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The Theoretical Study of Metal-Controlled Dehydrogenation Mechanism of MNHBH (M=Li,Na,K), a New Family of Hydrogen Storage Material 2

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Tong Li, and Jian-Guo Zhang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b10586 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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The Journal of Physical Chemistry

The Theoretical Study of MetalMetal-Controlled Dehydrogenation Mechanism of MN2H3BH3 (M=Li,Na,K) (M=Li,Na,K): A New Family of Hydrogen Storage Material Tong Lia, Jian-Guo Zhang*a aState

Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081 P. R.

China ABSTRACT: Metal hydrazineboranes (MHBs), as a kind of new hydrogen storage materials, show excellent hydrogen storage performance and dehydrogenation property. 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 the increasing of the metal radius, the energy difference of the isomers also increases. The dehydrogenation pathways of Lithium hydrazineborane (PathA) and Sodium hydrazineborane (PathB) show totally similar to each other in the dehydrogenation process despite of the energy barrier, as well as the comparison Path of A’ (for LiHB) and B’ (for NaHB). In contrast with LiHB and NaHB, the tautomeric reaction occurs in the Potassium hydrazineborane (KHB) firstly and the following dehydrogenation path is similar to 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.

release most of it content at approximately 363K. The electronic and the dehydrogenation mechanism of AB and MAB have been deeply theoretically and experimentally studied14-19. As the derivative of AB, hydrazineborane (HB, N2H4BH3) is a kind of new hydrogen storage material composed by a hydrazino and borane. The hydrogen content of hydrazineborane is 15.37 wt %, which is a little lower than AB’s. Thomas Hü gle etc, reported the synthesis and the thermal behavior of HB20, which indicated that the higher temperature can make HB release more hydrogen, and it can release 5.8wt % hydrogen at 100oC, 12 min. Hence, HB is a kind of potential hydrogen storage material. Similar to the reaction of AB with the metal hydrides, ball-milling the mixture of HB and metal hydrides can release the hydrogen and form the metalhydrazineborane (MHB, MN2H3BH3). MHBs show good performance in the hydrogen storage and have potential applications in the hydrogen storage field.

1. Introduction With the increasing environment pollution, the new energy, especially the clean energy becomes a significant research field. Hydrogen, as the lightest element in the world, shows a good performance in the clean energy field 1-3. It has a high combustion heat. However, the big problem of storage makes the obstacle. Therefore, the 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 high content of hydrogen (19.7wt %) 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 ture stable. Meanwhile, it can also decrease the hydrogen-release temperature and prevent the poison by-product, borane. Therefore, MAB is more environment-friendly than AB, but the hydrogen content is lower than AB which can S1

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LiN2H3BH3 (LiHB) is a kind of typical metal-hydrazineborane which is firstly reported by Wu et al21. It starts to release H2 below 70 oC, and the sharp gas releasing is above 3H2 per mol LiHB between 100 and 200 oC. The dehydrogenation process shows that LiHB can release 9.5 wt% of hydrogen (92% of the total weight loss). The minor amount by-product N2 and NH3 were also detected in the production. Same as the LiHB, NaN2H3BH3 (NaHB) is a fine hydrogen storage material with the hydrogen content of 8.85 wt% which is reported by Moury et al22. NaHB can release 1mol H2 at 95 oC during 5 min. In contrast to the pure HB (1mol hydrogen during 96min), the existence of Na can significantly decrease the dehydrogenation temperature. KN2H3BH3 (KHB) has the same properties with LiHB and NaHB. According to the research of Chua etc23, the KHB can also be synthesized by ball-milling the KH and the HB, but only the XRD pattern date was collected. Herein, the geometry structure and reasonable dehydrogenation pathways were investigated. Because of the different two N atoms in the MHB molecular, there is isomerism in the initial structures. Therefore, we designed multiple dehydrogenation pathways based on the different initial structures. First of all, metal can get the H atom to form metal hydrides with the BH3 group, it can improve the dehydrogenation ability of the MHB. As the lewis alkali, metal hydride carries the H- to from the hydrogen with the lewis acid N2H3BH2. Different attack position can also bring about the isomerism. Hence, the dehydrogenation of the MHBs is a complicated process by the metal-controlled, comprehensive mechanism can help to develop the field of the MHB hydrogen storage material.

stationary point 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 level26. The reaction rate constant was explored by the conventional transition state theory. Based on the geometries structures optimized by B3LYP/6-311G (3d, 2p) above, the single point energies (SPE) were calculated by the MP2/cc-Pvtz to obtain the minimum potential energy curve. For a further study of the electronic structure, the NBO analysis was made 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 28 Perdew-Burke-Ernzerhof (PBE) variant of the generalised 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.

3. Result and discussion 3.1 Crystal structure The crystal parameter used was taken from literatures. As shown in Figure1, different from the single molecule, the crystal structures are more reasonable with the consider of Madelung effect. The relevant parameters are show in Table1. N1 is N atom in the middle of the molecule, and N2 is the N atom by the side. The density of states was calculated to show the electronic structure (Figure SI1 and Figure SI2 in supporting information). Thermal parameters were also observed to analysis the thermal behaviours.

2. Computational methods All the geometries and electronic structure were calculated by the Gaussian 09 program24. Density functional theory (DFT) of B3LYP25 method with the basis set of 6-311G (3d, 2p) were used to analyze the geometries of the S3


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The FigureSI4 was shown in supporting information.

HB structure and the metal connects with HB by ionic bond. It’s similar with the DOS analysis result.

Figure1. The optimized crystal structure of LiHB and NaHB (Li and Na: purple,N:blue,B:pink,H:white)

Figure2. The comparison of the MHB isomer (Li:pink ， Na:red,K:blue,N:brown,B: purple,H:cyan)

Table 1 Crystal geometry parameters of the MHBs (unit: Å) LiHB2

NaHB 2

M-N1

M-N2

M-B

N1-N2

B-N2

2.081

3.019

3.242

1.498

1.574

2.402

3.282

3.313

1.464

Table 2 Molecular geometry parameters of the MHBs (unit: Å)

1.552

3.2 Geometry structure of single molecule and NBO analysis The initial molecular structures of LiHB and NaHB were gotten from the crystal structure and the molecular structure of KHB was designed in the similar way. The optimized structures MHB1 as well as another possible structure MHB2 we designed are shown in Figure2. MHB1 (M=Li, Na, K) can be regarded that the metal bonds with the N1 atom and the B atom, M-N-N-B forms a quadrilateral structure. It's 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, the MHB2 can be seen that the metal bonds with the N2 atom and the M-N-B forms a triangle structure. Isomers of MHBs show the different geometry and the energy which are also explored in Figure2. The significant geometry parameters are listed in Table 2. It can be seen that the metal bond the different N atoms. It can can slightly influence the geometry structure, but metal change makes little influence. Electrostatic potential analysis was also made to explore the initial reaction of the MHB molecule, which can be seen in the 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

M-N1

M-N2

M-B

N1-N2

B-N2

KHB1

2.480

3.183

2.922

1.464

1.601

KHB2

3.694

2.603

2.721

1.453

1.561

LiHB1

1.802

2.416

2.187

1.461

1.602

LiHB2

3.017

1.897

1.980

1.449

1.565

NaHB1

2.160

2.797

2.535

1.463

1.600

NaHB2

3.391

2.267

2.364

1.452

1.567

3.3 The dehydrogenation of LiHB As the slightest metal in the earth, Li can react with hydrazineborane to from the LiHB which is synthesized and discussed by Hui Wu etl. The potential dehydrogenation pathway is shown in Figure3 and Figure4. In the LA pathway, from LA1 to LTSA1, Li carries the H atom from N atom to form the LiH which can be regarded as the key point in the dehydrogenation progress. The energy barrier of the first step is 44.00kcal∙mol-1. With the loss of H atom, the lewis pair B-N accesses to each other and 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 hydrogen with a low energy barrier of only 0.47kcal∙ mol-1, indicating that this step can easily occur. With the loss of the hydrogen, the structure become 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

S4



energy

barrier

of

62.31kcal ∙ mol-1.

designed the 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 the energy barrier of 34.61kcal∙mol-1, it indicates that the formation of the LiH plays a important role in decreasing the energy barrier. In the LB’ pathway, LiH also forms the dehydrogenation bond with the H(N1) atom. The energy barrier of this step is 13.03kcal ∙ mol-1 which is twice than LB pathway, but it’s not too high to obstruct the reaction.

Figure3 Schematic energy profiles for the LA and LB pathways

Figure4.The dehydrogenation process of LA and LB pathway. (Li:pink,N:brown,B: purple,H:cyan)

The LB pathway starts from the initial structure of LiHB2, which energy barrier is 5.58kcal∙mol-1 lower than LiHB1. Similar with the LA pathway, the Li seizes an H atom from B atom with the energy barrier of 32.78kcal∙ mol-1. It’s 11.22 kcal∙mol-1 lower than LA pathway. From LB2 to LTSB2, the dehydrogenation bond of Li-H-…H+-N forms and one molecule of hydrogen releases with the energy barrier of 6.03kcal∙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 take place. Different from the LiAB, there are two N atoms in the LiHB molecule, which indicates the different dehydrogenation bond formed by the LiH with H(N1) and H(N2) should be discussed. As shown in the Figure 5 and Figure 6, LiH forms the dehydrogenation bond with the H(N1) atom with the energy barrier of 21.78kcal∙mol-1 in the LA’ pathway, it’s much higher than the same step in the LA pathway. For the comparison with LA pathway, we

Figure5 Schematic energy profiles for the LA’ and LB’ pathways

Figure6 The dehydrogenation process of LA and LB pathway. (Li:pink,N:brown,B: purple,H:cyan)

3.5 The dehydrogenation of NaHB The dehydrogenation process of NaHB is similar to that of LiHB. With the increasing of the radium of the metal, the energy difference of the isomers (NaHB1 and NaHB2) is also increased. As shown in Figure7 and Figure8, from NA1 to TSNA1, Na atom gets one H from N atom to form the metal hydrides. The energy barrier of this step is 70.62kcal ∙ mol-1, it indicates that the hydrogen transfer is difficult to occur. However, the energy barrier of the S5


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formation of the dehydrogenation bond (Na-H …H-N) is only 0.34kcal∙mol-1, such low energy barrier predicts that the hydrogen can release in a short time. From NA3 to NTSA3, the hydrogen release in a direct way without the help of Na atom but depended on the electrostatic interaction of the H-(B)…H+(N),

(shown in Figure9 and Figure 10). From NA2 to NTSA’2, NaH from the dehydrogenation bond with H(N1) atom with the energy barrier of 21.55kcal ∙ mol-1. Different from the LA’ pathway, the second dehydrogenation step in NA’ pathway is not the straight dehydrogenation, but forms the NaH first. The energy barrier is 24.35kcal∙mol-1. It explores that the formation of the MH can effectively decrease the dehydrogenation energy. The energy of the NB’ pathway is lower than NA’ pathway and the dehydrogenation barrier is only 6.23kcal∙mol-1. The structure of NB’3 is more stable, both of the formation of the NaH and straight dehydrogenation are difficult to occur.

the energy barrier is 64.61kcal∙mol-1, it show that the metal can obviously decrease the dehydrogenation barrier.

Figure7 Schematic energy profiles for the NA and NB pathways

Figure9 Schematic energy profiles for the NA’ and NB’ pathways Figure8 The dehydrogenation process of NA and NB pathway. (Na:red,N:brown,B: purple,H:cyan)

The whole energy barrier of the NB pathway is lower than 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 with NA pathway and the energy barrier is 33.61kcal∙ mol-1. It’s slightly higher than the same step in LA pathway which can estimate that the increasing of the metal ionic radius can also raise dehydrogenation energy barrier. From NB2 to NTSB2, the dehydrogenation bond from with the energy barrier of 3.37kcal∙mol-1 and one molar of hydrogen release. The energy of NB2 is a little higher than NA2’s. For the comparison to the NA and NB pathway, we designed NA’ and NB’ pathway which changes the dehydrogenation position

Figure10 the dehydrogenation process of NA’ and NB’ pathway (Na:red,N:brown,B: purple,H:cyan)

3.6 The dehydrogenation of KHB The dehydrogenation process of KHB is different from LiHB and NaHB. With the increasing of the metal radium, there is tautomers reaction from KHB1 to KHB2. As shown in Figure11, the energy barrier of the reaction is 48.08kcal∙mol-1. Comparing to the energy barrier of formation of the LiH and NaH in the LA and NA pathway (44.01kcal∙ mol-1 for LiHB, 70.62kcal∙mol-1 for NaHB), it can be indicated that the tautomers reaction S6



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Figure 12 Schematic energy profiles for the KA pathways (K:blue,N:brown,B: purple,H:cyan)

happens before the KH forms. Therefore, we discuss only one dehydrogenation pathway which initial structure is KHB2. From KA2 to KTSA2 (show in Figure12), K atom gets one H atom from BH3 group and forms the KH with the energy barrier of 37.40kcal∙mol-1. The structure of KA3 can be divided into two parts: one molecule of KH and one molecule of hydrazinborane (the bond length of K-N is 2.824Å) and the H- of the KH trends to access the H+ of the NH2NHBH2. Because of the short distance of two H atom, the dehydrogenation bond can be easily formed with the energy barrier of 2.06kcal∙ mol-1, it’s similar to the LiHB and NaHB. In structure KA4, K atom build the ionic bond with N1 atom, the bond length is 2.42 Å and the K atom can hardly get the H atom, the whole structure keeps stable.

3.7 The dehydrogenation rate calculation For further thermochemistry analysis, the free energy of activation (ΔG≠) was calculated to evaluated the energy barrier of the dehydrogenation step. The first step of the dehydrogenation of the different pathways were chosen because they all show the highest energy barrier. The relevant parameters are shown in Table3. it obviously that ΔG in NaHB is the lowest but in KHB are the highest. According to the formula: ݇ൌ

‫ܭ‬௕ ܶ ି୼ୋஷ ݁ ோ் ݄

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 the Table4. In fact, we also calculated the k of LA and NA pathway, but it’s ultra-low(

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