Kinetics Enhancement, Reaction Pathway Change, and Mechanism

Article pubs.acs.org/JPCC

Kinetics Enhancement, Reaction Pathway Change, and Mechanism Clarification in LiBH4 with Ti-Catalyzed Nanocrystalline MgH2 Composite Huaiyu Shao, Michael Felderhoff,* and Claudia Weidenthaler Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany ABSTRACT: A composite of 2LiBH4 + nano-MgH2* (Ti-catalyzed) shows significantly enhanced desorption kinetics compared to a conventional mixture of 2LiBH4 + MgH2. The desorption mechanism was studied in the temperature range between 304 and 383 °C and under different pressure conditions. Desorption temperatures are 50−70 °C lower compared to conventional 2LiBH4 + MgH2 mixtures. During the hydrogen release from a mixture of 2LiBH4 + nano-MgH2* at a hydrogen back-pressure of 0.4 MPa, MgB2 is formed and three different plateaus of equilibrium were detected. The reaction pathway is changed at 360 °C for the 2LiBH4 + MgH2 system when the nano-MgH2* is used.

1. INTRODUCTION Complex hydrides such as alanates, metal borohydrides, or amides are widely studied as solid-state storage materials for future hydrogen applications.1−3 Research was intensified especially after Bogdanović et al. showed significantly improved kinetics and cyclic performance by addition of TiCl3 catalyst to complex aluminum hydride (NaAlH4).4 LiBH4 is another possible candidate for hydrogen storage because of its high gravimetric hydrogen capacity (18.5 wt %) and volumetric capacity of 121 kg m−3.5 For simplification, the following decomposition process (eq 1) is adopted because high temperatures (above 550 °C) are required for the thermal decomposition of LiH. LiBH4 → B + LiH + 1.5H 2 (1)

reported a stepwise mechanism starting with the decomposition of MgH2 above 360 °C. This is followed by the decomposition of LiBH4 to LiH, B, and H2. Finally, the formation of Li−Mg alloys, MgB2, and H2 from Mg, LiH, and B is observed above 450 °C.34 Vajo et al. described the desorption from a 2LiBH4 + MgH2 composite below 360 °C as a simultaneous process at a back-pressure of at least 0.3 MPa according to eq 2. Above 360 °C, MgH2 decomposes to Mg metal, which then reacts with LiBH4 to form hydrogen, MgB2, and LiH as shown in eq 3.23,35 Bösenberg et al.36 studied the desorption mechanism of a catalyzed composite of 2LiBH4 + MgH2 at a back-pressure of 0.5 MPa up to 450 °C by the in situ XRD technique. Their results are summarized in eq 4.

In this case, the theoretical amount of released hydrogen is 13.9 wt %. The enthalpy change is 67 kJ mol−1 H2, which is still too large for the system to desorb hydrogen at low temperatures. To reduce the desorption temperature, several methods have been developed, such as destabilization and kinetics enhancement with metals6−9 and oxides,10 addition of Pt,11 fullerene,12 and halide,13−17 polymer catalysts,18 composite synthesis with LiNH2,19−21 Mg(AlH4)2,22 metal hydrides,23−28 or impregnation of porous materials.29−33 Vajo et al. reported the destabilization of the system by addition of MgH2 (eq 2).23 They reported an enthalpy value of only 40.5 kJ mol−1 H2 for the rehydrogenation reaction given in eq 2.

(4)

MgH2 + 2LiBH4 → MgB2 + 2LiH + 4H 2

(2)

Mg + 2LiBH4 → MgB2 + 2LiH + 3H 2

(3)

MgH2 + 2LiBH4 ⇌ Mg + 2LiBH4 + H 2 ⇌ MgB2 + 2LiH + H 2

Here we studied the properties of 2LiBH4 + nano-MgH2* composite under various conditions and clarified the reaction mechanism for this composite system. One key issue is to understand whether the reaction of the formation of MgB2 is simultaneously taking place with the dehydrogenation of LiBH4 or it is taken place afterward. The Ti-catalyzed MgH2 nanocrystalline sample (nano-MgH2*) was prepared by a homogeneously catalyzed synthesis method.37−39 The results were compared with the behavior of a 2LiBH4 + MgH2 composite, which is not nanocrystalline. Novel kinetic properties, cyclic performance, and a new reaction mechanism were observed and discussed in detail. Received: November 17, 2014 Revised: January 7, 2015 Published: January 7, 2015

However, the mechanism of desorption of the LiBH4/MgH2 composite is discussed controversially in the literature. Yu et al. © 2015 American Chemical Society

2341

DOI: 10.1021/jp511479d J. Phys. Chem. C 2015, 119, 2341−2348

The Journal of Physical Chemistry C

Article

Figure 1. X-ray diffraction patterns of 2LiBH4 + nano-MgH2* composites (a) after ball-milling, (b) after temperature-programmed desorption into vacuum, Tmax = 380 °C, and (c) after temperature-programmed desorption to 0.4 MPa back-pressure, up to 380 °C.

2. EXPERIMENTAL DETAILS Nanocrystalline Ti-catalyzed MgH2 (nano-MgH2*) was prepared by a homogeneously catalyzed synthesis method.37−39 For a typical synthesis procedure, 20 g of Mg powder (99.8% purity, −325 mesh, Alfa Aesar) and 1.46 g of anthracene (99.0% purity, Sigma-Aldrich) with a molar ratio of 100:1 (Mg to anthracene) were immersed in 100 mL of anhydrous THF, and 0.1 mL of ethyl bromide was added into the suspension. After 4 h stirring, 2.75 g of TiCl4·2THF was added. After an additional 30 min stirring, the suspension was filled into a highpressure autoclave, and a hydrogen pressure of 8 MPa was applied. The hydrogenation reaction at room temperature takes 1−2 days. After filtering the suspension, washing the residue three times with THF and pentane, and drying it at 70 °C for 5 h in vacuum, Ti-catalyzed nano-MgH2* was obtained. The composite samples of 2LiBH4 + MgH2 and 2LiBH4 + nano-MgH2* were produced by ball milling the mixture of LiBH4 (95% purity, ABCR Co.) with commercial MgH2 (98% purity, Alfa) or nano-MgH2*. A total weight of 0.5 g of the mixture was ball-milled for 3 h under 0.1 MPa argon. For milling, a planetary mill, PULVERISETTE 7, from Fritsch Co. was used. To prevent the contamination of the materials with abraded metallic particles (Fe, Cr, etc.), ball milling was performed in a milling vessel and with balls made of silicon nitride, Si3N4. The space volume of the vessel was 12 mL. For the milling process six balls with a diameter of 10 mm and a weight of 1.66 g each were used. This results in a ball-to-sample ratio of 20:1. The X-ray diffraction patterns for qualitative and quantitative analysis were collected on a Stoe STADI P transmission diffractometer in Debye−Scherrer geometry (Mo Kα1: 0.709 30 Å) with a primary monochromator and a linear position sensitive detector. For the measurements, the samples were filled into glass capillaries under argon, and the capillaries were sealed to prevent exposure to air before the measurements. The obtained XRD patterns were evaluated by comparison with entries in PDF-2 powder pattern database. To study heat effects during the hydrogen desorption process, TG/DTA experiments were performed using a

Netzsch STA 449 C thermobalance coupled with a Balzers Thermostar 442 mass spectrometer. The temperature program was set up from 30 to 550 °C with a heating rate of 10 K min−1. The measurements were done under an argon flow rate of 40 mL min−1. Samples were weighed and loaded into the crucibles in an argon atmosphere. Temperature-programmed desorption (TPD) and pressure composition isotherm (PCI) measurements were taken on an automatic apparatus PCTPro-2000 from Hy-Energy LLC. The TPD measurements of composite samples of 2LiBH4 + MgH2 and 2LiBH4 + nano-MgH2* were conducted from 30 to 380 °C into vacuum and at a hydrogen back-pressure of 0.4 MPa. For the measurement of desorption PCI curves first 2 MPa of hydrogen was applied to the samples with an amount of 0.2 g. Second, the system was heated up to the measurement temperatures without hydrogen release. For PCI measurements, the reservoir volume was ca. 12 mL while the sample holder free volume was ca. 13 mL. The maximum time for each aliquot is 100 min. The pressure change applied to the reservoir volume between each aliquot was 0.1 MPa. An equilibrium test during these PCI measurements was taken with a rate limit of 10−4 mass % min−1.

3. RESULTS 3.1. Desorption Behavior. Figure 1a shows the XRD pattern of a 2LiBH4 + nano-MgH2* composite after ball milling with the expected two phases LiBH4 and MgH2. In Figure 2, the TG-DTA curves of LiBH4, 2LiBH4 + MgH2 composite, and 2 LiBH4 + nano-MgH2* composite after milling are shown. The pure LiBH4 sample shows three endothermic peaks during the heating process, corresponding to the literature.40 The first peak at 121 °C is attributed to a phase transformation from the low-temperature LiBH4 structure (Pnma) to the high-temperature crystal structure (P63mc) as expressed in eq 5. The second peak at 285 °C is due to the melting of LiBH4 (eq 6). In the range from 300 to 550 °C only one desorption peak at 488 °C can be observed. This peak is ascribed to the decomposition of LiBH4 to LiH, B, and release of hydrogen (eq 1). The overall desorption content is 10.6 wt % (theoretical value 13.9 wt %). 2342



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Figure 2. TG-DTA curves of three samples (LiBH4, 2LiBH4 + commercial MgH2 composite, and 2LiBH4 + nano-MgH2* composite) measured between 30 and 550 °C with a heating rate of 10 K min−1 in Ar flow.

LiBH4(Pnma) → LiBH4(P 63mc)

(5)

LiBH4(P 63mc) → LiBH4(liq)

(6)

Figure 3. Temperature-programmed desorption (TPD) curves of 2LiBH4 + commercial MgH2 composite and 2LiBH4 + nano-MgH2* composite, decomposition into vacuum, Tmax = 380 °C.

The 2LiBH4 + MgH2 composite shows the same endothermic peaks at 120 and 285 °C like the pure LiBH4 compound. From 300 to 550 °C two main desorption peaks at 382 and at 466 °C can be observed. The first peak is due to desorption of hydrogen from MgH2 (in molten LiBH4) and the formation of Mg (eq 7). The second one is due to hydrogen desorption of LiBH4 and the formation of LiH and B. This desorption peak is lowered by 22 °C compared to the pure LiBH4. The desorption capacity of this composite is 10.0 wt % (theoretical value 11.5 wt %). MgH2(in liquid LiBH4) → Mg + H 2

(7)

The 2LiBH4 + nano-MgH2* composite also shows two endothermic peaks at temperatures below 300 °C. These two peaks again correspond to the structural phase transformation and melting of LiBH4. In the range from 300 to 550 °C two main endothermic peaks at 325 and 432 °C can be observed describing the decomposition of MgH2 and LiBH4. However, these two peaks from the 2LiBH4 + nano-MgH2* composite appear at much lower temperatures compared to desorption peaks from the 2LiBH4 + MgH2 composite. When using the catalyzed MgH2 nanocrystalline sample instead of commercial MgH2, the desorption temperature of MgH2 is lowered by 57 °C and of LiBH4 by 32 °C. The weight loss measured by TG is 12.3 wt % because the desorbed gas in the first heating cycle contains small amounts of organic species resulting from the catalyzed solution synthesis process.39 Figures 3 and 4 present the TPD measurements of 2LiBH4 + MgH2 and 2LiBH4 + nano-MgH2*, desorbed in vacuum and at a hydrogen back-pressure of 0.4 MPa. The insets in both figures show the amount of desorbed gas versus temperature during these measurements. Two different desorption processes can be observed in Figure 3, indicated by the slope of the curves. The first one is the decomposition of MgH2 (eq 7). It starts around 350 °C with a gas release of about 3 wt % hydrogen, followed

Figure 4. Temperature-programmed desorption (TPD) curves of 2LiBH4 + commercial MgH2 composite and 2LiBH4 + nano-MgH2* composite, 0.4 MPa hydrogen back-pressure, Tmax = 380 °C.

by the second desorption step. In this step, LiBH4 releases hydrogen and forms LiH and B (eq 1). This is confirmed by XRD result shown in Figure 1b. After hydrogen desorption from 2LiBH4 + nano-MgH2*, the sample consists mainly of Mg and LiH and small amounts of unreacted MgH2. The B-metal phase formed during the decomposition of LiBH4 usually cannot be detected by XRD technique due to the amorphous state of boron.41 From the inset in Figure 3 it can be seen that above 290 °C the 2LiBH4 + nano-MgH2* composite starts to desorb hydrogen dramatically. The second reaction starts around 320 °C. After 10 h of desorption, the composite has released 9.5 wt % of hydrogen. The desorption reaction is not completely finished as confirmed by XRD results shown in Figure 1b. Compared to the nanocrystalline material, the MgH2 phase in 2LiBH4 + MgH2 starts to release hydrogen above 360 °C 2343



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(see inset in Figure 3), and LiBH4 starts to desorb hydrogen above 370 °C. After 10 h desorption, this composite shows a loss of 8.6 wt % hydrogen (theoretical value 11.5 wt %). The comparison of the desorption results of these two composite samples shows a significantly reduced desorption temperature and a significantly enhanced kinetics of the nano-MgH2* compared to the commercial MgH2 material. Figure 4 shows the TPD results of both composites during the desorption process at a hydrogen back-pressure of 0.4 MPa. The formation of MgB2 during the decomposition of 2LiBH4 + MgH2 at a hydrogen back-pressure of about 0.3 MPa has been previously described in the literature.23,35 The first step is the hydrogen desorption from MgH2 producing Mg metal. This starts at about 365−375 °C for 2LiBH4 + commercial MgH2 and 2LiBH4 + nano-MgH2* composites (see Figure 4). At a hydrogen back-pressure of 0.4 MPa the temperature difference of starting hydrogen release of these samples is much smaller compared to desorption into vacuum. According to the van’t Hoff equation for the MgH2 desorption (discussed in a later section), temperatures above 340 °C are needed to reach an equilibrium pressure of 0.4 MPa. This means that the initial hydrogen atmosphere of 0.4 MPa suppresses the hydrogen desorption of MgH2 at temperatures below 340 °C because the issue here is the reduced thermodynamic driving force for dehydriding MgH2 at 0.4 MPa H2 pressure. The second step is the release of H2 from the Mg−LiBH4 mixture (eq 3) and the formation of MgB2 and LiH, verified by XRD (see Figure 1c) measured after the desorption of 2LiBH4 + nano-MgH2*. The XRD pattern shows mainly MgB2, LiH, and a small amount of unreacted LiBH4. From Figure 4, it can be seen that under these conditions the kinetics of the second step is obviously slower for 2LiBH4 + commercial MgH2. After 16 h desorption, 4.5 wt % hydrogen is released. The comparison of the desorption curves of a 2LiBH4 + nano-MgH2* composite into vacuum (in Figure 3) and at 0.4 MPa hydrogen back-pressure (Figure 4) shows obvious differences in kinetics. The kinetics for the desorption into vacuum is much faster and it takes only 6 h for this sample to release 9.5 wt % hydrogen. The desorption of the same material at a hydrogen back-pressure of 0.4 MPa takes 55 h to reach almost the same capacity. 3.2. Measurements of the Pressure−Composition Isotherms (PCI). The PCI desorption curves of the 2LiBH4 + nano-MgH2* composite measured at 383, 369, 357, 335, and 304 °C are shown in Figure 5. Three different plateaus can be observed during desorption processes at 383, 369, and 357 °C. During desorption from 2LiBH4 + MgH2, usually only one or two plateaus23,42 were detected. A third plateau was not reported before. At 383 °C, the desorption PCI measurement took 35 h and delivered 10.3 wt % hydrogen. At 369 and 357 °C, the PCI measurements took 44 and 48 h and released 9.6 and 9.8 wt % hydrogen. The PCI curve at 335 °C showed only two plateaus. The desorption PCI measurement at this temperature took 56 h with a desorption amount of 7.8 wt % hydrogen. At 304 °C, 3.0 wt % hydrogen are released in the first step after 13 h desorption. The second desorption step followed for additional 70 h. Under these conditions a precise measurement was not possible and is therefore not shown in Figure 5. The equilibrium pressures and temperatures of all plateaus displayed in Figure 5 are summarized in Table 1. The second desorption plateau at 335 °C is classified as plateau C due to the same reaction mechanism of these three plateau groups as deduced from the van’t Hoff equations. This is also supported

Figure 5. Desorption pressure−composition isotherm (PCI) curves of 2LiBH4 + nano-MgH2* composite measured at 383, 369, 357, 335, and 304 °C. The PCI desorption data were acquired from right to left along the horizontal axis.

Table 1. Equilibrium Pressure and Temperature of 2LiBH4 + nano-MgH2* Composite during Desorption Pressure− Composition Isotherm (PCI) Measurements plateau A

plateau B

plateau C

temp (°C)

equilib press. (0.1 MPa)

temp (°C)


temp (°C)


382.7 368.5 359.2 357.2 335.0 303.7

11.027 7.710 6.319 5.803 3.517 1.411

382.8 368.8 359.0 357.3

5.006 3.886 3.168 2.965

383.1 369.0 358.9 357.1 334.9

1.329 0.888 0.715 0.627 0.343

by XRD data as discussed later in this work. According to equilibrium pressure and temperature, the thus obtained van’t Hoff plot is shown in Figure 6. The calculated enthalpy and entropy values are presented in Table 2. The XRD measurements of the 2LiBH4 + nano-MgH2* composite performed after the PCI desorption were carried out in order to study the reaction mechanism (Figure 7). After three decomposition steps at 383, 369, and 357 °C, the main phases are MgB2 and LiH. However, after two desorption steps at 335 and 304 °C, the samples consists mainly of LiH and Mg (the B-metal phase is invisible by XRD). The first desorption step (plateau A) with a capacity of about 2.5 wt % can be observed during the desorption processes at all different desorption temperatures (Figure 6), resulting in the van’t Hoff equation with y = −9733x + 17.23. The values of x and y correspond to 1/T (K) and ln(P/0.1 MPa) in Figure 6. The enthalpy and entropy changes for plateau A are 80.9 kJ mol−1 H2 and 143.2 J K−1 mol−1 H2, respectively. These values are similar to results previously reported for the desorption of a nano-MgH2*(y = −9349x + 16.64).39 Considering the TPD results (Figures 3 and 4) as well as the XRD results (Figure 1), the reaction for plateau A is assigned to hydrogen desorption from MgH2 (eq 7). Small differences in both van’t Hoff equations may result from differences in desorption properties. A possible reason might be the different environments of the MgH2 in this study. In one case MgH2 is located in a solid state 2344



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assigned to hydrogen desorption from LiBH4 (eq 1). The reaction between 357 and 383 °C can be summarized by eq 1, and the formation of MgB2 from the metals is given in eq 8. Mg + 2B → MgB2

(8)

The crystalline products MgB2 and LiH obtained after the PCI desorption process are shown in Figure 7c−e. Two desorption steps, belonging to plateau A and plateau C, are observed for 2LiBH4 + nano-MgH2* at 335 and 304 °C. At the end of the reaction only Mg and LiH are detected (see Figure 7a,b). The reactions are given in eq 7 for plateau A and in eq 1 for plateau C. No plateau B can be observed under these experimental conditions.

4. DISCUSSION During desorption of hydrogen from a mixture of 2LiBH4 + nano-MgH2* into vacuum, only the plateaus A and C can be observed. This is in agreement with experimental results described in the literature. Plateau B (eq 3) cannot be observed under the experimental conditions at temperatures of 335 and 304 °C. This can be due to the fact that the formation of MgB2 accordingly to eq 3 cannot take place below 335 °C (see Figure 6). MgB2 is not formed after desorption at 304 and 335 °C even in the presence of Mg and B phases. It seems that below a temperature of 335 °C the formation of MgB2 from the elements can also not take place accordingly to eq 8. The existence of plateau C can be explained by a limited mass transfer of the solid Mg in the liquid LiBH4 phase, and no reaction (eq 3) between solid Mg and liquid LiBH4 is observed. This can be supported by the fact that the hydrogen content of plateau C depends on the temperature. At higher temperatures the content is less compared to lower temperatures, meaning that the mass transfer, necessary for the reaction given by eq 3, is faster. The enthalpy change of 92.9 kJ mol−1 H2 for the desorption of hydrogen from LiBH4 forming LiH and B metal is larger than the reported values in the range of 52−76 kJ mol−1 H2.2 Usually, PCI measurements never achieve complete equilibrium, especially for systems with poor kinetics. Mauron et al. calculated a value of 74 kJ mol−1 H2 from PCI measurements,43 but this value is extrapolated from equilibrium pressures measured at different hydrogen flow rates. The enthalpy change is largely higher compared to data extracted from experimental van’t Hoff plots because the desorption reaction is very slow. Agresti et al. reported a value of 78 ± 5 kJ mol−1 H2 by the same method.44 The enthalpy change values of 69.2 kJ mol−1 H2 for plateau B and 92.9 kJ mol−1 H2 for plateau C (Table 2) agree with the fact that the enthalpy change of desorption from Mg + 2LiBH4 to MgB2, LiH, and H2 should be much smaller than that from the decomposition of LiBH4 to LiH, B, and H2. This can be explained by the formation of the stable compound MgB2.

Figure 6. Van’t Hoff plot of three plateaus during desorption processes of 2LiBH4 + nano-MgH2* composite measured at 383, 369, 357, 335, and 304 °C.

environment; in the other case MgH2 is embedded in liquid LiBH4. The three PCI curves exhibit a flat plateau for plateau B (Figure 6). From the three equilibrium pressures and temperatures, the van’t Hoff equation is determined to be y = −8318x + 14.30. The enthalpy and entropy changes for plateau B are 69.2 kJ mol−1 H2 and 118.9 J K−1 mol−1 H2. At a lower temperature of 335 °C the second desorption plateau does not fit the van’t Hoff equation of y = −8318x + 14.30. However, it shows a good fit to the equation of plateau C (shown later). The plateaus belonging to plateau C are much steeper than those belonging to plateaus A and B, but a linear regression can be derived from the van’t Hoff plot. The calculated van’t Hoff equation for plateau C is y = −11176x + 17.31. The enthalpy and entropy changes for plateau C are 92.9 kJ mol−1 H2 and 143.9 J K−1 mol−1 H2. When 2LiBH4 + nano-MgH2* starts to desorb hydrogen at 380 °C at a back-pressure of 0.4 MPa, first MgH2 decomposes. The formed Mg metal reacts with LiBH4, producing MgB2 and LiH (Figures 4 and 1c). The second plateau (plateau B) at 383 °C shows an equilibrium pressure of 0.5 MPa (see Figure 5 and Table 1). The reaction equation for plateau B from 357 to 383 °C is caused by desorption of hydrogen from Mg + LiBH4 to form MgB2 and LiH (eq 3). 3.3. Desorption at Reduced Pressure. During desorption of hydrogen from 2LiBH4 + nano-MgH2* into vacuum at about 380 °C, first MgH2 decomposes, followed by desorption of hydrogen from LiBH4 and the formation of LiH and B (see Figures 3 and 1b). Also, the reaction deduced from plateau C is

Table 2. Van’t Hoff Equation, Enthalpy and Entropy Change, and Reaction Mechanism for Three Plateaus in Desorption Pressure−Composition Isotherm (PCI) Measurements of 2LiBH4 + nano-MgH2* Compositea plateau A plateau B plateau C a

van’t Hoff equation

ΔH (kJ mol−1 H2)

ΔS (J K−1 mol−1 H2)

reaction equation

y = −9733x + 17.23 y = −8318x + 14.30 y = −11176x + 17.31

80.9 69.2 92.9

143.2 118.9 143.9

MgH2 → Mg + H2 (in liquid LiBH4) Mg + 2LiBH4 → MgB2 + 2LiH + 3H2 2LiBH4 → 2B + 2LiH + 3H2

The values of x and y are calculated from 1/T (K) and ln(P/0.1 MPa), respectively, in Figure 6. 2345



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Figure 7. X-ray diffraction patterns of 2LiBH4 + nano-MgH2* composite samples after desorption pressure−composition isotherm (PCI) measurements at (a) 304, (b) 335, (c) 357, (d) 369, and (e) 383 °C.

attributed to plateau A and C according to the plateau pressure values. The reactions during desorption from 2LiBH4 + MgH2 at 360 °C is described by eqs 7 and 1. Plateau B (eq 3) takes place only when catalyzed nanocrystalline MgH2 is used. Hu et al. also studied the suppression effect of [B12H12]2− formation by using catalyzed MgH2.46 This different reaction mechanism leads to an improved dehydrogenation and rehydrogenation kinetics.39 For 2LiBH4 + nano-MgH2* it has been shown that MgB2 and LiH are the crystalline phases after desorption between 357 and 383 °C (Figure 6). For 2LiBH4 + MgH2 after desorption at 360 °C the main phases are MgB2, Mg, LiH, and LiBH4 (Figure 9a) according to eqs 7 and 1. The desorption amount is 8.8 wt %, which means that the desorption reaction (eq 1) is incomplete. This is supported by remaining amounts of LiBH4 (Figure 9a). The PCI curves of 2LiBH4 + nano-MgH2* show two plateaus A and C at 335 and 304 °C. The formation of MgB2 cannot be observed after desorption. However, the MgB2 phase is formed during desorption of 2LiBH4 + nano-MgH2* at the higher temperature of 360 °C (eq 1, Figure 9a) from the reaction of Mg and B (eq 8). It seems that this reaction is very slow because no formation of MgB2 is observed during decomposition into vacuum for 10 h at 380 °C (see Figures 2 and 1b). Shaw et al. studied the hydrogenation mechanism of 2LiH + MgB2 and reported a mechanism starting from produced Mg1−xLi2xB2 during decomposition. Hydrogenation from this compound leads to the formation of a 2LiBH4 + MgH2 composite.47 In this case, there should be two steps of hydrogenation. From Figure 9b,c, it can be concluded that there are no differences for the hydrogenation reaction, independently of whether starting from MgB2 and LiH or from LiH, B, and Mg. In both cases, MgH2 and LiBH4 phases are formed at 360 °C. After desorption of the rehydrogenated 2LiBH4 + nano-MgH2* sample, 8.8 wt % hydrogen is released. This is much higher compared to 4.8 wt % measured for a 2LiBH4 + MgH2 composite at the same temperature. The amount of absorbed hydrogen for both different composites is lower than the amount of released hydrogen during the decomposition. This may be due to the presence of residues of unreacted MgB2 and LiH (Figure 9b,c). However,

If eq 1 is subtracted two times from eq 3, eq 9 is obtained with a value for the enthalpy change of −71.1 kJ mol−1. This agrees very well with values from −71.6 to −73.2 kJ mol−1 for the enthalpy of formation of MgB2 in the temperature range 383−357 °C.45 Mg + 2B = MgB2 + 3· (69.2 − 92.9) kJ/mol = MgB2 − 71.1 kJ/mol

(9)

Analyzing desorption and absorption curves for 2LiBH4 + nano-MgH2* and 2LiBH4 + MgH2 at 360 °C (Figure 8), differences are apparent. 2LiBH4 + nano-MgH2* shows three obvious plateaus (plateau A, B, and C), corresponding to the reactions described by eqs 7, 3, 1, and 8. However, 2LiBH4 + MgH2 show only two plateaus. These two plateaus are

Figure 8. Desorption (open marks) and absorption (filled marks) pressure−composition isotherm (PCI) curves of 2LiBH4 + nanoMgH2* composite and 2LiBH4 + commercial MgH2 composite at 360 °C. 2346



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Figure 9. X-ray diffraction patterns of (a) 2LiBH4 + commercial MgH2 composite after desorption pressure−composition isotherm (PCI) measurement at 360 °C, (b) 2LiBH4 + commercial MgH2 composite after desorption and absorption PCI cycle at 360 °C, and (c) 2LiBH4 + nanoMgH2* composite after desorption and absorption PCI cycle at 360 °C.

Notes

higher pressures may lead to a complete hydrogenation to 2LiBH4 + MgH2.36

The authors declare no competing financial interest.

5. CONCLUSIONS Using nanocrystalline Ti-catalyzed MgH2, 2LiBH4 + nanoMgH2* shows significantly enhanced kinetics compared to a composite of 2LiBH4 + MgH2. Desorption temperatures at reduced pressure are 50−70 °C lower; desorption at 380 °C and a back-pressure of 0.4 MPa is much faster for 2LiBH4 + nano-MgH2* composite. MgB2 is formed after desorption. During the hydrogen desorption from 2LiBH4 + nanoMgH2*, the PCI measurement shows three plateaus at temperatures between 357 and 383 °C. The desorption reactions assigned to the three plateaus are MgH2 → Mg + H2, Mg + 2LiBH4 → MgB2 + 2LiH + 3H2, and 2LiBH4 → 2B + 2LiH + 3H2. The reaction Mg + B → MgB2 takes place after the third reaction. At 304 and 335 °C, two plateaus are found during desorption of 2LiBH4 + nano-MgH2* composite; the two corresponding desorption reactions are MgH2* → Mg + H2 and 2LiBH4 → 2B + 2LiH + 3H2. At 360 °C three different plateaus for desorption from 2LiBH4 + nano-MgH2* are observed, while for 2LiBH4 + MgH2 only two plateaus are detected. The reaction Mg + 2LiBH4 → MgB2 + 2LiH + 3H2 does not take place during desorption from 2LiBH4 + MgH2. However, this reaction takes place for 2LiBH4 + nano-MgH2* because of the significantly enhanced kinetics of catalyzed nanocrystalline MgH2 compared to noncatalyzed MgH2. After desorption, 2LiBH4 + nanoMgH2* shows improved rehydrogenation performance compared to 2LiBH4 + MgH2.

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ACKNOWLEDGMENTS

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REFERENCES

In addition to the basic funding of the Max-Planck-Society, this work was financially supported by the German-Chinese Sustainable Fuel Partnership (GCSFP). H.S. acknowledges the support from International Institute for Carbon-Neutral Energy Research (WPI-I2CNER) at Kyushu University in Japan.

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AUTHOR INFORMATION

Corresponding Author

*E-mail felderhoff@mpi-muelheim.mpg.de (M.F.). Present Address

H.S.: International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Japan. 2347



Article

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Kinetics Enhancement, Reaction Pathway Change, and Mechanism

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