Hydrogen absorption in modified intermetallic compound systems

Journal

of the Less-Common

HYDROGEN COMPOUND

HAYAO SUSUMU Department Tokiwadai, (Received

MetaLs, 89 (1983)

ABSORPTION SYSTEMS*

IMAMURA, TSUCHIYA

TOSHIO

of Industrial Chemistry, Ube 755 (Japan) February

251

IN MODIFIED

TAKAHASHI, Faculty

251

- 256

INTERMETALLIC

RAMIRO

of Engineering,

GALLEGUILLOS Yamaguchi

University,

I

and 2557

1, 1982)

Summary The modification of Mg,Ni by various organic compounds was investigated with a view to improving its hydrogen absorption properties. In particular Mg, Ni reversibly absorbed hydrogen under more moderate conditions when it had been modified with tetracyanoethylene or phthalonitrile. Studies of the treated materials using electron spin resonance and electronic spectra showed the formation of electron donor-acceptor (EDA) complexes owing to the high electron affinity of the organic species used. It is believed that the EDA complexes formed on the surface layer provide sites for hydrogen activation which is followed by the diffusion of excess hydrogen into the underlying intermetallic phase. This is consistent with the results of X-ray analysis and thermodynamic measurements.

1. Introduction A number of intermetallic compounds reversibly absorb large amounts of hydrogen under moderate conditions. This has aroused considerable interest in their application to hydrogen storage. In practice, however, some improvement of the hydrogen sorption properties of these materials is often required. The change in the hydrogen sorption properties produced by the partial replacement of transition metals or rare earth metals in intermetallics by other metals has been studied [l] . These changes are of fundamental interest. It is known that Mg,Ni can absorb hydrogen to form the hydride to an extent exceeding the capacity of liquid hydrogen by up to a factor of 1.3 on a per unit volume basis when it is brought into contact with hydrogen at about 20 atm and 300 “C [2], whereas it is inert to hydrogen at room tem-

*Paper presented at the International Symposium on the Properties of Metal Hydrides, Toba, Japan, May 30 -June 4, 1982. 0022-5088/83/0000-0000/$02.75

0 Eisevier

Sequoia/Printed

and Applications

in The Netherlands

252

perature and atmospheric pressure. We have recently found that Mg,Ni-based systems which have been treated with organic reagents with a high electron affinity are able to absorb hydrogen readily under more moderate conditions [3 - 61. This treatment constitutes a novel approach to the improvement of the hydrogen absorption capabilities of intermetallics. The present investigation is a further extension of this work to include the determination of the hydrogen absorption characteristics of Mg,Ni modified by various organic materials. The significance of the modification is discussed on the basis of the hydrogen absorption process.

2. Experimental

details

The intermetallic compound Mg,Ni was a commercial product obtained from the Nippon Yttrium Co. Ltd. The existence of the desired hexagonal structure was confirmed by X-ray diffraction analysis. Tetracyanoethylene (TCNE), phthalonitrile (PN), naphthacene and chloranil (obtained from Tokyo Kasei Co.) were used as modifiers without further purification. The hydrogen used was purified by passage through a molecular sieve and a liquid nitrogen trap. Modified Mg,Ni was prepared by reacting powdered Mg,Ni with the appropriate organic material in a dry tetrahydrofuran solvent at room temperature for 1 week. The mixture was then evacuated and a powdery product was obtained. The powder thus prepared was transferred to the apparatus for the hydriding and dehydriding measurements which was constructed of glass and included a high vacuum system. Thus the absorption and desorption processes could be conducted at pressures below 1 atm. Further details regarding the sample preparation and hydrogen absorption measurements have been described elsewhere [ 3 - 61. X-ray diffraction and electron spin resonance (ESR) techniques were used to examine the modified sample. The X-ray diffraction patterns were obtained using a Rigaku model SL-7H diffractometer with Cu Ka radiation. The ESR spectra were recorded using a JEOL-JES-ME X-band spectrometer with 100 KHz modulation. The field was calibrated using Mn2+-doped magnesium oxide powder.

3. Results The hydrogen absorption was measured at temperatures of about 22, 100 and 154 “C by admitting about 500 Torr of hydrogen after the samples had been degassed to about 10e5 Torr at 150 “c for 2 h. First it was confirmed that unmodified Mg,Ni did not absorb hydrogen under these conditions. However, the modified Mg,Ni readily absorbed copious quantities of hydrogen without activation by the conventional method of hydrogenation at high pressures and release at elevated temperatures. Typical hydrogen up-

253

II

5 time

Ch,

IO

19-2 2.n

I5

2.1

2.2

2.3

2.4

103/T

Fig. 1. Variation in the amounts of hydrogen absorbed with time: 0, TCNE-Mg*Ni; A, PN-MgzNi; 0, perylene-MgzNi [5, 61. The hydrogenation was performed in about 500 Torr of hydrogen at 22 “C () or at elevated temperatures (- ~ -). Fig. 2. Plot of log P us. l/T for PN-Mg,Ni-H.

TABLE Hydrogen

1 absorption

in Mg,Ni modified

with various

organic

compound?

Sampleb

]HJ/[MgzNi]

Anthracene-Mg2NiC Phenanthrene-MgzNiC Chrysene-MgzNiC Perylene-MgzNiC Naphthacene-MgzNi PN-MgzNi TCNE-Mg*Ni Chloranil-MgzNi

0.30 0.32 0.50 0.50 0.46 1.26 (2.94 at 154 oC)d 0.30 (2.27 at 100 oC)d 0.42

ofter 15 h

aThe measurements were made by admitting about 500 Torr of hydrogen at 22 “C. bThe samples were prepared by reacting a 1:l mixture of MgzNi and the organic pound. CRefs. 5 and 6. dThe values in parentheses were obtained from runs at elevated temperatures.

com-

take processes for the various systems, which were evaluated using volumetric techniques, are illustrated in Fig. 1 and the absorption data are summarized in Table 1. Desorption isotherms for the PN-Mg,Ni-H system were examined at temperatures of 154 - 199 “C and pressures of 9.8 - 61 Torr. The desorption kinetics were relatively fast and equilibrium at each experimental point was reached in less than 10 h. Well-defined plateaux were obtained and their pressures agreed well with the values estimated from the relation based on the Mg,Ni-H system [2] . In the rather limited temperature range in which

254

(cl

-A_ I

I

35

20

I

40

1

45

?.a

Fig. 3. X-ray after complete

diffraction patterns: removal of hydrogen.

(a) MgzNi;

(b) TCNE-MgzNiH1.2;

(c) TCNE-MgzNi

the isotherms were measured a plot of log P against l/T gave a straight line (Fig. 2) which obeyed the equation logP=-

3475 T

-I-6.271

The heat of dissociation of the hydride was calculated from the slope of the line in Fig. 2 as A@ = 15.8 kcal (mol HZ)-l which is very close to the value obtained in the unmodified system, i.e. Mg,Ni-H [2]. The desorption isotherms were almost unaffected by changing the organic modifier. The X-ray powder diffraction patterns of TCNE-Mg,NiH1.2 showed diffraction peaks with expanded lattice parameters (Fig. 3). When all the hydrogen was removed from the sample by evacuation at 230 “C the X-ray pattern was identical with that obtained before hydriding, indicating that there was no decomposition in the TCNE-Mg,Ni system and moreover that the original crystal structure of Mg,Ni was maintained in the bulk. Similar behaviour was observed in the other systems. 4. Discussion Mg,Ni did not absorb hydrogen at an observable rate under the conditions of this study. Therefore it is evident that the modification with the

255

organic materials produced an enhancement of the hydrogen affinity of Mg,Ni. In particular, a large increase in the hydrogen absorption capability of Mg,Ni treated with TCNE or PN was observed. The kinetic features of the hydrogen uptake were a function of the nature of the organic compounds with which Mg,Ni was combined. The results of X-ray and thermodynamic measurements, however, suggest that treatment with the organic compounds has little effect on the crystallographic and thermodynamic properties of the precursor Mg,Ni relative to the hydriding and dehydriding processes. Since hydrogen is absorbed dissociatively, it is clear that a sequence of events of considerable complexity is involved in the sorption process. The absorption process essentially consists of diffusion from the gas phase to the surface, cleavage of the hydrogen bond in the chemisorbed molecular hydrogen, interfacial diffusion and finally diffusion into the bulk. It is obvious that in the present system the ability to absorb hydrogen is determined by the surface processes and that the hydrogen molecule rapidly dissociates and is absorbed by bulk penetration. This is probably related to the observation that the modified systems also effectively catalyse the hydrogenation reaction of ethylene [ 41. The rapid dissociative absorption of hydrogen indicates that the materials have very active surfaces, although the factors responsible for the absorption process have not been adequately determined [ 7, 81. The chemical species which exist on the surface layers of these materials and which are involved in surface activity have recently been investigated. We examined the modified intermetallics using electronic spectra and ESR [ 5, 61. It was found that the reaction of intermetallics with organic materials resulted in the formation of electron donor-acceptor (EDA) complexes in which charge transfer occurred between the substances. ESR studies showed the presence of organic anion radicals as a result of the complexation. As shown in Fig. 4 the TCNE-Mg,Ni system exhibited an unresolved ESR signal from TCNE anion radicals with a g value of 2.0034 and an overall width of about 40 G. The results were similar to those obtained from the anthracene-SmMg, and

-

20 G

Fig. 4. ESR signal of organic radicals obtained after treatment of Mg,Ni with TCNE. The ESR measurements were made at room temperature.

256

perylene-Mg,Ni systems examined previously [ 5, 61. These observations strongly suggest that the formation of EDA complexes on the alloy surface as a result of charge transfer is responsible for the activation and subsequent absorption of hydrogen. This concept appears to be significantly supported by the fact that a series of condensed aromatic compounds treated with alkali metals, in which production of EDA complexes has been established, can activate hydrogen and absorb sufficient quantities to form metal hydrides [9] . Although the surface features of the modified Mg,Ni systems have not been established, it is probable that they possess an outer layer of the EDA species and that hydrogen absorption takes place via these active sites and reaches the underlying intermetallic phase by diffusion. Nuclear magnetic resonance measurements show that after hydriding the organic anion species are partially converted into the monohydrogenated state which returns to the original state after a short evacuation [ 61.

Acknowledgment This study

was assisted

by a grant from the Saneyoshi

Foundation.

References 1 F. A. Kuijpers, Philips Res. Rep., Suppl., 2 (1973) 31. H. H. van Mal, K. H. J. Buschow and F. A. Kuijpers, J. Less-Common Met., 32 (1973) 289. J. Shinar, D. Shaltiel, D. Davidov and A. Grayevsky, J. Less-Common Met., 60 (1978) 209. D. M. Gualtieri and W. E. Wallace, J. Less-Common Met., 55 (1977) 53. 2 J. J. Reilly and R. H. Wiswall, Jr., Znorg. Chem., 7 (1968) 2254. 3 H. Imamura and S. Tsuchiya, Chem. Commun., (1981) 567. 4 H. Imamura and S. Tsuchiya, J. Catal., 72 (1981) 383. 5 H. Imamura and S. Tsuchiya, Proc. 6th Japan-Soviet Catalysis Seminar, Osaka, October 5 - 7, 1981, p. 104. 6 H. Imamura, T. Takahashi and S. Tsuchiya, J. Catal., 77 (1982) 289. 7 W. E. Wallace, R. F. Karlicek, Jr., and H. Imamura, J. Phys. Chem., 83 (1979) 1708. 8 W. E. Wallace, A. Elattar, H. Imamura, R. S. Craig and A. G. Moldovan, in W. E. Wallace and E. C. Subbarao (eds.), The Science and Technology of Rare Earth Materials, Academic Press, New York, 1980, p. 329. 9 K. Tamaru, Catal. Rev., 4 (1971) 161. K. Tamaru and M. Ichikawa, Catalysis by Electron Donor-Acceptor Complexes, Kodansha, Tokyo, 1975.