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A First Principle Comparative Study on Chemisorption of H_{2} on C_{60}, C_{80}, and Sc_{3}N@C_{80} in Gas Phase and Chemisorption of H_{2} on Solid Phase C_{60}
Abstract
The chemisorptions of H_{2} on fullerenes C_{60} and C_{80}, endofullerene Sc_{3}C@C_{80} and solid C_{60} were comparatively studied. A chain reaction mechanism for dissociative adsorption of H_{2} on solid C_{60} is proposed under high pressure. The breaking of H–H bond is concerted with the formation of two C–H bonds on two adjacent C_{60} in solid phase. The adsorption process is facilitated by the application of high pressure. The initial H_{2} adsorption on two adjacent C_{60} gives a much lower barrier 1.36 eV in comparison with the barrier of adsorption on a single C_{60} (about 3.0 eV). As the stereo conjugate aromaticity of C_{60} is destructed by the initial adsorption, some active sites are created. Hence the successive adsorption becomes easier with much low barriers (0.6 eV). In addition, further adsorption can create new active sites for the next adsorption. Thus, a chain reaction path is formed with the initial adsorption dominating the whole adsorption process.
1. Introduction
From the beginning of the “fullerene area,” hydrogenated fullerenes have attracted widespread attention due to their potential application. It may not only be interesting for hydrogen storage [1–5], but also be used as an additive for lithium ion cells to significantly prolong the lifetime of these cells [6].
Experimentally, studies on the interaction between hydrogen and fullerenes have been focused on the chemical process of hydrogenation, with the products C_{60}H_{X} and C_{70}H_{X} being of fundamental interest as a model for other fullerene derivatives [7, 8]. A variety of chemical procedures have been devised to produce hydrogen radicals that could adsorb readily on these carbon atoms, using either reducing reagents [1, 9, 10], or catalysts [11]. In addition, direct hydrogenation of C_{60} and C_{70} has also been achieved without the usage of a catalyst by exposing solidphase fullerenes to highpressure hydrogen gas (0.5–30 kB) at elevated temperature (500–600 K) [4].
Theoretically, hydrogen storage based on fullerene materials has attracted many attentions. Using first principle calculations based on density functional theory, Sun and coworkers reported that each B_{36}C_{36} cage can store at most 18 hydrogen molecules at zero temperature [12]. They also find that an isolated Li_{12}C_{60} cluster where Li atoms are capped onto the pentagonal faces of the fullerene not only is very stable but also can store up to 120 hydrogen atoms in molecular form with a binding energy of 0.075 eV/H_{2} [13]. Zhao and coworkers report that a particular Scandium organometallic buckyballs can bind as many as 11 hydrogen atoms per transition metal, which gives the maximum retrievable H_{2} storage density 9 wt% [14]. Kang and coworkers reports that Nidispersed fullerenes are considered to be capable of storing 6.8 wt% H_{2}, with H_{2} desorption activation barrier of 11.8 kcal/mol, which is ideal for many practical hydrogen storage [15].
However, the mechanism for direct reactions between H_{2} and these fullerenes remains unexplained to the best of our knowledge. Chan et al. proposed the mechanism of H_{2} molecule dissociative chemisorption on the close cousin of fullerenes, carbon nanotubes, in solid phase under high pressure [16]. The breaking of the H–H bond is concerted with the formation of two C–H bonds on two adjacent carbon nanotubes in solid phase, facilitated by the application of high pressure which shortens the interstitial distance between nanotubes. The adsorption behavior gives some hints on H_{2} adsorption on fullerenes.
In this work, we proposed a chain reaction mechanism for H_{2} molecules dissociative adsorption on solid C_{60} under high pressure. In comparison, we also studied H_{2} adsorption on the most stable fullerenes C_{60} and C_{80} in gas phase as well as endofullerene Sc_{3}N@C_{80}.
2. Computational Method
The first principles total energy and electronic structure calculations were carried out within the framework of DFT [17] with a plane wave basis set and pseudopotentials for the atomic cores, as implemented in the Vienna ab initio simulation package (VASP) [18, 19]. The PW91 gradient correction was added to the local density exchangecorrelation functional and projector augmented wave (PAW) pseudopotentials [20, 21] were employed, with an energy cutoff of 400 eV for the planewave expansion as these approaches have successfully applied to similar systems [16]. The supercell is sampled with a 1 × 1 × 1 points mesh, generated by the MonkhorstPack algorithm. The convergence criteria were 1.0 × 10^{−4} eV for the SCF energy, 1 × 10^{−3} eV for total energy, and 0.05 eV/Å for atomic force, respectively.
A climbing image nudged elastic band method was used to locate the transition states [22–24]. The vibrational frequencies and normal modes were calculated by diagonalization of the massweighted force constant matrix, which was obtained using the method of finite differences of forces as implemented in VASP. The ions are displaced in the +/– directions of each Cartesian coordinate by 0.02 Å. There is only one imaginary frequency for all these structures, indicating that they are indeed the transition states in the potential energy surface.
The adsorption energies () for the adsorption of H_{2} on fullerenes were calculated by where , , and are the total energies of H_{2} adsorbed fullerene, total energies of fullerene (fullerene = C_{60}, C_{80}, Sc_{3}N@C_{80}), and the total energies of H_{2}, respectively. The larger adsorption energy indicates the stronger adsorption.
3. Results and Discussion
3.1. H_{2} Adsorption in Gas Phase
We firstly consider the H_{2} adsorption on fullerenes C_{60} and C_{80} and endofullerene Sc_{3}N@C_{80} to explore the possible adsorption media without any catalyst in gas phase. In the calculations, the interactions between H_{2} molecule and fullerenes (or endofullerenes) were modeled in a supercell of size 16.0 Å × 16.0 Å × 16.0 Å, with one point (gamma point). The energy barriers and reaction energies are listed in Table 1.

3.1.1. H_{2} Adsorption on C_{60} in Gas Phase
The icosahedral C_{60} consists of 12 pentagons and 20 hexagons. Hence the bonds can be categorized as two types, pentagonhexagon bonds (56 bond) and hexagonhexagon bonds (66 bond). Addition on adjacent sites such as 56 bond and 66 bond engenders isomers. Furthermore, addition can take place on nonadjacent sites, which would produce many isomers. For C_{60}H_{2}, the simplest fullerene dihydride, there are 23 isomers. However, there is only one isomer that has been characterized. Among all kinds of C_{60}H_{2} isomers, (1,2) addition products are considered as the most stable. To compare the adsorption difference, we investigated H_{2} adsorption on both 56 and 66 bonds. In addition, two adsorption modes are considered. One case is that the H–H bond of incoming H_{2} is to be considered to parallel the 66 bond (Structure A and Structure E in Figure 1). The other one is that the incoming H–H bond is to be considered to point perpendicularly to the C–C bond (Structure C and Structure G in Figure 1). Energetically, the total exothermic energy for the formation of the two C–H bonds in 66 bond addition production is 0.77 eV favorable than that for 56 bond addition (Table 1), indicating that 66 bond addition gives the most stable structure. The two adsorption modes result in two possible adsorption mechanisms. The parallel adsorption mode gives a concerted mechanism, in which two H atoms bonded to two C atoms (TS 1 in Figure 1), respectively. In contrast, the perpendicular adsorption mode gives a step mechanism (one H atom is adsorbed first, then the second one, TS 2 in Figure 1). The barriers for both concerted mechanism and step mechanism are so high that the reaction is very difficult to take place, although the barrier for the step mechanism is 0.11 eV favorable than that for the concerted mechanism.
3.1.2. H_{2} Adsorption on C_{80} in Gas Phase
C_{80} has the same I_{h} symmetry as C_{60}, so it can also be served as adsorption media, although experimentally no hydrite of C_{80} has been characterized. We carried out the same calculations as we did on C_{60} as shown in Figure 2. The 66 parallel and perpendicular adsorption modes have the same energy barrier of 2.47 eV whereas 56 parallel and perpendicular adsorption modes have the same barrier of 2.27 eV. Comparing to that of C_{60}, the calculated energy barrier is about 1 eV lower than that for C_{60} due to the less stability of C_{80}.
3.1.3. H_{2} Adsorption on Sc_{3}N@C_{80} in Gas Phase
As one of the most stable endofullerene, Sc_{3}N@C_{80} has become accessible in macroscopic quantities. Since the adsorption barrier for C_{80} has decreased more obviously than that for C_{60}, its stable derivative Sc_{3}N@C_{80} is expected to be a promising hydrogenation material. As shown in Figure 3, we considered two different adsorption modes: either adsorption to Sc bonded C or Sc nonbonded C. However, whichever atom H bonds, the calculated energy barriers are more than 3.5 eV, indicating that Sc_{3}N@C_{80} can hardly react with H_{2}. In addition, the overall exothermic energy is quite low at 0.1 eV, indicating that the reaction is not very favorable thermodynamically.
3.2. H_{2} Adsorption in Solid Phase
In contrast, the solid phase, composed of bundles of C_{60}, provides a unique chemical environment dependent on the external pressure and makes it much easier for H_{2} dissociative chemisorption on C_{60} bucky balls. In our calculations, the interactions between hydrogen molecule H_{2} and C_{60} were modeled in a supercell of size 9.55 Å × 9.55 Å × 13.50 Å, and an external pressure of ~50 kB was introduced.
3.2.1. Adsorption of the First H_{2} on Solid C_{60}
Under the pressure of pressure of ~50 kB, we explored the reactive trajectory of the first H_{2} dissociative chemisorption on C_{60} as shown in Figure 4. There are three steps involved: first, the incoming H_{2} dissociation and deposition on two adjacent C_{60}, from structure U to V, through transition structures (TS 11); second, the rotation of C_{60}; and finally, hydrogen migration through TS 12 to structure W, a 1,2addition product. Figure 5 gives the plot of the relative energies for the dissociative H_{2} chemisorption on solid C_{60}. From this figure, we can see that the initial dissociative chemisorption step has a barrier of 1.36 eV, while the barrier for the subsequent H migration is much lower at 0.5 eV. Compared to adsorption barrier on single C_{60} in gas phase, the barrier for H_{2} molecules adsorption on two adjacent C_{60} is quite low at 1.36 eV, which indicates that the reaction can easily take place at room temperature [16]. The energy barrier differences are mainly due to the rotation. In high pressure, C_{60} does not stand in his own position quietly but rotates around the center of mass randomly. The rotation deforms the H–H bond to help break the H–H bond, which leads to lower reaction barrier.
In addition, we have made some comparison between the first H_{2} chemisorption on solid C_{60} and solid (6,6) armchair carbon nanotube. The barrier difference for initial chemisorption is 0.14 eV. And the barrier difference for the H atom migration is 0.38 eV. Both the barrier differences are small. From this table, we can conclude that, energetically, there are no significant differences between the chemisorption on solid C_{60} and solid carbon nanotube under high pressure.
3.2.2. Adsorption of the Second H_{2} on Solid C_{60}
To explore whether it is possible to chemisorb more H_{2} on the solid C_{60} under high pressure, we investigate the reaction path for the addition of the second H_{2} on C_{60} as shown in Figure 6. The reaction mechanism is quite similar to the addition of the first one. In this process, the second H_{2} also dissociatively chemisorbs on two adjacent C_{60} in the initial step and then rotates slightly; one of the H atom migrates from one C_{60} to another and finally forms two 1,2addition products. The relative energies for the chemisorption of second H_{2} on solid C_{60} were shown in Figure 7. Compared to the first H_{2} chemisorption, the calculated barrier for the second H_{2} is 1.21 eV in the dissociation step, which is a littler lower than that for the first H_{2} adsorption. In addition, the overall process is also more exothermic, from 0.44 eV to 1.77 eV. This is mainly due to the fact that the first H_{2} molecule adsorption has already partially disrupted the conjugated system, so further addition is much easier.
In the first H_{2} adsorption process, we have mentioned that one of the H atoms will transfer from one C_{60} radical to another. There is another probability that H_{2} molecules react with the intermediate radical directly. We also investigate this kind of adsorption modes. For the C_{60}H intermediate, there are two active sites: site 2 and site 4. There are three probable adsorption modes: (1,2)(1,2) adsorption, (1,4)(1,4) adsorption, and (1,2)(1,4) adsorption. For (1,2)(1,2) adsorption, it means one H atom is adsorbed in site 2 and another H atom is also adsorbed in site 2 (Figure 8). The rules also apply for both (1,2)(1,4) adsorption and (1,4)(1,4) adsorption. The barriers of three category reactions are listed in Table 2. From this table, we can see that all the barriers are no more than 0.8 eV. They are much lower than the formerly calculated barrier 1.21 eV. The overall exothermic energies are also a little larger than the former calculated exothermic energy 1.77 eV. Based on these data, a conclusion can be drawn that the subsequent H_{2} molecules will easily react with C_{60} intermediate radicals. Thus the reaction is a chain reaction: once the first H_{2} is adsorbed, the H_{2} will be adsorbed one by one. There is no extra energy needed because the overall exothermic energy will compensate the energy which is needed to overcome the barrier. Herein the first H_{2} molecule adsorption has become the crucial step in the overall adsorption.

4. Conclusions
Based on the investigation of H_{2} molecules chemisorption on fullerenes C_{60} and C_{80} and endofullerene Sc_{3}C@C_{80}, we proposed a mechanism for H_{2} molecules adsorption on solid C_{60} under high pressure. Due to the rotation of C_{60}, the H_{2} molecule will easily chemisorb on two adjacent C_{60} under high pressure, which is more favorable than the H_{2} molecule adsorption on single C_{60}. The overall reaction is a chain reaction. The first H_{2} molecules adsorption is the crucial step in the overall H_{2} molecules adsorption process. Once the first H_{2} molecules is adsorbed on the C_{60}, the second and subsequent H_{2} will easily be adsorbed on the C_{60} due to the lower reaction barrier.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This work is supported by the Shenzhen Strategic Emerging Industries Special Fund Program of China (Grant nos. GGJS20120619101655715 and JCY20120619101655719), the Program for International Cooperation Projects of Shanxi Province (Grant no. 2010081018), and National Natural Science Foundation of China (Grant no. 51078252).
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Copyright © 2014 Hongtao Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.