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The anodic behavior of Pd electrodes in 1 M H2SO4
J, Electroanal. Chem., 1 2 5 ( 1 9 8 1 ) 3 4 7 - - 3 5 8
347
Elsevier S e q u o i a S.A., L a u s a n n e - - P r i n t e d in T h e N e t h e r l a n d s
THE ANODIC BEHAVIOUR OF Pd ELECTRODES IN 1114 H2SO4
K. G O S S N E R a n d E. M I Z E R A
Inst. Physik. Chemie der Universiffft, Theresienstrasse 3 7, D-8000 Munich (G.F.R.) (Received 1st D e c e m b e r 1 9 8 0 ; in revised f o r m 1 6 t h F e b r u a r y 1 9 8 1 )
ABSTRACT T h e a d s o r p t i o n o f o x y g e n o n Pd in 1 M H2SO 4 is s h o w n to c o m p e t e w i t h c o r r o s i o n of t h e s u b s t r a t e a n d w i t h d i f f u s i o n of o x y g e n i n t o t h e b u l k . A c o r r e c t d e s c r i p t i o n o f t h e a d s o r p t i o n m u s t a c c o u n t for a n o n - c o n t i n u o u s c h a n g e o f t h e o x y g e n coverage w i t h t h e p o t e n t i a l . A n a t t e m p t is m a d e t o find c o r r e l a t i o n s b e t w e e n t h e gas p h a s e a n d t h e electrochemical system.
INTRODUCTION
Palladium is well known to be one of the best catalysts for oxidations. In particular, the oxidation of CO on this metal has recently been intensively investigated. When such investigations are performed by electrochemical means, the electrode has to be cl~arged to a very positive region, where the oxidation of the substrate begins [ 1,2]. In cyclic voltammograms the anodic charge was found to be greater than the cathodic charge of the reverse cycle, even with measurements where no organic species was present in the electrolyte, which could give rise to an enhanced anodic current. The excess of the anodic over the cathodic charge was traced back to the corrosion of the substrate [3]. However, the extent to which corrosion and oxide formation govern the behaviour of Pd electrodes in the anodic range is as yet almost unknown. It is the aim of this paper to elucidate the processes occurring on Pd electrodes in the anodic range, since this will be a prerequisite for a better understanding of the oxidation reactions on these electrodes. EXPERIMENTAL
Unless otherwise stated, the experiments were performed at 22 + 1°C in 1 M H2SO4, prepared from conc. H2SO4 (quality 'Suprapur', Merck) and triply distilled water. Before starting an experiment the electrolyte was deoxygenated by vigorous bubbling with nitrogen. During the experiments a steady stream of nitrogen was passed over the solution. A commercial thermostatable cell (Metrohm) was used. A gold foil inserted into a glass tube with a coarse glass frit served as counter electrode. All potentials were measured against a mercury/ mercurous sulphate reference electrode, but are referred in the t e x t to the reversible hydrogen electrode. The polycrystalline Pd foils for the working electrodes 0022-0728/81/0000--0000/$02.50
© 1 9 8 1 Elsevier S e q u o i a S.A.
348
were supplied by Heraeus (99.99% purity). The respective geometric area (about 45 mm 2) was ascertained after a strong anodization after the end of an experiment. A black oxide layer was then formed from which the area having been in contact with the electrolyte could be derived. The electronic equipment consisted of a potentiostat (LB75 M, Bank Elektronik) driven by a VSG 72 sweep generator (Bank Elektronik). Voltammograms were recorded by a digital storage oscillograph (Nicolet, 1090A) from which they could be drawn with an X--Y recorder (HP 7004B). Integration of the currents involved was carried out simultaneously by a digital high-speed integrator (Time Electronics). The potentials where the current changed sign were taken as integration limits. The anodic charge Qa was obtained at the positive limit. At the negative potential limit Qnr, which indicates the quantity of charge not accessible to reduction during the reverse scan, could be read from the integrator display. Here, Qr was obtained from (Qa -- Qn~) and gives, after correction for doublelayer charging, the quantity of charge assignable to the oxide reduction. GENERAL
In this section some of the most important known features of the anodic behaviour of Pd in acidic solutions are briefly summarized. A voltammogram of the oxidation of Pd in 1 M H~SO4 is given in Fig. 1. A starting potential was chosen sufficiently positive to avoid difficulties that could arise by absorbed hydrogen. At 0.8 V the current rises during the anodic scan, indicating the beginning of oxidation of the substrate. Then the current decreases and remains nearly constant until at about +1.3 V it rises again, due to the beginning of oxygen evolution. At about +0.5 V the oxides formed are reduced during the cathodic scan, showing a single sharp reduction peak. A similar behaviour is also found with the other noble metals. The most characteristic property of noble metals in the positive potential range is the ability to form passivating oxides that prevent further dissolution of the metal, when the reversible potential of the respective Me/Me z÷ couple is exceeded. Metals which are n o t able to form passive oxides are usually unsuitable for anodic investigations in an aqueous medium, since the potential region between H2 evolution and metal dissolution is very small. The anodic current rise at +0.8 V can be traced back to the formation of Pd 2÷ ions according to [3] Pd ~ Pd 2÷ + 2 e-
(1)
At more positive potentials the reaction rate of reaction (1) will be increased, until finally the solubility product of the Pd species formed is locally exceeded and an oxide or hydroxide species is formed on the surface. Such a mechanism is supported by recent investigations of the corrosion rate of Pd in acidic medium [4,5], where the current was found to decrease again when the potential was further increased. The generally written oxide-forming reaction: Pd + H20 ~ PdO + 2 H ÷ + 2 emust therefore already imply reaction (1). The oxide species formed inhibits
(2)
349
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2
J
f
-2
J
0.5
L
1,0
Fig. 1. V o l t a m m o g r a m
1.5 U/V
o f t h e o x i d a t i o n o f P d in 1 M H 2 S O 4 . ( S c a n r a t e v = 1 V s -1 .)
reaction (1). Bulk PdO is know n to dissolve in acids only very slowly [6]. Although surface properties c a n n o t generally be correlated with bulk properties, the adsorbed oxygen layer surely also dissolves only at a low rate. This rate determines the chemical c o n t r i b u t i o n to the corrosion process according to PdO + 2 H ÷ ~ Pd 2÷ + H20
(3)
More positive potentials cause the oxide film to grow denser. Here, PdO was f o u n d to be a s e m i c o n d u c t o r with a band gap of 1.5 eV [7]. Therefore, the enhanced field strength within the film can give rise to f o r m a t i o n of Pd 4÷ ions, as was f o u n d by Kim et al. [8] in an ESCA study of the oxides electrochemically f o r m e d on Pd. T h e y f ound Pd 4÷ to form far below the electrochemical equilibrium potential of +1.47 V. At higher potentials PdO2 may be f o r m e d directly by Pd + 2 H20 ~ PdO2 + 4 H ÷ + 4 e-
(4)
Since the ionic radii of Pd 2÷ and Pd 4÷ are quite different (0.80 A and 0.65 A) [9] no h o m o g e n e o u s film can be e x p e c t e d when bot h kinds of ions are present to a certain extent. Moreover, the diffusion of Pd 4÷ t hrough the film will be facilitated. This m a y be a source of an additional dissolution of Pd ions. The strong increase of the surface roughness supports these statements. RESULTS
AND DISCUSSION
According to reactions (1)--(3) the total charge measurable during an anodic scan involves contributions f r om oxide f o r m a t i o n and corrosion. The respective portions greatly depend u p o n the selected experimental conditions. The dependence o f the anodically c o n s u m e d charge Qa on the scan rate v as well as on the anodic reverse potential U B is shown in Fig. 2. When U B is increased, Qa also increases. At the lower scan rates t h e dependence o f Q, on UB is more p r o n o u n c e d , partly because diffusion c o n t r o l of processes like corrosion and diffusion of o x y g e n into the bulk becomes less important, and partly because the evolution of oxygen c o m m e n c e s at lower potentials.
350
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d .
÷e
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'
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& uB/V
Fig. 2. D e p e n d e n c e of Qa on the scan rate and the anodic reverse potential s -1 ; (b) v = 40 mV
s -1; (c) v = 80 mV
s -1 ; (d) v = 200
mV
s -I ; (e) v = 400
U B. ( a ) v = 2 0 m V mV s -1 .
The anodically consumed charge Qa can be split into a portion Qr, accessible to reduction during the cathodic scan, and a portion Q,r that cannot be reduced under the same conditions. This is done in Fig. 3. Up to U B = 1.5 V the oxide coverage of the Pd electrode, as it is found from (Qa -- Qnr), increases almost linearly within the selected range (Fig. 3a). When U~ is made even more positive, a limiting value of the oxygen coverage is reached, in agreement with the measurements of Rand and Woods [ 2,10]. The values of Qnr could be taken directly from the integrator display. From Fig. 3b
o
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Fig. s -1 ;
i
i
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3 . ( a ) Reducible portion Qr, and (b) v - 40 mV s -1 ; (c) v = 80 mV
i
i
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i
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(b) non-reducible portion Qnr of Fig. s -1 ; (d) v = 200
mV
s -1 ; (e) v = 400
2 . ( a ) v -- 2 0 m V mV s -I .
351
it can be seen t h a t Q n r increases with UB, but the dependence is quite different with different scan rates. At 0.2 V s -1, Qnr was sometimes found to increase strongly when UB was chosen more positive than 1.5 V and when continuous cycling was applied. As this behaviour could n o t always be reproduced, at this scan rate the values are given only up to 1.45 V. The dependence of Qr upon the sweep rate applied is given in Fig. 4. Here, U B was chosen to be 1.45 V for 30 s. At this potential the oxide coverage is assumed to be a complete monolayer (see Fig. 5a). It can be seen that Qr is nearly independent of the sweep rate; only at small sweep rates can a small deviation be seen, owing to the reduction of some oxygen and some Pd 2÷ ions formed during the anodic scan.
Comparison with gas phase data Recently, a series of papers dealt with the interactions of oxygen with the Pd surface [ 11--13 ]. The most important findings are briefly reviewed in the following, to show to what extent electrochemical data can be compared with them and to test the extent to which c o m m o n conclusions are valid. (1) Oxygen has been found to be adsorbed quickly on to pure Pd with an initial sticking probability between 0.3 and 0.8 [ 11,14]. (2) Oxygen is absorbed by the bulk. From a fresh Pd sample the quantity of desorbable oxygen is greatly reduced compared with a sample already exposed to oxygen [ 11,12,14--17,18 ]. After sputtering of the t o p m o s t layers, when the LEED pattern indicated a pure Pd surface, the incorporation of oxygen into the bulk could still be derived from UPS spectra [ 11]. The absorption of oxygen greatly enhances the roughness of the surface (see Fig. 4 in ref. 17) and gives rise to a worse reproducibility of the desorption spectra, if care is n o t taken to
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Fig. 4. D e p e n d e n c e o f Qr o n the scan rate v ( U B = 1.45 V for 30 s). Fig. 5. (a) O x y g e n coverage as a f u n c t i o n o f the anodic reverse potential U B (v = 20 m V s - ] , potential stop at U B f o r 30 s). (b) S h i f t o f t h e o x y g e n d e s o r p t i o n p o t e n t i a l U D as a function o f U B. L e t t e r s d e n o t e t h e ranges r e f e r r e d t o in t h e t e x t .
352 pre-saturate the upper surface layers with oxygen. (3) Oxygen is adsorbed w i t h o u t activation up to 0 = 0.25 and forms regular adsorption patterns. On Pd(111) an ordered p2X2 structure up to 0 = 0.25 is reported [ 11]. Further exposure to oxygen leads to a continuous transition into a x/~ X xf3-/R 30 ° structure (up to 0 = 0.33). During this transition both 2 X 2 and the domains of the x/~ structure are found on the surface. (4) At higher oxygen pressures a surface oxide phase is formed. The study performed by Conrad et al. [ 11] is most interesting in this context for comparison with an electrochemical system. The authors generated a high effective oxygen pressure on the surface by decomposition of NO molecules, a m e t h o d analogous to the electrochemical m e t h o d by oxidation of water. In this way they achieved the formation of an epitaxial PdO(100) layer on the Pd(111) surface. This PdO c o m p o u n d is semiconducting and stable, even at high temperatures. (5) At very high temperatures a very stable PdO c o m p o u n d is formed, even at low oxygen pressures. This species does n o t react with CO and H2 and can be removed only by b o m b a r d m e n t with argon ions.
Test o f the usability o f gas phase data to the electrochemical system In electrochemistry the quantity analogous to the oxygen pressure is the potential applied, since at a given potential a certain a m o u n t of oxygen atoms is generated by the oxidation of water. On the other hand, the stability of the oxides formed is indicated by the temperature or the potential necessary for their desorption. From a plot of the shift of the potential UD of the oxygen reduction peak when the anodic reverse potential is steadily increased, it is therefore possible to draw conclusions about the stability of the respective oxide layer formed. Such a plot is given in Fig. 5b. It can be seen that the potential shift cannot be simply related to U B. At least five regions have to be distinguished, each of which is characteristic of a different behaviour of the oxygen adsorbed. Another interesting result is obtained, when the related coverage is plotted, as is done in Fig. 5a. According to ref. 2, a' plateau in the plot of the quantity of the oxide formed against the anodic reverse potential U B can be correlated to a monolayer of oxygen on the Pd surface. With our measurements this plateau was n o t so well defined as with t h o s e ' f o u n d by the authors cited. Nevertheless, a slight plateau is shown in Fig. 5a at about 1.45 V. If we assume 0 to be 1 at this potential, different slopes are f o u n d at 0 = 0.1 and 0 = 0.6. This may be taken together with the gas-phase results, to give an interpretation of the five different ranges of stability found in Fig. 5b.
Range (a) When U~ is increased in the range from 0.8 V to 0.9 V the peak potential of the oxygen reduction is shifted to more positive potentials. This can only be explained by a preferential adsorption of oxygen (or an OH species) to energetically favourable sites (dislocations, kinks, steps), so that the overvoltage necessary to remove it during the cathodic scan decreases when these preferential sites become used up.
353
In this range the charges involved decrease strongly with the number of cycles. This may be due either to a smoothing of the surface, caused by the dissolution of the less tightly bound Pd surface atoms, which are k n o w n to increase the surface roughness. The other possibility might be due to difficulties with the reduction of the oxygen bound strongly on to surface defect sites.
Range ( b) This range pertains up to about 1.05 V. From Fig. 5a 0 can be derived to be 0.25 at this potential. On Pd(111) the 2 × 2 structure is completed at this coverage, but this comparison can only be drawn with caution, for the reacting species will be different in electrochemical and gas-phase measurements and our measurements were performed on polycrystalline material. The reduction of the oxide occurs at about 0.80 to 0.82 V, irrespective of the anodic reverse potential. This means that the oxide formed in this range will be reduced at a high rate. A recent study of Bagotzky and Tarasevich [ 19] gave evidence t h a t the oxygen peaks at low anodic potentials are due to a discharge of OH ions to produce a less tightly bound MOH species. This would agree well with our findings.
Range (c) Now the oxygen coverage increases up to 0 -- 0.6 when UB is increased by only 100 mV. For the reduction of this oxide an appreciable overvoltage is necessary. The oxidation of an OH species according to MOH ~ MO + H ÷ + e-
(5)
or a dismutation process according to 2 MOH ~ MO + M + H20
(6)
to generate an MO species could reasonably explain the sudden increase of 0 in the electrochemical system. If an OH-covered surface loses the protons, additional places are available on the surface. These will be taken by other oxygen atoms so that 0 will increase strongly. Moreover, the oxygen will be more tightly bound to the surface than with the MOH species, which is in agreement with the experimental findings. Furthermore, the strong increase in 00 surely gives rise to strong repulsive interactions of the adsorbed oxygen atoms, so that place-exchange processes will occur to minimize the energy [20]. The reduction of this rearranged oxygen species proceeds only if some overvoltage is applied. Thus, the shift of UD in Fig. 5b can be reasonably explained.
Range (d) In this range the oxygen coverage increases linearly up to 0 = 1. The potential for the reduction also shifts almost linearly in the negative direction. The potential where 0 -- 1 is reached depends upon the time the electrode is allowed to stand at UB and its pre-conditioning. When the electrode is cycled several times up to the region where 0 -- 1, the electrode becomes passivated. The charge involved with each cycle was found to decrease until finally the picture given in Fig. 6 was obtained. The reduction potential of such a passivated electrode is shifted to more
354
negative potentials (Fig. 6). This p h e n o m e n o n is greatly dependent u p o n the sweep rate applied and could be observed best at a b o u t 0.2 V s -~ . As already mentioned, a non-reactive oxygen species was also found in the gas-phase experiments. Probably the application of high temperatures in the gas phase and of potential cycling in the electrochemical system have the same effect in this case.
Range (e) When the potential is increased even further, massive bulk oxidation begins. This can be derived from the sharply rising oxygen coverage of Fig. 5a. Bulk oxidation, which gives rise to a flat maximum in the cathodic curve of a voltammogram, is already found before the monolayer is completed. This is in agreement with ref. 5. The oxide formed is less stable than the oxide formed in range (d), since the slope of the cathodic potential shift is less inclined. This situation has no parallel with the gas-phase oxidations performed up to the present, since enormous oxygen pressures would be necessary.
Stability of the oxides formed To test the conclusions of the preceding sections, the stability of the oxides against the attack of hydronium ions was investigated. For this purpose the electrode was driven to the desired anodic reverse potential and the cathodic scan was stopped for various times in the passive region, where no net current passed further. At the stop potential the oxide formed at the (more positive) reverse potential was partly dissolved by the chemical attack of the solvent (according to reaction 3) and thus the charge accessible for reduction was reduced. The results are given in Fig. 7.
9O 0
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d .50 -2
~
o c
d X
X
X
X
X
:
X
b
-4 10 I
o
U/V
I
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4o tls
Fig. 6. V o l t a m m o g r a m o f a Pd e l e c t r o d e (v = 1 V s -1 ): (a) b e f o r e p a s s i v a t i o n ; (b) a f t e r passivation. Fig. 7. Decrease o f Qr w i t h time. (a) 0.1 M K2SO4, U B = 1.2 V; (b) 0.1 M K2SO4, U B = 1.6 V; (c) 5.5 M K2SO4, U B = 1.5 V; (d) 5.5 M K2SO4, UB = 1.2 V.
355 T h e o r d i n a t e values r e p r e s e n t t h e p o r t i o n o f t h e a n o d i c charge accessible to r e d u c t i o n . T h e r e f o r e , t h e values can be given in p e r c e n t a g e s a n d e x a c t k n o w l e d g e o f t h e e f f e c t i v e surface a r e a is u n n e c e s s a r y . In 0.1 M K2SO4 a b o u t 60% o f t h e positive-going scan can be r e d u c e d w h e n UB = 1.2 V. O n l y 35% can be r e d u c e d w h e n UB = 1.6 V, t h o u g h t h e o x i d e f o r m e d at t h e l a t t e r p o t e n t i a l s h o u l d e x h i b i t a greater stability. This is p a r t l y d u e t o t h e f o r m a t i o n o f a passive o x y g e n l a y e r at 1.6 V, a n d p a r t l y t o t h e a b s o r p t i o n o f s o m e o f t h e o x y g e n f o r m e d b y t h e bulk, as t h e e f f e c t i v e o x y g e n pressure is v e r y high at this p o t e n tial. In 5.5 M H2SO4 t h e e x p e c t e d b e h a v i o u r is t h e n f o u n d . A f t e r 60 s o n l y 38% can be r e d u c e d , w h e n UB = 1.2 V, w h e r e a s 53% are r e d u c e d w h e n UB = 1.5 V, t h o u g h at this higher p o t e n t i a l a higher p o r t i o n o f b u l k d i f f u s i o n will even decrease t h e real q u a n t i t y o f charge. T h e r e a c t i v i t y o f t h e d i f f e r e n t o x i d e s w i t h CO will be r e p o r t e d in a s u b s e q u e n t p a p e r .
Influence of oxygen bulk diffusion F r o m gas-phase m e a s u r e m e n t s , o x y g e n is well k n o w n t o be a b s o r b e d b y b u l k p a l l a d i u m [ 1 1 , 1 2 , 1 4 - - 1 7 ] . T h e degree o f a b s o r p t i o n was f o u n d t o b e g r e a t l y d e p e n d e n t u p o n t h e s u r f a c e r o u g h n e s s [ 1 7 , 1 8 ]. T h e r e f o r e , b u l k a b s o r p t i o n s h o u l d be far m o r e i m p o r t a n t w i t h v o l t a m m e t r i c m e a s u r e m e n t s , w h e r e t h e s u r f a c e is greatly r o u g h e n e d . E l e c t r o c h e m i c a l e v i d e n c e f o r b u l k o x y g e n a b s o r p t i o n c a n p o s s i b l y be d e r i v e d f r o m a c o m p a r i s o n o f t h e v a r i a t i o n o f t h e a n o d i c a n d c a t h o d i c charges Qa and TABLE 1 Influence of oxygen bulk diffusion. Changing of the charges involved with the number of cycles. (v = 0.1 V s -1, UB = 1.5 V, arbitrary units) Cycle
Qa
Qr
~Qa
~Qr
~Qr--~Qa
~(~Qr - ~ Q a )
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
718 646 645 648 651 654 656 658 661 663 665 667 668 669 670 671 672 673 674 675
352 392 414 429 440 450 459 465 472 478 483 489 492 495 498 500 503 506 509 511
--72 --1 +3 +3 +3 +2 +2 .+3 +2 +2 +2 +1 +1 +1 ÷1 +1 +1 +1 +1
+40 ÷22 +15 +11 +10 +9 +6 +7 +6 +5 +6 +3 +3 +3 +2 +3 +3 +3 +2
+112 +23 +12 +8 +7 +7 +4 +4 +4 +3 +4 +2 +2 +2 +1 +2 +2 +2 +1
112 135 147 155 162 169 173 177 181 184 188 190 192 194 195 197 199 201 202
356
000~
E 0
0
0 0
0 0 0.1
D
O O DDDDODDOODDDCDOD
J 1
I 4 (number
I g of
I 16 cy¢les)1/2
Fig. 8. D e p e n d e n c e o f F-(AQ r - - AQa) u p o n the square r o o t o f t h e n u m b e r o f cycles. Derived f r o m v o l t a m m o g r a m s w i t h U B = 1.5 V at 0.1 V s -1 . ( e ) a P d ; (•) ~-Au.
Qnr when the electrode is cycled continuously (Table 1). Here, Qa decreases during the first cycles, it then passes through a minimum and then rises again. The decrease may be due to the oxidation of impurities and of loosely bound Pd atoms during the first cycles, whereas the following increase will be caused by an enhanced surface roughness. The cathodic charge Qr increases continuously and to a much greater extent than can be ascribed only to the increase of the surface roughness. The difference (AQ~ - - A Q a ) for each two succeeding cycles gives the cathodic excess charge, which cannot be assigned to an increase of the surface roughness. Possibly the increase of Q~ can be led back to a gradual saturation of the upper surface layers with oxygen, so that oxygen bulk diffusion will play a less important role after some cycles and more surface oxygen atoms can be reduced during the negative-going scan. In Fig. 8 a plot of ~(AQ~ -- AQ,) against the square root of the number of cycles is given. As the scan rate varies linearly with time, this corresponds in some ways to a plot of t in. When the upper layers are saturated with oxygen the process becomes diffusion controlled as the values can be seen to approach nearly to a straight line after some cycles. In Fig. 8 the behaviour of a Pd electrode is compared to that of a gold electrode, which is n o t expected to absorb oxygen. No increase of ~(AQ~ - - A Q a ) could be found. This seems clearly to confirm the above-mentioned presumptions. CONCLUSIONS
When investigations are performed on Pd in the positive potential range, bulk diffusion of oxygen, corrosion and oxide formation will exert an influence on the currents involved, if this is done in an acid aqueous medium. The respective portions greatly depend upon the potential used. The stability of the oxides formed depends upon the positive reverse potential, the time the electrode is allowed to stand at this potential and the pH value. Different anions (in particular, C1-) will exert an influence equally well. In the literature there are conflicting views whether the oxygen on the surface must be considered as being ad-
357 sorbed or as a surface oxide. From our data and by comparison with gas-phase data we conclude that these two states cannot be sharply distinguished. Oxygen formed at lower potentials is easily reduced. If it also holds in the electrochemical system, that surface rearrangements within the oxygen layer similar to the phase transitions found in the gas phase will occur, a rather high surface mobility of oxygen atoms would be necessary. This would be possible only with adsorbed oxygen species. When the potential is made more positive, the oxygen layer formed exhibits a greatly increased stability until at least bulk oxide is formed. So the border line between adsorption and oxide formation cannot be drawn unambiguously. From Fig. 5 it can be derived that the adsorption of oxygen on the Pd surface cannot be described, if no allowance is made for a non-continuous behaviour of 0. Therefore, the attempts to describe it by taking into account decreasing heat of adsorption with increasing coverage may be successful only if they are confined to certain parts of the isotherm. An influence hitherto completely underestimated in electrochemical measurements on Pd electrodes is the diffusion of oxygen into the bulk of the electrode. This is somewhat surprising, because from gas-phase measurements it is well known that even low oxygen pressures suffice to cause this phenomenon. In electrochemistry the effective oxygen pressure on the surface is far higher. Therefore, bulk diffusion cannot be neglected. From these observations, difficulties in deriving t h e r m o d y n a m i c data, which are n o t or only little influenced by kinetic contributions, become evident. On the one hand, the scan rates used should be very low because of the very low exchange currents usually involved with oxide formation. On the other hand, bulk diffusion and dissolution are better avoided at higher scan rates. The behaviour of oxygen o n P d electrodes is surely even more complex than with gas-phase measurements. In view, however, of the surpassing importance of Pd electrodes for catalytic oxidations, further investigation of the reactivity of the oxides formed remains an important task in catalytic research. ACKNOWLEDGEMENTS The authors are indebted to the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie for financial support. They wish to t h a n k Mr. B. Struck from Time Electronics for his reconstruction of the high-speed integrator according to our special requirements.
REFERENCES 1 2 3 4 5 6 7 8 9
F . G . Will a n d C . A . K n o t t , Z. E l e k t r o c h e m . , 6 4 ( 1 9 6 0 ) 2 5 8 . D . A . J . R a n d and R. W o o d s , J. E l e c t r o a n a l . C h e m . , 31 ( 1 9 7 1 ) 2 9 . D . A . J . R a n d a n d R . W o o d s , J. E l e c t r o a n a l . C h e m . , 3 5 ( 1 9 7 2 ) 2 0 9 . J . F . L l o p i s , J . M . G a m b o a a n d L. V i c t o r i , E l e c t r o c h i m . A c t a , 17 ( 1 9 7 2 ) 2 2 2 5 . S.H. C a d l e , J. E l e c t r o c h e m . S o c . , 1 2 1 ( 1 9 7 4 ) 6 4 5 . C R C H a n d b o o k o f C h e m i s t r y a n d P h y s i c s , 59 ( 1 9 7 8 ) B 1 4 4 . I4. O k a m o t o a n d T. A s o , J a p . J. A p p l . P h y s . , 6 ( 1 9 6 7 ) 7 7 9 . K.S. K i m , A . F . G o s s m a n n a n d N. W i n o g r a d , A n a l . C h e m . , 4 6 ( 1 9 7 4 ) 1 9 7 . C R C H a n d b o o k o f C h e m i s t r y a n d P h y s i c s , 59 ( 1 9 7 8 ) F 2 1 3 .
358 10 D.A.J. Rand and R. Woods, Anal. Chem., 47 (1975) 1481. 11 H. C o n r a d , G. E r t l , J. K i l p p e r s a n d E . E . L a t t a , S u r f . Sci., 6 5 ( 1 9 7 7 ) 2 4 5 . 1 2 G. E r t l a n d J. K o c h in F. R i c c a ( E d . ) , A d s o r p t i o n - - D e s o r p t i o n P h e n o m e n a , A c a d e m i c Press, 1 9 7 2 , p. 3 4 5 . 13 G. E r t l a n d P. R a u , S u r f . Sci., 1 5 ( 1 9 6 9 ) 4 4 3 . 1 4 T. M a t s u s h i m a a n d J . M . W h i t e , S u r f . Sci., 6 7 ( 1 9 7 7 ) 1 2 2 . 1 5 Y . L . S a n d i e r a n d O . D . D u r i g o , J . P h y s . C h e m . , 73 ( 1 9 6 9 ) 2 3 9 2 . 16 J . S . Close a n d J . M . W h i t e , J. C a t a l . , 3 6 ( 1 9 7 5 ) 6 5 3 . 17 C.T. C a m p b e l l , D.C. F o y t a n d J . M . W h i t e , J. P h y s . C h e m . , 81 ( 1 9 7 7 ) 4 9 1 . 18 D . L . W e i s s m a n , M . L . S h e k a n d W.E. S p i c e r , S u r f . Sci., 9 2 ( 1 9 8 0 ) L 5 9 . 1 9 V.S. B a g o t z k y a n d M . R . T a r a s e v i c h , J. E l e c t r o a n a l . C h e m . , 1 0 1 ( 1 9 7 9 ) 1. 2 0 H. A n g e r s t e i n - K o z l o w s k a , B . E . C o n w a y , B. B a r n e t t a n d J. M o z o t a , J. E l e c t r o a n a l . C h e m . , 1 0 0 ( 1 9 7 9 ) 417.
347
Elsevier S e q u o i a S.A., L a u s a n n e - - P r i n t e d in T h e N e t h e r l a n d s
THE ANODIC BEHAVIOUR OF Pd ELECTRODES IN 1114 H2SO4
K. G O S S N E R a n d E. M I Z E R A
Inst. Physik. Chemie der Universiffft, Theresienstrasse 3 7, D-8000 Munich (G.F.R.) (Received 1st D e c e m b e r 1 9 8 0 ; in revised f o r m 1 6 t h F e b r u a r y 1 9 8 1 )
ABSTRACT T h e a d s o r p t i o n o f o x y g e n o n Pd in 1 M H2SO 4 is s h o w n to c o m p e t e w i t h c o r r o s i o n of t h e s u b s t r a t e a n d w i t h d i f f u s i o n of o x y g e n i n t o t h e b u l k . A c o r r e c t d e s c r i p t i o n o f t h e a d s o r p t i o n m u s t a c c o u n t for a n o n - c o n t i n u o u s c h a n g e o f t h e o x y g e n coverage w i t h t h e p o t e n t i a l . A n a t t e m p t is m a d e t o find c o r r e l a t i o n s b e t w e e n t h e gas p h a s e a n d t h e electrochemical system.
INTRODUCTION
Palladium is well known to be one of the best catalysts for oxidations. In particular, the oxidation of CO on this metal has recently been intensively investigated. When such investigations are performed by electrochemical means, the electrode has to be cl~arged to a very positive region, where the oxidation of the substrate begins [ 1,2]. In cyclic voltammograms the anodic charge was found to be greater than the cathodic charge of the reverse cycle, even with measurements where no organic species was present in the electrolyte, which could give rise to an enhanced anodic current. The excess of the anodic over the cathodic charge was traced back to the corrosion of the substrate [3]. However, the extent to which corrosion and oxide formation govern the behaviour of Pd electrodes in the anodic range is as yet almost unknown. It is the aim of this paper to elucidate the processes occurring on Pd electrodes in the anodic range, since this will be a prerequisite for a better understanding of the oxidation reactions on these electrodes. EXPERIMENTAL
Unless otherwise stated, the experiments were performed at 22 + 1°C in 1 M H2SO4, prepared from conc. H2SO4 (quality 'Suprapur', Merck) and triply distilled water. Before starting an experiment the electrolyte was deoxygenated by vigorous bubbling with nitrogen. During the experiments a steady stream of nitrogen was passed over the solution. A commercial thermostatable cell (Metrohm) was used. A gold foil inserted into a glass tube with a coarse glass frit served as counter electrode. All potentials were measured against a mercury/ mercurous sulphate reference electrode, but are referred in the t e x t to the reversible hydrogen electrode. The polycrystalline Pd foils for the working electrodes 0022-0728/81/0000--0000/$02.50
© 1 9 8 1 Elsevier S e q u o i a S.A.
348
were supplied by Heraeus (99.99% purity). The respective geometric area (about 45 mm 2) was ascertained after a strong anodization after the end of an experiment. A black oxide layer was then formed from which the area having been in contact with the electrolyte could be derived. The electronic equipment consisted of a potentiostat (LB75 M, Bank Elektronik) driven by a VSG 72 sweep generator (Bank Elektronik). Voltammograms were recorded by a digital storage oscillograph (Nicolet, 1090A) from which they could be drawn with an X--Y recorder (HP 7004B). Integration of the currents involved was carried out simultaneously by a digital high-speed integrator (Time Electronics). The potentials where the current changed sign were taken as integration limits. The anodic charge Qa was obtained at the positive limit. At the negative potential limit Qnr, which indicates the quantity of charge not accessible to reduction during the reverse scan, could be read from the integrator display. Here, Qr was obtained from (Qa -- Qn~) and gives, after correction for doublelayer charging, the quantity of charge assignable to the oxide reduction. GENERAL
In this section some of the most important known features of the anodic behaviour of Pd in acidic solutions are briefly summarized. A voltammogram of the oxidation of Pd in 1 M H~SO4 is given in Fig. 1. A starting potential was chosen sufficiently positive to avoid difficulties that could arise by absorbed hydrogen. At 0.8 V the current rises during the anodic scan, indicating the beginning of oxidation of the substrate. Then the current decreases and remains nearly constant until at about +1.3 V it rises again, due to the beginning of oxygen evolution. At about +0.5 V the oxides formed are reduced during the cathodic scan, showing a single sharp reduction peak. A similar behaviour is also found with the other noble metals. The most characteristic property of noble metals in the positive potential range is the ability to form passivating oxides that prevent further dissolution of the metal, when the reversible potential of the respective Me/Me z÷ couple is exceeded. Metals which are n o t able to form passive oxides are usually unsuitable for anodic investigations in an aqueous medium, since the potential region between H2 evolution and metal dissolution is very small. The anodic current rise at +0.8 V can be traced back to the formation of Pd 2÷ ions according to [3] Pd ~ Pd 2÷ + 2 e-
(1)
At more positive potentials the reaction rate of reaction (1) will be increased, until finally the solubility product of the Pd species formed is locally exceeded and an oxide or hydroxide species is formed on the surface. Such a mechanism is supported by recent investigations of the corrosion rate of Pd in acidic medium [4,5], where the current was found to decrease again when the potential was further increased. The generally written oxide-forming reaction: Pd + H20 ~ PdO + 2 H ÷ + 2 emust therefore already imply reaction (1). The oxide species formed inhibits
(2)
349
~
2
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f
-2
J
0.5
L
1,0
Fig. 1. V o l t a m m o g r a m
1.5 U/V
o f t h e o x i d a t i o n o f P d in 1 M H 2 S O 4 . ( S c a n r a t e v = 1 V s -1 .)
reaction (1). Bulk PdO is know n to dissolve in acids only very slowly [6]. Although surface properties c a n n o t generally be correlated with bulk properties, the adsorbed oxygen layer surely also dissolves only at a low rate. This rate determines the chemical c o n t r i b u t i o n to the corrosion process according to PdO + 2 H ÷ ~ Pd 2÷ + H20
(3)
More positive potentials cause the oxide film to grow denser. Here, PdO was f o u n d to be a s e m i c o n d u c t o r with a band gap of 1.5 eV [7]. Therefore, the enhanced field strength within the film can give rise to f o r m a t i o n of Pd 4÷ ions, as was f o u n d by Kim et al. [8] in an ESCA study of the oxides electrochemically f o r m e d on Pd. T h e y f ound Pd 4÷ to form far below the electrochemical equilibrium potential of +1.47 V. At higher potentials PdO2 may be f o r m e d directly by Pd + 2 H20 ~ PdO2 + 4 H ÷ + 4 e-
(4)
Since the ionic radii of Pd 2÷ and Pd 4÷ are quite different (0.80 A and 0.65 A) [9] no h o m o g e n e o u s film can be e x p e c t e d when bot h kinds of ions are present to a certain extent. Moreover, the diffusion of Pd 4÷ t hrough the film will be facilitated. This m a y be a source of an additional dissolution of Pd ions. The strong increase of the surface roughness supports these statements. RESULTS
AND DISCUSSION
According to reactions (1)--(3) the total charge measurable during an anodic scan involves contributions f r om oxide f o r m a t i o n and corrosion. The respective portions greatly depend u p o n the selected experimental conditions. The dependence o f the anodically c o n s u m e d charge Qa on the scan rate v as well as on the anodic reverse potential U B is shown in Fig. 2. When U B is increased, Qa also increases. At the lower scan rates t h e dependence o f Q, on UB is more p r o n o u n c e d , partly because diffusion c o n t r o l of processes like corrosion and diffusion of o x y g e n into the bulk becomes less important, and partly because the evolution of oxygen c o m m e n c e s at lower potentials.
350
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b
1.6
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÷e
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'
~'.4
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s -1; (c) v = 80 mV
s -1 ; (d) v = 200
mV
s -I ; (e) v = 400
U B. ( a ) v = 2 0 m V mV s -1 .
The anodically consumed charge Qa can be split into a portion Qr, accessible to reduction during the cathodic scan, and a portion Q,r that cannot be reduced under the same conditions. This is done in Fig. 3. Up to U B = 1.5 V the oxide coverage of the Pd electrode, as it is found from (Qa -- Qnr), increases almost linearly within the selected range (Fig. 3a). When U~ is made even more positive, a limiting value of the oxygen coverage is reached, in agreement with the measurements of Rand and Woods [ 2,10]. The values of Qnr could be taken directly from the integrator display. From Fig. 3b
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i
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i
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i
i
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s -1 ; (e) v = 400
2 . ( a ) v -- 2 0 m V mV s -I .
351
it can be seen t h a t Q n r increases with UB, but the dependence is quite different with different scan rates. At 0.2 V s -1, Qnr was sometimes found to increase strongly when UB was chosen more positive than 1.5 V and when continuous cycling was applied. As this behaviour could n o t always be reproduced, at this scan rate the values are given only up to 1.45 V. The dependence of Qr upon the sweep rate applied is given in Fig. 4. Here, U B was chosen to be 1.45 V for 30 s. At this potential the oxide coverage is assumed to be a complete monolayer (see Fig. 5a). It can be seen that Qr is nearly independent of the sweep rate; only at small sweep rates can a small deviation be seen, owing to the reduction of some oxygen and some Pd 2÷ ions formed during the anodic scan.
Comparison with gas phase data Recently, a series of papers dealt with the interactions of oxygen with the Pd surface [ 11--13 ]. The most important findings are briefly reviewed in the following, to show to what extent electrochemical data can be compared with them and to test the extent to which c o m m o n conclusions are valid. (1) Oxygen has been found to be adsorbed quickly on to pure Pd with an initial sticking probability between 0.3 and 0.8 [ 11,14]. (2) Oxygen is absorbed by the bulk. From a fresh Pd sample the quantity of desorbable oxygen is greatly reduced compared with a sample already exposed to oxygen [ 11,12,14--17,18 ]. After sputtering of the t o p m o s t layers, when the LEED pattern indicated a pure Pd surface, the incorporation of oxygen into the bulk could still be derived from UPS spectra [ 11]. The absorption of oxygen greatly enhances the roughness of the surface (see Fig. 4 in ref. 17) and gives rise to a worse reproducibility of the desorption spectra, if care is n o t taken to
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Fig. 4. D e p e n d e n c e o f Qr o n the scan rate v ( U B = 1.45 V for 30 s). Fig. 5. (a) O x y g e n coverage as a f u n c t i o n o f the anodic reverse potential U B (v = 20 m V s - ] , potential stop at U B f o r 30 s). (b) S h i f t o f t h e o x y g e n d e s o r p t i o n p o t e n t i a l U D as a function o f U B. L e t t e r s d e n o t e t h e ranges r e f e r r e d t o in t h e t e x t .
352 pre-saturate the upper surface layers with oxygen. (3) Oxygen is adsorbed w i t h o u t activation up to 0 = 0.25 and forms regular adsorption patterns. On Pd(111) an ordered p2X2 structure up to 0 = 0.25 is reported [ 11]. Further exposure to oxygen leads to a continuous transition into a x/~ X xf3-/R 30 ° structure (up to 0 = 0.33). During this transition both 2 X 2 and the domains of the x/~ structure are found on the surface. (4) At higher oxygen pressures a surface oxide phase is formed. The study performed by Conrad et al. [ 11] is most interesting in this context for comparison with an electrochemical system. The authors generated a high effective oxygen pressure on the surface by decomposition of NO molecules, a m e t h o d analogous to the electrochemical m e t h o d by oxidation of water. In this way they achieved the formation of an epitaxial PdO(100) layer on the Pd(111) surface. This PdO c o m p o u n d is semiconducting and stable, even at high temperatures. (5) At very high temperatures a very stable PdO c o m p o u n d is formed, even at low oxygen pressures. This species does n o t react with CO and H2 and can be removed only by b o m b a r d m e n t with argon ions.
Test o f the usability o f gas phase data to the electrochemical system In electrochemistry the quantity analogous to the oxygen pressure is the potential applied, since at a given potential a certain a m o u n t of oxygen atoms is generated by the oxidation of water. On the other hand, the stability of the oxides formed is indicated by the temperature or the potential necessary for their desorption. From a plot of the shift of the potential UD of the oxygen reduction peak when the anodic reverse potential is steadily increased, it is therefore possible to draw conclusions about the stability of the respective oxide layer formed. Such a plot is given in Fig. 5b. It can be seen that the potential shift cannot be simply related to U B. At least five regions have to be distinguished, each of which is characteristic of a different behaviour of the oxygen adsorbed. Another interesting result is obtained, when the related coverage is plotted, as is done in Fig. 5a. According to ref. 2, a' plateau in the plot of the quantity of the oxide formed against the anodic reverse potential U B can be correlated to a monolayer of oxygen on the Pd surface. With our measurements this plateau was n o t so well defined as with t h o s e ' f o u n d by the authors cited. Nevertheless, a slight plateau is shown in Fig. 5a at about 1.45 V. If we assume 0 to be 1 at this potential, different slopes are f o u n d at 0 = 0.1 and 0 = 0.6. This may be taken together with the gas-phase results, to give an interpretation of the five different ranges of stability found in Fig. 5b.
Range (a) When U~ is increased in the range from 0.8 V to 0.9 V the peak potential of the oxygen reduction is shifted to more positive potentials. This can only be explained by a preferential adsorption of oxygen (or an OH species) to energetically favourable sites (dislocations, kinks, steps), so that the overvoltage necessary to remove it during the cathodic scan decreases when these preferential sites become used up.
353
In this range the charges involved decrease strongly with the number of cycles. This may be due either to a smoothing of the surface, caused by the dissolution of the less tightly bound Pd surface atoms, which are k n o w n to increase the surface roughness. The other possibility might be due to difficulties with the reduction of the oxygen bound strongly on to surface defect sites.
Range ( b) This range pertains up to about 1.05 V. From Fig. 5a 0 can be derived to be 0.25 at this potential. On Pd(111) the 2 × 2 structure is completed at this coverage, but this comparison can only be drawn with caution, for the reacting species will be different in electrochemical and gas-phase measurements and our measurements were performed on polycrystalline material. The reduction of the oxide occurs at about 0.80 to 0.82 V, irrespective of the anodic reverse potential. This means that the oxide formed in this range will be reduced at a high rate. A recent study of Bagotzky and Tarasevich [ 19] gave evidence t h a t the oxygen peaks at low anodic potentials are due to a discharge of OH ions to produce a less tightly bound MOH species. This would agree well with our findings.
Range (c) Now the oxygen coverage increases up to 0 -- 0.6 when UB is increased by only 100 mV. For the reduction of this oxide an appreciable overvoltage is necessary. The oxidation of an OH species according to MOH ~ MO + H ÷ + e-
(5)
or a dismutation process according to 2 MOH ~ MO + M + H20
(6)
to generate an MO species could reasonably explain the sudden increase of 0 in the electrochemical system. If an OH-covered surface loses the protons, additional places are available on the surface. These will be taken by other oxygen atoms so that 0 will increase strongly. Moreover, the oxygen will be more tightly bound to the surface than with the MOH species, which is in agreement with the experimental findings. Furthermore, the strong increase in 00 surely gives rise to strong repulsive interactions of the adsorbed oxygen atoms, so that place-exchange processes will occur to minimize the energy [20]. The reduction of this rearranged oxygen species proceeds only if some overvoltage is applied. Thus, the shift of UD in Fig. 5b can be reasonably explained.
Range (d) In this range the oxygen coverage increases linearly up to 0 = 1. The potential for the reduction also shifts almost linearly in the negative direction. The potential where 0 -- 1 is reached depends upon the time the electrode is allowed to stand at UB and its pre-conditioning. When the electrode is cycled several times up to the region where 0 -- 1, the electrode becomes passivated. The charge involved with each cycle was found to decrease until finally the picture given in Fig. 6 was obtained. The reduction potential of such a passivated electrode is shifted to more
354
negative potentials (Fig. 6). This p h e n o m e n o n is greatly dependent u p o n the sweep rate applied and could be observed best at a b o u t 0.2 V s -~ . As already mentioned, a non-reactive oxygen species was also found in the gas-phase experiments. Probably the application of high temperatures in the gas phase and of potential cycling in the electrochemical system have the same effect in this case.
Range (e) When the potential is increased even further, massive bulk oxidation begins. This can be derived from the sharply rising oxygen coverage of Fig. 5a. Bulk oxidation, which gives rise to a flat maximum in the cathodic curve of a voltammogram, is already found before the monolayer is completed. This is in agreement with ref. 5. The oxide formed is less stable than the oxide formed in range (d), since the slope of the cathodic potential shift is less inclined. This situation has no parallel with the gas-phase oxidations performed up to the present, since enormous oxygen pressures would be necessary.
Stability of the oxides formed To test the conclusions of the preceding sections, the stability of the oxides against the attack of hydronium ions was investigated. For this purpose the electrode was driven to the desired anodic reverse potential and the cathodic scan was stopped for various times in the passive region, where no net current passed further. At the stop potential the oxide formed at the (more positive) reverse potential was partly dissolved by the chemical attack of the solvent (according to reaction 3) and thus the charge accessible for reduction was reduced. The results are given in Fig. 7.
9O 0
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Fig. 6. V o l t a m m o g r a m o f a Pd e l e c t r o d e (v = 1 V s -1 ): (a) b e f o r e p a s s i v a t i o n ; (b) a f t e r passivation. Fig. 7. Decrease o f Qr w i t h time. (a) 0.1 M K2SO4, U B = 1.2 V; (b) 0.1 M K2SO4, U B = 1.6 V; (c) 5.5 M K2SO4, U B = 1.5 V; (d) 5.5 M K2SO4, UB = 1.2 V.
355 T h e o r d i n a t e values r e p r e s e n t t h e p o r t i o n o f t h e a n o d i c charge accessible to r e d u c t i o n . T h e r e f o r e , t h e values can be given in p e r c e n t a g e s a n d e x a c t k n o w l e d g e o f t h e e f f e c t i v e surface a r e a is u n n e c e s s a r y . In 0.1 M K2SO4 a b o u t 60% o f t h e positive-going scan can be r e d u c e d w h e n UB = 1.2 V. O n l y 35% can be r e d u c e d w h e n UB = 1.6 V, t h o u g h t h e o x i d e f o r m e d at t h e l a t t e r p o t e n t i a l s h o u l d e x h i b i t a greater stability. This is p a r t l y d u e t o t h e f o r m a t i o n o f a passive o x y g e n l a y e r at 1.6 V, a n d p a r t l y t o t h e a b s o r p t i o n o f s o m e o f t h e o x y g e n f o r m e d b y t h e bulk, as t h e e f f e c t i v e o x y g e n pressure is v e r y high at this p o t e n tial. In 5.5 M H2SO4 t h e e x p e c t e d b e h a v i o u r is t h e n f o u n d . A f t e r 60 s o n l y 38% can be r e d u c e d , w h e n UB = 1.2 V, w h e r e a s 53% are r e d u c e d w h e n UB = 1.5 V, t h o u g h at this higher p o t e n t i a l a higher p o r t i o n o f b u l k d i f f u s i o n will even decrease t h e real q u a n t i t y o f charge. T h e r e a c t i v i t y o f t h e d i f f e r e n t o x i d e s w i t h CO will be r e p o r t e d in a s u b s e q u e n t p a p e r .
Influence of oxygen bulk diffusion F r o m gas-phase m e a s u r e m e n t s , o x y g e n is well k n o w n t o be a b s o r b e d b y b u l k p a l l a d i u m [ 1 1 , 1 2 , 1 4 - - 1 7 ] . T h e degree o f a b s o r p t i o n was f o u n d t o b e g r e a t l y d e p e n d e n t u p o n t h e s u r f a c e r o u g h n e s s [ 1 7 , 1 8 ]. T h e r e f o r e , b u l k a b s o r p t i o n s h o u l d be far m o r e i m p o r t a n t w i t h v o l t a m m e t r i c m e a s u r e m e n t s , w h e r e t h e s u r f a c e is greatly r o u g h e n e d . E l e c t r o c h e m i c a l e v i d e n c e f o r b u l k o x y g e n a b s o r p t i o n c a n p o s s i b l y be d e r i v e d f r o m a c o m p a r i s o n o f t h e v a r i a t i o n o f t h e a n o d i c a n d c a t h o d i c charges Qa and TABLE 1 Influence of oxygen bulk diffusion. Changing of the charges involved with the number of cycles. (v = 0.1 V s -1, UB = 1.5 V, arbitrary units) Cycle
Qa
Qr
~Qa
~Qr
~Qr--~Qa
~(~Qr - ~ Q a )
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
718 646 645 648 651 654 656 658 661 663 665 667 668 669 670 671 672 673 674 675
352 392 414 429 440 450 459 465 472 478 483 489 492 495 498 500 503 506 509 511
--72 --1 +3 +3 +3 +2 +2 .+3 +2 +2 +2 +1 +1 +1 ÷1 +1 +1 +1 +1
+40 ÷22 +15 +11 +10 +9 +6 +7 +6 +5 +6 +3 +3 +3 +2 +3 +3 +3 +2
+112 +23 +12 +8 +7 +7 +4 +4 +4 +3 +4 +2 +2 +2 +1 +2 +2 +2 +1
112 135 147 155 162 169 173 177 181 184 188 190 192 194 195 197 199 201 202
356
000~
E 0
0
0 0
0 0 0.1
D
O O DDDDODDOODDDCDOD
J 1
I 4 (number
I g of
I 16 cy¢les)1/2
Fig. 8. D e p e n d e n c e o f F-(AQ r - - AQa) u p o n the square r o o t o f t h e n u m b e r o f cycles. Derived f r o m v o l t a m m o g r a m s w i t h U B = 1.5 V at 0.1 V s -1 . ( e ) a P d ; (•) ~-Au.
Qnr when the electrode is cycled continuously (Table 1). Here, Qa decreases during the first cycles, it then passes through a minimum and then rises again. The decrease may be due to the oxidation of impurities and of loosely bound Pd atoms during the first cycles, whereas the following increase will be caused by an enhanced surface roughness. The cathodic charge Qr increases continuously and to a much greater extent than can be ascribed only to the increase of the surface roughness. The difference (AQ~ - - A Q a ) for each two succeeding cycles gives the cathodic excess charge, which cannot be assigned to an increase of the surface roughness. Possibly the increase of Q~ can be led back to a gradual saturation of the upper surface layers with oxygen, so that oxygen bulk diffusion will play a less important role after some cycles and more surface oxygen atoms can be reduced during the negative-going scan. In Fig. 8 a plot of ~(AQ~ -- AQ,) against the square root of the number of cycles is given. As the scan rate varies linearly with time, this corresponds in some ways to a plot of t in. When the upper layers are saturated with oxygen the process becomes diffusion controlled as the values can be seen to approach nearly to a straight line after some cycles. In Fig. 8 the behaviour of a Pd electrode is compared to that of a gold electrode, which is n o t expected to absorb oxygen. No increase of ~(AQ~ - - A Q a ) could be found. This seems clearly to confirm the above-mentioned presumptions. CONCLUSIONS
When investigations are performed on Pd in the positive potential range, bulk diffusion of oxygen, corrosion and oxide formation will exert an influence on the currents involved, if this is done in an acid aqueous medium. The respective portions greatly depend upon the potential used. The stability of the oxides formed depends upon the positive reverse potential, the time the electrode is allowed to stand at this potential and the pH value. Different anions (in particular, C1-) will exert an influence equally well. In the literature there are conflicting views whether the oxygen on the surface must be considered as being ad-
357 sorbed or as a surface oxide. From our data and by comparison with gas-phase data we conclude that these two states cannot be sharply distinguished. Oxygen formed at lower potentials is easily reduced. If it also holds in the electrochemical system, that surface rearrangements within the oxygen layer similar to the phase transitions found in the gas phase will occur, a rather high surface mobility of oxygen atoms would be necessary. This would be possible only with adsorbed oxygen species. When the potential is made more positive, the oxygen layer formed exhibits a greatly increased stability until at least bulk oxide is formed. So the border line between adsorption and oxide formation cannot be drawn unambiguously. From Fig. 5 it can be derived that the adsorption of oxygen on the Pd surface cannot be described, if no allowance is made for a non-continuous behaviour of 0. Therefore, the attempts to describe it by taking into account decreasing heat of adsorption with increasing coverage may be successful only if they are confined to certain parts of the isotherm. An influence hitherto completely underestimated in electrochemical measurements on Pd electrodes is the diffusion of oxygen into the bulk of the electrode. This is somewhat surprising, because from gas-phase measurements it is well known that even low oxygen pressures suffice to cause this phenomenon. In electrochemistry the effective oxygen pressure on the surface is far higher. Therefore, bulk diffusion cannot be neglected. From these observations, difficulties in deriving t h e r m o d y n a m i c data, which are n o t or only little influenced by kinetic contributions, become evident. On the one hand, the scan rates used should be very low because of the very low exchange currents usually involved with oxide formation. On the other hand, bulk diffusion and dissolution are better avoided at higher scan rates. The behaviour of oxygen o n P d electrodes is surely even more complex than with gas-phase measurements. In view, however, of the surpassing importance of Pd electrodes for catalytic oxidations, further investigation of the reactivity of the oxides formed remains an important task in catalytic research. ACKNOWLEDGEMENTS The authors are indebted to the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie for financial support. They wish to t h a n k Mr. B. Struck from Time Electronics for his reconstruction of the high-speed integrator according to our special requirements.
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