Imides for Hydrogen Storage: A

Oct 24, 2008 - Qiang Wang , Zhiqian Chen , Wenbin Yu , Yungui Chen and Yuan Li. Industrial & Engineering Chemistry Research 2010 49 (13), 5993-5996...
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18264

J. Phys. Chem. C 2008, 112, 18264–18269

Role of Amino Anion in Metal Amides/Imides for Hydrogen Storage: A First Principle Study Qiang Wang,† Yungui Chen,*,† Jinggang Gai,‡ Chaoling Wu,† and Mingda Tao† School of Materials Science and Engineering and State Key Laboratory of Polymer Materials Engineering, Sichuan UniVersity, Chengdu, 610065, People’s Republic of China ReceiVed: July 28, 2008; ReVised Manuscript ReceiVed: August 31, 2008

For favorable thermodynamics and the considerable kinetics of the hydrogen reaction, Li-Mg-N-H compounds hold promise as novel hydrogen storage materials for on-board usage in metal amide hydrogen storage systems. To improve their performance, much effort has been devoted to the fundamental properties of metal amides, such as electronic structure, and the energetics of their hydrogen reactions. Though understanding the amino anion transition in the hydrogen reaction is essential for further development, the role of the amino anion in metal amides/imides for hydrogen storage is still ambiguous. In this study, we investigated the electronic structures and chemical bonds of Mg(NH2)2, LiNH2, and Li2MgN2H2 by way of a first principle approach. Then, the H vacancy formation energies in LiNH2 and Li2MgN2H2 were estimated to comprehend the stability of the amino anion in metal amides/imides and its effect on the thermodynamics of the hydrogen reaction. Also, the transformation between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in LiNH2 and Li2MgN2H2 was calculated for a deeper understanding of the chemical interaction of the (2LiNH2 + MgH2) dehydriding reaction. Last, but not least, special attention is devoted to the calculation of the energetics of this reaction and its rehydrogenation. 1. Introduction Hydrogen storage technology for automobiles, hydrogen systems, and so on is critical for the development of hydrogenbased energy, and has attracted enormous interest in recent years. Among the hydrogen storage systems, metal amides and imides, following the first report by Chen et al. in 2002,1 have been extensively researched regarding their thermodynamics and kinetics of the hydrogen reaction,2-6 and further investigated for the mechanism of hydrogen desorption and absorption behaviors.7-10 Among these systems, the Li-Mg-N-H system has been intensively investigated due to the favorable thermodynamics and considerable kinetics of hydrogen reaction,11-16 which could be dehydrogenation from (2LiNH2 + MgH2)11-13 or (Mg(NH2)2 + 2LiH),14-16 and rehydrogenation to (Mg(NH2)2 + 2LiH), as shown in reactions 1 and 2, respectively.

2LiNH2 + MgH2 w Li2MgN2H2 + 2H2

(1)

Li2MgN2H2 + 2H2 S Mg(NH2)2 + 2LiH

(2)

Since the Li-Mg-N-H system holds promise as novel hydrogen storage materials, understanding the fundamental properties of metal amides and imides such as LiNH2, Mg(NH2)2, and Li2MgN2H2 are essential to improving their performance. In recent years, many researchers have reported the electronic structures and energetics of the (LiNH2 + LiH)17,18 and (Mg(NH2)2 + 2LiH)19-21 systems for hydrogen storage. Furthermore, the lowest energy crystal structure of Li2MgN2H2 has been determined by first principle theoretical study19,22 due to the disordered occupation position of Li and Mg atoms in the crystal structure, which was examined by synchrotron in * To whom correspondence should be addressed. Telephone: +86-2885466916. Fax: +86-28-85466916. E-mail: [email protected]. † School of Materials Science and Engineering. ‡ State Key Laboratory of Polymer Materials Engineering.

Figure 1. Unit cells of (a) Mg(NH2)2, (b) LiNH2, and (c) Li2MgN2H2. Purple, green, blue, and gray spheres represent Li, Mg, N, and H atoms, respectively.

situ X-ray diffraction and neutron diffraction studies.23 However, there has been little theoretical study on the chemical bonding in Mg(NH2)2 and Li2MgN2H2, let alone systematic investigation of the role of the amino anion in these amides/imides for hydrogen storage application. In the present work, we focused on the role of the amino anion in the hydrogen reaction. For this purpose, the electronic structures and chemical bonding in these metal amides/imides were investigated by density of states (DOS) and population analysis. To further study their thermal stabilities for hydrogen storage, comparative studies were made of the energy costs of H vacancy formation in LiNH2 and Li2MgN2H2. Moreover, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of each were studied in the dehydrogenation reaction, which is expressed by reaction 1. Finally, the reaction enthalpies of dehydrogenation from (2LiNH2 + MgH2) and then rehydrogenation to (Mg(NH2)2 + 2LiH), as shown in reactions 1 and 2, were calculated to estimate the stability of these amides/imides for hydrogen storage.

10.1021/jp806700h CCC: $40.75  2008 American Chemical Society Published on Web 10/25/2008

Amino Anion in Metal Amides/Imides

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TABLE 1: Crystal Structures of Mg(NH2)2, LiNH2, and Li2MgN2H2 from Values Calculated in the Present Work and Experimental Values calculated x

y

experimental z

x

a ) 10.7182, c ) 20.5284 Mg(NH2)2 (Å) Mg N H

0.3720 0.2990 0 0.2235 0.2390 0.0577 0.2788 0.2872

0.1045 0.750 0 0.4805 0.8049 0.9528 0.4237 0.5284

y

0.1892 0.3750 0.3844 0.2511 0.4028 0.3533 0.2228 0.2791

0.3729 (0.373) 0.2899 (0.287) 0 (0) 0.2279 (0.013) 0.229 (0.236) 0.058 (0.058) 0.287 (0.229) 0.286 (0.209)

b

N H

0 0 0 0.2234 0.2977 0.2225

z

x

y

0 0.5 0.5 0.2464 0.3358 0.1110

0 0.25 0.0092 0.1136 0.1225 0.1866

0 0 0 0.2284 0.31 0.23

N H a

0.5000 0.0039 0.2783 0.1382

c

0 0.5 0.5 0.2452 0.36 0.15

0 0.25 0.0042 0.1148 0.11 0.17

experimental

y

0.0000 0.2385 0.1344 0.0594

z

a ) 5.037, c ) 10.278

z

x

a ) 9.8537, b ) 4.9597, c ) 5.2056 Li2MgN2H2 (Å) Li/Mg

0.1881 (0.063) 3/8 (0.25) 0.3812 (0.257) 0.2477 (0.376) 0.4030 (0.271) 0.3514 (0.239) 0.2249 (0.104) 0.2761 (0.148)

c

calculated x

b

experimental

y a ) 5.041, c ) 10.580

LiNH2 (Å) Li

a

0.1049 (0.361) 3/4 (0) 0 (0.25) 0.4882 (0.023) 0.813 (0.061) 0.953 (0.201) 0.434 (0.177) 0.537 (0.285)

calculated x

z

a ) 10.3758 (10.37 ), c ) 20.062 (20.15 ) a

y

z

a ) 9.7871, b ) 4.9927, c ) 5.2019 d

0.2369 0.7367 0.0219 -0.0216

0 0.25 0.1357 0.0644

d

0.5 0 0.2785 0.1427

d

0.25 0.75 0.0000 -0.0440

Reference 29. b Reference 30. c Reference 31. d Reference 23.

2. Calculation Procedures The electronic structures of the relevant hydrides in the Li-Mg-N-H system were calculated with the CASTEP computer code,24 which implants density function theory with a plane wave basis set.25,26 The generalized gradient approximation (GGA) of Perdew et al. (PBE)27 with ultrasoft pseudopotential28 was employed to include the exchange correlation energy in the total energy. Projector-augmented wave potentials were employed for the elemental constituents; the H, Li, Mg, and N potentials contained one, three, eight, and five valence electrons, respectively. In all the calculations, kinetic energy cutoff of 340 eV was employed to guarantee a good convergence for the total energy and forces acting on the atoms. The convergence criteria for energy and displacement are 2 × 10-6 eV/atom and 10-3 Å, respectively. The residue force and stress in equilibrium geometry are on the order of 3 × 10-3 eV/Å and 0.01 GPa. For the Brillouin zone, the k points selected are 2 × 2 × 1 for Mg(NH2)2, 5 × 5 × 2, for LiNH2, and 3 × 5 × 5 for Li2MgN2H2, respectively. Density function theory as implanted in the Dmol3 program was used to investigate the orbitals involved in LiNH2 and Li2MgN2H2. The calculations for amides or imides were done with double numerable plus polarization (DNP) local basis, and the same exchange-correlation function (PBE) was employed in all calculations. As a result, the orbital interactions among the atoms in the compounds were elucidated; the orbital energies and the energy gaps between HOMO and LUMO were calculated by this all-electron method. Both the density functional theory code of CASTEP and Dmol3 are from Accelrys.

The unit cells used in the present work are presented in Figure 1. Single crystal X-ray diffraction (SCXRD) shows that Mg(NH2)2 is tetragonal and of the I41/acd symmetry. The lattice parameters are a ) 10.37 Å and c ) 20.15 Å,29 respectively. In a recent report,30 neutron powder diffraction (NPD) demonstrated the same space group for Mg(ND2)2 with little difference in the lattice parameters, which are a ) 10.3758 Å and c ) 20.062 Å, respectively. However, there are some notable differences in the crystal structure data and the atom positions, as listed in Table 1. In this study, we first used both of the above experimental structures to compute the total energy of Mg(NH2)2: -1589.2606 eV/formula unit for the former and -1589.2866 eV/formula unit for the latter, respectively. Thus the latter structure is more likely to be the stable structure, which is shown in Figure 1a and was then used for subsequent analysis. The crystal structure of LiNH2 is body-centered tetragonal with space group jI4, Figure 1b, where the lattice parameters are a ) 5.037 Å and c ) 10.278 Å,31 respectively. The Li atoms occupy the 2a (0, 0, 0), the 2c (0, 0.5, 0.25), and the 4f (0, 0, 0.0042) positions, respectively, and the N atom occupies the 8g (0.2284, 0.2452, 0.1148) position. The two H atoms, denoted as H1 and H2 in Figure 1b, are located at the 8g (0.31, 0.36, 0.11) and (0.23, 0.15, 0.17) positions, respectively. Four formula units are contained in the unit cell. The crystal structure of Li2MgN2H2 at ambient conditions is characterized as a tetragonal unit cell, which belongs to the space group No. 45 (Iba2), and the lattice parameters are a ) 9.7871 Å, b ) 4.9927 Å, and c ) 20.15 Å,23 respectively. The N atom and H atom are located at the 8c (0.1357, 0.2785, 0.0000) and

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Figure 2. Total and partial densities of states of Mg(NH2)2.

the 8c (0.0644, 0.1427, -0.0440) positions, respectively. The six H atoms, which point to the body center in the unit cell, are denoted as H1, and the other six H atoms are denoted as H2. The Li and Mg atoms randomly distribute at the 4b (0, 0.5, 0.25) and 8c (0.25, 0, 0.75) positions in an approximately 2:1 ratio. In our previous work, the lowest energy crystal structure of Li2MgN2H2 was calculated and the most stable structure of Li2MgN2H2 was obtained, where the Mg atom is located homogeneously at the 4b (0, 0.5, 0.25) and 8c (0.25, 0, 0.75) positions, as elaborated in Figure 1c. The present calculated geometry of Li2MgN2H2 is in reasonable agreement with other calculations19 and experimental findings,23 which are listed in Table 1. The crystal structures of correlated hydrides, such as MgH2 and LiH, were established as refs 32 and 33 elucidated, and their total energies were calculated as -1010.0926 and -206.7231 eV, respectively. The total energies of isolated molecules N2 and H2 were calculated by a cubic box with an edge of 10 × 10 × 10 Å, and are -543.2918 and -31.5632 eV, respectively. 3. Results and Discussion 3.1. Electronic Densities of States. The total and partial density of states (DOSs) of Mg(NH2)2 are plotted in Figure 2. There are four bonding peaks in the total DOS of Mg(NH2)2 below the Fermi level. In the energy range from -17 to -14 eV, the peak is dominated by the interaction between the H s and N 2s electrons, and the second peak located in the energy range from -7.5 to -5 eV is due to the hybridization of H s and N 2p electrons. The other two bonding peaks, in the energy range from -5 eV to the Fermi energy, are mainly contributed by the bonding of N 2p and Mg 3s, 2p electrons. The N-H bond, corresponding to the former two bonding peaks, is dominated by the interaction between H s and N 2s, 2p electrons. It has a small overlap in these orbitals, as located in the energy range from -3.5 to -2.5 eV. This indicates that the chemical nature of the N-H bond is strongly covalent. Similarly, the Mg-N bond denoted by the later two bonding peaks has many overlaps between the N 2s, 2p and Mg 3s, 2p orbitals in the energy range from -5 eV to the Fermi energy, which implies that the bonding between the N atom and the Mg atom is mainly covalent, too. The total and partial DOSs of LiNH2 are illustrated in Figure 3. From -16 to -5 eV below the Fermi energy, the two peaks, indicative of the N-H bond, are primarily due to the H s and N 2s electron interaction for the first peak and the H s and N 2p electron interaction for the second peak. The third bonding

Wang et al.

Figure 3. Total and partial densities of states of LiNH2.

Figure 4. Total and partial densities of states of Li2MgN2H2.

peak in the energy range from -5 to -2.5 eV is dominated by bonding of H s, Li 2p, and N 2s, 2p electrons, and the fourth bonding peak from -2.5 eV to Fermi energy is resulted by the interaction between Li 2s, 2p and N 2p electrons, respectively. In conjunction with the slight overlaps of H s, Li 2p, and N 2s, 2p electrons in the third peak, this indicates that the state of both the N-H bond and the Li-N bond are a mixture of covalence and ion. However, the former is primarily covalent and the latter is mainly ionic. There are one bonding peak and two groups of bonding peaks in the total DOSs of Li2MgN2H2, as shown in Figure 4. The bonding peak in the energy range from -15 to -13 eV below the Fermi level is dominated by the bonding of H s with N 2s electrons; only a small level of Mg 3s and Li 2s, 2p electrons can be identified in this region. From -6 to -4 eV, the group of bonding peaks is mainly due to the interaction among H s, N 2s, 2p, and Li 2p electrons. There is a small overlap among these atomic orbitals, which infers that the N-H bond and Li-N bond are covalent and ionic mixed. By the same token, the group bonding peaks in the energy range from -4 eV to the Fermi energy, which is mainly contributed by the N 2p electrons and only a little by Mg 3s, 2p and Li 2s, 2p electrons, has a small overlap in these orbitals. This indicates that the Li-N bond and the Mg-N bonds in this compound are mixed bonds of ion and covalence. Moreover, the DOS difference between H1 and H2 as noted in Figure 4 will result in the bond strength difference between N1-H1and N2-H2, and more details will be discussed in the next section.

Amino Anion in Metal Amides/Imides

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TABLE 2: Mulliken Charges and Overlap Population Analysis for Mg(NH2)2, LiNH2, and Li2MgN2H2 Mulliken charges Mg(NH2)2 LiNH2 Li2MgN2H2

average overlap population

Li1 (Li2)

Mg

N1 (N2)

H1 (H2)

N(N1)-H1

N(N2)-H2

Li1(Li2)-N1(N2)

Mg-N1(N2)

0.73 0.63 (0.18)

1.61 1.68

-1.31 -1.09 (-) -1.25 (-1.27)

0.25 0.18 (0.17) 0.08 (0.11)

0.74 0.81 0.87

0.76 0.79 0.79

0.08 0.27 (0.25)

-0.44 -0.40 (-0.55)

TABLE 3: H Vacancy Formation Energies in LiNH2 and Li2MgN2H2a energy (eV) vacancy H1 LiNH2 Li2MgN2H2 a

-2.75 -2.58

(-4.38,b

N-H (Å) vacancy H2

-2.35 ) c

N(N1)-H1

N(N2)-H2

vacancy H1

vacancy H2

1.030 1.035

1.032 1.038

1.030 1.042

1.037 (1.037c) 1.039

-1.83 (-4.20, -2.02 ) -2.35 b

c

Vacancies H1 and H2 represent the vacancy formation of H1 and H2 atoms in amide and imide, respectively. b Reference 17. c Reference

37.

3.2. Nature of Chemical Bonds. To further analyze the bond type and strength, the atomic Mulliken charges and average overlap population for bonds in Mg(NH2)2, LiNH2, and Li2MgN2H2 compounds are demonstrated in Table 2. For Mg(NH2)2, the Mg and H atoms are the main electron donors and the N atom is the only electron acceptor; their Mulliken charges are 1.61, 0.25, and -1.31, respectively. The average overlap populations of the N-H1 bond and the N-H2 bond are 0.74 and 0.76, respectively. This indicates that the bonding interactions of N-H are extremely covalent. On the other hand, the average overlap population of the Mg-N bond is -0.44; thus it proves the nature of chemical interaction of the Mg-N bond is antibonding states. For LiNH2, the atomic Mulliken charges of Li, N, H1, and H2 are 0.73, -1.09, 0.18, and 0.17, respectively. This implies that most electrons of the N atom accepted are from the Li atom and little from the H atom. The average overlap populations for N-H1, N-H2, and Li-N are 0.80, 0.81, and 0.08, respectively. This indicates the bonding of N-H is strongly covalent and the Li-N bond is primarily ionic. The small difference in average bond strength between N-H1 and N-H2 shows their different bond strengths, and this is mostly originated from the difference in their DOSs. For Li2MgN2H2, the atomic Mulliken charges in Table 2 elucidate that Li, Mg, and H atoms donate electrons while the N atoms accept electrons to form chemical bonding in this compound. The chemical bonds of N-H, Li-N, and Mg-N are mainly covalent, while the bond strengths of N1-H1 and N2-H2 are 0.87 and 0.79, respectively. This small difference, similar to that in LiNH2, implies that the bond strength in N1-H1 is stronger than that in N2-H2. In addition, the average overlap populations for Mg-N1 and Mg-N2 are calculated as -0.40 and -0.55, respectively. This fact suggests that their chemical bonding states are antibonding states. However, the Mulliken approach is known to overestimate charge separation and basis-set dependence, and it has caused some problems in all-metal clusters34 and polarity molecules.35 For LiNH2, it shows strongly ionic characteristics with this scheme in the present work and in another report.36 Associated with the slight overlaps in the total and partial DOSs of LiNH2, there may be some deviation between them. To further investigate the chemical bonds in LiNH2, a more reliable analysis with quantum theory of atoms in molecules (QTAIM) should be applied in future research. It is well-known that N-H bonds are broken in the process of dehydrogenation; thus it is interesting to understand the stability of [NH2]- and [NH]2- in amides/imides. In general, the N-H bond properties can be depicted by the cohesive energy, which is defined as the difference between the total

TABLE 4: HOMO and LUMO Orbital Energies of LiNH2 and Li2MgN2H2 cohesive energy (eV/atom)

orbital energy (eV)

HOMO-LUMO HOMO LUMO energy gap (eV)

LiNH2 -3.289 (-3.078,a -3.383b) -7.295 Li2MgN2H2 -3.315 (-3.4b) -7.145 a

-3.908 -4.678

3.387 2.467

Reference 17. b Reference 21.

energy of amide/imide and the sum of the energies of all the free atoms.17 In present work, this can be defined as follows:

1 f Ebond(H) ) Etot - Etot (H) - E(H2) 2

(3)

where Ebond(H) is the total interaction of the H atom with the other atoms in the amide or imide, Etot is the total energy of the whole system, Eftot(H) is the total energy of the system without the H atom, and E(H2) is the total energy of molecular H2. The calculated Ebond(H) values are listed in Table 3. The binding energies of N-H1 and N-H2 in LiNH2 are -2.75 and -1.83 eV, respectively. This indicates that the N-H2 bond will break up at first and dissociate the H2 atom during dehydrogenation. Similarly, the binding energies of H1 and H2 in Li2MgN2H2 are -2.58 and -2.35 eV, respectively. This small difference in the binding energies of H1 and H2 implies that the H2 will first dissociate from Li2MgN2H2 to form Mg3N2 product, and the H1 will be kept in the product of Li2NH, which was reported in the dehydrogenation reaction of the (3Mg(NH2)2 + 8LiH) system.14 Unfortunately, the energy cost of the N-H bond broken in Li2MgN2H2 is clearly higher than that of N-H2 in LiNH2, and this indicates an elevated temperature for the further hydrogen reaction in this system. Besides, the N1-H1 interaction distance changes from 1.032 to 1.037 Å with the vacancy H2 in LiNH2, and it changes from 1.035 to 1.039 Å for the vacancy H2 in Li2MgN2H2. Both interaction distances of N-H1 with the vacancy H2 in LiNH2 and Li2MgN2H2 increase, and thus the bond strength of N-H1 becomes weak. 3.3. HOMO-LUMO Energy Gap. Chemical interactions of amide anion [NH2]- and [NH]2- in LiNH2 (left) and Li2MgN2H2 (right) are depicted in Figure 5. Associated with specific molecular orbitals in LiNH2 and Li2MgN2H2, as shown in Figure 6, more detailed information regarding the bonding in amides/imides can be obtained. It is noted in Figure 5 that the HOMO in LiNH2 consists of purely N 2p, which donates an electron and accepts one electron to form a π orbital. For LUMO in LiNH2, a π antibond orbital near the N atom can be seen in Figure 6, and the N atom forms a σ* antibond with the

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Wang et al.

Figure 5. Schematic orbital energies for [NH2]- and [NH]2- in LiNH2 (left) and Li2MgN2H2 (right).

Figure 6. Isosurface plots of partial charge density of HOMO and LUMO in LiNH2 and Li2MgN2H2.

TABLE 5: Heats of Formation of Related Amides/Imides and Reaction Enthalpies of Their Hydrogen Reaction (1 eV ) 96.48 kJ/mol) Etot (eV/ formula unit)

e

Li2MgN2H2 Mg(NH2)2 LiNH2

-1938.1374 -1589.2866 -495.9579

LiH MgH2 Li Mg N2 H2 reaction 1 reaction 2

-206.7231 -1010.0926 -190.0205 -977.8659 -543.2918 -31.5632 -

∆H (kJ/mol) calculation

experiment

-518.6282 -482.6412 -263.2264 (-245.8,b -212.5c) -88.8581 -64.0241 35.9243 -70.8646

-351a -176d

a Reference 38. b Reference 18. c Reference 17. Reference 39. f Reference 11. g Reference 15.

-91d -74e 34f -44.1g d

Reference 1.

H s orbital, as illustrated in Figure 5. For Li2MgN2H2, the HOMO contains a π double bond of N 2p and the LUMO contains a π* double bond of N 2p, which are illustrated in Figure 5. The orbital energies of HOMO and LUMO in LiNH2 and Li2MgN2H2 are listed in Table 4. For LiNH2, the two orbital energies are -7.925 and -3.908 eV, respectively, and the HOMO-LUMO energy gap is 3.387 eV. For Li2MgN2H2, the HOMO-LUMO energy gap is 2.467 eV, which is the difference between the orbital energy of HOMO, -7.145 eV, and that of LUMO, -4.678 eV. The HOMO-LUMO energy gap of Li2MgN2H2 is smaller than that in LiNH2. Thus, this indicates

that the HOMO moves to a higher energy and the LUMO moves to a lower energy, while the LiNH2 reactant converts into the product of Li2MgN2H2, as shown in Figure 5. In addition, the cohesive energy of LiNH2 changes from -3.289 eV/atom to -3.315 eV/atom in Li2MgN2H2 after the reaction transition. This infers that Li2MgN2H2 will be more stable in the dehydrogenation reaction with LiH, and this finding agrees well with the large energy costs to create a H2 vacancy in Li2MgN2H2, as listed in Table 3. Meanwhile, large reaction enthalpy of the (Li2MgN2H2 + 2LiH) dehydrogenation reaction of another theoretical study21 is also confirmed by this finding. 3.4. Total Energy and Stability. In general, the enthalpies of formation of hydrides can be estimated by the difference among the total energies of bulk structures, which is depicted by eq 4. All the results were calculated at zero absolute temperature and the zero-point-energy corrections were not taken into account any of the formation enthalpies in this study.

z ∆H ) Etot(AxByHz) - xEtot(A) - yEtot(B) - Etot(H2) (4) 2 where ∆H is the formation enthalpy and Etot(X) is the total energy of the system X. The formation enthalpies of relevant hydrides were calculated and are listed in Table 5. Furthermore, the reaction enthalpies for reactions 1 and 2 were evaluated with these results. The formation enthalpies of Li2MgN2H2, Mg(NH2)2, and LiNH2 are calculated as -518.6282, -482.6412, and -263.2264 kJ/mol, respectively. Moreover, the calculated results of Mg(NH2)2 and LiNH2 are in reasonable agreement with the experimental values of -351 and -176 kJ/mol. Meanwhile, the evaluated results of LiH and MgH2 are -88.8581 and -64.0241 kJ/mol, which are very close to the corresponding experimental values of -91 and -75 kJ/mol. From the values of these formation enthalpies and total energies of the N2 and H2 molecules, the reaction enthalpies are estimated as 35.9243 and -70.8646 kJ/mol for reactions 1 and 2. These findings are considerably close to the experimental values of 34 and -44.1 kJ/mol, respectively. 4. Conclusions The electronic structure, chemical bonds, and energetics of metal amides/imides for hydrogen storage were intensively investigated by first principle computations. The role of the amino anion in these amides/imides for hydrogen storage was elucidated by population analysis, and was further elaborated by the energy costs of dissociating the H atoms from LiNH2 and Li2MgN2H2. In conjunction with the electronic structure and population analysis, the bonding interaction of N-H is strongly covalent in these amides/imides, while the metal-N bonds are strongly ionic in LiNH2 and covalent in Li2MgN2H2

Amino Anion in Metal Amides/Imides and Mg(NH2)2. Furthermore, the HOMO-LUMO energy gaps in LiNH2 and Li2MgN2H2 indicate that the HOMO orbital in LiNH2 moves upward and the LUMO orbital moves downward after the dehydrogenation reaction. This results in a smaller HOMO-LUMO energy gap in Li2MgN2H2 product than in LiNH2 reactant. Finally, the reaction enthalpies for reactions 1 and 2 are evaluated as 35.9243 and -70.8646 kJ/mol, respectively, and they are in considerable agreement with the corresponding experimental values of 34 and -44.1 kJ/mol. Acknowledgment. We are grateful to Dr. Yiming Wen for beneficial discussions. References and Notes (1) Chen, P.; Xiong, Z.; Luo, J.; Lin, J.; Tan, K. L. Nature 2002, 420, 302. (2) Xiong, Z.; Hu, J.; Wu, G.; Chen, P. J. Alloys Compd. 2005, 395, 209. (3) Liu, Y.; Xiong, Z.; Hu, J.; Wu, G.; Chen, P.; Murata, K.; Sakata, K. J. Power Sources 2006, 159, 135. (4) Hino, S.; Ichikawa, T.; Leng, H.; Fujii, H. J. Alloys Compd. 2005, 398, 62. (5) Hu, J.; Xiong, Z.; Wu, G.; Chen, P.; Murata, K.; Sakata, K. J. Power Sources 2006, 159, 116. (6) Xiong, Z.; Wu, G.; Hu, J.; Chen, P. J. Power Sources 2006, 159, 167. (7) Ichikawa, T.; Hanada, N.; Isobe, S.; Leng, H.; Fujii, H. J. Phys. Chem. B 2004, 108, 7887. (8) Leng, H.; Ichikawa, T.; Hino, S.; Nakagawa, T.; Fujii, H. Y. J. Phys. Chem. B 2005, 109, 10744. (9) Chen, P.; Xiong, Z.; Luo, J.; Lin, J.; Tan, K. L. J. Phys. Chem. B 2003, 107, 10967. (10) Chen, P.; Xiong, Z.; Yang, L.; Wu, G.; Luo, W. J. Phys. Chem. B 2006, 110, 14221. (11) Luo, W. J. Alloys Compd. 2004, 381, 284. (12) Luo, W.; Sickafoose, S. J. Alloys Compd. 2006, 407, 274. (13) Chen, Y.; Wu, C.-Z.; Wang, P.; Cheng, H.-M. Int. J. Hydrogen Energy 2006, 31, 1236. (14) Leng, H. Y.; Ichikawa, T.; Hino, S.; Hanada, N.; Isobe, S.; Fujii, H. J. Phys. Chem. B 2004, 108, 8763.

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