Structural and Redox Properties of Ce1-x

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Article 1-x

x

2-#

0.8

0.15

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2-#

Structural and Redox Properties of Ce ZrO and Ce Zr RE O (RE: La, Nd, Pr, Y) Solids Studied by High Temperature in situ Raman Spectroscopy Chrysanthi Andriopoulou, Antonios Trimpalis, Klito C. Petallidou, Anna Sgoura, Angelos M Efstathiou, and Soghomon Boghosian J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 23 Mar 2017 Downloaded from http://pubs.acs.org on March 23, 2017

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The Journal of Physical Chemistry

Structural and Redox Properties of Ce1-XZrxO2-δ and Ce0.8Zr0.15RE0.05O2-δδ (RE: La, Nd, Pr, Y) Solids Studied by High Temperature in situ Raman Spectroscopy

Chrysanthi Andriopoulou,1 Antonios Trimpalis,1 Klito C. Petallidou,2 Anna Sgoura,1 Angelos M. Efstathiou2 and Soghomon Boghosian 1,3*

1

Department of Chemical Engineering, University of Patras, Patras, GREECE

2

Department of Chemistry, University of Cyprus, Nicosia, CYPRUS

3

FORTH/ICE-HT, Patras, GREECE

( )

* to whom correspondance should be addressed; e-mail: [email protected]

1 ACS Paragon Plus Environment


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Abstract In situ Raman spectroscopy at temperatures up to 450oC is used to probe the structural and redox properties of Ce1-xZrxO2-δ solids (x = 0 – 0.8) prepared by the citrate sol – gel and co-precipitation with urea methods. The anionic sublattice structure of the solids is dependent on the preparation route. The composition effects exhibited by the Raman spectra are adequate for characterising the phases present and/or eventual phase segregations. For x=0.5 the pseudo-cubic t ′′ phase occurs for the solid prepared by the citrate sol-gel method, while phase segregation (cubic, tetragonal) is evidenced for the corresponding material prepared by the co-precipitation with urea method. A larger extent of defects and interstitial O atoms is evidenced for the materials prepared by the citrate sol-gel method. The well-known “defect” (“D”) band around 600 cm-1 for CeO2 as well as for Ce1-xZrxO2-δ consists of at least two components; “D1” above 600 cm-1 and “D2” below 600 cm-1. Doping of Ce0.8Zr0.2O2-δ with rare earth cations (La3+, Nd3+, Y3+, Pr3+) results in strengthening of the “D2” band that, however, is found to be insensitive under reducing conditions of flowing 5%H2/He at 450oC. A novel approach based on sequential in situ Raman spectra under alternating oxidizing (20%O2/He) and reducing (5%H2/He) gas atmospheres showed that the “D1” band is selectively attenuated under reducing conditions at 450οC and is therefore assigned to a metal – oxygen vibrational mode involving interstitial oxygen atoms that can be delivered under suitable conditions. A reversible temperature dependent evolution of the anionic sublattice structures of Ce1-xZrxO2-δ solids is evidenced by in situ Raman spectroscopy. The results are corroborated by powder XRD and oxygen storage capacity measurements and observed structure/function relationships are discussed. It is shown that at low temperatures (e.g. 450oC) the function of oxygen release and refill is based on a mechanism involving oxygen atoms in interstitial sites rather than


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on defects induced by hetrovalent M4+→ RE3+ doping, the latter improving the pertinent function at high (e.g. > 600oC) temperatures.



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1. INTRODUCTION

Cerium oxide, (CeO2, Fm3m cubic fluorite structure) has attracted great research and technological interest as a constituent of three-way catalysts (TWC),1 a key substrate for water-gas shift reaction catalysts,2 an oxide ion conductor solid electrolyte for fuel cells3 as well as in a large number of applications pertaining to industrial catalysis.1 This is owing to CeO2’s formidable redox properties allowing its rapid oxygen storage and delivery function as catalyst’s support, thereby classifying ceria as an oxygen storage material under dynamic working conditions.4 However, the oxygen storage capacity (OSC) of ceria is greatly attenuated by thermal stability issues. For remedying this weakness, different metal oxides have been used as additives,4-13 of which zirconia has been one of the most attractive ones in providing a very good balance in functional and stability grounds.11 The functionality of Ce1-xZrxO2-δ materials is largely defined by the phase conformation

and

most

importantly

by

the

anionic

(oxygen)

sublattice

structure/configuration. An elaborate phase diagram based on powder XRD results complemented by Raman spectroscopic measurements reported by Yashima et al14 is still addressed as a reliable benchmark. The structure evolves from monoclinic zirconia-like (P21/c) for x > 0.90 to cubic fluorite c (Fm3m) ceria-like for x < 0.15. At intermediate compositions, two more phases denoted t ′ and t ′′ , (i.e. tetragonal, belonging to P42/nmc) are shown to exist. Additionally, at temperatures exceeding 1473 K the monoclinic phase transforms into the tetragonal t form. The three tetragonal phases t , t ′ and t ′′ exhibit largely diversified tetragonality expressed by the c/a ratio (1.4, 1.01 and 1.0, respectively). Within the t ′′ and c phases the cations occupy exactly the same positions, thereby the two phases give rise to identical X-ray diffraction patterns and are often both referred to as c ′ . The mutual differences 4 ACS Paragon Plus Environment

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among the t ′′ and c phases pertain to their respective anionic sublattices. In the case of the t ′′ phase (tetragonal phase with no tetragonality, c/a = 1.0) the O atoms are slightly displaced from their (ideal) fluorite sites.14-16 However, the existing phase diagrams of the CeO2-ZrO2 system14,16 are not adequate for predicting the phases present in nanoparticles at the intermediate composition range because various synthesis protocols have given rise to different phases and/or different ranges of stability for the various phases.12,14,16-18 Notably, the degree of complexity in the CeO2-ZrO2 system can be higher in TWC applications when the mixed oxide is deposited on an alumina support.19 Raman spectroscopy is particularly sensitive in identifying structural alterations caused by O atom displacements due to inter alia the low mass and large polarizability of O atoms as well as in probing vibrational properties pertaining to intermediate range order. To the contrary, powder XRD is not sensitive to anionic sublattice structural transitions (like those occurring upon O atom displacement) because the X-ray atomic scattering factor of O is much smaller compared to that of Zr, Ce and RE’s cations, and therefore cannot distinguish the t ′′ from the c phase. Fluorite – type cubic structures (space group Fm3m, in which each metal atom, e.g. Ce, is surrounded by 8 O atoms forming the vertexes of a cube) have an exceptionally simple vibrational structure, i.e. exhibiting one single Raman band (F2g) observed typically in the vicinity of 465 cm-1 (depending on temperature). To the contrary, the selection rules preview ( Γvib = A1g + 2 B1g + 3E g ) typically 6 Raman active vibrational modes for a tetragonal phase. For the t ′′ phase, however, due to its lack of tetragonality some of the bands observed for typical tetragonal phases become degenerate and only four bands are typically detected.14,20,21 Raman spectra can be further complicated by band broadening resulting from disorder and defects caused by 5 ACS Paragon Plus Environment


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heterocationic exchange leading to short (rather than long) range periodicity in the anionic sublattice due to the heterogeneity and lack of periodicity pertaining to the cationic positions of the material’s matrix. Doping with heterovalent metal cations (e.g. trivalent rare earth cations, RE3+) results in further lowering of the symmetry and in the formation of defects caused by oxygen vacancies generated to compensate for the effective negative charge associated with a M4+→RE3+ exchange.21 The aim of the present work is to examine the effect of the preparation method on the structure and vibrational properties of Ce1-xZrxO2-δ crystalline solids. The comparison is undertaken for materials prepared by the citrate sol-gel and ureacoprecipitation methods. Additionally, the effect of substituting Zr4+ by RE3+ in Ce0.8Zr0.2O2-δ materials prepared by the citate sol-gel method (i.e. Ce0.8Zr0.15RE0.05O2δ,

where RE: La, Nd, Y, Pr) is investigated. The cationic sublattice structure is studied

by powder XRD analysis, whereas the structure of the anionic sublattice is probed by in situ Raman spectroscopy at temperatures up to 450oC. Also, oxygen storage capacity measurements were performed to investigate the surface and bulk redox processes of the CeO2-based materials. A novel approach for studying the redox properties of the studied materials by in situ Raman spectroscopy is used with a view to locate a Raman band due to a vibrational mode involving oxygen atoms that can be released by the materials under appropriate conditions.

2. EXPERIMENTAL SECTION 2.1 Synthesis of Ce1-xZrxO2-δ and Ce0.8Zr0.15RE0.05O2-δ Solids. Two preparation routes were followed for the synthesis of the solids studied. First, Ce1-xZrxO2-δ (x = 0.2, 0.35, 0.5 and 0.8) and Ce0.8Zr0.15RE0.05O2-δ (RE: La, Nd, Y and Pr) solids were prepared using the citrate sol-gel method (code: CA). Appropriate amounts of


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Ce(NO3)3.6H2O, ZrO(NO3)2, La(NO3)3.6H2O, Nd(NO3)3.6H2O, Y(NO3)3.6H2O or Pr(NO3)3.6H2O were dissolved in distilled water. Then, citric acid was added as a complexing agent of the metal ions.22,23 The ratio of the total molar amount of metal ions (M) to citric acid (CA), M:CA was 1:1.5. The resulting solution was subsequently heated to 70oC under stirring for assisting polymerization and condensation reactions. The solid was dried at 110oC for 17 h and then calcined in air stepwise at 350oC for 30 min, 400oC for 30 min and at 800oC for 6 h (5oC/min). Additionally, Ce1-xZrxO2-δ (x = 0.2, 0.5 and 0.8) solids were prepared using the urea co-precipitation method (code: U).23 The corresponding metal nitrates were dissolved in distilled and de-ionised water, while urea was then added in excess of 75 vol%. The resulting solution was heated at 80oC. The same process of drying and calcination was applied as in the case of citrate sol-gel synthesis method described above.

2.2 Powder XRD Analysis. Powder X-ray diffractograms were recorded in the 20-80o range (scan speed 2o/min) using a Shimadzu 6000 Series diffractometer (CuKa radiation, λ = 1.5418 Å). The lattice parameter was estimated according to the following Eq. (1) which holds for the fcc structure24:

a = d hkl h 2 + k 2 + l 2

(1)

where: h, k and l are the Miller indices of the given (hkl) crystallographic face. The lattice parameter was estimated using the (111) crystal face of CeO2.

2.3 BET Surface Area. The BET specific surface area (SSA, m2/g) of Ce1xZrxO2-δ

and Ce0.8Zr0.15RE0.05O2-δ (RE: La, Nd, Y and Pr) solids was estimated by

nitrogen adsorption-desorption isotherms at 77 K using a surface area and pore size analyzer (Micromeritics, Gemini model). Before measurements, the samples were 7 ACS Paragon Plus Environment


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degassed at 350oC for 2 h in N2 gas flow to remove adsorbed atmospheric water and most of adsorbed CO2 due to exposure to ambient air.

2.4 Oxygen Storage Capacity (OSC) Measurements. The H2-O2 pulse injection technique was used to measured the “oxygen storage capacity” (OSC) and “oxygen storage capacity complete” (OSCC) of Ce1-xZrxO2-δ and Ce0.8Zr0.15RE0.05O2-δ materials in the 500-800oC range. The apparatus and the experimental protocol used for OSC and OSCC measurements were previously reported.4 In the present work, only the OSCC quantity will be reported and discussed with respect to the Raman work since OSC reflects more “surface” (few top layers from the surface) oxygen vacant sites.4

2.5 In situ Raman Spectroscopy. The blue 491.5 nm line of a DPSS laser (from Cobolt, Sweden) operated at a power of 40 mW was used to excite the in situ Raman spectra at a right angle geometry and horizontal scattering plane; a cylindrical lens was utilised for slightly defocusing the incoming laser beam for reducing sample irradiance. A 0.85m Spex 1403 double monochromator was used to analyze the scattered light that was detected by -20oC cooled RCA photomultiplier tube interface with Labspec (J.-Y.) acquisition software. The optical cell used for recording in situ Raman spectra at temperatures up to 4500C under controlled flowing gas atmospheres was a homemade one.25,26 The Raman cell possesses a specially designed sample holder for mounting pressed wafer pellets of powder materials. The procedures for recording in situ Raman spectra at high temperatures under controlled gas flow have been described in detail elsewhere.25-27 A special protocol based on sequential in situ Raman spectra was


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applied for studying the redox properties of the materials.27 First, each sample is treated in situ under flowing 20%O2/He at 450oC for 1 h and the in situ Raman spectrum is recorded. Subsequently, the gas flow is switched to 5%H2/He and the sample is subjected to flowing 5%H2/He until steady state (i.e. for 2 h) and the in situ Raman spectrum at 450oC is then recorded. The sample is then re-subjected to flowing 20%O2/He for 1 h and its reinstatement to the initial (oxidized) condition is confirmed by recording the in situ Raman spectrum at 450oC under flowing 20%O2/He. Typically, the normalized Raman spectra are used for comparing the structural properties of the various samples. The normalization eliminates the effect of varying “path length” on the Raman data (i.e. variations in absorption of incident and scattered light by colored samples) as well as effects from eventual stray light and tail of the elastic Rayleigh line. When comparisons among spectra aim to focus on structural/vibrational effects, one finds it useful to disentangle the purely vibrational effects from those caused by temperature (e.g. Boltzmann distribution, depopulation of ground state, etc.). This is achieved by the so–called “reduction” procedure28,29 that expresses the Stokes-side reduced Raman intensity according to

ν~ I R (ν~ ) = ~ ~ 4 (ν 0 − ν )

−1

  1 ⋅ + 1 ⋅ I M (ν~ ) ~  exp( hcν / k B T ) − 1 

(2)

where I M (ν~ ) is the normalized experimental Raman intensity at the particular wavenumber ν~ , ν~0 is the excitation laser wavenumber and c, h and k B are the light velocity, the Planck and Boltzmann constants, respectively.



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3. RESULTS AND DISCUSSION 3.1 Influence of the Preparation Method on the Oxygen Sublattice Structure and Redox Properties of Ce1-xZrxO2-δ (x = 0 – 0.8) materials. 3.1.1 Raman spectra and oxygen sublattice structure. Figures 1 and 2 show the normalized Raman spectra obtained under dehydrated conditions for the Ce1xZrxO2-δ

materials. The spectra are recorded under flowing N2(g) at 300oC. Figure 1

shows spectra obtained for Ce1-xZrxO2-δ (x = 0, 0.2, 0.35, 0.5 and 0.8) prepared by the citrate sol-gel method (CA), while Figure 2 shows spectra obtained for Ce1-xZrxO2-δ (x = 0.2, 0.5 and 0.8) prepared by the co-precipitation method with urea (U). The spectrum of CeO2(CA) is included also in Figure 2 for comparison. The Raman spectrum obtained for pure CeO2 at 300oC features the characteristic F2g mode of the fluorite cubic structure (phase c) at 461 cm-1, the only Raman-allowed vibrational mode for the cubic fluorite structure. A 465 cm-1 value is usually reported for this mode that, however, is relevant to a perfect ceria lattice at room temperature. Volume and temperature contributions related to thermal expansion and anharmonic effects justify the slight red shift to 461 cm-1 (Fig.1). Additional weak bands can be discerned, as known30 at ca. 260 cm-1 (second order transverse acoustic mode, otherwise Raman inactive) and at ca. 590 cm-1 (defect induced mode, hereinafter referred-to as ′′D′′ band). The latter two weak bands result from punctual defects existing even in pure ceria that cause partial relaxation of symmetry rules.


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Ce1-xZrxO2-δ (citrate sol-gel) (e)

o

T = 300 C, N2(g)

t t

t

(d)

t

Normalised Raman Intensity

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448

x = 0.8,

t (c)

465 x = 0.5, 469

t''

F2g

x = 0.35, 466 F2g

c

(b)

x = 0.2,

c

461 F2g

x=0 CeO2, c

1000

800

x 0.4

D

(a)

600

400

Raman Shift, cm

200 -1

Figure 1. Composition dependence of in situ normalized Raman spectra obtained for Ce1-xZrxO2-δ solids (x = 0, 0.2, 0.35, 0.5 and 0.8) prepared by the citrate sol-gel method (CA). Laser wavelength, λ0 = 491.5 nm; laser power, w = 40 mW; spectral slit width, sww = 6 cm-1.



Ce1-xZrxO2-δ (urea) o

T = 300 C, N2(g)

F2g + t 464

(d)

t t

t

Normalised Raman Intensity

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x = 0.8,

c+t

463 F2g

(c)

t t

D

t

x = 0.5,

c+t

463 F2g D

x = 0.2,

c

461 F2g

x=0 CeO2, c

1000

(b)

800

D (a)

600

400

Raman Shift, cm

200 -1

Figure 2. Composition dependence of in situ normalized Raman spectra obtained for Ce1-xZrxO2-δ solids (x = 0.2, 0.5 and 0.8) prepared by the co-precipitation method with urea (U). The corresponding spectrum obtained for CeO2(CA) is included for comparison. Recording parameters: see Fig. 1 caption.


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The Raman spectra of Ce0.8Zr0.2O2-δ (CA) and Ce0.65Zr0.35O2-δ (CA) exhibit (Fig. 1) the F2g mode of a cubic phase (c) in gradually blue-shifting wavenumbers (466 and 469 cm-1, compared to 461 cm-1 for pure CeO2) and the ′′D′′ band with progressively higher intensity relative to the F2g band; the

ID

I F2 g ratio increases with

increasing x as long as the cubic c phase prevails, where I D and I F2 g denote the Raman band intensities of the ′′D′′ and F2g bands, respectively. The prevalence of a cubic phase (c) for these samples is confirmed also by powder XRD analysis (Section 3.3). The blue shift of the F2g band is justified by the lattice contraction caused by the Zr4+ ( i.r.Zr 4 + ,CN =8 = 0.84 Å) incorporation into the ceria matrix ( i.r.Ce4 + ,CN =8 = 0.97 Å). The gradual broadening of the F2g band on going from x = 0 (pure CeO2) to x =0.2 and x = 0.35 is justified by the concomitant lowering of the primary crystallite size (Table 1). The gradual increase of I D vs I F2 g is clearly due to progressive deformation of the anionic lattice resulting in the creation of defects and O vacancies. For example, it has been shown13 that with progressive lattice deformation, O atoms can relocate from the interior of the cation sublattice’s tetrahedral sites to the otherwise (ideally) empty interior of the cation sublattice’s octahedral sites (interstitial sites), thereby leaving self-charge balanced vacancies in the interiors of the tetrahedral sites.13 A clear change in the trends seen for x = 0 – 0.35 is observed for x = 0.5, i.e. for the Ce0.5Zr0.5O2-δ (CA) solid (Fig. 1). The main band, seen now at 465 cm-1 does not follow the progressive blue shift that would have been expected if Zr4+ had continued to be incorporated into the cubic matrix. Interestingly, the Raman spectrum (Fig. 1d) is the fingerprint of the t′′ phase.12,20,21 Whereas six Raman active bands are



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Table 1: Primary crystallite size (dc, nm), lattice constant (a, Å), cell volume (Å3) and SSA (m2 g-1) for the Ce1-xZrxO2-δ (x = 0.2, 0.35, 0.5 and 0.8) and Ce0.8Zr0.15RE0.05O2-δ (RE = La, Pr, Y and Nd) solids. Solid Ce0.8Zr0.2O2-δ Ce0.65Zr0.35O2-δ Ce0.5Zr0.5O2-δ Ce0.2Zr0.8O2-δ Ce0.8Zr0.2O2-δ Ce0.5Zr0.5O2-δ Ce0.2Zr0.8O2-δ Ce0.8Zr0.15La0.05O2-δ Ce0.8Zr0.15Pr0.05O2-δ Ce0.8Zr0.15Y0.05O2-δ Ce0.8Zr0.15Nd0.05O2-δ (a)

Synthesis Method CA CA CA CA U U U CA CA CA CA

dc (nm)

α (Å) (a)

9.5(a) 8.0(a) 7.3(a) 8.7(b) 9.0(a) 8.4(a) 8.7(a) 9.6(a) 8.8(a)

5.3714 5.3250 5.2843 5.3278 5.3766 5.3768 5.3640 5.3775

Cell volume (Å3) (a) 154.97 150.99 147.56 151.23 155.42 155.44 154.33 155.50

SSA (m2 g-1) 16.9 17.7 9.4 31.7 27.4 9.3 2.2 26.9 26.3 26.8 24.2

: Estimated based on the (111) face of CeO2

(b)

: Estimated based on the (101) face of t-ZrO2.

expected for a typical tetragonal phase, some of the modes become degenerate for phase t ′′ and thereby four bands are observed.12,20,21 The observed broadness of the bands relates to structural disorder upon heterocationic exchange (Ce4+→ Zr4+) that affects the order in the distribution of the various cations in the lattice. In Fig. 1d, one of the expected modes for the t ′′ phase at ca. 140 cm-1 is obscured by the tail of the Rayleigh wing. Powder XRD analysis cannot discern the t ′′ phase (c/a = 1.0) from the cubic c phase, to be illustrated in Section 3.3. The Raman spectrum obtained for the Ce0.2Zr0.8O2-δ (CA) solid (Fig.1e) is characteristic of a tetragonal phase (primarily t–ZrO2) in agreement with the powder XRD analysis (see Section 3.3). One of the six bands (expected for a t phase) is obscured by the tail of the Rayleigh wing, whereas two peaks overlap under the envelope of the ~620 cm-1 band.


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Contrary to the solids prepared by the citrate sol-gel method (CA) that exhibit phase homogeneity, the Ce1-xZrxO2-δ samples prepared by the co-precipitation method with urea (U) exhibit phase segregation for x>0.2 as evidenced by the Raman spectra shown in Fig. 2, in agreement with the powder XRD studies (Fig. S1). Thus, whereas for x = 0.2 (Fig. 2b) the prevailing phase is cubic where the F2g band at 463 cm-1 is slightly blue-shifted compared to the 461 cm-1 band observed for CeO2 due to Zr4+ incorporation into the ceria matrix, for x = 0.5 and 0.8 the cubic and tetragonal phases coexist. The intensity of the ′′D′′ band evolves with increasing x (from x =0 to 0.2 and 0.5), although the

ID

I F2 g

ratio (expressing the extent of structural defects and

occurrence of O vacancies) is markedly lower compared to the corresponding ratio observed for the counterpart samples prepared by the citrate sol-gel method (Fig. 1). Finally, it should be noted that factors affecting the F2g band position for counterpart solids of the same composition prepared by different methods (e.g. 466 cm-1 for Ce0.8Zr0.2O2-δ (CA) and 463 cm-1 for Ce0.8Zr0.2O2-δ (U)) include effects from lattice contraction/dilation, primary crystallite size and extend of tetragonal distortion of the cubic lattice manifested by the I D / I F2 g ratio.6 3.1.2. Redox properties of Ce1-xZrxO2-δ materials probed by in situ Raman spectra. In situ steady–state Raman spectroscopy at 450oC is used under oxidizing (flowing 20%O2/He) and reducing (flowing 5%H2/He) atmosphere in order to investigate the redox properties of the solids studied. Figure 3 shows the pertinent comparison between solids Ce0.5Zr0.5O2-δ, i.e. of identical composition but made following the two different preparation protocols, whereas Figure 4 shows the comparison between samples made by the same preparation method, i.e. by the citrate sol-gel method, but with different composition (i.e. x = 0.2 and x = 0.5). 15 ACS Paragon Plus Environment


800

600

400

200

Reduced Normalised Raman Intensity

0.16


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0.14

Ce0.5Zr0.5O2-δ (CA)

(A)

o

T = 450 C 0.12

~650

0.10

under 20%O2/He (g)

0.08

under 5%H2/He (g)

0.06 0.04

(a)

0.02

(b)

0.00

(c)

-0.02 0.16

F2g 459

Ce0.5Zr0.5O2-δ (U) o

0.12

T = 450 C

(B)

461

0.14

~645

F2g

0.10 0.08 0.06

(a)

461 459

under 20%O2/He (g)

480

(b) 440

400

under 5%H2/He (g) 0.04

(a) 0.02

(b) 0.00

(c)

-0.02

800

600

400

Raman Shift, cm

200

-1

Figure 3. Sequential in situ ′′reduced′′ and normalized Raman spectra obtained at 450oC for (A) Ce0.5Zr0.5O2-δ(CA) and (B) Ce0.5Zr0.5O2-δ(U) under flowing (a) 20%O2/He and (b) 5%H2/He; trace (c) is the outcome of subtraction of spectrum (b) obtained under H2 from spectrum (a) obtained under O2 in each case. Recording parameters: see Fig.1 caption.


Page 17 of 46

Reduced normalised Raman intensity

1000

800

600

0.16 0.14

466

o

0.12

200

F2g

Ce0.8Zr0.2O2-δ (CA) T = 450 C

400

D

(A) 464

0.10

615

0.08 0.06

(a) 0.04

(b)

~600 0.02

(c)

0.00

~630

Reduced normalised Raman intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.16 0.14


(B)

o

0.12

T = 450 C

D

0.10

465

635

463

0.08 0.06

(a) 0.04

(b) 0.02 0.00

1000

(c)

~650

800

600

400

200

-1

Raman shift (cm ) Figure 4. Sequential in situ ′′reduced′′ and normalized Raman spectra obtained at 450oC for (A) Ce0.8Zr0.2O2-δ(CA) and (B) Ce0.5Zr0.5O2-δ(CA) under flowing (a) 20%O2/He and (b) 5%H2/He; trace (c) is the outcome of subtraction of spectrum (b) obtained under H2 from spectrum (a) obtained under O2 in each case. Recording parameters: see Fig.1 caption.



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Figure 3A shows the sequential steady-state in situ Raman spectra obtained for Ce0.5Zr0.5O2-δ (CA) at 450oC under flowing 20%O2/He after 1 h treatment (spectrum (a)) and under flowing 5%H2/He after 2 h treatment (spectrum (b)). The Raman spectra are in “reduced” and normalized form (see Experimental part). Figure 3B shows the corresponding spectral information for Ce0.5Zr0.5O2-δ (U). The spectral features for the two samples having identical atom-% composition are markedly different, as discussed above. The most striking observation, however, is that at steady-state under reducing conditions (5%H2/He, 450oC), the intensity of the ′′D′′ band (i.e. at ca. 625 cm-1) is selectively attenuated. Most interestingly, the attenuation “mechanism” indicates that there are at least two components contributing to the ′′D′′ band envelop, of which the one pertains to a M–O (M = Ce, Zr) mode involving O atoms that are labile and can be released by the material under appropriate conditions (i.e. under flowing 5%H2/He at 450oC). This is best seen when one subtracts spectrum (b) obtained under 5%H2/He from spectrum (a) obtained under 20%O2/He gas flow, thereby obtaining trace (c) in each case. Trace (c) can be regarded as the spectroscopic fingerprint of the O atoms that have been released under the applied reducing conditions. At any case, the band intensities in traces (c) can provide a direct spectroscopic comparison of the amount of oxygen that can be detached under the applied conditions. Notably, re-subjection of each sample to flowing 20%O2/He at 450oC for 30 min results in the reinstatement of the ′′D′′ band envelop intensity, thereby signifying that an oxygen refill has taken place in each case. A slight red shift observed for the F2g band (from 461 to 459 cm-1) for the cubic phase extant in sample Ce0.5Zr0.5O2-δ (U) in Fig. 3B (inset) is justified by a partial reduction of Ce4+ to Ce3+ and a concomitant lattice expansion ( i.r.Ce3 + ,CN =8 = 1.13 Å vs i.r.Ce 4 + ,CN =8 = 0.97 Å).


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4 shows the sequential steady-state in situ Raman spectra (in “reduced” and normalized form) obtained at 450oC for Ce0.8Zr0.2O2-δ (CA) and Ce0.5Zr0.5O2-δ (CA) under flowing 20%O2/He after 1 h treatment (spectra marked by (a)) and under flowing 5%H2/He after 2 h treatment (spectra marked by (b)). Once again, the high wavenumber side of the ′′D′′ band is attenuated and traces marked by (c) that are derived after subtracting spectrum (b) from spectrum (a) for each sample case provide the fingerprint of the O atoms released under the applied conditions Following a resubjection of the samples in flowing 20%O2/He at 450oC for 30 min the spectral features are reinstated in their initial (prior to reduction) form. To the best of our knowledge, a selective sensitivity for band components contributing to the observed envelope of the so-called ′′D′′ (defect) band is evidenced for the first time. The ‘vacancy-interstitial-O-defect generation mechanism’ proposed by Mamontov et al13 is of relevance to interpret our observations. According to this mechanism, due to deformations and defects that arise also due to heterocationic doping (e.g. Zr4+→Ce4+), O atoms from the interior of cationic tetrahedral sites relocate to the otherwise (ideally) empty interior of the octahedral cationic sites, thereby creating O vacancies that are charge balanced from the O atoms occupying the interior of the octahedral sites (interstitial sites). Such interstitial O atoms may be available for release under suitable conditions. At this point it should be of relevance to examine if the above scenario is corroborated for pure ceria. Figure 5 shows the pertinent information, i.e. the sequential steady-state in situ Raman spectra (in ‘reduced’ and normalized form) obtained for CeO2 under flowing 20%O2/He after 1 h treatment (spectrum (a)) and under flowing 5%H2/He after 2 h treatment (spectrum (b)). Trace (c) is obtained after



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subtracting spectrum (b) from spectrum (a) whereas insets (A) and (B) provide focused views over the F2g and ′′D′′ bands, respectively. In spectrum Fig. 5(a), obtained under oxidizing conditions, the defect band is centered at ~590 cm-1 and its relative intensity, I D , is much weaker compared to the intensity of the F2g mode, I F2 g , thereby indicating a low extent of deformations and defects in pure CeO2 as seen also in Figs. 1 and 2 where a much higher

ID

I F2 g

ratio is seen for the Ce1-xZrxO2-δ solids.

Careful recording of the Raman spectrum with high accumulation time (in order to achieve a high S/N ratio) enables to discern that the ′′D′′ band (despite its low intensity) is attenuated upon subjection of CeO2 to reducing gas atmosphere (see Fig. 5(b) and inset (B)). Moreover, trace (c) reveals the fingerprint of the delivered oxygen by a weak band at ~620 cm-1 (best seen in inset (B), Fig. 5). Therefore, it turns out that also for pure CeO2 the ′′D′′ band comprises two components corresponding to different types of Cen+–O sites, of which the one involves O atoms that are detachable under the applied reducing conditions. Figure S2 (Supplementary Information) shows the deconvolution of the ′′D′′ band into its two components. Upon reduction (at steady state) the ′′D′′ band position appears apparently red-shifted due to the attenuation of its high wavenumber component. A final observation in Fig. 5 pertains to the F2g band position under the two gas atmospheres applied. Due to a partial Ce4+→Ce3+ reduction, the resulting lattice expansion together with an eventual primary particle size decrease contribute to the slight red shift that becomes evident both from the difference spectrum (c) and inset (A). Re-subjection of CeO2 to oxidizing atmosphere results in reinstatement of spectrum (a). The influence of defects in the Raman spectra of CeO2-based mixed oxides and the pertinent observation of two components within the ′′D′′ band envelope has 20 ACS Paragon Plus Environment

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F2g 458.5 457

(B)

(A)

458.5

(a) (b)

F2g 457

(c)

(a) (b) (c)

620 600

CeO2

400

590 620

480

460

440

420

o

T = 450 C

1000

800

(a) (b) (c)

600

400

200

-1

Raman Shift, cm

Figure 5. Sequential in situ ′′reduced′′ and normalized Raman spectra of CeO2 at 450oC under flowing (a) 20%O2/He and (b) 5%H2/He; trace (c) is the outcome of subtraction of spectrum (b) obtained under H2 from spectrum (a) obtained under O2. Recording parameters: see Fig.1 caption.

been addressed also earlier in the literature.31-34 Briefly, the band component near and above 600 cm-1 (D1) had been assigned to MO8 sites that did not contain O vacancy, whereas the band component below 600 cm-1 (D2) had been attributed to defect spaces involving an O vacancy. Our observation for the selective ability to reversibly release oxygen under reducing conditions from the MO8 sites that give rise to the D1 band component is indicative of a higher mobility and a looser bonding for the labile oxygen atoms that might be connected to the OSC of each sample. Notably, the D (i.e. D1 + D2) band depicts in a holistic manner the defect characteristics of the anionic



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sublattice configuration resulting from the activation of O mobility at elevated temperatures. For each Frenkel interstitial created13 contributing to the D1 component above 600 cm-1 there is a concurrent formation of a coordinatively unsaturated site, MO7, that contributes to the D2 counterpart component below 600 cm-1. Therefore, the removal of an interstitial O that is responsible for M-O modes giving rise to the high wavenumber component (D1) of the D band does not result in a parallel increase in the intensity of its counterpart component below 600 cm-1 (D2) because such a release is not creating a new coordinatively unsaturated site. 3.2 Effect of Rare Earth Doping on the Vibrational and Redox Properties of Ce0.8Zr0.15RE0.05O2-δ (RE = La, Pr, Y, Nd) solids. The effect of rare earth doping on the vibrational and redox properties of Ce1-xZrxO2-δ solids is addressed for one representative sample, namely Ce0.8Zr0.2O2-δ (CA), where doping with RE3+ took place resulting in samples with composition Ce0.8Zr0.15RE0.05O2-δ (RE = La, Pr, Y, Nd). Figure 6 shows the in situ reduced normalized Raman spectra obtained for the Ce0.8Zr0.15RE0.05O2-δ (CA) solids at 450oC under flowing 20%O2/He after 1 h of treatment. The corresponding Raman spectra of pure CeO2 (CA) and Ce0.8Zr0.2O2-δ (CA) are included for comparison. A red shift of the F2g band of the cubic phase c present is observed due to a lattice expansion taking place upon doping with La3+ ( i.r.CN =8 =1.16 Å), Y3+ ( i.r.CN =8 = 1.02 Å) and Nd3+ ( i.r.CN =8 = 1.11 Å) compared to the 466 cm-1 F2g band position for Ce0.8Zr0.2O2-δ. No appreciable shift could be measured for the F2g band position for Ce0.8Zr0.15Pr0.05O2-δ ( i.r.Pr 3+ ,CN =8 = 0.96 Å). The lattice expansion and the corresponding red shift is commensurate with the aforementioned ionic radii, i.r.RE3+ ,CN=8 , for the rare earth dopants, e.g the larger shift is measured for La3+ (i.e. from 466 to 461cm-1). 22 ACS Paragon Plus Environment

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Ce0.8Zr0.15RE0.05O2-δ (citrate sol gel)

o

F2g

T = 450 C under 20%O2/He 461

D2 ~595


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RE =

462

La

463

Nd Y

465

Pr 466 Ce0.8Zr0.2O2-δ 458 D1 ~625

800

600

CeO2

400

200 -1

Raman Shift, cm

Figure 6. In situ ′′reduced′′ normalized Raman spectra obtained under flowing 20%O2/He(g) at T = 450oC for Ce0.8Zr0.15RE0.05O2-δ solids (RE =La, Nd, Y and Pr) prepared by the citrate sol-gel method (CA). The corresponding spectra for CeO2 and Ce0.8Zr0.2O2-δ (CA) are included for comparison. Recording parameters: see Fig. 1 caption. 23 ACS Paragon Plus Environment


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Page 24 of 46

As seen in Fig. 6, doping with RE3+ results in the emergence of a band at ~595 cm-1 (marked as D2 in Fig. 6). For every two Zr4+ ions replaced by two RE3+ ions an O2- is removed from the matrix in order to compensate for the effective negative charge caused by the substitution and thereby a vacancy is created. This is manifested by the evolution of the D2 band (already extant also in pure CeO2, see section 3.1 and Fig. S2). Previously,31-34 in agreement with this observation, the D2 component (i.e. the low wavenumber component of the ′′D′′ band) had been assigned to defect sites that contain an O vacancy, i.e. to a vibrational mode involving a Mn+ ion with a coordinatively unsaturated site. One would then expect that the site giving rise to the D2 band would not be “willing” to easily deliver oxygen under reducing conditions. In order to shed light into this issue, we have recorded the in situ steady-state Raman spectra of Ce0.8Zr0.15RE0.05O2-δ (RE = La, Pr, Y, Nd) under flowing 5%H2/He after 2 h of treatment in each case and the results are shown in Figure 7 for Ce0.8Zr0.2O2-δ, Ce0.8Zr0.15La0.05O2-δ, Ce0.8Zr0.15Y0.05O2-δ and Ce0.8Zr0.15Nd0.05O2-δ. In all cases, it is evident that the ′′D′′ band intensity is lowered upon subjection of each sample to H2containing reducing gas atmosphere. The traces obtained following the subtraction of the spectrum obtained under hydrogen from the corresponding spectrum obtained under oxygen indicate that the intensity loss is incurred by the D1 component (i.e. the high wavenumber component of the ′′D′′ band). In all cases, the difference spectrum provides the fingerprint of the oxygen that has been released by the solid. It appears that the MO8 site giving rise to the D1 component in the 610–635 cm-1 range is the one capable of delivering oxygen under reducing conditions. Re-subjection of the samples to oxygen containing atmosphere (30 min under flowing 20%O2/He at 450oC) results in the refill of each sample with oxygen and reinstates the D1 band intensity. 24 ACS Paragon Plus Environment

Page 25 of 46

1000

800

600

400

200

1000

Ce0.8Zr0.2O2-ä

800

5% H2/He

F2g

Ce0.8Zr0.2O2-ä

D2

20% O2/He

5% H2/He

F2g

Ce0.8Zr0.15La0.05O2-ä

466

D

(B)

D1 Reduced Normalised Raman Intensity


200

o

20% O2/He

o

400

TT=450 = 480oCC

(A)

D2

TT=450 = 480oCC

600

Ce0.8Zr0.15La0.05O2-δ

D1

o

TT=450 = 480oCC

461

o

TT=450 = 480oCC

464

460 D

615

600

O2 ~600

800

O2 585

H2 O2-H2

~635

1000

600

400

200

1000

800

800

600

400

400

200

Raman shift (cm ) 1000

200

800

600

400

200

Ce0.8Zr0.15Nd0.05O2-ä

Ce0.8Zr0.15Y0.05O2-ä

(D)

o

T = 480o C

(C)

o

T T=450 = 480 oC C

T=450 C

D2

D2

D1

20% O2/He


20% O2/He

5% H2/He

Ce0.8Zr0.15Y0.05O2-δ

463

F2g

o

T = 480o C T=450 C

600

-1

-1

1000

H2 O2 - H2

~630

Raman shift (cm )


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5% H2/He

T T=450 = 480oCC

462

F2g

Ce0.8Zr0.15Nd0.05O2-ä o

D

D1

462 D

461

600

600

O2

585

800

600

H2

O2-H2

~625

1000

O2 ~570

H2

400

200

O2-H2

~610

1000

800

-1

Raman shift (cm )

600

400

200 -1

Raman shift (cm )



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 46

Figure 7: Sequential in situ ′′reduced′′ and normalized Raman spectra obtained at 450oC for (A) Ce0.8Zr0.2O2-δ(CA); (B) Ce0.8Zr0.15La0.05O2-δ(CA); (C) Ce0.8Zr0.15Y0.05O2δ(CA); and (D) Ce0.8Zr0.15Nd0.05O2-δ(CA) under flowing 20%O2/He and 5%H2/He gas as indicated by each spectrum. Peak analysis and curve fitting of recorded Raman spectra are also shown. Traces denoted by “O2-H2” are obtained by subtracting the spectrum obtained under H2 from the corresponding spectrum obtained under O2. Recording parameters: see Figure caption 1.


Page 27 of 46

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In order to quantitatively exploit the Raman band intensities, we performed a peak analysis and fit of the reduced spectra obtained for CeO2 (Fig. S2), Ce0.8Zr0.2O2-δ (Fig. 7a), Ce0.8Zr0.15La0.05O2-δ (Fig. 7b), Ce0.8Zr0.15Y0.05O2-δ (Fig. 7c), Ce0.8Zr0.15Nd0.05O2-δ (Fig. 7d) and Ce0.8Zr0.15Pr0.05O2-δ (not shown) with Gaussian bandshapes and the results of the peak analysis are shown in Fig. 7 and Fig. S2. A nonlinear Levenberg– Marquardt algorithm based method of regression was used for the analysis. Figure 8 shows the pertinent results, where the

ID

I F2 g ,

I D1

I F2 g and

I D2

I F2 g ratios are

compared. The relative intensity loss incurred by the ′′D′′ band upon subjection to reducing (H2) atmosphere is shown in Fig. 8(A) and the partition of the overall intensity loss to the D1 and D2 components is shown in Fig. 8(B) and 8(C), respectively. It is evident that the intensity loss (that occurs reversibly upon oxygen detachment) is incurred mainly by the defect sites that give rise to band D1. Previously,35 in situ Raman spectroscopy has been used to study the temperature dependence of the microstructure in Ce0.9Pr0.1O2-δ solid solution under oxidizing, inert and reducing atmospheres and pertinent variations in the relative band intensities were addressed and similar observations were reported when switching from oxidizing to reducing gas atmospheres. However, the exploitation of the band intensities was not combined either with a normalization protocol or with a procedure to disentangle the temperature dependent spectral features from thermal population/depopulation effects. The above results are considered being of significance for achieving a knowledge-based understanding of the function (oxygen storage, delivery and refill) of ceria-based mixed metal oxides using in situ Raman spectroscopy.



O2

0.5

H2

ID / IF2g

0.4

0.3

0.2

D vs F2g 0.1

(A)

0.0

CeO2 CeZrO2 CeZrLa CeZrPr CeZrY CeZrNd 0.5

Sample

O2 H2

0.4

ID1 / IF2g

D1 vs F2g 0.3

0.2

0.1

(B)

0.0

CeO2 CeZrO2 CeZrLa CeZrPr CeZrY CeZrNd Sample

0.5

O2 H2

0.4

D2 vs F2g 0.3

ID2 / IF2g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 46

(C)

0.2

0.1

0.0

CeO2 CeZrO2 CeZrLa CeZrPr CeZrY CeZrNd Sample

Figure 8. Plots of (A) I D I F2 g (ratio of integrated peak areas) for CeO2(CA), Ce0.8Zr0.2O2-δ(CA) and Ce0.8Zr0.15RE0.05O2-δ (CA) with RE=La,Pr,Y,Nd and of its partition to I D1 I F2 g and I D 2 I F2 g (plots (B) and (C)). The pertinent spectral data are shown in Fig. S1 and Fig. 7 (see text).


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3.3 Powder XRD Analysis and Cationic Sublattice Structure of Ce1-xZrxO2-δ and Ce0.8Zr0.15RE0.05O2-δ (RE = La, Pr, Y, Nd) Solids. Figure 9 presents powder XRD patterns of CeO2 and Ce1-xZrxO2-δ solids (x = 0.2, 0.35, 0.5 and 0.8) prepared by the citrate sol-gel method (CA). The powder XRD pattern of CeO2 (CA) phase presents the characteristic peaks of the fcc cubic fluorite structure. In particular, peaks of CeO2 were recorded at 28.7, 33.1, 47.5, 56.2, 59.2, 69.7, 76.7 and 79.3o 2θ which correspond to the (111), (200), (220), (311), (222), (400), (331) and (420) crystal planes. The powder XRD patterns of Ce1-xZrxO2-δ (x = 0.2, 0.35 and 0.5) were very similar to those of CeO2 and no diffraction peaks due to the t-ZrO2 phase were detected. On the other hand, the diffraction pattern of Ce0.2Zr0.8O2-δ solid (Zr/Ce=4.0) presents the characteristic peaks of t-ZrO2. In particular, the 2θ peaks recorded at 29.9, 34.6, 49.8 and 59.3o correspond to the (101), (110), (112) and (211) planes, respectively of t-ZrO2.36 The above results indicate that the fluorite cubic structure of Ce1-xZrxO2-δ solids prepared by the citrate sol-gel method is preserved for x≤0.5. This result is in harmony with the literature,37 where the limit of the formation of a cubic solid solution (c or t ′′ ) is reported to be that of x = 0.5. According to the Raman results (Fig. 1), for x = 0.2 and 0.35 the cubic phase prevails, whereas for x = 0.5 the

t ′′ phase is the prevailing one. It is obvious that the powder XRD technique cannot distinguish between the c and t ′′ phases. The diffraction peaks of Ce1-xZrxO2-δ (x = 0.2 - 0.5) solids appear shifted to higher 2θ angles compared to those of pure CeO2 (Fig. 9). This result shows the contraction of ceria lattice due to the incorporation of Zr4+ into the CeO2 fluorite structure, given the fact that the ionic radius of Zr4+ for 8-fold coordination (0.84 Ǻ) is lower than that of Ce4+ (0.97 Ǻ).


(e)

Page 30 of 46

Ce1-xZrxO2-ä (CA) CeO2 (311) (211)

(220) (112)

(110)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(111) (101) (200)


t-ZrO2

(d) (c) (b) (a)

20 25 30 35 40 45 50 55 60 65 70 75 80

2 theta

Figure 9: Powder XRD patterns of (a) CeO2 , (b) Ce0.8Zr0.2O2-δ, (c) Ce0.65Zr0.35O2-δ, (d) Ce0.5Zr0.5O2-δ and (e) Ce0.2Zr0.8O2-δ solids all prepared by the citrate sol-gel method (CA). . Figure S1 (Supplementary Information) shows powder XRD patterns of Ce1xZrxO2-δ

solids (x = 0.2, 0.5 and 0.8) prepared by the urea co-precipitation method (U).

For the Ce0.8Zr0.2O2-δ (U) solid the diffraction peaks were similar to those of pure CeO2 and shifted to higher 2θ angles. The diffraction pattern of Ce0.5Zr0.5O2-δ and Ce0.2Zr0.8O2-δ solids showed the same diffraction peaks as of pure CeO2 (shifted to higher 2θ angles) and additional peaks due to the t-ZrO2 phase. These results show that the urea co-precipitation method leads to phase segregation for x≥0.5,

in

agreement with the Raman studies (Fig. 2) and previous ones reported for the Ce0.5La0.5O2-δ solid.23 30 ACS Paragon Plus Environment

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Figure S3 (Supplementary Information) presents powder XRD patterns of Ce0.8Zr0.15RE0.05O2-δ (RE = La, Pr, Y, Nd) prepared by the citrate sol-gel method (CA). All powder XRD patterns present the same diffraction peaks with that of Ce0.8Zr0.2O2-δ. The diffraction peaks in most cases are shifted to lower 2θ angles, indicating that the incorporation of RE3+ (La3+, Y3+, Nd3+) into the Ce0.8Zr0.2O2-δ lattice leads to the expansion of its crystal lattice. This result is pronounced for RE3+ = La3+, Nd3+ given the fact that the ionic radii (for CN=8) of La3+ (1.16 Å) and Nd3+ (1.11 Å) are much larger than that of Ce4+ (0.97 Å). Table 1 summarizes the mean primary crystallite size (dc, nm), the lattice constant (α, Ǻ) and the cell volume (Å3) of CeO2 and Ce1-xZrxO2-δ solids (x = 0.2, 0.35, 0.5 and 0.8) prepared by the citrate sol-gel (CA) and the urea co-precipitation (U) methods. The Ce1-xZrxO2-δ solids lead to the formation of crystallite sizes in the 7.3-9.5 nm range. The crystallite size was found to decrease by a factor of 1.3 as Zr-content increased in the solid. The Ce0.2Zr0.8O2-δ (CA) solid presents a mean crystallite size for to the t-ZrO2 phase of 8.7 nm. The primary crystallite size of Ce0.5Zr0.5O2-δ (U) and Ce0.2Zr0.8O2-δ (U) were not estimated due to the overlapping of diffraction peaks of pure CeO2 or Ce1-xZrxO2-δ solid solution with those of t-ZrO2. All Ce1-iZrxO2-δ solids present lower lattice constant values (5.2843 – 5.3714 Å) than pure CeO2 (5.4057 Å), indicating the contraction of the ceria lattice, in agreement with values previously reported.37,38 The cell volume (Å3) obtained indicate the contraction of ceria lattice after Zr4+ introduction into the ceria lattice (CeO2: 157.96 Å3 vs. Ce1-xZrxO2-δ: 147.56 154.97 Å3). The mean primary crystallite size was found to be in the 8.4-9.6 nm range for the Ce0.8Zr0.15RE0.05O2-δ (RE= La3+, Pr3+, Nd3+, Y3+) solids, similar to that of Ce0.8Zr0.2O2-δ (9.5 nm). The lattice constant and cell volume parameter values were found to be slightly larger for the Ce0.8Zr0.15RE0.05O2-δ (RE= La3+, Pr3+, Nd3+) solids



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and slightly lower for the Ce0.8Zr0.15RE0.05O2-δ (RE= Y3+) compared to the Ce0.8Zr0.2O2-δ solid. 3.4 Oxygen Storage Capacity. Figure 10 compares the OSCC (Oxygen Storage Capacity Complete) obtained at 600oC (Fig. 10a) and 800oC (Fig. 10b) for Ce0.8Zr0.2O2-δ and Ce0.8Zr0.15RE0.05O2-δ (RE = La, Y, Nd and Pr) solids prepared by the citrate sol-gel method. At 600oC, Ce0.8Zr0.15Pr0.05O2-δ shows the highest OSCC value, whereas Ce0.8Zr0.15Nd0.05O2-δ the lowest one with respect to the four RE dopants investigated. On the other hand, after increasing the temperature to 800oC, all four dopants resulted in higher OSCC values compared to the Ce0.8Zr0.2O2-δ material. In particular, Ce0.8Zr0.15La0.05O2-δ exhibits the highest OSCC value with ~ 35 % increase with respect to Ce0.8Zr0.2O2-δ, the latter exhibiting the lowest OSCC at 800oC. Figure S4a presents results on the effect of Ce/Zr atom ratio on the OSCC of Ce1xZrxO2-δ

(x = 0.2, 0.35, 0.5 and 0.8) solids prepared by the citrate sol-gel method in the

temperature range of 500-800oC. The Ce0.65Zr0.35O2-δ(CA) solid presents the highest OSCC compared with the other solids (Fig. S4a). In particular, the OSCC increased by a factor of 1.55 when Ce0.65Zr0.35O2-δ (Zr/Ce=0.54) is compared with Ce0.2Zr0.8O2-δ (Zr/Ce=4.0) at 700oC. The Ce0.2Zr0.8O2-δ solid presents the lowest OSCC values among the samples prepared by the citrate sol-gel method. It appears that there is an optimum ratio of Ce/Zr that results in optimum OSCC and this has to do with the effect of Ce/Zr ratio on the creation of oxygen vacancies and interstitials within the solid of Zr-doped ceria. Figure S4b presents results on the effect of Ce/Zr atom ratio of OSCC for the Ce1xZrxO2-δ

(x = 0.2, 0.5 and 0.8) solids prepared by the urea co-precipitation (U) method.

After increasing the Ce:Zr ratio from 0.25 to 4.0 the OSCC increases steadily in the temperature range of 500-800oC.


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-1

OSCC (µmol O g )

2400 o

T = 600 C

2200

Ce0.8Zr0.2O2-δ

2000

Ce0.8Zr0.15RE0.05O2-δ

1800

3+

3+

3+

(a)

3+

RE = La , Y , Nd , Pr

1600 1400 1200 1000 800 600 CeZr

3+

3+

Y

Nd

3+

3+

Pr

La

Dopant 2400 2200

o

Ce0.8Zr0.2O2-δ

T = 800 C

(b)

Ce0.8Zr0.15RE0.05O2-δ

2000 -1

OSCC (µmol O g )

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1800 1600 1400 1200 1000 800 600 CeZr

3+

Y

3+

Nd

3+

Pr

3+

La

Dopant Figure 10: OSCC (µmol O g-1) measured by O2-H2 pulses at (a) 600oC and (b) 800oC for Ce0.8Zr0.2O2-δ and Ce0.8Zr0.15RE0.05O2-δ (RE = La3+, Y3+, Nd3+ and Pr3+) solids prepared by the citrate sol-gel (CA) method. 33 ACS Paragon Plus Environment


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It is to be remarked also that Ce1-xZrxO2-δ solids prepared by the citarte sol-gel method (CA) present larger OSCC (µmol O g-1) than the solids prepared by the coprecipitation method with urea (U) and that the difference in OSCC between counterpart samples prepared by the two methods increases with increasing temperature. Thus, the synthesis method affects bulk oxygen redox properties in the Ce1-xZrxO2-δ solids (e.g. concentration of oxygen vacancies and oxygen mobility). Similar results were obtained for Ce0.5La0.5O2-δ solids prepared by the CA and U methods.23 Notably, no significant difference for the amount of detachable oxygen at 450oC can be discerned by Raman spectroscopy for the Ce0.5Zr0.5O2-δ solids prepared by the two methods (Fig. 3) which is in agreement with the trends seen in Figs S4a and S4b.

3.5 Temperature Dependence of Oxygen Sublattice Structure in Ce1xZrxO2-δ

Solids. The anionic sublattice structure of the CexZr1-xO2-δ mixed metal

oxides is subjected to changes caused by oxygen mobility and migration within their lattice as outlined in the Introduction. Oxygen mobility and relocation13 results in many closely related anionic lattice site configurations that apart from breaking the selection rules for cubic symmetry and causing bands′ leaking due to disorder, it furthermore gives rise to broad continua rather than to well-formed isolated bands in the Raman spectra. Additionally, heterovalent cation substitution (i.e. M4+ → RE3+) creates further defects and vacancies within the anionic sublattice sites. Apart from the composition effects that – as described above (Section 3.1) – trigger the creation of defects and vacancies, temperature also appears to play a key role in the evolution of the anionic sublattice structure. Although this is a wellexpected phenomenon (i.e. slightly diversified T-dependent configurations for the 34 ACS Paragon Plus Environment

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anionic lattice structure are expected on account of diversified minima in lattice energy), it has not been to date captured by in situ molecular (e.g. Raman) spectroscopy. To the best of our knowledge, even though Raman spectroscopy is recognized as a powerful tool14 for probing structural properties of the anionic lattice of ceria-based mixed metal oxides, literature reports providing evidence based in molecular spectroscopy to support the T-dependent reversible evolution of the anionic sublattice of these materials are scarce. Herein we report, for the first time, the temperature-dependent reversible evolution of the anionic oxygen sublattice of the CexZr1-xO2-δ solids studied. Figure 11 shows the temperature-evolution of the steadystate in situ reduced and normalized Raman spectra obtained for e.g. the Ce0.5Zr0.5O2-δ solids, made by the two different synthesis routes used in the present work, on going sequentially from 50 to 200 and to 450oC under flowing 20%O2/He. Before recording the spectrum at 50oC, the solids are treated in situ at 450oC under flowing 20%O2/He for 1 h. The evolution of the anionic sublattice structure on going from 50oC to 450oC is manifested by: (a) an increase of the D band intensity relative to the F2g band (increase of the

ID

-1 I F2 g ratio); and (b) a blue shift (by 2-3 cm ) of the D band seen

near 630 cm-1. The evolution and mobility of the oxygen sublattice are activated by raising the temperature. The temperature rise facilitates oxygen diffusion within the lattice, thereby enabling a mechanism of increased relocations of oxygen atoms from their ideal fluorite sites to interstitial (Frenkel) sites.13,30 Notably, as evidenced by the in situ Raman results obtained under O2(g) and H2(g) containing atmospheres at 450oC (i.e. Figs 3, 4, 5 and 7), which show a selective attenuation of the high frequency component of the D band (D1 component) under reducing conditions, O atoms in interstitial sites are readily available for release and delivery under reducing gas conditions already starting from 450oC.

The aforementioned proposed 35

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mechanism accounts for the gradual increase of the

ID

I F2 g

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ratio with increasing

temperature and for the concurrent slight blue shift of the D band (Fig. 11), the latter being suggestive of a larger relative increase of the D1 component relative to its D2 counterpart with increasing temperature. This is interpreted to indicate that M – O modes associated with interstitial oxygen atoms (giving rise to the D1 component) give rise to stronger Raman bands compared to the respective modes in coordinatively unsaturated MO7 sites. However, it is noteworthy that in agreement with the proposed scenario, both components of the D band (D1 and D2) are strengthened with increasing temperature: oxygen relocation in interstitial sites results in strengthening of the D1 component and the concurrent creation of a self-charged balanced vacancy results in the creation of one coordinatively unsaturated MO7 site for each relocating oxygen atom, thereby justifying the strengthening of the D2 component (although to a somewhat lower extent compared to its counterpart D1, ascribed to a correspondingly lower Raman cross-section). Interestingly, the changes observed and the spectral variations in Figure 11 reflect purely the structural evolution of the oxygen sublattice since the ‘reduced’ representation of the Raman spectra (see Experimental section) disentangles the spectra from temperature effects (e.g. Boltzmann distribution, depopulation of ground state, etc). Moreover, the observed changes are reversible, i.e. after cooling in situ to 50oC the corresponding spectral features are reinstated indicating that internal diffusion oxygen pathways are reversible and that the cationic sublattice does not change during temperature cycling in the range of 50 to 450oC. The above are not trivial observations. For example, in a typical oxide material, e.g. TiO2-anatase (see Fig. S5) there is no temperature dependence in the relative band intensities on going from 50 to 450oC. The bands undergo the so-called



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473

under 20%O2/He (g)

(A)

633

(a) (b) (c)

635 465

o

50 C o

200 C o 450 C

Ce0.5Zr0.5O2-δ (U)

469

(B)

under 20%O2/He (g)

466 461 629

o

50 C o 200 C o 450 C

1000

632 x1.5

800

600

400

Raman Shift, cm

(a) (b) (c)

200

-1

Figure 11: Temperature dependence of in situ reduced and normalised Raman spectra obtained under flowing 20%O2/He for Ce0.5Zr0.5O2-δ solids made by: (A) the citrate sol-gel (CA) method; and (B) the co-precipitation method with urea (U) at temperatures as indicated by each spectrum. Recording parameters: see Figure 1 caption.



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thermal broadening that is typical for materials with well-defined crystal structures and this effect is attributed to a number of purely spectroscopic (non-structural) effects, e.g. anharmonicity. Moreover, in Figure S5 one can see that all bands undergo a red shift (i.e. shift to lower wavenumbers), a well-known effect in high temperature vibrational spectroscopy (due to anharmonicity, closer packing of excited vibrational levels and due to a volume contribution, e.g. lattice expansion). The mixed CexZr1xO2-δ

oxides exhibit only short (rather than long) range periodicity and extensive

defects. Slight compositional variations and/or variations in the extent of nano-scale heterogeneities are expected to affect the temperature-dependent behavior of their anionic sublattices. The occurrence of intracrystalline nanodomains embedded in the material matrix is known also for materials synthesized in laboratory (i.e. wellcontrolled) conditions.15 Given the technological relevance of the studied materials, it is worth pointing out that industrial-scale syntheses of e.g. two lots of such materials made by the same industrial protocol are not expected to be able to reproduce exactly similar anionic sublattice structures. 3.6 Structure/function relationships. Temperature appears to play a key role in the evolution of defects and vacancies in ceria-based mixed metal oxide solids. An increase in temperature activates the intrinsic mobility of O atoms in the lattice and facilitates the relocation of O atoms from the interior of tetrahedral cationic sites to the interior of octahedral cationic sites (Frenkel interstitials).13 The intensity increase of band D with rising temperature (Fig. 11) and most particularly the intensity increase of its D1 component above 600 cm-1 (assigned to M – O modes involving oxygen atoms in interstitial sites) is in agreement with the mentioned oxygen sublattice mobility. The O atoms in interstitial sites are available for reversible release/refill already at 450oC as evidenced by the sequential in situ Raman spectra 38 ACS Paragon Plus Environment

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(Figs. 3, 4, 5 and 7). On the contrary, O atoms in coordinatively unsaturated sites generated primarily upon heterovalent M4+ → RE3+ cation substitution that give rise to the D2 band component (extant intrinsically also in pure CeO2 due to occurrence of Ce3+ as well as in CeO2-ZrO2) are detached to a much lesser extent under reducing atmosphere at temperature of 450oC, as evidenced by the sequential in situ Raman spectra shown in Figs 3-5 and Fig.7 and the pertinent quantitative exploitation depicted in Fig.8. Significantly, the OSCC measurements (Fig. 10) corroborate the above and show that the beneficial effect of the M4+ → RE3+ cation substitution (generating oxygen vacancies and pertinent defects) is initiated above 600oC and becomes significant at 800oC. Therefore, the combination of results from the sequential in situ Raman spectra and the OSCC measurements indicate that oxygen release and refill at low temperatures (i.e. 450 – 600oC) take place primarily by means of a mechanism involving O atoms in interstitial sites.

4. CONCLUSIONS In situ Raman spectroscopy has been used to characterize the anionic oxygen sublattice structure of Ce1-xZrxO2-δ materials prepared by two different synthesis routes. The preparation method largely affects the identity of the phases present as well as the material phase homogeneity. The Raman spectra were adequate to show that the pseudo-cubic t ′′ phase occurs for the Ce0.5Zr0.5O2-δ material prepared by the citrate sol-gel method (CA), whereas the Ce0.5Zr0.5O2-δ counterpart made by the ureaco-precipitation (U) exhibited phase segregation (c and t phases). Powder XRD analysis could not discern the t ′′ phase occurring for the Ce0.5Zr0.5O2-δ(CA) material, because it is not able to differentiate the t ′′ phase from the cubic c phase. 39 ACS Paragon Plus Environment


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A band attributed to deformations/defects and O vacancies (named ′′D′′ band) occurring at ca. 600 cm-1 is shown to have at least two components (D1 slightly above 600 cm-1 and D2 below 600 cm-1), not only for the Ce1-xZrxO2-δ mixed oxide materials but also for pure CeO2 (shown for the first time). The high wavenumber component D1 is shown to be selectively sensitive under reducing conditions at 450oC (contrary to its D2 counterpart) and is attributed to defect MOx sites containing loosely bound oxygen that can be reversibly detached/refilled under appropriate conditions. A novel approach based on sequential acquisition of in situ Raman spectra is used for comparing ceria-based mixed oxides in functional grounds. Doping with rare earth ions (RE = La, Nd, Y, Pr) results in the evolution of band D2 that appears insensitive under reducing gas conditions at 450oC, thereby supporting the literature view of its assignment to units containing coordinatively unsaturated sites. For the first time, in situ Raman spectroscopy is used for perceiving the temperature dependent reversible evolution of the anionic sublattice structure of ceria-zirconia mixed metal oxides and for identifying the release/refill of oxygen from interstitial sites as being of primary significance at low (e.g. below 500oC) temperatures. In situ Raman spectroscopy and oxygen storage capacity studies are proven suitable techniques to investigate the bulk redox properties of CeO2/ZrO2based materials. The oxygen release/refill function at low temperatures (e.g. 450oC) is based primarily on the mobility/lability of oxygen atoms in interstitial sites.

Acknowledgments The European Regional Development Fund, the Republic of Cyprus and the Research Promotion Foundation of Cyprus are gratefully acknowledged for their financial support through the ΤΕΧΝΟΛΟΓΙΑ/ΘΕΠΙΣ/0311(ΒΕ) project. 40 ACS Paragon Plus Environment

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Supporting Information. Powder XRD patterns of Ce1-xZrxO2-δ (U) and Ce0.8Zr0.15RE0.05O2-δ (CA); in situ Raman spectra and peak analysis for CeO2 at 450oC under 20%O2/He(g) and under 5%H2/He(g); OSC at 500-800oC for Ce0.5Zr0.5O2-δ (CA) and Ce1-xZrxO2-δ (U);; in situ Raman spectra for TiO2 at 50 and 450oC under 20%O2/He(g)

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Structural and Redox Properties of Ce1-x

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