J. Phys. Chem. B 2006, 110, 21747-21750
21747
First-Principle Study of Adsorption of Hydrogen on Ti-Doped Mg(0001) Surface A. J. Du,†,‡ Sean C. Smith,*,†,‡ X. D. Yao,‡,§ and G. Q. Lu‡ Centre for Computational Molecular Science, Chemistry Building, and ARC Centre for Functional Nanomaterial, School of Engineering, The UniVersity of Queensland, QLD 4072, Brisbane, Australia, and School of Engineering, James Cook UniVersity, TownsVille, QLD 4811, Australia ReceiVed: May 28, 2006; In Final Form: August 6, 2006
Ab initio density functional theory (DFT) calculations are performed to study the adsorption of H2 molecules on a Ti-doped Mg(0001) surface. We find that two hydrogen molecules are able to dissociate on top of the Ti atom with very small activation barriers (0.103 and 0.145 eV for the first and second H2 molecules, respectively). Additionally, a molecular adsorption state of H2 above the Ti atom is observed for the first time and is attributed to the polarization of the H2 molecule by the Ti cation. Our results parallel recent findings for H2 adsorption on Ti-doped carbon nanotubes or fullerenes. They provide new insight into the preliminary stages of hydrogen adsorption onto Ti-incorporated Mg surfaces.
Introduction Among the metal hydrides under study as possible hydrogen storage media, magnesium hydride is one of the most promising candidates for automotive applications due to its very high capacity in the stoichiometric limit (7.6 wt %) and low cost.1-3 Unfortunately, the application is primarily limited by the relatively high requisite hydrogenation reaction temperature and slow kinetics. One of the possible reasons is that the hydrogen molecules do not readily dissociate on a Mg surface.4,5 Experimentally, many studies have been devoted to the catalytic effect on hydrogen adsorption of mixing transition metals into Mg hydride powder during ball milling.6-9 The transition metals have been considered to act as catalysts for accelerating hydrogen sorption, that is, enhancing the breaking up of molecular hydrogen into adsorbed atoms.10 However, the catalytic mechanism involved when these additives are used is still not clearly established. Theoretically, ab initio density functional theory (DFT) calculations have shown considerable predictive power for catalysis.11 Much insight can be gained from first-principle simulations for the process of designing alloy catalysts.12,13 There have been some theoretical calculations to study the role of transition metals in magnesium hydrides, focusing mainly on substituting one Mg atom with a Ti, Ni, or Fe atom in the bulk.14-16 The dissociative chemisorption of H2 onto a clean magnesium surface has been reported by several groups.17-19 We have recently carried out a preliminary investigation of the effect of incorporated titanium or carbon on the dissociative chemisorption of a single hydrogen molecule20,21 and the diffusion of atomic H on a Mg(0001) surface and into the first sublayer.22,23 Two theoretical groups have shown recently that transition metal atoms such as Sc and Ti coated on carbon fullerenes and nanotubes can bind molecular hydrogen with a binding energy of the order of 0.5 eV/H2 molecule and with gravimetric density * Corresponding author: fax 617-33654623; e-mail
[email protected]. † Centre for Computational Molecular Science, The University of Queensland. ‡ ARC Centre for Functional Nanomaterial, The University of Queensland. § James Cook University.
of up to 8 wt %.24,25 This can be understood by the facts that (i) Ti or Sc is first ionized due to charge transfer between carbon and Ti (Sc) and (ii) the transition metal cation then polarizes molecular H2. An interesting question that follows is whether additional hydrogen molecules (after the first) bind with a Ti atom incorporated into the Mg(0001) surface? No molecular adsorption state of H2 on pure Mg(0001) surface has been found.19 The situation that ensues upon incorporation of Ti into the Mg surface is very interesting as it could help to further clarify the role of transition metal catalysts for hydrogen sorption in relation to recent experimental findings. In this paper, the interactions of a series of H2 molecules with Ti-incorporated Mg(0001) surfaces were studied by ab initio DFT calculations, the nudged elastic band (NEB) method, and ab initio molecular dynamics (MD) simulations. Both the energetics and dynamics for the adsorption of H2 molecules on a Ti-incorporated Mg(0001) surface were systematically investigated. In the second section we outline our computational method. The third section presents the calculated results for the adsorption of hydrogen molecules onto a Ti-incorporated Mg(0001) surface. The last section draws relevant conclusions. Computational Methods All the calculations were performed with the plane-wave basis VASP code26,27 implementing the generalized gradient approximation (GGA) of PBE exchange correlation functional28 and the projector augmented wave method.29,30 The lattice constant of bulk Mg was calculated to be 3.191 Å, which is only 0.5% in error compared with the experimental value. The Ti-incorporated Mg(0001) surface was modeled by using a (3 × 3) surface unit cell with five layers of Mg atoms. In all calculations, atoms in the bottom layer are fixed and all the other atoms are allowed to relax freely. Three k points were used for the Brillouin zone sampling31 and the cutoff energy for plane waves was 312.5 eV. The vacuum space was up to 16 Å, which is large enough to guarantee a sufficient separation between periodic images. Additionally, ab initio MD simulations were performed to study the effect of finite temperature on the dissociation of H2 on the Ti-doped Mg(0001) surface. To determine dissociation barriers and minimum energy paths
10.1021/jp063286o CCC: $33.50 © 2006 American Chemical Society Published on Web 10/03/2006
21748 J. Phys. Chem. B, Vol. 110, No. 43, 2006
Du et al.
(MEP), the NEB method was used.32,33 This method involves optimizing a chain of images that connect the reactant and product state. Each image is allowed to move only in the direction perpendicular to the hypertangent. Hence the energy is minimized in all directions except for the direction of the reaction path. A damped molecular dynamics was used to relax ions until the force in each image is less than 0.02 eV/Å. Results and Discussion As reported in our previous work,21 the transition metalMg interface plays an important role in the dissociation of H2 on a Mg surface. Mg bulk has hexagonal structure and thus the Mg(0001) surface is most commonly used in existing studies.17-19 We design a Ti-incorporated Mg(0001) surface by substituting one surface Mg atom with a Ti atom (Mg44Ti). This process involves the creation of a Mg vacancy on Mg(0001) surface in the first step and subsequent occupation of the vacancy by a Ti atom. Taking these two processes together gives the adsorption energy at the surface-substitutional site:34
Eadsubs ) ETi/Mg(0001)-subs + EMg - ETi-atom - EMg(0001) where ETi/Mg(0001)-subs, EMg, ETi-atom, and EMg(0001) represent the total energies of the relaxed Ti-incorporated Mg(0001) surface, the bulk Mg atom, the isolated Ti atom, and the clean Mg(0001) slab, respectively. The formation energy is calculated to be -4.09 eV, which indicates that the Ti-incorporated Mg(0001) surface is thermodynamically stable. The reason we choose a model based on substituting one Mg atom with a Ti atom is 2-fold. First, the substituted Ti@Mg(0001) surface should occur commonly since the Mg vacancy will not be difficult to create during the high-energy ball milling experiment and may then easily be filled by a Ti atom. Second, the results can be used to directly compare with the existing results for H2 dissociation on the pure Mg(0001) surface and it will also be convenient for subsequent study of the diffusion of atomic H into bulk. First, we calculated the activation barrier for the dissociation of a single H2 molecule on pure Mg(0001) surface and Ti-incorporated Mg(0001) surface using the NEB method. The effective barrier for the dissociation of a hydrogen molecule on the Mg(0001) surface is calculated to be 1.05 eV, which is in good agreement with other theoretical calculations.17-19 This also reproduces the recombination barrier for desorption of H2 in relation to the experimentally observed TDS value (1.0 eV).20,35 On the Ti-doped Mg(0001) surface, however, a hydrogen molecule is found initially to physisorb on top of the Ti atom with an exothermicity of ca. 0.6 eV, followed subsequently by dissociation of the H2 with an activation barrier of only 0.103 eV. Figure 1 presents the energy profile for this complex reaction and displays a series of configurations along the MEP. Subsequently, we study the interaction of a second hydrogen molecule with the Ti-doped Mg(0001) surface, wherein two dissociated H atoms are already bound to the Ti atom (i.e., Mg44TiH2, the FS shown in Figure 1). In the NEB calculation, the IS is composed of one hydrogen molecule at a distance of 5 Å from the relaxed Mg44TiH2 surface. The relaxed FS (i.e., Mg44TiH4, FS in Figure 2) was found to comprise four H atoms bound to the surface-incorporated Ti atom. The length of Ti-H bonds increased by about 0.024 Å compared with Mg44TiH2. Similar to the minimum energy pathway for dissociation of the first hydrogen molecule, the second molecule moved initially toward the Ti atom and then dissociated nearly on top of it,
Figure 1. Energy profile and dissociation pathway for a hydrogen molecule on a Ti-incorporated Mg(0001) surface (i.e., Mg44Ti). (IS) Initial state; (LS) local minimum state; (TS) first transition state; (FS) final state. Black, gray, and small white balls represent Mg, Ti, and H atoms, respectively.
Figure 2. Energy profile and corresponding configurations along the dissociation pathway for second hydrogen molecule on a Ti-incorporated Mg(0001) surface (i.e., Mg44TiH2). Gray, black, and small white balls represent Mg, Ti, and H atoms, respectively.
around 1.50 Å from the Mg44TiH2 surface. Figure 2 presents the energy profile and corresponding configurations along the MEP for the dissociative chemisorption of the second hydrogen molecule on the Mg44TiH2 surface. The effective activation barrier is around 0.145 eV. We should emphasize that the contribution from spin polarization is not included in our calculation since it is computationally very demanding in NEB calculations. Significantly, we found that once the hydrogen molecule has dissociated on the Ti atoms, the system is nonmagnetic. Hence, while spin polarization may lower the total energy of system, the calculated activation barrier should not be greatly affected. Compared to the clean Mg(0001) surface, the activation barriers for the dissociation of both first and second hydrogen molecules on the Ti-incorporated surface are greatly reduced. This implies that the initial absorption rate of H2 molecules onto the doped surface should be significantly improved and can be understood in terms of the strong interaction between the molecular orbital of H2 and the metal d orbitals, arising from charge donation from the s orbital of H2 to the d orbital of Ti.36 Clearly, the dissociation barriers of two hydrogen molecules are only 4-6 times the thermal energy (around 0.025 eV) at
Adsorption of Hydrogen on Ti-Doped Mg(0001) Surface
J. Phys. Chem. B, Vol. 110, No. 43, 2006 21749
Figure 4. Three-dimensional isosurface plot of valence charge difference for Mg44TiH4-H2 system with an iso value of 0.01 e/Å3. It was shown that there is charge accumulation (red) and charge depletion (blue) within the studied system.
Figure 3. Prescribed final configuration obtained from the geometry optimization of third H2 on Mg44TiH4 surface: (a) top view; (b) side view. Gray, black, and small white balls represent Mg, Ti, and H atoms, respectively.
300 K. Furthermore, there is the possibility that energy associated with the exothermicity of the initial physisorption stage of the reaction path may aid in crossing the subsequent barrier to dissociation if not fully thermalized. On the basis of the Arrhenius equation alone, the rate of H2 dissociation should be considerable and the dissociation of hydrogen molecules on Ti-doped Mg(0001) surface may be expected to be activated at room temperature. To study the influence of temperature, we have performed ab initio MD simulations at 300K for Mg44Ti + H2 and Mg44TiH2 + H2 systems. Our MD simulations were based on the canonical (NVT) ensemble with the Nose algorithm.37 The time step was 0.5 fs and typical simulation lengths were 1 ps. Hydrogen molecules were initially positioned at 3 Å far from the top of the Ti atom on the surface. As would be expected on the basis of the computed MEPs, the spontaneous dissociation of both first and second H molecules onto Mg44Ti and Mg44TiH2 surfaces were observed within the time scale of 1 ps. As reported in the earlier studies,19 there is no molecular adsorption state of H2 on a pure Mg(0001) surface. However, upon starting a simulation with a third hydrogen molecule at a distance of 3 Å from the Mg44TiH4 surface, we observed the adsorption of H2 in molecular form after relaxation. In Figure 3, we present top and side views of the final configuration for
(Mg44TiH4-H2) obtained after geometry optimization. The molecular H2 is located at a distance around 2 Å from the top of the Ti atom with physisorption energy of 0.28 eV. To check whether additional hydrogen molecules could be attached onto Mg44TiH4, we then inserted eight hydrogen atoms around the Ti atom (i.e., Mg44TiH8) and performed geometry optimization. We found four of them still remain bound to the Ti atom, while the remaining H atoms form one physisorbed hydrogen molecule as above and one unbound hydrogen molecule far from the surface. To gain insight into the nature of the molecular adsorption state, the valence charge electron density difference (Fdiff) in a vacuum space is analyzed according to the following equation and plotted in Figure 4:
Fdiff ) FMgTiH4‚H2 - FMgTiH4 - FH2 Here FMgTiH4.H2 represents the valence charge density for the final configuration containing the Mg44TiH4 surface with the molecular adsorption state of H2. FMgTiH4 and FH2 represent the valence charge density for the Mg44TiH4 surface and an isolated H2 having the same position as the molecular adsorption state shown in Figure 3. Clearly, there is charge accumulation on the four H atoms on the Mg44TiH4 surface and charge depletion from the H2 molecule and neighboring Mg and Ti atoms. This indicates that there is significant charge transfer between them, ultimately leading to the Ti-H bond lengths for the Mg44TiH4-H2 complex being decreased by around 0.02 Å on average in comparison with the relaxed Mg44TiH4 structure. Evidently, the hydrogen molecule is polarized since the Ti atom has become effectively a Ti cation after chemisorption of two H2 molecules. This polarization mechanism governs the bonding of H2 to the Ti atom in molecular form. We note that this is similar to the previously demonstrated binding mechanism of
21750 J. Phys. Chem. B, Vol. 110, No. 43, 2006 H2 molecules on a Ni cation38 and H2 adsorption on Ti-decorated carbon nanotubes or fullerenes.24,25 The capacity of Ti to adsorb multiple hydrogen atoms is clearly significant, since it bears on the question of whether the incorporation of Ti would decrease the weight percentage storage capacity (due to the relatively large mass of Ti) or increase the capacity due to the binding affinity of Ti for hydrogen or, additionally, the possibility of a catalytic role in facilitating absorption of atomic H into the bulk. The requirements for effective catalysis clearly differ depending on the type of material one is considering: for nanotubes or fullerenes, surface physisorption is the dominant mechanism of storage, whereas for Mg nanocomposites, bulk chemisorption is the dominant mechanism. Hence, while the physical mechanism for molecular adsorption of H2 to the Ti site is similar in both cases, the ramifications for facilitating hydrogen storage are not necessarily the same in each case. It is important to note that the bulk hydrogenation of Mg involves not only the dissociation of hydrogen onto the Mg(0001) surface in the first step but also the subsequent diffusion of dissociated surface H atoms into the Mg matrix. The final states after chemisorption (i.e., the TiH2 or TiH4-like species) are very stable compared to the initial states. This of course contrasts with the energetics of chemisorption of hydrogen onto a pure Mg surface19,20 and implies that the chemisorbed H atoms are much more strongly bound to the Ti atom than to Mg atoms. Naively, one might expect that this implies a considerable diffusion barrier for motion of the chemisorbed H atoms from the surface Ti site into the bulk, since this will involve removing H atoms from the Ti where they are strongly bound into a sublayer where they interact primarily with Mg atoms. The activation energies for dehydrogenation of TiH2 to TiH and TiH to Ti are experimentally measured to be 74.79 and 131.921 kJ/mol,39 respectively, and the decomposition temperature of TiH2 was theoretically predicted to be quite high.40 A good catalyst clearly should not bind hydrogen too strongly. Hence, while the results presented herein do shed light on the nature of initial surface adsorption of hydrogen onto Ti sites in the Mg surface, they do not provide a full rationalization of the experimentally observed catalytic role of Ti for hydrogen sorption into ball-milled Mg nanocomposite materials. Further studies exploring various possible pathways for H diffusion from the Ti site into the bulk material will be required in order to probe for possible low-energy diffusion pathways that might then provide a robust theoretical explanation of the catalytic role. In a related context, we note that our recent studies22,23 have revealed a potential catalytic role that incorporated atomic carbon may play in facilitating hydrogen diffusion into the Mg bulk. Such synergetic effects have been also observed in recent ball-milling experiments for mixing Ti, graphite, or carbon nanotubes (5%) with Mg materials.23,41,42 Conclusions In summary, ab initio DFT calculations were performed to study the adsorption of H2 molecules on a Ti-doped Mg(0001) surface. We found that two hydrogen molecules will be dissociated on top of the Ti atom with very small activation barrierssaround 0.103 and 0.145 eV, respectively. Furthermore, binding of H2 onto Ti-doped Mg(0001) in molecular form is predicted for the first time. This phenomenon is attributed to the polarization of the H2 molecule by the Ti cation that results from the prior chemisorption events. Our results parallel recent findings for H2 adsorption onto Ti-decorated carbon nanotubes
Du et al. and fullerenes.24,25 These findings shed new light on the initial stages of hydrogen adsorption onto Ti surface sites in Mg nanocomposite materials. Acknowledgment. We acknowledge generous grants of high-performance computer time from both the Computational Molecular Science cluster computing facility at The University of Queensland and the Australian Partnership for Advanced Computing (APAC) National Facility. We also greatly appreciate the financial support by Australian Research Council through the ARC Centre for Functional Nanomaterials. References and Notes (1) Schwarz, R. B. MRS Bull. 1999, 24, 40. (2) Schlapbach, L.; Zuttel, A. Nature 2001, 414, 23. (3) Liang, G.; Huot, J.; Van Neste, A.; Schulz, R. J Alloys Compd. 1999, 292, 247. (4) Zaluska, A.; Zaluski, L.; Strom-Olsen, J. O. Appl. Phys. A: Mater. Sci. Process. 2001, 72, 157. (5) Von Zeppelin, F.; Reule, H.; Hirscher, M. J Alloys Compd. 2002, 330-332, 723. (6) Bobet, J. L.; Chevalier, B.; Darriet, B. J Alloys Compd. 2002, 330, 738. (7) Rivoirard, S.; de Rango, P.; Fruchart, D.; Charbonnier, J.; Vempaire, D. J Alloys Compd. 2003, 356-357, 622. (8) Liang, G. J Alloys Compd. 2004, 370, 123. (9) Hanada, N.; Ichikawa, T.; Fujii, H. J. Phys. Chem B 2005, 109, 7188. (10) Yavari, A. R.; de Castro, J. F. R.; Heunen, G.; Vaughan, G. J Alloys Compd. 2003, 353, 246. (11) Greeley, J.; Norskov, J. K.; Mavrikakis, M. Annu. ReV. Phys. Chem. 2002, 53, 319. (12) Greeley, J.; Mavrikakis, M. Nat. Mater. 2004, 3, 810. (13) Vang, R. T.; Honkala, K.; Dahl, S.; Vestergaard, E. K.; Schnadt, J.; Egsgaard. E.; Clausen, B. S.; Norskov, J. K., Besenbacher, F. Nat. Mater. 2005, 4, 160. (14) Shang, C. X.; Bououdina, M.; Song, Y.; Guo, Z. X. Int. J. Hydrogen Energy 2004, 29, 73. (15) Song, Y.; Guo, Z. X.; Yang, R. Phys. ReV. B 2004, 69, 094205. (16) Vegge, T.; Hedegaard-Jensen, L. S.; Bonde, J.; Munter, T. R.; Norskov, J. K. J Alloys Compd. 2005, 386, 1. (17) Norskov, J. K.; Houmoller, A. M. Phys. ReV. Lett. 1981, 46, 257. (18) Bird, D. M.; Clarke, L. J.; Payne, M. C.; Stich, I. Chem. Phys. Lett. 1993, 212, 518. (19) Vegge, T. Phys. ReV. B 2004, 70, 035412. (20) Du, A. J.; Smith, S. C.; Yao, X. D.; Lu, G. Q. J. Phys. Chem. B 2005, 109, 18037. (21) Du, A. J.; Smith, S. C.; Yao, X. D.; Lu, G. Q. J. Phys. Chem. B 2006, 110, 1814. (22) Du, A. J.; Smith, S. C.; Yao, X. D.; He, Y.; Lu, G. Q. J. Phys. Conf. Ser. 2006, 29, 167. (23) Yao, X.; Wu, C. Z.; Du, A. J.; Lu, G. Q.; Cheng, H. M.; Smith, S. C.; Zou, J.; He, Y. J. Phys. Chem. B 2006, 110, 11697. (24) Zhao, Y.; Kim, Y.-H.; Dillon, A. C.; Heben, M. J.; Zhang, S. B. Phys. ReV. Lett. 2005, 94, 155504. (25) Yildrim, T.; Ciraci, S. Phys. Rev. Lett. 2005, 94, 175501. (26) Kresse, G.; Furthmuller, J. Comput. Mater. Sci 1996, 6, 15. (27) Kresse, G.; Furthmuller, J. Phys. ReV. B 1996, 54, 11169. (28) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (29) Blochl, P. E. Phys. ReV. B 1994, 50, 17953. (30) Kresse, G.; Joubert, D. Phys. ReV. B 1999, 59, 1758. (31) Monkhorst, H. J.; Pack, J. D. Phys. ReV. B 1976, 13, 5188. (32) Henkelman, J.; Jo´nsson, H. J. Chem. Phys. 2000, 113, 9978. (33) Henkelman, J.; Uberuaga, B. P.; Jo´nsson, H. J. Chem. Phys. 2000, 113, 9901. (34) Kiejna, A. Phys. ReV. B 2003, 68, 235405. (35) Sprunger, P. T.; Plummer, E. W. Chem. Phys. Lett. 1991, 187, 559. (36) Nobuhara, K.; Kasai, H.; Dino, W. A.; Nakanishi, H. Surf. Sci. 2004, 566-568, 703. (37) Nose, S. Mol. Phys. 1984, 52, 255. (38) Niu, J.; Rao, B. K.; Jena, P. Phys. ReV. Lett. 1992, 68, 2277. (39) Bhosle, V.; Baburaj, E. G.; Miranova, M.; Salama, K. Mater. Sci. Eng., A 2003, 356, 190. (40) Grochala, W.; Edwards, P. P. Chem. ReV. 2004, 104, 1283. (41) Shang, C. X.; Guo, Z. X. J. Power Sources 2004, 129, 73. (42) Imamura, H.; Tabata, S.; Takesue, Y.; Sakata, Y.; Kamazaki, S. Int. J. Hydrogen Energy 2000, 25, 837.
