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Transition-Metal Decoration Enhanced Room-Temperature Hydrogen Storage in a Defect-Modulated Graphene Sheet A. Bhattacharya,† S. Bhattacharya,† C. Majumder,‡ and G. P. Das*,† Department of Materials Science, Indian Association for the CultiVation of Science, JadaVpur, Kolkata 700032, India, and Chemistry DiVision, Bhabha Atomic Research Centre, Mumbai 400085, India ReceiVed: January 10, 2010; ReVised Manuscript ReceiVed: May 3, 2010
Using first-principles density functional calculations, we show that a transition-metal (TM)-doped defected graphene sheet with periodic repetition of a C atom vacancy (Vc) can be used as a promising system for hydrogen storage. The TM atoms adsorbed above and below the defected site are found to have a strong bonding to the graphene sheet, thereby circumventing the problem of TM clustering, which is the main impediment for efficient hydrogen storage in nanostructure systems. The results reveal that, when the vacancymodulated graphene sheet is decorated on both sides by a combination of less than half-filled (TM1) and more than half-filled (TM2) elements, it results in the adsorption of molecular hydrogen with a binding energy lying in the desirable energy window. Among all the different TM1-TM2 combinations at a C vacancy site, Fe-Ti turns out to be the best choice where five H2 molecules get attached on each pair. To underscore the stability of these hydrogenated systems, we have performed an ab initio molecular dynamics simulation for a fully decorated defected graphene structure. The results show that, at room temperature, the system is stable with a gravimetric efficiency of 5.1 wt % of hydrogen, whereas desorption starts only at ∼400 K. 1. Introduction The safe and convenient storage of molecular hydrogen is a crucial target in the transition to a hydrogen-based energy economy.1 There are two main criteria for good hydrogenation. First, the system must adsorb a good weight percentage (DOE target is ∼6 wt %) of molecular hydrogen. Second, the binding energy (BE) of the adsorbed H2 molecules should ideally lie in the range of 0.2-0.7 eV/H2, that is, in between the range of physisorption and atomic chemisorption. To achieve this target, various carbon-based nanostructures, including planar graphene functionalized by metal atoms, are an active field of study in recent years.2-4 Reasonable high weight percentages of hydrogen adsorption (greater than the DOE target of 6 wt %) have been reported, but the desorption kinetics is still far from being satisfactory.3 Although functionalized nanotubes having a high surface-to-bulk ratio hold a great potential, there are several outstanding issues, such as metal clustering on the surface, endohedral versus exohedral storage, and the associated problem of crossing a high-energy barrier.4 By unfolding the nanotube, one gets a planar sheet that provides the advantage of utilizing both sides for metal decoration and, therefore, more efficient hydrogenation.2 However, there is a tendency of the TM atoms to form a cluster on the surface of the sheet because the TM-TM interaction is much stronger than the TM-host interaction.2,5 The problem of clustering of the adsorbed dopant drives toward the necessity to look for some alternative planar structure in which the dopant atom coalescence can be prevented, thereby resulting in an improved desorption kinetics. We achieve this by introducing vacancies. In this article, we report the results of our first-principles investigation for hydrogen storage on a graphene sheet where * To whom correspondence should be addressed. Phone: +91-33-2473 4971, ext. 202. Fax: +91-33-2473 2805. E-mail:
[email protected]. † Indian Association for the Cultivation of Science. ‡ Bhabha Atomic Research Centre.
native C atom vacancies have been created, and two different 3d TM atoms are absorbed on either side of each defect site. The insertion of TM dopant atoms above and below the defect resulted in a strong covalent bonding between the TM and the graphene sheet, which, in turn, prevents the possibility of metalclustering. 2. Computational Details The electronic structure and total energies are calculated with the Vienna ab initio Simulation Package (VASP),6 based on DFT.7,8 Projector-augmented wave (PAW) potentials9 are employed for the elemental constituents, viz. H, C and Sc, Ti, V, Mn, Fe and Co potentials, which contained one, four, three, four, five, seven, eight and nine valence electrons, respectively. The generalized gradient approximation (GGA) based calculations have been performed with the Perdew-Wang10,11 exchangecorrelation potential. The k-mesh is generated by the Monkhorst-Pack12 method, and all results are tested for convergence with respect to the mesh size. We have used a 4 × 4 × 1 k-mesh for fully relaxed geometry optimization and a 12 × 12 × 1 k-mesh for the DOS (density of states) plot. In all our calculations, self-consistency is achieved with a 0.1 meV convergence of total energy. For high precision calculations, we used a cutoff energy of 600 eV for the plane-wave basis. To obtain the optimized ground-state geometries,13,14 atomic forces are converged to less than 0.001 eV/Å by conjugated gradient (CG) minimization. We also performed ab initio molecular dynamics (MD) simulations in order to investigate the stability of the optimized structures and desorption of hydrogen molecules from TM1@C31@TM2. For Hirshfeld charge analysis, we have used Dmol3 code,15 where exchange and correlation terms are treated within the GGA functional by Perdew et al.16 Core electrons were treated in a nonrelativistic all-electron implementation of the potential. A double numerical quality basis set with a polarization function (DNP) was considered.
10.1021/jp100230c 2010 American Chemical Society Published on Web 05/13/2010
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Figure 1. Optimized structure (ball-and-stick model) of (a) a defected graphene sheet with a single native vacancy, (b) the Ti@C31@Fe system with five H2 molecules adsorbed at 300 K, as obtained by 2 ps ab initio MD simulations with a 1 fs time step and 2000 SCF runs, and (c) the 4Ti@C28@4Fe system showing full hydrogenation with 20 H2 molecules adsorbed at 300 K, obtained using the same MD simulations as mentioned above.
Figure 2. Binding energies of TM atoms located (i) at a hexagon-top position of a pure graphene sheet (solid circle) and (ii) on the defect site of the defected graphene sheet (solid square). The cohesive energies of the respective TM atoms (solid triangle) have been superposed for the sake of comparison.
3. Results and Discussion 3.1. TM-Doped Graphene Sheet with a C Vacancy. The primitive cell of graphene consists of two carbon atoms as the basis. We have used a 4 × 4 supercell comprising 32 carbon atoms placed in a triclinic crystal with dimensions of 9.8 Å × 9.8 Å × 12 Å. It is possible to realize experimentally lattice defects in planar nanostructures, such as graphene or boron nitride sheets.17 Recently, there have been reports on vacancygenerated defected graphene sheets.18 With a single C vacancy (Vc) introduced in the graphene sheet, the total number of C atoms in the sheet becomes 31 (Figure 1a). We have found that the binding energy (BE) of a single 3d TM dopant atom bonded to this vacancy-generated defect lies in the range of 4-8 eV (see the Supporting Information, Table SI-2), which is consistently higher than the corresponding cohesive energies19 of the respective TM atoms, as shown in Figure 2. The BEs of the same TM atoms placed on top of the hexagon of a pure graphene sheet without any vacancy have been estimated and are found to lie in the range of 1-1.4 eV (Figure 2), which are consistently lower than the corresponding cohesive energies of the TM atoms. Thus, the increased BE in the case of a defected graphene sheet leads to the prevention of metal clustering on the surface. In view of this, we have tried to do optimum TM coverage of the defected sheet. A 32-atom supercell of the graphene sheet can accommodate up to four such single C atom vacancies separated by a distance of ∼6 Å from each other. We have also verified from our MD simulation that a C28 sheet with four such vacancies remains stable up to a temperature of ∼1000 K. The formation energy/atom of a pure graphene sheet (C32) is Eform ) [Etotal(C32) - 32 × Etotal(C)]/32 ) 7.91 eV/atom, whereas that of a single defect-modulated graphene sheet (C31) is Eform ) [Etotal(C31) - 31 × Etotal(C)]/31 ) 7.75 eV/atom.
These energies are comparable, indicating the stability of the defected sheet. 3.2. Hydrogen Adsorption on TM@C31 for Single-Sided Doping (SSD). We now estimate the H adsorption in different 3d TMs anchored at the vacancy site on a single-sided-doped (SSD) defected graphene sheet. It is interesting to see that, when a TM atom with less than half-filled (LTH) d orbitals (let us call it TM1, e.g., Sc, Ti, V) is attached to the defected site of the SSD graphene sheet C31, H2 molecules get adsorbed with BE values ranging from 0.01 to 0.08 eV (see Figure 3 and the Supporting Information, Table SI-3), which is much below the physisorption range. Whereas when a TM atom with half-filled or more than half-filled (MTH) d orbitals (call it TM2, e.g., Mn, Fe, Co) is doped onto the same site, it adsorbs H2 molecules with a much higher BE ranging from 0.5 to 0.82 eV (see Figure 3 and the Supporting Information, Table SI-3). The difference of hydrogen adsorption capacity between the two classes of TM can be understood from the self-consistent density of states (DOS) analysis of SSD Fe@C31 and Ti@C31 systems, as discussed later.
Figure 3. Binding energies of consecutive hydrogen molecules adsorbed on the TM@C31 single-sided-decorated (SSD) system. Less than half-filled (LTH) TM atoms lead to physisorption, whereas more than half-filled (MTH) TM atoms lead to atomic chemisorption.
3.3. Hydrogen Storage of TMs in a Double-Sided-Doped (DSD) Graphene Sheet [TM1@C31@TM2]. On the basis of the above observation, we propose a novel conjecture of decorating the C atom vacancy from both the surfaces of the graphene sheet with a combination of two different TM atoms, one with LTH (TM1) and the other with MTH (TM2) configurations (Figure 1b). Our calculations reveal that the BE of the TM atoms at the Vc for this kind of double-sided doping (DSD) is ideal for the storage of molecular hydrogen. In the case of DSD, the BE of the TM atoms to the graphene sheet lies in the range of 4-5 eV, which eliminates the possibility of metal clustering at the surface. We use the nomenclature
TM-Doped Defected Graphene Sheet for H2 Storage
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TABLE 1: Comparison of Hydrogen Storage on TM Atoms at Defected Graphene Sites for Different TM1@C31@TM2 Combinations. The Combinations That Recorded Good Hydrogenation Are Only Mentioned no. of H2 molecules adsorbed to TM1 at 0 K
no. of H2 molecules adsorbed to TM2 at 0 K
BE of H2 molecules adsorbed to TM1 in ev
BE of H2 molecules adsorbed to TM2 in ev
desorption temperature of the H2 molecules attached to TM1 in K
desorption temperature of the H2 molecules attached to TM2 in K
Sc@C31@Sc
3
3
below 300 K
2
1
0.17 0.20 0.19 0.08
below 300 K
Ti@C31@Ti
below 300 K
below 200 K
Sc@C31@Ti
3
2
0.06 0.08
below 300 K
below 200 K
Sc@C31@Mn
3
3
500-750 K
2
3
below 300 K
400-950 K
Ti@C31@Co
2
3
0.18 0.21
250-350 K
200-850 K
final system: Ti@C31@Fe
2
3
0.24 0.42
0.36 0.65 0.72 0.31 0.69 0.87 0.19 0.45 0.81 0.28 0.49 0.69
below 300 K
Sc@C31@Co
0.18 0.18 0.17 0.14 0.18 0.11 0.18 0.16 0.19 0.17 0.24 0.14 0.17
350-500 K
400-700 K
system TM1@C31@TM2
TM1@C31@TM2 for such a system, for which the detailed adsorption and desorption properties of hydrogen molecules attached are listed in Table 1 (see Table SI-1 in the Supporting Information for more details). It is to be noted that, when the same TM atom is adsorbed above and below Vc, it does not yield a good hydrogenation. The hydrogen storage capacity of the TM dopants is mainly determined by the valence electronic configuration of the TM atoms in the combination. Table 1 shows that, among all the different TM1-TM2 combinations, Fe-Ti serves the best from the perspective of gravimetric efficiency as well as desorption behavior. Fe adsorbs three H2 molecules while Ti adsorbs two H2 molecules. The hydrogenation follows a Kubas-type interaction.20 The H2 molecules retain their molecular structure and do not dissociate to form metal dihydride. The binding energy of the H2 molecules is within 0.2-0.7 eV/H2, which is in the molecular chemisorption range. In a way, a combination of TMs that have a tendency to lose electrons to the defected host site and thereby become cationic, is preferred for hydrogen storage. In Ti@C31@Fe, both Ti and Fe prefer to lose their valence electrons to the carbon atom to attain zero and half-filled valence configurations, respectively. It has been observed that Sc@C31@Sc, Sc@C31@Fe, Ti@C31 @Fe, Sc@C31@Co, and Ti@C31@Co serve as prospective candidates for good hydrogenation. However, from the desorption point of view, Ti@C31@Fe (Table 1) turns out to be the best one. For full coverage, it is estimated that the 4Ti@C28@4Fe structure can store 5.1 wt % of hydrogen (Figure 1c). The desorption temperature of a hydrogen molecule is conventionally defined as the temperature at which the interaction between the H2 molecule and the system becomes negligible. We assume that, when the distance between the H2 molecule and the TM is significantly large (i.e., more than 3 Å), there is practically no interaction between the TM and H2 molecule. Our 2 ps ab initio molecular dynamics calculation with a time step of 1 fs and 2000 SCF runs using the Nose algorithm21 reveals that the system is stable at room temperature, whereas hydrogen desorption starts only at 400 K, (Table 1) and eventually, with an increase in temperature, other hydrogen molecules get released from the system.
The interaction between two such TM-modulated defected graphene sheets is interesting in view of the fact that there exists good interlayer bonding between two layers of pure graphene. If TM-modulated defected graphene sheets get close to each other, they continue to remain separated by a distance of ∼3.4 Å, as in case of pure graphite. We have carried out model calculations with two such TM-modulated layered structures of graphene sheets and found that the interaction of the TM with the vacancy, that is, Ti@Vc@Fe, is stronger compared with the interaction between the TMs facing two adjacent layers. It is to be noted that our calculation pertains to a single-layer metal-functionalized planar graphene sheet, which is rather difficult to synthesize, although serious attempts are being made to realize a free-standing graphene sheet. Incorporation of more than one layer is expected to affect the gravimetric efficiency. It is worth mentioning here that FeTiHx alloys constitute a family of bulk hydrides that has been well studied both experimentally and using first-principles calculations.22 However, a simple Ti-Fe heterodimer binds hydrogen in atomic form, with a binding energy of -1.8 eV for Fe-H and -1.6 eV for Ti-H. These fall in the atomic chemisorption range, and hence, it is not suitable for hydrogen storage. The interesting finding from our present study is that, on putting Fe and Ti on a defected graphene sheet, the desorption kinetics is substantially improved along with other desirable features for hydrogen storage. TABLE 2: Hirshfeld Charge Distribution Analysis of Various TM-Decorated Defected Graphene Structuresa System
C1
C2
C3
Ti
C31 Fe@C31 Ti@C31 Ti@C31@Fe Ti@C31@Fe+5H2
+0.005 -0.05 -0.128 -0.126 -0.11
+0.005 -0.05 -0.128 -0.15 -0.12
+0.005 -0.05 -0.128 -0.126 -0.11
+0.52 +0.50 +0.27
a
Fe +0.24 +0.18 -0.006
A negative sign implies electrons gained, whereas a positive sign implies electrons lost by the atom (unit of charge is electron).
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Figure 4. Total and partial (site-projected) densities of states (DOS) plots for Ti/Fe-decorated defected graphene sheets for the following cases: (a) three C atoms around the defect site, before and after Ti adsorption, (b) same as (a) for Fe adsorption, (c) Ti@C31 SSD after hydrogenation, (d) Fe@C31 SSD after hydrogenation, (e) Ti@C31@Fe DSD before hydrogenation, and (f) Ti@C31@Fe DSD after hydrogenation. In all the DOS plots, “0” marks the Fermi level (EF). All the unoccupied states lie above the EF, whereas the occupied states lie below it. See the text for details.
3.4. Electronic Structure and Charge Transfer for SSD [TM@C31] and DSD [TM1@C31@TM2]. From our Hirshfeld charge analysis, we observe that, in a pure graphene sheet [C32], all the C atoms are charge neutral and have a homogeneous charge state of zero. However, on removing one C atom and
Bhattacharya et al. thereby creating a defect (vacancy) in the graphene sheet, the three C atoms neighboring the defect site, call them C1, C2, and C3 (Figure 1a), attain a charge state of +0.005 each (Table 2). Thereafter, when the TM atoms are inserted at the defect site, they lose their valence electrons to the C atoms and thereby become cationic (Ti+ and Fe+). For DSD with Ti and Fe, the net charge state of Ti becomes +0.5, whereas that of Fe becomes +0.18 (Table 2). Now, when the hydrogen molecules are introduced, they lose electrons to the TM atoms and the C atoms at the defect (via TMs). The TM atoms gain electrons and attain a charge state of 0.27 and ∼0 for Ti and Fe, respectively. Most of the unoccupied Fe orbitals take part in hydrogen bonding, whereas only a part of the unoccupied Ti orbitals contribute. As a result of this, Ti adsorbs two H2 molecules by gaining some electronic charge but still shows a remnant charge state of +0.275. We have plotted the self-consistent total DOS of three C atoms neighboring the defect of C31 (3C@hole) and compared it with the same after Ti/Fe adsorption (Figure 4a,b) at the defect site (i.e., Ti@C31 and Fe@C31). It can be observed that, in both cases, there are large unoccupied peaks of the metal atoms (red color peaks for Ti and blue color peaks for Fe) appearing at the cost of the disappearance of the unoccupied peak (black color peak) of 3C@hole of pure C31. This implies an electron transfer from the metal to the C atoms neighboring the hole in the case of SSD. After full H2 adsorption in SSD Ti@C31 (Figure 4c), the unoccupied peaks of Ti are still retained, whereas in the case of SSD Fe@C31, the unoccupied peak nearly vanishes (Figure 4d). The point to be noted here is that the presence of an unoccupied level does not necessarily suggest the possibility of further electron transfer and hence H2 adsorption. Coming to DSD Ti@C31@Fe (Figure 4e), we observe that the unoccupied Ti peak increases (compared with SSD Ti@C31) and shifts closer to the Fermi level, while the unoccupied Fe peak decreases (compared with SSD Fe@C31), which desirably improves the provision of Ti in H2 (than SSD Ti@C31) adsorption and decreases the same for Fe (than SSD Fe@C31). After H2 adsorption (Figure 4f), electrons get transferred from H2 to the unoccupied Fe and Ti levels. From the charge density plots of Ti@C31@Fe (Figure 5), we observe that Fe is surrounded by a dense charge cloud passing through it and the dimpled carbon sheet. However, Ti is having a relatively much weaker charge cloud around it
Figure 5. Charge density contour plots (in units of electrons/(a.u.)3) for the DSD configuration of Ti@C31@Fe in two orthogonal planes: (a) passing through the graphene sheet and (b) passing through Ti and Fe perpendicular to the graphene sheet.
TM-Doped Defected Graphene Sheet for H2 Storage (Figure 5b). Comparing the cationic states of Ti+ and Fe+ in SSD versus DSD (Table 2), we observe that the differential change in Ti-charge (0.52 to 0.50) is much less compared to the change in Fe-charge (0.24 to 0.18). This indicates Fe+ becoming less cationic in the DSD configuration, which gets reflected in Figure 5b, by the dense electron cloud surrounding the Fe atom. It can be concluded that, in a DSD defected sheet with Ti and Fe, Ti loses most of its charge, while Fe loses a part of its charge to the neighboring C atoms. 4. Conclusion In summary, we have implemented DFT-based first-principles calculations to investigate the hydrogenation properties of defected graphene sheet modulated by 3d TM atoms. The TM dopant atoms adsorbed above and below the defect site were found to have an extra bonding to the sheet, which led to elimination of the clustering of the TM dopants. We observe that, when the combination is composed of a 3d TM1 of a valence electron configuration less than half filled and another 3d TM2 of a valence electron configuration more than the half filled, an optimum weight percent at room temperature can be realized. Among all possible TM1 and TM2 with which we have tried to decorate the defected graphene sheet, Fe-Ti qualifies as the best possible combination, satisfying both the criteria of a good gravimetric efficiency of hydrogen storage along with a favorable desorption kinetics. The fully decorated 4Ti@C28@4Fe structure was found to retain up to ∼5.1 wt % of H2 at room temperature. Hydrogen desorption started only at 400 K, whereas by 600 K, most of the H2 molecules were released without breaking the structure, resulting in an energetically reversible kinetics. Supporting Information Available: As Supporting Information, we provide the following: (1) Table SI-1 showing hydrogen BE and the corresponding desorption temperature for various TM1 and TM2 combinations anchored at the vacancy as TM1@C31@TM2. (2) Table SI-2 depicting the comparison of BE of TMs when placed on a single native C atom defect of a graphene sheet (C31) to that of when the respective TMs are
J. Phys. Chem. C, Vol. 114, No. 22, 2010 10301 placed on the hexagon of an intact graphene sheet (C32). (3) Table SI-3 showing the BE of hydrogen molecules absorbed to early and late TMs for an SSD configuration as TM@C31. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Schlappbach, L.; Zuttel, A. Nature 2001, 414, 353. (2) (a) Bhattacharya, S.; Majumder, C.; Das, G. P. J. Phys. Chem. C 2009, 113, 15783. (b) Durgun, E.; Ciraci, S.; Yildirim, T. Phys. ReV. B 2008, 77, 085405. (c) Ataca, C.; Aktu¨rk, E.; Ciraci, S. Phys. ReV. B 2009, 79, 041406R. (3) (a) Sun, Q.; Wang, Q.; Jena, P. Nano Lett. 2005, 5, 1273. (b) Yildirim, T.; Ciraci, S. Phys. ReV. Lett. 2005, 94, 175501. (4) Bhattacharya, S.; Majumder, C.; Das, G. P. J. Phys. Chem. C 2008, 112, 17487. (5) (a) Sun, Q.; Jena, P.; Wang, Q.; Marquez, M. J. Am. Chem. Soc. 2006, 128, 9741. (b) Chandrakumar, K. R. S.; Ghosh, S. K. Nano Lett. 2008, 8, 13. (6) (a) Kresse, G.; Hafner, J. Phys. ReV. B 1994, 49, 14251. (b) Kresse, G.; Furthmu¨ller, J. Comput. Mater. Sci. 1996, 6, 15. (7) Hohenberg, P.; Kohn, W. Phys. ReV. 1964, 136, B864. (8) Kohn, W.; Sham, L. Phys. ReV. 1968, 140, A1133. (9) Blo¨chl, P. E. Phys. ReV. B 1994, 50, 17953. (10) Perdew, J. P.; Wang, Y. Phys. ReV. B 1992, 45, 13244. (11) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Sing, D. J.; Fiolhais, C. Phys. ReV. B 1992, 46, 6671. (12) Monkhorst, H. J.; Pack, J. D. Phys. ReV. B 1976, 13, 5188. (13) Press, W. H.; Flannery, B. P.; Tenkolsky, S. A.; Vetterling, W. T. Numerical Recipes; Cambridge University Press: New York, 1976; Vol. 1. (14) Pulay, P. Chem. Phys. Lett. 1980, 73, 393. (15) (a) Delley, B. J. Chem. Phys. 1990, 92, 508. (b) Delley, B. J. Chem. Phys. 2000, 113, 7756. Dmol3 is available as a part of the Materials Studio Software from Accelrys http://www.accelrys.com. (16) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (17) (a) Chen, J.; Cullen, W. G.; Jang, C.; Fuhrer, M. S.; Williams, E. D. Phys. ReV. Lett. 2009, 102, 236805. (b) Jin, C.; Lin, F.; Suenaga, K.; Iijima, S. Phys. ReV. Lett. 2009, 102, 195505. (18) (a) Shevlin, S. A.; Guo, Z. X. J. Phys. Chem. C 2008, 112, 17456. (b) Singh, R.; Kroll, P. J. Phys: Condens. Matter 2009, 21, 196002. (19) Philipsen, P. H. T.; Baerends, E. J. Phys. ReV. B 1996, 54, 5326. (20) Kubas, G. J., Ed. Metal Dihydrogen and Bond Complexes Structure, Theory, and ReactiVity; Kluwer: Dordrecht, The Netherlands, 2001. (21) Nose, S. J. Chem. Phys. 1984, 81, 511. (22) Kinaci, A.; Aydinol, M. K. Int. J. Hydrocarbon Eng. 2007, 32, 2466.
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