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Journal of Alloys and Compounds 439 (2007) 243–248

Investigations on the desorption kinetics of Mm-doped NaAlH4

D. Pukazhselvan, M. Sterlin Leo Hudson, Bipin Kumar Gupta,M.A. Shaz, O.N. Srivastava ∗

Department of Physics, Banaras Hindu University, Varanasi 221005, India

Received 13 June 2006; received in revised form 17 August 2006; accepted 18 August 2006Available online 25 September 2006

bstract

This paper reports mischmetal (Mm) as an effective catalyst for fast desorption kinetics (3.7 wt% in 60 min in which ∼3.3 wt% observed in0 min and ∼5 wt% in 180 min at the desorption temperature of 150 ◦C for the 2 mol% Mm-doped material) and rehydrogenation (up to 35 cycles)f the light weight hydrogen storage material NaAlH4. In fact this catalyst Mm has been shown to be better than the presently known catalyst Ti.

or Mm, its presence when admixed in NaAlH4 is discernible. Also the higher reversible hydrogen storage capacity (4.77 wt% which is 86% of

he total reversible storage capacity) of the total is achievable in the 12th cycle. The possible modes of catalyst Mm for fast desorption kinetics haseen outlined and the most feasible mechanism in terms of weakening of Na and AlH4 bonding has been put forward.

2006 Elsevier B.V. All rights reserved.

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eywords: Complex hydrides; Catalysis; X-ray diffraction; Absorption/desorp

. Introduction

Hydrogen in many respects is an ideal energy carrier. It is theost abundant element in the universe and generally bound to

ther elements. It can be produced by variety of ways includinglectrolysis, chemical extraction, biological hydrogen produc-ion, photo dissociation of H2O, etc. The specific energy con-ent of hydrogen is 33.3 kWh/kg, nearly three times larger thanasoline or diesel [1]. However, problems especially with theydrogen storage do arise when applications in the real worldre considered. This is particularly true in the transportation areahere efficient onboard hydrogen storage is of major impor-

ance. For effective storage in vehicular applications, althoughntermetallic hydrides like FeTi, LaNi5, Mg2Ni, etc., possessravimetric storage capacity whose upper limit is ∼3 wt%, theequisite storage capacity is ∼6 wt%. Out of the various ther-odynamically permissible materials for reversible hydrogen

torage, sodium alanate is a promising candidate due to its high

ravimetric hydrogen density (5.55 wt% for first two steps) buthe thermal decomposition of NaAlH4 is kinetically challengingnd is almost not reversible. Recently NaAlH4 has been signifi-

∗ Corresponding author. Tel.: +91 542 2368468; fax: +91 542 2369889.E-mail address: hepons@yahoo.com (O.N. Srivastava).

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925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.jallcom.2006.08.060

inetics; Catalytic activity

antly improved as a possible hydrogen storage material by thection of catalysts [1–6]. However, the known effective catalystsre only few and in search for better catalysts and understandinghe role of catalysts for enhancing NaAlH4 desorption needs toe researched further.

The behavior of catalyzed NaAlH4 was first enlightened aspossible hydrogen storage material through the wet dopingethod of catalyst bearing precursors (e.g. Ti(OBu)4, Fe(OEt)4,

tc.) by Bogdanovic et al. [2]. Subsequently various catalystsave been introduced by the wet and dry doping methods, ofhich Ti-based catalysts enabled the ready release and reab-

orption of hydrogen [3–10]. However, these extensive researchfforts could not clear the puzzles of dispersed catalytic speciesn NaAlH4 matrix. Various studies have been carried out oni doped and bare alanates [11–15] for analyzing the loca-

ion and role of the Ti catalyst. However, these studies haveot been able to elucidate the said aspects of the catalyst Ti2–10,16–25,28–31]. Besides transition metals Ti, Zr and Fe,arbon nanotubes have also been found to enhance hydrogenesorption in NaAlH4 [24,25]. In our previous work we havehown that CNT admixed NaAlH4 exhibits improved dehydro-

enation as well as recyclability [25].

In this paper we described the hydrogenation/dehydro-enation behavior of inner transition elements (rare earth ele-ents) admixed NaAlH4. The inner transition elements taken

2 loys and Compounds 439 (2007) 243–248

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44 D. Pukazhselvan et al. / Journal of Al

s catalysts correspond to mischmetal Mm (Ce: 42 at.%, La:1 at.%, Nd: 18 at.%, Pr: 9 at.%). Performances of the Mm-oped NaAlH4 were analyzed through desorption kinetic mea-urements. The results obtained by us in the present investigationuggest that Mm represents a better catalysis than the earlieratalysts, e.g. Ti and CNT. Significant hydrogen rechargingapacity of the material was observed at lower charging tem-erature and pressure ranges. The 2 mol% Mm-doped NaAlH4aterial subjected for recharging at 110 atm and at 130 ◦C has

een carried through hydrogen charging discharging cycles upo 35 cycles and it reached its maximum storage capacity of.77 wt% in the 12th cycle. This is 86% of the total storageapacity (5.55 wt%) of the material.

. Experimental methods

The unpurified NaAlH4 (Aldrich) has been purified by following Shlenkethod [26]. The as received NaAlH4 was dissolved in THF (solubility limit in

etrahydrofuran is 162 g/l) and passed through a glass filter. NaAlH4 begins torystallize and got separated from the solution after this filtrate was concentratedn vacuum. Pentane/hexane was added to the solution and the mixture was stirredor 3 h under argon atmosphere, causing NaAlH4 to fully separate from theolution as a fine precipitate. The residue was washed with pentane and driednder vacuum overnight at room temperature. After drying, the purified NaAlH4

as obtained as a fine white powder.Bulk pieces of mischmetal (Mm) (∼0.5 mm) 99.9% (Leico, NY) were pow-

ered in an argon filled glove box. A miller similar to that used by us in ourrevious study of CNT admixed NaAlH4 was used. We found that chrome–nickeltainless steel miller of ∼500 rpm/min and milling vial of 150 ml volume withwo steel balls of 12.5 mm diameter and one steel ball of 4 mm diameter wasound optimum. The miller was kept and operated in an argon filled glove box.he fine powder of Mm catalyst in 1, 2, 3 and 5 mol% was mixed with suitablemount of purified NaAlH4 and ball milled for 1 h. Ti fine powder (2 mol%) wasoped in NaAlH4 by using the same miller and the procedure that employed foroping Mm in NaAlH4 was followed.

Absorption/desorption studies were carried out by using Sieverts type appa-atus. The sample was placed in a standardized steel reactor fabricated in ouraboratory [27]. The reactor was evacuated up to 10−2 to 10−3 Torr before insert-ng hydrogen (purity 99.9%). Rapid heating of the reactor having hydrogenressure at ∼1 atm up to the desorption temperature (150 ◦C) was accomplishedo desorb hydrogen from the sample. Hydrogen storage capacity of the catalyzedaAlH4 was determined with respect to the total weight of the sample includ-

ng the weight of the catalyst. Sample was recycled after hydrogenation at theemperature 130 ◦C and pressure 110 atm for 12 h.

The structural characterization of the samples have been carried outhrough X-ray diffraction technique employing Philips PW-1710 X-ray diffrac-ometer equipped with graphite monochromater employing Cu K� radiationλ = 1.54 A). The microstructural features have been monitored by Scanninglectron Microscopy (Philips, XL-20) using secondary electron imaging.

. Results and discussions

We explored the structural characterization of doped andndoped NaAlH4 samples in order to seek possible correla-ions between dehydrogenation/hydrogenation characteristicsnd their structural/microstructural features. XRD pattern of thehree different samples (a) NaAlH4 without catalysts, (b) 2 mol%

m:NaAlH4 and (c) 2 mol% Ti:NaAlH4 are compared in Fig. 1,

representative XRD pattern of the Mm-doped NaAlH4 is

hown in Fig. 1(b). A noteworthy result of this XRD patterns that in addition to diffraction peaks from NaAlH4, thoserom Mm as broad peaks are also found to be present. The

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aAlH4 with 2 mol% Mm catalyst; (c) NaAlH4 with 2 mol% Ti catalyst. Diffrac-ion patterns did not show any discernible lattice constant (a = 5.02 A and= 11.34 A) variation before and after doping.

iffraction peaks from NaAlH4 and Mm (dominant componentseing La and Ce) are indicated in the figure. This observations rather curious since it is known that the existence of wellefined catalyst bearing phases at low temperatures through X-ay diffraction has not clearly been observed in Ti:NaAlH4. Thebsence of Ti is taken to imply that Ti or Ti containing phasesay be present in amorphous state [20]. However, Weidenthaler

t al. [29] have proved the existence of catalyst bearing alloy inhe doped (Ti and Zr) NaAlH4 samples. In this interesting studyaAlH4 was first catalyzed and then filtered. In order to find

he possibility of catalyst combined phases, the filtrate was thenoncentrated for characterization studies, there the XRD studyroved the existence of Ti–Al alloy (TiAl3). In our investiga-ion, the presence of rather broad X-ray diffraction peak from

m suggests that the Mm fine crystalline particles/clusters areresent with the NaAlH4 matrix. The presence of Mm was invari-bly noticed in all the Mm-doped NaAlH4 samples investigatedy us and no significant lattice constant variation of NaAlH4 wasbserved before and after doping. These results suggest that Mmoes not get substituted for Na or Al but is present as a separatentity.

We have observed that Mm doping leads to enhanced hydro-en desorption behavior under near ambient temperature andressure ranges. It can be seen in Fig. 2 that the NaAlH4 sam-les doped by 1, 2, 3 and 5 mol% Mm kept under 1 atm hydrogenressure and temperature 150 ◦C leads NaAlH4 dissociationoupled with fast hydrogen liberation rate. It was observedhat all the samples gave peak hydrogen desorption after a cer-ain level showing slow kinetics (approximately 30 min afteraths reactor at 150 ◦C) regardless of what mole fraction of

m catalyst is being employed. After this level 1, 2, 3 andmol% Mm-doped NaAlH4 samples desorbed 0.87, 3.35, 2,nd 1.81 wt%, respectively, in the next 30 min. Particularly inmol% Mm-doped material, the rate of liberation correspond-

D. Pukazhselvan et al. / Journal of Alloys a

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ig. 2. Desorption kinetics of 1, 2, 3 and 5 mol% Mm catalyst-doped NaAlH4. Itan be seen that 2 mol% Mm:NaAlH4 leads better kinetics and storage capacity.

ng to 0.18 wt%/min has been obtained in the peak desorptionime (40–50 min, see Fig. 2). The intermediate phase Na3AlH6as separable here only by 1 mol% Mm-doped material where

he desorption rate decreased after releasing ∼3 wt%. The des-rption plot for 3 mol% Mm-doped material in this figure depictshat the hydrogen desorption from both NaAlH4 and Na3AlH6hases have been significantly improved at the desorption tem-erature 150 ◦C. Higher doping levels (5 mol% and higher) alsoesulted in good kinetics however the gravimetric hydrogen stor-ge capacity has been found to decrease.

Fig. 3 shows the desorption kinetics of NaAlH4 catalyzedy 2 mol% Mm, and 2 mol% Ti catalysts. These curves brings

ut the comparison of enhancement in desorption kinetics byhe catalyst Ti (2 mol%) as found by other works and the cat-lyst Mm (2 mol%) as obtained in the present work. As cane seen, a clear difference exists between the curve A (Ti as

ig. 3. Desorption kinetics reveals the superiority of Mm catalyst over Ti. Thei-doped sample release hydrogen from Na3AlH6 phase at higher temperature.

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nd Compounds 439 (2007) 243–248 245

he catalyst) and curve B (Mm as the catalyst). For the casef Ti, desorption at the temperature 150 ◦C proceeds to finallyield the storage capacity of 3.7 wt%. This corresponds tohe first step of the reaction 3NaAlH4 → Na3AlH6 + 2Al + 3H2∼3.7 wt%). On the other hand for the same concentration∼2 mol%) and temperature (∼150 ◦C) the Mm catalyst finallyeads to desorption giving storage capacity of ∼5 wt%. Thismplies that with Mm as catalyst, the NaAlH4 desorbs throughhe first as well as the second reaction. Therefore, the Mm at50 ◦C triggers the reaction NaAlH4 → Na3AlH6 and also theeaction Na3AlH6 → 3NaH + Al + (3/2)H2 (∼1.85 wt%). Theotal capacity therefore, corresponds to ∼5.5 wt%. It should beointed out that some experiments on desorption with doping ofaAlH4 with La alone (one of the major components of Mm)ere also done. However, the results on desorption of Mm wereetter than that of La. In view of this and the fact that Mm islso economically viable, i.e. much cheaper than the individ-al ingredients of Mm like La, Ce, Nd, etc., investigations wereonfined only on the Mm-doped NaAlH4.

As regards desorption time, it is ∼45 min for reaching thetorage capacity of ∼2.1 wt% for both Ti and Mm, later, i.e.eyond 60 min (2.68 wt%) for Ti-doped samples, lower desorp-ion rate ∼0.6 wt% was observed up to 80 min. However, Mmields comparatively higher desorption rate 3.7 wt% in the first0 min and then up to ∼1.3 wt% on desorption through a fur-her time period of ∼130 min. Based on the above, it can beaid that Mm is a better catalyst in regard to both the desorptiono reach higher storage capacity (∼5 wt%) at lower tempera-ure and also faster desorption kinetics. It should be mentionedhat with Ti powder as the catalyst, the second reaction com-

ences only when the temperature is increased up to 190 ◦C.s is evident from Fig. 3, beyond ∼3.2 wt% Ti-doped sampleives further 1.9 wt% hydrogen at the temperature of 190 ◦C. It ishus clear that Mm catalyst employed in the present investigationor increasing the desorption kinetics of NaAlH4 is better thanhe earlier catalyst Ti which has been used by several workers.

For NaAlH4 in addition to enhancing the desorption kinetics,nother important issue relates to recycling to get back the ini-ial material NaAlH4. The desirable characteristics should be toestore as close to the initial capacity as possible. This problemas not yet been fully solved [3,10,31]. Keeping this in viewnd in order to check on the recyclability of the Mm-dopedaAlH4, we carried out two different recharging runs. In onef them, whereas the temperature is kept constant at 130 ◦C, theharging pressure is varied as 40, 60, 80 and 100 atmosphere.n the other experiment the pressure is kept constant at 110 atmnd the temperature is varied as 50, 75, 100 and 130 ◦C. Theesorption temperature for both the above runs has been keptt 150 ◦C and the recharging time as 12 h. As can be seen fromig. 4a and b, the optimum recycling conditions corresponds

o pressure of 110 atm and temperature of 130 ◦C. The recycledaterial regains a capacity of ∼4.53 wt% in the first cycle. These

esults have been found to be reproducible and the repeated recy-

ling reveals that the capacity increases to ∼4.77 wt% in the 12thycle. Beyond 12th cycle the capacity decreased up to 16th cyclend reaches up to ∼4 wt%. Then beyond 16th cycle, it decreaseo 3.8 wt% at 35th cycle and no noticeable further capacity loss

246 D. Pukazhselvan et al. / Journal of Alloys and Compounds 439 (2007) 243–248

Fig. 4. Hydrogen restoring capacity of 2 mol% Mm-doped sample in lowercharging temperatures and pressures. (a) Charging temperature fixed as 130 ◦Cand pressure varied. (b) Charging pressure fixed to 110 atm and temperaturev(

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aried. Curve D (b) shows 81.6% of the total hydrogen capacity gets restored4.53 wt%) at 110 atm and 130 ◦C temperature.

as been observed. The above results on recyclability of NaAlH4ppears to be better than other reported results since a compar-tively better storage capacity of ∼4.77 wt% could be obtainedn our 12th recycling run.

Another feature bringing out the superiority of Mm for fasteresorption kinetics in NaAlH4 relates to the fact that even after5th cycle whereas the storage capacity has decreased from4.77 wt% (after 12th cycle) to ∼3.8 wt%, the kinetics remains

lmost the same in fact slightly better as for the first cycle upo about ∼70 min (capacity ∼3.67 wt%). This is clearly broughtut by Fig. 5(b).

As regards the role of catalyst Mm in enhancing the desorp-ion kinetics, and in improving the reversibility it may be oppor-

une to first recapitulate the case of catalyst Ti which has widelyeen studied by several workers [2–7,11–21,28–31]. In spite ofxtensive works, the location and exact role of Ti has not yet

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ig. 5. (a) Long term recycling of 2 mol% Mm-doped NaAlH4. The highesttorage capacity was observed in 12th cycle. (b) Comparison of the 1st, 2nd and5th cycles shows kinetics is relatively fast up to the last cycle.

ecome clear. Based on the theoretical [11–15] and experimentaltudies [16–21] carried out on Ti-doped NaAlH4, there appearso be four modes through which the catalyst may lead to the fastesorption in NaAlH4. These are (a) Ti is not substituted in theattice but forms amorphous Ti clusters which act as catalysts andeakens the bonding between Al and H in [AlH4]− complex and

hus releases hydrogen at moderate temperatures. (b) Formationf Ti–Al type alloy which provides catalytic activity and desta-ilizes [AlH4]− complex for the fast release of hydrogen. (c)ubstitution of Ti in place of Na or Al. Both substitutions show

ower hydrogen removal energy than that of pristine NaAlH413], Substitution in Na site plays a major role in lowering theydrogen removal energy due to the fact that the ionizationotential (IP) of Ti is higher (6.82 eV) than that of Na (5.13 eV)o it is energetically less favorable for Ti in place of Na toransfer one electron which it needs for stabilizing AlH [13,15].

d) The catalyst does not get substituted in the lattice but formshe crystalline clusters. The catalyst which exists in the form oftomic clusters donates (low IP than individual atom [32]) an

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lectron to Na+ ion thereby making it Na. The ionic componentf bonding between Na+ and AlH4

− gets weakened leading theestabilization of [AlH4]− and release of hydrogen at moderateemperatures.

In the present investigation, since as described earlier crys-alline Mm clusters is invariably found through XRD, since anyignature of formation of Mm-Al type alloy is not found andince separate existence of Na on dissociation is not found, itppears that the fourth, i.e. (d) option in the above is most rel-vant. It can, therefore, be taken that the most likely reasonor enhancement in low temperature desorption and kineticss that Mm gets incorporated in the form of atomic clusters.t should be mentioned that if Mm (Ce, La, Nd, etc.) existsn atomic form, their IP is higher than that of Na and hencehese cannot donate electron to Na+. It may however be pointedut that Mm in the present case does not exist in the form oftoms but atomic clusters. It is known that IP of atomic clus-ers (Mm) is lower than the IP of individual Mm atoms. Thus,or clusters of Ce atoms, i.e. (Cen), the IP may get loweredrom ∼5.5 to ∼4 eV [33]. The Mm atomic clusters unlike indi-idual atoms can, therefore, donate electrons to Na+, turningt into Na. The bond between Na+ and AlH4

− therefore getseakened. This leads to destabilization of AlH4

− and releasef hydrogen at moderate temperatures of ∼150 ◦C (the ion-zation potential (IP) of the main ingredients Mm: Ce and Lare 5.54 and 5.58 eV, whereas that of Ti it is 6.82 eV and fora is 5.14 eV). Therefore, the transfer of electron from Mm

tomic clusters (La, Ce) to Na+ is feasible. This is what has beenctually observed in the present investigation. Another relevantspect of Mm atomic cluster catalyst is that the experimentalvidence suggests that Mm remains as such and does not formny alloy with Al. Therefore, recycling capacity is expected toe better. This is in keeping with the experimental observationss described earlier. Further work focused on discerning theetails of Mm catalyst for enhancement of desorption kineticsnd recyclability of NaAlH4 is under progress and results wille forthcoming.

. Conclusion

In conclusion it can be thus said that Mm is an effective cat-lyst for enhancing the desorption kinetics of NaAlH4. Thus,t temperature of 150 ◦C for the 2 mol% Mm catalyst exist-ng in the form of atomic clusture-NaAlH4 material, in the first0 min it desorbs 0.32 wt%, in the next 30 min 3.35 wt% (i.e..67 wt% in the first 60 min) and in a time upon leaving 180 mint desorbs ∼5 wt%. Mm as atomic cluster thus appears to be

better catalyst than the presently known optimum catalysti. Unlike Ti, Mm atomic cluster become discernible throughRD when admixed in NaAlH4. The Mm catalyst exists in

he form of atomic clustures is capable of donating electrono Na+ which results in destabilization of NaAlH4, leading to

he release of hydrogen at ∼150 ◦C. As regards the regener-tion of desorbed NaAlH4, it is possible to revert back up to.77 wt% in the 12th cycle when charged it in the pressure10 atm and the temperature of 130 ◦C. The most feasible dehy-

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nd Compounds 439 (2007) 243–248 247

rogenation mechanism involving Mm atomic cluster catalysthrough lowering of bond strength between Na+ and AlH4

− haseen suggested.

cknowledgements

The authors are thankful to Professor A.R. Verma, Prof..N.R. Rao, Dr. R. Chidambaram and Professor Punjab Singh

or their encouragements. The authors gratefully acknowledgerof. R.S. Tiwari and Dr. S.S. Srinivasan for their helpful discus-ions and suggestions. We are also thankful to Mr. Vijay Kumaror his technical assistance. Financial assistance from MNES,GC and CSIR are also gratefully acknowledged.

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