On the hydriding and dehydriding kinetics of magnesium with a titanium dioxide admixture

Mat. R e s . B u l l . , Vol. 22, p p . 405~412, 1987. P r i n t e d in t h e USA. 0025-5408/87 $3.00 + .00 C o p y r i g h t (c) 1987 Pergamon J o u r n a l s L t d .

ON THE HYDRIDING AND DEHYDRIDING KINETICS OF MAGNESIUM WITH A TITANIUM DIOXIDE ADMIXTURE M. Khrussanova, M. Terzieva, P. Peshev and E. Yu. Ivanov ~ Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, f1040 Sofia, Bulgaria *Institute of Solid State Chemistry, Academy of Sciences of USSR, 63009fl Novosibirsk, USSR ( R e c e i v e d November 20, 1986; Communicated b y P. Hagenmuller)

ABSTRACT The paper deals with the hydriding and dehydriding kinetics at different temperatures and pressures of mixtures consisting of 90 wt% Mg and 10 wt% TiO 2 (futile) obtained (a) by mechanical alloying in a planetary mill and (b) by the usual procedure of grinding and homogenization. It is shown that the addition of TiO 2 improves the absorption and desorption characteristics of magnesium, this effect being more pronounced in the mechanically alloyed mixture. It is established that at all temperatures the highest absorption capacity of the samples corresponds to pressures close to the equilibrium one. The possible reasons for these effects are discussed. MATERIALS INDEX: magnesium, titanium dioxide, hydrogen storage media, mechanical alloys

Introduction Magnesium hydride, MgH2, is one of the most promising hydrides for hydrogen storage due to the high absorption capacity of magnesium and its low price. The direct synthesis of magnesium hydride should be carried out at high temperatures and pressures. For this reason, attempts have been made to facilitate this process. In ref. 1, for instance, it is shown that the use of mixtures of magnesium and different metals or alloys essentially facilitates hydriding. Still better results are obtained with alloys of magnesium and rare-earth metals where the presence of the rare earth accelerates the hydriding reaction, while the increase of the interface as a result of hydriding favours hydrogen desorption (2). However, the energy consumption during the preparation of alloys is considerable. This unfavourable circumstance can be coped with by applying the method of mechanical alloying where much less energy is needed and a much better contact between the particles is acquired. As is shown in a previous paper 405

406

M. K H R U S S A N O V A , et at.

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(3), the hydrogen capacity of a mixture of magnesium and cerium obtained by mechanical alloying, is practically the same as that of a binary CeMg12 alloy prepared by the classical method. Taking into account the catalytic properties of TiO2 in hydriding reactions, it was of interest to check the effect of this oxide on the hydriding kinetics of magnesium as well as to show the role of mechanical alloying by comparison of the absorptiondesorption properties of a mechanically activated Mg-Ti02 mixture with those of a mixture of the same composition prepared by the usual method. Experimental The initial substances were metallic magnesium and titanium dioxide (rutile) of a 99 % purity. Two kinds of mixtures with the composition 90 wt% Mg + 10 wt% TiO2 were prepared from them. The first one was obtained under argon in a planetary mill by mechanical alloying for 3 min with an acceleration of 60g whereas the second mixture was made by the usual procedure of grinding and homogenization in an agate mortar. The particles of the mixture obtained by mechanical alloying had the shape of platelets (q5 x 10 x 0.1 mm). The particles of the other mixture were roughly spherical (mean diameter of about 200 ~m). The absorption-desorption measurements were performed by a volumetric method after preliminary activation of the samples. The hydrogen capacity of the mechanically alloyed mixture was determined within the range 0.3-3 MPa at a constant temperature of 573, 598 and 623 K. The hydriding of the mixture having the same composition but prepared by the usual method was investigated at T = 623 K and pressures of 1-3 MPa. The x-ray phase analysis showed the appearance of MgH2 alone as a result of hydriding. Contrary to the mixture prepared by the usual method, where the activation is achieved in 3-4 absorption-desorption cycles, the activation of the mechanically alloyed mixture is slower due to the large initial size of the particles. Figure 1 presents the

FIG. I Dependence on time of the degree of conversion into a hydride (F) of a Mg(90~)Ti02(10~ ) mixture after different numbers of hydriding-dehydriding cycles

F x - 39 cyctes

0~

I

J35- cyctes

o4 0.3

o2 ~' o.1

\12-cycles T= 623K P= 3MPa

30

,

i

60

90

i

~20 t(min)

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MAGNESIUM

407

absorption curves during the gradual activation of this mixture. Obviously, a constant hydriding rate is attained after ~5 cycles and the shape of the curves is analogous to that for magnesium alloys containing a rare-earth element (2). Figures 2-4 show data from measurements of the hydrogen amount absorbed by the mixture obtained by mechanical alloying at a constant temperature of 573, 598 and 623 K, respectively, and different pressures. In all cases intersection of the curves

FIG. 2 Kinetic curves of hydriding at T = 573 K and different pressures of a Mg(90%) Ti02 (I0%) mixture obtained by mechanical alloying

weight °/° H absorbed

20 \0.3 MPo ~.o

K

o

3'0

io

9o

~o

tcm,ol

is observed, i.e. the samples exhibit a higher hydrogen capacity at pressures close to the equilibrium. This effect was also observed in a previous work during hydrogen storage by calcium-rich multiphase alloys with the general formula La2_xCaxMg17 (4).

weg I

H a 0sot bed

0.5 MPa

FIG. "2MPo

3

Kinetic curves of hydriding

at T = 598 K 2.01 ~

2

and different pressures of a Mg(90%) -

M PO

,.oI ,, 0

30

60

~io2(Io~)

mix-

ture obtained by mechanical alloying

gO

120 t(minl'

Figure 5 presents the results from the determination of the absorption properties of the Mg-TiO 2 mixture obtained in the usual way. The data of Tanguy et al. (I) on the hydriding of

408

M. K H R U S S A N O V A ,

et al.

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weight % H absorbed

3

1MPa 2MP(z 3MPo

3,C

FIG. 4 Kinetic curves of h y d r i d i n g at T = 623 K and different p r e s s u r e s of a Mg(90%) Ti02(I0%) mixture obtained by m e c h a n i c a l alloying

2,0

1.0

0

30

60

90

120

t(min)

weigh!% Hobsorbed 3.0

2.0

h 1.0

3/~........~.~

Mg

T=623K

~3MPo1618K)

0

i

i

30

60

~

i,

0

120 tin,n1

FIG. 5 K i n e t i c c u r v e s of h y d r i d i n g of: (o), a M g ( 9 0 % ) - T i O 2 ( I O % ) m i x t u r e o b t a i n e d in the u s u a l w a y at T = 623 K and d i f f e r e n t p r e s s u r e s ; (×), pure m a g n e s i u m at T = 618 K a n d P = 3 M P a a c c o r d i n g to ref. I p u r e m a g n e s i u m w i t h a l m o s t the same p a r t i c l e size at T = 6q8 K a n d P = 3 M P a are i n c l u d e d in the same f i g u r e . The r e s u l t s p r e s e n t e d in Figs. $ a n d 5 give i n f o r m a t i o n on the r o l e of b o t h t i t a n i u m d i o x i d e a n d the p r e p a r a t i o n m e t h o d of the m i x t u r e s . The c u r v e s in Figs. 6 a n d 7 o b t a i n e d at P = 0.2 M P a i l l u s t r a t e the d e s o r p t i o n a b i l i t y of the two k i n d s of m i x t u r e s . T a b l e q p r e s e n t s the d e s o r p t i o n t i m e s of h a l f of the a b s o r b e d h y d r o g e n

V o l . 22, No.

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weight ,/, H desor bed

MAGNESIUM

,••

409

613K

FIG. 6 Kinetic curves of h y d r o g e n d e s o r p t i o n at P = 0.2 MPa and different temperatures of a mixture obtained by mechanical alloying

30

20

~ 0

593 K

30

60

90

120

tlmin)

weight % ' H desor bed

FIG. 7 Kinetic curves of h y d r o g e n d e s o r p t i o n at P = 0.2 MPa and different temperatures of a mixture obtained in the usual way

~

663K

623K

2.0 613K

1.0

30

60

g0

120

t (min)

amount from the mixtures under i n v e s t i g a t i o n and from some magnesium alloys studied in previous papers of ours as well as some data of Darriet et al. (5-7)- Evidently, at 598 K the mixture prepared in the usual way shows no d e s o r p t i o n at all whereas at higher t e m p e r a t u r e s h y d r o g e n d e s o r p t i o n from it occurs considerably more slowly than from the m e c h a n i c a l l y alloyed mixture. Discussion C o m p a r i s o n of the a b s o r p t i o n curves in Figs. ~ and 5 shows that the mixture obtained by m e c h a n i c a l alloying has better h y d r o g e n a b s o r p t i o n p r o p e r t i e s than the mixture p r e p a r e d in the

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TABLE 1 Desorption Time of Half of the Hydrogen Amount Absorbed by Mg(90 wt%)-Ti02(10 wt%) Mixtures and by Some Magnesium Alloys ~ I / 2 (min) at a temperature of (K)

Sample 598

613

623

63

23

20

no desorption

80

55

CeMg12 (5)

105

26

-

8

La2Mg17 (6)

60

12

-

La2Mg16Ni

22

-

-

7

2

-

5 2 I

Mg(90 wt%)-Ti02(10 wt%) mechanically alloyed mixture Mg(90 wt%)-Ti02(10 wt%) mixture obtained in the usual way

(6)

Lal.8Cao.2Mg17

(7)

633

663

usual way. Electron microphotographs of the two kinds of mixtures before hydriding (Figs. 8 and 9) indicate the formation of a more intimate mixture in the case of mechanical alloying where a better contact betwesn Mg and Ti02 particles is achieved. Besides, mechanical alloying permits the formation of a large number of defects which also improve the absorptivity of the sample. And finally, electron microscopy observation of the activated samples after the necessary hydriding-dehydriding cycles has shown that activation causes negligible decrease in particle size of the mixture prepared in the usual way while the particle size of the

,

i

~

f

I

2S ~0 ISOI

FIG. 8 Electron microphotograph of a mechanically alloyed mixture before hydriding

FIG. 9 Electron microphotograph prior to hydriding of a mixture obtained in the usual way

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mechanically considerable

MAGNESIUM

411

alloyed mixture drastically decreases and becomes smaller than that of the other mixture.

The differences in absorption-desorption characteristics of the two kinds of mixtures can be satisfactorily explained taking into account the respective particle sizes, the degree of contact between the particles and the presence of different numbers of defects, while the improvement in hydrogen storage behaviour of magnesium after the addition of TiO 2 should be ascribed to the properties of this oxide. It is known that after heating in vacuum, under hydrogen or in an inert atmosphere, titanium dioxide becomes a n-semiconductor (8-10). Hence, under the activation conditions of the samples in the present paper, Ti02 will play a definite role during dissociative chemisorption of hydrogen on the surface of magnesium particles, all the more that hydrogen diffusion along the c-axis of futile is strongly facilitated (10). The process of mechanical alloying in an inert atmosphere favours the formation of n-Ti02 (or of Ti3+) on the particle surfaces. It should be pointed out that the catalytic effect of Ti02 is stronger with the mixture obtained by mechanical alloying than with the other mixture where the contact between particles is weaker. Thus, the surface sites on which titanium dioxide is localized, become nucleation centers. This is also confirmed by the experimental results given in Figs. 2-5. The intersection of the kinetic curves observed at three hydriding temperatures indicates that the hydriding process of the mixtures under consideration is associated with the formation and growth of magnesium hydride nuclei on the particle surface. The number of nuclei depends on whether the hydriding conditions are close to the equilibrium or not. This fact, which was for the first time established by Vigeholm et al. (11) on pure magnesium, was confirmed with magnesium alloys having a high magnesium concentration (La2-xCaxMg17) (4). As is evident from Fig. 2, this is also observed with relatively small differences between the experimental pressures as well as between them and the equilibrium pressure at a certain temperature. The assumption that Ti02 facilitates the nucleation of MgH 2 explains one more peculiarity of hydriding. As in the case of alloys of the type La2_xCaxMg17 , the overlapping of nuclei begins earlier in the presence-of Ti02. With mechanically alloyed samples hydriding stops at a conversion degree of about 50 %, which is significantly lower than that attained during the hydriding of pure magnesium with the same particle size (11). Similar results are also obtained during hydriding of a mixture of magnesium and 0 . 1 % nickel prepared by mechanical alloying (12). As was pointed out above, the difference in behaviour of the two mixtures is much more pronounced with respect to hydrogen desorption. The hydrogen desorption rate for the mechanically alloyed mixture is of the order of that for pure rare earth-magnesium alloys but is lower than that for substituted alloys containing Ca or Ni (e.g. La2_xCaxMg17 or La2Mg16Ni) whereas with the mixture prepared i n the usual way-desorption proceeds much more slowly. Belkbir et al. have established

(qS) that the decomposition

412

MAGNESIUM

Vol. 22, No.

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of magnesium hydride is associated with the appearance and growth of magnesium nuclei both on the surface and in the bulk of the particles, i.e. this is a topochemical reaction whose rate would depend on the size of the Mg/MgH 2 interface. That is why the higher rate of hydrogen desorption ~rom the mechanically alloyed mixture is probably due to the larger interface resulting from the more pronounced fragmentation of the sample during the hydriding. Nevertheless, this interface is smaller than that appearing during the hydriding of multicomponent alloys containing Ca and Ni, owing to which the latter have better desorption characteristics than the 90% Mg - 10% Ti02 mixture.

1. 2. 3-

4. 56. 7.

8. 9. 10.

References B. Tanguy, J. L. Soubeyroux, M. Pezat, J. Portier and P. Hagenmuller, Mat. Res. Bull. I__I, 1441 (1976). M. Khrussanova, Oomm. Dept. Chem., Bulg. Acad. Sci. (in press). E. Yu. Ivanov, B. Darriet, A. A. Stepanov, K. B. Gerasimov and I. G. Konstanchuk, Izv. Sib. Otdel. Acad. Nauk SSSR, Set. Khim. Nauk No 5, 30 (1984). M. Khrussanova, M. Terzieva and P. Peshev, J. Less-Common Metals (in press). B. Darriet, M. Pezat, A. Hbika and P. Hagenmuller, Mat. Res. Bull. 1_~4, 377 (1979). M. Khrussanova, M. Pezat, B. Darriet and P. Hagenmuller, J. Less-Common Metals 86, d53 (1982). M. Khrussanova, M. Terzieva, P. Peshev, K. Petrov, M. Pezat, J. P. Manaud and B. Darriet, Int. Jo Hydrogen Energy 10, 591 (1985). R. Yengar, M. Codell, J. Karra and J. Turkevich, J. Amer. Chem. Soc. 88, 5055 (1966). J. R. Harris and D. R. Rossington, J. Catalysis I__~#, 168 (1969). T. Schober and D. G. Westlake, Scr. Metallurgica 15, 9d5

(1981). 11. B. Vigeholm, J. Kj~ller, B. Larsen and A. S. Pedersen, J. Less-Common Metals 89, 135 (1983). 12. K. B. Gerasimov, E. Yu. Ivanov and V. V. Boldyrev~ Izv. Sib. Otdel. Akad. Nauk SSSR, Set. Khim. Nauk No 1, 27 (1985). 13. L. Belkbir, E. Joly and N. Gerard, Int. J. Hydrogen Energy 6, 285 (1981).