J. Phys. Chem. C 2007, 111, 4473-4476
4473
A Barrierless Process from Physisorption to Chemisorption of H2 Molecules on Light-Element-Doped Fullerenes ZhiGang Wang,*,†,‡ MingGuang Yao,§ ShouFu Pan,*,†,‡ MingXing Jin,† BingBing Liu,§ and HongXing Zhang‡ Institute of Atomic and Molecular Physics, Jilin UniVersity, Changchun 130012, People’s Republic of China, State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin UniVersity, Changchun 130023, People’s Republic of China, National Laboratory of Superhard Materials, Institute of Atomic and Molecular Physics, Jilin UniVersity, Changchun 130012, People’s Republic of China ReceiVed: December 19, 2006; In Final Form: January 17, 2007
We carried out the first-principle calculation on the basis of density functional theory (DFT) to investigate the effect of introducing C35B to the vicinity of a stable C35B-H2 system. The results indicate that C35B approaching C35B-H2 is an energy barrierless process, during which the H2 molecule has a tendency to dissociate and form a hydrogen bond with C35B. The result could be either a formation of a stable HBC35C34BCH molecule or a C35BH-HBC35 system via van der Waals (vdW) interaction. Both outcomes cause C35B to lose its hydrogen storage ability. We believe our findings are valuable for understanding the physical mechanisms of hydrogen storage using light-element-doped fullerenes.
Hydrogen has been recognized as a clean energy source. A great deal of effort, both theoretically and experimentally, has recently been devoted to searching for materials for hydrogen storage. Fullerenes and carbon nanotubes have been suggested as the most promising candidates. Their good stability, lightweight structure, and high surface-to-volume ratio make them ideal for hydrogen storage.1-6 Extensive investigations have shown that when the carbon atoms of carbon nanotubes and fullerenes are substituted by other atoms or when high-pressure technique is employed, the efficiency of hydrogen storage can be greatly improved.4-6 Practical applications require the hydrogen storage materials to have a large storage capacity and the ability to release hydrogen easily at ambient condition. Much attention was focused on how to maintain the stability of the stored hydrogen at ambient pressure and temperature. In a recent letter, Kim et al. proposed a nondissociative physisorption mechanism for boron-doped fullerene.4 They suggested that the H2 sorption process is barrierless, significantly simplifying the kinetics for the storage and making it possible to release H2 as fuel at ambient condition. Hence, the binding energy between the H2 molecules and the fullerenes can be modified by either substituting carbon atoms of the fullerenes with atoms of other elements or changing the number of the absorbed H2 molecules. It has been shown that the hydrogen molecule can adsorb on fullerene via van der Waals (vdW) force when the carbon atoms are substituted by atoms of other elements.4 As pointed out by Tournus et al.,7 such vdW interaction is also present among the fullerenes. From ref 4, we also learn that the H2 adsorption to the fullerene molecule doped with other atoms is barrierless. We believe, as we shall show later, that the fullerene sorption process to other fullerene molecules is also barrierless. In fact, * To whom correspondence should be addressed. E-mail:
[email protected] (Z. G. W.);
[email protected] (S. F. P.). Fax: +86431-85168816. † Institute of Atomic and Molecular Physics. ‡ Institute of Theoretical Chemistry. § National Laboratory of Superhard Materials, Institute of Atomic and Molecular Physics.
from ref 4 we can see that the electric interactions between the fullerene and the two H atoms are not equivalent, which may provide a condition for the “encroachment” of other fullerene molecules. Hence, the interaction among the large numbers of fullerene molecules and the interaction between the H2 molecule and the fullerene molecule must be taken into account when considering the mechanism of hydrogen storage in bulk fullerene materials. It is possible that the properties of vdW system formed by H2 and fullerene molecules may be changed by the encroachment of other fullerene molecules. If the interaction between other fullerene molecules and the vdW system takes place, the ideal model of the hydrogen storage using fullerene materials may have to be modified. In fact, Chan et al.8 early in 2001 reported that the breaking of the HH bond is concerted with the formation of two CH bonds on two adjacent carbon nanotubes in solid phase. Very recently, Sankaran and Viswanathan9 reported the dissociation and adsorption of H2 on carbon nanotubes. Therefore, if the interaction between other fullerene molecules and the vdW system studied in the present work results in the breaking of the HH bond, it would be an interesting and significant phenomenon for the hydrogen storage applications. On the basis of the discussion above, we ask ourselves such a question: when we introduce a fullerene molecule to the vdW system formed by H2 and fullerene, what influence does it have to a nodissociation hydrogen storage? To address this issue, we use the C35B fullerene,10,11 a very promising hydrogen storage material (reported by Kim et al.), to study the effect of introducing such a molecule to the vicinity of the C35B-H2 system. We used local density approximation (LDA)12-15 in density functional theory (DFT) method16,17 to study the intermolecular interaction in the present work, which was used extensively in the past and was discussed in details by Tournus et al. and Kim et al.4,7 To reduce the computational cost, we used DFT to handle such large systems. It has been shown that LDA method
10.1021/jp068723d CCC: $37.00 © 2007 American Chemical Society Published on Web 02/28/2007
4474 J. Phys. Chem. C, Vol. 111, No. 11, 2007
Wang et al.
Figure 2. Potential energy curve of the approach of a C35B to the C35B-H2 by using partial optimization. Figure 1. The sketch map of the approach of a C35B to the C35B-H2.
is more suitable than generalized gradient approximation (GGA)18 for computing the intermolecular interaction, especially for the vdW interaction between fullerene molecules. The GGA is known to underestimate binding energies. The electron wave functions were expanded in Gaussian function basis. The polarization function and the diffuse function19,20 were introduced into the basis set. We optimized the structure of C70B2H2 system using double-ζ basis.21 At the same time, we added a d-polarization function for carbon and boron atoms, respectively, while adding a p-polarization function for the hydrogen atoms (marked with 4-31g (d,p)). We used C35B-H2, C35B, C34BCH, and C35BH systems to describe the molecular structure of the dissociation limit on the potential energy surface. To avoid inaccurate structural optimizations of C35B-H2, C35B, C34BCH, and C35BH which might lead to system energy increase and affect the final conclusion, we employed a more expensive Mclean-Chandler triple-ζ basis for optimization.22 Because C35B-H2, C35B, C34BCH, and C35BH systems are much smaller compared to C70B2H2, the employment of this basis set is valid. Furthermore, we introduced two diffuse functions into the basis set, adding two d-polarization functions and one f-polarization function to the carbon and boron atoms, respectively, as well as two p-polarization functions and one d-polarization function to the hydrogen atom (marked with 6-311++g (2df, 2pd)).23 The geometry optimizations were performed via the Berny algorithm.24 The computation was carried out using Gaussian 03 program package.25 To calculate the energy of all stationary points, we also employed Mclean-Chandler triple-ζ basis. Here, we introduced two diffuse functions into the basis sets and added three d-polarization functions and one f-polarization function for carbon and boron atoms, respectively, as well as three ppolarization functions and one d-polarization function for hydrogen atom (marked with 6-311++g (3df, 3pd)). We used a big basis set to calculate the single-point energy of the system for reducing possible adverse effects on computational results. We also corrected the basis set superposition error (BSSE).26 With careful consideration, we selected a path to bring a C35B fullerene molecule close to the stable C35B-H2 system.27 Using partial optimization, we show that (Figure 2) the energy of C35B-H2-BC35 system decreases as the two B atoms gradually approach each other, with the system energy being the lowest
at a separation of 3.105 Å The system structure is very close to C2h symmetry when the distance of the two B atoms is 2.785 Å. More importantly, the HH distance of the hydrogen molecule increases monotonically as the C35B approaches C35B-H2 regardless of the value of the system energy. For the two configurations of the C35B-H2-BC35 system, one with a B-B distance of 3.105 Å and another of 2.785 Å, we further perform full geometry optimizations for the system. The yielded result of the system of C35B-H2-BC35 with a B-B distance of 2.785 Å has a C2h symmetry (Figure 3a), which is a vdW system of two C35BH molecules. The distance between the two B atoms is 4.243 Å and that between the two H atoms is 2.036 Å. The length of the HB bond is 1.221 Å, longer than that of a HB bond in an isolated C35BH molecule.23 The system energy of this structure (C35BH-HBC35) is 1.14 eV, lower than the sum of the energies of C35B-H2 and C35B, and is also 0.16 eV lower than the total energy of two isolated C35BH. The full optimization result for the C70B2H2 system with a B-B distance of 3.102 Å is a more stable HBC35C34BCH molecule with Cs symmetry (Figure 3b). The optimized structure can be regarded as two fractions, that is, C35BH and C34BCH, connected with a CC bond with sp3 hybridization having a bond length of 1.539 Å. Under this circumstance, the chemisorption of hydrogen to doped fullerene molecules takes place. The corresponding system energy is 3.63 eV, lower than the sum of system energies for C35B-H2 and C35B (Table 1). Comparing results of the full optimization performed on the two configurations described above, we find that in both cases the H2 molecule dissociates during the process as the system energy decreases and each of the two H atoms is seized by the fullerene molecules. However, there is also a difference between the two; in one case, it results in the formation of a vdW interacting C35BH-HBC35 system, whereas in the other case, it results in the formation of a C70B2H2 molecule with a lower system energy (i.e., HBC35C34BCH). We did not perform any molecular dynamics simulation, only the full optimization on the system geometry via the Berny algorithm. Our results for the system energy are as follows: E(C35BH2 + C35B) > E(C35BH-HBC35) > E(HBC35C34BCH). The most important result is that the system energy values for C35BH-HBC35 and HBC35C34BCH are 1.14 and 3.63 eV, respectively, lower than that of C35B-H2 + C35B. They are obviously larger than the binding energy between H2 and C35B on the
Process from Physisorption to Chemisorption of H2
J. Phys. Chem. C, Vol. 111, No. 11, 2007 4475
Figure 3. The full optimized geometries and corresponding highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) maps from the total electron density.
TABLE 1: The Energy Value of All Structures Is Calculated at LDA/6-311++g (3df, 3pd) Level C35B-H2 + C35B
HBC35C34BCH
C35BH-HBC35
C35BH*2
C35BH + C34BCH
0.0
-3.63
-1.14
-0.98
-1.73
basis of LDA level.28 It indicates that it is very difficult to store hydrogen with C35B fullerene. For the whole reaction process, the two C35B molecules behave like two forceps. One forceps fixes one end of the hydrogen molecule,4 and then another forceps approaches the hydrogen molecule from the opposite side and seizes the other end of the hydrogen molecule (see Figure 1). Finally, the two fullerene molecules break the HH bond and each seize a H atom. This is a barrierless process from physisorption to chemisorption. We suggest that the breaking of HH bond is caused by the energy release as C35B approaches C35BH2 which leads to an increase of the H-H distance, finally resulting in the chemisorption of H2 molecules in the hydrogen storage process. During this process, the whole system energy decreases monotonously. On the basis of the results described above, two problems should be noted in practical application: (1) How to prevent other C35B fullerene molecules from being close to the H2 molecules adsorbed on the C35B fullerene molecules and (2) how to prevent the breaking of HH bonds because of the approach of other fullerene molecules and the subsequent bonding of H with C35B fullerene molecules, which destroys the hydrogen storage capability of C35B fullerene molecules. In addition, because the C70B2H2 system is very large, we employed big basis sets for the structure optimization and performed path detection to ensure the reliability of the computation. Figure 1 shows the path which we selected to perform our calculation. Although it is not confirmed that the HBC35C34BCH is the lowest energy structure of the C70B2H2 system, we can regard the system energy of HBC35C34BCH molecule geometry obtained by our method as the upper limit. Our result indicates that the adsorption of H2 on light-element-doped fullerene changes from physisorption to chemisorption because of the breaking of H2 in the hydrogen storage process.
C34BCH*2 -2.48
Acknowledgment. This work was supported by Graduate Innovation Lab of Jilin University. The authors would like to thank Prof Keh-Jim Dunn for fruitful discussions and favorable help. The authors are also thankful for the support from the National Natural Science Foundation of China (10534010, 10374036, and 10374037) and the National Basic Research Program of China (2005 CB724400). References and Notes (1) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377-379. (2) Crabtree, G. W.; Dresselhaus, M. S.; Buchanan, M. V. Phys. Today 2004, 57, 39-44. (3) Lee, S. M.; Lee, Y. H. Appl. Phys. Lett. 2000, 76, 2877-2879. (4) Kim, Y. H.; Zhao, Y. F.; Williamson, A.; Heben, M. J.; Zhang, S. B. Phys. ReV. Lett. 2006, 96, 016102. (5) Mpourmpakis, G.; Froudakis, G. E.; Lithoxoos, G. P.; Samios, J. Nano Lett. 2006, 6, 1581-1583. (6) Chen, P.; Wu, X.; Lin, J.; Tan, K. L. Science 1999, 285, 91-93. (7) Tournus, F.; Charlier, J. C.; Me´linon, P. J. Chem. Phys. 2005, 122, 094315. (8) Chan, S. P.; Chen, G.; Gong, X. G.; Liu, Z. F. Phys. ReV. Lett. 2001, 87, 205502. (9) Sankaran, M.; Viswanathan, B. Carbon 2006, 44, 2816-2821. (10) Grossman, J. C.; Coˆte´, M.; Louie, S. G.; Cohen, M. L. Chem. Phys. Lett. 1998, 284, 344-349. (11) Piskoti, C.; Yarger, J.; Zettl, A. Nature 1998, 393, 771-774. (12) Hohenberg, P.; Kohn, W. Phys. ReV. 1964, 136, B864-B871. (13) Kohn, W.; Sham, L. J. Phys. ReV. 1965, 140, A1133-A1138. (14) Slater, J. C. Quantum Theory of Molecular and Solids; Vol. 4: The Self-Consistent Field for Molecular and Solids; McGraw-Hill: New York, 1974. (15) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 12001211. (16) Kohn, W.; Sham, L. J. Phys. ReV. 1965, 140, A1133-A1138. (17) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and Molecules; Oxford University Press: New York, 1989. (18) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B 1992, 46, 66716687.
4476 J. Phys. Chem. C, Vol. 111, No. 11, 2007 (19) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80, 3265-3269. (20) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. J. Comp. Chem. 1983, 4, 294-301. (21) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724-728. (22) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 56395648. (23) We employ LDA method on the basis of 4-31g (d,p), and 6-311++g (2df,2pd) basis sets to optimize the system geometry of C35BH2. The optimized geometries by the two basis sets are very consistent with each other. The difference between the two optimized results for the distance between B atom and H atom is about 10-3 angstrom and for the distance between two H atoms is about 10-4 angstrom. For such a big system like C70B2H2, only 550 basis functions are needed in 4-31g (d,p) basis set, but 1254 basis functions are required in 6-311++g (2df,2pd) basis set for optimization. Therefore, we can reduce a lot of computational cost when we use 4-31g (d,p) basis set. We also use 4-31g (d,p) and 6-311++g (2df,2pd) basis sets to optimize the C35BH molecule structure. The results are consistent with each other. The HB bond length is 1.217 Å by 4-31g (d,p) basis set and 1.219 Å by 6-311++g (2df,2pd). (24) Peng, C.; Ayala, P. Y.; Schlegel, H. B.; Frisch, M. J. J. Comp. Chem. 1996, 17, 49-56. (25) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Wang et al. Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03; Gaussian, Inc.: Pittsburgh, PA, 2003. (26) We calculate the single-point energy by using 6-311++g (3df, 3pd) basis set within LDA. The BSSE is based on ref 7. The detailed expression equation is
∆Etotal(C35B-H2) ) E(C35B-H2) - [E(C35B-H2ghost) + E (C35Bghost-H2)] ∆Etotal(C35BH-HBC35) ) E(C35BH-HBC35) - [E (C35BH-HBC35ghost) + E(C35BHghost-HBC35)] The results indicate that the systematic energy is -1351.832 hartree for C35B-H2 while the value is -2702.515 hartree for C35BH-HBC35 after BSSE. (27) As shown in Figure 1, both the two B atoms and the two H atoms are always located on one plane. The C35B molecule is brought gradually to the vicinity of C35B-H2 molecule along a direction which keeps a Cs symmetry for the whole system. If we keep the structure the same for each part of the system, the approach of C35B to C35B-H2 will result in the change of the system from Cs symmetry to C2h symmetry. During the process, we employ partial optimization method to optimize the geometrical structure of the B-H2-B part in the C70B2H2 system. (28) The binding energy in our work is 0.39 eV using LDA method on the basis of 6-311++g (3df, 3pd) basis set, in good agreement with the result reported in ref 4. The computational method for the total energy of the system and the individual energies of C35B and H2 is based on the method described in ref 26. The binding energies were obtained by using the following formula: Ebinding energy ) Etotal(C35B-H2) - [E(C35B) + E(H2)].
