Enhanced Oxygen Buffering by Substitutional and Interstitial Ni Point

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J. Phys. Chem. C 2010, 114, 10221–10228

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Enhanced Oxygen Buffering by Substitutional and Interstitial Ni Point Defects in Ceria: A First-Principles DFT+U Study Xinquan Wang,† Meiqing Shen,*,†,‡ and Jun Wang† Key Laboratory for Green Chemical Technology of State Education Ministry, School of Chemical Engineering & Technology, Tianjin UniVersity, Tianjin 300072, People’s Republic of China, and State Key Laboratory of Engines, Tianjin UniVersity, Tianjin 300072, People’s Republic of China

Stefano Fabris*,§,| DEMOCRITOS Theory@Elettra group, CNR-IOM Istituto Officina dei Materiali, Area Science Park SS14, Km 163, 5 BasoVizza, I-34012 Trieste, Italy, and SISSA Scuola Internazionale Superiore di Studi AVanzati, Via Bonomea 265, I-34136 Trieste, Italy ReceiVed: February 4, 2010; ReVised Manuscript ReceiVed: April 7, 2010

The defect chemistry and electronic structure of NiO/CeO2 solid solutions are studied by means of DFT+U calculations in the limit of low Ni doping. We consider four representative solid solutions in which the Ni atoms are present as substitutional and interstitial point defects in bulk crystalline CeO2, both in its stoichiometric form and in the presence of O vacancies. In all cases, Ni-doping significantly enhances the O buffering effect of ceria, controlled by O vacancy formation, but the actual microscopic mechanisms are different depending on the specific type and charge state of the point defects. The oxidation state of the Ni dopant is shown to univocally characterize the type of defect, whether interstitial (Ni+) or substitutional (Ni2+). Interstitial Ni+ defects result from a charge redistribution between the Ni and Ce cations that leads to the formation of characteristic Ni+-Ce3+ defect complexes. O release via vacancy formation in these interstitial solid solutions proceeds similarly as in pure CeO2, i.e., is mediated by electron localization processes reducing two Ce4+ ions to Ce3+. Quite differently, substitutional Ni2+ point defects yield unsaturated O 2p valence bands and the appearance of unoccupied gap states spatially localized on the O atoms neighboring the Ni defect. Consequently, O vacancy formation in substitutional solid solutions does not lead to reduction of Ce ions but to quenching of these gap states. Ab initio thermodynamics predict the substitutional solid solutions to be more stable than the interstitial ones by more than 2.4 eV over a wide range of pressures and temperatures. We demonstrate that these conclusions are robust with respect to the specific choice of the Hubbard U parameters accounting for the on-site electron Coulomb interaction on the Ni and Ce sites. 1. Introduction Ceria (CeO2) is one of the most important rare-earth oxide supports for metal nanoparticles in heterogeneous catalysts.1 This oxide takes an active catalytic role in several chemical reactions controlling applications in the fields of energy and environment, such as purification of exhaust gases,2 hydrogen production via the water-gas shift reaction, or selective CO oxidation.3 The promoting effect of cerium oxide is traced back to its oxygen storage capacity, an easy and reversible release of lattice oxygen that is assisted by the reduction of Ce4+ ions to Ce3+ following electron localization into Ce 4f states. Various studies have shown that the reducibility and catalytic activity of CeO2 can be considerably enhanced by doping with small amounts of transition metals.4 This is because the high cost of noble metal catalysts such as Pt and Pd constitutes an economic limit for their industrial applications, despite their catalytic efficiency toward important reactions, such as low * To whom correspondence should be addressed, [email protected] and [email protected]. † Key Laboratory for Green Chemical Technology of State Education Ministry, School of Chemical Engineering & Technology, Tianjin University. ‡ State Key Laboratory of Engines, Tianjin University. § DEMOCRITOS Theory@Elettra group, CNR-IOM Istituto Officina dei Materiali. | SISSA Scuola Internazionale Superiore di Studi Avanzati.

temperature CO oxidation.5 These considerations are at the basis of a considerable scientific and technological effort for introducing reactive transition metal ions into the ceria lattice, with the goal of identifying their role into the catalytic processes and engineering the defect chemistry accordingly.6-8 Recently more and more research is focusing on new catalysts containing cheaper transition metals with relatively high activity and fairly good stability.9 Particularly, CeO2-NiO catalysts have been studied in many hydrogenation and oxidation reactions taking advantage of the O-buffering effect.10 The synergistic effect of NiO and CeO2 is thought to be crucial for the high activities for CO oxidation and NO reduction11 as well as for enhancing methane combustion12 in Ni/CeO2 catalysts. We note that previous experimental studies demonstrated the enhanced O buffering of NiO/CeO2 solid solutions.13,14 These effects are generally interpreted in terms of differences in formal ionic charges, ionic sizes, and redox potentials of the Ni2+ and Ce4+ ions. As an example, by considering these formal ionic charges, the dispersion of Ni2+ ions into the CeO2 lattice as a substitutional dopant would require the presence of O vacancies to balance the electrostatics. However the oxidation state of the Ni ions, and the resulting catalytic activity, is determined by more complex phenomena. The efficiency of the catalysts depends, of course, on the number of active sites but

10.1021/jp101100f  2010 American Chemical Society Published on Web 05/13/2010

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also on the different nature of the cations, which can be present in multiple oxidation states.15 This can result from the reduction of the solid solution, i.e., on the formation of O vacancies under reaction conditions, but also on the specific nature of the solid solution, i.e., on the type of point defects formed by the Ni ions dispersed into the ceria lattice. Since Ni2+ has a much smaller ionic size compared to Ce4+, either it can substitute for a Ce4+ ion (substitutional site) in the fluorite lattice or it can occupy an interstitial site.16 The majority of the available experimental data (X-ray photoemission17 and absorption18) suggest that the most common ionic charge of Ni in NiO/CeO2 solid solutions is Ni2+. However we note that Ni+ species have also been reported.19 Correlating the charge state of the dopant with the specific type of defects (whether interstitial or substitutional, in the presence of O vacancies or of reduced Ce atoms nearby) as well as with changes in the O vacancy formation energy for the given solid solution would help to clarify the catalytic role of Ni point defects into the oxygen storage capacity of ceria-based catalysts. Despite the importance of such processes, there is still relatively little understanding of the underlying mechanism at the microscopic level. Although there are many experimental studies exploring the different charge states of Ni ions in Ni/ CeO2 systems17-19 there are very few first-principle studies addressing the same issue. Chafi et al. have investigated the interaction of nickel with ceria by means of DFT-GGA calculations,16 focusing in particular on the surface adsorption and interstitial defect sites. Their calculations were performed at the DFT-GGA level, which is often inadequate for describing the electronic structure of CeO2-based systems, particularly so in the presence of lattice defects and in nonstoichiometric materials. The calculated energetics suggests a driving force of 0.18 eV for the diffusion of an adatom from the (110) surface into the bulk to form an interstitial defect. For the bulk system, the existence of Ni2Ce has been predicted, in line with the experimental findings. However, there is no investigation on the enhanced mechanism of oxygen buffering in Ce-Ni-O solid solutions, although we note that similar solid solutions based on Au and Pd have been recently addressed.20 It is by now well established that standard DFT approaches do not allow for a correct description of the electron localization effects that are at the basis of its insulating nature, as well as of its O-buffering capacity.21 The addition of a model term, such as in the DFT+U methods, has been proved to solve this issue, and this approach has by now become very popular for firstprinciples calculations of ceria-based systems together with hybrid functionals that were recently employed.22 Similar difficulties for ab initio DFT methods are presented by NiO and have been addressed by the same DFT+U approaches.23 As a result, the NiO/CeO2 solid solution should also be calculated at the DFT+U level, with the additional complication that the Hubbard-U term would now act on two different sites and sets of orbitals, namely, the Ni d and Ce f ones, with the drawback of introducing additional empirical parameters in the ab initio simulations. In this paper we explore whether such approach, in which two different values for the U parameter are needed, is robust enough so that the main conclusions do not depend substantially on the specific choice of the parameters, as far as they are larger than a minimum value allowing for a correct description of electron localization. We can anticipate that for the specific case of NiO/CeO2 solid solutions, the main conclusions are rather insensitive on these parameters. Our starting point builds up on previous DFT+U studies of CeO2

Wang et al. and NiO that proposed values of the Hubbard U parameter equal to 4.5 and 4.6 eV for the Ce and Ni sites, correspondingly.23,24 In this paper we describe the defect chemistry and the electronic structure of NiO/CeO2 solid solutions in which the Ni atoms are dispersed in bulk ceria both as substitutional and as interstitial point defects. These two defects confer to the corresponding solid solution very different oxygen release capacity that we trace down to the different electronic properties of the two dopant sites. The calculated ab initio thermodynamics for defect formation allow us to conclude that the main species involved in the NiO/CeO2 solid solutions are the substitutional Ni point defects, which substantially enhance the O buffering of ceria. The paper is organized as follows: the computational details used in the DFT+U calculations as well as the approximations employed to determine the defect energetics are presented in section 2. The electronic and structural properties together with the relevant thermodynamics of four NiO/CeO2 model solid solutions are presented in section 3, where we consider both cases of interstitial and substitutional point defects. The dependency of the results on the computational parameters is discussed in section 3, where we also insert them in the context of the recent literature. The main conclusions are summarized in section 4. 2. Methods The spin-polarized density functional theory (DFT) calculations were performed in the framework of plane-wave and pseudopotentials by means of the Quantum ESPRESSO computer package.25 The Perdew-Burke-Ernzerhof (PBE) exchangecorrelation energy functional26 was used for the exchange correlation. The plane-wave basis set and the charge representation were limited by cutoffs of 30 and 300 Ry, respectively. The electron interaction with the ions were described with ultrasoft pseudopotentials27 constructed in the following electronic valence configurations: Ce, 5s2,5p6,4f1,5d1,6s2; Ni, 3d9,4s1; O, 2s2,2p4. In the present DFT+U work we have used the implementation of Cococcioni and de Gironcoli23 in which the occupation on the Ce f and Ni d states were calculated by means of two sets of projectors defined in terms of atomic-like functions. In line with previous analysis, we employ the value of U ) 4.5 and 4.6 eV for the Ce and Ni sites, respectively.23,24 The dependency of the calculated results on the value of the parameter U will be discussed in the text. The different Ni point defects in bulk CeO2 have been studied in the low concentration limit by means of 2 × 2 × 2 cubic supercells consisting of 96 lattice sites (Figure 1). The substitutional (Sub.) point defect was simulated by replacing one Ce with a Ni atom, while the interstitial (Int.) point defect by inserting a Ni atom at the center of the empty octahedral site in the O sublattice. These two systems will be denoted by means of CeNi(Sub.)O2 (Ce:Ni ) 31:1) for the substitutional (Figure 1a) and CeNi(Int.)O2 (Ce:Ni ) 32:1) for the interstitial defect (Figure 1b). In addition we have considered two additional model systems in which one O atom was removed from each of the supercells described above (see blue atom in Figure 1) leading to the stoichiometry denoted as CeNi(Sub.)O2-x and CeNi(Int.)O2-x, with x ) 1/32. The geometry has been relaxed by optimizing all internal structural parameters until the Hellman-Feynman forces on each ion were smaller than 0.01 eV/Å. In these calculations, integrals in the Brillouin zone were calculated by employing a Monkhorst-Pack28 2 × 2 × 2 mesh. Our aim is to simulate the electronic and structural properties of NiO/CeO2 solid solutions in the low Ni concentration limit. To this end, the volume of

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∆GfCeNi(Int.)O2-x(T, p) ) ECe32Ni(Int.)O63 - ENiO - 32ECeO2 + 2µO(T, p) ∆GfCeNi(Sub.)O2(T, p) ) ECe31Ni(Sub.)O64 - ENiO - 31ECeO2 µO(T, p) ∆GfCeNi(Sub.)O2-x(T, p) ) ECe31Ni(Sub.)O63 - ENiO - 31ECeO2 Figure 1. Supercells used to describe the Ni substitutional (a) CeNi(Sub.)O2-x and interstitial (b) CeNi(Int.)O2-x point defects in the NiO/CeO2 solid solutions. Red, green, and gray spheres represent the O, Ni, and Ce ions, respectively. The blue sphere (labeled as “V”) denotes the removed O atom leading to the oxygen vacancy. For clarity, the O atom nearest neighbors to the Ni defect are displayed in yellow.

the supercell was set equal to the calculated volume of pure ceria (5.48 Å, which is close to the experimental value of 5.41 Å). This is not a severe approximation since previous studies showed that relaxation of the external degrees of freedom in CeO2 supercells doped with several tetravalent ions results in calculated values of vacancy formation energy that differ only by 0.04-0.05 eV with respect to the corresponding values calculated employing the bulk lattice parameter.29 2.1. Thermodynamics. The free energy of formation of the model solid solutions described above will be determined on the basis of the calculated DFT+U total energies. To this end we consider the formation of the NiO/CeO2 solid solutions in terms of their individual oxides (NiO and CeO2) and of molecular oxygen. The explicit reactions describing the solid state formation for the specific Ni concentration used in the present study are CeNi(Int.)O2

NiO + 32CeO2 f Ce32Ni(Int.)O64 + 1/2O2 CeNi(Int.)O2-x

NiO + 32CeO2 f Ce32Ni(Int.)O63 + O2 CeNi(Sub.)O2

NiO + 31CeO2 + 1/2O2 f Ce31Ni(Sub.)O64 CeNi(Sub.)O2-x

NiO + 31CeO2 f Ce31Ni(Sub.)O63 The free energies of formation ∆Gf(T,p) corresponding to the different solid solutions are then approximated as

∆GfCeNi(Int.)O2(T, p) ) ECe32Ni(Int.)O64 - ENiO - 32ECeO2 + µO(T, p)

where, following the standard approximations employed in ab initio surface thermodynamics,30 the entropic contribution in the free energy of the solid oxides have been neglected, so that the dependency on T and p is determined by the chemical potential of the O atom only, µO. Since these are in equilibrium with an O2 atmosphere at the same T and p, the chemical potential of the O atom can be approximated as

µO(T, p) ) µO(T, p0) + ∆µO(T, p) ) µO(T, p0) + 1

/2kT ln(p/p0)

where the reference value for µO(T,p0) is chosen to be the total energy per O atom of an O2 molecule at 0 K. The calculated value of the latter is compensated for the known over binding of the molecular O2 predicted by the current approximations in the exchange and correlation functionals. Additionally, ENiO and ECeO2 are the total energies of pure oxides, while ECe32Ni(Int.)O63, ECe32Ni(Int.)O64, ECe31Ni(Sub.)O63, and ECe31Ni(Sub.)O64 are the total energies of supercells relating to interstitial/substitutional Ni point defect in CeO2 with and without an oxygen vacancy. 3. Results 3.1. Electronic Structure and Defect Charge Analysis. Our results show that the interstitial and substitutional Ni point defects in bulk ceria have very different effects on the electronic structure of NiO/CeO2 solid solutions. The former induce an electron transfer reducing one Ce ion to Ce3+ and resulting in Ni+ interstitial ions, the latter do not modify the formal 4+ valency of the Ce ions but lead to substitutional Ni2+ ions. In addition, also the electron reorganization due to the presence of O vacancies nearby these defects follows different mechanisms. These will be described and discussed in the following by analyzing the total density of electronic states (DOS), the atom decomposition obtained from projecting the density on atomic wave functions (PDOS), and the electron localization of the spin density. 3.1.1. Interstitial Ni Point Defect: CeNi(Int.)O2. The presence of a Ni interstitial point defect in bulk CeO2 drives a oneelectron charge transfer from the Ni to the Ce site, resulting in a pair of Ni+ and Ce3+ ions. This is clearly shown by the DOS and PDOS analysis for the CeNi(Int.)O2 system (Figure 2a) displaying a new one-electron gap state (see arrow) that results from the reduction of one Ce4+ ion to Ce3+ (the Ce-PDOS is represented by a blue line). There are two additional filled gap states at lower energies, just above the top of the valence band, originating from the Ni-O bonding as shown by their PDOS decomposition having mostly O p (red line) and Ni d (green line) character. By comparing the value of the Ni d Lo¨wdin charge calculated for the bulk NiO (8.32 e) and for the present CeNi(Int.)O2 system (8.73 e), we conclude that this Ni interstitial point defect is less ionic than a Ni2+ ion in its native metal oxide. We therefore identify the interstitial defect as a Ni+ ion.

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Figure 2. Interstitial Ni point defect in NiO/CeO2 solid solutions (CeNi(Int.)O2, top panels) and formation of an O vacancy neighboring the interstitial site (CeNi(Int.)O2-x, bottom panels) Total and projected densities of electronic states (a, c) together with the spin densities (b, d). The zero energy in the DOS is the Fermi level marked by the vertical dashed line.

TABLE 1: Lo¨wdin Charge Population for the Ni s, Ni d, and Ce f Orbitals, as Well as the Corresponding Spin Polarization (in µΒ) Calculated for Bulk NiO, CeO2, CeO2-x, and NiO-CeO2 Solid Solutionsa CeO2

CeO2-x

Nid Nis Cef

0.71 (1x Ce4+)

0.71 (30x Ce4+) 1.10 (2x Ce3+)

µNi µCe

0.0 (1x Ce4+)

0.0 (30x Ce4+) 0.99 (2x Ce3+)

NiO

CeNi(Int.)O2

CeNi(Int.)O2-x

CeNi(Sub.)O2

CeNi(Sub.)O2-x

8.32 (Ni2+) 0.50 (Ni2+)

8.73 (Ni+) 0.38 (Ni+) 0.71 (31x Ce4+) 1.09 (1x Ce3+) 0.95 (Ni+) 0.0 (31x Ce4+) 0.97 (1x Ce3+)

8.72 (Ni+) 0.41 (Ni+) 0.71 (29x Ce4+) 1.11 (3x Ce3+) 0.91 (Ni+) 0.0 (29x Ce4+) 0.96 (3x Ce3+)

8.21 (Ni2+) 0.44 (Ni2+) 0.71 (31x Ce4+)

8.24 (Ni2+) 0.45 (Ni2+) 0.71 (31x Ce4+)

1.36 (Ni2+) 0.0 (31x Ce4+)

1.69 (Ni2+) 0.0 (31x Ce4+)

1.61 (Ni2+)

a The resulting oxidation states of the cations are included in parentheses, together with the number of equivalent atoms (in boldface) in the supercell having that value of charge.

In the same analysis we also note small differences in the occupation of the Ni s state, which however do not affect our conclusion. Very small energy differences govern the relative position of the Ce3+ ion with respect to the interstitial Ni defect. We find that the lowest energy configuration is when the electron is far from the Ni interstitial (Figure 1b), but it is only 0.09 eV lower in energy than the configuration in which the Ni+ and Ce3+ are closest. 3.1.2. O Vacancy and Interstitial Ni Point Defect: CeNi(Int.)O2-x. The formation of an O vacancy near the interstitial Ni defect of the CeNi(Int.)O2 system presented above does not modify the occupation of the Ni impurity, which remains Ni+, instead it yields the reduction of two additional Ce4+ ions to Ce3+, similarly to the pure bulk case. The CePDOS (Figure 2c) and the charge population analysis (Table 1) provide evidence that the two excess electrons resulting from the O vacancy formation localize on two Ce ions (Figure 2d). Overall, this results in a complex defect cluster formed by the Ni interstitial, the O vacancy, and three compensating Ce3+ ions (one due to the presence of the Ni interstitial, two due to the presence of the O vacancy). The calculated Lo¨wdin charge for

the Ni ion does not modify substantially upon formation of an O vacancy (Table 1), therefore we conclude that the oxidation state of an interstitial Ni defect in reduced CeO2 is still Ni+. Also in this case, the precise localization site of these three excess electrons with respect to both the interstitial Ni and the O vacancy defects is determined by a very delicate energetics showing multiple minima. Accessing all the available defect configurations would require very large supercells and is beyond the scope of the present paper. We could stabilize a case in which all of the three Ce3+ ions are next nearest neighbor to the vacancy. However it is only 0.2 eV lower in energy than the configuration in which the Ce3+ ions were nearest neighbor to the O vacancy. The existence of these multiple defect configurations, differing in the localization site of the excess electrons are consistent with recent results for bulk and surfaces of pure ceria systems.22 Also these studies report multiple minima differing in the relative position of the excess electrons and the O vacancy. Both hybrid functionals and DFT+U approaches predict the next nearest neighboring minimum energy configuration for the Ce3+ ions and O vacancy. The energy difference between nearest and next nearest neighboring

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Figure 3. Substitutional Ni point defect in NiO/CeO2 solid solutions (CeNi(Sub.)O2, top panels) and formation of an O vacancy neighboring the substitutional site (CeNi(Sub.)O2-x, bottom panels) Total and projected densities of electronic states (a, c) together with the spin densities (b, d). The zero energy in the DOS is the Fermi level marked by the vertical dashed line.

Figure 4. Free energy of formation for CeNi(Int.)O2 and CeNi(Sub.)O2 solid solutions calculated with (a) U ) 4.5 eV and (b) U ) 2.5 eV values of the Hubbard U parameters acting on the Ce f and Ni d states.

position is however just 0.1 eV per Ce3+ ion. This value is consistent with our results obtained for the CeNi(Int.)O2 system. 3.1.3. Substitutional Ni Point Defect: CeNi(Sub.)O2. The density of electronic states for the system in which a Ni ion substitutes for a Ce ion (Figure 3a) is characterized by an unoccupied two-electron gap state, just ∼0.5 eV above the top of valence band. The PDOS analysis reveals that this state has mostly O p character and is spatially localized on the O atom nearest neighboring the Ni defect (Figure 3b). The origin of this state can be understood on the basis of a simple ionic model of the metal-oxide bonding. In a purely ionic picture, the metal atoms are fully oxidized so that all O atoms can reach their closed shell O2- ionic configuration. This condition cannot be achieved in this case because the highest oxidation state available to Ni is only 2+, lower than that one of the Ce4+ ion

that is substituted. As a result the total number of valence electrons resulting from the oxidation of the metals are not enough to fully saturate the O 2p bands, hence the appearance of the unoccupied two-electron gap state localized on the O atom neighboring the Ni defect. As it will be shown by the thermodynamics presented in the following section, the CeNi(Sub.)O2 structure can easily be reduced and is actually unstable in a large range of T and p. By comparing the calculated Lo¨wdin charges for NiO and for the substitutional CeNi(Sub.)O2 system (Table 1), we can conclude that the Ni defect can be associated with a Ni2+ ion and that all the Ce have their formal 4+ oxidation state. 3.1.4. Oxygen Vacancy and Substitutional Ni Point Defect: CeNi(Sub.)O2-x. The formation of an O vacancy close to a substitutional Ni defect (CeNi(Sub.)O2-x) makes available the

0.72 (31xCe4+) 0.77 (31xCe4+) 0.82 (31xCe4+) 0.87 (31xCe4+) 0.92 (31xCe4+) 0.94 (31xCe4+) 8.25 (Ni2+) 8.27 (Ni2+) 8.30 (Ni2+) 8.32 (Ni2+) 8.34 (Ni2+) 8.34 (Ni2+) 0.72 (31xCe4+) 0.76 (31xCe4+) 0.81 (31xCe4+) 0.87 (31xCe4+) 0.92 (31xCe4+) 0.94 (31xCe4+) 8.21 (Ni2+) 8.23 (Ni2+) 8.24 (Ni2+) 8.25 (Ni2+) 8.25 (Ni2+) 8.24 (Ni2+) 3+

(3xCe ) (29xCe4+) (3xCe3+) (29xCe4+) (3xCe3+) (29xCe4+) (3xCe3+) (29xCe4+) (3xCe3+) (29xCe4+) (32xCe4+)

Ce (f) Ni (d) Ce (f)

CeNi(Sub.)O2

Ni (d)

3+

(1x Ce ) (31xCe4+) (1xCe3+) (31xCe4+) (1xCe3+) (31xCe4+) (1xCe3+) (31xCe4+) (1xCe3+) (31xCe4+) (32xCe4+)

8.73 (Ni+) 8.72 (Ni+) 8.71 (Ni+) 8.70 (Ni+) 8.75 (Ni+) 8.74 (Ni+)

1.10 0.72 1.08 0.77 1.04 0.83 1.00 0.88 1.00 0.93 0.95

Ce (f)

CeNi(Int.)O2-x

Ni (d)

2.82 2.24 9.41 0.0 0.0

5.47

2.80 2.32 9.50 0.5 0.5

5.55

2.76 2.48 9.52 1.5 1.5

5.56

2.71 2.58 9.16 2.5 2.5

5.46

2.66 2.65 8.59 3.5 3.5

5.29

4.5 4.5

5.05

7.87

2.69

2.61

8.74 (Ni+) 8.72 (Ni+) 8.71 (Ni+) 8.70 (Ni+) 8.72 (Ni+) 8.72 (Ni+)

1.10 0.72 1.05 0.77 1.01 0.83 0.99 0.88 0.99 0.93 0.95

Ce (f)

CeNi(Int.)O2

Ni (d) ∆G(CeNi(Sub.)O2-x) ∆G(CeNi(Sub.)O2) ∆G(CeNi(Int.)O2-x) ∆G(CeNi(Int.)O2) U(Ni) U(Ce)

two excess electrons for filling the electron holes localized on the O atoms neighboring the Ni defect that were discussed in the previous section on the CeNi(Sub.)O2 system. Filling the two-electron gap state (that was unoccupied in the CeNi(Sub.)O2 system) lowers its energy down to the top of the valence band. As a result, the DOS of the defective CeNi(Sub.)O2-x system (Figure 3c) is now again consistent with that one of a large band gap insulator, the Fermi energy lying between the O 2p valence band and the unoccupied sharp band formed by the Ce 4f states. The charge population analysis (Table 1) provides evidence that the oxidation state of the substitutional Ni ion in the CeNi(Sub.)O2-x system is 2+ and that formation of an O vacancy does not involve the presence of reduced Ce3+ ions. We note that the DOS of the NiO/CeO2 solid solution in which Ni substitutes for the Ce ions in the presence of an equal amount of O vacancies can be seen as formed by overimposing the DOS of the individual bulk unreduced NiO and CeO2 oxides. This is also evident in the calculated Ni and Ce Lo¨wdin charges (Table 1) in the bulk NiO and CeO2, as well as in substitutional NiO/ CeO2 solid solutions. Additionally we also remark that the local crystal structure around the substitutional Ni site in the CeNi(Sub.)O2-x system closely resembles the one around the Ni site in bulk NiO. The six Ni-O bond lengths in the solid solution falls in the 2.122-2.160 Å range, which is quite compatible with the six corresponding bond legths in bulk NiO (2.167 Å). 3.2. Defect and Solid-Solution Thermodynamics. The methodology and approximations used to calculate the free energy of formation for the four NiO/CeO2 solid solutions containing Ni interstitial and substitutional defects have been defined in section 2.1. The results are presented in Figure 4 as a function of the variations in the chemical potential (∆µO) with respect to the reference state, µO(0,p0), in which we set p0 to 1 atm. The dependency of the free energy on T and p is shown on the top scale for three representative temperatures, T ) 300, 600, and 900 K. At the reference pressure, the solid solutions consisting of substitutional Ni2+ point defects result to be considerably more stable (by more than 2.3 eV) than those consisting of interstitial Ni+ ions. The calculated values of ∆Gf(0,p0) for the four solid solutions are reported in Table 2. In particular, the difference in the formation energies of the two substitutional CeNi(Sub.)O2 and CeNi(Sub.)O2-x systems is very small (0.08 eV), in within the error bar of our calculations. However, they display very different dependency on the chemical environment, so that it can be concluded that, in a wide range of O2 pressures and temperatures, the most stable NiO/CeO2 solid solution is the CeNi(Sub.)O2-x one, in which both Ni substitutional and O vacancy defects are present in equal numbers. The solid solutions containing interstitial Ni defects would become stable only at extremely low values of the chemical potential. In these conditions, due to the different O content of the two CeNi(Int.)O2 and CeNi(Int.)O2-x systems, the latter depends stronger on the O chemical potential than the former. As a result, even though at the reference pressure the formation energy of the CeNi(Int.)O2-x systems has the highest value (7.87 eV), by decreasing pressure it becomes more stable at a faster rate than the CeNi(Int.)O2 system, so that the critical pressure for the CeNi(Sub.)O2-x f CeNi(Int.)O2 transition is not far from the one for the CeNi(Int.)O2 f CeNi(Int.)O2-x transformation. The data presented in Figure 4a have also implications in the O-buffering capacity of NiO/CeO2 solid solutions, since the energy for O exchange at room temperature via the

Wang et al. CeNi(Sub.)O2-x

J. Phys. Chem. C, Vol. 114, No. 22, 2010 TABLE 2: Free Energy of Formation ∆Gf(0,p0) and Lo¨wdin Charges for the Interstitial and Substitutional Solid Solutions Calculated as a Function of the U Parameters Ranging from 0 to 4.5 eV

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Enhanced Oxygen Buffering CeNi(Sub.)O2-x T CeNi(Sub.)O2 transformation is very low, between 0.1 and 0.5 eV in the pressure range 1-10-6 atm at 300 K, and remains below 1 eV in that range of pressures even at 900 K. Quite importantly, this analysis of the relative stability of the solid solutions depends very weakly on the values of the Hubbard U parameters employed in the DFT+U calculations; hence the conclusions presented above are not affected by the particular choice of U values, as far as they are large enough to capture correctly electron localization effects in reduced ceria (i.e., U > 2 eV). To prove this, we include in Table 2 the values of the solid-solution formation free energy calculated with other values of U ranging between 4.5 and 0 eV. In the table, we report also the corresponding charge population analysis for the Ce and Ni sites. This charge analysis shows that a correct description of the electron localization on the Ce ions in the reduced systems (CeNi(Int.)O2 and CeNi(Int.)O2-x) requires values of U larger than ∼2 eV. Of course the absolute values of the free energies of formation are affected by this parameter, but most importantly the calculated energetics predict the same relative stability of the different solid solutions presented above for the case of U ) 4.5 eV. The main effect is a slight shift at lower values of ∆µO for the onset of stability field of the CeNi(Sub.)O2 phase, as shown by the free energy diagram calculated for U ) 2.5 eV and displayed in Figure 4b. Also in this case, the energy difference between the CeNi(Sub.)O2 and CeNi(Sub.)O2-x systems is very small (between -0.2 and 0.3 eV in the pressure range of 105-10-9 atm at 300 K), thus confirming that the enhanced O-buffering predicted by our DFT+U calculations does not result from a specific value of the U values but instead is valid for a wide range of values. 4. Conclusions In conclusion, the electronic structure predicted by our DFT+U calculations for NiO/CeO2 model solid solutions involving substitutional and interstitial Ni point defects, also in the presence of O vacancies, allow for rationalizing the different oxidation states for Ni and Ce experimentally reported, namely, Ni2+, Ni+, Ce4+, and Ce3+. The presence of substitutional Ni point defects is shown to result in Ni2+ ions, which are the most commonly observed defects in Ni-ceria systems.17,18 This experimental observations are quite compatible with the calculated thermodynamics: NiO/ CeO2 solid solutions involving substitutional Ni2+ defects are indeed predicted to be more stable than those involving interstitial Ni+ defects by more than 2.4 eV over a wide range of pressures and temperatures. Differently from the case of bulk stoichiometric CeO2, oxygen vacancy formation in the most stable substitutional solid solutions does not result in reduction of Ce ions nor in charge modifications of the Ni2+ ions. Instead, O release/uptake determines the appearance/ quenching of gap states formed by electron states localized on the O ions neighboring the Ni defect. This electronic effect is at the basis of the enhanced O-buffering effect of ceria induced by Ni defects in NiO/CeO2 solid solutions. Instead, all the analyzed solid solutions containing interstitial Ni point defects showed a charge redistribution among the metal cations resulting in the formation of characteristic Ni+-Ce3+ defect complexes. The mechanisms of oxygen vacancy formation in interstitial solid solutions are similar to those relevant for undoped CeO2 and involve the reduction of two further Ce cations to Ce3+. These solid solutions involving interstitial Ni ions are predicted to become thermodynamically more stable than those containing substitutional defects only in extreme

J. Phys. Chem. C, Vol. 114, No. 22, 2010 10227 conditions of temperature and O2 partial pressure. Finally, our analysis shows that these conclusions are robust with respect to the specific values of the U parameters used in the calculations and are valid for a wide range of U values. A minimum value of these is required to determine the full spatial localization of the electrons on the Ce and/or Ni sites but do not alter the charge transfer processes that leads to the different oxidation states of the interstitial and substitutional Ni point defects characterizing the NiO/CeO2 solid solutions. Acknowledgment. The authors are grateful for the financial support from the Program of Introducing Talents of Discipline to Universities of China, No. B06006. References and Notes (1) (a) Trovarelli, A. Catalysis by Ceria and Related Materials; Imperial College Press: London, 2000. (b) Perry Murray, E.; Tsai, T.; Barnett, S. A. Nature 1999, 400, 649. (c) Park, S.; Vohs, J. M.; Gorte, R. J. Nature 2000, 404, 265. (d) Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Science 2003, 301, 935. (e) Deluga, G. A.; Salge, J. R.; Schmidt, L. D.; Verykios, X. E. Science 2004, 303, 993. (2) (a) Kasˇpar, J.; Fornasiero, P.; Hickey, N. Catal. Today 2003, 77, 419. (b) Trovarelli, A. Catal. ReVsSci. Eng. 1996, 38, 439. (3) (a) Park, J. B.; Graciani, J.; Evans, J.; Stacchiola, D.; Ma, S.; Liu, P.; Nambu, A.; Sanz, J. F.; Hrbek, J.; Rodriguez, J. A. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 4975. (b) Rodriguez, J. A.; Ma, S.; Liu, P.; Hrbek, J.; Evans, J.; Pe´rez, M. Science 2007, 318, 1757. (4) (a) Wang, X. Q.; Rodriguez, J. A.; Hanson, J. C.; Gamarra, D.; Martinez-Arias, A.; Ferna´ndez-Garcı´a, M. J. Phys. Chem. B 2005, 109, 19595. (b) Bera, P.; Ca´mara, A. L.; Horne´s, A.; Martinez-Arias, A. J. Phys. Chem. C 2009, 113, 10689. (c) Jalowiecki-Duhamel, L.; Zarrou, H.; D’Huysser, A. Int. J. Hydrogen Energy 2008, 33, 5527. (d) Murugan, B.; Ramaswamy, A. V. J. Phys. Chem. C 2008, 112, 20429. (e) Kaneko, H.; Miura, T.; Ishihara, H.; Taku, S.; Yokoyama, T.; Nakajima, H.; Tamaura, Y. Energy 2007, 32, 656. (5) (a) Wilson, E. L.; Grau-Crespo, R.; Pang, C. L.; Cabailh, G.; Chen, Q.; Purton, J. A.; Catlow, C. R. A.; Brown, W. A.; de Leeuw, N. H.; Thornton, G. J. Phys. Chem. C 2008, 112, 10918. (b) Wilson, E. L.; Chen, Q.; Brown, W. A.; Thornton, G. J. Phys. Chem. C 2007, 111, 14215. (c) Putna, E. S.; Gorte, R. J.; Vohs, J. M.; Graham, G. W. J. Catal. 1998, 178, 598. (d) Golunski, S. E.; Hatcher, H. A.; Rajaram, R. R.; Truex, T. J. Appl. Catal., B 1995, 5, 367. (6) Li, P.; Chen, I. W.; Penner-Hahn, J. E. J. Am. Ceram. Soc. 1994, 77, 118. (7) Minervini, L.; Zacate, M. O.; Grimes, R. W. Solid State Ionics 1999, 116, 339. (8) Li, G.; Smith, R. L.; Inomata, H. J. Am. Chem. Soc. 2001, 123, 11091. (9) (a) Ciambelli, P.; Cimino, S.; Lisi, L.; Faticanti, M.; Minelli, G.; Pettiti, I.; Porta, P. Appl. Catal., B 2001, 33, 193. (b) Belessi, V. C.; Ladavos, A. K.; Pomonis, P. J. Appl. Catal., B 2001, 31, 183. (10) (a) Jalowiecki-Duhamel, L.; Zarrou, H.; D’Huysser, A. Catal. Today 2008, 138, 124. (b) Potdar, H. S.; Roh, H. S.; Jun, K. W.; Ji, M.; Liu, Z. W. Catal. Lett. 2002, 84, 95. (c) Wang, J. B.; Tai, Y. L.; Dow, W. P.; Huang, T. J. Appl. Catal., A 2001, 218, 69. (11) Wang, Y.; Zhu, A.; Zhang, Y.; Au, C. T.; Yang, X.; Shi, C. Appl. Catal., B 2008, 81, 141. (12) Shan, W.; Luo, M.; Ying, P.; Shen, W.; Li, C. Appl. Catal., A 2003, 246, 1. (13) Pino, L.; Vita, A.; Cipitı`, F.; Lagana`, M.; Recupero, V. Catal. Lett. 2008, 122, 121. (14) Jalowiecki-Duhamel, L.; Ponchel, A.; Lamonier, C.; D’Huysser, A. D.; Barbaux, Y. Langmuir 2001, 17, 1511. (15) (a) Yisup, N.; Cao, Y.; Feng, W. L.; Dai, W. L.; Fan, K. N. Catal. Lett. 2005, 99, 207. (b) Chary, K. V. R.; Rao, P. V. R.; Vishwanathan, V. Catal. Commun. 2006, 7, 974. (c) Li, Y.; Zhang, B. C.; Tang, X. L.; Xu, Y. D.; Shen, W. J. Catal. Commun. 2006, 7, 380. (16) Chafi, Z.; Keghouche, N.; Minot, C. Surf. Sci. 2007, 601, 2323. (17) Chettibi, S.; Wojcieszak, R.; Boudjennad, E. H.; Belloni, J.; Bettahar, M. M.; Keghouche, N. Catal. Today 2006, 113, 157. (18) (a) Kaneko, H.; Tamaura, Y. J. Phys. Chem. Solids 2009, 70, 1008. (b) Gonzalez-DelaCruz, V. M.; Holgado, J. P.; Perenˇ´ıguez, R.; Caballero, A. J. Catal. 2008, 257, 307. (19) Srinivas, D.; Satyanarayana, C. V. V.; Potdar, H. S.; Ratnasamy, P. Appl. Catal., A 2003, 246, 323. (20) (a) Farnesi Camellone, M.; Fabris, S. J. Am. Chem. Soc. 2009, 131, 10473. (b) Colussi, S.; Gayen, A.; Farnesi Camellone, M.; Boaro, M.; Llorca, J.; Fabris, S.; Trovarelli, A. Angew. Chem., Int. Ed. 2009, 48, 1. (c) Shapalov,

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