Adsorption and Dehydrogenation Behaviors of the NH 3 Molecule on

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Adsorption and Dehydrogenation Behaviors of the NH3 Molecule on the W(111) Surface: A First-Principles Study Ming-Kai Hsiao, Sheng-Ke Wu, and Hui-Lung Chen* Department of Chemistry and Institute of Applied Chemistry, Chinese Culture University, Taipei, 111, Taiwan ABSTRACT: The adsorption and dehydrogenation behaviors of ammonia on W(111) surface have been studied by employing spin-polarized density function theory calculations. In this work, three adsorption sites of the W(111) surface were considered, such as top (T), 3-fold-shallow (S), and 3-fold-deep (D) sites. The most stable structures of each NHx (x = 0−3) species on the W(111) surface have been predicted, and the corresponding dehydrogenation processes were found to be via two specific paths (A and B). In PATH A, the calculated activation energies for NHx (x = 1−3) dehydrogenations are 27.66 kcal/mol (for H2N−H bond activation), 32.66 kcal/mol (for HN−H bond activation) and 27.84 kcal/mol (for N−H bond activation), respectively, and the entire process is exothermic by 41.63 kcal/mol. On the other hand, in PATH B, the corresponding activation barriers are 35.97, 29.99, and 29.80 kcal/mol, respectively, and the entire process is 42.19 kcal/mol exothermic. To gain more insight into catalytic processes of the aforementioned conducts, the interaction nature between the adsorbate and substrate is analyzed via detailed electronic analysis.

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transition metal surfaces have been extensively discussed.11−39 In recent years, the dehydrogenation mechanism of NH3 catalyzed on the surface of a large number of metals has been studied via theoretical calculations, such as Fe,11−13 Co,13 Ni,13−18 Pd,18−20 Rh,21−23 Ir,24−27 Ru,28,29 Cu,30 and Pt.31−36 Among of them are those based on density function theory (DFT) calculations. It is a widely accepted that the dehydrogenation mechanism of NH3 consists of the following elementary steps: NH3(g) → NH3(a), NH3(a) → NH2(a) + H(a), NH2(a) → NH(a) + H(a), NH(a) → N(a) + H(a), N(a) + N(a) → N2(g), and H(a) + H(a) → H2(g), where the subscripts (g) and (a) denote the gas-phase and absorbed state, respectively. In the experiments, the recombinative desorption of N2 is the ratedetermining step (RDS) for NH3 catalytic decomposition on the single-crystal metal surfaces. However, N−H bond cleavage has also been found to be the RDS for NH3 decomposition in some cases.18 Most of these studies focus on body-centered cubic structure, crystals of large surface area, or single crystals. Besides this, there are several reports about the adsorption and dissociation of NH3 on the W surface.37−39 Grunze et al. studied the adsorption and decomposition of ammonia on a W(110) surface via XPS/UPS/LEED/SIMS spectrometer.39 They discuss the observed binding energy within a thermodynamic framework, employing the equivalent core approximation, and also reported the chemical absorption energy for different component, such as NH3, NH2, NH, N, and H. In this study, we performed DFT calculations to investigate NH3 decomposition on W(111) to shed light on individual bonding and cleavage processes. The adsorption geometries

INTRODUCTION In recent years, a large amount of petroleum has been used in the transportation industry. These petroleum products contain nitrogen in heterocyclic structures comprising pyridine and pyrrole ring systems.1,2 In the devolatilization process, the nitrogen of petroleum is divided between char-nitrogen and volatile nitrogen compounds, such as tar, NH3, and HCxN. When those compounds are burned, our environment becomes highly polluted in the atmosphere. On the other hand, the use of hydrogen fuel to replace gasoline, heating oil, natural gas, and other fuels, to reduce the formation of greenhouse gases, has attracted worldwide attention.3 Scientists express considerable concern in searching for sources and carriers of clean hydrogen. So far, a large amount of literature has been reported about regarding storage of hydrogen in a convenient, efficient, and safe manner. The technologies of conventional hydrogen production would lead to a huge amount of CO and CO2 as by products, which are known greenhouse gases and bring many disastrous environmental effects. Thus, ammonia is a carbon-free natural compounds and could be seen as a pollution-free onboard carrier of hydrogen being released by ammonia catalytic decomposition.4−6 Ammonia has a high hydrogen content (17.7 wt %) and high energy density (3000 Wh/kg). Therefore, NH3 provides a promising mode for the transfer and storage of hydrogen for on-site generation.7−9 Ammonia is one of the largest volume chemicals in the world and an infrastructure already exists.10 In recent years, the most extensively studied issue, the catalytic dehydrogenation of ammonia on transition metal surfaces, has become very important to industry and economics. A huge amount of experimental and theoretical investigations about the adsorption and dehydrogenation of NH3 on © XXXX American Chemical Society

Received: December 16, 2014 Revised: January 28, 2015

A

DOI: 10.1021/jp512506h J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Schematic presentation of W(111) surface used in the present studies: (a) side view and (b) top view. The T, D, and S represent top, deep, and shallow sites, respectively.

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 NH3, NH2, and NH Molecules NH3 r (Å) θ (deg) vasym vsym Do (kcal/mol) a

b

NH2

NH

calculated

experimental

calculated

experimental

calculated

experimental

1.022 106.8 3788.2 3606.6 101

1.017a 107.8a 3443.8b 3336.6b 99a

1.040 102.3 3367 3276 90

1.024a 103.4a 3301c 3219c 92f

1.023

1.038d

3173.1

3283.6e

82

84g

c

d

e

f

g

Reference 55. Reference 56. Reference 57. Reference 58. Reference 59. Reference 60. Reference 61.

The lateral cell has dimensions of a = b = 9.00 Å and c = 17.47 Å, which includes a vacuum region of thickness greater than 15 Å to ensure no interaction between the slabs. In this work, we calculate adsorption energies according to the following equation:

and energies, site preference, and relative stability of ammonia and its dehydrogenated species, as well as activation barriers, were systematically characterized.

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COMPUTATIONAL METHODS The electronic structure calculations were performed by employing periodic density functional theory (DFT) with the Vienna ab initio simulation package (VASP).40−44 The electron−ion interactions were described with the projector augmented wave (PAW) method.45,46 The generalized gradient approximation with a revised Perdew−Burke−Ernzerhof (GGA-rPBE) exchange correlation function was used in the current work.47,48 It was noted that the long-range dispersion interaction should be considered in the DFT calculation. The Brillouin zone is sampled with the Monkhorst−Pack grid.49 The calculations were performed with the (4 × 4 × 4) and (4 × 4 × 1) Monkhorst−Pack mesh k-points for bulk and surface calculations, respectively. A 400 eV cutoff energy, which allows convergence to 1 × 10−4 eV in total energy, is used. All calculations were carried out by using the spin-polarization method to describe the magnetic property of the W(111) surface model properly. The p(3 × 3) lateral cell of W(111) surface is modeled as periodically repeated slabs with six layers, as shown in Figure 1a. The bottom three atomic layers were kept frozen and set to the estimated bulk parameter, whereas the remaining layers were fully relaxed during the calculations.

Eadsorption = Eadsorbate‐surface − (Esurface + Egas‐phase molecule)

Here Eadsorbate‑surface, Esurface, and Egas‑phase molecule are the calculated electronic energies of adsorbed species on the W(111) surface, a clean W(111) surface, and a gas-phase molecule, respectively. A negative value of Eadsorption indicates an exothermic adsorption process. Vibrational frequencies of the adsorbed structures were analyzed by diagonalizing the Hessian matrix of selected atoms within the VASP approach. The nudged-elastic-band (NEB) method50−52 was applied to locate transition structures, and minimum energy pathways (MEP) were constructed accordingly.

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RESULTS AND DISCUSSION To ascertain the computational methods employed in this paper, we calculated properties of bulk tungsten, gas-phase NH3 molecule, and its fragments, NH2 and NH. Previously,53 Chen et al. have calculated the lattice constants of bulk tungsten at the PW91 and rPBE level of theory, and results showed 3.183 and 3.181 Å, respectively. These values agrees satisfactorily with the experimental value of 3.165 Å.54 Moreover, the bond B


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Figure 2. Located isomers of adsorbed NH3 on W(111) surface and their important geometry parameters calculated at the rPBE level of theory. The bond lengths are given in angstroms.

Table 2. Calculated Adsorption Energies, Relaxation Energies, Distortion Energies, Interaction Energies (kcal/mol), and Geometrical Parameters (Å) of Adsorbed NH3 Molecule on W(111) Surface adsorption site

adsorption energy

relaxation energy

WNH3(T-η1-N)-a WNH3(T-η1-N)-b WNH3(T-η1-N)-c WNH3(T,T-μ2-N,H)

−23.20 −22.47 −21.23 −19.62

3.02 3.27 1.70 2.33

0.96 −0.62

1.89 2.05

WNH3(S-η1-N)-a WNH3(S-η1-N)-b

distortion energy Top (T) 0.11 0.11 0.21 0.18 3-Fold-Shallow (S) 0.02 0.08

interaction energy

d(W−N)a

d(N−H)b

−26.33 −25.85 −23.14 −22.13

2.298 2.304 2.269 2.320

1.024/1.024/1.025 1.024/1.023/1.024 1.024/1.024/1.023 1.024/1.033/1.023

−0.95 −2.75

2.623 2.630

1.025/1.025/1.028 1.025/1.025/1.025

a

The shortest distance between the adsorbed atom (N) and the corresponding adsorption site of W surface. bThe bond lengths of NH1/NH2/NH3 are presented.

atom. At the 3-fold-deep site (D), the molecule adsorbs above the third-layer W atom. To discuss conveniently, we denote NH3/W(111), NH2/W(111), NH/W(111), N/W(111), and H/W(111) to represent the adsorption of NH3, NH2, NH, N, and H on the W(111) surface, respectively. The ammonia molecule is classified to the C3v symmetry and its electronic ground state belongs symmetry class 1A1 with the lone pair of electrons residing on the N atom. Therefore, the N atom of NH3 molecule could adsorb on a W(111) surface, and form several isomers. As results show in Figure 2, NH3/ W(111) structures are classified according to the adsorption sites which are written as follows: (1) WNH3(T-η1-N)-a, WNH3(T-η1-N)-b, WNH3(T-η1-N)-c, and WNH3(T,T-μ2N,H) adsorbed on the top site; (2) WNH3(S-η1-N)-a and WNH3(S-η1-N)-b belong to 3-fold-shallow site adsorption. As seen from Table 2, it is found that the energetically most stable structure is the adsorption at the top site, denoted as WNH3(Tη1-N)-a, with the adsorption energy of −23.20 kcal/mol. As compared to previous theoretical studies of ammonia adsorption on other transition metal surfaces,19,62−64 it is found that their computed results also reflected similar location of adsorption with our simulated outcomes. Among our favored

distance between neighboring tungsten atoms is calculated to be within the 2.750−2.754 Å range, which is also in satisfactory agreement with experimental value, 2.741 Å.54 The structures and frequencies of the isolated gas-phase molecule, NH3, and its fragments, NH2 and NH, are examined by embedding of isolated NH3 molecule, and its fragments, NH2 and NH, in a large unit cell of 25 × 25 × 25 Å3 dimensions. The calculated results and experimental properties of the molecule and its fragments are showed in Table 1. These data for ammonia molecule and its fragments, NH2 and NH, was fully compared with experimental properties, and all these results are in good agreement with the experimental values. First of all, we considered five chemical species for the fragments of ammonia decomposition by W(111) surface, such as NH3, NH2, NH, N, and H. To explore the site preference of adsorption for W(111) surface, each fragment have been placed in every possible locations. There are three adsorption sites on the W(111) surface considered in this work and characterized as top (T), 3-fold-shallow (S), and 3-fold-deep (D), as shown in Figure 1b. For the top site (T), the molecule adsorbs on the top of the first-layer W atom of W(111). At the 3-fold-shallow site (S), the molecule coordinates above the second-layer W C


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Figure 3. Located isomers of adsorbed NH2 and NH on W(111) surface and their important geometry parameters calculated at the rPBE level of theory. The bond lengths are given in angstroms.

Table 3. Calculated Adsorption Energies, Relaxation Energies, Distortion Energies, Interaction Energies (kcal/mol), and Geometrical Parameters (Å) of Adsorbed NH2 and NH Molecules on the W(111) Surface adsorption site WNH2(T-η1-N)-a WNH2(T-η1-N)-b WNH2(T,S-μ2-N) WNH(T-η1-N) WNH(T,S-μ2-N) WNH(T,T,S-μ3-N) WNH(T,T,S,D-μ4-N)

adsorption energy

relaxation energy

−94.78 −93.29 −93.38

8.39 10.08 8.39

−143.28 −142.90 −142.45 −144.37

9.37 2.45 1.42 0.47

distortion energy

For NH2 Molecule 1.13 1.02 0.67 For NH Molecule 0.56 0.42 0.37 0.38

interaction energy

d(W−N)a

d(N−H)b

−104.30 −104.39 −102.44

1.964 1.974 2.125

1.021/1.018 1.019/1.018 1.023/1.022

−153.21 −145.77 −144.24 −145.22

1.794 1.968 2.004 2.060

1.017 1.021 1.023 1.023

a

The shortest distance between the adsorbed atom (N) and the corresponding adsorption site of W surface. bThe bond lengths of NH1/NH2 are presented.

the slightly positive value of WNH3(S-η1-N)-a adsorption (0.96 kcal/mol) should be taken with caution, since there is theoretical evidence65 indicating that the DFT method used in this study might not be proper to represent long-range dispersion interaction (van der Waals interaction), and to the best of our knowledge the van der Waals complexes for NH3− W(111) interactions have not been experimentally reported. However, we assume that this kind of specific complex could be physisorbed intermediate,66 which is adsorbed by molecular adsorption and connectable to chemisorbed local minimum. For exploring the decomposition process of NH3 onto the W(111) surface, we then studied the optimized adsorption

adsorbed geometries, the distances between the nitrogen atom and the surface are about 2.298 Å. The N−H bond lengths of adsorbed NH3 are within the range of 1.023−1.025 Å which are close to the values of the isolated molecule (1.017 Å).55 Besides. the cone angle of H−N−H is slightly decreased to 106.3° as compared to gas-phase NH3 (107.8°).55 As a consequence, it is clear that the structure of the adsorbed NH3 is not significantly distorted by W(111) surface. Compared to the similar systems of NH3 on Fe,11 Rh,21 and Pt32 surfaces, it is found that our predicted W−N bond distance of WNH3(T-η1-N)-a is shorter than theirs indicating a stronger interaction between the NH3 and W(111) surface. However, D


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Figure 4. Located isomers of adsorbed N and H (in parentheses) on W(111) surface and their important geometric parameters calculated at the rPBE level of theory. The bond lengths are given in angstroms.

adsorption sites, those between top (T), shallow (S) and deep (D) sites- WX(T,S-μ2-X), and WX(T,T,S,D-μ4-X)-are much favored (N, ca. −168.3 ∼ −176.9 kcal/mol, and H, ca. −65.1 ∼ −68.4 kcal/mol) over the top site, WX(T-η1-X) (N, ca. −131.0 kcal/mol, and H, ca. −52.8 kcal/mol). According to the aforementioned results, it is found that the adsorption energy of the NHx (x = 0−3) species would increase as the number of hydrogen atom decreases. Similar calculated results were also observed by Novell-Leruth et al.,32 in which they suggested that the NHx fragments on Pt(100) and Pt(111) could reflect the increasing instability of the corresponding species in the gasphase. To more specifically understand this phenomenon, we also investigated the interaction force between the gas-phase molecule and surface via decomposing the adsorption energy into three principal components, such as relaxation energy, distortion energy, and interaction energy. The relaxation energy means energy difference between the nature and distorted surface. The distortion energy is the energy change between nature and distorted adsorbate. Finally, the interaction energy is defined as the energy alteration that takes place as the two components in their deformed geometries approach each other. As mentioned by Delbecq et al.,67 the relaxation, distortion, and interaction energies can be estimated on the basis of the following equations:

geometries/energies of NH2/W(111), NH/W(111), N/ W(111) and H/W(111) species. As expected, the coordination of NH2 and NH fragments adsorb on the W(111) surface could form several isomeric intermediates of NH2/W(111) and NH/ W(111) (shown in Figure 3). The calculated results (see Table 3) show that the possible coordinates of three isomers for the NH2 adsorbing on W(111) are WNH2(T-η1-N)-a, WNH2(Tη1-N)-b, and WNH2(T,S-μ2-N), with adsorption energies of −94.78, −93.29, and −93.38 kcal/mol, respectively. The N atom of NH2 favors the adsorption at the top site with the molecular C2-axis perpendicular to W(111) and the two hydrogen atoms pointing to the deep and shallow sites, simultaneously. For the geometry of WNH2(T-η1-N)-a with the highest adsorption energy, it is found that the bond length of N−W would dramatically decrease to 1.964 Å as compared to its original counterpart of NH3 adsorption (WNH3(T-η1-N)-a, 2.298 Å). In addition, the N−H bond lengths and H−N−H bond angle would decrease/increase to ca. 1.02 Å and 110.7°, respectively, as compared to the gaseous NH2 molecule (1.024 Å and 103.4°).55 For the NH fragment, it preferred to form the tetradentate construction, WNH(T,T,S,D-μ4-N), with an adsorption energy of −144.37 kcal/mol, which is better than the top or top/shallow sites by ca. 1.9 kcal/mol. For this preferable adsorption of WNH(T,T,S,D-μ4-N), the bond length of N−H would decrease to 1.023 Å as compared to 1.038 Å in the gas-phase.58 Besides, the distance of N−W of 2.060 Å is slightly larger than that of NH2/W(111) counterpart (1.964 Å). Finally, we also studied the adsorption of H and N atoms on the W(111). The N and H atoms adsorbing on W(111) lead to three different isomers: WX(T-η1-X), WX(T,S-μ2-X), and WX(T,T,S,D-μ4-X), where X can be either N or H (shown in Figure 4). As Table 4 indicates, the radical adsorbates (N and H atoms) adsorb strongly on the W(111) surface. Among many

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

E interaction = Eadsortion − Erelaxation − Edistortion

For example, as shown in Table 2, four larger adsorption energies are found for NH3 adsorbing around the top sites, implying that these kinds of adsorptions will be highly deformed by W(111) surface. As our expected, top site adsorption isomers have moderately higher distortion energies than that of the isomers adsorbed through the 3-fold-shallow sites (S). This indicates that the ammonia adsorption through the top site (T) can easily cause substantial distortion to the molecular NH3 structure. Consequently, for the gas-surface restructuring for WNH3(T-η1-N)-a, the contribution with the largest interaction energy (ca. −26.3 kcal/mol) demonstrates its greatest stability of all calculated NH3/W(111) conformations. However, one should note that our calculated relaxation and distortion energies are within DFT precision and because there are no experimental data available for comparison these results should be taken with caution. For the dehydrogenation processes of ammonia on a specific metal, the generally accepted mechanisms12,68,69 are as follows:

Table 4. Calculated Adsorption Energies and Geometrical Parameters (Å) of Adsorbed N and H Atoms on W(111) Surface adsorption energy WN(T-η1-N) WN(T,S-μ2-N) WN(T,T,S,D-μ4-N) WH(T-η1-H) WH(T,S-μ2−H) WH(T,T,S,D-μ4−H)

For N Atom −133.01 −168.30 −176.92 For H Atom −52.82 −68.38 −65.08

d(W−N or H)a 1.966 1.904 1.989 1.766 1.907 1.870

a

The shortest distance between the adsorbed atom (N or H) and the corresponding adsorption site of surface. E


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Figure 5. Calculated possible potential energy diagram for the decomposition of NH3 on the surface of W(111), where the numbers are the energies in kcal/mol and vi represents the imaginary frequency of that particular transition state.

NH3(g) + S → NH3(a)

(i)

NH3(a) → NH 2(a) + H(a)

(ii)

NH 2(a) → NH(a) + H(a)

(iii)

NH(a) → N(a) + H(a)

(iv)

mode was observed, and the distance of N−H bond for this dissociation is about 1.605 Å. In LM2, the first dehydrogenation process leads to NH2 formation at a top site and H adsorption between the top and shallow sites, which is also the most favorable configuration from our calculated results. We calculated also the next transition state TS2, for LM2 to form LM3 directly, but this process involves a greater energy barrier (32.66 kcal/mol) which is endothermic by 2.81 kcal/mol. The third dehydrogenation might occur from LM3 with a pertinently activated barrier, 27.84 kcal/mol for TS3, and forming N/3H/W(111), P1, in which the N is located between the top sites and the H atoms were adsorbed between top and shallow sites. The overall reaction NH3(g) + W(111) →LM1 → TS1 → LM2 → TS2 → LM3 →TS3 → P1 is calculated to be exothermic by 41.63 kcal/mol. The second path, PATH B, begins from LM4 passing through energy barrier 35.97 kcal/mol at TS4, producing LM5 that includes adsorbed NH2 on the top site position and adsorbed H between top and shallow sites; the process is exothermic by 3.21 kcal/mol. In the next transformation from LM5 to LM6 that involves the scission of another N−H bond from a NH2 adsorbate, we found a transition structure TS5, with a barrier height 29.99 kcal/mol and exothermic by 1.38 kcal/mol. Finally, the LM6 intermediate overcomes an activation barrier 29.80 kcal/mol at TS6 to break the third N−H bond and produce P2, with an exothermicity of 14.40 kcal/mol. This proposed path of reaction (NH3(g) + W(111) → LM4 → TS4 → LM5 → TS5 → LM6 →TS6 → P2) is totally exothermic by 42.19 kcal/mol. Although the LM4 is the most stable adsorption structure, the energy barrier for the first dehydrogenation process in PATH B (TS4) is obviously higher than PATH A (TS1) by 8.31 kcal/mol. One of the N−H bond length of adsorbed NH3 in LM1 is 1.033 Å which is longer than its gaseous ammonia molecule by 0.011 Å and its H atom approaches much nearer

where S signifies the catalytic metal surfaces. We constructed the potential-energy surface (PES) of NH3 + W(111) by mapping them with the NEB method, which is depicted in Figure 5. Among all tested conformations of NH3/ W(111) (shown in Figure 5), the structure WNH3(T,T-μ2N,H) has the smallest energy barrier for the first dehydrogenation of NH3 and WNH3(T-η1-N)-a is energetically the most stable configuration. Therefore, we discuss reaction paths separately for two initial intermediates WNH3(T,T-μ2-N,H) and WNH3(T-η1-N)-a, which are denoted as LM1 and LM4, respectively. In addition, the important geometric parameters of intermediates, transition structures, and products of these reactions are presented in Figure 6. As depicted in Figure 5, we categorized the examined reaction into two paths (A and B), corresponding to two possible products formation, which can be described as PATH A: R → LM1 → TS1 → LM2 → TS2 → LM3 → TS3 → P1 PATH B: R → LM4 → TS4 → LM5 → TS5 → LM6 → TS6 → P2

For PATH A, the first dehydrogenation of NH3 from LM1 produces NH2/H/W(111) (LM2) via transition structure TS1, with an energy barrier 27.66 kcal/mol and exothermic by 14.85 kcal/mol. At the transition state TS1, an imaginary frequency i1569.1 cm−1 corresponding to the N−H stretching vibration F


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Figure 6. Geometrical illustration of intermediates, transition states, and products for the NH3−W(111) interactions using the rPBE level of theory.

the W(111) surface so as to enhance the gas-surface interaction and to decrease the activation barrier. However, all of the N−H bond in the LM4 did not have this specific phenomenon. Therefore, the first dehydrogenation process for the LM1 will be much easier than that for the LM4 and we could suggest that the PATH A will be the dominant route. Among all NHx

dehydrogenation processes in PATH A, the dehydrogenation of NH2 is the highest barrier. If the thermal energy could overcome the barrier of TS2 (Ea = 32.66 kcal/mol), then all of the NHx dehydrogenation processes could take place on the surface of W(111). Although this activation energy of TS2 on the W(111) surface is slightly high, it is still lower than those G


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Figure 7. Illustration of charge-density difference for NH3 decomposition on W(111) via the proposed minimum-energy pathway: (a) before adsorption, (b) LM1, (c) LM2, (d) LM3, and (e) P1. The charge-density difference is calculated by using Δρdiff = ρ[surface + adsorbate] − ρ[surface] − ρ[adsorbate]. The values are effective charges which are calculated by Bader analysis program.

Furthermore, to be more explicit, we plotted the electronic local density of states (LDOS) of the system projected on the orbitals for the adsorbates, nitrogen and hydrogen species, as well as the W(111) substrate (shown in Figure 8). The degree of blending between adsorbate and substrate electronic states supports another evidence for clarifying the adsorption and dehydrogenation behaviors. Figure 8a shows the LDOS before the NH3−W(111) interaction. Parts b−e of Figure 8 correspond to the LDOS of LM1, LM2, LM3, and P configurations, respectively. In Figure 8a, the two maxima contributed by N atom (p orbital) and H atom (s orbital) near −2.0 and −7.5 eV exhibit the bonding orbitals of NH3. As the adsorptions and reactions proceed (see Figure 8, parts b and d), they apparently display stronger hybridization (specific states disappear unexpectedly and turn into broad in a range of −5.0 ∼ −10.0 eV) between the N (or H) atom (p or s orbital) and the Fe(111) surface (d orbital). We found also that there is one more split maximum at the left side of Figure 8d (compare to Figure 8c); in this region the overlap between N(p), H(s), and W(d) is more significant. The aforesaid results provide a rationalization of the larger molecular adsorption energy and mutual interaction reaction between the atom of adsorbate and the metal substrate. As illustrated in Figure 8e, noteworthy states coupling and pronounced broadening (much stronger hybridization) occur at the final dissociation stage of an adsorbed NH3 species into each favorable sites of the W(111) surface.

for NH3 dehydrogenations on Pt and Rh surfaces, where the rate determining activation energies of the reactions on those metal surfaces range from ca. 31.3 to 37.1 kcal/mol.23,36 As desorption processes, such as association reactions of H(a) + H(a) → H2(g) and N(a) + N(a) → N2(g), are expected to be driven by a large entropy term at raised temperatures. One can hence believe that this endothermic character for the final step desorption would be easily completed without further substantial endeavors. In order to explore interaction between ammonia and W(111) in detail, we investigated the charge-density difference for the NH3 adsorbed on the W(111) surface. Figure 7 shows diagrams of the contour surface of the electron density difference, Δρdiff = ρ[surface + adsorbate] − ρ[surface] − ρ[adsorbate], for each adsorbate/substrate system in the PATH A route. In addition, we analyze the DFT electron density of the aforementioned adsorbed intermediates and product (see in Figure 6 for atom labeling) with Bader’s method.70,71 According to these results, we can easily distinguish whether the interaction reflected in this polarization is mostly physical, i.e., involving only electrostatic and dispersion force, or contains mainly chemical contributions. From part a of Figure 7 to part b of7, the charge transferred between the W(111) surface (first layer) and the NH3 molecule is very slight (only 0.02 |e|). As the first dehydrogenation proceeds, 0.75 electron is transferred from the W(111) surface to the NH2 fragment (see Figure 7c, LM2). In the second and third dehydrogenation processes, the phenomena of charge-transfer become much obvious. This value increases to 1.34 and 2.08 electron for LM3 (from W(111) to the NH fragment, Figure 7d) and LM4 (from W(111) to the N atom, Figure 7e), respectively. As a result, one can therefore forecast that a extraordinary transfer of charge between the adsorbate and the substrate will play a pivotal role in accelerating the catalytic processes for NH3 dehydrogenation.

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CONCLUSION We have performed periodic DFT calculations to study the adsorptions and elementary steps for dehydrogenation of NHx(x = 1−3) on the W(111) surface. We have predicted completely the geometrical parameters, adsorption energies of the adsorbed NH3 and its dehydrogenated fragments. The results shows that isomers WNH3(T-η1-N)-a, WNH2(T-η1-N)a, WNH(T,T,S,D-μ4-N), WN(T,T,S,D-μ4-N), and WH(T,SH


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Figure 8. Local density of states (LDOS) for NH3 decomposition on W(111) along the proposed minimum-energy pathway: (a) before interaction and (b) LM1, (c) LM2, (d) LM3, and (e) P1. The azure, blue, and red lines represent W(d), N(p), and H(s), respectively. The dashed line represents the Fermi level.

μ2−H), are energetically favorable among all calculated structures of NH3/W(111), NH2/W(111), NH/W(111), and X/W(111) (for X = N and H atoms), respectively. In the catalytic process with the more favored route of PATH A, the NH3 dissociated processes are via a four-step mechanism and the entire process is exothermic by 41.63 kcal/mol. On the other hand, the NH3 desorption energy is lower than the energy barrier of the first dehydrogenation of NH3 on W(111) surface by ca. 8.0 kcal/mol. However, the competing reaction between desorption and the first dehydrogenation could be controlled by using the conditions of the applied pressure and temperature. All the information predicted about the reaction mechanism and the catalytic activity of various surface sites would be difficult to achieve with experimental measurements, indicating that the periodic DFT calculations might play a

essential role in the future strategic design of high-performance catalytic surfaces for the dissociation of NH3.

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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. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.-L.C. would like to acknowledge (1) the National Science Council, Republic of China, under Grant No. NSC 102-2113M-034-002-MY2 for financial support, (2) financial support by the Chinese Culture University, and (3) the National Center I


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for High-Performance Computing, Taiwan, for the use of computer time. In addition, we are deeply indebted to Professor M. C. Lin (from National Chiao Tung University (NCTU), Taiwan, and Emory University, Atlanta, GA) for persistent encouragement and instruction.

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