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The effect of thin palladium films on the kinetics of the anodic oxidation of Pd-H Electrodes
Electrochimica
Acta.
1970, Vol.
15, pp. 865 to 867.
SHORT
THE
Pcrgamon
Press.
Printed
in Northern
Ireland
COMMUNICATION
EFFECT OF THIN PALLADIUM FILMS ON THE KINETICS OF THE ANODIC OXIDATION OF Pd-H ELECTRODES* R. V. BUCUR and V. V. MORARNJ Institutefor Atomic Physics, Cluj, Rumania
FOR AN electrode reaction controlled exclusively by the diffusion across a thin, plane, finite film, the dependence of the maximum voltammetric current 1, on the potential scan rate u, is determined by the thickness of the finite film 1, following the relation1
x, = 2
(1 -
nFDCloS I
%9&d%
;I%) 2
[
1
(1)
where p,, = (nr;lRT)(z@/D), D is the diffusion coefficient, Cl0 is the initial concentration of the reactant, and S is the effective area of the electrode. For a large enough thickness (I w 100 pm), (I) reduces*B3 to the approximate form 1, M zN2, as in the semi-infinite diffusion case; for very thin films (PO < r2/4, uZ2/D < 6 x 104), (1) becomes I9 =
n2FvlCloS RT
-
(2)
This is an analogous form to the Hubbard-Anson equation, established for transport in thin layers of solution where diffusion can be neglected.4 Roe and Toni6 showed experimentally the direct dependence between I,, and o for a thin film of cadmium amalgam with 2 = 0.28 pm. The maximum voltammetric current as a function of the potential scan rate, between these two limiting situations, has the form 1, = uQwhere the exponent a can vary continuously from O-5 to 1.0. This is correct only if the mass transport in the thin film is diffusive. Thin film Pd-H electrodes show different behaviour. In Fig. 1 is plotted the a exponent values US palladium film thickness I for the oxidation reaction in H,SO, solution (II+ = 1-7 x lOa M, 25°C). The experimental work and the apparatus have been described earlier.‘j The palladium film was obtained by electrodeposition in phosphate solution7 at constant current I = 3 mA on a platinum support with a geometrical area of 1.36 cm2. Figure 1 shows that with decreasing thickness, a increases from O-5 to 1.0, but for I < 1 ,um this variation is not consistent with the theory. The effect of this anomalous variation of a is shown in Fig. 2: when I < 1 pm the maximum current I, decreases rapidly with decreasing 2. Curves are shown for * Manuscript received 14 December 1968. 865
Short communication
866
I.Or
I
0.91 \
H2S04, pi’]=
25T
1~74x10-3M
-----_
0.50--
0
I
I
I
6
I2
18
(I FIG,
1,
pm
Dependence of a on palladium film thickness I (I, M Y”). [Pd-H oxidation in H&O.; l-7 x lo-* M solution at 25”C.l
electrode
5 rfH = I~o4xlo-7
q-
atm
4 H,SO.+,
d
[Ii+]
=
I-74rl0-3M
25’ C
3
“0’ x ++
0
6
12 *
(,
I.8
24
pm
FIG. 2. Dependence of maximum current I= on the palladium film thickness 1, at various potential scan rates ~1. 1, palladized; 2, platinized palladium.
TABLE 1.
0.27 0.32 4.95
before heating 0.81 0.75 0.55
after heating 0.50 0.61 0.59
Short communication
867
three values of potential scan rate, o = 04, 0.2 and O-1 V/min for Pd-H electrodes with a constant hydrogen content, nH. = l-04 x lo-’ g-atom H. M.depends sensitively on the film structure of palladium. Table 1 summarizes a series of a values for three thickness before and after heating 5 min at ca 700°C. The effect of the thermal treatment is particularly marked for very thin films (I < O-5 pm) and practically non-existent for thicker films. The anomalous change of the exponent may be determined by an abrupt modification of the transport mechanism Up to I M 1 pm, diffusion transport prevails; of hydrogen through palladium. below this value, transport occurs by a strong interaction of the hydrogen with the Pd lattice, Work is in progress to clarify the anomaly. REFERENCES 1. E. SCHMIDT and H. R. GYGAX, Chimiu 16, 165 (1962). 2. W. T. DE VRIES and E. VAN DALEN, 3. eiectroanal. Chem. 8, 366 (1964);
3. 4. 5.
6. 7.
eiectroanai. Chem. 9,448 (1965). R. V. .BUCUR, I. GWACI and C. MIRON, J. electroanal. Chem. 13, 263 (1967). A. T. HUBBARD and F. C. ANSON, Analyt. Chem. 38, 58 (1966). D, K. ROE and J. E. A. TONI, Analyt. Chem. 37, 1503 (1965). R. V, BUCUR and V. V. MORARIU, J. eiectroanal. Chem., in press. V. V. OSTROUMOV, Zkjz. Khim. 31, 1812 (1957).
W. T. DE VRIES,J.
Acta.
1970, Vol.
15, pp. 865 to 867.
SHORT
THE
Pcrgamon
Press.
Printed
in Northern
Ireland
COMMUNICATION
EFFECT OF THIN PALLADIUM FILMS ON THE KINETICS OF THE ANODIC OXIDATION OF Pd-H ELECTRODES* R. V. BUCUR and V. V. MORARNJ Institutefor Atomic Physics, Cluj, Rumania
FOR AN electrode reaction controlled exclusively by the diffusion across a thin, plane, finite film, the dependence of the maximum voltammetric current 1, on the potential scan rate u, is determined by the thickness of the finite film 1, following the relation1
x, = 2
(1 -
nFDCloS I
%9&d%
;I%) 2
[
1
(1)
where p,, = (nr;lRT)(z@/D), D is the diffusion coefficient, Cl0 is the initial concentration of the reactant, and S is the effective area of the electrode. For a large enough thickness (I w 100 pm), (I) reduces*B3 to the approximate form 1, M zN2, as in the semi-infinite diffusion case; for very thin films (PO < r2/4, uZ2/D < 6 x 104), (1) becomes I9 =
n2FvlCloS RT
-
(2)
This is an analogous form to the Hubbard-Anson equation, established for transport in thin layers of solution where diffusion can be neglected.4 Roe and Toni6 showed experimentally the direct dependence between I,, and o for a thin film of cadmium amalgam with 2 = 0.28 pm. The maximum voltammetric current as a function of the potential scan rate, between these two limiting situations, has the form 1, = uQwhere the exponent a can vary continuously from O-5 to 1.0. This is correct only if the mass transport in the thin film is diffusive. Thin film Pd-H electrodes show different behaviour. In Fig. 1 is plotted the a exponent values US palladium film thickness I for the oxidation reaction in H,SO, solution (II+ = 1-7 x lOa M, 25°C). The experimental work and the apparatus have been described earlier.‘j The palladium film was obtained by electrodeposition in phosphate solution7 at constant current I = 3 mA on a platinum support with a geometrical area of 1.36 cm2. Figure 1 shows that with decreasing thickness, a increases from O-5 to 1.0, but for I < 1 ,um this variation is not consistent with the theory. The effect of this anomalous variation of a is shown in Fig. 2: when I < 1 pm the maximum current I, decreases rapidly with decreasing 2. Curves are shown for * Manuscript received 14 December 1968. 865
Short communication
866
I.Or
I
0.91 \
H2S04, pi’]=
25T
1~74x10-3M
-----_
0.50--
0
I
I
I
6
I2
18
(I FIG,
1,
pm
Dependence of a on palladium film thickness I (I, M Y”). [Pd-H oxidation in H&O.; l-7 x lo-* M solution at 25”C.l
electrode
5 rfH = I~o4xlo-7
q-
atm
4 H,SO.+,
d
[Ii+]
=
I-74rl0-3M
25’ C
3
“0’ x ++
0
6
12 *
(,
I.8
24
pm
FIG. 2. Dependence of maximum current I= on the palladium film thickness 1, at various potential scan rates ~1. 1, palladized; 2, platinized palladium.
TABLE 1.
0.27 0.32 4.95
before heating 0.81 0.75 0.55
after heating 0.50 0.61 0.59
Short communication
867
three values of potential scan rate, o = 04, 0.2 and O-1 V/min for Pd-H electrodes with a constant hydrogen content, nH. = l-04 x lo-’ g-atom H. M.depends sensitively on the film structure of palladium. Table 1 summarizes a series of a values for three thickness before and after heating 5 min at ca 700°C. The effect of the thermal treatment is particularly marked for very thin films (I < O-5 pm) and practically non-existent for thicker films. The anomalous change of the exponent may be determined by an abrupt modification of the transport mechanism Up to I M 1 pm, diffusion transport prevails; of hydrogen through palladium. below this value, transport occurs by a strong interaction of the hydrogen with the Pd lattice, Work is in progress to clarify the anomaly. REFERENCES 1. E. SCHMIDT and H. R. GYGAX, Chimiu 16, 165 (1962). 2. W. T. DE VRIES and E. VAN DALEN, 3. eiectroanal. Chem. 8, 366 (1964);
3. 4. 5.
6. 7.
eiectroanai. Chem. 9,448 (1965). R. V. .BUCUR, I. GWACI and C. MIRON, J. electroanal. Chem. 13, 263 (1967). A. T. HUBBARD and F. C. ANSON, Analyt. Chem. 38, 58 (1966). D, K. ROE and J. E. A. TONI, Analyt. Chem. 37, 1503 (1965). R. V, BUCUR and V. V. MORARIU, J. eiectroanal. Chem., in press. V. V. OSTROUMOV, Zkjz. Khim. 31, 1812 (1957).
W. T. DE VRIES,J.