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The properties of hydrogenated derivatives of the alkali atom coated clusters C6M6 (M = Li, Na): A density functional study
Accepted Manuscript The properties of Hydrogenated derivatives of the Alkali atom Coated Clusters C6M6(M=Li, Na):A density functional study Chunmei Tang, Fengzhi Gao, Zhenjun Zhang, Jing Kang, Jianfei Zou, Yan Xu, Weihua Zhu PII: DOI: Reference:
S2210-271X(15)00262-5 http://dx.doi.org/10.1016/j.comptc.2015.06.023 COMPTC 1857
To appear in:
Computational & Theoretical Chemistry
Received Date: Revised Date: Accepted Date:
26 April 2015 6 June 2015 23 June 2015
Please cite this article as: C. Tang, F. Gao, Z. Zhang, J. Kang, J. Zou, Y. Xu, W. Zhu, The properties of Hydrogenated derivatives of the Alkali atom Coated Clusters C6M6(M=Li, Na):A density functional study, Computational & Theoretical Chemistry (2015), doi: http://dx.doi.org/10.1016/j.comptc.2015.06.023
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The properties of Hydrogenated derivatives of the Alkali atom Coated Clusters C6M6(M=Li, Na):A density functional study Chunmei Tang*,a
Fengzhi Gaoa
Zhenjun Zhanga
a)
Jing Kanga
Jianfei Zoua
Yan Xua
Weihua Zhua
College of Science, Hohai University
Abstract：The generalized gradient approximation based on density functional theory is used to study the geometric structures, electronic properties and hydrogen storage abilities of alkali metal atom decorated C6M6(M=Li, Na). The most stable site for the Li and Na atoms is above the bridge site of the C-C bond of the C6 ring. It is calculated that C6Li6 and C6Na6 can most adsorb 24 and 12 H2 molecules respectively. The average adsorption energy for H2 of C6(M-nH2)6(M=Li, Na) are from 0.15 eV to 0.6eV, which are between physical adsorption and chemical adsorption (0.1-0.8eV), so they can realize the reversible adsorption of hydrogen. However, the average adsorption energy of H2 and the hydrogen
density of
C6(Li-4H2)6 are larger than those of C6(Na-2H2)6. Therefore, C6Li6 exhibits better hydrogen than that of C6Na6. The atomic orbital hybridization mechanism dominates the adsorption of H2 by C6M6(M=Li, Na). However, the desorption of H2 by C6(Li-4H2)6 can be realized easier than that by C6(Na-2H2)6 at the room temperature. The dynamic simulation, electronegativity, hardness, electrophilicity, and aromaticity of all the structures explore that C6M6(M=Li, Na) and their hydrogenation structures have considerable stabilities.
*
Supported by the Natural Science Foundation of China (Grant No. 11104062), Hohai University Innovation Rraining Project
(2013102941063). Corresponding author：e-mail：[email protected] 1
Keywords：C6; C6Li6; C6Na6; electronic property; hydrogen storage; density functional PACS: 74.25.Jb；71.15.Mb；81.05.u1．The introduction Because of the abundance, renewability, high efficiency, and environmentally friendly nature, hydrogen has been regarded as an ideal kind of energy, which has attracted much interest and has the potential to reduce our dependence on fossil fuels, helping to resolve the global warming issue[1, 2]. However, it is a great challenge to find a material suitable for hydrogen storage. A large number of recent studies have shown that the alkali metal atom decorated carbon materials are good candidates for hydrogen storage. For example, Li and Na decorated carbon nanotubes[3-4], C60(OM)12(M=Li, Na) [5], double Na modified graphene[6] can store 9.5, 13.45, 9.2, 9.78, 8.33 and 11.7 wt % hydrogen respectively, much higher than the target of 5.5 wt % by the year 2017 put forward by the department of energy(DOE)
[7]
.
Therefore, materials decorated by alkali metal atoms Li and Na should have remarkable hydrogen storage abilities.
Shimp et al.
[9]
have reported that C6Li6 can be experimentally produced by the Li steam
and benzene. Later, Baran et al.
[10]
have found that the conversion efficiency from C6Cl6 to
C6Li6 was more than 60%. Theoretically, Xie et al. [11] have putted forward the star structure of C6Li6. Giri et al. [12] have used the first-principle dynamic study to find that the star structure of 2
C6Li6 can quickly adsorb hydrogen under low temperature and room temperature, whereas the hydrogenated structure can release hydrogen quickly. However, the authors did not theoretically study the maximum number of adsorbed H2 and the interaction mechanism of C6(Li-nH2)6. In addition, the geometric structure, electronic properties, and hydrogen storage capacities of C6Na6 have not been reported until now. Therefore, we will study the geometric structures, electronic properties, and hydrogen storage abilities of hydrogenated derivatives of C6M6(M=Li, Na). Conceptual density functional theory (DFT) in conjunction with its various global reactivity descriptors like electronegativity [15]
[13]
(χ), hardness
[14]
(η), and electrophilicity
(ω) along with the associated electronic structure principles have been quite effective in
providing a meaningful rationale toward describing the stability and associated structural reactivities of the planer C6M6(M=Li, Na) clusters upon gradual binding with molecular hydrogen. An assessment of the stability of the H2-bound C6M6(M=Li, Na) cluster in terms of an aromaticity criterion offers important insights into the possible use of the C6M6(M=Li, Na) structure as an effective hydrogen storage material. The cage aromaticity of the bare as well as hydrogen bound C6M6(M=Li, Na) cluster has been evaluated from the nucleus independent chemical shift et al.
[17]
[16]
(NICS) values at any site computed by exploiting the procedure of Schleyer
and the NICS-rate defined by Noorizadeh et al.
[18]
The second part shows the
calculation details, the third part presents the results and discussion, and the conclusion is given in the fourth part. 2．Calculation details The numerical calculations are performed using the generalized gradient approximation (GGA) based on DFT [19]. The Perdue-Burke-Ernzerhof (PBE) functional[20] as implemented in 3
the DMol3 package[21] are used. The basis sets used in this work are the double numerical basis sets including d-polarization functionals (DNP)
[22]
. It is well known that GGA tends to
underestimate the binding energies(Ebs), whereas LDA tends to overestimate the Ebs. To avoid the underestimation of GGA and overestimation of LDA, the van der Waals (vdW) correction (standard DFT computations with empirical pair potentials, namely, DFT-D method) has been considered in our calculations. One dispersion correction put forward by Grimme[23] with its accompanied exchange-correlation functionals is used(PBE-Grimme). The importance of the dispersion correction in performing DFT simulations for the accurate modeling of hydrogen storage materials have been well documented, such as Pd doped graphene[24], transitional metal decorated C48B12 and C60Ca32[25] and so on. The electronic structure is obtained by solving the spin-polarized Kohn-Sham equation[26] self-consistently. Self-consistent field procedures are done with a convergence criterion of 10 -6 Hartree for the electron density. The convergence tolerances are 10-5 Hartree, 0.002 Hartree, and 0.005Å respectively for the energy, the max force, and the max displacement in the optimization. The ground-state structures are determined by their minimum energies, which are further verified by no imaginary frequency in the harmonic frequency calculations. The electronegativity (χ)
[13]
[14]
, hardness (η)
and electrophilicity (ω)
[15]
can provide
useful information that correlates with the thermodynamic stability and reactivity of any structure. These descriptors have been verified by the maximum hardness principle(MHP)
[27]
and the minimum electrophilicity principle(MEP)[28]. For an N-electron system, the χ, η, and ω can be defined in the following:
x (
E )V ( r ) N 4
(1)
2N ( ) 2 V (r ) N
(2)
2 x2 2 2 Here, E is the total energy of the N-electron system,
and v(r )
(3) are the chemical
potential and the external potential field respectively. Using the finite difference method, χ and η can be expressed as follows:
IA 2
(4)
IA
(5)
x
Here, I and A represent the vertical ionization potential and vertical electron affinity of the system respectively. I and A are computed in terms of the energies of the N-electron and N-1-electron systems. For an N-electron system with energy E(N), they may be expressed as follows:
I E ( N 1) E ( N )
(6)
A E ( N ) E ( N 1)
(7)
According to the literature, the aromaticity is the important basis to measure the stability of a molecule
[16]
. The widely used method to measure the molecular aromaticity
is the NICS. Schleyer et al.[17] have put forward the criterion for NICS, its main content is in the following: the probe atoms (Bq) locate at any site of the compound, the calculated nuclear magnetic resonance (NMR) is negative to the isotropic value. The negative and positive NMR separately indicates the aromaticity and antiaromaticity of the compound. If the NMR value is zero, the compound does not have aromaticity. The greater absolute 5
value of a negative NMR implies the stronger aromaticity. We define NICS(0) and NICS(1) as the NICS value respectively at the center and the site 1 Å distance to the center of the C6 ring. Lazzeretti et al. [29] have pointed out that NICS(1) is more accurate than NICS (0) to characterize the molecular aromaticity. If NICS (1) is greater than zero, the NICS-rate defined as NICS rate(r ) dNICS Limr 0 NICS (r r ) NICS (r ) by Noorizadeh et al. r
dr
[18]
can be used to further determine the aromaticity. The proportion of maximum
NICS-rate and minimum NICS-rate is defined as NRR
NICS rate(max) , when NRR> NICS rate(min)
0.5, the molecular aromaticity is strong, however, when NRR< 0.5, the molecule shows antiaromaticity. In order to verify the reliability of the method used in this text, we firstly adopt the same method to optimize C6H6. The calculated bond lengths of C-C and C-H, and the Eg of C6H6 are 1.37 Å, 1.07 Å and 12.9 eV respectively
[30]
, agree well with the experimental
value of 1.40 Å, 1.08 Å, and 13.1 eV[31]. The above results indicate that the method used in this paper is reliable. 3. Results and discussion We firstly theoretically determine the most stable sites for six Li and Na atoms above the C6 plane of the benzene. As well known, the big π bond makes each C atom in the flat benzene structure equivalent. There are two unequal positions in the benzene ring, the top site above the C atom and the bridge site above the C-C bond. In order to determine which position is more stable for the Li(Na) atom, six Li(Na) atoms are placed at these two sites respectively, as shown in figure 1 (a).
6
Table 1 presents the total energies (TEs), energy gaps (Egs), and Ebs of C6H6, C6M6-1 and C6M6-2(M=Li, Na). It is known from table 1 that C6M6-1 has the TE lower than that of C6M6-2, indicating C6M6-1 should be more thermodynamically stable. In addition, the Eg between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) is a useful quantity for examining the kinetic stability of a structure. A larger Eg corresponds to a higher strength required to perturb the electronic structure.[32] The Eg of C6M6-1 is larger than that of C6M6-2, hence, the more stable site for Li and Na should be above the bridge site of the C-C bond, forming the star structures of C6M6. Obviously, the Egs of C6M6-1(M=Li, Na) are comparable to some other hydrogen storage systems, such as C60[33] and B80[34]. Therefore, C6M6(M=Li, Na) should be more kinetically stable and can be synthesized on the experiment, agrees with the successful production of C6Li6[9, 10], therefore, C6Na6 are expected to be experimentally isolated in the future. The Eb of each alkali metal atom to the C6 structure is defined as follows[32] :
Eb EC6 6 * EM EC6 M 6
(8)
The Eb can reflect the binding strength of the metal atom to the substrate. The larger Eb indicates the stronger binding strength
[16]
. It is calculated that the Eb of C6M6-1 is larger
than that of C6M6-2, indicating C6M6-1 should have better stability. Moreover, the Eb of Li(Na) to the C6 surface is not only greater than that of Li-C2H4 (0.69 eV)
[35]
and
Li-graphene(1.10 eV)[36], but also larger than the experimental cohesive energy of Li (1.36eV) and Na(1.58 eV)[37]. Therefore, the C6M6(M=Li, Na) structures are rather stable, the problem of metal aggregative to form cluster is expected to be overcome, and 7
C6M6(M=Li, Na) will be stable for reversible hydrogen storage. Figure 2 clearly show that the hybridization between the 2p orbitals of Li and the p orbitals of C are responsible for the binding of Li atom to the C6 cluster. The hybridization of the py orbital of Li with the LUMO of C6 gives rise to the π-bonds. The NBO charge analysis of C6M6(M=Li, Na) explore that Li and Na have the average positive charges of 0.39e and 0.30e respectively, indicating the charge transfer from the alkali atom to carbon. When C6M6(M=Li, Na) adsorbs H2 molecules, charge will transfer from H to Li(Na). It can be known from figure 1(b) that the reactive points of C6Li6 mainly distribute on six Li atoms, whereas the reactivity points of C6Na6 mainly locate on six C atoms. Therefore, C6Li6 should have better hydrogen storage ability than that of C6Na6. Next, we further study the hydrogen storage capacities of C6M6(M=Li, Na). Figure 3 shows the optimized structures of C6(Li-nH2)6(n=1-5) and C6(Na-nH2)6(n=1-3). We calculate that the distance between the neighboring Li(Na) atoms are 3.06 Å(3.45 Å), which are considerably larger than experimental values of the Li2 dimer(2.67Å)[38] and the Na2 dimer(3.07Å)[39] respectively, indicating that dimerization is not a highly favorable process for the Li(Na) atom. Therefore, the problem of metal aggregative to form cluster is expected to be overcome, and the material will be stable for designing recyclable hydrogen materials. It is known from table 2 that the minimum bond length of C-M and average bond length of C-C in the C6M6(M=Li, Na) structures are almost unchanged in the adsorption process of H2, so these two structures can maintain their structure integrity when adsorbing H2. The shortest M-H bond length of C6(Li-5H2)6 and C6(Na-3H2)6 are 7.34 Å and 4.91 Å, 8
much longer than those of C6(Li-nH2)6(n=1-4) and C6(Na-nH2)6(n=1-2), indicating that the interaction between the fifth H2 and Li, the third H2 and Na are very weak. Therefore, each Li atom of C6Li6 can adsorb up to four H2 molecules, while each Na atom in C6Na6 adsorb only two H2 molecules, thus, C6Li6 shows better hydrogen storage capacity than that of C6Na6. Table 2 also shows the average adsorption energies(Eads) and the continuous adsorption energy(Ecs) of H2 of the C6(Li-nH2)6(n=1-5) and C6(Na-nH2)6(n=1-3) structures. The Ead is defined as[34]:
Ead
( EC6 M 6 6nEH 2 ) EC6 ( M nH 2 ) 2
（9）
6n
E… is the total energy of C6(M-nH2)6, C6M6 and H2 respectively, n is the number of the adsorbed H2 molecules. For C6(M-nH2)6(M=Li, Na), the Ead values are from 0.15eV to 0.6 eV, which are between physical adsorption and chemical adsorption (0.1-0.8eV) [35], so they can realize the reversible adsorption of H2. We can notice that the Ead values of C6(Li-nH2)6(n=1-5) are
from 0.35 to 0.60 eV,
much larger than those
of
C6(Na-nH2)6(n=1-3)(0.15 to 0.20 eV). Therefore, C6Li6 has the better hydrogen storage capacity than that of C6Na6. The Ec of H2 is defined as follows[40]:
Ec
EC6 [ M ( n1) H 2 ]6 6 EH 2 EC6 ( M nH2 )2 6
（10）
If the Ec is small or negative, the adsorption of H2 is difficult[40]. The calculated Ec of C6(Li-nH2)6(n=1-5) and C6(Na-nH2)6(n=1-3) gradually reduce, and achieve the 0.05 eV and 0.03 eV respectively at C6(Li-5H2)6 and C6(Na-3H2)6, so the continues loading of H2 become more difficult. Therefore, each Li atom of C6Li6 can adsorb up to four H2 9
molecules, while each Na of C6Na6 can only adsorb two H2 molecules. The calculated hydrogen storage density of C6(Li-4H2)6 is 29.60 wt %, much larger than that of C6(Na-2H2)6(10.26 wt %), however, they both are much higher than the 5.5 wt % target by the year 2017 proposed by the U.S. department of energy (DOE)[8]. In addition, the Egs gradually increase along with the adsorbed H2 molecules, further achieve the maximum value of 3.14 eV and 1.67 eV at C6(Li-4H2)6 and C6(Na-2H2)6 respectively. Therefore, the hollow orbitals of Li and Na atoms of C6(Li-4H2)6 and C6(Na-2H2)6 are fully occupied by electrons come from H2. Obviously, the Eg of C6(Li-4H2)6 are almost twice of that of C6(Na-2H2)6, exploring its better kinetic stability. An ideal hydrogen storage system not only has the good adsorption performance, but also has good hydrogen release ability at near-ambient conditions. In order to determine the hydrogen release abilities and the thermodynamic stabilities of C6(Li-4H2)6 and C6(Na-2H2)6, we carry out first-principle molecular dynamic simulation with 2fs time step at finite temperatures. Firstly, we study the thermal stabilities of C6(Li-4H2)6 and C6(Na-2H2)6 at T=77K. After 4 ps simulation, the structure still remains intact and no H2 molecule escapes out. We then study their thermal stabilities at the room temperature (T=300K). After 4 ps simulation, 12 H2 molecules escape from C6(Li-4H2)6 and only 2 H2 molecules escape from C6(Na-2H2)6. It can be expected that more H2 molecules will be released at the higher temperature，similar to the dynamic simulation of C6Li6-6H2 by Giri et al. [12] This indicates that the C6Li6 structure is more appropriate for hydrogen storage under near-ambient conditions. We now estimate the desorption temperature TD using the van’t Hoff equation[41] :
10
TD
H S ( ln p) 1 KB R
(11) kB)
R)
H S TD
[42]
TD
by computing the formation enthalpy for the first H2 absorbed structure and most H2 absorbed structure. The calculated TDL, TDH, and TD of C6Li6 and C6Na6 are shown in Table 3. It is obvious that C6(Li-4H2)6 can realize the desorption of H2 under
, while the hydrogen desorption of C6(Na-2H2)6 needs
higher temperature, which is consistent well with our previous dynamics research and that of Giri et al. [12] In order to further understand the adsorption mechanism of H2 by C6M6(M=Li, Na), figure 4 shows the partial density of states(PDOS), which are obtained by Lorentzian extension of the discrete energy levels, with weights given by the orbital populations of the levels, and a summation over them. The Fermi energy (Ef) is at 0 eV, shown by the black dotted line in the figure. Because of the DNP basis sets, the d orbitals of the structure and the p orbital of the H2 appear in the PDOS. It is known from figure 4 that the binding of Li(Na) to the C6 substrate makes the originally relatively concentrated energy levels of Li(Na) to split. The PDOS of H2 mainly distribute within the energy range from -15 eV to -10 eV, according to the bonding of H2, while the * antibonding of H2 mainly distributes near Ef. The dominant bonding state explores that the H-H bond do not break. For C6M6(M=Li, Na), it is important to note that the PDOS of the s and p levels are almost 11
unchanged after Li(Na) atom doping, however, there exists hybridization between the orbital density of H2 and the Li(Na) atom, thus, C6Li6 and C6Na6 mainly through the orbital hybridization to adsorb H2. In order to better understand the stabilities of the structures, we also calculate the electronegativity(χ),
hardness(η),
and
electrophilicity(ω)
of
(a)
C6H6,
C6Li6,
C6(Li-nH2)6(n=1-4), (b) C6H6, C6Na6, C6(Na-nH2)6(n=1-2) (-5 Å ~ 5 Å) presented in Table 2. We find that the calculated η and ω values of C6Li6 and C6(Li-nH2)6 are close to those of Giri et al.[12, 43] The χ, η and ω of C6M6(M=Li, Na) are lower than those of C6H6. We calculated that the electronegativity(χ) of H2 is smaller than those of C6M6(M=Li, Na), it is well known that the when the electronegativity difference between two parts of a molecule is larger than 1.5, the ionic bond will be formed.[44]We calculated that the electronegativity difference between H2 and C6Li6(C6Na6) is 2.11 and 1.98 respectively, so the Li-H2 bond and Na-H2 bond should be ionic, as same as the conclusion of Giri et al. [12], therefore, the Li(Na) atoms will be positively charged, the electron should transfer from the adsorbed H2 to the Li(Na) atoms, benefit for the hydrogen adsorption. This implies the importance of Li(Na)-doping onto the common cyclic hydrocarbons and their usage as potent media for hydrogen storage. The η and ω of C6(Li-nH2)6 firstly decrease and then increase along with the addition of H2. The χ of C6(Li-3H2)6 and the ω of C6(Li-2H2)6 reaches the minimum value. However, the η of C6(Na-nH2)6(n=1-2) gradually increases along with the adsorbed H2 molecules. When C6Na6 adsorbs 12 H2 molecules, the η and ω separately reaches the maximum value and the minimum value, indicating that the C6(Na-2H2)6 structure are the most stable. Thus, the H2 adsorption process of C6M6(M=Li, Na) conform to the maximum hardness principle[27] and the minimum electrophilicity principle [28]. 12
Next we study the aromaticity of all the structures. Figure 5 shows the scanning images of NICS values of (a)C6H6, C6Li6, C6(Li-nH2)6(n=1-4) and (b)C6H6, C6Li6, C6(Na-nH2)6(n=1-2) (-5 ~ 5 Å). It is well known that NICS(1) is more accurate than NICS(0) when determine the molecular aromaticity[30]. The calculated NICS (1) of C6Li6 and C6(Li-nH2)6(n=1-4) are respectively -6.1ppm, -7.89ppm, -9.54ppm, -11.2ppm and -11.4ppm. The calculated NICS (1) of C6Na6 and C6(Na-nH2)6(n=1-2) are separately -2.53ppm, -7.90ppm and -10.00ppm. When C6Li6(C6Na6) adsorbs 24(12) H2 molecules, the aromaticity achieves the maximum value, indicating their aromaticity are gradually increased with loads of H2 molecules. We can analyze that, different from C6H6, when six Li(Na) atoms are bound to the C6 ring, there exists strong hybridization between the Li(Na) centers and the C6 molecule, as shown in Figure 4, so the Li(Na) atoms will be positively charged. In the further continuous trapping of more H2 molecules, the strong p-ring current will
be
formed,
Therefore,
the
aromaticity
of
the
C6(Li-nH2)6(n=1-4)
and
C6(Na-nH2)6(n=1-2) will gradually increase on hydrogen adsorption. It is observed from figure 5 that C6Na6 and C6(Na-nH2)6 show different aromatic properties. Although the NICS(1) value of C6Na6 is smaller than zero, the NICS(r) value of C6Na6 become greater than zero when the r increases. We further calculate that the NICS-rate of C6Na6 is 26.3, much greater than the critical value of 0.5, exploring that C6Na6 should be aromatic. Therefore, the C6M6(M=Li, Na) structures and their hydrogenated structures have good stabilities. 4. Conclusion The bridge site above the C-C bond of the C6 plane of the benzene should be the most stable site for Li and Na. C6Li6 and C6Na6 can separately most adsorb 24 and 12 H2 13
molecules. The average adsorption energies of H2 for C6(M-nH2)6(M=Li, Na) are from 0.15eV to 0.6eV, which are between physical adsorption and chemical adsorption (0.1-0.8eV), so they can realize the reversible adsorption of hydrogen. However, the average adsorption energy of H2 and the hydrogen
density of C6(Li-4H2)6 are
larger than those of (Na-2H2)6, indicating C6Li6 has better hydrogen
than
that of C6Na6. The mechanism dominates the adsorption of H2 by the C6M6(M=Li, Na) structures should be the atomic orbital hybridization. On the other hand, the desorption of H2 for C6(Li-4H2)6 can be realized at the room temperature easier than that of C6(Na-2H2)6. C6M6(M=Li, Na) and their hydrogenation structures shows remarkable stabilities mainly through their dynamic simulation, electronegativity, hardness, electrophilicity, and aromaticity. Reference: [1] R. D. Cortright, R. R. Davda, J. A. Dumesic, Hydrogen from catalytic reforming of biomass-derived hydrocarbons in liquid water, Nature 418(20021)964-967. [2] K. R. S.Chandrakumar, S. K.Ghosh, Alkali-Metal-Induced Enhancement of Hydrogen Adsorption in C60 Fullerene: An ab Initio Study[J]. Nano. Lett, 8(2008)13-19. [3] W. Liu, Y. H. Zhao, E. J. Lavernia, Q. Jiang, Size-Dependent Deformation and Adsorption Behavior of Carbon Monoxide, Hydrogen, and Carbon on Pyramidal Copper Clusters[J]. J.Phys.Chem.C, 112(2008)7672-7677. [4] D. Habale, M. R.Sonawane, B. J. Nagare. First-Principle Study of Hydrogen Adsorption on Na-Coated Carbon Nanotubes[J]. IEEE(2011)1-6. [5] Q.Peng, G. Chen, H. Mizuseki, Y. Kawazoe Hydrogen storage capacity of C 60(OM)12 (M=Li and Na) clusters[J]. J. Chem. Phys. 131(2009)214505. [6] F. D. Wang, F. Wang, N. N. Zhang, Y. H. Li, S. W. Tang, H.Sun, Y. F. Chang, R. S. Wang, High-capacity hydrogen storage of Na-decorated graphene with boron substitution: First-principles calculations[J].Chem. Phys. Lett. 555(2013)212-216. [7] http://www.eere.energy.gov/hydrogenandfuelcells/storage/ [8] H. S. Huang, X. M. Wang, D. Q. Zhao, L. F. Wu, X. W. Huang, Y. C. Li, Hydrogen storage capacity of Y-coated Si@Al12 clusters, Acta Phys. Sin. 61 (2012) 073101. [9] L. A. Shimp, C. Chung, R. J. Lagow, The reaction of lithium vapor with benzene and halobenzenes[J]. Inorg. Chim. 14
Acta, 29(1978)7-81. [10] J. R. Baran, R. J. Lagow, New general synthesis for polylithium organic compounds[J]. J. Am. Chem. Soc, 112(1990)9415-9416. [11] Y. Xie, H. F. Schaefer. Hexalithiobenzene: a D6h equilibrium geometry with six lithium atoms in bridging positions[J]. Chem. Phys. Lett. 179(1991)63-567. [12] S. Giri, F. Lund, A. S. Núñez, A. T. Labbé, Can Starlike C6Li6 be Treated as a Potential H2 Storage Material?[J]. J. Phys. Chem. C 117(2013)5544-5551. [13] R. G. Parr, R. A. Donnelly, M. Levy, W. E. Palke Electronegativity: The density functional viewpoint[J]. J. Chem. Phys. 68(1978)3801. [14] R. G. Parr, R. G. Pearson. Absolute hardness: companion parameter to absolute electronegativity[J]. J. Am. Chem. Soc. 105(1983)7512-7516. [15] R. G. Parr, L.Szentpály, S.Liu, Electrophilicity Index[J]. J. Am. Chem. Soc.121(1999)1922-1924. [16] P. R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao, N. J. R. E. Hommes Nucleus-Independent Chemical Shifts: A Simple and Efficient Aromaticity Probe[J]. J. Am. Chem. Soc.118(1996)6317-6318. [17] P. Lazzeretti, Assessment of aromaticity via molecular response properties[J], Phvs. Chem. Chem. Phvs 6(2004)217-223. [18] S. Noorizadeh, M. Dardab A new NICS-based aromaticity index; NICS-rate[J]. Chem. Phys. Lett. 493(2010)376-380. [19] C. L. Tan, W. Cai, X. H. Tian, First-principles study on the effect of Hf content on martensitic transformation temperature of TiNiHf alloy[J]. Chin. Phys. 15(2006)2718. [20] San D., Dmol. Biosym. Technologies 1996, CA. [21] B. Delley, An all electron numerical method for solving the local density functional for polyatomic molecules[J]. Chem. Phys. 92(1990)508. [22] J. P. Perdew, K. Burke, M. Ernzerhof. Generalized Gradient Approximation Made Simple[J]. Phys. Rev. Lett. 77(1996)3865. [23] R. Fletcher [M] Practical Methods of Optimization 1(1980)193. [24] Ling Ma, Jian-Min Zhang, Ke-Wei Xu, Hydrogen storage on nitrogen induced defects inpalladium-decorated graphene: A first-principles study, Applied Surface Science 292 (2014) 921– 927. [25] Yi Gao, Xiaojun Wu, Xiao Cheng Zeng, Designs of fullerene-based frameworks for hydrogen storage, J. Mater. Chem. A 2(2014)5910–5914. [26] W. Kohn, L. J. Sham Self-Consistent Equations Including Exchange and Correlation Effects. [J] Phys. Rev 140 (1965)A1133. [27] E. Chamorro, P. K. Chattaraj, P. Fuentealba Variation of the Electrophilicity Index along the Reaction Path[J]. J. Phys. Chem. A 107(2003)7068-7072. [28] R. G. Pearson. Recent advances in the concept of hard and soft acids and bases[J]. J. Chem. Educ. 64(1987)561 [29] J. Aihara, Nucleus-independent chemical shifts and local aromaticities in large polycyclic aromatic hydrocarbons[J]. Chem. Phys.Lett. 365(2002)34-39. 15
[30] S. T. Howard, T. M. Krygowski. Benzenoid hydrocarbon aromaticity in terms of charge density descriptors[J]. J. Chem. 75(1997)1174-1181. [31] E.M.Spain, J. M. Behm, M. D. Morse The A1Σ+←X1Σ+ band system of CrMo[J]. Chem. Phys. Lett, 179(1991)411-416. [32] J. Aihara, Kinetic stability of carbon cages in non-classical metallfullerenes[J]. Chem. Phys. Lett. 343(2001)465-469. [33] Q. Wang, Q. Sun, P. Jena, Y. Kawazoe, Theoretical study of hydrogen storage in Ca-coated fullerenes, J. Chem. Theory Comput. 5(2009)374-379. [34] J. Phys. Chem. C 113( [35]
[36] G. Kucinskis, G. Bajars, J. Kleperis, Graphene in lithium ion battery cathode materials: A review, Journal of Power Sources, 240(2013)66-79. [37] C. Kittel, Introduction to Solid State Physics, 7th ed.(John Wiley & Sons, Inc., New York. 1996). [38] http://cccbdb.nist.gov/exp2.asp?casno=14452596 [39] http://cccbdb.nist.gov/exp2.asp?casno=25681792 [40] S.S. Han, W.A. Goddard. Lithium-Doped Metal-Organic Frameworks for Reversible H 2 Storage at Ambient Temperature. J. Am. Chem. Soc. 129(2007)8422-8423. [41] W. Zhou, T. Yildirim, E. Durgun, and S. Ciraci, Phys. Rev. B 76(2007) 085434. [42] 40Handbook of Chemistry and Physics, 75th ed., edited by D. R. Lide (CRC, New York, 1994). [43] S. Giri, B. Sateesh, A. Chakraborty, P. K. Chattaraj, Role of aromaticity and charge of a system in its hydrogen trapping potential and vice versa[J], Phys.Chem. Chem. Phys. 13(2011)20602−20614. [44] S. P. V. Ragué. "Introduction: Aromaticity". Chemical Reviews101 (2001)1115
16
Table 1：TEs, Egs, and Ebs of C6H6 and C6M6(M=Li and Na).
C6H6
C6Li6-1
C6Li6-2
C6Na6-1
TE(Ha)
---
-273.52
-273.19
-1201.54
Eg(eV)
12.9
1.20
0.40
1.11
0.40
Eb(eV)
---
2.86
2.56
2.53
2.34
17
C6Na6-2 -1201.30
Table 2:The A[C-C] and Min[H-M] of C6H6, C6M6, C6(Li-nH2)6(n≤5), and C6(Na-nH2)6 (n≤3), as well as the E ad , Ec, χ, η, and ω of C6H6, C6M6, C6(Li-nH2)6 (n≤5) and C6(Na-nH2)6 (n≤3). H2
C6H6
C6M6 Li
A[C-C]
---
1.37
---
E ad (eV)
Na
C6(M-H2)6 Li
C6(M-2H2)6
Na
Li
Na
C6(M-3H2)6 Li
Na
C6(M-4H2)6
C6(M-5H2)6
Li
Li
1.42 1.42
1.41 1.42
1.41
1.42
1.41 1.43
1.42
1.42
---
---
---
1.94 2.65
1.95
3.02
2.22 4.91
2.52
7.34
---
---
---
---
0.60 0.20
0.57 0.17
0.51 0.15
0.42
0.35
E c (eV)
---
---
---
---
0.60 0.20
0.24 0.15
0.19 0.03
0.16
0.05
χ η ω
1.01
3.67
3.12
2.99
1.71 0.79
1.55
0.19
1.69
1.90
---
14.08 7.35
4.38
4.98
5.61 5.47
5.55 6.80
4.98
5.50
---
0.04
0.52
0.10
0.27 0.06
0.22
0.30
0.33
---
(Å) Min[H-M]
(Å)
0.93
18
0.10
Table 3：TDL, TDH and TD of C6(Li-4H2)6 and C6(Na-2H2)6 (Unit：K). TDL
TDH
TD
C6(Li-4H2)6
121
261
234
C6(Na-2H2)6
394
431
413
19
Figure captions: Figure 1：(a) C6M6-1[six Li(Na) atoms locate above the bridge site of the C-C bond] and C6M6-2[six Li(Na) atoms is locate above the top site of the C atom] ； (b) The wavefunctions of the LUMOs of C6Li6 and C6Na6. Figure 2：The wavefunctions of LUMOs of C6, Li-2p, and C6Li. Figure 3：The optimized structures of C6(Li-nH2)6 (n=1-5) and (d)C6(Na-nH2)6 (n=1-3). Figure 4：The PDOS of (a) isolated Li, (b) Li in C6Li6, (c) Li in C6(Li-4H2)6, (d) isolated H2 molecule and that in the C6(Li-4H2)6 molecules, (e) isolated Na, (f) Na in C6Na, (g) Na in C6(Na-2H2)6, (h) isolated H2 and that of H2 in C6(Na-2H2)6. Figure 5：The scanning images of NICS of (a)C6H6, C6Li6, and C6(Li-nH2)6(n=1-4); (b)C6H6, C6Li6, and C6(Na-nH2)6(n=1-2) (-5~5 Å).
20
Figure 1, Tang et al.
21
Figure 2, Tang et al.
22
Figure 3, Tang et al.
23
Figure 4, Tang et al.
24
Figure 5, Tang et al.
25
Graphical abstract
It can be known from the figure that the reactive points of C6Li6 mainly distribute on six Li atoms, whereas the reactivity points of C6Na6 mainly locate on six C atoms. Therefore, C6Li6 should have better hydrogen storage ability than that of C6Na6. The calculated Ec explores that each Li of C6Li6 can most adsorb four H2 molecules, while each Na of C6Na6 can only adsorb two H2 molecules. The calculated hydrogen storage density of C6(Li-4H2)6 is 29.60 wt %, much larger than that of C6(Na-2H2)6(10.26 wt %), however, they both are much higher than the 5.5 wt % target at 2017 proposed by the U.S. department of energy (DOE).
26
Highlights 1、The most stable site for Li and Na in the C 6 plane of benzene is above the bridge site of the C-C bond 2、C6M6(M=Li, Na) are
4、C6M6(M=Li, Na) and their hydrogenation structures have considerable stabilities.
27
S2210-271X(15)00262-5 http://dx.doi.org/10.1016/j.comptc.2015.06.023 COMPTC 1857
To appear in:
Computational & Theoretical Chemistry
Received Date: Revised Date: Accepted Date:
26 April 2015 6 June 2015 23 June 2015
Please cite this article as: C. Tang, F. Gao, Z. Zhang, J. Kang, J. Zou, Y. Xu, W. Zhu, The properties of Hydrogenated derivatives of the Alkali atom Coated Clusters C6M6(M=Li, Na):A density functional study, Computational & Theoretical Chemistry (2015), doi: http://dx.doi.org/10.1016/j.comptc.2015.06.023
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The properties of Hydrogenated derivatives of the Alkali atom Coated Clusters C6M6(M=Li, Na):A density functional study Chunmei Tang*,a
Fengzhi Gaoa
Zhenjun Zhanga
a)
Jing Kanga
Jianfei Zoua
Yan Xua
Weihua Zhua
College of Science, Hohai University
Abstract：The generalized gradient approximation based on density functional theory is used to study the geometric structures, electronic properties and hydrogen storage abilities of alkali metal atom decorated C6M6(M=Li, Na). The most stable site for the Li and Na atoms is above the bridge site of the C-C bond of the C6 ring. It is calculated that C6Li6 and C6Na6 can most adsorb 24 and 12 H2 molecules respectively. The average adsorption energy for H2 of C6(M-nH2)6(M=Li, Na) are from 0.15 eV to 0.6eV, which are between physical adsorption and chemical adsorption (0.1-0.8eV), so they can realize the reversible adsorption of hydrogen. However, the average adsorption energy of H2 and the hydrogen
density of
C6(Li-4H2)6 are larger than those of C6(Na-2H2)6. Therefore, C6Li6 exhibits better hydrogen than that of C6Na6. The atomic orbital hybridization mechanism dominates the adsorption of H2 by C6M6(M=Li, Na). However, the desorption of H2 by C6(Li-4H2)6 can be realized easier than that by C6(Na-2H2)6 at the room temperature. The dynamic simulation, electronegativity, hardness, electrophilicity, and aromaticity of all the structures explore that C6M6(M=Li, Na) and their hydrogenation structures have considerable stabilities.
*
Supported by the Natural Science Foundation of China (Grant No. 11104062), Hohai University Innovation Rraining Project
(2013102941063). Corresponding author：e-mail：[email protected] 1
Keywords：C6; C6Li6; C6Na6; electronic property; hydrogen storage; density functional PACS: 74.25.Jb；71.15.Mb；81.05.u1．The introduction Because of the abundance, renewability, high efficiency, and environmentally friendly nature, hydrogen has been regarded as an ideal kind of energy, which has attracted much interest and has the potential to reduce our dependence on fossil fuels, helping to resolve the global warming issue[1, 2]. However, it is a great challenge to find a material suitable for hydrogen storage. A large number of recent studies have shown that the alkali metal atom decorated carbon materials are good candidates for hydrogen storage. For example, Li and Na decorated carbon nanotubes[3-4], C60(OM)12(M=Li, Na) [5], double Na modified graphene[6] can store 9.5, 13.45, 9.2, 9.78, 8.33 and 11.7 wt % hydrogen respectively, much higher than the target of 5.5 wt % by the year 2017 put forward by the department of energy(DOE)
[7]
.
Therefore, materials decorated by alkali metal atoms Li and Na should have remarkable hydrogen storage abilities.
Shimp et al.
[9]
have reported that C6Li6 can be experimentally produced by the Li steam
and benzene. Later, Baran et al.
[10]
have found that the conversion efficiency from C6Cl6 to
C6Li6 was more than 60%. Theoretically, Xie et al. [11] have putted forward the star structure of C6Li6. Giri et al. [12] have used the first-principle dynamic study to find that the star structure of 2
C6Li6 can quickly adsorb hydrogen under low temperature and room temperature, whereas the hydrogenated structure can release hydrogen quickly. However, the authors did not theoretically study the maximum number of adsorbed H2 and the interaction mechanism of C6(Li-nH2)6. In addition, the geometric structure, electronic properties, and hydrogen storage capacities of C6Na6 have not been reported until now. Therefore, we will study the geometric structures, electronic properties, and hydrogen storage abilities of hydrogenated derivatives of C6M6(M=Li, Na). Conceptual density functional theory (DFT) in conjunction with its various global reactivity descriptors like electronegativity [15]
[13]
(χ), hardness
[14]
(η), and electrophilicity
(ω) along with the associated electronic structure principles have been quite effective in
providing a meaningful rationale toward describing the stability and associated structural reactivities of the planer C6M6(M=Li, Na) clusters upon gradual binding with molecular hydrogen. An assessment of the stability of the H2-bound C6M6(M=Li, Na) cluster in terms of an aromaticity criterion offers important insights into the possible use of the C6M6(M=Li, Na) structure as an effective hydrogen storage material. The cage aromaticity of the bare as well as hydrogen bound C6M6(M=Li, Na) cluster has been evaluated from the nucleus independent chemical shift et al.
[17]
[16]
(NICS) values at any site computed by exploiting the procedure of Schleyer
and the NICS-rate defined by Noorizadeh et al.
[18]
The second part shows the
calculation details, the third part presents the results and discussion, and the conclusion is given in the fourth part. 2．Calculation details The numerical calculations are performed using the generalized gradient approximation (GGA) based on DFT [19]. The Perdue-Burke-Ernzerhof (PBE) functional[20] as implemented in 3
the DMol3 package[21] are used. The basis sets used in this work are the double numerical basis sets including d-polarization functionals (DNP)
[22]
. It is well known that GGA tends to
underestimate the binding energies(Ebs), whereas LDA tends to overestimate the Ebs. To avoid the underestimation of GGA and overestimation of LDA, the van der Waals (vdW) correction (standard DFT computations with empirical pair potentials, namely, DFT-D method) has been considered in our calculations. One dispersion correction put forward by Grimme[23] with its accompanied exchange-correlation functionals is used(PBE-Grimme). The importance of the dispersion correction in performing DFT simulations for the accurate modeling of hydrogen storage materials have been well documented, such as Pd doped graphene[24], transitional metal decorated C48B12 and C60Ca32[25] and so on. The electronic structure is obtained by solving the spin-polarized Kohn-Sham equation[26] self-consistently. Self-consistent field procedures are done with a convergence criterion of 10 -6 Hartree for the electron density. The convergence tolerances are 10-5 Hartree, 0.002 Hartree, and 0.005Å respectively for the energy, the max force, and the max displacement in the optimization. The ground-state structures are determined by their minimum energies, which are further verified by no imaginary frequency in the harmonic frequency calculations. The electronegativity (χ)
[13]
[14]
, hardness (η)
and electrophilicity (ω)
[15]
can provide
useful information that correlates with the thermodynamic stability and reactivity of any structure. These descriptors have been verified by the maximum hardness principle(MHP)
[27]
and the minimum electrophilicity principle(MEP)[28]. For an N-electron system, the χ, η, and ω can be defined in the following:
x (
E )V ( r ) N 4
(1)
2N ( ) 2 V (r ) N
(2)
2 x2 2 2 Here, E is the total energy of the N-electron system,
and v(r )
(3) are the chemical
potential and the external potential field respectively. Using the finite difference method, χ and η can be expressed as follows:
IA 2
(4)
IA
(5)
x
Here, I and A represent the vertical ionization potential and vertical electron affinity of the system respectively. I and A are computed in terms of the energies of the N-electron and N-1-electron systems. For an N-electron system with energy E(N), they may be expressed as follows:
I E ( N 1) E ( N )
(6)
A E ( N ) E ( N 1)
(7)
According to the literature, the aromaticity is the important basis to measure the stability of a molecule
[16]
. The widely used method to measure the molecular aromaticity
is the NICS. Schleyer et al.[17] have put forward the criterion for NICS, its main content is in the following: the probe atoms (Bq) locate at any site of the compound, the calculated nuclear magnetic resonance (NMR) is negative to the isotropic value. The negative and positive NMR separately indicates the aromaticity and antiaromaticity of the compound. If the NMR value is zero, the compound does not have aromaticity. The greater absolute 5
value of a negative NMR implies the stronger aromaticity. We define NICS(0) and NICS(1) as the NICS value respectively at the center and the site 1 Å distance to the center of the C6 ring. Lazzeretti et al. [29] have pointed out that NICS(1) is more accurate than NICS (0) to characterize the molecular aromaticity. If NICS (1) is greater than zero, the NICS-rate defined as NICS rate(r ) dNICS Limr 0 NICS (r r ) NICS (r ) by Noorizadeh et al. r
dr
[18]
can be used to further determine the aromaticity. The proportion of maximum
NICS-rate and minimum NICS-rate is defined as NRR
NICS rate(max) , when NRR> NICS rate(min)
0.5, the molecular aromaticity is strong, however, when NRR< 0.5, the molecule shows antiaromaticity. In order to verify the reliability of the method used in this text, we firstly adopt the same method to optimize C6H6. The calculated bond lengths of C-C and C-H, and the Eg of C6H6 are 1.37 Å, 1.07 Å and 12.9 eV respectively
[30]
, agree well with the experimental
value of 1.40 Å, 1.08 Å, and 13.1 eV[31]. The above results indicate that the method used in this paper is reliable. 3. Results and discussion We firstly theoretically determine the most stable sites for six Li and Na atoms above the C6 plane of the benzene. As well known, the big π bond makes each C atom in the flat benzene structure equivalent. There are two unequal positions in the benzene ring, the top site above the C atom and the bridge site above the C-C bond. In order to determine which position is more stable for the Li(Na) atom, six Li(Na) atoms are placed at these two sites respectively, as shown in figure 1 (a).
6
Table 1 presents the total energies (TEs), energy gaps (Egs), and Ebs of C6H6, C6M6-1 and C6M6-2(M=Li, Na). It is known from table 1 that C6M6-1 has the TE lower than that of C6M6-2, indicating C6M6-1 should be more thermodynamically stable. In addition, the Eg between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) is a useful quantity for examining the kinetic stability of a structure. A larger Eg corresponds to a higher strength required to perturb the electronic structure.[32] The Eg of C6M6-1 is larger than that of C6M6-2, hence, the more stable site for Li and Na should be above the bridge site of the C-C bond, forming the star structures of C6M6. Obviously, the Egs of C6M6-1(M=Li, Na) are comparable to some other hydrogen storage systems, such as C60[33] and B80[34]. Therefore, C6M6(M=Li, Na) should be more kinetically stable and can be synthesized on the experiment, agrees with the successful production of C6Li6[9, 10], therefore, C6Na6 are expected to be experimentally isolated in the future. The Eb of each alkali metal atom to the C6 structure is defined as follows[32] :
Eb EC6 6 * EM EC6 M 6
(8)
The Eb can reflect the binding strength of the metal atom to the substrate. The larger Eb indicates the stronger binding strength
[16]
. It is calculated that the Eb of C6M6-1 is larger
than that of C6M6-2, indicating C6M6-1 should have better stability. Moreover, the Eb of Li(Na) to the C6 surface is not only greater than that of Li-C2H4 (0.69 eV)
[35]
and
Li-graphene(1.10 eV)[36], but also larger than the experimental cohesive energy of Li (1.36eV) and Na(1.58 eV)[37]. Therefore, the C6M6(M=Li, Na) structures are rather stable, the problem of metal aggregative to form cluster is expected to be overcome, and 7
C6M6(M=Li, Na) will be stable for reversible hydrogen storage. Figure 2 clearly show that the hybridization between the 2p orbitals of Li and the p orbitals of C are responsible for the binding of Li atom to the C6 cluster. The hybridization of the py orbital of Li with the LUMO of C6 gives rise to the π-bonds. The NBO charge analysis of C6M6(M=Li, Na) explore that Li and Na have the average positive charges of 0.39e and 0.30e respectively, indicating the charge transfer from the alkali atom to carbon. When C6M6(M=Li, Na) adsorbs H2 molecules, charge will transfer from H to Li(Na). It can be known from figure 1(b) that the reactive points of C6Li6 mainly distribute on six Li atoms, whereas the reactivity points of C6Na6 mainly locate on six C atoms. Therefore, C6Li6 should have better hydrogen storage ability than that of C6Na6. Next, we further study the hydrogen storage capacities of C6M6(M=Li, Na). Figure 3 shows the optimized structures of C6(Li-nH2)6(n=1-5) and C6(Na-nH2)6(n=1-3). We calculate that the distance between the neighboring Li(Na) atoms are 3.06 Å(3.45 Å), which are considerably larger than experimental values of the Li2 dimer(2.67Å)[38] and the Na2 dimer(3.07Å)[39] respectively, indicating that dimerization is not a highly favorable process for the Li(Na) atom. Therefore, the problem of metal aggregative to form cluster is expected to be overcome, and the material will be stable for designing recyclable hydrogen materials. It is known from table 2 that the minimum bond length of C-M and average bond length of C-C in the C6M6(M=Li, Na) structures are almost unchanged in the adsorption process of H2, so these two structures can maintain their structure integrity when adsorbing H2. The shortest M-H bond length of C6(Li-5H2)6 and C6(Na-3H2)6 are 7.34 Å and 4.91 Å, 8
much longer than those of C6(Li-nH2)6(n=1-4) and C6(Na-nH2)6(n=1-2), indicating that the interaction between the fifth H2 and Li, the third H2 and Na are very weak. Therefore, each Li atom of C6Li6 can adsorb up to four H2 molecules, while each Na atom in C6Na6 adsorb only two H2 molecules, thus, C6Li6 shows better hydrogen storage capacity than that of C6Na6. Table 2 also shows the average adsorption energies(Eads) and the continuous adsorption energy(Ecs) of H2 of the C6(Li-nH2)6(n=1-5) and C6(Na-nH2)6(n=1-3) structures. The Ead is defined as[34]:
Ead
( EC6 M 6 6nEH 2 ) EC6 ( M nH 2 ) 2
（9）
6n
E… is the total energy of C6(M-nH2)6, C6M6 and H2 respectively, n is the number of the adsorbed H2 molecules. For C6(M-nH2)6(M=Li, Na), the Ead values are from 0.15eV to 0.6 eV, which are between physical adsorption and chemical adsorption (0.1-0.8eV) [35], so they can realize the reversible adsorption of H2. We can notice that the Ead values of C6(Li-nH2)6(n=1-5) are
from 0.35 to 0.60 eV,
much larger than those
of
C6(Na-nH2)6(n=1-3)(0.15 to 0.20 eV). Therefore, C6Li6 has the better hydrogen storage capacity than that of C6Na6. The Ec of H2 is defined as follows[40]:
Ec
EC6 [ M ( n1) H 2 ]6 6 EH 2 EC6 ( M nH2 )2 6
（10）
If the Ec is small or negative, the adsorption of H2 is difficult[40]. The calculated Ec of C6(Li-nH2)6(n=1-5) and C6(Na-nH2)6(n=1-3) gradually reduce, and achieve the 0.05 eV and 0.03 eV respectively at C6(Li-5H2)6 and C6(Na-3H2)6, so the continues loading of H2 become more difficult. Therefore, each Li atom of C6Li6 can adsorb up to four H2 9
molecules, while each Na of C6Na6 can only adsorb two H2 molecules. The calculated hydrogen storage density of C6(Li-4H2)6 is 29.60 wt %, much larger than that of C6(Na-2H2)6(10.26 wt %), however, they both are much higher than the 5.5 wt % target by the year 2017 proposed by the U.S. department of energy (DOE)[8]. In addition, the Egs gradually increase along with the adsorbed H2 molecules, further achieve the maximum value of 3.14 eV and 1.67 eV at C6(Li-4H2)6 and C6(Na-2H2)6 respectively. Therefore, the hollow orbitals of Li and Na atoms of C6(Li-4H2)6 and C6(Na-2H2)6 are fully occupied by electrons come from H2. Obviously, the Eg of C6(Li-4H2)6 are almost twice of that of C6(Na-2H2)6, exploring its better kinetic stability. An ideal hydrogen storage system not only has the good adsorption performance, but also has good hydrogen release ability at near-ambient conditions. In order to determine the hydrogen release abilities and the thermodynamic stabilities of C6(Li-4H2)6 and C6(Na-2H2)6, we carry out first-principle molecular dynamic simulation with 2fs time step at finite temperatures. Firstly, we study the thermal stabilities of C6(Li-4H2)6 and C6(Na-2H2)6 at T=77K. After 4 ps simulation, the structure still remains intact and no H2 molecule escapes out. We then study their thermal stabilities at the room temperature (T=300K). After 4 ps simulation, 12 H2 molecules escape from C6(Li-4H2)6 and only 2 H2 molecules escape from C6(Na-2H2)6. It can be expected that more H2 molecules will be released at the higher temperature，similar to the dynamic simulation of C6Li6-6H2 by Giri et al. [12] This indicates that the C6Li6 structure is more appropriate for hydrogen storage under near-ambient conditions. We now estimate the desorption temperature TD using the van’t Hoff equation[41] :
10
TD
H S ( ln p) 1 KB R
(11) kB)
R)
H S TD
[42]
TD
by computing the formation enthalpy for the first H2 absorbed structure and most H2 absorbed structure. The calculated TDL, TDH, and TD of C6Li6 and C6Na6 are shown in Table 3. It is obvious that C6(Li-4H2)6 can realize the desorption of H2 under
, while the hydrogen desorption of C6(Na-2H2)6 needs
higher temperature, which is consistent well with our previous dynamics research and that of Giri et al. [12] In order to further understand the adsorption mechanism of H2 by C6M6(M=Li, Na), figure 4 shows the partial density of states(PDOS), which are obtained by Lorentzian extension of the discrete energy levels, with weights given by the orbital populations of the levels, and a summation over them. The Fermi energy (Ef) is at 0 eV, shown by the black dotted line in the figure. Because of the DNP basis sets, the d orbitals of the structure and the p orbital of the H2 appear in the PDOS. It is known from figure 4 that the binding of Li(Na) to the C6 substrate makes the originally relatively concentrated energy levels of Li(Na) to split. The PDOS of H2 mainly distribute within the energy range from -15 eV to -10 eV, according to the bonding of H2, while the * antibonding of H2 mainly distributes near Ef. The dominant bonding state explores that the H-H bond do not break. For C6M6(M=Li, Na), it is important to note that the PDOS of the s and p levels are almost 11
unchanged after Li(Na) atom doping, however, there exists hybridization between the orbital density of H2 and the Li(Na) atom, thus, C6Li6 and C6Na6 mainly through the orbital hybridization to adsorb H2. In order to better understand the stabilities of the structures, we also calculate the electronegativity(χ),
hardness(η),
and
electrophilicity(ω)
of
(a)
C6H6,
C6Li6,
C6(Li-nH2)6(n=1-4), (b) C6H6, C6Na6, C6(Na-nH2)6(n=1-2) (-5 Å ~ 5 Å) presented in Table 2. We find that the calculated η and ω values of C6Li6 and C6(Li-nH2)6 are close to those of Giri et al.[12, 43] The χ, η and ω of C6M6(M=Li, Na) are lower than those of C6H6. We calculated that the electronegativity(χ) of H2 is smaller than those of C6M6(M=Li, Na), it is well known that the when the electronegativity difference between two parts of a molecule is larger than 1.5, the ionic bond will be formed.[44]We calculated that the electronegativity difference between H2 and C6Li6(C6Na6) is 2.11 and 1.98 respectively, so the Li-H2 bond and Na-H2 bond should be ionic, as same as the conclusion of Giri et al. [12], therefore, the Li(Na) atoms will be positively charged, the electron should transfer from the adsorbed H2 to the Li(Na) atoms, benefit for the hydrogen adsorption. This implies the importance of Li(Na)-doping onto the common cyclic hydrocarbons and their usage as potent media for hydrogen storage. The η and ω of C6(Li-nH2)6 firstly decrease and then increase along with the addition of H2. The χ of C6(Li-3H2)6 and the ω of C6(Li-2H2)6 reaches the minimum value. However, the η of C6(Na-nH2)6(n=1-2) gradually increases along with the adsorbed H2 molecules. When C6Na6 adsorbs 12 H2 molecules, the η and ω separately reaches the maximum value and the minimum value, indicating that the C6(Na-2H2)6 structure are the most stable. Thus, the H2 adsorption process of C6M6(M=Li, Na) conform to the maximum hardness principle[27] and the minimum electrophilicity principle [28]. 12
Next we study the aromaticity of all the structures. Figure 5 shows the scanning images of NICS values of (a)C6H6, C6Li6, C6(Li-nH2)6(n=1-4) and (b)C6H6, C6Li6, C6(Na-nH2)6(n=1-2) (-5 ~ 5 Å). It is well known that NICS(1) is more accurate than NICS(0) when determine the molecular aromaticity[30]. The calculated NICS (1) of C6Li6 and C6(Li-nH2)6(n=1-4) are respectively -6.1ppm, -7.89ppm, -9.54ppm, -11.2ppm and -11.4ppm. The calculated NICS (1) of C6Na6 and C6(Na-nH2)6(n=1-2) are separately -2.53ppm, -7.90ppm and -10.00ppm. When C6Li6(C6Na6) adsorbs 24(12) H2 molecules, the aromaticity achieves the maximum value, indicating their aromaticity are gradually increased with loads of H2 molecules. We can analyze that, different from C6H6, when six Li(Na) atoms are bound to the C6 ring, there exists strong hybridization between the Li(Na) centers and the C6 molecule, as shown in Figure 4, so the Li(Na) atoms will be positively charged. In the further continuous trapping of more H2 molecules, the strong p-ring current will
be
formed,
Therefore,
the
aromaticity
of
the
C6(Li-nH2)6(n=1-4)
and
C6(Na-nH2)6(n=1-2) will gradually increase on hydrogen adsorption. It is observed from figure 5 that C6Na6 and C6(Na-nH2)6 show different aromatic properties. Although the NICS(1) value of C6Na6 is smaller than zero, the NICS(r) value of C6Na6 become greater than zero when the r increases. We further calculate that the NICS-rate of C6Na6 is 26.3, much greater than the critical value of 0.5, exploring that C6Na6 should be aromatic. Therefore, the C6M6(M=Li, Na) structures and their hydrogenated structures have good stabilities. 4. Conclusion The bridge site above the C-C bond of the C6 plane of the benzene should be the most stable site for Li and Na. C6Li6 and C6Na6 can separately most adsorb 24 and 12 H2 13
molecules. The average adsorption energies of H2 for C6(M-nH2)6(M=Li, Na) are from 0.15eV to 0.6eV, which are between physical adsorption and chemical adsorption (0.1-0.8eV), so they can realize the reversible adsorption of hydrogen. However, the average adsorption energy of H2 and the hydrogen
density of C6(Li-4H2)6 are
larger than those of (Na-2H2)6, indicating C6Li6 has better hydrogen
than
that of C6Na6. The mechanism dominates the adsorption of H2 by the C6M6(M=Li, Na) structures should be the atomic orbital hybridization. On the other hand, the desorption of H2 for C6(Li-4H2)6 can be realized at the room temperature easier than that of C6(Na-2H2)6. C6M6(M=Li, Na) and their hydrogenation structures shows remarkable stabilities mainly through their dynamic simulation, electronegativity, hardness, electrophilicity, and aromaticity. Reference: [1] R. D. Cortright, R. R. Davda, J. A. Dumesic, Hydrogen from catalytic reforming of biomass-derived hydrocarbons in liquid water, Nature 418(20021)964-967. [2] K. R. S.Chandrakumar, S. K.Ghosh, Alkali-Metal-Induced Enhancement of Hydrogen Adsorption in C60 Fullerene: An ab Initio Study[J]. Nano. Lett, 8(2008)13-19. [3] W. Liu, Y. H. Zhao, E. J. Lavernia, Q. Jiang, Size-Dependent Deformation and Adsorption Behavior of Carbon Monoxide, Hydrogen, and Carbon on Pyramidal Copper Clusters[J]. J.Phys.Chem.C, 112(2008)7672-7677. [4] D. Habale, M. R.Sonawane, B. J. Nagare. First-Principle Study of Hydrogen Adsorption on Na-Coated Carbon Nanotubes[J]. IEEE(2011)1-6. [5] Q.Peng, G. Chen, H. Mizuseki, Y. Kawazoe Hydrogen storage capacity of C 60(OM)12 (M=Li and Na) clusters[J]. J. Chem. Phys. 131(2009)214505. [6] F. D. Wang, F. Wang, N. N. Zhang, Y. H. Li, S. W. Tang, H.Sun, Y. F. Chang, R. S. Wang, High-capacity hydrogen storage of Na-decorated graphene with boron substitution: First-principles calculations[J].Chem. Phys. Lett. 555(2013)212-216. [7] http://www.eere.energy.gov/hydrogenandfuelcells/storage/ [8] H. S. Huang, X. M. Wang, D. Q. Zhao, L. F. Wu, X. W. Huang, Y. C. Li, Hydrogen storage capacity of Y-coated Si@Al12 clusters, Acta Phys. Sin. 61 (2012) 073101. [9] L. A. Shimp, C. Chung, R. J. Lagow, The reaction of lithium vapor with benzene and halobenzenes[J]. Inorg. Chim. 14
Acta, 29(1978)7-81. [10] J. R. Baran, R. J. Lagow, New general synthesis for polylithium organic compounds[J]. J. Am. Chem. Soc, 112(1990)9415-9416. [11] Y. Xie, H. F. Schaefer. Hexalithiobenzene: a D6h equilibrium geometry with six lithium atoms in bridging positions[J]. Chem. Phys. Lett. 179(1991)63-567. [12] S. Giri, F. Lund, A. S. Núñez, A. T. Labbé, Can Starlike C6Li6 be Treated as a Potential H2 Storage Material?[J]. J. Phys. Chem. C 117(2013)5544-5551. [13] R. G. Parr, R. A. Donnelly, M. Levy, W. E. Palke Electronegativity: The density functional viewpoint[J]. J. Chem. Phys. 68(1978)3801. [14] R. G. Parr, R. G. Pearson. Absolute hardness: companion parameter to absolute electronegativity[J]. J. Am. Chem. Soc. 105(1983)7512-7516. [15] R. G. Parr, L.Szentpály, S.Liu, Electrophilicity Index[J]. J. Am. Chem. Soc.121(1999)1922-1924. [16] P. R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao, N. J. R. E. Hommes Nucleus-Independent Chemical Shifts: A Simple and Efficient Aromaticity Probe[J]. J. Am. Chem. Soc.118(1996)6317-6318. [17] P. Lazzeretti, Assessment of aromaticity via molecular response properties[J], Phvs. Chem. Chem. Phvs 6(2004)217-223. [18] S. Noorizadeh, M. Dardab A new NICS-based aromaticity index; NICS-rate[J]. Chem. Phys. Lett. 493(2010)376-380. [19] C. L. Tan, W. Cai, X. H. Tian, First-principles study on the effect of Hf content on martensitic transformation temperature of TiNiHf alloy[J]. Chin. Phys. 15(2006)2718. [20] San D., Dmol. Biosym. Technologies 1996, CA. [21] B. Delley, An all electron numerical method for solving the local density functional for polyatomic molecules[J]. Chem. Phys. 92(1990)508. [22] J. P. Perdew, K. Burke, M. Ernzerhof. Generalized Gradient Approximation Made Simple[J]. Phys. Rev. Lett. 77(1996)3865. [23] R. Fletcher [M] Practical Methods of Optimization 1(1980)193. [24] Ling Ma, Jian-Min Zhang, Ke-Wei Xu, Hydrogen storage on nitrogen induced defects inpalladium-decorated graphene: A first-principles study, Applied Surface Science 292 (2014) 921– 927. [25] Yi Gao, Xiaojun Wu, Xiao Cheng Zeng, Designs of fullerene-based frameworks for hydrogen storage, J. Mater. Chem. A 2(2014)5910–5914. [26] W. Kohn, L. J. Sham Self-Consistent Equations Including Exchange and Correlation Effects. [J] Phys. Rev 140 (1965)A1133. [27] E. Chamorro, P. K. Chattaraj, P. Fuentealba Variation of the Electrophilicity Index along the Reaction Path[J]. J. Phys. Chem. A 107(2003)7068-7072. [28] R. G. Pearson. Recent advances in the concept of hard and soft acids and bases[J]. J. Chem. Educ. 64(1987)561 [29] J. Aihara, Nucleus-independent chemical shifts and local aromaticities in large polycyclic aromatic hydrocarbons[J]. Chem. Phys.Lett. 365(2002)34-39. 15
[30] S. T. Howard, T. M. Krygowski. Benzenoid hydrocarbon aromaticity in terms of charge density descriptors[J]. J. Chem. 75(1997)1174-1181. [31] E.M.Spain, J. M. Behm, M. D. Morse The A1Σ+←X1Σ+ band system of CrMo[J]. Chem. Phys. Lett, 179(1991)411-416. [32] J. Aihara, Kinetic stability of carbon cages in non-classical metallfullerenes[J]. Chem. Phys. Lett. 343(2001)465-469. [33] Q. Wang, Q. Sun, P. Jena, Y. Kawazoe, Theoretical study of hydrogen storage in Ca-coated fullerenes, J. Chem. Theory Comput. 5(2009)374-379. [34] J. Phys. Chem. C 113( [35]
[36] G. Kucinskis, G. Bajars, J. Kleperis, Graphene in lithium ion battery cathode materials: A review, Journal of Power Sources, 240(2013)66-79. [37] C. Kittel, Introduction to Solid State Physics, 7th ed.(John Wiley & Sons, Inc., New York. 1996). [38] http://cccbdb.nist.gov/exp2.asp?casno=14452596 [39] http://cccbdb.nist.gov/exp2.asp?casno=25681792 [40] S.S. Han, W.A. Goddard. Lithium-Doped Metal-Organic Frameworks for Reversible H 2 Storage at Ambient Temperature. J. Am. Chem. Soc. 129(2007)8422-8423. [41] W. Zhou, T. Yildirim, E. Durgun, and S. Ciraci, Phys. Rev. B 76(2007) 085434. [42] 40Handbook of Chemistry and Physics, 75th ed., edited by D. R. Lide (CRC, New York, 1994). [43] S. Giri, B. Sateesh, A. Chakraborty, P. K. Chattaraj, Role of aromaticity and charge of a system in its hydrogen trapping potential and vice versa[J], Phys.Chem. Chem. Phys. 13(2011)20602−20614. [44] S. P. V. Ragué. "Introduction: Aromaticity". Chemical Reviews101 (2001)1115
16
Table 1：TEs, Egs, and Ebs of C6H6 and C6M6(M=Li and Na).
C6H6
C6Li6-1
C6Li6-2
C6Na6-1
TE(Ha)
---
-273.52
-273.19
-1201.54
Eg(eV)
12.9
1.20
0.40
1.11
0.40
Eb(eV)
---
2.86
2.56
2.53
2.34
17
C6Na6-2 -1201.30
Table 2:The A[C-C] and Min[H-M] of C6H6, C6M6, C6(Li-nH2)6(n≤5), and C6(Na-nH2)6 (n≤3), as well as the E ad , Ec, χ, η, and ω of C6H6, C6M6, C6(Li-nH2)6 (n≤5) and C6(Na-nH2)6 (n≤3). H2
C6H6
C6M6 Li
A[C-C]
---
1.37
---
E ad (eV)
Na
C6(M-H2)6 Li
C6(M-2H2)6
Na
Li
Na
C6(M-3H2)6 Li
Na
C6(M-4H2)6
C6(M-5H2)6
Li
Li
1.42 1.42
1.41 1.42
1.41
1.42
1.41 1.43
1.42
1.42
---
---
---
1.94 2.65
1.95
3.02
2.22 4.91
2.52
7.34
---
---
---
---
0.60 0.20
0.57 0.17
0.51 0.15
0.42
0.35
E c (eV)
---
---
---
---
0.60 0.20
0.24 0.15
0.19 0.03
0.16
0.05
χ η ω
1.01
3.67
3.12
2.99
1.71 0.79
1.55
0.19
1.69
1.90
---
14.08 7.35
4.38
4.98
5.61 5.47
5.55 6.80
4.98
5.50
---
0.04
0.52
0.10
0.27 0.06
0.22
0.30
0.33
---
(Å) Min[H-M]
(Å)
0.93
18
0.10
Table 3：TDL, TDH and TD of C6(Li-4H2)6 and C6(Na-2H2)6 (Unit：K). TDL
TDH
TD
C6(Li-4H2)6
121
261
234
C6(Na-2H2)6
394
431
413
19
Figure captions: Figure 1：(a) C6M6-1[six Li(Na) atoms locate above the bridge site of the C-C bond] and C6M6-2[six Li(Na) atoms is locate above the top site of the C atom] ； (b) The wavefunctions of the LUMOs of C6Li6 and C6Na6. Figure 2：The wavefunctions of LUMOs of C6, Li-2p, and C6Li. Figure 3：The optimized structures of C6(Li-nH2)6 (n=1-5) and (d)C6(Na-nH2)6 (n=1-3). Figure 4：The PDOS of (a) isolated Li, (b) Li in C6Li6, (c) Li in C6(Li-4H2)6, (d) isolated H2 molecule and that in the C6(Li-4H2)6 molecules, (e) isolated Na, (f) Na in C6Na, (g) Na in C6(Na-2H2)6, (h) isolated H2 and that of H2 in C6(Na-2H2)6. Figure 5：The scanning images of NICS of (a)C6H6, C6Li6, and C6(Li-nH2)6(n=1-4); (b)C6H6, C6Li6, and C6(Na-nH2)6(n=1-2) (-5~5 Å).
20
Figure 1, Tang et al.
21
Figure 2, Tang et al.
22
Figure 3, Tang et al.
23
Figure 4, Tang et al.
24
Figure 5, Tang et al.
25
Graphical abstract
It can be known from the figure that the reactive points of C6Li6 mainly distribute on six Li atoms, whereas the reactivity points of C6Na6 mainly locate on six C atoms. Therefore, C6Li6 should have better hydrogen storage ability than that of C6Na6. The calculated Ec explores that each Li of C6Li6 can most adsorb four H2 molecules, while each Na of C6Na6 can only adsorb two H2 molecules. The calculated hydrogen storage density of C6(Li-4H2)6 is 29.60 wt %, much larger than that of C6(Na-2H2)6(10.26 wt %), however, they both are much higher than the 5.5 wt % target at 2017 proposed by the U.S. department of energy (DOE).
26
Highlights 1、The most stable site for Li and Na in the C 6 plane of benzene is above the bridge site of the C-C bond 2、C6M6(M=Li, Na) are
4、C6M6(M=Li, Na) and their hydrogenation structures have considerable stabilities.
27