Density Functional Theory Study of Sn Adsorption on the CeO2

ARTICLE pubs.acs.org/JPCC

Density Functional Theory Study of Sn Adsorption on the CeO2 Surface Yun Zhao,† Botao Teng,*,† Zongxian Yang,*,‡ Yue Zhao,† Leihong Zhao,† and Mengfei Luo† †

Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China ‡ College of Physics and Information Engineering, Henan Normal University, Xinxiang, Henan 453007, China

bS Supporting Information ABSTRACT: The adsorption behaviors and electronic properties of Sn on the CeO2(111) surface were systematically investigated using the density functional theory (DFT) method. Our results suggested that Sn on the hollow site is more stable than that on the top oxygen site at the coverage of 0.25 ML, while Sn on the top oxygen site is the most stable configuration when the coverage of Sn is equal to or higher than 0.5 ML. Charge density difference (CDD) analysis indicates that electrons transfer from the Sn adatom to the substrate, leading to the reduction of Ce4+ to Ce3+ ion, which is in agreement with the experimental spectroscopy. The reduction degree of the substrate increases with the Sn coverage, which is well supported by the CDD and spin-resolved density of states (DOS) of the most stable xSn/CeO2(111) configurations. The adsorption of Sn can partially activate the surface oxygen of ceria. The tentative study of a probe molecule CO adsorption on the Sn/CeO2(111) surface indicates that CO adsorption is enhanced due to the strong tin
ceria interactions.

1. INTRODUCTION Due to the reversible transformation of the oxidation states of the cerium ions (from Ce4+ to Ce3+), ceria possesses high oxygen storage capacity (OSC) and has been widely applied to catalytic redox, solid oxide fuel cell, and water
gas shift reaction, etc.1
4 Studies have shown that the catalytic performance and the OSC of ceria might be considerably enhanced if adsorbed or doped with noble metal (NM) atoms.5
12 For example, Ce
Sn mixed oxides were reported to have high oxygen storage/release capacities compared with pure CeO2.13
17 Experimentally,
Skoda et al.18 reported that the strong tin
ceria interactions led to the partial Ce4+fCe3+ reduction, which was attributed to the charge transfer from Sn atoms to the Ce
O complex. Using X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS), Ma
sek et al. observed the formation of a new chemical Ce(Sn)+ state, which belongs to the SnCeO2 species.19 They suggested the formation of a mixed SnxCeO2 oxide which is more reactive than the CeO2 due to the high concentration of Ce3+ ions. Similar to the Sn
Ce
O complex,
Skoda et al.20 also reported the reduction of Ce4+ and the formation of a mixed Au
Ce
O surface, which agrees well with the density functional theory (DFT) calculations by Camellone and Fabris.21
23 The excellent works of Fabris’s group revealed the adsorption structures, electronic properties, and the possible catalytic effects of Au on the CeO2(111) surface, as well as the oxidation mechanism of CO on the AuxCe1
xO2 catalysts.21 Liu et al. systematically investigated the Au adsorption r 2011 American Chemical Society

behaviors on the stoichiometric and reduced CeO2(111) surface, and a formate mechanism of the WGS reaction was proposed for a Au4 cluster on the partially reduced CeO2(111) model.24 Compared with the extensive studies on the Au/CeO2 catalysts, little theoretical efforts were paid upon the tin
ceria interactions. In the present work, we performed a systematic investigation for the adsorption on the stoichiometric CeO2(111) surfaces using the density functional theory method. Then the CO adsorption behavior on the 1Sn/CeO2(111) model was tentatively calculated to explore the possible catalytic effects of Sn deposition on the ceria surface.

2. METHODS AND MODELS All calculations were performed on the basis of the plane-wave periodic density functional theory, using the Vienna ab initio simulation package (VASP) code.25 The electron exchange and correlation were treated within the generalized gradient approximation (GGA) using the Perdew
Wang 1991 (PW91) functional.26 The projector augmented plane-wave (PAW) method with a frozen-core approximation was used for the description of electron
ion interactions.27,28 The Ce-5s25p64f15d16s2, Sn5s25p2, and O-2s22p4 were treated as valence electrons. The cutoff energy was set as 400 eV for the plane wave basis set based Received: April 19, 2011 Revised: June 22, 2011 Published: July 22, 2011 16461

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Figure 1. Possible adsorption sites of Sn on the CeO2(111) surface (red balls, top oxygen atom; white balls, cerium atom; green balls, sublayer atom). The surface of the simulation cell is indicated by dashed lines.

on the convergence tests.29 The Brillouin-zone integrations were approximated by a sum over special k points chosen by the Monkhorst
Pack (MP) grid and the Gaussian smearing method with a sigma value of 0.1 eV.30 The Kohn
Sham equations were solved self-consistently, and the convergence criteria for the energy calculation and structure optimization were set to a selfconsistent field (SCF) tolerance of 1.0  10
4 eV and a maximum Hellmann
Feynman force tolerance of 0.03 eV/Å, which yield total energy convergence down to the 1 meV level. To correct the on-site Coulombic and exchange interaction of the strongly localized Ce-4f electrons for the partially reduced ceria surface,31
33 the DFT+U approach was used in our calculations, which was proved by Fabris et al.34 to be a unified modeling framework for pure CeO2, Ce2O3, and defective (CeO2
x) ceria structures. As reported in the literature,35
37 the value of U = 5 eV was used in the present work. The calculated bulk lattice constant (5.487 Å) with a MP grid of 5  5  5 agrees well with the experimental value (5.411 Å)38 and the calculated values in the literature.39,40 A slab with the bridge oxygen atoms exposed was used to model the CeO2(111) surface, which was the most stable √ surface in the low indices of ceria.41,42 The surface has a ( 3  2) replication. The vacuum space of 12 Å was set between the slabs to minimize their interaction. We systematically compared the adsorption energies of xSn on the CeO2(111) surface with six and nine layers, as shown in Table 1 of the Supporting Information. It is found that the differences of the adsorption energies on the CeO2(111) surface models with six and nine layers are generally less than 0.2 eV. Considering the efficiency and precision, the model with six atomic layers was used in the present work. The top three layers of the periodic slab were relaxed, and the bottom three layers were fixed in the corresponding bulk position, as shown in Figure 1. As shown in Figure 1, the possible adsorption sites are the top sites of surface oxygen (O-top) and cerium (Ce-top) atoms, the hollow site among the three surface oxygen atoms (hollow), the bridge sites of surface oxygen atoms (O
O-bridge), and oxygen and cerium atoms (O
Ce-bridge). Considering the possible adsorption sites of Sn on the CeO2(111) surface, the different initial structures were optimized. To obtain the interaction information between Sn and the CeO2(111) surface, as well as the Sn
Sn cohesive interaction effects on Sn adsorption behaviors, three adsorption energies were defined as follows. The total adsorption energy: Etads ¼ ExSn=slab
ðEslab þ xESn Þ

Figure 2. Optimized structures of Sn atoms on the CeO2(111) surface.

The average adsorption energy per adsorbate: Eav ads ¼

ExSn=slab
ðEslab þ xESn Þ x

The Sn
Sn cohesive energy per Sn atom:43 ESn
Sn ¼

ESnx þ xESn x

where ExSn/slab, Eslab, ESnx, and ESn are the energies of the xSn adsorbed system, the CeO2(111) slab, the isolated Snx cluster, and a single Sn atom, respectively; x is the number of the adsorbed Sn atoms; and Eads is negative for an exothermic adsorption. The more negative the adsorption energy, the stronger the adsorption.

3. RESULTS AND DISCUSSION 3.1. xSn (x = 1
4) Adsorption on the CeO2(111) Surface. To investigate the strong tin
ceria interactions with different Sn deposition amounts, the Sn adsorption behaviors on the CeO2(111) surface at 0.25, 0.5, 0.75, and 1 monolayer (ML) coverages were systematically calculated. The optimized structures were shown in Figure 2, and the corresponding structural parameters were listed in Table 1. Two stable configurations were found at 0.25 ML coverage. As shown in Figure 2(a), when a Sn atom adsorbs on the top site of a surface oxygen atom, the Sn
O bond is perpendicular to the surface with a bond length of 1.993 Å. The adsorption energy is
2.40 eV. A Sn atom adsorbed at the hollow site forms three Sn
O bonds of 2.204, 2.204, and 2.180 Å, as seen in Figure 2(b). The corresponding adsorption energy is
3.68 eV, indicating a much stronger interaction at the hollow site than that at the O-top site. Sn atoms initially adsorbed at the O
Ce bridge, O
O bridge, or Ce-top sites transformed into the hollow site automatically during the geometrical optimization. When two Sn atoms adsorb on the CeO2(111) surface, three optimized structures were obtained. The two adatoms might be both located on the top of surface oxygen atoms (Figure 2(c), 2Sn-2top) with two Sn
O bonds of 2.087 (Sn1
O) and 2.086 Å (Sn2
O). The corresponding total adsorption energy is
6.54 eV. A slightly weak adsorption configuration with one Sn atom 16462

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Table 1. Adsorption Energies (Eads), Sn
Sn Cohesive Energies (ESn
Sn), and Structural Parameters of Sn on the CeO2(111) Surface configurations

Etads/eV

Eav ads/eV

ESn
Sn/eV -

1Sn-top


2.40

-

1Sn-hollow


3.68

-

2Sn-2top


6.54


3.27

2Sn-br-top


6.37


3.19

dSn1
O/Å

dSn2
O/Å

dSn3
O/Å

dSn4
O/Å

1.993 2.204 2.204 2.180


1.41

2.087

2.086

2.221

2.161

2.179 2Sn-hollow-br


4.93


2.47

2.247

2.491

2.247 3Sn-3top


10.59


3.53

3Sn-br-2top


10.49


3.50


1.86

2.164 2.113

2.491 2.120

2.120

2.209

2.161

2.140

2.138

2.138

2.218 4Sn-4top


14.62


3.66


2.11

adsorbed on the O-top site and the other one located on the O
O bridge site (denoted as 2Sn-br-top) is shown in Figure 2(d). For this configuration, three Sn
O bonds of 2.161 (Sn2
O), 2.221, and 2.179 Å (Sn1
O) form, and its adsorption energy is
6.37 eV. In contrast, a much weaker configuration (2Sn-hollow-br) is found as shown in Figure 2(e), where the first Sn adsorbs at the hollow site with the formation of three Sn
O bonds (2.164
2.247 Å), and the second one is located at the neighbor O
O bridge site with the bond length of 2.491 Å. The corresponding adsorption energy is
4.93 eV. The average adsorption energies for the above configurations are
3.27,
3.19, and
2.47, which are all weaker than that of one Sn adsorbed at the hollow site. This might be due to the repulsive interactions between the adsorbed Sn atoms. The initial configuration (2Sn-2hollow) with two Sn atoms adsorbed at two hollow sites transforms into the 2Sn-2top one during optimization. When three Sn atoms are deposited on the CeO2(111) surface, the three adatoms might adsorb on the top of surface sites to form three Sn
O bonds (3Sn-3top, Figure 2(f)). The bond lengths are about 2.113
2.120 Å, and the corresponding adsorption energy is
10.59 (Etads) and
3.53 (Eav ads) eV. When two Sn atoms adsorb on the top of oxygen atoms and the third one locates the O
O bridge site, a slightly weaker adsorption configuration is found (3Sn-br-2top, Figure 2(g)). The structure has four Sn
O bonds of about 2.140
2.218 Å with an adsorption energy of
10.49 (Etads) and
3.50 (Eav ads) eV. When the deposited Sn atoms are up to four, only the 4Sn-4top configuration was obtained. All the Sn atoms adsorb on the O-top sites with the bond lengths of 2.119
2.138 Å, as shown in Figure 2(h). The corresponding adsorption energy is
14.62 (Etads) and
3.66 (Eav ads) eV. The configurations with three or four Sn atoms initially deposited at the hollow sites also transform into the 3Sn-3top and 4Sn-4top, respectively, after the geometrical optimization. From the calculated results above, it is very interesting to find that the Sn adatoms of the most stable configurations shift from the hollow site (1Sn-hollow, 0.25 ML) to the top site (xSn-xtop, x = 2
4). Furthermore, when the Sn coverage is higher than 0.5 ML, the average adsorption energies of Sn on the CeO2(111) surface increase with Sn coverage. This interesting variation can be explained by the interactions between Sn and the surface

2.119

2.132

Figure 3. Charge density difference of xSn/CeO2(111). The isosurface value is set as 0.005 e/Å3.

oxygen atoms (denoted as “Sn
O interaction”) as well as the interactions among Sn adatoms (denoted as “Sn
Sn interaction”) directly or indirectly (mediated by the surface). When the Sn coverage is low (0.25 ML), the distance between the adsorbed Sn atoms is longer than 6.7 Å, and the Sn
Sn interaction is negligible. Therefore, its most stable configurations must have the maximum electronic Sn
O interactions. According to the charge density difference of 1Sn-hollow (Figure 3 (a)) and 1Sn-top (Figure S1 of Supporting Information), the electronic interactions of the Sn
O bond (1Sn-top) are stronger than one of the three Sn
O bonds (1Sn-hollow), which is well consistent with the fact that the Sn
O bond of 1Sn-top (1.993 Å) is shorter than those of 1Sn-hollow (2.180
2.204 Å). However, the Sn/Os (surface oxygen) ratio is 1:4 for the 1Sn-hollow configuration, and the Sn adatom interacts with three surface oxygen atoms. Hence, the electronic Sn
O interactions of 1Sn-hollow are stronger than that of 1Sn-top. Correspondingly, the 1Sn-hollow configuration is more stable than that of 1Sn-top. When the coverage is up to 1 ML, the Sn/Os ratio is 1:1 for 16463

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Figure 4. Spin density for the xSn/CeO2(111) configurations. The isosurface value is set as 0.005 e/Å3.

4Sn-4top and 4Sn-hollow (if it exists). Hence, the average number of surface oxygen atoms that interacts per Sn adatom is only one. Therefore, the interactions between Sn adatoms and surface oxygen atoms adopt the relatively strong way, that is, the 4Sn-4top configuration, instead of the 4Sn-4hollow. In view of the Sn
Sn interactions, we can learn from the topview figures for the most stable configurations of the Supporting Information (Figure S2) that the Sn adatoms slightly deviate from the top site when the coverage is higher than 0.5 ML. This results in the Sn
Sn distances shortened to 3.223
3.589 Å in one repeated unit, which are shorter than the distance at the just top or hollow sites (3.880 Å). These deviations increase the cohesive interactions between Sn adatoms. The cohesive energies between Sn adatoms are
1.41,
1.86, and
2.11 eV for xSn-xtop (x = 2
4), respectively. Therefore, Eav ads increases with the Sn coverage (
3.27 eV for 2Sn-2top,
3.53 eV for 3Sn-3top, and
3.66 eV for 4Sn-4top). Similar results were obtained by Yang et al. on the Cux adsorption on the CeO2(111) surface.45 3.2. Electronic Properties. 3.2.1. Charge Density Difference. To further analyze the electronic interactions between adsorbate and the CeO2(111) upon adsorption, we calculated the charge density difference (CDD) for the typical xSn/CeO2(111) structures. The CDD was defined as ΔF = Fadsorbate/slab
Fslab
Fadsorbate, where Fadsorbate/slab is the charge density for the adsorbed system and Fslab and Fadsorbate are the charge densities for the noninteracting slab substrate and adsorbate, respectively. Therefore, the CCDs of different configurations measure the intuitionistic charge redistribution induced by the adsorption of Sn atoms. The corresponding CDDs of the most stable xSn/CeO2(111) configurations were shown in Figure 4, in which the yellow and the gray-blue part represent the accumulation and depletion electrons, respectively. As shown in the CDD of the 1Sn-hollow configuration (Figure 3(a)), electrons transfer from the Sn adatom to the substrate, which are mostly localized around the three surface O atoms and two Ce4+ ions. Therefore, the two Ce4+ ions were partially reduced to Ce3+ ones. The p electrons of surface oxygen atoms also partially feed back to the Sn adatom. For the 2Sn-top configuration (Figure 3(b)), each Sn adatom interacts with one

ARTICLE

surface oxygen atom, which leads to the electrons feeding back to each other. Different from the 1Sn-hollow configuration, the third Ce4+ between the two oxygen atoms was partially reduced. Similar electronic interactions were observed from the CDD of 3Sn-top and 4Sn-top configurations, seen in Figure 3(c) and (d), in which the number of the partially reduced Ce4+ ions is up to three and four, respectively. Therefore, we can conclude that the reduction degree of the ceria substrate increases with Sn coverage. To further verify the formation of Ce3+ ions during the reduction of ceria upon Sn adsorption, the spin density of the most stable xSn/CeO2(111) configurations was also shown in Figure 4, from which the formation of Ce3+ ions is clearly seen. These results are well consistent with the resonant photoelectron spectroscopy (RPES) of Sn deposited onto the CeO2(111) thin film, in which a giant 4f resonance enhancement of the Ce3+ species was obtained.18 Furthermore, the number of the spinpolarized Ce3+ ions increases with the Sn coverage on the ceria surface since more electrons transfer to the 4f orbital of cerium atoms. This will also be further proved by the following electronic density of states analysis. 3.2.2. Density of States. The typical density of states for 1Snhollow and 3Sn-3top configurations was shown in Figure 5. It can be learned from Figure 5(a) that Ce1 and Ce2 atoms are partially reduced to Ce3+ ions, with the electronic density of states peaks appearing between
1 and 0 eV in their corresponding local density of states plots. The other two (Ce3 and Ce4) are unreduced with only the narrow unoccupied 4f peaks existing above the Fermi level. Three Ce3+ ions were observed with partially occupied electronic density of states peaks appearing around the Fermi level as in Figure 5(b), indicating that the reduction degree of the substrate increases with Sn coverage, which is well consistent with the results of spin density results. 3.3. CO Adsorption on Sn/CeO2(111). As reported in the literature, only weak physisorption of CO occurs on the stoichiometric CeO2(111) surface.22,34,41,44 From the CDD of xSn/CeO2(111) (Figure 3), electrons of O
Ce bonds are partially lost compared with the stoichiometric CeO2(111) surface. Therefore, the O
Ce bonds are elongated compared with those of bulk CeO2 (2.376 Å), as shown in Table 2 of the Supporting Information. This indicates the partial activation of the surface oxygen due to the adsorption of Sn. Does the Sn adatom on the CeO2(111) surface enhance the CO adsorption? To explore its possible effects on CO adsorption, we tentatively calculate the adsorption behaviors of CO, which is often used as a probe molecule, on the 1Sn/CeO2(111) surface. The optimized structures were shown in Figure 6. When a Sn adsorbed on the hollow site, CO might adsorb at the Ce-top site as shown in Figure 6(a), with a C
Ce bond of 2.828 Å and the corresponding adsorption energy of
0.37 eV, which is slightly higher than that on the stoichiometric CeO2(111) surface. If CO adsorbs on the surface oxygen which is not in contact with the Sn adatom, a much stronger adsorption occurs with the formation of CO2 as shown in Figure 6(b). The corresponding adsorption is
1.39 eV. As shown in Figure 6(c), when Sn adsorbs on the O-top site, the C atom of CO interacts with Sn and one of the surface oxygen atoms. The Sn adatom deflects from the O-top site to form a SnCOcOs intermediate with the Sn
C and the C
Oc bond lengths of 2.417 and 1.238 Å. Similar results were observed by Camellone and Fabris on the Au/CeO2(111) surface.21 The corresponding adsorption (
2.54 eV) is much higher than that on the stoichiometric CeO2(111) surface. CO adsorbed on the top of the Sn 16464

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Figure 5. Density of states of xSn/CeO2(111) and the local density of states of selected Ce ions. (a) 1Sn-hollow and (b) 3Sn-3top. The vertical dashed line indicates the Fermi level at 0 eV.

Figure 6. CO configurations on the 1Sn/CeO2(111) surface (a) and (b) for 1Sn-hollow and (c) for 1Sn-top.

adatom is not stable and falls into the Ce-top or Os-top sites. In summary, the CO adsorption is enhanced due to the deposition of Sn on the CeO2(111) surface. This calculation is consistent with the experimental results that the Sn
Ce oxide is found to be a good catalyst.15 The high oxidation activity of Sn
Ce oxide compared with SnO2 and CeO2 suggests the existence of a synergetic effect between the two oxides for the CO oxidation reaction.46 Our results might partially explain the high CO oxidative reactivity of the SnOx/CeO2 catalyst compared with the pure SnO2 or CeO2 due to the strong interactions between Sn and ceria.

4. CONCLUSIONS The adsorption behaviors of Sn on the CeO2(111) surface with different coverages were systematically investigated by the density functional theory method. It suggests that strong chemisorption occurs when the Sn atoms adsorb on the CeO2(111) surface. At a low coverage (0.25 ML), the 1Sn-hollow configuration is more stable than that of 1Sn-top, while the xSn-xtop configurations are the most stable configurations at high coverages (equal to or higher than 0.5 ML). This interesting transformation can be attributed to the interactions between Sn and the surface oxygen atoms, as well as the cohesive interactions among Sn adatoms. Charge density difference analysis indicates that electrons transfer from the Sn adatom(s) to the substrate, leading to the reduction of cerium ions from Ce4+ to Ce3+ ions. The reduction degree of the substrate ceria increases with the Sn coverage. The adsorption of Sn can partially activate

the surface oxygen. The study of CO adsorption on the Sn/ CeO2(111) surface indicates that CO adsorption is enhanced due to the strong tin
ceria interactions.

’ ASSOCIATED CONTENT

bS

Supporting Information. Figure S1. Charge density difference of the 1Sn-top configuration. Figure S2. The top-view for the most stable xSn/CeO2(111) configurations. Table S1. Adsorption energies of the typical xSn on the CeO2(111) surface and CO on the 1Sn/CeO2(111) surface of the nine-layers model with the top six layers relaxed. Table S2. The O
Ce bond lengths for the typical xSn/CeO2(111) configurations. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. Tel.: +86-579-82282234. Fax: +86-57982282595 (Botao Teng). E-mail: [email protected]. Tel.: +86-373-3329346 (Zongxian Yang).

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No. 20903081) and the Natural Foundation of Zhejiang Province, China (Grant No. Y407163). Z.Y. also acknowledges support from the National Natural Science Foundation of China (Grant No. 10674042) and the 16465

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The Journal of Physical Chemistry C Innovation Scientists and Technicians Troop Construction Projects of Henan Province, China (Grant No. 104200510014).

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