First-Principles Investigation of the Molecular Adsorption and

Article pubs.acs.org/JPCC

First-Principles Investigation of the Molecular Adsorption and Dissociation of Hydrazine on Ni−Fe Alloy Surfaces Yan-Bin He,†,‡ Jian-Feng Jia,*,† and Hai-Shun Wu*,† †

School of Chemistry and Materials Science, Shanxi Normal University, Linfen 041004, China Pharmaceutical Department, Changzhi Medical College, Changzhi 046000, China

‡

S Supporting Information *

ABSTRACT: We have used density functional theory (DFT) with dispersion correction to investigate the adsorption and first dissociation step of hydrazine on Fe3Ni(111), FeNi(111), and FeNi3(111) surfaces. The calculations have shown that the FeNi3(111) surface offers the strongest binding of molecular hydrazine adsorbed on the top of an iron atom, whereas the strongest adsorption of NH2 fragment bridging between iron atoms is obtained on the Fe3Ni(111) surface; both molecular hydrazine and the NH2 fragment can be adsorbed strongly on the FeNi(111) surface.The first dissociation step of hydrazine on the FeNi(111) surface is found to be exothermic by −1.19 eV and presents an activation energy barrier of only 0.15 eV. A similar energy barrier is found on the Fe3Ni(111) surface, but a higher reaction energy of −1.47 eV is released on this surface. Furthermore, the electronic structures of the molecular and dissociative adsorptions are discussed, which is intended to shed some light on the binding nature of the adsorption and stability among conformations of the adsorbed molecule on these surfaces. It is expected that our results will provide useful information for the development of a catalyst for hydrazine dissociation.

1. INTRODUCTION The hydrazine (N2H4) decomposition reaction has recently been subjected to an increasing level of attention due to the possibility of hydrazine being used as a hydrogen storage medium in a possible hydrogen economy.1−3 Hydrazine is an extraordinary energy material with a CO-free source of hydrogen and contains hydrogen content as high as 12.5 wt %; it has been widely involved in the nuclear industry and aerospace science. However, to make efficient use of the hydrogen storage properties of hydrazine, finding highly selective and active catalysts for quick hydrogen release is of great importance for practical applications because the catalytic activity and selectivity strongly depend on the catalyst used. Experimental studies have shown that Rh and Rh alloys are highly active decomposition catalysts;4,5 however, they are expensive and limited in supply. Thus, there is a need to develop either less expensive alternatives or catalysts with higher activity. Bimetallic catalysts, especially those based on nonprecious, low-cost, and abundant metals, are in high demand for various interdisciplinary chemical processes, such as hydrogenation/ dehydrogenation, fuel-cell electrocatalysis, hydrazine and ammonia decomposition, and so forth6−10 in recent years. Activity enhancement occurs when Ni is alloyed with other transition metals, as has been explained experimentally by Xu and co-workers who have synthesized bimetallic nanoparticles of Ni−Rh,4,11 Ni−Pd,12 and Ni−Ir13 for chemical hydrogen © 2015 American Chemical Society

storage. In particular, they found that noble-metal-free NiFe nanoparticles with equimolar compositions of Ni and Fe exhibit excellent catalytic performance for complete decomposition of hydrous hydrazine.14 In addition, Manukyan and co-workers have designed a supported catalyst by reduction of Ni and Fe salts on copper nanoparticles, which also exhibited high catalytic conversion and enhanced selectivity of hydrogen evolution.15 Although current experimental studies have developed morphological control of nanoparticles, the catalytic processes involved are still mostly understood at an empirical level, and molecular insight would help to develop these techniques further. Presently, experimental information is not always sufficient, and accompanying theoretical calculations, such as density functional theory (DFT), can be helpful for clarifying some questions and provides a method to resolve the electronic structure of even rather complicated model systems with adequate accuracy at a reasonable computational cost.16 For instance, Leeuw and co-workers investigated the adsorption of molecular hydrazine on planar, low-index copper surfaces and defective copper surfaces.17−19 Theoretical studies on the molecular adsorption and dissociative mechanism of hydrazine on Ir(111),20 Ni(111),21 Fe(211),22 and Rh(111)23 have also Received: February 16, 2015 Revised: March 27, 2015 Published: March 31, 2015 8763

DOI: 10.1021/acs.jpcc.5b01605 J. Phys. Chem. C 2015, 119, 8763−8774

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The Journal of Physical Chemistry C

atomic forces were smaller than 0.02 eV/Å on each atom and employed a total energy convergence of 10−4 eV. To improve the description of long-range interactions, we employed the DFT-D3(BJ) method 38,39 of Grimme using the dftd3 program.40 Notice that the semiempirical method employed in our study to account for the vdW interactions is similar to that used to investigate ethanol and water adsorption on closepacked 3d, 4d, and 5d transition-metal surfaces by Tereshchuk et al.41 In this approach, the total adsorption energy, EPBE+D3 , is ads obtained by the sum of the self-consistent DFT total energy, D3 EPBE ads , with the van der Waals correction, Eads, such that

been reported. Unfortunately, to the best of our knowledge, few theoretical analyses have been carried out to systematically investigate the origin of the enhanced catalytic activity of N2H4 on Ni-based alloy catalysts. A detailed investigation on the origin at the atomic scale will facilitate designs to improve the decomposition of N2H4 catalysts. Previous research has shown that experimental measurements for Ni−Fe nanoparticles have indicated a uniform alloy composition without significant segregation of each component, and the major surface of NiFe nanoparticles are the fcc(111) plane.14 Thus, our study presents nickel−iron alloy surfaces with various molar ratios of Ni and Fe atoms, including Fe3Ni(111), FeNi(111), and FeNi3(111). These surfaces are used to analyze the interaction energy of adsorbent−adsorbate and the energy barrier of N−N bond breaking. To gain a better understanding of how hydrazine interacts with the catalyst, a dispersion-corrected DFT calculation has been performed on the nickel−iron alloy surfaces, because the inclusion of van der Waals (vdW) forces into DFT-GGA calculations often results in a large increase in binding energies that are in better agreement with experiment data.24−26 When considering dissociation of the hydrazine molecule, a number of mechanisms are possible. In this work, the N−N bond has been broken, as this provides the strongest binding for the dissociated NH2 fragments. Kinetically, breaking the N−N bond is expected to be the predominant mechanism for decomposition at lower temperatures due to a lower energy barrier, which is a key for the decomposition of hydrazine to hydrogen with 100% selectivity.20,23 Our results provide a better understanding at the atomic-level for the enhanced catalytic activity of Fe−Ni alloy catalysts toward the dissociation of hydrazine.

PBE + D3 PBE D3 Eads = Eads + Eads

(1)

The reaction paths have been generated by the nudged elastic band (NEB) method with the climbing-image modification (CI-NEB) to rigorously converge on saddle points, as implemented in VASP.42,43 The transition states (TS) have been confirmed by the saddle points obtained from the frequency calculations. Frequencies calculated using the finite displacement technique are used to compute the zero point energy (ZPE) corrections. The ZPE correction is calculated as ZPE = ∑i(1/2)hvi, where h is Planck’s constant and vi is the frequency of the ith vibrational mode of the adsorbate molecule. The vibrational modes were calculated explicitly under the frozen slab approximation. Spin polarization was taken into account, and the kinetic energy cutoff for the plane wave basis set was 400 eV. The bulk structure was optimized prior to building the surface systems. Fitting the change in energy to a change in lattice parameter with a Birch− Murnaghan equation of state yielded a lattice parameter where the energy was a minimum, which was relaxed further to give the optimized bulk lattice parameter and energy of the atoms in the bulk metal. The lattice parameter for bulk of FeNi, FeNi3, and Fe3Ni were calculated to be 3.567, 3.548, and 3.598 Å, respectively, which are in good agreement with the reported experimental values of 3.589,14 3.552,44 and 3.600 Å,45 respectively. A grid of 15 × 15 × 15 Γ-centered k-points was used in the bulk simulation. The presentations of induced charge density and electron localization functions (ELF) were visualized using VESTA.46 To characterize the surface stabilities of Fe and Ni alloy surfaces of various ratios, we computed the surface energy (γ), which is a measure of the thermodynamic stability of a given surface; the smaller its value, the more stable the surface. The unrelaxed surface energy (γu) is calculated as follows in eq 2

2. COMPUTATIONAL METHODS The three surfaces studied are the closed-packed Fe3Ni(111), FeNi(111), and FeNi3(111) surfaces. The stable configuration of the adsorbed molecules considered in this study was determined by exhausting a number of possible orientations on the surface of a four-layer slab with one molecule of adsorbate for every four surface atoms. From our calculations, we found that the adsorption energies of hydrazine on four- and five-layer slabs differ by only 0.01 eV. Moreover, the d band, and thus the electronic properties of the surface, has not changed considerably, as the thickness of the slab is increased. The adsorbate and the top two layers of the slab were fully relaxed in all directions, whereas the bottom two layers were held fixed at their bulk structure. The periodic images were separated by a vacuum region of more than 14 Å perpendicularly to the surface. The isolated hydrazine molecule in all three conformations was calculated by putting it inside the 20 × 20 × 20 Å supercell to minimize the interaction with the neighboring images in broken symmetry DFT calculations. All calculations were performed using a plane-wave density functional theory approach with the projector-augmented wave (PAW) method,27,28 as implemented in the Vienna ab initio simulation package (VASP).29−32 The exchange-correlation term was expressed using generalized gradient approximation (GGA) based on the Perdew−Burke−Ernzerhof (PBE) functional.33,34 Surface Brillouin zone integrations were performed on a grid of 5 × 5 × 1 Monkhorst−Pack k-points35 using Methfessel−Paxton smearing36 of σ = 0.2 eV. For all optimizations, the equilibrium geometries of the adsorbate were obtained by an iterative conjugate gradient method37 when the

γu =

Eslab,u − nE bulk (2)

2A

where Eslab,u is the energy of the unrelaxed slab, nEbulk is the energy of an equal number, n, of bulk atoms, and A is the surface area of one side of the slab. We have calculated the relaxed surface energy (γr) using eq 3 γr =

Eslab,r − nE bulk A

− γu

(3)

where Eslab,r is the energy of the relaxed slab with one side fixed in the optimized bulk geometry. The three major conformations of hydrazine in the gas phase: gauche, anti, and cis, are shown in Figure 1 (see Table S1 in the Supporting Information for the structural parameters of hydrazine). The gauche conformer is the lowest-energy structure with the anti and cis conformations 0.12 and 0.36 eV 8764


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multilayer relaxations indicated that the surface interlayer spacing, Δd12, is decreased with an increased ratio of nickel atoms in these alloys, which is in line with the order of surface energies. The magnetic moment per iron atom is increased with a decreased ratio of iron atoms in the Ni−Fe alloy, whereas the change of magnetic moment per nickel atom is not obvious, which again agrees with the experimental data.47 The site-projected electronic density of states (DOS) of the bulk and surface atoms (Figure 3) indicates the difference

Figure 1. Conformations of the hydrazine molecule: (a) gauche, (b) anti, and (c) cis (energies are relative to the gauche conformer). N = blue; H = white.

higher in energy, respectively. We started our investigation by placing the different N2H4 conformers in a number of initial configurations at a variety of positions on the FeNi(111), FeNi3(111), and Fe3Ni(111) surfaces to identify the system with the lowest energy. To measure the strength of surface− adsorbate binding, we calculated the adsorption energy by taking the difference between the total energy of the adsorption system with the energies of the relaxed clean slab and isolated species, Ex, according to eq 4 Eads = Esystem − (Eslab + Ex )

(4)

3. RESULTS AND DISCUSSION 3.1. Surface Properties. Figure 2 shows the structure of Ni−Fe alloy surfaces that we studied. The (111) surface is the Figure 3. DOS for (a) bulk Fe, (b) bulk Ni, (c) surface Fe, and (d) surface Ni d-band states. The positive and negative values of the DOS correspond to spin-up and spin-down states, respectively.

between the atomic environments of the nickel and iron atoms. In Figure 3, the distribution of spin-down states are pushed to higher energies for iron atoms (Figure 3a) and nickel atoms (Figure 3b) in the alloy bulks of Fe3Ni and FeNi, although both show a similar range of energies down to −5 eV below the Fermi level. On the other hand, the atom with a higher ratio in the alloy has a greater density (spin-up state) in the vicinity of the Fermi level for either the iron atom (Figure 3a) or nickel atom (Figure 3b). The DOS for the three Fe−Ni alloy surfaces in Figure 3c and d again show the same distribution of states for surface atoms. However, for Fe atoms in the FeNi3 and Fe3Ni surface (Figure 3c), the value of density is similar at 1 eV even though the ratio of iron atom on the FeNi3 surface is lower than on the Fe3Ni surface. 3.2. Molecular Adsorption. 3.2.1. Fe3Ni(111) Surface. A number of stable configurations are found for the adsorption of hydrazine molecules on the Fe3Ni surface. The location and orientation of the adsorbed molecules have a significant effect on the adsorbed structure and the relative stability of each adsorbed structure. The geometries and adsorption characteristics for the range of structures on the Fe3Ni(111) surface are given in Table 2, and the five low energy adsorption configurations are shown in Figure 4. The preferred adsorption structure on the Fe3Ni(111) surface releases an EPBE+D3 of ads 1.121 eV/N2H4 with the gauche conformation for N2H4. Gauche N2H4 bonds through a nitrogen atom to the surface with an Fe−N distance of 2.139 Å and an inclination angle of 31.7° (Figure 4a), and the N−N distance of 1.449 Å in this adsorption configuration is longer than in the gas gauche geometry. Marginally less favorable is the situation where N2H4 is adsorbed in the anti configuration, releasing energy of 1.092

Figure 2. Surface structures of (a) FeNi3, (b) FeNi, and (c) Fe3Ni alloys (distances are given in angstroms).

closed-packed obtained by cleaving the face-centered bulk plane. The surface nickel and iron atoms are therefore arranged in a hexagonal lattice with a separation of 2.509, 2.522, and 2.544 Å between nearest neighbor atoms for FeNi3(111), FeNi(111), and Fe3Ni(111), respectively. Table 1 summarizes Table 1. Surface Energies, Multilayer Relaxations, and Magnetic Moments of Ni−Fe Alloy Surfacesa −2

γ (J m ) Δd12 (%) Δd23 (%) μ/μB

Fe Ni

Fe3Ni(111)

FeNi(111)

FeNi3(111)

2.093 1.081 −0.575 2.679 0.650

1.998 0.739 −0.566 2.775 0.666

1.915 −1.672 −0.098 2.954 0.656

a Δdnm gives the difference from the bulk layer spacing for the n-m layer spacing, where 1 denotes the top layer.

the calculated surface energies, multilayer relaxations, and magnetic moments of the clean Ni−Fe alloy surfaces. These alloy surfaces show stabilities increasing in the order Fe3Ni(111) < FeNi(111) < FeNi3(111). Multilayer relaxations for the clean surfaces are calculated as a measure of the accuracy with which the model reproduces the surface structure. They are calculated as the percent difference in the surface interlayer spacing, Δdnm, from the layer spacing of the same orientation in the geometry-optimized bulk structure. The order of the 8765


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The Journal of Physical Chemistry C Table 2. Summary of Energy and Geometry for Hydrazine Adsorption Configurations on the Fe3Ni(111) Surface label

EPBE ads (eV)

EPBE+D3 (eV) ads

N−N (Å)

a b c d e

−0.741 −0.676 −0.552 −0.488 −0.305

−1.121 −1.092 −1.007 −0.905 −0.774

1.449 1.457 1.441 1.451 1.448

N−Ni (Å)

N−Fe (Å) 2.139 2.112 2.250

2.090 2.113

N−Fe (Å)

ϕHNNH (deg)a

θplane (deg)b

2.189

86.1 168.7 66.2 91.8 58.8

31.7 26.9 0.8 31.4 4.4

2.257

ϕ HNNH is the dihedral angle between hydrogen atoms that would overlap in a cis conformation. bθplane is the angle of inclination of the N−N bond to the surface plane. a

Figure 4. Adsorption geometries of hydrazine on the Fe3Ni(111) surface (shown looking down onto the surface, sideways, and at an angle). Neighboring hydrazine molecules have been omitted for clarity.

Figure 5. Adsorption geometries of hydrazine on FeNi(111) (shown looking down onto the surface, sideways, and at an angle). Neighboring hydrazine molecules have been omitted for clarity.

Table 3. Summary of Energy and Geometry for Hydrazine Adsorption Configurations on the FeNi(111) Surface label

EPBE ads (eV)

EPBE+D3 (eV) ads

N−N (Å)

a b c d e f

−0.729 −0.672 −0.584 −0.546 −0.366 −0.157

−1.122 −1.101 −0.999 −1.018 −0.846 −0.636

1.448 1.473 1.451 1.439 1.445 1.458

N−Ni (Å)

N−Ni Å

N−Fe (Å)

N−Fe (Å)

2.151 2.122 2.087 2.121 2.138

2.215 2.256 2.130

2.248

ϕHNNH (deg)a

θplane (deg)b

87.7 167.9 96.9 67.3 61.5 47.5

33.6 28.2 32.4 0.9 5.9 0.3

ϕHNNH is the dihedral angle between hydrogen atoms that would overlap in a cis conformation. bθplane is the angle of inclination of the N−N bond to the surface plane. a

eV/N2H4, with the N atom placed atop the iron atom with an Fe−N distance of 2.112 Å and the molecule positioned at an angle of 26.9° to the surface (Figure 4b). Adsorption geometries of Figure 4c and e obtained from the structure

optimizations are those in which the molecule bonds through both nitrogen atoms to the surface with the molecule almost parallel to the surface (0.8° for Figure 4c and 4.4° for Figure 4e). In addition, adsorption geometries of Figure 4d are similar 8766


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The Journal of Physical Chemistry C Table 4. Summary of Energy and Geometry for Hydrazine Adsorption Configurations at the FeNi3(111) Surface label

EPBE ads (eV)

EPBE+D3 (eV) ads

N−N (Å)

a b c d

−0.724 −0.645 −0.636 −0.249

−1.153 −1.109 −1.072 −0.748

1.448 1.471 1.451 1.449

N−Ni (Å)

2.074 2.162

N−Ni Å

2.117

N−Fe (Å)

ϕHNNH (deg)a

θplane (deg)b

2.169 2.145

93.7 168.2 96.8 55.8

30.9 26.9 29.9 1.52

ϕHNNH is the dihedral angle between hydrogen atoms that would overlap in a cis conformation. bθplane is the angle of inclination of the N−N bond to the surface plane. a

to the configuration of Figure 4a, which were optimized for investigating the effect of surface iron and the nickel atom about N2H4 adsorption on the Fe3Ni(111) surface. However, the adsorption configuration in Figure 4d shows a lower adsorption energy of −0.905 eV/N2H4 with the N atom placed atop the nickel atom at a Ni−N distance of 2.090 Å. As shown in Table 2, the van der Waals contribution is significant and has values in the range of −0.380 to −0.469 eV/N2H4. 3.2.2. FeNi(111) Surface. Hydrazine in the gauche conformation was placed on the clean FeNi(111) surface followed by optimization of the complete system. As shown in Figure 5, six N2H4 adsorption configurations with measurements for each arrangement are found in Table 3 to locate the top site or bridge site with adsorption energies of −0.636 to −1.122 eV/N2H4. The vdW contribution is significant and may even modify the nature of the interaction, as is found in the adsorption of hydrazine on the FeNi(111) surface (Figure 5f), where the adsorption changes from physisorption (−0.157 eV) to chemisorption (−0.636 eV). A similar effect has been found in the flat adsorption of N2H4 on the Cu(111) surface.19 The lowest energy structure is shown in Figure 5a, with of −1.122 eV/N2H4, which is similar to the adsorption EPBE+D3 ads energy of Figure 4a, and is found by optimizing the initial conformation in which the molecule in its gauche conformation on the FeNi(111) surface bonds through an iron atom with an Fe−N distance of 2.151 Å. The molecule rotates around its N− N bond and has a torsional angle of 87.7° relative to the cis conformer. However, in Figure 5b, the molecule of N2H4 rotates around the N−N bond toward the anti conformer with a torsional angle of 167.9° relative to the cis conformer and an increased N−N bond distance of 1.473 Å, where the adsorption of N2H4 has one N atom in a position slightly displaced from the top site on an Fe atom at an inclination angle of 28.2° to the surface. A local minimum energy structure is found when the gauche N2H4 is adsorbed through one N atom adsorbed on the top of a Ni atom at a distance of 2.087 Å; the molecule is inclined toward the FeNi(111) surface, of 0.999 eV/N2H4 (Figure 5c). releasing an EPBE+D3 ads In additional, we have also studied the structures in which the gauche molecule is bound through both nitrogen atoms to two Fe atoms (Figure 5d), an Fe and Ni atom (Figure 5e), and two Ni atoms (Figure 5f) on the FeNi(111) surface with adsorption energies of −1.018, −0.846, and −0.636 eV/N2H4, respectively, which demonstrated that the Fe atom provided a stronger adsorption site than the Ni atom. 3.2.3. FeNi3(111) Surface. A number of stable configurations are found when the hydrazine molecule is adsorbed on the FeNi3(111) surface. The geometries and stabilities of different adsorptions are summarized in Table 4. The strongest surface− hydrazine interactions (Figure 6a) are found on the FeNi3(111) surface. The lowest energy structure, releasing an adsorption ), which is the largest value energy of 1.153 eV/N2H4 (EPBE+D3 ads for the Fe3Ni(111), FeNi(111), and FeNi3(111) surfaces, but

Figure 6. Adsorption geometries of hydrazine on FeNi3(111) (shown looking down onto the surface, sideways, and at an angle). Neighboring hydrazine molecules have been omitted for clarity.

the adsorption energy of −0.724 eV/N2H4 (EPBE ads ) is minimum in the structures of Figure 4a, Figure 5a, and Figure 6a. For the lowest-energy adsorption structure on the FeNi3(111), FeNi(111), and Fe3Ni(111) surfaces, a plot of adsorption energies as a function of the ratio between the iron and nickel atoms reveals a clear trend in Figure 7. It is evident that the

Figure 7. Adsorption energies as a function of the ratio between surface iron and surface nickel atoms. The letters a, b, and c label the energies for the configurations corresponding to Figures 6a, 5a, and 4a, respectively.

stronger adsorption of N2H4 on the Ni-rich alloy surface is favorable. In the lowest energy structure of Figure 6a, gauche N2H4 bonds through one nitrogen atom to the top of the surface iron atom with a N−Fe distance of 2.169 Å and an inclination angle of 30.9°, which is a similar structure to those found on other Ni−Fe alloy surfaces. A less stable structure (Figure 6b) is obtained when one nitrogen is atop the Fe atom at a distance of 2.145 Å in the anti conformation. Cases in which the adsorption structure of the anti conformation has less energy by 0.02−0.04 eV than the gauche conformation are also found on the Fe3Ni(111) and 8767


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shown in Figure 8. The adsorption energy of −3.243 eV/NH2 (EPBE+D3 ) is greatest when the NH2 fragment is bonded to a 2ads fold hollow between the iron atoms (Figure 8a), and both N− Fe bonds have nearly equal distances. If the NH2 fragment is bonded to a 2-fold hollow of one iron and one nickel atom (Figure 8b), the adsorption energy is slightly less compared to Figure 8a, and the distances of the N−Ni bond and N−Fe bond are different. However, a similar geometry (Figure 8c) is found, which has adsorption energy and geometric parameters that are similar to those of Figure 8b, although the displacement of the NH2 fragment has placed it in a 3-fold hollow of hcp. A similar case occurs in Figure 8a and Figure 8d; displacement of the NH2 fragment has placed it in a 3-fold hollow of fcc, but the adsorption energy of the configuration in Figure 8d is reduced by ∼0.2 eV/NH2, and the geometrical parameters of the N−Fe bond are asymmetrical compared to those of Figure 8a. The adsorption of Figure 8e is weakest, where the NH2 fragment sits atop a surface iron atom and is raised by ∼0.6 Å away from the surface compared to the other geometries in Figure 8. 3.3.2. FeNi(111) Surface. A visual summary of the dissociated adsorption geometries on the FeNi(111) surface is presented in Figure 9 with the measurements for each

FeNi(111) surfaces. The result is in agreement with other theoretical studies of adsorption on various low index Cu17 and Fe(211) surfaces,22 but it is opposite for the adsorption of hydrazine on Pt(111)48 and Ni(111)21 surfaces. In the geometries of Figure 6c, one nitrogen atom of hydrazine is adsorbed atop the nickel atom in the gauche conformation, which has a similar geometry with Figure 6a, but it releases lower adsorption energy. The optimized structure of Figure 6d gives the weakest adsorption found on the FeNi3(111) surface. The geometry of N2H4 in Figure 6d with two nitrogen atoms bridging two surface nickel atoms has been optimized with an adsorption energy of −0.748 eV/N2H4 (EPBE+D3 ), which has a ads minor increase relative to the adsorption energy of Figure 5f because of the presence of more Ni atoms on the surface. 3.3. Dissociative Adsorption. From a set of starting geometries based on the same expected structures containing the adsorption sites of top, bridge, fcc, and hcp on the Fe3Ni(111), FeNi(111), and FeNi3(111) surfaces. Geometry optimizations of NH2 fragment have been carried out to find the lowest energy adsorbed structures. However, different low energy configurations are found when one NH2 fragment of dissociated hydrazine is adsorbed on the Fe3 Ni(111), FeNi(111), and FeNi3(111) surfaces, and the vdW contribution is weak for the adsorption of the NH2 fragment. The representative geometries are shown in the following text and figures on each Ni−Fe alloy surface. 3.3.1. Fe3Ni(111) Surface. A summary of the geometric parameters for NH2 fragment adsorption on the Fe3Ni(111) surface is given in Table 5 with images of the arrangements Table 5. Summary of Energy and Geometry for NH2 Fragment Adsorption Configurations at the Fe3Ni(111) Surface label

EPBE ads (eV)

EPBE+D3 ads (eV)

a b c d e

−2.991 −2.893 −2.888 −2.679 −2.556

−3.243 −3.160 −3.158 −3.003 −2.772

N−Ni (Å) 1.981 1.986

N−Fe (Å)

N−Fe (Å)

θ (deg)a

1.986 1.967 1.973 1.996 1.843

1.987

80.2 80.8 81.0 84.3

1.937

Figure 9. Adsorption geometries of the NH2 fragment on the FeNi(111) surface (1/4 ML coverage). Neighboring NH2 fragments have been omitted for clarity.

Table 6. Summary of Energy and Geometry for NH2 Fragment Adsorption Configurations on the FeNi(111) Surface

θ is the angle between the N atom and nearest Fe or Ni atom on the surface. a

label

EPBE ads (eV)

EPBE+D3 ads (eV)

N−Ni (Å)

a b c

−2.953 −2.868 −2.659

−3.209 −3.140 −2.916

1.986 1.962

N−Ni (Å)

1.961

N−Fe (Å)

N−Fe (Å)

θ (deg)a

2.000 1.987

2.006

76.1 80.2 78.0

θ is the angle between nitrogen atom and nearest Fe atom or Ni atom on the surface. a

arrangement in Table 6. Compared with the Fe3Ni(111) surface, only three low energy configurations are found when the NH2 fragment is adsorbed on the FeNi(111) surface, possibly due to the fact that it has the same proportion of iron and nickel atoms on this surface with the perfect symmetry. The NH2 fragment adsorption geometries on the FeNi(111) surface are those where the NH2 fragment interacts with the two surface atoms in the 2-fold hollow. In the strongest bound configuration (Figure 9a), the fragment is bonded in the 2-fold hollow equilibrium position of two iron atoms. The adsorption is weaker than the same configuration (Figure 8a) on the Fe3Ni(111) surface, and the angle of Fe−N−Fe is more acute. The NH2 fragment bonded in the 2-fold hollow of an iron and nickel atom (Figure 9b) leads to a strength of adsorption

Figure 8. Adsorption geometries of the NH2 fragment on the Fe3Ni(111) surface (1/4 ML coverage). Neighboring NH2 fragments have been omitted for clarity. 8768


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favored slightly. However, compared with the same configuration of Figure 8e on the Fe3Ni(111) surface, the adsorption energy for the configuration of Figure 10c reduced by 0.23 eV/ NH2, which further illustrates that the modified electronic structure of iron atom will increase the adsorption energy of molecules. Furthermore, for the adsorption configurations of Figures 10a, 9b, and 8b, where the NH2 fragment is adsorbed at the 2fold hollow between an iron and nickel atom, a plot of adsorption energies as a function of the ratio between surface iron and nickel atoms in Figure 11 reveals a trend that opposite to that in Figure 7, which suggests that the stronger adsorption of NH2 on the Fe-rich alloy would be helpful.

that is less than when NH2 is bound to two iron atoms. If the NH2 fragment is attached to the 2-fold hollow of two nickel atoms, the strength of adsorption is again reduced (Figure 9c). Meanwhile, both N−Ni bonds are symmetrical to the surface nickel atoms and are smaller than the parameters of other adsorbed geometries on this surface. Herein, the NH2 fragment in this geometry is closed by ∼0.1 Å away from the surface compared to that of Figure 9a. 3.3.3. FeNi3(111) Surface. Adsorption on the FeNi3(111) surface is summarized in the image of Figure 10 and the

Figure 11. Adsorption energies of the structures in Figures (a) 10a, (b) 9b, and (c) 8b as a function of the ratio between iron and nickel atoms.

Figure 10. Adsorption geometries of the NH2 fragment on the FeNi3(111) surface (1/4 ML coverage). Neighboring NH2 fragments have been omitted for clarity.

Because of the strong adsorption for both hydrazine and NH2 on the three Ni−Fe alloy surfaces, signs of displacement of the surface atoms under adsorption or dissociation are found. Table S2 in the Supporting Information summarizes the displacement of the surface atoms for the strongest adsorption of hydrazine and NH2 fragments on Fe3Ni(111), FeNi(111), and FeNi3(111) surfaces. For the molecule adsorptions on the three Fe−Ni alloy surfaces corresponding to the configurations of Figures 4a, 5a, and 6a, the surface Fe atoms that interact with the N atom of hydrazine are higher than the other surface atoms and the order of displacement is Fe3Ni(111) > FeNi(111) > FeNi3(111). For the NH2 adsorptions corresponding to the configurations of Figures 8a, 9a, and 10a, the order and magnitude of the surface atoms interactions with the N atom of NH2 are similar to molecule adsorption. However, greater displacement of surface atoms is found when two NH2 fragments are adsorbed on the Fe3Ni(111), FeNi(111), and FeNi3(111) surfaces; especially for the Fe3Ni(111) surface, all of the topmost atoms have a horizontal displacement of ∼3.0 Å. Displacement of surface atoms is not surprising as, for example, reconstruction of copper surfaces in the presence of NH2 fragments found that the adatom was displaced from the more favorable fcc 3-fold site to a 2-fold hollow.18 For an accurate description of the surface conditions and adsorption properties of a surface alloy, both the surface segregation and the ordering of the surface layers have to be considered. The surface segregation trend can be influenced by the surface energy of bimetallic surface alloys. For example, Au atoms have a tendency to segregate to the clean Pt(111) surface,49 because an alloy surface with a mixture of Au and Pt atoms would have lower surface energy than a pure Pt surface. Therefore, according to our DFT calculations, the surface

Table 7. Summary of Energy and Geometry for the NH2 Fragment Adsorption Configurations on the FeNi3(111) Surface label

EPBE ads (eV)

EPBE+D3 ads (eV)

N−Ni (Å)

N−Ni (Å)

a b c d

−2.793 −2.708 −2.301 −2.014

−3.070 −2.977 −2.543 −2.353

1.969 1.955

1.954

N−Fe (Å)

θ (deg)a

1.989

78.5 79.5

1.857 1.843

θ is the angle between the N atom and the nearest Fe or Ni atom on the surface. a

geometric data of Table 7. As the electronic structure of the iron atom between the FeNi3(111) surface and other Ni−Fe alloy surfaces are obviously different, adsorption geometries found on the FeNi3(111) surface always give weaker adsorption energies, which are reduced by ∼0.2 eV/NH2 relative to similar configurations on the Fe3Ni(111) and FeNi(111) surfaces. The favored adsorption mode of the NH2 fragment, as that found on the other surfaces, is through bridging two neighboring surface atoms. However, in this case, the strongest adsorption has the NH2 fragment bridged to the iron and nickel atoms with very slight asymmetry (Figure 10a), and another lessfavored adsorption has the NH2 fragment bridged to two nickel atoms symmetrically (Figure 10b). Furthermore, adsorption geometries of the NH2 fragment bonded on the top of the iron atom (Figure 10c) and nickel atom (Figure 10d) are also found on the FeNi3(111) surface, where the former configuration is 8769


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The Journal of Physical Chemistry C energy of Fe3Ni(111) is 2.093 J m−2, whereas the energy of FeNi3(111) is only 1.915 J m−2. When ignoring interactions between the Fe and Ni atoms in the surface, an alloy surface with more Ni atoms would have lower surface energy, which results in Ni atoms that have a tendency to segregate to the Fe−Ni alloy surface. Moreover, the surface segregation trend of solute atoms is significantly affected by the presence of an adsorbate, such as the segregation tendency of solute Ti atoms to the Pt surface, which is likely to change with the introduction of electronegative adsorbates to the surface because of the strong affinity of the electropositive Ti atom to bond with electronegative adsorbates.50 In this work, the adsorption energies of different adsorbates (N2H4 and NH2) on Fe- and Ni-segregated surfaces of ordered Fe3Ni and FeNi3 surface alloys have been calculated. The results show that the binding energies of N2H4 or NH2 are slightly larger for the adsorption site with an Fe nearestneighbor than for the other adsorption sites with a Ni nearestneighbor to the adsorptions in all Fe−Ni alloy surfaces, which suggests a preference for Fe binding to N2H4. This trend in binding energy can be understood from the perspective of the electronegativity of these elements. Fe has a slightly lower electronegativity (1.83) than Ni (1.91).51 The electropositive Fe prefers to bind to the electronegative N atom of adsorbates, and therefore, the strongest binding for every Fe−Ni alloy surface is N2H4 adsorbate on the Fe top site. However, as shown in Figure 7, the binding of N2H4 to the Fe top site of FeNi3 alloy surface is strongest, which suggests that Ni atoms segregating to the surface of an Fe−Ni alloy is favorable for the adsorption of N2H4. 3.4. Electronic Structure of Adsorption. Beyond inspection of the geometries and adsorption energies, the electronic structures of the configurations can be used to rationalize in more detail the differences between adsorbed structures. The site-projected electronic density of states (DOS) shows how the electronic structure is affected by interactions with the surface. We discuss here an overview of the electronic structure data for the N2H4 molecular and dissociative adsorptions on the three Ni−Fe alloy surfaces. To characterize the bond between the nitrogen and surface atoms, we carried out an analysis of the electron density, where the charge density difference was obtained by subtracting the sum of the charge densities of the molecule and the clean surface in the same geometry from the charge density of the total adsorbate system, as shown in eq 5 ρtransfer = ρsurf + mol − (ρsurf + ρmol )

(5)

3.4.1. Molecule Adsorption on the Three Alloy Surfaces. The electronic DOS will give further insight into the adsorption mechanism on the surface. Figure 12 shows the local DOS of the strongest molecule adsorption on the Fe3Ni(111) (a-1), FeNi(111) (b-1), and FeNi3(111) (c-1) surfaces. All of these adsorptions have the gauche conformation. In the gauche conformation, stabilization of HOMO dominates and results in overall stabilization of the gauche conformation as reported by Agusta et al.21 It also means that the gauche conformation allows the lone-pair in one NH2 group to reduce the repulsion. The DOS are projected on the p orbital of the bonded N atom and on the d orbital of the bonded Fe atom and are resolved to spin-up and spin-down directions. The two peaks at −3 and −5 eV are the states derived from adsorbate−substrate interactions, which can be described as two atomic orbitals interacting between HOMO of gauche conformation and the d2z orbital of

Figure 12. Projected electronic DOS of adsorbed atoms (1), induced charge density presentation by an isosurface of ±0.002 e/Å3, where yellow and blue denote gain and loss of electron density (2), and a slice through the hydrazine molecule and nearest Fe atom showing ELF contours with scale bar (no units) (3) for the strongest molecule adsorption on the (a) Fe3Ni(111), (b) FeNi(111), and (c) FeNi3(111) surfaces. The positive and negative values of the DOS correspond to spin-up and spin-down states, respectively.

the bonded surface of the Fe atom. In the perturbation scheme, the two orbitals perturb each other to give the states at −3 and −5 eV. The character of the perturbed states is a mixture of the two interacting orbitals. The mixing between HOMO and the d2z orbital is a first-order mixing,52 which corresponds to charge transfer between substrate and adsorbate. 8770


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The Journal of Physical Chemistry C Figure 12 also shows the induced charge density (a-2) with an isosurface of ±0.002 e/Å3 and electron localization function53,54 (ELF) slices (a-3) through the Fe−N bond of the adsorbed molecule on Fe3Ni(111) surfaces with geometries in the gauche conformation. This indicates that the charge is transferred mainly from N2H4 to the Fe atom in the uppermost layer of the Fe3Ni(111) surface, which can be categorized as a dative-type interaction and the charge contribution mostly comes from one side (in this case, the adsorbate). In addition, it is clear that hydrazine is bonded to the surface iron atom using one nitrogen atom and that the Fe−N bonds have an extensive region of localization in the ELF slices of Figure 12(a-3). The DOS, induced charge density and ELF slices, as shown in Figure 12b and Figure 12c, are similar with what shows in Figure 12a, which are in agreement with the similar adsorption energies on the three surfaces. 3.4.2. NH2 Fragment Adsorption on the Three Alloy Surfaces. Figure 13 shows the local DOS (a-1) of the strongest NH2 fragment adsorption on the Fe3Ni(111) surface. As described above, the NH2 fragment is bonded to a 2-fold hollow between two iron atoms (Figure 8a), and both N−Fe bonds have nearly equal distances. The most notable feature in the DOS for the adsorption of the NH2 fragment is that there is much less overlap of the nitrogen p-band states in the iron dband states than is found for molecular adsorption. The NH2 fragment bonded to the surface atoms, shown in Figure 13(a1), has a small overlap of the nitrogen states in the surface iron at −4 and −8 eV, and the iron states show a slight overlap in the nitrogen at −8 eV and above. The DOS for the NH2 fragment adsorption on the FeNi(111) and FeNi3(111) surfaces, as shown in Figure 13(b-1 and c-1), follows a similar trend as seen on the Fe3Ni(111) surface except for the smaller overlap of the d-band states in the nitrogen p-band states. However, note that the nitrogen atom of the NH2 fragment has interacted with the d-band states of iron and nickel atoms in Figure 13(c-1). The induced charge density (a-2) with an isosurface of ±0.005 e/Å3 and ELF slices (a-3) through the Fe−N bond of the NH2 fragment on the Fe3Ni(111) surface are also shown (Figure 13). This indicates that the charge is mainly transferred from NH2 to the region of the Fe−N bonds on the Fe3Ni(111) surface symmetrically. Meanwhile, in the ELF slices of Figure 13(a-3), it can be seen that the localized electrons involved in the bonding are not in two discrete “Fe−N bonds” but instead the region of localized electrons encompasses both interactions. Similar cases of charge transfer and regions of localized electrons are found in Figure 13(b-2 and b-3) on the FeNi(111) surface. However, it can be seen that the charge transfer and the regions of localized electrons in the bonds of Fe−N and Ni−N have slight asymmetry in Figure 13(c-2 and c3) on the FeNi3(111) surface. 3.5. Catalytic Activity of N2H4 Dissociation on the Three Fe/Ni(111) Surfaces. The nudged elastic band (NEB) method, with the climbing-image modification (CI-NEB) to rigorously converge on saddle points, was adopted here to determine the minimum energy paths (MEPs) for N2H4 dissociation on the Fe3Ni(111), FeNi(111), and FeNi3(111) surfaces. The CI-NEB method retains the information about the shape of the MEP and performs a rigorous convergence to a saddle point. Damped molecular dynamics was used to relax ions until the force in each image was less than 0.03 eV/Å. The activation energy barrier Ea is calculated by

Figure 13. Projected electronic DOS of adsorbed atoms (1), induced charge density presentation by an isosurface of ±0.005 e/Å3, where yellow and blue denote gain and loss of electron density (2), and a slice through the NH2 fragment and nearest atoms showing ELF contours with scale bar (no units) (3) for the strongest NH2 fragment adsorption on the (a) Fe3Ni(111), (b) FeNi(111), and (c) FeNi3(111) surfaces. The positive and negative values of the DOS correspond to spin-up and spin-down states, respectively.

Ea = E TS/surface − E IS/surface

(6)

and the reaction energy Er is calculated by Er = E FS/surface − E IS/surface 8771

(7) DOI: 10.1021/acs.jpcc.5b01605 J. Phys. Chem. C 2015, 119, 8763−8774

Article

The Journal of Physical Chemistry C where IS, TS, and FS are the initial, transition, and final states, respectively. The reaction energy denotes the amount of energy that a chemical reaction requires, where positive and negative values represent endothermic and exothermic reactions, respectively. In this section, we use the first dissociation step of hydrazine (N2H4 → 2NH2) as a representative to investigate the different catalytic activity of the Fe 3 Ni(111), FeNi(111), and FeNi3(111) surfaces because the activation energy of breaking the N−N bond is lower than the energy barrier for dehydrogenation in the extensive study.20,23 On the three alloy(111) surfaces, we found above that N2H4 prefers to chemisorb on the top site of the iron atom. Thus, we adopted the optimized states in which N2H4 is adsorbed on the top site of the iron atom as the initial state of the hydrazine dissociation path on the Fe3Ni(111), FeNi(111), and FeNi3(111) surfaces. Our first-principles calculations have shown that the optimum adsorption site for the NH2 fragment on the Fe3Ni(111) and FeNi(111) surfaces is the 2-fold hollow site between both iron atoms, whereas that for the NH2 radical on the FeNi3(111) surface is the 2-fold hollow site between an iron and nickel atom. Therefore, to obtain the final state of the N2H4 dissociation path, we optimized the lowest energy coadsorption geometries of two NH2 fragments on every Ni−Fe alloy surface. As shown in Figure 14, for the Fe3Ni(111) surface (Figure 14a), both NH2 fragments are placed at a 2-fold hollow between iron atoms. For the FeNi(111) surface (Figure 14b), one NH2 fragment is placed at a 2-fold hollow between the iron atoms, and the other fragment is placed at a 2-fold hollow between an iron and nickel atom. For the FeNi3(111) surface (Figure 14c), both NH2 fragments are placed at a 2-fold hollow between an iron and nickel atom. Finally, we calculated the energy barrier for the first dissociation step of hydrazine (N2H4 → 2NH2) with the optimized coadsorption geometry as the final state on each alloy surface. The interaction energy was defined as the difference between the energies of the coadsorbed configuration and infinite separation state (each adsorbate in a separate unit cell at its most stable position). The repulsive interaction energies of the final states are 0.579, 0.622, and 0.639 eV for the DFT+D3 calculations on the surfaces of Fe3Ni, FeNi, and FeNi3, respectively. It is seen that the coadsorbed systems are less stable with respect to the fragments in the infinite separation state due to repulsive interactions, which are enhanced with the increase in Ni atoms in the Fe−Ni alloys. Figure 14a displays the calculated energy barrier for the first dissociation step of the hydrazine molecule on the Fe3Ni(111) surface. The optimized geometries of the initial, transition, and final states are presented correspondingly. The initial state has the same geometry as in Figure 4a. In the transition state, although the distance of the N−N bond has not obviously changed, a change in the adsorption is found for the configuration with the molecule bridging two surface iron atoms; then, once the distance of the N−N bond has been stretched, the coadsorption energy of two NH2 fragments is increased quickly in the minimum energy path (MEP) for the dissociation of hydrazine. Finally, the distance between the broken N atoms in the final state is 2.748 Å after geometry optimizations. It can be seen from Figure 14a that the reaction energy barrier used to activate N2H4 is evaluated to be 0.17 eV, and the first dissociation process of hydrazine belongs to an exothermic reaction by −1.47 eV. As shown in Figure 14b, catalysis on the FeNi(111) surface is similar to the previous

Figure 14. Potential energy diagram of N2H4 dissociation on the (a) Fe3Ni(111), (b) FeNi(111), and (c) FeNi3(111) surfaces. The values in the middle are the activation energies with respect to adsorbed N2H4, including the ZPE correction. The values on the left are the reaction energies with respect to adsorbed N2H4. All values are expressed in electronvolts. The inset images, where neighboring molecules have been omitted for clarity, are the figures of the initial, transition, and final states.

case, but the reaction energy barrier has a lower value of 0.15 eV, and dissociation of the hydrazine N−N bond is less exothermic, calculated here to be −1.19 eV. On the FeNi3(111) surface, we can see from Figure 14c that the reaction energy is only −0.95 eV. However, the reaction needs to overcome an energy barrier of ∼0.68 eV to form the two decomposed NH2 fragments. In addition, the most notable feature in the catalysis of breaking the N−N bond on the FeNi3(111) surface is the adsorption configuration with a nearly cis-conformer in the transition state, which is different from the transition states found on the other surfaces and is the reason that the energy barrier in Figure 14c is higher than in Figure 14a and b.

4. CONCLUSION In summary, we present here a systematic density functional theory study of adsorption and the first dissociation step of 8772


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Generation from Aqueous Alkaline Solution of Hydrazine for Chemical Hydrogen Storage. Int. J. Hydrogen Energy 2014, 39, 9726−9734. (2) He, L.; Huang, Y.; Liu, X. Y.; Li, L.; Wang, A.; Wang, X.; Mou, C.-Y.; Zhang, T. Structural and Catalytic Properties of Supported Ni− Ir Alloy Catalysts for H2 Generation Via Hydrous Hydrazine Decomposition. Appl. Catal., B 2014, 147, 779−788. (3) Serov, A.; Padilla, M.; Roy, A. J.; Atanassov, P.; Sakamoto, T.; Asazawa, K.; Tanaka, H. Anode Catalysts for Direct Hydrazine Fuel Cells: From Laboratory Test to an Electric Vehicle. Angew. Chem., Int. Ed. 2014, 53, 10336−10339. (4) Singh, S. K.; Xu, Q. Complete Conversion of Hydrous Hydrazine to Hydrogen at Room Temperature for Chemical Hydrogen Storage. J. Am. Chem. Soc. 2009, 131, 18032−18033. (5) Singh, S. K.; Zhang, X.-B.; Xu, Q. Room-Temperature Hydrogen Generation from Hydrous Hydrazine for Chemical Hydrogen Storage. J. Am. Chem. Soc. 2009, 131, 9894−9895. (6) He, L.; Huang, Y.; Wang, A.; Liu, Y.; Liu, X.; Chen, X.; Delgado, J. J.; Wang, X.; Zhang, T. Surface Modification of Ni/Al2O3 with Pt: Highly Efficient Catalysts for H2 Generation Via Selective Decomposition of Hydrous Hydrazine. J. Catal. 2013, 298, 1−9. (7) Hansgen, D. A.; Vlachos, D. G.; Chen, J. G. Using First Principles to Predict Bimetallic Catalysts for the Ammonia Decomposition Reaction. Nat. Chem. 2010, 2, 484−489. (8) Huang, Y.; Chen, Z. X. Density Functional Investigations of Methanol Dehydrogenation on Pd−Zn Surface Alloy. Langmuir 2010, 26, 10796−10802. (9) Kattel, S.; Duan, Z.; Wang, G. Density Functional Theory Study of an Oxygen Reduction Reaction on a Pt3Ti Alloy Electrocatalyst. J. Phys. Chem. C 2013, 117, 7107−7113. (10) Ou, L. The Origin of Enhanced Electrocatalytic Activity of Pt− M (M = Fe, Co, Ni, Cu, and W) Alloys in PEM Fuel Cell Cathodes: A DFT Computational Study. Comput. Theor. Chem. 2014, 1048, 69−76. (11) Singh, A. K.; Yadav, M.; Aranishi, K.; Xu, Q. TemperatureInduced Selectivity Enhancement in Hydrogen Generation from Rh− Ni Nanoparticle-Catalyzed Decomposition of Hydrous Hydrazine. Int. J. Hydrogen Energy 2012, 37, 18915−18919. (12) Singh, S. K.; Iizuka, Y.; Xu, Q. Nickel-Palladium Nanoparticle Catalyzed Hydrogen Generation from Hydrous Hydrazine for Chemical Hydrogen Storage. Int. J. Hydrogen Energy 2011, 36, 11794−11801. (13) Singh, S. K.; Xu, Q. Bimetallic Nickel-Iridium Nanocatalysts for Hydrogen Generation by Decomposition of Hydrous Hydrazine. Chem. Commun. 2010, 46, 6545−6547. (14) Singh, S. K.; Singh, A. K.; Aranishi, K.; Xu, Q. Noble-Metal-Free Bimetallic Nanoparticle-Catalyzed Selective Hydrogen Generation from Hydrous Hydrazine for Chemical Hydrogen Storage. J. Am. Chem. Soc. 2011, 133, 19638−19641. (15) Manukyan, K. V.; Cross, A.; Rouvimov, S.; Miller, J.; Mukasyan, A. S.; Wolf, E. E. Low Temperature Decomposition of Hydrous Hydrazine over FeNi/Cu Nanoparticles. Appl. Catal., A 2014, 476, 47−53. (16) Kohn, W. Nobel Lecture: Electronic Structure of MatterWave Functions and Density Functionals. Rev. Mod. Phys. 1999, 71, 1253− 1266. (17) Daff, T. D.; Costa, D.; Lisiecki, I.; de Leeuw, N. H. Density Functional Theory Calculations of the Interaction of Hydrazine with Low-Index Copper Surfaces. J. Phys. Chem. C 2009, 113, 15714− 15722. (18) Daff, T. D.; de Leeuw, N. H. A Density Functional Theory Investigation of the Molecular and Dissociative Adsorption of Hydrazine on Defective Copper Surfaces. J. Mater. Chem. 2012, 22, 23210−23220. (19) Tafreshi, S. S.; Roldan, A.; Dzade, N. Y.; de Leeuw, N. H. Adsorption of Hydrazine on the Perfect and Defective Copper (111) Surface: A Dispersion-Corrected DFT Study. Surf. Sci. 2014, 622, 1−8. (20) Zhang, P. X.; Wang, Y. G.; Huang, Y. Q.; Zhang, T.; Wu, G. S.; Li, J. Density Functional Theory Investigations on the Catalytic

hydrazine on Fe3Ni(111), FeNi(111), and FeNi3(111) surfaces. Our study has shown that different ratios of iron atom to nickel atoms on the (111) alloy surface have a significant effect on the geometries and strengths of adsorption of hydrazine and NH2 fragments. The Fe3Ni(111) surface offers the weakest adsorption to the molecule, whereas the strongest adsorption energy is obtained on the FeNi3(111) surface, where the gauche conformation of hydrazine on the top of the iron atom is the preferred configuration. On the contrary, the FeNi3(111) surface offers the weakest adsorption to the NH2 fragment, whereas the highest adsorption energy is obtained on the Fe3Ni(111) surface, where the NH2 fragment is adsorbed on the 2-fold hollow between an iron and nickel atom. However, whether in the hydrazine or NH2 fragment, the FeNi(111) surface has a strong adsorption energy. Using the climbing-image nudged elastic band (CI-NEB) method based on the transition state theory, the energy barriers for the first dissociation step of hydrazine on the three alloy surfaces were calculated. On the Fe3Ni(111) surface, the first dissociation step of hydrazine released the largest reaction energy of 1.47 eV, and the lowest energy barrier of 0.15 eV is found on the FeNi(111) surface. In comparison, the first dissociation step of hydrazine on the FeNi3(111) surface is found to be exothermic by only −0.95 eV but presents an activation energy barrier of 0.68 eV. It can be concluded that the different ratio of iron to nickel atoms on the alloy surface not only changes the surface electronic structure but also varies the reaction energy barrier. It is expected that our results will provide useful information for the development of a catalyst for N2H4 dissociation. In our following work, we will further study hydrazine decomposition and reaction pathways on the Ni−Fe alloy surface, which will improve our insights into the mechanisms occurring in direct hydrazine fuel cells and in room temperature hydrogen generation from hydrazine for chemical hydrogen storage.

■

ASSOCIATED CONTENT

S Supporting Information *

Structural parameters of hydrazine and displacement of surface atoms upon adsorption or dissociation as discussed in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone & Fax: +86-3572052468. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21373131), Shanxi Scholarship Council of China, and the Program for New Century Excellent Talents in University (NCET-12-1035). Y.-B.H. thanks the LvLiang High Performance Computing Center for providing computational time.

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First-Principles Investigation of the Molecular Adsorption and

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