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Particle size and catalytic effect on the dehydriding of MgH2
Journal of Alloys and Compounds 399 (2005) 237–241
Particle size and catalytic effect on the dehydriding of MgH2 ´ R´ev´esz a,b,∗ , T. Spassov a D. F´atay a,b , A. b
a Department of Chemistry, University of Sofia “St. Kl. Ohridski”, Sofia, Bulgaria Department of General Physics, E¨otv¨os University, Budapest, H-1518, P.O. Box 32, Budapest, Hungary
Received 17 February 2005; accepted 28 February 2005 Available online 31 March 2005
Abstract Conventional polycrystalline -MgH2 was ball-milled under hydrogen-atmosphere to reach a nanocrystalline microstructure (∼10 nm), until the particle- and grain-size attained a constant value. Subsequently, 2 mol.% pre-milled Nb2 O5 (∼40 nm) was added to the MgH2 as a catalyst and the milling was continued. The effect of the particle- and grain-size as well as of the milling duration on the temperature of hydrogen desorption and on the dehydriding kinetics of the ball-milled MgH2 + Nb2 O5 powders was analyzed. The grain- and particle-size reduction decreases substantially the H-desorption temperature. The size-reduction occurs during the first 120 min of milling MgH2 , but subsequent milling with the catalyst results in additional slight MgH2 particle size decrease, due to the harder Nb2 O5 particles compared to the MgH2 ones. The decrease of the activation energy of dehydriding caused by the catalyst was observed after only 15 min of ball-milling. The catalytic effect is, however, suppressed at longer milling times, probably due to penetration of the catalytic particles into the MgH2 . © 2005 Elsevier B.V. All rights reserved. Keywords: Nanostructure; Ball-milling; X-ray diffraction
1. Introduction Due to a series of special characteristics of hydrogen storage in metals, such as capacity, light weight, low environmental impact, low cost and high natural abundance, many metal–hydrogen systems have been studied as hydrogenstorage materials, in which magnesium is considered as one of the most promising candidates mainly because of its high storage capacity (7.6 wt.% in MgH2 ) [1]. However, because of its high thermodynamic stability (H = −75 kJ/mol) [2], high hydrogen desorption temperature (higher than 400 ◦ C) and relatively poor hydrogen absorption/desorption kinetics at temperatures below 350 ◦ C, the application of MgH2 is impeded. To convert Mg completely to MgH2 requires more than 50 h at 350 ◦ C [3]. The poor kinetics of Mg-based alloys is generally attributed to the low H2 -dissociation ability of metallic Mg and the formation
∗
Corresponding author. Tel.: +36 1 372 28 23; fax: +36 1 372 2811. ´ R´ev´esz). E-mail address: [email protected] (A.
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.02.043
of hydrides at the surface, which hinders further H-transport into the matrix. Numerous attempts have been made to overcome these difficulties. For example, alloying improves significantly the thermodynamic and kinetic properties of magnesium, but decreases the hydrogen storage capacity. Metallic elements, such as V, Ti, Ni, Fe, Mn, Pd, Nb have been selected as additives to magnesium [4]. Recently, ball-milling (BM) and mechanical alloying (MA) have been developed for processing nanocrystalline Mg under a hydrogen gas environment (so called reactive mechanical milling (RMM) or reactive mechanical alloying (RMA)) in order to reach special advantages, such as improving the thermodynamic and kinetic properties of hydriding. By reducing the grain size of Mg to nanocrystalline dimensions the H-sorption kinetics are accelerated substantially [5–9]. Zaluska et al. have shown that nanocrystalline Mg with a crystalline size of 30 nm can absorb about 6 wt.% H in 120 min at 573 K and Mg2 Ni can absorb 2.1 wt.% H in 40 min at 473 K [10]. Furthermore, it was demonstrated that reducing the grain size of Mg to the nanoscale range,
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the hydrogen desorption temperature (Tdes ) is decreased by about 100 ◦ C [9,11,12]. It has also been noted that milling the hydride instead of the pure Mg, produces novel nanostructures with a high surface area beneficial for hydrogen sorption kinetics [13]. Further significant improvement of the hydriding properties was performed by adding metals [14–16] and oxides [11,12,17–19] as catalysts. Bormann and co-workers have shown that the H-sorption of ball-milled nanocrystalline Mg/MgH2 (∼10 nm) with 0.2 mol.% Nb2 O5 as catalyst takes place completely in less than 10 min at 300 ◦ C [11,12]. So far, the best kinetic results on hydrogenation have been reported by Barkhordarian et al. [12] showing that at 573 K, Mg with 0.5 mol.% Nb2 O5 can absorb 7 wt.% H in 60 s. Although excellent kinetics was found for higher temperature (523 K), no results at lower temperatures (lower than 523 K) have been reported. So far, contrary results were found on the influence of the microstructure on the maximum H-capacity of Mg and Mg alloys. Whereas nanocrystalline Mg2 Ni-based alloys reveal an improvement of H-capacity in the extremely fine nanocrystalline state (7–8 nm) [20,21], the capacity of nanocrystalline Mg/MgH2 (∼10 nm) is reduced by about 6% with respect to its coarser counterpart (grain size of about 20 nm and particle size of about 1 m) [22]. It has already been proved that the grain size determines the favorable H-sorption kinetics, but for Mg/MgH2 , unlike Mg2 Ni, this is accomplished by a decrease in the H-capacity [20–22]. The influence of the particle size on the H-sorption kinetics is, however, still not sufficiently studied. In our recent study [19], it was proved that in addition to the grain size, the particle reduction (to <1 m region) improves the H-sorption kinetics of nanocrystalline Mg/MgH2 powders. Mg/MgH2 powders with the same grain size (∼50 nm) and different particle size in the range 0.1–1 m were investigated. Furthermore, it was shown that the time of milling of the nanocrystalline MgH2 with a catalyst (Nb2 O5 ) influences the kinetics. Therefore, in the present study, fine nanocrystalline MgH2 powders (with particle size ≤2 m and grain size <20 nm) were produced and then milled additionally for different times with the catalyst. The influence of the milling time of the nanocrystalline MgH2 with the catalyst on the H-desorption kinetics and Tdes was investigated.
2. Experimental Commercial polycrystalline MgH2 (poly-MgH2 ) powder was ball-milled in a self-constructed planetary-type mill for 120 min under a H-atmosphere of 6 atm in order to obtain nanocrystalline material (nano-MgH2 ). Using a Hatmosphere prevents the decomposition of MgH2 during milling. The nano-MgH2 powder was further milled with 2 mol.% of pre-milled Nb2 O5 as catalyst for different times: 15, 40 and 90 min. Hereafter, these states will be denoted as nano-15, nano-40 and nano-90, respectively.
The evolution of the microstructure during milling was monitored by X-ray powder diffraction (XRD) with Cu K␣ radiation on a Philips X’pert powder diffractometer in θ–2θ geometry. The change of the microstructure during the BM process was systematically studied by characterization of X-ray lineprofile analysis. This evaluation technique is based on the separation of size and strain effects. Line-profile analysis was carried out by the convolutional multiple whole profile fitting procedure (CMWP). In this method, the whole measured diffraction pattern is fitted by the sum of a polynomial background and physically well established theoretical profile functions. It is assumed that the grains are spherical and have a lognormal size distribution: − ln (L/m)2 −1/2 −1 −1 σ L exp , (1) G(L) = (2π) 2σ 2 where σ and m are the variance and median of the distribution, respectively. The fitting procedure provides several fitting parameters, such as σ and m, volume-averaged grain sizes: D = m exp(2.5σ 2 ).
(2)
See elsewhere the detailed description of this analysis [23,24]. The morphology of the powders was monitored by scanning electron microscopy (SEM) on a JEOL 5510 apparatus. The H-sorption properties were measured by a PerkinElmer Thermogravimeter (TG) at different heating rates. The onset (Tonset ) and the inflection (Tinf ) of the desorption curves were obtained as the interception of leading edge and the baseline, and as the minimum of the derivative curve, respectively. Continuous heating experiments were performed at scan rates in the range of 5–20 K/min. From the shift of Tinf with increasing heating rate, the apparent activation energy (Eact ) of the hydride decomposition process was determined applying the Kissinger analysis [25].
3. Results and discussion Fig. 1 shows the XRD patterns of the poly-MgH2 , nanoMgH2 , nano-15, nano-40 and nano-90 powders. Significant line-broadening of the tetragonal -MgH2 peaks is clearly seen when the commercial MgH2 powder is ball-milled for 120 min indicating a drastic grain size refinement. The MgH2 phase is known as a low-pressure phase. In addition, the peaks of the high-pressure ␥-MgH2 -phase appears. However, their intensity is considerably lower than that of the phase. Adding the catalyst to MgH2 and subsequent milling results in only a slight additional line-broadening accompanied with the decrease of the relative peak intensities of the -MgH2 phase compared to the Nb2 O5 ones. However, after each milling time, the peaks of the hydride and the catalyst can be separated from each other, indicating that the
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Fig. 1. XRD patterns corresponding to the poly-MgH2 , nano-MgH2 , nano15, nano-40 and nano-90. The symbols (䊉), () and ( ) denote the Braggpeaks of the -MgH2 , ␥-MgH2 and Nb2 O5 phases, respectively.
mixing between the catalyst and hydride particles occurs on nanoscale, without any detectable reaction between the two constituents. Due to overlapping, the ␥-MgH2 phase is not detectable when the catalyst is added. The normalized lognormal distribution functions of the -MgH2 and Nb2 O5 , obtained by the CMWP method, are shown in Fig. 2a and b, respectively. It is seen that with increasing milling time the distribution shifts only slightly towards smaller values for the -MgH2 phase, indicating that grain-size refinement is negligible when the hydride is milled additionally together with the catalyst. The calculated D values for the -MgH2 grains in the nano-MgH2 , nano-40 and nano-90 states are 11, 9.5 and 9 nm, respectively. Similarly, no remarkable grain-size refinement occurs in the case of the catalyst (see Fig. 2b); D exhibits a slight decrease from 41 to 37 nm for nano-40 and nano-90, respectively. Fig. 3 shows the corresponding SEM images of the polyMgH2 , nano-MgH2 , nano-15 and nano-90 states. The commercial MgH2 powder exhibits particles with an average size of 40–50 m. After ball-milling for 120 min the MgH2 powder, the average size of the powder particles decreases drastically down to about 1 m. It was also detected that ball-milling the hydride with the catalyst results in some noticeable particle-size reduction after 15 min of ball-milling, probably due to the presence of the harder Nb2 O5 particles. In contrast, we found that milling only nano-MgH2 (without Nb2 O5 ) does not cause noticeable particle- and grain-size reduction. For the nano-40 and nano-90 states, a homogeneous dispersion of the catalyst on the MgH2 particles forms, while the particle size decreases only slightly as well as the average grain size reported above. TG profiles obtained at a heating rate of 10 K/min of the samples are shown in Fig. 4. In the polycrystalline state, the dehydrogenation reaction starts at around Tonset = 431 ◦ C, and it reaches its inflection point at Tinf = 445 ◦ C. The inset in Fig. 4 indicates that achieving the nanocrystalline state results in a remarkable decrease of Tonset down to 388 ◦ C for
Fig. 2. Grain-size histograms of the -MgH2 (a) and Nb2 O5 (b) phases as a function of the milling time.
the nano-MgH2 state. Less pronounced further decrease occurs when MgH2 is milled together with the catalyst (361 ◦ C for the nano-90 state). It is evident that the main decrease in nano-MgH2 , observed between the poly-MgH2 and nanoMgH2 states, is due to the substantial microstructural refinement both on micron and nanoscale. Since the milling of the hydride with the catalyst is not accompanied by a substantial particle- and grain-size reduction, the second decrease of Tonset is essentially smaller. The TG analysis of all polycrystalline and nanocrystalline states was carried out at different heating rates (5, 10 and 20 K/min) in order to obtain Eact for the H-desorption process (see Fig. 5). From the shift of Tinf with increasing heating rate, we can assume that the dehydrogenation process is thermally activated and the well-known Kissinger analysis [25] 2 is plotcan be applied. In this method the logarithm of β/Tinf ted as a function of 1/Tinf , where β is the heating rate. From the slope of the Kissinger plots (Fig. 5, inset), the activation energy of the dehydrogenation can be determined. The calculated values for the poly-MgH2 , nano-MgH2 , nano-15,
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Fig. 3. SEM images corresponding to the poly-MgH2 , nano-MgH2 , nano-15 and nano-90 states.
nano-40 and nano-90 states are plotted in Fig. 6. An effect of particle- and grain-size reduction on Eact can not be observed, the values of the poly-MgH2 and nano-MgH2 alloys are equal within the experimental error. Both values are in the range of the reported activation energies of decomposition for conventional polycrystalline -MgH2 material (120–142 kJ/mol [26–28]) as well as similar to that reported for ball-milled nanocrystalline MgH2 [29].
By adding the catalyst, it is found that there exists an optimal milling time when the activation energy is the lowest (83 ± 9 kJ/mol for the nano-15 powder). Hence, although the very short-time milling (15 min) leads to incomplete mixing of the hydride and catalyst particles, the catalytic effect of the Nb2 O5 is well pronounced. This value is slightly higher than those reported for nanocrystalline MgH2 + Nb2 O5 (65 kJ/mol) by Bormann and co-workers
Fig. 4. TG curves obtained at heating rate of 20 K/min for the poly-MgH2 , nano-MgH2 , nano-15, nano-40 and nano-90 states. The inset shows the decrease of Tonset as a function of milling time.
Fig. 5. TG curves obtained at different heating rates for the nano-40 powder. The inset shows the Kissinger plots for the poly-MgH2 , nano-MgH2 , nano15, nano-40 and nano-90 states.
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Acknowledgements The work has been supported by the EU HPRN-CT-200200208. The authors thank T. Klassen (GKSS, Germany) for the supply with the nanocrystalline MgH2 . The work has partially been supported by the Hungarian Scientific Research Fund (OTKA) under grants T043247 and T046990. The authors are indebted for T. Ung´ar and G. Rib´arik to their assistance in the XRD evaluation procedure.
References
Fig. 6. Eact values as a function of the milling time obtained from the Kissinger plots.
[12] and by Huot et al. [14] (63 kJ/mol), probably due to some unavoidable surface oxidation, mainly occurring during the TG analysis, despite of the protective pure Ar atmosphere. Longer milling times (40–90 min) result in subsequent increase of Eact , most probably due to the penetration of the catalyst particles into the interior of the hydride ones. Similar results were obtained in our recent study [19], showing that shorter milling time of MgH2 + Nb2 O5 leads to faster hydrogen absorption compared to long-time-milled MgH2 + Nb2 O5 . Another possible explanation of the deleterious effect of the milling duration on the H-sorption kinetics could be the partial reduction of the Nb2 O5 by Mg during the high-energy milling. This hypothesis is, however, less probable, since the XRD does not reveal noticeable reaction between Mg/MgH2 and Nb2 O5 .
4. Conclusions The dehydriding of ball-milled MgH2 with Nb2 O5 as catalyst was studied and the effect of the particle- and grainsize as well as of the milling duration on the temperature of hydrogen desorption and on the dehydriding kinetics was analyzed. The grain- and particle-size reduction decrease the H-desorption temperature. The size-reduction occurs during the first 120 min of MgH2 milling, but subsequent milling with the catalyst results in an additional slight MgH2 particle size decrease, due to the harder Nb2 O5 particles compared to the MgH2 . The catalytic effect of Nb2 O5 , expressed in lowering the activation energy of dehydriding, was observed after only 15 min of milling nano-MgH2 with the catalyst. This effect is, however, suppressed at longer milling, probably due to penetration of the catalytic particles into the MgH2 .
[1] R.C. Bowman, B. Fultz, MRS Bull. 27 (2002) 688. [2] K.H.J. Buschow, P.C.P. Bouten, A.R. Miedema, Rep. Prog. Phys. 45 (1982) 937. [3] J.-L. Bobet, C. Even, Y. Nakamura, E. Akiba, B. Darriet, J. Alloys Compd. 298 (2000) 279. [4] Y. Tsushi, H. Enoki, E. Akiba, J. Alloys Compd. 285 (1999) 298. [5] L. Zaluski, A. Zaluska, P. Tessier, J.O. Str¨om-Olsen, R. Schulz Mater. Sci. Forum 225 (1996) 853. [6] G. Liang, S. Boily, J. Huot, A. Van Neste, R. Schulz, J. Alloys Compd. 267 (1998) 302. [7] G. Liang, S. Boily, J. Huot, A. Van Neste, R. Schulz, J. Alloys Compd. 268 (1998) 302. [8] G. Liang, J. Alloys Compd. 370 (2004) 123. [9] W. Oelerich, T. Klassen, R. Bormann, J. Alloys Compd. 315 (2001) 237. [10] A. Zaluska, L. Zaluski, J.O. Str¨om-Olsen, J. Alloys Compd. 288 (1999) 217. [11] G. Barkhordarian, T. Klassen, R. Bormann, Scr. Mater. 49 (2003) 213. [12] G. Barkhordarian, T. Klassen, R. Bormann, J. Alloys Compd. 364 (2004) 242. [13] R. Schulz, J. Huot, G. Liang, S. Boily, G. Lalande, M.C. Denis, J.P. Dodelet, Mater. Sci. Eng. A 267 (1999) 240. [14] J. Huot, J.F. Pelletier, L.B. Lurio, M. Sutton, R. Schulz, J. Alloys Compd. 348 (2003) 319. [15] C.X. Shang, M. Bououdina, Z.X. Guo, J. Alloys Compd. 349 (2003) 217. [16] W. Oelerich, T. Klassen, R. Bormann, J. Alloys Compd. 322 (2001) L5. [17] Z. Dehouche, T. Klassen, W. Oelerich, J. Goyotte, T.K. Bose, R. Schulz, J. Alloys Compd. 347 (2002) 319. [18] J.-L. Bobet, S. Desmoulins-Krawiec, E. Grigorova, F. Cansell, B. Chevalier, J. Alloys Compd. 351 (2003) 217. ´ R´ev´esz, D. F´atay, D. Zander, T. Spassov, J. Metast. Nanocr. [19] A. Mater., in press. [20] S. Orimo, H. Fujii, Appl. Phys. A 72 (2001) 167. [21] T. Spassov, U. K¨oster, J. Alloys Compd. 279 (1998) 279. [22] N. Hanada, T. Ichikawa, S. Orimo, H. Fujii, J. Alloys Compd. 366 (2004) 269. [23] T. Ung´ar, J. Gubicza, G. Rib´arik, A. Borb´ely, J. Appl. Cryst. 34 (2001) 298. [24] G. Rib´arik, J. Gubicza, T. Ung´ar, Mater. Sci. Eng. A A387 (2004) 343. [25] H.E. Kissinger, Anal. Chem. 29 (1957) 1702. [26] A. Karty, J. Grunzweig-Genossar, P.S. Rudman, J. Appl. Phys. 50 (1979) 7200. [27] J.S. Han, M. Pezat, J.Y. Lee, J. Less-Common Met. 130 (1987) 395. [28] C.M. Stander, J. Inorg. Nucl. Chem. 39 (1977) 221. [29] J. Huot, G. Liang, S. Boily, A.V. Neste, R. Schulz, J. Alloys Compd. 293–295 (1999) 495.
Particle size and catalytic effect on the dehydriding of MgH2 ´ R´ev´esz a,b,∗ , T. Spassov a D. F´atay a,b , A. b
a Department of Chemistry, University of Sofia “St. Kl. Ohridski”, Sofia, Bulgaria Department of General Physics, E¨otv¨os University, Budapest, H-1518, P.O. Box 32, Budapest, Hungary
Received 17 February 2005; accepted 28 February 2005 Available online 31 March 2005
Abstract Conventional polycrystalline -MgH2 was ball-milled under hydrogen-atmosphere to reach a nanocrystalline microstructure (∼10 nm), until the particle- and grain-size attained a constant value. Subsequently, 2 mol.% pre-milled Nb2 O5 (∼40 nm) was added to the MgH2 as a catalyst and the milling was continued. The effect of the particle- and grain-size as well as of the milling duration on the temperature of hydrogen desorption and on the dehydriding kinetics of the ball-milled MgH2 + Nb2 O5 powders was analyzed. The grain- and particle-size reduction decreases substantially the H-desorption temperature. The size-reduction occurs during the first 120 min of milling MgH2 , but subsequent milling with the catalyst results in additional slight MgH2 particle size decrease, due to the harder Nb2 O5 particles compared to the MgH2 ones. The decrease of the activation energy of dehydriding caused by the catalyst was observed after only 15 min of ball-milling. The catalytic effect is, however, suppressed at longer milling times, probably due to penetration of the catalytic particles into the MgH2 . © 2005 Elsevier B.V. All rights reserved. Keywords: Nanostructure; Ball-milling; X-ray diffraction
1. Introduction Due to a series of special characteristics of hydrogen storage in metals, such as capacity, light weight, low environmental impact, low cost and high natural abundance, many metal–hydrogen systems have been studied as hydrogenstorage materials, in which magnesium is considered as one of the most promising candidates mainly because of its high storage capacity (7.6 wt.% in MgH2 ) [1]. However, because of its high thermodynamic stability (H = −75 kJ/mol) [2], high hydrogen desorption temperature (higher than 400 ◦ C) and relatively poor hydrogen absorption/desorption kinetics at temperatures below 350 ◦ C, the application of MgH2 is impeded. To convert Mg completely to MgH2 requires more than 50 h at 350 ◦ C [3]. The poor kinetics of Mg-based alloys is generally attributed to the low H2 -dissociation ability of metallic Mg and the formation
∗
Corresponding author. Tel.: +36 1 372 28 23; fax: +36 1 372 2811. ´ R´ev´esz). E-mail address: [email protected] (A.
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.02.043
of hydrides at the surface, which hinders further H-transport into the matrix. Numerous attempts have been made to overcome these difficulties. For example, alloying improves significantly the thermodynamic and kinetic properties of magnesium, but decreases the hydrogen storage capacity. Metallic elements, such as V, Ti, Ni, Fe, Mn, Pd, Nb have been selected as additives to magnesium [4]. Recently, ball-milling (BM) and mechanical alloying (MA) have been developed for processing nanocrystalline Mg under a hydrogen gas environment (so called reactive mechanical milling (RMM) or reactive mechanical alloying (RMA)) in order to reach special advantages, such as improving the thermodynamic and kinetic properties of hydriding. By reducing the grain size of Mg to nanocrystalline dimensions the H-sorption kinetics are accelerated substantially [5–9]. Zaluska et al. have shown that nanocrystalline Mg with a crystalline size of 30 nm can absorb about 6 wt.% H in 120 min at 573 K and Mg2 Ni can absorb 2.1 wt.% H in 40 min at 473 K [10]. Furthermore, it was demonstrated that reducing the grain size of Mg to the nanoscale range,
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the hydrogen desorption temperature (Tdes ) is decreased by about 100 ◦ C [9,11,12]. It has also been noted that milling the hydride instead of the pure Mg, produces novel nanostructures with a high surface area beneficial for hydrogen sorption kinetics [13]. Further significant improvement of the hydriding properties was performed by adding metals [14–16] and oxides [11,12,17–19] as catalysts. Bormann and co-workers have shown that the H-sorption of ball-milled nanocrystalline Mg/MgH2 (∼10 nm) with 0.2 mol.% Nb2 O5 as catalyst takes place completely in less than 10 min at 300 ◦ C [11,12]. So far, the best kinetic results on hydrogenation have been reported by Barkhordarian et al. [12] showing that at 573 K, Mg with 0.5 mol.% Nb2 O5 can absorb 7 wt.% H in 60 s. Although excellent kinetics was found for higher temperature (523 K), no results at lower temperatures (lower than 523 K) have been reported. So far, contrary results were found on the influence of the microstructure on the maximum H-capacity of Mg and Mg alloys. Whereas nanocrystalline Mg2 Ni-based alloys reveal an improvement of H-capacity in the extremely fine nanocrystalline state (7–8 nm) [20,21], the capacity of nanocrystalline Mg/MgH2 (∼10 nm) is reduced by about 6% with respect to its coarser counterpart (grain size of about 20 nm and particle size of about 1 m) [22]. It has already been proved that the grain size determines the favorable H-sorption kinetics, but for Mg/MgH2 , unlike Mg2 Ni, this is accomplished by a decrease in the H-capacity [20–22]. The influence of the particle size on the H-sorption kinetics is, however, still not sufficiently studied. In our recent study [19], it was proved that in addition to the grain size, the particle reduction (to <1 m region) improves the H-sorption kinetics of nanocrystalline Mg/MgH2 powders. Mg/MgH2 powders with the same grain size (∼50 nm) and different particle size in the range 0.1–1 m were investigated. Furthermore, it was shown that the time of milling of the nanocrystalline MgH2 with a catalyst (Nb2 O5 ) influences the kinetics. Therefore, in the present study, fine nanocrystalline MgH2 powders (with particle size ≤2 m and grain size <20 nm) were produced and then milled additionally for different times with the catalyst. The influence of the milling time of the nanocrystalline MgH2 with the catalyst on the H-desorption kinetics and Tdes was investigated.
2. Experimental Commercial polycrystalline MgH2 (poly-MgH2 ) powder was ball-milled in a self-constructed planetary-type mill for 120 min under a H-atmosphere of 6 atm in order to obtain nanocrystalline material (nano-MgH2 ). Using a Hatmosphere prevents the decomposition of MgH2 during milling. The nano-MgH2 powder was further milled with 2 mol.% of pre-milled Nb2 O5 as catalyst for different times: 15, 40 and 90 min. Hereafter, these states will be denoted as nano-15, nano-40 and nano-90, respectively.
The evolution of the microstructure during milling was monitored by X-ray powder diffraction (XRD) with Cu K␣ radiation on a Philips X’pert powder diffractometer in θ–2θ geometry. The change of the microstructure during the BM process was systematically studied by characterization of X-ray lineprofile analysis. This evaluation technique is based on the separation of size and strain effects. Line-profile analysis was carried out by the convolutional multiple whole profile fitting procedure (CMWP). In this method, the whole measured diffraction pattern is fitted by the sum of a polynomial background and physically well established theoretical profile functions. It is assumed that the grains are spherical and have a lognormal size distribution: − ln (L/m)2 −1/2 −1 −1 σ L exp , (1) G(L) = (2π) 2σ 2 where σ and m are the variance and median of the distribution, respectively. The fitting procedure provides several fitting parameters, such as σ and m, volume-averaged grain sizes: D = m exp(2.5σ 2 ).
(2)
See elsewhere the detailed description of this analysis [23,24]. The morphology of the powders was monitored by scanning electron microscopy (SEM) on a JEOL 5510 apparatus. The H-sorption properties were measured by a PerkinElmer Thermogravimeter (TG) at different heating rates. The onset (Tonset ) and the inflection (Tinf ) of the desorption curves were obtained as the interception of leading edge and the baseline, and as the minimum of the derivative curve, respectively. Continuous heating experiments were performed at scan rates in the range of 5–20 K/min. From the shift of Tinf with increasing heating rate, the apparent activation energy (Eact ) of the hydride decomposition process was determined applying the Kissinger analysis [25].
3. Results and discussion Fig. 1 shows the XRD patterns of the poly-MgH2 , nanoMgH2 , nano-15, nano-40 and nano-90 powders. Significant line-broadening of the tetragonal -MgH2 peaks is clearly seen when the commercial MgH2 powder is ball-milled for 120 min indicating a drastic grain size refinement. The MgH2 phase is known as a low-pressure phase. In addition, the peaks of the high-pressure ␥-MgH2 -phase appears. However, their intensity is considerably lower than that of the phase. Adding the catalyst to MgH2 and subsequent milling results in only a slight additional line-broadening accompanied with the decrease of the relative peak intensities of the -MgH2 phase compared to the Nb2 O5 ones. However, after each milling time, the peaks of the hydride and the catalyst can be separated from each other, indicating that the
D. F´atay et al. / Journal of Alloys and Compounds 399 (2005) 237–241
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Fig. 1. XRD patterns corresponding to the poly-MgH2 , nano-MgH2 , nano15, nano-40 and nano-90. The symbols (䊉), () and ( ) denote the Braggpeaks of the -MgH2 , ␥-MgH2 and Nb2 O5 phases, respectively.
mixing between the catalyst and hydride particles occurs on nanoscale, without any detectable reaction between the two constituents. Due to overlapping, the ␥-MgH2 phase is not detectable when the catalyst is added. The normalized lognormal distribution functions of the -MgH2 and Nb2 O5 , obtained by the CMWP method, are shown in Fig. 2a and b, respectively. It is seen that with increasing milling time the distribution shifts only slightly towards smaller values for the -MgH2 phase, indicating that grain-size refinement is negligible when the hydride is milled additionally together with the catalyst. The calculated D values for the -MgH2 grains in the nano-MgH2 , nano-40 and nano-90 states are 11, 9.5 and 9 nm, respectively. Similarly, no remarkable grain-size refinement occurs in the case of the catalyst (see Fig. 2b); D exhibits a slight decrease from 41 to 37 nm for nano-40 and nano-90, respectively. Fig. 3 shows the corresponding SEM images of the polyMgH2 , nano-MgH2 , nano-15 and nano-90 states. The commercial MgH2 powder exhibits particles with an average size of 40–50 m. After ball-milling for 120 min the MgH2 powder, the average size of the powder particles decreases drastically down to about 1 m. It was also detected that ball-milling the hydride with the catalyst results in some noticeable particle-size reduction after 15 min of ball-milling, probably due to the presence of the harder Nb2 O5 particles. In contrast, we found that milling only nano-MgH2 (without Nb2 O5 ) does not cause noticeable particle- and grain-size reduction. For the nano-40 and nano-90 states, a homogeneous dispersion of the catalyst on the MgH2 particles forms, while the particle size decreases only slightly as well as the average grain size reported above. TG profiles obtained at a heating rate of 10 K/min of the samples are shown in Fig. 4. In the polycrystalline state, the dehydrogenation reaction starts at around Tonset = 431 ◦ C, and it reaches its inflection point at Tinf = 445 ◦ C. The inset in Fig. 4 indicates that achieving the nanocrystalline state results in a remarkable decrease of Tonset down to 388 ◦ C for
Fig. 2. Grain-size histograms of the -MgH2 (a) and Nb2 O5 (b) phases as a function of the milling time.
the nano-MgH2 state. Less pronounced further decrease occurs when MgH2 is milled together with the catalyst (361 ◦ C for the nano-90 state). It is evident that the main decrease in nano-MgH2 , observed between the poly-MgH2 and nanoMgH2 states, is due to the substantial microstructural refinement both on micron and nanoscale. Since the milling of the hydride with the catalyst is not accompanied by a substantial particle- and grain-size reduction, the second decrease of Tonset is essentially smaller. The TG analysis of all polycrystalline and nanocrystalline states was carried out at different heating rates (5, 10 and 20 K/min) in order to obtain Eact for the H-desorption process (see Fig. 5). From the shift of Tinf with increasing heating rate, we can assume that the dehydrogenation process is thermally activated and the well-known Kissinger analysis [25] 2 is plotcan be applied. In this method the logarithm of β/Tinf ted as a function of 1/Tinf , where β is the heating rate. From the slope of the Kissinger plots (Fig. 5, inset), the activation energy of the dehydrogenation can be determined. The calculated values for the poly-MgH2 , nano-MgH2 , nano-15,
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Fig. 3. SEM images corresponding to the poly-MgH2 , nano-MgH2 , nano-15 and nano-90 states.
nano-40 and nano-90 states are plotted in Fig. 6. An effect of particle- and grain-size reduction on Eact can not be observed, the values of the poly-MgH2 and nano-MgH2 alloys are equal within the experimental error. Both values are in the range of the reported activation energies of decomposition for conventional polycrystalline -MgH2 material (120–142 kJ/mol [26–28]) as well as similar to that reported for ball-milled nanocrystalline MgH2 [29].
By adding the catalyst, it is found that there exists an optimal milling time when the activation energy is the lowest (83 ± 9 kJ/mol for the nano-15 powder). Hence, although the very short-time milling (15 min) leads to incomplete mixing of the hydride and catalyst particles, the catalytic effect of the Nb2 O5 is well pronounced. This value is slightly higher than those reported for nanocrystalline MgH2 + Nb2 O5 (65 kJ/mol) by Bormann and co-workers
Fig. 4. TG curves obtained at heating rate of 20 K/min for the poly-MgH2 , nano-MgH2 , nano-15, nano-40 and nano-90 states. The inset shows the decrease of Tonset as a function of milling time.
Fig. 5. TG curves obtained at different heating rates for the nano-40 powder. The inset shows the Kissinger plots for the poly-MgH2 , nano-MgH2 , nano15, nano-40 and nano-90 states.
D. F´atay et al. / Journal of Alloys and Compounds 399 (2005) 237–241
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Acknowledgements The work has been supported by the EU HPRN-CT-200200208. The authors thank T. Klassen (GKSS, Germany) for the supply with the nanocrystalline MgH2 . The work has partially been supported by the Hungarian Scientific Research Fund (OTKA) under grants T043247 and T046990. The authors are indebted for T. Ung´ar and G. Rib´arik to their assistance in the XRD evaluation procedure.
References
Fig. 6. Eact values as a function of the milling time obtained from the Kissinger plots.
[12] and by Huot et al. [14] (63 kJ/mol), probably due to some unavoidable surface oxidation, mainly occurring during the TG analysis, despite of the protective pure Ar atmosphere. Longer milling times (40–90 min) result in subsequent increase of Eact , most probably due to the penetration of the catalyst particles into the interior of the hydride ones. Similar results were obtained in our recent study [19], showing that shorter milling time of MgH2 + Nb2 O5 leads to faster hydrogen absorption compared to long-time-milled MgH2 + Nb2 O5 . Another possible explanation of the deleterious effect of the milling duration on the H-sorption kinetics could be the partial reduction of the Nb2 O5 by Mg during the high-energy milling. This hypothesis is, however, less probable, since the XRD does not reveal noticeable reaction between Mg/MgH2 and Nb2 O5 .
4. Conclusions The dehydriding of ball-milled MgH2 with Nb2 O5 as catalyst was studied and the effect of the particle- and grainsize as well as of the milling duration on the temperature of hydrogen desorption and on the dehydriding kinetics was analyzed. The grain- and particle-size reduction decrease the H-desorption temperature. The size-reduction occurs during the first 120 min of MgH2 milling, but subsequent milling with the catalyst results in an additional slight MgH2 particle size decrease, due to the harder Nb2 O5 particles compared to the MgH2 . The catalytic effect of Nb2 O5 , expressed in lowering the activation energy of dehydriding, was observed after only 15 min of milling nano-MgH2 with the catalyst. This effect is, however, suppressed at longer milling, probably due to penetration of the catalytic particles into the MgH2 .
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