- Email: [email protected]
The complex processes involved at Pd electrodes in 1 M H2SO4 in the potential range of oxygen electroadsorption-electrodesorption reactions
J. Electroanal. Chem., 157 (1983) 339-358
339
Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
THE COMPLEX PROCESSES INVOLVED AT Pd ELECTRODES IN 1 M H2SO 4 IN THE POTENTIAL RANGE OF OXYGEN ELECTROADSORPTION-ELECTRODESORPTION REACTIONS
A.E. BOLZ.~N, M.E. MARTINS and A.J. ARViA Instituto de Investiga~ciones Fisicoquimicas Terricas y Aplicadas - - I N I F T A , Casilla de Correo 16, Sucursal 4, 1900 La Plata (Argentina)
(Received 30th December 1982; in revised form 22nd March 1983)
ABSTRACT Pd electrodes in 1 M H2SO 4 show different electrochemical behaviour in the whole range of oxygen electroadsorption and the oxide layer formation, depending on the characteristics of the electrical perturbation during the initial potentiodynamic sweep of electrodissolution of Pd, and the O-electroadsorption takes place. In this case the electroreduction potentiodynamic profile exhibits the oxygen electrodesorption peak and a limiting current due to lad electrodeposition. Under these conditions the anodic to cathodic charge ratio is always greater than one. A different behaviour is observed after a prolonged potential cycling. The initial O-electroadsorption is activated and the Pd electrodissolution is practically suppressed. The charge is then equal to one and the oxygen electroreduction profile changes according to the history of the potential perturbation conditions. Results indicate the formation of different O-electroadsorbed oxide species, depending on the potential range covered during the experiments. The electrochemical reactions, including ageing effects and oxide film growth, are interpreted with a reaction formalism which, to a great extent, is similar to that already discussed for Pt electrodes in acid electrolytes.
INTRODUCTION T h e i m p o r t a n c e of n o b l e metals in heterogeneous catalysis a n d electrocatalysis is w i d e l y recognized. This has b e e n the reason for n u m e r o u s studies on the electroc h e m i c a l b e h a v i o u r of metals of R h a n d Pt g r o u p s [1,2], especially in a q u e o u s solutions in the p o t e n t i a l range o f the t h e r m o d y n a m i c stability of b u l k water. T h e conclusions f r o m those studies i n d i c a t e d c o m p l e x e l e c t r o s o r p t i o n processes involving either H o r O a d a t o m s . T h e i n t e r a c t i o n of these a d a t o m s with the m e t a l surface sites varies a c c o r d i n g to the c o n f i g u r a t i o n of the m e t a l lattice ( c r y s t a l l o g r a p h i c orientation, t y p e of surface defects, d i s t r i b u t i o n of surface crystallites [3-8] etc.) a n d with the electrolyte c o m p o s i t i o n [9-12]. This m e a n s that different types of m e t a l - h y d r o gen a n d m e t a l - o x y g e n bonds, can b e formed. Consequently, the e l e c t r o c a t a l y t i c activity of these m a t e r i a l s m u s t d e p e n d to a great extent u p o n the p h y s i c o c h e m i c a l p r o p e r t i e s of the first layer of a t o m s a n d molecules at the e l e c t r o d e / s o l u t i o n plane. I n this context, d a t a related to the electrochemical b e h a v i o u r of P d is relatively 0022-0728/83/$03.00
© 1983 Elsevier Sequoia S.A.
340 scarce [13-22] compared to other metals, particularly Pt and Au. Palladium exhibits a large absorption capacity for hydrogen [23] and its properties are adequate for use in organic electrosynthesis. Despite the electrochemical application of this metal, either pure or in alloys, the characteristics of the O-electrosorption processes are not fully understood. Thus, most of the reported data refer to stationary state conditions [14,19,20,21], or to the application of relaxation techniques [13,15-17,22] with a partial control of the different variables influencing those complex processes. Results reported in this paper offer a better understanding of the electrochemical behaviour of Pd in acid electrolytes, particularly in the potential range related to the O-electrosorption processes, and lead to the idea of a relationship between the activation of Pd for the O-electroadsorption process and the contribution of the proper electrooxidation of the base metal in defining the electrocatalyst characteristics. EXPERIMENTAL The working electrode consisted of a Pd wire (0.1 cm dia., 0.385 cm2 apparent area, Johnson Matthey & Co., 99.99% spectroscopically pure), which was polished with alumina (400 mesh)-water suspension and subjected to repetitive triangular potential sweeps (RTPS) between 0.25 and 1.46 V (NHE) at 0.3 V / s in 1 M H2SO4 until the stabilized RTPS current/potential profile was attained. The time required for this purpose was ca. 180 min. The auxiliary electrode was a Pd spiral of the same quality. A Hg/Hg2SO4/0.5 M H2SO4 reference electrode (E ° (NHE)= 0.680 V), conveniently shielded and connected through a Luggin capillary tip, was used. The potentials in the text are referred to the NHE scale. These electrodes were mounted in a conventional Pyrex glass three compartment electrolysis cell, with glass stopcocks lubricated with the same electrolyte solution. The electrolyte solution, 1 M H2SO4, was prepared with 98% H2SO4 (Merck p.a.) and triply distilled water. Runs were made at 25°C under a purified nitrogen atmosphere. The working electrode was perturbed with single linear and triangular potential sweeps (TPS), with repetitive triangular potential sweeps (RTPS), and with TPS combined with potential steps. Occasionally, perturbation programs consisting of triangularly modulated triangular potential sweeps were applied to the working electrode. This technique, used as described in the literature [24,25], provides qualitative and quantitative information about reversible steps participating in the electrochemical reaction. Conventional circuitry described in previous publications was used [26]. The upper (Es.a) and lower (Es,c) switching potentials, potential sweep rate (v) and other parameters of the different perturbation programs were varied at convenience, Electrolyte purity criteria were carefully satisfied [27].
341 RESULTS
The stabilized E / I profile. Influence of Es,a The potentiodynamic E l i profiles corresponding to the successive potential cYCling exhibit during the initial positive scan a broad current peak at ca. 1.28 V (peak I) and in the reverse scan a relatively symmetric cathodic peak (peak II) at ca. 0.7 V and an appreciable current contribution at 0.35 V. Afterwards, the overall anQdic charge increases, peak I decreases and an anodic current plateau is shown in the 0.95 V to 1.3 V range. Finally, the anodic profile obtained after a few potential cycles involves a smooth current peak at ca. 0.8 V and another one at ca. 1.1 V. Simultaneously, the cathodic charge increases, but the position of peak II remains unaltered, while the cathodic hump slightly decreases (Fig. la). The stabilized E / I profile that results after ca. 180 min of potential cycling (Fig. lb) comprises an overall charge that is lower than that involved in the initial profiles.
A
(o)
'~ OZ.
g
(b)
3
0
-0.2
-0.4
-0.6
-08
" -10
-
1.2
-1.4
o'.4
;.6
0'.8
1.'2
114
0'.4
016
d.8
lb
1'2
1;,
PotentiallY)
Fig. 1. RTPS E l i profiles at 0.2 V/s. Pd/1 M H2SO4; 25°C. (a) 1st to 13th RTPS E / I profiles. (b) E l i profile recorded after 180 min RTPS.
342 The anodic (Qa) to cathodic (Qc) charge ratio in the stabilized E / I profile approaches one. Those charges are mainly related to the O-electrosorption processes on palladium. The stepwise decrease of Es, a after the stabilized profile has b e e n attained (Fig. 2a) shows that for the lowest Es. a value the initial anodic process is considerably inhibited, but the electroreduction of the species anodically formed occurs at a potential more positive than that of peak II in Fig. 1. When Es, ~ becomes more negative and the shape of the electroreduction current peak changes, the height to width at half height ratio decreases. Furthermore, below 0.7 V, the cathodic current is practically equal to that expected for the discharge of the electrical double layer. Similarly, as Es. a decreases, there is a slight trend of the anodic current to increase above that of the stabilized E l i profile in the 1.1 V to 1.3 V range. On the other hand, when E~,a is stepwise increased, the changes of the E l i profiles are more drastic (Fig. 2b). As E~,a increases, the anodic profile tends to reproduce the characteristics of the first potential sweep already described in Fig. 1. Moreover, the maximum anodic current reached in the 1.1 V to 1.2 V range corresponds to the maximum cathodic limiting current recorded below 0.6 V. The anodic current in the 1.1 V to 1.2 V range involves a contribution related to the electrodissolution of Pd as Pd(II) is detected in solution. For the rest, the cathodic limiting current below 0.6 V corresponds to the electrodeposition of dissolved Pd(II). The voltammograms in both potential ranges are influenced by nitrogen bubbling. The corresponding currents increase under stirring. The cathodic limiting current is directly proportional to the concentration of Pd(II) in solution. The above described runs indicate that the activation of Pd electrodes concerning the O-electroadsorption process is remarkably influenced by the electrodissolution of the base metal. On this basis, the difference in the response of the E / I profile, according to the direction of change of Es,a, can be explained, considering that when E~,a decreases, the electrode surface moves from one fully covered by O-electroadsorbed species to a bare metal surface, but when E~,, increases, the surface is modified in the reverse direction. In the latter case, the response of the system should approach that of the metal under the first potential sweeps. Runs made under comparable switching potential conditions show that both the current associated with the electrodissolution of the base metal and that related to its electrodeposition from solution decrease with v because they involve, in the first case, a contribution of diffusion of reacting species out of the interface, and, in the second case, a diffusion of the reacting species to the metal surface. These changes are produced neither by probable impurities adsorbed during the preparation of the electrode surface, nor by impurities present in solution. These conclusions come from results obtained under comparable conditions on Pt and Au electrodes from the following experiments where Es, a was gradually changed either upwards or downwards during the RTPS. The dependence of both the anodic and the cathodic charge on E~,~ (Figs. 3 and 4) changes, whether runs are made with E~,a stepwise increasing, or decreasing. For E~,a decreasing, sigmoid type plots result and only at large v is a low slope smooth curve relationship obtained, probably approaching a straight line beyond 1.2 V. For
343 'r
r
i --
~o 0 ml
-02
0!!
•
- 0~ I1
-0.1 -02 -0.3 -O.l, t
-0.5
-0.
Q2
0~
o'~
,
i
08
~
i. ~,4
~'2 Potent(al//(
F i g . 2. Potentiodynamic
E/I profiles at
0.2 V/s.
Pd/I
M
H 2 S O 4, 2 5 ° C .
V'I
Es, a stepwise changed
downwards (a) and upwards (b).
increasing, the same behaviour is noticed, although the influence of v is more remarkable, the inflexion point of the charge/E~,~ plots corresponds to the potential range where the greatest contribution of Pd electrodissolution takes place. Qc exhibits a similar dependence on Es,a to Qa, but its magnitude, particularly at low v, is always smaller than that of Qa.
Es, a
344
(t))
(a) 1.6
t>
ev:03 v/s
c.J
+v=OZV/s Or:01 V/s 9V:O05V/s ~v:OOlV/=
E
% 0
q |-
|
o
o
,
o ~
0
4
0
~o
6
m
|
4
~
°° g t
l
db
0
÷
|
%
.r
°
0
+
0~
°.,
ID
6
o • ,I6
0.8
o
0o
P
Bo i
÷ •
4Y" 10
'
o
, i
12
I
p 1./.
Es,o/(V}
Fig. 3. Plots of Qa vs. Es, a. P d / l M n 2 s o 4. Es, a stepwise changed downwards (a) and uPwards (b). Scan rates are given in the figure.
Likewise, before attaining the stabilized Eli profile, the Q~/Q~ ratio is always greater than one and depends on v, particularly in the 1.1 V to 1.2 V range. Thus, for Es, a < 1.1 V, the potential of peak II changes very slightly with Es, a, but for Es,a > 1.1 V, it decreases linearly with Es, a, the slope of the straight line being independent of v (fig. 5). From these experiments the following conclusions are derived: (i) Initially the TPS El1 profile shows an inhibition of the O-containing film formation and a net contribution of Pd electrodissolution, particularly in the 1.1 V to 1.2 V potential range. (ii) Within the 0.75 V to 1.53 V potential range there is competition between these two reactions. (iii) The prolonged RTPS promotes the activation of the O-electroadsorption process and hinders Pd electrodissolution. (iv) From the electrochemical standpoint two limiting situations are achieved: one corresponds to the initial condition of the polished Pd electrode, probably related to an unstable metal surface. The second one can be associated with a stable surface condition resulting after a prolonged RTPS. The latter appears as the simplest for studying the O-electrosorption on Pd in fresh acid electrolyte.
345 i
r~
(o)
e v:O3 V/s + v : 0 2 V/s o v=0.1 v/s
",E u o
O v=oo5Vls
E
•~ v:O01V/s
0.(
0 0
0
O~ 0
+
02
el-
el-
+
0
o
6'
P
0
(b) (X( + A +
0 0.6
~
# 6
o
2 0
& 0
O.4 0 •
0.2
4-
• •
4-
o &
+
+
o
0o
,,
0
o o
•
+
o
t,
+ #o 0 0.8
i
110
Z
'
11/,
i
1.6
Es#/(V) Fig. 4. Plots of Qc vs. Es, a. P d / 1 M H2SO 4. Es, a stepwise changed downwards (a) and upwards (b). Scan rates are given in the figure.
Influence of the potential holding during the potentiodynamic run After the stabilized E l i profile is attained, the potentiodynamic E l i electroreduction profile changes considerably by holding the potential at E,, a certain time (~-) in the potential range of the O-containing layer. Thus, when E, = 0.963 V and
346 i
i
(a)
• v=03 V/$ + v=O.2 V/s o v=O.1 V/s 0 v=O~V/~ v=OD1V/$
I=I O,. l.U
0.6
o
9o
Oo
'~- ~i
.+
A
A
9o
0
,+ .÷O 0o
&
eF O o 90 e4
0o
el-
0.';
,+,
"
-0¢ 9o
t
(b)
o
A
o
+
+
0o
0.7
•
I
0.8
I
to
i
I
tz
I
0
I
A
I
1.4
t6 E s~a/( V )
Fig. 5. Plot of the cathodic current peak potential (Ep,ll) VS. Es.a. Pd/1 M H2SO4, 25°C. Es. a stepwise changed downwards (a) and upwards (b). Scan rates are given in the figure.
0 < • < 60 re_in (Fig. 6a), just in the region where I = 0, the electroreduction Eli profile run afterwards changes continuously with ~-. During the potential holding time there is a trend to a slight increase in the charge of the O-electroadsorbed species (QO) as ~"~ 0 and the reverse trend is observed when ~"~ 60 rain, while the limiting current (iL) of Pd electrodeposition increases to attain a stationary value. The initiation of the H-adatom formation is appreciably inhibited (0.22 V) as is that of the O-electrosorption. The inhibition of the latter is comparable to that found in the first TPS with a polished Pd electrode in a fresh electrolyte solution. Further-
347
i
(b)
~0.I
....z . -
/'
~.,°
~7~--~p
g
-O2
•
L/
f ~ ~ 15rain
.
.
.
.
.
.
.
'
'
5min i
Fig. 6. Potentiodynamic E/I profile resulting with TPS including a holding time (e') and E~. P d / l M H2SO4, 25°C. (a) E, = 0.963 V; (b) 1.018 V.
more, as Q O decreases, the potential of peak II shifts towards more negative values. The same run at E~ = 1.018 V, a potential where I * 0, shows, in principle, the same trend (Fig. 6b). Q¢O firstly increases together with i L and later approaches Qc in the stabilized El1 profile, while i L reaches the stationary value, From these runs one concludes that after the O-containing film has attained a certain critical thickness, it dissolves either chemically or electrochemically, yielding soluble Pd(II). The situation changes drastically again when 1.15 V < E, < 1.5 V, that is, when E~ slightly exceeds the potential range of Pd electrodissolution (Fig. 7). In this case, the anodic profile remains like that described for the stabilized Eli profile, but Qc firstly increases with ~" to attain a constant value after 30 rain or thereabouts. Simultaneously, peak II shifts gradually towards the negative direction and, finally, for T > 15 min, a clear splitting of peak II is recorded. The second peak lies within the potential range related to i L. Other changes in the negative profile are produced when E~ > 1.5 V (Fig. 7). Then, Q¢ increases with ~-, although in this case the cathodic peak remains sharp but accompanied by a hump on the negative potential side. Simultaneously, the peak potential becomes progressively more negative as Q¢ increases. In addition, the negative profile exhibits a new broad peak at ca. 1.17 V whose height increases with • . No appreciable changes in the anodic profile are noticed except the inhibition of the anodic current at Es.a after the potential holding at E~. The charge accumulated during the potential holding at 1.15 V < E~ < 1.6 V, measured through the Q¢ value, fits linear Q¢ vs. In ~" plots (Fig. 8), as is usually
348
,,~ 02
,
i
i
,
i N " ~ . . . "v~ ~ "~li\!A /
,
.........'li~Jii'l! /
~,oI-,.--.
7'Ai
-0.4
"~-!! -0.(
ftliijil i 6o min.~l ]Ii i '° m ' " ~ l t . J . ~ ,
L~L-'sP
IA A A ,
II \MI
\I
'
t
r~,.
-O.I 15rain
O~ (b)
-0.2
Es o ~ , _
\
E
-C~4 -0.6
Es, c
-08
°f V -'L~
['
________-.
-
-1.4
02
60 m | n ~
o14
~
i ~
1
o.6
-
1 rain 5rain lOmin 20min
30rain |
oa
I
~o
I
~z
J
~i~ ~.6 Potential/W)
Fig. 7. Potentiodynamic E/I profile resulting with TPS including a holding time (~) at E.. P d / l M H2SO 4, 25°C. (a) E~ = 1.38 V; (b) 1.53 V.
349
o t~ 1£
E o
1.2
oe E,:LS3~IIIIIII~. _ • T
I I
In(~/ls)) Fig. 8. Q¢ vs. In ~"plots. P d / l M H2SO4, 25°C; data taken from TPS E,r.
E/1 profiles at 0.25
V / s including
found in anodic film growth [28,29]. The slopes (b') of these plots change with E,, as found already for other metals [26]. Thus, for E, = 1.53 V, b' = 38 # C / d e c a d e , for E , = 1.38 V, b' = 10 # C / d e c a d e and for E, = 1.18 V, b' = 5 / t C / d e c a d e .
Potentiodynamic ageing perturbation program The electroreduction E l i profile run immediately after applying to the electrode the potentiodynamic ageing perturbation program (Fig. 9) depends strongly on the perturbation parameters (Es, c, E',¢, Es, a, v and ~-). At v = 0.25 V/s, considerable changes of the electroreduction profile are produced when E~,a = 1.53 V, Es, ¢ = 0.3 V and E~',¢ is fixed at a potential where 60 to 95% of the anodic film is electroreduced and rebuilt during each intermediate RTPS. Thus, when E'¢ = 0.66 V (Fig. 8a), after about 10 RTPS between E~c and E~,a, the charge of peak II decreases and simultaneously a new peak appears at ca. 0.45 V whose charge (Q'c') increases initially at the expense of the charge of peak II. Then, a net splitting of the electroreduction profile is observed. However, after 100 intermediate RTPS, peak II practically disappears and is replaced by a new current peak (II') at ca. 0.35 V, this potential value depending on z. The overall cathodic and anodic charges recorded after the IRTPS are greater than Q~ and Q~ is the stabilized E l i profile. However,
350
j.-
(a)
'~ 0.2
E
¢J
-02
E
-0.4
-06
Es,c
-0.8
- 10I ~5 __--.-- .-- T . . _ - . ~
(b) 0
! //!" -(12
I/l!
-0.4
10
P
0.5
E $,o
°
~
:!!
E's&
I'
Es,~ [ - - L - -
s:
-0.6
q3.5
I
1,0
-
1.5 -- J _ _ _
2O
t
Z5
-0.8 I|
I' -1.0
|
35
-12
¢0 02
0~
1.0
tz
1.~
1.6
Potentiol/(V]
Fig. 9. E / I profiles run with the potentiodynamic ageing program at 0.25 V / s . Pd/1 M H 2 S O 4 , 25°C. (a) E~.c corresponds to ca. 75% oxide removal and rebuilding. (1) 10, (2) 20, (3) 50, (4) 100 and (5) 200 cycles. (b) E ' c corresponds to ca. 95% oxide removal and rebuilding 400 cycles.
351 tlae potentiodynamic E l i profile resulting after continuing the RTPS between Es,~ and Es, a recovers the shape of the stabilized E l i profile. This suggests that the prolonged RTPS between E~',c and Es, a has produced a net increase of the electrode roughness. These effects depend remarkably on E~'c, namely, on the percentage of anodic charge involved in the electrochemical reaction (Fig. 8b) between E',c and Es, a. Thus, the estimated roughness factor resulting after 400 RTPS between E'c = 0.64 V, Es, a = 1.53 V and v = 0.25 V/s, as referred to the stabilized E l i profile, is about four. DISCUSSION
Fresh and stabilized Pd electrodes Under the absence of interference of H a and H-adatoms reactions, polycrystalline Pd offers the possibility to establish more dearly than in other noble metal electrodes the different processes participating within the potential range related to the thermodynamic stability of bulk water. It is already known that the stability and the reactivity of Pd surface oxides in the electrolyte depend to a great extent on the electrode history [13]. This is not only confirmed by the present results but it appears that a great deal of data concerning Pd electrode, particularly under non-stationary perturbation conditions, actually correspond to non-stabilized and non-controlled Pd electrode surfaces. The preceding results can, in principle, be interpreted in terms of two limiting types of behaviour. The first one corresponds to the fresh polycrystalline Pd electrode, which is mostly active for Pd electrodissolution and relatively inactive for O-electroadsorption. The second behaviour is that of the stabilized electrode as referred to in the preceding results. The stabilized electrode becomes progressively more inactive for Pd electrodissolution and remarkably more active for O-electroadsorption. In this case, the electrochemical response of Pd approaches that of polycrystalline Pt and Rh electrodes in aqueous acid electrolytes [30,31]. The Pd electrodissolution preferentially occurring with fresh electrodes, according to the reaction: Pd = pd2++ 2 e-
(1)
has been reported earlier by different authors [13,14-16]. The corrosion rate of Pd as Pd 2÷ in 0.2 M H2SO 4 already occurs at E ° ( N H E ) > 0.60 V and it depends on the applied potential [15]. The thermodynamic threshold potential of reaction (1) is E ° (NHE) = 0.95 V [32]. This value, however, lies very close to that of the overall reaction: Pd + H20 = PdO + 2 H + + 2 e-
(2)
which is E ° = 0.92 V [32]. Therefore, for a fresh polycrystalline electrode both processes (1) and (2) may occur simultaneously. On the other hand, the electrodeposition of Pd 2÷ begins at potentials where the O-containing layers are not
352 completely electroreduced. This means that a gradual change of the surface composition of the electrode should be accomplished during the RTPS in the 0.3 V to 1.53 V range, to attain, after a prolonged potential cycling, the stationary surface composition of the stabilized Pd electrode. The metal surface should change from a Pd surface to a PdxOy layer (x >> y). This agrees with the fact that PdO species inhibits the electrodissolution reaction of Pd to Pd 2÷ [13]. Therefore, the extent of the electrode surface change depends on whether PdO species are partially or completely electroreduced during each negative potential sweep, on the time scale of the potential perturbation, and on the contribution of Pd 2+ electrodeposition (Fig. 1). The latter process makes possible the occurrence of Pd atoms at the metal surface that are not a equilibrium in the metallic lattice, in part probably because of the presence of traces of oxide species. These facts should produce a change in the nature and activity of Pd electrodes from that of a clean Pd electrode to that of a P d - O alloy electrode. The formation of this type of alloy is accomplished, for instance, with Pt electrodes in aqueous acid electrolytes subjected to very drastic chemical or electrochemical treatments [33,34]. The condition of the Pd stabilized electrode is acquired, for instance, through a RTPS at 0.2 V / s (Es, c = 0.3 V and Es, a = 1.53 V) during ca. 180 min. One should expect, therefore, that the structure of the P d - O layer on Pd electrodes used as electrocatalysts will have a substancial influence on the course of any electrochemical reaction. PdO dissolves in acid very slowly [35] and this probably also applies for the PdO surface layer in the acid electrolyte. The reaction: PdO + 2 H ÷= pd2++ H 2 0
(3)
means a chemical contribution to the corrosion of Pd in the electrolyte. Changes in real Pd electrode area
The gradual changes of anodic charge produced during the RTPS can be associated with changes in the real area of Pd electrode. The electrode roughness can be estimated from the charge of the O-electroreduction current peak referred to that recorded in the first negative potential scan. Thus, at the beginning of each RTPS an increase of the electrode roughness is observed. This increase, which implies a roughness factor of about 1.4, after 12 potential cycles in the conditions indicated in Fig. l a, is quite likely to be produced in the short time range by the electrodeposition of Pd 2÷. The corresponding reaction can be written as follows: Pd 2+ + Pd x (PdO) + 2 e- = Pd*Pd(PdO)
(4)
where Pd* denotes an electrodeposited Pd atom misfitting the equilibrium position in the metal lattice, and x >>"1. After a prolonged RTPS, however, a decrease of the real electrode area in the long time range is observed (Fig. lb). This effect suggests a sintering of the Pd surface. Sintering effects have been observed in most freshly electrodeposited noble metal electrodes (electrometallized-metal, electrodes) [36,37].
353 The corresponding reaction is: Pd*Pdx (PdO) = Pdx+l (POO)
(5)
Sintering, which can also be explained through the formation of Pd* atoms, should occur only when a relatively large surface concentration of Pd* atoms has been produced. This explains why sintering is observed after the initial roughness increase. Surface roughening and sintering are no longer observed once the stabilized E l i profile has been achieved.
Electroformation of the oxygen-containing layer The electroformation of the oxygen-containing layer involves various stages whose contributions depend on the characteristics of the potential perturbation. In this respect, two main distinctions should be made depending on whether Qa is in the order of or greater than that of the PdO monolayer ( 4 2 0 / t C / c m 2) [17]. Thus, when Es. a < 1.1 V, Qa is smaller than that expected for the O-monolayer corresponding to one O-atom per Pd atom at the surface. The initiation of this process comprises the formation of Pd(OH) species according to the reaction: P d + H20 = Pd(OH) + H + + e-
(6)
Reaction (6) is formally the same, but involving other noble metals [38-40]. It corresponds to a fast process, as is clearly visualized through the TMTPS experiments run in the 0.3 V-1.46 V range (Fig. 10). The step following reaction (6) is related to the electroformation of the Pd(O) monolayer which can be formally written as follows: Pd(OH) = Pd(O) + H + + e-
(7)
The Pd(O) species also participates in ageing processes that are, in principle, similar to those described for other noble metals [41,42]. The ageing can be represented by the equation: Pd(O) ~ [Pd(O)]aged
(8)
The existence of two forms of Pd(O) is deduced from the splitting of the E l i electroreduction profiles under constant charge conditions observed when Es, a < 1.53 V (Figs. 7 and 9) after applying to the electrode one of the different ageing techniques. It should be emphasized that only under the constant electroreduction charge condition can one properly refer to the ageing of the O-electroadsorbed layer on Pd. The existence of different Pd(O) configurations at the electrode correlates, at least qualitatively, with the formation of a series of PdO ordered surface structures during the adsorption of oxygen from the gas phase on Pd single crystals, which can be transformed continuously by changing the coverage [43]. Reaction (8) probably occurs through a place exchange mechanism, as already pointed out for other noble metals [30]. In principle, reactions (1)-(6) are formally similar to the processes described for Pt in aqueous acid solutions under potentiodynamic perturbation conditions [42]. It is likely that at more positive
354
4.a
15 0.2
0,6
I ,0
1,4 Potential/(V)
Fig~ 10. Triangularly modulated single potential sweep run from Es, ¢ to Es, a. P d / l M H2SO4, 25°C. vbas~ = 1 V / s ; Omodulate d ~ 50 V//S; modulation amplitude 0,2 V; Es, ¢ = 0.3 V; Es. a = 1.46 V.
potentials the Pd(O) layer grows initially to a PdO phase film, which was found to be a semiconductor with a band gap of 1.5 eV [44].
Oxide layer thickening The thickening of the O-layer takes place at potentials between 0.96 V and 1.53 V. During the potentiodynamic run, the film growth is accompanied by a progressive increase in the electroreduction charge and simultaneously a gradual shift of the potential of the corresponding peak towards more negative values. The increase of the oxygen-containing film thickness should occur through a reaction such as: Pd,Pd(O) + (n + y ) H 2 0 ~ Pdx+,Or+,'n H 2 0 + 2y H++ 2 y e -
(9)
where y > x and n indicates the number of hydration water molecules in the film. According to reaction (9)', a non-stoichiometric oxide film is formed. The rate of film growth fits linear Q vs. In ~ relationships, as usaally found in the growth of anodic films [28,29]. Both the growth of the Pdx0 v film and its probable ageing are appreciably
355
enhanced when the electrode is subjected to the potentiodynamic ageing perturbation program. These effects admit the same explanation already given for Pt electrodes under comparable conditions in aqueous acid electrolytes [42]. Thus, the structural changes of surface oxide layers electroformed on Pd are associated with surface reconstruction as the initial step for the diffusion of oxygen into the bulk metal [18]. Surface oxide reconstruction is related to bulk oxidation of Pd [45]. The electrochemical penetration of oxygen underneath the first metal atom layers is coherent with the significant amount of oxygen that can be incorporated into bulk Pd at pressures far below the dissociation pressure of PdO [46]. On the other hand, the absorption of oxygen and diffusion into bulk Pd have been measured from the study of the interactions of oxygen gas with Pd surfaces [43,46-52]. In this respect, the charge balance indicates that bulk oxygen absorption into bulk Pd becomes possible. The interpretation given to the Pd oxide growth correlates with the fact that the spectra of the Pd-oxygen electrode system show, in principle, large quantities of excess oxygen in the form of adsorbed water, hydrated PdO or PdO 2, or the corresponding Pd(OH)2 or Pd(OH)4 [53]. ESCA results of electrochemically produced Pd oxide were interpreted in terms of PdO z formation through Pd 4+ ions [52]. The field strength increase within the film can assist the formation of Pd 4÷ ions, and consequently of PdO 2, at a potential lower than the equilibrium potential of the reaction: Pd + 2 H 2 0 = PdO 2 + 4 H + + 4 e-
( E ° = 1.47 V)
(10)
Finally, when the potential exceeds 1.2 V, a new oxide species is electroformed, as revealed through the electroreduction current peak located at 0.43 V (Fig. 7). The corresponding potential range, where the new species is formed, mostly involves the growth of a PdO 2 containing film. This is actually expected, because in this case the equilibrium potential of the reaction: PdO + H 2 0 = PdO2 + 2 H + + 2 e -
( E ° -- 1.263 V [32])
(11)
is exceeded. However, it is likely that PdO2-containing film also comprises species with peroxidic structures appearing just at the potential where the 02 evolution begins. The electroreduction current peak recorded at ca. 1.2 V can, in principle, be assigned to the electroreduction of that fraction of the oxide layer related to the peroxidic type species. Under these conditions, probably most of the oxide film still grows to Pd x + 1Oy+i stoichiometry as PdO 2 decomposes into PdO and 02 [19]. This type of decomposition reaction probably applies to most noble metal anodes in acid electrolytes at relatively high positive potentials [54,55]. The oxide layer structure formed at high positive potentials can be envisaged as a non-homogeneous complex film, as the ionic radii of Pd 2÷ and Pd 4+ ions are considerably different when both ionic species are film constituents. This inhomogeneity may assist the diffusion of Pd 4÷ through the oxide layer and, therefore, further contributing to Pd corrosion and increase of the surface roughness, as pointed out recently [13].
356 The PdO2-containing portion of the complex oxide film structure can be electroreduced either to PdO, according to: PdO 2 + 2 H ÷ + 2 e - = PdO + H20
( E ° / V = 1.263 - 0.059 pH)
(12)
or to Pd, through the following overall reaction: PdO 2 + 4 H + + 4 e - = Pd + 2 H20
( E ° / V = 1.04 - 0.059 pH)
(13)
Nevertheless, it should be noticed that the overvoltage for the electroreduction of PdO 2 is appreciably lower than that for PdO [20,21,56]. The structure of the complex electrochemical interface accomplished at high positive potentials can then be expressed as follows: Pd/PdxOy/PdO2/electrolyte, where a part of the peroxidic structure contributes to PdO 2 growth. The poor stability of bulk PdO 2 species under these conditions is well known [19]. The various O-electroadsorbed species and Pd oxides formed on anodized Pd at different potentials also exhibit different stabilities in contact with the acid electrolyte. The solubility of the different species can be studied through the charge decrease at open circuit after anodizing and ageing the Pd electrodes at different potentials [57]. The electrolytic activity of Pd in acid electrolyte is, therefore, remarkably dependent on the overall electrode history. This effect is to some extent comparable to that produced by the electrochemical reductions and heat treatments on the activity of Pt electrodes, as has been known for some time [58,59]. On the other hand, the limiting electrochemical behaviour of Pd electrodes are reflected through the behaviour of free Pd and oxidized Pd electrodes with respect to the 02 electroreduction reaction in alkaline electrolytes [60]. The electrolytic activity of Pd in these solutions was related to the content of hydrogen in Pd, so that the final loss of activity was caused by the electroformation of non-stoichiometric oxides or OH species on the electrode at sufficiently positive potentials [22]. In conclusion, the present results show that the electrochemical behaviour of Pd in the acid electrolyte, besides any anion influence and the H-adatom interface, is far more complex than was previously thought, due to the electrodissolution of the base metal and the electroformation of a variety of P d - O species that participate in different structural rearrangements. ACKNOWLEDGEMENTS INIFTA is sponsored by the Consejo Nacional de Investigaciones Cientificas y Trcnicas, the Universidad Nacional de La Plata and the Comisirn de Investigaciones Cientificas (Provincia de Buenos Aires). This work was partly supported by the Regional Program for the-Scientific and Technological Development of the Organization of American States.
357 REFERENCES 1 R. Woods in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 9, Marcel Dekker, New York, 1977. 2 G. Btlanger and A.K. Vijh in A.K. Vijh (Ed.), Oxides and Oxide Films, Vol. 5, Marcel Dekker, New York, 1977, p. 1. 3 A.T. Hubbard, R.M. Ishikawa and J. Katekaru, J. Electroanal. Chem., 86 (1978) 271. 4 E. Yeager, W.E. O'Grady, M.Y.C. Woo and P. Hagans, J. Electrochem. Soc., 125 (1978) 348. 5 A. Hamelin and J.P. Bellier, Surf. Sci., 78 (1978) 159. 6 D. Dickertmann, J.W. Schultze and K.J. Vetter, J. Electroanal. Chem. 55 (1974) 429. 7 K. Kinoshita, D.R. Ferrier and P. Stonehart, Electrochim. Acta, 23 (1978) 45. 8 P.N. Ross, J. Electroanal. Chem., 76 (1977) 139. 9 M.I. Florit, M.E. Martins and A.J. Arvia, J. Electroanal. Chem., 151 (1983) 209. l0 J. Clavilier and C. Nguyen Van Huong, J. Electroanal. Chem., 80 (1977) 101. 11 A. Angerstein-Kozlowska, B.E. Conway, B. Barnett and J. Mozota, J. Electroanal. Chem., 100 (1979) 417. 12 S. Shibata and M.P. Sumino, Electrochim. Acta, 26 (1981) 517. 13 K. Gossner and E. Mizera, J. Electroanal. Chem. 125 (1981) 347. 14 J.F. Llopis, J.M. Gamboa and J. Victori, Electrochim. Acta, 17 (1972) 2225. 15 S.H. Cadle, J. Electrochem. Soc., 121 (1974) 645. 16 D.A.J. Rand and R. Woods, J. Electroanal. Chem., 35 (1972) 209. 17 D.A.J. Rand and R. Woods, J. Electroanal. Chem., 31 (1971) 29. 18 T. Chierchie, C. Mayer and W. Lorenz, J. Electroanal. Chem., 135 (1982) 211. 19 J.P. Hoare, J. Electrochem. Soc., 112 (1965) 1129. 20 A. Hickling and G. Vrjosek, Trans. Faraday Sot., 57 (1961) 123. 21 R. Fortuny, J. Genesch and L. Victori, Afinidad 38 (1981) 91. 22 I. Rube~ and H. Bylikovb, Coll. Czech. Chem. Commun., 37 (1972) 715. 23 F.A. Lewis, The Palladium/Hydrogen System, Academic Press, London, 1967. 24 N.R. de Tacconi, J.O. Zerbino and A.J. Arvia, J. Electroanal. Chem., 79 (1977) 287. 25 B.E. Conway, H. Angerstein-Kozlowska, F.C. Ho, J. Klinger, B. Mac Dougall and S. Gottesfeld, Faraday Discuss. Chem. Soc., 56 (1973) 210. 26 M.E. Martins, R. Cbrdova O. and A.J. Arvia, Electrochim. Acta, 26 (1981) 1547. 27 B.E. Conway, H. Angerstein-Kozlowska, E. Criddle and W.B. Sharp, Anal. Chem. 45 (1973) 1331. 28 K. Ogura, S. Haruyama and K. Nagasaki, J. Electrochem. Soc., 118 (1971) 531. 29 S. Hadzi-Jordanov, H. Angerstein-Kozlowska, M. Vukovic and B.E. Conway, J. Electrochem. Soc., 125 (1978) 1471. 30 B.V. Tilak, B.E. Conway and H. Angerstein-Kozlowska, J. Electroanal. Chem., 48 (1973) 1. 31 C. Pallotta, N.R. de Tacconi and A.J. Arvia, Electrochim. Acta, 26 (1981) 261. 32 M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, Oxford, 1966. 33 J.P. Hoare, Electrochim. Acta, 26 (1981) 225. 34 A. Chialvo, W.E. Triaca and A.J. Arvia, J. Electroanal. Chem., 146 (1983) 93. 35 CRC Handbook of Chemistry and Physics, 57 (1977) B 138. 36 P. Stonehart and P.A. Zucks, Electrochim. Acta, 17 (1972) 2333. 37 A.C.C. Tseung and S.C. Dhara, Electrochim. Acta 20 (1975) 681. 38 H. Angerstein-Kozlowska, B.E. Conway and W.B.A. Sharp, J. Electroanal. Chem., 43 (1973) 9. 39 C.M. Ferro, A.J. Calandra and A.J. Arvla, J. Electroanal. Chem., 59 (1973) 239. 40 J.O. Zerbino, N.R. de Tacconi and A.J. Arvia, J. Electrochem. Soc., 125 (1978) 1266. 41 M.Y. Duarte, M.E. Martins and A.J. Arvia, An. Asoc. Quim. Argent., 70 (1982) 525. 42 M.E. Folquer, J.O. Zerbino, N.R. de Tacconi and A.J. Arvia, J. Electrochem. Soc., 126 (1979) 592. 43 G. Ertl and P. Ran, Surf. Sci., 15 (1969) 443. 44 H. Okamoto and T. Aso, Jap. J. Appl. Phys., 6 (1967) 779. 45 T.W. Orent and S.D. Bader, Surf. Sci., 115 (1982) 323.
358 46 C.T. Campbell, D.C. Foyt and J.M. White, J. Phys. Chem., 81 (1977) 491. 47 H. Conrad, G. Ertl, J. Kuppers and E.E. Latta, Surf. Sci., 65 (1977) 245. 48 G. Ertl and J. Koch in F. Ricca (Ed.), Adsorption-Desorption Phenomena, Academic Press, 1972, p. 345. 49 T. Matsushima and J.M. White, Surf. Sci., 67 (1977) 122. 50 Y.L. Sandier and O.D. Durigo, J. Phys. Chem., 73 (1969) 2392. 51 J.S. Close and J.M. White, J. Catal., 36 (1975) 653. 52 D.L. Weissman, M.L. Shek and W.E. Spicer, Surf. Sci., 92 (1980) L59. 53 K.S. Kim, A.F. Gossmann and N. Winograd, Anal. Chem., 46 (1974) 197. 54 C. Pallotta, N.R. de Tacconi and A.J. Arvia, J. Electroanal. Chem., 122 (1981) 367. 55 M.I. Florit, M.E. Martins and A.J. Arvia, J. Electroanal. Chem., 126 (1981) 255. 56 Y.A. Mazitov, K.I. Rozental and V.I. Veselovskii, Zh. Fiz. Khim., 38 (1964) 151. 57 A.E. Bolzhn, M.E. Martins and A.J. Arvla, in preparation. 58 S. Shibata and M.P. Sumino, Electrochim. Acta, 16 (1971) 1511. 59 T. Biegler, Aust. J. Chem., 26 (1973) 2571. 60 S. Kondo, K. Ohashi and S. Nagaura, Denki Kagaku, 37 (1969) 212.
339
Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
THE COMPLEX PROCESSES INVOLVED AT Pd ELECTRODES IN 1 M H2SO 4 IN THE POTENTIAL RANGE OF OXYGEN ELECTROADSORPTION-ELECTRODESORPTION REACTIONS
A.E. BOLZ.~N, M.E. MARTINS and A.J. ARViA Instituto de Investiga~ciones Fisicoquimicas Terricas y Aplicadas - - I N I F T A , Casilla de Correo 16, Sucursal 4, 1900 La Plata (Argentina)
(Received 30th December 1982; in revised form 22nd March 1983)
ABSTRACT Pd electrodes in 1 M H2SO 4 show different electrochemical behaviour in the whole range of oxygen electroadsorption and the oxide layer formation, depending on the characteristics of the electrical perturbation during the initial potentiodynamic sweep of electrodissolution of Pd, and the O-electroadsorption takes place. In this case the electroreduction potentiodynamic profile exhibits the oxygen electrodesorption peak and a limiting current due to lad electrodeposition. Under these conditions the anodic to cathodic charge ratio is always greater than one. A different behaviour is observed after a prolonged potential cycling. The initial O-electroadsorption is activated and the Pd electrodissolution is practically suppressed. The charge is then equal to one and the oxygen electroreduction profile changes according to the history of the potential perturbation conditions. Results indicate the formation of different O-electroadsorbed oxide species, depending on the potential range covered during the experiments. The electrochemical reactions, including ageing effects and oxide film growth, are interpreted with a reaction formalism which, to a great extent, is similar to that already discussed for Pt electrodes in acid electrolytes.
INTRODUCTION T h e i m p o r t a n c e of n o b l e metals in heterogeneous catalysis a n d electrocatalysis is w i d e l y recognized. This has b e e n the reason for n u m e r o u s studies on the electroc h e m i c a l b e h a v i o u r of metals of R h a n d Pt g r o u p s [1,2], especially in a q u e o u s solutions in the p o t e n t i a l range o f the t h e r m o d y n a m i c stability of b u l k water. T h e conclusions f r o m those studies i n d i c a t e d c o m p l e x e l e c t r o s o r p t i o n processes involving either H o r O a d a t o m s . T h e i n t e r a c t i o n of these a d a t o m s with the m e t a l surface sites varies a c c o r d i n g to the c o n f i g u r a t i o n of the m e t a l lattice ( c r y s t a l l o g r a p h i c orientation, t y p e of surface defects, d i s t r i b u t i o n of surface crystallites [3-8] etc.) a n d with the electrolyte c o m p o s i t i o n [9-12]. This m e a n s that different types of m e t a l - h y d r o gen a n d m e t a l - o x y g e n bonds, can b e formed. Consequently, the e l e c t r o c a t a l y t i c activity of these m a t e r i a l s m u s t d e p e n d to a great extent u p o n the p h y s i c o c h e m i c a l p r o p e r t i e s of the first layer of a t o m s a n d molecules at the e l e c t r o d e / s o l u t i o n plane. I n this context, d a t a related to the electrochemical b e h a v i o u r of P d is relatively 0022-0728/83/$03.00
© 1983 Elsevier Sequoia S.A.
340 scarce [13-22] compared to other metals, particularly Pt and Au. Palladium exhibits a large absorption capacity for hydrogen [23] and its properties are adequate for use in organic electrosynthesis. Despite the electrochemical application of this metal, either pure or in alloys, the characteristics of the O-electrosorption processes are not fully understood. Thus, most of the reported data refer to stationary state conditions [14,19,20,21], or to the application of relaxation techniques [13,15-17,22] with a partial control of the different variables influencing those complex processes. Results reported in this paper offer a better understanding of the electrochemical behaviour of Pd in acid electrolytes, particularly in the potential range related to the O-electrosorption processes, and lead to the idea of a relationship between the activation of Pd for the O-electroadsorption process and the contribution of the proper electrooxidation of the base metal in defining the electrocatalyst characteristics. EXPERIMENTAL The working electrode consisted of a Pd wire (0.1 cm dia., 0.385 cm2 apparent area, Johnson Matthey & Co., 99.99% spectroscopically pure), which was polished with alumina (400 mesh)-water suspension and subjected to repetitive triangular potential sweeps (RTPS) between 0.25 and 1.46 V (NHE) at 0.3 V / s in 1 M H2SO4 until the stabilized RTPS current/potential profile was attained. The time required for this purpose was ca. 180 min. The auxiliary electrode was a Pd spiral of the same quality. A Hg/Hg2SO4/0.5 M H2SO4 reference electrode (E ° (NHE)= 0.680 V), conveniently shielded and connected through a Luggin capillary tip, was used. The potentials in the text are referred to the NHE scale. These electrodes were mounted in a conventional Pyrex glass three compartment electrolysis cell, with glass stopcocks lubricated with the same electrolyte solution. The electrolyte solution, 1 M H2SO4, was prepared with 98% H2SO4 (Merck p.a.) and triply distilled water. Runs were made at 25°C under a purified nitrogen atmosphere. The working electrode was perturbed with single linear and triangular potential sweeps (TPS), with repetitive triangular potential sweeps (RTPS), and with TPS combined with potential steps. Occasionally, perturbation programs consisting of triangularly modulated triangular potential sweeps were applied to the working electrode. This technique, used as described in the literature [24,25], provides qualitative and quantitative information about reversible steps participating in the electrochemical reaction. Conventional circuitry described in previous publications was used [26]. The upper (Es.a) and lower (Es,c) switching potentials, potential sweep rate (v) and other parameters of the different perturbation programs were varied at convenience, Electrolyte purity criteria were carefully satisfied [27].
341 RESULTS
The stabilized E / I profile. Influence of Es,a The potentiodynamic E l i profiles corresponding to the successive potential cYCling exhibit during the initial positive scan a broad current peak at ca. 1.28 V (peak I) and in the reverse scan a relatively symmetric cathodic peak (peak II) at ca. 0.7 V and an appreciable current contribution at 0.35 V. Afterwards, the overall anQdic charge increases, peak I decreases and an anodic current plateau is shown in the 0.95 V to 1.3 V range. Finally, the anodic profile obtained after a few potential cycles involves a smooth current peak at ca. 0.8 V and another one at ca. 1.1 V. Simultaneously, the cathodic charge increases, but the position of peak II remains unaltered, while the cathodic hump slightly decreases (Fig. la). The stabilized E / I profile that results after ca. 180 min of potential cycling (Fig. lb) comprises an overall charge that is lower than that involved in the initial profiles.
A
(o)
'~ OZ.
g
(b)
3
0
-0.2
-0.4
-0.6
-08
" -10
-
1.2
-1.4
o'.4
;.6
0'.8
1.'2
114
0'.4
016
d.8
lb
1'2
1;,
PotentiallY)
Fig. 1. RTPS E l i profiles at 0.2 V/s. Pd/1 M H2SO4; 25°C. (a) 1st to 13th RTPS E / I profiles. (b) E l i profile recorded after 180 min RTPS.
342 The anodic (Qa) to cathodic (Qc) charge ratio in the stabilized E / I profile approaches one. Those charges are mainly related to the O-electrosorption processes on palladium. The stepwise decrease of Es, a after the stabilized profile has b e e n attained (Fig. 2a) shows that for the lowest Es. a value the initial anodic process is considerably inhibited, but the electroreduction of the species anodically formed occurs at a potential more positive than that of peak II in Fig. 1. When Es, ~ becomes more negative and the shape of the electroreduction current peak changes, the height to width at half height ratio decreases. Furthermore, below 0.7 V, the cathodic current is practically equal to that expected for the discharge of the electrical double layer. Similarly, as Es. a decreases, there is a slight trend of the anodic current to increase above that of the stabilized E l i profile in the 1.1 V to 1.3 V range. On the other hand, when E~,a is stepwise increased, the changes of the E l i profiles are more drastic (Fig. 2b). As E~,a increases, the anodic profile tends to reproduce the characteristics of the first potential sweep already described in Fig. 1. Moreover, the maximum anodic current reached in the 1.1 V to 1.2 V range corresponds to the maximum cathodic limiting current recorded below 0.6 V. The anodic current in the 1.1 V to 1.2 V range involves a contribution related to the electrodissolution of Pd as Pd(II) is detected in solution. For the rest, the cathodic limiting current below 0.6 V corresponds to the electrodeposition of dissolved Pd(II). The voltammograms in both potential ranges are influenced by nitrogen bubbling. The corresponding currents increase under stirring. The cathodic limiting current is directly proportional to the concentration of Pd(II) in solution. The above described runs indicate that the activation of Pd electrodes concerning the O-electroadsorption process is remarkably influenced by the electrodissolution of the base metal. On this basis, the difference in the response of the E / I profile, according to the direction of change of Es,a, can be explained, considering that when E~,a decreases, the electrode surface moves from one fully covered by O-electroadsorbed species to a bare metal surface, but when E~,, increases, the surface is modified in the reverse direction. In the latter case, the response of the system should approach that of the metal under the first potential sweeps. Runs made under comparable switching potential conditions show that both the current associated with the electrodissolution of the base metal and that related to its electrodeposition from solution decrease with v because they involve, in the first case, a contribution of diffusion of reacting species out of the interface, and, in the second case, a diffusion of the reacting species to the metal surface. These changes are produced neither by probable impurities adsorbed during the preparation of the electrode surface, nor by impurities present in solution. These conclusions come from results obtained under comparable conditions on Pt and Au electrodes from the following experiments where Es, a was gradually changed either upwards or downwards during the RTPS. The dependence of both the anodic and the cathodic charge on E~,~ (Figs. 3 and 4) changes, whether runs are made with E~,a stepwise increasing, or decreasing. For E~,a decreasing, sigmoid type plots result and only at large v is a low slope smooth curve relationship obtained, probably approaching a straight line beyond 1.2 V. For
343 'r
r
i --
~o 0 ml
-02
0!!
•
- 0~ I1
-0.1 -02 -0.3 -O.l, t
-0.5
-0.
Q2
0~
o'~
,
i
08
~
i. ~,4
~'2 Potent(al//(
F i g . 2. Potentiodynamic
E/I profiles at
0.2 V/s.
Pd/I
M
H 2 S O 4, 2 5 ° C .
V'I
Es, a stepwise changed
downwards (a) and upwards (b).
increasing, the same behaviour is noticed, although the influence of v is more remarkable, the inflexion point of the charge/E~,~ plots corresponds to the potential range where the greatest contribution of Pd electrodissolution takes place. Qc exhibits a similar dependence on Es,a to Qa, but its magnitude, particularly at low v, is always smaller than that of Qa.
Es, a
344
(t))
(a) 1.6
t>
ev:03 v/s
c.J
+v=OZV/s Or:01 V/s 9V:O05V/s ~v:OOlV/=
E
% 0
q |-
|
o
o
,
o ~
0
4
0
~o
6
m
|
4
~
°° g t
l
db
0
÷
|
%
.r
°
0
+
0~
°.,
ID
6
o • ,I6
0.8
o
0o
P
Bo i
÷ •
4Y" 10
'
o
, i
12
I
p 1./.
Es,o/(V}
Fig. 3. Plots of Qa vs. Es, a. P d / l M n 2 s o 4. Es, a stepwise changed downwards (a) and uPwards (b). Scan rates are given in the figure.
Likewise, before attaining the stabilized Eli profile, the Q~/Q~ ratio is always greater than one and depends on v, particularly in the 1.1 V to 1.2 V range. Thus, for Es, a < 1.1 V, the potential of peak II changes very slightly with Es, a, but for Es,a > 1.1 V, it decreases linearly with Es, a, the slope of the straight line being independent of v (fig. 5). From these experiments the following conclusions are derived: (i) Initially the TPS El1 profile shows an inhibition of the O-containing film formation and a net contribution of Pd electrodissolution, particularly in the 1.1 V to 1.2 V potential range. (ii) Within the 0.75 V to 1.53 V potential range there is competition between these two reactions. (iii) The prolonged RTPS promotes the activation of the O-electroadsorption process and hinders Pd electrodissolution. (iv) From the electrochemical standpoint two limiting situations are achieved: one corresponds to the initial condition of the polished Pd electrode, probably related to an unstable metal surface. The second one can be associated with a stable surface condition resulting after a prolonged RTPS. The latter appears as the simplest for studying the O-electrosorption on Pd in fresh acid electrolyte.
345 i
r~
(o)
e v:O3 V/s + v : 0 2 V/s o v=0.1 v/s
",E u o
O v=oo5Vls
E
•~ v:O01V/s
0.(
0 0
0
O~ 0
+
02
el-
el-
+
0
o
6'
P
0
(b) (X( + A +
0 0.6
~
# 6
o
2 0
& 0
O.4 0 •
0.2
4-
• •
4-
o &
+
+
o
0o
,,
0
o o
•
+
o
t,
+ #o 0 0.8
i
110
Z
'
11/,
i
1.6
Es#/(V) Fig. 4. Plots of Qc vs. Es, a. P d / 1 M H2SO 4. Es, a stepwise changed downwards (a) and upwards (b). Scan rates are given in the figure.
Influence of the potential holding during the potentiodynamic run After the stabilized E l i profile is attained, the potentiodynamic E l i electroreduction profile changes considerably by holding the potential at E,, a certain time (~-) in the potential range of the O-containing layer. Thus, when E, = 0.963 V and
346 i
i
(a)
• v=03 V/$ + v=O.2 V/s o v=O.1 V/s 0 v=O~V/~ v=OD1V/$
I=I O,. l.U
0.6
o
9o
Oo
'~- ~i
.+
A
A
9o
0
,+ .÷O 0o
&
eF O o 90 e4
0o
el-
0.';
,+,
"
-0¢ 9o
t
(b)
o
A
o
+
+
0o
0.7
•
I
0.8
I
to
i
I
tz
I
0
I
A
I
1.4
t6 E s~a/( V )
Fig. 5. Plot of the cathodic current peak potential (Ep,ll) VS. Es.a. Pd/1 M H2SO4, 25°C. Es. a stepwise changed downwards (a) and upwards (b). Scan rates are given in the figure.
0 < • < 60 re_in (Fig. 6a), just in the region where I = 0, the electroreduction Eli profile run afterwards changes continuously with ~-. During the potential holding time there is a trend to a slight increase in the charge of the O-electroadsorbed species (QO) as ~"~ 0 and the reverse trend is observed when ~"~ 60 rain, while the limiting current (iL) of Pd electrodeposition increases to attain a stationary value. The initiation of the H-adatom formation is appreciably inhibited (0.22 V) as is that of the O-electrosorption. The inhibition of the latter is comparable to that found in the first TPS with a polished Pd electrode in a fresh electrolyte solution. Further-
347
i
(b)
~0.I
....z . -
/'
~.,°
~7~--~p
g
-O2
•
L/
f ~ ~ 15rain
.
.
.
.
.
.
.
'
'
5min i
Fig. 6. Potentiodynamic E/I profile resulting with TPS including a holding time (e') and E~. P d / l M H2SO4, 25°C. (a) E, = 0.963 V; (b) 1.018 V.
more, as Q O decreases, the potential of peak II shifts towards more negative values. The same run at E~ = 1.018 V, a potential where I * 0, shows, in principle, the same trend (Fig. 6b). Q¢O firstly increases together with i L and later approaches Qc in the stabilized El1 profile, while i L reaches the stationary value, From these runs one concludes that after the O-containing film has attained a certain critical thickness, it dissolves either chemically or electrochemically, yielding soluble Pd(II). The situation changes drastically again when 1.15 V < E, < 1.5 V, that is, when E~ slightly exceeds the potential range of Pd electrodissolution (Fig. 7). In this case, the anodic profile remains like that described for the stabilized Eli profile, but Qc firstly increases with ~" to attain a constant value after 30 rain or thereabouts. Simultaneously, peak II shifts gradually towards the negative direction and, finally, for T > 15 min, a clear splitting of peak II is recorded. The second peak lies within the potential range related to i L. Other changes in the negative profile are produced when E~ > 1.5 V (Fig. 7). Then, Q¢ increases with ~-, although in this case the cathodic peak remains sharp but accompanied by a hump on the negative potential side. Simultaneously, the peak potential becomes progressively more negative as Q¢ increases. In addition, the negative profile exhibits a new broad peak at ca. 1.17 V whose height increases with • . No appreciable changes in the anodic profile are noticed except the inhibition of the anodic current at Es.a after the potential holding at E~. The charge accumulated during the potential holding at 1.15 V < E~ < 1.6 V, measured through the Q¢ value, fits linear Q¢ vs. In ~" plots (Fig. 8), as is usually
348
,,~ 02
,
i
i
,
i N " ~ . . . "v~ ~ "~li\!A /
,
.........'li~Jii'l! /
~,oI-,.--.
7'Ai
-0.4
"~-!! -0.(
ftliijil i 6o min.~l ]Ii i '° m ' " ~ l t . J . ~ ,
L~L-'sP
IA A A ,
II \MI
\I
'
t
r~,.
-O.I 15rain
O~ (b)
-0.2
Es o ~ , _
\
E
-C~4 -0.6
Es, c
-08
°f V -'L~
['
________-.
-
-1.4
02
60 m | n ~
o14
~
i ~
1
o.6
-
1 rain 5rain lOmin 20min
30rain |
oa
I
~o
I
~z
J
~i~ ~.6 Potential/W)
Fig. 7. Potentiodynamic E/I profile resulting with TPS including a holding time (~) at E.. P d / l M H2SO 4, 25°C. (a) E~ = 1.38 V; (b) 1.53 V.
349
o t~ 1£
E o
1.2
oe E,:LS3~IIIIIII~. _ • T
I I
In(~/ls)) Fig. 8. Q¢ vs. In ~"plots. P d / l M H2SO4, 25°C; data taken from TPS E,r.
E/1 profiles at 0.25
V / s including
found in anodic film growth [28,29]. The slopes (b') of these plots change with E,, as found already for other metals [26]. Thus, for E, = 1.53 V, b' = 38 # C / d e c a d e , for E , = 1.38 V, b' = 10 # C / d e c a d e and for E, = 1.18 V, b' = 5 / t C / d e c a d e .
Potentiodynamic ageing perturbation program The electroreduction E l i profile run immediately after applying to the electrode the potentiodynamic ageing perturbation program (Fig. 9) depends strongly on the perturbation parameters (Es, c, E',¢, Es, a, v and ~-). At v = 0.25 V/s, considerable changes of the electroreduction profile are produced when E~,a = 1.53 V, Es, ¢ = 0.3 V and E~',¢ is fixed at a potential where 60 to 95% of the anodic film is electroreduced and rebuilt during each intermediate RTPS. Thus, when E'¢ = 0.66 V (Fig. 8a), after about 10 RTPS between E~c and E~,a, the charge of peak II decreases and simultaneously a new peak appears at ca. 0.45 V whose charge (Q'c') increases initially at the expense of the charge of peak II. Then, a net splitting of the electroreduction profile is observed. However, after 100 intermediate RTPS, peak II practically disappears and is replaced by a new current peak (II') at ca. 0.35 V, this potential value depending on z. The overall cathodic and anodic charges recorded after the IRTPS are greater than Q~ and Q~ is the stabilized E l i profile. However,
350
j.-
(a)
'~ 0.2
E
¢J
-02
E
-0.4
-06
Es,c
-0.8
- 10I ~5 __--.-- .-- T . . _ - . ~
(b) 0
! //!" -(12
I/l!
-0.4
10
P
0.5
E $,o
°
~
:!!
E's&
I'
Es,~ [ - - L - -
s:
-0.6
q3.5
I
1,0
-
1.5 -- J _ _ _
2O
t
Z5
-0.8 I|
I' -1.0
|
35
-12
¢0 02
0~
1.0
tz
1.~
1.6
Potentiol/(V]
Fig. 9. E / I profiles run with the potentiodynamic ageing program at 0.25 V / s . Pd/1 M H 2 S O 4 , 25°C. (a) E~.c corresponds to ca. 75% oxide removal and rebuilding. (1) 10, (2) 20, (3) 50, (4) 100 and (5) 200 cycles. (b) E ' c corresponds to ca. 95% oxide removal and rebuilding 400 cycles.
351 tlae potentiodynamic E l i profile resulting after continuing the RTPS between Es,~ and Es, a recovers the shape of the stabilized E l i profile. This suggests that the prolonged RTPS between E~',c and Es, a has produced a net increase of the electrode roughness. These effects depend remarkably on E~'c, namely, on the percentage of anodic charge involved in the electrochemical reaction (Fig. 8b) between E',c and Es, a. Thus, the estimated roughness factor resulting after 400 RTPS between E'c = 0.64 V, Es, a = 1.53 V and v = 0.25 V/s, as referred to the stabilized E l i profile, is about four. DISCUSSION
Fresh and stabilized Pd electrodes Under the absence of interference of H a and H-adatoms reactions, polycrystalline Pd offers the possibility to establish more dearly than in other noble metal electrodes the different processes participating within the potential range related to the thermodynamic stability of bulk water. It is already known that the stability and the reactivity of Pd surface oxides in the electrolyte depend to a great extent on the electrode history [13]. This is not only confirmed by the present results but it appears that a great deal of data concerning Pd electrode, particularly under non-stationary perturbation conditions, actually correspond to non-stabilized and non-controlled Pd electrode surfaces. The preceding results can, in principle, be interpreted in terms of two limiting types of behaviour. The first one corresponds to the fresh polycrystalline Pd electrode, which is mostly active for Pd electrodissolution and relatively inactive for O-electroadsorption. The second behaviour is that of the stabilized electrode as referred to in the preceding results. The stabilized electrode becomes progressively more inactive for Pd electrodissolution and remarkably more active for O-electroadsorption. In this case, the electrochemical response of Pd approaches that of polycrystalline Pt and Rh electrodes in aqueous acid electrolytes [30,31]. The Pd electrodissolution preferentially occurring with fresh electrodes, according to the reaction: Pd = pd2++ 2 e-
(1)
has been reported earlier by different authors [13,14-16]. The corrosion rate of Pd as Pd 2÷ in 0.2 M H2SO 4 already occurs at E ° ( N H E ) > 0.60 V and it depends on the applied potential [15]. The thermodynamic threshold potential of reaction (1) is E ° (NHE) = 0.95 V [32]. This value, however, lies very close to that of the overall reaction: Pd + H20 = PdO + 2 H + + 2 e-
(2)
which is E ° = 0.92 V [32]. Therefore, for a fresh polycrystalline electrode both processes (1) and (2) may occur simultaneously. On the other hand, the electrodeposition of Pd 2÷ begins at potentials where the O-containing layers are not
352 completely electroreduced. This means that a gradual change of the surface composition of the electrode should be accomplished during the RTPS in the 0.3 V to 1.53 V range, to attain, after a prolonged potential cycling, the stationary surface composition of the stabilized Pd electrode. The metal surface should change from a Pd surface to a PdxOy layer (x >> y). This agrees with the fact that PdO species inhibits the electrodissolution reaction of Pd to Pd 2÷ [13]. Therefore, the extent of the electrode surface change depends on whether PdO species are partially or completely electroreduced during each negative potential sweep, on the time scale of the potential perturbation, and on the contribution of Pd 2+ electrodeposition (Fig. 1). The latter process makes possible the occurrence of Pd atoms at the metal surface that are not a equilibrium in the metallic lattice, in part probably because of the presence of traces of oxide species. These facts should produce a change in the nature and activity of Pd electrodes from that of a clean Pd electrode to that of a P d - O alloy electrode. The formation of this type of alloy is accomplished, for instance, with Pt electrodes in aqueous acid electrolytes subjected to very drastic chemical or electrochemical treatments [33,34]. The condition of the Pd stabilized electrode is acquired, for instance, through a RTPS at 0.2 V / s (Es, c = 0.3 V and Es, a = 1.53 V) during ca. 180 min. One should expect, therefore, that the structure of the P d - O layer on Pd electrodes used as electrocatalysts will have a substancial influence on the course of any electrochemical reaction. PdO dissolves in acid very slowly [35] and this probably also applies for the PdO surface layer in the acid electrolyte. The reaction: PdO + 2 H ÷= pd2++ H 2 0
(3)
means a chemical contribution to the corrosion of Pd in the electrolyte. Changes in real Pd electrode area
The gradual changes of anodic charge produced during the RTPS can be associated with changes in the real area of Pd electrode. The electrode roughness can be estimated from the charge of the O-electroreduction current peak referred to that recorded in the first negative potential scan. Thus, at the beginning of each RTPS an increase of the electrode roughness is observed. This increase, which implies a roughness factor of about 1.4, after 12 potential cycles in the conditions indicated in Fig. l a, is quite likely to be produced in the short time range by the electrodeposition of Pd 2÷. The corresponding reaction can be written as follows: Pd 2+ + Pd x (PdO) + 2 e- = Pd*Pd(PdO)
(4)
where Pd* denotes an electrodeposited Pd atom misfitting the equilibrium position in the metal lattice, and x >>"1. After a prolonged RTPS, however, a decrease of the real electrode area in the long time range is observed (Fig. lb). This effect suggests a sintering of the Pd surface. Sintering effects have been observed in most freshly electrodeposited noble metal electrodes (electrometallized-metal, electrodes) [36,37].
353 The corresponding reaction is: Pd*Pdx (PdO) = Pdx+l (POO)
(5)
Sintering, which can also be explained through the formation of Pd* atoms, should occur only when a relatively large surface concentration of Pd* atoms has been produced. This explains why sintering is observed after the initial roughness increase. Surface roughening and sintering are no longer observed once the stabilized E l i profile has been achieved.
Electroformation of the oxygen-containing layer The electroformation of the oxygen-containing layer involves various stages whose contributions depend on the characteristics of the potential perturbation. In this respect, two main distinctions should be made depending on whether Qa is in the order of or greater than that of the PdO monolayer ( 4 2 0 / t C / c m 2) [17]. Thus, when Es. a < 1.1 V, Qa is smaller than that expected for the O-monolayer corresponding to one O-atom per Pd atom at the surface. The initiation of this process comprises the formation of Pd(OH) species according to the reaction: P d + H20 = Pd(OH) + H + + e-
(6)
Reaction (6) is formally the same, but involving other noble metals [38-40]. It corresponds to a fast process, as is clearly visualized through the TMTPS experiments run in the 0.3 V-1.46 V range (Fig. 10). The step following reaction (6) is related to the electroformation of the Pd(O) monolayer which can be formally written as follows: Pd(OH) = Pd(O) + H + + e-
(7)
The Pd(O) species also participates in ageing processes that are, in principle, similar to those described for other noble metals [41,42]. The ageing can be represented by the equation: Pd(O) ~ [Pd(O)]aged
(8)
The existence of two forms of Pd(O) is deduced from the splitting of the E l i electroreduction profiles under constant charge conditions observed when Es, a < 1.53 V (Figs. 7 and 9) after applying to the electrode one of the different ageing techniques. It should be emphasized that only under the constant electroreduction charge condition can one properly refer to the ageing of the O-electroadsorbed layer on Pd. The existence of different Pd(O) configurations at the electrode correlates, at least qualitatively, with the formation of a series of PdO ordered surface structures during the adsorption of oxygen from the gas phase on Pd single crystals, which can be transformed continuously by changing the coverage [43]. Reaction (8) probably occurs through a place exchange mechanism, as already pointed out for other noble metals [30]. In principle, reactions (1)-(6) are formally similar to the processes described for Pt in aqueous acid solutions under potentiodynamic perturbation conditions [42]. It is likely that at more positive
354
4.a
15 0.2
0,6
I ,0
1,4 Potential/(V)
Fig~ 10. Triangularly modulated single potential sweep run from Es, ¢ to Es, a. P d / l M H2SO4, 25°C. vbas~ = 1 V / s ; Omodulate d ~ 50 V//S; modulation amplitude 0,2 V; Es, ¢ = 0.3 V; Es. a = 1.46 V.
potentials the Pd(O) layer grows initially to a PdO phase film, which was found to be a semiconductor with a band gap of 1.5 eV [44].
Oxide layer thickening The thickening of the O-layer takes place at potentials between 0.96 V and 1.53 V. During the potentiodynamic run, the film growth is accompanied by a progressive increase in the electroreduction charge and simultaneously a gradual shift of the potential of the corresponding peak towards more negative values. The increase of the oxygen-containing film thickness should occur through a reaction such as: Pd,Pd(O) + (n + y ) H 2 0 ~ Pdx+,Or+,'n H 2 0 + 2y H++ 2 y e -
(9)
where y > x and n indicates the number of hydration water molecules in the film. According to reaction (9)', a non-stoichiometric oxide film is formed. The rate of film growth fits linear Q vs. In ~ relationships, as usaally found in the growth of anodic films [28,29]. Both the growth of the Pdx0 v film and its probable ageing are appreciably
355
enhanced when the electrode is subjected to the potentiodynamic ageing perturbation program. These effects admit the same explanation already given for Pt electrodes under comparable conditions in aqueous acid electrolytes [42]. Thus, the structural changes of surface oxide layers electroformed on Pd are associated with surface reconstruction as the initial step for the diffusion of oxygen into the bulk metal [18]. Surface oxide reconstruction is related to bulk oxidation of Pd [45]. The electrochemical penetration of oxygen underneath the first metal atom layers is coherent with the significant amount of oxygen that can be incorporated into bulk Pd at pressures far below the dissociation pressure of PdO [46]. On the other hand, the absorption of oxygen and diffusion into bulk Pd have been measured from the study of the interactions of oxygen gas with Pd surfaces [43,46-52]. In this respect, the charge balance indicates that bulk oxygen absorption into bulk Pd becomes possible. The interpretation given to the Pd oxide growth correlates with the fact that the spectra of the Pd-oxygen electrode system show, in principle, large quantities of excess oxygen in the form of adsorbed water, hydrated PdO or PdO 2, or the corresponding Pd(OH)2 or Pd(OH)4 [53]. ESCA results of electrochemically produced Pd oxide were interpreted in terms of PdO z formation through Pd 4+ ions [52]. The field strength increase within the film can assist the formation of Pd 4÷ ions, and consequently of PdO 2, at a potential lower than the equilibrium potential of the reaction: Pd + 2 H 2 0 = PdO 2 + 4 H + + 4 e-
( E ° = 1.47 V)
(10)
Finally, when the potential exceeds 1.2 V, a new oxide species is electroformed, as revealed through the electroreduction current peak located at 0.43 V (Fig. 7). The corresponding potential range, where the new species is formed, mostly involves the growth of a PdO 2 containing film. This is actually expected, because in this case the equilibrium potential of the reaction: PdO + H 2 0 = PdO2 + 2 H + + 2 e -
( E ° -- 1.263 V [32])
(11)
is exceeded. However, it is likely that PdO2-containing film also comprises species with peroxidic structures appearing just at the potential where the 02 evolution begins. The electroreduction current peak recorded at ca. 1.2 V can, in principle, be assigned to the electroreduction of that fraction of the oxide layer related to the peroxidic type species. Under these conditions, probably most of the oxide film still grows to Pd x + 1Oy+i stoichiometry as PdO 2 decomposes into PdO and 02 [19]. This type of decomposition reaction probably applies to most noble metal anodes in acid electrolytes at relatively high positive potentials [54,55]. The oxide layer structure formed at high positive potentials can be envisaged as a non-homogeneous complex film, as the ionic radii of Pd 2÷ and Pd 4+ ions are considerably different when both ionic species are film constituents. This inhomogeneity may assist the diffusion of Pd 4÷ through the oxide layer and, therefore, further contributing to Pd corrosion and increase of the surface roughness, as pointed out recently [13].
356 The PdO2-containing portion of the complex oxide film structure can be electroreduced either to PdO, according to: PdO 2 + 2 H ÷ + 2 e - = PdO + H20
( E ° / V = 1.263 - 0.059 pH)
(12)
or to Pd, through the following overall reaction: PdO 2 + 4 H + + 4 e - = Pd + 2 H20
( E ° / V = 1.04 - 0.059 pH)
(13)
Nevertheless, it should be noticed that the overvoltage for the electroreduction of PdO 2 is appreciably lower than that for PdO [20,21,56]. The structure of the complex electrochemical interface accomplished at high positive potentials can then be expressed as follows: Pd/PdxOy/PdO2/electrolyte, where a part of the peroxidic structure contributes to PdO 2 growth. The poor stability of bulk PdO 2 species under these conditions is well known [19]. The various O-electroadsorbed species and Pd oxides formed on anodized Pd at different potentials also exhibit different stabilities in contact with the acid electrolyte. The solubility of the different species can be studied through the charge decrease at open circuit after anodizing and ageing the Pd electrodes at different potentials [57]. The electrolytic activity of Pd in acid electrolyte is, therefore, remarkably dependent on the overall electrode history. This effect is to some extent comparable to that produced by the electrochemical reductions and heat treatments on the activity of Pt electrodes, as has been known for some time [58,59]. On the other hand, the limiting electrochemical behaviour of Pd electrodes are reflected through the behaviour of free Pd and oxidized Pd electrodes with respect to the 02 electroreduction reaction in alkaline electrolytes [60]. The electrolytic activity of Pd in these solutions was related to the content of hydrogen in Pd, so that the final loss of activity was caused by the electroformation of non-stoichiometric oxides or OH species on the electrode at sufficiently positive potentials [22]. In conclusion, the present results show that the electrochemical behaviour of Pd in the acid electrolyte, besides any anion influence and the H-adatom interface, is far more complex than was previously thought, due to the electrodissolution of the base metal and the electroformation of a variety of P d - O species that participate in different structural rearrangements. ACKNOWLEDGEMENTS INIFTA is sponsored by the Consejo Nacional de Investigaciones Cientificas y Trcnicas, the Universidad Nacional de La Plata and the Comisirn de Investigaciones Cientificas (Provincia de Buenos Aires). This work was partly supported by the Regional Program for the-Scientific and Technological Development of the Organization of American States.
357 REFERENCES 1 R. Woods in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 9, Marcel Dekker, New York, 1977. 2 G. Btlanger and A.K. Vijh in A.K. Vijh (Ed.), Oxides and Oxide Films, Vol. 5, Marcel Dekker, New York, 1977, p. 1. 3 A.T. Hubbard, R.M. Ishikawa and J. Katekaru, J. Electroanal. Chem., 86 (1978) 271. 4 E. Yeager, W.E. O'Grady, M.Y.C. Woo and P. Hagans, J. Electrochem. Soc., 125 (1978) 348. 5 A. Hamelin and J.P. Bellier, Surf. Sci., 78 (1978) 159. 6 D. Dickertmann, J.W. Schultze and K.J. Vetter, J. Electroanal. Chem. 55 (1974) 429. 7 K. Kinoshita, D.R. Ferrier and P. Stonehart, Electrochim. Acta, 23 (1978) 45. 8 P.N. Ross, J. Electroanal. Chem., 76 (1977) 139. 9 M.I. Florit, M.E. Martins and A.J. Arvia, J. Electroanal. Chem., 151 (1983) 209. l0 J. Clavilier and C. Nguyen Van Huong, J. Electroanal. Chem., 80 (1977) 101. 11 A. Angerstein-Kozlowska, B.E. Conway, B. Barnett and J. Mozota, J. Electroanal. Chem., 100 (1979) 417. 12 S. Shibata and M.P. Sumino, Electrochim. Acta, 26 (1981) 517. 13 K. Gossner and E. Mizera, J. Electroanal. Chem. 125 (1981) 347. 14 J.F. Llopis, J.M. Gamboa and J. Victori, Electrochim. Acta, 17 (1972) 2225. 15 S.H. Cadle, J. Electrochem. Soc., 121 (1974) 645. 16 D.A.J. Rand and R. Woods, J. Electroanal. Chem., 35 (1972) 209. 17 D.A.J. Rand and R. Woods, J. Electroanal. Chem., 31 (1971) 29. 18 T. Chierchie, C. Mayer and W. Lorenz, J. Electroanal. Chem., 135 (1982) 211. 19 J.P. Hoare, J. Electrochem. Soc., 112 (1965) 1129. 20 A. Hickling and G. Vrjosek, Trans. Faraday Sot., 57 (1961) 123. 21 R. Fortuny, J. Genesch and L. Victori, Afinidad 38 (1981) 91. 22 I. Rube~ and H. Bylikovb, Coll. Czech. Chem. Commun., 37 (1972) 715. 23 F.A. Lewis, The Palladium/Hydrogen System, Academic Press, London, 1967. 24 N.R. de Tacconi, J.O. Zerbino and A.J. Arvia, J. Electroanal. Chem., 79 (1977) 287. 25 B.E. Conway, H. Angerstein-Kozlowska, F.C. Ho, J. Klinger, B. Mac Dougall and S. Gottesfeld, Faraday Discuss. Chem. Soc., 56 (1973) 210. 26 M.E. Martins, R. Cbrdova O. and A.J. Arvia, Electrochim. Acta, 26 (1981) 1547. 27 B.E. Conway, H. Angerstein-Kozlowska, E. Criddle and W.B. Sharp, Anal. Chem. 45 (1973) 1331. 28 K. Ogura, S. Haruyama and K. Nagasaki, J. Electrochem. Soc., 118 (1971) 531. 29 S. Hadzi-Jordanov, H. Angerstein-Kozlowska, M. Vukovic and B.E. Conway, J. Electrochem. Soc., 125 (1978) 1471. 30 B.V. Tilak, B.E. Conway and H. Angerstein-Kozlowska, J. Electroanal. Chem., 48 (1973) 1. 31 C. Pallotta, N.R. de Tacconi and A.J. Arvia, Electrochim. Acta, 26 (1981) 261. 32 M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, Oxford, 1966. 33 J.P. Hoare, Electrochim. Acta, 26 (1981) 225. 34 A. Chialvo, W.E. Triaca and A.J. Arvia, J. Electroanal. Chem., 146 (1983) 93. 35 CRC Handbook of Chemistry and Physics, 57 (1977) B 138. 36 P. Stonehart and P.A. Zucks, Electrochim. Acta, 17 (1972) 2333. 37 A.C.C. Tseung and S.C. Dhara, Electrochim. Acta 20 (1975) 681. 38 H. Angerstein-Kozlowska, B.E. Conway and W.B.A. Sharp, J. Electroanal. Chem., 43 (1973) 9. 39 C.M. Ferro, A.J. Calandra and A.J. Arvla, J. Electroanal. Chem., 59 (1973) 239. 40 J.O. Zerbino, N.R. de Tacconi and A.J. Arvia, J. Electrochem. Soc., 125 (1978) 1266. 41 M.Y. Duarte, M.E. Martins and A.J. Arvia, An. Asoc. Quim. Argent., 70 (1982) 525. 42 M.E. Folquer, J.O. Zerbino, N.R. de Tacconi and A.J. Arvia, J. Electrochem. Soc., 126 (1979) 592. 43 G. Ertl and P. Ran, Surf. Sci., 15 (1969) 443. 44 H. Okamoto and T. Aso, Jap. J. Appl. Phys., 6 (1967) 779. 45 T.W. Orent and S.D. Bader, Surf. Sci., 115 (1982) 323.
358 46 C.T. Campbell, D.C. Foyt and J.M. White, J. Phys. Chem., 81 (1977) 491. 47 H. Conrad, G. Ertl, J. Kuppers and E.E. Latta, Surf. Sci., 65 (1977) 245. 48 G. Ertl and J. Koch in F. Ricca (Ed.), Adsorption-Desorption Phenomena, Academic Press, 1972, p. 345. 49 T. Matsushima and J.M. White, Surf. Sci., 67 (1977) 122. 50 Y.L. Sandier and O.D. Durigo, J. Phys. Chem., 73 (1969) 2392. 51 J.S. Close and J.M. White, J. Catal., 36 (1975) 653. 52 D.L. Weissman, M.L. Shek and W.E. Spicer, Surf. Sci., 92 (1980) L59. 53 K.S. Kim, A.F. Gossmann and N. Winograd, Anal. Chem., 46 (1974) 197. 54 C. Pallotta, N.R. de Tacconi and A.J. Arvia, J. Electroanal. Chem., 122 (1981) 367. 55 M.I. Florit, M.E. Martins and A.J. Arvia, J. Electroanal. Chem., 126 (1981) 255. 56 Y.A. Mazitov, K.I. Rozental and V.I. Veselovskii, Zh. Fiz. Khim., 38 (1964) 151. 57 A.E. Bolzhn, M.E. Martins and A.J. Arvla, in preparation. 58 S. Shibata and M.P. Sumino, Electrochim. Acta, 16 (1971) 1511. 59 T. Biegler, Aust. J. Chem., 26 (1973) 2571. 60 S. Kondo, K. Ohashi and S. Nagaura, Denki Kagaku, 37 (1969) 212.