Superoxide and Peroxide Species on CeO2(111), and Their Oxidation

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Superoxide and Peroxide Species on CeO2(111), and Their Oxidation Roles Yun Zhao,† Bo-Tao Teng,*,† Xiao-Dong Wen,‡ Yue Zhao,† Qiao-Ping Chen,† Lei-Hong Zhao,† and Meng-Fei Luo† †

Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States

‡

S Supporting Information *

ABSTRACT: The electronic properties and oxidation roles of superoxide and peroxide species on the small and big CeO2(111) surfaces with an oxygen vacancy are systematically investigated utilizing density functional theory. When the CeO2 surface is partially reduced, a surface oxygen vacancy forms, and two Ce4+ ions of substrate are reduced to Ce3+ ones. If O2 adsorbs at the top site of Ce3+ ion, which is close to the surface oxygen vacancy, it slips into the vacancy. Then, two 4f electrons of Ce3+ ions feedback to the π2p* orbital of O2, and a diamagnetic peroxide species forms. When O2 adsorbs at the top site of Ce3+ ion apart from the surface oxygen vacancy, superoxide species with three different orientations are obtained. Meanwhile, only one Ce3+ ion is oxidized to Ce4+ since one 4f electron transfers to the π2p* orbital of O2. The migration barrier from O2−(η2-1) to O22− is 0.35 eV, indicating that superoxide might easily transform into peroxide with elevating temperature. CO is directly oxidized to CO2 by the superoxide without energy barrier, while a carbonate forms when CO reacts with peroxide. A high desorption barrier of the carbonate to form a gas CO2 molecule indicates that peroxide species might play the dominant role at relatively high temperature.

1. INTRODUCTION Ceria-based catalysts have received much attention owing to their excellent oxygen storage capacity (OSC), and have been widely applied into three-way catalysts, solid oxide fuel cells, water gas shift (WGS) reaction, and so forth.1−4 Superoxide (O2−) and peroxide (O22−) are two of the most important oxygen species in oxidation reactions on ceria-based catalysts, which have been extensively studied in experiment. They are generally characterized by electron paramagnetic resonance (EPR), Raman, and FT-IR spectroscopies.5−11 These experimental results also motivated the related theoretical studies of oxygen species on ceria-based catalysts. Using a superoxide radical (O 2 − ) as adsorbate, Huang and Fabris12 first investigated the adsorption behaviors and electronic properties of superoxide on both CeO2(111) and (110) surfaces. Since the oxidative species is mainly derived from O2 instead of free O2− in practical catalysts, the identification of superoxide on ceriabase catalysts, as well as its catalytic role is still unclear. On the basis of the O−O bond length and the Raman spectroscopy, Choi et al.8 observed peroxide and superoxide-like species on the partially reduced ceria catalyst by a peroidical density functional theory (DFT) calculation using a nine-layer CeO2(111) supercell. Li et al.13 found a peroxide on CeO2(111) with a surface oxygen vacancy, and a superoxide species that binds to a surface Ce3+ ion with a sublayer oxygen vacancy in a periodic slab model using DFT+U approch. Nolan14 reported that a superoxide exists on the doped © 2012 American Chemical Society

CeO2(110), while a peroxide forms on the undoped surface by DFT+U calculation. From the adsorption behaviors of O2 on ceria nanoparticles, Preda et al.15 reported a theoretical work of superoxide species formed by direct interaction between O2 and the low-coordinated Ce3+ ions of ceria nanoparticles. Zhu et al.16 observed a superoxide species on a CeO2(110) surface that was partially reduced by its supported Au3 cluster. However, the important oxidation role of supreoxide was not involved in their work. By far, most of experimental results have suggested that oxygen vacancies play a key role in the formation of superoxide and peroxide species, and the two species take part in the subsequent oxidative reaction.1,17 In the experimental side, the identification of O2− and O22− species on ceria catalysts are generally based on the fingerprint of Raman and IR spectra: 1050−1150 cm−1 for O2− species, and 850−950 cm−1 for O22−.5,7,9,18,19 In theory, O2− and O22− species are often distinguished by the O−O bond length, sometimes combined with the vO−O frequency of IR spectroscopy. As well-known, the electronic configuration of the paramagnetic O2− should be that only one unpaired electron exists in the π2p* orbital, as shown in the schematic molecular orbital (see Scheme 1); while the electronic structure of O22−, a diamagnetic species, can be Received: February 19, 2012 Revised: July 10, 2012 Published: July 14, 2012 15986

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Scheme 1. Schematic Molecular Orbital Diagram of O2−a

As reported in our previous work, the calculated bulk lattice constant of 5.487 Å and the surface with the bridge oxygen atoms exposed are used in the present work.38 To explore the possible oxygen species on the partially reduced CeO2(111) surface, two different superlattice sizes, p(2 × √3) and p(2√3 × 3) slab models are used in the present work, as shown in Figure 1. The corresponding k-point meshes are set to 2 × 2 ×

a

The dashed lines indicate the primary parentage of the orbitals; the 2s-2p mixing is included on the orbital representations of the σ and σ* orbitals. Note that the second-order orbital splitting on π* (occupied and half-occupied π*) does not show in the scheme. Figure 1. Slab models of the partially reduced CeO2(111) surface. (Red ball for O, gray ball for Ce, blue dotted ball for oxygen vacancy.) (a,b) p(2√3 × 3); (c,d) p(2 × √3). The surface of the simulation cell is indicated by dashed lines.

formed after transferring two electrons from ceria substrate. However, the corresponding electronic transferring mechanism between a periodical ceria substrate and the π2p* orbital of O2 were seldom reported. On the other hand, although experiments show that superoxide species exhibit much higher activity toward CO oxidation than peroxides at low temperatures,9,10,20,21 the different catalytic roles of O2− and O22− species in oxidation on ceria-based catalysts have rarely been compared in theoretical work. In the present work, to provide more theoretical information for experimental studies, we systematically investigate the formation mechanism and electronic properties of superoxide and peroxide species on a partially reduced CeO2(111) model catalyst. Using CO as a probe molecule, the different catalytic roles of superoxide and peroxide species are also compared in detail using DFT calculation.

1 and 3 × 4 × 1, respectively. According to the adsorption energies of O2 on CeO2(111) surface with different force tolerances and thickness in Table S1 of Supporting Information, a six-layer model with a force tolerance of 0.03 eV/Å is used in our calculation by considering the precision and efficiency. The vacuum space is set to 12 Å between the slabs to minimize their interaction. The adsorption energy is defined as follows: Eads = E(adsorbate/slab) − [E(slab) + E(adsorbate)]

where E(adsorbate/slab) is the total energy of a slab model with adsorbate; E(slab) and E(adsorbate) are energies of the slab and an isolated adsorbate, respectively. Therefore, a negative Eads value means exothermic adsorption, while a positive one means endothermic adsorption. The more negative the adsorption energy, the stronger the adsorption.

2. METHODS AND MODELS All calculations employed in this work are based on the plane wave/pseudopotential approach using Vienna Ab-initio Simulation Package (VASP). 22 The electron exchange and correlation are treated within the generalized gradient approximation using the Perdew−Wang 1991 (PW91) functional.23 The Kohn−Sham one-electron wave functions are expanded in a plane wave basis with an energy cutoff of 400 eV on the basis of convergence test,24 using projector augmented wave (PAW) potentials.25,26 The cerium 5s, 5p, 5d, 4f, 6s and the oxygen 2s, 2p electrons are treated as valence electrons. The Brillouin-zone integrations are approximated by a sum over special k points using the Monkhorst−Pack scheme. The Gaussian smearing method is set to 0.1 eV.27 It was reported that the on-site Coulombic and exchange interactions of the strongly localized Ce-4f electrons are important for the partially reduced ceria surface,28−30 and the DFT+U method is suggested for the system with 4f electrons.31 On the basis of the theoretical tests of U value in the literature, 5 eV is used herein.32−37 The Kohn−Sham equations are solved selfconsistently, and the self-consistent field energy tolerance is set to 1.0 × 10−4 eV. The maximum Hellmann−Feynman force tolerance of 0.03 eV/Å is used for structural optimization.

3. RESULTS AND DISCUSSION 3.1. O 2 Adsorption on the Paritally Reduced CeO2(111). Both EPR and Fourier transform infrared (FTIR) results suggested that O2 binds to the surface cerium ion when oxygen vacancies are formed on the CeO2 surface.39 Then, the superoxide species are observed at low temperature. However, few theoretical studies proposed the O2− species on the partially reduced CeO2 surface. The oxygen species obtained in a periodic slab is peroxide, including our previous work.38 Can superoxide be formed on the partially reduced CeO2 surface? To explore the question, we systematically calculate the O2 adsorption behaviors on the partially reduced CeO2(111) slabs with different sizes. The optimized structures are shown in Figure 2, and Table 1 shows the corresponding geometric parameters. When O2 adsorbs in the p(2√3 × 3 slab, it stands at the top site of Ce3+ ion to form a η1 configuration with an adsorption energy of −0.30 eV, as shown in Figure 2a. The corresponding 15987

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When O2 adsorbs in the p(2 × √3) slab, only one stable configuration is obtained, as seen in Figure 2e\, which shows O2 also occupies the surface oxygen vacancy of the partially reduced CeO2(111) surface. The O−O bond length is 1.428 Å with an adsorption energy of −3.25 eV. The corresponding O− O vibration frequencies of the two configurations calculated herein are 909 and 921 cm−1, respectively, which are consistent with the value observed for O22− species from FT-IR spectroscopy.7,18 On the basis of the adsorption energies of O2 adsorbed at the partially reduced CeO2(111) surface, the adsorption energies of superoxides are about −0.30 to −0.38 eV, which is much lower than those of peroxides. This indicates that superoxides are less stable than peroxides, and might be easy to transform into the more stable peroxide species, which will be further proved by the potential energy surface calculation of migration from superoxide to peroxide. 3.2. Electronic Property of Superoxide and Peroxide. According to the adsorption energies, the O−O bond length, and the corresponding frequencies of O−O vibration, the oxygen species in Figure 2a−c are very close to those of the superoxide species reported in the literature, while the configurations in Figure 2d−e are similar to peroxide species.8,12,13,18,39,40 However, the key criterion to determine a superoxide radical anion (O2−) on ceria-based catalysts should be one electron of the substrate transferring to one of the halffilled π2p* orbitals of O2, while a peroxide (a diamagnetic oxygen species) can be formed after transferring two 4f electrons of Ce3+ of the substrate to the adsorbed O2. To further identify the oxygen species above, the spin densities of the substrates and O2 adsorbed on substrates are calculated and shown in Figures 3 and 4, respectively. As shown in Figure 3a, there is one surface oxygen vacancy in the p(2√3 × 3) slab, and two Ce3+ ions with two characteristically unpaired electrons localized at their 4f orbital, indicating a partially filled 4f state. Therefore, the two Ce4+ ions are reduced to Ce3+ ones with the electronic configuration of 4f1, which agrees well with the literature,9 as well as the results from hybrid DFT calculation by Nolan.41,42 A similar result is observed for the partially reduced p(2 × √3) slab. The main difference of the spin densities for the two slabs are that the two Ce3+ ions in the p(2 × √3) slab are both close to the oxygen vacancy, while they are distributed apart from the oxygen vacancy in the p(2√3 × 3) slab. It was reported that multiple electronic configurations might coexist on the partially reduced CeO2.42 Li et al.13 suggested that there are six possibly electronic Ce3+ configurations on a big supercell; the configuration with two first-neighbor Ce3+ ions is the least stable, and that with two second-neighbor Ce3+ ions is the most favorable.42 The electronic configuration on the p(2√3 × 3) slab herein is the same as the most stable one reported by Li et al. However, only one electronic configuration in the p(2 ×

Figure 2. Different oxygen species on the partially reduced CeO2(111) surface with different sizes. Superoxide adsorbed at the p(2√3 × 3): (a) η1, (b) η2-1, (c) η2-2; Peroxide adsorbed at the p(2√3 × 3) (d) and p(2 × √3) (e). Pink ball for adsorbed O2.

O−O bond length is 1.307 Å with a vibration frequency of 1133 cm−1. When O2 lies at the top site of Ce3+ ion, two stable structures can be obtained, in which the O−O bond is parallel to the x or y axes. There are two Ce−O bonds forming for both configurations, denoted as η2-1 and η2-2, as shown in Figure 2b,c. The adsorption energies of the two η2 configurations are −0.38 and −0.31 eV, respectively. O2 might also fall into the surface oxygen vacancy and bind to its neighbor Ce ions, as shown in Figure 2(d). It gives the adsorption energy as high as −2.80 eV with the O−O distance of 1.430 Å, indicating the high activity of the surface vacancy.

Table 1. Adsorption Energies and Geometric Parameters of O2− and O22− Species on the Partially Reduced CeO2(111) Surface species

p(2 ×

O22−

3)

p(2 3 × 3)

O2−

η1 η2-1 η2-2 O22−

Eads (eV)

dO−O(Å)

νO−O (cm−1)

−3.25

1.428

921

−0.30 −0.38 −0.31 −2.80

1.307 1.352 1.351 1.430

1133 1080 1107 909

15988

νO−O,exp(cm−1)

dCe−O (Å) 2.258 2.411 2.419

1126

883

2.219 2.275 2.371 2.268 2.357 2.448 2.448 2.267

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Figure 3. Spin density of the partially reduced CeO2(111) of p(2√3 × 3) (a) and p(2 × √3) (b) models. The isosurface value is set as 0.005 e/Å3. Red ball for O, gray ball for Ce, and blue dotted ball for oxygen vacancy.

slab, it falls into the oxygen vacancy after geometric optimization. It might be attributed to the fact that the two Ce3+ ions are both near the oxygen vacancy, and O2 is readily prone to slip into the more stable site. However, the transformation is not observed in the p(2√3 × 3) slab due to the distribution of Ce3+ apart from the oxygen vacancy. There are no spin electrons for peroxide species on the CeO2(111) surface. Correspondingly, their spin density figures are the same as their geometric structures (see Figure 2d,e). To put it another way, no spin densities exist for the two structures, indicating that the corresponding oxygen species are diamagnetic. Therefore, the two 4f electrons of Ce3+ ions are transferred to the π2p* orbital of the adsorbed O2. Hence, we concluded that O2 adsorbed in the vacancy forms a typical peroxide species. According to the results obtained above, we can reasonably deduce that the electronic formation mechanism of superoxide and peroxide species on the partially reduced CeO2 surface. When the CeO2 surface is partially reduced, in the meantime, the surface oxygen vacancy forms and leads to reducing from two Ce4+ to Ce3+. When O2 adsorbs at the top site of the Ce3+ ion, which is close to the surface oxygen vacancy, it slips into the vacancy and a diamagnetic peroxide species forms since two 4f electrons of Ce3+ ions feedback to the half-filled π2p* orbital of O2. A magnetic superoxide species can be obtained only if O2 adsorbs at the top site of the Ce3+ ion apart form the surface oxygen vacancy accompanying the transfer of one 4f electron from CeO2 surface to the π2p* orbital of O2. 3.3. Migration from Superoxide to Peroxide. To further compare the different properties of the two oxygen species, we calculate the potential energy surface for the transformation from O2− to O22‑ (Figure S1 of the Supporting Inforamtion). The representative transition state of the migration from O2−(η2-1) to O22− is shown in Figure 5. The migration energy barrier is 0.35 eV, indicating that superoxide might easily transform into peroxide with elevating temperature. This conclusion is consistent with the experimental results observed by Li et al.18 who suggested that the IR peak of O2− decreases with the temperature increase from 200 to 473 K, while the O22− peak simultaneously increases. 3.4. Oxidation Roles of Superoxide and Peroxide. To shed light on the different oxidation roles of O2− and O22‑ species on the CeO2 catalyst, we explore the adsorption and reaction behaviors of CO as a probe molecule. The optimized structures of CO reacting with superoxide and peroxides are shown in Figure 6. When CO approaches the superoxide radical anion, it is directly oxidized to CO2 without an activation barrier. After CO reacts with η1 or η2 superoxide species, the released energies are

Figure 4. Spin density for superoxide on the partially reduced CeO2(111) surface. (a) η1, (b) η2-1, (c) η2-2. The isosurface value is set as 0.01 e/Å3.

√3) slab (Figure 3b) is obtained herein, which might be due to the small size of p(2 × √3). As shown in the spin density of O2 adsorbed on the partially reduced CeO2(111) substrates (see Figure 4), one 4f electron of the two Ce3+ ions transfers from the Ce3+ ion to the π2p* orbital of O2 when it locates at the top site of the Ce3+ ion in the p(2√3 × 3) slab. Correspondingly, the Ce3+ ion is oxidized to a Ce4+ one. Therefore, the superoxide species on the partially reduced CeO2(111) surface is unambiguously confirmed by analysis of the electronic structure; it provides a solid support for the experimental EPR5,43 and FT-IR spectra.7,18 However, when O2 adsorbs at the top site of Ce3+ ion in the p(2 × √3) 15989

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Figure 7. Transition state of CO2 desorption from a carbonate. Figure 5. Transition state of the migration from O2−(η2) to O22−.

of catalysts with high performance. To prepare nano CeO2 support with more structural defects47 and to introduce noble metals supported on48 or doped into49,50 ceria to lower the oxygen vacancy formation energy are two of the most important methods that have been widely used in catalyst development.

6.03, 5.96, and 6.02 eV, respectively. The structures of both products are similar, so only one is shown in Figure 6a. After oxidation, the catalyst surface recovers the initial stoichiometric one. Therefore, it can be deduced that the superoxide species has a high oxidative reactivity at relatively low temperature. It is well consistent with the experimental result that CO oxidation by O2− occurs at 213 K.9 When CO reacts with the peroxide species, a carbonate intermediate forms with an adsorption energy of 4.09 eV, as shown in Figure 6b. The carbonate must overcome a high desorption barrier of 0.77 eV to form a gas CO2 molecule, as shown in Figure 7. This is consistent with the experimental results that carbonate desorbs at relatively high temperature on ceria catalysts.44,45 Therefore, we deduce that peroxide species might play a dominant role at relatively high temperature. Here, we have to make a note. Although CO can be directly oxidized by superoxide at low temperature, which is experimentally observed in the literature9 and theoretically verified herein, we might not deduce that a partially reduced CeO2 oxide itself is an excellent catalyst that makes the oxidation occur continuously. After CO oxidation by O2−, the surface recovers the initially stoichiometry without oxygen vacancy; the oxidation of CO does not occur continuously only if a new vacancy forms. However, the oxygen vacancy formation energy for the stoichiometric CeO2(111) surface is 2.89 eV calculated in the present work, which is consistent with the value reported in the literature.46 Therefore, forming oxygen vacancies is a key step for creating active sites. That is the reason why in experiment, one should pretreat the samples by H2 at high temperature to obtain the partially reduced ceria catalyst.18 Therefore, to design a catalytic material with a low vacancy formation energy is the direction for the development

4. CONCLUSION The electronic formation mechanisms of superoxide and peroxide species are systematically investigated via calculating O2 adsorption behaviors on two partially reduced CeO2 surface with different sizes. The results suggest that the surface oxygen vacancy forms and leads to reducing from Ce4+ to Ce3+ ions on a partially reduced CeO2(111) surface. If O2 adsorbs at the top site of Ce3+ ion, which is close to the surface oxygen vacancy (a small supercell, p(2× √3)), it slips into the vacancy, and a diamagnetic peroxide species forms, since two 4f electrons of Ce3+ ions feedback to the two half-filled π2p* orbital of O2. Superoxide species with three different orientations are obtained only if O2 adsorbs at the top site of Ce3+ ion apart form the surface oxygen vacancy (a big superlattice, p(2√3 × 3)). Simultaneously, only one Ce3+ ion is oxidized to Ce4+. The relatively low migration barrier from O2−(η2-1) to O22− (0.35 eV) indicates that superoxide might easily transform into peroxide with elevating temperature. CO can be directly oxidized to CO2 by the superoxide without an energy barrier, while a carbonate forms when CO reacts with peroxide. The high desorption barrier of the carbonate to form a gas CO2 molecule implies that peroxide species might play the dominant role at relatively high temperature.

Figure 6. CO reaction with superoxide (a) and peroxide (b) species. Purple ball for O and gray ball for C of CO. 15990

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ASSOCIATED CONTENT

S Supporting Information *

Table S1: Adsorption energies of O2, the O−O bond lengths of O2− and O22− species on a partially reduced p(2√3 × 3) model with different force tolerances and layers. Figure S1: Potential energy surface for the migration from O2− to O22−. This information is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel: +86-579-82282234; Fax: +86-57982282595. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant No. 20903081) and the Natural Foundation of Zhejiang Province, China (Grant No.Y407163). X.-D.W. gratefully acknowledges a Seaborg Institute Fellowship (the LDRD program at LANL). The Los Alamos National Laboratory is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under Contract DE-AC5206NA25396.

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