential shift from 0.05 to 0.85 V is denoted by Q o .) When equilibrium set in after 60-90 min at about 0.85 V, the cell was rinsed with a deoxygenated supporting electrolyte and then the cyclic voltammogram of the Pt electrode covered with Ru was determined in ruthenium-ion-free supporting electrolyte (curve 2 in Fig. 1). On the basis of Fig. 1 it may be stated that the deposited Ru inhibits hydrogen adsorption slightly, the double-layer region broadens and the oxygen (OH-) adsorption character of the Pt is strongly changed. Such a strong effect of metal deposition on the adsorption characteristics of platinized Pt can be explained only by surface processes. It follows from the above, that the deposited Ru must be in the adsorbed state. A study of the dissolution of ruthenium deposited onto platinized platinum In the investigation of the electrochemical properties of a ruthenized Pt electrode irreversible oxidation and dissolution of the Ru coating must be taken into consideration [7,8]. In our experience, dissolution of the Ru deposited onto platinized Pt could be detected after lo-15 cycles between potential limits of 0.05 and 1.15 V. If the upper potential limit of the cyclic voltammetric measurement is high enough, continuous dissolution of the deposited Ru can be observed and finally the initial Pt surface is restored (Fig. 3). In the experiment illustrated in Fig. 3 the Pt electrode was first kept at 0.05 V in the presence of 50 mM RuCl, . l-3 H,O until a charge of 5 Q” was used for Ru deposition; then the cell was rinsed and refilled
Fig. 3. Dissolution of ruthenium from the Pt surface in 0.5 M H2S04 potential limits of 0.05 and 1.5 V. Sweep rate: 48 mV/s.
during
continuous
cycling between
with deoxygenated 0.5 M H,SO,. After this procedure a cyclic voltammetric measurement was carried out between 0.05 and 1.5 V, the results of which are given in Fig. 3. Ruthenium deposition at 0.05 V After determination of the cyclic voltammogram of the Pt electrode (curve 1 in Fig. 4) its potential was scanned to 0.05 V. At this potential 1 ml ruthenium solution was introduced into the main compartment of the cell and ruthenium deposition was continued until a quantity of electricity of 1 Q” was consumed. After this procedure the cell was rinsed and refilled with deoxygenated ruthenium-ion-free supporting electrolyte and then the cyclic voltammogram of the electrode covered with Ru was determined (curve 2 in Fig. 4). This experiment was repeated, but now the electrode was maintained at 0.05 V until a charge of 4 Q” (curve 3 in Fig. 4) or of 10 Q” (curve 4 in Fig. 4) was consumed for ruthenium deposition. It should be noted that when a charge of 10 Q” was used for ruthenium deposition, a visible Ru coating was formed on the electrode. In spite of the substantial amount of Ru on the Pt, the two peaks of hydrogen desorption characteristic of Pt were still visible and the amount of hydrogen adsorbed was practically independent of the amount of Ru deposited. From this phenomenon the
237
IlmA 20
Fig. 4. (1) Voltammogram of Pt electrode in 0.5 M H2S04 solution; voltammogram of Pt electrode covered with an amount of Ru Q” (2), 4 Q” (3) and 10 Q” (4) in 0.5 M H,SO, supporting electrolyte. Sweep rate: 37 mV/s.
conclusion might be drawn that no adsorbed Ru layer was formed, but merely that a Ru coating was deposited in the form of three-dimensional nuclei in the outer layer of the platinum black substrate. On the other hand, the oxygen desorption peak typical for Pt gradually disappeared as if Ru adsorption had taken place. This contradiction might be explained if it is assumed that the presence of Ru nuclei on the Pt surface changed the oxygen adsorption character of the Pt surface free of Ru. An alternative explanation might be that the Ru crystals deposited onto the 100 and 110 crystal faces of polycrystalline Pt in a similar manner as silver and palladium [9,12], covering the Pt surface almost completely, are not uniform in their thermodynamic character, and consequently, hydrogen adsorption on the Ru surface may result in two peaks. Ruthenium deposition at different potentials The same amount of Ru was deposited onto a platinized Pt electrode at potentials of 0.00, 0.05, 0.20 and 0.60 V. The potentiostatically deposited Ru coatings were examined in a ruthenium-ion-free supporting electrolyte (Fig. 5). On the basis of the experimental data it can be stated that the valence state of the Ru coatings deposited between the potential limits of 0 and 0.2 V are almost identical, since the I-V profiles are independent of the deposition potential. However, the I-V profiles of the electrode covered with Ru above this potential limit are gradually changed. New oxidation reduction peaks appear at - 0.75 V in
238 IlmA 20I
Fig. 5. (1) Voltammogram of Pt electrode covered with an amount of Ru deposited voltammogram of Pt electrode saturated with Ru at 0.6 V. Sweep rate: 37 mV/s
at 0.05 ir Q”;
(2)
the oxidation branch and - 0.5 V in the reduction branch of the cyclic voltammogram (curve 2 in Fig. 5). With the help of the cyclic voltammogram of a Ru electrode covered with RuO, as a result of heat treatment, it could be verified that the peaks are probably due to the formation and reduction of RuO, [13]. Short anodization at - 1.35 V of a platinized Pt electrode covered with Ru at 0.05 V results in a cyclic voltammogram as if the Ru had been deposited at 0.6 V. On the basis of the above results, it is clear that the oxidation state of the Ru layers deposited above - 0.2 V is gradually increased and the oxidation-reduction process taking place at - 0.75 V (and - 0.5 V in the negative direction) is preceded by comparatively slow oxidation processes. Products of these slow reactions are oxidized further (and faster) at - 0.75 V. Finally, it must be emphas;. ed that a Ru layer deposited at 0.6 V under these circumstances could not be reduced to the oxidation state characterized by a Ru coating deposited at 0.05 V. This observation verified the above idea about the relatively slow initial oxidation of Ru deposited on platinized Pt at low potentials. On the mechanism of ruthenium deposition via ionization of hydrogen aakorbed on platinized platinum On the basis of the above results it may be concluded that the very first stage of the mechanism is Ru deposition with simultaneous ionization of adsorbed hydrogen, consequently shifting the potential in the positive direction. According to our earlier results [3-51, at the beginning (below the Nemst potential) of the spontaneous formation of an adsorbed metal layer, mostly bulk deposition ought to have occurred in the outer layer of the Pt black because the rate-determining step at this stage is diffusion of Ru ions from the bulk of the supporting electrolyte. However, no sharp separation of bulk and underpotential deposition could be experienced
239
here; therefore, both of them have to be taken into account in the first step of the mechanism: H,+H++e-
0)
Ru”+ + n e- + Ru,,
(2)
Run+ + n e- 4 Ru adS
(3)
Comparison of curve 2 in Fig. 1 with curve 2 in Fig. 4 shows that the spontaneous increase of the potential results in some rearrangement [14], and reoxidation of Ru deposited at smaller potentials Ru bulk+ Ru”+ + a e-
(4)
Ru,,
(5)
+ Rub,dS+ b e-
Depending on the actual potential of the electrode, the valency of the ruthenium ions formed by these reactions may be in the range l-4 because the final value of the spontaneous potential rise (Fig. 2) is in the potential range of RuO, formation
PINaturally, electrons produced by reactions (4) and (5) are also consumed additional ruthenium ion deposition, which leads to increased coverage: Run+ + x e- --, Ru$ix)+
in (6)
Naturally, in this case also the charge number of the deposited ruthenium ions depends on the electrode potential and might be in the range l-3. Considering the standard potentials of different redox processes of ruthenium [8] and the valence state of the ruthenium salt [ll] used in the experiments, it is highly improbable that any ruthenium species with a charge higher than 4 takes part in the formation of the adsorbed Ru layer, because catalytic disproportionation was not observed in this case [15]. Taking into account the standard potentials of Ru 20, (E o = 0.738 V) and RuO, (E o = 0.937 V) formation [8], it can be expected that their formation might overlap; therefore, they could be mixed on the Pt surface. It follows that the oxidation peak at - 0.75 V and the reduction peak at - 0.5 V in Fig. 1 can very likely be attributed to the oxidation and reduction of adsorbed Ru(II1) and Ru(IV) species. It seems to us that this idea is supported by the concept of the relatively slow initial oxidation of Ru. This process is very likely the formation of Ru(I1) species which are oxidized further and faster at - 0.75 V. REFERENCES 1 2 3 4 5 6
S.H. Cadle and S. Bruckenstein, Anal. Chem., 43 (1971) 1858. D.A.J. Rand and R. Woods, J. Electroanal. Chem, 44 (1973) 83. S. Szab6 and F. Nagy, J. Electroanal. Chem., 85 (1977) 339. S. Szabb and F. Nagy, Isr. J. Chem., 18 (1979) 162. M. Watanabe and S. Motoo, J. Electroanal. Chem., 60 (1975) 267. S. Hati-Jordanov, H. Angerstein-Kozlowska, M. Vukovik and B.E. Conway, J. Phys. Chem., 81 (1977) 2271.
240 7 S. Had&Jordanov, H. Angerstein-Kozlowska, M. Vukovif and B.E. Conway, J. Electrochem. Sot., 125 (1978) 1471. 8 R.O. Lezna, N.R. DeTacconi and A.J. Arvia, J. Electroanal. Chem., 151 (1983) 193. 9 S. Szabo and F. Nagy, J. Electroanal. Chem., 70 (1976) 357. 10 J.A. Rard, Chem. Rev., 85 (1985) 1. 11 E.A. Seddon and K.R. Seddon, The Chemistry of Ruthenium, Elsevier, Amsterdam, 1984, p. 159. 12 R.G. Barradas, S. Fletcher and S. Szabb, Can. J. Chem., 56 (1978) 2029. 13 D. Michell, D.A.J. Rand and R. Woods, J. Electroanal. Chem., 89 (1987) 11. 14 N. Furuya and S. Motoo, J. Electroanal. Chem., 72 (1976) 165. 15 S. Szabb, J. Electroanal. Chem., 172 (1984) 359.
J. Electroanal. Chem., 230 (1987) 233-240 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
INVESTIGATION OF RtJTJdENIUM DEPOSITION ONTO A PLATINIZED PLATINUM ELECIRODE IN SULFURIC ACID MEDIA
S. SZABd and I. BAKOS Central Research Institute for Chemistry, Hungarian Academy P. 0. Box I7 (Hungary)
of Sciences, H-1525 Budapest,
(Received 22nd July 1986; in revised form 15th January 1987)
ABSTRACT
Ruthenium deposition onto platinized F’t electrode in 0.5 M H,SO, solution is investigated, The Z-V profiles of the Pt electrode covered with Ru depend on the potential of Ru deposition. This phenomenon is explained by the increasing degree of oxidation of the Ru layers deposited at higher potentials. Oxidation of Ru deposited at low potentials begins with comparatively slow processes. A mechanism for Ru deposition via ionization of hydrogen adsorbed on platinked Pt electrode is proposed.
INTRODUCTION
In the past, several papers have been devoted to the investigation of underpotential deposition of noble metals onto noble metal substrates [l-4], but until now no paper has been published on the underpotential deposition of Ru onto a Pt surface. Several papers deal with the electrochemical and electrocatalytic properties of ruthenized Pt electrodes [5-81. These electrodes can be regarded, however, as Ru electrodes since the electrodeposited Ru layer is thick enough to cover the Pt. surface completely. The only exception is an observation of Watanabe and Motoo [5], who used a Pt electrode covered with Ru adatoms as a Ru/Pt electrocatalyst but failed to study the adsorption of ruthenium onto the Pt surface. Their attempt to calculate ruthenium coverage based on OH- adsorption is questionable in the light of later results [6-81. The purpose of our paper is to give an account of the work carried out to describe the spontaneous formation of a ruthenium layer on the surface of a platinized Pt electrode covered with preadsorbed hydrogen. The characteristics of the platinized Pt electrode covered with electrodeposited Ru are also dealt with. 0022-0728/87/$03.50
0 1987 Elsevier Sequoia S.A.
234 EXPERIMENTAL
All experiments were carried out in the same three-compartment cell described earlier [9]. A characteristic feature of our cell is that cell solutions can be replaced with the exclusion of air. It allows the study of the properties of a Pt electrode covered with deposited metal in the absence of depositing ions. The 0.5 M H,SO, supporting electrolyte was prepared from Merck reagents with triply distilled water as described earlier [9]. Purified nitrogen was bubbled through the cell to deoxygenate the solutions. 1 g RuCl, - l-3 H,O (Ventron GmbH) was dissolved in 100 cm3 1 M HCl and this solution was used as a source of Ru ions. The concentration at the beginning of Ru deposition was 0.5-l mM which, however, never dropped below 0.4 mM in the course of experiments. It must be taken into consideration, however, in the discussion of the results that the average valence state of ruthenium in this compound is near +4 [lO,ll]. The polycrystalline platinum electrode was platinized as described previously [9]. Its apparent surface area was ca. 2 cm’ and the roughness factor was ca. 250. All potentials are referred to the hydrogen electrode immersed into the same supporting electrolyte in the main compartment of the cell. RESULTS AND DISCUSSION
Ruthenium deposition via ionization of hydrogen a&orbed on platinized platinum First the cyclic voltammogram of the Pt electrode was determined in supporting electrolyte (curve 1 in Fig. 1) then the electrode was saturated with hydrogen by
Fig. 1. (1) Voltammogram of Pt electrode in 0.5 M H,S04 solution; (2) voltammogram of Pt electrode covered with ruthenium deposited via the ionization of preadsorbed hydrogen in 0.5 M H,SO, free of ruthenium ions. Sweep rate: 37 mV/s.
235
I
20
LO
t/mm
6o
Fig. 2. Variation of electrode potential during the deposition of ruthenium adsorbed on platinized platinum under open-circuit conditions.
via ionization
of hydrogen
strong cathodic polarization. When, under open-circuit circumstances, the potential reached 0.05 V owing to hydrogen desorption, 0.5 cm3 of Ru solution was introduced into the main compartment of the cell. It caused a potential shift in the positive direction (Fig. 2). (The amount of charge corresponding to this potential shift from 0.05 to 0.85 V is denoted by Q o .) When equilibrium set in after 60-90 min at about 0.85 V, the cell was rinsed with a deoxygenated supporting electrolyte and then the cyclic voltammogram of the Pt electrode covered with Ru was determined in ruthenium-ion-free supporting electrolyte (curve 2 in Fig. 1). On the basis of Fig. 1 it may be stated that the deposited Ru inhibits hydrogen adsorption slightly, the double-layer region broadens and the oxygen (OH-) adsorption character of the Pt is strongly changed. Such a strong effect of metal deposition on the adsorption characteristics of platinized Pt can be explained only by surface processes. It follows from the above, that the deposited Ru must be in the adsorbed state. A study of the dissolution of ruthenium deposited onto platinized platinum In the investigation of the electrochemical properties of a ruthenized Pt electrode irreversible oxidation and dissolution of the Ru coating must be taken into consideration [7,8]. In our experience, dissolution of the Ru deposited onto platinized Pt could be detected after lo-15 cycles between potential limits of 0.05 and 1.15 V. If the upper potential limit of the cyclic voltammetric measurement is high enough, continuous dissolution of the deposited Ru can be observed and finally the initial Pt surface is restored (Fig. 3). In the experiment illustrated in Fig. 3 the Pt electrode was first kept at 0.05 V in the presence of 50 mM RuCl, . l-3 H,O until a charge of 5 Q” was used for Ru deposition; then the cell was rinsed and refilled
Fig. 3. Dissolution of ruthenium from the Pt surface in 0.5 M H2S04 potential limits of 0.05 and 1.5 V. Sweep rate: 48 mV/s.
during
continuous
cycling between
with deoxygenated 0.5 M H,SO,. After this procedure a cyclic voltammetric measurement was carried out between 0.05 and 1.5 V, the results of which are given in Fig. 3. Ruthenium deposition at 0.05 V After determination of the cyclic voltammogram of the Pt electrode (curve 1 in Fig. 4) its potential was scanned to 0.05 V. At this potential 1 ml ruthenium solution was introduced into the main compartment of the cell and ruthenium deposition was continued until a quantity of electricity of 1 Q” was consumed. After this procedure the cell was rinsed and refilled with deoxygenated ruthenium-ion-free supporting electrolyte and then the cyclic voltammogram of the electrode covered with Ru was determined (curve 2 in Fig. 4). This experiment was repeated, but now the electrode was maintained at 0.05 V until a charge of 4 Q” (curve 3 in Fig. 4) or of 10 Q” (curve 4 in Fig. 4) was consumed for ruthenium deposition. It should be noted that when a charge of 10 Q” was used for ruthenium deposition, a visible Ru coating was formed on the electrode. In spite of the substantial amount of Ru on the Pt, the two peaks of hydrogen desorption characteristic of Pt were still visible and the amount of hydrogen adsorbed was practically independent of the amount of Ru deposited. From this phenomenon the
237
IlmA 20
Fig. 4. (1) Voltammogram of Pt electrode in 0.5 M H2S04 solution; voltammogram of Pt electrode covered with an amount of Ru Q” (2), 4 Q” (3) and 10 Q” (4) in 0.5 M H,SO, supporting electrolyte. Sweep rate: 37 mV/s.
conclusion might be drawn that no adsorbed Ru layer was formed, but merely that a Ru coating was deposited in the form of three-dimensional nuclei in the outer layer of the platinum black substrate. On the other hand, the oxygen desorption peak typical for Pt gradually disappeared as if Ru adsorption had taken place. This contradiction might be explained if it is assumed that the presence of Ru nuclei on the Pt surface changed the oxygen adsorption character of the Pt surface free of Ru. An alternative explanation might be that the Ru crystals deposited onto the 100 and 110 crystal faces of polycrystalline Pt in a similar manner as silver and palladium [9,12], covering the Pt surface almost completely, are not uniform in their thermodynamic character, and consequently, hydrogen adsorption on the Ru surface may result in two peaks. Ruthenium deposition at different potentials The same amount of Ru was deposited onto a platinized Pt electrode at potentials of 0.00, 0.05, 0.20 and 0.60 V. The potentiostatically deposited Ru coatings were examined in a ruthenium-ion-free supporting electrolyte (Fig. 5). On the basis of the experimental data it can be stated that the valence state of the Ru coatings deposited between the potential limits of 0 and 0.2 V are almost identical, since the I-V profiles are independent of the deposition potential. However, the I-V profiles of the electrode covered with Ru above this potential limit are gradually changed. New oxidation reduction peaks appear at - 0.75 V in
238 IlmA 20I
Fig. 5. (1) Voltammogram of Pt electrode covered with an amount of Ru deposited voltammogram of Pt electrode saturated with Ru at 0.6 V. Sweep rate: 37 mV/s
at 0.05 ir Q”;
(2)
the oxidation branch and - 0.5 V in the reduction branch of the cyclic voltammogram (curve 2 in Fig. 5). With the help of the cyclic voltammogram of a Ru electrode covered with RuO, as a result of heat treatment, it could be verified that the peaks are probably due to the formation and reduction of RuO, [13]. Short anodization at - 1.35 V of a platinized Pt electrode covered with Ru at 0.05 V results in a cyclic voltammogram as if the Ru had been deposited at 0.6 V. On the basis of the above results, it is clear that the oxidation state of the Ru layers deposited above - 0.2 V is gradually increased and the oxidation-reduction process taking place at - 0.75 V (and - 0.5 V in the negative direction) is preceded by comparatively slow oxidation processes. Products of these slow reactions are oxidized further (and faster) at - 0.75 V. Finally, it must be emphas;. ed that a Ru layer deposited at 0.6 V under these circumstances could not be reduced to the oxidation state characterized by a Ru coating deposited at 0.05 V. This observation verified the above idea about the relatively slow initial oxidation of Ru deposited on platinized Pt at low potentials. On the mechanism of ruthenium deposition via ionization of hydrogen aakorbed on platinized platinum On the basis of the above results it may be concluded that the very first stage of the mechanism is Ru deposition with simultaneous ionization of adsorbed hydrogen, consequently shifting the potential in the positive direction. According to our earlier results [3-51, at the beginning (below the Nemst potential) of the spontaneous formation of an adsorbed metal layer, mostly bulk deposition ought to have occurred in the outer layer of the Pt black because the rate-determining step at this stage is diffusion of Ru ions from the bulk of the supporting electrolyte. However, no sharp separation of bulk and underpotential deposition could be experienced
239
here; therefore, both of them have to be taken into account in the first step of the mechanism: H,+H++e-
0)
Ru”+ + n e- + Ru,,
(2)
Run+ + n e- 4 Ru adS
(3)
Comparison of curve 2 in Fig. 1 with curve 2 in Fig. 4 shows that the spontaneous increase of the potential results in some rearrangement [14], and reoxidation of Ru deposited at smaller potentials Ru bulk+ Ru”+ + a e-
(4)
Ru,,
(5)
+ Rub,dS+ b e-
Depending on the actual potential of the electrode, the valency of the ruthenium ions formed by these reactions may be in the range l-4 because the final value of the spontaneous potential rise (Fig. 2) is in the potential range of RuO, formation
PINaturally, electrons produced by reactions (4) and (5) are also consumed additional ruthenium ion deposition, which leads to increased coverage: Run+ + x e- --, Ru$ix)+
in (6)
Naturally, in this case also the charge number of the deposited ruthenium ions depends on the electrode potential and might be in the range l-3. Considering the standard potentials of different redox processes of ruthenium [8] and the valence state of the ruthenium salt [ll] used in the experiments, it is highly improbable that any ruthenium species with a charge higher than 4 takes part in the formation of the adsorbed Ru layer, because catalytic disproportionation was not observed in this case [15]. Taking into account the standard potentials of Ru 20, (E o = 0.738 V) and RuO, (E o = 0.937 V) formation [8], it can be expected that their formation might overlap; therefore, they could be mixed on the Pt surface. It follows that the oxidation peak at - 0.75 V and the reduction peak at - 0.5 V in Fig. 1 can very likely be attributed to the oxidation and reduction of adsorbed Ru(II1) and Ru(IV) species. It seems to us that this idea is supported by the concept of the relatively slow initial oxidation of Ru. This process is very likely the formation of Ru(I1) species which are oxidized further and faster at - 0.75 V. REFERENCES 1 2 3 4 5 6
S.H. Cadle and S. Bruckenstein, Anal. Chem., 43 (1971) 1858. D.A.J. Rand and R. Woods, J. Electroanal. Chem, 44 (1973) 83. S. Szab6 and F. Nagy, J. Electroanal. Chem., 85 (1977) 339. S. Szabb and F. Nagy, Isr. J. Chem., 18 (1979) 162. M. Watanabe and S. Motoo, J. Electroanal. Chem., 60 (1975) 267. S. Hati-Jordanov, H. Angerstein-Kozlowska, M. Vukovik and B.E. Conway, J. Phys. Chem., 81 (1977) 2271.
240 7 S. Had&Jordanov, H. Angerstein-Kozlowska, M. Vukovif and B.E. Conway, J. Electrochem. Sot., 125 (1978) 1471. 8 R.O. Lezna, N.R. DeTacconi and A.J. Arvia, J. Electroanal. Chem., 151 (1983) 193. 9 S. Szabo and F. Nagy, J. Electroanal. Chem., 70 (1976) 357. 10 J.A. Rard, Chem. Rev., 85 (1985) 1. 11 E.A. Seddon and K.R. Seddon, The Chemistry of Ruthenium, Elsevier, Amsterdam, 1984, p. 159. 12 R.G. Barradas, S. Fletcher and S. Szabb, Can. J. Chem., 56 (1978) 2029. 13 D. Michell, D.A.J. Rand and R. Woods, J. Electroanal. Chem., 89 (1987) 11. 14 N. Furuya and S. Motoo, J. Electroanal. Chem., 72 (1976) 165. 15 S. Szabb, J. Electroanal. Chem., 172 (1984) 359.