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MgH2 system for hydrogen storage properties
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 2 9 2 e3 2 9 7
Available online at www.sciencedirect.com
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Influence of metal oxide on LiBH4/2LiNH2/MgH2 system for hydrogen storage properties Huiping Yuan*, Xugang Zhang, Zhinian Li, Jianhua Ye, Xiumei Guo, Shumao Wang, Xiaopeng Liu, Lijun Jiang General Research Institute for Nonferrous Metals, No. 2 Xinjiekou Wai Street, Beijing 100088, China
article info
abstract
Article history:
In this study, various nanoscale metal oxide catalysts, such as CeO2, TiO2, Fe2O3, Co3O4, and
Received 22 July 2011
SiO2, were added to the LiBH4/2LiNH2/MgH2 system by using high-energy ball milling.
Received in revised form
Temperature programmed desorption and MS results showed that the LieMgeBeNeH/
10 November 2011
oxide mixtures were able to dehydrogenate at much lower temperatures. The order of the
Accepted 11 November 2011
catalytic effect of the studied oxides was Fe2O3 > Co3O4 > CeO2 > TiO2 > SiO2. The onset
Available online 10 December 2011
dehydrogenation temperature was below 70 C for the samples doped with Fe2O3 and Co3O4 with 10 wt.%. More than 5.4 wt.% hydrogen was released at 140 C. X-ray diffraction
Keywords:
indicated that the addition of metal oxides inhibited the formation of Mg(NH2)2 during ball
Hydrogen storage material
milling processes. It is thought that the changing of the ball milling products results from
LiNH2
the interaction of oxide ions in metal oxide catalysts with hydrogen atoms in MgH2. The
LiBH4
catalytic effect depends on the activation capability of oxygen species in metal oxides on
MgH2
hydrogen atoms in hydrides.
Metal oxide catalyst
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
At present, one of the largest obstacles for hydrogen-fueled vehicles is the lack of effective on-board hydrogen storage media. Materials with high gravimetric and volumetric hydrogen capacity and fast discharge/recharge kinetics under moderate temperature and pressure conditions need to be obtained to solve this problem. In the past 10 years, the complex hydride materials [1], such as alanates [2,3], amides [4,5], borohydrides [6,7], and their combinations with magnesium hydrides [8e10], have been largely focused on and investigated. However, the unfavorable thermodynamics and kinetics and the low hydrogen capacity still limit their applications. Mixed complex hydrides (LiBH4 and LiNH2) with conventional hydride (MgH2) in a molar ratio of 1:2:1 have been
recently reported by Yang et al. [11]. This is a “self-catalyzing” system with comparatively fast kinetics, low desorption temperature, and almost complete suppression of NH3 release. It was found that the mixture could release up to 8.6 wt.% hydrogen for the temperature starting at 423 K via multiple reactions. Niemann et al. reported that the nanoscale engineering through ball milling reduced the onset hydrogen release temperature of the mixture to 353 K [12]. The dehydrogenation and rehydrogenation processes and the effect of the stoichiometry on the hydrogen storage properties of this ternary complex hydride system have also been investigated [13]. To further improve the hydrogen storage behavior, Srinvasan et al added various nanosized metals to this ternary complex hydride system in concentrations ranging from 2 to 10 mol%. It was found that the additives of Co and Ni could lower the hydrogen release temperature by 75e100 C on the
* Corresponding author. Tel.: þ86 01 82241238. E-mail address: [email protected] (H. Yuan). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.11.065
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 2 9 2 e3 2 9 7
major decomposition step [14]. Keeping these aspects in view, in this study we will try to add other catalysts to this multinary complex hydride system to further enhance the hydrogen desorption properties, such as the hydrogen desorption temperature and the kinetics. Metal oxides have been proved to be effective catalysts for a number of solid-state hydrides absorption/desorption reactions [15,16]. There are wide possible ranges of metal oxide composites. They possess good capacity of valence alternation. Some metal oxide nanoframeworks also serve additional roles beyond catalysis through interactions with hydrides [17,18]. In the present study, the addition of nanosized metal oxides, such as CeO2, TiO2, Fe2O3, Co3O4, and SiO2, to the LiBH4/2LiNH2/MgH2 system was investigated. These nanosized additives were chosen as they possessed large surface areas, having effective interactions with the hydride system. The structural features and the hydrogen desorption properties of this system with and without the addition of metal oxides were compared. The catalytic effects of different metal oxides on this system were discussed.
2.
Experimental methods
The compounds LiBH4 and LiNH2 were purchased from SigmaeAldrich with a purity of 95%. They were used as received. The nanoscale metal oxides were purchased from Beijing Nachen with a purity of 99.9%. The received metal oxides were heated at 150 C for 2 h under vacuum to remove the surface adsorbed water. The particle sizes of CeO2, TiO2, Fe2O3, Co3O4, and SiO2 are 20 nm, 10 nm, 30 nm, 20 nm, and 30 nm, respectively. The MgH2 was home-made. Magnesium powder was sealed in a stainless steel vial for high-energy ball-milling under 3 MPa H2 pressure for 40 h, and then it was rehydrogenated for three times to get high purity MgH2. The rehydrogenation was performed at 350 C under 50 bar hydrogen. The particle size of MgH2 is 0.3 mm. The investigated composites of LiBH4, LiNH2, and MgH2 in a molar ratio of 1:2:1 with an addition of 10 wt.% metal oxides were prepared by high-energy ball milling the corresponding chemicals under an argon atmosphere of 1 bar using a Spex-8000M apparatus. The ball to powder weight ratio was about 25:1. LiBH4 and LiNH2 were first milled for 5 h, producing the quaternary structure hydride Li4BN3H10, and then MgH2 and various metal oxides (CeO2, TiO2, Fe2O3, Co3O4, and SiO2) were added and milled for an additional 5 h. All the sample preparation processes were performed in an Ar-filled glove-box with water vapor and oxygen concentration both less than 1 ppm. The phase structures were measured by X-ray diffraction (XRD) using X’Pert Pro MPD diffractometer with CuKa radiation. The temperature dependence of the decomposition was monitored by a combined system of a simultaneous thermal analysis NETZSCA STA409PC and a mass spectrometer (MS) HiDEN HPR20. Samples were heated under the flow of purified Ar at 30 mL/min. The volumetric desorption measurements were performed on a standard Sievert’s type apparatus. Temperature-programmed-desorption (TPD) tests were carried out on a home-built apparatus [19]. Precise pressure measurement (0.3&) and temperature control (0.1 C) were accomplished using a high-precision pressure transducer and
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an electric oven, respectively. The purities of hydrogen gas and argon gas are 99.999% and 99.9% respectively. About 400 mg of sample was used each time. The IR absorption spectra were collected in diffuse reflectance infrared Fourier transform mode at resolutions of 4 cm1. The samples were mixed with paraffine in an appropriate proportion to ensure cutting off air.
3.
Results and discussion
The dehydrogenation behaviors of the post-milled LiBH4/ 2LiNH2/MgH2 system with and without metal oxides were investigated using TPD apparatus. The desorption curves obtained at a heating rate of 1 C min1 were shown in Fig. 1. The pristine sample showed a clear two-step dehydrogenation process starting at 170 C and 325 C. The desorption process was the same with the results reported by Yang et al. [11]. According to their research, in low temperature desorption process, Li4BN3H10 melts and reacts with MgH2 to form Li2Mg(NH)2 and LiBH4, and then reversible reaction happens between Mg(NH2)2 and LiH. In high temperature desorption process, Li2Mg(NH)2 reacts with LiBH4 to form Li3BN2, Mg3N2, and H2. The samples doped with CeO2, TiO2, and SiO2 showed similar curve types with the pristine sample. However, a new hydrogen release process starting at 70 C appeared when the sample doped with Fe2O3 or Co3O4. It indicates a new reaction happened. At the same time, the temperature of the second desorption process moved to 220 C, which was 100 C lower than the pristine sample. The total capacity of the hydrogen releasing decreased to 6.9e7.9 wt.% after the addition of different metal oxides. They were lower than the capacity of 8.6 wt.% for the pristine sample. The MS results of the hydrogen desorption accords well with the TPD results. As can be seen in Fig. 2, there appears a new dehydrogenation peak below 120 C after the addition of metal oxides. The order of the onset temperature of the new peak for the studied metal oxides was Fe2O3 < Co3O4 < CeO2 < TiO2 < SiO2. The effects of the oxide additives on the second
Fig. 1 e Hydrogen kinetic desorption date as a function of the temperature at a heating rate of 1 C minL1 under 1 bar of hydrogen for LiBH4/2LiNH2/MgH2 composites with and without metal oxides.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 2 9 2 e3 2 9 7
Fig. 2 e MS results of hydrogen as a function of the temperature at a heating rate of 5 C minL1 for LiBH4/ 2LiNH2/MgH2 composites with and without metal oxides. Insert shows the enlarged view of the first hydrogen release peak.
hydrogen release peak were different from the first peak. The sample doped with CeO2 exhibited the lowest hydrogen release temperature at 169 C, which was 13 C lower than the pristine sample at 182 C. In addition, not all the oxide additives brought out lower temperatures for the third hydrogen release peak. The samples doped with Fe2O3 and Co3O4 showed very low hydrogen release peaks at about 234 C. The hydrogen release peak of Fe2O3 doped sample was sharper than the peak of Co3O4 doped sample, indicating the more rapid desorption rate. It is obvious that the addition of metal oxides can remarkably enhance the hydrogen desorption at low temperature range. Metal oxide doped samples all exhibited the hydrogen release peak below 120 C, which was not observed on metal or other compound catalyzed samples. It suggests that this hydrogen desorption reaction is induced by the existence of oxygen species. The samples collected after the ball-milling and the dehydrogenation at 350 C for 60 min were characterized using the XRD method. The identity and relative abundance of the post-milled phases changed when the pristine samples were doped with metal oxides. As shown in Fig. 3(a), diffraction patterns assigned to new phases of Mg(NH2)2, Li4BN3H10, and the residual MgH2 were observed in the post-milled pristine sample, which was the same with the results previously reported for the LiBH4/2LiNH2/MgH2 composites. After the addition of metal oxides, the major composites contained only Li4BN3H10 and MgH2. This suggests that metal oxide additives inhibit the formation of Mg(NH2)2 during the ballmilling processes. The peaks marked with different symbols in Fig. 3(a) exhibited the existence of Co3O4, CeO2, TiO2, and SiO2. Fe2O3 was partially reduced to FeO. At the same time, MgO was detected in the samples doped with Co3O4, Fe2O3, and CeO2. We think some metal oxides may react with MgH2 to form MgO and be partially reduced. The phase changes of the ball-milled samples have also been proved by IR spectroscopy. As can be seen in Fig. 4, the
Fig. 3 e X-ray patterns of LiBH4/2LiNH2/MgH2 composites with and without metal oxides. (a) Samples after the ball milling, (b) Samples after the dehydrogenation at 350 C for 60 min.
pristine sample showed the signature of NeH stretching vibration of Mg(NH2)2 at 3328 and 3272 cm1 and Li4BN3H10 at 3296 and 3242 cm1. However, after the addition of metal oxides, the observation of NeH stretching vibration of Li4BN3H10 were at 3300 and 3242 cm1, and NeH stretching vibration of Mg(NH2)2 could not be observed. The formation of Mg(NH2)2 was inhibited. In addition, the signature of NeH bending vibration of the post-milled pristine sample located at 1572 cm1. The wave-number shifted to 1550 cm1 after the addition of metal oxides. The negative shift of the wavenumber suggests the weakening of NeH bonds, which leads to the lower hydrogen desorption temperatures. However, no apparent differences were detected between the samples doped with different metal oxides. Therefore, we think oxygen species affect the products during the ball milling process.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 2 9 2 e3 2 9 7
Fig. 4 e FTIR spectra of the ball-milled LiBH4/2LiNH2/MgH2 composites with and without metal oxides.
On the basis of the analysis by Yang et al. [11], the reactions happened during ball milling and upon subsequent heating, but before the onset of hydrogen release are as follows: 3LiNH2 þ LiBH4 /Li4 BN3 H10
(1)
2Li4 BN3 H10 þ 3MgH2 /3MgðNH2 Þ2 þ 2LiBH4 þ 6LiH
(2)
According to XRD and IR results, reaction (2) was inhibited by the addition of various metal oxides. The ball milling products contained Li4BN3H10, MgH2, metal oxides, and a little MgO. As was reported in the literature, when MgH2 was mixed with metal oxides, there were strong interactions between them [20,21]. The fast sorption kinetics of MgH2/MxOy-systems could be introduced at the surface of the metal oxide particles during high-energy ball milling [21]. Before the dissociation of hydrogen molecules can take place, hydrogen atoms with higher atomization energy in MgH2 tend to adsorb at the surface of metal oxides. Although the content of the metal oxide was small in our study, they are with nano particle sizes, contain high density defects, and have large surface areas. This interaction keeps the MgH2 from reacting with Li4BN3H10 during the ball milling process. In Fig. 3(b) the products of the pristine sample contained Li3BN2, Mg3N2, LiMgBN2, and Phase X [11] after the dehydrogenation at 350 C for 60 min. With the addition of metal oxides, the content of MgO with different phase structures (30-0794 and 87-0652) increased and a new phase LiBO2 appeared for SiO2 doped sample. The results also indicate the strong interactions between MgH2 and metal oxides. In addition, the DSC curves (Fig. 5) showed the first hydrogen release process was exothermic for the LiBH4/2LiNH2/MgH2 composites doped with different metal oxides. The endothermic peak corresponding to the phase transformation of LiBH4 disappeared. It was reported in the literature the dehydrogenation of various metal catalyzed LiB0.33N0.67H2.67 was exothermic [22,23]. Li4BN3H10 can release hydrogen at a relatively low temperature of about 120 C, when it was added with NiCl2 [24]. Therefore, we think that the exothermic peak may originate from the desorption of Li4BN3H10. It was reported the characters of MgH2, such as its crystallite size, remarkably affected the hydrogen release
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Fig. 5 e DSC results of the ball-milled LiBH4/2LiNH2/MgH2 composites with and without metal oxides (at a heating rate of 5 C minL1).
temperature of the ternary LiNH2/MgH2/LiBH4 hydrogen storage system [25]. Based on the analysis above, the metal oxide may interact with MgH2 and enhance the desorption kinetics together with MgH2. Hirate et al. have found the atomization energy of oxide ions multiplied by their number in the molecular formula decide the activation capability of metal oxides on hydrogen atoms in hydrogen storage materials [26]. The experiments and theoretical calculations showed that transition metal oxides were better catalysts than non-transition metal oxides and iron oxides were better catalysts than other transition metal oxides, such as TiO2. These results are in accordance with our experiments. Therefore, we think that the atomization energy of the oxide ion is a good parameter to estimate the catalytic effect of metal oxide on the hydrogen desorption reaction of this system. The samples doped with Fe2O3, Co3O4, and CeO2 exhibited predominant desorption properties and were chosen to be further investigated. The temperatures of the prior two hydrogen release peaks at different heating rates (1, 2, 3, and 5 C min1) were acquired using the DSC measurements. The reaction kinetic parameters were determined by using the KissingereAkahiraeSunose approach [10]. ln b=T2p ¼ lnðAR=Ea Þ Ea =RT In the above equation, Tp is the peak temperature, A is the pre-exponential factor, b is the heating rate, Ea is the activation energy, and R is the gas constant. The activation energy and the pre-exponential factor can be calculated from the slope and the intercept of the fitted line (see Fig. 6). The Kissinger-plots have good linearity for the LiBH4/2LiNH2/MgH2 composites and the samples doped with Fe2O3, Co3O4, and CeO2. An addition of 10 wt.% metal oxides leads to a decrease of the activation energy and an increase of the reaction rate constant for the first hydrogen release process. The activation energy Ea of the corresponding exothermic peak is with the order of Fe2O3 < Co3O4 < CeO2. The rate constant k at 80 C was
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 2 9 2 e3 2 9 7
Fig. 7 e Hydrogen desorption isotherms (at 140 C to vacuum) of the LiBH4/2LiNH2/MgH2 composites and the samples doped with Fe2O3, Co3O4, and CeO2. Insert shows the enlarged view for the time before 150 min.
Fig. 6 e Kissinger plots of the LiBH4/2LiNH2/MgH2 composites and the samples doped with Fe2O3, Co3O4, and CeO2. (a) Kissinger plots derived from the low temperature exothermic peak, (b) Kissinger plots derived from the high temperature endothermic peak.
determined by the Arrhenius equation (k ¼ Aexp(Ea/RT)). They are 30.35, 6.56, and 3.75 min1 for the Fe2O3, Co3O4, and CeO2 doped samples, respectively. However, the decreases of the kinetic barrier were not observed on all these three oxide doped samples for the second hydrogen release process. Activation energy of the endothermic peak is with the order of CeO2 < Pristine sample < Fe2O3 < Co3O4. Because of the effect of the exothermic reaction, the reactants and the mechanism of the follow-up hydrogen desorption reaction changed, which altered the activation energy. The rate constant k is 1.98 103, 2.66 103, 3.32 103, and 7.29 103 min1 for the pristine sample, the Co3O4 doped sample, the Fe2O3 doped sample, and the CeO2 doped sample, respectively. The catalytic effect of the metal oxide can accelerate the kinetics of the two hydrogen desorption processes in low temperature region. Fig. 7 shows the hydrogen desorption isotherms of the LiBH4/2LiNH2/MgH2 composites doped with Fe2O3, Co3O4 and CeO2. At the initial stage (time < 25 min) the hydrogen desorption rates of the samples doped with Fe2O3 and Co3O4 were faster than the rates of the sample doped with CeO2 and the pristine sample. With the increase of the time, the
hydrogen desorption rate of the sample doped with CeO2 became the fastest. This change is accordant with the results above. The samples doped with Fe2O3 and Co3O4 can release as high as 5.4 wt.% hydrogen at 140 C in 20 h. Ferric oxide and Co3O4 can remarkably accelerate the hydrogen desorption rate and increase the hydrogen release capacity in low temperature region. But it is a pity that only 1.8 wt.% of the capacity were reversible. The reaction mechanism needs to be further investigated to evaluate the interaction of metal oxides with this system.
4.
Conclusions
To further improve the hydrogen storage performance of the LieMgeBeNeH system, various nanosized metal oxides were added as the catalysts. It was found that the metal oxides could enhance the kinetics and decrease the temperatures of the hydrogen desorption reactions in low temperature region. Among them, Fe2O3 and Co3O4 exhibited the remarkable catalytic effects. The samples doped with Fe2O3 and Co3O4 can start to decompose at below 70 C and release 5.4 wt.% hydrogen at 140 C. The interactions of oxygen species in metal oxides with hydrogen atoms in MgH2 is in favor of decreasing the hydrogen desorption temperatures of this system. The catalytic effect depends on the activation capability of the oxygen species in metal oxides on the hydrogen atoms in hydrides.
Acknowledgements The authors wish to acknowledge the financial supports provided by International S&T Cooperation Program of China (2010DFA52180) and the National Basic Research Program of China (Grant No. 2010CB631305) under the Ministry of Science and Technology of China.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 2 9 2 e3 2 9 7
references [14] [1] Demirci UB, Akdim O, Miele P. Ten-year efforts and a no-go recommendation for sodium borohydride for on-board automotive hydrogen storage. International Journal of Hydrogen Energy 2009;34:2638e45. [2] Verkuijlen MHW, Gao JB, Adelhelm P, van Bentum PJM, de Jongh PE, Kentgens APM. Solid-state NMR studies of the local structure of NaAlH4/C nanocomposites at different stages of hydrogen desorption and rehydrogenation. Journal of Physical Chemistry C 2010;114:4683e92. [3] Lohstroh W, Roth A, Hahn H, Fichtner M. Thermodynamic effects in nanoscale NaAlH4. Chem Phys Chem 2010;11: 789e92. [4] Chen P, Xiong ZT, Luo JZ, Lin JY, Tan KL. Interaction of hydrogen with metal nitrides and imides. Nature 2002;420: 302e4. [5] Wang JH, Liu T, Wu GT, Li W, Liu YF, Araujo CM, et al. Potassium-modified Mg(NH2)2/2LiH system for hydrogen storage. Angewandte Chemie International Edition 2009;48: 5828e32. [6] Ravnsbœk D, Filinchuk Y, Cerenius Y, Jakobsen HJ, Besenbacher F, Skibsted J, et al. A series of mixed-metal borohydrides. Angewandte Chemie International Edition 2009;48:6659e63. [7] Mao JF, Guo ZP, Poh CK, Ranjbar A, Guo YH, Yu XB, et al. Study on the dehydrogenation kinetics and thermodynamics of Ca(BH4)2. Journal of Alloys and Compounds 2010;500: 200e5. [8] Sheppard DA, Paskevicius M, Buckley CE. Hydrogen desorption from the NaNH2eMgH2 system. Journal of Physical Chemistry C 2011;115:8407e13. [9] Wang WJ, Li WJ, Li QZ. Theoretical study of the interaction between LiNH2 and HMgH. International Journal of Quantum Chemistry 2011;111:675e81. [10] Hu JJ, Liu YF, Wu GT, Xiong ZT, Chu YS, Chen P. Improvement of hydrogen storage properties of the LieMgeNeH system by addition of LiBH4. Chemistry of Materials 2008;20:4398e402. [11] Yang J, Sudik A, Siegel DJ, Halliday D, Drews A, Carter RO, et al. A self-catalyzing hydrogen-storage material. Angewandte Chemie International Edition 2008;47:882e7. [12] Hattrick-Simpers JR, Maslar JE, Niemann MU, Chiu C, Srinivasan SS, Stefanakos EK, et al. Raman spectroscopic observation of dehydrogenation in ball-milled LiNH2eLiBH4eMgH2 nanoparticles. International Journal of Hydrogen Energy 2010;35:6323e31. [13] Sudik A, Yang J, Siegel DJ, Wolverton C, Carter RO, Drews AR. Impact of stoichiometry on the hydrogen storage properties
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Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Influence of metal oxide on LiBH4/2LiNH2/MgH2 system for hydrogen storage properties Huiping Yuan*, Xugang Zhang, Zhinian Li, Jianhua Ye, Xiumei Guo, Shumao Wang, Xiaopeng Liu, Lijun Jiang General Research Institute for Nonferrous Metals, No. 2 Xinjiekou Wai Street, Beijing 100088, China
article info
abstract
Article history:
In this study, various nanoscale metal oxide catalysts, such as CeO2, TiO2, Fe2O3, Co3O4, and
Received 22 July 2011
SiO2, were added to the LiBH4/2LiNH2/MgH2 system by using high-energy ball milling.
Received in revised form
Temperature programmed desorption and MS results showed that the LieMgeBeNeH/
10 November 2011
oxide mixtures were able to dehydrogenate at much lower temperatures. The order of the
Accepted 11 November 2011
catalytic effect of the studied oxides was Fe2O3 > Co3O4 > CeO2 > TiO2 > SiO2. The onset
Available online 10 December 2011
dehydrogenation temperature was below 70 C for the samples doped with Fe2O3 and Co3O4 with 10 wt.%. More than 5.4 wt.% hydrogen was released at 140 C. X-ray diffraction
Keywords:
indicated that the addition of metal oxides inhibited the formation of Mg(NH2)2 during ball
Hydrogen storage material
milling processes. It is thought that the changing of the ball milling products results from
LiNH2
the interaction of oxide ions in metal oxide catalysts with hydrogen atoms in MgH2. The
LiBH4
catalytic effect depends on the activation capability of oxygen species in metal oxides on
MgH2
hydrogen atoms in hydrides.
Metal oxide catalyst
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
At present, one of the largest obstacles for hydrogen-fueled vehicles is the lack of effective on-board hydrogen storage media. Materials with high gravimetric and volumetric hydrogen capacity and fast discharge/recharge kinetics under moderate temperature and pressure conditions need to be obtained to solve this problem. In the past 10 years, the complex hydride materials [1], such as alanates [2,3], amides [4,5], borohydrides [6,7], and their combinations with magnesium hydrides [8e10], have been largely focused on and investigated. However, the unfavorable thermodynamics and kinetics and the low hydrogen capacity still limit their applications. Mixed complex hydrides (LiBH4 and LiNH2) with conventional hydride (MgH2) in a molar ratio of 1:2:1 have been
recently reported by Yang et al. [11]. This is a “self-catalyzing” system with comparatively fast kinetics, low desorption temperature, and almost complete suppression of NH3 release. It was found that the mixture could release up to 8.6 wt.% hydrogen for the temperature starting at 423 K via multiple reactions. Niemann et al. reported that the nanoscale engineering through ball milling reduced the onset hydrogen release temperature of the mixture to 353 K [12]. The dehydrogenation and rehydrogenation processes and the effect of the stoichiometry on the hydrogen storage properties of this ternary complex hydride system have also been investigated [13]. To further improve the hydrogen storage behavior, Srinvasan et al added various nanosized metals to this ternary complex hydride system in concentrations ranging from 2 to 10 mol%. It was found that the additives of Co and Ni could lower the hydrogen release temperature by 75e100 C on the
* Corresponding author. Tel.: þ86 01 82241238. E-mail address: [email protected] (H. Yuan). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.11.065
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 2 9 2 e3 2 9 7
major decomposition step [14]. Keeping these aspects in view, in this study we will try to add other catalysts to this multinary complex hydride system to further enhance the hydrogen desorption properties, such as the hydrogen desorption temperature and the kinetics. Metal oxides have been proved to be effective catalysts for a number of solid-state hydrides absorption/desorption reactions [15,16]. There are wide possible ranges of metal oxide composites. They possess good capacity of valence alternation. Some metal oxide nanoframeworks also serve additional roles beyond catalysis through interactions with hydrides [17,18]. In the present study, the addition of nanosized metal oxides, such as CeO2, TiO2, Fe2O3, Co3O4, and SiO2, to the LiBH4/2LiNH2/MgH2 system was investigated. These nanosized additives were chosen as they possessed large surface areas, having effective interactions with the hydride system. The structural features and the hydrogen desorption properties of this system with and without the addition of metal oxides were compared. The catalytic effects of different metal oxides on this system were discussed.
2.
Experimental methods
The compounds LiBH4 and LiNH2 were purchased from SigmaeAldrich with a purity of 95%. They were used as received. The nanoscale metal oxides were purchased from Beijing Nachen with a purity of 99.9%. The received metal oxides were heated at 150 C for 2 h under vacuum to remove the surface adsorbed water. The particle sizes of CeO2, TiO2, Fe2O3, Co3O4, and SiO2 are 20 nm, 10 nm, 30 nm, 20 nm, and 30 nm, respectively. The MgH2 was home-made. Magnesium powder was sealed in a stainless steel vial for high-energy ball-milling under 3 MPa H2 pressure for 40 h, and then it was rehydrogenated for three times to get high purity MgH2. The rehydrogenation was performed at 350 C under 50 bar hydrogen. The particle size of MgH2 is 0.3 mm. The investigated composites of LiBH4, LiNH2, and MgH2 in a molar ratio of 1:2:1 with an addition of 10 wt.% metal oxides were prepared by high-energy ball milling the corresponding chemicals under an argon atmosphere of 1 bar using a Spex-8000M apparatus. The ball to powder weight ratio was about 25:1. LiBH4 and LiNH2 were first milled for 5 h, producing the quaternary structure hydride Li4BN3H10, and then MgH2 and various metal oxides (CeO2, TiO2, Fe2O3, Co3O4, and SiO2) were added and milled for an additional 5 h. All the sample preparation processes were performed in an Ar-filled glove-box with water vapor and oxygen concentration both less than 1 ppm. The phase structures were measured by X-ray diffraction (XRD) using X’Pert Pro MPD diffractometer with CuKa radiation. The temperature dependence of the decomposition was monitored by a combined system of a simultaneous thermal analysis NETZSCA STA409PC and a mass spectrometer (MS) HiDEN HPR20. Samples were heated under the flow of purified Ar at 30 mL/min. The volumetric desorption measurements were performed on a standard Sievert’s type apparatus. Temperature-programmed-desorption (TPD) tests were carried out on a home-built apparatus [19]. Precise pressure measurement (0.3&) and temperature control (0.1 C) were accomplished using a high-precision pressure transducer and
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an electric oven, respectively. The purities of hydrogen gas and argon gas are 99.999% and 99.9% respectively. About 400 mg of sample was used each time. The IR absorption spectra were collected in diffuse reflectance infrared Fourier transform mode at resolutions of 4 cm1. The samples were mixed with paraffine in an appropriate proportion to ensure cutting off air.
3.
Results and discussion
The dehydrogenation behaviors of the post-milled LiBH4/ 2LiNH2/MgH2 system with and without metal oxides were investigated using TPD apparatus. The desorption curves obtained at a heating rate of 1 C min1 were shown in Fig. 1. The pristine sample showed a clear two-step dehydrogenation process starting at 170 C and 325 C. The desorption process was the same with the results reported by Yang et al. [11]. According to their research, in low temperature desorption process, Li4BN3H10 melts and reacts with MgH2 to form Li2Mg(NH)2 and LiBH4, and then reversible reaction happens between Mg(NH2)2 and LiH. In high temperature desorption process, Li2Mg(NH)2 reacts with LiBH4 to form Li3BN2, Mg3N2, and H2. The samples doped with CeO2, TiO2, and SiO2 showed similar curve types with the pristine sample. However, a new hydrogen release process starting at 70 C appeared when the sample doped with Fe2O3 or Co3O4. It indicates a new reaction happened. At the same time, the temperature of the second desorption process moved to 220 C, which was 100 C lower than the pristine sample. The total capacity of the hydrogen releasing decreased to 6.9e7.9 wt.% after the addition of different metal oxides. They were lower than the capacity of 8.6 wt.% for the pristine sample. The MS results of the hydrogen desorption accords well with the TPD results. As can be seen in Fig. 2, there appears a new dehydrogenation peak below 120 C after the addition of metal oxides. The order of the onset temperature of the new peak for the studied metal oxides was Fe2O3 < Co3O4 < CeO2 < TiO2 < SiO2. The effects of the oxide additives on the second
Fig. 1 e Hydrogen kinetic desorption date as a function of the temperature at a heating rate of 1 C minL1 under 1 bar of hydrogen for LiBH4/2LiNH2/MgH2 composites with and without metal oxides.
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Fig. 2 e MS results of hydrogen as a function of the temperature at a heating rate of 5 C minL1 for LiBH4/ 2LiNH2/MgH2 composites with and without metal oxides. Insert shows the enlarged view of the first hydrogen release peak.
hydrogen release peak were different from the first peak. The sample doped with CeO2 exhibited the lowest hydrogen release temperature at 169 C, which was 13 C lower than the pristine sample at 182 C. In addition, not all the oxide additives brought out lower temperatures for the third hydrogen release peak. The samples doped with Fe2O3 and Co3O4 showed very low hydrogen release peaks at about 234 C. The hydrogen release peak of Fe2O3 doped sample was sharper than the peak of Co3O4 doped sample, indicating the more rapid desorption rate. It is obvious that the addition of metal oxides can remarkably enhance the hydrogen desorption at low temperature range. Metal oxide doped samples all exhibited the hydrogen release peak below 120 C, which was not observed on metal or other compound catalyzed samples. It suggests that this hydrogen desorption reaction is induced by the existence of oxygen species. The samples collected after the ball-milling and the dehydrogenation at 350 C for 60 min were characterized using the XRD method. The identity and relative abundance of the post-milled phases changed when the pristine samples were doped with metal oxides. As shown in Fig. 3(a), diffraction patterns assigned to new phases of Mg(NH2)2, Li4BN3H10, and the residual MgH2 were observed in the post-milled pristine sample, which was the same with the results previously reported for the LiBH4/2LiNH2/MgH2 composites. After the addition of metal oxides, the major composites contained only Li4BN3H10 and MgH2. This suggests that metal oxide additives inhibit the formation of Mg(NH2)2 during the ballmilling processes. The peaks marked with different symbols in Fig. 3(a) exhibited the existence of Co3O4, CeO2, TiO2, and SiO2. Fe2O3 was partially reduced to FeO. At the same time, MgO was detected in the samples doped with Co3O4, Fe2O3, and CeO2. We think some metal oxides may react with MgH2 to form MgO and be partially reduced. The phase changes of the ball-milled samples have also been proved by IR spectroscopy. As can be seen in Fig. 4, the
Fig. 3 e X-ray patterns of LiBH4/2LiNH2/MgH2 composites with and without metal oxides. (a) Samples after the ball milling, (b) Samples after the dehydrogenation at 350 C for 60 min.
pristine sample showed the signature of NeH stretching vibration of Mg(NH2)2 at 3328 and 3272 cm1 and Li4BN3H10 at 3296 and 3242 cm1. However, after the addition of metal oxides, the observation of NeH stretching vibration of Li4BN3H10 were at 3300 and 3242 cm1, and NeH stretching vibration of Mg(NH2)2 could not be observed. The formation of Mg(NH2)2 was inhibited. In addition, the signature of NeH bending vibration of the post-milled pristine sample located at 1572 cm1. The wave-number shifted to 1550 cm1 after the addition of metal oxides. The negative shift of the wavenumber suggests the weakening of NeH bonds, which leads to the lower hydrogen desorption temperatures. However, no apparent differences were detected between the samples doped with different metal oxides. Therefore, we think oxygen species affect the products during the ball milling process.
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Fig. 4 e FTIR spectra of the ball-milled LiBH4/2LiNH2/MgH2 composites with and without metal oxides.
On the basis of the analysis by Yang et al. [11], the reactions happened during ball milling and upon subsequent heating, but before the onset of hydrogen release are as follows: 3LiNH2 þ LiBH4 /Li4 BN3 H10
(1)
2Li4 BN3 H10 þ 3MgH2 /3MgðNH2 Þ2 þ 2LiBH4 þ 6LiH
(2)
According to XRD and IR results, reaction (2) was inhibited by the addition of various metal oxides. The ball milling products contained Li4BN3H10, MgH2, metal oxides, and a little MgO. As was reported in the literature, when MgH2 was mixed with metal oxides, there were strong interactions between them [20,21]. The fast sorption kinetics of MgH2/MxOy-systems could be introduced at the surface of the metal oxide particles during high-energy ball milling [21]. Before the dissociation of hydrogen molecules can take place, hydrogen atoms with higher atomization energy in MgH2 tend to adsorb at the surface of metal oxides. Although the content of the metal oxide was small in our study, they are with nano particle sizes, contain high density defects, and have large surface areas. This interaction keeps the MgH2 from reacting with Li4BN3H10 during the ball milling process. In Fig. 3(b) the products of the pristine sample contained Li3BN2, Mg3N2, LiMgBN2, and Phase X [11] after the dehydrogenation at 350 C for 60 min. With the addition of metal oxides, the content of MgO with different phase structures (30-0794 and 87-0652) increased and a new phase LiBO2 appeared for SiO2 doped sample. The results also indicate the strong interactions between MgH2 and metal oxides. In addition, the DSC curves (Fig. 5) showed the first hydrogen release process was exothermic for the LiBH4/2LiNH2/MgH2 composites doped with different metal oxides. The endothermic peak corresponding to the phase transformation of LiBH4 disappeared. It was reported in the literature the dehydrogenation of various metal catalyzed LiB0.33N0.67H2.67 was exothermic [22,23]. Li4BN3H10 can release hydrogen at a relatively low temperature of about 120 C, when it was added with NiCl2 [24]. Therefore, we think that the exothermic peak may originate from the desorption of Li4BN3H10. It was reported the characters of MgH2, such as its crystallite size, remarkably affected the hydrogen release
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Fig. 5 e DSC results of the ball-milled LiBH4/2LiNH2/MgH2 composites with and without metal oxides (at a heating rate of 5 C minL1).
temperature of the ternary LiNH2/MgH2/LiBH4 hydrogen storage system [25]. Based on the analysis above, the metal oxide may interact with MgH2 and enhance the desorption kinetics together with MgH2. Hirate et al. have found the atomization energy of oxide ions multiplied by their number in the molecular formula decide the activation capability of metal oxides on hydrogen atoms in hydrogen storage materials [26]. The experiments and theoretical calculations showed that transition metal oxides were better catalysts than non-transition metal oxides and iron oxides were better catalysts than other transition metal oxides, such as TiO2. These results are in accordance with our experiments. Therefore, we think that the atomization energy of the oxide ion is a good parameter to estimate the catalytic effect of metal oxide on the hydrogen desorption reaction of this system. The samples doped with Fe2O3, Co3O4, and CeO2 exhibited predominant desorption properties and were chosen to be further investigated. The temperatures of the prior two hydrogen release peaks at different heating rates (1, 2, 3, and 5 C min1) were acquired using the DSC measurements. The reaction kinetic parameters were determined by using the KissingereAkahiraeSunose approach [10]. ln b=T2p ¼ lnðAR=Ea Þ Ea =RT In the above equation, Tp is the peak temperature, A is the pre-exponential factor, b is the heating rate, Ea is the activation energy, and R is the gas constant. The activation energy and the pre-exponential factor can be calculated from the slope and the intercept of the fitted line (see Fig. 6). The Kissinger-plots have good linearity for the LiBH4/2LiNH2/MgH2 composites and the samples doped with Fe2O3, Co3O4, and CeO2. An addition of 10 wt.% metal oxides leads to a decrease of the activation energy and an increase of the reaction rate constant for the first hydrogen release process. The activation energy Ea of the corresponding exothermic peak is with the order of Fe2O3 < Co3O4 < CeO2. The rate constant k at 80 C was
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Fig. 7 e Hydrogen desorption isotherms (at 140 C to vacuum) of the LiBH4/2LiNH2/MgH2 composites and the samples doped with Fe2O3, Co3O4, and CeO2. Insert shows the enlarged view for the time before 150 min.
Fig. 6 e Kissinger plots of the LiBH4/2LiNH2/MgH2 composites and the samples doped with Fe2O3, Co3O4, and CeO2. (a) Kissinger plots derived from the low temperature exothermic peak, (b) Kissinger plots derived from the high temperature endothermic peak.
determined by the Arrhenius equation (k ¼ Aexp(Ea/RT)). They are 30.35, 6.56, and 3.75 min1 for the Fe2O3, Co3O4, and CeO2 doped samples, respectively. However, the decreases of the kinetic barrier were not observed on all these three oxide doped samples for the second hydrogen release process. Activation energy of the endothermic peak is with the order of CeO2 < Pristine sample < Fe2O3 < Co3O4. Because of the effect of the exothermic reaction, the reactants and the mechanism of the follow-up hydrogen desorption reaction changed, which altered the activation energy. The rate constant k is 1.98 103, 2.66 103, 3.32 103, and 7.29 103 min1 for the pristine sample, the Co3O4 doped sample, the Fe2O3 doped sample, and the CeO2 doped sample, respectively. The catalytic effect of the metal oxide can accelerate the kinetics of the two hydrogen desorption processes in low temperature region. Fig. 7 shows the hydrogen desorption isotherms of the LiBH4/2LiNH2/MgH2 composites doped with Fe2O3, Co3O4 and CeO2. At the initial stage (time < 25 min) the hydrogen desorption rates of the samples doped with Fe2O3 and Co3O4 were faster than the rates of the sample doped with CeO2 and the pristine sample. With the increase of the time, the
hydrogen desorption rate of the sample doped with CeO2 became the fastest. This change is accordant with the results above. The samples doped with Fe2O3 and Co3O4 can release as high as 5.4 wt.% hydrogen at 140 C in 20 h. Ferric oxide and Co3O4 can remarkably accelerate the hydrogen desorption rate and increase the hydrogen release capacity in low temperature region. But it is a pity that only 1.8 wt.% of the capacity were reversible. The reaction mechanism needs to be further investigated to evaluate the interaction of metal oxides with this system.
4.
Conclusions
To further improve the hydrogen storage performance of the LieMgeBeNeH system, various nanosized metal oxides were added as the catalysts. It was found that the metal oxides could enhance the kinetics and decrease the temperatures of the hydrogen desorption reactions in low temperature region. Among them, Fe2O3 and Co3O4 exhibited the remarkable catalytic effects. The samples doped with Fe2O3 and Co3O4 can start to decompose at below 70 C and release 5.4 wt.% hydrogen at 140 C. The interactions of oxygen species in metal oxides with hydrogen atoms in MgH2 is in favor of decreasing the hydrogen desorption temperatures of this system. The catalytic effect depends on the activation capability of the oxygen species in metal oxides on the hydrogen atoms in hydrides.
Acknowledgements The authors wish to acknowledge the financial supports provided by International S&T Cooperation Program of China (2010DFA52180) and the National Basic Research Program of China (Grant No. 2010CB631305) under the Ministry of Science and Technology of China.
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