Raman Spectra of Polycrystalline CeO2: A Density Functional Theory

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Raman Spectra of Polycrystalline CeO: A Density Functional Theory Study Christian Schilling, Alexander Hofmann, Christian Hess, and M. Veronica Ganduglia-Pirovano J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06643 • Publication Date (Web): 25 Aug 2017 Downloaded from http://pubs.acs.org on August 26, 2017

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The Journal of Physical Chemistry

Raman Spectra of Polycrystalline CeO2: A Density Functional Theory Study

Christian Schilling1, Alexander Hofmann2, Christian Hess1*, M. Verónica GandugliaPirovano3*

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Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität

Darmstadt, Alarich-Weiss-Str. 8, 64287 Darmstadt, Germany 2

Umicore AG & Co. KG, Rodenbacher Chaussee 4, 63457 Hanau, Germany

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Instituto de Catálisis y Petroleoquímica-Consejo Superior de Investigaciones Científicas,

Marie Curie 2, 28049 Madrid, Spain

ACS Paragon Plus Environment

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Abstract Cerium oxide is an important material for catalytic and fuel cell applications. We present an ab initio density functional theory (DFT) study of the vibrational properties of ceria focusing on the interpretation of Raman spectra of polycrystalline powder samples, with vibrational bands in the frequency region between 250 and 1200 cm-1. The model systems include the oxidized CeO2 as well as the reduced CeO2-x and Ce2O3 bulk materials together with the CeO2(111) and oxygen defective CeO2-x(111) surfaces. The experimentally observed band at 250 cm-1 is assigned to a surface mode of the clean CeO2(111) surface, in agreement with our Raman spectra of ceria (CeO2) powders with varying crystal size (Filtschew, A.; Hofmann, K.; Hess, C., J. Phys. Chem. C 2016, 120, 6694). The reduced model systems display signature vibrational bands in the 480−600 cm-1 region associated to the presence of oxygen defects and reduced Ce3+ ions. In the high frequency region between 800 and 900 cm-1, characteristic peroxide (O22−) stretching vibrations at the oxidized and defective ceria surfaces are obtained, and a systematic study with respect to the peroxide coverage provides the basis for a correlation between the position of the peroxide stretching mode and its adsorption geometry and concentration. The present theoretical analysis allows for a consistent description of the experimental Raman spectra of polycrystalline ceria. The outlined approach serves as a reference for the description of vibrational properties of other metal oxides.


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1. Introduction

Ceria (CeO2) is of importance for a variety of applications in, e.g., catalysis, solid oxide fuel cells, and oxygen membranes.

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Its popularity originates partly from its use in three-way

catalytic converters for gasoline powered vehicles, 3 which contain a ceria-based solid solution that provides and removes oxygen during the stages of rich and lean air/fuel ratios, respectively, extending the operating window of the three-way catalyst. 4 5 Ceria can be used as active phase for, e.g., partial alkyne hydrogenation.

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Moreover, its unique redox

properties associated with the oxygen mobility and oxygen storage/release capacity makes it also an interesting active support material during CO oxidation,

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7 8 9 10 11

water gas shift,

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for noble metal particles (Au/Pt/Rh)

or hydrogen evolution from ethanol.

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Furthermore, ceria serves as support of vanadia catalysts for selective oxidation reactions yielding a superior activity. 13 14 15 16 17 For catalytic reactions involving ceria-based systems, the ease with which the Ce valence shifts between Ce4+ and Ce3+ is crucial. The creation of a neutral oxygen vacancy is accompanied by the formation of a pair of Ce3+ ions. Many experimental

18 19 20 21 22

and theoretical

23 24 25 26 27 28 29

efforts have been made in order to

understand the structure of oxygen vacancies and their interactions on ceria surfaces. Nonetheless, most of the experimental studies on the low-index ceria surfaces such as CeO2(111), have often been performed under ultra-high vacuum conditions using surface sensitive techniques,

18 19 20 21 22

and extrapolation to typical operating pressures of working

catalysts is often not straightforward. In an attempt to bridge the so-called pressure-gap,

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Raman spectroscopy, which is a non-destructive method, can be applied to study active catalysts under a broad range of reaction conditions concerning pressure and temperature. 31 32 33

Raman spectra of single crystals of CeO2 34 35 and doped CeO2 36 were previously recorded as well as the Raman spectrum of polycrystalline Ce2O3.

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The phonon dispersion curve for


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CeO2 was calculated based on a simple rigid-ion model (DFT)-based calculations.

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Moreover, particle-size

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or on density functional theory

and trivalent doping

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effects in

polycrystalline ceria samples were studied using Raman spectroscopy by following the changes in the F2g band at 464 cm-1 and also those in a broad feature appearing at ~570 cm-1 in case of the doped systems. A neutral oxygen vacancy is introduced into the ceria lattice upon substitution of a pair of Ce4+ ions by trivalent cations such as La3+, Pr3+ and Nd3+. The feature at ~570 cm-1 is attributed to the presence of oxygen vacancies, based on a simple model calculation. 40 41 Moreover, the formation of O22− peroxide species upon oxygen adsorption on reduced ceria has been used to probe surface oxygen vacancy sites as investigated by Raman spectroscopy 42 43 44 45 46 and density functional theory (DFT) calculations. 47 48 49 50 However, the Raman activity of the peroxide stretching mode has not been yet calculated and a systematic study of peroxides as a function of coverage is lacking in the literature. Furthermore, Raman spectroscopy has been applied to characterize powder ceria samples under in situ/operando conditions spectroscopic techniques.

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in combination with other optical

Recently, Lohrenscheit et al.

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and X-ray

employed operando Raman

spectroscopy to evidence the participation of oxygen vacancies in the CO oxidation over Au/CeO2 catalysts. Additionally, Filtschew et al.

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discussed the Raman spectra of

polycrystalline ceria samples including the variation of vibrational properties with crystal size and synthetic route. Based on their analysis, a Raman band at 250 cm-1 was related to a ceria surface Ce-OH species. Although Raman spectroscopy is an important technique for the characterization of oxide-based catalytic materials, combined experimental and theoretical studies of the Raman spectra of oxidized and partially reduced ceria bulk and surfaces, where, in addition, charged O2 species can be adsorbed at surface vacancy sites, are relatively scarce 47

and limited to the calculation of vibrational frequencies. That is, no calculated Raman (or

IR) intensities of such systems have been reported yet, despite their importance for the interpretation of experimental Raman spectra. ACS Paragon Plus Environment

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In this work, we present a detailed DFT study on the Raman spectra of the CeO2 and Ce2O3 bulk phases and the CeO2(111) surface including Raman shifts and intensities. The partially reduced CeO2-x bulk and CeO2(111) surface are also considered. In addition, the characteristic peroxide stretching mode is calculated as a function of coverage and chemical environment. The theoretical results are discussed in regard to recently published Raman spectra of polycrystalline CeO2 powder samples of varying crystal size (J. Phys. Chem. C 2016, 120, 6694) as well as earlier related works.


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2. Theoretical and Experimental Methods

2.1. Models and Computational Details

Bulk properties of fluorite type CeO2 (Fm3തm) and hexagonal Ce2O3 (A-type P3തm1) were determined for the primitive cells (see Figure 1 A) and C)) containing one stoichiometric formula unit (CeO2 and Ce2O3, respectively). We applied spin-polarized DFT in the DFT+U approach

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with the generalized gradient corrected approximation (GGA) by Perdew, Burke

and Ernzerhof (PBE), 57 as well as the hybrid functional by Heyd, Scuseria and Ernzerhof (HSE06), 58 as implemented in the Vienna ab initio simulation package (VASP, version 5.3.5, http://www.vasp.at/). 59 60 61 62 63 The Ueff value of 4.5 eV 64 was used for the Ce 4f states. The Kohn-Sham equations were solved employing the projected augmented wave (PAW) method. 65 The Ce (5s, 5p, 6s, 4f, 5d) and O (2s, 2p) valence electrons were treated with a plane wave cutoff of 400 eV. To determine the CeO2 and Ce2O3 lattice constants for the conventional unit cells, as given in Table 1, the crystalline bulk structures (see Figure 1 B) and C)) were fully relaxed employing the stress tensor with 800 eV cutoff. Residual forces were smaller than 10-6 eV/ Å (10-4 eV/ Å for HSE06), and the total energy was converged up to 10-8 eV (10-6 for HSE06). The Brillouin zone was sampled using a (6×6×6) and (6×6×3) Monkhorst-Pack

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grid for primitive CeO2 and Ce2O3, respectively. To model oxygen

vacancies in bulk CeO2, a 2×2×2 expansion of the conventional cubic cell (Ce32O64) was employed, with a (2×2×2) Monkhorst-Pack grid sampling of the Brillouin zone. We considered one oxygen vacancy (Ce32O63) and a vacancy pair of third-neighbor vacancies oriented along the [111] direction (Ce32O62), with Ce3+ configurations according to Murgida et al.

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We performed ionic relaxations for a fixed lattice constant until residual forces were

reduced to 0.01 eV/Å, with a total energy precision of 10-6 eV. This setup is used in all relaxations performed in this work, unless it is specifically stated otherwise. In addition, in ACS Paragon Plus Environment

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order to account for the volume expansion due to the presence of excess charge in the defective Ce32O63 and Ce32O62 cells, for the isolated vacancy (Ce32O63), we first relaxed the volume using symmetry but without any constraints regarding the localization of the excess charge, whereas for the vacancy pair (Ce32O62), we first fitted the energy over a certain volume range to the Murnaghan equation of state. At each fixed cell volume (with cubic cell shape), the structure was optimized using symmetry and no constraints regarding the localization of the excess charge with a 800 eV energy cutoff. In a second step the most stable Ce3+ configurations

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were imposed to the Ce32O63 and Ce32O62 structures at their calculated

equilibrium lattice constant. The CeO2(111) surface was modeled using a supercell (see Figure 1 D) and E)) containing a 3 O-Ce-O trilayers (TL) slab with calculated CeO2 bulk equilibrium lattice constant. 10 Å of vacuum was added to the slab to avoid interaction between surfaces. Moreover, reduced CeO2-x(111) surfaces with surface or subsurface oxygen defects, were modeled with (2×2) periodicity with Ce3+ configurations according to Murgida et al. 25 Furthermore, chemisorbed peroxide (O22−) species were modeled on the CeO2-x(111) surface [O22−/ CeO2-x(111)] for a wide range of coverages using (4×4), (3×3), (2×2), (1×2), and (1×1) surface periodicities with (2×2×1), (2×2×1), (3×3×1), (6×3×1), and (6×6×1) Monkhorst Pack grids, respectively. All surfaces were allowed to relax with the bottom O-Ce-O trilayer and the surface unit cell kept fixed during geometry optimization.


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Figure 1. A) Primitive and B) conventional cubic representation of CeO2. C) A-Type Ce2O3 D) Top and E) side view of the oxidized CeO2(111) surface with (2×2) periodicity. Grey and blue balls represent Ce4+ and Ce3+ cations, red balls bulk oxygen, and yellow and green balls represent surface and subsurface oxygen atoms. This color code is used throughout the text.


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2.2. Vibrational analysis

In order to calculate the mass-weighted Hessian matrix required for a vibrational frequency calculation in the harmonic approximation, the force-constants were computed either from numerical differentiation using the finite difference approach with ±0.015 Å displacements, or from a density functional perturbation (or linear response) theory (DFPT) implemented in the VASP package.

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technique, both

The latter is not available when hybrid functionals are

employed and thus it was used with PBE+U, whereas the finite differences approach was used with both PBE+U and HSE06. Table 1 shows that the position of the first order allowed F2g Raman band in crystalline CeO2, as calculated with the DFPT and finite differences approaches, differs by less than 0.1 cm-1. To evaluate the normal mode position with a finite differences approach with PBE+U or hybrid functional framework (HSE06) a tight selfconsistent field (SCF) energy convergence criterion of 10-8 eV is used. As soon as the normal modes are calculated, it is possible to model their associated Raman and infrared (IR) activity as follows. Infrared spectroscopic activity of normal modes can be determined on the basis of the displacement vector and calculated Born effective charges. 68 70 In VASP and with the PBE+U functional, Born effective charges and the macroscopic dielectric tensor can be computed using DFPT. Raman scattering activity requires computing the change in the macroscopic dielectric tensor with respect to each normal mode, and thus relates to the third derivative of the energy with respect to atomic positions (see Supporting Information).

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In practice, the dielectric matrix is calculated for two structures where the

atoms are displaced by +0.005 Å and −0.005 Å along each normal mode vector, respectively, and derivatives are obtained with a finite differences approach (see Supporting Information). Successive calculations of Raman activities of normal modes are done using a python script written by Fornari and Stauffer.

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Due to the computational demand for calculating Raman

intensities, i.e., for a structure with ܰ௔௧ atoms and thus with 3ܰ௔௧ normal modes, 3ܰ௔௧ ×2 ACS Paragon Plus Environment

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additional DFPT runs each consisting of six linear response calculations are necessary (a total of 36ܰ௔௧ linear response calculations, see Supporting Information), only those modes lying within the 250 to 1200 cm-1 frequency region of observed Raman shifts were considered for intensity calculations. To account for spectral line broadening, the intensity of the normal mode was multiplied by a Gaussian function with a 10 cm-1 full width at half maximum (FWHM). The phonon dispersion and the density of vibrational states (DOS(ω)) were calculated within the PBE+U framework using the PHONOPY package

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with a (4×4×4) expansion of the

primitive ceria lattice. The finite difference method is applied to structures with appropriately displaced atoms. The energy convergence criterion for all single-point calculations, needed for the construction of dynamical matrices, was set to 10-8 eV. For the non-analytic term correction, the dielectric constant and Born effective charge of Ce and O were taken from a DFPT calculation (ε= 6.667, Z*(Ce)=5.524, Z*(O)=−2.765), which is in line with previously published data obtained with DFT(PBEsol)+U(5 eV).

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The experimental value of ε=5.31

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is overestimated by all calculations.


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2.3. Experimental Raman measurements

A detailed description of the ceria samples and the Raman setup is given in reference 55. This contribution does not contain new experimental results but the discussion refers repeatedly to previously published data.

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Thus, in the following, the used samples and Raman setup will

be briefly outlined. CeO2 powder samples were synthesized from decomposition of Cerium(III)nitrate hexahydrate at temperatures between 500 and 1000°C (AF500 and AF1000, respectively) in an electric furnace leading to an increase in crystal size with higher decomposition temperature. The commercial samples are abbreviated as follows. Fluka: Fluka >99.0%; SA 99.995%: Sigma Aldrich, 99.995% trace metal basis; SA < 25 nm: Sigma Aldrich, 0.5 ML, vacancies are in nearest-neighbor positions in the oxygen layer, and localization of excess electrons in energetically much less favored nearest-neighbor cation sites is also unavoidable.


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The highest value of +‫ܧ‬௩௔௖,୓ is obtained for a coverage of 0.5 ML [2.70 eV]. The reduced

+‫ܧ‬௩௔௖,୓ value for 1 ML [2.27 eV] as compared to 0.5 ML [2.70 eV] is the result of significant lattice relaxations that stabilize the highly reduced 1 ML structure; upon removal of the surface oxygen layer, the initially subsurface oxygen atoms strongly relax outward leaving a vacant O layer behind

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and a subsequent reconstruction with a change in the stacking

(Figure S1 D) further stabilizes the 1 ML structure by about 0.3 eV 85. The resulting structure corresponds to 1 quintuple layer (QL) of hexagonal (A-type) Ce2O3 on 1 trilayer (TL) of CeO2(111). For ‫ܧ‬௔ௗ௦,୓ , a gradual increase starting from 0.06 ML [0.28 eV] to 0.5 ML [0.36 eV] and a maximum for 1 ML [0.5 eV] is observed. This clearly reflects the repulsive interactions between adsorbed peroxide species.

The formation of superoxide species, O2−, by the transfer of only one electron to the chemisorbed oxygen molecule, is also possible. We considered one configuration at 0.25 ML coverage, as shown in Figure 11 E), and found the superoxide species to be by about 0.6 eV less stable than the corresponding peroxide species (see Table S3). A Bader charge analysis for the chemisorbed superoxide (12.73 e, O−O: 1.345 Å) and peroxide (configuration a, 13.19 e, O−O: 1.446 Å) species, compared to that for gas phase O2 (11.98 e, O−O: 1.234 Å), reflected the expected transfer of one and two electrons, respectively; the Bader charge for 2×O2− lattice oxygen atoms is 14.42 e.

The vibrational analysis of the O22−/CeO2-x(111) systems is performed after relaxation of the structures and fixing all but the peroxide species to their positions in order to reduce computational demand. The difference between such a constraint frequency calculation and a calculation in which surface atoms were included, changed the frequency of the Raman active


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O−O stretching mode by less than 0.1% and its intensity by less than 1%, as determined for the (2×2) structure in configuration a. For O22− coverages between 0.06 and 0.25 ML, the peroxide vibrations in the two configurations, a and b, are predicted to be located 39 to 38 cm-1 apart (see Figure 13 and Table S4), and the calculated intensity for configuration a is higher than that for b. Specifically, at a coverage of 0.06 ML, the normal mode analysis revealed a peroxide vibration at 893 cm-1 for configuration a and at 932 cm-1 for configuration b, with the intensity of the former being about four times higher than that of the latter. Huang et al.

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predicted

that at 0.25 ML coverage the rotation between a and b configurations is quasi barrier free (20 meV). Hence, an asymmetric peroxide band composed of two peaks due to configurations a and b is expected to be observed in experiment. Moreover, both peaks are expected to slightly shift to higher Raman shifts for a coverage increase from 0.06 to 0.25 ML (see Figure 13 and Table S4). At a coverage of 0.5 ML, the O−O stretching mode of structures b-1 and b-2, are located at 940 cm-1 and at 952 cm-1, respectively. The highest Raman shift for a peroxide vibration, i.e., 973 cm-1, is predicted for full monolayer coverage. Based on our findings we propose a blue-shift of the peroxide vibration as a function of coverage as illustrated in Figure 13. For comparison of theory and experiment, we propose a linear correlation using two bands, namely, the 893 and 932 cm-1 bands which are associated to the experimentally observed bands at 830 and 860 cm-1, respectively (see Figure 2 and ref. 43, and compare to section 4.3 below). The relative intensities of both sets of bands is comparable. A line with slope 1.30 and y-intercept −186 cm-1 has been obtained. A detailed discussion of the dependence of peroxide Raman shifts on coverage as well as a thorough comparison of theoretical and experimental Raman results will be given in the discussion section.


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Figure 11. O22−/CeO2-x(111) surface with various periodicities. Top view of A) peroxide species leaning towards Ce4+ (configuration a) with (3×3) periodicity and B) peroxide species leaning towards Osub (configuration b). Top and side views of C) configuration a with (2×2) periodicity, D) configuration b with (2×2) periodicity, and E) considered superoxide structure with (2×2) periodicity. Top view of F) configuration b-1 (rectangle) and G) configuration b-2, (parallelogram) with (1×2) periodicity. H) Top view of configuration b with (1×1) periodicity. The unit cell is indicated by black lines and doubled for the (1×2) and (1×1) structures for clarity. Violet balls: absorbed peroxide species. ACS Paragon Plus Environment

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Figure 12. Adsorption energies of a single O atom forming a peroxide species on the clean , CeO2(111) surface, ‫ܧ‬௔ௗ௦,୓ , and of O22− species on a defective CeO2-x(111) surface, ‫ܧ‬௔ௗ௦,୓మష మ as well as the vacancy healing energy, −‫ܧ‬௩௔௖,୓ , for the most stable peroxide structures (indicated), and as a function of peroxide coverage, from 0.06 ML to 1 ML. As illustrated for a coverage of 1ML, ‫ܧ‬௔ௗ௦,୓మష = −‫ܧ‬௩௔௖,୓ + ‫ܧ‬௔ௗ௦,୓ . For details see text. మ


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1000

910

O2/Ovac=1 config. a 2

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O2/Ovac=1 config. b 2

980

900

O2/Ovac1000nm) ceria nanoparticles with the calculated vibrational bands of the indicated model systems. In the following the results will be discussed focusing first on bulk properties, and then on surface properties. Finally, the peroxide results will be discussed in the context of the available literature.


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Figure 15. Comparison of the experimental Raman spectra of 25-30 nm and >1000 nm ceria particles with the calculated bands of the indicated ceria model systems. As discussed in the text, the outlined approach allows for a consistent description of the experimental Raman spectra. The calculated Raman shifts were scaled by a factor of 1.06 except for O22−/CeO2-x(111). The O22− calculated shifts are correlated with the experimental ones by assigning the calculated 893 cm-1 and 932 cm-1 vibrations to the observed 830 cm-1 and 860 cm-1 bands (cf. Figure 13). For simplicity, only peroxide the mode and the shifts corresponding to the lowest (0.06 ML, a and b structures) and to the highest (1 ML) O22− coverages are displayed.


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4.1. Raman modes of bulk CeO2 and CeO2-x

As already mentioned (Section 3.1), the vibrational frequency of the first order allowed F2g Raman band in CeO2 was determined for the primitive cell (three atoms). The HSE06 framework with the finite differences approach (450.8 cm-1) reproduces the experimental position of the F2g band at 464 cm-1, as measured for single crystals, 34 polycrystalline powders,

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thin films, 87 and

better than the PBE+U approach (437.3 cm-1 with DFPT and

437.2 cm-1 with the finite differences technique). Thus, the PBE+U vibrational frequencies of the oxidized and reduced bulk an ceria surfaces, are multiplied by a scale factor of 1.06 (464 cm-1/437 cm-1) when compared to experiments (Figure 15). Calculated frequencies of overtones (Section 3.2), as derived from the density of vibrational states, account for the experimentally observed weak bands for ceria single crystals and large ceria nanocrystals located at 250 cm-1, 402 cm-1, 595 cm-1, 950 cm-1, and 1170 cm-1 (see Figure 2 and Figure 15). However, the strong increase in intensity observed at 250 cm-1 and 402 cm-1 with decreasing crystal size (see Figure 2 and Figure 15) cannot be correlated to overtones. In fact, these bands are related to the surface termination of the nanocrystals rather than the bulk as discussed in the next section. Previously, the decrease in size of ceria nanoparticles had been observed to correlate with the increase in the ceria lattice constant as well as a red-shift of the F2g band position, and a relation to the presence of oxygen vacancies was suggested. 39 For example, for a particle size change from 25 nm to 6.1 nm, a 1.4% volume expansion and a 5 cm-1 red-shift were observed. 39 Our results (Section 3.4), i.e., the calculated F2g red-shift as a result of the lattice expansion of reduced bulk ceria (see Figure 7), support the experimental observations. We show that, for example, a volume expansion of 1.2 % for a slight bulk reduction (Ce32O62), in comparison to the fully oxidized bulk (Ce32O64), results in a 2.5 cm-1 red-shift. Although Ce2O3 in the cubic crystal structure (C-Type) has not been calculated, the hexagonal Ce2O3 ACS Paragon Plus Environment

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structure (A-Type) with a Eg vibration at 409.2 cm-1 (PBE+U) may provide a lower limit for the F2g shift, i.e., ~27 cm-1. Upon an increase in temperature from -200°C to 600°C, a red-shift of the F2g band from 464 cm-1 to 449 cm-1 was experimentally observed. 39 A red-shift of the F2g band was also reported for Au/CeO2 catalysts upon reduction in CO 82 and under reaction conditions of CO oxidation. 53

Thus, if particle size changes and temperature effects are avoided, the observed F2g shift

could serve as a semi quantitative measure of the increase in oxygen vacancy concentration in bulk ceria.

Bands in the 480-600 cm-1 region are generally associated with the presence of oxygen vacancies, 40 41 42 that are accompanied by the formation of reduced Ce3+ sites (see Figure 2). Bands in this region are more intense for smaller ceria nanocrystals, suggesting that smaller particles possess an increased intrinsic capability towards oxygen vacancy formation. This is in line with the calculated decrease in vacancy formation energy for ceria nanocrystals as compared to extended surfaces.

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Moreover, it has previously been shown that for ceria

nanocrystals the intensity of a band at 560 cm-1 increases with trivalent doping that favors oxygen vacancy formation.

40

Furthermore, oxygen defect-induced bands in doped ceria

nanocrystals have been identified within the above-mentioned frequency region using UV resonance Raman spectroscopy. 41 The calculated Raman spectra of reduced bulk ceria (Section 3.3) offer an explanation for the origin of the experimentally observed bands within the 480-600 cm-1 region. A detailed analysis of the structure and the Raman active vibrational modes reveals that the coordination cube around Ce4+/3+ ions in CeO2-x strongly influences the spectral properties. We show that in the Raman spectra of bulk CeO2-x a clear distinction can be made between Raman bands arising from Ce3+ in direct proximity to the oxygen defect, characterized by a calculated band at 500 cm-1, and those originating from Ce3+ located in the second coordination sphere of the ACS Paragon Plus Environment

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oxygen vacancy, giving rise to a band at 480 cm-1. Moreover, the 550 cm-1 region can be assigned to the Ce3+O7VO coordination (i.e., Ce3+ reduction close to a defect), whereas the 525 cm-1 region is attributed to the Ce4+O7VO coordination cube. As mentioned, scaling the calculated vibrational frequencies by a factor of 1.06 can account for the underestimation of the F2g band position in the PBE+U framework thus providing an interpretation of the experimentally observed behavior of the defect band region (see Figure 15).


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4.2. Raman modes of the CeO2(111) and CeO2-x(111) surfaces

Previous combined Raman and diffuse IR experiments of polycrystalline ceria have suggested that the 250 cm-1 band, which has an increased intensity for smaller ceria nanoparticles (Figure 2 and Figure 15), could be assigned to a surface Ce-OH stretching vibration.

55

The

calculations of the Raman spectrum of the CeO2(111) surface (Section 3.5) enable a detailed analysis and reveal that the band at 250 cm-1 (predicted at 225 cm-1 with PBE+U) is dominated by the O−Ce longitudinal stretching of atoms in the outermost layers. Thus, rather than the presence of surface hydroxyl it is the varying surface/bulk ratio of the ceria nanocrystals that is responsible for the observed intensity changes of the 250 cm-1 band. Moreover, the intensity of a Raman band at 402 cm-1 has also been observed to increase for smaller nanocrystals. This band is assigned to a transversal Ce-O stretching vibration of the topmost O-Ce layer, which is predicted at 363 cm-1 (see Figure 8). The calculations of the Raman spectra of the CeO2-x(111) surface with 0.25 ML of surface or subsurface oxygen vacancy (Section 3.6) reveal that the intensity of the O−Ce longitudinal stretching vibration is damped upon surface oxygen vacancy formation. Thus, this mode could be used to monitor the degree of CeO2(111) surface oxidation (see Figure 10).


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4.3. Peroxide vibrations: O22−/CeO2-x(111) with O22−/Ovac=1 and O22−/Ovac=0.5

Adsorbates are omnipresent on ceria surfaces. In particular, peroxide-related vibrational bands are of great interest regarding the characterization of ceria defects as well as in the context of catalysis, and thus have previously been investigated experimentally theoretically modeled

42 43 44 45 46

and

47 48 49 50

. As already described in Sections 3.7 and 3.8, we modeled

O22− species on the defective CeO2-x(111) surface for two different O22−/Ovac ratios, namely, O22−/Ovac=1 and O22−/Ovac=0.5. For 0.06 ML to 0.25 ML peroxide coverage and O22−/Ovac=1, we find two stable configurations, in which one peroxide O fills the vacancy and the second one either leans to a neighboring surface Ce4+ (configuration a see Figure 11A) or towards a neighboring subsurface O (configuration b see Figure 11B). Our calculations for 0.25 ML (O22−/Ovac=1) are in-line with those by Huang et al. 50 regarding structure and relative stability of both configurations. However, in the context of vibrational analysis, Huang et al.

50

only

considered superoxide species. The adsorption of peroxide species and its vibrational structure was also studied by Choi et al.,

47

using a surface slab with (1×2) periodicity (0.5

ML) and the PW91 functional, which is known not to be able to properly describe the localization of excess charge in reduced ceria systems.

77

Thus, the reported O22− binding

energy, ‫ܧ‬௔ௗ௦,୓మష , cannot be compared to our calculations for 0.5 ML (Figures 11F and G, మ Table S3). Nevertheless, Choi et al.

47

found a difference in bond length of 0.011 Å and a

frequency shift from 960 cm-1 to 978 cm-1 as well as a lowering of the energy by 50 meV for structure (1×2)-b-1 (i.e., peroxides forming a rectangle, Figure 11F) as compared to structure (1×2)-b-2 (i.e., peroxides forming a parallelogram, Figure 11G) that is in general reproduced by our calculations. However, our calculated frequency shift difference is only 12 cm-1, based on the peroxide frequencies of 940 cm-1 and 952 cm-1 for structure (1×2) b-1 and structure (1×2) b-2, respectively (Table S4).


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Furthermore, Zhao et al. x(111)

48

considered peroxide species in configuration type-a on CeO2-

with PW91+U(5 eV) and (2√3×3) [0.083ML] and (2×√3) [0.25 ML] periodicities,

which are comparable to our (3×3) [0.11 ML] and (2×2) [0.25 ML] calculations, respectively. In particular, for the 0.25 ML coverage, our calculated values for the O−O bond length and the associated stretching frequency are by 0.015 Å larger and by 21 cm-1 smaller, than the corresponding values reported by Zhao et al. 48 Regarding the effect of nearby surface oxygen vacancies on the properties of O22− species on the defective CeO2-x(111) surface (O22−/Ovac=0.5), our results show that the O22− formation energy, the length of the O−O bond, and the associated Raman shift are sensitive to the location of the Ce3+ ions relative to the formed peroxide (see Figure 13, Table S4). This is inline with the findings by Kullgren et al.

49

who studied the formation of peroxide species on

CeO2-x(111) with a nearby subsurface oxygen vacancy with (3×3) periodicity.

In the following, first a brief summary of the main experimental findings on peroxide adsorption on ceria will be given. We will then propose an assignment of the experimental features based on the results of our calculations. Raman spectra recorded upon O2 adsorption at 93 K on polycrystalline ceria, which had previously been reduced in H2 at 673 K, are characterized by an asymmetric peroxide feature consisting of a main band located at 831 cm-1 and a shoulder at around 860 cm-1. 42 O2 adsorption on polycrystalline ceria prereduced with CO at 473 K resulted in a comparable spectral behavior. 43 However, for higher CO pre-reduction temperatures (573 K and 673 K), an additional band at 877 cm-1 plus a blue-shifted shoulder were observed. 43 Contributions of peroxide features at around 830 cm-1 were also reported by Daniel et al.

45

and by us (see the asymmetric peroxide feature at 830

cm-1 in Figure 2 red, blue, and pink line). Please note that in the latter case the ceria samples were not pre-reduced prior to the Raman characterization and therefore a low coverage of


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peroxides is expected. A peroxide peak at 831 cm-1 was also observed in ex situ Raman 44

measurements after CO oxidation over CeO2 and Au/CeO2,

whereas operando Raman

measurements during room temperature CO oxidation over Au/CeO2, revealed four components at 832 cm-1, 847 cm-1, 866 cm-1, and 881 cm-1.

46

Finally, we note that a peak at

831 cm-1 with a shoulder at ~860 cm-1 as well as a blue-shifted band at 874 cm-1 were also observed for commercial ceria samples pre-reduced at 773 K in H2.

45

Besides, CeO2

nanorods and nanooctahedrons pre-reduced at 673K exhibit two peaks at ~830 cm-1 and ~860 cm-1, whereas the component at ~860 cm-1 shifts to 851 cm-1 for CeO2 nanocubes. 42 Based on the Raman active vibrations of peroxide species calculated in this study (Section 3.7 and Figure 13), we propose the observed asymmetric peroxide band with peaks at 830 cm-1 and 860 cm-1 (see Figure 2 and Figure 3 in ref. 43) to originate mainly from the existence of two possible stable peroxide structures at coverages up to 0.25 ML (configurations a and b in Figure 11), which results in O−O stretching frequencies separated by ~39 cm-1 (Figure 13). Please note that the calculated peroxide frequencies are blue-shifted by ~63-70 cm-1 with respect to experimental values for coverages between 0.06 ML and 0.25 ML (see Figure 13 and Table S4), in agreement with previous reports. 47

We will now address the intensity of the peroxide-related vibrations. Experimentally, the relative intensities of the 830 cm-1 and 860 cm-1 peaks observed for powder catalysts vary with peroxide coverage. For example, for our non pre-reduced samples (see Figure 2), a very low peroxide coverage is expected and the band at 830 cm-1 is dominant, namely, the lower frequency mode is found to be ~10 times more intense than the higher frequency mode. Moreover, a factor of ~3 has been observed for pre-reduction with H2 at 673 K, and almost comparable intensities for pre-reduction with a better reducing agent such as CO at the same temperature (see Figure 3 in ref. 43). The theoretical models (see Table S4), can account for this behavior as the relative intensity decreases from a factor ~4 to ~2.5 as the coverage ACS Paragon Plus Environment

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increases from 0.06 ML (bands at 893 cm-1 and 932 cm-1) to 0.25 ML (bands at 900 cm-1 and 938 cm-1).

In experiments, further blue-shifted Raman features are commonly observed such as bands at 877 cm-1 43 or 874 cm-1 45 for bare CeO2 and the band at 881 cm-1 for Au/CeO2, 46 all of which were reported for higher degrees of (pre-)reduction and may represent aggregated peroxide species. Indeed, the theoretical models for high O22− coverage (≥ 0.5 ML, Figure 11, Table S4) provide support for this interpretation. In particular, for coverage ≥ 0.5 ML blue-shifts of 47 cm-1 (0.5 ML (1×2)-b-1, Figure 11F), 59 cm-1 (0.5 ML (1×2)-b-2, Figure 11G), and 80 cm1

(0.5 ML (1×1)-b, Figure 11H) are predicted, with respect to the most intense band at 893

cm-1 for 0.06 ML. As mentioned above, the presence of nearby Ce3+ ions is not expected to give rise to additional bands outside the 830−860 cm-1 range. Thus, the 847 cm-1 band observed for Au/CeO2 catalysts

46

may originate from peroxide species located close to a Ce3+ ion

(compare Section 3.8, Figure 14 and Table S4), associated with nearby oxygen vacancies or charge transfer from a metal particle to CeO2.


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5 Summary and Conclusion

Based on new theoretical results, we critically discuss the assignment of Raman bands

Raman Spectra of Polycrystalline CeO2: A Density Functional Theory

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