Transition Metal Decorated Porphyrin-like Porous Fullerene

Article pubs.acs.org/JPCC

Transition Metal Decorated Porphyrin-like Porous Fullerene: Promising Materials for Molecular Hydrogen Adsorption K. Srinivasu and Swapan K. Ghosh* Theoretical Chemistry Section, Department of Atomic Energy, Bhabha Atomic Research Centre and Homi Bhabha National Institute, Mumbai 400 085, India ABSTRACT: Transition metal decorated carbon materials like fullerenes and nanotubes have been studied extensively for hydrogen adsorption applications. However, the weaker metal binding energy makes these materials unsuitable for making a stable hydrogen storage material. Here, using ab initio based density functional theory (DFT) calculations, we have studied the hydrogen adsorption in transition metal doped porphyrinlike porous fullerene, C24N24. This porous fullerene is generated through truncated doping of 24 carbon atoms by 24 nitrogen atoms, and the resulting fullerene contains six N4 cavities with a cavity diameter of 3.708 Å. These N4 cavities are found to bind with transition metal atoms (Sc, Ti, and V) very strongly, and the binding energies are found to be considerably larger than (nearly double) the corresponding metal cohesive energies. These transition metal sites are found to adsorb molecular hydrogen through well-known Kubas-type interactions. The calculated adsorption energies of molecular hydrogen around the different metal sites are found to be in the range of −9.0 to −3.0 kcal/mol, which is considered to be the enthalpy range required for ambient condition hydrogen storage. In the case of C24N24Sc6, the maximum hydrogen adsorption capacity is found to be ∼5.1 wt % with a total of 24 H2 molecules adsorbed around the metal doped fullerene.

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INTRODUCTION Fossil fuel dependent energy systems not only lead to the near future energy crises due to the depleting resources and everincreasing energy demand but also devastate the earth’s atmosphere by increasing CO2 content. Hydrogen energy is considered to be perhaps the best alternative to the presently existing energy systems because of its renewable and environmental friendly nature.1 However, bringing the dream of transition to hydrogen economy to reality highly depends on two major factors, viz., the cost-effective hydrogen generation and its storage in a safe and economic mode. Until now, many materials have been explored to make an efficient onboard hydrogen storage material. Apart from storing hydrogen as gas and liquid, two other hydrogen storage schemes studied extensively are the chemical hydrogen storage where the hydrogen is stored in atomic form like in the case of metal hydrides and the physical hydrogen storage where hydrogen is stored in molecular form like in the cases of porous framework materials. As each material is having its own advantages as well as drawbacks, searching for a new and better material for hydrogen storage is an important area of contemporary research. During the past two decades, a large number of materials, viz. metal hydrides, amino boranes, carbon- and boron-based nanomaterials, porous framework materials, etc., have been tested for hydrogen storage.2−6 However, none of the materials are capable of reaching the targets set by the Department of Energy (DOE).7 A family of carbon nanomaterials like graphene, carbon nanotubes, fullerenes, and porous carbon materials are extensively studied for hydrogen storage, and it is © 2012 American Chemical Society

found that the simple carbon surfaces cannot hold hydrogen efficiently as the driving factor in binding molecular hydrogen is simple van der Waals interactions. However, their metal decorated counterparts are found to have reasonably good hydrogen adsorption enthalpies.8 Light metals like alkali and alkaline earth metals supported on carbon nanomaterials are found to have improved adsorption enthalpies in the range 3.0−4.0 kcal/mol per H2.9−12 Transition metal decorated carbon nanomaterials are found to have hydrogen adsorption energies that are suitable for ambient temperature hydrogen storage.13,14 On the other hand, these materials have a drawback of being associated with very high metal cohesive energies which leads to the clustering of metals.15 In recent studies it was shown that this clustering problem can be avoided through the substitutional doping of base material with heteroatoms like boron.16 Recently, we have shown that Si−Li binary clusters can adsorb hydrogen more efficiently, and still there are many metal decorated carbon and silicon clusters which could be explored further for their hydrogen adsorption characteristics.17 Very recently, Luo et al.18 developed a liquid phase hydrogen storage material which can be operated at ambient conditions. Doping with heteroatoms like boron and nitrogen is well demonstrated in carbon materials to alter their properties.19 Boron and nitrogen substituted fullerenes, CNTs, and graphenes have already been synthesized.20 Recently, Lee et Received: May 16, 2012 Revised: September 27, 2012 Published: November 15, 2012 25184

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al.21 reported the synthesis of porphyrin-like CNT through plasma-enhanced chemical vapor deposition, where four carbon atoms are substituted by truncation doping of four nitrogen atoms. Usachov et al.22 also synthesized the nitrogen doped graphene from s-triazine molecules. Cao et al.23 could synthesize the porous 1:1 graphitic carbon nitride and carbon nitride nanotubes consisting of all s-triazine (C3N3) rings connected through C−C bond from the reaction of cyanuric chloride with sodium at 230 °C using NiCl2 as a catalyst. Inspired from these experimental studies, we attempt here to design a new porous fullerene, C24N24, consisting of eight striazine rings connected through C−C bonds. This modified fullerene is found to have six N4 cavities which are similar to those present in porphyrin with a cavity diameter of 3.708 Å and can be expected to bind transition metal atoms effectively which also avoid the clustering problem. Transition metal porphyrin systems are well studied and are very important in many biological processes, photocatalysis, etc., and recently many porphyrin-based functional materials were proposed for different applications.24 Ryou et al.25 studied the hydrogen adsorption in calcium and titanium doped porphyrin systems and showed that in Ca−porphyrin systems the adsorption is through electric polarization and that in the case of Ti−porphyrin it is through Kubas26 type of interaction. As each cavity in C24N24 resembles the porphyrin N4 cavity, here we have studied the hydrogen adsorption in transition metal (Sc, Ti, and V) decorated C24N24. Interestingly, we find that the transition metal binding energy in all the cases is considerably larger (nearly double) than the cohesive energy of the corresponding metal. Scandium decorated species is found to hold a larger amount of hydrogen as compared to the titanium and vanadium decorated systems, and the corresponding gravimetric capacity is found to be 5.1 wt %.

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Article

RESULTS AND DISCUSSION First we discuss the electronic structure of the porous fullerene considered for the present study. The carbon nitride fullerene C24N24 is generated through truncated doping of 24 carbon atoms by 24 nitrogen atoms. How the truncated doping is carried out is shown schematically in Figure 1. One of the six

Figure 1. Schematic presentation of truncation substitutional doping of C60 to generate C24N24.

cavities generated is shown in the picture where two carbons connecting the five-membered rings (indicated using a green ellipse) are removed and the four carbons (marked by blue circles) connecting these two carbons are replaced with nitrogen atoms. As there are six such equivalent C−C bonds, a total of 12 carbons are removed and 24 carbons are replaced by nitrogens which generates C24N24 as shown in Figure 1 with six N4 cavities that resemble the porphyrin cavities. This modified fullerene is found to be composed of eight equivalent s-triazine rings connected through C−C bonding. The optimized geometry of C24N24 is found to have octahedral symmetry similar to C60 with C−C and C−N bond length of 1.555 and 1.341 Å, respectively. These C−C bond lengths are found to be longer as compared to that observed35 in C48N12 while the corresponding C−N bonds are shorter in C24N24 as compared to that in C48N12. To calculate the Hessian, we have reoptimized the geometry of C24N24 at the B3LYP/6-31G* level of theory. The Hessian calculated analytically for this fullerene shows that there are no imaginary frequencies and the minimum vibrational frequency is 61.35 cm−1, which indicates that the structure is stable and is at one of the minima on the potential energy surface. The harmonic vibrational spectrum generated from the Hessian calculation is reported in Figure 2. The peaks around 800 cm−1 are due to the s-triazine ring inplane to out-of-plane motion, and the strong peak around 1400−1600 cm−1 corresponds to the C−N and CN stretching mode. The peak at 1195 cm−1 is found to correspond to the C−C single bond stretching. All our reported vibrational frequencies are consistent with the earlier reported s-triazine based structures.36 The calculated formation energy per atom in C24N24 is found to be −7.40 eV, which is less than that of C60 (−8.66 eV) calculated at the same level of theory, and this could be because of the more open structure of C24N24 with six cavities. From the band decomposed charge density pictures reported in Figure 3, it can be seen that the major contribution to highest valence band is coming from nitrogen orbitals and the lowest conduction band is composed of both carbon and nitrogen orbitals. The calculated vertical ionization potential, electron affinity, and HOMO−LUMO gap of C24N24 are found to be 8.31, 3.17, and 2.48 eV, respectively, which are comparable to those of C60 (7.39, 2.06, and 2.87 eV), indicating the stability of the fullerene considered.

COMPUTATIONAL DETAILS

We have used the first-priciples based Vienna ab initio Simulation Package (VASP)27 for all the electronic structure calculations. Projector augmented wave (PAW)28 potentials were used with a kinetic energy cutoff of 550 eV for all the elemental constituents (C, N, Ti, Sc, V, and H). Density functional calculations were carried out using the generalized gradient approximation (GGA) of Perdew−Burke−Ernzerhof (PBE)29 exchange-correlation potential. All the structures were fully optimized with an energy convergence for total energy in the self-consistent field iteration as 1 × 10−6 eV and keeping the unit cell unchanged until the Hellmann−Feynman force components on each elemental constituent is less than 0.01 eV Å−1. A vacuum region of about 18 Å between the neighboring images was considered to avoid the image interactions. To calculate the Hessian, we have used the Becke’s three-parameter exchange functional and Lee−Yang− Parr correlation (B3LYP)30 DFT functional as implemented in the GAMESS31 software. Bader32 charge density analysis has been performed to calculate the atomic charges. Graphical software Molden33 and Gabedit34 were used in generating the initial geometries as well as the reported pictures. The formation energy (per atom) of C24N24 has been calculated using the formula Eform = (1/48)(E(C24N24) − 24E(C) − 24E(N)) 25185

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Figure 2. Harmonic vibrational spectra of C24N24 generated from the Hessian.

Ti, and V are found to be 2.117, 2.044, and 1.989 Å, respectively. The metal binding energies calculated using the PBE method are found to be −188.0, −187.6, and −172.8 kcal/ mol per metal atom for Sc, Ti, and V, respectively. As all these metal binding energies are considerably large (nearly double) as compared to the corresponding metal cohesive energies,13a,37 the problem of metal aggregation to form cluster is expected to be overcome, and the material will be stable for designing a recyclable hydrogen storage material. The atomic charges on each metal site, calculated through Bader charge density analysis, are reported in Table 1. The positive charge accumulated on all the transition metal sites shows that there is a charge transfer from metal to C24N24 which could also be observed from the overlap of metal orbitals with the nitrogen orbitals shown in Figure 5. The magnetic moments calculated on each metal atom in the metal complexes C24N24−M6 are found to be 0.04, 1.11, and 2.21 Bohr magnetons for Sc, Ti, and V, respectively, which are also reported in Table 1. From the variation in C−N bond lengths reported in Table 1, it can be observed that there is an elongation in the C−N bond lengths from 1.34 to 1.39 Å on metal complexation which could be because of loss of π-bonding interaction between the carbon and nitrogen. At the same time, the C−C bond length is shortened from 1.55 to 1.4 Å after the metal decoration. To check the possibility of the dimerization of C24N24−TM complexes, we have optimized the dimer geometries of C24N24−TM−TM−C24N24 for all three kinds of metal decorated fullerenes. The calculated results show that the dimerization energies, −3.41, −10.42, and −12.38 kcal/mol for Sc, Ti, and V complexes, respectively, are very less as compared to the metal binding energies. The calculated TM−TM distances are also found to be around 3.71, 3.73, and 3.31 Å

Figure 3. Band decomposed charge density isosurfaces of (a) highest valence band and (b) lowest conduction band of C24N24.

From the structure of C24N24, one can see that there are six porphyrin-like N4 cavities which can be decorated with transition metals (TM) similar to the prophyrin−TM complexes. We considered three different metals, viz. scandium (Sc), titanium (Ti), and vanadium(V), for decorating the N4 sites. All the optimized geometries of the metal decorated C24N24 are shown in Figure 4. The TM−N bond lengths for Sc,

Figure 4. Optimized geometries of C24N24TM6 (TM = Sc, Ti, and V).

Table 1. Calculated Metal Binding Energies (per Metal Atom) in kcal/mol, Bond Lengths in Å, and Charge on Metal Atoms in au bond lengths (Å) system C24N24 C24N24−Sc6 C24N24−Ti6 C24N24−V6 a

metal binding energya (kcal/mol) 188.01 187.65 172.80

TM−N

C−C

C−N

charge on metal atoms

magnetic moment per metal atom (μB)

2.117 2.044 1.989

1.555 1.410 1.397 1.398

1.341 1.396 1.394 1.390

+1.78 +1.41 +1.31

0.04 1.11 2.21

The metal binding energy is calculated as BE(M) = (1/6)(E(C24N24−M6) − 6E(M) − E(C24N24)). 25186

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Figure 5. Calculated total density of states (total) and projected density of states on nitrogen (N), carbon (C), and titanium (Ti) atoms of C24N24Ti6.

Table 2. Calculated Hydrogen Interaction Energies (per Hydrogen Molecule) in kcal/mol and Metal−Hydrogen Bond Lengths in Å system C24N24−Sc6(H2)6 C24N24−Sc6(H2)12 C24N24−Sc6(H2)18 C24N24−Sc6(H2)24 C24N24−Ti6(H2)6 C24N24−Ti6(H2)12 C24N24−Ti6(H2)18 C24N24−V6(H2)6 C24N24−V6(H2)12 C24N24−V6(H2)18

interaction energy per H2 (kcal/mol) −5.38a −3.64a −3.13a −2.49a −7.35a −5.81a −4.06a −8.90a −5.44a −5.08a

−5.38b −1.90b −2.11b −0.57b −7.35b −4.27b −0.56b −8.90b −1.98b −4.36b

Figure 6. Optimized geometries of C24N24Sc6(H2)6n (n = 1−4).

shortest M−H bond length (Å)

molecular hydrogen is found to be 0.771 Å, which is slightly elongated as compared to the free H2 molecule (0.750 Å) calculated at the same level of theory. The molecular hydrogen adsorption mechanism on transition metal systems has been well explored and is explained through Kubas type of interaction. According to this mechanism, there will be electron transfer from occupied σ orbitals of hydrogen to the metal d orbitals and back-electron transfer from metal d orbitals to hydrogen σ* orbital. To check this, we have plotted the density of states as shown in Figure 7 where one can see the overlap of

2.32 2.34 2.36 2.36 2.00 2.07 2.1 1.85 1.86 1.86

Interaction energy is calculated as ΔE(H 2 ) = (1/6n)(E(C24N24Ti6(H2)6n) − E(C24N24Ti6) − 6n E(H2)). bInteraction energy is calculated as ΔE(H2) = (1/6)(E(C24N24Ti6(H2)6n) − E(C24N24Ti6(H2)6(n−1)) − 6E(H2)). a

in the Sc, Ti, and V cases, respectively, and are considerably large as compared to the metal dimers.38 These results show that dimerization is not a highly favorable process for all the cases. We have also studied the C24N24−TM2 structures by placing the metal atoms on opposite faces of the fullerene to verify the energy difference between the ferromagnetic and antiferromagnetic states. The C24N24Sc2 structure is found to be diamagnetic in nature whereas the other two are paramagnetic. In the case of C24N24Ti2, the energy difference between the ferromagnetic and antiferromagnetic states is calculated to be 3.28 kcal/mol, whereas the corresponding energy difference in C24N24V2 is found to be 2.89 kcal/mol. As the transition metals decorated on suitable host materials are known to trap hydrogen with suitable adsorption enthalpies for room temperature hydrogen storage applications, we have studied the hydrogen adsorption properties of all the three different metal decorated porous fullerenes (C24N24−Sc6, C24N24−Ti6, and C24N24−V6) considered here. The optimized structures of systematically hydrogenated C24N24−Sc6 using the PBE method are given in Figure 6. The interaction energy per molecular hydrogen in C24N24−Sc6(H2)6, where each metal site adsorbs one molecular hydrogen as shown in Figure 6, is found to be −5.4 kcal/mol. The H−H bond length in the adsorbed

Figure 7. Calculated total density of states (total) and projected density of states on titanium (Ti) and hydrogen (H) atoms of C24N24Ti6(H2)6.

hydrogen σ orbitals with metal d orbitals (at ∼9.0 eV) and also σ* hydrogen orbitals with metal d orbitals near the Fermi level. The second and third hydrogen molecules per metal site are found to be adsorbed with interaction energy of −3.6 and −3.1 kcal/mol per hydrogen molecule, respectively. In the case of C24N24Sc6(H2)24, where each metal is allowed to interact with four H2 molecules, the fourth one is found to move away from the metal site as shown in Figure 4. Though the fourth H2 is far from the metal site (∼3.0 Å), the interaction energy per 25187

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molecular hydrogen in C24N24Sc6(H2)24 is found to be −2.5 kcal/mol, which shows that the fullerene surface also plays a role in holding the molecular hydrogens. With a total of 24 molecular hydrogens adsorbed around the C24N24Sc6 fullerene, one has a hydrogen gravimetric density of ∼5.1 wt % at 0 K, which is very close to the target of 5.5 wt % set by DOE for the year 2015. In the case of Ti decorated fullerene, C24N24Ti6, the first molecule of hydrogen per metal is found to get adsorbed with an interaction energy of −7.3 kcal/mol, which is quite higher as compared to the Sc decorated fullerene case. The Ti−H bond length is also found to be shorter (2.00 Å) as compared to the Sc−H distance (2.32 Å). The H−H bond length in the adsorbed hydrogen is found to be elongated to 0.809 Å from 0.750 Å in the free H2 molecule. This larger elongation as compared to that observed in the Sc case is due to the strong interaction between Ti and molecular hydrogen which may be attributed to the availability of more number of d electrons in Ti as compared to Sc. The second molecular hydrogen is adsorbed with an interaction energy of −5.8 kcal/mol per molecular hydrogen. On introducing the third molecular hydrogen per metal site, one of the three hydrogen molecules is found to move far (∼3.5 Å) away, which shows that Ti can adsorb only two molecules of hydrogen which is consistent with the earlier reports on Ti−porphyrin complexes.25 The adsorption energy per molecular hydrogen in C24N24Ti6(H2)18 is found to be −4.06 kcal/mol, and the corresponding gravimetric density is calculated to be 3.8 wt % at 0 K. In the case of C24N24V6 also, we have hydrogenated the vanadium centers systematically, and the calculated hydrogen adsorption energy per molecular hydrogen in C24N24V6(H2)6 is found to be −8.9 kcal/mol. This adsorption energy is still higher as compared to both Sc and Ti metal decorated cases which might be attributed to the higher d orbital population in vanadium as compared to the other two metals. The V−H distance is found to be 1.85 Å, which is the shortest among all the M−H distances studied here, and the variation is also consistent with the calculated adsorption energies. The adsorption energy per H2 in the case of C24N24V6(H2)12 is calculated to be −5.4 kcal/mol. On introducing the third molecular hydrogen per metal site, one of the molecules is found to move far away from the metal site quite similar to the Ti case. However, the hydrogen adsorption energy per H2 is calculated to be −5.08 kcal/mol, which could be because of the contribution from the fullerene surface in binding the molecular hydrogen. With a total of 18 molecular hydrogen adsorbed around the metal doped fullerene, the hydrogen gravimetric density is 3.7 wt % at 0 K for this species. Although the scandium decorated fullerene is found to adsorb hydrogen with higher gravimetric capacity, the adsorption energies are less as compared to the other two metal decorated systems. In the case of titanium and vanadium decorated C24N24, the hydrogen adsorption energies are quite good and are also near to the required adsorption energies proposed for ambient temperature hydrogen storage. Hence, though the gravimetric capacity is a little less (3.8 wt %) as compared to the scandium decorated system (5.1 wt %), titanium and vanadium decorated porous fullerenes appear to be better candidates as constituents of hydrogen storage materials. More interestingly, as the one-dimensional nanotubes and two-dimensional graphitic carbon nitride sheets consisting of s-triazine rings are already synthesized,22 it could be possible

to synthesize the model fullerene considered in this study as well.

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CONCLUSIONS In summary, we have designed here a novel porous carbon nitride fullerene (C24N24) consisting of eight s-triazine rings with six N4 cavities which resemble the porphyrin cavities. The porous fullerene is found to have the same symmetry as that of C60. All the three metals considered in this study (Sc, Ti, and V) are found to bind to the N4 cavities with high interaction energies which are nearly double the cohesive energies of the corresponding metals, and hence the clustering problem associated with the transition metals can be avoided. These transition metal sites are found to trap molecular hydrogen through Kubas type of interaction, and the hydrogen adsorption energies are also found to be suitable for ambient temperature hydrogen storage.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the BARC computer center for providing the high performance parallel computing facility. This work has been supported by the INDO-EU project HYPOMAP in the area of Computational Materials Science. The work of S.K.G. is also supported through Sir J. C. Bose Fellowship from DST, India.

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp3047517 | J. Phys. Chem. C 2012, 116, 25184−25189