Significantly Improved Dehydrogenation of LiAlH4 Destabilized by

May 17, 2012 - Isobe , S.; Yao , H.; Wang , Y. M.; Kawasaki , H.; Hashimoto , N.; Ohnuki , S. Int. J. Hydrogen Energy 2010, 35, 7563– 7567. [Crossre...
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Significantly Improved Dehydrogenation of LiAlH4 Destabilized by MnFe2O4 Nanoparticles Fuqiang Zhai,† Ping Li,† Aizhi Sun,† Shen Wu,† Qi Wan,† Weina Zhang,† Yunlong Li,† Liqun Cui,† and Xuanhui Qu*,†,‡ †

School of Material Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China

‡

S Supporting Information *

ABSTRACT: The effects of nanosized MnFe2O4 additive on the dehydrogenation properties of LiAlH4 prepared by ball milling were investigated for the first time. It was found that the LiAlH4 + 7 mol % MnFe2O4 sample started to decompose at 62 and 119 °C for the first two dehydrogenation stages and released 7.45 wt % hydrogen, which is 88 and 71 °C lower than those of as-received LiAlH4, respectively. The isothermal dehydriding kinetics show that the doped LiAlH4 sample could release about 4.7 wt % hydrogen in 70 min at 90 °C. Furthermore, the first two dehydrogenation steps could be finished within 80 min with 7.44 wt % hydrogen released at 120 °C, whereas as-received LiAlH4 only released about 0.5 wt % hydrogen for the same temperature and time. From differential scanning calorimetry (DSC) and Kissinger desorption kinetics analyses, the apparent activation energies, Ea, of the doped sample were 66.7 kJ/mol for the first dehydrogenation stage and 75.8 kJ/mol for the second dehydrogenation stage, resulting in decreases of 40.2% and 58.1% compared with those of as-received LiAlH4, which are much higher than those of LiAlH4 doped with other reported catalysts calculated by Kissinger method. Through X-ray diffraction (XRD) and Fourier transform infrared (FTIR) observations, in situ formed Fe0.9536O and amorphous Mn or Mn-containing phases together provide a synergetic catalytic effect for the remarkably improved dehydrogenation properties of LiAlH4.

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INTRODUCTION Because of the environment and energy crises, it is critical for us to find a new energy carrier for mobile and stationary applications. Hydrogen is considered as one of the ideal energy carrier candidates for various applications in order to improve the adverse effects of fossil fuels on the environment and reduce the dependence on fossil fuels.1,2 However, searching for efficient and cost-effective hydrogen storage methods has become the major obstacle to a hydrogen-based economy. Currently there are three major hydrogen storage forms: high pressurized, cryogenic liquid, and solid-state hydrogen storage. High pressurized and cryogenic liquid hydrogen storage as relatively traditional ways are both unfavorable due to their extremely utilized conditions.3 Meanwhile, solid-state storage hydrogen has become a promising option due to its high volumetric hydrogen capacity and safety considerations.4,5 Since Bogdanovic and Schwichardi found that doping TiCl3 catalyst could drastically improve the hydrogen storage performances of NaAlH4,6 the solid-stage complex hydrides (LiAlH4, NaAlH4, LiBH4, et al.) are viewed as potential hydrogen storage materials depending on their high gravimetric and volumetric hydrogen storage capacity.7−21 Among these complex hydrides, LiAlH4 has attracted more and more attention due to its relatively larger theoretical hydrogen © 2012 American Chemical Society

capacity. Upon heating, LiAlH4 would decompose according to the following three stages:22 3LiAlH4 → Li3AlH6 + 2Al + 3H 2 (5.3 wt % H 2 , 150−175 °C)

(R1)

Li3AlH6 + 2Al → 3LiH + 3Al + 3/2H 2 (2.6 wt % H 2 , 180−220 °C)

(R2)

3LiH + 3Al → 3LiAl + 3/2H 2 (2.6 wt % H 2 , >400 °C)

(R3)

Usually, researching the dehydrogenation of LiAlH4 only needs to consider the first two dehydrogenation steps because R3 occurs at extreme high temperature compared with the application temperature. Although LiAlH4 possesses the superiority in inherent hydrogen storage capacity, high onset desorption temperature and relatively slow hydrogen desorption kinetics still limit the applications of LiAlH4 on-board. In order to overcome these Received: March 21, 2012 Revised: May 17, 2012 Published: May 17, 2012 11939

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In order to further analyze the dehydrogenation performances and calculate the dehydriding kinetics of the doped LiAlH4 sample by using the Kissinger method, the simultaneous differential scanning calorimetry/thermogravimetric analysis (DSC/TGA) measurements were performed using a NETZSCH STA 449C under a flow of 50 mL/min highpurity Ar. About 5 mg of sample was sealed into a 50 mL alumina crucible in the glovebox, and then was heated at different heating rates (6 °C/min, 9 °C/min, and 12 °C/min) from 35 to 300 °C, respectively. The morphology of the as-received and doped samples was observed by scanning electron microscopy (SEM, ZEISS EVO 18, Germany). Before observation, the samples were sealed into a vessel containing Ar overpressure in the glovebox, and then transferred to the SEM chamber to prevent oxidation and moisture affection. The phase structures of samples before and after dehydrogenation were carried out using a MXP21VAHF Xray diffractometer (XRD with Cu Ka radiation, 40 kV, 200 mA) over the angle from 10° to 90° (2θ) with a scanning velocity of 0.02° per step. Before XRD measurement, the samples were uniformly coated with parafilm to prevent the oxidation during testing. Fourier transform infrared spectroscopy (FTIR) analyses of the as-received and doped samples after ball milling were conducted by using a Bruker Vector 22 FTIR spectrometer. About 0.5 mg of sample and 100 mg dried KBr were mixed and hand-milled in a mortar in a glovebox, and then the mixture was pressed to be sliced for the FTIR test. The tested samples were analyzed in attenuated total reflectance mode (ATR) using the Pike MIRacle accessory equipped with a Ge crystal. The FTIR spectra were recorded from 800 cm−1 to 2000 cm−1 with a spectral resolution of 4 cm−1.

obstacles, there has been a great deal of effort to improve the dehydrogenation properties of LiAlH4 by adding different kinds of catalysts. So far, the investigated catalysts for LiAlH4 have included (1) pure metal such as V,23 Ni,23−25 Fe,23,26,27,30 Ti,1,27−29 Sc,27 and Al;30 (2) alloys such as Ti3Al, TiAl3, Al3Fe, and Al22Fe3Ti8;27,30,31 (3) C species such as carbon black,23 carbon nanofibers,32 and TiC;33 (4) metal halides such as AlCl3,23 FeCl3,23 TiCl3·1/3AlCl3,28,34 VBr3,35 HfCl4,36 ZrCl4,36 LaCl3,37 VCl3,32 TiCl3,38 ZnCl2,38 NiCl2,39 TiF3,40 NbF5,41 MnCl2,42 and K2TiF6;43 (5) other catalysts such as TiH2,23 Ce(SO4)2,44 VCl3&CNFs,32 and SWCNT-metallic.45 To our best knowledge, among the various mentioned catalysts for LiAlH4, there are very few reports on LiAlH4 doped with metal oxide catalysts. Metal oxides have been widely used as catalyst for Mg-based hydrogen storage materials.46−48 For LiAlH4, until now, only three kinds of metal oxides (TiO2,49 Nb2O550 and Cr2O3,50) were reported as catalysts for development of the hydrogen storage performance of LiAlH4, and these reports were just published in 2011. Therefore, metal oxide catalysts are becoming a new researching focus for LiAlH4 in order to further significantly improve the hydrogen storage properties of LiAlH4. As far as we know, no studies have been reported on LiAlH4 doped with Mn ferrite (MnFe2O4). Meanwhile, Varin et al. demonstrated the superior effects of Mn2+ and amorphous Mn on improving the dehydrogenation properties of LiAlH4.42 It has also been confirmed that Fe3+ and Fe could provide favorable effects on enhancing the dehydrogenation properties of LiAlH4.23,26,27,30 Therefore, it is reasonable to believe that MnFe2O4 shows extremely great potential as a catalyst to advance the hydrogen storage properties of LiAlH4 through combining these positive factors together to form metal oxide. In this work, we utilized MnFe2O4 nanoparcticles as catalyst precursors and investigated the effects of MnFe2O4 additive on the hydrogen storage properties of LiAlH4 prepared by ball milling.

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RESULTS AND DISCUSSION Dehydrogenation Properties Analysis. Figure 1 exhibits the nonisothermal dehydrogenation properties of as-received LiAlH4, as-milled LiAlH4, and LiAlH4 doped with 1 mol %, 5 mol %, 7 mol %, and 9 mol % MnFe2O4 nanoparticles. It is obvious that adding MnFe2O4 nanopowders significantly improved the dehydriding properties of LiAlH4, resulting in a drastic reduction of the onset desorption temperature, not only

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EXPERIMANTAL SECTION Fabrication. LiAlH4 (≥95% pure) was purchased from Sigma Aldrich Co., and MnFe2O4 (≥99.99% pure, 20 nm) was obtain from the institute of functional materials at the University of Science and Technology Beijing and prepared by the sol−gel method. LiAlH4 and MnFe2O4 nanopowders were used as received with no additional purification. All experimental operations were performed in a glovebox filled with high-purity Ar in order to avoid oxidation and moisture. About 1 g of LiAlH4 was mixed with different proportions of MnFe2O4 nanoparticles, and then the mixture was load into a stainless milling vial with a ball to powder weight ratio of 15:1. Ball milling was carried out in a high-energy Spex mill at the rate of 400 rpm for 30 min. During the ball milling process, the steel vial should rest for 5 min after milling 10 min to prevent its heating. Characterization. The dehydrogenation and rehydrogenation properties of as-received LiAlH4 and the doped samples were measured by a Sieverts-type pressure−composition− temperature (PCT) apparatus. Typically, 0.5 g of sample was loaded into the sample vessel, and then was heated at a heating rate of 6 °C/min under a controlled vacuum of 0.1 atm. According to the theoretical dehydriding temperature of LiAlH4,22 the heating temperature range of PCT apparatus was set from room temperature to 250 °C.

Figure 1. Thermal desorption curves of as-received LiAlH4, as-milled LiAlH4, and LiAlH4 + 1 mol %, 5 mol %, 7 mol %, and 9 mol % nanosized MnFe2O4. 11940

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for the first step (R1), but also for the second step (R2). As shown in Figure 1, the onset desorption temperatures for the doped samples were all equal to or less than 80 °C, indicating a remarkable decrease compared with that of as-received LiAlH4. For as-received LiAlH4, it started to decompose at 150 °C for the first stage and at 190 °C for the second stage, with a total of 7.63 wt % hydrogen released. Compared with as-received LiAlH4, the onset desorption temperature of as-milled LiAlH4 both slightly decreased by 17.5 °C for the first two dehydrogenation steps due to the activation introduced to the LiAlH4 matrix by mechanical milling.24,40 When doping MnFe2O4 nanoparticles to the LiAlH4 matrix, the onset desorption temperature of LiAlH4 obtained a further remarkable decrease. For the 1 mol % MnFe2O4-doped sample, the dehydrogenation process started at 80 °C for the first stage and initiated at 133 °C for the second stage. Further increasing the additive amount, the 5 mol % doped sample began to decompose at 77 °C and finished at 126 °C for the first stage. Compared with the onset desorption temperature of asreceived LiAlH4, adding 1 mol % and 5 mol % MnFe2O4 caused a decrease in the onset desorption temperature of 70 and 73 °C for the first step, and 57 and 64 °C for the second step, respectively. During the first two dehydrogenation processes, the sample with 1 mol % MnFe2O4 released about 7.53 wt % hydrogen, whereas 7.49 wt % hydrogen was desorbed for the 5 mol % doped sample, which are both close to the theoretical hydrogen content of pristine LiAlH4. Further adding MnFe2O4 to 7 mol %, the first two dehydrogenation stages initiated at 62 and 119 °C, respectively, resulting in further decreasing compared to the 1 mol % and 5 mol % doped samples and a reduction of 88 and 71 °C lower than those of as-received LiAlH4. Meanwhile, the 7 mol % doped sample released 7.45 wt % in the first two dehydrogenation steps, respectively. With the addition of MnFe2O4 up to 9 mol %, the onset dehydrogenation temperature declined to 60 and 116 °C for the first two stages, which indicates an extremely large decline compared with that of LiAlH4 with various catalysts reported in the literature.33−37,41,43,50 However, the desorption hydrogen content for the 9 mol % doped sample was only 3.72 wt % and 0.73 wt % in the first two dehydrogenation steps, signifying a drastic reduction in the released hydrogen capacity after doping an excess amount of MnFe2O4 nanoparticles. This phenomenon could be explained by the fact that the excessive catalytic effect was brought so that amount of hydrogen was released during ball milling. Therefore, through comprehensively considering the above analyses, the LiAlH4 + 7 mol % MnFe2O4 sample exhibited optimal dehydrogenation performances, including the onset dehydrogenation temperature and the released hydrogen capacity, and utilizing 7 mol % as the optimal adding content of MnFe2O4 nanoparticles allowed us to analyze the catalytic effect and mechanism of MnFe2O4 in the following tests. In order to further compare the thermal decomposition performances of LiAlH4 with and without catalyst, Figure 2 shows the DSC/TGA curves of as-received LiAlH4 and the 7 mol % doped sample within the temperature range of 35−300 °C at the heating rate of 6 °C/min. As shown in Figure 2, the DSC plot of as-received LiAlH4 included four characteristic peaks in the first two dehydrogenation stages corresponding to two exothermic peaks and two endothermic peaks. The first exothermic peak appeared at 154.8 °C, corresponding to the interaction between LiAlH4 and surface hydroxyl impurities, and the first endothermic peak occurred at 166.9 °C due to the

Figure 2. DSC/TGA profiles of as-received LiAlH4 and LiAlH4 + 7 mol % MnFe2O4 within the temperature range of 35−300 °C at a heating rate of 6 °C/min.

melting of LiAlH4.51 Furthermore, the second exothermic peak at 184.4 °C was assigned to the dehydrogenation step of liquid LiAlH4 according to the first dehydrogenation stage (R1), and the second endothermic peak at 240.9 °C was attributed to the dehydrogenation step of Li3AlH6 according to R2.52 However, for the doped sample, there were only two characteristic peaks in the DSC curve shown in Figure 2, suggesting that the thermal events for the doped sample was reduced from four to two by adding MnFe2O4 in the first two dehydrogenation processes. First, the exothermic event began to occur at about 70 °C, and the exothermic peak appeared at 128.2 °C, indicating that the MnFe2O 4-doped LiAlH4 started to decompose prior to its melting so that the first endothermic peak for the doped sample corresponds to the decomposition of solid state LiAlH4. Subsequently, the endothermic event initiated at about 150 °C,and the endothermic peak at 186 °C was assigned to the dehydrogenation step of Li3AlH6, which is similar with the DSC results of LiAH4 doped with various catalysts published in the relative literature.30,33,34,40,41,43,49,50 Through analyzing the above DSC results, the remarkable reduction on the peak temperature reveals that the dehydrogenation properties of LiAlH4 were significantly improve by adding nanosized MnFe2O4 dopant. However it is noteworthy that the onset dehydrogenation temperature tested by DSC is quite higher than that measured by PCT, and a similar phenomenon also appeared in the reported literatures,41,43,49 which is mainly due to the different decomposition atmospheres for the tested samples in DSC (1 atm argon) and PCT (0.1 atm vacuum) measurement, resulting in the different driving forces during the desorption process. Meanwhile, through simultaneous TGA measurement, the total amount of hydrogen released for the 7 mol % doped sample achieved 7.45 wt %, as shown in Figure 2, which is in good agreement with that measured by PCT and quite larger than that of LiAlH4 doped with various catalysts reported in the literature.26,28,33,37,38,41,43,45,46,50 The 7 mol % complete dehydrogenated sample was rehydrogenated at 150 °C under 7.5 MPa hydrogen pressure, but reversibility of this system cannot be observed due to the extremely low formation enthalpy of LiAlH4.43,53 In order to make the isothermal test temperature close to the practical operating temperature of PEM fuel cells, three valuable temperatures (90, 120, and 150 °C) for LiAlH4 were chosen to carry out isothermal dehydrogenation measurement. Figure 3 represents the isothermal dehydrogenation kinetics of 11941

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possess the magnetic property to hinder the SEM equipment imaging. Through the above analysis on microstructure, the dehydriding performances of the doped LiAlH4 sample obtained a drastic advance after ball milling due to the decrease in particle size, and crystallite size results in introducing a high surface defect density and creating more grain boundaries. Moreover, a high density of nanosized catalyst particles forms a large amount of nucleation sites at the surface of the LiAlH4 matrix, and the interface mobility of the transformed phase could conduct along the two-dimensional network of grain boundaries, which all leads to the surface activation and larger surface area of LiAlH4 particles. In order to further analyze the dehydrogenation mechanism of LiAlH4 after doping MnFe2O4 nanoparticles in terms of exo/ endothermic characteristics, the apparent activation energy (Ea) of as-received LiAlH4 and the MnFe2O4-doped sample for the first two decomposition steps were obtained by the Kissinger method calculation.53 The Ea values of as-received LiAlH4 for the first two dehydrogenation steps are 111.6 and 180.7 kJ/mol, respectively, while the Ea of the MnFe2O4-doped sample for the first two dehydrogenation steps are calculated to be 66.7 and 75.8 kJ/mol, respectively. Therefore, there is a remarkable reduction of 44.9 and 104.9 kJ/mol in Ea for the two dehydrogenation steps of LiAlH4, indicating that the dehydrogenation kinetics of LiAlH4 obtained a significant improvement by doping MnFe2O4 nanoparticles. For showing the catalytic effect of MnFe2O4 on the dehydrogenation of LiAlH4 in the field of Ea, the comparison of Ea of LiAlH4 doped with different catalysts calculated by the Kissinger method is listed in Table 1. Through comparing with various reported catalysts, the decline rate of the Ea of the doped sample for the first two dehydrogenation steps both obtained the maximum of 40.2% and 58.1% after doping MnFe2O4 nanopowders, which signifies the superiority of MnFe2O4 on improving the dehydriding kinetics of LiAlH4 compared with other reported catalysts. The FTIR spectra of as-milled LiAlH4, and the 1 mol %, 7 mol %, and 9 mol % doped LiAlH4 samples after ball milling are shown in Figure 5. It is clear that there is an IR absorption peak at 1473 cm−1 for the doped samples. Moreover, the intensity of the IR absorption peak at 1473 cm−1 gradually became stronger with increasing MnFe2O4 content, which could be ascribed to the appearance of the Al−H stretching mode of Li3AlH6.35,55 However, no IR absorption peak existed at the same position in the FTIR spectra of the as-milled LiAlH4 in Figure 5. For asmilled LiAlH4, there are two regions of active infrared vibrations of the Al−H bonds: (1) Al−H stretching modes between 1600 and 1800 cm−1; (2) Li−Al−H bending modes

Figure 3. Isothermal dehydrogenation kinetics of (a) as-received LiAlH4 at 120 °C, and LiAlH4 + 7 mol % MnFe2O4 at (b) 90 °C, (c) 120 °C, and (d) 150 °C. (1) presses the first dehydrogenation step; (2) presses the second dehydrogenation step.

as-received LiAlH4 at 120 °C and the 7 mol % doped sample at 90, 120, and 150 °C, respectively. For as-received LiAlH4, only 1.5 wt % hydrogen was released at 120 °C after 180 min dehydrogenation, indicating the poor desorption kinetics of pristine LiAlH4. However, the doped sample could release 4.7 wt % hydrogen within 70 min at 90 °C, indicating the first dehydrogenation step of LiAlH4 almost completed. When heated at 120 °C, the doped sample could finish the first two dehydrogenation steps within 80 min and release about 7.44 wt % hydrogen, whereas as-received LiAlH4 only released less than 0.5 wt % hydrogen for the same temperature and time. Further increasing temperature up to 150 °C, only 32 min was required to complete the first two dehydrogenation steps. Therefore, there is a significant improvement for the dehydrogenation kinetics of as-received LiAlH4 by adding MnFe2O4 nanopowders. Dehydrogenation Mechanism. Figure 4a shows the SEM micrograph of as-received LiAlH4. Microscopically, the microstructure of as-received LiAlH4 powders was shown as an irregular rod, and their particle size was larger than 100 μm. However, the MnFe2O4-doped sample consisted of regular globular particles, and their particle size was distributed between 5 and 20 μm, as shown in Figure 4b. It is worth mentioning that the embedded MnFe2O4 particles could not be observed at the surface of LiAlH4 matrix through the SEM image because the original crystallite size of MnFe2O4 is extremely small (only 20 nm), and the MnFe2O4 particles

Figure 4. SEM micrographs of (a) as-received LiAlH4 and (b) LiAlH4 + 7 mol % MnFe2O4 after ball-milling. 11942

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Table 1. Comparison of Ea of LiAlH4 Doped with Different Catalysts Calculated by the Kissinger Method step 1

step 2

Ea (kJ/mol)

Ea (kJ/mol)

catalyst

before doping

after doping

decline rate (%)

before doping

after doping

decline rate (%)

references

K2TiF6 Nb2O5 TiC TiCl3·1/3AlCl3 MnFe2O4

116.2 86 86 81 111.6

78.2 64.5 59 89 66.7

32.7 25 31.4 0 40.2

133 101 101 108 180.7

90.8 79 70 103 75.8

31.7 21.8 30.7 4.6 58.1

43 50 33 28 this work

Figure 5. FTIR spectra of (a) as-milled LiAlH4 and (b) 1 mol %, (c) 7 mol %, and (d) 9 mol % MnFe2O4-doped LiAlH4 after ball milling.

Figure 6. XRD patterns for as-milled LiAlH4 and LiAlH4 + 1 mol %, 7 mol %, and 9 mol % MnFe2O4 after ball milling.

between 800 and 900 cm−1.43,54,55 Meanwhile, for Li3AlH6, its active infrared vibration exhibits the Al−H stretching modes between 1500 cm−1 and 1400 cm−1.35,55 Therefore, it is reasonable to conclude that the doped LiAlH4 sample incurred partial decomposition and formed Li3AlH6 during the ball milling process, and the decomposition reaction was conducted more severely with increasing MnFe2O4 amount. In order to unveil what reaction happened between LiAlH4 and MnFe2O4 during the ball milling, Figure 6 represents the XRD patterns for as-milled LiAlH4 and the LiAlH4 doped with 1 mol %, 7 mol %, and 9 mol % MnFe2O4 after ball milling. For the as-milled LiAlH4 shown in Figure 6, all diffraction peaks corresponded to LiAlH4 except for the diffraction peaks at 21.4° and 23.8° ascribing to parafilm, and no additional diffraction peaks were detected, indicating that LiAlH4 remains stable during the ball milling process.25,30,41,43 The stability of LiAlH4 could also be confirmed by the same dehydriding capacity for as-received and as-milled LiAlH4 (Figure 1), and the FTIR spectra of as-milled LiAlH4 (Figure 5). However, the weak additional diffraction peaks of microcrystalline aluminum and Li3AlH6 started to appear after doping 1 mol % MnFe2O4. At the same time, the diffraction peaks of Li2Fe3O4 (JCPDS# 37-1432) were observed at 18° and 34.8°, and the diffraction peaks at 36.3°, 42.3°, 61.2° and 73.8° corresponded to Fe0.9536O (JCPDS# 74-1880) for the doped sample in Figure 6, which demonstrates that the reaction between LiAlH4 and MnFe2O4 took place during ball milling by forming a ternary Li−Fe oxide and Fe oxide species with a reduced valence state.

A similar decomposition reaction occurred between LiAlH4 and Nb2O5,50 in which a complete reduction of Nb2O5 happened and subsequently formed LiNbO3 and NbO2. Further raising the MnFe2O4 amount, peaks of the decomposition products including Al, Li3AlH6, Li2Fe3O4, and Fe0.9536O gradually enhanced, while peaks of LiAlH4 continuously declined, suggesting that LiAlH4 reacted with MnFe2O4 and incurred partial decomposition during the ball milling process, and the decomposition became more severe with increasing MnFe2O4 amount. It is slightly surprising that the diffraction peaks of Li3AlH6 were not observed for the 9 mol % doped sample in Figure 6, which is in accordance with the results of Liu et al.40 and Ismail et al.41 In their reports, the LiAlH4 doped with 4 mol % TiF3 or 5 mol % NbF5 did not produce Li3AlH6 diffraction peaks in their XRD diffraction patterns, while the Li3AlH6 phase appeared in LiAlH4 doped with a lower catalyst amount. For all the doped samples, the diffraction peaks of MnFe2O4 were not detected. It is difficult for MnFe2O4 nanoparticles to be distinguished from the noisy background signal, mainly because their extremely small particle size and relatively lower adding proportion results in their intrinsically weak X-ray signal compared to LiAlH4 powders. This phenomenon is in accordance with the reported literature that TiF3-,40 NbF5-,41 and TiO249-doped LiAlH4 as additives also cannot be detected for those samples after ball milling. It is noteworthy that, although LiAlH4 reacted with MnFe2O4 and formed Li2Fe3O4 and Fe0.9536O as the decomposition products, there were no 11943

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sites at the surface of the LiAlH4 matrix for dehydrogenation products. With the doped LiAlH4 samples after dehydrogenation at 250 °C, the reaction between LiAlH4 and MnFe2O4 has readily completed. The Fe0.9536O phase also appears in the XRD pattern of dehydrogenation products, and the diffraction peak of Fe0.9536O gradually strengthens with further raising the additive amount. These finely dispersed dehydrogenated products may contribute to kinetic desorption improvement by serving as the active sites for nucleation and creation of the dehydrogenated product by shortening the diffusion distance of the reaction ions. Therefore, it is reasonable to conclude that in situ formed Fe0.9536O and amorphous Mn or Mn-containing phases together provide a synergetic catalytic effect for the remarkably improved dehydrogenation kinetics of LiAlH4.

diffraction peaks of Mn or Mn-containing phases (JCPDS# 330887/32-0637) in Figure 6. It is believed to be mainly due to Mn or Mn-containing phases being in an amorphous state.42 In order to determine the phase structures of the doped LiAlH4 samples in the dehydrogenation process, XRD scans were performed on the as-milled LiAlH4 as well as on the 1 mol %, 7 mol %, and 9 mol % MnFe2O4-doped samples after dehydrogenation at 250 °C, as shown in Figure 7. For as-milled

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CONCLUSIONS In summary, the dehydrogenation performances of LiAlH4 were remarkably improved by doping MnFe2O4 nanoparticles. The onset desorption temperature of the 7 mol % doped sample initiated at 62 and 119 °C for the first two dehydrogenation stages and released 7.45 wt % hydrogen, indicating a substantial reduction of 88 and 71 °C compared with those of as-received LiAlH4. Furthermore, the doped LiAlH4 could release about 4.7 wt % hydrogen within 70 min at 90 °C. When increasing temperature up to 120 °C, the doped sample could complete the first two dehydrogenation steps within 80 min with 7.44 wt % hydrogen released, while only 0.5 wt % hydrogen was desorbed for as-received LiAlH4 at the same temperature and time. This demonstrates that doping MnFe2O4 to LiAlH4 could significantly reduce the decomposition temperature and enhance the desorption kinetics of LiAlH4. Through analyzing the DSC and Kissinger curves, the Ea values of as-received LiAlH4 were calculated to be 111.6 and 180.7 kJ/mol for the first two dehydrogenation steps, while the Ea of the doped sample decreased to 66.7 and 75.8 kJ/mol after doping MnFe2O4, resulting in decline rates of 40.2% and 58.1%, obtaining the maximum among those of LiAlH4 doped with other reported catalysts calculated by the Kissinger method. Meanwhile, FTIR and XRD characterizations for the doped samples reveal that LiAlH4 reacted with MnFe2O4 during ball milling by forming a ternary Li−Fe oxide, Fe oxide species with a reduced valence state, and amorphous Mn or Mn-containing phases. These in situ formed Fe0.9536O and amorphous Mn or Mn-containing phases together have a synergetically catalytic effect on drastically improving the dehydrogenation kinetics of LiAlH4. From the above experimental analyses, it is reasonable to conclude that MnFe2O4 is an effective dopant for significantly improving the dehydrogenation performance of LiAlH4.

Figure 7. XRD spectra for as-milled LiAlH4 and the LiAlH4 + 1 mol %, 7 mol %, and 9 mol % MnFe2O4 samples after dehydrogenation.

LiAlH4, the XRD spectra shows that the dehydrogenated sample only consisted of Al and LiH as dehydrogenation products without containing LiAlH4, Li3AlH6, and other additional phases, indicating LiAlH4 had completed the first two dehydrogenation steps when heated up to 250 °C. By contrast, for the doped samples, their XRD patterns imply that there were not only Al and LiH but also Li2Fe3O4 and Fe0.9536O as the dehydrogenated products, and peaks of Fe0.9536O phases gradually strengthened with raising MnFe2O4 amount. With respect to the significantly improved dehydrogenation properties of LiAlH4, in situ formed Fe0.9536O acts as a catalyst for the first two dehydrogenation steps of LiAlH4. Meanwhile, although no diffraction peaks of Mn or Mn-containing phases could be identified for the doped samples in Figure 7, it is reasonable to believe that Mn or Mn-containing phases may also play an important catalytic role for LiAlH4 during the dehydrogenation process.42 From the above analyses, the effect of doping MnFe2O4 on the dehydrogenation properties of LiAlH4 could be concluded as follows: First, MnFe2O4 reacts with LiAlH4 during mechanical milling by forming a ternary Li−Fe oxide, Fe oxide species with a reduced valence state, and amorphous Mn or Mn-containing phases, which has suggested that Fe or Mn species metals can be reduced to their lower oxidation state by increasing the high local temperatures during the ball milling process.23,27,30,31,42 The localized impact of composite powders during ball milling makes the localized temperature increase, resulting in the reaction between MnFe2O4 and LiAlH4 and the partial decomposition of MnFe2O4. These newly formed decomposition products could act as catalysts to facilitate the dehydrogenation steps of LiAlH4 mainly because they could create surface activation and form a large amount of nucleation

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ASSOCIATED CONTENT

S Supporting Information *

Figure of DSC curves of as-received LiAlH4 at heating rates of 6 °C/min, 9 °C/min, and 12 °C/min; figure of the Kissinger plots of as-received LiAlH 4 for the first and second dehydrogenation step; figure of DSC curves of LiAlH4 + 7 mol % MnFe2O4 at heating rates of 6 °C/min, 9 °C/min, and 12 °C/min; and figure of the Kissinger plots of the 7 mol % doped sample for the first two dehydrogenation steps. This material is available free of charge via the Internet at http:// pubs.acs.org. 11944

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel:+86-10-62332700; Fax: +8610-62334311. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to acknowledge the financial support from the National High-Tech R&D Program (863 Program) of China (2006AA05Z132).

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dx.doi.org/10.1021/jp302721w | J. Phys. Chem. C 2012, 116, 11939−11945