Feb., 1958
HYDROGEN OVERVOLTAGE MEASUREMENTS ON NICKEL AND PALLADIUM
229
AN ELECTROCHEMICAL INVESTIGATION OF HYDROGEN PRODUCING REACTIONS CATALYZED BY NICKEL AND NICKELPALLADIUM CATHODES BY JAMES P. HOAREAND SIGMUND SCHULDINER U . 8. Naval Research Laboratory, Washington $6,D. C. Received September $3, 1967
Hydrogen overvoltage measurements in acid solution of nickel, palladium and a aerie's of nickel-palladium alloys were made. The hydrogen producing mechanisms are discussed. It was found that the steady-state, open-circuit potential values for palladium and the high palladium alloys which contained dissolved hydrogen were all positive to a Pt/H2 electrode in the same solution. The nickel-palladium alloys containing between 32 and 99 atomic $7, nickel gave zero potential against a Pt/H2 reference in the same solution. B use of nickel-palladium-hydrogen ternary alloys it is possible to vary the number of holes per atom in the d-band from agout zero to 0.6. The relative catalytic activity of these metals for the hydrogen producing reaction is shown to be related t o this number which in turn is determined by the hydrogen solubility.
Introduction I n an investigation of the hydrogen-producing reactions on gold-palladium alloys, the authors' were able to show the effect of the gold content on the catalytic activity of these alloys. In that study, the added gold contributed s-electrons to the dband of the alloy thus reducing the number of holes per atom to zero as the gold content increased to an atom ratio -0.6. After cathodic electrolysis of alloys of Au/Pd atomic ratios below 0.6, the holes in the d-band of each alloy were filled with electrons from dissolved hydrogen. Thus, for this entire series of ternary Au-Pd-H alloys, the d-band for each composition was filled with electrons and the fundamental difference between the alloys, other than the gold content, was the difference in the number of protons in interstitial positions in each lattice. I n the present study, it was felt that an investigation in which the number of holes in the d-band of a ternary hydrogen alloy system could be varied would give further information concerning the effect of electronic configuration on the catalytic activity for the hydrogen-producing reaction. This could be accomplished by determining the rate constants for a series of nickel-palladium alloys. Nickel-palladium alloys form a series of solid solutions with face-centered cubic lattices over the entire composition range. Mott and Jones2 gave evidence that the number of holes per atom in the d-band of each of these metals was near 0.6. Wohlfarth* concluded that the number of holes per atom in the d-band remained constant a t 0.6 over the entire range of nickel-palladium alloys. Tverdovski and Vert4 have shown that the solubility of hydrogen in nickel-palladium alloys is a function of the nickel content and from pure palladium to a Ni/Pd atom ratio of about 0.3 the hydrogen solubility decreases from an atom ratio H/Pd of 0.54 to almost zero. Hence by use of this series of ternary alloys (Ni-Pd-H) as cathodes for hydrogen-producing reactions the number of holes per atom in the d-band can be varied from about zero to 0.6. (1) S, Schuldiner and J. P. Hoare, THISJOURNAL, 61, 705 (1957). (2) N. F. Mott and H. Jones, "Properties of Metals and Alloys," Oxford Un. Press, London, 1936, pp. 190400. (3) E. P. Wohlfarth, J . Phva. Cham. Solids, 1, 35 (1956). (4) L. P. Tverdovski and Z. L. Vert, Doklady A k . Navk., S S S R , 88, 805_(1953).
Experimental Pure nickel, palladium and a series of nickel-palladium alloys of the following atomic percentages of nickel were used? 10.0, 17.7, 25.0, 32.0, 44.3, 57.4, 64.7, 74.0, 80.7, 85.0, 94.3, 98.0 and 99.0. All measurements were made in electrolytically purified 2 N sulfuric acid solutions stirred vigorously with purified hydrogen. Measurements were also made on pure nickel in 1 N hydrochloric acid solution. The experimental details were the same as those given in the investigation of gold-palladium alloys.' Before overvoltage measurements were taken all the metal electrodes were cathodized long enough to saturate them with hydrogen .1*6 The solution temperature was 30 f lo. All potential measurements are referred to a Pt/H2 electrode in the same solution. Individual points could be reproduced to within 1 2 mv.
Hydrogen Overvoltage on Pure Palladium and Nickel.-The Tafel b slope of 0.04 (see Fig. la) on pure palladium indicates that the electrochemical desorption of atomic hydrogen on a cathode surface sparsely covered with hydrogen was rate determining. The region in which the overvoltage is independent of current density indicates a hydrogen-producing mechanism which does not involve atomic hydrogen as an intermediate. These mechanisms have been discussed in detail in earlier papers. 1,6,7 The pure nickel cathode (see Fig. lb) gave a Tafel 0.12 slope in both sulfuric and hydrochloric acid solution; however, the io (current density value of the extrapolated Tafel curve a t zero overvoltage) value in sulfuric acid was about two orders of magnitude higher than that in hydrochloric acid solution. Similar results were reported by Jefferys, Yeager and Hovorka.8 This difference in the io coincides with the relative solubility of nickel in the two acids. A sample of 99.99% nickel was found to dissolve in 2 N sulfuric acid at a rate of 7 X 10-6 g./hr., and in 1N hydrochloric acid at a rate of 1 X (5) The 10.0 to 94.3% alloys were obtained from Dr. A. I. Schindler of the Metallurgy Division, U. 6. Naval Research Laboratory. The nickel used in the alloys was 99% pure containing traoes of copper, iron, cobalt and molybdenum. The palladium was 99.9% pure. Dr. J. A. Dreesen of the Carnegie Institute of Technology furnished the 98.0 and 99.0% alloys. The nickel wed in these two samples was 99.99% and the palladium was 99.98% pure. The pure nickel and the pure palladium electrodes used in this study were both 99.99%. (6) J. P. Hoare and S. Schuldiner. J. Elsetrochem. SOC.,101, 485 (1955). (7) S. Schuldiner and J. P. Hoare, International Colloquium on
Reference Electrodes and Structure of the Double Layer, Paris, France, October, 1956. (8) R. Jefferye, E. Yeager and F. Hovorka, paper presented before the Electrochemical Society Meeting in Ban Francisoo, April 30 to May 3, 1956.
230
JAMES P. HOAREAND SIGMUND SCHULDINER
Vol. 62
T=30+I0
I
LOG APPARENT CURRENT DENSITY (A/cm2).
Fig. la.-Hydrogen
overvoltage on Pd, Ni and Ni-Pd alloys.
low5 g./hr. Because of the higher solubility in HCl, the open circuit potential was about -0.07 v. vs. Pt/H2 whereas it was about -0.01 v. in 2 N H2S04. There are two likely rate-determining steps in the over-all hydrogen-producing reaction which can account for the b slope of 0.12. These are the slow discharge of hydrogen ions and the electrochemical desorption from a surface which is virtually covered with a layer of atomic h y d r ~ g e n . ~ An interpretation by the authors117 of the experimental results of OikawaIO indicates that the slow discharge step is rate-controlling on nickel. On the basis of experimental data, Lukovtsev, Levina and Frumkinll as well as Bockris and PotterI2 came t o the same conclusion. On the other hand, Conway and BockrisI3 on the strength of a theoretical approach came to the conclusion that the electrochemical desorption step was rate determining. It is not possible to distinguish between the two mechanisms by the data presented. If it is assumed that the electrochemical desorption is rate controlling then it follows, since the measured Tafel slope is 0.12, that the nickel surface must be virtually covered with a layer of atomic hydrogen. According t o BeeckI4 the heat of adsorption of hydrogen on a nickel surface exposed to hydrogen only is reduced to about half its volume as the surface becomes covered with hydrogen. If it is assumed that the heat of adsorption of hydrogen on nickel exposed t o the gas phase is unaltered by the (9) R. Parsons, Trans. Faraday Soc., 47, 1332 (1951). (10) M.Oikawa, Bull. Chem. SOC.,Japan, 28, 626 (1955). (11) P. Lukovtsev, S. Levina and A. Frumkin, Acta Physicochim. USSR, 11, 21 (1939); P. Lukovtsev and S. Levina, Zhurn. F i z . Khim. U S S R , 29, 1508 (1955). (12) J. O'M. Bockris and E. C. Potter, J . Chem. Phys., 20, 614 (1952).
(13) B. E. Conway and J. O'M. Bockris, Naturwisa., 19, 446 (1956); International Colloquium on Reference Electrodes and Structure of the Double Layer, Paris, October, 1956; J . Chem. Phys., 46, 532 (1957). (14) 0. Beeok, Disc. Faraday Soo., 8 , 118 (1950).
addition of aqueous solutions, then several conclusions concerning the coverage of the nickel with hydrogen and the rate-determining steps are possible. If this assumption holds and if either the electrochemical desorption or slow discharge were rate determining then it would appear that nickel would be highly covered with hydrogen, even a t very low current densities. This would be so because if the fraction of the nickel covered with hydrogen was low the heat of adsorption of hydrogen would be higher than for either platinum or p a l l a d i ~ m . ~ ~ * ~ ~ Then using the relationship of Horiuti and Polanyi15 the rate constant for the discharge step on nickel would be larger than on either platinum or palladium. It would then follow that either the 0.04 or 0.03 Tafel slope should be found for a nickel cathode since palladium and platinum in strong acid solution have hydrogen producing mechanisms which are controlled by the electrochemical desorption with a 0.04 slopes and an atomic combination step with a 0.03 slope,16respectively. Since a 0.12 slope is observed on nickel, it would then follow that if the basic assumption is correct, the surface must be largely covered with atomic hydrogen. It is also possible, if the heat of adsorption of hydrogen is greatly reduced, because of large surface coverage, that the slow discharge step could become slower than the electrochemical desorption step. This is because the rate of the slow discharge step would decrease with decreasing heat of adsorption of hydrogen,15 while the electrochemical desorption step would increase. l ~ ~ , ~ ~ Hydrogen 0vervoltage on Nickel-Palladium Alloys. Tafel 0.03-0.04 Range.-The experimental overvoltage, 11, vs. log current density, relationships for the Ni-Pd series are shown in Fig. l a and Fig. lb. From these results it can be seen that starting (15) 3 . Horiuti and M. Polanyi, Acta Physicochim. U.S.S.R., 8 , 505 (1935). (16) 8.Schuldiner, J . Electrochem. Soe., 9@,488 (1952).
4
Feb., 1958
HYDROGEN OVERVOLTAGE MEASUREMENTS ON NICKEL AND PALLADIUM
23 1
T 2N= 3 0H2S04 r1°
-0
-0. v,
I_J
-> 0
F
-C
-OX
Fig. 1b.-Hydrogen
overvoltage on Pd, Ni and Ni-Pd alloys.
with pure palladium there is a region in which the Tafel b slope is 0.04 and as the nickel content increases the slope decreases to 0.03 and then returns to the 0.04 value. The 0.04 slope can be interpreted as indicating that the rate-determining step is the electrochemical desorption on a surface which is sparsely covered with atomic hydrogen.6 Where the combination of hydrogen atoms is rate controlling a slope of 0.03 is Slopes of values in between 0.03 and 0.04 give evidence for a situation in which the combination and electrochemical mechanisms are about equally rate determining. Tafel 0.12 Range.-With the exception of pure palladium, all of the overvoltage curves enter a region in which the Tafel b slope is equal to 0.12. As in the ca?e of pure nickel there are two likely rate-controlling steps that can account for such a slope. These are the slow discharge of hydrogen ions and the electrochemical desorption of atomic hydrogen from a surface which is virtually covered with a layer of atomic hydrogen. It is felt that the
.
slow discharge step is more likely the rate-determining one, As the current density is increased, the number of hydrogen atoms adsorbed on the surface would also increase and the heat of adsorption of the hydrogen would decrease. Under these conditions the rate of the atomic desorption steps would increase while the rate of the discharge step would decrease. It is possible that a point could be reached where the slow discharge becomes rate controlling. The experimental data for all the alloys show a change t o a 0.12 slope a t a current density of about 0.3 amp./ cm.2and a t an overvoltage value of about -0.075 v. This invariance of the transition point values of current density and potential with composition of alloys could possibly indicate that the increase in the number of hydrogen atoms on the surface from zero current to the polarization value a t the start of the 0.12 slope is about the same. Because the rate of change of the heat of adsorption with surface coverage may be a complicated function of the
JAMESP. HOAREAND SIGMUND SCHULDINER
232
composition of the alloy and because the required data are unavailable, it is not possible a t this time to make a quantitative analysis of this problem. If it is assumed that the electrochemical desorption is rate determining in the 0.12 range, then it would be expected that a more gradual change from the lower to the higher slope would occur than is observed in Fig. 1. This is because a change from a sparsely covered to a saturated surface would not take over a short current density range." It also may be noted that because of the high hydrogen ion concentration in the 2 N sulfuric acid solution, the change to the 0.12 slope could not be explained on the basis of change of reactant from hydronium ion to water as was found for platinum,'5 and palladium6 in less acid solutions. It is interesting to note that for the alloys with nickel compositions from 44 to 65 atomic % ' there is an anomalous inflection in the overvoltage curves from the lower to higher Tafel slopes. This inflection has been checked repeatedly so that it appears to be a real effect. This would indicate that a possible change in the active electrode material occurs rather than just a change in the rate-controlling step. At the present time no reasonable explanation for this effect can be suggested. Linear q us. i Range.-At low current densities a range is found in which the overvoltage is linearly dependent on the current density. The rate constants, -di/ds, for the over-all hydrogen-producing reactions and the extrapolated zero current overvoltage values were determined in this range and are given in Table I. TABLE I RATECONSTANTS FOR Pd, Ni-Pd ALLOYSAND Ni IN 2 N HISOl DETERMINED IN THE RANGEIN WHICH q WAS LINEARLY DEPENDENT ON CURRENT DENSITY Composition, atomic % '
Pure Pd 10% Ni l8y0Ni 25% Ni 32% Ni 44% Ni 57% Ni 65% Ni 74% Ni 81% Ni 85% Ni 94% Ni 98% Ni 99% Ni Pure Ni
-di/dq, mho6
4.00 2.50 1.66 0.32 .05 .05 .07 .15 .08 -13 .10 .50 .25 * 25 .02
Extrpld. zero current overvoltages,
Reveraible open-circuit potential, V.
V.
-0.004 .021 .017
-
-
.010
.o .o .o .o .o .o .o .o .o .o
.017
+0.050 .037 .018 .007
+ + +
-
.o .o .o .o .o .o .o .o .o .o
.004
The overvoltages a t extrapolated zero currents for Pd, the 10 to 25% Ni alloys, and Ni are all other than zero; however, all of the other Ni-Pd alloys do give zero values. This can be explained on the basis of the solubility of hydrogen in these metals. As shown by Tverdovski and Vert,4 hydrogen does dissolve in the Ni-Pd alloys containing up to 25 atomic % nickel. It already has beenshownl that for palladium which had been electrolytically(17) H, W, Salrberg and S. Schuldiner, ibid., 104, 319 (1957).
Vol. 62
charged with hydrogen, the linear q us. i relationship when extrapolated to zero current does not go through the origin. This is because on electrolytic-charging the hydrogen dissolves in the metal beyond an H/Pd atomic ratio of 0.6. The 0.6 H/ Pd ratio represents the &phase of the alloy and it is known that the reversible potential for this alloy against a Pt/H2 electrode in the same solution is zero volt. When the hydrogen content of the alloy is increased beyond 0.6, then the potential will become negative to zero volt. In a series of three papers,18 a full discussion of this problem is given and it is shown that palladium electrolyticallycharged with hydrogen does not form an equilibrium hydrogen electrode and does not remain a t zero upon open circuit. The q us. i relationship for palladium is obtained by extrapolating back to zero current from a metal charged with hydrogen beyond the 0.6 ratio. This extrapolation shows that at zero current there is a partial pressure of hydre gen at the electrode interface which is greater than atmospheric. If the electrode is permitted to remain at zero current, hydrogen leaves the metal until a potential 50 mv. positive to a Pt/H2 electrode is obtained.6J8 It is suggested that a similar explanation holds for the 10 to 25% Ni alloys. Hydrogen does not dissolve appreciably in the 32 to 99% Ni alloys. Since the open-circuit potential for these alloys is equal to zero, it is evident that these metals form equilibrium hydrogen electrodes. Pure nickel does not extrapolate to zero volt because nickel dissolves in the solution a t low current densities. Open-circuit Potentials.-Steady-state, open-circuit potentials were obtained for all the electrodes in hydrogen saturated solution after cathodic polarization. These values were checked by a determination after anodic polarization. Upon opening the circuit, with the exception of pure nickel, the steady-state potential was reached within a few hours. This potential value did not change during an interval of 15 hours. I n a trial run with the 98y0 alloy, the open-circuit potential remained constant for four days. The results are shown in the last column of Table I. The potential values for the pure palladium and high palladium alloys which contained dissolved hydrogen were all positive to zero volt. These potentials may be owing to the formation of a phase in the Ni-Pd-H alloys which is analogous t o a-Pd-H and for which the potential-determining reaction involves an equilibrium between hydrogen ions in solution and hydrogen in the metal.ls The open-circuit reversible potentials for the nickel-palladium alloys between 32 and 99 atomic % nickel were all equal to zero. This showed that these alloys were single phase systems on which the only potential-determining reaction occurring was the same as on a Pt/H2 electrode. For pure nickel, the open-circuit wtential was negative to the Pt/H2 electrode. This was because nickel dissolved in the acid solution and the (18) 9. Schuldiner, G . W. Castellan and J. P. Hoar6 J . Chcm. Phva. in press; G . W. Castellan, J. P. Hoare and 8.Schuldiner, ibid., in press; J. P. Hoare, S. Schuldiner and G. W. Castellan, ibid., in press.
1
I
HYDROGEN OVERVOLTAGE MEASUREMENTS ON NICKELAND PALLADIUM
Feb., 1958
4.0k
4:
DIAMAGNETIC
0.54
I ‘k I.o FERROMAGNETIC
j
t
0
20
0
0.25
-
U
40 60 ATOMIC% Ni 0.67 150 Ni/S.d ATOM RATIO.
5 4.0
03
Fig. 2.--Effect of nickel content on the rate constant of the over-all hydrogen-producing reaction in the range in which 9 is linearly dependent on current density. Also shown, a8 a broken line, is the relationship found by Tverdovski and Vert4 between nickel content and hydrogen solubility (as proton/Pd atom ratios).
potential was determined by the activity of nickel ions. The potential of the.pure nickel was not a steady-state value since the nickel ion concentration slowly increased with time. The potential values for the alloys with less than 100 atomic yo nickel indicate that the solubility of the nickel-palladium alloys in hydrogen-saturated sulfuric acid solution was negligible. A solubility determination was made for the 94y0 nickel alloy. It was found that in a period of 24 hours no detectable amount of the metal went into solution. However, in acid solutions exposed to air, the high nickel content alloys did dissolve to some extent and negative potentials were obtained. Catalytic Activity.-It has been pointed out by the authors6s7that since there is a linear relationship between the overvoltage and the current density in the low current region, the ratio K = -di/dq was a measure of the catalytic activity of the hydrogen producing reaction. Figure 2 shows the relationship between the catalytic activity, the composition of the nickel-palladium alloys, and the solubility of hydrogen in these metals. The solubility curve for these alloys was determined by Tverdovski and Vert4and is shown as a broken line in Fig. 2. There is an essentially linear relationship between hydrogen solubility and catalytic activity since both of these factors depend on the per cent. of nickel in the alloy in about the same way. It is also interesting to note that at about the point where
233
the hydrogen solubility is negligible (and where the catalytic activity levels off) the magnetic susceptibility of the alloys changes from paramagnetic to ferromagnetic. The higher the hydrogen content, the lower would be the magnetic susceptibility since electrons from the hydrogen fill positive holes in the d-band of the The catalytic activity also decreases with an increase in the number of positive holes in the d-band. This is because the heat of adsorption of atomic hydrogen increases as the number of holes increases.14 Since the ratedetermining step is the electrochemical desorption of chemisorbed atomic hydrogen, the catalytic activity decreases as the heat of adsorption of atomic hydrogen increases.’JJ3 i Using Wohlfarth’ss suggestion that the number of holes per atom in the d-band remained constant at 0.6 for all the Ni-Pd alloys, it can be concluded that for the Ni-Pd-H system, from an atomic nickel content of zero to 25%, the number of holes per atom will vary from about zero to about 0.G depending on the hydrogen solubility. Above 25% nickel the number of holes can be considered to remain constant a t 0.6. It is interesting to note that when a small amount of palladium is alloyed with nickel, the catalytic activity for the hydrogen producing reaction is greater than in the case of pure nickel or alloys of nickel content from 30 to 85 atomic %. I n addition, these low palladium alloys have Tafel slopes of 0.04 as in the case of the high palladium alloys which dissolve hydrogen. These facts strongly indicate tthat a significant preferential segregation of pure palladium or high palladium alloys may occur on the surface of the 94, 98 and 99% nickel alloys. This may be due to two different reasons. The first probably is the difference of surface tensions of palladium and nickel. The surface tensions of these metals are unknown. However, since heats of sublimation would be related in the same way as surface tensions, and since the heat of sublimation of nickel is about 8 kcal.lg higher than that of palladium, it is possible that there will be a preferential increase of palladium concentration on the surface. The second reason is that during the activation procedure for these alloys, especially the cleaning with nitric acid, there may be a preferential solution of nickel and a concentrating of palladium on the surface. These effects evidently are not significant for the 30 to 85% nickel content alloys. Acknowledgments.-The authors are indebted to Drs. J. C. White and G. W. Castellan of the Electrochemistry Branch, and to Drs. A. I. Schindler and E. I. Salkovite of the Metallurgy Division, U. S. Naval Research Laboratory, for their suggestions and advice during the course of this work. (19) W. M. Latimer “The Oxidation States of the Elements and Their Potentials in Aqueous Solutions,” 2d ed., Prentice-Hall, Inc., New York, N. Y., 1952, pp 199. 203.
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