Computational Investigation on Adsorption and Dissociation of the

Dec 13, 2010 - Our calculations with spin-polarized density functional theory were carried out to characterize the adsorption and dissociation of the ...
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J. Phys. Chem. C 2011, 115, 521–528

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Computational Investigation on Adsorption and Dissociation of the NH3 Molecule on the Fe(111) Surface Ren-Jie Lin,† Feng-Yi Li,† and Hui-Lung Chen*,‡ Department of Chemistry, National Chung Hsing UniVersity, Taichung 402, Taiwan, and Department of Chemistry and Institute of Applied Chemistry, Chinese Culture UniVersity, Taipei, 111, Taiwan ReceiVed: September 20, 2010; ReVised Manuscript ReceiVed: NoVember 8, 2010

Our calculations with spin-polarized density functional theory were carried out to characterize the adsorption and dissociation of the NH3 molecule on the Fe(111) surface. The molecular structures and adsorbate/substrate interaction energies of NH3/Fe(111), NH2/Fe(111), NH/Fe(111), N/Fe(111), and H/Fe(111) configurations were predicted. In these calculations, four adsorption sites, such as top (T), bridge (B), 3-fold-shallow (S), and 3-fold-deep (D) sites, of the Fe(111) surface, were considered. It was shown that the barriers for the stepwise NH3 dissociation reaction, NH3(g) f N(a) + 3H(a), are 28.32 kcal/mol (for H2N-H bond activation), 28.49 kcal/mol (for HN-H bond activation), and 25.34 kcal/mol (for N-H bond activation), and the entire process is 20.08 kcal/mol exothermic. To gain insight into the catalytic activity of the Fe(111) surface for the dehydrogenation of NH3, the interaction nature between adsorbate and substrate is also analyzed by the detailed electronic analysis. Introduction Hydrogen is considered to be a desirable fuel for several reasons, among which hydrogen is the least polluting fuel to be used in an internal-combustion engine, and it can also react in a highly efficient hydrogen/oxygen fuel cell to produce electricity.1-3 As an important industrial chemical, hydrogen is also a clean fuel: the absence of objectionable combustion products is the greatest motivation for its application as a transportation fuel. Hydrogen generated from straightforward decomposition of ammonia (NH3) represents an attractive alternative to hydrocarbons for fuel cells.4,5 NH3, as one of the largest volume chemicals in the world, has already been produced on a large scale, and there exists an infrastructure for its distribution. One of the important potential applications of NH3 is its utilization as a source and storage substance for hydrogen fuel cells.6-8 Due to its high hydrogen content (17.7% by weight) and high energy density (3000 Wh/ kg), NH3 can be delivered and stored easily and therefore provides a promising mode of hydrogen storage for its on-site generation.9-11 Because of these and other technological, economical, and environmental factors, numerous reports have described the decomposition and oxidation reactions of ammonia.12-15 Recently, NH3 decomposition on transition metal surfaces has attracted steady increasing attention due to not only those reasons mentioned above but also its great industrial and economic significance. Several materials composed of transition metal have been investigated as catalysts for NH3 decomposition reactions, including metals,12,13,16-20 alloys,14 and metal oxides.21,22 Most of these studies focus on body-centered cubic (bcc) structure, crystals of large surface area, or single crystals.23 Błon´ski et al24 computed the properties of bcc iron surfaces, such as structural energetics, surface relaxation, and magnetic properties. Among those surfaces they investigated, the Fe(110) surface is the most stable one, followed by the Fe(100) surface, and the Fe(111) * Corresponding author. E-mail: [email protected]. Tel.: +8862-28610511 ext 25313. Fax: +886-2-28614212. † National Chung Hsing University. ‡ Chinese Culture University.

surface is the least stable one.24 However, the (111) facet on a small Fe crystallite is thought to have high catalytic activity from the very open surface structure,25 and this unique crystal face possesses the highest turnover rate.26 To the best of our knowledge, no theoretical study regarding the mechanism of NH3 decomposition on the Fe(111) surface is thoroughly reported. Here we report our findings with periodic density functional theory (DFT) for the adsorption and dissociation behaviors of NH3 on the Fe(111) surface. We believe that this understanding is essential in the future study of rational design of the surface catalytic model in the decomposing NH3 molecule.

Computational Methods The periodic density functional theory (DFT) calculations with the projector augmented wave (PAW) method27,28 were carried out by using the Vienna ab initio simulation package (VASP).29-33 The Generalized Gradient Approximation with revised Perdew-Burke-Ernzerhof (GGA-rPBE) exchangecorrelation functional was used.34,35 The Brillouin zone is sampled with the Monkhorst-Pack grid.36 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 performed by using the spin-polarization method to describe the magnetic property of the Fe(111) surface model properly. The p(3 × 3) lateral cell of the Fe(111) surface is modeled as periodically repeated slabs with six layers. The bottom three atomic layers were kept frozen and set to the estimated bulk parameters, whereas the remaining layers were fully relaxed during the calculations. The lateral cell has dimensions a ) b ) 12.11 Å and c ) 19.78 Å, which includes a vacuum region of thickness greater than 15 Å to ensure no interaction between the slabs. We calculated adsorption energies according to the following equation

10.1021/jp1089883  2011 American Chemical Society Published on Web 12/13/2010

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Figure 1. Schematic presentation of the Fe(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, while the middle of two top sites near each other is considered as a bridge site and labeled as B.

Eadsorption ) E[surface + adsorbate] - (E[surface] + E[adsorbate]) where E[surface + adsorbate], E[surface], and E[adsorbate] are the calculated electronic energies of adsorbed species on the Fe(111) surface, a clean Fe(111) surface, and a gas-phase molecule, respectively. Vibrational frequencies of the adsorbed structures were analyzed by diagonalizing the Hessian matrix of selected atoms within the VASP approach. The nudgedelastic-band (NEB) method37-39 was applied to locate transition structures, and minimum energy pathways (MEP) were constructed accordingly. Results and Discussion To ascertain the computational approaches used in this paper, we calculated properties of lattice parameters and the magnetic moment for bulk Fe. Previously,40 we have computed the lattice parameter of bulk Fe by using rPBE (with consideration of spin polarization) and found it to be 2.836 Å, which approaches the experimental value 2.866 Å41 more closely than for the PW91 and PBE levels, 2.818 and 2.817 Å, respectively. At the rPBE level, the calculated magnetic moment of bulk Fe40 is 2.23 µB and agrees satisfactorily with the experimental value, 2.22 µB.41

Lin et al. In addition, it has been shown previously42 that the predicted chemisorption energies of small molecules on some metal surfaces at the rPBE functional are in better agreement with the experimental values than the PW91-calculated. For example, the PW91 functional gives too large chemisorption energies numerically by about 14 kcal/mol, while the rPBE functional proves to be more accurate (less than 5 kcal/mol discrepancy) for CO on Ni(111) and Pd(111) surfaces.42 Furthermore, the selection of the rPBE level yields not only reliable geometries but also adsorption energies, which has been examined and confirmed in the analogous systems of H2 adsorption and CO and H2 coadsorption on the Fe(111) surface.43 There are five chemical species in the NH3 decomposition reaction on the Fe(111) surface, i.e., NH3, NH2, NH, N, and H. We used the above chemical species to explore the possible adsorption sites on the Fe(111) surface. There are four adsorption sites on the Fe(111) surface considered in this study and characterized as top (T), bridge (B), 3-fold-shallow (S), and 3-fold-deep (D), as shown in Figure 1. For the top site (T), the molecule adsorbs on the top of the first-layer Fe atom of Fe(111). At the bridge site (B), the molecule adsorbs above the center of the Fe-Fe bond of the two nearest Fe sites. At the 3-fold-shallow site (S), the molecule coordinates above the second-layer Fe atom. At the 3-fold-deep site (D), the molecule adsorbs above the third-layer Fe atom. To facilitate the discussion, we denote NH3/Fe(111), NH2/Fe(111), NH/Fe(111), N/Fe(111), and H/Fe(111) to represent the adsorption of NH3, NH2, NH, N, and H on the Fe(111) surface, respectively. Due to the C3V symmetry, the electronic ground state of the NH3 molecule belongs to symmetry class 1A1 with the unpaired electron residing on the N atom. As expected, the NH3 molecule can adsorb on a Fe(111) surface in several isomeric forms. The resulting NH3/Fe(111) structures (shown in Figure 2) can be classified according to the adsorption sites. FeNH3(T-η1-N)-a and FeNH3(T-η1-N)-b belong to top site adsorption. FeNH3(Bµ2-N, H) is bridge site adsorption. FeNH3(S-η1-N)-a and FeNH3(S-η1-N)-b belong to 3-fold-shallow site adsorption. FeNH3(D-η1-N) is 3-fold-deep site adsorption. As shown in Table 1, our results indicate that FeNH3(T-η1-N)-a is energetically the most stable among all calculated NH3/Fe(111) structures

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

NH3 Molecule on the Fe(111) Surface

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TABLE 1: Calculated Adsorption Energy, Relaxation Energy, Distortion Energy, Interaction Energy (kcal/mol), and Geometrical Parameters (Å) of the Adsorbed NH3 Molecule on the Fe(111) Surface adsorption site top (T) FeNH3(T-η1-N)-a FeNH3(T-η1-N)-b bridge (B) FeNH3(B-µ2-N, H) 3-fold-shallow (S) FeNH3(S-η1-N)-a FeNH3(S-η1-N)-b 3-fold-deep (D) FeNH3(D-η1-N)

adsorption energy

relaxation energy

distortion energy

interaction energy

d(Fe-N)a

d(N-H)b

-16.25 -13.97

0.16 0.57

0.18 0.12

-16.60 -14.66

2.118 2.150

1.021/1.023/1.021 1.023/1.023/1.023

-12.81

0.81

0.12

-13.75

2.170

1.022/1.031/1.022

-0.26 0.87

2.06 2.25

0.09 0.03

-2.41 -1.41

2.472 2.503

1.023/1.025/1.025 1.024/1.022/1.025

1.82

0.14

0.04

1.64

3.951

1.022/1.023/1.020

a

b

The shortest distance between the adsorbed atom (N) and the corresponding adsorption site of the Fe surface. The bond length of NH1/ NH2/NH3 is presented.

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

with an adsorption energy of -16.25 kcal/mol. FeNH3(T-η1-N)-a also possesses the shortest Fe-N bond and three evidently extended N-H bonds, which indicate a strong interaction between the NH3 and the catalyst. However, one should notice that the calculated FeNH3(S-η1-N)-b and FeNH3(D-η1-N) adsorption energies (slightly positive values) are within DFT precision and should be taken with caution. Experimental investigation of the ammonia adsorption has been reported in detail by Ertl and co-workers.44-46 They examined the catalytic process of an ammonia molecule onto Fe(100), Fe(110), and Fe(111) surfaces, and the observed adsorption energies were in the range of -10 ∼ -17 kcal/mol, in reasonable agreement with our predicted values, -16.25 and -13.97 kcal/mol, for isomers FeNH3(T-η1-N)-a and FeNH3(T-η1-N)-b, respectively. On the other hand, computational study (DFT, B3LYP method) of the ammonia adsorption onto the Fe(111) surface performed by Satoh et al.47 predicted that the adsorption through the top site of the Fe(111) surface gives the largest optimized adsorption energy with a value of -21.77 kcal/mol and the shortest bond distance (Fe-N) of 2.20 Å. To be more specific, we also performed a detailed

analysis by decomposing the adsorption energy into three principal components, i.e., the relaxation energy of the Fe(111) surface, the distortion energy of NH3, and the interaction energy between NH3 and the Fe(111) surface, as described by Delbecq et al.48 The relaxation, distortion, and interaction energies were calculated according to the following equations

Erelaxation ) E[distorted surface] - E[surface] Edistortion ) E[distorted adsorbate] - E[adsorbate] Einteraction ) Eadsorption - Erelaxation - Edistortion The relaxation and distortion energies (Erelaxation and Edistortion) are the energetic costs for bringing the surface and the adsorbate from their equilibrium geometry to the geometry they have in the ultimate system, while the interaction

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TABLE 2: Calculated Adsorption Energy, Relaxation Energy, Distortion Energy, Interaction Energy (kcal/mol), and Geometrical Parameters (Å) of the Adsorbed NH2 and NH Fragments on the Fe(111) Surface adsorption energy

relaxation energy

distortion energy

interaction energy

d(Fe-N)a

d(N-H)b

for NH2 molecule FeNH2(T-η1-N)-a FeNH2(T-η1-N)-b FeNH2(T,S-µ2-N)

-55.06 -53.44 -64.80

0.84 1.12 1.96

1.07 0.44 0.70

-56.96 -54.99 -67.46

1.850 1.868 1.989

1.016/1.018 1.022/1.020 1.021/1.022

for NH molecule FeNH(T-η1-N) FeNH(T,S-µ2-N) FeNH(T,T,S-µ3-N) FeNH(T,T,S,D-µ4-N)

-71.80 -89.89 -96.43 -95.62

1.68 1.85 5.19 2.35

0.34 0.38 0.17 0.31

-73.82 -92.12 -101.79 -98.28

1.622 1.816 1.907 1.903

1.024 1.024 1.030 1.028

adsorption site

a The shortest distance between the adsorbed atom (N) and the corresponding adsorption site of the surface. b The bond length of NH1/NH2 is presented.

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

energy (Einteraction) is the energy alteration that takes place as the two components in their deformed geometries approach each other. As shown in Table 1, it is found that the top site adsorbed NH3 geometries have two larger adsorption energies. Hence, one can predict easily that these two adsorbed NH3 on top-site positions will be highly deformed by Fe(111), leading to the enhancement of their distortion energies. As expected, we found that both FeNH3(T-η1-N)-a and FeNH3(Tη1-N)-b isomers possess moderately higher distortion energies than an isomer adsorbed through the 3-fold-shallow site (S) and/or the 3-fold-deep site (D). This indicates that the ammonia adsorption through the top site (T) can create substantial distortion to the NH3 molecular structure. Consequently, for the gas-surface restructuring for FeNH3(T-η1N)-a, the contribution with the larger interaction energy (ca. -16.6 kcal/mol) explains its greatest stability of all calculated NH3/Fe(111) structures. Understanding the nature of Fe-NH2, Fe-NH, Fe-N, and Fe-H interactions and structures and energetics of NH2/Fe(111), NH/Fe(111), N/Fe(111), and H/Fe(111) species is significant for exploring the decomposition of NH3 onto the Fe(111) surface. As expected, the coordination of NH2 and NH fragments onto the Fe(111) surface leads to formation of NH2/Fe(111) and NH/Fe(111) intermediates that might exist in several isomeric forms (shown in Figure 3). Our calculations (see Table 2) show that the isomer FeNH2(T,S-µ2-N) with NH2 coordinated to Fe

through the N atom is energetically the most favored conformation among all calculated structures of NH2/Fe(111). The shortest Fe-N bond length of bidentate FeNH2(T,S-µ2-N) is 1.989 Å with adsorption energy of -64.80 kcal/mol. Different from NH2, the NH fragment is likely to form the tridentate isomer FeNH(T,T,S-µ3-N). The adsorption energy of the tridentate adsorbate FeNH(T,T,S-µ3-N) is -96.43 kcal/mol, and the Fe-N bond length is around 1.91 Å. The N-H bond length (1.030 Å) in NH/Fe(111) is somewhat shorter than that in its gas phase (1.036 Å).49 The coordination of N and H atoms on Fe(111) leads to N/Fe(111) and H/Fe(111) intermediates, respectively, as shown in Figure 4. These species might exist in five different isomers: FeX(T-η1-X), FeX(B-µ4-X), FeX(T,S-µ2-X), FeX(S-η1-X), and FeH(D-η1-H), where X can be either N or H. As indicated in Table 3, the radical adsorbates of N and H atoms adsorb strongly to the Fe(111) surface. Among many adsorption sites, those between top (T) and shallow (S) as well as bridge (B) sitessFeX(T,S-µ2-X) and FeX(B-µ4-X)sare favored (N, ca. -115.3 ∼ -131.4 kcal/mol, and H, ca. -59.9 ∼ -61.3 kcal/ mol) over the top site, FeX(T-η1-X) (N, ca. -91.2 kcal/mol, and H, ca. -47.0 kcal/mol). The generally accepted mechanism20,50,51 involved in the stepwise dissociation of H atoms from the NH3 molecule on a specific metal catalyst is as follows

NH3 Molecule on the Fe(111) Surface

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TABLE 3: Calculated Adsorption Energy, Relaxation Energy, Distortion Energy, Interaction Energy (kcal/mol), and Geometrical Parameters (Å) of Adsorbed N and H Atoms on the Fe(111) Surface adsorption site

adsorption energy

d(Fe-N or H)a

for N atom FeN(T-η1-N) FeN(B-µ4-N) FeN(T,S-µ2-N) FeN(S-η1-N)

-91.17 -131.40 -115.28 -112.75

1.559 1.783 1.720 1.812

for H atom FeH(T-η1-H) FeH(B-µ4-H) FeH(T,S-µ2-H) FeH(S-η1-H) FeH(D-η1-H)

-47.01 -59.82 -61.28 -59.92 -52.06

1.588 1.699 1.662 1.610 1.664

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

NH3(g) + S f NH3(a)

(i)

NH3(a) f NH2(a) + H(a)

(ii)

NH2(a) f NH(a) + H(a)

(iii)

NH(a) f N(a) + H(a)

(iv)

where S represents the metal catalyst. The FeNH3(T-η1-N)-a conformer is energetically the most stable among all conformers of NH3/Fe(111) under study, and therefore we chose it as the initial structure (denoted as LM1) to study the decomposition of NH3 on Fe(111) at a 1/9 monolayer (ML) coverage. To investigate the details of these reactions, we constructed potential-energy surfaces by mapping them with the NEB method, which is depicted in Figure 5. The important geometric parameters of intermediates, transition structures, and products of these reactions are presented in Figure 6. The resulting profile reveals that the formation of LM1 is an exothermic reaction by -16.25 kcal/mol and occurs smoothly along the MEP by shortening the two N-H bonds from 1.052 to 1.023 Å and the Fe-N bond from 4.063 to 2.118 Å, without a well-defined transition state. The first dehydrogenation of NH3 from LM1 is also exothermic (-3.10 kcal/mol with respect to reactants, NH3(g) + Fe(111) surface) and produces NH2/H/ Fe(111), LM2, via transition structure TS1, with an energy barrier of 28.32 kcal/mol. In LM2 (see Figure 5), the adsorbed NH2 group remains on the top site position, and the H atom is located between the top and shallow sites, which is the most stable conformation according to our calculated results. The next transformation from LM2 to LM3 is an endothermic reaction (17.55 kcal/mol with respect to reactants) and involves the further scission of the N-H bond from the NH2 adsorbate, and we located a transition structure TS2, with a barrier of 28.49 kcal/mol. The final dehydrogenation is from LM3 to LM4, which includes FeN(T-η1-N) plus three FeH(T, S-µ2-H) fragments, with a pertinent energy barrier, 25.34 kcal/mol for TS3. This is an endothermic reaction (26.44 kcal/mol with respect to reactants). As for LM4 (see Figure 5), the adsorbed N atom is on the top site position, which is not the most stable one among N/Fe(111) structures in this study. The N atom could thus readily diffuse to the more stable site, FeN(B-µ4-N), to form another N/3H/Fe(111) product, P. This process is found to be 20.08 kcal/mol exothermic and with a small energy barrier

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

(4.32 kcal/mol) at transition structure TS4. According to our results, we propose that the overall reaction is NH3(g) + Fe(111) f FeNH2(T-η1-N)-a (LM1) f LM2 f LM3 f LM4 f P, an exothermic reaction (20.08 kcal/mol with respect to reactants). Among all NHx dehydrogenation processes, the dehydrogenation of NH2 has the highest barrier. If thermal energy could overcome the barrier of TS2 (Ea ) 28.49 kcal/mol), then all of the NHx dehydrogenation processes could take place on the surface of Fe(111). Although this barrier on the Fe(111) surface is relatively high, it is still lower than those for NH3 dehydrogenation on Pt and Rh surfaces, where the ratedetermining barriers of the reactions on these metal surfaces range from ca. 31.3 to 37.1 kcal/mol.13,52 Figure 7 shows diagrams of the contour surface of the electron density difference, ∆Fdiff ) F[surface + adsorbate] - F[surface] - F[adsorbate], for each adsorbate/substrate system in the path of NH3 dehydrogenation onto the Fe(111) surface. In addition, Bader’s method53 was also used to analyze the DFT electron density of the aforementioned adsorbed intermediates (see the inset in Figure 6 for atom labeling). Judging from these analyses, one can distinguish whether the interaction reflected in this polarization is mostly physical, i.e., involving only electrostatic and dispersion forces, or comprises principally chemical contributions. To examine the changes from Figure 7a to 7b, it is found that the charge transfer between the surface Fe atoms (first layer) and the NH3 molecule is rather subtle. As the reaction proceeds (see Figures 7c), 0.53 electron is transferred from Fe(111) to the NH2 fragment in LM2, but this value increases to 0.68 and 0.73 electron for LM3 (from Fe(111) to the NH fragment, Figure 7d) and LM4 (from Fe(111) to the N atom, Figure 7e), respectively. As a consequence, one can hence predict that a conspicuous transfer of charge between the adsorbate and the substrate will play a crucial role in accelerating the catalytic processes for NH3 dehydrogenation. After the atomic nitrogen species become adsorbed at their most stable site (Figure 7f), the effective charge becomes somewhat positive (0.24 |e|). This phenomenon corresponds to the back-donation of the lone-pair electron of the nitrogen atom to the empty d-orbital of a Fe atom, which would lead to the enhancement of their mutual interaction between adsorbate and Fe(111). Furthermore, to be more meticulous, we plotted the electronic local density of states (LDOS) of the system projected on the

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

Figure 7. Illustration of charge-density difference for NH3 decomposition on Fe(111) via the proposed minimum-energy pathway: (a) before adsorption, (b) LM1, (c) LM2, (d) LM3, (e) LM4, (f) P. The charge-density difference is calculated by using ∆Fdiff ) F[surface + adsorbate] - F[surface] - F[adsorbate]. The isosurfaces are set at 0.02 eÅ3. The values are effective charges which are calculated by the Bader analysis program.

orbitals for the adsorbed nitrogen and hydrogen species, as well as the Fe(111) substrate (shown in Figure 8). The degree of mixing between adsorbate and substrate electronic states serves as another piece of evidence for explaining the complicated reaction scenario. Figure 8a shows the LDOS before the NH3-Fe(111) interaction. Figures 8b-8f correspond to the LDOS of LM1, LM2, LM3, LM4, and P configurations, respectively. These results apparently demonstrate that the stronger interaction (additional states emerge unexpectedly in the range -5.0 ∼ -10.0 eV) between the N (or H) atom and the Fe(111) surface. As these reactions progress (from Figure

8a to 8e), the presence of common peaks in the N (or H) and Fe PDOS suggests strong mixing of the two sets of electronic states with the characteristics of extraordinary hybridation. As illustrated in Figure 8f, noteworthy broadening occurs (strong hybridation) at the final dissociation stage of adsorbed N and H atoms into the most favorable sites of the Fe substrate. Conclusion In summary, our calculated results demonstrate that the Fe(111) surface exhibits catalytic activity to decompose NH3.

NH3 Molecule on the Fe(111) Surface

J. Phys. Chem. C, Vol. 115, No. 2, 2011 527 molecule dehydrogenation on the Fe(111) surface, which are calculated by the VASP program. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 8. Local density of states (LDOS) for NH3 decomposition on Fe(111) along the proposed minimum-energy pathway: (a) before interaction, (b) LM1, (b) LM2, (c) LM3, (d) LM4, and (f) P. The black, blue, and red lines represent Fe(d), N(p) and H(s), respectively. The dashed line represents the Fermi level.

The data show that isomers FeNH3(T-η1-N)-a, FeNH2(T,Sµ2-N), FeNH(T,T,S-µ3-N), FeN(B-µ4-N), and FeH(T,S-µ2H), are energetically favored among all calculated structures of NH3/Fe(111), NH2/Fe(111), NH/Fe(111), and X/Fe(111), for X ) N and H atoms, respectively. The catalytic process is likely to proceed via a four-step mechanism, which agrees satisfactorily with experimental preditions, and the system must, however, receive heat for desorption of molecular dihydrogen. This information with regard to the reaction mechanism, the catalytic activity of various surface sites, and the relevance of the surface structure would be otherwise arduous to achieve with experimental measurements, indicating that the periodic DFT calculations might play a significant role in the reasonable explication of NH3 dissociation behavior in heterogeneous catalysis. Acknowledgment. H.-L. Chen would like to acknowledge the (1) National Science Council, Republic of China, under Grant Number NSC 98-2113-M-034-002-MY2, for the financial support; (2) the financial support by Chinese Culture University; and (3) National Center for High-performance Computing, Taiwan, for the use of computer time. In addition, we are deeply indebted to Professor M. C. Lin (from NCTU, Taiwan, and Emory University, USA) for persistent encouragement and instruction. Supporting Information Available: Table S1∼Table S10: Optimized geometries in Cartesian for the mechanism of NH3

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