Influence of Ti and Al Dopants on the Dehydrogenation Characteristics

Article pubs.acs.org/JPCC

Influence of Ti and Al Dopants on the Dehydrogenation Characteristics of Mg(BH4)2: Electronic Structure Mechanisms B. Shi, Y. Song,* J. H. Dai, and H. Z. Yu School of Materials Science and Engineering, Harbin Institute of Technology at Weihai, 2 West Wenhua Road, Weihai 264209, China ABSTRACT: We studied the influence of Ti and Al dopants on the dehydrogenation properties of Mg(BH4)2 via first principles calculations. The dopants only affect the electronic structure of matrix in their vicinity as they use various mechanisms to improve the dehydrogenation properties of Mg(BH4)2. Ti prefers to occupy the Mg site, whereas Al usually substitutes for the B atom and tends to form AlH4. However, one of the Al−H bonds is weak and will be broken during dehydrogenation. In Ti-doped systems, dehydrogenation properties of Mg(BH4)2 change depending on which site the Ti atom occupies. If it substitutes for Mg, the interaction between the Ti and the B atoms is weaker than the Mg−B interaction in the undoped Mg(BH4)2. When Ti substitutes for a B atom, it can only hold two H atoms and so is likely to generate a metal hydride such as TiH2. The other two H atoms are held by weak bonds and so easily may be released during dehydrogenation. This means that Ti is a good candidate to improve the dehydrogenation properties of Mg(BH4)2.

1. INTRODUCTION Magnesium borohydrideMg(BH4)2was first synthesized in 19501 and is now being considered as a hydrogen storage medium due to its relatively large hydrogen capacity of 14.8 mass %. However, the structure of unsolvated Mg(BH4)2 still remains uncertain as it is difficult to obtain crystalline products of a high enough quality for diffraction measurements. Some theoretical and experimental investigations have been performed in recent years.2−12 Total-energy density functional theory (DFT) calculations suggest that the Pmc21 type ion arrangement of Mg(BH4)2 is most likely, as it has the lowest total energy among the 28 possible arrangements.2 P3̅m1, P2/c, I4̅m2, F222, and P6122 symmetries were also reported at low temperatures.3−6 The (α phase) P6122 symmetry was confirmed experimentally by Filinchuk et al., and the (β phase) Fddd and P63 symmetries were identified at high temperatures.7,8 Numerous studies have attempted to identify the dehydrogenation pathway of unsolvated Mg(BH4)2.13−26 One possible dehydrogenation process of Mg(BH4)2 is shown below,17 α‐Mg(BH4)2 → β‐Mg(BH4)2

with a hydrogen release of 11 wt % at 638 K. Reaction 4 is the dissociation of MgH2 at 733 K. The high thermodynamic stability of Mg(BH4)2 means that these dissociation temperatures are too high for practical applications. Doping is one way to reduce the thermodynamic stability (and so the dissociation temperature) of this compound. The initial decomposition temperature can be reduced to 119 K if the Mg(BH4)2 is adsorbed into the voids of pretreated, activated carbon with a pore diameter less than 2 nm.16 Li et al. mixed Mg(BH4)2 with Ti-based additives, such as Ti, TiB2, TiH2, TiO2, and TiCl3, and found that the dehydriding temperature of α-Mg(BH4)2 catalyzed by TiCl3 was significantly reduced from 535 to 361 K.24 Newhouse et al. found that mixing Mg(BH4)2 with 5 mol % TiF3 and ScCl3 could increase hydrogen yields and the desorption rate at 573 K.14 Yu et al. found that a 1:1 mol ratio mixture of Mg(BH4)2 and LiNH2 starts dehydrogenation at 433 K and can achieve a total weight loss of 7.2 wt % up to 573 K.25 However, there are few studies of the electronic structure and bonding characteristics of doped Mg(BH4)2. It is essential that the mechanisms by which additives affect a hydride's hydrogenation and dehydrogenation reactions are clarified if the hydrides are to be modified for practical applications. We therefore performed first-principles calculations to investigate the hydrogen−metal interactions and the stability of Ti- and Aldoped Mg(BH4)2.

(1)

β‐Mg(BH4)2 → 1/6MgB12H12 + 5/6MgH2 + 13/6H 2 (2)

→MgH2 + 2B + 3H 2

(3)

→Mg + 2B + 4H 2

(4)

The α to β phase transition occurs at approximately 460 K. At approximately 593 K, the β phase produces the intermediate compounds MgB12H12 and MgH2 and releases 5.9 wt % hydrogen. The MgB12H12 further dissociates into MgH2 and B © 2012 American Chemical Society

Received: December 20, 2011 Revised: May 10, 2012 Published: May 16, 2012 12001

dx.doi.org/10.1021/jp212289u | J. Phys. Chem. C 2012, 116, 12001−12007

The Journal of Physical Chemistry C

Article

2. SIMULATION METHOD The electronic structure and total energy of pure and Ti- and Al-doped Mg(BH4)2 were calculated by the generalized gradient approximation of Perdew−Burke−Ernzerhof using SIESTA (Spanish Initiative for Electronic Simulations with Thousands of Atoms).27,28 A cutoff energy of 4000 eV and a 3 × 3 × 1 k-mesh were used, and two consecutive energies that differed by less than 0.1 meV were considered self-consistent in the electronic structure calculations. α-Mg(BH4)2 has a hexagonal structure (space group P6122) with the lattice parameters a = 1.0354 nm and c = 3.7055 nm.7 Its primitive cell contains 30 formula units of 11 atoms each shown in Figure 1. Full relaxations of pure α-Mg(BH4)2 gave

lattice parameters as a = 1.0178 nm and c = 3.7052 nm and distances of 0.245, 0.210, and 0.125 nm between the nearest Mg−B, Mg−H, and B−H atoms. These values are very close to experimental findings.7

3. RESULTS AND DISCUSSION 3.1. Occupation Behavior and Hydrogen Dissociation Energy. The M-doped Mg(BH 4 ) 2 is denoted as Mg30−xB60−yMx+y+z, where (x = 1, y = 0, z = 0) and (x = 0, y = 1, z = 0) when M substitutes for a Mg atom at the (0.478, 0.503, 0.588) site and a B atom at the (0.538, 0.495, 0.524) site. (x = 0, y = 0, z = 1) means that M occupies the (0.105, 0.210, 0.250) interstitial site (Figure 1). The concentration of dopant in these systems was about 0.3 at %. Full relaxation and spin polarization calculations were performed. The occupation energy of the dopant in Mg(BH4)2 as defined below determines which site the dopant occupies. Eoccu = E(Mg 30 − xB60 − yMx + y + z) − E(Mg 30B60) − [(x + y + z)E(M) − xE(Mg) − yE(B)]

(5)

Here, E(X) denotes the total energy of system X. Total energies of −56.321 eV for Al, −93.874 eV for Ti, −23.88 eV for Mg, and −77.09 eV for B were calculated under the same framework. The calculated lattice parameters of the doped systems, the coordinates, and the occupation energies of the dopants are listed in Tables 1−3. In general, doping changes the symmetry of Mg(BH4)2 slightly, which causes a small difference between lattice parameters a and b. The atomic radius of the dopants Ti and Al is smaller than that of Mg (0.145 and 0.143 nm vs 0.160 nm), which means the volume of the lattice is reduced if Ti or Al substitutes for Mg. However, if the dopant occupies an interstitial site, the opposite happens: the dopant distorts the atoms surrounding it as it pushes them away. The lattice volume is reduced if Ti substitutes for B but increases if Al is substituted for it. The most stable system occurs when Ti substitutes for a Mg atom as the occupation energy of −1.896 eV (obtained via spin polarization calculations) is the lowest of the considered systems. The next most stable system occurs when Al is substituted for a B atom; this system is also the easiest Al-doped system to produce. To further clarify the effects of the dopant on the dehydrogenation behavior of Mg(BH4)2, we calculated the dissociation energy by taking a H atom away from the compound:

Figure 1. Unit cell of Mg(BH4)2 with a = b = 1.0178 nm and c = 3.7052 nm. The green, beige, and white balls denote Mg, B, and H atoms. Yellow and violet balls show the substitution positions of the Mg and B. The red ball denotes the initial location of the dopant at an interstitial site.

Ed = [E(XHn − 1) + 1/2E(H 2)] − E(XHn)

(6)

Here, X stands for Mg30−xB60−yMx+y+z, and n = 240 is the total number of H atoms in a unit cell of Mg(BH4)2. The energy E(XHn−1) of the dissociated system was estimated by taking a

Table 1. Mg Substituted Mg(BH4)2; Lattice Parameters, Coordinates, and Occupation Energy Eoccu of Dopant in Mg(BH4)2; and Hydrogen Dissociation Energy Eda lattice parameters (nm) dopant

a

b

c

u

v

w

Eoccu (eV)

Ed (eV)

Ti

1.0159 (1.0123) 1.0163 (1.0165)

1.0181 (1.0186) 1.0146 (1.0146)

3.6985 (3.7091) 3.6997 (3.7017)

0.482 (0.481) 0.493 (0.493)

0.505 (0.487) 0.514 (0.514)

0.585 (0.585) 0.587 (0.588)

0.055 (−1.896) 3.229 (2.950)

2.175 (2.191) 1.391 (1.625)

Al a

coordinates of dopant

Numbers in brackets are obtained with spin polarization calculations. 12002



Article

Table 2. B-Substituted Mg(BH4)2; Lattice Parameters, Coordinates, and Occupation Energy Eoccu of Dopant in Mg(BH4)2; and Hydrogen Dissociation Energy Eda lattice parameters (nm) a

b

c

u

v

w

Eoccu (eV)

Ed (eV)

Ti

1.0168 (1.0174) 1.0204 (1.0204)

1.0049 (1.0053) 1.0201 (1.0201)

3.7294 (3.7287) 3.7288 (3.7288)

0.579 (0.579) 0.601 (0.601)

0.390 (0.389) 0.487 (0.488)

0.524 (0.524) 0.526 (0.526)

2.413 (1.041) 1.284 (1.271)

0.448 (0.206) 1.917 (1.405)

Al a


dopant

Numbers in brackets are obtained with spin polarization calculations.

H atom from the final structure of the considered systems while keeping the geometry of the unit cell and the coordinates of the remaining atoms unchanged. The energy of a hydrogen molecule, E(H2), was estimated as −31.292 eV using a supercell of the same size containing two H atoms 0.074 nm apart. This result is very close to the value of −31.617 eV calculated using the FP-LAPW method.29 The dissociation energies for the considered systems are listed in Tables 1−3. Tables 1−3 show that spin polarization significantly reduces the occupation energy of Ti in Mg(BH4)2, most notably when Ti substitutes for a Mg atom. This is consistent with the finding that including spin polarization in the energy calculations of a Ti-doped Na3AlH6 system reduces the total energy.30 However, spin polarization only weakly affects the stability of Al-doped systems. The absolute difference of the occupation energies of these systems obtained with and without spin polarization is less than 0.3 eV. The values obtained for the hydrogen dissociation energy are mostly greater if spin polarization is included, except for the B-substituted systems. The overall geometry of the unit cell and the positions of dopants in Mg(BH4)2 were almost identical regardless of the spin polarization mainly due to the low concentration of the dopant (0.3 at %) and the large unit cell sizes. The analysis and discussion below are based on the calculations with spin polarization for the Ti-doped systems and without spin polarization for the other systems. When Ti substitutes for a Mg atom with an occupation energy of −1.896 eV, the Ti causes little geometric distortion and the hydrogen dissociation energy Ed of 2.191 eV is very close to undoped Mg(BH4)2's 2.199 eV. The lowest Ed (0.206 eV) occurs when Ti substitutes for a B atom in Ti's secondmost favorable site among the considered systems. As a Ti atom is larger than a B atom, the H atoms in the substituted TiH4 are pushed away from Ti atom, distorting the TiH4's structure. This means the Ti−H bonds are stretched more than the B−H bonds; the H−Ti−H angles of around 76.5° were much smaller than the H−B−H angles of 107−116°. Also, as Ti and B atoms have different valences, the Ti−H bonds have unequal lengths. The Ti only holds two H atoms, leaving the other two nearly free, which in turn dramatically reduces Ed. When Ti occupies the interstitial spaces, it breaks the symmetry of the nearby BH4 group, leaving one H atom symmetrically nonequivalent to the other three, and so gives a hydrogen dissociation energy much less than that of the undoped system. However, this system is unlikely to occur in practice as its positive occupation energy of 2.967 eV is too large. Of the Al-doped systems, the system where Al occupies the interstitial spaces had the lowest hydrogen dissociation energy and yet the largest occupation energy, while the system where Al substituted for the B atom had the lowest occupation energy of 1.271 eV. This substitution produces Mg(AlH4)2 with a geometry distinctly different to that of the Mg(BH4)2. The

bonds between the Al and the H atoms are between 0.159 and 0.166 nm, while the bonds between the B and the H atoms are 0.1239 and 0.1252 nm. The AlH4 forms a tetragonal symmetry with a H−Al−H angle of 90°, while the original H−B−H angles were between 107 and 116°. The Mg−Al bond length of about 0.281 nm is slightly longer than the 0.245 nm Mg−B bond. This indicates that H atoms around Al dopant are dramatically distorted, weakening the Al−H bonds and resulting in a relatively low hydrogen dissociation energy of 1.405 eV (Table 2). The differences between the geometry of the generated Mg(AlH4)2 “molecule” and the Mg(AlH4)2 compound are also interesting: the Mg(AlH4)2 compound's Al−H bond length of 0.167 nm is shorter than that of the Mg(AlH4)2 “molecule”, and the H−Al−H angles in the Mg(AlH4)2 compound are around 110°. These differences imply a stronger interaction between Al and H atoms in Mg(AlH4)2 compound than in the produced Mg(AlH4)2 “molecule”. In the following sections, we seek to clarify the mechanisms that dopants influence the dehydrogenation properties of Mg(BH4)2 by analyzing the electronic structures of the undoped Mg(BH4)2 and for each dopant the systems with the lowest occupation energy (Ti substituting for an Mg atom and Al substituting for a B atom) and the lowest hydrogen dissociation energy (Ti substituting for a B atom and Al occupying the interstitial sites). 3.2. Electronic Structure of Undoped Mg(BH4)2. The electronic structure of the undoped Mg(BH4)2 is shown in Figure 2. The total density of states (DOSs) is similar to the

Figure 2. Electronic structures of Mg(BH4)2. (a) Total and partial densities of states and (b) the CDD on the (110) plane. 12003



Article

Figure 3. Electronic structures of Mg(BH4)2 with Ti substituting for an Mg atom. Total and partial densities of states, (a) with and (b) without the spin polarization, and (c) the CDD on the (110) plane.

Figure 4. Electronic structures of Mg(BH4)2 with Ti substituting for a B atom. Total and partial densities of states, (a) with and (b) without the spin polarization, and (c) the CDD on the (110) plane.

Here, ρsys is the system's charge density and ρatom is the charge i density of the individual atoms that make up the system. ρatom is i evaluated using a pseudostructure in which the i-th atom is kept in place while the other atoms are removed. The lattice parameters and symmetry of this pseudostructure are the same as the original supercell. Figure 2b is the CDD on the (110) plane of the undoped Mg(BH4)2. Dashed and solid lines represent the positive and negative values of the CDD. If the CDDs around two adjacent atoms have the same sign, we expect them to be covalently bonded, and different signs imply ionic bonding. It is clear that bonding is covalent between the B and the H atoms but is ionic between the Mg atom and the BH4. 3.3. Ti doped Mg(BH4)2. Figure 3a shows the total and partial DOSs when Ti substitutes for a Mg atom. The PDOSs of the Mg atom are not significantly different from those of the undoped Mg(BH4)2 and so are not illustrated in Figure 3a. In this system, Ti is surrounded by four BH4 groups, and its

VASP calculations performed by Vajeeston et al.2 The valence bands consist of two crystal-filed groups separated by 2.0 eV, and there is a 6.3 eV gap between the valence and the conduction bands. The H s and Mg s, p electrons contribute to both groups in the valence bands, but the B s electrons are concentrated in the lower energy region (from −10.9 to −8.7 eV), and the B p electrons are concentrated in the upper energy region (from −7.0 to −3.0 eV). This implies covalent sp hybridization between the B and the H atoms. In addition, the partial DOSs of this system show that there are far more of Mg's electrons than B's or H's electrons in the conduction bands. The B and H atoms share their valence electrons and produce covalent bonds. The charge difference distribution (CDD) further clarifies the bonding characteristics of this system: ρCDD = ρsys −

∑ ρiatom i

(7) 12004



Article

nearest neighboring H atoms are labeled H1, H2, and H3. There is a distinct difference of the Ti d electrons between the spin up and the spin down. Figure 3a shows that the spin polarization significantly stabilizes this system as it amplifies the bonding peak of the Ti d electrons with upward spin and moves it to a lower energy. This could explain why the Ti's occupation energy is dramatically reduced with spin polarization (Table 1). However, the spin polarization only weakly influences the PDOSs of B and H. The total DOS is similar to that of the undoped system as it has two bonding energy windows (one from −11.7 to −9.6 eV and the other from −7.3 to −4.0 eV). In the lower energy region, the total DOS is dominated by s electrons from the H and the B atoms, while the Ti's 3d electrons interact with B p and H s electrons in the higher energy region. The Ti−B bond is the strongest of these bonds with 0.246 nm between the two atoms and is similar to the 0.238 nm Ti−B bond in TiB2. The amplitude of H1 peaks is greater than that of the H2 peaks, and the shape of PDOS of the H3 s orbital is different to that of the H s orbitals in the undoped Mg(BH4)2, all of which illustrate the weakness of the bonds between the Ti and the H atoms. This is consistent with the Ti−H1 (0.211 nm), Ti−H2 (0.212 nm), and Ti−H3 (0.214 nm) bond lengths, whose values are slightly larger than the 0.203 nm Mg−H bond in Mg(BH4)2. The CDD on the system's (110) plane is plotted in Figure 3c and shows significant charge differences around the Ti atom. The charge difference between Ti and H atoms is larger than the difference between the Mg and the H atoms. Figure 4a shows the total and partial DOSs of the system where Ti substitutes for a B atom in Mg(BH4)2. Interestingly, the peaks of spin up PDOSs of Ti d electrons have a much greater amplitude than the spin down PDOSs, whereas the spin down PDOSs of H s electrons have a much greater amplitude than the spin up PDOSs. Figure 4a,b shows how the Ti d orbital attracts the H s orbital toward the Fermi energy, which may reduce the system's stability. Ti d electrons bond with H1 and H2 s electrons in the energy region between −2.5 and −0.4 eV. The corresponding CDD in Figure 4c highlights the overlapping distributions of the Ti and H atoms, which could be the source of the Ti−H bonds. The distances between the Ti and the H atoms are around 0.185 nm, which makes them longer than the B−H bonds in undoped Mg(BH4)2 but shorter than the 0.1928 nm bonds in TiH2. However, the Mg−H bonds in the doped system are shortened to around 0.198 nmonly 1% longer than those in MgH2 hydride (0.1955 nm). H atoms may therefore bond to either the Ti or the Mg atom to generate TiH2 or MgH2. 3.4. Al-Doped Mg(BH4)2. Figure 5a shows the total and partial DOSs when Al substitutes for a B atom in Mg(BH4)2. As there were no significant differences between the spin up and the spin down PDOSs, Figure 5a only plots the spin up PDOS. A few features stand out: the Al s and p orbitals are completely disjointed, with peaks at −8.0 and −4.0 eV, respectively. The Al s electrons mainly interact with the Mg s and p electrons in the region between −8.5 and −7.5 eV. In addition, the PDOSs of H1, H2, and H3 s orbitals are almost identical (with only a slight difference for the H1 s orbital), and the overlap with the Al p orbital at −4.0 eV contributes to the Al−H bonds. Also, the main peak of the H4 s orbital is about −0.5 eV less than the peaks of other three H atoms and overlaps with the second peak of the Al p orbital at −4.5 eV. This implies that the Al−H4 bond is much weaker than the Al−H1, Al−H2, and Al−H3 bonds due to the smaller peak of Al p electrons and a longer

Figure 5. Electronic structures of Mg(BH4)2 with Al substituting for a B atom. (a) Total and partial densities of states and (b) the CDD on the (110) plane.

length (0.1665 vs 0.1590−0.1650 nm). Figure 5b shows the CDD on the (110) plane of this system. The slight differences between the CDD of the doped AlH4 and the undoped BH4 do not affect the characteristics of the bonds between the metal and the H atoms. Al and H atoms are covalently bonded, whereas the Mg−Al bond is ionic, as is the Mg−B bond in the undoped system. The Al atom of the Al interstitially doped Mg(BH4)2 is initially surrounded by two BH4 groups at the (0.105, 0.210, 0.250) site (the B atoms are labeled B1 and B2 in Figure 6a). The Al atom is between 0.258 and 0.298 nm from its nearest H atoms. After the relaxation, it moves to the (0.103, 0.205, 0.250) site, making the distances to the now-distorted H atoms between 0.251 and 0.318 nm. The positions of surrounding B atoms also change slightly, reducing the distance between the Al and the B1 atoms from 0.313 to 0.304 nm and stretching the

Figure 6. Electronic structures of Mg(BH4)2 with Al occupying an interstitial site. (a) Total and partial densities of states and (b) the CDD on the (110) plane. 12005



Article

Table 3. Interstitially Doped Mg(BH4)2; Lattice Parameters, Coordinates, and Occupation Energy Eoccu of Dopant in Mg(BH4)2; and Hydrogen Dissociation Energy Eda lattice parameters (nm) a

b

c

u

v

w

Eoccu (eV)

Ed (eV)

Ti

1.0213 (1.0187) 1.0279 (1.0207)

1.0216 (1.0162) 1.0246 (1.0173)

3.7140 (3.7029) 3.7250 (3.7210)

0.105 (0.107) 0.104 (0.103)

0.208 (0.219) 0.209 (0.205)

0.250 (0.250) 0.250 (0.250)

4.168 (2.967) 3.380 (3.127)

1.517 (1.215) 0.911 (1.127)

Al a


dopant

Numbers in brackets are obtained with spin polarization calculations.

distance between the Al and the B2 atoms from 0.369 to 0.375 nm. These distortions may explain why the occupation energy of this system is so high and may also weaken the bonding between the B and the H atoms, in turn giving the system the low hydrogen dissociation energy shown in Table 3. Figure 6a shows the total and partial DOSs of the Al interstitially doped Mg(BH4)2. The large distance between the Al and the Mg atoms means that the Mg's PDOS is not affected by doping, and so, Mg is omitted from Figure 6a. The peaks in the total DOS are divided into two groups, −12.5 to −10.4 eV and −8.2 to −5.0 eV. In the lower energy region (−12.5 to −10.4 eV), the peaks are dominated by s electrons from the H and B atoms, but in the upper energy region (−8.2 to −5.0 eV), the Al s electrons bond with the H1 s and B1 p electrons and the Al p electrons bond with the H2 s and B2 s electrons. We also note that as the PDOS of Al p electrons is distributed mainly above the Fermi energy, this system is less stable than other systems considered here. The corresponding CDD in Figure 6b shows that the Al is almost isolated and the CDD does not change significantly beyond the vicinity of the Al atom, which means that Al is unlikely to occupy this site.

Mg(BH4)2, the energy cost to dope this site with Al (1.271 eV) means that Al is not a viable way of influencing the dehydrogenation properties of Mg(BH4)2. Our results show that when Ti substitutes for B, the dissociation energy is reduced to 0.206 eV, and as the energy cost to perform the Ti doping is relatively low (the occupation energy is 1.041 eV), we believe Ti is a good candidate to improve the dehydrogenation properties of Mg(BH4)2.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Basic Research Programme of China, Grant No. 2011CB606400-G; the Natural Science Foundation of Shandong, China, Grant No. ZR2010BM034; and the Fundamental Research Funds for the Central Universities, Grant No. HIT.NSRIF.2009144.

■

4. CONCLUSIONS We investigated the influence of Ti and Al dopants on the stability and hydrogen dissociation of Mg(BH4)2 via electronic structure and total energy calculations using the GGA-PBE method within SIESTA. The effect of spin polarization on the stability and electronic structures was shown to strongly stabilize the Ti-doped systems but had little effect on the other systems considered here. Dopants mainly influence matrix atoms in their vicinity. The mechanisms by which dopants improve the dehydrogenation properties of Mg(BH4)2 depend on which sites the dopants occupy. Ti atoms prefer to substitute for the Mg atoms, but this only weakly affects the hydrogen dissociation energy. However, the hydrogen dissociation energy was dramatically reduced when a Ti atom substitutes for a B atom as the Ti atom can only hold two H atoms, so the other two H atoms are nearly free and are easily released during the dehydrogenation. In the Al-doped systems, the lowest occupation energy appeared when an Al atom substitutes for a B atom, but the lowest hydrogen dissociation energy occurred when the Al atom occupied an interstitial site as the Al dopant distorts the positions of its surrounding H atoms. However, the high energy distribution of the Al p electrons (above the Fermi energy) and low number of overlaps between the orbitals of the Al and the host atoms means that this system is unlikely to occur in practice. When an Al atom substitutes for a B atom, it nonequivalently interacts with its surrounding four H atoms and creates one Al−H bond that is much weaker than other three. Although this system's hydrogen dissociation energy is about half that of the undoped

REFERENCES

(1) Wiberg, E.; Bauer, R. Z. Naturforsch. B 1950, 5, 397−398. (2) Vajeeston, P.; Ravindran, P.; Kjekshus, A.; Fjellvåg, H. Appl. Phys. Lett. 2006, 89, 071906. (3) Nakamori, Y.; Miwa, K.; Ninomiya, A.; Li, H.; Ohba, N.; Towata, S.; Züttel, A.; Orimo, S. Phys. Rev. B 2006, 74, 045126. (4) Ozolins, V.; Majzoub, E. H.; Wolverton, C. Phys. Rev. Lett. 2008, 100, 135501. (5) Voss, J.; Hummelshøj, J. S.; Łodziana, Z.; Vegge, T. J. Phys.: Condens. Matter 2009, 21, 012203. (6) Dai, B.; Sholl, D. S.; Johnson, J. K. J. Phys. Chem. C 2008, 112, 4391−4395. (7) Filinchuk, Y.; Č erný, R.; Hagemann, H. Chem. Mater. 2009, 21, 925−933. (8) George, L.; Drozd, V.; Saxena, S. K.; Bardaji, E. G.; Fichtner, M. J. Phys. Chem. C 2008, 113, 486−492. (9) Zhou, X. F.; Qian, Q. R.; Zhou, J.; Xu, B.; Tian, Y.; Wang, H. T. Phys. Rev. B 2009, 79, 212102. (10) van Setten, M. J.; de Wijs, G. A.; Fichtner, M.; Brocks, G. Chem. Mater. 2008, 20, 4952−4956. (11) Bil, A.; Kolb, B.; Atkinson, R.; Pettifor, D. G.; Thonhauser, T.; Kolmogorov, A. N. Phys. Rev. B 2011, 83, 224103. (12) Łodziana, Z.; van Setten, M. J. Phys. Rev. B 2010, 81, 024117. (13) Chłopek, K.; Frommen, C.; Léon, A.; Zabara, O.; Fichtner, M. J. Mater. Chem. 2007, 17, 3496−3503. (14) Newhouse, R. J.; Stavilla, V.; Hwang, S.; Klebanoff, L. E.; Zhang, J. Z. J. Phys. Chem. C 2010, 114, 5224−5232. (15) van Setten, M. J.; Lohstroh, W.; Fichtner, M. J. Mater. Chem. 2009, 19, 7081−7087. (16) Fichtner, M.; Zhao-Karger, Z.; Hu, J.; Roth, A.; Weidler, P. Nanotechnology 2009, 20, 204029. 12006



Article

(17) Li, H. W.; Miwa, K.; Ohba, N.; Fujita, T.; Sato, T.; Yan, Y.; Towata, S.; Chen, M. W.; Orimo, S. Nanotechnology 2009, 20, 204013. (18) Ozolins, V.; Majzoub, E. H.; Wolverton, C. J. Am. Chem. Soc. 2008, 131, 230−237. (19) Matsunaga, T.; Buchter, F.; Miwa, K.; Towata, S.; Orimo, S.; Züttel, A. Renewable Energy 2008, 33, 193−196. (20) Matsunaga, T.; Buchter, F.; Mauron, P.; Bielman, M.; Nakamori, Y.; Orimo, S.; Ohba, N.; Miwa, K.; Towata, S.; Züttel, A. J. Alloys Compd. 2008, 459, 583−588. (21) Li, H. W.; Kikuchi, K.; Sato, T.; Nakamori, Y.; Ohba, N.; Aoki, M.; Miwa, K.; Towata, S.; Orimo, S. Mater. Trans. 2008, 49, 2224− 2228. (22) Li, H. W.; Kikuchi, K.; Nakamori, Y.; Ohba, N.; Miwa, K.; Towata, S.; Orimo, S. Acta Mater. 2008, 56, 1342−1347. (23) Hanada, N.; Chłopek, K.; Frommen, C.; Lohstroh, W.; Fichtner, M. J. Mater. Chem. 2008, 18, 2611−2614. (24) Li, H. W.; Kikuchi, K.; Nakamori, Y.; Miwa, K.; Towata, S.; Orimo, S. Scr. Mater. 2007, 57, 679−682. (25) Yu, X. B.; Guo, Y. H.; Sun, D. L.; Yang, Z. X.; Ranjbar, A.; Guo, Z. P.; Liu, H. K.; Dou, S. X. J. Phys. Chem. C 2010, 114, 4733−4737. (26) Yang, J.; Zhang, X.; Zheng, J.; Song, P.; Li, X. Scr. Mater. 2011, 64, 225−228. (27) Soler, J. M.; Artacho, E.; Gale, J. D.; García, A.; Junquera, J.; Ordenjón, P.; Sánchez-Portal, D. J. Phys.: Condens. Matter. 2002, 14, 2745−2779. (28) Ordejón, P.; Artacho, E.; Soler, J. M. Phys. Rev. B 1996, 53, 10441. (29) Song, Y.; Guo, Z. X.; Yang, R. Phys. Rev. B 2004, 69, 094205. (30) Pozzo, M.; Alfè, D. Int. J. Hydrogen Energy 2011, 36, 15632− 15641.

12007


Influence of Ti and Al Dopants on the Dehydrogenation Characteristics

Recommend Documents