The hydrogen electrode reaction mechanism on palladium and its relevance to hydrogen sorption

Surface Technology, 4 ( 1 9 7 6 ) 277 - 290

277

© 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 H Y D R O G E N E L E C T R O D E REACTION MECHANISM ON PALLADIUM AND ITS RELEVANCE TO H Y D R O G E N SORPTION*

M. E N Y O a n d T. M A O K A

The Research Institute for Catalysis, Hokkaido University, Sapporo (Japan) ( R e c e i v e d D e c e m b e r 1, 1 9 7 5 )

Summary The mechanism of the hydrogen electrode reaction on palladium in 2N H2SO4 has been investigated mainly through polarization behaviour and analysis of overpotential decay transients. The V o l m e r - H e y r o v s k y mechanism is concluded with the first step a b o u t 20 times more rapid than the second step. Hydrogen diffusion in solution away from the electrode was kinetically important only on highly activated electrodes. Relevance of the mechanism to hydrogen sorption into palladium during cathodic electrolysis is discussed.

1. Introduction Some metals placed in aqueous solution, or in a gas phase with an appreciable degree of humidity, may absorb hydrogen. Occasionally, the hydrogen dissolved causes changes in the mechanical properties of the metal, typically the phenomenon called hydrogen embrittlement. It is widely accepted that this is connected with the hydrogen electrode reaction (HER) taking place on the surface, usually coupled with metal corrosion. There may be two possible sources of hydrogen, hydrogen molecules which are products of the HER, and hydrogen adatoms or the like which are reaction intermediates; it is believed that the latter are more condUcive to hydrogen absorption. In this case, the intermediate must be supplied from H2 or H÷B (B = H 2 0 or OH- ) and, accordingly, one has to look at the p h e n o m e n o n of hydrogen absorption from a kinetic rather than an equilibrium point of view. The relation between hydrogen absorption and the kinetics of the H E R has been discussed [1]. Briefly, the (maximum) activity of hydrogen absorbed in a metal is to be related to the activity of the hydrogen intermediate of the HER, presumably the hydrogen adatom H(a). In the steady state of the

*This p a p e r was p r e s e n t e d at t h e C h e m i c a l Society, F a r a d a y Division, discussion o n H y d r o g e n in Metals, h e l d at B i r m i n g h a m University (Gt. Britain), J a n u a r y 5 - 7, 1976.

278 -~(mV) - ~ (mY) 6

(b)

80

~On

loff

(0) 60

--!i.... -i}:ii-iiii .....

40 20 0

, 0

I

2

,t (rain) 3

0

t (sec] o

{o

20

30

Fig. 1. T y p i c a l g a l v a n o s t a t i c c a t h o d i c t r a n s i e n t s : (a) at low c u r r e n t density, i = 3.29 × 10 - 5 A / c m 2 ; ( b ) at high c u r r e n t d e n s i t y , i = 3.1 x 10 - 3 A / c m 2. 2N H2SO4, 25 °C, e l e c t r o d e p o s i t e d Pd e l e c t r o d e ( o n 0.3 m m ~ Ag wire).

HER (but the quasi-equilibrium state of the absorption process), one may express PH(metal) = PH(a)

(1)

where Pn(a) is in turn related to the detailed kinetics of the HER. Palladium has been extensively used in studies of hydrogen sorption in electrochemical systems. Obviously, its high hydrogen solubility and high electrocatalytic activity for the HER renders the metal one of the best for examining the problem in question. Frumkin and Aladjalova [2] reported extensive studies on the HER on Pd, particularly the time course of the overpotential after switching on or off the polarization current on one face of a Pd foil and " d i f f u s i o n " of overpotential to the non-polarized back side of the membrane. They concluded from their observations that n~ in the decay transient (Fig. 1) corresponds to the overpotential c o m p o n e n t for the discharge reaction (the Volmer process), H+B + e -~ H(a) + B

(2)

and n~ to the Nernstian concentration polarization due to accumulation of H2 in the solution in the vicinity of the electrode with simultaneous accumulation of hydrogen absorbed in the Pd. In other words, n~ is understood as being due to an insufficient rate of diffusion of H2 in solution away from the electrode. They considered that H2 is formed by the recombination process (the Tafel process) 2H(a) -~ H 2 but with a negligible c o m p o n e n t of overpotential for this step [3]. The conclusion of slow H2 diffusion was supported later by a number of investigators [4 - 6]. The plot of z~ against the logarithm of the polarization current i is shown in Fig. 2. It gives a slope of approximately 30 mV/decade, in good h a r m o n y with the above conclusion, but it then tends to show a limiting overpotential at high current density.

(3)

279

(mY)

/[~

3OO .....

Pt

250 200 150

/

/

-2

-I

I

I00 5O Iogi

0 -5

-4

-3

0

i

(h/cm z)

Fig. 2. P o l a r i z a t i o n c u r v e s o n Pt a n d e l e c t r o d e p o s i t e d activation).

Pd e l e c t r o d e s (Pt, a f t e r a n o d i c

Vetter [7] takes the appearance of a limiting V~ as an indication that the process responsible is n o t hydrogen diffusion but the Heyrovsky process of H2 formation: H(a) + H÷B + e-~ H2 + B

(4)

He demonstrated t h a t the behaviour can quantitatively be accounted for on the basis of the equation derived for the Volmer-Heyrovsky mechanism• On the other hand, others who maintain the diffusion theory suggest that such a " s a t u r a t i o n " overpotential is due to saturation of hydrogen in Pd: there would be a practical limit on the hydrogen concentration, C~(pd), in Pd and hence a limiting value of ~ , which would be expressed as - ~ = (1/f)ln(c~(pd),eq)

(5)

where f - F/RT and CH(pd),eq is the concentration in a system at equilibrium with 1 arm hydrogen. It is intended in the present work to improve our understanding of hydrogen sorption in Pd. The work mainly concerns the elucidation of the HER mechanism on Pd through polarization studies and analysis of overpotential decay transients. The relevance of the mechanism to hydrogen absorption into Pd will then be discussed briefly.

2. Experimental Pd foil and Pd electrodeposits on Ag wire were used as the test electrodes. The foil used was thin ( ~ 1 0 um) so that the time constant of the hydrogen

280 71 (mY) 80

60

40

20

o

;

,'o

,;

2'°

t (sec)

Fig. 3. D e p e n d e n c e of overpotential decay upon polarization time; electrodeposited Pd, i = 0.636 m A / c m 2.

absorption process would not be too long. The electrodeposition (~0.8 C/cm 2 at 20 p A / c m 2) was carried out from a dilute (~1 × 10 4 mol) PdSO4 solution in 1N H2SO4. The polarization behaviour was studied in 2N H2SO4 and the temperature was kept at 25 °C by means of a water thermostat, except for the temperature dependence studies. The cell and electrodes were cleaned with hot chromic acid mixture overnight or, in the cases with Ag substrate wires, with hot 2N NaOH. The hydrogen was purified by a Pd/Ag membrane hydrogen purifier and introduced into the cell via a liquid N 2 trap followed by a trap containing the same solution as used in the experiment. The overpotential transients were followed by an ordinary oscilloscope, a chart recorder, or a transient recorder (Biomation 8100) with a preamplifier.

3. Results and discussion 3.1. Overpotential c o m p o n e n t 7~ 3.1.1~ 71 versus 71 As already reported in the literature [2], .~1 (in the build-up curve) and 7~ (in the decay curve) are nearly equal to each other at low current densities (Fig. l(a)), but nl is definitely larger than n~ at high current densities (Fig. l(b)). It is suggested in the literature [8, 9] that this indicates an increase in the catalytic activity of the Pd electrode for the Volmer process, probably due to lattice expansion of the Pd caused by dissolution of hydrogen in the Pd. This interpretation is plausible, as seen in Fig. 3, where 7] decreases with duration of the polarization. However, this cannot be a complete interpretation because the variation n] with time appears to be discontinuous at t = 0. It is likely that the difference between 71 and 7~ is partly caused by the fact that 71 must have sufficient magnitude to meet the applied current, whereas this is not required of 7~.

281 2

3

x\ x

~"XXx~xx x

I O0 ~

0

-4

i

I

-3

-2

"N,

i

-I

log t

Fig. 4. T e s t o f e q n . ( 6 ) f o r t h e V o l m e r p r o c e s s ; e l e c t r o d e p o s i t e d

3.1.2. Decay

Pd, i = 0 . 1 1 8 A / c m 2.

transient

It is generally argued among various investigators that 7~ ,corrected for the ohmic pseudo-overpotential, is the overpotential component corresponding to the Volmer process. This view is well supported in this work through an analysis of the initial part (~10 msec) of the overpotential decay. Assuming that this decay represents discharge of the electrode double layer capacitance C through the Volmer process, viz. d 7 / d t = I i t/C, and that the rate of discharge during decay is the same with the steady state current at the corresponding 7 value, viz. 7 = a - - b log I i J, we readily obtain (7 is negative for the cathodic polarization } 77 = a + b log (10 ('0-a)/b + ( 2 . 3 0 3 / b C ) t }

(6)

where a = b log io, v, b is the Tafel slope, io,v is the exchange current density of the Volmer process, and 7o is the steady state overpotential before the current interruption. The plot of eqn. (6) is shown in Fig. 4. At relatively large values of t. the - - ~ vs. log t curve is linear with a slope b = 78 mV, which compares well with the slope of the Tafel line at a moderate overpotential region (cf. Fig. 2). Also, the plot of the whole logarithmic term with those values of b and i0,v evaluated from the Tafel line gives an excellent line, yielding a slope , and intercept values which are close to those used in calculating the term. The interpretation of ~?~ can therefore be taken as satisfactory. This also indicates, in the high overpotential region in which the analysis was conducted, that the HER is practically controlled by the Volmer process. Analysis presented below indicates that the Volmer pr.ocess is relatively rapid; io,V is roughly 20 times the i o of the overall HER. That the Tafel line of 120 mV slope at high current densities may appear in spite of such a value of io,v has been discussed elsewhere [1, 10].

282

3. 2. Overpotential component 7?2 3.2.1. Diffusion of hydrogen across the Brunner-Nernst layer 3.2.1.1. Comparison with active Pt electrode. It is repeatedly proposed in the literature that the overpotential c o m p o n e n t 72 is due to slow hydrogen removal f r o m the electrode by a diffusion process. This is probably certain for electrodes of high electrocatalytic activity, e.g. palladized Pd [5], but n o t necessarily so for Pd of ordinary activity. This is particularly i m p o r t a n t in this work where we hope d to investigate the HER mechanism and hence we preferred to e m p l o y electrodes with ordinary degrees of activity. The t h e o r y can be tested first by comparison of 7~ with the analogous quantity obtained on highly activated Pt (or Pd) electrodes. Although the time constant o f the overpotential decay on Pt is much shorter than on Pd, because there is no large hydr oge n reservoir as with Pd, one can still distinguish 72 in the time region below 1 sec. With a Pt wire electrode of nearly the same dimension as the Pd-deposited Ag wire, it was observed that ~ after anodic activation was much smaller than on Pd (Fig. 2). The decay transient of 7~ on Pt was analysed. The decay with time of c o n c e n tr atio n Co o f hydrogen in the vicinity of the electrode is [ 11] c0=c~+-~ff-,=0

( 2 n + 1 ) 2 exp

452

t

where c o is the initial value of c0, c~ is the bulk concentration, D is the diffusion coefficient and 5 is the thickness of the diffusion layer. Neglecting terms higher than the second and employing a Nernst equation for a concentration cell to express 72 in accordance with the present model, we obtain 8c o D~ 2 ln{exp(--2fT~) -- 1} = In -- t (7) ~2C~

4~ 2

Application of eqn. (7) to the decay transient generally yielded satisfact o r y linearity. Also, the value of 5 ~ 0.005 cm (Fig. 5) obtained f r o m the slope can be accepted as reasonable [ 12] . Hence, it is concluded t hat 72 on the active Pt electrode represents (the m a x i m u m limit of) the rate of hydr oge n diffusion across the B r u n n e r Nernst layer under the present experimental conditions. It t hen follows t hat 7~_ on Pd, which is much larger, c a n n o t be due to the hydrogen diffusion process.

3. 2.1.2. Heat of activation. The t e m p e r a t u r e dependence studies indicated that the heat o f activation corresponding to 7~, as obtained from the polarization resistance, is roughly 7 kcal on ordinary Pd electrodes, but decreases to as low as 3 kcal on a highly active Pd electrode obtained by a strong electrodeposition current (Fig. 6). The latter value is reasonable for the diffusion process but the f or m e r value is hardly so.

283 log ( I / R )

-/0

-15 (xlO-3cm) × x

x

X X

-20

X

X

-J3

I

-2

I -I

I 0

log i (Alcnn z )

3

I 3.,5

I(llT]xl03 4.0

Fig. 5. Dependence of thickness of Brunner-Nernst layer upon polarization current; Pt wire electrode (0.3 mm ¢), 2N H2SO4, 25 °C. Fig. 6. Arrhenius plot of reaction admittance corresponding to ~2; electrodeposited Pd, R in ~ cm2. 3.2.1.3. Limiting value o f ~ . This experimentally observed p h e n o m e n o n indicates a saturation value of Co/C~ at high polarization currents. This has often been explained in connection with limiting concentration of hydrogen dissolved in Pd. However, this view is in doubt since what directly determines ~ in the diffusion theory is Co in the solution, and not the hydrogen concentration in Pd, and also Co would increase with increase of polarization current irrespective of attainment of the hydrogen concentration limit in Pd. The above arguments altogether suggest that ~ , at least its major part, should be attributed to an electrocatalytic process taking place on the electrode surface.

3. 2. 2. Reaction control 3.2.2.1. The Volmer-Tafel mechanism. If the formation of H2 from 2H(a) is not sufficiently rapid [13, 14], it may cause appreciable values of overpotential to remain at the beginning of the overpotential decay, viz. 77~. Then, ~ will decay with time according to the relation -

-

--2F 2 dnH(Pd) - - d t A /O'T I ( aaH(a),eq l l ( a ) ) -- 1 I -

-

(8)

where nH(Pd) is the number of moles of H(Pd), A is the electrode surface.area, all(a) is the activity of H(a) and /O,Wis the exchange current density of the Tafel process.

284

The i s o t h e r m of h y d r o g e n a b s o r p t i o n in Pd is r e p o r t e d by F r u m k i n and his c o l l a b o r a t o r s to be logarithmic [15] : CH(Pd) = Cn(ed),l + K log P ( c m 3 H 2 / c m 3 Pd, P in atm)

(9)

with Cn(Pd),1 = 956 and K = 68 (by i n t e r p o l a t i o n o f their data to 25 °C). Stackelberg and Bischoff [9] also r e p o r t e d the same f u n c t i o n a l f o r m with C H ( P d ) , 1 = 850 and K = 63. We will e m p l o y this empirical relation in the following analysis. F/H(Pd ) in half thickness l o f Pd foil is

Al

273 T

r/H(Pd) -- 2.24 × 104

CH(Pd) ---- ]~lCH(Pd)

(moles)

or, with eqn. (9),

(Kkl/2.303) lnP

l/H(Pd) = F/H(Pd), 1 +

(10)

When the a b s o r p t i o n and a d s o r p t i o n equilibria are b o t h established, PH:

= 2PH(Pd) = 2PH(a)

or

lnP = 21n(aH(Pd)/aH(Pd),eq) = 2 ln(aH(a)/aH(a),eq)

(11)

where the subscript " e q " signifies equilibrium at 1 arm. A c c o r d i n g l y , f r o m the last t w o equations, rimed) = nH(ed)n + 2 (Kkl/2.303) ln(aH(a)/an(a),eq)

(12)

We shall assume t h a t such equilibria are m a i n t a i n e d during the V~_d e c a y . We also assume t h a t the V o l m e r process is practically in equilibrium during the d e c a y ; o n l y a very small a m o u n t o f i o n i z a t i o n of H(a) w o u l d be n e e d e d to adjust the e l e c t r o d e p o t e n t i a l t o f o l l o w the slow variation o f all(a). We t h e n have F

aH(a)/aH(a),eq = exp(--f~ 2 )

( 13 )

F r o m eqns. (12) and {13), r/H(Pd ) = k l C H ( P d ) , 1 - -

2(Kk1/2.303)fi?2

(14)

E q u a t i o n (8) is n o w c o n v e r t e d to 4F 2

Kkl

d~72 -

A

2.303 R T

-

dt

- /OuT {exp(--2fv~) -- 1}

Solving in

I -- e x p ( 2 f ~ , o ) t

1 -- exp(2f~2)

= k 't

(15)

where ~2,o is the initial value o f V~ and k' = 2 . 3 0 3 io,wA/4FKkl

(sec - 1 )

(16)

285 V-T

o x z~ [] ®

20

I 7 6 x IO-S(A/cm z ) I . 7 6 x IO -4 0 . 7 1 x 1 0 -3 I 7 6 x 1 0 -2 7 0 5 x l O -2

- ~ ] (mV)

ox

OR A

180

~ _

5

z~

0X z~

120

I0 Ox ~ o ~o

,o

IO

5

[ t (sec) 20

15

0

I 2

064 309

( m A / c m z)

3,

3077

2

Q L rO

0

I

20

L

50

I

t ( sec)

40

Fig. 7. Test o f eqn. (15) for t h e V o l m e r - T a f e l m e c h a n i s m : 1 - - exp(2f~?'2,0) YV-T -= In 1 --

exp(2fn~,o)

Fig. 8. S u p e r - p o l a r i z a t i o n behaviour in c a t h o d i c galvanostatic transients; e l e c t r o d e p o s i t e d Pd.

Equation (15) was tested (Fig. 7). Linearity of the log term vs. t relation was fair but constancy of the slope with variation of polarization current was not very satisfactory as compared with the case of Fig. 9 presented below. The diffusion model can also be analysed analogously. Starting with 2F -

-

A

dnH(Pd ) -

-

dt

C0 - - C~ =

D

-

-

5

employing eqn.(10)and - - 2 f ~ = In Co/C~, and assuming Henry's law for dissolution of H2 in aqueous solution, we obtain In

1 -- exp(2f~,o) _ t 1 -- e x p ( 2 / ~ )

r

where r - klS/Dc~. This equation has the same form as eqn. (15). Thus, although the diffusion model should not be applicable in this case, the diffusion equation closely simulates the V~ decay.

3.2.2.2. The Volmer-Heyrovsky mechanism. Vetter [7] derived the following relation for the Volmer-Heyrovsky process: , l+Mexp(--f~) exp(--f~?2"°) = exp(--f~ ) + M

M = 1 iio,v +io,H) 2- , - / O , H ~),V

(17)

where ~,v and/O,H are the exchange current densities of the processes. Equation (17) assumes a limiting value of ~ at large overpotentials:

+-[r~,~m = In M

(18)

286 which explains the "saturation" of r~,0. The value of io,v/io, H was found from the observation of---02,Um ~ 42 mV by Clamroth and Knorr [4] to be either 10.1 or 0.1. We may exclude io,v/io, H = 0.1 because, if this is the case, it can be shown that the activity of the hydrogen adatom, and hence the a m o u n t of hydrogen absorption in Pd, should d e c r e a s e with increase of cathodic overpotential [ 1 ] ; this contradicts the experimental observations. These authors also demonstrated a good fit of eqn. (17) with the experimental 72,0 vs. log i relation employing io,v/io,H = 10.1. The V o l m e r - H e y r o v s k y mechanism was supported in this work for the following reasons. (a) The 72,0 vs. log i relation, including its limiting behaviour, was roughly explained by eqn. (17), with io,v/io, H ~ 20. (b) "Super polarization" was observed during cathodic galvanostatic transients at high current densities (Fig. 8); such a behaviour is expected in this mechanism [7, 16]. (c) Interpretation of the ~ decay curve on the basis of this mechanism was found to be satisfactory (see below). Decay transients of rT~ are derived as follows. Based on io,v/io, H ~ 20, we may take the Volmer process to be practically in equilibrium during the r~ decay. Then, 2F dnH(Pd) { "H(a) ) A d~-- - i0'H exp ( - - a f ~ ) -- exp ((1 -- a ) f ~ } \ aH(a),eq The solution for a = 1/2, employing eqns. (12) and (13), is In

(exp(--frT~/2) + 1} {exp(--f~,0/2) -" 1} {exp(--f~/2) -- 1} (exp(--f~,0/2) + 1}

(19)

+

+2 (tan-1 exp (--fv~_/2) - - t a n - 1 exp (--fr~'2,o/2)} = k ' t

(20)

where k' is the same as in eqn. (16), except that i0, T is n o w replaced by iO,H. A typical plot of eqn. (20.) is shown in Fig. 9. The linearity was fair and, in contrast with Fig. 7, the slope was maintained constant, independent of variation of the polarization current, over a very wide range. The plot deviates from the initial straight line after a b o u t 10 sec. It is considered likely that this is due to neglect of concentration polarization in the analysis, which must not, in fact, be completely negligible. This gives rise to a somewhat higher reverse rate of the Heyrovsky process or to a decreased rate of the ~ decay. This will have some importance, especially at lower values of rT~/~,o or at later portions of the decay. 3. 3. H y d r o g e n o v e r p o t e n t i a l a n d h y d r o g e n a b s o r p t i o n 3. 3.1. A f f i n i t y d i s t r i b u t i o n a m o n g various p r o c e s s e s

As mentioned in the Introduction, eqn. (1), the equilibrium activity of hydrogen in Pd is determined by the steady state activity of H(a) which is in turn determined by the kinetics of the HER'. If the HER occurs through the sequence

287

o I 7 6 x 1 0 -5 (,&/cm 2) x I 7 6 x r O -4 ,,, 0 7 t x l O "3 ® 7 0 5 x l O -2

20

0 o~

I0

0

/ 0

A

5

o"

I

I

I0

15

I t (sec) 20

Fig. 9. T e s t o f e q n . ( 2 0 ) f o r t h e V o l m e r - H e y r o v s k y

mechanism

( e x p ( - - f v ~ / 2 ) + 1} (exp(--f~2,0/2) -- 1} Y V _ H ~ In

{exp(--f,72,0/2)+ I}

( e x p ( - - f ~ / 2 ) - I}

+2 {tan - 1 e x p ( - - f n ~ / 2 ) -

(+ e)

t a n - 1 exp(--fT?~.0/2))

(+ H+ + e)

H +

]

+

Volmer

> H(a)

+-----Ag~

~I

> H2,0 Heyrovsky

~H 2 diffusion

H(Pd) >l,

,

I

>E

I

--Ag2

I

all(a) will be determined by the combined processes (i.e. of Heyrovsky and diffusion). We may write

--Ag2 =

(PH(a) -I-pH + + ~te - - p H 2 , ° ) + (PH2 ~ - - P H 2 , ~ )

---- ]2H(a) + ,t/H* + /2e - - /2H2,~

but at equilibrium 0 = PH(a),eq + PH ÷ + Pe,eq - - P H 2 , ~

Hence, defining p~ - -

~te,eq = - -

all(a)

--Ag 2 = RT I n - -

F~

F~

(2

aH(a),eq

Therefore, regardless of the relative magnitudes of the free energy changes of the Heyrovsky and diffusion processes, all(a) is simply related to Ag2. In other words, in the discussion of steady state absorption of hydrogen in Pd, we may treat the case as if the diffusion is included in the Heyrovsky process. The problem of hydrogen absorption into Pd in the electrochemical system can be visualized by considering an analogy to a gas phase system.

288 We consider a hydrogen pressure P which is hypothetically in equilibrium with the hydrogen overpotential [1]. Briefly, one may say that p H 2 = 2pma ), or eqn. (11). Hence, from eqn. (21), --Ag 2 = ( R T / 2 ) l n P - - e~?

(22)

As discussed previously [ 1], we have 2raFT? Ag2 - - - , m+l

m = Agl/Ag2

(23)

and m+l

= 2f~ ( l n

meq+exp(f~) meq + exp(---fv)

)-1

(24)

where rr~q -~ ( Ag2/ Agl )eq = io,v/io,H

(25)

Hence Ag2 RT

- 2fv--ln

m~q + exp(f~) meq + exp(--f~)

Introducing this relation into eqn. (22), we obtain P= (meqexp(--f~)+l)2 rr~q + exp(--fv)

(26)

At sufficiently high cathodic overpotentials at which exp(--fv ) > ) rn~q > > exp(fv ) P attains a limiting value [1] : Pure = (meq) 2

(27)

According to Vetter [7], at high overpotentials --fr~jim-~ in ~-

1)f

meq + - meq

(28)

Hence, for the value --~,lim ~ 60 mV as observed above, we obtain m~,q ~ 20 or P ~ 400 atm. This means that the hydrogen concentration in Pd during strong electrolysis would be such as is in absorption equilibrium ~vith an hydrogen atmosphere of approximately 400 atm. It should be noted that this is higher than the value one anticipates directly from the ~.um value on the basis of a Nernstian equation. 3. 3.2. C o m p a r i s o n w i t h o t h e r metals The mechanism of HER concluded above on Pd is different from that on other catalytically active metals, Pt, Rh, Ag, Au and Ni, where the VolmerTafel mechanism was concluded from deuterium tracer studies. (The same mechanism might also be applicable to Fe [1] .) For the Volmer-Tafel mechanism, it can be shown that the v~ component represents the affinity value of the Tafel process and hence the hydrogen

289 c o n c e n t r a t i o n in t h e metal during the electrolysis w o u l d reach such a m a g n i t u d e as is in e q u i l i b r i u m with a P value which is d i r e c t l y calculable f r o m ~ , 0 b y means o f a Nernstian relation. This means that, at the same ~2.0 values o n Pd and o n the o t h e r metals q u o t e d above, the Pd is m o r e hydrogen-absorbing.

4. S u m m a r y T h e m e c h a n i s m o f the h y d r o g e n e l e c t r o d e r e a c t i o n ( H E R ) o n Pd p r e p a r e d b y e l e c t r o d e p o s i t i o n o n Ag wire was investigated mainly t h r o u g h observations o f p o l a r i z a t i o n b e h a v i o u r and analysis o f the galvanostatic o v e r p o t e n t i a l transients. T h e initial (< 10 msec) rapid o v e r p o t e n t i a l decay, 7'1, u p o n t e r m i n a t i n g t h e p o l a r i z a t i o n c u r r e n t was satisfactorily e x p l a i n e d o n the basis o f the V o l m e r process, H ÷ + e -* H(a). T h e rest o f the o v e r p o t e n t i a l , ~ , d e c a y s slowly. On highly e l e c t r o c a t a l y t i c a l l y active Pd or Pt electrodes, ~ is a t t r i b u t e d t o a slow diffusion process o f H2 away f r o m the e l e c t r o d e , as o f t e n suggested in the literature, b u t this s h o u l d n o t be the case o n Pd electrodes with o r d i n a r y m a g n i t u d e s o f activity. Thus, t h e process responsible for ~ o n such e l e c t r o d e s was c o n s i d e r e d to be r e a c t i o n c o n t r o l l e d . The V o l m e r - H e y r o v s k y ( V - H ) m e c h a n i s m (H ÷ + e -* H(a), H(a) + H ÷ + e -* H2) was p r o p o s e d on the basis o f the following evidence. ( l ) A saturation b e h a v i o u r o f ~ at high c u r r e n t densities c o u l d be a c c o u n t e d for w i t h an e q u a t i o n derived b y V e t t e r for the V - H mechanism. (2) " S u p e r p o l a r i z a t i o n " was observed during build-up o f c a t h o d i c galvanostatic transients at high c u r r e n t densities, as e x p e c t e d with this mechanism. (3) The r e q u i r e m e n t s o f this m e c h a n i s m were well satisfied b y the d e c a y transients. The relevance o f the H E R m e c h a n i s m to h y d r o g e n a b s o r p t i o n during c a t h o d i c p o l a r i z a t i o n o f Pd was discussed. It was s h o w n t h a t for t h e V - H m e c h a n i s m the limiting h y d r o g e n pressure, P~m, which is h y p o t h e t i c a l l y equivalent t o the h y d r o g e n o v e r p o t e n t i a l , is related to the ratio o f the e x c h a n g e c u r r e n t densities o f the V o l m e r and H e y r o v s k y processes, meq - io.v/io,H, by Pnm = rn~. On the Pd used in this w o r k meq was evaluated t o be a b o u t 20 f r o m the s a t u r a t i o n value o f ~ ~- 60 mV. Thus, we o b t a i n e d Pnm -~ 400 atm. T h e figure would, h o w e v e r , be m u c h smaller o n e l e c t r o c a t a l y t i c a l l y active Pd, such as palladized electrodes.

References 1 2 3 4

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