Hydrogen interaction in palladium alloys

Journal

of the Less-Common

HYDROGEN

Metals,

INTERACTION

154 (1989)

285

285

- 294

IN PALLADIUM

ALLOYS

M. SHAMSUDDIN Department University, (Received

of Metallurgical Engineering, Varanasi-221 005 (India) October

Institute

of Technology,

Banaras

Hindu

3, 1988)

Summary Thermodynamic data, reported in the literature, on the solubility of hydrogen in palladium alloys, obtained by the equilibrium-calorimetric technique, have been used in discussing the nature of the hydrogen interaction. Both metal-hydrogen and hydrogen-hydrogen interactions in these alloys have been discussed in terms of Sieverts’ constants and interaction parameters. With the aid of Sieverts’ constant and the self-interaction parameter for hydrogen, it has been clearly demonstrated that the nature of the hydrogen-hydrogen interaction in palladium alloys changes from attractive to repulsive at a proportion of gold, silver or manganese of around 30 - 40 at.%.

1. Introduction Historically, palladium-hydrogen happens to be the first system which attracted research activities after Graham [l] had discovered the absorption of hydrogen in palladium and palladium-silver alloys. In fact, the Pd-H system [2] was the most closely studied because of the high solubility and mobility of hydrogen in the f.c.c. palladium lattice as well as the superconducting [3] nature of the hydrogen-rich P-phase. Despite the exhaustive literature covering both experimental and theoretical studies, the nature of the hydrogen interaction in palladium alloys is not yet well understood. This aspect has recently been discussed in a review by McLellan and Yoshihara [4], but the significance of the salient features of the hydrogen interaction was somewhat obscured because of the length of the paper. The present communication is an attempt at an understanding of the interaction of hydrogen in palladium alloys based on the thermodynamic data reported by Kleppa and coworkers [ 5 - 121. The thermodynamic data used in this paper were determined by a combined equilibrium-calorimetric technique [ 13, 141, which, with each sample of palladium alloy, the equilibrium pressure of hydrogen and the partial molar enthalpy of the solution of hydrogen in the alloy were measured simultaneously. 0022-5088/89/$3.50

@ Elsevier

Sequoia/Printed

in The Netherlands

286

2. Theoretical

background

From the equilibrium pressure of hydrogen (deuterium) p over an alloy, the partial molar free energy A&m,) is obtained from the relation AGHW

(1)

=~RT~~PH~(D~)

The relationship x and the equilibrium ing to the equation 103x -= l/2 P where

a +

between the solubility of hydrogen in the alloy sample pressure p may be represented by Sieve&’ law accord-

bp1’2

(2)

nH

X= nPd

+ GM

(M z Au, Ag, Cu, Ni, Co, Fe, Mn). The coefficients a and b in eqn. (2), known as Sieverts’ constants, may be used to calculate [63 the excess partial molar free energy A@& and its derivative with X: AGE(n,(

x -

0) = -RT

In ~(760)~‘~

(3) (4)

The observed linear relation fin(nf

partial

molar

enthalpies

NH(D)

are

represented

= c + d;rc

by the (5)

The interaction between two or more dissolved solutes centrations in palladium can be examined through the effect activity coefficient of the other. Wagner [15] first introduced an interaction parameter, which for the effect of an alloying the activity coefficient of hydrogen is expressed as

at dilute conof one on the the concept of element M on

a In vn eM-- Hax&l

(6)

where vn and XM are respectively the activity coefficient of hydrogen and the atomic fraction of the alloying element M in the Pd-H-M system. A negative value of E$ means attraction between M and H atoms, whereas a positive value is indicative of M-H repulsion. The self-interaction parameter of hydrogen E: is defined as a In Etn axH in

which xn refers to the atomic fraction

of hydrogen

in the alloy.

287

3. Discussion When palladium is alloyed with gold, silver or manganese (up to 33 at.% Mn) the lattice parameter is increased, and when it is alloyed with copper, nickel, cobalt or iron, the lattice parameter is reduced. This correlates with the fact that alloying palladium with gold, silver and manganese makes the whereas alloying it with copper, enthalpy of solution more exothermic, nickel, cobalt and iron makes the enthalpy of solution less exothermic or indeed endothermic, depending on the alloy composition. The solubility of hydrogen (deuterium) in palladium alloys obeys Sieve&’ law, expressed by eqn. (2). Typical plots for hydrogen in palladium-gold alloys are shown in Fig. 1. In this expression a and b are constants at constant temperature. The coefficient a is always positive and its numerical value is a direct measure of the strength of the interaction between hydrogen and the metallic solvent at high dilution; a low value shows a weak interaction and hence low solubility. Numerically this will be reflected both in a correspondingly large positive value of AC”, (--RT In ~(760)“~ when x + 0) and in a small negative or positive value of A& (3~+ 0). The second coefficient b reflects the effective hydrogen-hydrogen interaction in the solution. This coefficient may have a positive, negative or zero

0

5

10

15

20

25

l/2 TF

Fig. 1. x/p”2

(torr)

vs. plf2 for hydrogen

in I’d-Au

alloys at 555 and 700 K.

288

value. A positive b indicates hydrogen-hydrogen attraction, a negative value repulsion and a value of zero means that the solution is “ideal” according to Henry’s law. The values of the coefficients a and b for palladium and its alloys at 555 K and 700 K have been listed in Table 1 and for 555 K also plotted in Figs. Z(a) and 2(b). For comparison, coefficients for deuterium at 555 K have also been incorporated in the table. From Table 1 and Fig. 2(a) it is seen that the coefficient a is positive for palladium and all its alloys. The value is larger for Pd-Au, Pd-Ag and Pd-Mn (up to 33 at.% Mn) than for pure palladium. This correlates with the fact that when palladium is alloyed with gold, silver and manganese (up to 33 at.%), the lattice expands and more hydrogen enters the lattice, exhibiting thereby more exothermicity in the partial molar enthalpy of solution of hydrogen. By contrast a decrease in the value of a for Pd-Cu, Pd-Ni, PdCo and Pd-Fe alloys is consistent with the contraction of the palladium lattice and hence low solubility and less exothermicity in the partial molar TABLE 1 Summary of thermodynamic dium and palladium alloysb Solvent

data a for dilute solutions of hydrogen (deuterium) in palla-

700 K, Hz

555 K, Hz IO30 (Torr-1’2)

106b (Torr-‘)

Pd Pdo.soAuo.Io Pdo.7sAuo.x Pd0.60Au0.,,,

0.500 0.640 0.743 0.624

Pdo.aoAgo.lo Pd o.7sAg0.2s Pd o.aoAgo.w

0.914 2.042 3.12

7.8 21.9 -15.7

Pdo~aaCuo,,o 0.404 Pdo.7sCuo.x 0.269 Pd0.60Cu0.w 0.139

0.99 0.23 0.05

Pdo.~Nio.re Pd 0.7sNio.zs Pdo.wCoo_Io Pde.~Feo.ro Pdo.~Mno.ra Pdo.szMlb!rs Pdo.67Mno.33 Pde.~Mn,.~ Pde.soMno.so

0.229 0.068 0.201 0.177 0.503 0.574 0.537 0.033 0.008

2.60 3.00 2.15 -2.74

0.52 0.00 0.30 9.30 1.716 3.34 -8.82 0.26 0.00

555 K, Dz

555 K, Hz

1 03a (Torr-1’2)

106b (TOI--‘)

lo3 a (Torr-1’2)

I06b (Torr-‘)

0.373 0.422 0.410 0.306

0.90 0.54 0.02 -0.50

0.333 0.438 0.523 0.453

1.50 1.36 0.80 -1.60

-8.86 -4.03 -2.48 +9.13

0.572 0.964 1.19

2.0 3.7 -1.4

0.631 1.403 2.240

3.2 10.0 -7.4

-6.89 -2.68 +20.49

-

-

0.271 0.181 0.097

0.56 0.33 0.02

-1.79 -0.06 (0)

0.192 0.069 0.342 0.310 0.154 0.021 0.004

0.08 0.00 0.629 0.695 0.00 0.00 0.00

0.156 0.044 0.136 0.126 0.342 0.404 0.371 -

0.13 0.00

-1.07 0.00

:z 0:439 0.759 -4.29 -

4

(-14) (-8) -1.05 -10.04 +30.15 (-5) 0.00

aBased on refs. 5 - 12. bThe equilibrium pressures (in Torr) are a function of the alloy composition (npd + nM) according to the relation 103x = ap112 + bp.

x = nH/

289

j

10 (a)

O

0

30

20 Atomic

% M

,

10

30

20 Atomic

(b)

j$(

,

0

% M

Fig. 2(a). a us. alloy composition for solutions of hydrogen in palladium alloys at 555 (b) b us. alloy composition for solutions of hydrogen in palladium alloys at 555 K.

K;

enthalpy. In all the cases the solubility (i.e. a) of hydrogen as well as of deuterium decreases with an increase in temperature (Table 1). The values of changes in a and volume on alloying listed in Table 2 show that no convincing quantitative agreement can be established except the change in a depends on the amount of expansion or contraction. The degree of exothermicity expressed as ~H(anoyj - fin,,,, is directly related to the amount of expansion or contraction of the palladium lattice. The values of the interaction parameters listed in Table 3 show the effect of the alloying elements, i.e. gold, silver, copper, nickel, cobalt, iron

TABLE Some

2 relevant

data on palladium

alloys

Alloy

AVa ( cm3 mol-‘)

Aab (TorrP1’2)

A&llOY - A&, (kcal molP’)

Pdo.sAuo.1 Pdo.sAgo.1 Pdo.sCuo.1 Pda.9Nia.r Pdo.sCoo.1 Pda.sFec. 1 Pda.sMne.1

0.133 0.162 -0.163 -0.170 -0.224 -0.102 0.010

0.140 0.414 -0.096 -0.271 -0.299 -0.323 0.003

-0.71 -0.89 0.16 0.65 0.63 0.40 -0.45

a valley b aalloy

-

vPd. aPd.

290 TABLE

3

Atomic

interaction

Interaction parameter 4?

in palladium

Alloying

alloys

additions

(M)

AU

Ag

cu

Ni

CO

Fe

Mn

-2.36

-6.42

2.63

8.58

9.70

10.96

-0.18

and manganese (dilute concentration) on the activity coefficient of hydrogen in palladium. In terms of interatomic attraction the negative values of ept e$ and EF may be understood on the basis of a very simple model, illustrate by the palladium-hydrogen-gold or silver or manganese solutions. In Pd-H solutions each hydrogen atom is surrounded by a number of palladium atoms which share the Pd-H bonding energy, The bonding of gold, silver and manganese with hydrogen is stronger than the Pd-H bonding and this leads to two results: (i) the ratio Au:Pd, Ag:Pd or Mn:Pd is greater in the nearest neighbourhood of hydrogen atoms than in the bulk of the solution and (ii) the hydrogen atoms become more firmly bonded as the gold or silver or manganese concentration increases. The values of the interaction parameters (Table 3) demonstrate that the effect of silver on hydrogen is greater than that of gold while that of manganese is much lower. The extent of this effect depends on the amount of expansion on alloying (Table 2) which is due to the repulsion of palladium and gold or silver or manganese (around 33 at.%) atoms in the nearest neighbourhood. Copper, nickel, cobalt and iron show a positive effect, i.e. the activity coefficient of hydrogen in pa~adium increases on alloying with these elements. Each atom of copper, nickel, cobalt or iron is probably bonded, like hydrogen, to a nearest-neighbour shell of palladium atoms. Bonding of palladium atoms to copper, nickel, cobalt or iron leaves less palladium to be bonded with hydrogen and raises the activity coefficient of the hydrogen. This competition for palladium atoms increases the activity coefficient of each competing species so long as the effect is not overshadowed by stronger intersolute attractive forces as in Pd-H-Cu, Pd-H-Ni, Pd-H-Co and Pd-HFe solutions. In his early studies on solutions of hydrogen in palladium and its alloys Sieverts [16] showed that in pure palladium the coefficient b was positive at lower temperatures but approached zero at higher temperatures. Similar behaviour has been observed in these alloys. Figure 1 very clearly demonstrates that the coefficient b (i.e. slope of plots of X/PI/~ us. p”*) decreases and then changes its sign from positive to negative with an increase in gold content from 10 to 40 at.% at both the temperatures 555 and 700 K. A similar trend has been observed in A@n us. x plots (Fig. 3). From Fig. 2(b) it is evident that b changes its sign from positive to negative at around 30 40 at.% Au, Ag or Mn. This means that the hydrogen-hydrogen interaction

291

I

1

t

555K,

4.6 -

Pd-40

at.%

Hz

_

Au

3.8 Pd - 25 ot.%

1

Au

I

I

3.63.2 -

Fig. 3.

I’d-25

at.%

Au

Agn US.x for hydrogen

in Pd-Au

alloys at 555 and 700 R.

changes from attractive to repulsive in this concentration range. Table 1 represents a similar trend for hydrogen as well as deuterium. The numerical values of b for Pd-Cu, Pd-Ni and Pd-Co at 555 and 700 K are quite low, indicating that these solutions are nearly ideal. In these alloys b approaches zero with an increasing quantity of alloying elements and an increase in temperat~e (Table l), for hydrogen as well as for deuterium. In the Pd-Mn system the nature of the interaction changes again from repulsive to attractive near 40 at.% Mn. The successive changes from attractive to repulsive and then repulsive to attractive in Pd-Mn alloys indicate the complex nature of the system in the composition range 30 - 40 at.% Mn, exhibiting order-disorder transformations. Phutela and Kleppa [lo] have made a detailed analysis of this solid state transformation based on the thermodynamic properties of hydrogen in Pd-Mn alloys. The change in the nature of the hydrogen-hydrogen interaction from attractive to repulsive in palladium alloys at around 30 - 40 at.% Au, Ag and Mn is also evident from the change in sign of the self-interaction parameters of hydrogen EE. From Table 1 it is clear that E: is negative or positive respectively for the positive or negative values of b. A positive value of b or negative value of E: indicates hydrogen-hydrogen attraction. A negative b or positive E: means repulsion. A zero value of b or E: shows that the solution

292

is ideal in the sense of Henry’s law. This ideal behaviour has been observed for the solubility of hydrogen and deuterium in Pd0_7sNi0.25 and for hydrogen in Pd,,Mn,, at 555 K and in Pd0.67Mn0.s3, Pdo.$lno.4 and Pd,,Mn,, at 700 K (Table 1). It is interesting to make a more direct comparison between the data shown in Figs. Z(a) and 2(b). The effect of alloying additions on A(?“, (x -+ 0) and aA@$/ax presented in Figs. 4(a) and 4(b) demonstrates the complete consistency between our own results. It may be noted from Fig. 4(b) that aA@/ax, which principally reflects the ratio b:a2, changes its sign from negative to positive near 30 at.% Au, 40 at.% Ag and 30 at.% Mn. In Pd-Mn the derivative again changes sign from positive to negative on approaching 40 at.% Mn. In Pd-Ni the derivative aA@$jax approaches zero at 25 at.% Ni in comp~ison with its negative value at 10 at.% Ni. The derivative of the enthalpy of hydrogen &&fax in Pd-Au, Pd-Ag and Pd-Mn alloys changes sign from negative to positive at 30 - 40 at.% Au, Ag or Mn as is evident from Fig. 5(b). Figs. 4(a) and 5(a) show that AGE and mn vary in a similar manner. However, there is one significant difference in that while the values of AGz (x -+ 0) increase in the sequence Ag < Au < (Mn) < Cu < Ni < Co < Fe, the co~espond~g sequence for A& (x + 0) is Ag < Au < (Mn) < Cu < Fe < Co = Ni, The enthalpies of solution of hydrogen in Pd,.$?ie.l and Pd,.&o,,, at 555 K (Fig. 5(a)) indicate that endothermic enthalpies of solution undoubtedly will be found for proportions of palladium below 75 at.% [ 121. Such behav-

I

2.

I

10 Atomic

t

I

20

30 %

M

I 40

(b)

Atomic

% hi

Fig. 4. (a) AGE (x + 0) us. alloy composition for solutions of hydrogen in palladium alloys at 555 K; (b) aAt?&& vs. alloy composition for solutions of hydrogen in palladium alloys at 555 K.

LoE

‘0

y” _-

E II’ a

60

I

I

10

20

(a)

Atomic

I

I

-32t

30 %

M

,

,

10 Atomic

04’

Fig. 5. (a) A& (x + 0) us. alloy composition for x us. alloy composition alloys at 555 K; (b) a-,/a dium alloys at 555 K.

,

20 %

30

i, 40

M

solutions of hydrogen in palladium for solutions of hydrogen in palla-

iour has actually been observed for deuterium for hydrogen in this alloy at 700 K [ 121.

in PdO.,,Ni,.,,

at 555

K and

4. Conclusions (1) The more exothermic heat of solution of hydrogen (deuterium) in Pd-Au, Pd-Ag and Pd-Mn (up to 33 at.% Mn) is in conformity with the expansion of the palladium lattice on alloying with gold, silver or manganese and the negative values of the interaction parameters &“, cig and EE”. (2) The less exothermic or endothermic values of the heat of solution of hydrogen (deuterium) in Pd-Cu, Pd-Ni, Pd-Co and Pd-Fe alloys are in conformity with the contraction of the palladium lattice on alloying with copper, nickel, cobalt or iron and the positive values of the interaction parameters cg”, eF, e$’ and ~2. (3) The hydrogen-hydrogen interaction in Pd-Au, Pd-Ag and Pd-Mn alloys changes from attractive to repulsive at around 30 - 40 at.% Au, Ag and Mn. Both Sieverts’ constant b and the self-interaction parameter of hydrogen confirm this change-over. The nature of the interaction in Pd-Mn alloys is more complex owing to the order-disorder transformation in the system.

294

Acknowledgment The author is grateful to Professor O.J. Kleppa of the University of Chicago for introducing him to the subject of the thermodynamics of metalhydrogen systems. We is thankful to Professor V. B. Tare for valuable suggestions in preparing the manuscript.

References 1 T. Graham, Philos. Trans. H. Sot. London, 156 (1966) 415. 2 E. A. Lewis, The Pa&dium Hydrogen System, Academic Press, London, 1967. 3 T. Skoskiewicz, Ber. Bunsenges. Phys. Chem., 76 (1972) 874; Phys. Status Solidi, K123 (1972) 6. 4 R. B. McLellan and M. Yoshihara, Acta MetaEI., 35 (I) (1987) 197. 5 G. Boureau, 0. J. Kleppa and P. Dantzer, J. Chem. Phys., 64 (1974) 5247. 6 G. Boureau and 0. J. Kleppa, J. Chem. Phys., 65 (1976) 3915. 7 C. Picard, 0. J. Kleppa and G. Boureau, J. Chem. Phys., 70 (1979) 2710. 8 0. J. Kleppa, M. Shamsuddin and C. Picard, J. Chem. Phys., 71 (1979) 1656. 9 M. Shamsuddin and 0. J. Kleppa, J. Chem. Phys., 71 (1979) 5154. 10 R. C. Phutela and 0. J. Kleppa, J. Chem, Phys., 75 (1981) 4095. 11 R. C. Phutela and 0. J. Kleppa, J. Chem. Phys., 76 (1982) 1525. 12 M. Shamsuddin and 0. J. Kleppa, J. Chem. Phys., 80 (1984) 3760. 13 0. J. Kleppa, M. E. Melnichak and T. V. Charm, J. Chem. Thermodyn., 5 (1973) 595. 14 G. Boureau and 0. J. Kleppa, J. Chem. Thermodyn., 9 (1977) 543. 15 Carl Wagner, Thermodynamics of Alloys, Addison-Wesley, Reading, MA, 1952. 16 A. Sieverts, 2. Phys. Chem., 88 (1914) 451.