Iron catalyzed hydriding of magnesium

Scripta METALLURGICA

Vol. 16, pp. 285-286, 1982 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

IRON CATALYZED HYDRIDING OF MAGNESIUM

J.-M. Welter ~ and P.S. Rudman ~ ~Institut fur Festk6rperforschung, Kernforschungsanlage JHlich, 5170 J~lich, Germany ~ D e p t . of Physics Technion-Israel Institut of Technology,

Haifa,

Israel

(Received December 3, 1981)

Introduction The reaction Mg + H 2 ÷ MgH 2 is generally difficult to nucleate because the surface is contaminated by MgO or Mg(OH) 2 which either inhibits the dissociative adsorption H2 + 2Had, or the surface-to-bulk transfer Had + ~. Once nucleated, MgH 2 then grows rather slowly with the dominant rate limiting step being the diffusion of H through the growing MgH 2 layer (I). Dissociative adsorption on the MgH 2 surface may also limit the growth rate (2). Thus both the nucleation and growth kinetics of MgH 2 may be helped by a hydrogen dissociative adsorption catalyst on the Mg surface. One approach to the solution of this problem is the incorporation of a catalyst in a homogeneous alloy as exemplified by Mg2Ni which, in common with many other readily hydrided alloy phases, is composed of a strong oxide-former (Mg) and a weak oxide-former (Ni). Under normal hydriding conditions the residual oxygen partial pressure is capable of oxidizing the strong oxide-former, but incapable of oxidizing the weak oxide-former. Considering the Mg2Ni example, thermodynamic calculations predict (2) that the surface reaction Mg2Ni + 02 ÷ 2MgO + Ni will occur, with Ni metal clusters serving as the dissociative adsorption catalyst. This reaction was identified by Schlapbach and co-workers (3) using X-ray photoemission and Auger electron spectroscopy. Another approach to Dosition a dissociative adsorption catalyst on the surface is to employ a two-phase composite: a hydride-former phase + a catalyzer phase. Thus, for example, Eisenberg et al. (4) employed electroplated Ni on Mg. This was not very successful because of identified problems of adhesion and distribution of the Ni and probably also because reaction between Mg and Ni produces a blocking phase MgNi 2. Whenever in a multiphase system the phases are not in equilibrium with each other, reactions that produce interfering phases are possible. In order to obtain an intimately mixed and adherent catalyzer phase, with only equilibrium phases present, Rudman and co-workers (2) employed a eutectic mixture of Mg (hydride-former phase) and Mg2Cu (catalyzer phase). Here we present preliminary results for another two-phase approach which employs iron particles as a dissociative adsorption catalyst for hydriding magnesium. Material Preparation Iron does not form intermetallic compounds with magnesium. Furthermore it has very little solubility in either solid or liquid magnesium and thus a fine distribution of iron particles cannot be obtained by precipitating them out from a Mg-Fe solution. Thus we started with fine iron powder (20 ... 40 u m diam.) which was simply sprinkled among magnesium chips. The mixture was pressed to increase

285 0036-9748/82/030285-02503.00/0 Copyright (c) 1982 Pergamon Press Ltd.

HYDRIDING OF Mg

286

V01.

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its density and then melted in a graphite crucible under argon. The melting technique employed was described previously (5). FIG. I is an optical micrograph of a section through the ingot. As can be seen, this technique did not succeed in producing an even dispersion of iron particles throughout the ingot. Most of the particles are ineffectively located in clusters. Thus, although the starting material was ~5 a/o Fe, the effective composition was much less. Furthermore the sponge like nature of the powder is clearly revealed. There is no doubt that the catalytic effect can be increased considerably by a more uniform dispersion of finer iron particles. Fine filings from the ingot were employed in the hydriding experiments. The hydriding reactor and instrumentation are essentially as described previously (5). Experimental Results FIG. 2 presents an aspect of isobaric/isothermal (15 • 105 Pa/400 °C) hydriding kinetics: it shows the fraction of Mg hydrided as a function of time, for Fe-catalyzed Mg and for uncatalyzed Mg of identical preparation but without the Fe addition. Prior to this recorded run both materials had undergone 20 hydriding/dehydriding cycles. The iron addition resulted in significantly faster hydriding kinetics. Following these runs the samples were exposed to air for a day and then the hydriding kinetics measurements were repeated. This resulted in somewhat slower hydriding kinetics but more significantly, only about 20 % of the uncatalyzed Mg could now be hydrided while for the Fe-catalyzed material essentially 1OO % of the Mg could still be hydrided and within a few hydriding/dehydriding cycles the hydriding kinetics were restored. We cannot yet say whether the catalytic effect of iron is due to a change in the nucleation and/or growth characteristics. We do conclude that if a technique of improved dispersion of iron particles in the magnesium melt can be perfected, then magnesium catalyzed with iron particles should exhibit very useful hydriding kinetics. References I. Z. Luz, J. Genossar and P.S. Rudman, J. Less-Common Metals 73, 113 (1980). 2. J. Genossar and P.S. Rudman, Z. f~r Physik. Chemie Neue Folge 116, 215 (1979). 3. A. Seller, L. Schlapbach, Th. von Waldkirch, D. Shaltiel and F. Stucki, J. Less-Common Metals 73, 193 (1980). 4. F.G. Eisenberg, D.A. Zagnoli and J.J. Sheridan III, J. Less-Common Metals 74, 323 (1980). 5. A. Karty, J. Genossar and P.S. Rudman, J. Appl. Phys. 50, 7200 (1979).

"~ 1.0 Fe doped M g = ~ , ~ , ~

~'~

/

+''~''4'~" ~Pure Mg

2 I

0 FIG.

l...-.~,---'+

1

I

2

I

I

3 ~ Time [hr]

I

5

I

Optical micrograph of a section through Mg - 5 % Fe melt etched with 5 % HNO 3 in ethanol showing the Fe particles.

FIG.

2

Hydriding kinetics at 15 of H 2 and 400 °C.

105 Pa

6