Graphyne and Graphdiyne: Versatile Catalysts for Dehydrogenation of

Sep 27, 2013 - ... at Weihai, 2 West Wenhua Road, Weihai 264209, Shandong, China ... The interaction of light metal complex hydrides with GP and GD is...
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Graphyne and Graphdiyne: Versatile Catalysts for Dehydrogenation of Light Metal Complex Hydrides Huize Yu,† Aijun Du,*,‡ Y. Song,*,† and Debra J. Searles§ †

School of Materials Science and Engineering, Harbin Institute of Technology at Weihai, 2 West Wenhua Road, Weihai 264209, Shandong, China ‡ School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, QLD 4001, Brisbane, Australia § Centre for Theoretical and Computational Molecular Science, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, QLD 4072, Brisbane, Australia S Supporting Information *

ABSTRACT: The interaction between new two-dimensional carbon allotropes, i.e., graphyne (GP) and graphdiyne (GD), and light metal complex hydrides LiAlH4, LiBH4, and NaAlH4 was studied using density functional theory (DFT) incorporating long-range van der Waals dispersion correction. The interaction of light metal complex hydrides with GP and GD is much stronger than that with fullerene because of the well-defined pore structure of GP and GD. Such strong interactions greatly affect the degree of charge donation from the alkali metal atom to AlH4 or BH4, consequently destabilizing the Al− H or B−H bonds. Compared to the isolated light metal complex hydride, the presence of GP or GD can lead to a significant reduction of the hydrogen removal energy. Most interestingly, the hydrogen removal energies for LiBHx on GP and with GD are found to be lowered at all the stages (x from 4 to 1), whereas the H-removal energy in the third stage is increased for LiBH4 on fullerene. In addition, the presence of uniformly distributed pores on GP and GD is expected to facilitate the dehydrogenation of light metal complex hydrides. The present results highlight new interesting materials to catalyze light metal complex hydrides for potential application as media for hydrogen storage. Because GD has been successfully synthesized in a recent experiment, we hope the present work will stimulate further experimental investigations in this direction.

1. INTRODUCTION Of the known systems for hydrogen storage, complex metal hydrides such as NaAlH4, LiBH4, and LiAlH4 are considered as suitable hydrogen storage media because of the large gravimetric hydrogen content of these materials.1 However, these compounds are highly stable; thus, catalysts are required to lower the hydrogen desorption temperature and to facilitate hydrogen release.2 Consequently, there has been considerable interest in finding possible destabilization strategies that would facilitate release of molecular hydrogen at low temperatures. Now it has been established that a small amount of Ti dopant in NaAlH4 facilitates accelerated hydrogen release under moderate conditions, a discovery which revitalized the research into light metal complex hydrides as potential hydrogen storage materials.3 Currently a fundamental understanding of how the Ti-catalyst works is still lacking and remains a topic of debate.4 Recently, carbon nanomaterials have been proposed as potential “true catalysts” for light metal complex hydrides because they retain their structure and are not consumed in the reaction. Berseth et al. combined results of experiment and density functional theory to show that the curvature of a carbon nanostructure plays a significant role in the hydrogenation and dehydrogenation of NaAlH4.5 A recent experiment has now © 2013 American Chemical Society

shown that fullerenes can act as catalysts for both hydrogen uptake and release in lithium borohydride (LiBH4).6,7 Scheicher et al. have studied the energy to remove hydrogen by state-of-the-art DFT method, and it was found that the underlying catalytic mechanism of fullerene can be understood by the fact that the higher electron affinity of C60 caused the Na or Li to donate more of its electronic charge to the carbon substrate, thus destabilizing the BH4 complex, which ultimately leads to a reduction in the H-removal energy.8 In the studies by Scheicher et al. and Berseth et al., insight into how well a material will perform as a catalyst was investigated by examining the effect of the substrates on the energy required to add and/or remove hydrogen atoms from the alkali metal complex hydrides in a stepwise manner. We use this approach here to study some new materials, which are expected to perform well as catalysts. A new type of carbon allotrope (graphyne, GP or graphdiyne, GD) with the combination of sp and sp2 hybridization through the incorporation of the acetylenic linkages (−CC−) has Received: June 20, 2013 Revised: September 19, 2013 Published: September 27, 2013 21643

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Figure 1. The geometry of (a) GP and (b) GD. The possible sites for LiAlH4, NaAlH4, and LiBH4 to locate on are labeled I−IV. Site I is on the mid of the hexagonal carbon rings, site II on the mid hollow of diacetylene, site III on the top of a carbon atom in diacetylene, and site IV on the mid hollow of triangle hole constructed by hexagonal carbon rings and diacetylene.

Table 1. Geometry of LiAlHx (x = 4 to 1) with and without GP and GD bond length (nm) Li−Al

Al−H

Li−subs Al−C

isolated with GP with GD isolated with GP with GD GP GD GP GD

x=4

x=3

x=2

x=1

0.223 0.232 0.231 0.158−0.167 0.159−0.166 0.159−0.167 0.130 0.131 0.417 0.419 0.441 0.441

0.249 0.281 0.329 0.160−0.169 0.159−0.164 0.159−0.163 0.112 0.054 − − − −

0.248 0.262 0.308 0.163−0.182 0.157−0.163 0.158−0.162 0.126 0.054 0.204 0.204 0.207 0.214

0.250 0.275 0.266 0.162 0.164 0.162 0.102 0.073 0.206 0.206 − −

been proposed and successfully synthesized experimentally.9,10 Because of uniformally distributed pores that are characteristic of these materials, they have been proposed as potential membranes for hydrogen purification, and this has been demonstrated computationally.11 Generally, porous carbon nanostructures are expected to have a strong interaction with alkali metal atoms, which can reduce the negative charge on the Al/BH4 and weaken the bonding between Al/B and H.12 Qian et al.7 have shown that carbon sheet edges in graphene nanofibers enhance hydrogen release from NaAlH4, therefore the porous materials might be expected to have similar effects. Sun et al. demonstrate that Li atoms on graphdiyne have a very high binding energy, promising novel applications in lithium ion batteries.13 Stimulated by these studies, an intriguing question, however, is whether the alkali atoms in complex metal hydrides still have strong interactions with graphynes. If so, will the strong interaction affect the removal energy of hydrogen atoms from GP/GD-supported light metal complex hydrides? To explore this question, we report below a series of first principles calculations to study the interaction between light metal complex hydrides and graphyne family members (GP and GD). We find the energies for the stepwise removal of up to three hydrogen atoms from GP/GD supported LiAlH4, NaAlH4, and LiBH4 species have been substantially reduced, suggesting GP and GD may be efficient catalysts for hydrogen storage in light metal complex hydrides.

exchange-correlation functional.14,15 The generalized gradient approximation (GGA) was used, considering van der Waals interactions resulting from dynamical correlations between fluctuating charge distributions16 for LiAlH4, NaAlH4, and LiBH4 species with and without GP and GD. The cutoff energy was fixed at 500 eV. VESTA was used to visualize the difference in charge density when the complex metal hydride interacts with the carbon substrate.17 The lattice constant is calculated to be 0.689 nm for GP and 0.946 nm for GD. The acetylenic chains in our model are 0.400 and 0.660 nm for GP and GD, respectively. Supercells are chosen as 3 × 3 unit cells of the GP with a size of 2.066 nm × 2.066 nm and 2 × 2 unit cells of the GD with a size of 1.894 nm ×1.894 nm, respectively, as shown in Figure 1a,b. Calculations were performed under periodic boundary conditions utilizing cubic supercell boxes with a height of 2 and 3 nm for the GP and GD, respectively.

3. RESULTS AND DISCUSSIONS 3.1. Geometry of LiAlHx, LiBHx, and NaAlHx Clusters. To evaluate the catalytic effect of the GP and GD on the dehydrogenation of LiAlH4, NaAlH4, and LiBH4, the geometries of LiAlHx, NaAlHx, and LiBHx (x = 1 to 4) with and without the GP and GD were determined (details are stated in the Supporting Information). The locations of LiAlH4, NaAlH4, or LiBH4 on the substrates GP and GD were determined by minimizing the total energy of the systems with the hydride clusters initially located at four different positions on the substrates as illustrated in Figure 1. Site IV is the most stable position for location of the hydride clusters for both GP and GD.

2. COMPUTATIONAL METHOD The geometries and relative energies of the different carbon nanostructures (GP and GD) with LiAlH4, NaAlH4, and LiBH4 species reported in this work were estimated via VASP using the projector augmented wave (PAW) method with the PBE 21644

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Table 2. Geometry of LiBHx (x = 4 to 1) with and without GP and GD bond length (nm) Li−B

B−H

Li−subs B−C

isolated with GP with GD isolated with GP with GD GP GD GP GD

x=4

x=3

x=2

x=1

0.192 0.199 0.198 0.120−0.125 0.121−0.125 0.121−0.125 0.126 0.123 0.386 0.388 0.409 0.458

0.206 0.214 0.221 0.119−0.123 0.121−0.124 0.120−0.124 0.137 0.148 0.172 − 0.172 −

0.222 0.229 0.236 0.122 0.120−0.124 0.121−0.124 0.123 0.098 0.165 0.165 0.161 0.169

0.209 0.254 0.492 0.121 0.119 0.118 0.112 0.017 0.149 0.149 0.148 0.150

Table 3. Geometry of NaAlHx (x = 4 to 1) with and without GP and GD bond length (nm) Na−Al

Al−H

Na−subs Al−C

isolated with GP with GD isolated with GP with GD GP GD GP GD

x=4

x=3

x=2

x=1

0.258 0.264 0.288 0.159−0.167 0.160−0.166 0.160−0.167 0.126 0.083 − 0.382 0.385 0.383 0.386

0.285 0.300 0.331 0.160−0.165 0.157−0.161 0.159−0.162 0.162 0.025 − − − − −

0.290 0.302 0.315 0.163 0.162−0.169 0.157−0.161 0.171 0.020 − 0.204 0.215 − −

0.302 0.386 0.303 0.176 0.163 0.157 0.163 −0.026 − 0.208 0.208 0.208 0.209

are almost identical. However, the bonding between LiAlHx (x = 1 to 3) and GD is stronger than that between LiAlHx (x = 1 to 3) and GP. For the isolated LiAlHx clusters, the Li−Al distance increases with a decrease in the number of hydrogen atoms in cluster. Both the catalysts GP and GD increase the Li−Al distance, implying that the catalysts weaken the bonding strength between Li and Al. It seems that the effect of the GP and GD on the Al−H bonds in the AlHx groups is weak or has an inverse direction to the effect on the Li−Al distance. These results indicate that substrates enhance the bonding strength between Al and H in LiAlHx (x = 2 and 3) clusters which might facilitate the hydrogen absorption of complex metal hydrides. We note that the substrates also promote the formation of the Al−C bond. Two Al−C bonds with equal (or slightly different) lengths of 0.204 nm (0.207 and 0.214 nm) are formed associating with the closing of LiAlHx cluster to the GP (or GD) for the cases of x = 2. This agrees with the experimental measurements of Al−C bond lengths of 0.190−0.222 nm in similar systems.18 However, chemical bonding of Al−C is not observed in LiAlH4 with substrates; the shortest distances between Al and C atoms are observed to be 0.417 nm for LiAlH4 with GP and 0.441 nm for LiAlH4 with GD. For the isolated LiBHx clusters, the Li−B bond length is increased with the decrease of the number of H atoms in the cluster until x = 2 and then slightly reduced in LiBH cluster, while the length of the B−H bonds is almost unchanged with the number of hydrogen atoms (Table 2). The situation changes when the LiBHx cluster is adsorbed on the GP and

The calculated structural information of the most stable systems are summarized in Tables 1−3, and the final configurations of LiAlH4, NaAlH4, and LiBH4 on the GP and GD are illustrated in Figures 2 and 3, respectively. Table 1 gives

Figure 2. Metal complex hydrides on the GP substrate. (a) LiAlH4, (b) LiBH4, and (c) NaAlH4. Black, purple, white, pink, light pink, and red balls stand for C, Li, H, Al, B and Na atoms, respectively.

Figure 3. Metal complex hydrides on the GD substrate. (a) LiAlH4, (b) LiBH4, and (c) NaAlH4. Black, purple, white, pink, light pink, and red balls stand for C, Li, H, Al, B and Na atoms, respectively.

the geometries of the LiAlHx (x = 1 to 4) clusters with and without the GP and GD substrates. LiAlH4 interacts almost equally with GP and GD as indicated by the fact that the distance between the Li atom and the substrates GP and GD 21645

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GD. The Li−B bond length continuously increases with a decrease in the number of hydrogen atoms in the cluster, and the amplitude of the stretch in LiBHx with GD is larger than that in LiBHx with GP. However, it seems that the length of B− H bonds remains unchanged in LiBHx (x = 4 to 1) with GP or GD. The distance between the Li atom and substrates has its largest value when x = 3 and its smallest value for x = 1 in both LiBHx with GP and GD, but the Li atom of LiBHx (x = 2 and 1) is much closer to the GD, implying that the catalytic effect of GD on the two clusters is stronger than that of GP. B−C bonds were also detected in LiBHx (x = 3 to 1) with substrates (Table 2): one with a bond length of 0.172 nm for x = 3 with both GP and GD and two with equal bond lengths of 0.165 nm for x = 2 and of 0.149 nm for x = 1 with GP but with different bond lengths of 0.161 and 0.169 nm for x = 2 and of 0.148 and 0.150 nm for x = 1 with GD, which reasonably agree with the experimental value of 0.144 nm.19 The NaAlHx clusters also show similar features (Table 3). The bond length of Na−Al bonds in the isolated NaAlHx clusters increases with x from 4 to 1, while the Al−H bond lengths are almost unchanged except that for x = 1. The Al−H bond length is about 7% longer than with x = 4. The catalysts GP and GD stretch the Na−Al bond length and shrink the Al− H bonds for x = 3 to 1, and the effect of GD on the shrinking of the Al−H bonds is stronger than that of GP. The distance between NaAlHx clusters and substrates varies with respect to x. Generally, interaction between NaAlHx cluster and GD is stronger than that between the NaAlHx cluster and GP, making the NaAlHx cluster closer to the GD. It even “inserts” into the GD for x = 1 (indicated by the negative value in Table 3). This indicates that the carbon atoms attract the Na atom more strongly than the Li atom, which may be because the valence electrons are less strongly bound to the atomic core in Na than that in Li. Through interaction with the substrates, electrons within the NaAlHx clusters are redistributed, which may generate Al−C bonds. Table 3 also shows that two Al−C bonds with lengths of 0.204 and 0.215 nm are formed for NaAlH2 on GD and four Al−C bonds, all with bond lengths of 0.208 nm, are formed for NaAlH with GD, which agrees with the experimental measurements of 0.190−0.222 nm.18 In this regard, the catalytic effect of GD on the complex metal hydride is stronger than that of GP. After optimization of LiAlHx, LiBHx and NaAlHx on GP/GD with x = 1 to 3, it is found that the BHx and AlHx units do not lie directly above the Li or Na atoms, except for NaAlHx with GP for x = 1 to 3. Compared to the isolated LiAlHx, LiBHx, and NaAlHx, the bond strength of the alkali metal atom and AlHx/ BHx is weakened by the substrates GP and GD, which may originate from the interaction between the alkali metal atom and the carbon atom. The influence of GD on the bond strength of the alkali metal atom and AlHx/BHx is stronger than that of GP. 3.2. Binding Energy and Hydrogen Removal Energy. 3.2.1. Binding Energy. To evaluate the stability of the systems considered, the binding energy was estimated using the following definition: ΔE b = E(A) + E(B) − E(A + B)

Table 4. Binding Energy (eV) of Metal Complex Hydrides on the Substrates GP, GD, and C60 substrate hydride

GP

GD

C60

LiAlH4 LiBH4 NaAlH4

1.36 0.63 0.89

0.71 0.75 0.86

− 0.50 0.68

stable energetically. The most stable configuration is LiAlH4 with GP followed by NaAlH4 with GP. The binding energies in Table 4 indicate that the interactions between LiBH4, NaAlH4 and GP, GD are stronger than those between LiBH4, NaAlH4 and C60, which may indicate that GP and GD have a much greater possibility for catalysis of the complex metal hydrides. 3.2.2. Hydrogen Removal Energy. The catalytic effect of the GP and GD on the dehydrogenation of metal complex hydrides considered here can be clarified by evaluating the hydrogen removal energy from the hydrides with the GP and GD. The hydrogen removal energy is defined as ΔE h = E(MHx − 1) + E(H) − E(MHx)

(2)

Here MHx stands for LiAlHx, LiBHx, or NaAlHx cluster and x = 4, 3, and 2. The energy of a hydrogen atom, E(H), is estimated as 1.12 eV using a 2 × 2 × 2 nm3 supercell containing one hydrogen atom. This approach is supported by earlier research that indicates that this relatively small supercell is representative of the properties of its crystal.20 The results of the H-removal energies of the systems considered are listed in Table 5 and one Table 5. H-Removal Energies (eV) of Metal Complex Hydrides with and without the Substrates (x = 4, 3, 2) x isolated LiAlHx with GP with GD isolated LiBHx with GP with GD isolated NaAlHx with GP with GD

4

3

2

3.89 1.84 1.48 4.64 2.49 2.18 3.70 2.33 1.28

2.81 2.43 2.09 3.68 2.47 2.62 2.49 3.65 2.12

3.31 3.77 3.80 4.19 3.05 2.62 3.20 1.98 3.25

example, the LiBHx (x = 4 to 2), is illustrated in Figure 4 with the results for LiBHx with C60.8 It can be seen that the catalytic effect of the GP and GD on the reduction of the hydrogen removal energy from LiBHx is stronger than that of C60. Interestingly, it is noted that removing the second H is the easiest step for LiBH4, i.e., the step from LiBH3 to LiBH2, on C60 and GP but not on GD. Removal of the second hydrogen requires the most energy for LiBH4 on GD, which may prevent release of hydrogen in this situation. The lower hydrogen removal energy gained in the present calculation indicates that GP and GD as substrates are more effective than C60 for facilitating the release of hydrogen from LiBH4. From Table 5, it is seen that the strongest stabilization due to the removal of H atoms among the three complex hydrides is the LiBHx. The energy of removing one H atom from the LiBH2 cluster is 4.19 eV, which is consistent with the energy required to add one H atom to LiBH cluster reported by Scheicher et al.8 The hydrogen removal energies of LiBH4 with

(1)

Here E(A + B) stands for the total energy of the A + B system. E(A) and E(B) stand for the total energies of the isolated metal complex hydride and GP/GD, respectively. The results are listed in Table 4. All systems show positive binding energies, indicating that the metal complex hydrides with GP/GD are 21646

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Figure 4. Hydrogen removal energy of LiBHx (x = 4, 3, and 2) with and without carbon substrates. Dark blue line stands for isolated LiBHx. The pink line stands for substrate C60, while the green and light blue lines stand for the GP and GD substrates, respectively.

distance between the two atoms, but interaction between Al and C atoms in both LiAlH3 and NaAlH3 is found to be relatively weak. The interaction between B/Al and C atoms may be the mechanism by which the GP promotes the hydrogen removal from the metal complex hydrides studied here. The impact of GP on the hydrogen removal energy in the LiAlH4 system can be explained by the decrease in distance between the Li and the substrate after removing one hydrogen atom. For LiBH4, although the B−H bond length is unchanged after removal of one hydrogen atom, the interaction strength between B and C makes the Li atom move away from the substrates to strengthen the interaction between Li and BH4 clusters, which may increase the barrier for hydrogen release. B. Dehydrogenation with GD Catalyst. The hydrogen removal energies of LiAlHx, LiBHx, and NaAlHx (x = 4, 3, 2) with and without GD are plotted in Figure 5. In LiAlHx or NaAlHx + GD systems, the catalytic effect of the GD on the reduction of the hydrogen removal energy is stronger than that of the GP for x = 4 and 3, while in the LiBHx + GD system, it is stronger for x = 4 but weaker for x = 3 (see Table 5). This characteristic is consistent with the geometric characteristics shown in Tables 1−3. The distance between alkali metal M (M = Li, Na) and GD decreases to 0.054 nm from 0.131 (M = Li) and to 0.025 nm from 0.083 nm (M = Na) in the MAlH3 + GD systems, and the distance between Li and GD in LiBH3 is stretched to 0.148 nm from 0.123 nm, which could weaken the catalytic effect of the GD. The Al−H bond of LiAlH3 and NaAlH3 decrease compared to the former system, while the B−H bond of LiBH3 remains unchanged, which is similar to the case of GP. The removal of hydrogen atoms from the x = 4 systems also weakens the bonding interaction between alkali metal and Al or B hydride ligands as the distance between the alkali metal and Al or B atoms increases to 0.329 nm from 0.231(Li−Al), to 0.221 nm from 0.198 nm (Li−B), and to 0.331 nm from 0.288 nm (Na− Al). This may increase the possibility of generating the B−C bond with the bond length of 0.172 nm in LiBH3 + GD system. Although no Al−C bond is detected in MAlH3 + GD systems (M = Li, Na), the alkali metal M moves closer to the GD surface than in the LiBH3 + GD system. The GD shows the strongest catalytic effect on the hydrogen removal energy in NaAlH4 system because of the decrease in the distance between Na atom and substrate after removing one

GP and GD are 2.49 and 2.18 eV, respectively, which are lower than the energy reported by Scheicher et al.8 The energy of removing one H atom from isolated NaAlH4 is 3.70 eV, which agrees with the value of 3.80 eV reported by Berseth et al.5 The hydrogen atom removal energy of NaAlH4 with GP and GD are 2.33 and 1.28 eV, respectively, which is lower than the values of 2.85 eV for NaAlH4 supported on C60, 3.60 eV supported on graphene, and 2.95−3.07 eV supported on a range of nanotubes reported by Berseth et al.5 It is interesting to note that the binding energies of two hydrogen atoms to LiAlHx, LiBHx, and NaAlHx are smaller than the H2 cohesive binding energy in some cases (e.g., for LiAlH4 → LiAlH2 + H2 on GD), indicating that the binding energy is so low in this case that the ABHx could spontaneously release hydrogen. We now turn to the main part of our investigation, namely the stepwise dehydrogenation in LiAlHx, LiBHx, and NaAlHx on the substrates of GP and GD. A. Dehydrogenation with GP Catalyst. The hydrogen removal energies are plotted against the hydrogen content in MAlHx (M = Li, Na) and LiBHx (x = 4 to 2) with and without GP in Figure 5. The hydrogen removal energy decreased in most cases (exceptions being LiAlH2 and NaAlH3), and the distances between alkali metal atoms and the hydride ligands are increased in all the GP catalyzed systems comparing to the uncatalyzed system. However, the distance between the metal complex hydrides and the substrate GP decreases from 0.130 to 0.112 nm in LiAlH3 + GP, but increases from 0.126 to 0.137 nm in LiBH3 + GP and to 0.162 nm in NaAlH3 + GP (Tables 1−3). Furthermore, the configurations of the ligands are different. The plane containing the three H atoms is parallel to the GP in LiAlH3 + GP, almost parallel to the GP in LiBH3 + GP, and perpendicular to the GP in NaAlH3 + GP. The GP has the strongest catalytic effect on the reduction of the barrier of hydrogen dissociation for LiBH4 and the weakest effect for NaAlH4. The difference in the hydrogen removal energies of the pure and the GP-catalyzed metal complex hydrides (x = 4) ΔEre is 2.15, 2.05, and 1.37 eV for LiBH4, LiAlH4, and NaAlH4, respectively, as shown in Table 5 and Figure 5. This catalytic effect is also evident for LiBH3 (ΔEre = 1.21 eV) and LiAlH3 (ΔEre = 0.38 eV), but not for NaAlH3 (an inverse effect was illustrated with ΔEre = −1.16 eV). Interaction between the B and C atoms is strong, resulting in a dramatic reduction in 21647

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and AlH4 cluster and therefore the interactions between Al and H atoms within the AlH4 cluster. 3.3. Electronic Structure. Results of ab initio calculations were also used to consider the electron redistribution that occurs when the complex metal cations interact with GP and GD and therefore provide more insight into the nature of this interaction. We found that significant charge transfer (or charge redistribution) occurs between the carbon structures and complex metal clusters as shown in Figure 6. In Figure 6a,b, the charge density is shown for the isolated GP and GD. In Figure 6c−h, the enhancement and depletion of the charge density due to the interaction of the substrate and alkali metal atom are shown. Clearly there is a complicated redistribution; however, there are some important similarities in all cases but one (NaAlH4). The charge density in the plane of the carbon substrate near the alkali metal atom increases, as does the charge density on the carbon atoms of the acetylenic chains closest to the alkali metal atom when the complex metal hydrides are absorbed on the substrates. The charge density decreases between the alkali metal atom and the Al/B atom. This behavior suggests development of a strong interaction of the Li/Na with the carbon substrate and a decrease in the interaction with Al/B. In addition, the charge density between the Al/B and one of the hydrogen atoms furthest from the Li/ Na decreases, which is consistent with a reduction in the removal energy of an H atom from the ABH4 (A = Li, Na; B = B, Al) complex metal hydrides that is observed. NaAlH4 behaves differently. In this case, there is a larger charge transfer from Na to AlH4. We also see a decrease in the hydrogen removal energy, but it is not as large as in the other cases. When the structures of NaAlH4 and NaAlH3 on GP are compared, it appears that the hydrogen that is furthest from the Na atom remains bonded to the Al, and one of the closer H atoms is preferentially removed. We also observe that the ABH4 is tilted when it bonds to GD, whereas the AB bonds normal to the surface of GP. This appears to enhance the effect of the carbon substrate on the strength of the bond to the hydrogen atom that is far from the alkali metal atom and close to the GD and could be the reason why it is easier to remove the H atom when we have a GD substrate.

4. CONCLUSIONS Carbon nanomaterials show great promise as catalysts for hydrogen desorption and sorption in light metal complex hydrides. The present study provides an insight into the interaction of LiAlH4, LiBH4, and NaAlH4 with nanostructure carbons (GP and GD). It has been shown that carbon nanostructures, traditionally thought of as hydrogen storage materials, can act as catalysts for hydrogenation/dehydrogenation of light metal complex hydrides such as LiAlH4, NaAlH4, and LiBH4. Using density functional theory under the generalized gradient approximation with correct treatment of the van der Waals interaction for exchange and correlation, the electronic structures of LiAlHx, NaAlHx, and LiBHx (x = 4 to 1) units with and without carbon substrates are calculated. Intermediates through which carbon substrates may catalyze the hydrogen desorption/sorption from/in light metal complex hydrides were considered. The mechanism that leads to a sizable reduction of the energy required to remove hydrogen atom from LiAlH4, NaAlH4, and LiBH4 relies on an enhanced stabilization of the product state. The catalytic mechanisms of

Figure 5. The hydrogen removal energy of LiAlHx, NaAlHx, and LiBHx (x = 4 to 1) without (the black line) and with GP and GD (the red and blue lines, respectively). (a) LiAlHx, (b) LiBHx, and (c) NaAlHx.

hydrogen atom, resulting in the largest reduction in hydrogen removal energy. The origin of this behavior is the strong interaction between the Na atom and carbon atoms in the substrate, which weakens the interaction between the Na atom 21648

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Figure 6. Charge density of the optimized isolated (a) GP and (b) GD on the (001) plane. (c−h) Change in the charge density when the metal complex hydrides are added, calculated by the equation ρtotal − ρsubstrate − ρmetal complex hydride, where ρ stands for the charge density. Isosurfaces with a charge density of 0.0025 are shown. (c) LiAlH4 with GP, (d) LiBH4 with GP, (e) NaAlH4 with GP, (f) LiAlH4 with GD, (g) LiBH4 with GD, and (h) NaAlH4 with GD.

or B and C atoms. Second, the decrease in the distance between the alkali metal atoms and substrates facilities the catalytic effect of substrates on the hydrogen removal from the light metal complex hydrides. Third, the affinity between alkali metal

GP and GD on the hydrogen release from LiAlH4, NaAlH4, and LiBH4 depend on three points. First, the sideways orientation between the alkali metal atoms and the complex hydrides AlH4 and BH4 units ensures the possibility of interaction between Al 21649

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The Journal of Physical Chemistry C

Article

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atoms and C atoms results in an increase in the electron density around the C atom with the removal of hydrogen, which results in decreasing energy for removal of hydrogen from the hydrides. Certainly, there are many more questions to be answered before we have a complete understanding of the catalyzing effects of carbon nanomaterials on light metal complex hydrides. We hope that the present work can provide a valuable guide to the experimental investigation for the role of carbon nanomaterials in catalysis.

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ASSOCIATED CONTENT

S Supporting Information *

The ground state structures of LiAlHx, NaAlHx, and LiBHx with GP and GD. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.S.). *E-mail: [email protected] (A.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was undertaken with the assistance of resources provided at the NCI National Facility systems at the Australian National University, which is supported by the Australian Commonwealth Government through a Queensland Cyber Infrastructure Foundation partner share grant, as well as access to facilities within the Australia Institute of Bioengineering Nanotechnology, the University of Queensland. The authors are grateful for the support of H.Y. through the China Scholarship Council. The authors thank the National Basic Research Programme of China (Grant 2011CB606400-G), the Natural Science Foundation of Shandong, China (Grant ZR2010BM034), and the Fundamental Research Funds for the Central Universities (Grant HIT.NSRIF.2009144). A.D. greatly appreciates financial support of the Australian Research Council QEII Fellowship.

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REFERENCES

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dx.doi.org/10.1021/jp406081v | J. Phys. Chem. C 2013, 117, 21643−21650