Abstract
The chemisorptions of H2 on fullerenes C60 and C80, endofullerene Sc3C@C80 and solid C60 were comparatively studied. A chain reaction mechanism for dissociative adsorption of H2 on solid C60 is proposed under high pressure. The breaking of H–H bond is concerted with the formation of two C–H bonds on two adjacent C60 in solid phase. The adsorption process is facilitated by the application of high pressure. The initial H2 adsorption on two adjacent C60 gives a much lower barrier 1.36 eV in comparison with the barrier of adsorption on a single C60 (about 3.0 eV). As the stereo conjugate aromaticity of C60 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 wide-spread 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 C60HX and C70HX 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 C60 and C70 has also been achieved without the usage of a catalyst by exposing solid-phase fullerenes to high-pressure 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 B36C36 cage can store at most 18 hydrogen molecules at zero temperature [12]. They also find that an isolated Li12C60 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/H2 [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 H2 storage density 9 wt% [14]. Kang and coworkers reports that Ni-dispersed fullerenes are considered to be capable of storing 6.8 wt% H2, with H2 desorption activation barrier of 11.8 kcal/mol, which is ideal for many practical hydrogen storage [15].
However, the mechanism for direct reactions between H2 and these fullerenes remains unexplained to the best of our knowledge. Chan et al. proposed the mechanism of H2 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 H2 adsorption on fullerenes.
In this work, we proposed a chain reaction mechanism for H2 molecules dissociative adsorption on solid C60 under high pressure. In comparison, we also studied H2 adsorption on the most stable fullerenes C60 and C80 in gas phase as well as endofullerene Sc3N@C80.
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 exchange-correlation functional and projector augmented wave (PAW) pseudopotentials [20, 21] were employed, with an energy cutoff of 400 eV for the plane-wave 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 Monkhorst-Pack 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 mass-weighted 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 H2 on fullerenes were calculated by where , , and are the total energies of H2 adsorbed fullerene, total energies of fullerene (fullerene = C60, C80, Sc3N@C80), and the total energies of H2, respectively. The larger adsorption energy indicates the stronger adsorption.
3. Results and Discussion
3.1. H2 Adsorption in Gas Phase
We firstly consider the H2 adsorption on fullerenes C60 and C80 and endofullerene Sc3N@C80 to explore the possible adsorption media without any catalyst in gas phase. In the calculations, the interactions between H2 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. H2 Adsorption on C60 in Gas Phase
The icosahedral C60 consists of 12 pentagons and 20 hexagons. Hence the bonds can be categorized as two types, pentagon-hexagon bonds (5-6 bond) and hexagon-hexagon bonds (6-6 bond). Addition on adjacent sites such as 5-6 bond and 6-6 bond engenders isomers. Furthermore, addition can take place on nonadjacent sites, which would produce many isomers. For C60H2, the simplest fullerene dihydride, there are 23 isomers. However, there is only one isomer that has been characterized. Among all kinds of C60H2 isomers, (1,2) addition products are considered as the most stable. To compare the adsorption difference, we investigated H2 adsorption on both 5-6 and 6-6 bonds. In addition, two adsorption modes are considered. One case is that the H–H bond of incoming H2 is to be considered to parallel the 6-6 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 6-6 bond addition production is 0.77 eV favorable than that for 5-6 bond addition (Table 1), indicating that 6-6 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. H2 Adsorption on C80 in Gas Phase
C80 has the same Ih symmetry as C60, so it can also be served as adsorption media, although experimentally no hydrite of C80 has been characterized. We carried out the same calculations as we did on C60 as shown in Figure 2. The 6-6 parallel and perpendicular adsorption modes have the same energy barrier of 2.47 eV whereas 5-6 parallel and perpendicular adsorption modes have the same barrier of 2.27 eV. Comparing to that of C60, the calculated energy barrier is about 1 eV lower than that for C60 due to the less stability of C80.
3.1.3. H2 Adsorption on Sc3N@C80 in Gas Phase
As one of the most stable endofullerene, Sc3N@C80 has become accessible in macroscopic quantities. Since the adsorption barrier for C80 has decreased more obviously than that for C60, its stable derivative Sc3N@C80 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 Sc3N@C80 can hardly react with H2. In addition, the overall exothermic energy is quite low at 0.1 eV, indicating that the reaction is not very favorable thermodynamically.
3.2. H2 Adsorption in Solid Phase
In contrast, the solid phase, composed of bundles of C60, provides a unique chemical environment dependent on the external pressure and makes it much easier for H2 dissociative chemisorption on C60 bucky balls. In our calculations, the interactions between hydrogen molecule H2 and C60 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 H2 on Solid C60
Under the pressure of pressure of ~50 kB, we explored the reactive trajectory of the first H2 dissociative chemisorption on C60 as shown in Figure 4. There are three steps involved: first, the incoming H2 dissociation and deposition on two adjacent C60, from structure U to V, through transition structures (TS 11); second, the rotation of C60; and finally, hydrogen migration through TS 12 to structure W, a 1,2-addition product. Figure 5 gives the plot of the relative energies for the dissociative H2 chemisorption on solid C60. 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 C60 in gas phase, the barrier for H2 molecules adsorption on two adjacent C60 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, C60 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 H2 chemisorption on solid C60 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 C60 and solid carbon nanotube under high pressure.
3.2.2. Adsorption of the Second H2 on Solid C60
To explore whether it is possible to chemisorb more H2 on the solid C60 under high pressure, we investigate the reaction path for the addition of the second H2 on C60 as shown in Figure 6. The reaction mechanism is quite similar to the addition of the first one. In this process, the second H2 also dissociatively chemisorbs on two adjacent C60 in the initial step and then rotates slightly; one of the H atom migrates from one C60 to another and finally forms two 1,2-addition products. The relative energies for the chemisorption of second H2 on solid C60 were shown in Figure 7. Compared to the first H2 chemisorption, the calculated barrier for the second H2 is 1.21 eV in the dissociation step, which is a littler lower than that for the first H2 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 H2 molecule adsorption has already partially disrupted the conjugated system, so further addition is much easier.
In the first H2 adsorption process, we have mentioned that one of the H atoms will transfer from one C60 radical to another. There is another probability that H2 molecules react with the intermediate radical directly. We also investigate this kind of adsorption modes. For the C60H 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 H2 molecules will easily react with C60 intermediate radicals. Thus the reaction is a chain reaction: once the first H2 is adsorbed, the H2 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 H2 molecule adsorption has become the crucial step in the overall adsorption.
4. Conclusions
Based on the investigation of H2 molecules chemisorption on fullerenes C60 and C80 and endofullerene Sc3C@C80, we proposed a mechanism for H2 molecules adsorption on solid C60 under high pressure. Due to the rotation of C60, the H2 molecule will easily chemisorb on two adjacent C60 under high pressure, which is more favorable than the H2 molecule adsorption on single C60. The overall reaction is a chain reaction. The first H2 molecules adsorption is the crucial step in the overall H2 molecules adsorption process. Once the first H2 molecules is adsorbed on the C60, the second and subsequent H2 will easily be adsorbed on the C60 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).