20
Downloaded by UNIV MASSACHUSETTS AMHERST on October 2, 2012 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0167.ch020
Palladium-Hydrogen: The Classical Metal-Hydrogen System TED B. FLANAGAN Department of Chemistry, University of Vermont, Burlington, VT 05401 W. A. OATES Department of Metallurgy, University of Newcastle, Newcastle, New South Wales, Australia
The current status of the palladium-hydrogen system is discussed with emphasis on thermodynamics of hydrogen solution. New pressure-composition-temperature data have been obtained in the dilute region of primary solubility, i.e., before hydride-phase formation. From these data, partial excess entropies of solution have been obtained and are shown to be independent of hydrogen content. These data suggest that the hydrogen-hydrogen interaction mainly is enthalpic in origin; the interaction enthalpies have been obtained over a wide temperature range. These new data are discussed relative to existing models for this system.
P
a l l a d i u m - h y d r o g e n occupies a u n i q u e position a m o n g m e t a l - h y d r o g e n systems. T h e r e have been hundreds of publications on this system since the discovery b y G r a h a m i n 1866 (J) that p a l l a d i u m can absorb large amounts of hydrogen. Interest i n p a l l a d i u m - h y d r o g e n has c o n t i n u e d p a r t l y because of its great experimental convenience. T h e r e are no oxidation problems w i t h p a l l a d i u m , a n d the h y d r i d e phase can be f o r m e d without m e c h a n i c a l disintegration of the m e t a l , although extensive plastic d e f o r m a t i o n occurs (2). B y contrast, m a n y of the other hydrogen-absorbing metals are disrupted massively upon h y d r i d e formation. Recently, interest i n p a l l a d i u m - h y d r o g e n has intensified because techniques of neutron physics were a p p l i e d to this system (3) a n d sup e r c o n d u c t i v i t y i n p a l l a d i u m - h y d r o g e n w i t h very h i g h H : P d ratios (4) was discovered. 0-8412-0390-3/78/33-167-283/$05.00/0 © American Chemical Society In Transition Metal Hydrides; Bau, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1978.
284
TRANSITION M E T A L HYDRIDES
Review of Pressure-Composition-Temperature
Relationships
Downloaded by UNIV MASSACHUSETTS AMHERST on October 2, 2012 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0167.ch020
O n e of the most useful features of m e t a l - h y d r o g e n systems are their pres s u r e - c o m p o s i t i o n - t e m p e r a t u r e data, P-C-T. Such relationships for p a l l a d i u m - h y d r o g e n are shown i n F i g u r e 1. F o r compositions a n d temperatures w i t h i n the envelope, two solid phases coexist, as required by the phase rule. T h e lower hydrogen-content α-phase represents solution of hydrogen into the metal, a n d the higher hydrogen-content β-phase is the h y d r i d e . B o t h α a n d β are
H/Pd Figure
I.
Pressure-composition-temperature ships for palladium-hydrogen
relation (43)
In Transition Metal Hydrides; Bau, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1978.
20
285
Palladium-Hydrogen
FLANAGAN AND OATES
nonstoichiometric a n d , as pointed out b y Anderson (5), this was the first n o n stoichiometric system for w h i c h detailed t h e r m o d y n a m i c data were available. T h e phase d i a g r a m is k n o w n at m u c h lower temperatures (2) than shown i n F i g u r e 1, but it cannot be complete since the t h i r d law requires that nonstoichiometry cannot exist at 0°K; however, reasonable extrapolation of the β-phase boundary does not meet this requirement. There is probably a eutectoid reaction at quite low temperatures; some indications of an anomalous transformation have been observed (6, 7). Downloaded by UNIV MASSACHUSETTS AMHERST on October 2, 2012 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0167.ch020
A n early a p p l i c a t i o n (1952) of neutron diffraction (8) revealed that the hydrogen atoms occupy the octahedral interstices within the fee palladium lattice in the β-phase.
M o r e recent neutron scattering data have indicated that these
same interstices are occupied i n the α phase (9). Thus, the α and β phases differ only i n their hydrogen content and i n the lattice parameters of the c o n t a i n i n g p a l l a d i u m matrix.
F o r p a l l a d i u m - h y d r o g e n w i t h i n the two-phase envelope,
two distinct fee lattice parameters can be measured.
Baranowski et al. (10) have
shown that the v o l u m e expansion of p a l l a d i u m per added h y d r o g e n atom is the same for both α a n d β phases, i.e., the partial molar v o l u m e of hydrogen, V H , is independent of hydrogen content up to H : P d = η ^ 0.75 ( V H = 1 6 5 c m m o l 3
H
_ 1
).
Indeed, the same value of V H is found for fee p a l l a d i u m alloys a n d for
other fee systems that absorb hydrogen into octahedral interstices (JO). In the α-phase, it is found that at very low hydrogen concentrations η
