dehydrogenation properties of MgH2

JOURNAL OF RARE EARTHS, Vol. 33, No. 3, Mar. 2015, P. 334

Effect of LaFeO3 on hydrogenation/dehydrogenation properties of MgH2 ZHANG Wei (张 伟)1, CHENG Ying (程 颖)2, LI Yongheng (李永恒)2, DUAN Zhichen (段智琛)2, LIU Jian (刘 坚)2,* (1. Hebei Key Laboratory of Applied Chemistry, College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China; 2. State Key Laboratory of Heavy Oil, College of Science, China University of Petroleum-Beijing, Beijing 102249, China) Received 11 June 2014; revised 8 December 2014

Abstract: LaFeO3 was used to improve the hydrogen storage properties of MgH2. The MgH2+20 wt.%LaFeO3 composite was prepared by ball milling method. The composite could absorb 3.417 wt.% of hydrogen within 21 min at 423 K while MgH2 only uptaked 0.977 wt.% hydrogen under the same conditions. The composite also released 3.894 wt.% of hydrogen at 623 K, which was almost twice more than MgH2. The TPD measurement showed that the onset dissociation temperature of the composite was 570 K, 80 K lower than the MgH2. Based on the Kissinger plot analysis of the composite, the activation energy Edes was estimated to be 86.69 kJ/mol, which was 36 kJ/mol lower than MgH2. The XRD and SEM results demonstrated that highly dispersed LaFeO3 could be presented in MgH2, benefiting the reduction of particle size and also acting as an inhibitor to keep the particles from clustering during the ball-milled process. Keywords: magnesium hydride; hydrogen storage composite; hydrogen properties; hydrogenation kinetics; rare earths

Energy poverty is a great challenge in the world due to the excessive energy utilization and waste of the non-renewable coal and petroleum. Yet in other ways, the traditional resource which is not inexhaustible and irreversible may be a good reason for exploring new materials such as the hydrogen storage materials to solve the mentioned issues[1–4]. Magnesium hydride, especially MgH2, has drawn attentions to substitute the traditional resources owing to its high abundance but low cost. As a solid hydrogen storage material, MgH2 has many advantages such as excellent reversibility and cycle performance. However, its practical application is hampered due to the high dissociation temperature and thermodynamics stability[5,6]. In recent years, many groups have devoted to avoid the disadvantages of magnesium hydride through the formation of ternary hydride or even the catalytic activity phase to improve its hydrogen kinetics. Therefore, many methods were invented such as preparation of thin particle size by mechanically ball-milling, modification of the surface of the matrix and addition of some metal element catalysts, such as transition metal fluorides[7–9], transition metal oxides[10,11]. Friedrichs et al.[12] reported that the addition of Nb2O5 contributed to improving the hydrogen/dehydrogen behaviors of MgH2 as a consequence of the intimate contact between MgH2 and Nb2O5. Furthermore, it was found that the hydrogen desorption kinetics was much remarkably improved by adding Cr2O3 or

Nb2O5 as co-catalyst to the MgH2[13]. In addition, the rare earth perovskite-type oxides that belong to the ABO3 compounds have been widely used as catalytic materials in the field of fuel cell electrodes[14,15]. The catalytic activity of the perovskite-type oxides can be comparable to some noble metal catalysts in certain conditions[16]. Deng et al.[17] reported that LaCrO3 electrodes showed remarkable electrochemical reversibility and might be a promising candidate for future electrochemical materials. As a catalyst, the perovskite-type oxides have not been applied for hydrogen desorption of MgH2. Our group had confirmed the Fe and La was quite reactive, showing better catalytic during ball-milled with MgH2. Therefore, in this work, LaFeO3 was selected as a new additive to ball-mill with MgH2, and its effects on the absorption/desorption performances of MgH2 were systemically investigated.

1 Experimental MgH2 was purchased from the Sigma Alfa Co., and LaFeO3 was synthesized by stearic acid combustion method[18].The synthesized LaFeO3 was mixed with pure MgH2 in a mass ratio of 1:4. Then the mixture was put into a stainless steel container corresponding to some stainless steel balls and with a ball to powder ratio of 20:1. The ball-milling was undertaken by setting the planetary miller at a speed of 500 r/min for 2 h, and 5

Foundation item: Project supported by National Natural Science Foundation of China (21376261, 50971112, 51001043, 21173270), Natural Science Foundation of Hebei Province (E2010001170) and Scientific Research Foundation for Returned Overseas Chinese Scholars of Hebei Province (20100501) * Corresponding author: LIU Jian (E-mail: [email protected]; Tel.: +86-10-89732326) DOI: 10.1016/S1002-0721(14)60422-1

ZHANG Wei et al., Effect of LaFeO3 on hydrogenation/dehydrogenation properties of MgH2

min rest for each 30 min cycle. Ball milling was performed under Ar atmosphere to prevent the residual air and oxygen from forming MgO and other contaminants. All the experiment on handling these samples were performed at the argon glove with H2O and O2 amount <1 ppm. The effect of LaFeO3 on the dissociation temperature of MgH2 during dehydrogenation was investigated by means of temperature-programmed-desorption (TPD). Five hundred milligrams of sample were loaded in the sample chamber and then put into the furnace. The test temperature was set to increase from room temperature to 773 K with a heating rate of 10 K/min. The hydrogen sorption kinetics was performed at different temperatures with an initial pressure of 3 and 0.001 MPa of hydrogen, respectively. The phase composition and crystallite size were characterized by X-ray diffraction technology using Cu Kα radiation with a scanning speed of 4(º)/min from 10º to 80º. SEM was used to investigate the structural morphology of pure MgH2.

2 Results and discussion 2.1 Phase composition and structural characterization Fig. 1 exhibits the XRD patterns of MgH2+20 wt.% LaFeO3 composite at different hydrogenation stages. In Fig. 1(1), for the as-milled sample, the most intensive diffraction peak is indexed to the β-MgH2 phase and metastable γ-MgH2 phase. The formation of the metastable phase γ-MgH2 is a consequence of the mechanically ball-milling and the intensive strain[19,20]. After milling, LaFeO3 can still be detected with high diffractions and broad peaks, indicating that LaFeO3 exhibits amorphous phenomenon with the particle size significantly reduced in the milling process. In Fig. 1(2), after hydrogenation under 3 MPa of H pressure, no diffraction peak of metastable γ-MgH2 is detected, while the diffraction peak of LaFeO3 is accordingly identified. Therefore, it indicates that there may exist a transformation between metastable γ-MgH2 and β-MgH2 phase. In the case of dehydrided

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sample of Fig. 1(3), β-MgH2 phase disappears with only Mg and LaFeO3 present, which indicates that there is a dehydrogenation reaction from β-MgH2 phase to Mg phase during the dehydrogenation process. During the whole process, the XRD pattern of the MgH2+20 wt.%LaFeO3 composite exhibits the peak corresponded to LaFeO3, which indicates the high stability of this composite at high temperature in the presence of MgH2. SEM was employed to further study the structure of the composites. Fig. 2 shows the SEM images of ballmilled MgH2+20 wt.%LaFeO3 composite and pure MgH2. The particle sizes depicted in the SEM images are decreased owing to the high energy mechanically ball-milling. It is apparent that the grain size of MgH2 ball-milled with LaFeO3 is smaller than that of pure MgH2, demonstrating that the addition of LaFeO3 is beneficial to decreasing the particle diameter of MgH2. Fig. 3 shows the SEM images of MgH2+20 wt.%LaFeO3 and MgH2 after hydrogen absorption at 623 K. The particles of the composite after hydrogenation are homogeneous and highly dispersed, while the particles of the pure MgH2 aggregates into large cluster. According to the Scherrer’s law, the average particle size of the MgH2+LaFeO3 composite was calculated about 26.7 nm,

Fig. 1 X-ray patterns of MgH2+20 wt.%LaFeO3 composite (1) Ball-milling; (2) Hydrogenation; (3) Dehydrogenation

Fig. 2 SEM micrographs of MgH2+20 wt.%LaFeO3 composite (a) and MgH2 ball-milled for 2 h (b)

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Fig. 3 SEM micrographs of MgH2+20 wt.%LaFeO3 composite (a) and MgH2 hydrogenated at 623 K (b)

smaller than that of MgH2 (27.5 nm). It shows that LaFeO3 can not only promote to decrease the particle size but also serve as an inhibitor to limit the formation of the large cluster and lead to more favorable hydrogen kinetics. 2.2 Dehydrogenation properties of MgH2+20 wt.% LaFeO3 composite The effect of LaFeO3 on the dissociation temperature for the MgH2 after dehydrogenation was investigated under a heating rate of 10 K/min. As shown in Fig. 4, it is obvious that the hydrogen desorption temperature for the composite is about 685 K compared to 770 K of pure MgH2 when 4 wt.% of hydrogen is released. Furthermore, it can be seen that the onset decomposition temperature for MgH2+20 wt.%LaFeO3 composite is about 570 K, which is decreased by about 80 K than pure MgH2. This shows that the desorption properties of MgH2 are further improved by the addition of LaFeO3. And also, the effect of LaFeO3 on dehydrogenation performance of pure MgH2 was studied. Fig. 5 presents the dehydrogenation curves of MgH2+20 wt.%LaFeO3 composite, MgH2 and LaFeO3 at 623 K. Under the identical conditions, MgH2 with LaFeO3 releases 3.894 wt.%

Fig. 4 Temperature-pressure-desorption curves of the MgH2+ 20 wt.%LaFeO3 composite and MgH2

Fig. 5 Dehydrogenation curves of the MgH2+20 wt.%LaFeO3 composite and MgH2 and LaFeO3 at 623 K

of hydrogen within 21 min while pure MgH2 only releases 2.319 wt.%. While the LaFeO3 exhibited no ability of hydrogen desorption at the same condition, showing that the MgH2 is highly activated owing to the addition of LaFeO3. It indicates that the addition of LaFeO3 is beneficial to the desorption capacity of MgH2. In addition, MgH2+20 wt.%LaFeO3 composite shows faster hydrogen kinetics than MgH2. Fig. 6 presents the hydrogen absorption curves of MgH2+20 wt.%LaFeO3 composite and MgH2 and LaFeO3 at both 423 and 473 K under the same hydrogen pressure of 3 MPa. The LaFeO3 exhibited no ability of hydrogen absorption at the same condition, but the hydrogenation kinetics of MgH2 can be enhanced by increasing the reaction temperatures. At 423 K, MgH2+20 wt.%LaFeO3 composite absorbs 3.417 wt.% hydrogen in 21 min, which is about 2.44 wt.% higher than pure MgH2. At 473 K, MgH2+20 wt.%LaFeO3 composite can uptake 4.57 wt.% of hydrogen after 21 min, which is also higher not only than pure MgH2 under the same condition but also much higher than that of the composite at lower temperature (423 K). With the increasing of reaction temperature, the activity of LaFeO3 was improved. It was favorable for the H diffusion in the matrix, thus further enhancing the hydrogen capacity

ZHANG Wei et al., Effect of LaFeO3 on hydrogenation/dehydrogenation properties of MgH2

Fig. 6 Hydrogenation curves of the MgH2+20 wt.%LaFeO3 composite and MgH2 and LaFeO3 under the pressure of 3 MPa at different temperatures

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Fig. 7 DTA curves of MgH2+20 wt.%LaFeO3 composite (a) and MgH2 (b) with a heating rate of 5, 10, 15, 20 K/min

of MgH2+20 wt.%LaFeO3 composite. 2.3 Active energy of MgH2+20 wt.%LaFeO3 composite The dehydrogenation behaviors of the composite were further investigated by the DTA. Fig. 7 shows the DTA curves of MgH2+20 wt.%LaFeO3 composite (Fig. 7(a)) and MgH2 (Fig. 7(b)) at different heating rates. Hanada et al. [21] claimed that the behavior of H-desorption in MgH2 with other catalysts follows the first-order reaction model. To investigate the fast H-sorption kinetics upon the addition of LaFeO3, the activation energies of samples in the desorption process were calculated from the Kissinger plot equation[22]: (1) d [ln(α / Tm2 )] / d (1/ Tm ) = − Ea / R where α is the heating rate (K/min), Tm is the temperature for the maximum desorption rate (K), and R is the gas constant of 8.314 J/(mol·K). As shown in Fig. 8 of the Kissinger plot, the activation energy Edes of MgH2+20 wt.%LaFeO3 and MgH2 are calculated to be 86.69 and 122.29 kJ/mol, respectively. It is obvious that LaFeO3 plays an important role in decreasing the energy barrier in the desorption process of MgH2.

3 Conclusions The hydrogenation and dehydrogenation behaviors of

Fig. 8 Kissinger’s plot for the hydrogen desorption reaction for MgH2+20 wt.%LaFeO3 composite and MgH2 at various heating rates

MgH2 were remarkably improved by the addition of LaFeO3. Some value results are as follows: (1) MgH2+20 wt.%LaFeO3 composite could absorb 3.417 wt.% of hydrogen within 21 min at 423 K, while only 0.977 wt.% of hydrogen was uptaked in pure MgH2 under the same conditions. In addition, the dehydrogen ability of the composite was almost twice than that of MgH2. (2) The onset dissociation temperature for MgH2+20 wt.%LaFeO3 composite was about 570 K, which was

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about 80 K lower than that of MgH2. (3) From the Kissinger plot, the activation energy Edes was 86.69 kJ/mol for the composite, 36 kJ/mol lower than that of MgH2. (4) Based on the XRD and SEM results, the highly dispersed LaFeO3 in pure MgH2 could not only be beneficial to the reduction of the particle size but also act as an inhibitor to keep the particles from clustering during the ball-milled process, thus leading to the improvement in the hydrogenation-dehydrogenation properties of MgH2.

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