Hydrogenation mechanism of small fullerene cages

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Hydrogenation mechanism of small fullerene cages A.A. EL-Barbary a,b,* a b

Physics Department, Faculty of Education, Ain Shams University, Cairo, Egypt Physics Department, Faculty of Science, Jazan University, Jazan, Saudi Arabia

article info

abstract

Article history:

Ab initio DFT (density functional theory) is used to investigate the hydrogenation energy

Received 7 May 2015

and hydrogenation mechanism of Cn and CnHn fullerene cages from n ¼ 20 to n ¼ 60. All

Received in revised form

calculations have been performed using G03W package, with B3LYP exchange-functional

28 September 2015

and applying basis set 6e31G(d, p). It is found that the most stable hydrogenation sites

Accepted 25 October 2015

on the Cn fullerene cages are

Available online 12 November 2015

and

and

sites and on the CnHn fullerene cages are

,

sites. The calculations show that the required energy to initiate the hydrogen

migration on the surface of Cn fullerene cages between two metastable structures of C54H is Keywords:

~1.5 eV and on the surface of CnHn fullerene cages between metastable and stable struc-

Hydrogenated fullerene

tures of the C60H61 fullerene cage is ~2.35 eV. Also, it is found that the energy release from

DFT

hydrogen migration is always enough to direct the hydrogenation process towards the

Hydrogenation barrier

most stable structures and it reduces the number of hydrogen atoms bonded to the

Hydrogen storage

fullerene cage via forming H2 molecules. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Many experimental and theoretical attempts are made for enhancement the hydrogen storage in carbon materials [1e10]. In particular, endohedral and exohedral hydrogen storage capacity of C60 and metal decorated C60 have been investigated using various experimental techniques and theoretical methods [11e15]. The hydrogen storage in many systems as Ti6C24B24 [16], C48B12M12 (M ¼ Fe, Co, Ni) [17], M32B80 (M ¼ Ca and Sc) [18] Li9C60 [19], Li12C60 [20], Ti doped fullerene [21], fullerene-like Co3C [22] Ni-dispersed fullerenes [23] sodium intercalated fullerenes [24] Be@C60 [25] have exhibited remarkable hydrogen adsorption capacity. Also, by the incorporation of C into (BN)12 fullerene [26,27] theoretical investigation shows that the hydrogenation reaction on carbon doped cluster is both thermodynamically favored and kinetically feasible under ambient.

Hydrogenated C60 fullerene cages were synthesized soon after the development of industrial methods to produce significant quantities of these materials [28e39]. Experimentally, hydrogenated C60 fullerene cages are synthesized by many methods as the direct non-catalytic hydrogenation, producing C60H218 [40], the reaction of gaseous hydrogen with C60Pd4.9, producing C60H226 [35], the catalytic hydrogenation in toluene solution in the presence of Ru/C, producing C60H3648 [41], the radical hydrogenation with promoter C2H5I, producing C60H36 [42], the reduction with lithium in ammonia in the presence of BuOH, producing C60H1836 [33], the reduction in toluene solution through hydroborating or hydrozirconating producing C60H24 [31], the hydrogen transfer on the fullerene from the dihydroanthracene, producing C60H1836 [43]. Also, the interaction of C60 with hydrogen atoms has also been intensively studied theoretically [44e47] showing that the theoretical predictions of the stability of hydrogenated fullerene cage C60H60.

* Physics Department, Faculty of Education, Ain Shams University, Cairo, Egypt. E-mail address: [email protected]. http://dx.doi.org/10.1016/j.ijhydene.2015.10.102 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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It is clear that the theoretical predictions of the hydrogenated fullerene cage of C60H60 are always larger than the actual experimental production, C60Hn (n < 60). Also, the X-ray studies have shown that the hydrogenated fullerene cage C60H36 is characterized by the molecular destruction caused by the break of bonds [32,46,48]. It should be emphasized that the experimental methods for hydrogenated fullerene cages have not yet to give the maximum hydrogen content for some unknown reasons. Therefore our main task in this paper is to clarify these unknown reasons. One of the experimental studies has shown that the degree of hydrogenation and the mechanism of this process are related to the structure of the fullerene cages and, in particular, to the pentatomic rings forming the fullerene cages [49]. Hence, the small fullerene cages Cn, n ¼ 20 to 60, are chosen to investigate the effect of pentatomic rings on the hydrogenation mechanism. All the possible hydrogenation sites inside and outside each fullerene cage from C60 cage to C20 cage are investigated. Also, the hydrogen migration energies between different hydrogenation sites are calculated.

Methodology All calculations are performed with DFT as implemented within G03W package [50e53], using B3LYP exchange-functional [54,55] and applying basis set 6e31G(d, p). All obtained structures are fully optimized where the Cn fullerene cages are optimized under symmetric constraint however CnH and CnHnþ1 are optimized without any constraint. All the optimized geometric structures are confirmed with frequency calculations where all vibrational frequencies are found to be positive. In this work, the hydrogen adsorption energies of depositing one hydrogen atom on the Cn and CnHn fullerene cages from n ¼ 20 to n ¼ 60 are investigated. The hydrogen adsorption H energy (EH ads ) is calculated as Eads ¼ E(CcageH) e (E(Ccage) þ E(H)) [56], where E(CcageH) the energy of hydrogenated cage, E(Ccage) the energy of fullerene cage and E(H) the energy of a hydrogen atom. The energy barrier is calculated as the energy difference between the initial structure and the saddle point structure [57]. The saddle point structure is obtained by orthogonality constraints where all atoms is stepped a long a specified 3N vector (the difference between the relaxed initial and final structures) and allowed to relax at each step point in all directions orthogonal to this vector.

Results Geometric structures All the small fullerene cages are formed by two types of CeC bonds, a double bond character with average bond length 1.39 Å and a single bond character with average bond length 1.45 Å. C60 cage is a truncated icosahedron of the point-group symmetry (Ih) and sixty carbon atoms are arranged in 20 sixmembered and 12 five-membered rings. C58 cage possesses a C3V symmetry and fifty eight atoms are arranged in 17 sixmembered, 13 five-membered and 1 seven-membered rings. C56 cage has D2 symmetry and fifty six atoms arranged in 18

six-membered and 12 five-membered rings. C54 cage has C2V symmetry and fifty four atoms which are arranged in 16 sixmembered and 12 five-membered rings. C52 cage possesses a C2 symmetry and fifty two atoms are arranged in 16 sixmembered, and 12 five-membered rings. C50 cage has D5h symmetry and fifty atoms arranged in 15 six-membered and 12 five-membered rings. C48 cage possesses D3 symmetry and forty eight atoms which are arranged in 14 six-membered and 12 five-membered rings. C46 cage has C2 symmetry and forty six atoms are arranged in 13 six-membered and 12 fivemembered rings. C44 cage has D3h symmetry and forty four atoms arranged in 12 six-membered and 12 five-membered rings. C42 cage possesses D3 symmetry and forty two atoms are arranged in 11 six-membered and 12 five-membered rings. C40 cage has D2 symmetry and forty atoms arranged in 10 sixmembered and 12 five-membered rings. Finally, C20 cage is a truncated icosahedron of the point-group symmetry (Ih) and twenty carbon atoms are arranged in zero six-membered and 12 five-membered rings, see Fig. 1. All the sixty carbon atoms of the C60 cage are equivalent, therefore, only one adsorbing site is present; namely site, the vertex between two hexagons and one pentagon [58,59]. For the C58 cage, there are four different adsorption sites; namely site (the vertex between two hexagons and one heptagon), site (the vertex between one pentagon, one hexagon and site (the vertex between two pentagons and one heptagon), site (the vertex between two pentagons and one heptagon), site. For the rest of small fullerene cages, one hexagon) and site (the there are four different adsorbing sites; namely site (the vertex between vertex between three pentagons), site and the site, see Fig. 2. three hexagons), the

Hydrogen adsorption First hydrogenation outside and inside the Cn fullerene cages The first hydrogenation (adsorption) energy outside and inside the small fullerene cages is calculated and is shown in Tables 1,2. All the adsorbed hydrogen atoms outside and inside the small fullerene cages are bonded to the carbon atoms with average bond length 1.1  A. For outside hydrogenation, the bonds between the hydrogenated carbon atom and its first three neighbor carbon atoms are single bonds with average bond site, site, site and length 1.55  A, 1.54  A, 1.54  A, 1.52  A at site, respectively. Also, for inside hydrogenation the bonds between the hydrogenated carbon atom and its first three neighbor carbon atoms are single bonds with average bond site, site, site and length 1.50  A, 1.50  A, 1.48  A, 1.50  A at site, respectively. Therefore, one can report that the first hydrogen deposition atom outside and inside the Cn fullerene cage leads to convert all the bonds between the hydrogenated carbon atom and its first neighbor carbon atoms to single bonds, see Fig. 3. Also, it is noticed that the hydrogen deposition is accompanied by puckering the bonded carbon atom outside the surface of fullerene cages. This observation is reported previously [60] and leads to improve the stability of fullerene cages. From Tables 1,2, the adsorption energy of the first hydrogenation outside the small fullerene cages is always lower than the adsorption energy inside the small fullerene cages. Also, it is found that the most stable hydrogenation sites are at and sites outside the surface of fullerene cages, agrees

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Fig. 1 e Fully optimized structures of a) C60 cage, b) C58 cage, c) C56 cage, d) C54 cage, e) C52 cage, f) C50 cage, g) C48 cage, h) C46 cage, i) C44 cage, j) C42 cage, k) C40 cage and l) C20 cage. The yellow rings represent the adjacent pentagon rings. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

with the previous experimental observation [26]. In addition, the most stable hydrogenated fullerene cage is found to be C20H20 with hydrogenation energy 2.80 eV, agrees well with previous theoretical calculations [60] indicating that the C20H20 was the most stable partially hydrogenated fullerene.

Hydrogenation inside and outside the fully hydrogenated fullerene cages, CnHn The study of the first hydrogen deposition on the unhydrogenated Cn fullerene cages is shown that the outside

hydrogenation is always energetically favored than the inside hydrogenation. Now, the deposition of one hydrogen atom inside and outside the fully hydrogenated fullerene cages from C20H20 to C60H60 will be also investigated. After fully optimization, the deposited hydrogen atom inside the hydrogenated CnHn fullerene cage is always broken its bond with carbon atom and moved to be located at the center of fullerene cages, independent on the hydrogenation sites, see Fig. 4. However, when a hydrogen atom is deposited on the surface of hydrogenated CnHn fullerene cages, there are two

Fig. 2 e View of different optimized adsorbing sites for hydrogen adsorption outside the small fullerene cages. White atoms are represented the hydrogen atoms and the gray atoms are represented the carbon atoms. The pink, blue and yellow rings represent the heptagon rings, the hexagon rings and the pentagon rings, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Table 1 e The calculated first hydrogenation (adsorption) energies inside and outside the fullerene cages, from C20H to C60H. Energy is given by eV. EH ads /eV

CnH H-inside the cage site C20H C40H C42H C44H C46H C48H C50H C52H C54H C56H C60H

site

e 1.42 1.95 2.21 1.95 1.95 1.96 1.99 2.10 2.20 e

H-outside the cage

site

e 1.83 2.24 2.03 1.67 1.80 2.11 1.76 2.29 2.54 2.47

site

site

0.94 1.48 1.62 1.96 e e e e e e e

e 2.04 2.15 1.96 2.89 2.11 2.57 2.16 2.04 2.39 e

site

e 0.40 0.31 0.53 0.22 0.31 0.45 0.43 0.73 0.61

site

e 0.60 0.30 0.10 0.02 0.40 0.01 0.40 0.33 0.32 0.22

site 2.80 2.10 1.60 1.10 e e e e e e e

e 2.01 1.59 0.98 0.32 1.17 0.70 0.98 1.29 1.01 e

Table 2 e The calculated first hydrogenation (adsorption) energies inside and outside the C58H fullerene cage. Energy is given by eV. EH ads /eV

CnH H-inside the cage site C58H

1.87

site 1.63

site 1.36

H-outside the cage site 2.48

site 2.18

mechanisms for hydrogenation process depending on the hydrogenation sites. In the first mechanism, when the , , and sites, the hydrogen atom is located at the hydrogen atom is found to be bonded to carbon atom with CeH bond length of 1.2  A. This mechanism causes the sharing bond between pentagon-pentagon rings or between pentagon-hexagon rings to break, see Fig. 5(b,c,d,g), and is consistent with experimental findings [32,46,48]. However, in the second mechanism when the hydrogen atom is located at

site 0.71

site 0.00

site 1.10

site 0.23

site 1.00

, and sites, the hydrogen atom is found to be the bonded to one hydrogen atom of CnHn cage. This mechanism leads to form H2 molecule and reduce the number of hydrogen atoms of CnHn cage by one to be CnHn1, see Fig. 5(a,e,f), agrees with experimental observations [31,33,35,41e43]. From Tables 3,4, the calculations show that the hydrogenation energy of depositing one hydrogen atom outside the hydrogenated CnHn fullerene cages is always more lower than the hydrogenation energy of depositing one hydrogen atom

Fig. 3 e View of different optimized adsorbing sites for hydrogen adsorption outside and inside the small fullerene C42H , b) , c) d) and the inside hydrogenation sites are e) , f) , g) cage. The outside hydrogenation sites are a) h). .

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Fig. 4 e The obtained fully optimized structures of a) fully hydrogenated C42H42 cage and b) deposition of one hydrogen atom inside hydrogenated fullerene C42H42 cage.

inside the CnHn fullerene cages. Also, it is found that the first mechanism of depositing one hydrogen atom outside CnHn fullerene cages leads to an increase of hydrogenation energy comparing with the hydrogenation energy of depositing one hydrogen atom outside the Cn fullerene cages. For example, the outside hydrogenation energies at the site of the C42H43 and C44H45 cages comparing with C42H and C44H cages are increased from 1.60 eV to 2.49 eV and from 1.10 eV to 2.38 eV, respectively. However, the second mechanism of depositing one hydrogen atom outside CnHn fullerene cages leads to lower the hydrogenation energy comparing with the hydrogenation energy of depositing one hydrogen atom outside the Cn fullerene cages. For example, the outside hysite of C40H41 cage comparing drogenation energy at the with C40H cage is lowered the hydrogenation energy from site of the C58H58 0.40 eV to 2.01 eV and at the comparing with C58H cages from 0.71 eV to 1.73 eV. This finding is consistent with the previous experimental studies

that had shown that the degree of hydrogenation and the hydrogenation mechanism are related to the structure of the fullerene [49]. Finally, one can report that the hydrogen atoms can be trapped outside the CnHn fullerene cage creating metastable structures as at the , , and sites or can pick one hydrogen from the CnHn fullerene cages, creating H2 mole, and sites and leaving the fullerene cules as at the cages as CnHn1, see Fig. 5. The latter structures are the most stable structures and therefore the hydrogenated carbons are exposed to loss their hydrogen atoms through forming H2 molecules. To be more quantitative, the migration barrier between the metastable and stable structures will be calculated. The C54 fullerene cage is chosen to study the migration energy of one hydrogen atom on the surface of Cn cages. The migration energies between the two metastable structures at and sites and also between the metastable structure at

Fig. 5 e View of optimized hydrogenation sites of depositing one hydrogen atom outside the hydrogenated CnHn cages. White circles are represented the hydrogen atoms and the gray circles are represented the carbon atoms. The red circles are represented the adsorption sites on carbon atoms. Numbers 5,6 and 7 are referred to the numbers of carbon atoms in each ring. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Table 3 e The calculated hydrogenation (adsorption) energy of depositing one hydrogen atom inside and outside the CnHn fullerene cages. Energy is given by eV. EH ads /eV

CnHnþ1 H-inside the cage site C20H21 C40H41 C42H43 C44H45 C46H47 C48H49 C50H51 C52H53 C54H55 C56H57 C60H61

site

e 2.43 2.41 2.40 2.39 2.38 2.38 2.38 2.38 2.37 e

H-outside the cage

site

e 2.43 2.41 2.42 2.43 2.38 2.38 2.38 2.38 2.37 2.37

site

site

2.57 2.43 2.41 2.40 e e e e e e e

e 2.43 2.41 2.40 2.39 2.38 2.38 2.38 2.38 2.37 e

site

e 2.01 0.10 0.13 0.15 0.28 0.34 0.37 0.59 1.10 e

site

e 2.26 2.20 2.26 2.27 2.32 2.31 2.36 2.34 2.29 2.29

site 2.11 2.14 2.49 2.38 e e e e e e e

e 2.24 2.32 1.88 1.93 1.92 1.94 2.02 2.38 1.95 e

Table 4 e The calculated hydrogenation (adsorption) energy of depositing one hydrogen atom inside and outside the C58H59 fullerene cages. Energy is given by eV. EH ads /Ev

CnHnþ1 H-inside the cage site C58H59

0.74

site 0.74

site 2.38

H-outside the cage site 2.38

site and the stable structure at site are calculated and are shown in Fig. 6. All the stable, metastable and saddle point structures are depicted as ball-and- stick models and are included in Fig. 6. Also, the geometric parameter of the stable, metastable and saddle point structures are their relative

site 3.89

site 1.73

site 1.18

site 1.91

site 2.27

site 2.38

energy with respect to the energy of stable structure is shown in Table 5. In Fig. 6, the DE is the energy difference with respect to the site. In the stable energy of the most stable structure at the and metastable structures, the hydrogen atom is bonded to

Fig. 6 e The two paths of hydrogen atom migration on the surface of C54 fullerene cage. The first path from (a) the metastable site to (b) the metastable structure at site. The second path from the metastable structure at site to (c) structure at site. (d) and (e) are the saddle point structures. the most stable structure at

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Table 5 e The calculated relative energy E(eV) with respect to the stable structure and the key geometry parameters of the structures shown in Fig. 6. Structures

E/eV SS MS1 MS2 SP1 SP2

0 1.62 2.02 3.50 3.52

C1eH 1.05 1.07 1.07 1.26 1.22

Å Å Å Å Å

SS refers to the most stable structure at site, MS1 refers to the metastable structure at site, SP1 and SP2 refer to the saddle point structures.

the carbon atom with single bond y 1.1  A, see Fig. 6(aec) and Table 5. For the saddle point structures, Fig. 6(dee), the hydrogen atom is bonded to two carbon atoms, with bond lengths y1.2  A and y1.5  A. It is found that the energy difand ference between the two metastable structures at the the sites is about 0.4 eV and the energy barrier to achieve and the transformation from metastable structures at the site is 1.5 eV, close to the previous theoretical energy at the barrier 1.45 eV of hydrogen migration on C60 cage [57]. However, the energy difference between the metastable structures site and the most stable structure at the site is at the 1.62 eV. To achieve the hydrogen migration from the metastable structure at site to the most stable structure at the site, the required energy is 1.88 eV, however the calculated energy release from the first and second migrations is found to be 1.9 eV and 3.5 eV, respectively. Therefore, it is required only 1.5 eV to initiate the hydrogen migration between the two metastable structures and it will release 1.9 eV which is enough to direct the hydrogen migration to the stable structure and to gain an energy release of 3.5 eV. The energy release (3.5 eV) is enough to direct all the sites. metastable structures to the stable structure at the This finding agrees with the previous experimental explanation of forming the C60H36 fullerene cage where 36 is the minimum number of hydrogen atoms required to leave unconjugated double bonds in each of the hexagons [34]. The C60H60 cage is chosen to study the hydrogen migration energy of depositing one hydrogen atom on the surface on CnHn fullerene cages. There are two mechanisms for hydrogen deposition outside the C60H60 cages depending on the hydrogenation sites, as mention above. The migration energy of one site and the hydrogen atom between the stable structure at site are calculated and are shown metastable structure at in Fig. 7. The stable, metastable and saddle point structures are depicted as ball-and- stick models and are included in Fig. 7. Also, the geometric parameter of the stable, metastable and saddle point structures are their relative energy with respect to the energy of stable structure is shown in Table .6. In Fig. 7, the DE is the energy difference with respect to the energy of the stable structure fullerene cage. It is found that the energy difference between the metastable structure C60H61 and the stable structure C60H59þH2 is about 2.47 eV. Also, the energy barrier to achieve the transformation from the metastable structure to the stable structure is 2.35 eV while the calculated energy release from this transformation is about 4.82 eV. Therefore, the required energy barrier to

C1eC2

C1eC3

C1eC4

1.55  A  1.53 A 1.43  A  1.53 A 1.51  A

1.51  A 1.53  A 1.43  A 1.43  A 1.44  A

1.55  A 1.53  A 1.53  A 1.53  A 1.40  A

site, MS2 refers to the metastable structure at

initiate the hydrogen transformation from the metastable structure C60H61 to the stable structure C60H59þH2 is 2.35 eV and the energy release is 4.82 eV. This energy release (4.82 eV) is enough to continue reducing the hydrogen atoms bonded to the fullerene cage via forming H2 molecules. This finding could be considered as one possibility to explain the previous experimental results of producing the C60Hn where n is always less than 60 [31,33,35,40e44] and the experimental observation of reducing the hydrogenation weight upon prolonged hydrogenation [61].

Conclusion DFT calculations is performed to investigate the hydrogen migration on Cn and CnHn fullerene cages. It is found that the most stable sites for hydrogen deposition on the Cn fullerene and sites. Also, it is found that the cages are at the hydrogen deposition on the CnHn fullerene cages can be trapped outside the fullerene cages creating metastable , , and sites or can pick one structures at the hydrogen atom from the CnHn fullerene cages and form H2 molecules at the , and sites via converting the CnHn fullerene cages to CnHn1 fullerene cages. In addition, the

Fig. 7 e The hydrogen migration paths on the C60H60 fullerene cage from (a) the metastable structure to (b) the stable structure and (c) is the saddle point structure.

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Table 6 e The calculated relative energy E(eV) with respect to the stable structure and the key geometry parameters of the structures shown in Fig. 7. Structures

E/eV SS MS SP

0 2.47 4.82

H1eH2

C1eH1

C1eH2

C1eC2

C1eC3

C1eC4

0.75 Å 1.37 Å 0.85 Å

6.50  A 1.08  A 1.06  A

7.25  A 1.11  A 2.36  A

1.49  A 1.55  A 1.43  A

1.49  A  1.55 A 1.43  A

1.47  A 2.87  A 1.40  A

SS refers to the stable structure, MS refers to the metastable structure and SP refers to the saddle point structure.

hydrogen deposition is found to be accompanied with puckering the carbon atoms outside the surface of the fullerene cages in order to improve their stability. It is noticed that the energy required to initiate the hydrogen migration on the Cn fullerene cages between the two metastable structures of C54H is 1.5 eV and the hydrogen migration will release 1.9 eV which is enough to direct all the metastable structures to the stable structure with energy release of 3.5 eV. This energy release (3.5 eV) is enough to deposit all the hydrogen atoms only at the stable site. The required energy barrier to initiate the hydrogen migration on the CnHn fullerene cages between the metastable and stable structures of the C60H60 fullerene cage is found to be 2.35 eV and the calculated energy release is 4.82 eV. This energy release is enough to continue reducing the number of hydrogen atoms bonded to the fullerene cage via forming H2 molecules and this could be used to explain why n is always less than 60 in the experimental hydrogenated C60Hn cage.

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