Journal of Nanomaterials

Volume 2014 (2014), Article ID 758985, 8 pages

http://dx.doi.org/10.1155/2014/758985

Research Article

First-Principle Study of H₂ Adsorption on LaFeO₃(110) Surface

Yu-Hong Chen,1,2 Bing-Wen Zhang,¹ Liang Yang,¹ Mei-Ling Zhang,¹ Cai-Rong Zhang,1,2 Long Kang,² and Yong-Chun Luo²

¹School of Science, Lanzhou University of Technology, Lanzhou 730050, China

²State Key Laboratory of Gansu Advanced Non-Ferrous Metal Materials, Lanzhou University of Technology, Lanzhou 730050, China

Received 15 January 2014; Revised 6 May 2014; Accepted 12 May 2014; Published 21 July 2014

Academic Editor: Jinlong Jiang

Copyright © 2014 Yu-Hong Chen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The adsorption of H₂ molecule on LaFeO₃(110) surface was studied by first-principle calculations. Based on the adsorption sites, adsorption energies, and electronic structures, it can be found that one H atom can be adsorbed on O atom and form –OH with the O atom, which is the most stable structure. One H atom can be adsorbed on one Fe atom, which makes Fe³⁺ turn to Fe²⁺. Two H atoms can form H₂O molecule with O atom, which makes it possible to form oxygen vacancy on the surface.

1. Introduction

Nickel-metal (NI-MH) batteries have been widely studied for its favorable property such as high capacity, fast charge and discharge rate, environmental compatibility, and long stable periods [1]. Cathode material has a great effect on the performance of NI-MH batteries. Traditional negative materials are hydrogen storage alloys including AB₅, AB₂, AB, and Mg-based alloys [2, 3]. Although AB₅-type alloys have been widely applied in various devices, it has high cost and low capacity. Therefore, a lot of researches have been carried out to develop new negative materials to reduce the cost and improve the capacity. In recent years, Perovskite ABO₃ has attracted a lot of attention as potential negative materials [4, 5].

The capacity of perovskite ABO₃ as cathode material is much higher than that of traditional materials. Mandal et al. [6] have developed an unprecedented intake of hydrogen by BaMnO₃/Pt to the extent of 1.25 w.% at moderate temperatures (190–260°C) and ambient pressure. Esaka et al. [7] have proposed perovskite-type oxides (A = Sr or Ba, M = rare earth element) prepared by a conventional solid-state reaction method as innovative electrode materials for Ni-MH batteries. Deng et al. [8] have reported that the reversible capacity of perovskite was more than 500 mAh/g at a discharge current density of 31.25 mA/g when the temperature rises to 333 K, which is much higher than that of AB₅ alloys [9]. Considering the high capacity and abundance of La, perovskite LaFeO₃ has been studied widely as potential negative materials for Ni-MH batteries. Deng et al. [10] have reported that before the 20th cycle, the discharge capacity of LaFeO₃ keeps steady at about 80 mAh/g, 160 mAh/g, and 350 mAh/g at 298 K, 313 K, and 333 K, respectively. However, the electrochemical hydrogen storage mechanism of the perovskite oxide still remains uncertain, and it requires more investigation to know how the H atoms combine with the perovskite oxide. In this research, we employed density functional theory (DFT) to investigate the slab character of LaFeO₃ and hydrogen adsorption on the slab aiming to explain the adsorption mechanism of reaction and provide theoretical guidance for correlative experiment.

2. Models and Computational Methods

2.1. Models

In this paper, we employed the slab model which is widely used in surface calculation. The structure of LaFeO₃ belongs to orthorhombic crystal system, whose space group is Pbnm [11]. As for LaFeO₃(110) slab, there are two kinds of models which we defined as Model I and Model II. Taking accuracy and speed in consideration, we simulate the LaFeO₃(110) surface with a 4-layer model and for further discussion a 7-layer model will be studied for improving and comparison. The structure parameter of H₂ molecule which is about to be adsorbed is 0.741 Å and the thickness of vacuum is set at 20 Å. When the model is set up, 8 favorable sites are taken in calculation, which is shown in Figure 1. H₂ molecule is put on all the sites separately parallel to the crystal surface.

(a)

(b)

Figure 1: Vertical view of all symmetrical adsorption sites: (a) Model I (b) Model II. ((A): O long bridge; (B): O short bridge; (C): O top; (D): O cave; (E): La top; (F): La bridge; (G): Fe top; (H): O top).

2.2. Computational Methods

The first-principle calculations are performed with Cambridge serial total energy package (CASTEP) in the framework of density functional theory (DFT), which is based on the plane-wave pseudopotential. The exchange and correlation energy were treated within the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE). The electron-ion interaction is described with ultrasoft pseudopotential (UUSP). After testing the model with -point grid set , it is decided to use the supercells consisting of 5 × 5 × 1 unit cell for the doped system calculations. All the calculations are carried out non-spin-polarized with a kinetic energy cutoff of 340 eV, and the convergence criteria for energy and displacement are 2 × 10⁻⁶ eV/atom and 10⁻³ Å, respectively. The calculated structure parameters of H₂ molecule are 0.746 Å, which agrees well with experimental results [12].

Moreover, the structure parameters of LaFeO₃(110) surface is , which is the same as experimental results.

3. Results and Discussions

3.1. Decision of the Adsorption Sites of H₂ Molecule on LaFeO₃(110) Surface

Table 1 lists the energies and structure parameters of all possible sites. It is defined that , , , and are the shortest distances of H atoms, H atom and O atom, H atom and Fe atom, and H atom and La atom, respectively. The dissociation energy of H₂ molecule is defined as the following equation:

and represent the energy of H₂ molecule and H atom, respectively. The adsorption energy of H₂ molecule is defined in the following equation:

and represent the energy of clean crystal surface and the surface with two adsorbed H atoms, respectively. According to the definitions above, it can be inferred that the structure of LaFeO₃(110)/H₂ is stable when is positive, and the bigger the is, the more stable the structure will be; it means that the H₂ molecule has been dissociated when is positive.

Table 1: Energies and structure parameters of possible sites.

LaFeO₃ (110) 

surface
Possible adsorption

sites
/Å
/Å
/Å
/Å
/eV
E/eV

Free H₂
0.746
—
—
—
—
4.502

Model I
O long bridge (a)
3.431
0.980
2.389
3.456
3.130
−1.854

O short bridge (b)
3.471
0.982
2.373
3.505
3.507
−1.878

O top (c)
1.613
0.981
2.202
3.877
2.389
1.220

O cave (d)
0.760
17.335
18.359
18.677
0.370
4.495

Model II
La top (e)
0.765
18.411
19.500
19.213
0.357
4.494

La bridge (f)
2.968
2.677
1.571
2.853
1.898
−1.542

Fe top (g)
0.760
16.978
17.477
18.359
0.248
4.495

O top (h)
4.784
2.229
1.587
2.966
1.829
−2.186

According to the results in Table 1, the adsorption of H₂ molecule on sites a, b, c, h, and f belong to chemical adsorptions, and they are physical adsorptions when H₂ molecule is put on the other sites.

All the chemical adsorption sites are shown in Figure 2. The data in the figure are representative of distances between H atoms, whose unit is Å. By analysing the data in Table 1 and adsorption in Figure 2, we can find the following.

(1)On both site a and site b, the H₂ molecule is dissociated into two H atoms and then each H atom is absorbed by O atom, which is a chemical adsorption.(2)On both site h and site f, after dissociation of the the H₂ molecule, each H atom is absorbed by Fe atom, which is chemical adsorption.(3)On site c, after adsorption, the structure parameters ( = 1.613 Å, = 1.007 Å) are close to the experimental parameters that Gu et al. [13] have reported ( = 1.545 Å, = 0.978 Å). We can infer that H₂O molecule is formed, which makes it possible to form an oxygen vacancy if the H₂O molecule gets rid of the crystal surface [14].

(a)

(b)

(c)

Figure 2: All the sites where a chemical adsorption occurs. Model I: (a) O short bridge; (b) O top. Model II: (F) La bridge.

Considering the accuracy, a model consisting of 7 layers of atoms is adopted. Table 2 lists the structure parameters and energies of the crystal surface. According to the data in the table, the surface with H atoms absorbed on O atoms is more stable than that with H atoms absorbed on Fe atoms and both are chemical adsorptions.

Table 2: Structure parameters and energies of the crystal surface.

Initial position of H₂
/Å
/Å
/Å
/Å
/eV
E/eV

O top in Model II
2.968
2.677
1.571
2.853
1.898
−1.542

O short bridge in Model I
3.647
0.976
2.332
3.367
4.713
−1.956

3.2. The Adsorption Mechanism of H₂ Molecule on O Short Bridge in Model I

The following discussions are about the properties on O short bridge in Model I including Mulliken charge population, density of states, and electron localization function.

3.2.1. Mulliken Charge Population

It shows the information of interaction between the H₂ molecule and the crystal surface by analysing Mulliken charge population. Table 3 lists the Mulliken Charge population before and after adsorption, respectively. The H atoms get positive charges and the crystal surface is negatively charged after adsorption. The change of charges on the surface takes 69.5% of total change, which indicates that the H₂ molecules mainly interact with the atoms on the surface. Moreover, surface potential falls after adsorption, which means the structure is more stable.

Table 3: Mulliken population on O short bridge in Model I.

Layer
Atom
Charges before adsorption
Charges

after adsorption

H

H

0.40

0.40

First
O

O

O

O
−0.61

−0.61

−0.61

−0.61
−0.68

−0.82

−0.82

−0.64

Second
O

La

Fe
−0.65

1.68

0.77
−0.65

1.65

0.76

Third
O
−0.69
−0.69

Fourth
O

Fe

La
−0.70

1.23

0.28
−0.70

1.21

0.29

3.2.2. Density of States (DOS)

The information of interaction between atoms can be obtained by analysing the change of valance states and energies. Figure 3 lists density of states of H₂ molecule adsorption on O short bridge in Model I before and after adsorption, respectively. By analysing the data, we can find that the energy of highest occupied states (HOS) is between −6.5 eV and 3.0 eV, which is mainly caused by O 2p electrons and Fe 3d electrons. In contrast, the energy of HOS has changed to that which is between −8.0 eV and 0.5 eV after adsorption, which is mainly caused by H 1s electron, O 2p electrons, and Fe 3d electrons. The HOS of electrons moved to deeper levels after adsorption. That is to say, the structure becomes more stable after adsorption. And the H₂ molecule is dissociated absolutely after adsorption. In addition, H 1s not only overlaps but also resonates with O 2s and O 2p; it means that H atom forms–OH with O atom after adsorption.

(a)

(b)

Figure 3: DOS on O short bridge in Model I (a) before adsorption and (b) after adsorption.

3.2.3. Electron Localization Function (ELF)

Properties of bonds between atoms can be analysed by ELF. The range of ELF is the interval from 0 to 1. The closer ELF to 1, the deeper the electron localization degree. On the other hand, if ELF is close to zero, the electrovalent bond will be strong.

Figure 5(a) lists the ELF of LaFeO₃(110) surface with H atoms absorbed on O atoms and on the initial position of O short bridge in Model I. It is obvious that the ELF is close to 1, which means the covalent bond between the H atom and the O atom is very strong.

3.3. Adsorption Mechanism of H₂ Molecule on O Top in Model II

The following discussions are about the properties on O top in Model II including Mulliken charge population, density of states, and electron localization function.

3.3.1. Mulliken Charge Population

Table 4 lists the Mulliken charge population before and after adsorption, respectively. The H atoms get negative charges and the crystal surface gets positive charges after adsorption. The Fe atoms, which absorb H atoms, lose charges after adsorption. It makes Fe³⁺ turn to Fe²⁺. And that agrees with what Hoffmann et al. [15] have reported on change of valance states of B in perovskite ABO₃. Surface potential falls after adsorption, which means that the structure is more stable. Moreover, it is observed that the La atoms got charges after adsorption, which indicates that the valance states of La atoms have changed.

Table 4: Mulliken population on O top in Model II.

Layer
Atom
Charges before adsorption
Charges after adsorption

H

H

−0.31

−0.31

First
O

Fe

Fe

Fe

Fe

La
−0.72

0.42

0.42

0.42

0.42

1.37
−0.74

0.46

0.48

0.31

0.34

1.45

Second
O
−0.74
−0.73

Third
O

Fe

La
−0.69

0.62

1.49
−0.69

0.62

1.51

Fourth
O
−0.70
−0.70

3.3.2. Density of States (DOS)

Figure 4 lists the density of states of H₂ molecule adsorption on O top in Model II before and after adsorption, respectively. By analysing the data, we get three conclusions. First, the energy of HOS of H atoms is decreased after adsorption and in contrast the energy of HOS of Fe atoms is increased after adsorption, which means that the H atoms get more stable and the Fe atoms get more excited after adsorption. Second, the peaks of density of states falls after adsorption, which means the energy of the surface is lower and the structure is more stable after adsorption. Third, after adsorption, the DOS of Fe atoms have changed, which means the charge population of Fe atoms have changed.

(a)

(b)

Figure 4: DOS on O top in Model II (a) before adsorption and (b) after adsorption.

(a)

(b)

Figure 5: ELF on O top in two models: (a) Model I; (b) Model II.

3.3.3. Electron Localization Function (ELF)

Figure 5(b) lists the ELF of LaFeO₃(110) surface with H atoms absorbed on Fe atoms with the initial position of the H₂ molecule being O top in Model II. It is obvious that the ELF between the H atom and the Fe atom is about 0.5, which means that a metallic bond is formed after adsorption. And that agree with what Mandal et al. [6] have reported on the change of valance states of Mn.

4. Conclusions

The adsorption H₂ on LaFeO₃(110) surface is studied. H₂ is put on one symmetrical adsorption site, parallel to the surface, and then is calculated with first-principle methods. After analysing and discussing the results, we can find the following.

(1)H atom can be absorbed on O atom and form –OH. There is a strong covalent bond between them after adsorption. The adsorption energy is 3.507 eV, which is higher than that on other sites.(2)Two H atoms can form H₂O molecule with an O atom. The adsorption energy is 1.220 eV, which is lower than that when H atom is absorbed on O atom or Fe atom. An Oxygen vacancy will be formed on the surface if the H₂O molecule gets rid of the surface.(3)H atoms can be absorbed on Fe atom and forms a metallic bond with it, which not only makes Fe³⁺ turn to Fe²⁺ but also makes the H atoms more stable and the Fe atoms more excited.(4)Although no H atom is absorbed on La atom, it is observed that the charge population has changed after adsorption, which indicates that the valance states of La may have changed.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors gratefully acknowledge the financial support from the Basic Scientific Research Foundation for Gansu Universities of China (Grant no. 05-0342), the Science and Technology Project of Lanzhou City (Grant no. 2011-1-10), and the Doctoral Foundation of Lanzhou University of Technology (Grant no. BS200901).

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