Metal-doped sodium aluminium hydrides as potential new hydrogen storage materials

Journal of Alloys and Compounds 302 (2000) 36–58

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Metal-doped sodium aluminium hydrides as potential new hydrogen storage materials q Borislav Bogdanovic´ a , *, Richard A. Brand b , Ankica Marjanovic´ a , Manfred Schwickardi a , ¨ a Joachim Tolle a

¨ Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mulheim ¨ Max-Planck-Institut f ur an der Ruhr, Germany b ¨ GH Duisburg, D-47048 Duisburg, Germany Department of Physics, Gerhard-Mercator-Universitat Received 1 August 1999; received in revised form 11 October 1999; accepted 25 October 1999

Abstract Thermodynamics and kinetics of the reversible dissociation of metal-doped NaAlH 4 as a hydrogen (or heat) storage system have been investigated in some detail. The experimentally determined enthalpies for the first (3.7 wt% of H) and the second dissociation step of Ti-doped NaAlH 4 (3.0 wt% H) of 37 and 47 kJ / mol are in accordance with low and medium temperature reversible metal hydride systems, respectively. Through variation of NaAlH 4 particle sizes, catalysts (dopants) and doping procedures, kinetics as well as the ´ M. cyclization stability within cycle tests have been substantially improved with respect to the previous status [B. Bogdanovic, Schwickardi, J. Alloys Comp. 253–254 (1997) 1]. In particular, using combinations of Ti and Fe compounds as dopants, a cooperative (synergistic) catalytic effect of the metals Ti and Fe in enhancing rates of both de- and rehydrogenation of Ti / Fe-doped NaAlH 4 within 57 ¨ cycle tests, reaching a constant storage capacity of |4 wt% H 2 , has been demonstrated. By means of Fe Mossbauer spectroscopy of the Ti / Fe-doped NaAlH 4 before and throughout a cycle test, it has been ascertained that (1) during the doping procedure, nanosize metallic Fe particles are formed from the doping agent Fe(OEt) 2 and (2) already after the first dehydrogenation, the nanosize Fe particles with NaAlH 4 present are probably transformed into an Fe–Al-alloy which throughout the cycle test remains practically unchanged.  2000 Elsevier Science S.A. All rights reserved. Keywords: Reversible metal hydride hydrogen storage materials; Metal-doped sodium aluminium hydride; Transition metal catalyzed hydride dissociation; ¨ Metal hydrides, thermodynamic properties; Mossbauer spectroscopy

1. Introduction The development of low- and / or medium-temperature reversible hydrides having higher than hitherto known gravimetric hydrogen storage capacities and lower prices per unit stored hydrogen is one of prerequisites for their technical applications [1–3]. Complex hydrides of light metals Li, Na and Al, such as LiAlH 4 (10.5 wt% H) and NaAlH 4 (7.4 wt% H) have been considered for this purpose [4]. Although the two-step thermal dissociation of solid NaAlH 4 to NaH, Al and H 2 , as well as the corresponding reverse reactions (Eq. (1a,b)) have been known for some time [5–8] (cf. Sections 2 and 4.1.2), there was in the past no known attempt to apply them as a reversible hydrogen storage system. q According to the invited lecture presented at the Symposium on ‘New Protium Function’, Tokyo, December 17–18, 1998, Japan. *Corresponding author. Fax: 149-208-306-2980. ´ E-mail address: [email protected] (B. Bogdanovic)

A considerable progress in this direction was therefore the recognition that the complex hydrides of Na, (Li) and Al, which were usually regarded as non-reversible, can be made reversible by including selected transition or rare earth metals as catalysts [9,10].1

1

The work on doped alkali metal aluminium hydrides as hydrogen storage materials (Refs. [9,10] and the present article) has its origin in the observation that solutions of Mg 2 Cl 3 AlH 4 ?6THF in THF, prepared according to the Eq. (a) [11], decompose with evolution of hydrogen and deposition of an Al-mirror much more readily, when for the THF

2 MgH 2* 1 AlCl 3 → Mg 2 Cl 3 AlH 4 ? 6THF

(a) MgH *2 is catalytically prepared magnesium hydride [12]. preparation of MgH *2 the titanium instead of the chromium homogeneous catalyst [12] is used. As a consequence, a systematic study of the catalytic effect of transition metals, especially of titanium, on the rate and extent of hydrogen evolution from complex alanates in solution and in the solid state was undertaken. Decomposition of LiAlH 4 in solution with evolution of hydrogen caused by Ti-catalysts was at that time known from the literature [13,14] and from our own work [15,16].

0925-8388 / 00 / $ – see front matter  2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00663-5

208C

B. Bogdanovic´ et al. / Journal of Alloys and Compounds 302 (2000) 36 – 58

The catalysts for reaction Eq. (1), as well as for the corresponding reversible dissociation of Na 3 AlH 6 (Eq. (2)) and Na 2 LiAlH 6 (Eq. (3)), are generated by reaction of alkali metal alanates in an organic solvent, or in absence of solvents, with small amounts (e.g., 1–2 mol%) of some specific compounds of the mentioned metals (the doping reaction) [9,10]. (a)

NaAlH 4á1 / 3Na 3 AlH 6 1 2 / 3Al 1 ( b)

H 2 (g) áNaH 1 Al 1 3 / 2H 2 (g) ( 3.7 wt%H )

(1)

37

THF (DME)

4 NaH 1 AlBr 3 (AlCl 3 )

THF 5 tetrahydrofuran

→

NaAlH 4 1 3NaBr (NaCl)

DME 5 dimethylether (4)

3NaAlH 4 1 AlCl 3 → 3NaCl 1 4AlH 3

(5a)

4AlH 3 1 4NaH → 4NaAlH 4

(5b)

3NaAlH 4 1 AlCl 3 1 4NaH → 4NaAlH 4 1 3NaCl

(5)

(5.5 wt%H )

Na 3 AlH 6 á3NaH 1 Al 13 / 2H 2 (g)

(2)

( 3.0 wt%H)

Na 2 LiAlH 6 á2NaH 1 LiH 1 Al 13 / 2H 2 (g)

(3)

( 3.5 wt%H)

The described systems [9], still have disadvantages of unsatisfactory dehydrogenation / rehydrogenation rates, and therefore the necessity to operate them at relatively high temperatures (| $1508C) and pressures (150 bar), as well as insufficient cyclic stability. The intention of the present study was therefore to investigate the influence of variation of catalysts and of the extent of doping, as well as of other factors which could affect rates, extent and repeatability of the reactions Eq. (1a,b) (Section 4.4). From the thermodynamic standpoint it was desirable to determine experimentally the hydrogen dissociation pressures of both dissociation steps at temperatures as low as possible (Section 4.3). Via investigation of the doping reaction and of its reaction products and of the latter under cycling conditions, it was expected to obtain information on the nature of the catalysts for the reactions equation (1a,b) (Section 4.4.6).

2. Review of methods for the synthesis of NaAlH 4 , Na 3 AlH 6 and Na 2 LiAlH 6 The solid complex hydrides NaAlH 4 , Na 3 AlH 6 and Na 2 LiAlH 6 serve as basic materials for the new hydrogen storage systems considered here. NaAlH 4 was formerly industrially produced via the direct synthesis from the elements (see below) [17] and is presently supplied as a laboratory reagent [18].

2.1. Synthesis of NaAlH4 starting from sodium hydride and aluminium halides

Zakharkin and Gavrilenko [21] prepared NaAlH 4 from NaH and AlCl 3 in benzene, using Al(C 2 H 5 ) 3 as a catalyst (Eqs. (6a,b), (5b) and (6)). The benzene soluble catalyst was separated from NaAlH 4 and NaCl by filtration. NaAlH 4 was separated from NaCl by extraction with THF. Al(C 2 H 5 ) 3 1 NaH → Na[Al(C 2 H 5 ) 3 H] 3Na[Al(C 2 H 5 ) 3 H] 1 AlCl 3 → AlH 3 1 3Al(C 2 H 5 ) 3 1 3NaCl

(6b)

AlH 3 1 NaH → NaAlH 4

(5b)

4NaH 1 AlCl 3 → NaAlH 4 1 3NaCl

(6)

2.2. Direct syntheses of NaAlH4 In the early 1960s Ashby [22,23] and Clasen [24] described, following Ziegler’s [25] trialkylaluminium synthesis, the ‘direct synthesis’ of NaAlH 4 , starting from activated aluminium, Na or NaH and hydrogen under pressure (Eq. (7)). As a reducing agent, NaAlH 4 can replace LiAlH 4 effectively in most organic reductions [26]. NaH 1 Al 1 3 / 2H 2

THF / 1408C / 150 bar

→

NaAlH 4

(7)

According to Eq. (7), in the presence of Al(C 2 H 5 ) 3 , NaAlH 4 can also be prepared in hydrocarbons [27]. In this preparation Al with 0.1% Ti was used, but no catalytic effect of Ti was noted. A systematic investigation on the preparation of NaAlH 4 after Eq. (7), whereby reaction conditions, catalysts and solvents were varied, led to an optimized procedure [28]. In a patent disclosure from 1969 a synthesis of NaAlH 4 is described, whereby 1,4-diazabicyclo[2.2.2]-octane (dabco) and triphenylmethane were used as catalysts Eq. (8) [29]. toluene / dabco / Ph 3 CH / 175 bar, 1508C

The first two procedures for the preparation of NaAlH 4 (Eqs. (4) and (5)), following the synthesis of LiAlH 4 [19], were described in 1955 by Finholt et al. [20]. In both synthetic variants (Eqs. (4) and (5)), sodium halides are obtained as side products.

(6a)

NaH 1 Al 1 2H 2

→

NaAlH 4

(8)

In 1974 Dymova et al. [8] succeeded in realizing the synthesis of NaAlH 4 from the elements in absence of solvents by carrying out the process in the melt (Eq. (9)).

B. Bogdanovic´ et al. / Journal of Alloys and Compounds 302 (2000) 36 – 58

38

NaH(l) 1 Al 1 2H 2

p.175 bar, T ,2808C

→

3. Experimental details NaAlH 4 (l)

(9)

3.1. Starting materials More recently the same authors [30] described the preparation of NaAlH 4 by grinding solid NaH and AlH 3 in a ball-mill.

2.3. Syntheses of NaAlH4 with the help of alkali metal fluorides A synthesis of NaAlH 4 from NaF, Al and hydrogen in toluene (Eq. (10)) was disclosed in a patent specification [31]. toluene / ( C 2 H 5 ) 3 Al

6NaF 1 4Al 1 6H 2

→

150 bar, 1508C

3NaAlH 4 1 Na 3 AlF 6 (10)

A laboratory method for the preparation of NaAlH 4 with the help of NaF exists [32]. NaF is reacted with (C 2 H 5 ) 3 Al to give the complex Na[Al(C 2 H 5 ) 3 F] [33,34] (Eq. (11)). Thereupon LiAlH 4 is allowed to react with the Na[Al(C 2 H 5 ) 3 F]; NaAlH 4 separates thereby from the solution, while Li[Al(C 2 H 5 ) 3 F] remains dissolved (Eq. (12)). toluene / 1008C /

→

NaF 1 (C 2 H 5 ) 3 Al

Na[Al(C 2 H 5 O 3 F] 1 LiAlH 4

Na[Al(C 2 H 5 ) 3 F]

toluene / RT /

→

(11)

3.2. Purification, crystallisation and characterization of NaAlH4

NaAlH 4 ↓ 1

Li[Al(C 2 H 5 ) 3 F]

(12)

2.4. Syntheses of Na3 AlH6 and Na2 LiAlH6 Na 3 AlH 6 can be prepared from NaAlH 4 and NaH in heptane (Eq. (13)) [27]. A direct synthesis of Na 3 AlH 6 from the elements is also known [5] (Eq. (14)). C 7 H 16 / 140 bar, 1008C

→

NaAlH 4 1 2NaH 3Na 1 Al 1 3H 2

Na 3 AlH 6

toluene /AlEt 3 / 350 bar, 1658C

→

(13)

Na 3 AlH 6

(14)

Na 2 LiAlH 6 can be prepared according to the same procedure as given for Na 3 AlH 6 (Eq. (15)) [9], or from LiAlH 4 and NaH in toluene under H 2 pressure (Eq. (16)) [35]. toluene /AlEt 3 / 350 bar, 1658C

NaAlH 4 1 NaH 1 LiH

→

Na 2 LiAlH 6 (15)

toluene

LiAlH 4 1 2NaH → Na 2 LiAlH 6

NaAlH 4 was supplied by the Chemetall (Frankfurt, Germany) or by Ethyl Corporation (Richmond, VA, USA). The hydrogen was 99.9%, Messer-Grießheim. The Titetra-n-butylate (Ti(OBu) 4 ), Aldrich, 99% was distilled in high vacuum before use. Anhydrous FeCl 2 :ALFA. Feethylate (Fe(OEt) 2 ), prepared according to the electrochemical method in high purity [37] was available. All reactions and operations with air-sensitive materials (metal hydrides, dopants, doped metal hydrides) were performed under argon using Schlenk technique and airand water-free solvents. THF was boiled for several hours over magnesium anthracene?3THF and subsequently distilled under argon. Toluene, ether and pentane were distilled over NaAl(C 2 H 5 ) 4 . Vacuum, 0.1–0.2 mbar; high vacuum, 10 23 mbar. Elemental analyses, Dornis and Kobe ¨ (Mulheim an der Ruhr, Germany); XRD, Stoe diffractometer, STUDI, 2 P with PSD, Cu Ka radiation; Scanning electron microscopy (SEM), ISI-60 apparatus; 57 Fe ¨ Mossbauer spectroscopy, Wissel drive, 57 Co in Rh source; computer program NORMOS, Wissel; IR spectra, Nicolet 7199 FT-IR. The specific surface areas were measured by the BET method with nitrogen.

(16)

Both Na 3 AlH 6 and Na 2 LiAlH 6 were obtained recently [36] by grinding mixtures of NaAlH 4 and NaH or LiH in the solid state.

3.2.1. NaAlH4 precipitated from THF solution by pentane (designed as NaAlH4 ( p)) Fifty grams of commercial NaAlH 4 (Chemetall) in 500 ml of THF (solubility of NaAlH 4 in THF: 162 g / l at 208C (m.p. 1788C) [17]) were stirred during 3 h and filtered through a glass frit. The filtrate was concentrated in vacuo to the volume of |100 ml, whereby NaAlH 4 starts to separate from the solution. Under rigorous stirring 400 ml of pentane are now added to the THF solution causing NaAlH 4 to separate from the solution as a fine precipitate. The suspension is stirred for 1 h, filtered and the NaAlH 4 washed 2 times with each 80 ml of pentane. After drying in vacuo, 39.5 g (79%) of NaAlH 4 were obtained as a fine colorless powder. Elemental analysis of NaAlH 4 (p): Na 42.66 (calc. 42.75), Al 49.79 (49.96), H 7.51 (7.47)%. IR spectrum (KBr) is in accordance with Ref. [9], and shows no THF absorptions. XRD of NaAlH 4 (p), Fig. 1, is accordance with the diffraction data file. SEM in Fig. 3b. A thermovolumetric analysis [38] of a sample (1.23 g) of NaAlH 4 (p) (20–2708C, 48C / min; Fig. 4) delivered upon thermal dissociation 815 ml of H 2 (208C, 1 bar), corresponding to 5.52 wt. H 2 (calc. 5.55 wt% H 2 ). NaAlH4 precipitated by pentane in fine form (designed as NaAlH 4 ( pf )) (Fig. 3c) was prepared in the same way, except that the THF solution of NaAlH 4 was poured in the stirred pentane. Preparation of NaAlH4 by precipitation from THF

B. Bogdanovic´ et al. / Journal of Alloys and Compounds 302 (2000) 36 – 58

39

Fig. 1. X-ray powder diagram of NaAlH 4 (p).

solution by ether (designed as NaAlH 4 (e)) (Fig. 3a) has been described earlier [9].

3.3. Doping procedures 3.3.1. Doping of NaAlH4 for the screening tests [10] Each |1.3-g portion of NaAlH 4 (e) was suspended in 20 ml of ether, and to each of the stirred suspensions was added 5 mol% (based on NaAlH 4 ) of the respective metal compound (Table 2, Section 4.4.1). After 20–60 min (completion of H 2 -evolution), the solvent was evaporated and the residues dried in vacuo. Further procedure is described in Section 4.4.1 and the results are given Table 2. 3.3.2. Doping of NaAlH4 with Ti- and Fe-alcoholates and FeCl2 and their mixtures; recording of H2 evolution at different temperatures 3.3.2.1. Doping of NaAlH4 ( p) with 2 mol% of Ti( OBu)4 in toluene at 358 C ( Expt. 1) A total of 2.44 g (45.1 mmol) of NaAlH 4 (p) was suspended in 40 ml of toluene, and to the stirred suspension at room temperature were added 0.30 ml (0.88 mmol) of Ti(OBu) 4 by means of a pipette. The flask was connected to a 200-ml automatic gas burette [38] and the suspension then stirred at 358C (oil bath) until the H 2 evolution ceased (|35 h; Fig. 5a, —). Gas evolution: 41 ml H 2 (1.94 mol H 2 / mol Ti). The solvent was evaporated and the residue dried under vacuum. Weight of the Tidoped NaAlH 4 , 2.73 g. The doping of NaAlH 4 (p) with 2

mol% of Ti(OBu) 4 in toluene at different temperatures (Fig. 5a) was carried out as described for Expt. 1. A parallel experiment (Expt. 2) was carried out using the same amount of starting materials, except that, after cessation of H 2 evolution, the brown suspension was filtered, the precipitate washed with toluene and pentane and dried in vacuo, leaving 2.54 g of a black solid. The filtrate was evaporated to dryness in vacuo, giving a colorless solid (|0.2 g) whose IR spectrum [39] in the range of 3200–850 cm 21 was superimposable with the IR spectrum of an authentic sample of Ti(OBu) 4 ; since in the course of the doping reaction (Eq. (22)) Ti is completely precipitated, and on the basis of the IR spectrum, the presence of NaAlH 4 [9] could be excluded, the solid is evidently Na /Al-butylate.

3.3.2.2. Doping of NaAlH4 ( p) with 2 mol% of Fe( OC2 H5 )2 or FeCl2 in toluene at 358 C ( Expt. 3) The doping of NaAlH 4 (p) with 2 mol% of Fe(OC 2 H 5 ) 2 or FeCl 2 in toluene was carried out as described for Expt. 1. The course of H 2 -evolution with time is represented in Fig. 5b.

3.3.2.3. Doping of NaAlH4 ( p) with each 1 mol% of Ti( OC4 H9 )4 and Fe( OC2 H5 )2 in toluene at 358 C ( Expt. 4) The experiment, starting from 2.47 g (45.9 mmol) of NaAlH 4 (p), 0.16 ml (0.47 mmol) of Ti(OBu) 4 and 70 mg (0.47 mmol) of Fe(OEt) 2 in 40 ml of toluene, was carried out as described for Expt. 1, except that after the completed doping reaction (Fig. 5c, ) the reaction mixture

40

B. Bogdanovic´ et al. / Journal of Alloys and Compounds 302 (2000) 36 – 58

was filtered (as in Expt. 2). Weight of the (Ti1Fe)-doped NaAlH 4 (p), 2.49 g (92%).

3.3.3. Doping of NaAlH4 with 2 mol% of Ti( OBu)4 in ether and with 0.5, 1.0, 2.0 and 4.0 mol% of Ti( OBu)4 in toluene at room temperature Each 2–4-g sample of NaAlH 4 (p) was suspended in 40–50 ml of the solvent, and to the stirred suspensions were added the corresponding amounts (0.1–0.3 ml) of Ti(OBu) 4 by means of a pipette. The reaction flask (100 ml volume) was then connected to a gas burette and the suspensions stirred at 208C until the H 2 evolutions stopped, which took 4–5 h for the doping in ether and 48–72 h for the doping in toluene. H 2 evolutions for the doping in ether and in toluene were 1.0 and 1.9–2.4 mol H 2 / mol Ti, respectively. The solvents were evaporated and the brown solids dried for 1–2 h in vacuum. The dehydrogenation of the obtained Ti-doped samples of NaAlH 4 are represented in Figs. 10 and 11a,b, and Sections 4.4.2–4.4.3. 3.3.4. Doping of NaAlH4 for the cycle tests 3.3.4.1. Doping of NaAlH4 with 2 and 4 mol% of Ti( OBu)4 and with each 1 and 2 mol% of Ti( OBu)4 and Fe( OEt)2 ( Fig. 13 a) The doping of NaAlH 4 (p) (2.5–4.5 g) with 2 and 4 mol% of Ti(OBu) 4 in 40–50 ml of toluene was carried out at 45–558C as described in Section 3.3.2, Expt. 1 and with each 1 and 2 mol% of Ti(OBu) 4 and Fe(OEt) 2 at 358C as described in the same section, Expt. 2. 3.3.4.2. Doping of NaAlH4 with each 1 or 2 mol% of Ti( OBu)4 and FeCl2 and with each 1 mol% of Ti( OBu)4 and NiCl2 in toluene at 508 C ( Fig. 13 b) Each 1.4–1.7-g sample of NaAlH 4 (p) was suspended in 30 ml of toluene and the corresponding amounts of Ti(OBu) 4 (by a pipette) and solid FeCl 2 or NiCl 2 added. The reaction vessels were connected to a gas burette [38] and the reaction mixtures stirred at 508C until the H 2 evolutions stopped (|5 h). The H 2 evolutions corresponded to 3.1–3.5 mol H 2 / mol (Ti1Fe or Ni) (theor. 3.0 mol H 2 / mol (Ti1Fe or Ni)). Toluene was evaporated and the residues dried for 2 h in vacuum.

3.4.2. Direct dissociation pressure measurements For direct dissociation pressure measurements, 11.20 g of NaAlH 4 (e) were doped with 1.3 mol% of b-TiCl 3 in 35 ml of diethyl ether as described in Ref. [9]. A total of 11.33 g of the thus Ti-doped NaAlH 4 was transferred in an autoclave (32 ml volume) of the type depicted in Fig. 1 of Ref. [9], and the autoclave placed in the electrically heated oven of the equipment. The autoclave was tightly closed and stepwise heated up to definite temperatures, and at each step the temperature was kept constant until the complete constancy of pressure was attained. At temperatures below 1008C it took several hours until equilibrium was established; above 1008C, constant pressure was reached much more rapidly. The same procedure was applied also for the measurement of the dissociation pressure of a sample of Na 3 AlH 6 (5.60 g) doped with 2 mol% of b-TiCl 3 in 15 ml of diethyl-ether [9]. The temperature / pressure values resulting from the measurements are listed in Table 1; the corresponding T /p curves are represented graphically in Fig. 9b (—) and set against those from the PCI measurements (Fig. 9a). 3.5. Calculation of reaction enthalpies (DH) for the reversible dissociation of Ti-doped NaAlH4 (s) and ( l) ( Eqs. (1 a,b)) from the PCI data 3.5.1. Calculation of DH for the Ti-doped NaAlH4 (s) ( Eq. (1 a, s)) from PCI desorption and absorption data Desorption and absorption pressure values ( p [bar]) corresponding to temperatures from 150 to 1838C (taken at H /Al51.4–2.0 from Fig. 6a, (– j –) and (– s –); ln p and 1000 / K values in parentheses): 60.4, 65.1 (4.101, 4.176) / 150 (2.364); 79.9, 83.7 / 160; 102.7, 105.7 / 170; 133.1, 134.0 (4.891, 4.898) / 183 (2.193). A plot of ln p against 1000 / K (Fig. 7, 150–1838C, – j – and – s –) results in approximately straight lines. Calculation of DH 5 R ln( p2 /p1 ) / 1 /T 2 2 1 /T 1 [51] from ln p and 1 /T values at 150 and 1838C: Table 1 Temperature / pressure value pairs resulting from direct dissociation pressure measurements on Ti-doped NaAlH 4 and Na 3 AlH 6 NaAlH 4 (1.3 mol% Ti)

3.4. Determination of pressure-combination isotherms 3.4.1. Determination of pressure-composition isotherms ( PCIs) of Ti-doped NaAlH4 The equipment and the procedure for determination of PCIs of Ti-doped NaAlH 4 have been previously described [9]. For determination of the PCI curves (Fig. 6a,b), each 9–14-g sample of NaAlH 4 (p) was doped with 2 mol% of Ti(OBu) 4 in 60 ml of toluene at 458C (until evolution of 2.0 mol of H 2 / mol Ti took place) as described in Section 3.3.2, Expt. 1. Numerical data resulting from PCI determinations can be found in Ref. [39].

Na 3 AlH 6 (2 mol% Ti)

T (8C)

p (bar)

T (8C)

p (bar)

T (8C)

p (bar)

17 40 60 81 101 121 140 160 168

0 0.4 2.0 7.0 15.8 31.2 53.0 85.8 106.5

174 179 184 189 194 199 203 211

132.4 136.9 139.8 142.6 145.3 147.9 150.5 154.1

18 40 60 80 100 120 140 159 180 194 211 225

0 0.2 0.4 0.6 1.4 8.2 9.2 10.6 13.6 20.4 32.4 46.6

B. Bogdanovic´ et al. / Journal of Alloys and Compounds 302 (2000) 36 – 58

41

DHDES 5 38.4 kJ / mol DHABS 5 35.1 kJ / mol

3.6. First dehydrogenations

3.5.2. Calculation of DH for the Ti-doped NaAlH4 ( l) ( Eq. (1 a, l)) from PCI measurements Desorption and absorption pressure values ( p [bar]) corresponding to temperatures of 1998C (taken at H /Al5 1.3–1.6 from Fig. 6a, – j – and – s –) and 2118C (taken from Refs. [9,10]; ln p and 1000 / K values in parentheses): 144.6, 146.2 / 199; 152.2, 153.7 (5.025, 5.035) / 211 (2.066). A plot of ln p against 1000 / K (Fig. 7, 183– 2118C, – j – and – s –) results in straight lines. Calculation of DH from ln p and 1 /T values at 183 and 2118C:

General procedure: 0.6–0.7-g samples of the respective doped materials (Sections 3.3.2–3.3.4) were weighed in the glass vessel of 25 ml volume depicted in Fig. 2 and the vessel connected to an automatic gas burette of 1-l volume (Fig. 2). A Ni–Cr–Ni thermocouple, whose point was immersed in the layer of the sample, allowed recording of the sample temperature during the thermovolumetric measurements [38]. The glass vessel was inserted in the oven which had been previously heated up and was kept at the desired temperature during measurements. The progression of H 2 evolution from the samples, together with the sample temperature, were recorded on a two-channel recorder.

DHDES 5 8.77 kJ / mol DHABS 5 8.96 kJ / mol

3.5.3. Calculation of DH for the Ti-doped Na3 AlH6 ( Eq. 1 b) from PCI measurements Desorption and absorption pressure values ( p [bar]) corresponding to temperatures from 199 and 2448C (taken at H /Al50.5 from Fig. 6a, – j – and – s –; ln p and 1000 / K values in parentheses): 22.2, 23.1 (3.100, 3.140) / 199 (2.119); 63.7, 64.2 (4.154, 4.162) / 244 (1.934). Calculation of DH from ln p and 1 /T values at 199 and 2448C: DHDES 5 47.37 kJ / mol DHABS 5 45.93 kJ / mol

3.6.1. Influence of the particle size of NaAlH4 and of the doping medium on the first dehydrogenation of Tidoped NaAlH4 The curves of dehydrogenation at 1208C of NaAlH 4 (p) doped with 2 mol% of Ti(OBu) 4 in ether and in toluene (Section 3.3.3), together with that from dehydrogenation of NaAlH 4 (e) doped in ether [9], are reproduced in Fig. 10 (Section 4.4.2). Amounts of hydrogen desorbed from

Fig. 2. Equipment used for thermovolumetric measurements [38].

42

B. Bogdanovic´ et al. / Journal of Alloys and Compounds 302 (2000) 36 – 58

samples of NaAlH 4 (p) (Fig. 10) doped in toluene and ether were 3.21 and 3.03 wt%, respectively.

3.6.2. Progression of first dehydrogenation of NaAlH4 ( p) doped with Ti( OBu)4 , with Fe( OEt)2 or with their combination in toluene The curves of the first dehydrogenation of NaAlH 4 (p) doped with 2 (at 120, 160 and 1808C), 1 (at 120 and 1608C) and 0.5 mol% of Ti(OBu) 4 (at 1208C) are reproduced in Fig. 11a (Section 4.4.3). In Fig. 11b the curves of the first dehydrogenations at 1608C of NaAlH 4 (p) doped with 2 mol% of Ti(OBu) 4 and with 2 mol% of Fe(OEt) 2 (Section 3.3.2) are set against that of NaAlH 4 (p) doped with both 1 mol% of Ti(OBu) 4 and Fe(OEt) 2 ; the desorption curve of NaAlH 4 (p) doped with 2 mol% of Fe(OEt) 2 at 1808C is also represented in the diagram. 3.7. Cycle tests 3.7.1. Cycle tests using NaAlH4 doped with 2 and 4 mol% of Ti( OBu)4 and with each 1 and 2 mol% Ti( OBu)4 and Fe( OEt)2 The equipment used for carrying out cyclic tests in this series (Fig. 13a, Section 4.4.4) has been previously described (Fig. 1 in Ref. [9]). The cyclic tests using 2.2–2.8 g of the samples doped according to the Section 3.3.4.1 were performed in an open system, i.e., during dehydrogenations, hydrogen was desorbed against normal pressure and the progress of hydrogen evolution was measured by means of the automatic gas burette; during hydrogenations, fresh hydrogen was taken from the hydrogen pressure cylinder. Dehydrogenations were implemented by heating up the oven of the equipment during |20 min to the desired dehydrogenation temperature (see caption of the Fig. 13a), after which the temperature was kept constant until the end of H 2 evolution. Hydrogenations were performed by cooling down the autoclave to |508C and then pressurizing it with hydrogen. The autoclave was subsequently heated up (208C / min) to the desired hydrogenation temperature and kept at that temperature until the hydrogenation was completed. After lowering the autoclave temperature to ,608C, it was carefully depressurized to normal pressure and than connected to the automatic gas burette, in order to start the next dehydrogenation. During de- and rehydrogenations, the progress of the H 2 evolution (automatic gas burette [9]) and the pressure drop in the autoclave respectively, together with the inner temperature of the sample (thermocouple), were recorded by means of a two-channel recorder. 3.7.2. Investigation of a ( Ti 1 Fe)-doped NaAlH4 in the ¨ course of a cycle test via 57 Fe Mossbauer spectroscopy A sample (2.42 g) of NaAlH 4 (p) was doped with each 2 mol% of Ti(OBu) 4 and Fe(OEt) 2 in toluene at 358C as described for Expt. 4 of Section 3.3.2; after completion of

H 2 evolution (3.1 mol H 2 / mol (Ti1Fe), Eq. (22a)) the toluene was evaporated in vacuum, giving 2.64 g of solid (Ti1Fe)-doped NaAlH 4 . For recording of 57 Fe MB spectra before and during the cycle test, samples of (Ti1Fe)doped NaAlH 4 (150–200 mg, Fe content 3–4 mg) were filled in an acrylic glass dish (1 cm diameter) in a glove box, the dish closed with a fitting acrylic stopper and the stopper sealed with an adhesive. MB spectrum of the (Ti1Fe)-doped NaAlH 4 at room temperature and at 4 K: (Fig. 15a,b). For MB spectrum after a sample (1.4 g) of (Ti1Fe)-doped NaAlH 4 has been dehydrogenated at 1808C (Section 3.6), see Fig. 16a. A sample (2.48 g) of (Ti1Fe)-doped NaAlH 4 was subsequently subjected to 11 de- and rehydrogenation cycles (dehydrogenation, 1608C; rehydrogenation, 140–95 bar, 1048C) and finally dehydrogenated at 1908C: MB spectrum at 4.2 K (Fig. 16b) and at room temperature (Fig. 17a).

3.7.3. Cycle tests using NaAlH4 doped with each 1 and 2 mol% of Ti( OBu)4 and FeCl2 and with each 1 mol% of Ti( OBu)4 and NiCl2 Samples of 1.3–1.4 g of NaAlH 4 (p) doped with Ti, Ti and Fe, or Ti and Ni according to Section 3.3.4.2 were transferred into, and weighed in a cylindrical glass vessel which served for hydrogenations and which was provided with a Ni / Cr–Ni thermocouple; during dehydrogenation measurements the point of the thermocouple was immersed in the layer of the sample, as shown in Fig. 1. The glass vessel has on the top a second opening for inlet of argon. The glass vessel (diameter 2.8 cm, volume 50 ml) was constructed so as to tightly fit in a high-grade steel autoclave used for hydrogenations; the autoclave was provided with an electrical heating and a temperature regulating system. The first and the following dehydrogenations in this series of cycle tests were carried out at 1808C in the same way as described above for first dehydrogenations (Section 3.6). After each dehydrogenation the thermocouple was removed from the glass vessel, the vessel closed in the autoclave and |100 bar of H 2 pressed onto the autoclave. The autoclave was then heated up to 1008C (resulting in an increase of H 2 pressure to about 120 bar) and kept at that temperature for 17 h (unless otherwise stated). Pressure drop due to hydrogenations was 3–4 bar or less. The results of the cycle tests are graphically represented in Fig. 13b and discussed in Section 4.4.4.

4. Results and discussion

4.1. Purification, crystallisation and characterisation of solid NaAlH4 As a starting material for all subsequent investigations (Sections 4.1.1–4.4.6) purified crystalline NaAlH 4 was employed.

B. Bogdanovic´ et al. / Journal of Alloys and Compounds 302 (2000) 36 – 58

4.1.1. Preparation of crystalline NaAlH4 of different particle sizes and shapes For the purpose of purification, commercial NaAlH 4 was dissolved in THF and NaAlH 4 precipitated from the solution by addition of ether [24] or pentane (see Section 3). The different kinds of isolation of NaAlH 4 from the THF solution allow NaAlH 4 of different particle sizes and shapes to be prepared, as demonstrated by means of scanning electron microscopy (SEM) investigations:

43

• through ether precipitation (Fig. 3a) NaAlH 4 (e) is obtained in the form of relatively large crystals of 50625 mm size and 0.8 m 2 / g; this implies a low rate of nucleation and uniform crystal growth [40]; • precipitation by pentane (Fig. 3b) results in NaAlH 4 (p) as a conglomerate of particles of average 10–20 mm size (specific surface |2.5 m 2 / g) with a few ones in the range of 50 mm (NaAlH 4 ( p)); and • still finer NaAlH 4 ( pf ) particles (5–10 mm particle

Fig. 3. Scanning electron microscopy (SEM) images of NaAlH 4 crystals obtained by precipitation of NaAlH 4 from tetrahydrofuran (THF) solutions by addition of ether (a) or pentane (b), or by pouring THF solutions of NaAlH 4 into pentane (c).

44

B. Bogdanovic´ et al. / Journal of Alloys and Compounds 302 (2000) 36 – 58

size, Fig. 3c) can be obtained by pouring solutions NaAlH 4 in THF into pentane. As it turned out, charges of NaAlH 4 (p) or ( pf ) deliver, after doping with Ti, a storage material with considerable higher de- and rehydrogenation rates at 1208C in comparison to the Ti-doped material prepared from NaAlH 4 (e) [9] (see Section 4.4.2 and Fig. 10). Hence, in experiments described in the following, unless otherwise stated, NaAlH 4 of(p) or ( pf ) quality was applied.

4.1.2. A thermovolumetric analysis of undoped NaAlH4 In the literature the two-step thermal dissociation of solid undoped NaAlH 4 (Eq. (1a,b), left to right) has already been described several times, whereby hydrogen evolution measurements, differential thermal analysis (DTA; [41–44]), thermogravimetry [6,45] and the differential scanning calorimetry (DSC) [45] have been applied. For characterization of metal hydride samples as hydrogen storage materials, we routinely apply the thermovolumetric method [9,38]. The course of the thermal dissociation of a sample of solid undoped NaAlH 4 ( pf ) is represented in Fig. 4. The first and the second dissociation step of NaAlH 4 (3.7 and 5.5 wt% H) are clearly discernible by a distinct bend in the hydrogen evolution curve ( ). At |1858C a strong endothermic effect in the temperature curve (—) is registered, which arises from fusion of NaAlH 4 (Eq. (17)) [45]. The hydrogen evolution of the first dissociation step begins only at the sample temperature of |2308C (Eq. (18)) [45]; thereby liquid NaAlH 4 is transformed into solid a-Na 3 AlH 6 and finely dispersed Al. The hydrogen evolution, which is at maximum rate at |2408C, is associated with only a weak

endothermic effect; according to Ref. [45], the effect is caused by the (weakly endothermic) dissolution of aNa 3 AlH 6 in liquid NaAlH 4 and not by the dissociation enthalpy of NaAlH 4 , which is almost thermoneutral. At 2528C a-Na 3 AlH 6 with a monoclinic structure undergoes a weakly endothermic phase transition with formation of b-Na 3 AlH 6 with a cubic structure (Eq. (19)) [45,46]. Hydrogen evolution of the second step (Eq. (20)) takes place above |2658C and is, according to Ref. [45], to the extent of 41.5 kJ / mol endotherm (cf. Section 4.3.2). NaAlH 4 (s)áNaAlH 4 (l) DH 5 23.2 kJ / mol [45]

(17)

NaAlH 4 (l)á1 / 3a-Na 3 AlH 6 (s) 1 2 / 3Al 1 H 2 (g) DH, very low [45]

(18)

a-Na 3 AlH 6 (s)áb-Na 3 AlH 6 (s) DH 5 3.6 kJ / mol [45] (19) b-Na 3 AlH 6 (s)á3NaH(s) 1 Al 1 3 / 2H 2 (g) DH 5 41.5 kJ / mol [45]

(20)

The thermal dissociation of NaH into Na(l) (m.p. 97.88C) and H 2 (g) takes place above 4258C (Eq. (21)) [47] and is at around 6608C followed by melting of Al [41,42]. NaH(s)áNa(l) 1 1 / 2H 2 (g) DH 5 57 kJ / mol [47]

(21)

4.2. Doping of NaAlH4 with metal compounds or their combinations 4.2.1. Screening of doping agents [10] The doping of NaAlH 4 (e) with different metal compounds is described in Section 3.3.1 and the results of the screening tests are discussed in Section 4.4.1 (Table 2). 4.2.2. Investigation of the wet-chemical doping reactions of NaAlH4 with Ti- and Fe-alcoholates and FeCl2 and their mixtures by monitoring of the H2 -evolution [39] As it turned out (Section 4.4.2), the doping of NaAlH 4 with titanium compounds in toluene results in storage materials with considerable higher H 2 dis- and recharging rates [39] than that from doping in ether [9]. In the experiments described in the following, if not otherwise stated, doping procedures were therefore carried out in toluene, in which NaAlH 4 is virtually insoluble. The progression of H 2 evolution in the course of doping of NaAlH 4 (p) in toluene with Ti-alcoholates at different temperatures is represented in Fig. 5a. At 35 or 458C, the H 2 evolution is virtually completed after delivery of 2 mol H 2 / mol Ti, which as already reported [9],2 points to the

Fig. 4. Thermovolumetric analysis of a sample of undoped NaAlH 4 (p); temperature program, 20→270, 48C / min, thereafter 2708C; —, temperature within the sample; , hydrogen evolution versus heating time.

2

On the other hand it is known that the reduction of TiCl 3 by MgH 2 or LiAlH 4 proceeds with formation of HTiCl (Ti(12) oxidation state) [16,48,49].

B. Bogdanovic´ et al. / Journal of Alloys and Compounds 302 (2000) 36 – 58

45

Fig. 5. Progression of hydrogen evolution with time for the doping of NaAlH 4 (p) in toluene at 358C (unless otherwise stated) with each 2 mol% of the dopant: (a) —, Ti(OBu) 4 ; / / / / / / / /, Ti(OEt) 4 ; (b) — — —, Fe(OEt) 2 ; — - - —, FeCl 2 ; (c) , Ti(OEt) 4 1Fe(OEt) 2 ; - - - -, addition of the curves ‘—’ and ‘— — —’, (d) , Ti(OBu) 4 1FeCl 2 ; - - -, addition of the curves ‘—’ and — - - —.

reduction of Ti(14) in Ti(OBu) 4 to ‘Ti(0)’ (Eq. (22)). At 558C (Fig. 5a) the H 2 evolution proceeds beyond the stage of 2 mol H 2 / mol Ti, although at a much lower rate. This suggests a ‘Ti(0)-catalyzed’ dissociation of NaAlH 4 present in large excess, since NaAlH 4 at 558C and normal pressure is thermodynamically unstable with respect to the decomposition to Na 3 AlH 6 and Al and H 2 (Eq. (23)) (cf. Section 4.3.3 and Fig. 9a,b). A comparable Ti-catalyzed dissociation of LiAlH 4 in ether is known from the literature [13–16]. toluene

xNaAlH 4 1 Ti(OBu) 4 → (x 2 1)NaAlH 4 1 NaAl(OBu) 4 1 ‘Ti(0)’ 1 2H 2 (g)

(22)

x550–100

xNaAlH 4 1 Ti(OBu) 4 1 Fe(OC 2 H 5 ) 2 → (x 2 1.5)NaAlH 4 1 1.5NaAl(OBu) 4 x 5 50–100

1 ‘Ti(0)’ 1 ‘Fe(0)’ 1 3H 2 (g)

(22a)

‘Ti(0)-catalyst’

NaAlH 4

Fig. 5c ( ). Since NaAlH 4 is present in a great excess, it can be expected that the reduction of both alcoholates in mixture will proceed independently from each other; in this case the H 2 evolution curve should coincide with the sum curve of H 2 evolution of both alcoholates (- - - -). Fig. 5c shows however, that the reduction of the mixture of both alcoholates (—) takes place at a much lower rate than expected for the independent reduction of both of them (- - - -). This indicates an interaction of alcoholates during the process of their reduction to the zerovalent stage (Eq. (22a)).

→

toluene / 558C

1 / 3Na 3 AlH 6 1 2 / 3Al 1 H 2 (g)

(23)

From Fig. 5b it can be seen that the H 2 evolution for NaAlH 4 (p) doped with Fe(OEt) 2 (— — —) is much more rapid for the same doped with FeCl 2 (— - - —), and that also the H 2 evolution proceeds beyond release of 1 mol H 2 / mol Fe (expected for the Fe(12)→Fe(0)-reduction). NaAlH 4 was also doped with combinations of catalytically active metals. The selection in favor of Ti–Fe and Ti–Ni combinations (Sections 4.4.3–4.4.4) was made primarily because Ti–Fe and Ti–Ni intermetallics are wellknown low temperature reversible hydrogen storage materials [1]. The progress of H 2 release with time for the doping of NaAlH 4 (p) with Ti(OBu) 4 and Fe(OEt) 2 is represented in

In contrast to this behavior, in the case of doping of NaAlH 4 (p) with a mixture of Ti(OBu) 4 and FeCl 2 (Fig. 5d), there is apparently no such an interaction, since the curve of H 2 evolution ( ) nearly coincides with the sum curve of H 2 evolutions of each of the separate compounds (- - - -).

4.3. Thermodynamics of the Ti-doped NaAlH4 -system 4.3.1. PCI and direct dissociation pressure measurements In the present study PCIs of the Ti-doped NaAlH 4 system for both modes in the temperature range from 104 to 2448C are reported (for earlier measurements, see Refs. [7,9]). The PCIs of Ti-doped NaAlH 4 are represented in

46

B. Bogdanovic´ et al. / Journal of Alloys and Compounds 302 (2000) 36 – 58

Fig. 6a,b. All the PCI curves reveal two application favourable properties of the system: a low hysteresis and an almost negligible plateau slope. For direct measurements of hydrogen dissociation pressures, samples of NaAlH 4 (e) and of Na 3 AlH 6 were doped with 1.3 and 2.0 mol% of b-TiCl 3 respectively [9]. The doped samples in an autoclave were stepwise heated up to definite temperatures (measured was the inner temperature of the samples) and at each step the temperature was kept constant until the complete constancy of

pressure was reached (see Section 3.4.2, Table 1). The amounts of the samples and the (dead) volume of the autoclave were so chosen that, in spite of the hydrogen desorption, a substantial amount of the hydrides remained. The thus determined hydrogen dissociation pressures of Ti-doped NaAlH 4 (e) and Na 3 AlH 6 as a function of temperature in the temperature range of 20–211 and 2258C proved to be in satisfactory agreement with those resulting from PCI measurements (see Fig. 9b, — and – – j – –, Section 4.3.3). Of particular interest is the fact that already at temperatures as low as 60 and 808C the dissociation pressures of NaAlH 4 of 2.0 bar and of Na 3 AlH 6 of 0.6 bar, respectively, can be directly measured. This is evidence that even these non-optimized Ti-doped samples of NaAlH 4 (cf. Section 4.4.2) can desorb hydrogen at temperatures below 1008C.

4.3.2. Determination of thermodynamic parameters of the first and second dissociation steps of Ti-doped NaAlH4 The absorption and desorption equilibrium pressure data taken from the middle of the two plateaus (Fig. 6a,b) appear in the van’t Hoff plots (Fig. 7) as separate straight lines, representing the temperature dependence of the dissociation pressures of NaAlH 4 and Na 3 AlH 6 , respectively. The line representing the NaAlH 4 dissociation reveals at |1838C a bend which nearly coincides with the melting point of NaAlH 4 at about 135 bar of H 2 pressure [50] 3 . Thus, above 1838C we are dealing with liquid

Fig. 7. Evaluation of thermodynamic parameters for the two-step reversible dissociation of Ti-doped NaAlH 4 . Fig. 6. (a) Pressure-composition isotherms for Ti-doped NaAlH 4 at different temperatures; (b) left part of (a) at higher resolution.

3 The lowering of the melting point of Ti-doped NaAlH 4 by the products of the doping reaction is hereby not taken into account.

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Ti-doped NaAlH 4 , whose dissociation enthalpy (see below) differs a lot from that of the same solid material. From the slopes of the straight lines in Fig. 7 following the van’t Hoff equation, enthalpy changes associated with the first and second dissociation step of NaAlH 4 (s) (Eqs. (1a, s) and (1b)) are calculated to be 37 and 47 kJ / mol (average values from desorption and absorption data, Section 3), respectively; for the dissociation of NaAlH 4 into NaH, Al and H 2 (Eq. (1a,b)), taking into account that only one-third of the molar amount of Na 3 AlH 6 is involved in the process, gives a dissociation enthalpy of 53 kJ / mol. For the reversible dissociation of Ti-doped, liquid NaAlH 4 (Eq. (1a, l)) an enthalpy of 9 kJ / mol is determined. From the difference between the enthalpy values for the dissociation of solid and liquid Ti-doped NaAlH 4 a fusion enthalpy of 28 kJ / mol (Eq. (24)) can be obtained. NaAlH 4 (s)á1 / 3Na 3 AlH 6 1 2 / 3Al 1 H 2 (g) DH 5 37 kJ / mol

(1a, s)

Na 3 AlH 6 1 2Alá3NaH 1 3Al 1 3 / 2H 2 (g) DH 5 47 kJ / mol

(1b)

(cf. Eq. (20), Ref. [45], 41.5 kJ / mol) NaAlH 4 (s)áNaH 1 Al 1 3 / 2H 2 (g) DH 5 53 kJ / mol

(1)

NaAlH 4 (l)á1 / 3Na 3 AlH 6 1 2 / 3Al 1 H 2 (g) DH 5 9 kJ / mol NaAlH 4 (s)áNaAlH 4 (l) DH 5 23 kJ / mol

(1a, l) (24)

47

(cf. Eq. (17), Ref. [45], 23.2 kJ / mol).

4.3.3. Comparison of van’ t Hoff plots of Ti-doped NaAlH4 and Na3 AlH6 systems with those of known intermetallic and elemental hydrides As proposed by Buchner [51], the temperature at which the (hydrogen) dissociation pressure of a metal hydride reaches 1 bar H 2 , can be utilized to classify reversible metal hydrides as low temperature (LT; 1 bar H 2 below 508C), medium temperature (MT; 1 bar H 2 between 50 and 2008C) and high temperature metal hydrides (HT; 1 bar H 2 at above 2008C). In Fig. 8, van’t Hoff plots of the doped NaAlH 4 and Na 3 AlH 6 systems (Fig. 7) are reproduced together with those of various hitherto known intermetallic and elemental hydrides. The majority of well-known metal hydrides, such as FeTiH 22x and LaNi 5 H 6 , belong to the group of LT metal hydrides and several important ones, MgH 2 , Mg 2 NiH 4 and Mg 2 FeH 6 , are HT metal hydrides. In this connection it is interesting to note in Fig. 8 that in the application quite relevant medium temperature region only few hydrides, namely PdH x , LaNi 4 Al and LaNi 3.5 Al 1.5 [52], are known. Ti-doped Na 3 AlH 6 system (Fig. 8; Eq. (2)) reaches 1 bar of hydrogen pressure at |1008C, and is thus one of few known MT hydrides. For thermodynamic reasons application-relevant operational temperatures of the doped Na 3 AlH 6 system should therefore lie above 1008C. On the other hand, from Fig. 8 it can be seen that the straight line representing the first dissociation step of NaAlH 4 (Eq. (1a)) crosses the 1-bar H 2 pressure line at

Fig. 8. Comparison of van’t Hoff plots of NaAlH 4 and Na 3 AlH 6 systems with those of various intermetallic and elemental hydrides. Adapted from Ref. [1].

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|308C and parallels very closely the van’t Hoff plot of LaNi 5 H 6 . Ti-doped NaAlH 4 should therefore behave as a typical LT hydride. However, because of its still not yet achieved ideal kinetic properties (see Sections 4.4.2– 4.4.3), this hydride system can presently be operated only at temperatures above 100–1208C. In comparison to LaNi 5 H 6 , Ti-doped NaAlH 4 (Eq. (1a)) has, however, more than twice higher hydrogen storage capacity (see below). Fig. 9a gives a general view of the possibilities for operation of these new hydride systems, neglecting their kinetic properties. The two curves in Fig. 9a divide the linear P–T map in three, or more precisely, four areas: in

Fig. 9. (a) Pressure–temperature relations for the Ti-doped NaAlH 4 system; (– j –) desorption pressure; (– s –) absorption pressure; (*) hydrogen storage capacity of Na 3 AlH 6 in absence of Al. (b) Comparison of hydrogen dissociation pressures for the Ti-doped NaAlH 4 system from PCI (a, – – –) and direct measurements (—).

the stronger shaded area solid (s) NaAlH 4 is thermodynamically stable; above 1838C is the area of stability of liquid (l) NaAlH 4 . Na 3 AlH 6 is stable inside the slightly shaded area and the blank area represents the region of stability of NaH /Al mixtures under hydrogen pressure. Crossing of the borders between the three (or four) areas in the P–T map in one or other direction is necessarily associated with absorption of heat and release of hydrogen, or, in the reverse direction, with release of heat and absorption of hydrogen (marked in Fig. 9a by arrows). Thus, crossing the left curve from left to right results in the release of (maximum) 3.7 wt% H and requires 37 kJ / mol of heat. Starting from Na 3 AlH 6 (that is in absence of Al, Eq. (2)), and crossing the second border from left to right, is associated with evolution of maximum 3.0 wt% H and absorption of 47 kJ / mol of heat. If both borders are crossed in the same direction, 5.5 wt% of hydrogen can maximally be released, whereby 53 kJ / mol of heat have to be fed to the system. The doped NaAlH 4 system thus offers the possibility to be utilized both as a whole, or the two dissociation steps can be applied separately for hydrogen or heat storage. In order to utilize only the first dissociation step for hydrogen storage (Eq. (1a)) the H 2 discharging temperature should be kept below or around 1008C, at which temperatures the hydrogen dissociation pressure of Na 3 AlH 6 (Fig. 9a) is still very low, while the dissociation of NaAlH 4 is already thermodynamically strongly favored.4 As outlined above, utilization of the doped Na 3 AlH 6 system (Eq. (2)) for the purpose of reversible thermochemical heat storage, heat transformations, etc., is of particular interest since, for it the system is operable under moderate H 2 pressures in the medium temperature region (e.g., 150–2508C; Fig. 9a). An obvious disadvantage of the doped NaAlH 4 system (Eq. 1a,b) as a hydrogen storage system is that pressure and temperature conditions for charging and discharging of hydrogen across both dissociation steps differ considerably from each other: in order to discharge hydrogen until the NaH1Al stage at normal pressure, the discharge temperature must be raised above the 1008C level; on the other hand, the high dissociation pressure of NaAlH 4 opposes charging of the system with hydrogen back to the NaAlH 4 stage, so that relatively high hydrogenation pressures must be applied. This lowers the energy efficiency of the system. In principle it should be possible to tailor the thermodynamic properties of the system in such a way that the two curves in Fig. 9a come closer together, thus eliminating the problem. It has already been shown [9] that the dissociation pressure of the doped Na 3 AlH 6 can be lowered to the extent of 20 bar by substitution of one of the Na atoms of the compound by Li.

4

This operation mode should in principle enable to reach the target set by the Japanese ‘New Sunshine Program’ [53]: 3 wt% of H 2 , released at a temperature of 1008C.

B. Bogdanovic´ et al. / Journal of Alloys and Compounds 302 (2000) 36 – 58

More promising at present is perhaps the ‘catalytic option’: the difference between the dissociation pressures of NaAlH 4 and Na 3 AlH 6 (Fig. 9a) diminishes as the temperature is lowered, and thus conditions necessary for hydrogen dis- and recharging approach each other. The possibility to operate the system at low(er) temperatures depends primarily upon the activity of the applied catalysts (dopants). Improvement of the catalytic activity of the dopants would thus lessen a major disadvantage of this hydrogen storage system. With a perfect catalyst of the kind available, it should be possible to discharge hydrogen from the system up to the NaH1Al stage under normal pressure at a temperature slightly above 1008C and to recharge it with hydrogen at 308C and hydrogen pressures above 1 bar (marked by the arrow at the bottom of the diagram Fig. 9a). Also for this reason further improvement of the catalyst’s activity is of primary importance (see next Section 4.4).

4.4. Kinetic properties and cyclic stability of metal doped NaAlH4 4.4.1. Screening of the catalysts (dopants) [10] The following procedure was used to select metal compounds to be used as dopants for NaAlH 4 : after doping of NaAlH 4 with the respective metal compounds (Section 3.3.1 and Table 2), the doped NaAlH 4 was dehydrogenated (2708C) and the amount of hydrogen desorbed established (1, dehydrogenation). The mixture was then hydrogenated and again dehydrogenated under the same conditions as before (2, dehydrogenation). The ratio of the amounts of hydrogen evolved within the (2) and the (1) dehydrogenation (in %), expressed as ‘degree of rehydrogenation’ Table 2), is used as a rough measure of the catalytic activity of the applied dopant. As shown in the Table 2, a high catalytic activity is exerted by Ti, Zr and V compounds and several of the rare earth chlorides; FeCl 2 and NiCl 2 were accordingly somewhat less active as dopants. As it turned out, the kinetics of hydrogen de- and

49

Fig. 10. Influence of particle size of NaAlH 4 and of the doping medium on dehydrogenation of Ti-doped NaAlH 4 ; progression of first dehydrogenation at 1208C for NaAlH 4 (e) and (p) doped with each 2 mol% of Ti(OBu) 4 : - - - - -, NaAlH 4 (e) (Fig. 3a), doped in ether (taken from Ref. [9]); — — —, NaAlH 4 (p) (Fig. 3b), doped in ether; —, NaAlH 4 (p), doped in toluene.

reabsorption of solid doped NaAlH 4 (Eq. (1a,b)) depends not only upon the kind and amount of the metal compound used as a dopant, but also upon morphology and size of NaAlH 4 particles used, the doping procedure (kind of the solvent used for doping or doping in solid state [10]) and, in particular, upon combination of two different metal dopants [39].

4.4.2. Effect of particle size of NaAlH4 and of the doping medium on the rate of dehydrogenation of Tidoped NaAlH4 With respect to the rate and extent of dehydrogenation, NaAlH 4 (p) crystals (Section 4.1.1, Fig. 3b) are highly favored over NaAlH 4 (e) crystals (Fig. 3a) when both are doped with Ti(OBu) 4 in ether (Fig. 10, — — — and - - - -). A further increase of dehydrogenation rate is found

Table 2 Degrees of rehydrogenation a of dehydrogenated NaAlH 4 as a function of the dopant Dopant b

1st thermolysis c (% by weight of H 2 )

2nd thermolysis c (% by weight of H 2 )

Degree of rehydrogenation (%)

Dopant b

1st thermolysis c (% by weight of H 2 )

2nd thermolysis c (% by weight of H 2 )

Degree of rehydrogenation (%)

– TiCl 4 ?-TiCl 3 HTiCl?0.5THF Ti(OBu) 4 Cp 2 TiCl 2 ZrCl 4 Cp 2 ZrCl 2 VCl 3 Cp 2 VCl 2

5.52 4.51 4.75 5.00 4.23 4.48 4.71 4.40 4.81 4.47

0.55 2.85 2.96 3.07 2.60 2.34 2.59 2.89 2.65 2.11

10 63 62 61 61 52 55 66 55 47

NbCl 3 YCl 3 LaCl 3 CeCl 3 PrCl 3 NdCl 3 SmCl 3 FeCl 2 NiCl 2 ?1.5THF

4.59 4.59 4.56 4.53 4.51 4.54 4.42 4.65 4.69

1.91 2.20 2.62 2.47 2.64 3.10 2.77 2.13 2.24

42 48 57 54 59 68 63 46 48

a

Hydrogenation conditions: 1208C / 150–130 bar of H 2 / 24 h. 5 mol% each, based on NaAlH 4 . c From room temperature to 2708C at 48C / min; then 2708C until the H 2 evolution was completed. b

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B. Bogdanovic´ et al. / Journal of Alloys and Compounds 302 (2000) 36 – 58

out when NaAlH 4 (p) crystals are doped by Ti(OBu) 4 in toluene instead of in ether (Fig. 10, —). Remarkable in the last experiment is the completion of dehydrogenation (3.4 wt% H) at a temperature of only 1208C within 3–4 h. Because of these results, in the experiments discussed in the following, if not otherwise stated, NaAlH 4 (p), doped in toluene was used.

4.4.3. Effect of the doping agents Ti( OBu)4 and Fe( OEt)2 and of their concerted action on the rates of hydrogen de- and reabsorption by the doped NaAlH4 : evidence for a synergistic Ti /Fe effect It was thought that doping of NaAlH 4 with Ti–Fe or Ti–Ni combinations (cf. Section 4.2.2) the intermetallics TiFe or TiNi might be formed in the course of the doping procedure, or during cycle tests (Section 4.4.4), and may act as a catalyst for de- and rehydrogenation processes [39]. Zidan et al. [54] have recently been able to demonstrate the beneficial effect of combined Ti–Zr doping on the rate of dehydrogenation of NaAlH 4 to the NaH1Al stage. Fig. 11a shows that the rate of the first dehydrogenation of NaAlH 4 (p) doped with Ti(OBu) 4 increases with the amount of the dopant and temperature. In Fig. 11b the progressions of the first dehydrogenations of NaAlH 4 (p) at 1608C doped with 2 mol% of Ti(OBu) 4 (—) and with 2 mol% of Fe(OEt) 2 (/ / / / / / / / ) is contrasted with that of the same doped with both 1 mol% of Ti(OBu) 4 and Fe(OEt) 2 ( ). As it can be seen from the diagram, the Ti / Fe-doped NaAlH 4 (p) is significantly more rapidly dehydrogenated than that doped with the double amount of single dopants. Additionally, the diagram shows that Fe(OEt) 2 as a catalyst is much less efficient than Ti(OBu) 4 . Even more pronounced is the possible cooperative Ti / Fe effect after several de- and rehydrogenation cycles in cycle tests (cf. Section 4.4.4, Fig. 13a): the Ti-doped NaAlH 4 (p) (Fig. 11c, 4 mol%, — - - —) requires for completion of dehydrogenation at 1408C |3 h, while the Ti / Fe-doped NaAlH 4 (p) (each 2 mol%, — — —) requires for the same only 1.25 h. Additionally, both at 140 and 1608C the dehydrogenation rate of the Ti / Fe-doped NaAlH 4 (p) (each 1 mol%, ) is about as high as that of NaAlH 4 (p) doped with the double amount of Ti(OBu) 4 (2 mol%, —), although, as shown above, Fe(OEt) 2 is much less active as a dehydrogenation catalyst than Ti(OBu) 4 . Some qualitative features concerning rates of dehydrogenations in cycle tests using NaAlH 4 (p) doped with 1 and 2 mol% each of Ti(OBu) 4 and FeCl 2 and 1 mol% each of Ti(OBu) 4 and NiCl 2 (Section 4.4.4, Fig. 13b) can be inferred from recorded dehydrogenations. Using the named dopants, the dehydrogenation rate at 1808C decreases in the order 2 mol% Ti / Fe.1 mol% Ti / Fe.2 mol% Ti.1 mol% Ti / Ni, the dehydrogenations being completed within roughly 20, 40 and 60 min, respectively. In order to evaluate the effect of single or combined Tiand Fe-dopants of the rate of hydrogen reabsorption of doped NaAlH 4 (p) in dehydrogenated (NaH1Al) state

Fig. 11. Progression of dehydrogenation of NaAlH 4 (p), doped in toluene; (a, b), first dehydrogenation; (c), cycle tests (Fig. 13a); - - - -, — — —, ——, — - - —, doping with 0.5, 1, 2 and 4 mol% of Ti(OBu) 4 respectively; / / / / / / / /, doping with 2 mol% of Fe(OEt) 2 ; , , doping with each 1 and 2 mol% of Ti(OBu) 4 and Fe(OEt) 2 , respectively.

during cycle tests (Section 4.4.4), several checks were carried out. At 1708C / 150 bar hydrogen pressure (Fig. 12a) the hydrogenation rate increases in the order: NaAlH 4 (e) doped with 2 mol% of Ti(OBu) 4 in ether (— — —),NaAlH 4 (p), 2 mol% Ti(OBu) 4 , toluene (—),

B. Bogdanovic´ et al. / Journal of Alloys and Compounds 302 (2000) 36 – 58

51

Fig. 12. (a–c) Progression of hydrogen absorptions in cycle tests (Fig. 13a) for dehydrogenated NaAlH 4 (p) doped in toluene (unless otherwise stated): —, — - - —, 2 and 4 mol% of Ti(OBu) 4 , respectively; - - - -, 2 mol% Fe(OEt) 2 ; , , 1 and 2 mol% each of Ti(OBu) 4 and Fe(OEt) 2 respectively; — — —, doping with 2 mol% of Ti(OBu) 4 in ether (taken from Ref. [9]); (d): —, 2 mol% each of Ti(OBu) 4 and Fe(OEt) 2 (Fig. 13a, – h –).

NaAlH 4 (p), 4 mol% Ti(OBu) 4 , toluene (— - - —). The diagram in Fig. 12b shows that Fe(OEt) 2 as a dopant (2 mol%, - - - -) is an order of magnitude less efficient than Ti(OBu) 4 (2 mol%, —) in catalyzing the rehydrogenation of NaAlH 4 (p). In contrast to this, the concerted catalytic action of each 1 mol% of Ti(OBu) 4 and Fe(OEt) 2 as dopants ( ) exceeds significantly the catalytic effect of 2 mol% of Ti(OBu) 4 as a dopant (—). Fig. 12c represents hydrogen absorptions versus time for dehydrogenated samples of NaAlH 4 (p) doped with 2 and 4 mol% of Ti(OBu) 4 (— and — - - —) and with 1 and 2 mol% of both Ti- and Fe-alcoholates ( and ) of 16–26 cycles lasting cycle tests (Fig. 13a). Following qualitative features of rehydrogenations can clearly be inferred from the diagram of Fig. 12c: 1. the sample doped with 4 mol% of Ti (— - - —) is more rapidly hydrogenated than that doped with 2 mol% of Ti (—). Likewise, hydrogenation of the sample doped with 2 mol% of Ti and Fe ( ) is more rapid than that of the sample doped with 1 mol% of Ti and Fe ( ). Hence it follows that the hydrogenation rate increases with the amount of applied both mono- or bimetallic catalysts, which means that we are indeed dealing with catalyzed reactions.

2. The sample doped with 1 mol% of Ti and Fe ( ) is more rapidly hydrogenated than that doped with 2 mol% of Ti (—); the same holds true also for the comparison 2 mol% of Ti and Fe ( ) versus 4 mol% Ti (— - - —). We have now additionally to take into account, that the activity of Fe as a catalyst for de- and rehydrogenations of only Fe-doped NaAlH 4 , as shown above, is considerably lower than that of Ti as a dopant. Since in each of the cases investigated the activity of the applied Ti / Fe catalysts exceeds the activity of single Ti or Fe catalysts, a cooperative (synergistic) catalytic effect of the metals Ti and Fe in enhancing rates of both de- and rehydrogenation of the Ti / Fe-doped NaAlH 4 system can be taken as established. The synergistic effect, however, appears to be more pronounced with the combination of Ti- and Fealcoholates than with Ti(OBu) 4 / FeCl 2 or NiCl 2 combinations as dopants. The diagram in Fig. 12d shows progression of hydrogen absorptions in the course of a cycle test carried out with a sample of NaAlH 4 (p) doped in toluene with 2 mol% each of Ti(OBu) 4 and Fe(OEt) 2 (Section 4.4.4, Fig. 13a, h) at different temperatures and pressures. From Fig. 12d it can be seen that the initial rate of hydrogenation (first 5–10

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B. Bogdanovic´ et al. / Journal of Alloys and Compounds 302 (2000) 36 – 58

Fig. 13. (a) Cyclic stability tests for NaAlH 4 (p) doped in toluene (hydrogenation time for T #1208C, 17 h and for T .1208C, 2–5 h; de- and rehydrogenation curves in Figs. 11 and 12, respectively); (– s –) 1 mol% each of Ti(OBu) 4 and Fe(OEt) 2 (dehydrogenation temperature in 1–20, 23 and 25th cycle 1608C, in 24th cycle 1408C and in 21–22nd cycle 1808C); (– d –) 2 mol% of Ti(OBu) 4 (deh. temp. in 1–9, 11–13, 15–16, 18–19, 21–24 and 26th cycles, 1808C, in 10, 14 and 17th cycles 1608C, in 20 and 25th cycles 150 and 1408C respectively); (– h –) each 2 mol% of Ti(OBu) 4 and Fe(OEt) 2 (deh. temp. in 1–8, 10–12 and 14–16th cycles 1608C, in 9 and 13th cycles 140 and 1308C, respectively); (– j –) 4 mol% of Ti(OBu) 4 (deh. temp. in 1 and 14th cycles 1808C, in 5th cycle 1408C and in 2–4, 6–13 and 15th cycles 1608C; the low hydrogen capacities attained in cycles 9 and 11 are due to reaching of equilibrium conditions). (b) Cyclic stability tests for NaAlH 4 (p) doped in toluene (dehydrogenation at 1808C / normal pressure; T Hyd 51008C, PHyd 5120 bar, 17 h): (– ^ –) 1 mol% each of Ti(OBu) 4 and FeCl 2 ; (– 앳 –) 1 mol% each of Ti(OBu) 4 and NiCl 2 (in 7th cycle only 1.5 h hydrogenation time); (– ♦ –) each 2 mol% of Ti(OBu) 4 and FeCl 2 (in 4, 5 and 6th cycles, hydrogenation time restricted to 2.5, 5 and 10 h, respectively); (– 3 –) 2 mol% of Ti(OBu) 4 .

min) at 1048C decreases proportionally with decreasing hydrogen pressure (135.84.64.45 bar). Applying an initial hydrogen pressure of 45 bar the hydrogen absorption stops at a pressure of 23 bar, reaching only 2.9 wt% H, because under these conditions the equilibrium (see Fig. 9a) is nearly attained. Hydrogenations of the same material (Fig. 12d) under hydrogen pressures of 140–150 bar at temperatures as low as 88, 70 and 508C or under 65–40 bar at 888C are possible.

4.4.4. Cyclic stability tests conducted upon the NaAlH4 system doped with Ti as well as with combined Ti /Fe and Ti /Ni catalysts [39] Cyclic stability is one of the major criteria for applicability of metal / metal hydride systems for reversible hydrogen storage. According to some preliminary results [9], Ti-doped Na 3 AlH 6 and Na 2 LiAlH 6 systems exhibited somewhat higher cyclic stability than the Ti-doped NaAlH 4 system.

B. Bogdanovic´ et al. / Journal of Alloys and Compounds 302 (2000) 36 – 58

Within the present study, some cyclic tests were carried out on NaAlH 4 (p) doped with Ti(OBu) 4 and with Ti(OBu) 4 / Fe(OEt) 2 , FeCl 2 and NiCl 2 combinations (Fig. 13a,b), the doping being conducted in toluene instead of the earlier described in ether [9]. Differing from the previous results [9], NaAlH 4 (p) doped with 2 mol% of Ti(OBu) 4 in toluene (Fig. 13a, – d –) showed during 26 cycles no significant decrease of storage capacity. In comparison to the sample doped with 2 mol% of Ti (– d –), the NaAlH 4 (p) sample doped with 1 mol% each of Ti and Fe (Fig. 13a, – s –) reaches, during 25 cycles, an average storage capacity of 4 wt% H 2 . According to cycle tests including four to 11 cycles, similar results can be achieved also with NaAlH 4 (p) doped with Ti(OBu) 4 – FeCl 2 or –NiCl 2 combinations (Fig. 13b).

4.4.5. Investigation of the Ti-doped NaAlH4 in the course of a cyclic test by scanning electron microscopy ( SEM) and energy dispersive X-ray analysis ( EDX) Within investigations of Ti-doped NaAlH 4 as a hydrogen storage material, SEM and EDX analyses of the material subjected to a cycle test have been carried out. Fig. 14a shows an SEM micrograph of a particle of Ti-doped NaAlH 4 (4 mol% Ti; Section 4.4.4; Fig. 13a, – j –) after the 17th dehydrogenation in 10 000 magnification. Because of the hydrogen desorption process the particle is highly porous and is composed of grown together crystallites of less than 5 mm size. From the particle in Fig. 14a EDX analyses in the form of so-called Na (b) and Al mappings (c) were carried out. The black areas in the Fig. 14b show the distribution of the

53

element Na in the sample, which exists there in the form of NaH; in Fig. 14c, black areas are representative for the Al distribution. The areas of high concentration of each of the metals, which are of |3 mm size, are distinctly separated from each other. Fig. 14a–c shows that the doped material in dehydrogenated state is segregated into NaH and Al phases.

4.4.6. Investigation of the Fe species present in the Ti / Fe-doped NaAlH4 before and during a cycle test via 57 ¨ Fe Mossbauer spectroscopy ¨ For the purpose of 57 Fe Mossbauer (MB) spectroscopic investigation, a sample of NaAlH 4 (p), doped with 2 mol% each of Ti(OBu) 4 and Fe(OEt) 2 , was subjected to 11 deand rehydrogenation cycles (Eq. (25)). After the doping, the MB spectrum at room temperature (Fig. 15a and Table 3) displays a non-magnetic (or ‘superparagamagnetic’, see below) behavior and has a large quadrupole splitting (D 5 1.00 mm / s). This feature points to a highly asymmetric arrangement of (Fe) atoms around Fe; the large line width indicates a low level of atomic order. The MB spectrum Fig. 15a is completely different from that of Fe(OEt) 2 at ambient temperature (Fig. 15c, Table 3), from which it follows that after the doping reaction Fe(OEt) 2 is no longer present. The MB spectrum of the same sample measured at 4.2 K (Fig. 15b) reveals a sextet characteristic of magnetic splitting. Magnetic hyperfine fields are broadly distributed with a maximum at 32.98 Tesla; the isomer shift (d 50.27 mm / s) is close to that of bcc Fe (Fig. 15d). Since the magnetic hyperfine fields of the MB spectrum of the doped

Fig. 14. SEM investigation of dehydrogenated Ti-doped NaAlH 4 (p): (a) SEM image of dehydrogenated NaAlH 4 (p) doped with 4 mol% of Ti(OBu) 4 , after a 17-cycle test represented in Fig. 13a, (– j –) (magnification, 310 000); EDX mappings of Na (b) and Al (c).

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B. Bogdanovic´ et al. / Journal of Alloys and Compounds 302 (2000) 36 – 58

Fig. 15. (a) 57 Fe MB spectrum of NaAlH 4 (p) doped with each 2 mol% of Ti(OBu) 4 and Fe(OEt) 2 at room temperature; (b) the same sample measured at 4.2 K; (c) MB spectrum of Fe(OEt) 2 at room temperature; (d) MB spectrum of bcc Fe at room temperature.

sample at 4.2 K is almost in agreement with that of bcc Fe (Fig. 15d, 33.00), it is an independent evidence that during the doping procedure (cf. Eq. (22a)) Fe(OEt) 2 is reduced to the zerovalent (metallic) stage. The disappearance of magnetic properties of Fe at ambient temperature (Fig. 15a in comparison to Fig. 15b) can be interpreted assuming nanosize Fe particles, which

as such possess ‘superparamagnetic’ properties. The ‘superparamagnetism’ of extremely small (nano) particles is an apparent paramagnetism of magnetically ordered small particles caused by thermal fluctuations of magnetisation domains direction; such fluctuations take place randomly during the life time of the nuclear excited states. On the basis of MB spectroscopy it is possible to

Table 3 57 ¨ Fe Mossbauer spectroscopy data of Fe species present in Ti / Fe-doped NaAlH 4 before and during a cycle test, and of related materials Sample

Temp. of measurement

Isomer shift (relative to bcc Fe at 208C) (mm / s)

Quadrupole splitting DEQ or quadrupole line shift 2e (magnetic spectra) (mm / s)

Hyperfine magnetic field (Tesla)

Fig. no.

NaAlH 4 after doping with 2 mol% each of Ti(OBu) 4 and Fe(OEt) 2

208C 4.2 K

0.32 0.27

1.00 – –

– 32.98 (broad lines)

15a 15b

Fe(OEt) 2

208C

1.18 0.96 0.51

2.93 1.64 –

– – –

15c

bcc Fe

4.2 K

0.00

–

33.00

15d

Ti / Fe-doped NaAlH 4 after dehydrogenation (1808C)

4.2 K

0.30

0.54

–

16a

Ti / Fe-doped NaAlH 4 after 11 cycles and a dehydrogenation

4.2 K 208C

0.31 0.19

0.53 0.56

–

16b 17a

Fe–Ti–Al t 2 -alloy (Fe 25.1, Ti 44.2, Al 30.7 at% [55])

4.2 K

0.08

0.32

–

17b

t 2 1Al 13 Fe 4 -alloy (Fe 25.6, Ti 9.8, Al 64.6 at% [55])

208C

0.14

0.34

NaAlH 4 doped with 2 mol% of Fe(OEt) 2 , after dehydrogenation

4.2 K

0.30

0.55

17c –

–

B. Bogdanovic´ et al. / Journal of Alloys and Compounds 302 (2000) 36 – 58

differentiate between macroscopic and very small (nano) particles.5 Accordingly, the ratio of integral areas of ambient and low temperature (4.2 K) MB spectra can be taken as an indication of particle sizes of investigated materials. In the case of large particles, this ratio is expected to be of the order of 0.9; nanoparticles often exhibit a much lower ratio than 1. The ratio of integral areas of MB spectra at room temperature and at 4.2 K for Fe(OEt) 2 is 0.06, for Ti / Fe-doped NaAlH 4 0.34 and for bcc Fe it is ca. 0.9. Hence it follows, that both Fe(OEt) 2 particles and Fe particles in the doped material must be very small. On this basis also the broad magnetic distribution in the sample at 4.2 K (Fig. 15b), and the large quadrupole splitting in the spectrum at room temperature (Fig. 15a), can be explained. In summary, in the course of the doping procedure Fe(OEt) 2 is reduced to nanosize Fe particles which are probably embedded in a NaAl-hydridealkoxide matrix (Eq. (22a)). The Ti / Fe-doped NaAlH 4 sample (Fig. 15a,b) was subsequently subjected to 11 de- and rehydrogenation cycles test under variable cycling conditions (Eq. (25)). Fig. 16 shows the MB spectra of the Ti / Fe-doped sample at 4.2 K, taken after the first dehydrogenation (spectrum

Fig. 16. (a) 57 Fe MB spectrum of NaAlH 4 (p) doped with 2 mol% each of Ti(OBu) 4 and Fe(OEt) 2 after the first dehydrogenation (4.2 K); (b) MB spectrum of the same sample measured after 11 de- and rehydrogenation cycles and a dehydrogenation (4.2 K). 5 The area of a MB spectrum is a measure of the Debye–Waller ¨ (Lamb–Mossbauer) effects f. Surface atoms on small particles have a lower value of f than bulk atoms. A lower f value leads to a stronger temperature-dependence of the area as well as to a lower value. Thus the ratio of the area between 4.2 K (liquid helium) and room temperatures is an indication of the particle size (ratio of surface to bulk atoms). The presence of an electric field gradient (EFG) also attests to the predominance of surface atoms. The EFG is zero for a bulk site in cubic ¨ symmetry [D.P.E. Dickson, F.J. Berry, Mossbauer Spectroscopy, Cambridge University Press, Cambridge UK, 1986.].

55

(a)) and after 11 cycles and a dehydrogenation (spectrum (b)). MB spectra of the sample in hydrogenated and dehydrogenated state differ only insignificantly from each other. Already after the first dehydrogenation (1808C) the MB spectrum changes completely with respect to the spectrum before dehydrogenation, since instead of the broad sextet (Fig. 15b), an asymmetrical doublet (Fig. 16a) appears. After the twelfth dehydrogenation the MB spectrum (Fig. 16b) changes only a little with respect to the spectrum after first dehydrogenation, except that the doublet becomes more symmetrical. The symmetrization of the doublet after 11 cycles may suggest an ordering process, whereby the surroundings of Fe atoms become more uniform, but still not cubic. Comparison of the areas of spectra at 4.2 K (Fig. 16b) and at room temperature indicates that the Fe alloy particles have increased in size somewhat (increase of area ratios from 0.34 to 0.73), but are still very small. NaAlH 4 (Ti / Fe) 0.02

1608C

á

1048C / 140 bar

2H 2

[(NaH 1 Al)(Ti / Fe) 0.02 ] 1 3 / (25)

The disappearance of magnetic Fe features in the spectrum of Fig. 16a,b points to the formation of a nonmagnetic Fe alloy, which could be an Fe alloy with Ti, with Al or with both Ti and Al. Formation of Fe–Ti alloys can be definitely excluded on the basis of comparison of the spectrum with the MB spectrum of the two known Fe–Ti alloys, FeTi and Fe 2 Ti [55]. In order to decide between the presence of a binary Fe–Al, or a ternary Fe–Ti–Al alloy, the MB spectrum of the sample after the cycle test (Fig. 17a) was compared with MB spectra of available ternary [56] Fe–Ti–Al alloys having different compositions (Fig. 17b,c). According to their MB spectra (Fig. 17b,c), the ternary Fe–Ti–Al alloys are not identical with the sample in question (Fig. 17a). The closest resemblance to the spectrum of Fig. 17a, however, was found in the case of the spectrum of the alloy having the lowest Ti content (4.4% Ti, Fig. 17c), although also this spectrum, according to its isomer shift (0.14) and quadrupole splitting, still differed significantly from the spectrum of the sample under investigation (Fig. 17a). Since a metallurgically prepared binary Fe–Al alloy was not available, a sample was prepared by doping NaAlH 4 (p) only with Fe(OEt) 2 (2 mol%); this sample, after dehydrogenation, displayed a MB spectrum which was practically superimposable with that of the investigated sample (Fig. 17a and Table 3). These results point towards formation of an Fe–Al rather than a Fe–Ti–Al alloy as a result of doping NaAlH 4 with Fe- and Ti-alcoholates and subsequent hydrogen dis- and rechargings (Eq. (25)). Formation of a sodium containing Fe–Al alloy cannot be excluded, but is improbable, since neither Al nor Fe form alloys with Na [57,58]. The results discussed above leave the question about the

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B. Bogdanovic´ et al. / Journal of Alloys and Compounds 302 (2000) 36 – 58

NaAlH 4 (Eq. (25)). Thus, in the sequence of doping of NaAlH 4 with both dopants (Eq. (22a)) and subsequent cycling (Eq. (25)), it can be assumed that an Fe–Al and additionally a Ti–Al alloy are formed side by side.

5. Conclusions and outlook

5.1. Features of doped sodium aluminium hydrides as reversible hydrogen storage materials Doped sodium aluminium hydrides differ from hitherto known reversible metal hydride–metal systems [2–4,51] in several important aspects:

Fig. 17. Comparison of the 57 Fe MB spectrum of Ti / Fe-doped NaAlH 4 sample after the cycle test with those of some ternary Fe–Ti–Al alloys [55]. (a) MB spectrum of the Ti / Fe-doped sample after the cycle test (Fig. 16b), measured at room temperature; (b) MB spectrum of a Fe–Ti–Al t 2 alloy with Fe 25.1, Ti 44.2 and Al 30.7 at% (4.2 K); (c) MB spectrum of a Fe–Ti–Al alloy with Fe 26.2, Ti 4.4 and Al 69.4 at% (208C).

nature of the Ti-doped phase present in the reversible Ti / Fe-doped NaAlH 4 system (Eq. (25)) unanswered. For the possible formation of a Ti–Al alloy from Ti(OBu) 4 and NaAlH 4 in the course of the doping reaction (Eq. (22a)) and subsequent de- and rehydrogenations (Eq. (25)), there exists at present only indirect evidence on the basis of the literature. Haber et al. [59] described recently a ‘chemical synthesis’ of Ti–Al alloys through reduction of TiCl 3 with LiAlH 4 in boiling mesitylene and subsequent heating of the reaction products to 5508C. Depending on the molar ratio of the reactants (Eqs. (26) and (27)), two different Ti-aluminides, TiAl and TiAl 3 , could be prepared and identified by XRD. 2TiCl 3 1 3LiAlH 4 → 2TiAl 1 AlCl 3 1 3LiCl 1 6H 2 (g)

• Hydrogen storage is based on catalyzed thermal dissociation and reconstitution of complex light metal hydrides (Eqs. (1)–(3)) and not on incorporation of H atoms in interstitial positions of crystal lattices of metals, intermetallics and alloys. • In dehydrogenated state, not a metal, intermetallic nor an alloy but an intimate conglomerate of two nonmiscible phases, a metal hydride (NaH) and a metallic phase (Al), exists; in the process of the charging of the system with hydrogen (Eqs. (1)–(3), from right to left), the two phases react with each other and hydrogen in the presence of a catalyst (dopant) producing the homogeneous complex hydride phase. • The efficiency of the systems depends to a great deal upon the activity and stability of the applied catalysts (dopants). • According to its thermodynamic characteristics, the doped Na 3 AlH 6 system (Eq. (2)) belongs to the class of the so-called medium temperature metal hydrides [51], of which only a few representatives are known. The system thus offers new chances for thermal utilizations (heat storage, heat pumps) in the medium temperature (150–2508C) region. • Shortcomings of the doped systems (Eqs. (1) – (3)) in comparison to typical low temperature reversible metal hydrides are their inability to be operated at ambient temperatures (which can possibly be eliminated via improved catalysts or systems of the new type) and the as-yet unknown long-term cyclic stability. In considering possible applications of the new systems, safety aspects (behavior upon exposure to air, etc.) should be included.

(26) TiCl 3 1 3LiAlH 4 → TiAl 3 1 3LiCl 1 6H 2 (g)

5.2. Accessibility of materials (27)

The reactions in boiling mesitylene (Eqs. (26) and (27)) are comparable to the doping reactions of NaAlH 4 with Ti(OBu) 4 or Ti(OBu) 4 / Fe(OEt) 2 combinations (Eqs. (22) and (22a)), and the subsequent heating of the reaction products corresponds to cycle tests of Ti- or Ti / Fe-doped

The basis for these new hydrogen storage materials are low price, industrially available raw materials, sodium hydride and Al powder. Known processes for the production of NaAlH 4 (or Na 3 AlH 6 ) from these raw materials and hydrogen exist and have been practised on industrial scale [17]. The doping procedure can be simplified by

B. Bogdanovic´ et al. / Journal of Alloys and Compounds 302 (2000) 36 – 58

conducting it in dry state [10], but further improvements of methods for the preparation of metal doped alkali metal aluminium hydrides are desirable.

5.3. Prospects for forthcoming research Presently, research work on metal-doped alkali metal aluminium hydrides and their hydrogen dis- and recharging reactions appears attractive at least from the fundamental point of view, since new information may be obtained concerning the use of catalysts. Further development of catalysts (or catalyst combinations), which would enable the systems in question to operate at lower temperatures, and to understand their mode of action is needed. By partial substitution of Na in NaAlH 4 or in Na 3 AlH 6 by other alkali, or other metals — as shown by the example of Na 2 LiAlH 6 [9,10] — it should be possible to tailor the thermodynamic and also to influence the kinetic properties of the systems. In spite of favourable perspectives, there is still a lot of work to be done — especially the conducting of long-term cyclic stability tests — until the practicability of doped alkali metal aluminium hydrides as hydrogen storage materials is proved or disproved. It is therefore perhaps premature to discuss at present their possible applications, although their possible use as (economic) hydrogen (energy) sources of relatively high gravimetric and volumetric density as required for instance for fuel cells is obvious.

Acknowledgements The authors thank Professor M. Groll, University of Stuttgart, Germany, for useful informations, Dr M. Palm, ¨ Max Planck Institute for Iron Research, Dusseldorf, Germany, for samples of Fe–Ti–Al alloys, Dr G. Sandrock, SunaTech, Inc. Ringwood, NJ, USA, for useful comments and encouragement and Dr B. Tesche and his associates, ¨ Kohlenforschung, Mulheim-Ruhr, ¨ Max-Planck-Institut fur Germany, for SEM investigations. We are obliged to Chemetall GmbH, Frankfurt, Germany, and Ethyl Corporation, Richmond, VA, USA, for gifts of sodium aluminium hydride. Financial support by the Fonds der Chemischen Industrie, Frankfurt, Germany, is gratefully acknowledged.

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