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Ab Initio Study of the Hydrogenation of Mg with Incorporated Ti Atom on the Surface
1
Recent studies show that a single Ti atom coated on a single-walled nanotube (SWNT) and C 60 fullerene could bind up to four hydrogen molecules [1-2]. One Ti atom substituted one Mg atom on Mg(0001) surface could decrease the dissociation barrier of H 2 to 0.10 eV [3]. How about the interaction of many H 2 molecules with Ti incorporated Mg(0001) surface? The improved hydrogenation kinetics has experimentally been observed by mixing Ti, Carbon Nanotube and Mg materials during ball milling [4]. Ab Initio Ab Initio Study of the Hydrogenation Study of the Hydrogenation of Mg with Incorporated Ti Atom on of Mg with Incorporated Ti Atom on the Surface the Surface Aijun Du and Sean C. Smith Centre for Computational Molecular Science, Chemistry Building, The University of Queensland, Qld 4072 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. The authors also greatly appreciate the financial support by Australian Research Council through the ARC Center for Functional Nanomaterials. Introduction Computational Details Results & Discussions (continued) Results & Discussions Conclusions References Acknowledgements Magnesium hydride (MgH 2 , 7.6 wt%) is a very promising approach for H 2 energy carrier in mobile vehicles. Unfortunately, the application is primarily limited by the high hydrogenation reaction temperature and slow kinetics. In this work, we report new results from DFT studies of the dissociative chemisorption of molecular H 2 on a Ti- incorporated Mg(0001) surface. It was found that two hydrogen molecules can dissociate on top of Ti atom with very small activation barrier (around 0.10 eV). Additionally, molecular adsorption of H 2 on Ti atom on Mg(0001) surface is also observed. These results parallel recent findings for H 2 adsorption on a Ti-decorated carbon nanotube [1] and clustered Ti on C 60 fullerenes [2]. It can be seen clearly from Figure 2 that there is a only a very small It can be seen clearly from Figure 2 that there is a only a very small barrier for the dissociation of first H barrier for the dissociation of first H 2 molecule on Ti-incorporated molecule on Ti-incorporated Mg(0001) surface. Mg(0001) surface. AIMD at room temperature did observe the spontaneous dissociation of H AIMD at room temperature did observe the spontaneous dissociation of H 2 on Ti-incorporated Mg(0001) surface. on Ti-incorporated Mg(0001) surface. Compared to pure Mg(0001) surface, the activation barrier of H Compared to pure Mg(0001) surface, the activation barrier of H 2 decreased from 1.15 eV to 0.10 eV. decreased from 1.15 eV to 0.10 eV. Strong interaction between the molecular orbital of H Strong interaction between the molecular orbital of H 2 and the metal and the metal d d orbital orbital of Ti. [1 T.Yildirim and S. Ciraci, Phys.Rev.Letts., 94 (2005) 175501. [2] Qiang Sun, Qian Wang, Puru Jena and Yoshiyuki Kawazoe, J.Am.Chem.Soc, 127 (2005) 14582. [3] Aijun Du, S.C.Smith, X.D.Yao and G.Q.Lu, J.Phys.Chem.B, 109 (2005) 18037. [4] X. Yao, C. Z. Wu, Aijun Du, G. Q. Lu and Sean C. Smith J.Phys.Chem.B, (2006) in press. [5] Kresse, G.; Furthmuller, J. Comput. Mater. Sci 1996, 6, 15. Kresse, G.; Furthmuller, J. Phys.Rev.B 1996, 54, 11169. [6] Blochl, P.E. Phys.Rev.B 1994, 50, 17953. Kresse, G.; Joubert, D. Phys.Rev.B 1999, 59, 1758. [7] J.Henkelman and H.Jónsson, J.Chem.Phys., 2000, 113, 9978; 2000, 113, 9901. [8] Aijun Du, S.C.Smith, X.D.Yao and G.Q.Lu , J. Phys. Conference Series, 29 (2006) 167. Figure 4 Final configuration for the third H 2 molecule on Mg 44 TiH 4 surface. Molecular adsorption state of H 2 is first observed. The adsorption energy of hydrogen molecule is around 0.29 eV. Figure 2 Energy profile for the dissociation of first H 2 molecule on Ti-incorporated Mg(0001) surface. The black, grey and small white balls represent Mg, Ti and H atoms, respectively. IS, LS1, TS1, LS2 and FS represent the initial state, first local minimum state, first transition state, second local minimum state and final state, respectively. Figure 3 Energy profile for the dissociation of the second H 2 molecule on Mg 44 TiH 2 surface. The green, grey and pink balls represent Mg, Ti and H atoms, respectively. Figure 1a Figure 1b Ab initio PAW Method: All the calculations were performed using VASP code [5] implementing GGA of PBE exchange correlation functional. Projector Augmented Wave (PAW) method [6] is used to describe the electronic-ion-core interaction. Ti- incorporated Mg(0001) surface was modeled as a (3×3) surface unit cell with 5 layers of Mg atoms. Only gamma point calculation is performed. The vacuum space is at least 15 Å, which is enough to guarantee a sufficient separation between periodic images. Nudged Elastic Band Method : The NEB method is used [7] to get Minimum Energy Path (MEP). A damped molecular dynamics was used to relax ions until the force in each image are less than 0.02 eV/Å. Ab initio Molecular Dynamics Simulation: NVT ensemble (T=300K) with Nose thermostat. Deuterium is used instead of hydrogen during AIMD. MD time step is 0.5 fs. The formation of Ti-incorporated Mg(0001) surface involves the creation of a Mg vacancy on Mg(0001) surface in the first step and the vacancy is then occupied by a Ti atom. Taking them together gives the adsorption energy (=1/9) at the surface substitutional site. The formation energy of Ti@Mg(0001) surface is calculated by ) 0001 ( ) 0001 ( / Mg atom Ti Mg subs Mg Ti subs ad E E E E E Where E Ti/Mg(0001)-subs , E Mg , E Ti-atom and E Mg(0001) represent the total energy of the relaxed Ti-incorporated Mg(0001) surface, the bulk Mg atom, the isolated spin- polarized Ti atom and the clean Mg(0001) slab, respectively. The adsorption energy is calculated to be -4.09 eV, which indicated that Ti-incorporated Mg(0001) surface is thermodynamically stable. The activation barrier for the dissociation of the second H The activation barrier for the dissociation of the second H 2 molecule on Ti- molecule on Ti- Mg(0001) surface (Mg Mg(0001) surface (Mg 44 44 TiH TiH 2 ) is also small (0.145 eV) and we observe the ) is also small (0.145 eV) and we observe the spontaneous dissociation of second H spontaneous dissociation of second H 2 on surface by using AIMD at 300K.. on surface by using AIMD at 300K.. P Polarization mechanism where the charge on the Ti cation polarizes the H 2 . Further studies on the activation barrier for the diffusion of atomic H from Further studies on the activation barrier for the diffusion of atomic H from surface to bulk show the macroscopically improvements in hydrogenation surface to bulk show the macroscopically improvements in hydrogenation kinetics is still very limited. Other catalyst such as carbon is expected to kinetics is still very limited. Other catalyst such as carbon is expected to be play an important role for in the atomic H diffusion process [8]. be play an important role for in the atomic H diffusion process [8]. (a) (b)
description
Ab Initio Study of the Hydrogenation of Mg with Incorporated Ti Atom on the Surface Aijun Du and Sean C. Smith Centre for Computational Molecular Science, Chemistry Building, The University of Queensland, Qld 4072. Introduction. Results & Discussions (continued). - PowerPoint PPT Presentation
Transcript of Ab Initio Study of the Hydrogenation of Mg with Incorporated Ti Atom on the Surface
Recent studies show that a single Ti atom coated on a single-walled nanotube (SWNT) and C60
fullerene could bind up to four hydrogen molecules [1-2].
One Ti atom substituted one Mg atom on Mg(0001) surface could decrease the dissociation barrier of H2 to 0.10 eV [3].
How about the interaction of many H2 molecules with Ti incorporated Mg(0001) surface?
The improved hydrogenation kinetics has experimentally been observed by mixing Ti, Carbon Nanotube and Mg materials during ball milling [4].
Ab InitioAb Initio Study of the Hydrogenation of Mg Study of the Hydrogenation of Mg with Incorporated Ti Atom on the Surfacewith Incorporated Ti Atom on the Surface
Aijun Du and Sean C. Smith
Centre for Computational Molecular Science, Chemistry Building, The University of Queensland, Qld 4072
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. The authors also greatly appreciate the financial support by Australian Research Council through the ARC Center for Functional Nanomaterials.
Introduction
Computational Details
Results & Discussions (continued)
Results & Discussions
Conclusions
References
Acknowledgements
Magnesium hydride (MgH2, 7.6 wt%) is a very promising approach for H2 energy carrier in mobile vehicles. Unfortunately, the application is primarily limited by the high hydrogenation reaction temperature and slow kinetics. In this work, we report new results from DFT studies of the dissociative chemisorption of molecular H2 on a Ti-incorporated Mg(0001) surface. It was found that two hydrogen molecules can dissociate on top of Ti atom with very small activation barrier (around 0.10 eV). Additionally, molecular adsorption of H2 on Ti atom on Mg(0001) surface is also observed. These results parallel recent findings for H2
adsorption on a Ti-decorated carbon nanotube [1] and clustered Ti on C60 fullerenes [2].
It can be seen clearly from Figure 2 that there is a only a very small barrier for the dissociation of It can be seen clearly from Figure 2 that there is a only a very small barrier for the dissociation of first Hfirst H22 molecule on Ti-incorporated Mg(0001) surface. molecule on Ti-incorporated Mg(0001) surface.
AIMD at room temperature did observe the spontaneous dissociation of HAIMD at room temperature did observe the spontaneous dissociation of H22 on Ti-incorporated on Ti-incorporated
Mg(0001) surface.Mg(0001) surface.
Compared to pure Mg(0001) surface, the activation barrier of HCompared to pure Mg(0001) surface, the activation barrier of H22 decreased from 1.15 eV to 0.10 eV. decreased from 1.15 eV to 0.10 eV.
Strong interaction between the molecular orbital of HStrong interaction between the molecular orbital of H22 and the metal and the metal dd orbital orbital of Ti.
[1 T.Yildirim and S. Ciraci, Phys.Rev.Letts., 94 (2005) 175501.
[2] Qiang Sun, Qian Wang, Puru Jena and Yoshiyuki Kawazoe, J.Am.Chem.Soc, 127 (2005) 14582.
[3] Aijun Du, S.C.Smith, X.D.Yao and G.Q.Lu, J.Phys.Chem.B, 109 (2005) 18037.
[4] X. Yao, C. Z. Wu, Aijun Du, G. Q. Lu and Sean C. Smith J.Phys.Chem.B, (2006) in press.
[5] Kresse, G.; Furthmuller, J. Comput. Mater. Sci 1996, 6, 15. Kresse, G.; Furthmuller, J. Phys.Rev.B 1996, 54, 11169.
[6] Blochl, P.E. Phys.Rev.B 1994, 50, 17953. Kresse, G.; Joubert, D. Phys.Rev.B 1999, 59, 1758.
[7] J.Henkelman and H.Jónsson, J.Chem.Phys., 2000, 113, 9978; 2000, 113, 9901.
[8] Aijun Du, S.C.Smith, X.D.Yao and G.Q.Lu , J. Phys. Conference Series, 29 (2006) 167.
Figure 4 Final configuration for the third H2 molecule on Mg44TiH4 surface. Molecular adsorption state of H2 is first observed. The adsorption energy of hydrogen molecule is around 0.29 eV.
Figure 2 Energy profile for the dissociation of first H2 molecule on Ti-incorporated Mg(0001) surface. The black, grey and small white balls represent Mg, Ti and H atoms, respectively. IS, LS1, TS1, LS2 and FS represent the initial state, first local minimum state, first transition state, second local minimum state and final state, respectively.
Figure 3 Energy profile for the dissociation of the second H2 molecule on Mg44TiH2 surface. The green, grey and pink balls represent Mg, Ti and H atoms, respectively.
Figure 1a Figure 1b
Ab initio PAW Method:
All the calculations were performed using VASP code [5] implementing GGA of PBE exchange correlation functional. Projector Augmented Wave (PAW) method [6] is used to describe the electronic-ion-core interaction. Ti-incorporated Mg(0001) surface was modeled as a (3×3) surface unit cell with 5 layers of Mg atoms. Only gamma point calculation is performed. The vacuum space is at least 15 Å, which is enough to guarantee a sufficient separation between periodic images. Nudged Elastic Band Method:
The NEB method is used [7] to get Minimum Energy Path (MEP). A damped molecular dynamics was used to relax ions until the force in each image are less than 0.02 eV/Å. Ab initio Molecular Dynamics Simulation:
NVT ensemble (T=300K) with Nose thermostat. Deuterium is used instead of hydrogen during AIMD. MD time step is 0.5 fs.
The formation of Ti-incorporated Mg(0001) surface involves the creation of a Mg vacancy on Mg(0001) surface in the first step and the vacancy is then occupied by a Ti atom. Taking them together gives the adsorption energy (=1/9) at the surface substitutional site. The formation energy of Ti@Mg(0001) surface is calculated by
)0001()0001(/ MgatomTiMgsubsMgTisubsad EEEEE
Where ETi/Mg(0001)-subs, EMg, ETi-atom and EMg(0001) represent the total energy of the relaxed Ti-incorporated Mg(0001) surface, the bulk Mg atom, the isolated spin-polarized Ti atom and the clean Mg(0001) slab, respectively. The adsorption energy is calculated to be -4.09 eV, which indicated that Ti-incorporated Mg(0001) surface is thermodynamically stable.
The activation barrier for the dissociation of the second HThe activation barrier for the dissociation of the second H22 molecule on Ti-Mg(0001) surface (Mg molecule on Ti-Mg(0001) surface (Mg4444TiHTiH22) is ) is
also small (0.145 eV) and we observe the spontaneous dissociation of second Halso small (0.145 eV) and we observe the spontaneous dissociation of second H22 on surface by using AIMD on surface by using AIMD
at 300K..at 300K..
PPolarization mechanism where the charge on the Ti cation polarizes the H2.
Further studies on the activation barrier for the diffusion of atomic H from surface to bulk show the Further studies on the activation barrier for the diffusion of atomic H from surface to bulk show the macroscopically improvements in hydrogenation kinetics is still very limited. Other catalyst such as carbon macroscopically improvements in hydrogenation kinetics is still very limited. Other catalyst such as carbon is expected to be play an important role for in the atomic H diffusion process [8].is expected to be play an important role for in the atomic H diffusion process [8].
(a) (b)
