Neutral and charged boron-doped fullerenes for CO2 adsorption

Summary Recently, the capture and storage of CO2 have attracted research interest as a strategy to reduce the global emissions of greenhouse gases. It is crucial to find suitable materials to achieve an efficient CO2 capture. Here we report our study of CO2 adsorption on boron-doped C60 fullerene in the neutral state and in the 1e −-charged state. We use first principle density functional calculations to simulate the CO2 adsorption. The results show that CO2 can form weak interactions with the BC59 cage in its neutral state and the interactions can be enhanced significantly by introducing an extra electron to the system.


Introduction
The continuous dependence on fossil fuel combustion for the generation of energy has dramatically increased the atmospheric CO 2 concentrations over the last century. Despite concerns for global climatic changes and many attempts to sustainably generate energy, fossil fuel combustion continues to be the main source of electricity while releasing 13 Gt of CO 2 [1] to the atmosphere each year. Therefore CO 2 capture and storage (CCS) technology is a promising solution to reduce atmospheric CO 2 emissions [2]. Solvent absorption that is based on amines is the most common technology for the capture of CO 2 . However this method is criticized for its very high energy consumption and operational limitations such as corrosion, slow uptake rates, foaming and large equipment. Hence there is a huge interest in solid adsorbent materials for CCS [3][4][5][6]. In past few years metal organic frameworks (MOFs) have emerged as solid CO 2 adsorbent materials due to their tuneable chemical and physical properties.
Particularly, there is growing interest for metal free carbonbased nanomaterials for gas adsorption. Carbon-based nanomaterials such as fullerene, carbon nanotubes and graphene offer excellent thermal and chemical stability as CO 2 adsorbents [7,8]. Heterofullerenes are fullerene structures in which one or more cage carbon atoms are substituted by heteroatoms [9]. In addition to the properties mentioned above, which are inherent to carbon-based nanomaterials, heterofullerenes also offer excellent tuneable chemical and physical properties [10]. Gas adsorption on heterofullerenes is an appealing subject. B. Gao et al. [11] studied CO 2 adsorption on calcium decorated C 60 fullerene and F. Gao et al. [12] studied O 2 adsorption on nitrogen-doped fullerene.
Boron-doped C 60 fullerenes are one of the most structurally stable heterofullerenes [9]. Guo et al. synthesized B-doped C 60 fullerenes for the first time, in microscopic amounts by laser vaporisation [13]. Zou et al. [14] demonstrated the synthesis of B-doped C 60 fullerene by using radio frequency plasma-assisted vapour deposition. Recently Dunk et al. [15] introduced a method to produce BC 59 directly from exposing C 60 fullerene to boron vapour. Wang et al. [16] stated that substituting a single C atom of the C 60 fullerene with a B atom does not cause a significant distortion in the cage structure. The net change in the dihedral angle due to the doping is only 1.6% and Kurita et al. [17] predicted that due to the similarity between the C-B bond and the C-C bond, the changes in the bond lengths are less than 5%. Therefore the BC 59 fullerene has a similar structural and thermal stability as C 60 fullerene. Despite the numerous study results, which confirm the structural stability of B-doped C 60 fullerene, very little studies have been done on applications of B-doped fullerene. Here, for the first time we report a study about the CO 2 adsorption on B-doped C 60 fullerene, in which a single C atom is replaced with a B atom.
Sun et al. [8] predicted an enhanced CO 2 adsorption on 1e −and 2e − -charged boron nitride sheets and nanotubes, which show very little chemical affinity towards CO 2 in their neutral state. Also Sun et al. [18] showed that chemical interactions between boron-carbon nanotubes (B 2 CNT) and CO 2 can be enhanced by introducing extra electrons to the system. The enhanced interaction of CO 2 with adsorbent materials by electron injection has been further proved by Jiao et al. [19]. Therefore, we will investigate the CO 2 adsorption on BC 59 fullerene in both the neutral and the 1e − -charged states.

Computational Details
First-principles density functional theory (DFT) calculations were carried out to study CO 2 adsorption on the BC 59 cage. The BC 59 structure was fully optimized in the given symmetry. The calculations were carried out at B3LYP [20][21][22] level of theory while using the split valance polarized basis set 6-31G(d). B97d [23,24] with the same basis set was used for calculations when non-covalent interactions are predominant. The CO 2 adsorption on BC 59 was studied in the neutral state and in the 1e − -charged state. The electron distribution and transfer were analysed with Mulliken population analysis method [25].
The adsorption energies were calculated using the following equation. (1) where E ads is the adsorption energy, is the total energy of the BC 59 cage with a CO 2 molecule adsorbed and and are the energies of the isolated BC 59 cage and CO 2 molecule, respectively. For a favourable adsorption the calculated adsorption energy should have a negative value. To provide more accurate results for the chemisorption energy the counterpoise corrected energy [26,27] was also calculated.
The transition state was located by using the synchronous transit-guided quasi-Newton (STQN) method [28,29], which was then fully optimized by using the Berny algorithm at the B3LYP/6-31G(d) level. The optimized transition structure was used for IRC calculations at the same level of theory [30,31]. All calculations were carried out by using the Gaussian 09 package [32]. The GaussView 5 package [33] was used to visualize the optimized molecular structures, molecular orbitals and charge distributions.

Results and Discussion
The substitution of a C atom in the C 60 fullerene by a B atom causes a charge transfer between C and B atoms, which results in an unbalanced charge distribution in the fullerene cage. The unbalanced charge distribution forms B-C complex sites for the adsorption of CO 2 ( Figure 1). Here we considered two possible sites for the CO 2 adsorption: the B-C atomic site between two hexagonal rings (HH B-C site) and two identical B-C sites between a hexagonal ring and pentagonal ring (HP B-C site).

Adsorption of CO 2 on uncharged BC 59 fullerenes
According to our simulation results, the CO 2 molecules can only form weak interactions with BC 59 cage in its neutral state. The physisorption energy is a weak −2.04 kcal/mol (−4.1 kcal/mol for B97D/6-31G(d) calculations) and the weak interactions are mainly van der Waals interactions between the CO 2 molecule and the adsorbent. The CO 2 physisorbed configuration is shown in Figure 2

Effects of charges on the structure
Kim et al. [34] predicted that C 59 B − should be a stable entity because of the isoelectronic configuration with C 60 . This claim is further validated by experimental observations by Dunk et al. [15]. The Mulliken charge analysis and the electron density distributions of the lowest unoccupied molecular orbitals (LUMO) are adopted to assess the influence of changing the charge state of BC 59 . Figure 3 shows that the LUMO of the neutral BC 59 is noticeably concentrated on the B atom and the neighbouring C atoms. Furthermore experimental results of Guo et al. [13] showed that boron doping creates an electron defficient site at the B atom. This suggests that an additional electron added to the system will be accepted by the B atom. This hypothesis is consistent with theoretical predictions of Kurita et al. [17] and Xie et al. [35], who stated that the doped B atom in C 60 fullerene acts as an electron acceptor. The comparison of the Mulliken population analysis of the neutral and the 1e − -state of BC 59 proves that the negative charge introduced to the system is essentially accepted by the B atom. The Mulliken atomic charge of the B atom in the BC 59 structure in the neutral state has changed from 0.138 to 0.012 upon the introduction of the negative charge, while as shown in Figure 4 the charges on the C atoms are not changed significantly. CO 2 adsorption on BC 59 fullerene in the 1e −state Next we studied the CO 2 adsorption on a 1e − -charged BC 59 cage. The results confirm that the negatively charged BC 59 fullerene exhibits a stronger interaction with CO 2 . Unlike the neutral BC 59 , for which the interaction with CO 2 molecule was only physical, here the charged BC 59 forms a substantial chemical interaction with CO 2 causing the molecule to undergo significant structural deformations. A stable CO 2 adsorption is observed at the HH B-C site. The chemisorption energy of −15.41 kcal/mol (−64.48 kJ/mol) (−13.48 kcal/mol with BSSE correction) agrees well with the ideal range of chemisorption energy (40-80 kJ/mol) for a good CO 2 adsorbent [36].  The higher adsorption energy and the significant distortions in the structure confirm a stronger interaction between CO 2 molecule and negatively charged BC 59 than its neutral state. These interactions can be explained due to the Lewis acidity of CO 2 , which prefers to accept electrons [18]. On the other hand the B atom of the BC 59 becomes less positively charged upon the addition of an extra electron. Therefore it becomes more likely to donate electrons to the CO 2 molecule leading to stronger interactions between the two molecules. Figure 6 shows the minimum energy pathway for the adsorption from the physisorbed state to the chemisorbed configur-ation. We performed frequency calculations on the optimized transition structure, which confirms that it is a first order saddle point and hence an actual transition structure. From this figure, the activation barrier for the chemisorption is estimated to be 13.25 kcal/mol (55.43 kJ/mol). The low barrier of the reaction indicates that the reaction is energetically favourable. For the desorption step, the removal of the added charge will decrease the stability of the bond between CO 2 and the doped fullerene. The thermodynamic analysis of the reaction shows that the CO 2 chemisorption is spontaneous only for temperatures less than 350 K. Therefore we suggest a method of manipulating the charge state and the temperature of the system for adsorbent recycling. Charging the system can be achieved by electrochemical methods, electrospray, and electron beam or gate voltage control methods [8].

Conclusion
By using DFT calculations we have studied the adsorption mechanisms of CO 2 on a C 60 fullerene cage, in which a single C atom is substituted by a B atom. Our calculation results show that the BC 59 cage, in its neutral state, shows a low chemical interaction with CO 2 molecule, which only physisorbs with E ads = −2.04 kcal/mol. However CO 2 adsorption on the BC 59 can be significantly enhanced by injecting negative charges into the structure. The CO 2 molecule chemisorbs on the 1e − -charged BC 59 with E ads = −15.41 kcal/mol. This study suggests that we can conclude 1e − -charged BC 59 cage structure is a promising CO 2 adsorbent.