VoL. 134, No. 8
SACV MEASUREMENT
OF THE POLARIZATION
23. C. Kato, J. E. Castle, B. G. Ateya, and H. W. Pickering,
ibid., 127, 1897 (1980).
24. C. Kato and H. W. Pickering, ibid., 131, 1219 (1984).
25. C. Kato, H. W. Pickering, and J. E. Castle, ibid., 131,
1274 (1984).
26. L. M. Callow, J. A. Richardson, and J. L. Dawson, Br.
Corros. J., 11, 123 (1976).
27. F. L. LaQue, J. Am. Soc. Nav. Eng., 29, 54 (1941).
28. H. H. Uhlig, Trans. Electrochem. Soc., 85, 307 (1944).
29. H. H. Uhlig, Z. Elektrochem., 62, 700 (1958).
30. P. F. Effertz, W. Fichte, and P. F o r c h h a m m e r , VGB
Kraftwerkstechnik, 2, 54 (1974).
31. W. C. Stewart and F. L. LaQue, Corrosion (Houston), 8,
259 (1952).
32. P. S. Keir, M. J. Pryor, and P. R. Sperry, This Journal,
114, 777 (1967).
33. S. R. J. S a u n d e r s and M. J. Pryor, ibid., 115, 1037
(1968).
34. R. F. North and M. J. Pryor, Corros. Sci., 8, 147 (1968).
35. H. Gabel, J. A. Beavers, J. B. Woodhous, and E. N.
Pugh, Corrosion (Houston), 32, 253 (1976).
36. J. Holliday and H. W. Pickering, This Journal, 120, 470
(1973).
RESISTANCE
1957
37. C. Wagner and W. Traud, Z. Elektrochem., 44, 391
(1938).
38. M. Stern, Corrosion (Houston), 14, 440t (1958).
39. W. C. Stewart and F. L. LaQue, ibid., 8, 259 (1952).
40. C. Kato, M.S. Thesis, The P e n n s y l v a n i a State University, University Park (1978).
41. J. M. Popplewell, R. J. Hart, and J. A. Ford, Corros.
Sci., 13, 295 (1973).
42. H. Hack, H. Shih, and H. W. Pickering, in "Surfaces Inhibition, and Passivation," E. McCafferty and R. J.
Brodd, Editors, p. 355, The Electrochemical Society
S o f t b o u n d P r o c e e d i n g s Series, P e n n i n g t o n , N J
(1986).
43. H. Hack and H. W. Pickering, In preparation.
44. D. D. Macdonald, " T r a n s i e n t T e c h n i q u e s in Electrochemistry," pp. 12, 295, P l e n u m Press, N ew York
(1977).
45. E. Gileadi, E. Kirowa-Eisner, and J. Penciner, "Interfacial Electrochemistry," p. 75, Addison-Wesley Publishing Co., Inc., L o n d o n (1975).
46. J. O'M. Bockris and A. K. N. Reddy, "Modern Electrochemistry," p. 1028, P l e n u m Press, New York (1970).
47. H. Shih and H. W. Pickering, To be published.
Adsorbed Hydrogen on Iron in the Electrochemical Reduction of
Protons
An FTIR Study
J. O'M. Bockris,* J. L. Carbajal,* B. R. Scharifker,* and K. Chandrasekaran*
Department of Chemistry, Texas A & M University, College Station, Texas 77843
ABSTRACT
This paper reports an investigation of the adsorption of hydrogen on iron during the hydrogen evolution reaction. The
solution pH was 8.4. The investigation was carried out by means of FTIR spectroscopy. Fe-H vibrations were identified at
2060 and 980 c m - ' ; and were compared with the values of estimates for stretching and bending vibrations in Fe-H, respectively. The peak intensities varied with electrode potential. Assuming that the area u n d er the peak at 2060 c m -~ is proportional to the hydrogen coverage, 0s, on iron, a slope of F/6RT was obtained for the plot of 0 vs. "q where ~ is the overpotential. C o r r e s p o n d i n g e x p e r i m e n t s with D~O showed peaks w h i ch are relatable to those for h y d r o g e n in an e x p e c t e d
manner. An indirect estimate of the OHvalue suggested this to be about 0.3 at -0.9V. The results are consistent with a slow
discharge-atomic combination route for hydrogen evolution.
The d e t e r m i n a t i o n of the surface c o n c e n t r a t i o n on noble metals of h y d r o g e n taking part as an i n t e r m e d i a t e in
t he e l e c t r o c h e m i c a l r e d u c t i o n of p r o t o n s to h y d r o g e n
m o l e c u l e s has b e e n m e a s u r e d by c o u l o m e t r i c m e t h o d s
s i n c e a b o u t 1955 [cf. B r e i t e r et al. (1-3)], and m o r e recently by Chevillot et al. (4) and Woods et al. (5).
On n o n - n o b l e metals, t h e d e t e r m i n a t i o n of the interm e d i a t e c o n c e n t r a t i o n of h y d r o g e n radicals [ i m p o r t a n t
in r e s p e c t to considerations of r a t e - d e t e r m i n i n g step and
p a t h w a y (6)] is difficult to carry out by c o u l o m e t r i c methods b ecau s e of co-dissolution of the m e ta l substrate (7).
In the p res en t paper, it is s h o w n that infrared spectrosc o p y leads to t h e i d e n t i f i c a t i o n of a d s o r b e d H and D as
i n t e r m e d i a t e s in H~ and D~ evolution; and allows a rough
e s t i m a t e of f r a c t i o n a l o c c u p a n c y of th e e l e c t r o d e surface.
Earlier w o r k in the e x a m i n a t i o n o f the concentration of
hydrogen on non-noble metals.--Bockris and Kita (8) derived a relation b e t w e e n p s e u d o c a p a c i t a n c e and (}. They
c o n c l u d e d that at the corrosion potential of iron for
pH = 4, t h e h y d r o g e n c o v e r a g e was a b o u t 3%. K i m and
Wilde (9) applied the d o u b l e pulse galvanostatic m e t h o d
(10) to t h e d e t e r m i n a t i o n of h y d r o g e n c o v e r a g e on iron
o v e r a r a n g e of p o t e n t i a l s , o b t a i n i n g 5-12% c o v e r a g e at
pH = 4. Correspondingly, Flitt and Bockris (7, 11) found
1-10% coverage within the same range of potentials using
t he same m e t h o d but for lower pH ranges.
D o u b l e pulse galvanostatic m e a s u r e m e n t s on iron are
subject to large uncertainties c o n n e c t e d with t h e correc*Electrochemical Society Active Member.
tion for the hydrogen which originates in the interior of
the iron but takes part in the dissolution. An alternative
approach is therefore desirable.
Experimental
The w o r k i n g electrode used was a highly polished iron
rod (99.9995% pure), 1 cm diam and 3 cm long, c o n n e c t e d
to a c o p p e r rod 1 cm diam and 10 cm long clad with heat
s h r i n k a b l e Teflon tube. Th e e x p o s e d area was 0.78 c m ~.
T h e e l e c t r o d e surface was p o l i s h e d with 0.25 txm diam o n d paste.
Th e c o u n t e r e l e c t r o d e was a Pt wire 1 m m d i a m and 5
c m long. Th e cell u s e d for t h e F T I R s t u d y is d e s c r i b e d
e l s e w h e r e (12). T h e w o r k i n g e l e c t r o d e p o t e n t i a l was
m e a s u r e d against a m e r c u r y / m e r c u r o u s sulfate r ef ere nc e
el ect r o d e p r o v i d e d with a Vycor tip. Potentials are on the
NH scale.
T h e s o l u t i o n s w e r e p r e p a r e d f r o m p y r o l i t i c a l l y distilled water. T h e w a t e r had a c o n d u c t a n c e of less t h a n
10 -6 m h o s c m - ' . Th e s o l u t i o n c o n s i s t e d of 0.0375M sodium tetraborate anhydrous, ultrapure, mixed with
0.15M o r t h o r h o m b i c boric acid. Th e m i x t u r e m a d e a
buffer solution o f p H = 8.4. D e u t e r i u m oxide, 100% isotopic purity, was used in t h e e x p e r i m e n t . Before measurements, the solution was further purified by pre-electrolysis at 0.1 m A / c m ~ for at least 24h, m a i n t a i n e d u n d e r N2
bubbling.
Nitrogen gas was purified by passing it t h r o u g h a colu m n c o n t a i n i n g c o p p e r m e t a l t u r n i n g s h e a t e d to a b o u t
350~176
and t h e n t h r o u g h two s u c c e s s i v e traps filled
with m o l e c u l a r sieves.
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1958
J. E l e c t r o c h e m . Soc.: E L E C T R O C H E M I C A L
A potentiostat driven by a waveform generator and an
X - Y r e c o r d e r w e r e u s e d for c o n t r o l l i n g a n d m o n i t o r i n g
the electrode potential. Synchronization between applic a t i o n o f t h e p o t e n t i a l s t e p a n d t h e b e g i n n i n g o f d a t a coll e c t i o n w a s m a d e b y t a p p i n g a T T L (logic gate) s i g n a l accompanying the beginning of current data collection at
t h e D i g i l a b A/D c o n v e r t e r a n d a p p l y i n g t h e s i g n a l to trigg e r t h e a p p r o p r i a t e p o t e n t i a l s t e p at t h e p o t e n t i o s t a t .
A Digilab FTS-20E FTIR spectrometer
system
equipped with a Data General Nova 4 computer was
u s e d . A r e f l e c t i o n a t t a c h m e n t ( H a r r i c k M o d e l V R A S5D)
w i t h r e t r o m i r r o r a c c e s s o r y ( R M A - 4 D G ) w a s u s e d to
g u i d e t h e i n c i d e n t I R b e a m to t h e e l e c t r o d e s u r f a c e a n d
t h e r e f l e c t e d b e a m b a c k to t h e d e t e c t o r . T h e a n g l e of inc i d e n c e w a s 40 ~ A M o l e c t r o n I G P Z 2 8 ( C a m b r i d g e P h y s i cal S c i e n c e s ) g o l d g r i d p o l a r i z e r w a s u s e d to p l a n e p o l a r ize t h e b e a m . T h e F T I R d e t e c t o r w a s a m i d r a n g e I R f a s t
d e t e c t o r , w i t h m e r c u r y c a d m i u m t e l l u r i d e 77 K s o l i d state units.
The cell was washed overnight with chromic mix. It
was then washed and rinsed with conductivity water and
f i l l e d w i t h b o r a t e b u f f e r s o l u t i o n to p r e t r e a t t h e e l e c trode for lh at -0.740V NHE. The procedure assures an
i r o n s u r f a c e f r e e o f o x i d e s (13). M e a n w h i l e , t h e I R d e t e c tor was stabilized and cooled off with liquid nitrogen.
A f t e r l h o f e l e c t r o d e p r e t r e a t m e n t at - 0 . 7 4 0 N H S , t h e solution was replaced and a new (pre-electrolyzed) solution
added. The electrode was then positioned in the cell
c l o s e to t h e I R t r a n s p a r e n t w i n d o w ( Z n S e ) , l e a v i n g a t h i n
film o f s o l u t i o n (1 # m ) b e t w e e n it a n d t h e w i n d o w . T h e
b u b b l i n g o f N2 c o n t i n u e d t h r o u g h t h e e x p e r i m e n t . N2
purging was continuously administered to remove the
outside chamber water vapor and carbon dioxide.
T h e c e l l w a s a l i g n e d to o b t a i n t h e m a x i m u m r e f l e c t a n c e f r o m t h e w o r k i n g e l e c t r o d e . T h e n , ca. 30 r a i n w a s
allowed to purge the external chamber, increasing the
flow o f N2 i n t o t h i s c h a m b e r w h i l e h o l d i n g c o n s t a n t t h e
potential of the electrode (-0.50V NHE).
T h e e l e c t r o d e w a s t h e n p o l a r i z e d at s u c c e s s i v e p o t e n t i a l s i n t h e a d s o r p t i o n r a n g e . A t o t a l o f 500-1000 s c a n s
w e r e c o l l e c t e d at e a c h p o t e n t i a l . All s p e c t r a a t e a c h potential were added and signal averaged to improve the
S / N ratio. T h e s p e c t r a r e p o r t e d r e p r e s e n t t h e d i f f e r e n c e
b e t w e e n t h e s p e c t r a at a g i v e n c a t h o d i c p o t e n t i a l a n d
t h a t at a r e f e r e n c e p o t e n t i a l , - 0 . 5 V N H E (see D i s c u s s i o n
s e c t i o n ) . All s p e c t r a w e r e m e a s u r e d w i t h p a r a l l e l p o l a r i z e d I R r a d i a t i o n (14). T h e r e s u l t a n t s p e c t r a a r e t h u s
termed p-polarized differential spectra, and give inform a t i o n a b o u t t h e a d s o r b e d layer.
Real surface area of the iron electrode.--It is n o t p o s s i b l e to u s e a c o u l o m e t r i c h y d r o g e n d e s o r p t i o n m e t h o d o n
i r o n d u e to c o d i s s o l u t i o n o f t h e s u b s t r a t e (7). A n e s t i m a tion of the roughness factor was made in comparison
with electrodes of known surface area. The roughness
f a c t o r for t h e i r o n e l e c t r o d e p r e p a r e d b y c h e m i c a l v a p o r
d e p o s i t i o ' n m e t h o d w a s d e t e r m i n e d to b e 8.5 u s i n g v a r i o u s m e t h o d s (15). T h e p o l i s h e d i r o n e l e c t r o d e (0.25 ~ m
d i a m o n d p a s t e ) of t h e p r e s e n t e x p e r i m e n t s h o u l d h a v e a
r o u g h n e s s f a c t o r less t h a n t h a t o f t h e c h e m i c a l v a p o r deposited electrode. Metal electrodes prepared by the same
p r o c e d u r e , i.e., p o l i s h e d w i t h d i a m o n d p a s t e , 0.25 ~ m ,
showed a roughness factor of three which was determ i n e d b y c a p a c i t a n c e m e a s u r e m e n t (12). H e n c e , it s e e m s
reasonable to accept a roughness factor of three for the
o x i d e - f r e e i r o n e l e c t r o d e , p r e p a r e d as d e s c r i b e d .
Results
SCIENCE
A u g u s t 1987
AND TECHNOLOGY
I 5x10"3
g
o
<
2400
2000
1600
Wavenumbers
1200
800
Fig. 1. Differential IR spectra in absorbance of Fe-H vibration at
- 0 . 9 V NHE in borate buffer solution.
surface species corresponding
to it up to 0 = 0.5. The relative area of the peaks at 2060 and 980 cm -I are shown in
Fig. 3a and b as a function of electrode potential. The
peak area, A, varies logarithmically with electrode potential a n d t h e s l o p e ~VI6 log A = -0.33.
3. Effect of isotopes on the absorption s p e c t r u m . - - W h e n
a 50% D 2 0 - H 2 0 m i x t u r e w a s u s e d as a s o l v e n t , t h e p e a k
IFi0.2
Fe- H
-0.9 V(NHE)
-0.7
LU
O
Z
<
m
rr
0
o9
m
<
-0.6
-0.5
1. Absorption spectrum of the iron-electrolyte interface
during the hydrogen evolution reaction.--The a b s o r p t i o n
spectrum of iron in borate buffer solution at -0.9V NHE
is s h o w n i n Fig. 1. T h r e e p e a k s : 2060, 1600, a n d 980 c m 1
a r e p r e d o m i n a n t . T w o s m a l l p e a k s at 1450 a n d 1260 c m -1
a r e also o b s e r v e d .
2. Potential dependence of peak heights f o r H-containing s o l u t i o n s . - - T h e a b s o r b a n c e i n t e n s i t y o f t h e v a r i o u s
p e a k s c h a n g e s w i t h e l e c t r o d e p o t e n t i a l (Fig. 2). T h e a r e a
u n d e r a p e a k is p r o p o r t i o n a l to t h e c o n c e n t r a t i o n o f t h e
2300 2200 2100 2 0 0 0
WAVENUMBERS
1 9 0 0 1800 1700
/ c m -1
Fig. 2. Differential IR spectra in absorbance of Fe-H vibration at different potentials in H20 borate buffer solution.
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Vol. 134, No. 8
ADSORPTION
OF HYDROGEN
ON
IRON
1959
1000
9 100
-0.5
~"
T
Z
<
-0,4
f/3
13_
rr
O
Q.
-
10
-
~> -0,2
9
-0.1
I
lOO
I
lOOO
-0.5
-0.6
-0.7
-0.8
-0.9
V/NHE
IR Peak A r e a / A r b i t r a r y Units
Fig. 3. Coverage-potential plot for hydrogen on iron electrode, borate buffer solution (a, left) 2060 cm -~ and (b, right) 980 cm -~
o b s e r v e d a t 2060 c m -1 i n p u r e a q u e o u s s o l u t i o n w a s
a c c o m p a n i e d b y a c o r r e s p o n d i n g p e a k a t 1485 c m 1
(Fig. 4). T h e p e a k a t 1600 c m -1 w a s a c c o m p a n i e d b y a
p e a k a t 1200 c m -1. T h e p e a k at 980 w a s n o t a c c o m p a n i e d
b y a n e x p e c t e d p e a k a t 680 c m - ' ( o u t o f t h e s p e c t r o m e t e r
r a n g e ) (Fig. 4).
t h e p e a k s h i f t s to l o w e r f r e q u e n c i e s ( b a t h o c h r o m i c ) , z.e.,
2075 c m -L (21). S i m i l a r l y , P o n s et al. s h o w e d (22) t h a t acet o n i t r i l e s t r o n g l y a d s o r b s o n P t a t all p o t e n t i a l s p o s i t i v e
t o - 0 . 3 V N H E , as w a s e v i d e n c e d b y t h e a p p e a r a n c e o f
t h e C = N f u n d a m e n t a l s t r e t c h a t 2350 c m -1, w h i c h is w e l l
4. Potential dependence of peak height for D-containing solutions.--The r e l a t i v e a r e a o f t h e p e a k a t 1485 c m -1
is s h o w n i n Fig. 5 as a f u n c t i o n o f e l e c t r o d e p o t e n t i a l .
T h i s p e a k a r e a i n t h e p r e s e n c e of D~O v a r i e s l o g a r i t h m i c a l l y w i t h e l e c t r o d e p o t e n t i a l a n d t h e s l o p e o f t h e l o g A-~
p l o t w a s - 0 . 3 4 V d e c a d e 1.
5. Tafel slopes.--The T a f e l p l o t for t h e i r o n - b o r a t e
b u f f e r a t p H 8.4 is s h o w n i n Fig. 6. T h i s T a f e l s l o p e w a s
t h e s a m e b e f o r e a n d a f t e r F T I R e x p e r i m e n t s , i.e., t h e
Z n S e w i n d o w o f t h e D i g i l a b s p e c t r o m e t e r is n o t t h e
s o u r c e o f m a t e r i a l p o t e n t i a l l y a d s o r b e d o n t h e electrodes.
Discussion
Reproducibility ofpeaks.--The r e p r o d u c i b i l i t y o f t h e
p e a k s a t 2060 c m -1 w a s 11 c m -1, p e a k m a x i m u m i n t h e
p r e s e n c e o f H is t h e n r e g a r d e d 2060 -+ 11 c m -1 (cf. R e s u l t s
s e c t i o n a n d Fig. 2). T h e c o r r e s p o n d i n g p e a k m a x i m u m
i n t h e p r e s e n c e o f D~O h a d a r e p r o d u c i b i l i t y o f -+5 c m - ' .
I t is a t 1485 -+ 5 c m i.
The Fe-H(a~s~frequencies.--There a r e n o r e c o r d e d d e t e r m i n a t i o n s o f t h e f r e q u e n c y o f v i b r a t i o n for h y d r o g e n ads o r b e d o n i r o n . H o w e v e r , B a r c l a y (16) d i s c u s s e d t h i s
question and took the Fe-H vibration from inorganic
c o m p l e x e s i n s o l u t i o n (17-18).
Table I reproduces the absorption frequencies for
s o m e i r o n h y d r i d e s i n s o l u t i o n (17). T h e a v e r a g e v a l u e
f r o m t h i s t a b l e is 1885 -+ 25 c m ', o b t a i n e d h e r e f o r h y d r o g e n a d s o r b e d o n i r o n . H o w e v e r , it is k n o w n t h a t as a
result of adsorption, the absorbance maximum may be
a f f e c t e d i n r e s p e c t t o a s h i f t in f r e q u e n c y ( s u r f a c t o c h r o m i s m ) o r a s a c h a n g e i n i n t e n s i t y (19). T h e s h i f t s i n
frequency may be to lower or higher frequencies, and
t h e i r m a g n i t u d e f r o m t e n s to h u n d r e d s o f w a v e n u m b e r s
(2O).
T h u s , for e x a m p l e , S C N - i n s o l u t i o n h a s a p e a k m a x i m u m a t 2150 c m -1, b u t i n t h e a d s o r b e d s t a t e o n p l a t i n u m ,
<:
_
-0.65
-0.5
I
I
t
I
P
1
]
J
1800
1700
1600
1500
1400
1300
1200
1100
Wavenumbers
Fig. 4. Differential IR spectra of Fe-D vibration at different potentials in H20/D20 mixture (1:1 ) containing borate buffer solution.
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J. Electrochem. Soc.: E L E C T R O C H E M I C A L
1960
SCIENCE
AND TECHNOLOGY
A u g u s t 1987
Table I. Survey of iron-hydrogen bond frequencies in complexes a
Complex
-0.5
7Z
~" -0.4
HFe(CO)~ +
t_HFe(CO)(depe)2+ b
t-HFe(N~)(depe)~ +
HFe-(--C2H4--PhPCH2CH2P[Ph]~)-(diphos)
C--H2Fe(PF3)4
t-HzFe(P[OEt]3)4
H2FeN2(PEt~Ph)2
H2Fe(diphos)2C6H5
E
-0,3
9
~/cm1900
1875
1870
1893
1935
1912
1855
1840
-0 2
a This table is an extended version of that published by Barclay,
(16).
b (depe) = Diethylphosphine ethane.
-0 1
10
100
IR Peak Area/Arbitrary Units
Fig. 5. Coverage (IR peak area)-overpotential for deuterium adsorbed on iron.
s h i f t e d t o t h e b l u e f r o m t h e b u l k 2150 c m - ' f u n d a m e n t a l
( h y p s o c h r o m i c ) (24).
K a e s i g a n d S a i l l a n t (17) g a v e t h e I R f r e q u e n c y for several hundred metal hydrides. The stretching frequency
l i e s a r o u n d 1900 -+ 300 c m -1 f o r all t h e t r a n s i t i o n m e t a l
h y d r i d e c o m p l e x e s . I n v i e w o f t h e a b o v e e v i d e n c e (inc l u d i n g t h a t of T a b l e I), t h e a b s o r b a n c e a t 2060 c m ' appears to be consistent with adsorbed hydrogen bonded
to iron.
The frequency of a vibrational transition can be calcul a t e d f r o m f o r c e c o n s t a n t s a n d r e d u c e d m a s s , i.e.
-
vl -
1
~
2~c
V
/
k
[1]
~1
w h e r e vl is t h e w a v e n u m b e r o f t h e a b s o r b a n c e m a x i m u m , c is t h e v e l o c i t y o f light, k a n d g, a r e f o r c e c o n s t a n t
and reduced mass, respectively. When one of the atoms
is i s o t o p i c a l l y l a b e l e d , t h e f o r c e c o n s t a n t r e m a i n s t h e
same for both the compounds. Thus, for an isotopically
labeled compound
~/
k
2~rc V
~
-
i
v2-
[2]
then
tl,2
v2
-
V
[3]
~1
W h e n D~O is u s e d i n s t e a d o f w a t e r , a n a b s o r b a n c e m a x i m u m a t 1485 c m -1 is o b s e r v e d . T h e r a t i o o f f r e q u e n c i e s
-0.5
-0 4
W"
"r -0,3
7
>
m
-0.2
~
9
-0.1
0.0
-7
I
-6
I
-5
I
-4
I
-3
Current Dens~ty/(A/cm ~)
Fig. 6. Current-potential plot for hydrogen evolution reaction on iron
electrode.
(2060/1485 = 1.39) a g r e e s w i t h t h e r e d u c e d m a s s r a t i o o f
F e - D a n d F e - H (1.4).
A further confirmation of the identity of the wave
n u m b e r s o b s e r v e d in t h e e x p e r i m e n t s o f t h i s p a p e r , as
b e i n g a s s o c i a t e d w i t h a d s o r b e d H, (2060 a n d 980 c m -1)
c a n b e m a d e b y c o m p a r i s o n w i t h t h e w a v e n u m b e r s obs e r v e d for h y d r o g e n a d s o r b e d o n o t h e r t r a n s i t i o n m e t a l s
f r o m t h e g a s p h a s e . A b r o a d p e a k c e n t e r e d at 2105 c m -1
h a s b e e n a t t r i b u t e d to t h e s t r e t c h i n g v i b r a t i o n s o f h y d r o gen (from the gas phase) adsorbed on polycrystalline
p l a t i n u m , ( P t - H ) (23). W h e n h y d r o g e n is r e p l a c e d b y
d e u t r i u m , t h e p e a k m a x i m u m s h i f t s (23) t o 1512 c m -1.
The shift in frequency agrees with the frequency calculated based on the relevant reduced masses, and can be
compared with the similar change noted in the present
w o r k . H y d r o g e n a d s o r b e d o n p o l y c r y s t a l l i n e n i c k e l also
h a s a n a b s o r b a n c e m a x i m u m (24) a t 1926 c m -1. T h u s , it
s e e m s r e a s o n a b l e t o a s s i g n t h e 2060 c m -1 a b s o r b a n c e
m a x i m u m to t h e s y m m e t r i c s t r e t c h i n g v i b r a t i o n o f Fe-H.
T h e full w i d t h a t h a l f m a x i m u m for F e - H a b s o r b a n c e at
2060 c m -1 is 350 c m -1. T h i s l a r g e h a l f - w i d t h m a y b e c o m p a r e d w i t h t h o s e o f o t h e r m e t a l h y d r i d e s . T h e full w i d t h
at half maximum for hydrogen adsorbed on platinum
(23) f r o m t h e g a s p h a s e is 170 c m -1. H y d r o g e n b o n d i n g
broadens the absorption spectrum, Thus, a larger value
f o r F e - H i n s o l u t i o n is e x p e c t e d . S i n c e d e u t e r i u m h y d r o g e n b o n d s l e s s t h a n h y d r o g e n , t h e full w i d t h a t h a l f m a x i m u m f o r F e - D is 80 c m 1, i.e., s m a l l e r t h a n t h a t o f Fe-H.
The highest absorbance intensity for terminally
b o n d e d F e - H o b s e r v e d i n t h e p r e s e n t e x p e r i m e n t is
0.018. T e r m i n a l l y b o n d e d P t - H i n t h e gas p h a s e s h o w s a n
a b s o r b a n c e o f 0.2 (23). T h e l o w e r a b s o r b a n c e v a l u e rec o r d e d for F e - H i n t h e s o l u t i o n m a y b e d u e to l o w a n g l e
of incidence used in this investigation.
T h e a b s o r b a n c e m a x i m u m a t 1600 c m i m a y b e d u e to
O - H b e n d i n g v i b r a t i o n s o f t h e w a t e r m o l e c u l e (25). T h e
w e a k a b s o r b a n c e m a x i m a a t 1450 a n d 1260 c m -1 c a n b e
c o m p a r e d w i t h t h e o b s e r v a t i o n s o f S c h a r i f k e r et al. (26)
w h o f o u n d p e a k f r e q u e n c i e s o f 1400 a n d 1150 c m -1 f o r
a d s o r b e d b o r a t e i o n s o n iron.
A r e l a t i v e l y s t r o n g a b s o r p t i o n a t 980 c m -1 m a y b e assigned to asymmetric stretching vibrations of Fe-H.
Asymmetric stretching vibrations of hydrogen adsorbed
o n i r o n i n t h e g a s p h a s e h a s b e e n o b s e r v e d a t 1060 c m -1
u s i n g e l e c t r o n e n e r g y l o s s s p e c t r o s c o p y (27). C o a d s o r b e d w a t e r s h i f t s t h i s a b s o r b a n c e to l o w e r f r e q u e n c i e s .
H e n c e , it is r e a s o n a b l e to a s s u m e t h a t t h e a s y m m e t r i c
s t r e t c h i n g v i b r a t i o n ~ o f h y d r o g e n a d s o r b e d o n i r o n i n sol u t i o n o c c u r a t 980 c m 1. A s y m m e t r i c s t r e t c h i n g v i b r a tions of M-H for multinuclear hydride complexes occur
i n t h i s r e g i o n (17). T h i s 980 c m -1 p e a k d i s a p p e a r s w h e n
D 2 0 is u s e d i n s t e a d o f H~O. A s i n t h e c a s e o f 2060 c m -1
peak, this absorbance maximum must be shifted to
lower frequencies upon deutration. The expected
absorbance maximum based on reduced mass calculat i o n s (28) is 700 c m -1, w h i c h is b e y o n d t h e r a n g e o f t h e instrument used.
A n estimate of surface coverage.--Calculation o f a n absolute hydrogen coverage on the iron electrodes may be
attempted from FTIR results obtained during cathodic
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Vol. 134, No. 8
ADSORPTION
OF HYDROGEN
p o l a r i z a t i o n . T h e f o r m a l i s m u s e d is c u r r e n t l y a p p l i e d to
predict the vibration frequency of species adsorbed on
t h e s u r f a c e (29). T h e p r e s e n t a p p r o a c h d o e s n o t p r e d i c t a
frequency, rather it calculates the extinction coefficient
and consequently the surface coverage from the IR peak
intensity.
I t is k n o w n t h a t (30)
3
~ he
-05
-04
/
I
Z
/
-0.3
o~,
*'~'/ ~
4m~Ce. A
///
-0.2
I~12 -
w h e r e m~ is t h e m a s s o f t h e e l e c t r o n , ~ is t h e w a v e n u m b e r o f t h e a d s o r p t i o n p e a k , c is t h e v e l o c i t y o f light, (l~) is
t h e t r a n s i t i o n d i p o l e m o m e n t , h is P l a n k ' s c o n s t a n t , e t h e
e l e c t r o n i c c h a r g e , eo is t h e d i e l e c t r i c c o n s t a n t o f t h e m e d i u m , a n d A is t h e a b s o r p t i v i t y .
T h e a b s o r p t i v i t y A c a n b e r e l a t e d to t h e a b s o r p t i o n coe f f i c i e n t at a g i v e n f r e q u e n c y , ~, b y A' = Iv ~2 ~,d~, w h e r e ~,
.f J
O
-01
00
I
-6
-7
;
-5
l
~d~ = ~ . . . . V2c0'2 - r,)
{
-4
I
-3
Current Denslty/(A/cm ~)
a n d v2 r e p r e s e n t s t h e b r e a d t h of t h e a b s o r p t i o n b a n d .
T h e v a l u e o f t h e t r a n s i t i o n d i p o l e m o m e n t (~), is n o t
k n o w n for Fe-H. It is a q u a n t i t y w h i c h g e n e r a l l y h a s to
be calculated by means of quantum mechanical conside r a t i o n s . H e r e , w e r e f e r to c a l c u l a t i o n s m a d e for t h e Si-H
b o n d f o r w h i c h t h e v a l u e o f ~ is 2100 c m -1 (cf. t h e v a l u e
o b t a i n e d h e r e o f 2060 cm-1). I n t h e c a l c u l a t i o n s f o r t h e
t r a n s i t i o n d i p o l e m o m e n t o f Si-H, t h e v a l u e o b t a i n e d w a s
1.33 D e b y e / A ( 3 1 ) . U s i n g a v a l u e o f 1.5A for t h e a p p r o p r i a t e F e - H b o n d l e n g t h (32), o n e o b t a i n s (#F~-a) = 1.99D.
U s i n g t h i s v a l u e o f (1~) i n Eq. [4], o n e o b t a i n s A ( t h e i n t e g r a t e d a b s o r p t i o n coefficient). F r o m t h e e x p r e s s i o n
A =
1961
>
me~C
8mr2
ON IRON
[51
1
a . . . . c a n b e o b t a i n e d , ;2 - ; , = 500 c m ' (el. Fig. 2), t h e r e f o r e ~m~n = 8 X 10 (~c m 2 m o l - L
The concentration of adsorbed species at the interface
w a s o b t a i n e d b y c o n s i d e r a t i o n o f t h e e x t i n c t i o n coefficient, e = amean/2.303 , a n d t h e o p t i c a l p a t h t h r o u g h t h e
a d s o r b a t e , d = ~2//cos 0, w h e r e l is t h e F e - H b o n d l e n g t h
(1.5 x 10 -s c m ) a n d r t h e a n g l e o f i n c i d e n c e o f t h e I N
radiation.
T h e e x p e r i m e n t a l l y m e a s u r e d a b s o r b a n c e , a, a t - 0 . 9 V ,
w a s 1 . 8 x 10 -2, a t a n a n g l e o f i n c i d e n c e r o f 40: T h u s ,
f r o m a = eCd, w h e r e C is t h e c o n c e n t r a t i o n o f a b s o r b i n g
m o l e c u l e s , C is f o u n d to b e 0.15 m o t c m ~. T h e v a l u e o f F
c a n b e c o r r e s p o n d i n g l y o b t a i n e d f r o m t h e v a l u e of t h e
Fe-I-I d i s t a n c e , l, i.e:, 17. = C. 1 = 2 x 3 x 10 -~ m o l c m 2.
S i n c e F o ~ = 2.7 x 10 "~m o l c m -2 a n d t h e r o u g h n e s s f a c t o r
o f t h e s u r f a c e is 3, t h e n 0 = F/F3max = 0.28. N o t e t h a t t h i s
v a l u e c o r r e s p o n d s t o t h e s u r f a c e s i t u a t i o n at t h e h i g h e s t
c u r r e n t d e n s i t i e s Used (5 x 10 -.~ A/em-~).
Correction of O,,~ by 0r~f.--The l e a s t n e g a t i v e p o t e n t i a l
that was used in the FTIR spectroscopic investigation
was -0.5V NHE. Speetroseopic results at more anodie
p o t e n t i a l s w e r e n o t r e p r o d u c i b l e p r e s u m a b l y d u e to dissolution of the iron electrode. Thus, the implicit assumption that subtraction of the reference spectrum taken at
- 0 . 5 V N H E c o r r e s p o n d e d to zero H c o v e r a g e m u s t b e reexamined.
T h e c o r r o s i o n p o t e n t i a l for t h e i r o n e l e c t r o d e h a s b e e n
r e p o r t e d i n t h e p H r a n g e o f 0 to 5 (7). A n e x t r a p o l a t i o n o f
t h i s p l o t s h o w e d a c o r r o s i o n p o t e n t i a l o f - 0 . 5 2 V N H E at
t h e p H o f 8.4 u s e d i n t h e p r e s e n t w o r k . T h u s , t h e r e f e r e n c e p o t e n t i a l u s e d i n t h i s i n v e s t i g a t i o n ( - 0 . 5 V ) is effectively the corrosion potential. The fractional hydrogen
c o v e r a g e (0) a t t h e c o r r o s i o n p o t e n t i a l i n p H = 8.4 w a s
earlier estimated by Boekris and Kita (pseudocapacit a n e e m e a s u r e m e n t s ) to b e 0.03 (8).
Thus, 0.03 h a s t o b e a d d e d to t h e v a l u e s for t h e h y d r o gen coverage calculated here using an assumption of
0 = 0 a t 0.5V N H E . T h e c o r r e c t e d e o v e r a g e as a f u n c t i o n
of overpotential (obtained from the knowledge of the
v a l u e 0.28 + 0.03 a t - 0 . 9 V N H E ) is s h o w n in Fig. 8.
Correction of the Tafel slope for c h a n g e of pH at the
interface.--There is n e e d t o c o r r e c t t h e h y d r o g e n o v e r p o t e n t i a l for t h e c h a n g e of p H at t h e i n t e r f a c e d u e to t h e
Fig. 7. Tofel correction
p r o d u c t i o n of e x c e s s o f O H - t h e r e d u r i n g h y d r o g e n evol u t i o n (see A p p e n d i x ) . T h e r e a c t i o n m e c h a n i s m s h o w s
a n a p p a r e n t i n d e p e n d e n c e o f p H , b e c a u s e h y d r o g e n disc h a r g e is c o m i n g f r o m w a t e r . N e v e r t h e l e s s , t h e p r o d u c tion of OH- may alter the interfacial environment and
consequently the reversible potential and the working
potential have not been measured under similar condit i o n s a n d h e n c e a c o r r e c t i o n is n e c e s s a r y . T h e c a l c u l a t i o n is m a d e i n A p p e n d i x .
T h i s r a t e o f p r o d u c t i o n o f O H - at t h e i n t e r f a c e a t a c u r r e n t d e n s i t y ca. 10 -3 A c m -2 is s o m e f o u r o r d e r s of m a g n i t u d e f a s t e r t h a n t h e r a t e o f n e u t r a l i z a t i o n , i.e., of t h e r e a c t i o n t i m e o f t h e b u f f e r . T h i s b r i n g s a p H c h a n g e f r o m 8.4
to 11.4 w h i c h h a s to b e a c c o u n t e d for. F i g u r e 7 s h o w s t h e
T a f e l l i n e c o r r e c t e d for t h e s e c h a n g e s . T h u s , t h e corr e c t e d T a f e l l i n e h a s a g r a d i e n t o f 0.180V d e c a d e -1 (corr e c t e d f r o m Fig. 6).
Area under the peaks for H and D adsorption.--By
m e a s u r i n g p e a k a r e a s for H a n d D a d s o r p t i o n f r o m Fig. 1
a n d 3, o n e f i n d s a r a t i o ofAH/AD = 6. I f t h e m o d e o f e v o l u t i o n o f h y d r o g e n is a r a t e - d e t e r m i n i n g p r o t o n d i s c h a r g e
(6), f o l l o w e d b y a s u r f a c e r e c o m b i n a t i o n r e a c t i o n , it c a n
b e s h o w n t h a t (33)
kH
kD
( a~+ l]ln
w h e r e D i s a n d C o m r e p r e s e n t p r o t o n d i s c h a r g e a n d rec o m b i n a t i o n of H a n d D, r e s p e c t i v e l y , 1 a n d aH+/aD+ is t h e
r a t i o o f t h e s u m of t h e c o n c e n t r a t i o n o f i o n s d i s c h a r g i n g
H o v e r t h e s u m of t h e c o n c e n t r a t i o n o f t h o s e d i s c h a r g i n g
D.
'k. is the rate constant for 2Haa,--* H2 (gas); k D is that for Ha,, +
Daa, --* HD.
03
0 24
"~
o
0
018
_
012
0 O6
0I
02
-03
-04
-0.5
-O 6
Overpotenhal/V(NHE)
Fig. 8. Coverage-overpotential relation for hydrogen adsorbed on
iron corrected for reference potential.
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1962
J. E l e c t r o c h e m . Soc.: E L E C T R O C H E M I C A L
SCIENCE
In order to obtain this quantity, one can use an equat i o n d e r i v e d b y G o l d (34) w h i c h r e l a t e s t h e s u m o f t h e
concentrations the ions needed for the H discharge and
D d i s c h a r g e , r e s p e c t i v e l y , to t h e e q u i l i b r i u m c o n s t a n t o f
the reaction
2D~O + + 3H20 ~ 2H.~O + + 3D20
[7]
T h i s e q u a t i o n is
August1987
f o r w a r d r e a c t i o n r a t e s for d i s c h a r g e a n d r e c o m b i n a t i o n
reactions are
id = Fk~CH2o(1 - 0) e x p ( - ~ V F / R T ) e x p (-~f(0))
ir
=
Fk202 e x p (2~f(0))
= Ell,
[8]
a n d t h e v a l u e f o r t h e e q u i l i b r i u m c o n s t a n t o f [7] (=L) is
9.0 (35-37). T h e c o n c e n t r a t i o n of D20, a n d H 2 0 for a g i v e n
H / D c o m p o s i t i o n o f t h e s o l u t i o n w a s c a l c u l a t e d (38) f r o m
t h e v a l u e o f t h e e q u i l i b r i u m c o n s t a n t K = 3.8 of t h e r e a c tion
H20 + D20 ~ 2HDO
[16]
3[H.~O] + 2[H2DO]
+ [HD20]
3[D~O]
+ [DH~O]
=
2[H~O]
+ [HDO]
2[D20]
+ [HDO]
= LII~;
[1O]
F u r t h e r , for p r o t o n d i s c h a r g e o n Fe, it h a s b e e n f o u n d [1]
that
N o v a l u e e x i s t s f o r (kH/kD) ...... t, o n P e b u t s u c h a v a l u e
for P t w a s m e a s u r e d (6) as 3. I n v i e w o f t h e f a c t t h a t t h e
q u a n t i t y c o n c e r n e d is a n i s o t o p i c ratio, t h e v a l u e m a y b e
a p p l i e d t o t h e F e s u r f a c e w i t h o u t s i g n i f i c a n t l o s s o f accuracy.
T h u s , f r o m Eq. [6], a n d u s i n g t h e v a l u e of aH+/aD+ g i v e n
b y [10]
0n
0D
-
1.5
[12]
T h i s c o m p a r e s w i t h t h e e x p e r i m e n t a l v a l u e o f 6 (cf. Fig. 2
a n d 4).
T h i s a p p a r e n t d i s c r e p a n c y i n v o l v e s t h e i m p l i c i t assumption that the relative spectral intensities per bond
i n F e - H a n d F e - D a r e t h e s a m e . I t is k n o w n (39, 40) t h a t
O-H b o n d s g i v e I R i n t e n s i t i e s w h i c h a r e f o u r t i m e s
g r e a t e r t h a n t h o s e o f O-D b o n d s . T h u s , 1.5 • 4 = 6, i n
a g r e e m e n t w i t h t h e o b s e r v e d r a t i o of p e a k a r e a s (cf. Fig.
2 a n d 4).
M e c h a n i s m o f h y d r o g e n e v o l u t i o n r e a c t i o n on i r o n at
p H 8.4.--Views in the literature concerning the mechanism of hydrogen evolution reaction on iron are not
u n i f i e d t h o u g h s u g g e s t i v e of s l o w d i s c h a r g e r e a c t i o n foll o w e d b y a c o m b i n a t i o n o f h y d r o g e n (41-42). T h i s m e c h a n i s m c a n b e d i s t i n g u i s h e d f r o m t h e a l t e r n a t e ( f a s t discharge of proton followed by the rate-determining
electrochemical desorption mechanism) by the 0 value.
In the case of rate-determining proton discharge this
s h o u l d b e l o w (0 < < 1) w h e r e a s i n t h e c a s e o f t h e r a t e determining electrochemical desorption mechanism, 0
a p p r o a c h e s u n i t y ( 4 1 ) . T h e s p e c t r o s c o p i c r e s u l t s (0 =
0.03-0.31) s u p p o r t t h e first of t h e s e t w o m e c h a n i s m s .
H i g h T a f e I s l o p e . - - T h e T a f e l s l o p e o b t a i n e d (0.18) is
h i g h e r t h a n t h a t p r e d i c t e d for t h e s l o w d i s c h a r g e m e c h a n i s m f o l l o w e d b y f a s t r e c o m b i n a t i o n (6)
H 2 0 + M(e)
sl~
M-H + O H -
[13]
f(O)
3~
k2 CH2o
3RT
[18]
VF
3RT
[19]
S u b s t i t u t i n g i n [16] a n d r e a r r a n g i n g
ir = F k 2 0 2 e x p
1
k1(1-0)
3~-k~
~
T h e r e f o r e , dV/d l o g ir = - 3 R T / F = - 0 . 1 8 V .
Coverage potential relation.--It c a n b e s h o w n (33) t h a t
for a coupled discharge-recombination hydrogen evolution mechanism, under Langmuir conditions, the value
e x p e c t e d f o r d l o g O/d~ is F/4RT. I n f a c t , w h a t w a s obs e r v e d h e r e is F/5.6RT. N o w it is o f i n t e r e s t to n o t e t h a t
f o r t h e m e c h a n i s m s u g g e s t e d , bTempkln = 3 R T / F w h i l e
bLa,~muir = 2RT/F. C o r r e s p o n d i n g l y , (~ i n i/~V)Lan~muir =
F/4RT; a n d t h e r e f o r e , it is n o t u n e x p e c t e d t h a t (5 i n
i/aV)T,m,k~, = F/6RT. I n s u p p o r t of t h i s i n d i c a t i o n , it c a n
b e s h o w n (43) t h a t t h e v a l u e of ~ i n O/~V i n t h e T e m p k i n
c a s e is 1/(4RT/F + 3r/F) w h e r e r is t h e T e m p k i n coefficient. Such coefficients have been given by Conway and
G i l e a d i (44). T h e y r a n g e f r o m 2-5 k J / t o o l . T a k i n g r as 3
k J/tool and substituting in the equation given provides
3.07 f o r t h e c a l c u l a t e d c o e f f i c i e n t f o r i n O/V; t h e e x p e r i m e n t a l v a l u e F/5.6RT is 3.08.
Acknowledgments
T h e a u t h o r s w o u l d l i k e to t h a n k Dr. V. J o v a n c i c e v i c
a n d Dr. P. Z e l e n a y for h e l p f u l d i s c u s s i o n s . T h i s w o r k
w a s p a r t l y s u p p o r t e d b y t h e W e l c h F u n d u n d e r g r a n t no.
55213.
M a n u s c r i p t s u b m i t t e d J u n e 23, 1986; r e v i s e d m a n u s c r i p t r e c e i v e d F e b . 20, 1987.
T e x a s A & M U n i v e r s i t y assisted in m e e t i n g the publication costs o f this article.
Appendix I. Correction on the Tafel slope for a pH change at the
interface
To c a l c u l a t e t h e c h a n g e i n p H i n t h e d o u b l e l a y e r i n t h e
a b s e n c e o f a b u f f e r b u t i n t h e p r e s e n c e o f a h y d r o g e n evolution reaction which increases the pH in the double
layer, o n e m a y w r i t e (45)
loath -
DnF
5
[(CoH-)DL -- (CoH)B]
f~
2M+H2
[14]
This can be explained in terms of an activated coupled
discharge mechanism under Tempkin conditions. The
[A-l]
w h e r e ~ = 0.01 c m in t h e e s t i m a t e d d i f f u s i o n l a y e r t h i c k n e s s for " m i l d s t i r r i n g " (i.e., H2 b u b b l i n g in s o l u t i o n ) , D
is t h e d i f f u s i o n c o n s t a n t i n c m 2 s 1, n is t h e c h a r g e o n t h e
i o n s c o n c e r n e d , F is t h e F a r a d a y c o n s t a n t i n c o u l o m b s
t o o l -I, a n d C is t h e c o n c e n t r a t i o n of s p e c i e s a t t h e d o u b l e
l a y e r i n m o l c m -2.
T h u s , if t h e b u l k p H is 8.4, Coil = 10 -5."o m o l 1 a n d t h e
a b o v e e q u a t i o n s h o w s t h a t , i n t h e a b s e n c e o f b u f f e r act i o n s , it w o u l d i n c r e a s e t o 10 -3.5 t o o l 1-1 a t 10 -3 A c m -2,
a n d h e n c e s i g n i f i c a n t c o r r e c t i o n for t h e v a l u e o f t h e o v e r potential should be made.
It is k n o w n (46) t h a t t h e e q u i l i b r i u m i n a b o r a t e b u f f e r
s o l u t i o n is g i v e n b y
B(OH)~ + H 2 0 ~ B ( O H ) c + H § ( p K = 9)
M-H+M-H
[17]
To o b t a i n t h e d e p e n d e n c e of 0 as a f u n c t i o n of V, t h e ass u m p t i o n is m a d e t h a t t h e v a r i a t i o n o f t h e t e r m 0~/1 - 0
w i t h p o t e n t i a l is n e g l i g i b l e c o m p a r e d w i t h t h a t of t h e exp o n e n t i a l t e r m e x p [36f(0)]. T h u s , s o l v i n g f o r f(0)
[9]
I n o u r e x p e r i m e n t s (50-50 m i x t u r e s o f H 2 0 a n d D 2 0 b y
v o l u m e ) , [H20] = 14.8, [D20] = 12.0, a n d [ H D O ] = 26.07
tool d m 3. T h e r a t i o o f t h e a c t i v i t i e s of h y d r a t e d p r o t o n s
to t h a t o f h y d r a t e d d e u t e r o n s is t h e n g i v e n b y
[15]
Equating these rates,
e x p [3~f(0)] = k~/k~ e x p ( - ~ V F / R T ) C , 2 o
(3[I-IaO +] + 2[H2DO +] + [HD20])(2[D30] + [HDO])
(3[DaO] + 2[HD20] + [H.~DO])(2[H20] + [HDO])
+ 2[D2HO]
AND TECHNOLOGY
[A-2]
T h e r a t e o f r e c o m b i n a t i o n is v e r y f a s t (kR = 10 TM s -1
mo1-1) a n d kD = 10s -~ (47). H e r e , kD is t h e r a t e o f d i s s o c i a tion of boric acid in the buffer and eventually the rated e t e r m i n i n g s t e p i n t h e n e u t r a l i z a t i o n o f O H . T h u s , for a
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Vol. 134, No. 8
ADSORPTION
boric acid c o n c e n t r a t i o n of 0.15M, the rate of proton prod u c t i o n will be 1.5 mol 1-1 s -1.
The rate of f o r m a t i o n of OH- in the d o u b l e layer at a
t y p i c a l c u r r e n t density of 10 -3 A c m -2 is given by
i
10-3 A/cm 2
- 5 x 10-8 m o l c m - ~ s - ~
nF
2 x 10~
5x10 8
- 103=moll
is-,=
1"6
x 103
3 x 10-8
Hence, the rate of p r o d u c t i o n of OH- is three orders of
m a g n i t u d e faster than the rate of neutralization at the interface (1.5 mol 1-1 s-l).
The c h a n g e in h y d r o g e n o v e r p o t e n t i a l can be calculated u s i n g the Nernst equation. Thus, for a current density of 10 -3 A cm -~, (Co.-)B = 10 -5.6 a n d (Co.)D,, = 10-3sL
Hence, h~ = -0.123V similarly for i = 10 -4 A cm -2 h~ =
-0.066V a n d for i = 10 -s A cm 2 An = -0.0206. F i g u r e 7
shows the i-V curve corrected for this effect, with a new
slope of ca. 3RT/F.
REFERENCES
1. M. Breiter, C. Knorr, and W. Volkl, Z. Electrochem., 59,
681 (1955).
2. M. Breiter, Ann. N. Y. Acad. Sci., 101, 709 (1963).
3. M. Breiter, J. Electroanal. Chem. InterfaciaI Electrochem., 90, 425 (1978).
4. J. Chevillot, J. Farcy, C. H i n n e n , and A. Rousseau,
ibid., 64, 39 (1975).
5. B. Baker, D. Rand, and R. Woods, ibid., 97, 189 (1979).
6. J. O'M. Bockris and D. Koch, J. Phys. Chem., 65, 1941
(1961).
7. H. Flitt and J. O'M. Bockris, Int. J. Hydrogen Energy, 7,
411 (1982).
8. J. O'M. Bockris and H. Kita, This Journal, 108, 679
(1961).
9. C. Kim and B. Wilde, ibid., 118, 202 (1971).
10. M. Devanathan, J. O'M. Bockris, and W. Mehl, J. Electroanal. Chem. Interfacial Electrochem., 1, 143 (1959).
11. H. Flitt, Ph.D. Thesis, University of South Australia
(1980).
12. M. A. Habib and J. O'M. Bockris, This Journal, 132, 108
(1985).
13. J. O'M. Bockris, M. Genshaw, V. Brusic, and H. Wroblowa, Electrochim. Acta, 16, 1859 (1971).
14. R. Greenler, J. Chem. Phys., 44, 310 (1966).
15. V. Jovancicevic, J. O'M. Bockris, J. L. Carbajal, P.
Zelenay, and T. Mizuno, This Journal, 133, 2219
(1986).
16. D. Barclay, J. Electroanal. Chem. Interfacial Electrochem., 44, 310 (1966).
17. H. Kaesz and R. Saillant, Chem. Rev., 72, 231 (1932).
18. A. Ginsberg, in "Transition Metal Chemistry," Vol. 1,
R. I. Carlin, Editor, Edward Arnold, London (1965).
19. J. deBoer, Z. Physik. Chem. (Frankfort am Main), B18,
49 (1932).
20. H. Leftin and M. Hobson, Jr., in "Advances in Catalysis," Vol. 14, D. Eley, H. Pines, and P. Weisz, Editors,
OF HYDROGEN ON IRON
1963
Academic Press, New York (1963).
21. S. Pons, J. Electroanal. Chem. InterfacialElectrochem.,
150, 495 (1983).
22. S. Ports, T. Davison, A. Bewick, and P. Schmidt, ibid.,
125, 237 (1982).
23. W. A. Pliskin and R. P. Eischens, Z. Phys. Chem., N.F.
24, 11 (1960).
24. H. Ibach and D. I. Mills, "Electron Energy Loss Spectroscopy and Surface Vibrations," p. 277, Academic
Press, London (1982).
25. G. Socrates, "Infrared Characteristics Group Frequencies," John Wiley & Sons, Inc., New York (1980).
26. B. R. Scharifker, M. A. Habib, J. L. Carbajal, and
J. O'M. Bockris, Surf. Sci., 173, 97 (1986).
27. A. M. Baro and W. Erley, Surf. Sci., 112, L 759 (1981).
28. G. Barrow, "Introduction to Molecular Spectroscopy,"
McGraw-Hill Inc., New York (1962).
29. S. Korzeniewski, R. B. Shirts, and S. Ports, J. Phys.
Chem., 89, 2297 (1985).
30. P. Atkins, "Physical Chemistry," p. 585, W. H. Freem a n Co., San Francisco (1978).
31. D. Steele, "Theory of Vibrational Spectroscopy,"
p. 127, W. B. Saunders Co., London (1971).
32. Table of Interatomic Distances and Configuration in
Molecules and Ions, L. E. Sutton, Editor, The Chemical Society, Budington House, London (1958).
33. J. O'M. Bockris, in "Modern Aspects of Electrochemistry," Vol. 1, J. O'M. Bockris and B. Conway, Editors,
P l e n u m Press, New York (1954).
34. V. Gold, Trans. Faraday Soc., 64, 2770 (1967).
35. R. A. M. O'Ferrall, G. W. Koeppl, and A. J. Kresge, J.
Am. Chem. Soc., 93, 9 (1971).
36. E. Lee Purlee, J. Am. Chem. Soc., 81, 263 (1959).
37. P. Salomaa and V. Aalto, Acta Chem. Scand., 2 0 , 2035
(1966).
38. J.W. Pyper, R. S. Newbury, and G. W. Barton, J. Chem.
Phys., 46, 2253 (1967).
39. D. Eisenberg and W. K a u z m a n n , "The Structure and
Properties of Water," Oxford University Press,
London (1965).
40. K. Kretzschner, J. Sass, A. Bradshaw, and S. Holloway, Surf. Sci., 115, 183 (1983).
41. M. Enyo, in "Comprehensive Treatise of Electrochemistry," Vol. 7, B. Conway, J. O'M. Bockris, E. Yeager,
S. Khan, and R. White, Editors, P l e n u m Press, New
York (1983).
42. M. Enyo, Electrochim. Acta, 18, 155 (1973).
43. J. L. Carbajal, Ph.D. Thesis, Texas A&M University,
College Station, Texas (1985).
44. E. Gileadi and B. E. Conway, in "Modern Aspects of
Electrochemistry," Vol. 3, J. O'M. Bockris and B. E.
Conway, Editors, Butterworths, London (1964).
45. A. Bard and I. Faulkner, "Electrochemical Methods,"
John Wiley & Sons, New York (1980).
46. R. Messner, C. Baes, Jr., and F. Sweeton, Inorg. Chem.,
11, 537 (1972).
47. M. Eigen, W. Kruse, G. Maass, and I. DeMaeyer, "Progress in Reaction Kinetics," Vol. 2, The Macmillan
Co., New York (1964).
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