First-Principles Investigation of Dehydrogenation on Cu-Doped MgH2

Article pubs.acs.org/JPCC

First-Principles Investigation of Dehydrogenation on Cu-Doped MgH2 (001) and (110) Surfaces Hai-Chen Wang,† Dong-Hai Wu,† Liu-Ting Wei,† and Bi-Yu Tang*,†,‡ †

School of Chemistry and Chemical Engineering, Guangxi University, Nanning, 530004, China Department of Physics, Xiangtan University, Xiangtan, Hunan Province, 411105, China

‡

S Supporting Information *

ABSTRACT: The Cu-doping effects on dehydrogenation of MgH2 (001) and (110) surfaces were investigated by using firstprinciples calculations. On the basis of the calculation results of total energy, the Cu dopant prefers to occupy the interstitial site on both surfaces rather than substitute a Mg atom, and the Cu dopant bonds with four adjacent H ions to form a CuH4 cluster. The electronic structures show that Cu−Mg and Cu−H interactions strongly weaken Mg−H interactions, which leads to a reasonable expectation of decreased dehydrogenation temperature. The kinetic barriers for Cu-doped (001) and (110) surfaces are, respectively, reduced to 1.83 and 1.48 eV, showing that improved kinetics can be expected due to the much lower desorption barriers on both Cu-doped surfaces. The present results are beneficial to resolve the disputes between previous reports on the catalytic effects of Cu-doping in the MgH2 system. possesses the poorest desorption kinetic properties.8 On the other hand, MgH2 doped with Cu, Al, and Zn is activated, and the addition of Cu can both lower the dehydrogenation temperature to 270 °C and accelerate the desorption speed of MgH2.15 Moreover, the first-principles calculations of Al-, Ti-, Fe-, Ni-, Cu-, and Nb-doped MgH2 systems showed that these alloying elements thermodynamically destabilize the magnesium hydride with decreasing order of effect from Cu, Ni, Al, Nb, and Fe to Ti.16 Zeng and co-workers17 also performed firstprinciples calculations of 3d transition-metal-doped MgH2 systems and reported that the Cu dopant causes the best result in improving Mg−H dissociation due to the larger average Mg−H distance as well as strong Cu−H interaction. The disagreement between these mentioned experimental and theoretical discoveries urged us to study the destabilization and catalytic effects of Cu-doping on the MgH2 system. Because the hydrogen desorption of MgH2 mainly occurs on surfaces or interfaces, the details about energetics and electronic structures of the doped MgH2 surface require a full investigation.

1. INTRODUCTION Because of its clean combustion products and sustainability, hydrogen energy has become one of the most potential candidates to replace fossil fuels. However, the transportation of hydrogen energy must overcome the challenges from developing safe and cost-effective storage methods. Solid hydrides are promising high content hydrogen storage systems. Among them, MgH2 has attracted wide attention in the last two decades because of its high hydrogen storage capacity (7.9 wt %), together with the low cost and low density of magnesium. Unfortunately, the relatively high dehydrogenation temperature and poor dehydrogenation kinetics hampered practical applications of MgH2.1−3 There were dozens of attempts to improve the dehydrogenation behavior of MgH2. It is revealed that alloying MgH2 with transition metals can lower the dehydrogenation temperature as well as speed up the H2 desorption.4−13 Hanada et al.14 studied the thermodynamics properties of nanoparticle Fe, Co, Ni, and Cu catalyzed MgH2 and found that, with increasing 3d-electron numbers, the dopants reduce the desorption peak temperatures, except for Cu. The investigation of desorption kinetics of Al-, Ti-, Fe-, Ni-, Cu-, and Nb-doped MgH2 showed that the Cudoped MgH2 system with the largest formation enthalpy © XXXX American Chemical Society

Received: February 26, 2014 Revised: June 3, 2014

A

dx.doi.org/10.1021/jp502031h | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. Top (a) and side (b) views of MgH2 (001) surface slab model. Green (blue) spheres denote Mg (H), orange ball denotes substitution site, and the two gray balls (labeled as Int. 1 and Int. 2, respectively) denote two interstitial sites.

Figure 2. Top (a) and side (b) views of MgH2 (110) surface slab model. Green (blue) spheres denote Mg (H), orange ball denotes substitution site, and gray ball (labeled as Int.) denotes interstitial site.

2. COMPUTATIONAL DETAILS All calculations were carried out with density function theory (DFT) based on the PW91 exchange-correlation function,23 and a plane-wave basis set with the projected augmented wave method,24 as implemented in the Vienna Ab initio Simulation Package (VASP).25,26 The (001) surface is simulated by a (2 × 2) slab model with 6 layers containing 72 (Figure 1) atoms, and the (110) surface is simulated by a (2 × 2) slab model with 8 layers containing 96 atoms (Figure 2) after convergence checking of layer thickness (see Table S1 in the Supporting Information). After adequate testing, the vacuum in the z direction was set as 14 Å to avoid interactions between replicated slabs. The energy cutoff was set as 550 eV in all calculations, a 7 × 7 × 7 Monkhorst−Pack k-mesh was used in bulk MgH2 optimization and a 3 × 3 × 1 Monkhorst−Pack kmesh was used in slab calculations. The convergence of total energy was 0.1 meV·cell−1. For the ionic relaxation of slab models, the absolute magnitude of force on each atom was below 0.02 eV·Å−1. The kinetic effect of Cu-doping was investigated by a onestep direct dehydrogenation method27 in which two neighboring H atoms bounded weakly to the surface were simultaneously desorbed to form a H2 molecule. The variation of total

The density functional theory (DFT) calculations of low index MgH2 (001) and (110) surfaces showed that the MgH2 (110) surface is more stable and has a lower desorption energy barrier than the MgH2 (001) surface.18 DFT calculations of Al-, Ti-, Mn-, and Ni-doped MgH2 (001) and (110) surfaces illustrated the site preferences and destabilization mechanisms of different dopants in both the surfaces.19,20 The combination of doping and strain effects of Al, Si, and Ti on the MgH2 (001) surface was found to have maximal improvement in dehydrogenation enthalpy (ΔH).21 On the basis of DFT calculations, Wang et al.22 showed that Ti-doping reduces the kinetic energy barrier of H2 desorption by 0.41 eV (a 22% drop) and causes a concerted mechanism of synchronized diffusion of H atoms on MgH2 (110) surfaces. With the inspiration of these works, we used first-principles calculations to investigate thermodynamics and desorption kinetics of Cudoped (001) and (110) MgH2 surfaces. By analyzing the electronic structures and desorption barriers of the doped system, we tried to provide more evidence to clarify the disputes between previous reports on the thermodynamic destabilization and catalytic effects of Cu-doping on the MgH2 system B



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magnesium ions indicate that the bonds between magnesium and hydrogen are mainly ionic, which is consistent with other DFT calculations.17,18,27 Figure 3b,d displays the charge density difference between the surface and the superposition of atomic density. The increase of electron density around H atoms and the region of charge depletion around Mg atoms also indicate the ionic character of the Mg−H bond. By using the Bader charge analysis in molecule AIM theory,29−32 it is found that, on the (001) MgH2 surface, the number of electrons around Mg and H atoms is, respectively, 0.44 and 1.78, as shown in Table 2. The ionic charges of Mg and H can be represented as Mg1.56+ and H0.78−, which imply that Mg−H bonds are strong ionic. In addition, on the (110) surface, there are 0.41 and 1.79 electrons around Mg and H atoms, respectively, also indicating the stronger ionic character of Mg−H bonds. Figure 4 shows the charge density of the Cu-doped MgH2 (001) surface. The three-dimensional charge density isosurfaces in Figure 4a reveal that the Cu dopant bonds with four H ions to form a CuH4 cluster. As shown in Table 2, the Cu−H bond length is 1.62 Å, shorter than 1.66 Å calculated in a bulk doping system,17 indicating stronger Cu−H interactions. The H1− Cu−H2 and H1−Cu−H3 bond angles are 108.5° and 129.0°, respectively. Therefore, the geometry of this cluster is close to a tetrahedron. The 2D projection in Figure 4b indicates covalent Cu−H bonds, similar to the cases in the Cu-doped bulk MgH2 system.17 Additionally, the charge density difference displayed in Figure 4c shows that there are indeed increasing density areas along the Cu−H bond direction, which also implies the covalent nature of Cu−H bonds. Comparing with a free Cu atom, which has a 3d104s1 configuration, the number of electrons around the dopant Cu changes slightly to 11.03, as shown in Table 2. After Cu-doping, the Bader charge on H1 decreases significantly, whereas that on Mg1 dramatically increases. Therefore, the electron transfer from H to Mg is obviously weakened. The increase of Bader charge on the Cu atom is within numerical errors, which implies that the change of Cu net charge is imperceptible. These changes indicate that the electron transfer is from H ions to the Cu atom through Cu−H interactions and then from Cu back to Mg through Mg−Cu interactions. The electron transfer from Mg to H atoms is partially reversed by Cu−H and Cu−Mg interactions. As shown in Table 2, the Mg−H bond length increases after Cu-doping, similar to the lengthening trends of Mg−H bonds observed in a Cu-doped bulk MgH2 system.17 Therefore, the Mg−H bonds are weakened by Cu-doping. The charge density of the Cu-doped MgH2 (110) surface is displayed in Figure 5, in which the bridging (in-plane) H atoms are labeled as BH (PH) in the topmost layer and Mg atoms connected with BH (PH) are labeled as BM (PM). The Cu dopant also bonds with four H atoms to form a CuH4 cluster, with Cu−H bond lengths ranging from 1.71 to 1.60 Å. The PH−Cu−PH bond angle is 149.3°, and the BH−Cu−BH bond angle is 85.4°, so the geometry of this cluster is more like a “seesaw” than a tetrahedron. The Cu-doping distorts the (110) surface, and the PM1 atom moves inward significantly to additionally bond with the BH2 atom. As shown in Table 3, the length of PM1−BH2 bond is 2.16 Å, which means that the bond is relatively weak. The Cu−H covalent bonds shown in Figure 5b,c also imply strong Cu−H interactions. Again, the Bader charge of Cu ion is 11.12. As shown in Table 3, the electron numbers of BH (PH)

energy was plotted as a function of the perpendicular distance of the two desorbing H atoms from the surface; thus, the hydrogen desorption energy barriers (Eact) were obtained.

3. RESULTS AND DISCUSSION 3.1. Cu Dopant Site Preference and Occupation Energy. The rutile-type crystal structure of α-MgH2 has a tetragonal symmetry (P42/mnm) with experimentally measured lattice parameters of a = 0.4501 nm and c = 0.3010 nm.28 The optimized lattice parameters of bulk MgH2 are a = 0.4505 nm and c = 0.3005 nm, being in very good agreement with available experimental and other theoretical results.18−20,28 The calculated surface energies (γ) of optimal MgH2 (001) and (110) surfaces are, respectively, 0.052 and 0.035 eV·Å−2, being consistent with other calculations.18−20 The smaller γ of the (110) surface than that of the (001) surface indicates that the (110) surface is more stable than the (001) surface, which is in accordance with previous investigation.18 For Cu-doped MgH2, both interstitial and substitution sites were considered in both (001) and (110) surfaces. There are one substitution and two symmetrically nonequivalent interstitial sites on the (001) surface, and one substitution and one interstitial site on the (110) surface. The site preference of Cu and the stability of doped surfaces relative to pure MgH2 surfaces can be identified by the occupation energies (Eocc), which were calculated via the following definition16,19 Eocc = Edoped − Epure − [nECu − mEMg ]

(1)

where (n = 1, m = 1) stands for substitution sites, (n = 1, m = 0) stands for interstitial sites, Edoped and Epure are, respectively, total energies for doped and clear surfaces, and ECu and EMg are, respectively, total energies per atom in fcc-Cu and hpc-Mg. The occupation energies of all sites labeled in Figures 1 and 2 are listed in Table 1. For a clearer understanding of the positions of Table 1. Occupation Energies (Eocc, in eV) and Coordinates after Relaxation for Different Sites fractional coordinates sites (001) (001) (001) (110) (110)

Sub. Int. 1 Int. 2 Sub. Int.

Eocc

x

y

z

2.061 1.290 1.864 2.391 1.964

0.500 0.750 0.730 0.250 0.506

0.500 0.500 0.731 0.250 0.370

0.328 0.330 0.389 0.417 0.424

different doping sites, one can refer to the atomic coordinates of the (001) and (110) slab models in Tables S2 and S3 in the Supporting Information. From Table 1, one can note that all occupation energies are positive, which implies that the surfaces are destabilized by Cu-doping. Obviously, the Cu dopant prefers to occupy interstitial site Int. 1 and site Int. on (001) and (110) surfaces, respectively. Moreover, the doping energy of Cu on the (110) surface is larger than that on the (001) surface, indicating that it is harder to dope Cu in the (110) surface of MgH2. In following calculations and investigations, only preferred sites would be considered. 3.2. Geometry and Charge Density. The valence electron densities of (001) and (110) MgH2 surfaces are shown in Figure 3. In Figure 3a,c, the strongly localized electrons around the hydrogen ions and the low densities on C



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Figure 3. Projection of (a) total electron density and (b) electron density difference on the MgH2 (001) surface and the projection of (c) total electron density and (d) electron density difference on the MgH2 (110) surface. In (a) and (c), the green (blue) balls denote Mg (H) atoms, and in (b) and (d), the red (blue) contours denote regions of charge buildup (depletion).

H s states; near the Fermi energy, the interactions between Mg p and H s states are dominant, similar to reported calculations.17,19,27 Figure 7 shows the density of states of the Cu-doped (001) surface. The amount of spin-polarization in the Cu-doped (001) surface is minor due to the high symmetric distribution of spin-up and spin-down parts of the DOS. Therefore, only the spin-up part is plotted for the projected DOS, whereas, for the total DOS, both the spin-up and spin-down parts are drawn. Because of Cu-doping, the Fermi level shifted about 3 eV upward to higher energy, and the bottom of the spin-up conduction band (CB) is below it. Obviously, the states in the bottom of the CB are mainly contributed by Mg s, Mg p, and H s states. The occupation of these states indicates that the Mg− H bonds are much easier to break on the Cu-doped (001) MgH2 surface due to the easy mobility of the valence electron in the CB.27 The valence band (VB) can be separated into three parts. The lower part from −10.0 to −7.0 eV is mainly contributed by hybridization of Mg s, Mg p, and H s states. Comparing with the DOS of the pure MgH2 (001) surface, the peaks of strong Mg−H s−s hybridizations in the lower part shift below −8.0 eV, and the magnitude of these peaks decreases significantly. In the middle part from −7.0 to −3.0 eV in Figure 7, Cu−H d−s hybridization peaks dominate from −7.0 to −6.0 eV and from −4.0 to −3.0 eV, while the rest of this part is mainly contributed by Mg−Cu s−d hybridization. This clear picture of Cu−Mg hybridization is the evidence of the Cu−Mg interactions discussed above. Moreover, the hybridizations between Mg and H states dramatically drop in the middle part. Similar trends of reduced Mg−H p−s hybridizations were reported in a Cu-doping bulk MgH2 system.16,17 The reduced

Table 2. Bader Charges and Bond Length (in Å) for Pure and Cu-Doped (001) Surfaces of MgH2 Mg1 bader charge

bond length

pure doped

0.44 0.53

pure doped

H1

H1

1.78 1.78 1.57 1.57 Mg1−H1 1.83 1.98

Mg2

H1

Cu

0.41 1.78 0.42 1.57 11.03 H1−Mg2 H1−Cu 2.01 2.33

1.62

ions decrease to 1.61 (1.58) and the ones of BM (PM) atoms increase to 0.43 (0.47), indicating the similar electron transfer mechanism on the (110) surface as discussed above. Moreover, it can be seen from Table 3 that the BM−BH1 bond and especially the PM1−PH bond are largely lengthened. Comparing with the (001) surface, the Mg−H bond lengths are longer on the (110) surface. Therefore, the Mg−H bonds on the (110) MgH2 surface are more weakened after Cudoping. 3.3. Density of States. The density of states (DOS) of pure MgH2 (110) and (001) surfaces is shown in Figure 6. The energy gaps between the valence and conduction bands are 2.88 eV in the (001) surface and 2.86 eV in the (110) surface, respectively. Comparing with the gap about 4.2 eV in bulk MgH2, the gaps in (001) and (110) surfaces are narrower. Similar narrowed gaps of the MgH2 (001) surface are reported by other calculations.19 The diminished gaps imply that Mg−H bonds are more susceptible to dissociation.27 Moreover, the gap in the (110) surface is 0.02 eV lower than that in the (001) surface, indicating that Mg−H bonds are easier to dissociate on the (110) surface. In both surfaces, the lower part of the valence band is mainly contributed by interactions of Mg s and D



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Figure 4. Charge density for the top two layers of the Cu-doped MgH2 (001) surface: (a) Side view of 3-D isosurfaces with values of 0.37 e·Å−3 (yellow) and 0.10 e·Å−3 (gray). (b) Projection along z direction. The green, red, and blue balls denote Mg, Cu, and H atoms, respectively. (c) Projection of charge density difference along z direction. The red (blue) contours denote regions of charge buildup (depletion).

Figure 5. Charge density for the topmost two layers of the Cu-doped MgH2 (110) surface. Bridging H atoms (in-plane H) are labeled as BH (PH) in the first layer, and the Mg atoms bonded to BH (PH) are labeled as BM (PM): (a) Top view for 3-D isosurfaces with values of 0.38 e·Å−3 (yellow) and 0.13 e·Å−3 (gray). (b) Projection of charge density along z direction. The green balls denote Mg, the blue balls denote H, and the red ball denotes Cu. (c) Projection of charge density difference along z direction. The red (blue) contours denote regions of charge buildup (depletion).

Table 3. Bader Charges and Bond Length (in Å) for Pure and Cu-Doped (110) Surfaces of MgH2 bader charge

bond length

pure doped pure doped

BM

BH1

PM1

0.41 0.43 BM−BH1

1.78 1.61 PM1−PH

0.42 0.46 PM2−PH

1.81 1.58 Cu−BH1

0.42 0.47 Cu−PH

11.12 PM1−BH2

1.94 1.92

1.94 2.74

1.71

1.6

3.35 2.16

1.86 1.98

hybridizations between Mg and H states cause weakened ionic Mg−H bonds. During the bond weakening, a fraction of electron transfer might be reversed as discussed above, leading to increased electron density around Mg atoms and decreased

PH

PM2

Cu

electron density around H atoms. This interpretation is supported, to a large extent, by the above Bader charge data. Comparing with the pure (001) MgH2 surface, the band gap located in the third part from −3.0 eV to the Fermi level is E



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Figure 6. Density of states of the pure MgH2 (001) surface (a) and (110) surface (b), in which the Fermi level is set at zero energy.

Figure 7. Density of states of the Cu-doped (001) MgH2 surface. The Fermi level is set at zero energy and marked by the vertical dotted line. The labels of atoms are the same as those in Figure 4.

Figure 8. Density of states of the Cu-doped (110) MgH2 surface. The Fermi level is set at zero energy and marked by the vertical dotted line. The labels of atoms are the same as those in Figure 5.

narrowed from 2.88 to 2.74 eV after Cu-doping. The drop of gap means that exciting an electron into the CB for dissociating the Mg−H bond would require less energy. Therefore, the Mg−H bonds are easier to break after Cu-doping. The density of states calculated for the Cu-doped (110) MgH2 surface is shown in Figure 8. Similar to the situation in the Cu-doped (001) surface, the spin polarization effect is weak due to the high symmetry of spin-up and spin-down parts. The bottom of the CB is also below 0.0 eV because the Fermi level upwardly shifts about 3 eV to higher energy region. Again, the occupied bottom of the CB indicates that the dissociation of Mg−H bonds is more susceptible.

The lower part of the VB ranging from −10.0 to −8.0 eV also arises from Mg s and H s states. The similar downward shift and lowered peaks of Mg−H s−s hybridizations can also be observed, indicating the weakened Mg−H bonds after Cudoping. In the middle part from −8.0 to −3.0 eV, Cu−H d−s hybridization peaks are observed from −8.0 to −6.0 eV and from −4.0 to −3.0 eV, whereas Mg−Cu s−d hybridization peaks dominate the rest. It seems that Cu d states insert in the middle part and then hybridize with Mg p and H s states separately in a different energy region. The electrons in Mg−H p−s hybridization states are pushed to Cu−Mg d−p and Cu−H F



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Figure 9. Energy profiles and the transition states for desorption of two H atoms from pure and Cu-doped MgH2 (a) (001) surfaces and (b) (110) surfaces. The green, red, blue, and orange balls represent Mg atoms, to be desorbed H atoms, other H atoms, and Cu atoms, respectively.

Table 4. Activation Energy Barriers (in eV) for H2 Desorption from Pure Cu-Doped (001) and (110) Surfaces of MgH2 pure (001) DFT calculations

this work a

Cu-doped (001)

pure (110)

Cu-doped (110)

1.83

1.78a 1.83c 1.90d 2.25

1.48

2.86a 2.86b 2.75d 2.90

Reference 18. bReference 27. cReference 22. dReference 33.

weakly bound to the surface simultaneously desorb to form molecular hydrogen. Similar to the studies in refs 18 and 22, the H1 (H3) atoms on the (001) surface and the BH1 (PH) atoms on the (110) surface were chosen as desorbing hydrogen atoms due to their destabilized bonds with adjacent Mg atoms. The initial reactant states (RS) were chosen as the relaxed equilibrium configurations of these pure and doped surfaces. The final product states (FS) were made by removing a pair of H atoms from the surface and reintroducing them as a H2 molecule at a distance of more than 5.0 Å from the surface. Then, a chain of images lying in between the RS and FS geometries were constructed and relaxed at their fixed masscenter height of the desorbing hydrogen molecule.27 It must be emphasized that the contribution from zero-point energy (ZPE) is not included in the one-step dehydrogenation calculations. The reason is that ZPE correction is relatively insignificant since it can be treated as a constant offset in the MgH2 system.18 The variations of total energy as functions of the distance of the two desorbing H atoms from the surfaces are plotted in Figure 9. It can be seen that the energies of the FS along the one-step dehydrogenation pathways on pure (001) and (110) surfaces of MgH2 are 1.60 and 1.59 eV, respectively. The values are also close to the reported results.18 The transition states during the one-step dehydrogenation process of pure MgH2 (001) and (110) surfaces are similar to that of nudged elastic band (NEB) calculations.18 The activation barriers (Eact) are the energies of transition states (TS) relative to the energy of RS. As listed in Table 4, the Eact are 2.90 and 2.26 eV for (001) and (110) surfaces of pure MgH2, respectively. The values are close to previous NEB calculations18,22,33 and one-step dehydrogenation calculations.27 The barrier of the (001) surface is higher than that of the (110) surface, consistent with other calculation results.18 The lower Eact of the (110) surface is due to the narrower band gap in DOSs. The energies of the FS of Cu-doped (001) and (110) surfaces of MgH2 are 1.20 and 0.64 eV, respectively. Compared with pure surfaces, the lowered energy of the FS is associated

d−s hybridization states. The Mg−H hybridizations in this region are significantly weakened. As shown in Table 3, it is reasonable to deduce that Cu−H interactions cause electron transfer from the H ion to the Cu atom and Cu−Mg interactions cause electron transfer from the Cu atom back to the Mg ion. The “cooperation” effect of Cu−H and Cu−Mg interactions results in a fraction electron transfer from H to Mg through the Cu dopant, which acts as the “bridge”, consistent with the increase of electron density on Mg atoms and the decrease on H atoms. Obviously, the Mg−H bonds are weakened by Cu-doping because of the reduced magnitude of Mg−H hybridization peaks. Moreover, the drop of magnitude of hybridization peaks of BH1 is more than that of PH, which means that BM1−BH1 bonds are more destabilized than PM2−PH bonds. In the higher energy region from −3.0 eV to the Fermi level, the band gap drops to 2.1 eV. The reduction is more dramatic than in the (001) MgH2 surface with Cudoping, causing Mg−H bonds to be much easier to break. In both surfaces, the mixing of Cu d and H s states in the valence band pushes Mg−H s−s hybridization peaks toward a lower energy range and lowers the magnitude of them. In addition, Cu−Mg d−p strong hybridizations also decrease the Mg−H p−s mixing. Clearly the d states of Cu played a crucial role in weakening the Mg−H interactions and Cu-doping largely destabilizes the MgH2 surfaces. Thus, the decrease of dehydrogenation temperature reported in ref 15 can be predicted. Furthermore, the narrowed gaps and occupied bottom of the CB together indicate that the Mg−H bonds are more susceptible to dissociate. 3.4. Activation Energy Barrier. Although the Mg−H bonds are significantly weakened and easier to break after Cudoping, the existence of band gaps in the DOSs of Cu-doped MgH2 surfaces reminds us that the kinetic barriers might still be relatively large to hamper the dehydrogenation process. Therefore, the kinetics properties of Cu-doped MgH2 surfaces need to be investigated. To calculate the desorption energy barriers on pure and Cu-doped surfaces, the one-step direct dehydrogenation method27 was adopted, in which two H atoms G



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Figure 10. Variations of DOSs and band gaps for MgH2 surfaces along H2 desorption paths from S1 (near RS) through TS to S5 (near FS): (a) pure (001), (b) Cu-doped (001), (c) pure (110), and (d) Cu-doped (110) surfaces. The Fermi levels are set at zero energy and marked by vertical dotted lines.

with interactions between H-vacancies in the Cu-doping system.18 Cu-doping dramatically reduces Eact to 1.83 eV (176 kJ·mol−1) and 1.48 eV (143 kJ·mol−1) for (001) and (110) surfaces, respectively. Both surfaces show considerably dropped energy barriers. Although the one-step dehydrogenation pathway gives up-limit estimates27 about the energy barriers, the barriers of Cu-doped surfaces calculated by the one-step dehydrogenation are still lower than the barriers of pure MgH2 surfaces calculated by the NEB method. Therefore, the decrease of energy barriers can be expected in Cu-doped (001) and (110) MgH2 surfaces. To further reveal the mechanism behind the decrease of Eact, the variations of density of states along desorption paths are plotted in Figure 10. Again, because the amount of spin-polarization in Cu-doped surfaces is minor, only spin-up DOSs are plotted. It can be seen that, for pure and doped surfaces, along the desorption path from S1 (near RS) to TS, the band gaps are narrower, the system energies are higher, and thus the “activation energies” are lower, although the critical gap reduction for pure and doped surfaces occurs, respectively, above and below the Fermi level. Furthermore, from TS to S5 (close to FS), the opposite tendency is exhibited. Therefore, Figure 10 further reveals a clear correlation of band gap reduction with reduction of the activation energy for desorption, and this correlation of gap reduction with decrease of the “activation energy” is consistent with the discussion of DOS above. Therefore, the reduction of calculated desorption barriers of Cu-doped surfaces is caused by the upward shift of the Fermi level into the bottom of the CB and the decrease of energy gaps. The geometry rearrangement for TSs of doped surfaces is much greater than that of pure ones, as shown in Figure 9. The Cu atoms move inward in the transition states of both surfaces,

whereas the Cu atom moves 0.468 Å deeper in the (001) surface. Other DFT investigations showed similar movinginward trend of a Ti dopant on a MgH2 (110) surface.22 The TS obtained through the one-step dehydrogenation method may not be considered as the real TS. However, by adequate relaxation and testing of configurations near the TS, our “TS” would be very close to the real one in both geometry and energy, and for pure MgH2 (001) and (110) surfaces, our present calculation results are comparable with NEB calculations.18,22,27,33 Comparing with the reported 22% (0.41 eV) reduction of Tidoping on a (110) MgH2 surface,22 Cu-doping has a much better catalytic effect. Because the PW91-GGA functional usually underestimates the band gaps,27 the degree of drop of kinetic barriers is inevitably affected. However, GGA brings about the similar underestimation of gaps for both pure and doped surfaces, and the decreasing tendency of barriers on Cudoped surfaces is reliable and reasonable, indicating that Cudoping could improve the dehydrogenation properties of MgH2. The present Eact for Cu-doped (110) and (001) surfaces of MgH2 are close to the experimental result of 160 ± 11 kJ·mol−1 in a MgH2/MgCu2 system.34 However, it should be emphasized that the experimental measurements reflect an average effect over many microscopic processes involved in the dehydrogenation process, such as hydrogen atom or vacancy diffusion, phase nucleation and growth, surface desorption, etc. Although making direct comparison with these experimental results is not adequate, our work shows that the significantly reduced desorption barriers are obtained by Cu-doping. Therefore, the calculation results of our study are helpful to reveal the effect of Cu-doping on the dehydrogenation kinetics from MgH2. Additionally, it is reasonable to expect that the H



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(5) Larsson, P.; Araújo, C. M.; Larsson, J. A.; Jena, P.; Ahuja, R. Role of Catalysts in Dehydrogenation of MgH2 Nanoclusters. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 8227−8231. (6) Song, M. Y.; Kwon, S. N.; Kwak, Y. J.; Park, H. R. Improvement of Hydrogen-Storage Properties of MgH2 by Addition of Li3N, LiBH4, Fe and/or Ti. Mater. Res. Bull. 2013, 48, 74−78. (7) Liang, G.; Huot, J.; Boily, S.; Neste, A. V.; Schulz, R. Catalytic Effect of Transition Metals on Hydrogen Sorption in Nanocrystalline Ball Milled MgH2−Tm (Tm=Ti, V, Mn, Fe and Ni) Systems. J. Alloys Compd. 1999, 292, 247−252. (8) Shang, C. X.; Bououdina, M.; Song, Y.; Guo, Z. X. Mechanical Alloying and Electronic Simulations of (MgH2+M) Systems (M=Al, Ti, Fe, Ni, Cu and Nb) for Hydrogen Storage. Int. J. Hydrogen Energy 2004, 29, 73−80. (9) Choi, Y. J.; Lu, J.; Sohn, H. Y.; Fang, Z. Z. Hydrogen Storage Properties of the Mg−Ti−H System Prepared by High-Energy−HighPressure Reactive Milling. J. Power Sources 2008, 180, 491−497. (10) Choi, Y. J.; Lu, J.; Sohn, H. Y.; Fang, Z. Z.; Rönnebro, E. Effect of Milling Parameters on the Dehydrogenation Properties of the Mg− Ti−H System. J. Phys. Chem. C 2009, 113, 19344−19350. (11) Yu, X. B.; Guo, Y. H.; Yang, H.; Wu, Z.; Grant, D. M.; Walker, G. S. Improved Hydrogen Storage in Magnesium Hydride Catalyzed by Nanosized Ti0.4Cr0.15Mn0.15V0.3 Alloy. J. Phys. Chem. C 2009, 113, 5324−5328. (12) Zahiri, B.; Danaie, M.; Tan, X.; Amirkhiz, B. S.; Botton, G. A.; Mitlin, D. Stable Hydrogen Storage Cycling in Magnesium Hydride, in the Range of Room Temperature to 300 °C, Achieved Using a New Bimetallic Cr-V Nanoscale Catalyst. J. Phys. Chem. C 2011, 116, 3188− 3199. (13) Zhou, C.; Fang, Z. Z.; Ren, C.; Li, J.; Lu, J. Effect of Ti Intermetallic Catalysts on Hydrogen Storage Properties of Magnesium Hydride. J. Phys. Chem. C 2013, 117, 12973−12980. (14) Hanada, N.; Ichikawa, T.; Fujii, H. Catalytic Effect of Nanoparticle 3d-Transition Metals on Hydrogen Storage Properties in Magnesium Hydride MgH2 Prepared by Mechanical Milling. J. Phys. Chem. B 2005, 109, 7188−7194. (15) Milanese, C.; Girella, A.; Bruni, G.; Berbenni, V.; Cofrancesco, P.; Marini, A.; Villa, M.; Matteazzi, P. Hydrogen Storage in Magnesium−Metal Mixtures: Reversibility, Kinetic Aspects and Phase Analysis. J. Alloys Compd. 2008, 465, 396−405. (16) Song, Y.; Guo, Z.; Yang, R. Influence of Selected Alloying Elements on the Stability of Magnesium Dihydride for Hydrogen Storage Applications: A First-Principles Investigation. Phys. Rev. B 2004, 69, 094205. (17) Zeng, X. Q.; Cheng, L. F.; Zou, J. X.; Ding, W. J.; Tian, H. Y.; Buckley, C. Influence of 3d Transition Metals on the Stability and Electronic Structure of MgH2. J. Appl. Phys. 2012, 111, 093720. (18) Du, A.; Smith, S. C.; Yao, X. D.; Lu, G. Q. Ab Initio Studies of Hydrogen Desorption from Low Index Magnesium Hydride Surface. Surf. Sci. 2006, 1854−1859. (19) Dai, J. H.; Song, Y.; Yang, R. Intrinsic Mechanisms on Enhancement of Hydrogen Desorption from MgH2 by (001) Surface Doping. Int. J. Hydrogen Energy 2011, 36, 12939−12949. (20) Dai, J. H.; Song, Y.; Yang, R. First Principles Study on Hydrogen Desorption from a Metal (=Al, Ti, Mn, Ni) Doped MgH2 (110) Surface. J. Phys. Chem. C 2010, 114, 11328−11334. (21) Hussain, T.; Sarkar, A. D.; Maark, T. A.; Sun, W.; Ahuja, R. Strain and Doping Effects on the Energetics of Hydrogen Desorption from the MgH2 (001) Surface. Europhys. Lett. 2013, 101, 27006. (22) Wang, L.-L.; Johnson, D. D. Hydrogen Desorption from TiDoped MgH2 (110) Surfaces: Catalytic Effect on Reaction Pathways and Kinetic Barriers. J. Phys. Chem. C 2012, 116, 7874−7878. (23) Perdew, J. P.; Wang, Y. Accurate and Simple Analytic Representation of the Electron-Gas Correlation Energy. Phys. Rev. B 1992, 45, 13244−13249. (24) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953−17979.

diffusion of hydrogen atoms and vacancies through bulk MgH2 and interfaces may also significantly influence the overall kinetic properties of hydrogen release. Therefore, further investigation is needed.

4. CONCLUSION In this article, the Cu-doped MgH2 (001) and (110) surfaces are investigated by employing DFT calculations. Because of the lower occupation energy, the Cu dopant prefers to occupy the interstitial site rather than substitute a Mg atom. The Cu atom bonds with four adjacent H atoms to form tetrahedral and seesaw CuH4 clusters on (001) and (110) surfaces, respectively. The electron density and Bader charge show that the Cudoping lengthens the Mg−H bond and reverses a fraction of electron transfer from Mg to H atoms. Therefore, the Mg−H bonds are significantly weakened after Cu-doping. The density of states further demonstrates that the Cu−Mg and Cu−H interactions together weaken the Mg−H interaction. Therefore, the expectation of dropped dehydrogenation temperature is reasonable due to the destabilization effects of Cu-doping. The one-step dehydrogenation method calculation shows that Cudoping dramatically reduces Eact to 1.83 eV (176 kJ·mol−1) and 1.48 eV (143 kJ·mol−1) for (001) and (110) surfaces, respectively. Therefore, the catalytic effect of Cu-doping on dehydrogenation kinetics of the MgH2 system can also be expected. The present investigation shows that Cu-doping can improve both thermodynamics and kinetics of MgH2. To gain more detailed insights into the kinetic improvement effect of Cu-doping, further investigations on vacancy diffusion and phase nucleation in the Cu-doped MgH2 system are still needed.

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ASSOCIATED CONTENT

S Supporting Information *

The convergence test on slab thickness and the atomic coordinates of (001) and (110) slab models. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 731 58292195. Fax: +86 731 58292468. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the NSFC (51071053) is gratefully appreciated. REFERENCES

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First-Principles Investigation of Dehydrogenation on Cu-Doped MgH2

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