Adsorption and dissociation of ammonia on small iron clusters

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e7

Available online at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/he

Adsorption and dissociation of ammonia on small iron clusters Xilin Zhang a, Zhansheng Lu a,*, Dongwei Ma b, Zongxian Yang a,* a b

College of Physics and Electronic Engineering, Henan Normal University, Xinxiang 453007, China School of Physics, Anyang Normal University, Anyang 455000, China

article info

abstract

Article history:

The stepwise dehydrogenation of NHx on small iron clusters is investigated from the

Received 8 August 2014

density functional theory (DFT) calculations. The results indicate that the fewer the H

Received in revised form

atoms of the NHx (x ¼ 0e3) species, the higher the adsorption energies of NHx on the same

31 October 2014

Fe cluster are. Also the larger the cluster size, the stronger the adsorption for the same NHx

Accepted 1 November 2014

species is. The catalytic activity of small Fe clusters for NH3 dehydrogenation is compar-

Available online xxx

atively studied, and the rate-limiting steps for the reactions are addressed. It is found that the rate-limiting steps for the dehydrogenation of NHx on the Fe and Fe3 are that for the NH

Keywords:

decomposition, and those on the Fe2 and Fe4 are that for the NH2 dissociation. The barriers

Ammonia decomposition

for the rate-limiting steps are 1.06, 1.49, 1.43 and 1.51 eV for the dehydrogenation of NHx on

Small iron clusters

the Fe, Fe2, Fe3 and Fe4 clusters, respectively. The results suggest that the small Fe clusters

DFT

can be regarded as the potential candidates for NH3 dehydrogenation reactions and can serve as a reference for further investigations on the catalytic activity of small Fe clusters supported by various catalytic materials. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Ammonia (NH3) is considered to be an important industrial chemical owing to its wide potential applications, which is known as a nitrogen feedstock for agriculture [1] and a refrigerant [2,3]. More importantly, it has also been used as a source and storage substance for hydrogen fuel cells [4e7]. Hydrogen generated from single step decomposition of ammonia represents an attractive alternative to hydrocarbons for hydrogen fuel cells [6,8,9] and also is a straightforward process with CO-free product gases N2 and H2. At the same time, NH3 is an undesirable byproduct of industrial catalytic reactions [10], which can cause pollution of lakes and

seas, and its emissions into the environment need to be avoided. Because of these and other technological, economical, and environmental factors, numerous researches on the decomposition and oxidation reactions of ammonia have been performed [4e6]. It is often reported that the iron nanoparticles play a key role in hydrolytic dehydrogenation of ammonia for chemical hydrogen storage [11e13]. For example, Yan and co-worker [11] have recently developed a simple but efficient method for preparing amorphous Fe nanoparticles with high catalytic activity for the generation of H2 from ammonia-like molecule. Ammonia decomposition on iron particles of various grain sizes with an ammonia stream has been observed by Nishimaki and co-works [12]. Lanzani and

* Corresponding authors. E-mail addresses: [email protected] (Z. Lu), [email protected] (Z. Yang). http://dx.doi.org/10.1016/j.ijhydene.2014.11.003 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Please cite this article in press as: Zhang X, et al., Adsorption and dissociation of ammonia on small iron clusters, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.11.003

2

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e7

Laasonen [14] studied the bonding and dissociation of NH3 and its fragment on a nanosized icosahedral Fe55 cluster using spin-polarized density functional theory (DFT) calculations. Their results suggest that the catalytic activity of iron surfaces towards ammonia-like molecules is enhanced when the metal is in the nanostructured phase. Some researches also showed that the catalytic properties of small iron clusters show large deviation with cluster size. It is testified by the results of Sun et al. [15,16], who observed that the iron dimer has a stronger catalytic effect on the activation of methane, a species of ammonia-like molecule, than the iron atom by DFT calculations. They also studied the activation reactions of methane on Fe4 cluster with different spin states. However, whether or not the iron clusters with different size have similar phenomenon on the activation of NH3 is undefined owning to the lack of researches on the catalytic effect of smaller iron clusters, such as iron atom, iron dimer, Fe4 and so on. Motivated by this, we employ the first-principles DFT calculation to investigate the NH3 adsorption and dissociation on smaller Fe clusters (from Fe1 to Fe4) and catalytic activity dependence on the cluster size. The simulation results indicate that the iron atom has the highest catalytic effect on the NH3 dehydrogenation, which is followed by Fe3. The Fe4 and iron dimer have the relative lower activation, while also comparable with the Fe55. We also find that the catalytic effect of Fe clusters is stronger than some noble and transition metals surfaces, such as Au(111) [17], Pt(111) [18] and Fe(111) [19,20], indicating that the smaller iron clusters can also be regarded as excellent candidates for NH3 dehydrogenation reactions. Through the comparison of the activated dehydrogenation of NH3 by iron clusters of different sizes, even atomic scale, and iron bulk, the study will provide useful information to tune the relative reaction rates of the different steps during the activation of NH3 by controlling the cluster size, which is very important for understanding the NH3 dehydrogenation process from both environmental protection and the continuous supply of clean hydrogen gas for fuel cells.

Computational details The periodic DFT calculations are performed using the Vienna Ab-initio Simulation Package (VASP) [21,22]. For improving the calculation efficiency, core electrons are replaced by the projector augmented wave (PAW) pseudo-potentials [23] and the generalized gradient approximation of the Perdew, Burke, and Ernzernhof (PBE) functional [24] is used for the exchange and correlation. The KohneSham orbitals are expanded using plane-waves with the well converged cutoff energy of 450 eV and the convergence criterion for the electronic selfconsistent iteration is set to 105 eV. The iron clusters and the adsorbates are free to relax until the self-consistent forces ˚ 1. Spin polarization is taken into account drop below 0.02 eV A in the calculations and the Gaussian smearing method is employed to determine electron occupancies with a smearing parameter of 0.2 eV. With the purpose of avoiding the interactions due to the artificial periodicity, a vacuum layer of ˚ is used to separate the periodic images in the direction 15 A

perpendicular to the surface. The Brillouin zone integration is performed with a 1  1  1 G-centered MonkhorstePack (MP) grid. The different K-points, including 3  3  1 and 5  5  1, are tested for comparison and only tiny changes in energy are found. The most stable geometric structures of iron clusters are obtained firstly based on the above calculation parameter ˚. settings. The optimized Fe2 dimer has a bond length of 2.02 A The result is well comparable with previous theoretical [25] and experimental works [26]. The equilateral triangle is the most stable configuration for Fe3 cluster, which is in agreement with the previous DFT calculation [27,28]. However, the ˚ ) is slightly longer than the result of Alebond length (2.24 A ˚ ) [29]. The linear structure for the Fe3 cluster many et al. (2.14 A is found to be less stable (higher in energy by about 2.00 eV) than the equilateral triangle configuration. For Fe4, the tetrahedral structure is lower in energy than the rhombic structure by 0.16 eV, which agrees with the result of Chen et al. [27]The ˚ ) of the present calculation are longer than bond lengths (2.31 A those obtained by Yin et al. [30] Although all the possible geometric configurations of iron clusters are discussed here, we will mainly present the results based on the most stable geometric structures of iron clusters in the following. The geometric configurations of NHx in gas phase are also obtained. Take the NH3 as an example, we find that the NeH ˚ , which is in good agreement with the previbonds are 1.02 A ous results [31]. Our calculated HeNeH cone angle (106.33 ) is the same as the calculated value of Xie et al. [31]. The climbing image nudged elastic band method (CI-NEB) [32,33], a method for finding saddle points and minimum energy paths between known reactants and products, is employed to investigate the saddle points and minimum energy paths for NHx dissociation on the iron clusters. In this work, the spring force between adjacent images is set to ˚ 1. Images are optimized until the forces on each atom 5.0 eVA ˚ 1. The energy barriers are calculated drop below 0.02 eVA using the initial state as a reference. Vibrational frequency analysis is carried out for all the optimized geometries and the transition states. It is found that the reactant complexes have all real frequencies, while each transition state has a single imaginary frequency. The detailed results can be found in the Supporting information. The adsorption and coadsorption energies are defined by the formulas (1) and (2), respectively. Eads ¼ ECluster þ ENHx  EðNHx =ClusterÞ

(1)

Ecoads ¼ ECluster þ EH þ ENHx  E½ðNHx þHÞ=Cluster

(2)

The EðNHx =ClusterÞ and E½ðNHx þHÞ=Cluster are the spin-polarized total energies for the optimized equilibrium configurations of iron cluster with a single molecule (NHx) and two coadsorbed molecules (NHx and H), respectively. ECluster is the spinpolarized total energy for the optimized bare iron cluster, and ENHx and EH are the spin-polarized total energies of the corresponding isolated gas molecules in the ground state. The x represents 0e3 in (1) and 0e2 in (2), respectively. Bader charge analysis method [34] is used to evaluate the atomic charges, which are further used to calculated the number of electrons transferred in the adsorption process.

Please cite this article in press as: Zhang X, et al., Adsorption and dissociation of ammonia on small iron clusters, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.11.003

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e7

Results and discussion Adsorption of reaction intermediates As well-known, the initial adsorption manner of a molecule on the catalyst greatly affects the subsequent reactions. To have a better understanding on the catalytic characteristics of the Fe clusters toward NHx, the adsorption properties of NHx (x ¼ 0e3) and H species are investigated firstly. The most stable adsorption configurations are shown in Fig. 1 (all figures generated using the VESTA package) [35], with the corresponding adsorption energies and geometrical parameters summarized in Table 1. As shown in Fig. 1 and Table 1, the NH3 molecule prefers to be adsorbed at the top site (Top) of an iron atom of all the Fe clusters studied with the N atom bonding to the iron atom and the H atoms pointing outward. The adsorption energies increase stepwise along with the cluster size (from Fe to Fe4). Accordingly the shortest NeFe distance is shortened. In order to provide a rational interpretation of the adsorption energies, the d-band center of the Fe clusters are calculated. In this paper, the d-band center is taken to be the average value of the d-band centers of all the Fe atoms. We find that the d-band centers become gradually closer to the Fermi level for the Fe,

3

Fe2, Fe3, and Fe4 clusters. It is known that the lower the d-band center, the weaker the adsorption is. The d-band center of Fe is the farthest to the Fermi level, the adsorption energy for the adsorbed species is therefore the lowest. The geometric structure parameters of the adsorbed molecule are shown in Table 1. As for the adsorbed NH2, the FeeFe bridge site (Bri) is the preferred adsorption site, with the N atom directly connected to the FeeFe bond, and the HeH bond of NH2 almost perpendicular to the FeeFe bond. The bond lengths of the Fe clusters and the NH2 are all shown in Fig. 1. The interaction between the metals and the N atoms leads to the rearrangement of the bonds in the adsorbed molecules (NH3 and NH2) and hence the increase of the HeNeH cone angle [19]. From Table 1, we also find that the adsorption energies of NH2 are quit larger than that of NH3 on the Fe clusters, which are in line with the shortened NeFe bonds and enlarged HeNeH cone angle. The results of Bader charge analysis shown in Table 2 indicate that the charges transferred between NH3 and Fe clusters are very niggardly, yet the charges transferred in the NH2 systems are higher by about an order of magnitude than the corresponding values in the NH3 systems. According to the calculated results, the NH prefers to be adsorbed at the top site of Fe with the N connected to the Fe

Fig. 1 e The optimized geometric structures of NHx (x ¼ 0e3) and H adsorbed on the Fe clusters. Please cite this article in press as: Zhang X, et al., Adsorption and dissociation of ammonia on small iron clusters, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.11.003

4

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e7

Table 1 e Adsorption energy (Ead) of NHx (x ¼ 0e3) on Fen (n ¼ 1e4), and the HeNeH cone angle (∠HNH( )) after the NHx adsorption. Species

NH3 NH2 NH N H

Fe

Fe-dimer

Fe-trimer

Fe-tetramer

Ead

Site

:HNH

Ead

Site

:HNH

Ead

Site

:HNH

Ead

Site

:HNH

0.90 2.96 3.78 4.05 2.65

Top Top Top Top Top

107.92 109.09 e e e

1.06 3.42 4.61 5.07 2.24

Top Bri Bri Bri Top

107.38 107.45 e e e

1.16 3.68 5.12 5.90 2.45

Top Bri Bri Hol Bri

107.47 107.48 e e e

1.35 3.85 5.11 6.16 2.99

Top Bri Hol Hol Hol

107.51 108.24 e e e

directly, and the H atom pointing outward. For Fe2 and Fe3 clusters, the bridge sites are also the optimum adsorption sites for NH. However, the NeH bond is oblique and coincides with the NeFe2 and NeFe3 planes, respectively, as shown in Fig. 1. This may be because both the H atom and the Fe atoms which are directly connected with the N donate electrons to the N atom, then an electrostatic repulsion is developed between the negative N atom and Fe atoms. The stronger electronegativity of N leads to the stronger electrostatic repulsion between the Fe and H, as well as a farther FeeH distances and different adsorption patterns. The adsorption characteristics of N and H on the iron clusters are also calculated and listed in Fig. 1. The adsorption of N on the Fe clusters has the largest adsorption energies and charge transfers, as list in Tables 1 and 2. The results indicate that N prefers to be adsorbed at the sites that can form as many as NeFe bonds. However, this rule is not applicable for the adsorption of H. We find from Fig. 1 that the H prefers to be adsorbed at the top site of Fe and Fe2, while it prefers to be adsorbed at the bridge site and hollow site on the Fe3 and Fe4, respectively. Moreover, on all the iron clusters studied, the fewer H atoms of the NHx (x ¼ 0e3) species, the higher the adsorption energies of NHx are. In this case, the N moiety of NHx is coordinated with more iron atoms, forming shorter FeeN bonds.

The stepwise dissociation of ammonia In this section, we will mainly present the results of NH3 dehydrogenation reactions on the Fe clusters. The catalytic activity of Fe clusters for each step of dehydrogenation reaction is comparably studied. The geometrical configurations of the initial states (IS), the transition states (TS) and the final states (FS) of all the reactions are shown in Fig. 2, and the calculated reaction barriers as well as the reaction heats are listed in Table 3. For the first step of dehydrogenation (Step 1), NH3 is adsorbed firstly at the top site on the iron clusters and further

dissociated into NH2 and H. For the Fe cluster, the NH2 moiety moves slightly closer to the Fe atom and the exfoliation H is also attached to the Fe atom. For the Fe2, Fe3 and Fe4 clusters, we also find that the NH2 moiety is located at the top site for the transition state, and H moves towards the bridge site. Then the reaction proceeds by the H further occupying the bridge position and the NH2 keeping at the top site (on Fe2 and Fe3) or moving to the bridge site (on Fe4). The FeeH and FeeN distances in the product states are in the ranges of ˚ ) and (1.84e1.99 A ˚ ), respectively. The energy bar(1.61e1.74 A riers for the dissociation of H in step 1 and the correspondingly reaction heats are all shown in Table 3. The results show that the reaction heat released on Fe is slightly higher than those on Fe2 and Fe3, which may be responsible for the lower dehydrogenation barrier on Fe. On the other hand, we also find that the coadsorption energies of NH2 and H are higher than the sum of the separated adsorption energies for all the clusters except for the Fe, as shown in Table 4, which may be also responsible for the lower dehydrogenation barrier on Fe than that on other clusters. For the second dehydrogenation step (Step 2), the most stable adsorption structures of NH2 are taken as the initial state (IS2), and the TS2 and FS2 represent for the transition states and the final states, respectively. Correspondingly, the energy barriers and the reaction heats are listed in Table 3, which show that the trend of the reaction barriers on the different Fe clusters for the second dehydrogenation step is the same as that for the first dehydrogenation reaction step. We also find that the second dehydrogenation reactions have higher activation barriers than the first step on the corresponding iron clusters and the reactions are endothermic, instead of exothermic. Moreover, the adsorption energy and the reaction barrier of NH2 on Fe are all smaller than those on the Fe2, Fe3 and Fe4 clusters. This may be due to the similar adsorption configurations of reactant, intermediate and product states on Fe. The similar phenomenon was also observed on the Pt surface by Santen et al [36].

Table 2 e The results of Bader charge analysis in the processes of NH3 dehydrogenation on iron clusters. The positive and negative values represent the electrons accumulation and depletion on NHx or NHx/H (The columns of NHx/H represents the charge accumulation in the moieties of NHx and H in the most stable coadsorption structures), respectively. The N and H in the last two columns are the charges of the adsorbed N and H, respectively.

Fe1 Fe2 Fe3 Fe4

NH3

NH2/H

NH2

NH/H

NH

N/H

N

H

0.02 0.01 0.05 0.05

0.53/0.56 0.55/0.55 0.52/0.50 0.69/0.47

0.48 0.66 0.59 0.55

0.60/0.31 0.88/0.36 0.94/0.51 0.95/0.49

0.69 0.89 0.94 1.16

0.64/0.24 1.06/0.49 1.13/0.25 1.38/0.44

0.74 1.02 1.12 1.30

0.35 0.48 0.42 0.39

Please cite this article in press as: Zhang X, et al., Adsorption and dissociation of ammonia on small iron clusters, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.11.003

5

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e7

Fig. 2 e The geometric structures of initial states (ISx), transition states (TSx) and the final states (FSx) in the stepwise dehydrogenation reactions of NH3 on the Fe clusters. x (¼1e3) denotes the steps of the dehydrogenation reactions of NH3. The inserted numbers in the figures are the bond lengths and the spheres with golden-yellow, silver-white and light-pink colors represent the Fe, N and H atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

For the step of NH decomposition into N and H, the IS, TS and FS are accordingly summarized in the three rightmost columns in Fig. 2. Based on our calculated results, we find that the NeH bond experiences distortion, elongation or broken on the Fe, Fe2 and Fe4 clusters, and finally reach the FS. However, the NH adsorbate diffuses firstly from the bridge site to the hollow site before being cleaved on the Fe3. The calculated barrier for NH diffusion on Fe3 is 0.2 eV, with a small reaction heat of about 0.13 eV (endothermic). Following the tendency that “dissociation reactions tend to access product-like or late transition states”, the metastable configuration with the NH located at the hollow site of Fe3 favors the NeH bond breaking. Their energy barriers and reaction heats are shown in Table 3. The results indicate that the dehydrogenation process on Fe is more facile than those on Fe2, Fe3 and Fe4, which is similar to the second dehydrogenation step. Once the NH decomposes completely, the adsorption preference of the isolated N or H becomes more important. From Table 1, we find that the adsorption of N is quit stronger than that of H. In another words, N tends to occupy the active sites preferable and leads to the catalytic deactivation. So the

further study about how to remove the remaining N needs to be carried out. The Bader charge analysis in the process of ammonia dehydrogenation is performed and the results are shown in Table 2. It is found that the Fe atoms in the clusters lost/get very limited number of electrons (0.01e0.05 e) when ammonia is adsorbed. Then the Fe clusters donate more electrons to the ammonia and facilitate the breaking of the NeH bond, and finally the first H is exfoliated completely from NH3, accompanied by a charge lost of about 1.10 e from the Fe clusters. Once the H is removed from the NHx, the formed NHx1 can get additional electrons from the Fe clusters (as shown in Table 2), and facilitates the further dehydrogenation steps. Similar charge transfer phenomenon as that in the first dehydrogenation step is also found in the second and the third steps. It should be pointed out that, in the catalysis theory, the overall barrier is more important rather than a particular barrier, and the barrier of the rate-limiting step should be equal to the highest particular one in the reaction process. For the integrated dehydrogenation reaction, the rate-

Table 3 e The results of energy barriers (Eb, eV) and reaction heats (DH, eV) for the NH3 stepwise dehydrogenation reactions on the Fe clusters. Reactions NH3* þ * / NH2* þ H* NH2* þ * / NH* þ H* NH* þ * / N* þ H*

Fe

Fe2

Fe3

Fe4

Eb

DH

Eb

DH

Eb

DH

Eb

DH

0.53 1.03 1.06

0.52 0.69 0.63

1.00 1.49 1.24

0.18 0.23 0.47

0.89 1.33 1.43

0.30 0.09 0.40

0.93 1.51 1.43

0.71 0.07 0.05

Please cite this article in press as: Zhang X, et al., Adsorption and dissociation of ammonia on small iron clusters, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.11.003

6

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e7

Table 4 e The coadsorption energy (Ecoads) and the corresponding sum of the separated adsorption energies (shown in the parentheses) of NHx and H (x ¼ 0e2) on Fe clusters. Conf. represents the coadsorption configuration. Conf. NH2 þ H NH þ H NþH

Fe

Fe2

Fe3

Fe4

6.23 (6.61) 4.59 (6.43) 7.01 (6.70)

6.04 (5.66) 5.50 (6.85) 7.99 (7.31)

6.27 (6.13) 5.90 (7.57) 8.56 (8.35)

6.87 (6.84) 6.23 (8.10) 9.03 (9.15)

Henan Province (Grant No. 2011038), Foundation for the Key Young Teachers of Henan Normal University and Start-up Foundation for Doctors of Henan Normal University. Parts of the simulations are performed on resources provided by the high-performance computing center of College of Physics and Electronic Engineering in Henan Normal University.

Appendix A. Supplementary data limiting steps for Fe and Fe3 are the NH decomposition, and those for Fe2 and Fe4 are the NH2 dissociation processes. Their barriers for the rate-limiting steps are 1.06, 1.49, 1.43 and 1.51 eV for Fe, Fe2, Fe3 and Fe4, respectively. The results indicate that the monatomic Fe is a relatively better catalyst for ammonia decomposition, which is followed by Fe3, Fe2 and Fe4.

Conclusions The first-principles calculations are performed to determine the preferred adsorption sites and the adsorption energies of NHx (x ¼ 0e3) and H species on small iron clusters. The results show that the NH3 and NH2 are preferable to be adsorbed at the top and bridge sites of the Fe clusters, respectively. The most stable adsorption sites for NH are the bridge sites on the Fe2 and Fe3, but the hollow site on Fe4. We also find that the N atom prefers to be adsorbed at the sites with as many as Fe neighbors. Yet the rule is not applicable for the adsorption of H, which prefers to be adsorbed at the top (on Fe and Fe2), bridge (on Fe3) and hollow (on Fe4) sites, respectively. The catalytic activity of the small Fe clusters on the dehydrogenation reactions of the NH3 and the rate-limiting steps for the NH3 stepwise dehydrogenation are also studied. The calculated results indicate that the monatomic iron atom has the highest catalytic activity for the NH3 stepwise dehydrogenation, which is followed by the Fe3, Fe2 and Fe4 clusters. For the integrated reaction, the rate-limiting steps for Fe and Fe3 are the NH decomposition, and those for Fe2 and Fe4 are the NH2 dissociation processes. The energy barriers for the ratelimiting steps are 1.06, 1.49, 1.43 and 1.51 eV for Fe, Fe2, Fe3 and Fe4, respectively, suggesting that the small Fe clusters can be regarded as the excellent candidates for the NH3 dehydrogenation reactions. The results provide useful information to tune the relative reaction rates of the different steps during NH3 dehydrogenation by controlling the sizes of the iron clusters, which can serve as a reference for investigating the catalytic activity of small Fe clusters supported by other materials.

Acknowledgment This work is supported by the National Natural Science Foundation of China (Grant No. 11174070, 11147006, 11474086 and 51401078). Z. Lu also acknowledges the support from the China Postdoctoral Science Foundation funded project (Grant No. 2012M521399) and Postdoctoral Research Sponsorship in

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.11.003.

references

[1] Jones AV. Access to chemistry. Royal Society of Chemistry; 1999. [2] Selim A, Elsayed M. Performance of a packed bed absorber for aqua ammonia absorption refrigeration system. Int J Refrig 1999;22:283e92. [3] Yang L, Du K. An optimizing method for preparing natural refrigerant: ammonia-water nanofluids. Integr Ferroelectr 2013;147:24e33. [4] Van Hardeveld R, Van Santen R, Niemantsverdriet J. Formation of NH3 and N2 from atomic nitrogen and hydrogen on rhodium (111). J Vac Sci Technol A 1997;15:1558e62. ga-Mariadassou G, Shin C-H, Bugli G. Tamaru's model for [5] Dje ammonia decomposition over titanium oxynitride. J Mol Catal A Chem 1999;141:263e7. [6] Choudhary T, Sivadinarayana C, Goodman D. Catalytic ammonia decomposition: COx-free hydrogen production for fuel cell applications. Catal Lett 2001;72:197e201. [7] Lan R, Irvine JT, Tao S. Ammonia and related chemicals as potential indirect hydrogen storage materials. Int J Hydrogen Energy 2012;37:1482e94. [8] Li L, Hurley JA. Ammonia-based hydrogen source for fuel cell applications. Int J Hydrogen Energy 2007;32:6e10. [9] Li Y, Liu S, Yao L, Ji W, Au C-T. Core-shell structured iron nanoparticles for the generation of COx-free hydrogen via ammonia decomposition. Catal Commun 2010;11:368e72. [10] Koebel M, Elsener M, Kleemann M. Urea-SCR: a promising technique to reduce NOx emissions from automotive diesel engines. Catal Today 2000;59:335e45. [11] Yan JM, Zhang XB, Han S, Shioyama H, Xu Q. Ironnanoparticle-catalyzed hydrolytic dehydrogenation of ammonia borane for chemical hydrogen storage. Angew Chem Intern Ed 2008;47:2287e9. [12] Nishimaki K, Ohmae S, Yamamoto T, Katsura M. Formation of iron-nitrides by the reaction of iron nanoparticles with a stream of ammonia. Nanostruct Mater 1999;12:527e30. [13] Chein R-Y, Chen Y-C, Chang C-S, Chung J. Numerical modeling of hydrogen production from ammonia decomposition for fuel cell applications. Int J Hydrogen Energy 2010;35:589e97. [14] Lanzani G, Laasonen K. NH3 adsorption and dissociation on a nanosized iron cluster. Int J Hydrogen Energy 2010;35:6571e7. [15] Sun Q, Li Z, Wang M, Du A, Smith SC. Methane activation on Fe4 cluster: a density functional theory study. Chem Phys Lett 2012;550:41e6. [16] Sun Q, Li Z, Du A, Chen J, Zhu Z, Smith SC. Theoretical study of two states reactivity of methane activation on iron atom and iron dimer. Fuel 2012;96:291e7.

Please cite this article in press as: Zhang X, et al., Adsorption and dissociation of ammonia on small iron clusters, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.11.003

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e7

[17] Liu R, Shen W, Zhang J, Li M. Adsorption and dissociation of ammonia on Au (111) surface: a density functional theory study. Appl Surf Sci 2008;254:5706e10. [18] Offermans W, Jansen A, Van Santen R. Ammonia activation on platinum {111}: a density functional theory study. Surf Sci 2006;600:1714e34. [19] Duan X-Z, Ji J, Qian G, Fan C, Zhu Y, Zhou X-G, et al. Ammonia decomposition on Fe (110), Co (111) and Ni (111) surfaces: a density functional theory study. J Mol Catal A Chem 2012;357:81e6. [20] Lin R-J, Li F-Y, Chen H-L. Computational investigation on adsorption and dissociation of the NH3 molecule on the Fe (111) surface. J Phys Chem C 2010;115:521e8. [21] Kresse G, Furthmu¨ller J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a planewave basis set. Comput Mater Sci 1996;6:15e50. [22] Kresse G, Furthmu¨ller J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys Rev B 1996;54:11169. [23] Kresse G, Joubert D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys Rev B 1999;59:1758. [24] Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phys Rev Lett 1996;77:3865. [25] Gutsev GL, Bauschlicher CW. Electron affinities, ionization energies, and fragmentation energies of Fen clusters (n ¼ 2e6): a Density Functional Theory study. J Phys Chem A 2003;107:7013e23. [26] Purdum H, Montano P, Shenoy G, Morrison T. Extended-Xray-absorption-fine-structure study of small Fe molecules isolated in solid neon. Phys Rev B 1982;25:4412.

7

[27] Chen J, Wang C, Jackson KA, Pederson MR. Theory of magnetic and structural ordering in iron clusters. Phys Rev B 1991;44:6558. [28] Ballone P, Jones R. Structure and spin in small iron clusters. Chem Phys Lett 1995;233:632e8. guez O, Alemany M, Rey C, Ordejo  n P, Gallego L. Density[29] Die functional calculations of the structures, binding energies, and magnetic moments of Fe clusters with 2 to 17 atoms. Phys Rev B 2001;63:205407. [30] Yu S, Chen S, Zhang W, Yu L, Yin Y. Theoretical study of electronic structures and magnetic properties in iron clusters (n  8). Chem Phys Lett 2007;446:217e22. [31] Huang W, Lai W, Xie D. First-principles study of decomposition of NH3 on Ir (100). Surf Sci 2008;602:1288e94. [32] Henkelman G, Jonsson H. Improved tangent estimate in the nudged elastic band method for finding minimum energy paths and saddle points. J Chem Phys 2000;113:9978e85.  nsson H. A climbing image [33] Henkelman G, Uberuaga BP, Jo nudged elastic band method for finding saddle points and minimum energy paths. J Chem Phys 2000;113:9901e4. [34] Arnaldsen A, Tang W, Henkelman G. Bader charge analysis. 2012. [35] Momma K, Izumi F. VESTA: a three-dimensional visualization system for electronic and structural analysis. J Appl Cryst 2008;41:653e8. [36] Offermans W, Jansen A, Van Santen R, Novell-Leruth G, Ricart J, Perez-Ramirez J. Ammonia dissociation on Pt {100}, Pt {111}, and Pt {211}: a comparative density functional theory study. J Phys Chem C 2007;111:17551e7.

Please cite this article in press as: Zhang X, et al., Adsorption and dissociation of ammonia on small iron clusters, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.11.003