First-Principles Design of Iron-Based Active Catalysts for Adsorption

Article pubs.acs.org/JPCC

First-Principles Design of Iron-Based Active Catalysts for Adsorption and Dehydrogenation of H2O Molecule on Fe(111), W@Fe(111), and W2@Fe(111) Surfaces Ming-Kai Hsiao, Yi-Chun Hsieh, and Hui-Lung Chen* Department of Chemistry and Institute of Applied Chemistry, Chinese Culture University, Taipei, 111, Taiwan S Supporting Information *

ABSTRACT: The adsorption and dehydrogenation of water on Fe(111), W@Fe(111), and W2@Fe(111) surfaces have been studied via employing the first-principles calculations method based on the density functional theory. The three adsorption sites of the aforesaid surfaces, such as top (T), 3fold-shallow (S), and 3-fold-deep (D), were considered. The most favorable structure of all OHx (x = 0−2) species on the surfaces of Fe(111), W@Fe(111), and W2@Fe(111) have been thoroughly predicted and discussed. Our calculated results revealed that the adsorbed configurations of FeH2O(Tη1-O)-b, W@FeH2O(T-η1-O)-a, and W2@FeH2O(T-η1-O)-a possess energetically the most stable structure with their corresponding adsorption energies of −8.08, −13.37, and −18.61 kcal/mol, respectively. In addition, the calculated activation energies for the first dehydrogenation processes (HO-H bond scission) of H2O on Fe(111), W@Fe(111), and W2@Fe(111) surfaces are 24.40, 12.62, and 9.97 kcal/mol, respectively. For second dehydrogenation processes (O−H bond scission), the corresponding activation energies of OH on Fe(111), W@Fe(111), and W2@Fe(111) surfaces are 39.35, 22.69, and 26.24 kcal/ mol, respectively. Finally, the entire dehydrogenation courses on the varied Fe(111), W@Fe(111), and W2@Fe(111) surfaces are exothermic by 20.08, 41.35, and 59.30 kcal/mol, respectively. To comprehend the electronic properties of its nature of interaction between the adsorbate and substrate, we calculated the electron localization functions, local density of states, and Bader charges; the results were consistent and explicable. has attracted much attention.9−16 Numerous reports focused on an explanation of the mechanism of the interaction between water and the surface. In the theoretical field, the first-principles calculation method is a powerful tool to study the mechanism of chemical reaction at the molecular level. In particular, periodic density function theory calculations have become a useful approach to study the adsorption and dissociation reaction of atoms and molecules on the metal surfaces. Recently, many authors became devoted to exploring the adsorption and decomposition mechanism of H2O on various surfaces, such as Pd,12,13 Au,14,15 Cu,16 and γ-Al2O3(110).17 Jiang et al.16 found that the dissociation of OH group is the most difficult step in water dehydrogenation process, which is the rate-determining step. Błoński et al. performed the calculations of properties of bcc iron surfaces, such as structural energies, surface relaxation and magnetic properties; the sequence of stability of these examined surfaces was (110) > (100) > (111).18 However, Spencer et al. have experimentally shown that the relative rates of ammonia formation on Fe(111), Fe(100), and Fe(110) were 418:25:1 at 798 K under a total pressure of 20 atm, which demonstrates the

1. INTRODUCTION In the environmental pollution and fossil fuel crisis, alternative energy sources play a very important role.1 Hydrogen energy has attracted much attention in the world, because it is expected to be a clean, recyclable energy source with a low polluting nature and a highly efficient fuel to produce electricity.2−4 Therefore, hydrogen energy could replace fossil fuel and would be employed for vehicles and portable devices within a few decades.5,6 The method used to store and produce hydrogen with a convenient and higher efficiency is very important. However, several conventional methods for preparing hydrogen gas lead to a large output of COx as byproducts. Scientists are interested in finding a carbon-free natural compound to prepare hydrogen gas. Fortunately, water is one of them, and it is also pollution-free onboard and also the product of combustion of hydrogen fuel. H2O is a hydrogen-rich compound and has 11.1% hydrogen carrying capacity. According to that, water is more attractive to useful development of a hydrogen energy economy. Therefore, direct catalytic decomposition of H2O is a efficient recycling method and provides production of carbon monoxide free hydrogen. As people known, water is one of the most prominent and important substances on our planet. It plays important roles in the world, and could be used with many physical and chemical processes.7,8 During the past several decades, water dissociation © XXXX American Chemical Society

Received: July 25, 2016 Revised: October 23, 2016 Published: October 24, 2016 A

DOI: 10.1021/acs.jpcc.6b07455 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C E interaction = Eadsorption − Erelaxation − Edistortion

(111) facet on a small crystallite of Fe possessing a large catalytic activity due to its open surface structure.19 According to these observations, we applied periodic density functional theory to investigate the adsorption and dehydrogenation of H2O molecule on each (111) facet of iron-based catalyst, where Fe(111) represents the pure iron surface, W@Fe(111) represents the monolayer coating of tungsten metals on Fe(111) surface, and W2@Fe(111) counterpart represents double-layer coating of tungsten metals on Fe(111) surface. The adsorption geometries/energies, site preferences, relative stabilities of H2O and its dehydrogenated species, as well as activation barriers were systematically characterized and analyzed. To the best of our knowledge, there are no theoretical papers fully studying the dehydrogenation of H2O on varied metallic surfaces with body-centered-cubic lattice. We believe that this understanding is essential in the future study to rational design of the catalytic surface model in the dehydrogenation of H2O molecule.

and relaxation energies (Edistortion and Erelaxation) represent the deformation energies for the adsorbate and the surface, respectively, for bringing them to interact to form the final equilibrium adsorption geometry from their respective equilibrium geometry separated with an indefinite distance. The interaction energy (Einteraction) is the binding energy between the adsorbate and surface in their deformed geometries used to form the stable adsorption. A negative value of ΔEads represents an exothermic process of adsorption. Vibrational frequencies of the adsorbed structures were analyzed by diagonalizing the Hessian matrix of selected atoms with the harmonic approximation using the finite difference displacements of 0.05 Å. Transition states were carefully located (with creating at least eight images between the reactant and product) using the climbing nudged elastic band method (CI-NEB) method,32,33 and then further optimized to the actual saddle point using quasi-Newton algorithm. The finally relaxed transition states were confirmed through frequency analysis, for which only one imaginary frequency was found for each of them.

2. COMPUTATIONAL METHODS The adsorption and dehydrogenation behaviors of H2O on the M(111) surface, where M = Fe, W@Fe, and W2@Fe, have been investigated by the spin-polarized density functional theory (DFT)20 performed by using the Vienna ab initio simulation package (VASP) program.21−25 All the electron−ion interactions were described with the projector augmented wave (PAW) method,26,27 and the exchange-correlation effects were described with the generalized gradient approximation with the revised Perdew−Burke−Ernzerhof (GGA-rPBE) method.28,29 The electronic states were expanded by using the plane wave basis set with cutoff energy 400 eV, which allows convergence to 1 × 10−4 eV in total energy. The Brillouin zone is sampled with the Monkhorst−Pack grid.30 The calculations were performed with the (4 × 4 × 4) and (4 × 4 × 1) Monkhorst−Pack mesh k-points for bulk and surface calculations, respectively. Each slab consisted of six layers of atoms with the surface represented by a 2 × 2 unit cell, while the bottom three atomic layers were kept fixed and the remaining layers were fully movable during the calculations. In addition, the slabs were separated in the direction perpendicular to the surface by a vacuum region greater than 15 Å to make sure there is no interaction between the slabs. In this work, we calculate adsorption energies according to the following equation:

3. RESULTS AND DISCUSSION 3.1. Accuracy of the Method and Model. To ensure the reliability of the computational method in our work, we first calculated some significant characters of bulk iron, tungsten, gas-phase H2O, and OH molecules. Previously, Chen et al.34 have already predicted the lattice parameter of bulk iron at GGA-rPBE level of theory, and their result represented 2.836 Å, which approaches the experimental value 2.866 Å. For the bulk tungsten, the lattice parameters of bulk at GGA-rPBE functional have also been reported by Chen et al.,35 and the result shows 3.181 Å, which is closely with the experimental value of 3.165 Å. Moreover, the structure parameters, vibrational frequencies and dissociation energies of relevant isolated gas-phase molecules, H2O and OH are calculated by enclosing them into a large unit cell of 25 × 25 × 25 Å3 dimensions. As shown in Table 1, all our calculated results are Table 1. Calculated and Experimental Values of the Geometrical Parameters (Bond Lengths in Å and Angles in deg), Vibrational Frequencies (in cm−1), and Dissociation Energies (in kcal/mol) of Gaseous Phase H2O and OH Molecules

ΔEads = E[surface + adsorbate]

H2O

− (E[surface] + E[adsorbate]) r (Å) θ (deg) vasym vsym vband D0 (kcal/mol)

in which E[surface + adsorbate], E[surface], and E[adsorbate] are the calculated electronic energies of adsorbed species on surfaces, a clean surfaces, and a gas-phase molecule, respectively. In order to understand the adsorption of the OHx (x = 1, 2) species on surfaces, the adsorption energy was also divided into three components, i.e., the relaxation energies of the surfaces, the distortion energies of OHx, and the interaction energies between OHx and the surfaces, as proposed by Delbecq et al.31 The relaxation, distortion and interaction energies due to the OHx adsorption were estimated based to the following equations,

a

b

OH

calculated

experimental

calculated

experimental

0.972 104.4 3793.6 3673.8 1610.7 115.90

0.956a 105.2a 3755.7b 3657.0b 1594.7b 117.91a

0.988

0.970c

3591.8

3737.8c

104.14

101.28c

c

Reference 36. Reference 37. Reference 38.

in general good agreement with the experimental observations.36−38 Consequently, above examined results indicate that our chosen method and model are suitable for describing the behaviors of H2O adsorption and dehydrogenation on varied M(111) surfaces, where M = Fe, W@Fe, and W2@Fe. 3.2. Adsorptions of H2O and Its Dehydrogenated Fragments. For exploring the dehydrogenation processes of

Erelaxation = E[distorted surface] − E[surface] Edistortion = E[distorted adsorbate] − E[adsorbate] B


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The Journal of Physical Chemistry C H2O onto the varied M(111) surfaces, first we investigated four important adsorbed species, such as H2O, OH, O, and H, respectively. Three probable adsorption states on the varied M(111) surfaces by varying the conformation of aforementioned species are considered, such as the top (T), 3-foldshallow (S), and 3-fold-deep (D), as shown in Figure 1(b). To

could promote adsorption and activation behaviors between H2O and varied M(111) surfaces. To locate the H2O molecule on Fe(111), W@Fe(111) and W2@Fe(111) surfaces, we placed a H2O molecule at various sites on them. The structures of H2O adsorptions on the varied surfaces of M(111) are shown in Figure 2, and the relevant

Figure 2. Optimized adsorption structures of H2O molecule on Fe(111), W@Fe(111), and W2@Fe(111) surfaces and their important geometrical parameters calculated at the GGA-rPBE level of theory. The bond lengths are given in angstroms.

calculated results are tabulated in Table 2. First of all, the resulting H2O/Fe(111) structures are classified according to the adsorption site, which are denoted as follows: FeH2O(T-η1O)-a, FeH2O(T-η1-O)-b, and FeH2O(T-η1-O)-c, with adsorption energies of −7.94, −8.08, and −5.77 kcal/mol, respectively. It should be mentioned that a number of initial geometries were tried, but we were only able to locate three stable adsorption configurations as shown herein. Among them, the first adsorption type, FeH2O(T-η1-O)-a, represents the H2O molecule lying nearly parallel to Fe(111) surface with its two H atoms in the direction above the shallow sites; the second type, FeH2O(T-η1-O)-b, represents the H2O molecule lying parallel to Fe(111) surface also with its two H atoms in the direction above the deep sites; and the third type, FeH2O(T-η1-O)-c, represents the H2O molecule lying vertical to the Fe(111) surface with its two H atoms pointing up to vacuum space. Apparently, the energetically most stable structure of FeH2O(T-η1-O)-b conformer is the adsorption at the top site via the O atom of H2O with an O−Fe bond distance of 2.206 Å. The O−H bond lengths and the angle of H−O−H are 0.973, 0.975 Å and 104.46°, respectively, which are close to the values of the isolated water molecule (0.956 Å and 105.2°), indicating that the structure of the adsorbed H2O is not significantly distorted by Fe(111) surface. As compared to previously theoretical studies of H2O adsorption on other transition metal surfaces,12−16 their computed results also demonstrated that the H2O adsorbed parallel to the substrate. Second, for H2O/W@Fe(111) system, we also found three similar stable adsorption structures, which are W@FeH2O(Tη1-O)-a, W@FeH2O(T-η1-O)-b, and W@FeH2O(T-η1-O)-c, with the adsorption energies of −13.37, −12.26, −10.98 kcal/

Figure 1. Graphical representations and the corresponding electronic localization function (ELF) contour maps of Fe(111), W@Fe(111) and W2@Fe(111) surfaces used in the present study: (a, c, e, g) side views and (b, d, f, h) top views, respectively.

discuss more conveniently, we labeled H2O/M(111), OH/ M(111), O/M(111), and H/M(111) to represent the adsorptions of H2O, OH, O, and H on the surfaces of varied M(111), respectively. The electronic localization function (ELF) contour diagrams of Fe(111), W@Fe(111), and W2@ Fe(111) surfaces were also shown in parts c and d, parts e and f, and parts g and h of Figure 1 for side and top views, respectively. As in parts c and d of Figure 1, the ELF contour diagram of Fe(111) apparently shows that the ELF value is weak at the top sites, and the electrons were found to be highly localized at shallow and deep sites due to the higher ELF value. However, after modifying by tungsten metals into Fe(111) surfaces, W@Fe(111), and W2@Fe(111), both ELF contour diagrams apparently illustrate that the ELF values are close to zero at shallow and deep sites, indicating that their electron distributions are both highly localized top sites, as shown in parts e and f and parts g and h of Figure 1. These results could reflect the face that the surfaces of W@Fe(111) and W2@ Fe(111) in each top site would donate electrons to the gaseous molecule as they adsorbed and make the surface oxidized and potentially electron donors. Consequently, all these charge transferring phenomena between gaseous molecule and metals C


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The Journal of Physical Chemistry C

Table 2. Calculated Adsorption Energies, Relaxation Energies, Distortion Energies, Interaction Energies (kcal/mol), and Geometrical Parameters (Å) of Adsorbed H2O Molecules on Varied Metal Surfaces (Fe(111), W@Fe(111), and W2@Fe(111)) adsorption site

adsorption energy

relaxation energy

−7.94 −8.08 −5.77

0.22 0.19 1.15

W@FeH2O(T-η1-O)-a W@FeH2O(T-η1-O)-b W@FeH2O(T-η1-O)-c

−13.37 −12.26 −10.98

1.09 0.81 1.18

W2@FeH2O(T-η1-O)-a W2@FeH2O(T-η1-O)-b W2@FeH2O(T-η1-O)-c

−18.61 −17.32 −15.39

0.34 0.30 0.33

FeH2O(T-η1-O)-a FeH2O(T-η1-O)-b FeH2O(T-η1-O)-c

distortion energy For Fe Surface −0.22 −0.15 0.27 For W@Fe Surface −0.13 −0.05 0.71 For W2@Fe Surface −0.10 −0.23 0.63

interaction energy

d(M−O)a

d(O−H)b

∠(H−O−H)

−7.94 −8.12 −7.19

2.152 2.206 2.213

0.979/0.977 0.973/0.975 0.971/0.970

105.67 104.46 107.63

−14.33 −13.02 −12.87

2.278 2.273 2.245

0.979/0.980 0.981/0.979 0.975/0.974

106.94 106.80 110.81

−18.85 −17.38 −16.35

2.223 2.229 2.244

0.983/0.985 0.979/0.979 0.974/0.970

106.43 105.71 110.40

a

The shortest distance between the adsorbed atom (O) and the corresponding adsorption site of metal surface (Fe(111), W@Fe(111) or W2@ Fe(111)). bThe bond lengths of OH1/OH2 are presented.

Figure 3. Local density of states (LDOS) for H2O adsorption on Fe(111), W@Fe(111) and W2@Fe(111) surfaces: (a−c) before adsorptions and (d−f) energetically the highest adsorption structures, FeH2O(T-η1-O)-b, W@FeH2O(T-η1-O)-a, and W2@FeH2O(T-η1-O)-a, respectively. The dashed line represents the Fermi level.

H2O/W@Fe(111) system, suggesting that the structures of the adsorbed H2O will not effectively distorted by W-modified Fe(111) surfaces. As compared to the similar systems of H2O on Pd(111),13 Au(111),14 and Cu(111)16 surfaces, it is found that our predicted adsorption energies of W@FeH2O(T-η1-O)a and W2@FeH2O(T-η1-O)-a are definitely higher than theirs (ca. −5.07, −2.58, and −2.77 kcal/mol for Pd(111), Au(111), and Cu(111), respectively), indicating the stronger interactions are observed between H2O and iron-based M(111) surfaces. To understand these phenomena more specifically, we consider the concept mentioned by Delbecq et al.31 We investigated the interaction force between the gas-phase

mol, respectively. Third, three comparable stable isomers of H2O adsorbed on the W2@Fe(111) counterpart were also predicted, which are denoted as W2@FeH2O(T-η1-O)-a, W2@ FeH2O(T-η1-O)-b, and W2@FeH2O(T-η1-O)-c with adsorption energies of −18.61, −17.32, and −15.39 kcal/mol, respectively. All these results are in good accord with our aforementioned observations of ELF that the adsorbed H2O on W@Fe(111) and W2@Fe(111) surfaces will be more active on top sites than on shallow and deep sites by possessing higher ELF values. We also noted that the O−H bond of the adsorbed H2O on both W@Fe(111) and W2@Fe(111) surfaces only elongate ca. 0.5−1.0% from the optimized O−H bond length in D


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The Journal of Physical Chemistry C molecule and surface via decomposing the adsorption energy into three principal components, such as relaxation energy, distortion energy, and interaction energy, respectively. The results of these calculations are also shown in Table 2. For example, three larger adsorption energies are found for H2O adsorbing onto W2@Fe(111) around the top sites, implying that these kinds of adsorptions will be deformed by W2@ Fe(111) surface. The energy of mutual interaction for W2@ FeH2O(T-η1-O)-a is ca.1.5 and 2.5 kcal/mol larger than those of W2@FeH2O(T-η1-O)-b and W2@FeH2O(T-η1-O)-c counterparts, indicating that the gas-surface restructuring for configuration of W2@FeH2O(T-η1-O)-a could strongly stabilize its whole structure. Furthermore, to be more specific, we investigated the electronic local density of states (LDOS) of the system projected on the orbitals for the adsorbate of H2O and each iron-based M(111) substrate (shown in Figure 3). The degree of hybridization for its electronic state between adsorbate and substrate would provide us another piece of evidence to realize the adsorption and dehydrogenation behaviors. Parts a−c of Figure 3 show the LDOS before the H2O interaction with the Fe(111), W@Fe(111) and W2@Fe(111) surfaces, respectively. Parts d−f of Figure 3 represent the LDOS of energetically the highest adsorption structures, FeH2O(T-η1-O)-b, W@FeH2O(T-η1-O)-a, and W2@FeH2O(T-η1-O)-a, respectively. In parts a−c of Figure 3, the four peaks contributed by H2O near left sides of the Fermi level exhibit three bonding orbitals (2a1, 1b2, and 3a1) and one nonbonding orbital (1b1). As each adsorption proceeds (shown in parts d−f of Figure 3), the Fermi levels of each H2O will be all shifted right after adsorption, which illustrates the increase of charge density in each adsorbed H2O from W-modified Fe(111) surfaces. It is also found that one specific state (1b1) reduced abruptly after adsorptions of H2O on each surface, indicating that the larger molecular adsorption energies will be found through their stronger hybridization mutually. In addition, the values of the LDOS integrated overlap area (A) between the H2O adsorbate and the varied M(111) surfaces follow the order of W2@Fe(111) (0.347) > W@Fe(111) (0.327) > Fe(111) (0.302), consistent with the calculated adsorption energies of H2O and indicating a larger overlap area between H2O and the varied M(111) surfaces correlates with a greater adsorption energy. To explore the minimum energy pathways (MEPs) for the dehydrogenation mechanisms of H2O onto varied M(111) surfaces, we at the same time studied the optimized adsorption geometries/energies of each OH, H, and O dehydrogenated fragment. The coordination of OH fragment onto each M(111) surface leads to the formation of several stable isomeric structures, which are shown in Figure 4, and the relevant calculated results are tabulated in Table 3. On the Fe(111) surface, the three more stable isomers are FeOH(T-η1-O), FeOH(T,S-μ2-O), and FeOH(T,T,S-μ3-O), with corresponding adsorption energies of −79.44, −83.05, and −82.66 kcal/mol, respectively. Regarding the most stable structure of FeOH(T,Sμ2-O), the O atom of OH fragment favors to adsorb between the top and shallow sites and the H atom points to the deep site of Fe(111). Besides, similar calculated results of OH adsorptions were also found for W@Fe(111) and W2@Fe(111) surfaces, showing that the three stable isomers are W@ FeOH(T-η1-O)-a, W@FeOH(T-η1-O)-b, and W@FeOH(T,T,S-μ3-O) (with adsorption energies of −107.01, −106.46, and −87.04 kcal/mol), and W2@FeOH(T-η1-O)-a, W2@ FeOH(T-η1-O)-b, and W2@FeOH(T,T,S-μ3-O) (with adsorp-

Figure 4. Optimized adsorption structures of OH molecule on Fe(111), W@Fe(111), and W2@Fe(111) surfaces and their important geometrical parameters calculated at the GGA-rPBE level of theory. The bond lengths are given in angstroms.

tion energies of −108.94, −108.66, and −89.60 kcal/mol), respectively. Consequently, the order of OH adsorption energy is accordingly W2@Fe(111) > W@Fe(111) > Fe(111), which correlates positively with our aforementioned observations of ELF results: the more that the ELF value is gained from metal surfaces, the higher in adsorption energy will be obtained. Similar phenomena also appeared for the adsorptions of O and H atoms on varied M(111) surfaces. As expected, it is found that W2@Fe(111) possess the highest adsorption energies for O and H adsorptions. All adsorption models are depicted in Figures 5 and 6, and the O and H adsorption characteristics are provided in Tables 4 and 5. Among them, the adsorbate O atom can adsorb strongly on Fe(111) surface, and the most favorable adsorption site is FeO(T,S-μ2-O)-a, with adsorption energy of −129.21 kcal/mol. For the W@Fe(111) and W2@ Fe(111) counterparts, however, the most favorable adsorbed sites for O atom are W@FeO(T-η1-O) and W2@FeO(T,S-μ2O), with adsorption energies of −161.86 and −161.61 kcal/ mol, respectively. In addition, the atomic H adsorbate could also adsorb strongly on these varied M(111) surfaces. Among these kinds of adsorptions, the H adsorbate onto Fe(111), W@ Fe(111), and W2@Fe(111) surfaces were favored to adsorb with FeH(T,S-μ2-H), W@FeH(T-η1-H), and W2@Fe(T,S-μ2H) configurations, with adsorption energies of −61.28, −59.95, and −65.85 kcal/mol, respectively. 3.3. Reaction Mechanisms and Potential Energy Surfaces for H2O Dehydrogenation. The dehydrogenation mechanisms of H2O on a transition metal are commonly accepted as follows: H 2O(g) + S → H 2O(a)

(i)

H 2O(a) → OH(a) + H(a)

(ii)

OH(a) → O(a) + H(a)

(iii)

Here S indicates the catalytic metal surface. The potential energy diagrams (shown in Figure 7) of H2O dehydrogenation on each M(111) surface were mapping by CI-NEB method, E


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Table 3. Calculated Adsorption Energies, Relaxation Energies, Distortion Energies, Interaction Energies (kcal/mol), and Geometrical Parameters (Å) of Adsorbed OH Molecules on Varied Metal Surfaces (Fe(111), W@Fe(111), and W2@Fe(111)) adsorption site

adsorption energy

relaxation energy

interaction energy

d(M−O)a

d(O−H)

−3.65 −3.50 −3.55

−76.50 −83.08 −82.11

1.831 1.961 2.110

0.974 0.975 0.977

−3.57 −3.71 −3.58

−109.09 −108.42 −87.40

1.923 1.921 2.170

0.972 0.975 0.978

−3.65 −3.63 −3.67

−107.17 −107.00 −88.12

1.922 1.920 2.274

0.975 0.974 0.978

distortion energy

For Fe Surface FeOH(T-η1-O) FeOH(T,S-μ2-O) FeOH(T,T,S-μ3-O)

a

−79.44 −83.05 −82.66

W@FeOH(T-η1-O)-a W@FeOH(T-η1-O)-b W@FeOH(T,T,S-μ3-O)

−107.01 −106.46 −87.04

W2@FeOH(T-η1-O)-a W2@FeOH(T-η1-O)-b W2@FeOH(T,T,S-μ3-O)

−108.94 −108.66 −89.60

0.71 3.52 3.00 For W@Fe Surface 5.64 5.67 3.94 For W2@Fe Surface 1.88 1.97 2.19

The shortest distance between the adsorbed atom (O) and the corresponding adsorption site of varied metal surfaces.

Figure 6. Optimized adsorption structures of H atom on Fe(111), W@Fe(111), and W2@Fe(111) surfaces and their important geometrical parameters calculated at the GGA-rPBE level of theory. The bond lengths are given in angstroms.

Figure 5. Optimized adsorption structures of O atom on Fe(111), W@Fe(111), and W2@Fe(111) surfaces and their important geometrical parameters calculated at the GGA-rPBE level of theory. The bond lengths are given in angstroms.

Table 4. Calculated Adsorption Energies (kcal/mol) and Geometrical Parameters (Å) of Adsorbed O Atom on Varied Metal Surfaces (Fe(111), W@Fe(111), and W2@Fe(111))

and the geometrical illustrations of all adsorbed intermediates and transition states using the GGA-rPBE level of theory are all portrayed in Figure 8 and Figures S1−S3 (Supporting Information). For the Fe(111) surface, as the FeH2O(T-η1O)-b conformer is energetically the most stable among all the calculated structures of H2O/Fe(111) system, we chose it as an initial structure (denoted as Fe-IM1) to study the dehydrogenation of H2O on Fe(111). The first dehydrogenation of H2O from Fe-IM1 into adsorbed OH and H producing Fe-IM2 requires a activation barrier of 24.40 kcal/mol (Fe-TS1) with an exothermicity of 11.72 kcal/mol. At the transition state FeTS1, an imaginary frequency i1302.8 cm−1 corresponding to the O−H stretching vibration mode was observed, and the distance of O−H bond for Fe-TS1 is around 1.317 Å (shown in Figure S1). In the Fe-IM2 structure, both the coadsorbate of OH and H are located between top and shallow sites position. Finally, the Fe-IM2 intermediate overcomes an activation barrier of 39.35 kcal/mol at Fe-TS2 (with an imaginary frequency of i1217.15 cm−1) to break the second O−H bond and produce Fe−P, O/2H/Fe(111), with an overall

adsorption site

adsorption energy

For Fe Surface −129.21 −128.38 −128.36 For W@Fe Surface W@FeO(T-η1-O) −161.86 W@FeO(T,S-μ2-O) −155.42 W@FeO(T,T,S-μ2-O) −155.44 For W2@Fe Surface W2@FeO(T-η1-O) −161.22 W2@FeO(T,S-μ2-O) −161.61 W2@FeO(T,T,S-μ2-O) −161.53 FeO(T,S-μ2-O)-a FeO(T,S-μ2-O)-b FeO(T,T,S-μ2-O)

d(M-O)a 1.799 1.863 1.858 1.746 2.047 2.041 1.744 2.061 2.056

a

The shortest distance between the adsorbed atom (O) and the corresponding adsorption site of varied metal surfaces.

F


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Similar reaction mechanisms were also predicted for W@ Fe(111) and W2@Fe(111) counterparts, and it is found that the adsorption behaviors of H2O molecule on these surfaces are much more facile and possess higher adsorption energies of 13.37 (W@Fe-IM1) and 18.61 (W2@Fe-IM1) kcal/mol, respectively, as compared to Fe(111). For adsorbed intermediate H2O to produce adsorbed OH and H, the calculated activation barriers of O−H bond dissociation are 12.62 (W@ Fe-TS1) and 9.97 (W2@Fe-TS1) kcal/mol, respectively, on W@Fe(111) and W2@Fe(111) surfaces. At the transition states of W@Fe-TS1 and W2@Fe-TS1, two imaginary frequencies of i1371.97 and i762.18 cm−1 were observed, corresponding to the vibration modes of O−H stretching, and the distances of the O−H bond during these dehydrogenation processes are around 1.30 and 1.20 Å, respectively. The tendency of these barriers directly correlate with the interaction of the H2O molecule sticking onto the varied M(111) surfaces; the stronger is the interaction of this H2O molecule with the surface, the more easily the O−H bond of adsorbed H2O breaks, such that the barrier for dissociation of the O−H bond is the least (9.97 kcal/ mol) on W2@Fe(111). Among them, W2@Fe-IM2 was found to be highly exothermic (ca. −32.5 kcal/mol) as compared to Fe(111) and W@Fe(111) counterparts. In addition, the second dehydrogenations of H2O from W@Fe-IM1 and W2@Fe-IM2 into coadsorbed O and 2H, producing W@Fe−P and W2@ Fe−P, requires activation barriers of 22.69 (via W@Fe-TS2) and 26.24 kcal/mol (via W2@Fe-TS2), with overall exothermicities of 41.35 and 59.30 kcal/mol, respectively. On the basis of the aforementioned PESs for the dehydrogenations of H2O on varied M(111) surfaces, the second dehydrogenation process of breaking the O−H bond has the highest activation barrier. If the thermal energy could overcome the barrier height of M-TS2 (39.35, 22.69, and 26.24 kcal/mol for Fe(111), W@Fe(111), and W2@Fe(111) surfaces, respectively, where the rate determining barriers of the reactions are given), then all of the OHx dehydrogenation processes could take place on these metallic and bimetallic surfaces. Although these activation energies of M-TS2 on the W@Fe(111) and W2@Fe(111) surfaces are slightly high, they are still lower than those for H2O dehydrogenations on

Table 5. Calculated Adsorption Energies (kcal/mol) and Geometrical Parameters (Å) of Adsorbed H Atom on Varied Metal Surfaces (Fe(111), W@Fe(111), and W2@Fe(111)) adsorption site

adsorption energy

For Fe Surface −47.01 −59.92 −61.28 For W@Fe Surface W@FeH(T-η1-H) −59.95 W@FeH(S-η1-H) −57.81 W@FeH(T,T,S-μ3-H) −58.29 For W2@Fe Surface W2@FeH(T-η1-H) −64.48 W2@FeH(S-η1-H) −65.65 W2@FeH(T,S-μ2-H) −65.85 FeH(T-η1-H) FeH(S-η1-H) FeH(T,S-μ2-H)

d(M-H)a 1.588 1.610 1.662 1.600 1.623 1.645 1.762 1.855 1.850

a

The shortest distance between the adsorbed atom (H) and the corresponding adsorption site of varied metal surfaces.

Figure 7. Calculated possible potential energy diagrams (in kcal/mol) for the dehydrogenation reactions of H2O molecule on the Fe(111), W@Fe(111), and W2@Fe(111) surfaces.

exothermicity of 20.78 kcal/mol, in which the O and 2H are all located between top and shallow sites.

Figure 8. Geometrical illustrations of all intermediates (IM1−IM2), transition states (TS1−TS2) and products (P) for the dehydrogenation reactions of H2O molecule on the Fe(111), W@Fe(111), and W2@Fe(111) surfaces. G


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Figure 9. Illustration of charge-density difference for the IM1 of adsorbed H2O on each Fe(111), W@Fe(111), and W2@Fe(111) surface: (a) FeIM1, (b) W@Fe-IM1, and (c) W2@Fe-IM1, respectively.

Pd(100),12 Pd(111),13 Au(111),14 Au(100),15 Cu(111),16 and γ-Al2O3(110)17 surfaces, where the rate determining activation energies of the reactions on these metal surfaces range from ca. 29.9 to 66.6 kcal/mol. As a consequence, according to our present calculations and a lot of comparisons with other similar works, we could conclude that the W@Fe(111) surface could exhibit a larger catalytic activity to decompose H2O. However, since the simulated program (such as CASTEP package) and functionals (such as GGA-PW91 and GGA-PBE functionals) of these published studies are somewhat different from those in our present work, we think the existence of uncertainty should be taken into account regarding these comparisons. In addition, it is found that the remaining high exothermic O atom will eventually cover the whole surface in each M(111) system, which could lead to block the reaction sites and obstruct the further dehydrogenation reactions of other H2O molecules. Recently, Lin et al.39 have theoretically explored the CO oxidation mechanism on the W(111) surface, in which they described the highly adsorbed O atom could be successfully removed through the pathway of Eley−Rideal (ER) mechanism (O(ads) + CO(gas) → CO2(gas)), with a pertinent activation barrier of ca. 30.3 kcal/mol. However, since there is no experimental observation available for this related issue, the importance of such recycling needs to be experimentally confirmed. 3.4. Electronic Analyses of Each Intermediate in H2O Dissociation. To further analyze the electronic interactions between adsorbate of H2O and substrates of varied M(111) surfaces, we also calculated the charge density difference (CDD) for each M-IM1 structure. Figure 9 shows the plots of the contour surface of CDD, Δρdiff = ρ[surface + adsorbate] − ρ[surface] − ρ[adsorbate], for each adsorbate/substrate system of the H2O/M(111): (a) Fe-IM1, (b)W@Fe-IM1, and (c) W2@Fe-IM1, in which the blue and yellow parts represent the accumulation and depletion electrons, respectively. In addition, we analyze the DFT electron density of the aforementioned structure by utilizing the Bader charge analysis40,41 with a program designed by Henkelman et al.42 As shown in the CCD from Figure 9, the charge transferred behaviors from the Fe(111), W@Fe(111), and W2@Fe(111) surfaces to the H2O adsorbate were observed, they are 0.02 |e|, 0.04 |e|, and 0.06 |e|, respectively. Therefore, it was shown that the W2@Fe(111) surface not only facilitates chemical interaction between H2O and W metals but also induces favorable electronic charge distribution at the interface with H2O. These phenomena are in excellent agreement with our aforementioned observations of ELF and LDOS results that the W2@Fe(111) surface would donate electrons to the adsorbed H2O(a) much easier than those of Fe(111), and W@Fe(111) counterparts. As a

consequence, one can hence believe that a remarkable transfer of charge between the adsorbate and substrate will play a crucial role in accelerating the catalytic processes for H2O dehydrogenation.

4. CONCLUSION In summary, the dehydrogenation behaviors of H2O molecule on the metallic surface of Fe(111) and bimetallic surfaces of W@Fe(111) and W2@Fe(111) have been investigated with periodic DFT calculations. The possible adsorption sites, geometrical parameters, and adsorption energies of H2O, OH, O and H on varied M(111) surfaces are thoroughly characterized. The origins of discrepancy in activity and reactivity are revealed in terms of adsorption structures, charge distributions and activation energies analysis of some key adsorbed intermediates. Our calculated results showed that the dehydrogenation processes of H2O on varied M(111) surfaces will first form the adsorbed intermediates of M-IM2, and the calculated activation barriers of HO−H bond scission are 24.40, 12.62, and 9.97 kcal/mol for Fe(111), W@Fe(111) and W2@ Fe(111) surfaces, respectively. In addition, for the second dehydrogenation processes of OH on varied M(111) surfaces, the energy barriers are 39.35, 22.69, and 26.24 kcal/mol, respectively. On the basis of the results given above, we could conclude that the W@Fe(111) surface will be more effectively to dissociate H2O than those of Fe(111) and W2@Fe(111) counterparts. All the information predicted by theoretical approaches would be laborious to achieve with experimental operations, indicating that our systematic investigations through DFT calculations may apply to enable screening of promising candidates with considerably catalytic efficiency for hydrogen gas formation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b07455. Structures and important parameters of intermediates, transition states and products for H2O dehydrogenation processes on each surface of Fe(111), W@Fe(111), and W2@Fe(111), shown by using the GGA-rPBE level of theory (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(H.-L.C.) E-mail: [email protected]. Telephone: +886-2-28610511 ext 25313. Fax: +886-2-28617006. H


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The Journal of Physical Chemistry C Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by (1) the Ministry of Science and Technology in Taiwan, with Grant Number of MOST 1052113-M-034-002, and (2) the Research Foundation of the Chinese Culture University, and (3) the use of CPU time from the National Center for High-Performance Computing, Taiwan. H.-L.C. would like to take this opportunity to express the deepest gratitude to H.-L.C.’s former postdoctoral supervisor, Prof. Minghuey Shieh, for her unremitting instructions. Without her persistent encouragement, guidance and past support in postdoctoral position, H.-L.C. could not have had the present achievement and level of scholarship in PCCU.

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