Article Cite This: J. Phys. Chem. A 2018, 122, 1344−1349
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Theoretical Study of the Metal-Controlled Dehydrogenation Mechanism of MN2H3BH3 (M = Li, Na, K): A New Family of Hydrogen Storage Material Tong Li and Jian-Guo Zhang* State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, People’s Republic of China
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S Supporting Information *
ABSTRACT: Metal hydrazineboranes (MHBs), as a kind of new hydrogen storage materials, show excellent hydrogen storage performance and dehydrogenation properties. Herein, we designed multiple dehydrogenation pathways to compare the metal-controlled effect. Quantum chemistry theory is used to calculate the crystal structure for determining the molecular structure. With an increase of the metal radius, the energy difference of the isomers also increases. The dehydrogenation pathways of lithium hydrazineborane (path A) and sodium hydrazineborane (path B) appear totally similar to each other in the dehydrogenation process despite the energy barrier, as well as the comparison paths A′ (for LiHB) and B′ (for NaHB). In contrast with LiHB and NaHB, the tautomeric reaction occurs in the potassium hydrazineborane (KHB) first, and the following dehydrogenation path is similar to that of the LiHB and NaHB. It explores the hydrogen-release properties of the different metal hydrazineboranes and also indcates the affection of the metal in the metal hydrazineboranes hydrogen-storage system.
1. INTRODUCTION With increasing environmental pollution, new energy, especially clean energy, has become a significant research field. Hydrogen, as the lightest element in the world, shows good performance in the clean energy field.1−3 It has a high combustion heat. However, the big problem of storage is an obstacle. Therefore, research of safe and high-capacity hydrogen storage material is more and more important. Ammoniaborane (AB, NH3BH3) is a kind of potential hydrogen storage material with thermal stability and a high content of hydrogen (19.7 wt %).4−10 Reaction between AB and metal hydride can produce a new hydrogen storage material, metal ammoniaborane (MAB, MNH2BH3).11−13 Metal replaces one hydrogen to become stable. Meanwhile, it can also decrease the hydrogen-release temperature and prevent the poisonous byproduct, borane. Therefore, MAB is environmentally friendlier than AB, but the hydrogen content is lower than that of AB, which can release most of its content at approximately 363 K. The electronic and dehydrogenation mechanisms of AB and MAB have been deeply theoretically and experimentally studied.14−19 As the derivative of AB, hydrazineborane (HB, N2H4BH3) is a kind of new hydrogen storage material composed of a hydrazino and borane. The hydrogen content of HB is 15.37 wt %, which is a little lower than that of ABs. Thomas Hügle et al. reported the synthesis and the thermal behavior of HB,20 which indicated that the higher temperature can make HB release more hydrogen and it can release 5.8 wt % hydrogen at 100 °C in 12 min. Hence, HB is a kind of potential hydrogen storage material. Similar to the reaction of AB with the metal hydrides, © 2018 American Chemical Society
ball-milling the mixture of HB and metal hydrides can release the hydrogen and form a metal hydrazineborane (MHB, MN2H3BH3). MHBs show good performance in the hydrogen storage and have potential applications in the hydrogen storage field. LiN2H3BH3 (LiHB) is a kind of typical MHB that was first reported by Wu et al.21 It starts to release H2 below 70 °C, and the sharp gas release is above 3H2 per mol of LiHB between 100 and 200 °C. The dehydrogenation process shows that LiHB can release 9.5 wt % of hydrogen (92% of the total weight loss). Minor amounts of byproducts N2 and NH3 were also detected in the production. Same as the LiHB, NaN2H3BH3 (NaHB) is a fine hydrogen storage material with a hydrogen content of 8.85 wt %, which was reported by Moury et al.22 NaHB can release 1 mol of H2 at 95 °C during 5 min. In contrast to the pure HB (1 mol of hydrogen during 96 min), the existence of Na can significantly decrease the dehydrogenation temperature. KN2H3BH3 (KHB) has the same properties as LiHB and NaHB. According to the research of Chua et al.,23 the KHB can also be synthesized by ball-milling the KH and the HB, but only the XRD pattern data was collected. Herein, the geometry structure and reasonable dehydrogenation pathways were investigated. Because of the two different N atoms in the MHB molecule, there is isomerism in the initial structures. Therefore, we designed multiple dehydrogenation Received: October 26, 2017 Revised: January 1, 2018 Published: January 11, 2018 1344
DOI: 10.1021/acs.jpca.7b10586 J. Phys. Chem. A 2018, 122, 1344−1349
Article
The Journal of Physical Chemistry A pathways based on the different initial structures. First of all, metal can get the H atom to form metal hydrides with the BH3 group, and it can improve the dehydrogenation ability of the MHB. As the Lewis alkali, metal hydride carries the H− to and from the hydrogen with the Lewis acid N2H3BH2. A different attack position can also bring about isomerism. Hence, the dehydrogenation of the MHBs is a complicated process, but a metal-controlled, comprehensive mechanism can help to develop the field of the MHB hydrogen storage materials.
Table 1. Crystal Geometry Parameters of the MHBs (units: Å) LiHB2 NaHB2
M−N1
M−N2
M−B
N1−N2
B−N2
2.081 2.402
3.019 3.282
3.242 3.313
1.498 1.464
1.574 1.552
parameters were also observed to analyze the thermal behavior, shown in Figure SI4 in the Supporting Information. 3.2. Geometry Structure of a Single Molecule and NBO Analysis. The initial molecular structures of LiHB and NaHB were obtained from the crystal structure, and the molecular structure of KHB was designed in a similar way. The optimized structures MHB1 as well as another possible structure MHB2 that we designed are shown in Figure 2.
2. COMPUTATIONAL METHODS The geometries and electronic structure were calculated by Gaussian 09.24 Density functional theory (DFT) of the B3LYP25 method with the basis set of 6-311G (3d, 2p) was used to analyze the geometries of the stationary points and the transition states. For a further analysis, the frequency and the thermal constant were also predicted. To identify the correction of the transition state, the intrinsic reaction coordinate (IRC) was applied at the same level.26 The reaction rate constant was explored by conventional transition state theory. On the basis of the geometric structures optimized by B3LYP/6-311G (3d, 2p) above, the single-point energies (SPE) were calculated by MP2/cc-Pvtz to obtain a minimum potential energy curve. For further study of the electronic structure, NBO analysis was conducted to explore the charge and the Wiberg bond of the MHBs. The Dmol3 code27 of Materials Studio 5.0 software was employed to calculate the periodic crystal structure. The Perdew−Burke−Ernzerhof (PBE)28 variant of the generalized gradient approximation (GGA)29 was used to calculate the exchange−correlation energy. The convergence criteria required the total energy of the system to converge to 1.0 × 10−5 Ha, with the remaining forces on the atoms being less than 0.002 Ha/Å and the displacements of atoms being less than 0.001 Å. In this calculation, the DNP orbital basis set was chosen to explore all electrons, while 3 × 2 × 2 k-points were used to sample the Brillouin zone for LiHB and NaHB. The smearing width in the calculation was set to 0.05 Ha for easier convergence.
Figure 2. Comparison of the MHB isomer (Li: pink; Na: red; K: blue; N: brown; B: purple; H: cyan).
MHB1 (M = Li, Na, K) can be considered that the metal bonds with the N1 atom and the B atom, M−N−N−B forms a quadrilateral structure. It is obvious that the M−N bond and the M−B bond are shorter than that in the crystal structure, but the N−N bond and the N−B bond show no difference with the internal molecular effect. Different from MHB1, MHB2 can be seen as the metal bonding with the N2 atom, and the M−N−B forms a triangle structure. Isomers of MHBs show a different geometry and energy, which are also explored in Figure 2. The significant geometry parameters are listed in Table 2. It can be Table 2. Molecular Geometry Parameters of the MHBs (units: Å) KHB1 KHB2 LiHB1 LiHB2 NaHB1 NaHB2
3. RESULT AND DISCUSSION 3.1. Crystal Structure. The crystal parameter used was taken from the literature. As shown in Figure 1, different from
M−N1
M−N2
M−B
N1−N2
B−N2
2.480 3.694 1.802 3.017 2.160 3.391
3.183 2.603 2.416 1.897 2.797 2.267
2.922 2.721 2.187 1.980 2.535 2.364
1.464 1.453 1.461 1.449 1.463 1.452
1.601 1.561 1.602 1.565 1.600 1.567
seen that the metal bonds with the different N atoms. It can slightly influence the geometry structure, but the metal change has little influence. Electrostatic potential analysis was also made to explore the initial reaction of the MHB molecule, which can be seen in Figure SI3. Meanwhile, the AdNDP method30 was also used to explore the chemical bond of the single molecular structure. The result shows that there are all covalent bonds in the HB structure and the metal connects with HB by an ionic bond. It is similar to the DOS analysis results. 3.3. Dehydrogenation of LiHB. As the slightest metal in the earth, Li can react with HB to from the LiHB, which is synthesized and discussed by Hui Wu et al. The potential dehydrogenation pathway is shown in Figures 3 and 4. In the LA pathway, from LA1 to LTSA1, Li carries the H atom from the N atom to form the LiH, which can be regarded as the key
Figure 1. Optimized crystal structure of LiHB and NaHB (Li and Na: purple; N: blue; B: pink; H: white).
the single molecule, the crystal structures are more reasonable considering the Madelung effect. The relevant parameters are shown in Table 1. N1 is the N atom in the middle of the molecule, and N2 is the N atom on the side. The density of states was calculated to show the electronic structure (Figures SI1 and SI2 in the Supporting Information). Thermal 1345
DOI: 10.1021/acs.jpca.7b10586 J. Phys. Chem. A 2018, 122, 1344−1349
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The Journal of Physical Chemistry A
Figure 3. Schematic energy profiles for the LA and LB pathways. Figure 5. Schematic energy profiles for the LA′ and LB′ pathways.
Figure 4. Dehydrogenation process of LA and LB pathways (Li: pink; N: brown; B: purple; H: cyan). Figure 6. Dehydrogenation process of LA and LB pathways (Li: pink; N: brown; B: purple; H: cyan).
point in the dehydrogenation progress. The energy barrier of the first step is 44.00 kcal·mol−1. With the loss of a H atom, the atoms of the Lewis pair B−N access each other to build the triangle structure (N−N−B), and the whole structure can be divided into two parts. One part is LiH, and the other is the ring of N−N−B. Then, the LiH carries the negative H−(Li) to access the positive H+(N) to form one molecule of hydrogen with a low energy barrier of only 0.47 kcal·mol−1, indicating that this step can easily occur. With the loss of hydrogen, the structure becomes stable and the Li bond with two N atoms can hardly get one more H atom from the BH2 group. The second dehydrogenation step depends on the electrostatic interaction of the H−(B) and H+(N), which shows a high energy barrier of 62.31 kcal·mol−1. The LB pathway starts from the initial structure of LiHB2, whose energy barrier is 5.58 kcal·mol−1 lower than that of LiHB1. Similar to the LA pathway, the Li seizes a H atom from the B atom with an energy barrier of 32.78 kcal·mol−1. It is 11.22 kcal·mol−1 lower than the LA pathway. From LB2 to LTSB2, the dehydrogenation bond of Li−H−···H+−N forms and one molecule of hydrogen releases with an energy barrier of 6.03 kcal·mol−1. In the structure LB3, the Li is far from the B atom; therefore, it is difficult to get the H(B) atom, and the dehydrogenation reaction is hard to carry out. Different from the LiAB, there are two N atoms in the LiHB molecule, which indicates that the different dehydrogenation bond formed by the LiH with H(N1) and H(N2) should be discussed. As shown in the Figures 5 and 6, LiH forms a dehydrogenation bond with the H(N1) atom with an energy barrier of 21.78 kcal·mol−1 in the LA′ pathway; it is much higher than the same step in the LA pathway. For comparison with the LA pathway, we designed a direct dehydrogenation mechanism based on the electrostatic interaction of the H+(N) and H−(B). The H+(N) and H−(B) approach each other and release one molar hydrogen with an energy barrier of 34.61 kcal·mol−1; it indicates that formation of the LiH plays a important role in decreasing the energy barrier. In the LB′
pathway, LiH also forms a dehydrogenation bond with the H(N1) atom. The energy barrier of this step is 13.03 kcal· mol−1, which is twice that of the LB pathway, but it is not too high to obstruct the reaction. 3.4. Dehydrogenation of NaHB. The dehydrogenation process of NaHB is similar to that of LiHB. With an increase of the radius of the metal, the energy difference of the isomers (NaHB1 and NaHB2) is also increased. As shown in Figures 7
Figure 7. Schematic energy profiles for the NA and NB pathways.
and 8, from NA1 to TSNA1, the Na atom gets one H from a N atom to form the metal hydrides. The energy barrier of this step
Figure 8. Dehydrogenation process of NA and NB pathways (Na: red; N: brown; B: purple; H: cyan). 1346
DOI: 10.1021/acs.jpca.7b10586 J. Phys. Chem. A 2018, 122, 1344−1349
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The Journal of Physical Chemistry A is 70.62 kcal·mol−1; it indicates that hydrogen transfer is difficult to occur. However, the energy barrier of the formation of the dehydrogenation bond (Na−H···H−N) is only 0.34 kcal· mol−1; such a low energy barrier predicts that the hydrogen can release in a short time. From NA3 to NTSA3, the hydrogen releases in a direct way without the help of a Na atom but depends on the electrostatic interaction of the H−(B)···H+(N); the energy barrier is 64.61 kcal·mol−1, and it shows that the metal can obviously decrease the dehydrogenation barrier. The whole energy barrier of the NB pathway is lower than the NA pathway, which should be regarded as the main dehydrogenation pathway of NaHB. From NB1 to NTSB1, Na gets one H atom, which is similar to the NA pathway, and the energy barrier is 33.61 kcal·mol−1. It is slightly higher than the same step in the LA pathway, which can estimate that the increase of the metal ionic radius can also raise the dehydrogenation energy barrier. From NB2 to NTSB2, the dehydrogenation bond forms with an energy barrier of 3.37 kcal·mol−1 and 1 mol. of hydrogen release. The energy of NB2 is a little higher than that of NA2. For comparison to the NA and NB pathways, we designed NA′ and NB′ pathways, which change dehydrogenation position (shown in Figures 9 and 10). From NA2 to
Figure 11. Schematic energy profiles for the isomerization reaction of KHB (K: blue; N: brown; B: purple; H: cyan).
barrier of the reaction is 48.08 kcal·mol−1. Compared to the energy barrier of formation of the LiH and NaH in the LA and NA pathway (44.01 kcal·mol−1 for LiHB, 70.62 kcal·mol−1 for NaHB), it can be indicated that the tautomer reaction happens before the KH forms. Therefore, we discuss only one dehydrogenation pathway, whose initial structure is KHB2. From KA2 to KTSA2 (shown in Figure 12), the K atom gets one H atom from the BH3 group and forms KH with an energy
Figure 9. Schematic energy profiles for the NA′ and NB′ pathways.
Figure 12. Schematic energy profiles for the KA pathways (K: blue; N: brown; B: purple; H: cyan).
barrier of 37.40 kcal·mol−1. The structure of KA3 can be divided into two parts, one molecule of KH and one molecule of hydrazineborane (the bond length of K−N is 2.824 Å), and the H− of the KH tends to access the H+ of the NH2NHBH2. Because of the short distance of the two H atoms, the dehydrogenation bond can be easily formed with an energy barrier of 2.06 kcal·mol−1; it is similar to the LiHB and NaHB. In structure KA4, the K atom builds an ionic bond with the N1 atom; the bond length is 2.42 Å, and the K atom can hardly get the H atom; the whole structure remains stable. 3.6. Dehydrogenation Rate Calculation. For further thermochemistry analysis, the free energy of activation (ΔG⧧ ) was calculated to evaluate the energy barrier of the dehydrogenation step. The first step of the dehydrogenation of the different pathways was chosen because they all show the highest energy barrier. The relevant parameters are shown in Table 3. It obviously that ΔG in NaHB is the lowest, but in KHB, it is the highest. According to the formula
Figure 10. Dehydrogenation process of NA′ and NB′ pathways (Na: red; N: brown; B: purple; H: cyan).
NTSA′2, NaH from the dehydrogenation bonds with a H(N1) atom with an energy barrier of 21.55 kcal·mol−1. Different from the LA′ pathway, the second dehydrogenation step in the NA′ pathway is not the straight dehydrogenation but forms the NaH first. The energy barrier is 24.35 kcal·mol−1. It shows that the formation of the MH can effectively decrease the dehydrogenation energy. The energy of the NB′ pathway is lower than that of NA′ pathway, and the dehydrogenation barrier is only 6.23 kcal·mol−1. The structure of NB′3 is more stable; both the formation of the NaH and straight dehydrogenation are difficult to carry out. 3.5. Dehydrogenation of KHB. The dehydrogenation process of KHB is different from that of LiHB and NaHB. With the increase of the metal radius, there is a tautomer reaction from KHB1 to KHB2. As shown in Figure 11, the energy 1347
DOI: 10.1021/acs.jpca.7b10586 J. Phys. Chem. A 2018, 122, 1344−1349
The Journal of Physical Chemistry A
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Table 3. Gibbs Free Energy of MHB (units: kJ/mol) ΔG
LiHB1
NaHB1
KHB
85.52
77.09
106.85
Article
AUTHOR INFORMATION
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[email protected]. ORCID
⧧ KT k = b e−ΔG / RT h
Jian-Guo Zhang: 0000-0003-0175-2274 Notes
The authors declare no competing financial interest.
where kb is the Boltzmann constant, R is the universal gas constant, h is the Planck constant, and T is the temperature. The reaction reaction rate constant k was calculated for corresponding pathways. The results are listed in Table 4. In
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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Natural Science Foundation of China (NSFC 21071019) and the 111 project (B17004) in China.
Table 4. Dehydrogenation Rate of MHB (units: s−1) LiHB NaHB KHB
80 °C
90 °C
100 °C
110 °C
120 °C
1.65 29.11 0.001
3.78 61.68 0.003
8.29 125.64 0.009
17.48 246.75 0.02
35.50 468.55 0.05
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fact, we also calculated the k values of the LA and NA pathways, but it is ultralow (
