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Effect of chemisorbed carbon monoxide on the rate of hydrogen dissolution in palladium electrodes at potentials of the α phase
J. Electroanal. Chem., 109 ( 1 9 8 0 ) 2 5 3 - - 2 6 0
253
© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
E F F E C T OF CHEMISORBED CARBON MONOXIDE ON THE RATE OF H Y D R O G E N DISSOLUTION IN PALLADIUM ELECTRODES AT POTENTIALS OF THE a PHASE
M.W. B R E I T E R
General Electric Corporate Research and Development, Schenectady, N Y 12345 (U.S.A.) (Received 1st S e p t e m b e r 1 9 7 9 ; in revised form 19th N o v e m b e r 1 9 7 9 )
ABSTRACT Cathodic c u r r e n t - - p o t e n t i a l curves were measured on a s m o o t h palladium wire electrode, covered to different degrees with c h e m i s o r b e d c a r b o n m o n o x i d e , in 0.5 M H2SO4 at sweep rates b e t w e e n 0.01 and 30 V s -1 in the p o t e n t i a l range b e t w e e n 0.6 and 0.08 V. In the absence o f COad the net r e a c t i o n o f h y d r o g e n dissolution at potentials o f the ~ phase was d i f f u s i o n - c o n t r o l l e d at sweep rates <0.1 V s -1. The dissolution rate was r e d u c e d t o a b o u t half at small sweep rates with increasing coverage of COad up to a b o u t 0.6. A f u r t h e r increase o f the coverage led t o a considerable decrease o f the dissolution rate. However, the dissolution rate did n o t b e c o m e negligibly small at s a t u r a t i o n coverage with COad.
INTRODUCTION
The dissolution and adsorption of hydrogen under equilibrium conditions was studied on smooth palladium electrodes at potentials of the ~ phase in sulfuric acid solution [ 1 ] and other solutions [ 2] previously. Pulse techniques and voltammetric measurements were used. The following sequence of processes is involvedH + + e - ~ Had
(i)
Had ~ Hdis ,s
(2)
Hdis,s ~ Hdis,b
(3)
Here Had designates an adsorbed hydrogen atom, Hdis,s is a hydrogen atom disSolved directly below the surface and Hdi s,b denotes H atoms below the first atomic layers of Pd atoms from the surface. Reaction (i) is the Volmer reaction, reaction (2) is a reaction without charge transfer for the transition from the adsorbed state into a dissolved state directly below the surface and reaction (3) is diffusion. T w o different approaches [3,4] to the study of individual reactions of the above scheme were reported recently [3,4]. In the first approach, thin m e m branes of palladium were employed. Hydrogen was deposited cathodicaliy at potentials of the a phase on one side and removed anodicaUy on the other side in sulfuric acid solution. The rate of hydrogen deposition was obtained as a
254
function of the potential on anodically activated electrodes in the absence or presence of a poison (As203). In the second approach, voltammetric sweeps in a large range of sweep rates and impedance measurements were chosen as techniques. The correlation between the results of the present study and those of refs. 3 and 4 is discussed later. If chemisorbed species which block a part of the surface are present, the rate of the net process of hydrogen dissolution is affected because reactions (1) and (2) are sensitive to these species. Carbon monoxide molecules which axe strongly bonded [ 5] on palladium electrodes at potentials of the a phase were used in this investigation as blocking species. The main reason for the choice of COaa is that the coverage can be produced and characterized in a relatively simple way. Cathodic sweeps, starting at a potential where the electrode is essentially free of hydrogen, were analyzed to assess the effect of CO~a on the rate of the net reaction of hydrogen dissolution.
Experimental The measurements were made in a Pyrex glass vessel of conventional design. Purified 0.5 M H2SO4 was used as electrolyte at room temperature (22 + 1 ° C). The electrode potential U was measured and is given vs. a hydrogen electrode in 0.5 M H2SO4. The electrolyte was stirred with purified nitrogen at 1 cm 3 s -~ if not stated otherwise. The palladium wire electrode construction has already been described [ 1 ]. The Pd electrode was located in the axis of a Pd wire spiral which served as the counter electrode in the voltammetric measurements of cathodic current density--potential curves (i--U curves). For the chemisorption of carbon monoxide the test electrode was kept at 0.6 V and the Pd spiral at 0.8 V. The solution was stirred with CO at 1 cm 3 s -1 . Two potentiostatic circuits with counter electrodes in separate compartments accomplished this. After 600 s the stirring with CO of AC grade was stopped. After stirring with purified nitrogen 1000 s the electrolyte was replaced once under exclusion of air with fresh electrolyte, presaturated with nitrogen. The test electrode and the palladium spiral stayed at open circuit during the latter washing procedure. The saturation coverage with COaa was achieved in this way on the test electrode while the carbon monoxide coverage of the spiral was essentially zero. Experimental evidence for these two statements was derived from anodic charging curves. These curves displayed an arrest (compare Fig. 1) for the test electrode, the length of w h i c h d i d not increase any more with time of stirring beyond about 600 s. This arrest which is due to the oxidation of CO~d to CO2 is absent on the spiral. The determination of the transition time rco according to TC O ---- Ta n - - T c a t h
(4)
is illustrated in Fig. 1. The cathodic transition time r¢~th is due to the reduction of the oxygen layer which had been formed simultaneously with the oxidation of COaa to CO2 during the preceding anodic charging curve. Partial anodic oxidation of the saturation coverage allowed various degrees of coverage on the test electrode to be attained. The coverage with CO~d was
255 1.21.0
0.8
F"
:=' 0.6 0.4 02. 0
1
.
2
I
4 t/s
1
I
6
8
Fig. 1. A n o d i c and c a t h o d i c charging curves t a k e n w i t h +0.38 m A c m -2 on a s m o o t h palladiu m wire e l e c t r o d e in 0.5 M H2SO4 u n d e r n i t r o g e n stirring. C a r b o n m o n o x i d e was prea d s o r b e d at 0.6 V.
always determined at the end of the voltammetric m e a s u r e m e n t as the ratio: 0co = Q c o / s Q c o
(5)
Here Qco designates the anodic charge for the o x i d a t i o n of a given coverage of COad, and sQco corresponds to the saturation coverage. More details a b o u t the potential dependence of the coverage with CO~d and the o x i d a t i o n of CO~d can be f o u n d in a separate paper b y the a u t h o r [ 6 ]. A conventional circuit, consisting of a p o t e n t i o s t a t and a f u n c t i o n generator, allowed to take the voltammetric i--U curves. The single cathodic sweep starting at 0.6 V and ending at 0.08 V was triggered manually. The i--U curves were p h o t o g r a p h e d from the screen of an oscilloscope. Sweep rates b e t w e e n 10 m V s-' and 30 V s -1 were e m p l o y e d at the same coverage with COad. Then the coverage was changed, and the m e a s u r e m e n t s were repeated. RESULTS
The cathodic i--U curves are similar to the cathodic p o r t i o n of cyclic i--U curves [1,7] on s m o o t h palladium in 0.5 M H2SO4. Examples are n o t shown here for this reason. At first, voltammetric i--U curves were t a k e n at different sweep rates in the absence of COad. The test electrode was always kept for 1800 s at 0.6 V b e t w e e n t w o consecutive m e a s u r e m e n t s of i--U curves in order to ensure the absence of dissolved H atoms f o r m e d during the preceding sweep. Dissolved h y d r o g e n is removed rapidly at 0.6 V. A semilogarithmic plot of the ratio of current density to sweep rate v vs. potential is shown for 0 co = 0 in Fig. 2. The sweep rate which is c o n s t a n t along any of the five curves in Fig. 2 varied b e t w e e n 0.01 and 10 V s -~. As is usual, the current was d e t e r m i n e d at a given potential as t h e difference b e t w e e n the recorded current and the respective current on the base line. The extension of the horizontal part of the i--U curve b e t w e e n 0.6 and 0.35 V served as base line. The charging of the double layer is corrected for in this a p p r o x i m a t e way. A correction for the ohmic potential
256 0,~
-
Q25
020
015 0.t0
-
005
n J ~l,,,,i i0-5
n ~ nlnnJ,l
5.10-5 i0-4
n
[ Jln,~l
5.10-4 i0-3
t
~
510-3 10-2
jl~J,~l
, ~ ,lJJ~Jl
5-10-2 IO-I
5-I0-I I
ilvf/ ~-~:~-~ Fig. 2. Semilogarithmic plot of t h e ratio of current density to sweep rate vs. p o t e n t i a l at different sweep rates in the absence of c h e m i s o r b e d c a r b o n m o n o x i d e on the test electrode" (o) 0 . 0 1 V s - 1 ; ( A ) 0 . 0 3 V s - Z ; ( [ ] ) 0 . 1 V s - 1 ; ( v ) 1 V s - 1 ; ( o ) 10 V s -z. 0~30Q25 0.20 0.15 010 0:05
t
, ~ I,JJ,l
10.6
510 -6
J I0 -5
n , l[nlJf 510 .5
l 10-4
~ J l n]u.L
L A~J
5"10 - 4 10-3
J nJl~l
,
510 .3 10.2
j
J t nl~,l 510 -2
I0 -I
Fig. 3. Semilogarithmic plot of the ratio i/v vs. t h e p o t ent i al at different sweep rates in the presence of the s a t u r a t i o n coverage of COad" (o) 0.01 V s - l ; (A) 0.03 V s -z ; (G) 0.1 V s -z ; (q) 1 V s - 1 ; ( o ) 1 0 V S -1 .
030-
020
010 0
i 10.5
i
eitJJil 5"10.5
I 10.4
1 1~]1111 510 -4
I I0 -3
I i laliiJ 510 -3
1_3 I0-2
i Jllllt 510 "2 I0 -I
1
1 t JllaZ] 5"10"1
I
li/vl/s.~ 1:m2 )
Fig. 4. Semilogarithmic plot of the ratio i/v vs. the p o t e n t i a l at 0.01 Vs -1 and different coverages with COad" (O) 0CO = 0; (El) •CO = 0.34; (V) 0CO = 0.67; (O) 0CO = 0.74; (A) 0CO = 0.92; (m) 0CO = 0.95; (V) 0CO = 1.
257 5 5 I0a'
F
o
i0 a 5~oa
~'E 5"j0-3
5.f0-4
5 I0-5
10-5
i 0
~L 02
J
• 04
J
J
06
I
t 08
J
1 ~0
Oc0
Fig. 5. Semilogarithmie p l o t of t h e ratio i/v vs. the carbon m o n o x i d e coverage at 0.1 V for different sweep rates: (o) 0.01 V s - l ; (A) 0.1 V s - l ; (o) 1 V s - l ; (v) 10 V s -z .
drop between the test electrode and the tip of the Luggin capillary of the reference electrode was not applied. A similar plot is represented for 0 co = 1 in Fig. 3. For coverages <1 the l o g l i / v l - U curves are located between the curves for the same sweep rate in Fig. 2 and Fig. 3. The shape of these curves is similar to those in Fig. 3. A semilogarithmic plot of ]i/vl vs. U is given for v = 0.01 V s -L and different values of ~ co in Fig. 4. Owing to the small currents at low sweep rates there are few points at these rates in Fig. 4. The logarithm of li/vl at 0.1 V is plotted vs. the coverage with carbon monoxide at different sweep rates in Fig. 5. This figure allows the retarding effect of CO~d on the dissolution reaction at constant potential to be estimated. DISCUSSION
Extent of diffusion control of the dissolution reaction The rate of the dissolution reaction is influenced by the sweep rate and coverage with CO~d (compare Figs. 2 and 3). In the absence of CO~d the logli/v]- U curves may be represented by straight parallel lines between about 0.25 and 0.1 V at sweep rates up to 0.1 V s -~ (compare Fig. 2). Such a dependence is to be expected [8] if the equilibrium of steps 1 and 2 of the dissolution reaction is practically established. The derivation [8] of the diffusion-controlled rate of
258 hydrogen dissolution may be transferred to the present case with slight modifications. The dependence of the concentration sCH of atomic hydrogen directly below the surface u p o n the potential is given [ 2] by
U = --(RTg/F) In(sCH/A )
(6)
Here A is a constant, determined experimentally, and R, T and F have their usual meanings; g is defined [ 2 ] as g = In aH/ln cH
(7)
As a first approximation (error <1%) eqn. (13a) in ref. 8 was derived for the a m o u n t of hydrogen diffusing into the bulk of the metal during a cathodic potential sweep. Under consideration of eqn. (6) as b o u n d a r y condition, it follows from eqn. (13a) of ref. 8:
R T g In {F2A ] / D R T ~ --____ R T g ln _Iil U=~ \gRTr vF / F v
(8)
According to eqn. (8) the slope of U vs. logli/vl should be the same as the slope of U vs. log sCH. Experimentally, a slope of 83 mV decade -~ is obtained from the straight line for 0 co = 0 in Fig. 2. This value compares well with 81 mV decade -~ found [2] from the linear dependence between U and log sCH. The constant D designates the diffusion coefficient of hydrogen in the a phase. The preceding results are in agreement with previous studies [ 4]. They do not agree with the results in ref. 3. It was stated there that the Volmer reaction can be studied at potentials of the u phase on thin membranes of palladium, activated anodically. A Tafel-type dependence was found [ 3 ] between the rate of hydrogen dissolution and electrode potential. The slope was 113 mV decade -~ of current density. It is suggested that the difference between the results of the present study and those in ref. 3 is due to a different surface state. It appears that the palladium membrane did not approach the active state of the Pd wire. The latter conclusion is confirmed by the fact that the value of SCH, c o m p u t e d according to eqn. (9),
i = (DF/d) sCu
(9)
for a m e m b r a n e of thickness d from the lower curve in Fig. 1 of ref. 3 amounts to 4.7 × 10 -s mol cm -3 at U = 0.1 V. In contrast, SCH has a value of 4.5 × 10 -4 tool cm -3 for the Pd wire electrode in sulfuric acid [ 1 ] and about 2 X 10 -4 mol cm -3 in gas phase studies [9], extrapolated to r o o m temperature. The presence of activation polarization leads to the lower value of sCu in ref. 3. It is very likely that the hydrogen coverage of the surface is also much lower for the membrane than for the wire. It should be pointed out here that a Tafel-type dependence between U and i results if diffusion in the bulk of a thin m e m b r a n e is the rate-controlling step of the dissolution reaction. The combination of eqns. (6) and (9) leads to:
U = --(RTg/F) ln(lil d/DFA)
(lo)
In this case the slope is equal to 2.3 R T g / F in a plot of U vs. loglil. The distinction between activation polarization and diffusion control is clear when the
259 value of sCH is considerably smaller t h a n t h e equilibrium value, as in the above example. Figure 2 d e m o n s t r a t e s t h a t t h e logti/vI--U curves are n o t straight lines at v > 0.1 V s -~. The equilibrium of reactions (1) and (2) canno longer be considered established at the larger sweep rates. Since the dissolution reaction is diffusion controlled at low sweep rates in the absence of COad, the effect of different coverages on the shape of the logli/vl ~ U curves is s h o w n for 0.01 V s-' in Fig. 4. These curves are close to each o t h e r up to a coverage of 0.34 and their slope is practically the same b e t w e e n a b o u t 0.25 and 0.13 V. When the coverage with CO~d increases, the logIi/vl--U curves are shifted to the left and display a slight curvature. Owing to the narrowness of the potential range at large coverages it is n o t certain if the nearly linear shape of the logli/vI--U curve is meaningful at 0co > 0.9. It follows from t h e results in Fig. 4 t h a t the dissolution reaction remains largely diffusion-controlled up to a b o u t 0co = 0.34. Even at 0co = 0.67 the retarding effect of the c a r b o n m o n o x i d e coverage is only a b o u t 0.5 at c o n s t a n t potential.
Retardation o f the dissolution reaction at constant potential A plot of li/vt vs. 0co is shown for U = 0.1 V at different sweep rates in Fig. 5. The respective plot for U = 0.2 V looks similar and is t h e r e f o r e n o t given. The results in Fig. 5 confirm the previous conclusion t h a t the inhibition of the dissolution reaction by c a r b o n m o n o x i d e adsorption is relatively small, up to 0 co = 0.6 at small sweep rates. The s u b s e q u e n t decrease of the dissolution rate b e t w e e n 0 co = 0.6 and 1 is m u c h larger. At sweep rates above 0.1 V s -~ the r e t a r d a t i o n of the dissolution rate is steeper b e t w e e n 0co = 0 and 0.6 t h a n at v < 0.1 V s - ' , b u t it is also followed by a larger decrease b e t w e e n 0co = 0.6 and 1. The results in Fig. 5 have a c o m m o n feature with those in Fig. 7 of ref. 10. The rate of the anodic o x i d a t i o n of molecular h y d r o g e n at c o n s t a n t potential (rl = 0.05 V) was p l o t t e d as a f u n c t i o n of c a r b o n m o n o x i d e coverage in sulfuric acid solution at r o o m t e m p e r a t u r e . The r e t a r d a t i o n of the electrochemical reaction rate is relatively small up to a b o u t 0 co = 0.6 and increases rapidly afterwards with increasing coverage. There is, however, a significant difference in the order of m a g n i t u d e of t h e r e t a r d a t i o n at high coverages. The rate of molecular h y d r o g e n oxidation decreases by a b o u t a factor of 10 -2 there. In contrast, the rate of h y d r o g e n dissolution decreases b y a b o u t a factor of 5 at 0.01 V s -1. It is suggested t h a t the l a t t e r difference reflects the situation t h a t t w o sites are required for the reaction
H2 ~ 2 Had
(11)
while only one site is involved in reactions (1) or (2). The r e t a r d a t i o n of react i o n (11) should be larger t h a n t h a t of reactions (1) or (2). The results in Figs. 3 and 5 reveal t h a t the dissolution rate is n o t negligibly small at 0co - 1. In fact, the ratio li/vl has a b o u t the same value at u - 0.1 V for 0co = 1 and v - 0.01 V s -1 and 0co - 0 and v -- 1 V s -~. It is suggested t h a t the said result is related to the relatively large n u m b e r of bridged b o n d e d mole-
260
cules of carbon monoxide on palladium. The ratio 2sQH/sQco, which gives the average number of adsorption sites per COad, is close to 1.5 for the palladium wire electrodes of this study. Two interpretations are possible" (1) The dissolution reaction can still occur in the case of two-site adsorption of COad. (2) The surface is not completely covered by CO,d, even under the conditions that a saturation coverage is established experimentally. A distinction between these two possibilities is not feasible on the basis of the present results. It would be desirable to separate the retarding effect of CO~d into partial effects for reactions (1) and (2). This might be accomplished by determining the hydrogen coverage in the presence of COad as a function of potential. Unfortunately, the technique used in this work does not give the hydrogen coverage by an extrapolation of Qc - v'/2 curves to infinite sweep rate, where u
Qc(U) = f
i(t) dt
(12)
0.6
The extrapolation procedure assumes that the a m o u n t of dissolved hydrogen depends linearly upon v '~2 while the a m o u n t of adsorbed hydrogen is independent of sweep rate. This is not correct according to the results in Figs. 2 and 4. REFERENCES
1 2 3 4 5 6 7 8 9 10
M.W. Breiter, J. E l e c t r o a n a l . C h e m . , 81 ( 1 9 7 7 ) 2 7 5 . M.W. Breiter, J. E l e c t r o a n a l . C h e m . , 90 ( 1 9 7 8 ) 4 2 5 . E . G . Daft, K. B o h n e n k a m p a n d H.-J. Engell, Z. P h y s . C h e m . , N.F., 1 0 8 ( 1 9 7 7 ) 33. M.W. Breiter, Z. P h y s . C h e m . N.F., 1 1 2 ( 1 9 7 8 ) 1 8 3 . A.B. F a s m a n , S.N. H o v i k o v a , a n d D.B. S o k o l s k y , Zh. Fiz. K h i m . , 40 ( 1 9 6 6 ) 5 5 6 . M.W. Breiter, J. E l e c t r o a n a l . C h e m . , 101 ( 1 9 7 9 ) 3 2 9 . F . G . Will a n d C.A. K n o r r , Z. E l e c t r o c h e m . Bet. B u n s e n g e s , P h y s . C h e m . , 6 4 ( 1 9 6 0 ) 2 5 8 . B.J. B e n D a n i e l a n d F . G . Will, J. E l e c t r o c h e r n . Soc., 114 ( 1 9 6 7 ) 9 0 9 . J . O . C l e w e l e y , J . F . L y n c h a n d T.B. F l a n a g a n , J. C h e m . Soc. F a r a d a y I, 73 ( 1 9 7 7 ) 4 9 4 . M.W. Breiter, J. E l e c t r o a n a l . C h e m . , 65 ( 1 9 7 5 ) 6 2 3 .
253
© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
E F F E C T OF CHEMISORBED CARBON MONOXIDE ON THE RATE OF H Y D R O G E N DISSOLUTION IN PALLADIUM ELECTRODES AT POTENTIALS OF THE a PHASE
M.W. B R E I T E R
General Electric Corporate Research and Development, Schenectady, N Y 12345 (U.S.A.) (Received 1st S e p t e m b e r 1 9 7 9 ; in revised form 19th N o v e m b e r 1 9 7 9 )
ABSTRACT Cathodic c u r r e n t - - p o t e n t i a l curves were measured on a s m o o t h palladium wire electrode, covered to different degrees with c h e m i s o r b e d c a r b o n m o n o x i d e , in 0.5 M H2SO4 at sweep rates b e t w e e n 0.01 and 30 V s -1 in the p o t e n t i a l range b e t w e e n 0.6 and 0.08 V. In the absence o f COad the net r e a c t i o n o f h y d r o g e n dissolution at potentials o f the ~ phase was d i f f u s i o n - c o n t r o l l e d at sweep rates <0.1 V s -1. The dissolution rate was r e d u c e d t o a b o u t half at small sweep rates with increasing coverage of COad up to a b o u t 0.6. A f u r t h e r increase o f the coverage led t o a considerable decrease o f the dissolution rate. However, the dissolution rate did n o t b e c o m e negligibly small at s a t u r a t i o n coverage with COad.
INTRODUCTION
The dissolution and adsorption of hydrogen under equilibrium conditions was studied on smooth palladium electrodes at potentials of the ~ phase in sulfuric acid solution [ 1 ] and other solutions [ 2] previously. Pulse techniques and voltammetric measurements were used. The following sequence of processes is involvedH + + e - ~ Had
(i)
Had ~ Hdis ,s
(2)
Hdis,s ~ Hdis,b
(3)
Here Had designates an adsorbed hydrogen atom, Hdis,s is a hydrogen atom disSolved directly below the surface and Hdi s,b denotes H atoms below the first atomic layers of Pd atoms from the surface. Reaction (i) is the Volmer reaction, reaction (2) is a reaction without charge transfer for the transition from the adsorbed state into a dissolved state directly below the surface and reaction (3) is diffusion. T w o different approaches [3,4] to the study of individual reactions of the above scheme were reported recently [3,4]. In the first approach, thin m e m branes of palladium were employed. Hydrogen was deposited cathodicaliy at potentials of the a phase on one side and removed anodicaUy on the other side in sulfuric acid solution. The rate of hydrogen deposition was obtained as a
254
function of the potential on anodically activated electrodes in the absence or presence of a poison (As203). In the second approach, voltammetric sweeps in a large range of sweep rates and impedance measurements were chosen as techniques. The correlation between the results of the present study and those of refs. 3 and 4 is discussed later. If chemisorbed species which block a part of the surface are present, the rate of the net process of hydrogen dissolution is affected because reactions (1) and (2) are sensitive to these species. Carbon monoxide molecules which axe strongly bonded [ 5] on palladium electrodes at potentials of the a phase were used in this investigation as blocking species. The main reason for the choice of COaa is that the coverage can be produced and characterized in a relatively simple way. Cathodic sweeps, starting at a potential where the electrode is essentially free of hydrogen, were analyzed to assess the effect of CO~a on the rate of the net reaction of hydrogen dissolution.
Experimental The measurements were made in a Pyrex glass vessel of conventional design. Purified 0.5 M H2SO4 was used as electrolyte at room temperature (22 + 1 ° C). The electrode potential U was measured and is given vs. a hydrogen electrode in 0.5 M H2SO4. The electrolyte was stirred with purified nitrogen at 1 cm 3 s -~ if not stated otherwise. The palladium wire electrode construction has already been described [ 1 ]. The Pd electrode was located in the axis of a Pd wire spiral which served as the counter electrode in the voltammetric measurements of cathodic current density--potential curves (i--U curves). For the chemisorption of carbon monoxide the test electrode was kept at 0.6 V and the Pd spiral at 0.8 V. The solution was stirred with CO at 1 cm 3 s -1 . Two potentiostatic circuits with counter electrodes in separate compartments accomplished this. After 600 s the stirring with CO of AC grade was stopped. After stirring with purified nitrogen 1000 s the electrolyte was replaced once under exclusion of air with fresh electrolyte, presaturated with nitrogen. The test electrode and the palladium spiral stayed at open circuit during the latter washing procedure. The saturation coverage with COaa was achieved in this way on the test electrode while the carbon monoxide coverage of the spiral was essentially zero. Experimental evidence for these two statements was derived from anodic charging curves. These curves displayed an arrest (compare Fig. 1) for the test electrode, the length of w h i c h d i d not increase any more with time of stirring beyond about 600 s. This arrest which is due to the oxidation of CO~d to CO2 is absent on the spiral. The determination of the transition time rco according to TC O ---- Ta n - - T c a t h
(4)
is illustrated in Fig. 1. The cathodic transition time r¢~th is due to the reduction of the oxygen layer which had been formed simultaneously with the oxidation of COaa to CO2 during the preceding anodic charging curve. Partial anodic oxidation of the saturation coverage allowed various degrees of coverage on the test electrode to be attained. The coverage with CO~d was
255 1.21.0
0.8
F"
:=' 0.6 0.4 02. 0
1
.
2
I
4 t/s
1
I
6
8
Fig. 1. A n o d i c and c a t h o d i c charging curves t a k e n w i t h +0.38 m A c m -2 on a s m o o t h palladiu m wire e l e c t r o d e in 0.5 M H2SO4 u n d e r n i t r o g e n stirring. C a r b o n m o n o x i d e was prea d s o r b e d at 0.6 V.
always determined at the end of the voltammetric m e a s u r e m e n t as the ratio: 0co = Q c o / s Q c o
(5)
Here Qco designates the anodic charge for the o x i d a t i o n of a given coverage of COad, and sQco corresponds to the saturation coverage. More details a b o u t the potential dependence of the coverage with CO~d and the o x i d a t i o n of CO~d can be f o u n d in a separate paper b y the a u t h o r [ 6 ]. A conventional circuit, consisting of a p o t e n t i o s t a t and a f u n c t i o n generator, allowed to take the voltammetric i--U curves. The single cathodic sweep starting at 0.6 V and ending at 0.08 V was triggered manually. The i--U curves were p h o t o g r a p h e d from the screen of an oscilloscope. Sweep rates b e t w e e n 10 m V s-' and 30 V s -1 were e m p l o y e d at the same coverage with COad. Then the coverage was changed, and the m e a s u r e m e n t s were repeated. RESULTS
The cathodic i--U curves are similar to the cathodic p o r t i o n of cyclic i--U curves [1,7] on s m o o t h palladium in 0.5 M H2SO4. Examples are n o t shown here for this reason. At first, voltammetric i--U curves were t a k e n at different sweep rates in the absence of COad. The test electrode was always kept for 1800 s at 0.6 V b e t w e e n t w o consecutive m e a s u r e m e n t s of i--U curves in order to ensure the absence of dissolved H atoms f o r m e d during the preceding sweep. Dissolved h y d r o g e n is removed rapidly at 0.6 V. A semilogarithmic plot of the ratio of current density to sweep rate v vs. potential is shown for 0 co = 0 in Fig. 2. The sweep rate which is c o n s t a n t along any of the five curves in Fig. 2 varied b e t w e e n 0.01 and 10 V s -~. As is usual, the current was d e t e r m i n e d at a given potential as t h e difference b e t w e e n the recorded current and the respective current on the base line. The extension of the horizontal part of the i--U curve b e t w e e n 0.6 and 0.35 V served as base line. The charging of the double layer is corrected for in this a p p r o x i m a t e way. A correction for the ohmic potential
256 0,~
-
Q25
020
015 0.t0
-
005
n J ~l,,,,i i0-5
n ~ nlnnJ,l
5.10-5 i0-4
n
[ Jln,~l
5.10-4 i0-3
t
~
510-3 10-2
jl~J,~l
, ~ ,lJJ~Jl
5-10-2 IO-I
5-I0-I I
ilvf/ ~-~:~-~ Fig. 2. Semilogarithmic plot of t h e ratio of current density to sweep rate vs. p o t e n t i a l at different sweep rates in the absence of c h e m i s o r b e d c a r b o n m o n o x i d e on the test electrode" (o) 0 . 0 1 V s - 1 ; ( A ) 0 . 0 3 V s - Z ; ( [ ] ) 0 . 1 V s - 1 ; ( v ) 1 V s - 1 ; ( o ) 10 V s -z. 0~30Q25 0.20 0.15 010 0:05
t
, ~ I,JJ,l
10.6
510 -6
J I0 -5
n , l[nlJf 510 .5
l 10-4
~ J l n]u.L
L A~J
5"10 - 4 10-3
J nJl~l
,
510 .3 10.2
j
J t nl~,l 510 -2
I0 -I
Fig. 3. Semilogarithmic plot of the ratio i/v vs. t h e p o t ent i al at different sweep rates in the presence of the s a t u r a t i o n coverage of COad" (o) 0.01 V s - l ; (A) 0.03 V s -z ; (G) 0.1 V s -z ; (q) 1 V s - 1 ; ( o ) 1 0 V S -1 .
030-
020
010 0
i 10.5
i
eitJJil 5"10.5
I 10.4
1 1~]1111 510 -4
I I0 -3
I i laliiJ 510 -3
1_3 I0-2
i Jllllt 510 "2 I0 -I
1
1 t JllaZ] 5"10"1
I
li/vl/s.~ 1:m2 )
Fig. 4. Semilogarithmic plot of the ratio i/v vs. the p o t e n t i a l at 0.01 Vs -1 and different coverages with COad" (O) 0CO = 0; (El) •CO = 0.34; (V) 0CO = 0.67; (O) 0CO = 0.74; (A) 0CO = 0.92; (m) 0CO = 0.95; (V) 0CO = 1.
257 5 5 I0a'
F
o
i0 a 5~oa
~'E 5"j0-3
5.f0-4
5 I0-5
10-5
i 0
~L 02
J
• 04
J
J
06
I
t 08
J
1 ~0
Oc0
Fig. 5. Semilogarithmie p l o t of t h e ratio i/v vs. the carbon m o n o x i d e coverage at 0.1 V for different sweep rates: (o) 0.01 V s - l ; (A) 0.1 V s - l ; (o) 1 V s - l ; (v) 10 V s -z .
drop between the test electrode and the tip of the Luggin capillary of the reference electrode was not applied. A similar plot is represented for 0 co = 1 in Fig. 3. For coverages <1 the l o g l i / v l - U curves are located between the curves for the same sweep rate in Fig. 2 and Fig. 3. The shape of these curves is similar to those in Fig. 3. A semilogarithmic plot of ]i/vl vs. U is given for v = 0.01 V s -L and different values of ~ co in Fig. 4. Owing to the small currents at low sweep rates there are few points at these rates in Fig. 4. The logarithm of li/vl at 0.1 V is plotted vs. the coverage with carbon monoxide at different sweep rates in Fig. 5. This figure allows the retarding effect of CO~d on the dissolution reaction at constant potential to be estimated. DISCUSSION
Extent of diffusion control of the dissolution reaction The rate of the dissolution reaction is influenced by the sweep rate and coverage with CO~d (compare Figs. 2 and 3). In the absence of CO~d the logli/v]- U curves may be represented by straight parallel lines between about 0.25 and 0.1 V at sweep rates up to 0.1 V s -~ (compare Fig. 2). Such a dependence is to be expected [8] if the equilibrium of steps 1 and 2 of the dissolution reaction is practically established. The derivation [8] of the diffusion-controlled rate of
258 hydrogen dissolution may be transferred to the present case with slight modifications. The dependence of the concentration sCH of atomic hydrogen directly below the surface u p o n the potential is given [ 2] by
U = --(RTg/F) In(sCH/A )
(6)
Here A is a constant, determined experimentally, and R, T and F have their usual meanings; g is defined [ 2 ] as g = In aH/ln cH
(7)
As a first approximation (error <1%) eqn. (13a) in ref. 8 was derived for the a m o u n t of hydrogen diffusing into the bulk of the metal during a cathodic potential sweep. Under consideration of eqn. (6) as b o u n d a r y condition, it follows from eqn. (13a) of ref. 8:
R T g In {F2A ] / D R T ~ --____ R T g ln _Iil U=~ \gRTr vF / F v
(8)
According to eqn. (8) the slope of U vs. logli/vl should be the same as the slope of U vs. log sCH. Experimentally, a slope of 83 mV decade -~ is obtained from the straight line for 0 co = 0 in Fig. 2. This value compares well with 81 mV decade -~ found [2] from the linear dependence between U and log sCH. The constant D designates the diffusion coefficient of hydrogen in the a phase. The preceding results are in agreement with previous studies [ 4]. They do not agree with the results in ref. 3. It was stated there that the Volmer reaction can be studied at potentials of the u phase on thin membranes of palladium, activated anodically. A Tafel-type dependence was found [ 3 ] between the rate of hydrogen dissolution and electrode potential. The slope was 113 mV decade -~ of current density. It is suggested that the difference between the results of the present study and those in ref. 3 is due to a different surface state. It appears that the palladium membrane did not approach the active state of the Pd wire. The latter conclusion is confirmed by the fact that the value of SCH, c o m p u t e d according to eqn. (9),
i = (DF/d) sCu
(9)
for a m e m b r a n e of thickness d from the lower curve in Fig. 1 of ref. 3 amounts to 4.7 × 10 -s mol cm -3 at U = 0.1 V. In contrast, SCH has a value of 4.5 × 10 -4 tool cm -3 for the Pd wire electrode in sulfuric acid [ 1 ] and about 2 X 10 -4 mol cm -3 in gas phase studies [9], extrapolated to r o o m temperature. The presence of activation polarization leads to the lower value of sCu in ref. 3. It is very likely that the hydrogen coverage of the surface is also much lower for the membrane than for the wire. It should be pointed out here that a Tafel-type dependence between U and i results if diffusion in the bulk of a thin m e m b r a n e is the rate-controlling step of the dissolution reaction. The combination of eqns. (6) and (9) leads to:
U = --(RTg/F) ln(lil d/DFA)
(lo)
In this case the slope is equal to 2.3 R T g / F in a plot of U vs. loglil. The distinction between activation polarization and diffusion control is clear when the
259 value of sCH is considerably smaller t h a n t h e equilibrium value, as in the above example. Figure 2 d e m o n s t r a t e s t h a t t h e logti/vI--U curves are n o t straight lines at v > 0.1 V s -~. The equilibrium of reactions (1) and (2) canno longer be considered established at the larger sweep rates. Since the dissolution reaction is diffusion controlled at low sweep rates in the absence of COad, the effect of different coverages on the shape of the logli/vl ~ U curves is s h o w n for 0.01 V s-' in Fig. 4. These curves are close to each o t h e r up to a coverage of 0.34 and their slope is practically the same b e t w e e n a b o u t 0.25 and 0.13 V. When the coverage with CO~d increases, the logIi/vl--U curves are shifted to the left and display a slight curvature. Owing to the narrowness of the potential range at large coverages it is n o t certain if the nearly linear shape of the logli/vI--U curve is meaningful at 0co > 0.9. It follows from t h e results in Fig. 4 t h a t the dissolution reaction remains largely diffusion-controlled up to a b o u t 0co = 0.34. Even at 0co = 0.67 the retarding effect of the c a r b o n m o n o x i d e coverage is only a b o u t 0.5 at c o n s t a n t potential.
Retardation o f the dissolution reaction at constant potential A plot of li/vt vs. 0co is shown for U = 0.1 V at different sweep rates in Fig. 5. The respective plot for U = 0.2 V looks similar and is t h e r e f o r e n o t given. The results in Fig. 5 confirm the previous conclusion t h a t the inhibition of the dissolution reaction by c a r b o n m o n o x i d e adsorption is relatively small, up to 0 co = 0.6 at small sweep rates. The s u b s e q u e n t decrease of the dissolution rate b e t w e e n 0 co = 0.6 and 1 is m u c h larger. At sweep rates above 0.1 V s -~ the r e t a r d a t i o n of the dissolution rate is steeper b e t w e e n 0co = 0 and 0.6 t h a n at v < 0.1 V s - ' , b u t it is also followed by a larger decrease b e t w e e n 0co = 0.6 and 1. The results in Fig. 5 have a c o m m o n feature with those in Fig. 7 of ref. 10. The rate of the anodic o x i d a t i o n of molecular h y d r o g e n at c o n s t a n t potential (rl = 0.05 V) was p l o t t e d as a f u n c t i o n of c a r b o n m o n o x i d e coverage in sulfuric acid solution at r o o m t e m p e r a t u r e . The r e t a r d a t i o n of the electrochemical reaction rate is relatively small up to a b o u t 0 co = 0.6 and increases rapidly afterwards with increasing coverage. There is, however, a significant difference in the order of m a g n i t u d e of t h e r e t a r d a t i o n at high coverages. The rate of molecular h y d r o g e n oxidation decreases by a b o u t a factor of 10 -2 there. In contrast, the rate of h y d r o g e n dissolution decreases b y a b o u t a factor of 5 at 0.01 V s -1. It is suggested t h a t the l a t t e r difference reflects the situation t h a t t w o sites are required for the reaction
H2 ~ 2 Had
(11)
while only one site is involved in reactions (1) or (2). The r e t a r d a t i o n of react i o n (11) should be larger t h a n t h a t of reactions (1) or (2). The results in Figs. 3 and 5 reveal t h a t the dissolution rate is n o t negligibly small at 0co - 1. In fact, the ratio li/vl has a b o u t the same value at u - 0.1 V for 0co = 1 and v - 0.01 V s -1 and 0co - 0 and v -- 1 V s -~. It is suggested t h a t the said result is related to the relatively large n u m b e r of bridged b o n d e d mole-
260
cules of carbon monoxide on palladium. The ratio 2sQH/sQco, which gives the average number of adsorption sites per COad, is close to 1.5 for the palladium wire electrodes of this study. Two interpretations are possible" (1) The dissolution reaction can still occur in the case of two-site adsorption of COad. (2) The surface is not completely covered by CO,d, even under the conditions that a saturation coverage is established experimentally. A distinction between these two possibilities is not feasible on the basis of the present results. It would be desirable to separate the retarding effect of CO~d into partial effects for reactions (1) and (2). This might be accomplished by determining the hydrogen coverage in the presence of COad as a function of potential. Unfortunately, the technique used in this work does not give the hydrogen coverage by an extrapolation of Qc - v'/2 curves to infinite sweep rate, where u
Qc(U) = f
i(t) dt
(12)
0.6
The extrapolation procedure assumes that the a m o u n t of dissolved hydrogen depends linearly upon v '~2 while the a m o u n t of adsorbed hydrogen is independent of sweep rate. This is not correct according to the results in Figs. 2 and 4. REFERENCES
1 2 3 4 5 6 7 8 9 10
M.W. Breiter, J. E l e c t r o a n a l . C h e m . , 81 ( 1 9 7 7 ) 2 7 5 . M.W. Breiter, J. E l e c t r o a n a l . C h e m . , 90 ( 1 9 7 8 ) 4 2 5 . E . G . Daft, K. B o h n e n k a m p a n d H.-J. Engell, Z. P h y s . C h e m . , N.F., 1 0 8 ( 1 9 7 7 ) 33. M.W. Breiter, Z. P h y s . C h e m . N.F., 1 1 2 ( 1 9 7 8 ) 1 8 3 . A.B. F a s m a n , S.N. H o v i k o v a , a n d D.B. S o k o l s k y , Zh. Fiz. K h i m . , 40 ( 1 9 6 6 ) 5 5 6 . M.W. Breiter, J. E l e c t r o a n a l . C h e m . , 101 ( 1 9 7 9 ) 3 2 9 . F . G . Will a n d C.A. K n o r r , Z. E l e c t r o c h e m . Bet. B u n s e n g e s , P h y s . C h e m . , 6 4 ( 1 9 6 0 ) 2 5 8 . B.J. B e n D a n i e l a n d F . G . Will, J. E l e c t r o c h e r n . Soc., 114 ( 1 9 6 7 ) 9 0 9 . J . O . C l e w e l e y , J . F . L y n c h a n d T.B. F l a n a g a n , J. C h e m . Soc. F a r a d a y I, 73 ( 1 9 7 7 ) 4 9 4 . M.W. Breiter, J. E l e c t r o a n a l . C h e m . , 65 ( 1 9 7 5 ) 6 2 3 .