Oxygen Uptake of Tb–CeO2: Analysis of Ce3+ and Oxygen Vacancies

Article pubs.acs.org/JPCC

Oxygen Uptake of Tb−CeO2: Analysis of Ce3+ and Oxygen Vacancies Anita M. D’Angelo,† Amelia C. Y. Liu,‡ and Alan L. Chaffee*,† †

Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC), School of Chemistry, and ‡Monash Centre for Electron Microscopy and School of Physics and Astronomy, Monash University, Clayton, VIC 3800, Australia S Supporting Information *

ABSTRACT: In response to concerns about the world’s generation of greenhouse gases, there are incentives to develop lower energy alternatives to cryogenic air separation. Oxygen sorbents are one such alternative in which high oxygen uptakes are achieved through engineering materials with a high number of oxygen vacancy defects. For a series of Tb−CeO2 mixed oxides, the oxygen uptake was determined using thermogravimetric analysis (TGA). Electron energy loss spectroscopy (EELS) was used to investigate the relative oxygen vacancy concentration and complement those findings obtained using TGA. Gas switching experiments conducted at 700 °C show that a higher oxygen uptake was obtained when 30% Tb was incorporated into the lattice (76 μmol·g−1) in comparison to when 10% Tb was introduced (21 μmol·g−1). The O IB/IC ratio, indicative of oxygen vacancies, was found to increase with increasing Tb content, and Raman spectroscopy demonstrated that these defects were introduced with the addition of Tb. The increase in oxygen uptake with increasing Tb was attributed to the introduction of Tb generating vacancies and increasing the materials reduction ability. It was also observed that for a standard CeO2 sample the Ce IM5/IM4 and O IB/IC ratios varied depending on whether spectra were obtained from a collection of smaller crystallites or a single crystallite.

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INTRODUCTION Ceria (CeO2) is widely utilized in catalysis,1 gas sensors,2 and fuel cells3 due to its redox ability allowing it to change between Ce4+/Ce3+ valence states and ability to uptake/release oxygen reversibly. These properties may be exploited so that through changing the surrounding oxygen partial pressure, CeO2 can be used as a reversible oxygen adsorbent for air separation. Currently air separation is primarily carried out cryogenically and requires a higher energy consumption than adsorbent based systems; however, the oxygen purity is limited to 95% using nitrogen selective adsorbents.4 Consequently oxygen selective adsorbents may offer the potential to provide higher oxygen purity with a reduced energy penalty, relative to current practice. As the oxygen uptake is dependent on the presence of stable Ce3+ and vacancies, the oxygen uptake of CeO2 can be increased by maximizing these active species. Vacancies act as potential adsorption sites as materials possessing a greater number of defects have a higher oxygen storage capacity (OSC). For example, in the work of Mamontov et al.5 the oxygen vacancy concentration in CeO2 was 10% at 600 °C which decreased to 5% when the material was aged at ∼760 °C. © XXXX American Chemical Society

The oxygen vacancy concentration was correlated to significant changes in the OSC, where before the decrease in vacancy concentration at 750 °C, the OSC was 217 μmol·g−1 and decreased to 62 μmol·g−1 at 800 °C. Theoretically, for every vacancy that is present, there are two Ce3+ required to balance the charge. To encourage the formation of reactive species, the inclusion of a dopant can be used to introduce lattice defects, the nature of which varies with dopant and its concentration. For Sm doped CeO2, at 98.5%), Tb(NO3).5H2O (Aldrich, 99.9%), and urea (15× the total ion concentration, Sigma-Aldrich, 99−100.5%) in deionized water (0.1 mol L−1 solution). With vigorous stirring, the solution was heated to 90 °C for 8 h and the precipitate filtered and washed with water and then ethanol before drying in an oven at 90 °C overnight. Calcination was carried out under a nitrogen atmosphere, heating at 2 °C min−1 to 700 °C and then holding for 2 h. To prepare CeO2, Ce(NO3)3·6H2O (Aldrich, 99%) was used as the cerium salt, and the procedure for the Tb−CeO2 mixed oxides was followed. Transmission Electron Microscopy. TEM images were obtained on a JEOL JEM-2100F FEG TEM operated at 200 kV. Samples were prepared by dispersing in n-butyl alcohol (ChemSupply, > 99%) and ultrasonicating before dipping a holey carbon film (400 mesh Cu) into the solution. EELS results were acquired in TEM diffraction mode from a Gatan 776 Enfina 1000 parallel detection EELS spectrometer using a 40 μm condenser aperture, spot size 1, dispersion 0.1 eV/ channel, a 3 mm collection aperture, and a 20 cm camera length. The collection angle was β ≈ 21 mrad and α ≈ 13 mrad. Under these conditions the fine structure of the O and Ce edges was observed to be stable indicating that beam damage was minimized.19 Each spectrum was obtained by converging the beam onto a cluster of ∼10−20 crystallites in random orientation and collecting an EEL spectrum in diffraction mode using an acquisition time of 5 s summing 20 spectra. In total ∼10 spectra were obtained from different areas on each sample. The probability of plural scattering was minimized by sampling from a thin layer of ∼5 nm nanoparticle crystallites. The relative thickness of areas sampled in this way was estimated to be 0.2 of the inelastic mean-free-path from the low loss spectra.20 Thus, to minimize electron dose, only the core loss spectra were recorded from each area. This sampling method minimized cation reduction and the creation of oxygen vacancies that can occur through use of an intense probe and prolonged beam exposure. It is not impossible though that some reduction may still occur due to the reducing nature of the microscope environment. Furthermore, obtaining data from a collection of crystallites in random orientation also allowed for any orientation dependence variation to be eliminated.21 After background subtraction using the power-law technique, the integrated white line intensities were determined using a 16 eV window for Ce M5,4 and a 2 eV window for the O IA,B,C edges. Due to the low thickness of material, plural scattering was not removed. The consistent method of sampling and uniform crystallite size minimized any variation in plural scattering and ensured the integrity of comparative results. The integrated peak intensities were used to account for peak and instrumental broadening 22 and reduce errors.23 Errors associated with the white line ratios were evaluated by calculation of the standard error of the mean, allowing for the average ratio to be calculated after taking into account the standard deviation and quantity of crystallite clusters sampled. The experimental method used in this work is consistent with previous analyses.11,24,25 Raman Spectroscopy. Defects in the materials were investigated using a Renishaw Invia Raman spectrograph with an excitation wavelength of 633 nm from a HeNe source.

The analysis of the Ce and Tb valence states in Tb−CeO2 materials have been analyzed by other authors using low vacuum techniques such as EELS, X-ray absorption spectroscopy (XAS), and X-ray photoelectron spectroscopy (XPS). Oxidation states of Ce cations can be derived from EELS data by calculating the ratio between the M5 and M4 white lines, where an increase in the Ce M5/M4 ratio indicates an increase in the Ce3+ component.9 An increase in the vacancy concentration can also be determined from an increase in the O IB/IC ratio; the edges are due to transitions from the O 1s → O 2p band hybridized with the Ce 5d-eg (peak “B”) and 5d-t2g (peak “C”) levels.10,11 Without the use of standards, qualitative information can be obtained by comparison of the relative change in specimen ratios.12 EELS studies have been carried out on Ce1−xTbxO2−x sintered materials with %Tb = 0.1, 0.25, and 0.5 by Ye et al.7 They reported the Ce M5/M4 ratio increased from 0.66 for CeO2 to 0.72 when the %Tb = 0.1, 0.25, and 0.5. The O IB/IC ratio increased from 0.64 for CeO2 to 0.77 for Tb = 0.1, 0.80 for Tb = 0.25 and 0.90 for Tb = 0.5. Balaguer et al.13 reported that XPS experiments indicated there was a higher amount of surface Tb4+ in cobalt (2 mol %) doped Ce0.8Tb0.2O2 (60.5%) compared to undoped Ce0.8Tb0.2O2 (54%). From X-ray absorption near edge structure (XANES) measurements at 10−8−10−9 Torr, a Ce0.9Tb0.1O2 material was concluded to possess mostly Tb cations in the +3 oxidation state, while Ce0.5Tb0.5O2 had a greater Tb3+ amount than TbO1.7. It was concluded that the number of oxygen atoms located near Tb is lower for Ce0.5Tb0.5O2 compared to ́ Ce0.9Tb0.1O2. Martinez-Arias et al.14 reported in XPS studies 4+ that the amount of Tb , where f(Tb4+) = (Tb4+/(Tb3+ + Tb4+), at the particle surface decreased from 0.60 to 0.35 for Ce0.8Tb0.2O2 to Ce0.5Tb0.5O2, respectively. Also, Li et al.15 reported the XPS data obtained for Ce1−xTbxO2 materials with %Tb = 0.1, 0.17, 0.29, and 0.38 and suggested that only Tb3+ cations were present. Tb cations may also preferentially adopt the +3 oxidation state in CeO2 to reduce the lattice strain energy as the ionic radius of Tb3+ (1.04 Å) is closer to the size of Ce4+ (0.97 Å) compared to Tb4+ (0.88 Å).16 EELS has been used quantitatively to determine the valence states of cations in metal oxides. Various methods, including the white line ratio method, have been studied by Tan et al.12 for determining oxidation states of cations in V-, Mn-, and Fe based oxides. In their work, different white line ratios were obtained for oxides with Fe in the +3 oxidation state, and the ratio was shown to increase with increasing sample thickness. Schmid and Mader17 used EELS to determine the cation oxidation state in Mn and Fe doped ZnO films through calculation of the white line ratio and the energy difference between the two white line positions. A series of reference materials were analyzed, and the white line ratios were plotted as a function of oxidation state. They found that only Mn2+ replaced Zn2+ cations in the films, whereas Fe3+ cations were postulated to form nanoclusters. Müller et al.18 also used the energy difference between Co white lines to determine the Co valence state in a Ba0.5Sr0.5Co0.8Fe0.2O3−δ perovskite. The aim of this study was to establish qualitative trends between the relative change in bulk vacancy and Ce 3+ concentration, oxygen uptake, and %Tb content for a series of Tb−CeO2 oxide materials with Tb = 10, 20, and 30%. The oxygen uptake was determined using TGA. EELS and Raman spectroscopies were used to further understand the defect nature of these materials which can lead to high oxygen uptakes necessary for oxygen sorbents. B

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allows it to uptake more oxygen. The increase in Tb can be attributed to result in an increase in the oxygen uptake and also increase the materials ability to lose oxygen, i.e., undergo reduction. Analysis of Vacancies Using Raman. Vacancy introduction from Tb introduction was investigated using Raman spectroscopy. The laser penetration depth has been shown to vary with wavelength for rare earth oxides due to optical absorption.31 A 633 nm laser was selected to reduce absorption and increase the penetration depth based on diffuse reflectance UV−vis data (Figure S2), consequently providing both surface and bulk information. Figure 2 shows that the Raman spectra of the Tb−CeO2 mixed oxides possess a peak at 460 cm−1 due to the

Samples were prepared by focusing the laser on the powder using a ∼1 μm spot size and laser power of 1 mW. Spectra were obtained between 2000 and 200 nm. Peak fitting of the defect mode was carried out using a Lorentzian function in OriginLab.26 Thermogravimetric Analysis. To quantify oxygen uptake, gas switching experiments were carried out using a Mettler Toledo TGA/DSC 1. Samples were weighed (∼15 mg) into an alumina crucible (150 μL) and heated to 700 °C in a flow of nitrogen (35 mL·min−1) before instrument air was introduced (35 mL·min−1) after which nitrogen was then reintroduced. The weight increase when the gas was switched to instrument air was used to calculate the oxygen uptake.

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RESULTS AND DISCUSSION Oxygen Uptake Using TGA. The oxygen uptake of the prepared powders was measured at 700 °C in gas switching experiments using TGA. When the gas flow was switched from nitrogen to air an uptake of 21 μmol·g−1 was observed for Tb = 10% which increased to 45 μmol·g−1 for Tb = 20% and 76 μmol·g−1 for Tb = 30%. When the gas was then switched from air back to nitrogen, a weight decrease was observed as oxygen was desorbed. Figure S1 shows that the loss of oxygen occurs gradually indicating the rate of oxygen uptake was greater than the rate at which oxygen is able to be removed from the lattice. Oxygen uptake was concluded to be fully reversible at this temperature as, after the gas was switched back to nitrogen, the weight returned to its original level. Results are summarized in Figure 1 showing the relationship between uptake and Tb level;

Figure 2. Raman spectra of Tb−CeO2 with Tb = 10, 20, and 30% showing the F2g mode at 460 cm−1 and defect band characteristic of vacancies. The bottom spectra of 30% Tb−CeO2 show the fitted peaks (green), the cumulative fit peak (black), baseline (blue), and original spectra (red).

symmetrical stretch of the fluorite structure (F2g mode) and a wide asymmetric defect band centered at ∼585 cm−1.32,33 For a Gd doped CeO2 system the defect mode can be resolved into peaks; the peak at 550 cm−1 is attributed to defect spaces containing oxygen vacancies and the peak at 600 cm−1 to a MO8-complex with an eightfold coordinated dopant cation and no oxygen vacancies.34 In the present case, peak fitting using a Lorentzian function suggests the peak to be comprised of modes at 570 and 600 cm−1.35 A band at ∼590 cm−1 in CeO2 nanocrystal spectra was observed by Wu et al.36 and proposed to be due to vacancy-interstitial (Frenkel-type) defects. Anion Frenkel defects have been shown to have the lowest disorder formation energy in comparison to Frenkel or Shottky defects.37 This defect band is not present for pure CeO2 indicating it was introduced with the addition of Tb. The present spectra also show that the F2g mode becomes broader with increasing Tb content, a feature which has been attributed to an increase in vacancy concentration and lattice strain in doped CeO2 and CeO2−δ nanoparticles.32,38 Thus, the creation of vacancies through the addition of Tb is supported by the presence of this defect band in Tb−CeO2 spectra. Analysis of Vacancies Using EELS. The background subtracted Ce M5,4 EELS spectra of the purchased CeO2 and CePO4 collected in TEM diffraction mode are shown in Figure 3a. Both CeO2 and CePO4 were used as representative materials where all cations were in the +4 and +3 oxidation states, respectively. Both profiles consist of two prominent edges (M5 and M4) that arise from spin−orbital splitting of 3d

Figure 1. Oxygen uptake of the Tb−CeO2 oxides at 700 °C as determined by gas switching experiments using TGA.

it is clear that as the amount of Tb is increased the oxygen uptake also increased. Within the lattice Tb cations can be considered to exist in both the +3 and +4 oxidation states.27 The presence of Tb3+ introduces vacancies which can uptake oxygen; however, complete oxidation of all Tb3+ cations may be difficult.28 Prior to heat treatment, the material with the highest Tb content was considered to possess the greatest portion of Tb4+ cations, which can undergo reduction and generate Tb3+. The ability of Tb4+ cations to be reduced under these conditions arises as oxygen can be readily removed from the lattice under a flow of inert gas, as well as in oxidizing atmospheres.8 Slow reduction is possible below 500 °C; however, the rate of reduction has been shown to increase above 500 °C.29 At room temperature a pO2 of 10−30 atm is required to reduce Ce4+ cations in CeO2.30 As there is a greater quantity of Tb4+ cations in the 30% material, which can undergo reduction, this in turn C

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Figure 3. (a) Ce M5,4 white lines of standard CeO2 and CePO4. (b) Ce IM5/IM4 and O IB/IC intensity ratios of the CeO2 and Tb−CeO2 mixed oxides.

Figure 4. (a) TEM image of Ce0.9Tb0.1O2 and (b) Ce0.7Tb0.2O2.

Figure 5. (a) Ce M5,4 white lines of the Tb−CeO2 mixed oxides with Tb = 10, 20, and 30%. (b) O K (1s) near-edge fine structure of a standard CeO2 and CePO4.

the relative proportion of valence states for Ce. 11,41 Consequently, CeO2, with a greater portion of Ce4+, possesses a Ce IM5/IM4 ratio lower than that of CePO4. Furthermore, changes in the white line ratio are also affected by changes in the local structure surrounding the metal cation; for example, the Mn L3/L2 ratio was shown to vary for perovskite manganese oxides containing Mn with the same oxidation state.42 The integrated Ce M5,4 edges were used to calculate the Ce IM5/IM4 white line ratios, and the data are presented in Figure 3b and summarized in Table S1. The data plotted with error bars are included in Figure S3 and Figure S4. Under the sampling conditions employed, the Tb white line ratio could

core electron states, 3d5/2 (M5), and 3d3/2 (M4) and are the result of transitions to unoccupied 4f states (3d104f n → 3d94f n+1 39 ). The M4 edge intensity decreases as the proportion of cations in the +3 oxidation state increases. In the initial 4f level there is one electron for Ce3+ (nf = 1, where for Ce4+, nf = 0) to result in the observed decrease in M4 edge intensity.22 In addition, the CePO4 profile shows a small shoulder on the lower energy side of both M5 and M4 edges whereas CeO2 possess satellites on the higher energy sides of these two edges, which are the result of transitions to 4f states.40 These Ce M5,4 edges are indicative of f shell occupancy and are hence sensitive to changes in valence state, so that they can be used to indicate D

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Figure 6. (a) O IA/IC intensity ratios of the Tb−CeO2 mixed oxides with Tb = 10, 20, and 30%. (b) O K (1s) near-edge fine structure of the Tb− CeO2 mixed oxides.

Figure 5b. In the ground state CeO2 has a fully occupied O 2p band which hybridizes with the metal 4f orbitals. As these are partially occupied with some p-symmetry, this results in transitions from O 1s to the p-like component of the metal 4f orbitals.47 In the CeO2 spectra, the observed feature labeled “A” is the result of these transitions. With an increasing amount of Ce3+, the lowest 4f level is fully occupied so transitions are not possible and result in a decrease in the intensity of feature A. Consequently the presence of the peak A in CeO2 is evidence of cations in the +4 oxidation state, and this feature is nonexistent when all cations are in the +3 oxidation state, as for CePO4. The integral intensity ratio between the two features (IA/IC) can be used to indicate the Ce4+ fraction as a decrease in the feature A results from an increase in the density of the 4f state. By increasing the %Tb in the material, the O IA/IC decreases as there is a reduction in the fraction of Ce4+.7 The integrated IA/IC for standard CeO2 was calculated as 0.56 and decreases with increasing doping (Figure 6a). In the CeO2 spectra, the peaks marked “B” and “C” arise from transitions from O 1s to the O 2p band hybridized with the Ce 5d-eg and 5d-t2g levels, respectively.10 The O K (1s) near-edge fine structure is sensitive to changes in the local structure surrounding the metal cation.48 Doping alters the metal-O bond length, distorting the crystal structure and bonding configuration, which consequently affects the O 1s → 2p (hybridized with 5d-eg and 5d-t2g) transition.49 The local environment surrounding the Tb cation differs compared to that of the Ce cation. An increase in the intensity ratio between the two peaks labeled B and C can be used to indicate the presence of oxygen vacancies.25 In a similar Ce1−xPrxO2−δ material, vacancies were reported to result in a decrease in the peaks labeled A and C as the amount of Pr increased.50 The O fine structure changes in the presence of oxygen vacancies as the metal cation−oxygen anion charge transfer is altered.51 Spectra of the Tb−CeO2 mixed oxides are presented in Figure 6b, and with increasing Tb all the features in the near-edge region (A, B, and C) broaden and the feature A noticeably decreases in intensity. Broadening of the fine-structure features has been reported to be due to an increase in vacancy concentration.52 The IB/IC ratio was determined for standard CeO2 as 0.67, synthesized CeO2 as 0.74, and for the oxides with Tb = 10, 20, and 30% levels the IB/IC ratio were 0.80, 0.84 and 0.85, respectively. This increase in IB/IC was attributed to an increase in vacancies as for every two Ce3+ that are present, there is theoretically one vacancy for charge balance.

not be measured due to the high binding energy of Tb (M4 = 1275 eV, M5 = 1241 eV).43 The pure Ce4+ and Ce3+ states of the calibration specimens CeO2 and CePO4 gave Ce IM5/IM4 ratios of 0.69 and 0.83, respectively. There are however discrepancies within the existing literature of the Ce IM5/IM ratio for CeO2, which likely arise from variations in the Ce valence state due to differences in the crystallite size and surface terminations.44,45 The effect of crystallite size on the Ce IM5/IM4 ratio is discussed further in the text below. In our work, as data was taken from a variety of crystal clusters, and the average crystallite size for all materials ranged from 8 to 10 nm (Figure 4a and Figure 4b), variations in the Ce IM5/IM4 ratio due to size differences were considered to be minimal. When Tb is incorporated into the lattice, regardless of the quantity, all profiles possess the white lines characteristic of Ce predominantly in the +4 oxidation state (Figure 5a).19 Through the introduction of Tb into CeO2, the Ce IM5/IM4 ratio increases from 0.67 for synthesized CeO2 to 0.74 when Tb = 10%. These values are similar to those obtained by Ye et al.25 for a series of for Tb doped CeO2 samples, who found that Ce IM5/IM4 ratio increased from 0.67 for pure CeO2 to 0.72 when Tb = 10%. The Ce IM5/IM4 ratios of the Ce M5,4 edge for Tb− CeO2 mixed oxides with Tb = 10, 20, and 30% were 0.74, 0.75 and 0.75, respectively. These ratios are higher than that of CeO2 and may be attributed to modification of the local environment surrounding the Ce cations due to the introduction of Tb. We attribute the increase in the Ce IM5/ IM4 ratio following Tb addition to an increase in the amount of Ce3+, as did Sharma et al.46 for CeO2 doped with Eu. In their work, the addition of 10% Eu resulted in the generation of Ce3+ from the reduction of Ce4+. Higher amounts of %Eu were proposed to result in Eu3+ cations replacing both Ce4+ and Ce3+ cations. The trend in the Ce IM5/IM4 ratio observed in this work is in line with previous measurements by Garvie and Buseck19 that were obtained following removal of the plural scattering component. In their work a qualitative increase in the Ce IM5/IM4 ratio from 0.66 to 0.75 corresponded to a decrease in the Ce4+ fraction from 100% to 67% and suggests a substantial increase in vacancy concentration. Our results however show that there is an increase in the Ce3+ fraction when Tb is introduced, but no significant increase in the Ce3+ fraction when the %Tb is increased from 10% to 30%. The O K (1s) near-edge fine structure of CeO2 shows three features present at ∼533 eV (a), 535 eV (b), and 540 eV (c) in E

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to the type of crystallite facet.55 Smaller crystallites are also prone to beam damage/reduction to result in a greater amount of cations in the +3 oxidation state.56 In this study we sampled from a collection of crystallites using a spread beam to allow for the average ratio to be determined and so that the effects from beam damage, nanoparticle size, and morphology would be eliminated. This resulted in more reproducible results with lower variance. The higher Ce IM5/IM4 ratio of 0.69 for the purchased CeO2, compared to 0.67 for the synthesized CeO2, can be attributed to a greater +3 component.

It has been suggested for a similar Gd−CeO2 system that electrical charge balance cannot be used to solely explain the introduction of oxygen vacancies from doping. Excess vacancies are thought to be stabilized by a combination of low association energies, high dopant concentrations, and distribution of dopant cations in the lattice.53 For all Tb samples, the IB/IC was higher than the CeO2 indicating that the introduction of Tb increases the vacancy concentration. Although the increase in vacancies is gradual, overall, in comparison to the standard CeO2 and synthesized CeO2, when Tb is introduced into the lattice there is an increase in the oxygen vacancy concentration. Ye et al.25 also determined the O IB/IC ratio which increased from 0.64 for pure CeO2 to 0.77 and 0.9 for 10% and 50% Tbdoped CeO2, respectively. These results emphasize the defect nature of Tb−CeO2 oxides which lead to their oxygen uptake behavior. Effect of Sampling Area on Ce and O Intensity Ratios. The CeO2 data presented here were calculated from data obtained from a collection of sub-10 nm crystallites (Figure 7,

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CONCLUSIONS Terbium−CeO2 mixed oxides have the potential to be used as reversible oxygen sorbents due to their ability to change oxidation states with a change in oxygen partial pressure. The ability of these materials to uptake oxygen is governed by the concentration of defect sites/vacancies in which an increase in vacancies results in an increase in the oxygen uptake. In this study, TGA was used to determine the oxygen uptake of a series of Tb−CeO2 mixed oxides. At 700 °C the oxygen uptake increased with increasing Tb content; an uptake of 21 μmol·g−1 was obtained for the 10% Tb and increased to 76 μmol·g−1 when 30% Tb was incorporated into the lattice. Through Raman spectroscopy the presence of defects were found to be the result of Tb introduction into the CeO2 structure and suggested the existence of both metal−oxygen complexes with and without vacancies. EELS was used to qualitatively investigate the change in Ce3+ and oxygen vacancy concentration through calculation of the respective Ce IM5/IM4 and O IB/IC ratios. Our results show that oxygen vacancies are introduced by Tb and the vacancy concentration increases with increasing Tb content. There however was no apparent increase in the Ce3+ concentration with increasing %Tb. By incorporating more Tb into the structure, the vacancy concentration was found to increase. Thus, oxygen uptakes exhibited by these materials can be attributed to an increase in Tb cations which can undergo reduction and create vacancies. An increase in the presence of these active species, when higher levels of Tb are introduced into the lattice, can allow for higher oxygen uptakes. Through determining the vacancy sites available for oxygen uptake, these results may be used to improve and optimize material design for oxygen separation applications.

Figure 7. TEM image distinguishing a collection of sub-10 nm crystallites (region a) from single crystallites (regions b and c).

region a) rather than a large or small single octahedral crystallite (Figure 7, region b and c). All the CeO2 crystallites appeared to have either octahedral morphology (eight {111} facets) or truncated octahedral morphology (eight {111} and six {100} facets).54 Depending on the region sampled, the Ce IM5/IM4 and O IB/IC ratios varied. For example, if spectra were obtained from a collection of ∼10−20 small crystallites, the Ce IM5/IM4 ratio was 0.69, while for the spectra obtained from a large or small single crystallite it was determined as 0.80. A ratio of 0.80 was similar to that of 0.83 obtained for CePO4 indicating a higher amount of cations in the +3 state. The IB/IC ratio determined from the collection of crystallites and single crystals was 0.67 and 0.92, respectively. Differences in the Ce IM5/IM4 and O IB/IC ratios may be attributed to variations between average crystallite sizes and contributing to variability in the measured valence state. Turner et al.44 used EELS to determine the valence state in CeO2 truncated octahedral particles and found that smaller 4 nm particles had a greater Ce3+ component in comparison to 30 and 60 nm particles. Bulk CeO2 was found to have the highest amount of Ce4+, and the smallest particles (2 nm) were predominantly in a reduced state with most cations in the +3 state. The extent of reduction was also shown to vary according

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b04063. TGA, DR UV−vis, EELS ratio graphs with error bars, and tabulated data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: alan.chaff[email protected]. Telephone: +61 3 9905 4626. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support provided by the Australian Government through its Cooperative Research Centre program and through the Australian National Low F

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Emissions Coal Research Development (ANLEC R&D) scheme. ANLEC R&D is supported by Australian Coal Association Low Emission Technology Limited and the Australian Government through the Clean Energy Initiative. We also gratefully acknowledge the facilities within the Monash Centre for Electron Microscopy and Finlay Shanks for the Raman spectroscopy data.

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DOI: 10.1021/acs.jpcc.6b04063 J. Phys. Chem. C XXXX, XXX, XXX−XXX