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Pressure-composition isotherms in the palladium-copper-hydrogen system
Solid State Communications,Vol. 15, PP. 407—409, 1974.
Pergamon Press.
Printed in Great Britain
PRESSURE-COMPOSITION ISOTHERMS IN THE PALLADIUM—COPPER—HYDROGEN SYSTEM R. Burch and R.G. Buss Department of Chemistry, The University, Whiteknights, Reading, England (Received 15 February 1974 by C. W. McCombie)
Pressure-composition isotherms have been determined at 303K for palladium—copper alloys containing 5, 10, 15 and 20 at.% copper. It has been found that the solubility of hydrogen at 1 atmosphere pressure decreases linearly as the copper content of the alloys is increased.
PRESSURE-composition isotherms in palladium alloy—hydrogen systems exhibit two characteristic properties which are of theoretical interest. These are the variations with alloy composition of the relative partial molar enthalpy of hydrogen at infinite dilution, and the limiting solubility at high pressures of gaseous hydrogen. A number of palladium alloy systems have been investigated,’ and several theoretical models have been proposed.25 These theories differ essentially in the relative importance which is placed upon electronic or elastic interactions in the metal during absorption of hydrogen. Thus, for palladium— silver alloys it has been suggested that the dependence of the relative partial molar enthalpy of hydrogen on alloy composition is due either to elastic3 or electronic effects.4 Furthermore, for alloys of palladium with metals such as nickel it has been suggested5 that the solubility limit at high pressures of hydrogen may be dependent on the lattice parameter of the alloy rather than on the electron—atom ratio which appears to dominate in a number of other alloy systems. None of the alloy systems which have been investigated so far enable a clear distinction to be made between these two alternatives. In the palladium—silver alloy system, for example, there is a simultaneous increase in the lattice parameter and in the electron—atom ratio of the alloys, either of which effects could account for the increased enthalpy of solution,
ratio increases in exactly the same way as in palladium— silver alloys. For these reasons a comparison between the pressure—composition isotherms for Pd Cu and PdAg alloys would be very informative. Only a single investigation of the absorption of hydrogen by PdCu alloys has been reported,6 and it was found that copper was apparently donating two electrons per atom to palladium. This result is surprising in view of the similar magnetic susceptibilities of PdCu and PdAg alloys. Because of the theoretical importance of these systems it was decided to investigate the absorption of hydrogen by a series of PdCu alloys.
A more useful series of alloys would be palladium— copper alloys, in which the lattice parameter decreases as the copper content increases while the electron—atom 407
The alloys were prepared by melting accurately weighed amounts of the Specpure metals (Johnson Matthey and Co. Ltd.) under high purity argon in an arc furnace. The alloy buttons were annealed under argon for 1 week at 800°C and then rolled into the form of a thin strip about 1 m X 5 mm X 0.2 mm. The composition of the alloys was calculated from the initial weights since the weight loss on heating was negligible. Full details of the absorption procedure will appear elsewhere7 but the method consisted of activating the samples by electroplating with palladium black and then mounting the samples in a Sieverts-type apparatus where they absorbed measured quantities of hydrogen. By subsequently removing known quantities of hydrogen from the gas phase it was possible to obtain desorption pressure-composition isotherms for each of the alloys. The approach to equilibrium was monitored continuously by making simultaneous measurements of the resistance of the samples, and
408
ISOTHERMS IN THE PALLADIUM—COPPER—HYDROGEN SYSTEM
reaction was only considered complete when the resistance became constant. The desorption isotherms at 303K for alloys containing 5, 10, 15 and 20 at.% copper are shown in Fig. 1. Each isotherm is the combination of at least two separate experiments. The previously reported isotherms6 at 297K are shown for comparison in Fig. 2.
~orr 100
Vol. 15, No.2
discrepancies are possible. Firstly, the alloys used by the earlier workers were prepared by co-deposition from solution and the samples may not have been homogeneous. Secondly, their absorption technique involved the absorption of hydrogen from hydrogensaturated solutions. This technique is very dependent on having highly active samples. However, we have found that PdCu alloys are extremely difficult to activate, and samples have to be reactivated at frequent intervals. We believe, therefore, that the discrepancies in the two sets of data can be attributed to the fact that the samples used by the previous workers were not sufficiently active for true equilibrium to be attained.
06 07
H /Me 05 1
____________________________________________________________________________
0
02
04 H~ /Me
06
08
03
FIG. 1. Desorption pressure—composition isotherms at 303K for pattadium—copper alloys. X, 5%Cu; 0,10%Cu;-l- 15%Cu; A,20%Cu;—,Pd.
i000
e
7// 100 /torr 10
1
01
02
d c
04
b
a
02
________________________________ 0
5
10
15 % Cu
20
25
FIG. 3. Solubility of hydrogen at 303K and 760 torr as a function of alloy composition (at.% copper). 0
~EEEEEEEEEEE 03
04 H “Me
05
06
FIG. 2. Absorption pressure—composition isotherms at 297K from reference 6. a,4%Cu; b,8%Cu; c,12%Cu; d,15%Cu;e,2l%Cu. There are clearly large differences between the two sets of data. At least two reasons for these
present plateau Since results pressures our data x from cannot represents reference be compared desorption 6. two isotherms the since absorption can be the compared pressure—composition isotherms. in theHowever, single phase the relationships (awith orsets j3)the regions of inearlier data these regions are identical during both absorption and desorption. The differences in the data can be illustrated best by plotting the solubility at, say, 760 torr against alloy composition. This is shown in Fig. 3. From this type of plot the earlier workers had obtained the relationship H
2Cu +
Pd+Cu
=
constant
Pd+Cu
and hence had concluded that copper was divalent in CuPd alloys. However, the present data more closely fit the relationship
Vol. 15, No.2
ISOThERMS IN THE PALLADIUM—COPPER—HYDROGEN SYSTEM H
Cu
Pd + Cu + Pd + Cu
409
ratio increases as the copper content of the alloy =
constant
which indicates that copper is approximately monovalent. This is in agreement with the magnetic susceptibility data which have been referred to earlier, The present results are significant as regards current theoretical interpretations of the pressure—composition isotherms. Firstly because they indicate that the equffibrium pressure at any hydrogen to metal atomic
increases, which is in contrast to the behaviour observed in the PdAg system. And secondly, because the solubifity at 1 atmosphere is rather similar to the solubiity in an equivalent PdAg alloy. The theoretical implications of these results will be discussed more fully elsewhere.7 Acknowledgements RGB wishes to thank the Science Research Council for the award of a post. graduate studentship. —
REFERENCES 1.
LEWIS F.A., The Palladium Hydrogen System, Academic Press, London and New York (1967).
2.
SIMONS J.W. and FLANAGAN T.B., Can. J. Chem., 43, 1665 (1965).
3.
BRODOWSKY H., Z. Phys. Chem. (N.F.) 44,129(1965).
4.
EBISUZAKI Y. and O’KEEFFE M.,J. Phys. Ozem., 72,4695 (1968).
5.
BURCH R., Trans. Faraday Soc., 66, 736 (1970).
6.
KARPOVA R.A. and TVERDOVSKII I.P., Zhur. fiz. Khim. 33, 1393 (1959).
7.
BURCH R. and BUSS R.G., to be published.
Druck—Komposition Isotherme bei 303K fur Pd—Cu Legierungen mit 5, 10, 15 and 20 At.% Kupfer wurden bestimmt. Die Loslichkeit von Wasserstoff unter 1 Atmosphare Druck vermindent sich linear mit steigendem Kupfer Inhalt der Legierung.
Pergamon Press.
Printed in Great Britain
PRESSURE-COMPOSITION ISOTHERMS IN THE PALLADIUM—COPPER—HYDROGEN SYSTEM R. Burch and R.G. Buss Department of Chemistry, The University, Whiteknights, Reading, England (Received 15 February 1974 by C. W. McCombie)
Pressure-composition isotherms have been determined at 303K for palladium—copper alloys containing 5, 10, 15 and 20 at.% copper. It has been found that the solubility of hydrogen at 1 atmosphere pressure decreases linearly as the copper content of the alloys is increased.
PRESSURE-composition isotherms in palladium alloy—hydrogen systems exhibit two characteristic properties which are of theoretical interest. These are the variations with alloy composition of the relative partial molar enthalpy of hydrogen at infinite dilution, and the limiting solubility at high pressures of gaseous hydrogen. A number of palladium alloy systems have been investigated,’ and several theoretical models have been proposed.25 These theories differ essentially in the relative importance which is placed upon electronic or elastic interactions in the metal during absorption of hydrogen. Thus, for palladium— silver alloys it has been suggested that the dependence of the relative partial molar enthalpy of hydrogen on alloy composition is due either to elastic3 or electronic effects.4 Furthermore, for alloys of palladium with metals such as nickel it has been suggested5 that the solubility limit at high pressures of hydrogen may be dependent on the lattice parameter of the alloy rather than on the electron—atom ratio which appears to dominate in a number of other alloy systems. None of the alloy systems which have been investigated so far enable a clear distinction to be made between these two alternatives. In the palladium—silver alloy system, for example, there is a simultaneous increase in the lattice parameter and in the electron—atom ratio of the alloys, either of which effects could account for the increased enthalpy of solution,
ratio increases in exactly the same way as in palladium— silver alloys. For these reasons a comparison between the pressure—composition isotherms for Pd Cu and PdAg alloys would be very informative. Only a single investigation of the absorption of hydrogen by PdCu alloys has been reported,6 and it was found that copper was apparently donating two electrons per atom to palladium. This result is surprising in view of the similar magnetic susceptibilities of PdCu and PdAg alloys. Because of the theoretical importance of these systems it was decided to investigate the absorption of hydrogen by a series of PdCu alloys.
A more useful series of alloys would be palladium— copper alloys, in which the lattice parameter decreases as the copper content increases while the electron—atom 407
The alloys were prepared by melting accurately weighed amounts of the Specpure metals (Johnson Matthey and Co. Ltd.) under high purity argon in an arc furnace. The alloy buttons were annealed under argon for 1 week at 800°C and then rolled into the form of a thin strip about 1 m X 5 mm X 0.2 mm. The composition of the alloys was calculated from the initial weights since the weight loss on heating was negligible. Full details of the absorption procedure will appear elsewhere7 but the method consisted of activating the samples by electroplating with palladium black and then mounting the samples in a Sieverts-type apparatus where they absorbed measured quantities of hydrogen. By subsequently removing known quantities of hydrogen from the gas phase it was possible to obtain desorption pressure-composition isotherms for each of the alloys. The approach to equilibrium was monitored continuously by making simultaneous measurements of the resistance of the samples, and
408
ISOTHERMS IN THE PALLADIUM—COPPER—HYDROGEN SYSTEM
reaction was only considered complete when the resistance became constant. The desorption isotherms at 303K for alloys containing 5, 10, 15 and 20 at.% copper are shown in Fig. 1. Each isotherm is the combination of at least two separate experiments. The previously reported isotherms6 at 297K are shown for comparison in Fig. 2.
~orr 100
Vol. 15, No.2
discrepancies are possible. Firstly, the alloys used by the earlier workers were prepared by co-deposition from solution and the samples may not have been homogeneous. Secondly, their absorption technique involved the absorption of hydrogen from hydrogensaturated solutions. This technique is very dependent on having highly active samples. However, we have found that PdCu alloys are extremely difficult to activate, and samples have to be reactivated at frequent intervals. We believe, therefore, that the discrepancies in the two sets of data can be attributed to the fact that the samples used by the previous workers were not sufficiently active for true equilibrium to be attained.
06 07
H /Me 05 1
____________________________________________________________________________
0
02
04 H~ /Me
06
08
03
FIG. 1. Desorption pressure—composition isotherms at 303K for pattadium—copper alloys. X, 5%Cu; 0,10%Cu;-l- 15%Cu; A,20%Cu;—,Pd.
i000
e
7// 100 /torr 10
1
01
02
d c
04
b
a
02
________________________________ 0
5
10
15 % Cu
20
25
FIG. 3. Solubility of hydrogen at 303K and 760 torr as a function of alloy composition (at.% copper). 0
~EEEEEEEEEEE 03
04 H “Me
05
06
FIG. 2. Absorption pressure—composition isotherms at 297K from reference 6. a,4%Cu; b,8%Cu; c,12%Cu; d,15%Cu;e,2l%Cu. There are clearly large differences between the two sets of data. At least two reasons for these
present plateau Since results pressures our data x from cannot represents reference be compared desorption 6. two isotherms the since absorption can be the compared pressure—composition isotherms. in theHowever, single phase the relationships (awith orsets j3)the regions of inearlier data these regions are identical during both absorption and desorption. The differences in the data can be illustrated best by plotting the solubility at, say, 760 torr against alloy composition. This is shown in Fig. 3. From this type of plot the earlier workers had obtained the relationship H
2Cu +
Pd+Cu
=
constant
Pd+Cu
and hence had concluded that copper was divalent in CuPd alloys. However, the present data more closely fit the relationship
Vol. 15, No.2
ISOThERMS IN THE PALLADIUM—COPPER—HYDROGEN SYSTEM H
Cu
Pd + Cu + Pd + Cu
409
ratio increases as the copper content of the alloy =
constant
which indicates that copper is approximately monovalent. This is in agreement with the magnetic susceptibility data which have been referred to earlier, The present results are significant as regards current theoretical interpretations of the pressure—composition isotherms. Firstly because they indicate that the equffibrium pressure at any hydrogen to metal atomic
increases, which is in contrast to the behaviour observed in the PdAg system. And secondly, because the solubifity at 1 atmosphere is rather similar to the solubiity in an equivalent PdAg alloy. The theoretical implications of these results will be discussed more fully elsewhere.7 Acknowledgements RGB wishes to thank the Science Research Council for the award of a post. graduate studentship. —
REFERENCES 1.
LEWIS F.A., The Palladium Hydrogen System, Academic Press, London and New York (1967).
2.
SIMONS J.W. and FLANAGAN T.B., Can. J. Chem., 43, 1665 (1965).
3.
BRODOWSKY H., Z. Phys. Chem. (N.F.) 44,129(1965).
4.
EBISUZAKI Y. and O’KEEFFE M.,J. Phys. Ozem., 72,4695 (1968).
5.
BURCH R., Trans. Faraday Soc., 66, 736 (1970).
6.
KARPOVA R.A. and TVERDOVSKII I.P., Zhur. fiz. Khim. 33, 1393 (1959).
7.
BURCH R. and BUSS R.G., to be published.
Druck—Komposition Isotherme bei 303K fur Pd—Cu Legierungen mit 5, 10, 15 and 20 At.% Kupfer wurden bestimmt. Die Loslichkeit von Wasserstoff unter 1 Atmosphare Druck vermindent sich linear mit steigendem Kupfer Inhalt der Legierung.