Mechanistic Investigation of the Catalytic Decomposition of Ammonia

Article pubs.acs.org/JPCC

Mechanistic Investigation of the Catalytic Decomposition of Ammonia (NH3) on an Fe(100) Surface: A DFT Study Sang Chul Yeo,† Sang Soo Han,‡ and Hyuck Mo Lee*,† †

Department of Materials Science and Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea Center for Computational Science, Korea Institute of Science and Technology (KIST), Hwarangno 14-gil 5, Seongbuk-Gu, Seoul 136-791, Republic of Korea

‡

S Supporting Information *

ABSTRACT: Catalytic decomposition of ammonia (NH3) is a promising chemical reaction in energy and environmental applications. Density functional theory (DFT) calculations were performed to clarify the detailed catalytic mechanism of NH3 decomposition on an Fe(100) surface. Specifically, the elementary steps of the mechanism were calculated for the general dehydrogenation pathway of NH3. The adsorption of two types of ammonia dimers (2NH3), locally adsorbed NH3 and hydrogen-bonded NH3, were then compared, revealing that locally adsorbed NH3 is more stable than hydrogen-bonded NH3. By contrast, the dehydrogenation of dimeric NH3 results in a high energy barrier. Moreover, the catalytic characteristics of NH3 decomposition on a nitrogen (N)-covered Fe surface must be considered because the recombination of nitrogen (N2) and desorption have an extremely high energy barrier. Our results indicate that the catalytic characteristics of the NH3 decomposition reaction are altered by N coverage of the Fe surface. This study primarily focused on energetic and electronic analysis. Finally, we conclude that Fe is an alternative catalyst for the decomposition of NH3 in COx-free hydrogen production. NH3 decomposition follows a dehydrogenation mechanism.8 The rate-limiting step for NH3 decomposition on an Fe catalyst remains controversial. Lanzani and Laasonen suggested that the rate-limiting step is the first dehydrogenation step (NH3 → NH2 + H), which occurs in a nanosized Fe cluster.23 Lin et al.39 investigated the adsorption and dissociation of NH3 on Fe(111) and found that the third dehydrogenation reaction (NH → N + H) is the rate-limiting step. Conversely, the major rate-limiting step in catalytic NH3 decomposition is reported as the associative desorption of N2 on the surface.28 While these findings have provided insight into the decomposition of NH3 on an Fe surface, the intermediate species and a detailed catalytic mechanism have not yet been elucidated. A comprehensive DFT study would facilitate the determination of the complete reaction pathway of NH3 decomposition on Fe-based catalysts. Maier et al.40 reported a combined experimental (scanning tunneling microscopy)−theoretical (DFT) approach, which was used to demonstrate that one NH3 molecule (called αammonia) is chemisorbed to the surface via formation of an N−Ru chemical bond and another NH3 molecule (β-ammonia) is hydrogen bonded to the α-ammonia in the adsorption of an ammonia (NH3) dimer on Ru(0001). Similar dimer config-

1. INTRODUCTION Ammonia (NH3) is a toxic compound that can cause environmental pollution. However, NH3 can also provide COx-free hydrogen (H2) production through the decomposition of the NH3 molecule, which is a promising technology in a hydrogen economy.1−6 Thus, the catalytic performance and surface chemistry of NH3 decomposition have received considerable attention. A fundamental understanding of catalysts for the decomposition of NH3 has become very important.7−15 In general, Ru and Ir are the most active catalysts for the decomposition of NH3.16−22 However, they are expensive, and their supplies are limited. Less expensive alternatives (such as Ni-, Co-, and Fe-based catalysts) with good catalytic activity are needed.23−28 Consequently, NH3 decomposition on a variety of metal surfaces has been extensively studied. Several fundamental studies concerning NH3 decomposition have been performed using materials such as Ir29−32 and Ru.21,33−37 Recently, Duan et al.28 studied NH3 decomposition on inexpensive alternative catalysts, such as Ni(111), Co(111), and Fe(110) surfaces, using density functional theory (DFT) calculations. These authors compared the catalytic reactivity, including the adsorption of NH3 and the energy barrier for the N2 recombination reaction, and also experimentally determined that Fe nanoparticles on carbon nanofibers are highly active and stable due to the supporting effect.38 © 2014 American Chemical Society

Received: November 6, 2013 Revised: February 15, 2014 Published: February 18, 2014 5309

dx.doi.org/10.1021/jp410947d | J. Phys. Chem. C 2014, 118, 5309−5316

The Journal of Physical Chemistry C

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urations were also determined for Ni(111)41 and Pt(111).42 These findings suggest that the adsorption of dimeric NH3 on an Fe surface will be similar, although this reaction has not been studied. In addition, according to Li et al.,43 N atoms that are strongly adsorbed on the Fe surface inhibit the NH 3 decomposition reaction. These authors used a plasma-driven catalyst to increase the NH3 conversion from 7.8% to 99.9%. These findings indicate the importance of considering the effects of an N-covered Fe surface on NH3 decomposition reactions. In this study, we investigated the catalytic performance of an Fe(100) surface in the decomposition reaction of monomeric NH3. To clarify the exact mechanism, we calculated the elementary steps, including the adsorption energy (Ead), energy barrier (Eb), and reaction energy (ΔE). The results indicated that the catalytic properties of monomeric NH3 dehydrogenation on Fe are similar to those on the most active catalysts, such as Ru and Ir.29,30,32,44 Subsequently, we investigated the adsorption of two types of dimeric ammonia (2NH3) on the Fe surface: locally adsorbed NH3 and hydrogen-bonded NH3. Locally adsorbed NH3 was more stable than hydrogen-bonded NH3. Thus, we confirmed that the catalytic activity in the decomposition reaction of 2NH3 is altered due to locally adsorbed NH3. We also performed a quantitative analysis of the effect of surface coverage using energetic and electronic analyses, thereby reconsidering NH3 decomposition as a function of the surface coverage of N on the Fe surface. This study contributes to the fundamental understanding of the structural, energetic, and catalytic properties of Fe catalysts for NH3 decomposition and explains why Fe-based catalysts present major disadvantages for application in hydrogen (H2) production.

To determine the transition states and energy barriers for NH3 decomposition reactions on the Fe surface, we employed the climbing image nudged elastic band method (CI-NEB) and accordingly constructed the minimum energy path.56 Moreover, we calculated the density of state to investigate electronic structures for each geometry.57 To estimate the catalytic activity, the adsorption energy (Ead), energy barrier (Eb), and reaction energy (ΔE) were calculated for each reaction. In addition, to analyze the electronic structure, we evaluated the energy of the d-band center, εd, for the 4d electronic state. These equations are provided in the Supporting Information.

3. RESULTS AND DISCUSSION 3.1. Determination of Adsorption Intermediates. To investigate NH3 decomposition reactions, we considered an Fe(100) surface, which is the stable Fe surface.28,39,58 For the Fe(100) surface, we employed a (3 × 3) supercell containing a five-layer slab in which the bottom two layers were fixed and all the other atoms are allowed to relax freely. The vacuum layer was up to 15 Å, as shown in Figure 1. Hollow (h), bridge (b), and top (t) sites were considered as adsorption sites for NH3, NH2, NH, N, and H, as summarized in Table 1.

2. COMPUTATIONAL METHODS We performed DFT calculations within a plane wave basis set using the Vienna Ab-initio Software Package (VASP) to investigate the atomic and electronic structures of the Fe surfaces, in which the project augmented wave method45−48 with a frozen-core approximation was used. The spin-polarized DFT calculation was performed using the generalized gradient approximation of the Perdew−Burke−Ernzerhof (PBE) exchange-correlation functional.49 We studied adsorption and dissociation of NH3 on Fe(100) surface. In a previous study,50 the Norskov group reported that the PBE functional well describes the experimental data of N adsorption on Fe(100) surface in DFT calculation. And the type of PBE functional is believed to give more accurate energies than the type of PW91, especially in the case of adsorption of atoms and molecules, energetically.51 As well, the Cater group reported that the physical properties of Fe system computed with PBE functional is in good agreement with the experiment values.52−54 Therefore, the PBE functional is able to correctly describe our system. Moreover, an energy cutoff of 400 eV was used, and a (5 × 5 × 1) Monkhorst−Pack grid was considered for k-point sampling. The geometries were optimized using a conjugate gradient method until the maximum force in every degree of freedom was less than 0.03 eV/Å. For the relaxation of the electronic degrees of freedom, we achieved a total energy convergence of up to 10−6 eV. This DFT calculation provides a lattice parameter of 2.83 Å, a bulk modulus of 1.76 Mbar, a cohesive energy of 5.16 eV, and a magnetic moment of 2.21 μB for the bcc Fe; this value of the magnetic moment is close to reported values.55

Figure 1. Structural configuration of a 3 × 3 supercell containing five layers of Fe(100). The h in a black circle denotes a hollow site, b denotes a bridge site, and t denotes a top site. These sites are the preferred adsorption sites for (a) NH3, (b) NH2, (c) NH, (d) N, and (e) H. The purple, blue, and white spheres represent Fe, N, and H atoms, respectively.

An NH3 species is preferentially adsorbed at the top sites with a binding energy of −0.92 eV through the formation of an Fe−N chemical bond with C3v symmetry because a lone pair electron from the N atom interacts with the d-orbital of an Fe atom. Here, the N−H and Fe−N bond distances are 1.03 and 2.17 Å, respectively. Upon adsorption, the H−N−H bonding angle is 107.7°, similar to the value obtained in a previous theoretical study of NH3 adsorption on Fe(110).28 Grunze et al.7 experimentally reported that the adsorption energies of NH3 on Fe(100) are in the range of −0.43 to −0.74 eV, in reasonable agreement with our calculated values. An NH2 5310



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Table 1. Adsorption Properties of NHx (with x = 0−3) Intermediates on Fe(100)a species NH3 NH2 NH N H NH3 (molecule)

adsorption site

Ead (eV)

dN−H (Ǻ )

dN−Fe (Ǻ )

∠HNH (deg)

top bridge hollow hollow bridge

−0.92 −3.01 −5.22 −7.35 −0.64

1.026 1.024 1.037

2.173 1.515 0.857 0.432

107.7 107.8

1.021

106.5

a

The energy was calculated with respect to the corresponding species in the gas phase and with respect to the NHx (with x = 0−3) species and the H atom.

species prefers to adsorb at the bridge site with an adsorption energy of −3.01 eV and N−H and Fe−N bond distances of 1.02 and 1.52 Å, respectively. Furthermore, an NH species is adsorbed at the hollow site with a binding energy of −5.22 eV and N−H and Fe−N bond distances of 1.04 and 0.86 Å, respectively. The preferential adsorption site for an N atom is a hollow site with a binding energy of −7.35 eV, while that for an H atom is a bridge site with a binding energy of −0.64 eV.55 Previously,59 we reported that an N2 molecule prefers a hollow site with an adsorption energy of −1.03 eV, with an N−N distance that is 1.243 Å longer than that of N2 in the gas phase. The adsorption energies of NHx (with x = 1−3) on Fe(100) exhibit a trend of NH > NH2 > NH3. This trend can be analyzed using projected density of states (PDOS) and d-band center theory, which indicates the trend in the reactivity of metals or the effects of alloying, surface configuration, and adsorbate−adsorbate interactions.60 The d-band center is calculated as shown in the Supporting Information. The DOS for the d-band center of the Fe atoms that are in contact with the NH3 species and the p-band of the nitrogen atom in the NHx (with x = 0−3) intermediate states were determined for the NH3, NH2, and NH species. The Fermi level was set to 0 (Figure S1). The black line denotes a clean Fe(100) surface prior to the adsorption of each species. By contrast, the red line indicates the adsorption conditions on the surface. We focused on the d-band center (εd) of each species (NH3, NH2, and NH) on the Fe(100) surface and determined εd = −0.87, −0.57, and −0.02, respectively. The d-band center of each species on the Fe(100) is close to the Fermi level, in the order NH3 < NH2 < NH. These features are similar to those describing the adsorption of NHx (with x = 0−3) species on Fe(100), as shown in Figure S1. 3.2. Monomeric Ammonia (NH3) Decomposition on Fe(100). To obtain a reaction pathway diagram for NH3 decomposition on Fe(100), we first investigated the relative elementary reaction steps or the NH3 dehydrogenation, as shown in Figure 2. The sequence of intermediate steps for NH3 decomposition can be written as follows: NH3* + * → NH 2* + H*

(1)

NH 2* + * → NH* + H*

(2)

NH* + * → N* + H*

(3)

Figure 2. Reaction pathway diagram of ammonia (NH3) decomposition on Fe(100). TS indicates the transition state.

reaction of NH3, which is important to estimate the catalytic activity. The initial and final configurations were determined at the lowest energy barrier from the calculations for the different reaction pathways. The upper three reaction steps for the NH3 decomposition reaction can also be considered geometrical changes, and they were defined as the initial, transition, and final states (IS, TS, and FS, respectively) of the elementary steps. Reaction 1 begins with the stable configuration at the top site, followed by dehydrogenation into NH2 at the bridge site and the presence of H at the bridge site. This step has an energy barrier of 0.95 eV and is exothermic by −0.26 eV. For the second reaction 2, NH2 begins with a stable configuration at the bridge site, followed by dehydrogenation into NH at a hollow site and H at the bridge site. This step has an energy barrier of 1.14 eV and is exothermic by −0.29 eV. For the final reaction 3, NH begins with a stable configuration at a hollow site, followed by dehydrogenation into N at a hollow site and H at the bridge site. This step has an energy barrier of 0.78 eV and is exothermic by −0.46 eV. On the basis of these results, we concluded that the rate-liming step for the decomposition of NH3 over an Fe(100) surface is the decomposition of NH2 to NH and H (reaction 2) because this step shows the largest energy barrier. To provide detailed information on the origin of the reaction energy barrier of the elementary reaction steps, we resolved the reaction energy barrier (Eb) by dividing it into the following six terms: def IS TS E b = ΔEsub + ΔEAB + EAB + E int − EATS − E BTS

(4)

where ΔEsub represents the energy of the substrate, which changes from its IS to the TS on the barrier (ΔEsub = ΔETS sub − def ΔEIS ), and ΔE is the deformation energy of the reactant sub AB (AB), which reflects the effect of the structural deformation of AB from the IS to the TS. A and B represent the adsorbates such as NHx (with x = 0−3) species and the H atom, respectively. IS and TS indicate initial state and transition state. This effect is also reflected in the way in which the N−H bonds are separated. EIS AB is called the binding energy of AB at the IS and is equal to the absolute adsorption energy of AB at the IS. ETS int represents the interaction energy between A and B at the TS. These four terms positively contribute to the energy TS barrier. Two other terms, ETS A and EB , negatively affect the

Here, the asterisk (*) denotes an empty site, and an NHx (with x = 0−3) intermediate species is adsorbed onto the Fe surface. We focused on the rate-limiting step for the decomposition 5311



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Table 2. Energy Barriers and Contribution Factors (eV) for the Elementary Reactionsa reactions

ΔEsub

ΔEdef AB

EIS AB

ETS int

ETS A

ETS B

Eb

NH3 → NH2 + H NH2 → NH + H NH → N + H NH3 + NH3 → NH3 + NH2 + H NH3 + NH2 + H → NH3 + NH + 2H NH3 + NH + 2H → NH3 + N + 3H

0.03 0.05 0.19 0.09 0.12 0.26

3.56 2.21 1.28 3.65 2.73 1.55

0.92 3.01 5.22 0.48 2.36 3.34

1.26 1.98 2.57 1.61 2.23 2.92

2.38 3.69 5.86 2.41 3.68 4.63

2.44 2.42 2.62 2.45 2.43 2.65

0.95 1.14 0.78 0.97 1.33 0.79

a The energy was calculated with respect to the elementary reaction steps. A and B represent the adsorbates of the NHx (with x = 0−3) species and the H atom, respectively. IS and TS indicate initial state and transition state.

TS energy barrier. ETS A (EB ) represents the binding energy of the A (B) intermediate species in the TS without considering B (A). These values are shown in Table 2. To analyze the direct NH3 IS TS TS decomposition reaction, the four terms ΔEdef AB, EAB, Eint , and EA def should be studied in detail. The value of ΔEAB decreases, TS TS whereas those of EIS AB, Eint , and EA increase as the direct dehydrogenation process of NH3 proceeds. In particular, the relatively large interaction energy (ETS int ) at the TS leads to a higher energy barrier. The relatively small binding energies of TS EIS AB and EA lead to lower energy barriers. In addition, ΔEsub and ETS do not have any less effect on the energy barrier (Eb), B which is not as easily altered as the other terms. 3.3. Dimeric Ammonia (2NH3) Decomposition on Fe(100). For the dimeric ammonia (2NH3) decomposition, we considered the adsorption of two types of NH3 dimers. Preadsorbed NH3 leads to an additional locally adsorbed NH3 at the top site. Alternatively, an additional hydrogen-bonded NH3 is possible, as shown in Figure 3. To clarify the

NH3* + NH 2* + H* → NH3* + NH* + 2H*

(6)

NH3* + NH* + 2H* → NH3* + N* + 3H*

(7)

where the asterisk (*) denotes an empty site or an NHx (with x = 0−3) intermediate species adsorbed onto the Fe surface. In reaction 5, the locally adsorbed NH3 dimer has a binding energy of −0.48 eV as seen in Table 2, and the reaction begins with locally adsorbed NH3 on the top site of Fe. The locally adsorbed NH3 begins with the stable configuration at the top site, followed by dehydrogenation into NH2 at the bridge site and the presence of H at the bridge site. The energy barrier for the first decomposition reaction is 0.97 eV (see Table 2 or Figure 4), which is slightly higher than 0.95 eV for reaction 1 in

Figure 4. Reaction pathway of dimeric ammonia (2NH3) decomposition on Fe(100).

Figure 3. Configuration and energy difference between hydrogenbonded and locally adsorbed ammonia for dimeric ammonia (2NH3).

the mechanism of monomeric NH3 dehydrogenation. Moreover, the reaction is exothermic by −0.44 eV. For reaction 6, the first dehydrogenated NH2 begins with a stable configuration at the bridge site, followed by dehydrogenation into NH at a hollow site and H at the bridge site. This step has an energy barrier of 1.33 eV and is exothermic by −0.32 eV. In the final reaction step, NH begins with a stable configuration at a hollow site, followed by dehydrogenation into N at a hollow site and H at the bridge site. The energy barrier is 1.15 eV, and the reaction is exothermic by −0.43 eV. Overall, the reaction energy barrier of each step in the decomposition of dimeric NH3 is higher than for the decomposition of monomeric NH3, as shown in Figure 4. The rate-limiting step of dimeric NH3 dehydrogenation is therefore the second dehydrogenation reaction, which is identical to the dehydrogenation reaction of monomeric NH3.

decomposition of dimeric NH3, we determined the stability of the configuration between hydrogen-bonded and locally adsorbed ammonia. The energy state of locally adsorbed ammonia is lower than that of hydrogen-bonded ammonia as shown in Figure 3. Therefore, the decomposition of dimeric NH3 likely occurs through the dehydrogenation of locally adsorbed ammonia. To obtain a reaction pathway diagram for dimeric NH3 decomposition on Fe(100), we first investigated the relative elementary reaction steps. The sequence of elementary steps for the decomposition can be written as follows: NH3* + NH3* → NH3* + NH 2* + H*

(5) 5312



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To investigate the energy barrier of the elementary steps in detail, we also considered the contribution of the energy barrier, as shown in Table 2. To analyze the locally adsorbed NH3 decomposition reaction, we analyzed the following terms IS TS def of ΔEdef AB, EAB, and EA . We observed that the value of ΔEAB increased for dimeric NH3 compared with monomeric NH3, whereas the values of EIS AB decreased as the NH3-hindered decomposition reaction proceeded. Moreover, the relatively strong binding energies of ETS int lead to higher energy barriers. By contrast, the higher energies of ETS A contribute to lower energy barriers. In addition, ΔEsub and ETS B do not have any less effect on the energy barrier (Eb), which is not as easily altered as the other terms. Consequently, the preadsorbed NH3 is constrained for the dehydrogenation of locally adsorbed NH3 at the top site. 3.4. Dependence of Ammonia Decomposition on NCovered Fe(100). The catalytic characteristics of metal surfaces are highly dependent on the adsorbates used to cover the surface. An energy barrier for the N2 recombination reaction, which is the highest energy barrier, of 2.17 eV has been reported by us.59 Moreover, the reaction is highly endothermic (1.03 eV), which is consistent with previous theoretical and experimental results.24,61,62 Therefore, nitrogen exists on the Fe surface and subsequently forms an N-covered surface. Thus, a quantitative analysis of the surface coverage effect via energetic and electronic analyses is required. We examined the adsorption energy and dissociation reaction to determine the effects of surface coverage on Fe(100). Initially, we optimized various N-precovered surfaces (θ = 0, 0.22, 0.56, 0.78, and 1.00 ML when θ is coverage and ML stands for monolayer) and analyzed the first dehydrogenation (NH3 → NH2 + H) elementary step of NH3. Quantitatively, we classified the precovered surfaces according to the number of Fe−N bonding events. As the N coverage increased, the number of Fe−N bonding events increased. Hammer investigated the N2 adsorption and dissociation at a Ru(0001) surface coated with N, O, or H using DFT63 and measured the N2 dissociation energy barrier and reaction energy for different levels of surface coverage of the adsorbate (N, O, or H). As discussed by this author, these energy changes are caused by the effects of N precoverage on the surface electronic structure, which affects the bonding strength in a manner suggested by previous theoretical and experimental results.64,65 To confirm the modification of the electronic structure due to surface precoverage, we calculated the d-band for the DOS of the Fe atom as the nitrogen surface coverage shifted. Moreover, the calculated d-band centers (εd) for an Fe atom with different numbers of Fe−N bonds, as shown in Figure 5, were as follows: −0.87 > −0.92 > −1.15 > −1.28 > −1.37 for θ = 0, 0.22, 0.56, 0.78, and 1.00 ML, respectively. Next, the adsorption energies (Ead) of NH3 on various N-precovered Fe(100) surfaces were calculated and are summarized in Table 3. The adsorption site of the NH3 molecule is located at the Fe top site, which establishes several Fe−N bonds, as shown in Figure 6. As the N precoverage increases, the NH3 adsorption energy decreases to −0.92, −0.72, −0.45, −0.41, and −0.39 eV. The first dehydrogenation reaction for the decomposition of NH3 at various levels of nitrogen surface coverage begins with a previously adsorbed NH3 at the Fe top site. The configurations of the initial, transition, and final states are shown in Figure 6. To determine the change in the catalytic reactivity as the N precoverage changes on the surface, the energy barrier (Eb) and the reaction energy (ΔE) were calculated using the CI-NEB method. The dissociation energy barrier (Eb) increased as the

Figure 5. Fe d-projected density of states for an Fe atom with different numbers of Fe−N bonds on Fe(100). The dotted line represents a Fermi level of 0.

Table 3. Coverage Dependence of the Adsorption and Dissociation Properties of Ammonia (NH3) on Fe(100) N coverage (ML)

no. of bonding Fe−Na

Ead (eV)

dN−H (Ǻ )

dN−Fe (Ǻ )

Eb (eV)

ΔE (eV)

0 0.22 0.56 0.78 1.00

0 1 2 3 4

−0.92 −0.72 −0.45 −0.41 −0.36

1.026 1.024 1.023 1.023 1.022

2.173 2.165 2.158 2.203 2.224

0.94 1.12 1.28 1.76 2.15

−0.44 0.46 0.78 1.31 1.94

a

Denotes the number of nitrogen atoms near the Fe atom.

Figure 6. Reaction pathway and configuration of the initial, transition, and final states for an ammonia-initiated dehydrogenation reaction on various nitrogen-precovered surfaces (θ = 0, 0.22, 0.56, 0.78, and 1.00 ML where θ is coverage and ML stands for monolayer).

surface nitrogen concentration increased, with values of 0.94, 1.12, 1.28, 1.76, and 2.15 eV. The variation in the energy barrier depends on the number of Fe−N bonds on the Fe surface, which depends on the electronic structure. Therefore, the electronic structure of the reaction is expected to correlate with 5313



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inexpensive, Fe-based catalyst that can promote NH 3 decomposition for COx-free hydrogen production. Clarification of the exact mechanism and descriptions of the adsorption energy, the energy barrier, and the reaction energy of the NHx (with x = 0−3) intermediate species are important. We identified the exact catalytic decomposition reaction of NH3 on Fe(100). Subsequently, we investigated the adsorption of two types of ammonia dimers (2NH3) and concluded that decomposition probably occurs through locally adsorbed NH3 dimers. According to our results, the dehydrogenation of a hydrogen-bonded NH3 dimer leads to a higher energy barrier than that for monomeric NH 3 and that the second dehydrogenation reaction is the rate-limiting step. To analyze the energy barrier, we distributed the contribution factors of the elementary steps. Moreover, the investigation of the catalytic activity of an Fe surface precovered with N revealed a reduction in the catalytic activity as the N coverage on the Fe surface increased. In conclusion, the development of a method to remove nitrogen from the surface may facilitate the design of Fe catalysts with high activity for NH3 decomposition, thereby enabling the development of an Fe catalyst with similar performance as the most active Ru and Ir catalysts.

the adsorption energy and the energy barrier of the dissociation reaction. To confirm the effect of fully N-covered Fe(100) on the catalytic properties, we calculated the adsorption properties of NHx (with x = 0−3) intermediates on 1.00 ML N-covered Fe(100). Our results indicated that all species are adsorbed at the top site. The adsorption properties were worse than for clean Fe(100), as summarized in Table 4. These results indicate Table 4. Adsorption Properties of NHx (with x = 0−3) Intermediates on 1.00 ML N-Covered Fe(100)a species NH3 NH2 NH N H NH3 (molecule)

adsorption site

Ead (eV)

dN−H (Ǻ )

dN−Fe (Ǻ )

∠HNH (deg)

top top top top top

−0.36 −1.49 −1.73 −2.66

1.022 1.023 1.035

2.224 1.882 1.723 1.554

108.17 109.63

1.021

106.48

a

The energy was calculated with respect to the corresponding species in the gas phase and with respect to the NHx (with x = 0−3) species and the H atom.

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ASSOCIATED CONTENT

S Supporting Information *

that the energy barrier of complete ammonia dehydrogenation was higher than for decomposition on a clean surface. In addition, the reaction energy (ΔE) indicates an endothermic reaction, as shown in Figure 7. Consequently, to improve the

The coordinates and thermodynamic data for the structural models reported in this paper. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +82-42-350-3334; Fax +82-42350-3310 (H.M.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by POSCO and a grant from the National Research Foundation of Korea (NRF) funded by the Korean government (MEST) (2011-0028612) and by the Future-based Technology Development Program (Nano Fields), which is funded through the NRF by the Ministry of Education, Science and Technology (2009-0082472). S.S.H. is grateful for the financial support from the Korea Institute of Science and Technology (Grant 2E24630).

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Figure 7. Reaction pathway diagram of ammonia (NH3) decomposition on fully N-covered Fe(100).

REFERENCES

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catalytic performance of the Fe catalyst, a method to remove N surface coverage on the Fe surface should be considered. Several experiments7,24,62 examined adsorption and dissociation behaviors of the NH3 molecules and intermediates onto Fe(100), Fe(110), and Fe(111) surfaces and showed that as the concentration of chemisorbed nitrogen on Fe surfaces increases, the chemical activities of the Fe surfaces for NH3 dissociation are reduced, which is similar to our theoretical finding.

4. CONCLUSION In summary, we clarified the mechanism of the NH 3 decomposition reaction using DFT calculations to develop an 5314



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Mechanistic Investigation of the Catalytic Decomposition of Ammonia

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