Dehydriding kinetics of mechanically alloyed mixtures of magnesium with some 3d transition metal oxides

Int..1. tt.l'drogen Fnergy, Vol. 16, No. 4, pp. 265 270, 1991.

0360 t199 91 $3.00 + 0.00 Pergamon Press plc. ~: 1991 International Association for Hydrogen Energy.

Printed in Great Britain.

D E H Y D R I D I N G K I N E T I C S OF M E C H A N I C A L L Y A L L O Y E D M I X T U R E S OF M A G N E S I U M WITH SOME 3d T R A N S I T I O N METAL OXIDES M . TERZIEVA, M . KHRUSSANOVA and P. PESHEV Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1040 Sofia, Bulgaria

( Received j b r publication 21 Not,ember 1990)

Abstract The dehydriding kinetics of mechanicallyalloyed mixtures of 90 wt% Mg and 10 wt% of F% O~ or MnO> respectively, have been investigated. It is estabished that with both mixtures the dehydriding rate is limited by an intrinsic process. In the case of Mg Fe203, when the reaction transformed function F < 0.4, this process is the surface conversion tq--,~, the desorption being hindered probably by the formation of Mg2FeH~. Depending on temperature and pressure, the rate controlling step of dehydriding for mixtures of 90wt% Mg + 10wt% MnO, and the same F values can be either the fl ~ phase transition or the hydrogen diffusion through the :~ phase layer.

INTRODUCTION In previous papers [1-3] it has been shown that mechanically alloyed mixtures of magnesium with some 3d transition metal oxides have very good absorptiondesorption properties towards hydrogen. It has been established that during mechanical alloying and hydriding, the partial reduction of added oxide leads to the formation of clusters of the corresponding transition metal [3,4]. From the literature it is known that these clusters facilitate the dissociative hydrogen chemisorption, thus accelerating the hydriding~zlehydriding process [5, 7]. The main product of the hydriding of mechanically alloyed mixtures is magnesium hydride. Hydrogen desorption from this hydride is a complex process including intrinsic processes, gas mass flow and heat transfer, as follows: (A) Intrinsic processes consisting of the stages of (i) fi ~ phase transition on the surface, (ii) hydrogen diffusion through the c~ phase layer formed, (iii) hydrogen transition from the absorbed to the chemisorbed state, (iv) possible diffusion of hydrogen at the boundary between the grains of the metal and the magnesium oxide as well as surface diffusion of hydrogen atoms on the metal surface in the presence of metal particles which may serve as sites of hydrogen chemisorption, and (v) associative chemisorption of hydrogen on the metal surface, (B) Transport of hydrogen molecules from the particle surfaces to the gas phase by ordinary, Knudsen or forced flow.

(C) Heat exchange associated with a decrease of temperature during desorption, which leads to a drop of the equilibrium pressure, i,e. of the driving force of desorption. In the literature there is no definite opinion about which of these stages is rate controlling during the dehydriding of magnesium and its alloys, Thus, Stander [8] and Han et al. [9] are of the opinion that dehydriding of magnesium hydride is limited by the 1t~:~ phase transition. According to Karty et al. [10], the rate controlling step of the decomposition of this hydride in the presence of a Mg:Cu catalyst is the diffusion of hydrogen through the ~ phase. Lupu et aL [1 I] think that in the Mg2Ni-H 2 system at pressures considerably lower than the equilibrium, dehydriding is limited by hydrogen diffusion through the ~ phase, while at pressures close to equilibrium the [3--*~ conversion becomes rate controlling. The studies of Song [12] on the same system and on a system in which there is, in addition to Mg2Ni, also < 5 wt% free nickel [13], indicate that depending on the temperature and pressure of dehydriding, both Knudsen and ordinary diffusion of hydrogen through the pores, channels and cracks of the particles and the nucleation of the ~ phase can be rate determining. The purpose of the present paper was to study the dehydriding kinetics of mechanically alloyed mixtures of magnesium with 10 wt% of Fe20~ or MnO z in order to elucidate the rate-controlling step of the process from the viewpoint of nucleation and growth. According to Rudman [14], this theory represents the best description of the hydriding and dehydriding processes of metals and alloys.

265

266

M. TERZIEVA et al. EXPERIMENTAL

F '

Mixtures with the composition 90 wt% Mg + l0 wt% Fe203 and 90 wt% Mg + l0 wt% MnO2 were prepared by mechanical alloying in a planetary mill under argon with an acceleration of 60 g for 5 min.

176bar

06 T=S33K

//

The desorption characteristics of the samples were determined after preliminary activation of 1 g samples, which led to equilibrium particle sizes (r ~ 350/~m for the mixture containing Fe203 and r ~ 800 mm for the mixture with MnO 2). The corresponding specific surface areas as determined by the BET method were 6 and 2 m 2 g ~, respectively. The evolved hydrogen amount was measured by a volumetric method [15] at pressures of 1.30 and 4.03 bar and temperatures of 623 and 633 K for the Mg-Fe203-H 2 system, and pressures of 1.45-3.88 bar and temperatures of 608 and 623 K for the Mg-MnO2-H2 system. The samples were preliminarily hydrided for 1 h at P = 20 bar. In this case, F values of 0.43 and 0.65 were attained at temperatures of 623 and 633 K, respectively, for the Fe203 containing mixture, whereas for the sample with MnO2 and temperatures of 608 and 623 K, F = 0.45 and 0.47, respectively. The function F is defined as the ratio between the moles of adsorbed (desorbed) hydrogen for a time t and the theoretically determined moles of hydrogen in MgH 2 which correspond to the amount of magnesium in the given sample, The temperature was maintained constant with an accuracy of + 1 K, the thermocouple being placed at the bottom of the reactor. The temperature decrease at the beginning of the reaction was about 2 K, which did not substantially affect the driving force of the process and the correctness of the conclusions based on neglect of the effect of heat transfer, RESULTS A N D DISCUSSION Figures 1-4 shows the kinetic curves of dehydriding F = f ( t ) obtained at the above temperatures and differ-

F 90*/*Mg-10%Fe203

T=623K 0,3 / 0.2

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~ ,~l.75bor

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w , , 0 30 60 t(min] Fig. 1. Dependence of F on t during dehydriding of 90% Mg + 10 wt% Fe203 mixture at T = 623 K and different pressures,

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20

30

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t(min)

Fig. 2. Dependence of F on t during dehydriding of a mixture of 90wt% Mg and 10 wt% Fe203 at 633 K and different pressures. ent hydrogen pressures for the two mechanically alloyed mixtures. From these data it is possible to obtain the dependences d F / d t = r e ( p ) . For the FezO3-containing mixture, F values between 0.05 and 0.1 and dehydriding temperatures of 623 and 633 K, respectively, these dependences are presented in Fig. 5. However, it can be assumed that the character of the dependences will be preserved at significantly higher F values due to the almost linear shape of the F = f ( t ) curves (Figs 1 and 2). Figure 6 shows the d F / d t = 6P(P) dependences for the MnO2 containing sample at 623 K and different F values. Obviously, their character is the same as that of the curves for the sample with Fe203. Similar curves are also obtained after processing of the experimental data in Fig. 3 concerning the mixture of 90wt% Mg + 10 wt% MnO2 at 608 K. In the literature [16] there are data on the shape of the d F / d t = 5~(P ) curves for different rate controlling steps of the dehydriding process. Comparison with the above results shows that for the mixtures of magnesium with Fe203 or MnO2, the rate controlling step can be the fl ~:~ phase transition or the hydrogen diffusion through the c~ phase layer. As is known, the dehydriding of magnesium is associated with the appearance and growth of ~ phase nuclei and can be described by the Johnson-Mehl-Avrami (JMA) equation:

where r/ and K are constants which can be determined from the experimental data. Karty et al. [10] have found the r/values for different rate-controlling steps of magnesium dehydriding. However, on the basis of these values alone it is impossible to judge the rate-controlling step of the process. A more reliable interpretation of the experimental data is possible if the dependence of K on pressure and temperature is known. On the basis of data

MECHANICALLY ALLOYED MIXTURES OF MAGNESIUM

9

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03

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.

10

20

,

30

i

40

i

50

,

r

60

i

70

i

80

.

90 t(rnir4

Fig. 3. Dependence o f F on t during dehydriding of a mixture of 90wt% Mg + 10 wt% MnO, at T = 608 K and different pressures. from the literature concerning the dehydriding reaction when diffusion is rate-controlling, K ~ [1 - (P/Peq) 1'2] [17], while with the /~--,~ phase transformation as a rate-determining step, K ~ ln(Peq/P) [18].

'c ~N~ 0.010

1. The mechanically alloyed mixture of 90 wt% Mg and 10 wt% Fe20 ~

,

Figures 7 and 8 present the dependences lg [ - l n ( 1 - F)] = f (lg t) for the Fe2 03 containing mixture, which are obtained after processing of the experimental data according to the JMA equation. The points lie on straight lines with a slope of about 1. In this case, the rate controlling step may be both two-dimensional diffusion of hydrogen through the growing magnesium phase and /~--*~ phase transition at the interface. The dependences of the rate constant K on the pressure at temperatures of 623 and 633 K are given in Figs 9 and 10. The K vs ln(Peq,,'P) linear dependence found confirms the

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Fig. 4. Dependence of F on t during dehydriding of 90 wt% Mg + 10 wt% MnO 2 at T = 623 K and different pressures.

0

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Fig. 6. Dependence dF/dt = 5f(P) for a mixture of 90 wt°,/o Mg + 10 wt% MnO 2 at T = 623 K.

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M. TERZIEVA et al. 05

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• Fig. 7. Dependence lg [ - l n ( l - F ) ] =fOg t) for a mixture of 90 wt% Mg + 10wt% Fe_,O3 at T = 623 K and different pressures, assumption that the dehydriding is limited by the//--*~ conversion. From the Arrhenius dependence of K on T it is possible to determine the apparent activation energy of dehydriding according to the equation

FEa/T,- T~ K(T2) _ K(T, ) e x P L ~ L ~ ) ] , when the values of K are known for two temperatures and ln(Peq/P)= const. The value of 275 _+ 2 kJ mol -m obtained is almost twice as high as that for magnesium hydride determined by Han et al. [9] when the chemical transformation on the surface is rate-controlling. This enhanced E, value is probably due to the formation of 0

05

10

.

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Fig. 9. Dependence of K on [1--(P/Peq) 1,'2]for a mixture of 90 wt% Mg + 10 wt% Fe203 at temperatures of 623 and 633 K.

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a second hydride which is more stable than the magnesium hydride. As has been established in Ref. [3], during the hydriding Fe203 is reduced to Fe. Under the conditions of the experiment (T ~< 633 and P = 20 bar), formation of Mg2FeH~ according to the reaction x - 4 2MgH2 + Fe + ~ - - H 2~ Mg2 Felix is possible. The E, of hydriding for the Mg-Fe-H 2 system and the enthalpy of formation of ternary hydride are, according to Ref. [19], 214+_15 and 8 6 + 6 k J m o l - l , respectively. In Ref. [20] it is found that AH = - 9 8 kJ mol ~. The E, value of dehydriding obtained by us - (275 + 2 kJ mol ~) for Mg + Fe 203 does not contradict the supposition that the hindered desorption is due to the presence of the ternary hydride Mg 2FeH~.

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1.0

1.5

[n(Peq/P)

Fig. lO. DependenceofKonln(P~/P)foramixtureof9Owt% Mg + I0 wt% Fe203 at temperatures of 623 and 633 K.

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Figures 11 and 12 show the dependences lg [ - ln(1 - F )] = f (lg t) for the MnO2 containing mixture at 603 and 623 K. It is evident that, depending on the conditions (T and P), and on the reacted fraction, the experimental points can lie on one, two or three straight lines with different slopes. At the lower temperature (608 K) and a pressure close to the equilibrium (3.12 bar), the points are situated on a straight line with q ~ 2. With increasing AP, when P = 2.82 bar, 1 < r / < 2 up to a F v a l u e of 0.17, after which r / ~ 1. When P < 2.82 bar, three straight Nines are observed: (i) at 1 < q < 2 up to F values of 0.14, 0.13 and 0.12 and pressures of 2.36, 1.01 and 1.45 bar, respectively: (ii) at r/ ~ 1 up to F ~ 0.40, os

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269

and (iii) at q ~ 0.5, F > 0.40, i.e. the r/ value decreases with the drop of A P as well as with advancing reaction (increasing F value). At the higher temperature ( T = 623 K), there is an analogous change in the r/value. In this case, r / ~ 3/2 at a pressure near the equilibrium, while with increasing AP the value reaches I With advancing dehydriding, r/also decreases from about 1 to r / ~ 0 . 5 for F > 0 . 4 0 . This shows that either the rate-controlling step changes, or, in the case of the higher temperature, there is alteration in the dimensionality of the hydrogen diffusion. Unfortunately, in the case of this mixture the K vs pressure dependences (for q ~ 1, Figs 13, 14) give no explicit answer to this question because they support both assumptions, i.e. both dependences are linear. We are of the opinion that the/3 - ~ transition on the surface is the more probable rate controlling step of the reaction at low temperatures and pressures close to the equilibrium, while the diffusion becomes rate-limiting later, after the formation of the ~ phase and at pressures far from the equilibrium. This assumption is in agreement with the conclusion of Lupu et al. [I 1]. With further hydriding, when the F value exceeds 0.4, perhaps the one-dimensional hydrogen diffusion through the magnesium layer becomes rate-controlling (q ~ 0.5 since at such F values there is already a continuous a phase layer). CONCLUSION The results obtained lead to the conclusion that the dehydriding rate of mechanically alloyed mixtures of 9 0 w t % M g + 1 0 w t % Fe203 (or MnO2)depends on the occurrence of an intrinsic process. In the case of the 90 wt% Mg + 10 wt% Fe203 mixture, the surface/3--*~ transition is the rate controlling step up to F = 0.4 and the desorption is hindered by the probable formation of Mg, F e H , .

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Fig. 12. Dependence lg [ - l n ( 1 - F ) ] = f ( l g t) for a mixture of 90 wt% Mg + 10 wt% MnO~ at T = 623 K and different pressures.

12 11- P/Pe~) ]

05

Fig. 13. Dependence of K on It - (p/p~q)~/2] for a mixture of 90 wt% Mg + I0 wt% MnO2 at temperatures of 608 and 623 K.

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M. TERZIEVA et al. "~~

~ ~ _~

90% Mg-lOO/oMnO~

2.0

~ =1 .

/

~ 50 4.0

/

1.5 ~

1.0

05

3.0 •

-608K

o- 623K

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1.0 1.0 tn(Peq/P)

1.5

Fig. 14. Dependence of Kon ln(P+q/P)for a mixture of 90 wt% Mg + 10 wt% MnO 2 at temperatures of 608 and 623 K. Depending on the temperature and the pressure, the

rate-controlling step of the dehydriding within the same range of F values for the 90 wt% Mg-t- 10 wt% MnO2 mixture may be the phase transition or hydrogen diffusion through the ct phase layer. REFERENCES 1. M. Khrussanova, M. Terzieva, P. Peshev and E. Ivanov, Mater. Res. Bull. 22, 405 (1987).

2. M. Khrussanova, M. Terzieva, P. Peshev, I. Konstanchuk and E. Ivanov, Z. Phys. Chem. N.F. 164, 1261 (1989). 3. M. Khrussanova, M. Terzieva, P. Peshev, I. Konstanchuk and E. Ivanov, J. Less-Common Metals+, submitted. 4. P. Peshev, M. Khrussanova, D. Chakarov, M. Terzieva and Ts. Marinova, Mater. Res. Bull. 24, 207 (1989). 5. L. Schlapbach, A. Seiler and F. Stucki, Mater. Res. Bull. 13, 697 (1978). 6. A. Seiler, L. Schlapbach, Th. vonValdkirch, D. Shaltiel and F. Stucki, J. Less-Common Metals 73, 193 (1980). 7. J. Jacob and M. Polak, Mater. Res. Bull. 16, 131l (1981). 8. C. M. Stander, J. Inorg. Nucl. Chem. 39, 221 (1977). 9. J. S. Han, M. Pezat, J. Y. Lee, J. Less-Common Metals 130, 395 (1987). 10. A. Karty, J. Grunzweig-Genossar, R. S. Rudman, J. Appl, Phys. 50, 7200 (1979). 11. D. Lupu, A. Biris, G. Mihailescu, R. Sarbu and D. Vonica, Int. J. Hydrogen Energy 13, 685 (1988). 12. M. Y. Song, B. Darriet, M. Pezat, J. Y. Lee and P. Hagenmuller, J. Less-Common Metals 118, 235 (1986). 13. M. Y. Song, J. Less-Common Metals 157, 155 (1990). 14. P. S. Rudman, J. Less-Common Metals 89, 93 (1983). 15. B. Tanguy, J. L. Soubeyroux, M. Pezat, J. Portier and P. Hagenmuller, Mater. Res. Bull. 11, 1441 (1976). 16. C. N. Park and J. Y. Lee, J. Less-Common Metals 91, 189 (1983). 17. P. S. Rudman, J. Appl. Phys. 50, 7195 (1979). 18. S. Tanaka, J. D. Clewley, T. B. Flanagan, J. Less-Common Metals 56, 137 (1977). 19. I. G. Konstanchuk, E. Yu. Ivanov, B. Darriet, V. V. Boldyrev and P. Hagenmuller, lzv. Sib. Otd. Akad. Nauk SSSR, Set. Khirn. Nauk, No. 3, 29 (1986). 20. J. J. Didisheim, P. Zolliker and K. Yvon, Inorg. Chem. 23, 1953 (1984).