Catalytic effect in the hydrogenation of Mg and Mg compounds: Surface analysis of MgMg2Ni and Mg2Ni

Mat. Res. Bull. Vol. 14, pp. 1235-1246, 1979. Printed in the USA. 0025-5408/79/091235-12502.00/0 Copyright (c) Pergamon P r e s s Ltd.

CATALYTIC EFFECT IN THE HYDROGENATION OF Mg AND Mg COMPOUNDS: SURFACE ANALYSIS OF Mg-Mg2Ni AND Mg2Ni +

Laboratorium Institut

L. Schlapbach, D. Shaltiel fur Festk~rperphysik ETH, CH-8093

P. Oelhafen fHr Physik der Universit~t

Basel,

Zfirich

CH-4056

Basel

(Received July 26, 1979; Communicated by A. Rabenau)

ABSTRACT By means of XPS and AES we have analysed the chemical states and the concentrations of Mg and Ni at the surface of the hydrogen storage materials Mg-Mg2Ni eutectic alloy and Mg2Ni intermetallic compound. Mg2Ni decomposes at the surface into Mg oxide and metallic Ni. Molecular hydrogen can be dissociated at the metallic Ni precipitations or at the metallic subsurface layer. Thus the proposed model describes the catalytic effect of Mg2Ni on the hydrogenation [i] of Mg by surface decomposition and the ability to supply atomic hydrogen. With a similar model we described already successfully catalytic effects at the surface of LaNi 5 and FeTi [2]. It is likely that it is applicable to other Mg compounds, too.

+work under

the National

permanent address: versity, J e r u s a l e m

Racah

Research

Program

Institute

1235

"Energy"

of Physics,

Hebrew Uni-

1236

L. SCHLAPBACH, et a__..!l.

Vol. 14, No. 9

Introduction Many intermetallic compounds absorb large quantities of hydrogen. As to the reversible storage of hydrogen the best known compounds are FeTi, LaNi5, Mg2Ni and ZrFe2_xMn x and some other AB 2 compounds [1,3]. Pure Mg forms the binary hydride MgH 2 with the high hydrogen content of 7.6 w% and would therefore be an attractive material for the storage of hydrogen, had it not two significant disadvantages: First, the formation of MgH 2 directly from Mg and molecular hydrogen gas without a catalyst is extremely slow or impossible even at high temperatures and pressure [4]. Secondly, the hydride is too stable; a temperature of 3OOOC is required to reach a desorption pressure of 1 bar [i]. These disadvantages might be eliminated by using Mg compounds or alloys. From 1967 on Reilly and Wiswall [5] investigated Mg2Ni and Mg2Cu [i]. They found that Mg2Ni reacts readily with hydrogen forming reversibly the ternary hydride Mg2NiH 4. The analogous compound Mg2Cu is reported to decompose upon hydrogenation into MgH 2 and MgCu2, also reversible. Alloys of Mg with Mg2Ni or with Mg2Cu exhibit two plateaus in the hydrogen p r e s s u r e - c o m p o s i t i o n isotherms. The lower plateau is due to the reaction of Mg with H 2 Mg + H 2 ~ MgH 2 Reilly and Wiswall concluded that the presence of Mg2Ni or Mg2Cu has a catalytic effect on the hydrogenation of Mg. Gales and Grumel [6] confirmed that the rate of reaction of the formation of MgH 2 is much larger in Mg2Cu than in pure Mg. Lateron Wiswall and Reilly [7] observed that the addition of 5 w% Ni to Mg accelerates the formation of MgH 2. Rudman and coworkers [8] studied the kinetics of the hydrogenation of the Mg-Mg2Cu eutectic alloy at conditions that only the Mg phase was hydrided and dehydrided, i.e. that the Mg2Cu phase played the role of the catalyst only. They concluded that the hydrogenation and dehydrogenation are rate limited by the diffusion of the hydrogen through a growing layer of MgH 2 or Mg, respectively. They assume as a working hypothesis for the catalytic role of Mg2Cu that it exhibits an external surface which is reduced and cleaned of oxides by hydrogen during the hydriding reaction. They support their hypothesis with thermodynamic considerations about the oxide formation. This paper deals with the question of how the surface of Mg2Ni or in analogy that of Mg2Cu and possibly other Mg compounds could be prevented from oxidation and its meaning for the different steps in the formation or decomposition of the hydride.

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Wagner, cited by Wicke et al. [9] noticed already in 1932 that the d i s s o c i a t i o n of m o l e c u l a r hydrogen and the recombination of atomic hydrogen are very important steps in the hydrogenation process. D i s s o c i a t i v e c h e m i s o r p t i o n of hydrogen occurs on many d-metals [iO]. Pd, Nb and V absorb and desorb hydrogen if their surface is metallic. They lose their a b s o r p t i o n and even the d e s o r p t i o n c a p a b i l i t y if the surface is o x i d i z e d [ii] . Small p a r t i c l e s of d-metals, e.g. Ni, belong to the best hydrogenation catalysts as long as they are kept metallic. Recently we have analyzed the surface of LaNi 5 and FeTi by means of X-ray p h o t o e m i s s i o n spectroscopy (XPS), Auger electron s p e c t r o s c o p y (AES) and investigations of the magnetic properties [2,12,13]. In both compounds surface segregation and decomposition occurs due to thermodynamic reasons and the presence of a small amount of oxygen [14]. In a thin surface layer La (Ti) diffuses to the surface and binds the oxygen present as impurity forming an oxide. This keeps the remaining Ni (Fe) metallic. It forms s u p e r p a r a m a g n e t i c particles of about 6000 Ni atoms or 2000 Fe atoms, resp. The d i s s o c i a t i o n of the m o l e c u l a r hydrogen in the a b s o r p t i o n process and the r e c o m b i n a t i o n of the atomic hydrogen in the d e s o r p t i o n process then p r o b a b l y occur at the metallic Ni or Fe p r e c i p i t a t i o n s or possible on the metallic subsurface layer of the intermetallic compound. With each cycle of h y d r o g e n a t i o n the segregation and d e c o m p o s i t i o n continues, fresh Ni (Fe) p r e c i p i t a t e s and a fresh subsurface layer is formed. This is a s e l f r e s t o r i n g m e c h a n i s m of the active surface. Wallace [15] used this model of a d e c o m p o s e d surface for a d e s c r i p t i o n of the kinetics of the hydrogen absorption of LaNi 5. We suppose that a similar surface d e c o m p o s i t i o n occurs in CaNi 5 and that the formation of ferromagnetic p a r t i c l e s upon h y d r o g e n a t i o n (Yagisawa and Yoshikawa [16]) has to be explained by a d e c o m p o s i t i o n into Ca oxide or hydroxide and Ni. Mg builds up a thin p r o t e c t i v e oxide layer and therefore can hardly d i s s o c i a t e m o l e c u l a r hydrogen. We think that in alloys or intermetallic compounds of Mg with Ni, Cu [5,7], Y and rare earth m e t a l s [17,18,19], and Li, Zr and Th [20] surface segregation and d e c o m p o s i t i o n prevent the formation of a compact oxide layer. This would explain the better h y d r o g e n a t i o n kinetics of the Mg compounds and alloys m e n t i o n e d above. T h e r e f o r e we started to analyze the surface of Mg compounds. In this paper we shall report about room t e m p e r a t u r e XPS and AES investigations of the M g - M g 2 N i eutectic alloy and on p r e l i m i n a r y results of Mg2Ni. In a forthcoming paper v. W a l d k i r c h et al. [21] are going to d e s c r i b e the results on Mg2Cu. A c c o r d i n g to our knowledge the surface of Mg2Ni has never been analysed before, whereas AES and XPS spectra of Mg in evap o r a t e d M g 2 C u were investigated by Fuggle et al. [22].

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Vol. 14, No. 9

Experimental We prepared the Mg2Ni intermetallic compound and the Mg-Mg2Ni eutectic alloy [23] (11.3 at% Ni, i.e. Mg to Ni atomic ratio = 7.8 : i) by premelting Mg (99.95 at%, Atomergic) and Ni (99.998 at%, Koch Light) in MgO crucibles at 0.7 bar Ar by RF heating. After premelting the Mg2Ni sample contained easily detectable amounts of Mg and MgNi 2. The sample was then remelted and quenched in a water cooled Cu levitation crucible of a modified oxford type at 0.7 bar Ar. X-ray diagrams of the quenched sample still contained the most intensive lines of MgNi 2 ((201) and (107) of the rel. intensity i00) but with very low intensity The lines (iO1) and (004) with rel. intensities 90 and 80 were not detectable. X-ray diagrams of the Mg-Mg2Ni showed no reflexions from MgNi 2. The eutectic alloy was investigated at room temperature on a L e y b o l d - H e r a e u s spectrometer at pressure p < i-i0-i0 mbar. MgKe radiation (1253.6 eV) was used. The energy scale was calibrated against the Au 4f levels [24]. Together with the eutectic Mg-Mg2Ni sample we inserted samples of pure Mg and Ni to be used as standards. The p r e l i m i n a r y results on Mg2Ni were measured on an older type of instrument at p ~ 3-10 -9 mbar without baking out. We worked with the XPS core levels Mg2s~2, Mg2P~2,3/2, Ni3P~2,3/2, Ni2P~2, Ni2P3/2 and 0 is~2 with the escape depth 20-30 ~ and with the very surface sensitive low energy Auger lines MgO (32 eV), Mg (45 eV) and Ni (61 eV) with the escape depth z 5 ~ [25]. The samples were fractioned (Mg2Ni) or filed (Mg-Mg2Ni) in air before inserting. In the UHV system the samples were cleaned by Ar + bombardment with a sputtering rate of z 20 ~/min (i0 ~A/cm 2, 3 kV). XPS and AES Results Pure Mg and Ni In the upper part of Fig. 1 and Fig. 2 the XPS reference spectra of pure, metallic (Ar + bombarded) Mg and Ni are shown. The shape and position of the peaks Mg2s, Mg2p and Ni2p agrees well with published spectra for metallic Mg [26,27] and metallic Ni [28,29,30]. The position of the Ni3p peak coincides with a plasmon peak [26] of Mg. A c c o r d i n g to Fuggle [27], Kowalczyk et al. [31] and Halder and Alonso [32] the Mg2s (88.6 eV) and Mg2p (49.6 eV) levels of metallic Mg are shifted to about 1.5 eV higher binding energy in oxidized Mg. The exposure of Ni to oxygen shifts the 2p3/2 level from 852.6 eV for metallic Ni to 853 ÷ 853.5 eV for Ni with adsorbed oxygen [28,30,33]. The formation of Ni oxide is indicated by a peak broadening on the high binding energy side

i

1 50

I 60

1

ENERGY

] 80

FIG.

BINDING

] 70

il 90

=-

eV

7.8:1

7.3:1

6.6: I

Mq:Ni

~ERTED

XPS spectra of the Mg2p, Mg2s and Ni3p core levels of pure Ni and Mg c l e a n e d by Ar + bombardment (upper part) and of M g - M g 2 N i eutectic alloy as inserted, c l e a n e d and exposed to oxygen

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I 880

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2

ENERGY

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;

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IOOL

AS INSERTED

PURE Ni (At')

i

XPS spectra of the Ni2p core levels of pure Ni c l e a n e d by Ar + b o m b a r d m e n t (upper part) and of M g - M g 2 N i eutectic alloy as inserted, c l e a n e d and exposed to o x y g e n

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>

D~

~Q

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O

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L. SCHLAPBACH, et al.

Vol. 14, No. 9

for small amounts of oxide and the growing of a Ni oxide peak some eV higher than the 2P3/2 peak of metallic Ni. For totally passivated Ni finally the 2P3/2 level is shifted to 856.5 eV [2 4 and corresponds largely to Ni(OH) 2 [ 3 4 • From the position of the Mg2s, 2p and Ni2p peaks we are able to distinguis h between metallic and oxidized Mg and N~. To determine the Mg to Ni ratio in the surface layer of z 20 A we integrated the peak height of the Mg2s, Mg2p and Ni3p levels, subtracted the background and divided by calculated cross sections [34]. For this procedure the Mg plasmon peak had to be subtracted from the Ni3p peak. Mg-Mg2Ni

eutectic

alloy

From the spectra of the Mg-Mg2Ni alloy (Fig. i) as inserted we conclude that the surface contains mostly oxidized Mg and nearly no Ni. The Mg to Ni ratio is about 30:1 or larger. The corresponding Ni2p peak in Fig. 2 indicates that the Ni is l a r g e ly in the metallic state. The peak broadening towards higher binding energy accounts for some oxidized Ni, but shape and position of the peak are completely different from those of Ni passivated at air [29]. After cleaning the sample by Ar + bombardment during one hour all the Mg and Ni is metallic, the Mg to Ni ratio being 6.6:1. Compared to the bulk concentration (7.8:1) the Mg concentration is too low, probably indicating a preferential sputtering of Mg. The sample was then exposed step by step to i, 3, i0 and i00 L 02 (i L = 1 Langmuir = 10 -6 Torr sec). The Ni peaks decreased and the Mg to Ni ratio increased from 6.6:1 for the sputtered surface to 6.8, 7.3, 7.8 and 8.1:1 after i, 3, i 0 and iOO L 02 resp. From Fig. 1 the broadening of the Mg2s and 2p levels towards higher binding energy, i.e. towards the oxide position, is evident. After iO L 02 about 25% Mg is oxidized. However the Ni, as follows clearly from the 2p peaks shown in Fig. 2, remains metallic. The deconvolution of the Ni3p peak (Fig. i) from the o v e r l a p p i n g Mg plasmon peak yields the Ni peak at 67 eV in agreement with that of pure metallic Ni. The 0 is peak appeared at 531 eV and corresponds to Mg oxide [22]. The oxidized surface of the Mg-Mg2Ni sample was further analysed by means of AES, using very surface sensitive, low kinetic energy lines of Mg, MgO and Ni, and then Ar + sputtered during 0.5, 0.5, i, 2, 5 and 5 min. and analysed after each sputter intervall. The results are put together to a depth profile in Fig. 3. The AES peak heights were calibrated with those of our pure Mg and Ni samples which are in good agreement with the AES handbook [35]. The oxygen exposed surface exhibits within the AES escape depth of z 5 ~ the (Mg + MgO) to Ni ratio 16:1 compared to 7.8:1 for the bulk and 8.1:1 determined for the same surface with XPS with the escape depth z 20 ~. There-

Vol. 14, No. 9

SURFACE ANALYSIS

1241

61 _ 4 ~i

o~ ° -12o :T 0

[]

0 IX

I~

• (Mg+Mg 0):I~ 4 z

'

O~o~o__ !

0 0

o~

5 I0 SPUTTERING TIME FIG.

AES depth profile of Mg-Mg2Ni to 114 L oxygen.

0 15MIN

3

eutectic

alloy after the exposure

fore the first few atomic layers must be largerly enriched in Mg. After s p u t t e r i n g ~ 40 ~ (2 min) the ratio 3.5:1 is reached. This ratio remains constant with further sputtering. Compared with the bulk c o n c e n t r a t i o n this ratio is too low, w h i c h indicates again that Mg is p r e f e r e n t i a l l y sputtered. The preferential s p u t t e r i n g has of course a larger effect on the AES c o n c e n t r a t i o n tion ratio because of the smaller escape depth. The surface of the o x y g e n exposed sample contains mostly MgO and nearly no Ni or metallic Mg. With sputtering the peaks of Ni and metallic Mg appear and increase rapidly together with the d e c r e a s i n g of the MgO peak. Mg2Ni So far we have investigated the surface of Mg2Ni by means of XPS only. The surface c o m p o s i t i o n of the Mg2Ni sample as inserted (fractioned in air, not baked out) is Mg:Ni z 20:1. The separation of the Mg2p and 2s peaks for metallic and oxidized Mg is c l e a r l y seen in Fig. 4. About 80% of the Mg is oxidized. The oxygen is peak is located at 531.5 eV and has

1242

L. SCHLAPBACH, et al.

i

I

i

Mg2p

i

Ni 3p

Vol. 14, No. 9

I Mg2s[

/

At*5' I

I

L

I 80

BINDING

I 9 0 eV

ENERGY

FIG.

4

XPS spectra of the Mg2p, Mg2s and Ni3p core levels of Mg2Ni air exposed and after 5 min c l e a n i n g by Ar + bombardement.

shoulders at 530 eV and 532.5 eV indicating a m i x t u r e of Mg oxide, hydroxide and Mg w i t h adsorbed oxygen and water [22]. F r o m the shape and p o s i t i o n (852.8 eV and 869.9 eV) of the Ni2p peaks we conclude that the small amount of Ni present is to a large extent in the m e t a l l i c state. Ar + sputtering of the sample inc r e a s e d the Ni c o n c e n t r a t i o n up to the Mg to Ni ratio z 2:1. A possible p r e f e r e n t i a l sputtering of Mg was p r o b a b l y c o m p e n s a t e d by further Mg surface segregation due to insufficient v a c u u m conditions. The Mg2s and 2p and Ni2p peaks c o r r e s p o n d to the metallic state for both elements. The shoulder of the Mg2s peak towards higher b i n d i n g energy accounts for some o x i d i z e d Mg. W i t h i n 12 hours (20°C, creased again to 4:1.

3-10 -9 mbar)

the Mg to Ni ratio

in-

Discussion In our analysis we did not take into account effects due to d i f f e r e n t escape depth e.g. for Mg and MgO [25] or m a t r i x effects of Auger electrons [36]. The possible chemical shift of the XPS peaks Mg2s and Mg2p between m e t a l l i c Mg in pure Mg and in Mg2Ni is not larger than 0.2 eV and therefore within the e x p e r i m e n t a l error. This agrees with Fuggle's [22] investigations of Mg in Mg2Cu. Possible chemical effects on the AES spectrum are still under i n v e s t i g a t i o n [21]. We have shown that the air exposed M g - M g 2 N i eutectic alloy and Mg2Ni intermetallic compound exhibits a strongly Mg enriched surface, the Mg being oxidized. The Ni however, in contrast to air exposed, p a s s i v a t e d Ni is largely in the m e t a l l i c state. There are t h e r m o d y n a m i c reasons [37] for the surface

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SURFACE ANALYSIS

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segregation of m e t a l l i c Mg in Mg2Ni. The segregation we observed after the exposure of the clean surface to oxygen is due to the lowering of the surface energy upon the o x i d a t i o n and is under the present e x p e r i m e n t a l c o n d i t i o n s much stronger than the segregation of metallic Mg. Based on the above results and the analogy to similar inv e s t i g a t i o n s on LaNi 5 and FeTi [2,12,13,14] we propose the following model to describe the catalytic effect of Mg2Ni on the h y d r o g e n a t i o n of Mg: Pure m e t a l l i c Mg exposed to air as well as in high v a c u u m or in the purest available hydrogen gas at room t e m p e r a t u r e immediately forms a p r o t e c t i v e oxide layer. This oxide layer prevents the d i s s o c i a t i o n of m o l e c u l a r hydrogen and therefore the formation of Mg hydride. In Mg2Ni surface segregation and d e c o m p o s i t i o n occurs. Mg diffuses to the surface and binds the oxygen p r e s e n t as impurity, k e e p i n g hereby the Ni metallic. That m e c h a n i s m is expected to be m u c h more effective around 300°C, i.e. at the temperature of h y d r o g e n a t i o n and deh y d r o g e n a t i o n of Mg2Ni. The material then consists of a decomposed surface of p a r t i c l e s of oxidized Mg and particles of m a i n l y metallic Ni (possibly MgNi2) supported on the metallic subsurface layer of the intermetallic compound. The hydrogen m o l e c u l e s can p e n e t r a t e the d e c o m p o s e d surface and can be dissociated either on the metallic Ni particles or on the metallic subsurface layer. The atomic hydrogen enters into the lattice and forms the hydride. In M g - M g 2 N i alloys (e.g. eutectic alloy or only Mg with some percent Ni, w h i c h forms always Mg2Ni) the Mg2Ni is present in the form of p r e c i p i t a t i o n s in the Mg matrix. The precipitations at the surface of the Mg-Mg2Ni alloy exhibits surface dec o m p o s i t i o n and enables the d i s s o c i a t i o n of the m o l e c u l a r hydrogen. The atomic hydrogen enters the Mg2Ni p r e c i p i t a t i o n s and diffuses through Mg2Ni into the Mg matrix and forms Mg hydride, in a n a l o g y to Rudmans [8] model for Mg-Mg2Cu. Recently started investigations of the magnetic p r o p e r t i e s of Mg2Ni and Mg-Mg2Ni support this model. F u r t h e r m o r e surface c o r r o s i o n of MgNi 2 together with the formation of magnetic Ni colloids was already noticed in 1958 by Bernus and Bode [38]. The catalytic effect of Mg2Ni on the h y d r o g e n a t i o n of Mg d i s c o v e r e d by Reilly and Wiswall [5] is therefore not an effect of Mg2Ni itself, but of the d e c o m p o s i t i o n of Mg2Ni with the formation of m e t a l l i c Ni p a r t i c l e s and a m e t a l l i c subsurface layer, a c c e s s i b l e to m o l e c u l a r hydrogen. It is probable that the model is also a p p l i c a b l e to explain the fairly rapid h y d r o g e n a t i o n of Mg in alloys and compounds with Cu [5,8], Y [17], rare earth metals [18,19], Li, Zr and Th [20]. I n v e s t i g a t i o n s on Mg2Cu going to be reported by Th. yon W a l d k i r c h et al. [21] point to an analogous process. M e t a l l i c Ni p a r t i a l l y covered with o x y g e n is still catalytically active but with a reduced rate of reaction [39]. At

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Vol. 14, No. 9

3OO°C the reduction of oxides by hydrogen [8,13] and the vaporization of some Mg might produce additional metallic surface area With each hydrogenation cycle further oxygen is delivered as impurity of the hydrogen. At the moment we cannot say whether the segregation and d e c o m p o s i t i o n goes on in order to find the additional oxygen - i.e. a continuous degradation of the material - or whether the hydrogen reduces again part of the oxidized metal. Investigations of the magnetic properties and their variation with the number of hydrogenation cycles will clarify this point. In addition to the magnetic investigations the surface analysis of Mg compounds around the temperature of hydrogenation are being prepared. Acknowled@ement We would like to thank H.C. Siegmann, A. Seiler, and Th. von W a l d k i r c h for many helpful discussions.

F. Stucki

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