Thermodynamics and Kinetics of Hydrogen Absorption in Rare Earth

22 Thermodynamics and Kinetics of Hydrogen Absorption in Rare Earth-Cobalt (R Co and RCo ) and Rare Earth-Iron (RFe ) Compounds 2

7

3

Downloaded by PENNSYLVANIA STATE UNIV on September 25, 2013 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0167.ch022

3

A. GOUDY, W. E. WALLACE, R. S. CRAIG, and T. TAKESHITA Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260

Pressure-composition isotherms were obtained for some rare earth intermetallic hydrides of the type R Co (R = a rare earth). The compounds absorbed large quantities of hydrogen, giving the limiting composition R Co H at 100 atm. This solubility exceeds that of the RCo compounds but is less than that of the RCo compounds. Pr Ni and ErNi were included in this study for comparison. Kinetic studies of hydrogen desorption were conducted on the following representative compounds: Dy Co , Gd Co , DyCo , ErCo , DyFe , and ErFe . Desorption was second order, indicating the recombination of hydrogen atoms on the metal surface to form molecular hydrogen as the likely rate-determining step. Activation energies for this process range from 15-36 kcal/mol. Hydride stability varies systematically with atomic number of the constituents. 2

2

7

7

9

5

3

2

2

7

2

7

7

3

3

3

3

3

I

n

1969 Zijlstra a n d Westendorp ( I ) reported that the rare earth intermetallic S111C05 extensively absorbed hydrogen. T h e y subjected this m a t e r i a l , the most p o w e r f u l permanent magnet k n o w n , to metallographic examination, polishing and acid-etching it to reveal its grain structure. T h e y found that the magnetic properties of S m C o that h a d been processed this w a y were degraded sharply. T h e effect was traced to hydrogen that was absorbed into the lattice d u r i n g the acid etch. Almost simultaneously, N e u m a n n (2) observed that L a N i , w h i c h has a C a C u structure like S m C o , absorbs hydrogen. Shortly thereafter, V a n V u c h t , Kuijpers, a n d B r u n i n g (3,4) found that L a N i a n d some structurally related rare earth intermetallics ( R C o , etc., where R represents a rare earth) absorb hydrogen extensively, and the absorption (or desorption) occurs 5

5

5

5

5

5

0-8412-0390-3/78/33-167-312/$05.00/0 © American Chemical Society

In Transition Metal Hydrides; Bau, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

22.

Rare Earth-Cobalt

GOUDY E T AL.

313

and -Iron Compounds

extremely r a p i d l y . ( F o r i n f o r m a t i o n about the stoichiometries, structures, a n d properties of this extensive class of materials, see Ref. 5.) that the proton density is ~ 6 Χ 1 0

2 2

T h e solubility is such

c m " or roughly 50% greater than the proton 3

density of l i q u i d h y d r o g e n (for a n i n t e r m e t a l l i c w i t h H

2

gas at ~ 2 a t m at

25°C). As impressive as the h y d r o g e n solvent capacity of L a N i s a n d similar m a t e rials is, the r e m a r k a b l e feature of these materials is their r a p i d exchange of h y drogen between the h y d r i d e a n d gaseous phases under m i l d temperature a n d


pressure conditions.

F o r L a N i , 95% of the dissolved hydrogen is released within 5

5 m i n at 50° C if the restraining pressure is d r o p p e d f r o m 3 to 1 a t m (3).

In

contrast, U H or Y H , w h i c h have higher proton densities, r e a d i l y release h y 3

3

drogen only at elevated temperatures.

Since hydrogen is absorbed dissociatively,

the gas can exist, at least fleetingly, as monatomic hydrogen on the metals surface. T h i s suggests that the surfaces of the rare earth intermetallics are quite active, w h i c h indicates their possible use as heterogeneous catalysts.

Some rare earth

intermetallics (6, 7, 8, 9) were shown recently to be very effective as synthetic NH

a n d as methanation catalysts.

3

Since the rare earth intermetallics can contain large amounts of h y d r o g e n at pressures ressure a n d temperature of the system s k n o w n volume. T h e t i m e r e q u i r e d or the system to reach e q u i l i b r i u m usually varied f r o m 0.5 to 2.0 hr at pressures above 1 a t m to 12 hr at lower pressures. A b s o r p t i o n pressure-composition isotherms were established b y m e t e r i n g f i x e d amounts o f h y d r o g e n into the system a n d d e t e r m i n i n g the pressure after e q u i l i b r i u m . In each case, the sample chamber was isolated before h y d r o g e n

Î

T a b l e I.

System

P l a t e a u Pressures of R 2 C 0 7 - H Systems Temperature (°C)

Plateau Pressure (atm) a + β

Plateau Pressure (atm) β-r a

Ce Co -H

50 100 150

0.10 0.80 4.40

— — —

Pr Co -H

150 175 200

0.10 0.29 0.70

1.47 3.50 7.65

Nd Co -H

125 150 175

0.027 0.101 0.300

0.84 2.45 5.80

Gd Co -H

75 100 125

0.031 0.122 0.405

2.20 5.35 10.26

Tb Co -H

50 75 101

0.0265 0.090 0.300

Dy Co -H

50 74.6 101

0.078 0.25 0.78

2.25 5.90 14.0

Ho Co -H

50 75 101

0.225 0.69 1.86

4.1 11.5 26.0

Er Co -H

50 75 101

0.44 1.20 3.20

8.0 18.5 36.0

2

7

2

7

2

7

2

2

7

7

2

7

2

2

7

7

1.28 3.80 9.95


316

TRANSITION M E T A L HYDRIDES

was a d d e d to the r e m a i n i n g portions of the system, then reopened so the entire system c o u l d equilibrate. Thermodynamic

Results

Pressure-composition isotherms for the D y C o 7 - H system, w h i c h are typical 2

(17) of the R 2 C 0 7 - H systems, are shown i n F i g u r e 1.

Except for C e C o 7 - H , two

plateau regions were f o u n d i n a l l systems studied.

T h i s indicates that three

2

crystallographically distinguishable hydrides exist a n d c a n be designated as a ,


β, a n d 7.

T h e α phase is the solid solution phase based upon R C o 7 , a n d 7 is the 2

100

I0h Ε ο

IOI°C

Χ

/

Ο

/

/

f

o\

0.01

4

Hydrogen

6

Concentration

Figure 1. Pressure-composition isotherms for the Dy Co-j-H system. Hydrogen concentration is expressed as atoms of Η per formula unit of Dy Co . X designates absorption; Ο designates desorption. 2

2

7


22.

Rare Earth-Cobalt

GOUDY E T AL.

and -Iron

3 "T~


100 ι

317

Compounds

100

10

ε

4 5 "C

ο

ο- • ο — ο — ο — ο — j ? /

R2C07 > RC05.

100

0.001

ι

1

1

1

1

1

1

1

1

1

«

1

1

1

1

'

2.2

2.6 1/T

Figure 3.

Thus, the capacity

χ

3.0

io3

( K"

1

3.4

)

Temperature dependence on the plateau (a + β) for the R Coj-H systems

pressure

2



2.2

2.6 1/T χ

Figure 4.

3.0

ιο3

3.4

(Κ"' )

Temperature dependence on the plateau pressures (β + y) for the R 2 C 0 7 - H systems

for hydrogen absorption and hydride stability increases w i t h increasing rare earth content of the hydride.

L u n d i n et al. (18) noted a correlation between stability

a n d the size of the interstitial hole where hydrogen presumably is located. Absorption experiments for Nd2Co7 a n d D y C o 7 show that the q u a n t i t y 2

ΔΡ/Ρ is small ( R C o > R N i . 3

3

3

Since the unit

3

c e l l size of E r T diminishes i n the order, E r F e > E r C o > E r N i , one m i g h t at 3

3

3

3

tribute the systematics to the v a r y i n g size of the interstitial sites o c c u p i e d by hydrogen.

H o w e v e r , electronic factors are also involved.

Second, the h y d r i d e

stabilities of the R C o 7 compounds are greater than those for the corresponding 2

R Ni 2

7

compounds since the plateau pressures of P r N i are m u c h greater t h a n 2

7

those for P r C c 7 . 2

Effect of Hydrogénation on Lattice

Parameters

X - r a y diffraction patterns of the metal hydrides were obtained to determine the effect of dissolved hydrogen on the lattice dimensions a n d to c o n f i r m that the sample h a d not undergone decomposition. III.

Results are presented i n T a b l e

T h e R C c 7 materials are similar crystallographically to the R C c materials 2

i n the f o l l o w i n g respects: (1) B o t h f o r m three hydrides (α, β, a n d 7) upon hydrogénation. (2) B o t h systems retain their r h o m b o h e d r a l or hexagonal s y m m e t r y u p o n h y d r i d i n g . T h e R C 0 5 - H systems, on the other hand, were found to be degraded


22.

GOUDY ET AL.

Rare Earth-Cobalt

and -Iron

321

Compounds

f r o m the hexagonal C a C u s structure to an o r t h o r h o m b i c structure u p o n h y driding. (3) E x p a n s i o n takes place m a i n l y along the c - d i r e c t i o n u p o n f o r m a t i o n of the β-hydride, but m a i n l y i n the basal plane u p o n f o r m a t i o n of the 7 - h y d r i d e . F o r the RC05 hydrides, expansion is i n the basal plane w i t h no appreciable change i n the c-parameter for either the β- or 7 - h y d r i d e . Kinetic Features


D e s o r p t i o n kinetics were measured i n the two-phase region a n d , i n a few cases, i n the single-phase region.

T h e rate of h y d r o g e n evolution f r o m the h y

d r i d e was d e t e r m i n e d gasometrically as a f u n c t i o n of time.

The compound

(~2-4 g) was sealed i n a copper sample chamber and immersed i n a Hoskins tube furnace controlled to w i t h i n ± 0 . 2 5 ° C b y a Paktronics, Inc. temperature controller. T h e temperature of the sample was measured w i t h a copper-constantan ther m o c o u p l e p l a c e d i n contact w i t h the sample chamber.

Temperatures were

chosen so that the e q u i l i b r i u m vapor pressure i n the two-phase region was i n excess of 1 a t m ( ~ 5 - 1 0 atm).

A l l samples were activated before the experiment

b y sequentially absorbing and desorbing hydrogen at least three times to ensure T a b l e III.

Lattice Parameters a n d P r o t o n Densities i n R 2 C 0 7 - H Systems H/cm c/a

V(i4 )

%AV

24.496 24.986

4.952 4.990

519.2 542.5

4.5

5.058 5.081 5.312

24.508 26.300 26.014

4.845 5.176 4.897

543.0 588.0 635.7

5.053 5.069 5.268

24.427 26.286 25.919

4.834 5.186 4.920

540.1 584.9 622.9

—

—

8.3 15.3

1.9 4.2

Gd Co Gd Co H .6 Gd Co H .

5.017 5.012 5.199

36.309 39.043 38.624

7.237 7.790 7.429

791.4 849.3 904.1

7.3 14.2

1.8 3.9

Tb Co Tb Co H2. Tb Co H .

6

5.007 5.011 5.175

36.269 38.964 38.482

7.244 7.776 7.436

787.4 847.3 892.5

Dy Co Dy Co H .6 Dy Co H .4

4.988 4.984 5.169

36.151 38.699 38.314

7.248 7.765 7.412

778.9 832.5 886.5

Ho2Co

4.989

36.172

7.250

4.963

35.886

7.231

Compound

a (il)

c(A)

3

4.497 5.007

Pr Co Pr Co7H2.5 Pr Co7H58

Ce2Co7 Ce2Co7H . 5

2

7

2

2

Nd Co Nd Co H . Nd Co H . 2

7

2

7

2

7

2

7

6

2

2

7

2

7

2

7

2

2

5

7

2

7

2

7

2

6

7

2

7

2

2

7

6

Er Co 2

α

9

7

7

7

3

Χ ΙΟ

3

2 2

Α

— 5.9

—

— 1.7 3.7

8.3 17.1

—

—

—

7.6 13.3

1.9 4.4

— 6.9 13.8

— 1.9 4.3

779.7

—

—

765.5

—

—

These can be compared with the proton density in liquid hydrogen, 4.2 Χ 10 L / c m . 22


2

322


a constant active surface.

H y d r o g e n was then allowed to f l o w freely f r o m the

sample into a gas-measuring buret.

Since the sample size was small c o m p a r e d

w i t h the copper container, a n d was bathed i n H

2

gas (a good heat conductor),

it appeared that there was an adequate rate of heat transfer to the sample w h i c h m i n i m i z e d the temperature gradient between the sample's interior a n d the container w a l l .

(This was i n f e r r e d

o m the general performance of the

e q u i p m e n t a n d the internal consistency of the data.) T h e reaction order for decomposition of metal h y d r i d e was determined by


f i t t i n g the e x p e r i m e n t a l data to a rate equation of the general f o r m : — dt

-kC

(1)

n

where C is the amount of hydrogen i n the h y d r i d e phase, k is the desorption rate constant, a n d η is the order of the reaction.

In this rate equation, the reverse

reaction, i.e., absorption, was not i n c l u d e d .

T h e desorption reaction was c o n

d u c t e d against a pressure of 1 a t m , w h i c h is m u c h below the plateau pressures, so absorption can be neglected. In C

0

1/C -

F o r η = 1 a n d η = 2, E q u a t i o n 1 becomes

- In C = kt

for η = 1

(2)

1 / C o = kt

for η = 2,

(3)

where C o is the initial quantity of hydrogen i n the desorbing hydride phase. e x p e r i m e n t a l data were e x a m i n e d using these expressions.

The

T h e second-order

plots fit the data more satisfactorily than the first-order plots.

I

'

I

1

142.8,

·/

132.1

// / /

"

ο

A

io

7

ιό,* »/V

3

7

j

127.1

y

122.2

/

/

s

/ •

·'

/ ,

/

ι

I

I

100

200

ι

300

T i m e (sec ) Figure 5. Rate of hydrogen release from Dy2Co-j-H teau. Cis a measure of the hydrogen concentration intermetallic (in liters at 25°C and 1

on the a + β pla remaining in the atm).


22.

Rare Earth-Cobalt and -Iron

GOUDY E T AL.

323

Compounds

40


Ο

20

600 Time (sec ) Figure 6. Rate of hydrogen release from Dy Co -H. For short time intervak, the process converts the y form into the β form. For longer times, the material only consists of the β form. The break in the curve corresponds to the disappearance of the y form. 2

7

Desorption kinetics are d e t e r m i n e d for D V 2 C 0 7 , Gd2Cc>7, E r C o 3 , D y C o 3 , E r F e , a n d D y F e . T h e results are illustrated b y the D y C o 7 - H system (see Figures 5 and 6). Q u a n t i t y C is the volume of hydrogen, measured at 1 a t m and at 25°C, that is retained i n the sample at time t. C is proportional to the hydrogen concentration i n the h y d r i d e . T h e data shown i n F i g u r e 5 are for the system i n the a + β two-phase region, whereas those i n F i g u r e 6 involve the two-phase β 3

3

2

+ 7 region i n the early reaction stages and the single-phase β region i n the latter stages. T h e break i n the curve corresponds to movement f r o m the two-phase to the single-phase region. T h e curve plotted i n Figure. 5 is continuous since the variation i n hydrogen content cannot sufficiently remove all of the β phase. L o g k is linear w i t h 1 / T (see F i g u r e 7). T h e slope indicates an activation energy of 23 k c a l / m o l for the D y C o 7 - H (β —• a) system. 2

K i n e t i c data for a l l the R C o - H systems studied are s u m m a r i z e d i n T a b l e I V . T h e observed behavior i n a l l cases indicated a second-order process. T h e activation energies exceed the AH values given i n T a b l e II, as expected of the endothermal nature of the process. In most cases, E is two or more times as large as AH. M o r e work is necessary to establish f u l l details of the dehydrogenation process since a sequence of steps is involved. H y d r o g e n migration i n the lattice, conversion of one phase into another, recombination of atomic hydrogen at the surface of the metal, and release of molecular hydrogen into the gas phase should be considered. D i f f u s i o n of hydrogen most likely is not rate controlling. T h e second-order nature of the process and the magnitude of the activation energy indicate that the process is not diffusion controlled. H y d r o g e n diffusion i n metals 2

7

a


324


Table IV.

System Gd Co -H

E (kcal/mol) a

Temperature(°C)

k(net) Χ 10 (l- sec~ ) l

l

36

160.0 165.0 172.1 178.0 186.0

5.61 12.04 23.4 43.2 59.5

Gd Co -H (β + y)

18

68.2 75.0 81.1 87.8 99.2

4.15 12.8 19.6 29.1 47.4

Dy Co -H

23

122.2 127.1 132.1 137.1 142.8

9.4 11.9 22.1 28.4 37.2

Dy Co -H (β + y)

15

43.0 49.0 55.0 61.2 68.3

3.87 7.84 11.3 16.9 26.5

ErCc-3

20

103.5 109.0 115.4 120.7 126.2

1.10 2.10 3.28 4.38 5.21

ErCc-3 (0 + 7)

18

53.0 58.2 65.2 69.1 73.0

.519 .949 1.68 2.13 2.56

DyCc-3

25

139.9 145.6 151.6 156.1 161.2

1.18 2.45 3.77 4.80 5.50

DyCc-3 (β + y)

16

79.1 86.1 93.6 100.0 107.0

0.57 1.28 2.00 2.47 3.49

ErFe

23

159.4 163.9 169.4

7.02 11.63 14.85

2

7

(α +


K i n e t i c D a t a for the D e s o r p t i o n o f H y d r o g e n f r o m Selected R a r e E a r t h Intermetallics

2

β)

7

2

7

(α +

2

β)

7

(α +

(α +

β)

β)

3


5

22.

Rare Earth-Cobalt and -Iron

GOUDY ET AL.

Table IV. System

E (kcal/mol)


DyFe

Continued Temperature{°C)

a

25

3

325

Compounds

k(net) Χ 10 (l- sec~ ) l

l

175.0 180.1 186.5

20.32 24.23 35.38

194.2 200.0 204.7 209.4 216.8

2.57 4.44 5.76 7.39 7.90

-3.0

-3.5

CP

ο

-4.0

h

-4.5 2.9

3.0

3.1

(1/Txl0 ) 3

K"

3.2

1

Figure 7. Temperature dependence on the rate constant for hydrogen release from the Dy Co -H system on the β -\- y plateau 2

7


5

326

TRANSITION METAL HYDRIDES

involves an activation energy from 3-12 kcal/mol (19). The second-order nature of the desorption process suggests that atomic hydrogen recombination at the surface is rate controlling.


Literature Cited 1. Zijlstra, H., Westendorp, F. F., Solid State Commun. (1969) 7, 857. 2. Neumann, H. H., Ph.D. thesis, Technische Hochschule, Darmstadt, Germany, 1969. 3. Van Vucht, J. H. N., Kuijpers, F. Α., Bruning, H. C. A. M., Philips Res. Rep. (1970) 25, 133. 4. Kuijpers, F. Α., Ph.D. thesis, University of Delft, Holland, 1973. 5. Wallace, W. E., "Rare Earth Intermetallics," Academic, New York, 1973. 6. Takeshita, T., Wallace, W. E., Craig, R. S., J. Catal. (1976) 44, 236. 7. Coon, V. T., Takeshita, T., Wallace, W. E., Craig, R. S., J. Phys. Chem. (1976) 80, 1878. 8. Elattar, Α., Takeshita, T., Wallace, W. E., Craig, R. S., Science (1977) 196, 1093. 9. Wallace, W. E., Elattar, Α., Takeshita, T., Coon, V., Bechman, C. Α., Craig, R. S., "Proceedings of the 2nd International Conference on the Electronic Structure of the Actinides," J. Mulak, W. Suski, R. Troć, Ed., p. 357, Polish Academy of Sci ences, Warsaw, 1977. 10. Van Mal, H. H., Buschow, Κ. H. J., Miedema, A. R., J. Less-Common Met. (1976) 49, 473. 11. Bechman, C. Α., Goudy, Α., Takeshita, T., Wallace, W. E., Craig, R. S., Inorg. Chem. (1976) 15, 2184. 12. Takeshita, T., Wallace, W. E., Craig, R. S., Inorg. Chem. (1974) 13, 2282 and 2283. 13. Raichlen, J. S., Doremus, R. H., J. Appl. Phys. (1971) 42, 3166. 14. Boser, O., J. Less-Common Met. (1976) 46, 91. 15. Lundin,C.Ε., Lynch, F. Ε., Denver Research Institute, University of Denver, AFOSR Contract No. F44620-74-C-002 (ARPA Order 2552), January 1975, First Annual Report; May 1976, Final Report. 16. Gruen, D. M., Mendelsohn, M., ADV. CHEM. SER. (1978) 167, 327. 17. Goudy, Α., Ph.D. thesis, University of Pittsburgh (1976). 18. Lundin, C. E., Lynch, F. E., Magee, C. B., J. Less-Common Met. 19. Birnbaum, Η. K., Wert, C. Α., Ber. Busenges, Phys. Chem. (1972) 76, 806. RECEIVED July 19, 1977. Work was partially assisted by a contract with the Energy Research and Development Administration.


Thermodynamics and Kinetics of Hydrogen Absorption in Rare Earth

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