Hydrogen Interaction with the Al Surface Promoted by Subsurface

Article pubs.acs.org/JPCC

Hydrogen Interaction with the Al Surface Promoted by Subsurface Alloying with Transition Metals Feng Zhang,† Yan Wang,† and M. Y. Chou†,‡,* †

School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332, United States Institute of Atomic and Molecular Science, Academia Sinica, Taipei 10617, Taiwan

‡

ABSTRACT: Dissociative chemisorption of H2 on the Al surface is a crucial step in the regeneration of promising hydrogen-storage materials such as alane and alanates. We show from first-principles calculations that transition metals such as V and Nb can act as effective catalysts for H2 interaction with Al(100). When located at subsurface sites, V and Nb can reduce the activation barrier for H2 dissociation by significantly larger values than the well-studied catalyst Ti. In addition, the binding energy of a H atom on the surface can be enhanced by as much as 0.4 eV when V or Nb is introduced in the sublayers of Al(100). The diffusion barrier for the adsorbed hydrogen is reduced by ∼0.1 eV, showing an increased hydrogen mobility. The mechanism of promoting the metal surface reactivity by subsurface alloying with transition metals proposed in this work may serve as a new possible scheme for catalytic reactions on the metal surface.

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INTRODUCTION With its abundance and high gravimetric hydrogen capacity, aluminum is a promising medium for on-board hydrogen storage, a major step in the development of hydrogen-powered vehicles.1 Aluminum hydride (alane) AlH3 contains 10.1 wt % H and is thermodynamically metastable at room temperature with a reasonably long lifetime. At T ∼ 100 °C, the temperature suitable for the operation of hydrogen fuel cells, AlH3 readily decomposes into Al and H2. However, the low cohesive energy of bulk AlH 3 makes its regeneration from Al and H 2 unattainable under practical conditions. On the other hand, it has been demonstrated by a careful study using complementary scanning tunneling microscopy (STM) and surface infrared (IR) measurements that mobile AlH3 oligomers can be easily formed on the Al surface if atomic hydrogen is available.2 Recently, a new route was proposed for the regeneration of bulk AlH3, in which the AlH3 oligomers on the Al surface react with a solution containing an electron donor, L (for example, amine or ether), to form an intermediate adduct AlH3−L. Bulk AlH3 can be obtained with the decomposition of AlH3−L.3−5 Alternatively, hydrogen can be stored in complex alanates Mx(AlH4)y, where M is generally an alkali or alkali-earth metal. Mx(AlH4)y has a lower gravimetric density of H than AlH3, but is more stable than AlH3. For example, sodium alanate, NaAlH4, releases 5.3 wt % H in the following two-step reaction: NaAlH4 ↔ 1/3 Na3AlH6 + 2/3 Al + H2 ↔ NaH + Al + 3/2 H2. A small amount of catalyst, such as Ti, was found to greatly improve the kinetics so that the reaction can take place under moderate temperature and pressure conditions in both directions.6 This breakthrough has stimulated extensive experimental studies focusing on the microstructure and reaction kinetics on these compounds.7 Again, the dissociation of H2 and consequent formation of AlH3 oligomers on the Al surface is the key step in the regeneration of NaAlH4, since it has been reported that NaAlH4 can be formed by mixing the © 2012 American Chemical Society

AlH3 with NaH or Na3AlH6 without using a catalyst or hydrogen overpressure.8−10 Therefore, the regeneration of both AlH3 and Mx(AlH4)y can easily take place if a suitable catalyst can be found to facilitate H2 chemisorption on the Al surface. Indeed, it has been shown that a transition metal such as Ti can effectively catalyze the formation of AlH3-triethylenediamine adduct,3 as well as the regeneration of NaAlH4.6 Several theoretical calculations have also been performed to understand the effect of Ti on the dissociative adsorption of H2 on the Al surface.10−14 It was found that certain configurations with Ti located at the topmost layer of the Al surface can dissociate molecular hydrogen with ultralow activation barriers,11−13 However, these configurations turn out to be unstable compared with those with Ti located at subsurface sites.10,13,14 Furthermore, Ti in the top layer binds H atoms so strongly that the effective transport of adsorbed H atoms on the Al surface, a step necessary in the formation of AlH3 oligomers, is hindered. On the other hand, when Ti is located only at subsurface sites, the dissociation barrier for molecular H2 on Al(100) is found to decrease to ∼0.7 eV,13,14 which in general agrees with the experimentally obtained energy barrier for H2 uptake in the rehydrogenation of Ti-doped NaAlH4.15 This also suggests that H2 dissociation remains the rate-limiting process, even with the presence of Ti. Reference 10 also reported a structure containing only second-layer Ti atoms that has a very low energy barrier of 0.26 eV for H2 dissociation, but this seems to be inconsistent with experimental findings15 and energetics results from other calculations.13,14 Even with the help of Ti, the reaction conditions for the regeneration of either AlH3 or Mx(AlH4)y are still not completely optimal. Searching for possibly better catalysts for Received: June 28, 2012 Revised: August 8, 2012 Published: August 12, 2012 18663

dx.doi.org/10.1021/jp306377j | J. Phys. Chem. C 2012, 116, 18663−18668

The Journal of Physical Chemistry C

Article

H2 sorption is still an ongoing effort.16,17 In this work, we report that other transition metals, such as V and Nb, could have promising catalytic effects for H adsorption on Al(100). In addition, we provide a better understanding of how the catalyst effect depends on the position of the catalyst in the periodic table through a systematic examination of early transition-metal catalysts Sc, Ti, V, and Nb. We show in the following that the intriguing mechanism of promoting the metal surface reactivity by subsurface alloying with transition metals explains the effects of these catalysts.

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COMPUTATIONAL DETAILS We have performed calculations based on density-functional theory (DFT) with a plane-wave basis as implemented in the Vienna ab initio Simulation Package.18,19 For the exchangecorrelation functional, the generalized gradient approximation (GGA) in the form proposed by Perdew and co-workers20 is used. The electron−ion interaction is described by Vanderbilt ultrasoft pseudopotentials.21 The surfaces are modeled by periodic slabs containing 10 or 12 (001) atomic layers of Al separated by a vacuum region of 23 Å. Atomic H adsorption and H2 dissociation are studied in a 2 × 2 unit cell (in reference to the cubic unit cell of Al) with eight metal atoms per layer. The k-space integrals are evaluated using a 4 × 4 grid for the 2 × 2 unit cell in the DFT calculation and an 8 × 8 grid for evaluating the density of states (DOS). The plane-wave cutoff energy is 425 eV, the total-energy convergence is 10−5 eV, and the force convergence in structural relaxation is 0.01 eV/Å.

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Figure 1. Unit cells (1 × 1) of surface slabs of Al(100) doped with transition metals. Light and dark atoms represent Al and the transition metal dopant, respectively. The dopant coverage in configurations A− G is 1 ML; that in configuration α is 0.5 ML. Configuration β shows the combination of a slab with 1 ML coverage of dopant and a pure Al(100) slab for energy comparison (see text).

RESULTS AND DISCUSSIONS

Energetics and Atomic Configurations of Modified Al(100). We first determine the atomic configuration when a low level of transition metals is added to the Al surface. Previous studies have shown that the Al(100) configurations with Ti located at subsurface sites are more stable than that with Ti exposed at the topmost layer.10,13,14 In this section, we confirm that the subsurface sites are preferred for other transition metal additives, such as Sc, V, and Nb. We further examine the stability of surface configurations consisting of a local geometry of a bulk alloy against several nonalloying configurations near the surface. For the slabs with a local alloy geometry, it is reasonable to assume that the surface alloy takes the form of an Al-rich phase since the concentration of the dopant is low. As in ref 14, the trialuminide phase is selected for all the transition metal dopants in this work. ScAl3, TiAl3 and NbAl3 are the first stable alloys on the Al-rich side in the phase diagrams of Al−Sc, Al−Ti, and Al−Nb, respectively. VAl3 is also a stable alloy, although higher Al-composition phases exist in the Al−V phase diagram. Figure 1 shows the (1 × 1) unit cells of the surface-slab configurations considered in this study. In configurations A−D, the dopants and surrounding Al atoms form a local geometry of the trialuminide phase in either a D022 (A, C) or L12 (B, D) pattern, whereas no corresponding local geometry of a stable bulk alloy exists for configurations E−G. Configurations A−G all contain an equivalent 1 ML of dopants. Thus, their relative stability can be established by directly comparing the total energies, which are given in Table 1, where the total energy for configuration A is set to be the reference. Either configuration C or configuration D has the lowest energy, depending on the dopants studied. This indicates that the dopants prefer to form a local alloy structure with the neighboring Al atoms and to stay

at subsurface sites. For Sc, configuration D with a L12 local geometry has the lowest energy, but for other dopants, the D022 pattern (configuration C) is most stable. This preference of local geometry is consistent with the corresponding bulk trialuminide. Configurations A and B with the dopants exposed in the top layer have a significantly higher energy, which can be attributed to the fact that transition metals usually have high surface energy.22 Interestingly, the L12 local geometry (configuration B) is preferred for all the dopants when they are exposed on the top layer. This is because the interaction between dopants in the first and third layers is enhanced as a result of the shorter separation between them in the L12 pattern.23 Configurations E−G are unstable as a result of the lack of a local-alloying structure, although they also have all dopants located at subsurface sites (some of the dopants in configuration G are at an even deeper layer than in configuration C or D). To investigate the effect of hydrogen adsorption on the stability of the different configurations, we also calculated the total energy of configurations A−G with a hydrogen atom at the most stable site on the surface in a unit cell. The results are also included in Table 1. In general, the above conclusions for the bare surface slabs still hold when hydrogen is adsorbed. Configuration α with only one layer containing 0.5 ML of dopant atoms is another configuration that has been studied previously.13 Here, we further examine its stability against a combination of a slab with two dopant layers (configuration C or D) and a pure Al slab. Configuration β illustrates this combination, using configuration C as an example. In calculations related to Sc, the more stable configuration D is used instead. To determine whether alloying is favorable, we 18664



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Table 1. Total Energy (in units of eV per 1 × 1 unit cell) of Configurations A−G in Figure 1 Measured with Respect to Configuration Aa bare slabs dopant

A

B

C

D

Sc Ti

0 0

−0.50 −1.12

−0.81 −1.11

V Nb

0 0

−0.39 −0.44 −0.56b −1.20c −0.28 −0.19

Sc Ti V Nb

0 0 0 0

−0.15 −0.25 −0.18 0.13

F

G

ΔE = Eα − 1/2Eβ

0.39 −0.55 −1.16c

−0.41 −0.90

0.21 0.17

−0.62 −0.43

0.15 0.26

−0.03 −0.64 −0.51 −0.44

0.06 0.02 0.03 0.14

E 1.42 0.13 0.32c

−0.93 −0.91 −0.37 −0.70 −0.98 −0.80 0.46 −0.33 With 1 Hydrogen Atom Adsorbed Per Unit Cell −0.24 −0.47 1.69 0.63 −0.90 −0.87 0.12 −0.30 −0.89 −0.81 −0.64 −0.63 −1.02 −0.74 0.16 −0.36

a In the last column, ΔE = Eα−1/2Eβ measures the energy gain in the alloyed configurations. bResults from ref 23, which used the same 12-layer slabs to model the Al surface. cResults from ref 13, in which only four layers were used to model the Al surface in these calculations.

examine ΔE, defined as Eα − 1/2Eβ, where Eα is the total energy of configuration α, and Eβ is the sum of the total energies of the two surface slabs in configuration β. As shown in the last column of Table 1, ΔE remains positive for all the dopants for both empty and H adsorbed slabs. This indicates that configuration α is less stable than the combination of configuration D (C) and a pure Al slab for Sc (Ti, V, and Nb) additives. Therefore, in the following calculations, only configuration D for Sc and configuration C for other dopants will be considered. Single Hydrogen Adsorption and Diffusion on Al(100) with Subsurface Alloying. In this section, we study in detail hydrogen adsorption on different sites of the Al(100) surface modified by subsurface transition metals, as in configuration C or D. These include sites on top of Al (top site T), between two Al atoms (bridge site B), and surrounded by four Al atoms (hollow site H). When Al(100) is doped by subsurface transition metals, H1 (H2) represents the hollow site above a second-layer dopant (Al) atom. The energy of an adsorbed hydrogen atom Eads is defined with respect to that in the gas phase: ⎛ ⎞ n Eads = ⎜Eslab + nH − Eslab − E H 2⎟ /n ⎝ ⎠ 2

Table 2. Energy Defined in Eq 1 (in units of eV) of One Hydrogen Atom Adsorbed on a (2 × 2) Cell of Pure and Modified Al(100) adsorption site T B H1 H2

pure 0.29 0.08 0.50

Scmodified

Timodified

Vmodified

Nbmodified

0.09 −0.04 0.19 0.51

−0.12 −0.05 0.31 0.55

−0.33 −0.23 0.23 0.88

−0.31 −0.28 0.30 0.87

the least favorable and is locally unstable. The binding of hydrogen on both the bridge and top sites is enhanced when Al(100) is modified by subsurface transition metals, with the binding of the top site being enhanced most. For Sc, hydrogen still prefers the bridge site, and the adsorption energy is −0.04 eV, but the top site becomes more favorable for other transition metals with adsorption energies of −0.12, −0.33, and −0.31 eV for Ti, V, and Nb, respectively. The successive decrease of Eads with subsurface Sc, Ti, and V shows that the catalytic effect is enhanced by the increasing number of valence d electrons in the catalyst. However, the similar Eads for V- and Nb-modified surfaces indicates that the change from 3d to 4d valence electrons has a minimal influence on the energy of the adsorbed hydrogen. To understand the influence of subsurface transition metal catalysts on the electronic structure of the Al surface, Figure 3 compares the DOS projected on a surface Al atom for pure Al(100) and modified Al(100) with subsurface Sc, Ti, V, and Nb without hydrogen adsorbed. We use the L12 local arrangement for Sc and D022 for other three transition metals. The same radius of 1.4 Å for Al is used for the projection of DOS of all systems. Compared with pure Al(100), when the transition metal dopants are below the surface, the Al s states are slightly shifted to the lower energy, resulting in an increased Al s population from −9 to −5 eV. In addition, a more pronounced increase of Al p states is observed below the Fermi level. To see how these changes in the electronic states of surface Al promote hydrogen adsorption, we compare the projected DOS for pure and modified Al(100) (using Ti as an example) with adsorbed hydrogen, as shown in Figure 4. Al(1) [Al(2)] represents the surface Al atom that is closest to (farthest from) the H atom. For the pure Al(100) surface, the adsorption of H at the bridge site has a lower energy than the

(1)

where Eslab and Eslab+nH are the total energies of the empty slab and of the slab with n hydrogen atoms adsorbed, respectively. EH2 is the chemical potential of H2 in the gas phase. Here, we use the total energy of H2 calculated with GGA at T = 0. At finite temperature and pressure, the contributions from TS and PV terms in Eslab and Eslab+nH will largely cancel out, since the surface slab is almost unchanged during H adsorption. EH2 does significantly depend on temperature and pressure; however, it does not affect the relative stability of H atoms adsorbed on Al(100) with different catalysts, since the same EH2 value will be used to calculate Eads on different surfaces. Table 2 shows the adsorption energy of a hydrogen atom on the pure and the modified Al(100) surfaces. We used a 2 × 2 unit cell in the calculation so that the interaction between hydrogen atoms in neighboring cells is negligible. For an isolated hydrogen atom, the most stable adsorption site on pure Al(100) is the bridge site with an energy of 0.08 eV with respect to the gas phase. The top site is a metastable adsorption site with a higher Eads of 0.29 eV. The hollow site is 18665



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top site. As shown in Figure 4, the bridge-site H interacts mainly with the Al s orbital, while the top-site H interacts more with the Al p state. With the presence of subsurface Ti, the population of the s states ranging from −9 to −5 eV has slightly increased. The s state of bridge-site H also peaks at this energy interval. At the same time, the interaction of Ti d and Al p states evidenced from a common peak in the projected DOS for Ti d and Al p below the Fermi level causes a drastic increase of Al p population, which in turn leads to an enhanced binding between top-site H and Al. Indeed, when Ti is present below the surface, the top site becomes most stable for adsorbed hydrogen. No direct interaction is found between the subsurface Ti (or any other catalysts) and adsorbed H. Therefore, the role of the subsurface catalysts is to modify the properties of the Al surface, resulting in an enhanced binding of adsorbed H on the surface. The smaller difference in Eads between the top and bridge sites on modified Al(100) than on the pure Al surface as shown in Table 2 suggests that the diffusion barrier for the adsorbed H atom on the Al surface is also reduced with catalysts at the subsurface. This is confirmed in Figure 5, where the energy of

Figure 2. Four different sites for hydrogen adsorption on modified Al(100) by subsurface transition metals (top view). The arrows denote possible diffusion paths for the adsorbed H atom.

Figure 3. Projected DOS on a surface Al atom for pure Al(100) and Al(100) modified by subsurface Sc, Ti, V, and Nb. The Fermi level is at E = 0.

Figure 5. Energy (Eads) of an adsorbed H atom defined in eq 1 along the diffusion path.

the H atom is plotted along the diffusion path on Al(100) with and without subsurface catalysts. To construct the diffusion path, the horizontal position of the H atom is fixed on a straight line connecting the initial and final states while the height of the H atom is free to relax. Only the diffusion path with the lowest energy barrier is shown in Figure 5. Except for the Sc-modified surface, the diffusion path passes through nearest-neighboring bridge and top sites successively (path 1 in Figure 2). For the Al surface with subsurface Sc, H diffuses along path 2 in Figure 2 from one bridge site to its nearest-neighboring bridge site without going through the top sites. The energy barriers are 0.14, 0.09, 0.13, and 0.09 eV for a Sc-, Ti-, V-, and Nb-modified Al surface, respectively. All of them are significantly smaller than the diffusion barrier of 0.22 eV on pure Al(100). It should be noted that the actual diffusion barriers could be smaller than those calculated in this work because of the constraint on the horizontal position of the H atom. However, for the pure Al surface, there is no room for such reduction, since the calculated diffusion barrier is almost equal to the

Figure 4. Projected DOS for the pure and Ti-modified Al(100) surfaces with adsorbed H. Al(1) is the surface atom closest to H, and Al(2) is the farthest one in the 2 × 2 unit cell. The Fermi level is at E = 0.

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energy of about 0.9 eV, as calculated in previous studies.12−14 With catalysts at the subsurface, the most stable dissociated state is state I (Figure 7d). A possible starting point for the H2 molecule to reach state I would be from above the hollow site (H2 in Figure 2), as shown in Figure 7a. If we let the H2 unit approach the surface in the vertical plane denoted by the solid line in Figure 7a with the H2 unit parallel to the surface and centered above the H2 hollow site, it gives a high dissociation barrier of ∼1.0 eV, comparable to the value of 0.9 eV on pure Al(100). This is because the surface charge density above the hollow site is too low to give any catalytic effects. To identify other dissociation routes with lower barriers, we set the initial H2 molecule on top of a less symmetrical site, which is shifted from the H2 site to a nearest-neighboring bridge site, as schematically shown in Figure 7b, then we sample the potential energy surface by varying the separation of the two hydrogen atoms and the distance of the H2 unit from the surface. A similar method has been used to study the interaction between H2 and Al(110) surface.24 At each sampling point, we fix the H−H separation and the H2-surface distance and allow all the other degrees of freedom to relax.25 In particular, the center-ofmass of the H2 unit is free to move horizontally along the dashed line in Figure 7b to find the position with the lowest energy. The potential energy surface sampled in this way is shown in Figure 8, which shows a possible path for H2 dissociation. The energy of the H2 unit at the transition state defines the energy barrier for the dissociation, which is about 0.8, 0.6, 0.4, and 0.2 eV with the catalyst Sc, Ti, V, and Nb located at subsurface sites, respectively. Further analysis of the transition state show that for the Al(100) surface with different catalysts, the horizontal position of the H2 unit is close to the bridge site, as shown in Figure 7c, where the surface charge density is significantly larger than the H2 site (Figure 7a).

difference between the H energies at the top site and at the bridge site (see the top panel of Figure 5), which sets a lower bound for the diffusion barrier between these two sites. Therefore, more accurate calculations with the constraint lifted only further strengthen the conclusion that subsurface alloying with transition-metal catalysts reduces the diffusion barrier for adsorbed hydrogen. Pairing of Two Neighboring H Adatoms. The natural dissociated state for a H2 molecule has two hydrogen atoms adsorbed on two neighboring adsorption sites. For pure Al (100), we find that the two hydrogen atoms essentially do not interact with each other either on two neighboring bridge or top sites. Thus, the energy for each hydrogen is still welldescribed by the values in Table 2. However, with transition metals below the surface, the two hydrogen atoms at two bridge sites across an H2 site (state I in Figure 6) experience a fairly

Figure 6. Energy of each hydrogen atom defined in eq 1 along the linearly interpolated diffusion path connecting states I and state II on the pure Al(100) surface and the Al(100) surface modified by subsurface Sc, Ti, V, Nb. The top views for states I and II are also shown.

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strong attractive interaction mediated by the Al surface. This effectively reduces Eads and gives rise to a more stable state than that with two hydrogen atoms on top sites (state II in Figure 6). The interaction between the hydrogen atoms in state II is weak with or without sublayer dopants, since the energy of each hydrogen atom in state II is very close to that of a single isolated hydrogen atom at the top site (see Figure 6 and Table 2). Figure 6 also shows Eads of each hydrogen atom along a linear path connecting states I and II. It indicates that the energy required to separate the stable hydrogen pair in state I is relatively high for the Sc- and Ti-modified surfaces: 0.29 and 0.17 eV, respectively. However, this energy is only ∼0.05 eV for V- and Nb-modified surfaces, indicating a better H mobility. Dissociation of a H2 Molecule on Modified Al(100). In the following, we study the effect of the transition metal catalysts on the dissociation of a H2 molecule on the Al(100) surface. H2 dissociation on pure Al(100) requires an activation

CONCLUSION We have systematically studied hydrogen adsorption on Al(100) modified by four transition metalsSc, Ti, V, or Nbnear the surface from first-principles calculations to address a critical catalytic reaction for hydrogen storage systems. Instead of being exposed in the top layer, the transition metal atoms prefer to stay at subsurface sites and take a local structure of a stable Al-rich alloy in the bulk with or without hydrogen adsorption. Such unique arrangement of the transition metal catalysts on Al(100) enhances the interaction of hydrogen with surface Al atoms by modifying the electronic states of surface Al. As a result, the hydrogen adsorption energy decreases from 0.08 eV for pure Al(100) to −0.04, −0.12, −0.33, and −0.31 eV for Al(100) with subsurface Sc, Ti, V, and Nb, respectively. The change of hydrogen adsorption energy depends strongly on the number of valence d electrons in the

Figure 7. Top views of (a) an initial state with the H2 molecule on top of a hollow site, (b) an initial state with the H2 molecule between the hollow site and the bridge site, (c) the transition state with the H2 unit located near the bridge site, and (d) the final dissociated state. The solid and dashed lines denote the vertical reflection planes. 18667



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(2) Go, E.; Thuermer, K.; Reutt-Robey, J. Surf. Sci. 1999, 437, 377− 385. (3) Graetz, J.; Chaudhuri, S.; Wegrzyn, J.; Celebi, Y.; Johnson, J. R.; Zhou, W.; Reilly, J. J. J. Phys. Chem. C 2007, 111, 19148−19152. (4) Graetz, J. Chem. Soc. Rev. 2009, 38, 73−82. (5) Wong, B. M.; Lacina, D; Nielsen, I. M. B.; Graetz, J.; Allendorf, M. D. J. Phys. Chem. C 2011, 115, 7778−7786. (6) Bogdanović, B.; Schwickardi, M. J. Alloys Compd. 1997, 253, 1−9. (7) Jensen, C; Wang, Y; Chou, M. Y. In Solid-State Hydrogen Storage; Walker, G., Ed.; Woodhead Publishing Limited: Cambridge, England, p 381. (8) Finholt, A. E.; Barbaras, G. D.; Barbaras, G. K.; Urry, G.; Wartik, T.; Schlesinger, H. I. J. Inorg. Nucl. Chem. 1955, 1, 317−325. (9) Mamula, M.; Hanslik, T Collect. Czech. Chem. Commun. 1967, 32, 884−891. (10) Chaudhuri, S.; Graetz, J.; Ignatov, A.; Reilly, J. J.; Muckerman, J. T. J. Am. Chem. Soc. 2006, 128, 11404−11415. (11) Chaudhuri, S.; Muckerman, J. T. J. Phys. Chem. B 2005, 109, 6952−6957. (12) Du, A. J.; Smith, S. C.; Lu, G. Q Chem. Phys. Lett. 2007, 450, 80−85. (13) Chen, J.-C.; Juanes-Marcos, J. C.; Al-Halabi, A; Olsen, R. A.; Kroes, G.-J. J. Phys. Chem. C 2009, 113, 11027−11034. (14) Wang, Y.; Zhang, F.; Stumpf, R.; Chou, M. Y. Phys. Rev. B 2011, 83, 195419:1−5. (15) Luo, W.; Gross, K. J. J. Alloys Compd. 2004, 385, 224−231. (16) Li, L.; Qiu, F.; Wang, Y.; Wang, Y.; Liu, G.; Yan, C.; An, C.; Xu, Y.; Song, D.; Jiao, L.; Yuan, H. J. Mater. Chem. 2012, 22, 3127−3132. (17) Hu., J; Ren, S.; Witter, R.; Fichtner, M. Adv. Energy Mater. 2012, 2, 560−568. (18) Kresse, G; Furthmüller, J. Phys. Rev. B 1996, 54, 11169−11186. (19) Kresse, G; Furthmüller, J. Comput. Mater. Sci. 1996, 6, 15−50. (20) Perdew, J. P; Chewary, J. A; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C Phys. Rev. B 1992, 46, 6671− 6687. (21) Vanderbilt, D. Phys. Rev. B 1990, 41, 7892−7895. (22) Venables, J. A. Introduction to Surface and Thin Film Process; Cambridge University Press: Cambridge, U.K., 2000. (23) Spišaḱ , D; Hafner, J. Surf. Sci. 2005, 582, 69−78. (24) Gundersen, K; Jacobsen, K. W.; Nørskov, J. K.; Hammer, B. Surf. Sci. 1994, 304, 131−144. (25) The atomic relaxation is subject to the constraint of the reflection symmetry shown in Figure 7b.

Figure 8. Energy variation of an H2 unit approaching the Al (100) surface modified by Sc, Ti, V, and Nb as a function of the distance from the surface and the H−H separation. The energy is in reference to that of an isolated H2. Contour values are in units of eV, with positive and negative values denoted by solid and dashed lines, respectively.

transition metal, but only weakly on which shell the valence electrons are located. Another advantage of subsurface alloying is to avoid the direct interaction of hydrogen with the transition metal catalysts, which would be strong enough to make both hydrogen and the catalysts inactive for further reactions. In addition, the mobility of adsorbed hydrogen is improved with the diffusion barrier reduced by ∼0.1 eV, when the catalyst V or Nb is located at the second layer. The catalysts at subsurface sites also decrease the dissociation barrier for a hydrogen molecule on Al(100). The reduction in the dissociation barrier increases with the total number of d electrons in the catalyst. The Nb-modified Al(100) surfaces produces a dissociation barrier as low as 0.2 eV. This mechanism with dopants in a subsurface-alloyed configuration to act as catalysts for surface reactions may be a new scheme to consider for other surface catalytic systems.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Awards DEFG02-97ER45632 and DEFG02-05ER46229.

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REFERENCES

(1) Tollefson, J. Nature 2010, 464, 1262−1264. 18668


Hydrogen Interaction with the Al Surface Promoted by Subsurface

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