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Catalytic effect of niobium oxide on hydrogen storage properties of mechanically ball milled MgH2
ARTICLE IN PRESS
Physica B 383 (2006) 49–50 www.elsevier.com/locate/physb
Catalytic effect of niobium oxide on hydrogen storage properties of mechanically ball milled MgH2 Nobuko Hanada, Takayuki Ichikawa, Hironobu Fujii Materials Science Center, N-BARD, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan
Abstract We examined a catalytic effect of niobium oxide (Nb2O5) on the hydrogen storage properties of MgH2 prepared by mechanical ball milling method. The MgH2 composite doped with 1 mol% Nb2O5 by ball milling for 20 h desorbed hydrogen up to 6 mass% in the temperature range from 200 to 250 1C at the heating rate of 5 1C/min under a purified helium flow. After dehydrogenation at 200 1C, the product showed remarkable hydrogen absorption kinetics. A large amount of gaseous hydrogen up to 4.5 mass% was absorbed even at room temperature under 1 MPa hydrogen pressure within 15 s and finally its capacity reached up to 5 mass%. Furthermore, the valence state of Nb2O5 doped in MgH2 was examined by X-ray absorption near edge structure (XANES) measurement. The results indicated that additive Nb2O5 was reduced by MgH2 during mechanical milling. This suggests that the Nb compound, in which the valence state of Nb atom is less than 5+, acts as a catalyst for the hydrogen absorbing/desorbing kinetics. r 2006 Elsevier B.V. All rights reserved. PACS: 61.10.Ht; 81.05.Je; 84.60.Ye Keywords: Hydrogen storage materials; Magnesium hydride; Catalytic effect; Mechanical milling
Magnesium is one of the attractive hydrogen storage materials, because it has high hydrogen capacity of 7.6 mass%. But the reaction speed of hydrogen absorption/ desorption reaction is too low for practical use. The surface modification of Mg by additives is needed because Mg does not have active surface for H2 molecule. We have investigated the hydrogen storage properties of the MgH2 composites with a small amount of some transition metals and metal oxides produced by ball milling method [1–4]. Among those additives, the MgH2 composite with niobium oxide (Nb2O5) exhibited the most superior hydrogen desorption properties. Barkhordarian et al. also reported that Nb2O5 revealed the best catalytic effect on the hydrogen storage properties of Mg among the oxide catalysts in their trials [5,6]. The MgH2 catalyzed with 0.2 mol% by ball milling for 100 h desorbs 7 mass% hydrogen within 150 s under vacuum and absorbs 7
Corresponding author. Tel.: +81 82 424 5744; fax: +81 82 424 7486.
E-mail address: [email protected] (T. Ichikawa). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.03.051
mass% hydrogen with in 60 s under 0.84 MPa hydrogen pressure [5]. In this paper, we present our recent results of the hydrogen absorption properties and the X-ray absorption near edge structure (XANES) measurements for MgH2 catalyzed with 1 mol% Nb2O5. The milling time for 20 h, which was regarded to be the optimized treatment in our trials, was necessary to obtain the suitable catalytic effect on hydrogen desorption properties for the MgH2 composite with 1 mol% Nb2O5 [2,3]. The composite milled for 20 h desorbed 6 mass% hydrogen in the temperature range from 200 to 250 1C at the heating rate of 5 1C/min under a purified helium flow atmosphere. The ball milled product was dehydrogenated under a high vacuum condition for 8 h at 200 1C (catalyzed Mg) and rehydrogenated under a pure hydrogen gas of 1 MPa for 8 h at 200 1C (catalyzed MgH2). Fig. 1 shows hydrogen absorption properties of the catalyzed Mg at 3 different temperatures, room temperature RT (20 1C), 150 and 200 1C. We notice that a large amount of hydrogen up to about 4.5 mass% is absorbed within 15 s even at room temperature under 1 MPa hydrogen gas atmosphere.
ARTICLE IN PRESS N. Hanada et al. / Physica B 383 (2006) 49–50
50
1.2
150°C RT
1.0
4 H2 abs. (mass%)
H2 absorption(mass%)
5
250°C
Normalized absorbance
6
3 2 1
250°C
6
150°C 5 RT
milling for 20 h dehydrogenation rehydrogenation Nb
0.8 0.6
Nb2O5 0.4
4 0
5000
0 0
10
20 time (sec)
10000 time(sec) 30
15000
0.2 40
Fig. 1. The amount of hydrogen absorption as a function of reaction time for the catalyzed Mg at room temperature RT (20 1C), 150 and 250 1C under 1 MPa hydrogen.
At 150 and 250 1C, gaseous hydrogen more than 5.0 mass% is absorbed under 1 MPa within 30 s. Finally, the total amount of the hydrogen absorption reaches up to 5.0, 5.7 and 5.7 mass%, respectively, at RT, 150 and 250 1C. Furthermore, the catalyzed MgH2 after rehydrogenation desorbed 5 mass% hydrogen at 160 1C within 100 min under a purified helium flow [4]. From the Kissinger plot, the activation energy for hydrogen desorption was estimated to be 71 kJ/mol H2, which is comparable to enthalpy change DH74 kJ/mol H2 of hydrogen desorption reaction from MgH2. This indicates that the activation energy of hydrogen desorption is sufficiently decreased by the catalytic effect of Nb2O5. To investigate the additive condition in MgH2, the valence state of Nb2O5 doped in MgH2 was examined by XANES measurement, which was performed at BL19 on SPring-8. Fig. 2 shows the XANES profiles of Nb K-edge for the ball milled product, the catalyzed Mg after dehydrogenation and the catalyzed MgH2 after rehydrogenation. In this figure, the XANES profiles for commercial Nb and Nb2O5 are also shown as references. We notice that the Nb K-edge XANES patterns for all products are located between Nb
0.0 18970
18980
18990 19000 Energy [eV]
19010
19020
Fig. 2. Nb K-edge XANES profiles for the ball milled product, the catalyzed Mg and the catalyzed MgH2. The XANES profiles for Nb metal and Nb2O5 are also shown as the references.
phase and Nb2O5 one. This result suggests that the additive Nb2O5 is reduced by MgH2 during the ball milling and becomes other Nb compound, in which the valence state of the Nb atom is less than 5+. Therefore, it seems likely that the Nb compound acts as a catalyst for the hydrogen sorption reactions in the product for the dissociation of molecular hydrogen and recombination of hydrogen atoms. This work was supported by the Grant-in-Aid for COE Research (No. 13CE2002) of the Ministry of Education, Sciences and Culture of Japan.
References [1] [2] [3] [4] [5] [6]
N. Hanada, et al., J. Phys. Chem. B 109 (2005) 7188. T. Ichikawa, et al., Mater. Trans. 46 (2005) 1. N. Hanada, et al., J. Alloys Compd. 404–406 (2005) 716. N. Hanada, et al., J. Alloys Compd., in press. G. Barkhordarian, et al., Scr. Mater. 49 (2003) 213. G. Barkhordarian, et al., J. Alloys Compd. 364 (2004) 242.
Physica B 383 (2006) 49–50 www.elsevier.com/locate/physb
Catalytic effect of niobium oxide on hydrogen storage properties of mechanically ball milled MgH2 Nobuko Hanada, Takayuki Ichikawa, Hironobu Fujii Materials Science Center, N-BARD, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan
Abstract We examined a catalytic effect of niobium oxide (Nb2O5) on the hydrogen storage properties of MgH2 prepared by mechanical ball milling method. The MgH2 composite doped with 1 mol% Nb2O5 by ball milling for 20 h desorbed hydrogen up to 6 mass% in the temperature range from 200 to 250 1C at the heating rate of 5 1C/min under a purified helium flow. After dehydrogenation at 200 1C, the product showed remarkable hydrogen absorption kinetics. A large amount of gaseous hydrogen up to 4.5 mass% was absorbed even at room temperature under 1 MPa hydrogen pressure within 15 s and finally its capacity reached up to 5 mass%. Furthermore, the valence state of Nb2O5 doped in MgH2 was examined by X-ray absorption near edge structure (XANES) measurement. The results indicated that additive Nb2O5 was reduced by MgH2 during mechanical milling. This suggests that the Nb compound, in which the valence state of Nb atom is less than 5+, acts as a catalyst for the hydrogen absorbing/desorbing kinetics. r 2006 Elsevier B.V. All rights reserved. PACS: 61.10.Ht; 81.05.Je; 84.60.Ye Keywords: Hydrogen storage materials; Magnesium hydride; Catalytic effect; Mechanical milling
Magnesium is one of the attractive hydrogen storage materials, because it has high hydrogen capacity of 7.6 mass%. But the reaction speed of hydrogen absorption/ desorption reaction is too low for practical use. The surface modification of Mg by additives is needed because Mg does not have active surface for H2 molecule. We have investigated the hydrogen storage properties of the MgH2 composites with a small amount of some transition metals and metal oxides produced by ball milling method [1–4]. Among those additives, the MgH2 composite with niobium oxide (Nb2O5) exhibited the most superior hydrogen desorption properties. Barkhordarian et al. also reported that Nb2O5 revealed the best catalytic effect on the hydrogen storage properties of Mg among the oxide catalysts in their trials [5,6]. The MgH2 catalyzed with 0.2 mol% by ball milling for 100 h desorbs 7 mass% hydrogen within 150 s under vacuum and absorbs 7
Corresponding author. Tel.: +81 82 424 5744; fax: +81 82 424 7486.
E-mail address: [email protected] (T. Ichikawa). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.03.051
mass% hydrogen with in 60 s under 0.84 MPa hydrogen pressure [5]. In this paper, we present our recent results of the hydrogen absorption properties and the X-ray absorption near edge structure (XANES) measurements for MgH2 catalyzed with 1 mol% Nb2O5. The milling time for 20 h, which was regarded to be the optimized treatment in our trials, was necessary to obtain the suitable catalytic effect on hydrogen desorption properties for the MgH2 composite with 1 mol% Nb2O5 [2,3]. The composite milled for 20 h desorbed 6 mass% hydrogen in the temperature range from 200 to 250 1C at the heating rate of 5 1C/min under a purified helium flow atmosphere. The ball milled product was dehydrogenated under a high vacuum condition for 8 h at 200 1C (catalyzed Mg) and rehydrogenated under a pure hydrogen gas of 1 MPa for 8 h at 200 1C (catalyzed MgH2). Fig. 1 shows hydrogen absorption properties of the catalyzed Mg at 3 different temperatures, room temperature RT (20 1C), 150 and 200 1C. We notice that a large amount of hydrogen up to about 4.5 mass% is absorbed within 15 s even at room temperature under 1 MPa hydrogen gas atmosphere.
ARTICLE IN PRESS N. Hanada et al. / Physica B 383 (2006) 49–50
50
1.2
150°C RT
1.0
4 H2 abs. (mass%)
H2 absorption(mass%)
5
250°C
Normalized absorbance
6
3 2 1
250°C
6
150°C 5 RT
milling for 20 h dehydrogenation rehydrogenation Nb
0.8 0.6
Nb2O5 0.4
4 0
5000
0 0
10
20 time (sec)
10000 time(sec) 30
15000
0.2 40
Fig. 1. The amount of hydrogen absorption as a function of reaction time for the catalyzed Mg at room temperature RT (20 1C), 150 and 250 1C under 1 MPa hydrogen.
At 150 and 250 1C, gaseous hydrogen more than 5.0 mass% is absorbed under 1 MPa within 30 s. Finally, the total amount of the hydrogen absorption reaches up to 5.0, 5.7 and 5.7 mass%, respectively, at RT, 150 and 250 1C. Furthermore, the catalyzed MgH2 after rehydrogenation desorbed 5 mass% hydrogen at 160 1C within 100 min under a purified helium flow [4]. From the Kissinger plot, the activation energy for hydrogen desorption was estimated to be 71 kJ/mol H2, which is comparable to enthalpy change DH74 kJ/mol H2 of hydrogen desorption reaction from MgH2. This indicates that the activation energy of hydrogen desorption is sufficiently decreased by the catalytic effect of Nb2O5. To investigate the additive condition in MgH2, the valence state of Nb2O5 doped in MgH2 was examined by XANES measurement, which was performed at BL19 on SPring-8. Fig. 2 shows the XANES profiles of Nb K-edge for the ball milled product, the catalyzed Mg after dehydrogenation and the catalyzed MgH2 after rehydrogenation. In this figure, the XANES profiles for commercial Nb and Nb2O5 are also shown as references. We notice that the Nb K-edge XANES patterns for all products are located between Nb
0.0 18970
18980
18990 19000 Energy [eV]
19010
19020
Fig. 2. Nb K-edge XANES profiles for the ball milled product, the catalyzed Mg and the catalyzed MgH2. The XANES profiles for Nb metal and Nb2O5 are also shown as the references.
phase and Nb2O5 one. This result suggests that the additive Nb2O5 is reduced by MgH2 during the ball milling and becomes other Nb compound, in which the valence state of the Nb atom is less than 5+. Therefore, it seems likely that the Nb compound acts as a catalyst for the hydrogen sorption reactions in the product for the dissociation of molecular hydrogen and recombination of hydrogen atoms. This work was supported by the Grant-in-Aid for COE Research (No. 13CE2002) of the Ministry of Education, Sciences and Culture of Japan.
References [1] [2] [3] [4] [5] [6]
N. Hanada, et al., J. Phys. Chem. B 109 (2005) 7188. T. Ichikawa, et al., Mater. Trans. 46 (2005) 1. N. Hanada, et al., J. Alloys Compd. 404–406 (2005) 716. N. Hanada, et al., J. Alloys Compd., in press. G. Barkhordarian, et al., Scr. Mater. 49 (2003) 213. G. Barkhordarian, et al., J. Alloys Compd. 364 (2004) 242.