A First-Principles Study - American Chemical Society

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Adsorption of NO Molecule on Spinel-Type CuFe2O4 Surface: A First-Principles Study Zhi Jiang,† Wenhua Zhang,‡ Wenfeng Shangguan,*,† Xiaojun Wu,*,‡ and Yasutake Teraoka§ †

Research Center for Combustion and Environment Technology, Shanghai Jiao Tong University, Shanghai, 200240, China CAS Key Laboratory of Materials for Energy Conversion, Department of Material Science and Engineering, and Hefei National Lab for Physical Science at the Microscale, University of Science and Technology of China, Hefei, Anhui, 230026 China § Faculty of Engineering Sciences, Kyushu University, Fukuoka 816-8580, Japan ‡

ABSTRACT: The electronic properties of spinel-type CuFe2O4 material and the adsorption behavior of NO molecule on CuFe2O4 (100) surface were studied by using density functional theory method with on-site correction for Coulomb interaction (DFTþU). Our studies suggest that the ground state of CuFe2O4 bulk has an inverse spinel structure, which is a magnetic semiconductor. On the inverse spinel-type CuFe2O4 (100) surface, NO molecule prefers to adsorb on the top site of surface Fe atom with the formed N
Fe bond. The adsorption energy, electronic properties, and structures were investigated to provide an initial understanding to the catalysis of NO molecule over CuFe2O4 surface.

’ INTRODUCTION Spinel-type transition-metal oxides comprise a class of compounds with a general formula of AB2O4, such as the mineral spinel MgAl2O4, Fe3O4, CuCo2O4, and so forth. These materials usually possess unique electronic, magnetic, and optical properties suitable for many technical applications, including superconductors, magnetic core, humidity sensors, and so forth.1
8 In particular, some spinel-type transition-metal oxides have excellent catalytic properties, which have attracted much fundamental and applied research attention from the past decade.9
26 Current environmental catalysis research has shown that the spinel-type transition-metal oxide catalysts are effective for nitrogen oxide (NOx) decomposition/reduction reaction,9
19 and some may have excellent catalytic activity for simultaneous catalytic removal of NOx and diesel soot particulates.16
19 Spinel-type CuFe2O4 is one of the most excellent spinel-type oxides for simultaneous catalytic removal of nitrogen oxides and diesel particulates.17
19 Our previous kinetics studies of soot
NO over spinel-type CuFe2O4 indicated that below 550 °C, the concentration of CO2 was higher than that of N2 and that the soot
NO reaction over spinel-type CuFe2O4 might be controlled by the NO adsorption step at this temperature stage.18,19 The adsorption property is closely related with the nature of the catalysis, and knowledge of adsorbate adsorption may serve as a basis for the development of a comprehensive mechanism for the simultaneous catalytic removal process of nitrogen oxides and diesel particulates. In recent years, small molecule adsorption on other spinel-type oxide surfaces has gradually intrigued researchers’ attention.9,10,21,22,26 Yin et al. have studied the adsorption of NH3, NO2, and NO on CuAl2O4 (100) surface.10 Xu et al. have studied the adsorption of NO, CO, O2, and the subsequent reaction between CO and NO on the spinel-type CuCr2O4 r 2011 American Chemical Society

(100) surface.10,21,22 Piskorz et al. studied the decomposition of N2O over the surface of Co3O4 cobalt spinel with density functional theory (DFT) method.26 These studies provide some basic understanding to chemical reaction on spinel-type catalysts’ surfaces. To the best of our knowledge, however, theoretical work has not been undertaken in understanding the adsorption of NO molecule on the CuFe2O4 surface, a typical catalyst for simultaneous removal of nitrogen oxides and diesel particulates. In this paper, we report an electronic structure study of NO adsorption on one typical low index plane of the spinel-type CuFe2O4, the (100) surface. Using the density functional theory (DFT) method, with the Cu and Fe 3d states treated with on-site correction for Coulomb interaction (DFTþU), we explored the electronic properties of spinel-type CuFe2O4 material and the adsorption behavior of NO molecule on CuFe2O4 (100) surface under an ideal situation. The results presented here provide an initial understanding of the catalysis mechanism of NO molecule over the CuFe2O4 surface. To our knowledge, this is the first theoretical study of NO adsorption on spinel-type CuFe2O4 surface.

’ MODELS AND METHOD The calculations were performed by using spin-polarized DFT method as implemented in Vienna ab initio simulation package.27,28 To account for the correlated 3d orbital of Cu and Fe atoms, the calculations were performed at DFTþU level using Perdew, Burke, and Ernzerhof functional (PBE) generalized gradient approximation (GGA) scheme.29
32 Here, we chose 5.0 and 4.0 eV for the U term to describe Cu and Fe 3d states, respectively.32
34,39 Received: April 14, 2011 Revised: May 26, 2011 Published: May 30, 2011 13035

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Figure 1. The optimized (a) spinel and (b) inverse spinel structures of CuFe2O4. The red, green, and brown represent O, Fe, and Cu atoms, respectively.

Table 1. Lattice Constants and Average Binding Energies of Spinel and Inverse Spinel Structures of CuFe2O4 on the Basis of DFT and DFTþU Methods, Respectively method
) a, b, c (Å R, β, γ (o) Ebinding (eV)

spinel

inverse spinel

DFT

5.777, 5.777, 5.777

5.976, 5.976, 5.676

DFTþU

5.917, 5.917, 5.917

6.059, 6.059, 5.912

DFT

60.00, 60.00, 60.00

61.64, 61.64, 57.03

DFTþU

60.00, 60.00, 60.00

60.80, 60.80, 57.57

DFT

4.704

4.630

DFTþU

3.719

3.992

The interaction between ions and electrons is described by using the frozen-core projector augmented wave method.35,36 The plane-wave basis set cutoff energy is 500 eV, and the energy convergence was set to 1  10
5 eV. The lattice constants and positions of ions of the CuFe2O4 bulk are optimized at the same time, and the Hellmann
Feynman force per atom is less than 0.01 eV/Å. The k-meshes of 5  5  5 and 11  11  11 within Monkhorst-Pack k-points scheme were adopted for the structural optimization and the electronic structure calculation of the CuFe2O4 unit cell containing 14 atoms, respectively.37 We chose the 2  2 (100) surface of CuFe2O4 to simulate the adsorption of NO. Only Γ point was used when studying the adsorption of NO on the CuFe2O4 surface to save the computational resource. Dipole correction to the total energy is considered in the calculation.38

’ RESULTS AND DISCUSSION One of the interesting features of spinel-type transition-metal oxides is that there is a wide range of cation distributions within the structure, including normal spinel structure, inverse spinel structure, and the intermediate phase between them.7 Previous studies have suggested that CuFe2O4 usually has an inverse spinel structure and that the redistribution of the cations can tune the electronic and magnetic properties of CuFe2O4.39
42 Here, only ideal normal and inverse spinel structures of CuFe2O4 were studied to verify the method we used. The optimized normal and inverse spinel structures of bulk CuFe2O4 are shown in Figure 1. The normal CuFe2O4 spinel structure has a face-center-cubic

Figure 2. The electronic band structures of the inverse spinel CuFe2O4 with spin-polarized DFTþU method. The major and minor spin bands are plotted with black and red lines, respectively. The Fermi energy level was plotted with a dotted line. T, W, R, G, and X represent (0.5,
0.5, 0.0), (0.75,
0.25,
0.25), (0.5, 0.0, 0.0), and (0.5, 0.5,
0.5) k-point in the first Brillouin zone, respectively.

(fcc) lattice with space group Fd3m, where the tetrahedral hollow sites (A site) in an fcc close-packed oxygen sublattice are occupied by Cu atoms and where the octahedral hollow sites (B site) are occupied by Fe atoms in the unit cell as shown in Figure 1a. In an ideal inverse CuFe2O4 spinel structure, the tetrahedral hollow sites are occupied by Fe atoms while the octahedral hollow sites are occupied by equal Cu and Fe atoms as shown in Figure 1b. A large supercell should be used with random occupation of Fe and Cu atoms at octahedral hollow sites to simulate the real structure of inverse spinel CuFe2O4.7 Both in the normal and inverse spinel structures, O atoms are positioned in the center of a tetrahedral structure with four neighbors of Fe or Cu atoms. In Table 1, the optimized lattice parameters and average binding energy per atom of normal and inverse CuFe2O4 spinel structures based on DFT and DFTþU methods are summarized. Different magnetic orders of transition-metal atoms are considered to find the magnetic ground states for both normal and inverse CuFe2O4 spinel structures. The average binding energy is defined as Eb = (Etotal
nCuECu
nFeEFe
nOEO)/(nCu þ nFe þ nO), where Etotal is the total energy of CuFe2O4 per unit cell, Eatom is the total energy of atom, and n is the number of atoms within the unit cell. It can be found that DFTþU predicted slightly larger lattice constants than DFT method. The calculated binding energy suggested that the inverse spinel structure is more stable than the normal one with DFTþU method, while DFT method gives the contrary result. Thus, only DFTþU method was adopted to perform the following studies. In Figure 2, the electronic band structures of the inverse spinel structure of CuFe2O4 bulk were plotted with spin-polarized DFTþU method. The inverse spinel CuFe2O4 is a spin-polarized semiconductor with an indirect band of 1.429 eV. The profiles of spin charge density, total density of states (DOS), and partial DOS (PDOS) projected on Cu, Fe, and O atoms are plotted in Figure 3. From the plotted spin charge density distribution, the Fe atoms located at A and B sites are coupled with anti-ferromagnetic method at the ground state. The 13036

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Figure 3. (a) The isovalue surface of the spin electronic density distribution of inverse spinel CuFe2O4 in the unit cell. The blue and yellow represent (0.03 au. (b) The total DOS and partial DOS for major and minor spin of CuFe2O4. The Fermi level was plotted with a red dotted line.

Figure 4. (a) and (b) are the front and top views of 2  2 (100) surface of CuFe2O4 inverse structure before the atomic relaxation, respectively. (c) and (d) are the front and top views of 2  2 (100) surface of CuFe2O4 inverse structure after the atomic relaxation, respectively.

local magnetic moments of Fe atoms at A and B sites are
4.08 and 4.17 μB, respectively. Cu atoms and Fe atoms at B sites are coupled in ferromagnetic method, and the local magnetic moment of Cu atom is 0.62 μB. The magnetic order consistent with previous work,42 where the magnetic moments of ions within the same sites (A or B site) are ferromagnetically ordered and those between different sites (A and B sites) are antiferromagnetically ordered O atoms, has a weak magnetic moment of 0.068 μB. The total magnetic moment of CuFe2O4 is 1.98 μB per unit cell. From the calculated DOS and PDOS, the occupied states just below the Fermi level are mainly contributed by the coupling of d electrons from Cu and Fe atoms and p electrons from O atoms, while the

unoccupied states just above the Fermi level are mainly contributed by p orbitals of O atoms. As shown in Figure 4, we built 2  2 (100) surface from the inverse spinel structure of CuFe2O4 to study the adsorption behavior of NO molecule on the surface. Thirteen atomic layers plus a vacuum layer of 20 Å thickness were chosen to simulate the half-infinite structure of the surface. When performing optimization of surface structure, the atoms in the bottom seven atomic layers are fixed at their bulk positions (the atoms within the black box as shown in Figure 4a), while other atoms are relaxed without any constraint. The magnetic orders of the atoms are the same as those inside the perfect bulk. After the geometric relaxation, the surface atoms have reconstruction compared with their original bulk positions as shown in Figure 4b. Compared with the bulk structure, the surface O atoms shift up 0.82 Å, while surface Cu and Fe atoms shift down 0.54 Å. The sixth atomic layer counting from the top layer has a very tiny vertical shift of about 0.06 Å indicating that the atomic positions in this atomic layer are very close to their bulk positions. The total magnetic moment is about 32.41 μB. In the following, we studied the adsorption of NO molecule on the reconstructed CuFe2O4 surface. Four initial structures were considered, including (1) NO molecule was adsorbed on the top of the surface Cu atom with the N atom of the molecule toward the Cu atom (ON
Cu), (2) NO molecule was adsorbed on the top of the surface of Cu atom with the O atom of the molecule toward the Cu atom (NO
Cu), (3) NO molecule was adsorbed on the surface Fe atom with the N atom toward the Fe atom (ON
Fe), and (4) NO molecule was adsorbed on the surface Fe atom with the O atom toward the Fe atom (NO
Fe). All atoms in the surface have the same initial magnetic order as that in bulk when performing geometric optimization. The optimized structures of four situations are presented in Figure 5, and the adsorption energies of NO molecule on the surface, the charge distribution over the NO molecule, and the geometric parameters of the four optimized structures are summarized in Table 2. The adsorption energy is defined as Eads = E(CuFe2O4 þ NO)
E(CuFe2O4)
E(NO), where E(system) is the total energy of the corresponding system. From the calculated adsorption energy, all configurations have negative values of adsorption energy suggesting that the adsorption of NO molecule on CuFe2O4 (100) surface is exothermic. The NO molecule 13037

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Figure 5. Optimized configurations of NO adsorption on CuFe2O4 surface (a) ON
Cu, (b) NO
Cu, (c) ON
Fe, (d) NO
Fe. The blue ball represents nitrogen atom.

Table 2. Calculated Adsorption Energy of NO Molecule on the Surface, Equilibrium N
M or O
M (M = Cu or Fe) Bond Length, N
O Bond Length within the NO Molecule, the Charge Distribution over the NO Molecule, and the O
N
M or N
O
M Bond Angles ON
Cu

NO
Cu

ON
Fe

NO
Fe


1.05


0.53


1.98


0.33

N
M or O
M (Å)

1.76

1.93

1.79

1.86

N
O (Å)

1.19

1.21

1.19

1.21

chargeNO (e)

1.70

1.66

1.60

2.43

Eads (eV)

O
N
M (°) N
O
M (°)

149.31

166.74 135.21

170.04

prefers to adsorb on the surface Cu or Fe atom with the N atom connected to the surface atom. The most favorable configuration is that NO molecule adsorbs over the surface Fe atom (NO
Fe) forming a N
Fe bond. The calculated adsorption energy is
1.98 eV. The adsorption of NO molecule over the surface Fe atom with the formed O
Cu bond (NO
Fe) has the smallest adsorption energy of about
0.33 eV. In the ON
Fe configuration, the N
Fe bond length is 1.78 Å. Because of the interaction between the NO molecule and the CuFe2O4 surface, about 1.60 e charge is transferred from the NO molecule to the surface resulting in the weak N
O bond within the molecule. The charge analysis result is obtained with the Bader method.43 The N
O bond length is elongated to 1.19 Å, which is longer than that of the N
O bond in the free NO molecule (1.17 Å at the same calculation level). The total

Figure 6. Molecular energy levels of NO molecules of two spin components: up (black), spin-up and down (red), spin-down. For spin-up component, the π orbital labeled is two-degenerated states.

magnetic moment of NO
Fe configuration is about 31.96 μB, which is a little smaller than that of pure CuFe2O4 surface. In all other configurations, NO molecules are charged with positive charges with N
O bond elongated compared with that in the free NO molecule. To understand the interaction between NO molecule and CuFe2O4 surface, the electronic structure and the molecular orbital of NO molecule are plotted in Figure 6, and the density of states projected on NO molecule and the 3d orbital of Fe atom bound to NO molecule of the ON
Fe configuration are plotted in Figure 7. For NO molecule, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) π* can be composed to Np and Op, and the coefficients of Np and Op are 0.393:0.294 for HOMO and 0.397:0.283 for LUMO. That is, Np contributes more to HOMO and LUMO, 13038

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Figure 7. Partial density of states (PDOS) of adsorbed NO molecule and the 3d states of directly contacted Fe atom; (a) and (b) represent spin-up and spin-down components, respectively.

which indicates that the N-end of NO molecule prefers to make bonds with other atoms. The σ and π orbitals of the NO molecule are defined as π = Npx þ Npy þ Opx þ Opy and σ = Ns þ Npz þ Os þ Opz, where Ab means the b type atomic orbital of element A.44 Upon the spin-up component, there is strong hybridization between NO 2π* orbital and Fe 3d states. Upon the spin-down component, there is strong hybridization between NO 1π orbital and Fe 3d states because of the empty NO 2π* orbital. Thus, an electron is donated from NO π orbitals to Fe 3d states. Also, the hybridization between NO σ orbital and Fe 3d states is negligible, which indicates a weak back-donation process between NO and Fe for the high oxidation state of Fe atom. Thus, the strong adsorption mostly comes from the hybridization of π orbitals and 3d states, and electrons are transferred from NO to Fe3þ.

’ CONCLUSION In conclusion, we studied the ideal adsorption situation of NO molecule on the surface of CuFe2O4 with DFTþU method. Our studies reveal that the on-site Coulomb interaction within Cu and Fe 3d states should be considered when dealing with CuFe2O4, and the inverse spinel structure of CuFe2O4 is energetically favorable. In the inverse spinel CuFe2O4, Fe atoms occupied the tetrahedral hollow site and the octahedral hollow sites equally, while Cu atoms occupied the remaining octahedral hollow sites. The Fe atoms at tetrahedral and octahedral sites are coupled in antiferromagnetic method with local magnetic moment of about 4 μB. The Cu atoms and the Fe atoms at the octahedral sites are coupled in ferromagnetic method, and the local magnetic moment of Cu atoms is 0.62 μB. NO molecule prefers to adsorb on the top site of surface Fe atom with the formed N
Fe bond over the (100) surface of the inverse spinel CuFe2O4. The adsorption is exothermic with the adsorption energy of about
1.98 eV. Because of the interaction between the NO molecule and the surface, about 1.60 e charges are transferred from the molecule to the surface, and the N
O bond within the molecule is elongated. The strong adsorption mostly comes from the hybridization of π orbitals (NO) and 3d states (Fe3þ), and electrons are transferred from NO to Fe3þ, which will lead to the further NO activation/reduction reaction.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (W.S.), [email protected] (X.W.).

’ ACKNOWLEDGMENT The financial support of the Natural Science Foundation of China (NSFC) through NSFC program (50906050, 50222203, 11004180), National Key Basic Research Program (Grant No. 2011CB921400), and China Postdoctoral Science Foundation (20060400167) are gratefully acknowledged. This work is supported by Shanghai Supercomputer Center. ’ REFERENCES (1) Grimes, N. W. Phys. Technol. 1975, 6, 22. (2) Galasso, F. S. Structure and Properties of Inorganic Solids; Pergamon: New York, 1970. (3) Johnston, D. C.; Prakash, H.; Zachariasen, W. H.; Viswanathan, R. Mater. Res. Bull. 1973, 8, 777. (4) Gorter, E. W. Philips Res. Rep. 1954, 9, 295. (5) Regtien, P. P. L. Sens. Actuators 1981, 2, 85. (6) Gusmano, G.; Monteperelli, G.; Traversa, E.; Mattogno, G. J. Am. Ceram. Soc. 1993, 76, 743. (7) Wei, S. H.; Zhang, S. B. Phys. Rev. B 2001, 63, 045112. (8) Perron, H.; Mellier, T.; Domain, C.; Roques, J.; Simoni, E.; Drot, R.; Catalette, H. J. Phys.: Condens. Matter 2007, 19, 346219. (9) Walsh, A.; Yan, Y.; Al-Jassim, M. M.; Wei, S. H. J. Phys. Chem. C 2008, 112, 12044. (10) Yin, X. L.; Han, H. M.; Kubo, M.; Miyamoto, A. Theor. Chem. Acc. 2003, 109, 190. (11) Xu, X. L.; Chen, W. K.; Li, J. Q. J Mol. Struct.: THEOCHEM 2008, 860, 18–23. (12) Fierro, G.; Dragone, R.; Ferraris, G. Appl. Catal., B: Environ. 2008, 78, 183. (13) Fierro, G.; Ferraris, G.; Dragone, R.; Lo Jacono, M.; Faticanti, M. Catal. Today 2006, 116, 38. (14) Kim, T. W.; Song, M. W.; Koh, H. L.; Kim, K. L. Appl. Catal., A: Gen. 2001, 210, 35. (15) Chen, L.; Horiuchi, T.; Mori, T. Appl. Catal., A: Gen. 2001, 209, 97. (16) Fino, D.; Russo, N.; Saracco, G.; Specchia, V. J. Catal. 2006, 242, 38. (17) Shangguan, W. F.; Teraoka, Y.; Kagawa, S. Appl. Catal., B: Environ. 1996, 8, 217. 13039

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