Vacancy Generation and Oxygen Uptake in Cu-Doped Pr-CeO2

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Vacancy Generation and Oxygen Uptake in Cu-Doped Pr-CeO2 Materials using Neutron and in Situ X‑ray Diffraction Anita M. D’Angelo,† Nathan A. S. Webster,‡ and Alan L. Chaffee*,† †

Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC), School of Chemistry, Monash University, Clayton, Victoria 3800, Australia ‡ CSIRO Mineral Resources, Private Bag 10, Clayton South, Victoria 3169, Australia S Supporting Information *

ABSTRACT: The oxygen uptake ability of Pr-CeO2-based oxygen carriers, catalysts, and solid oxide fuel cells can be attributed to 3+ cation generation and the presence of vacant oxygen sites. Oxygen occupancies of CeO2, Pr-CeO2, and 5% Cu-doped Pr-CeO2 were investigated using neutron diffraction and related to the oxygen uptake as determined using thermogravimetric analysis (TGA). The presence of vacant tetrahedral oxygen sites at room temperature did not correspond to low-temperature oxygen uptake. The materials did not uptake oxygen at 420 °C, but oxygen uptake was observed at 600 °C, which indicated that a minimum temperature needs to be met to generate sufficient vacancies/3+ cations. Variations in the lattice parameter as a function of temperature were revealed using in situ X-ray diffraction (XRD). With increasing temperature the lattice parameter increased linearly due to thermal expansion and was followed by an exponential increase at ∼300−400 °C as cations were reduced. Despite segregation of Cu into CuO at high dopant concentration, at 600 °C a higher O2 uptake was obtained for Ce0.65Pr0.20Cu0.15O2−δ (120 μmol g−1), in comparison to Ce0.75Pr0.2Cu0.05O2−δ (92 μmol g−1), and was higher than that for Ce0.8Pr0.2O2−δ (55 μmol g−1). Both Pr and Cu introduce vacancies and promote the O2 uptake of CeO2.

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oxidation.6 The pO2 value is higher than that required for oxidation of Ce3+ to Ce4+, which at room temperature is 10−30 atm.7 Reversible oxygen uptake is achieved as Pr can switch between the 4+ and 3+ oxidation states with reasonable changes in pO2.8 However, a change in unit cell size is expected. The cubic fluorite structure of Pr-CeO2 also ensures that the crystal lattice can undergo a high degree of oxygen nonstoichiometry without structural loss.8 Pr facilitates oxygen exchange between the gas and solid phase9 and encourages vacancy formation and surface reducibility, thus improving the OSC of CeO2.10 These properties allow us to exploit the differences in the size of Pr4+ (0.96 Å) and Pr3+ (1.13 Å)11 cations to observe expansion of the unit cell under reducing conditions and contraction under oxidizing conditions. Neutron diffraction can be used to determine the presence of vacancies in bulk CeO2 through the refinement of oxygen occupancy factors. Studies on CeO2 materials are, however, generally focused on the structural effects of doping with Zr and Y, as these materials have catalytic applications. Links have been drawn between the defect concentration and oxygen storage capacity of a ZrCeO2 material by Mamontov et al.12

INTRODUCTION Maximizing the oxygen storage capacity (OSC) or oxygen uptake is a primary requirement and driving force in CeO2based solid oxide fuel cell (SOFC) and catalyst development. Materials exhibiting high OSC can be synthesized by introducing oxygen vacancy defects, which are typically achieved through the addition of lower valent substituents.1 Dopant cations weaken or form undercoordinated bonds with oxygen ions, thus lowering the reduction temperature and increasing the OSC/oxygen uptake.2 In particular, the addition of Pr to CeO2 introduces Pr3+ cations, which have the potential to be oxidized and create vacancy defects. These vacancies are primarily located around the Pr3+ cations where a local distortion is created, weakening the oxygen bonds surrounding Pr3+ and facilitating lattice oxygen availability and reduction.3 The ability to remove oxygen from the lattice (i.e., lower the reduction temperature) is enhanced.4 Consequently, the energy required for oxygen removal from the crystal lattice is reduced as the oxygen vacancy formation energy is higher for undoped CeO2 than for CeO2 doped with a trivalent or tetravalent ion.5 This is possible with the addition of Pr to CeO2, as the Pr4+ cation is more readily reduced in comparison to Ce4+. At 700 °C and an oxygen partial pressure (pO2) of 10−1 atm, Ce0.8Pr0.2O2−δ possesses a mixture of both Pr3+ and Pr4+ cations and requires pressures of pO2 >10−7 atm for Pr3+ © XXXX American Chemical Society

Received: June 23, 2016

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DOI: 10.1021/acs.inorgchem.6b01499 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

XRD allowed for direct measurement of lattice expansion/ contraction of a Cu-doped Pr-CeO2 mixed oxide at 600 °C. Further insights into vacancy generation were obtained through room-temperature neutron diffraction by refinement of the oxygen occupancies. Understanding the structural changes that occur under oxidizing and reducing conditions of Pr-CeO2based oxides can be used to develop materials which exhibit both fast adsorption/desorption kinetics and high OSC, both of which are necessary requirements for oxygen carriers, catalysts, and solid oxide fuel cells.

They found that by increasing the aging temperature from 500 to 850 °C there was an increase in the OSC. For pure CeO2 the defect concentration decreased when there was an increase in the aging temperature. Analysis of Y-CeO2 at 900 °C and between 10−1 and 10−18 atm by Li et al.13 showed that there was a linear relationship between the vacancy concentration and lattice parameter. Biphasic models have also been used for defect structure analysis of Y-CeO2, indicating that the mesoscopic structure can consist of Y2O3 C-type structure domains.14 Oxygen diffusion in CexZr1−xO2 materials at high temperatures have been extensively studied by Yashima.15 At 750 °C oxygen diffusion in Ce0.5Zr0.5O2 was found to occur along a curved path in the ⟨100⟩ and ⟨110⟩ directions, and movement in the ⟨111⟩ direction was inhibited as anions would be too far away from the cations, making them unstable. Anion diffusion was also shown to occur more readily than for cations due to their higher atomic displacement parameter.16 Although diffusion is unfavorable in the ⟨111⟩ direction, at 1559 °C, anions may begin to shift in this direction to the more spacious 4b (1/2, 1/2, 1/2) site.17 Neutron diffraction investigations into the effect of the rare earths Tb and Pr appear to be limited; however, analysis of a series of Tb-CeO2 and Pr-CeO2 materials was carried out by Coduri et al.18 The refined occupancies of the materials with % Pr = 12.5, 15, 37.5, and 50 in CeO2 were 0.995, 0.989, 0.998, and 0.994, respectively, and it was concluded that the disorder in these materials was low. Generally work on the rare earths has focused on Yb, Y, Nd, and La, and their addition to CeO2 has been shown to increase lattice disorder using the pair distribution function (PDF) technique.19 Furthermore, the structural changes of La-doped CeO2 have been investigated under oxidizing and reducing conditions.20 Oxides analyzed using neutron scattering that contain Pr, Cu, and Ce cations generally focus on the material’s superconductivity property.21 In addition to neutron diffraction, the structural properties of Pr-CeO2 and Cu-CeO2 materials have been studied using XRD: for example, Pu et al.22 used in situ XRD to determine the change in lattice parameter of Ce0.9Pr0.1O2−δ as a function of temperature under O2, He, and H2 atmospheres. Defects in CeO2-based materials may be analyzed using Raman spectroscopy as, in their work, vacancy concentrations were determined by comparison of the defect band to F2g mode intensities. Similar defect analysis methods have been used by other authors to study Pr-CeO2 and Ce0.9Pr0.05Cu0.05O2−δ materials using Raman spectroscopy.23,24 Oxygen occupancy refinement of a series of Cu-CeO2 materials, with data obtained from a synchrotron X-ray source, showed that the addition of Cu increases the vacancy concentration.25 To the best of our understanding there has been little work on the study of oxygen vacancies in Pr-CeO2 materials and, more specifically, Cu/Prcodoped CeO2 systems by refinement of oxygen occupancy factors using neutron diffraction. The aim of this study was to establish a link among vacancy generation and oxygen uptake and lattice expansion/contraction for Pr-CeO2 materials. Cu was also added as a dopant to create greater lattice strain and lower the vacancy formation energy in comparison to a tetrahedrally coordinated cation.2 This understanding is essential to improve the OSC/oxygen uptake of these systems. The oxygen uptake of a series of Cudoped Pr-CeO2 materials was determined by N2−air−N2 gas switching experiments conducted using thermogravimetric analysis (TGA). To explain the observed oxygen uptake of the materials and evaluate the reversibility of absorption, in situ

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EXPERIMENTAL SECTION

Preparation of Cu-Doped Pr-CeO2 Mixed Oxides. Cu-doped Pr-CeO2 containing 15 and 20 mol % Pr were doped with Cu at 5, 10, and 15 mol % using Cu(NO3)2·3H2O (Sigma-Aldrich, 99−104%). All materials were synthesized by coprecipitation from (NH4)2Ce(NO3)6 (Aldrich, >98.5%), Pr(NO3)·5H2O (Aldrich, 99.9%), and Cu(NO3)2· 3H2O (Sigma-Aldrich, 99−104%) by dissolving with urea (15× the total ion concentration, Sigma-Aldrich, 99−100.5%) in deionized water (total ion concentration 0.1 mol L−1). The solution was stirred and heated at 90 °C for 8 h before filtering the precipitate, washing with water and then ethanol, and drying overnight in an oven. Materials were calcined for 2 h at 700 °C under a nitrogen atmosphere by heating from ambient temperature at 2 °C min−1. Electron Microscopy. Images from transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM)-bright field (BF), and high-angle annular dark field (HAADF) were obtained using a FEI Tecnai G2 F20 S-TWIN FEG TEM instrument operated at 200 kV. Samples dispersed in n-butyl alcohol were ultrasonicated before dipping an ultrathin holey carbon film into the solution. Thermogravimetric Analysis. Oxygen uptakes were measured using a Mettler Toledo TGA/DSC 1 instrument. Samples (∼10 mg) were heated at a rate of 10 °C min−1 under a flow of N2 (35 mL min−1) to the analysis temperature (420 or 600 °C). After 100 min under N2, the gas flow was switched to instrument air (35 mL min−1) for 100 min before reintroducing N2. The average oxygen uptake was calculated from the observed weight increase when the gas was switched to instrument air. Samples were analyzed more than once to ensure reproducibility. X-ray Diffraction. In situ XRD data were collected using an Inel Equinox 3000 instrument fitted with a CPS 120 position sensitive detector and a Mo tube operated at 40 kV and 40 mA. Samples were packed into a 0.7 mm quartz capillary and stoppered with glass wool before heating to 600 °C at 10 °C min−1 under a flow of N2. After ∼160 min the gas was switched to instrument air and then, after 100 min, N2 was reintroduced. Data sets were collected continuously during heating and isothermal treatment at 1 min intervals. The evolution of Pr-CeO2 reflection positions as the experiments progressed was visualized by stacking the data sets to produce plots of accumulated data with temperature plotted vs 2θ and viewed down the intensity axis. Data sets were refined using an “hkl phase” in Topas,12 in order to determine the fcc (space group Fm3m ̅ ) unit cell parameter as a function of temperature and time. To determine the diffractometer zero and instrumental effects on peak width and shape, data were collected and refined using a Si internal standard (NIST 640C line position standard). Neutron Diffraction. Data were collected on the ECHIDNA highresolution powder diffractometer at the Australian Nuclear Science and Technology Organisation (ANSTO)26 at room temperature and using a wavelength of ∼1.3 Å and Q (accessible d range, Q = 2π/d) range of 0.34 Å−1 to −9.57 Å−1. Samples were contained within 1/4 in. vanadium cans (1.5 mL). The diffractometer wavelength and instrument zero were refined using a LaB6 standard (NIST standard reference material 660b). All refinements were carried out using GSAS27 with EXPGUI.28 The oxygen occupancy factors, fractional coordinates, and thermal parameters of the cations and anions were refined simultaneously. B

DOI: 10.1021/acs.inorgchem.6b01499 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. STEM-BF images of (a) Ce0.70Pr0.15Cu0.15O2−δ with red arrows indicating surface steps and (b) Ce0.70Pr0.2Cu0.1O2−δ. STEM-HAADF images of (c) Ce0.65Pr0.2Cu0.15O2−δ, (d) Ce0.80Pr0.15Cu0.05O2−δ, (e) Ce0.75Pr0.2Cu0.05O2−δ, and (f) Ce0.75Pr0.15Cu0.1O2−δ with d spacings corresponding to the cubic fluorite structure.

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RESULTS AND DISCUSSION Morphology Characterization. STEM images were obtained to investigate the morphology of the synthesized Cu-doped Pr-CeO2 mixed-metal oxides. As shown in Figure 1a, crystallites were multifaceted with most being ∼10 nm in size. Figure 1b,c shows that aggregates were formed. Surface steps can be seen on some crystallites (Figure 1a) which are generally comprised of low-coordinated and weakly bound atoms.29 Oxygen atoms at these sites may be easier to remove, thus assisting in the reducibility and reactivity of the Cu-doped PrCeO2 materials.30−32 A d spacing of 0.31 nm (Figure 1d,e) is visible and is in agreement with the (111) reflections of the CeO2 cubic fluorite lattice (space group Fm3m ̅ ). Figure 1f shows a d spacing of 0.19 nm, which may be due to either the (220) reflections of CeO2 or a lattice spacing of the segregated CuO. TGA Determination of Oxygen Uptake. Gas switching experiments (N2−air−N2, Figure S1 in the Supporting Information) were carried out to determine the oxygen uptake of the Cu-doped Pr-CeO2 materials at 420 and 600 °C. For all prepared materials, increasing the analysis temperature resulted in an increase in the oxygen uptake; the material with the lowest content of both Pr and Cu (Ce0.80Pr0.15Cu0.05O2−δ) exhibited an uptake of 15 μmol g−1 at 420 °C and 62 μmol g−1 at 600 °C. This was attributed to both vacancies present through Pr cation introduction and the increase in Pr3+ due to increasing temperature. Through density functional theory (DFT) calculations Ahn et al.3 determined that the reduction energy of two Pr4+ cations was 1.83 eV, while 4.38 eV was needed to reduce two Ce4+ cations, indicating that Pr4+ is preferentially reduced over Ce4+. In these materials Pr promotes the O2 uptake of CeO2. Within the lattice Pr readily substitutes for Ce atoms as Ce and Pr cations have similar ionic radii (0.97, 1.14, 0.96, and 1.13 Å for Ce4+, Ce3+, Pr4+, and Pr3+, respectively).11 Electronically and structurally the lattice is modified as vacancy-generated electrons localize around Pr4+ and distort the lattice; this has been attributed to the Jahn−Teller distortion of PrO2−δ.33,34

Charge compensation vacancies act as potential oxygen adsorption sites that are generated with increasing temperature as further Pr4+ → Pr3+ reduction occurs. Abel et al.35 reported for a Pr0.8Zr0.2O2−y material an increase in the Pr3+ content from 25 to 75 atom % as the temperature was increased from 700 to 1200 °C. In this work, oxygen exchange and diffusion may also be encouraged by the addition of Pr to CeO2. Figure 2 shows that oxygen uptake only occurred at 600 °C with little to no increase when the experiment was carried out

Figure 2. Oxygen uptake of Cu-doped Pr-CeO2 at 420 and 600 °C.

at 420 °C. These results imply that Cu acts as a promoter of PrCeO2 to increase the uptake only after a thermodynamic or kinetic barrier has been overcome; oxygen diffusion is limited at the lower temperature. At 600 °C there was an increase in oxygen uptake with increasing Cu content; the uptake was 62 μmol g −1 for Ce0.80 Pr0.15 Cu 0.05O 2−δ, 76 μmol g −1 for Ce0.75Pr0.15Cu0.10O2−δ, and 86 μmol g−1 for Ce0.70Pr0.15Cu0.15O2−δ. Similarly to Pr, the oxygen uptake promoting effect of increasing Cu is thought to result from the generation of charge compensation vacancies.36 Greater lattice distortion and longer bond lengths allow for easier oxygen removal and consequently a higher OSC.37 C

DOI: 10.1021/acs.inorgchem.6b01499 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Addition of Cu2+ to CeO2 can distort the lattice as Cu2+ forms a close to square-planar coordination where cations attempt to maintain the Jahn−Teller distorted 4-fold coordination of CuO.25,38 Greater lattice distortion has been observed for dopants which adopt a square-planar (Pd and Pt ∼1.2 Å from the perfect Ce lattice position) rather than a tetrahedral configuration (Zn and Be ∼1.05 Å).39 In this coordination the three undercoordinated O atoms result in a system with the lowest oxygen vacancy formation energy. Consequently Cu2+ has an O-vacancy formation energy (1.24 eV) comparable to those of Pt2+ (1.20 eV) and Pd2+ (1.22 eV), which is lower than those for Be2+, Mg2+, Ca2+, Sr2+, Ba2+, Ni2+, and Zn2+ cations.2 Alayoglu et al.40 attributed the Pt-promoted redox ability and reversible expansion and contraction of CeO2 to the formation of Ce3+ and vacancies. The Cu content dependence on oxygen uptake indicates that greater lattice strain allows oxygen to be released from the lattice more readily. Although this is considered an effect of Pr4+ reduction, the presence of Ce3+ cannot be excluded as the reduction of Ce4+ to Ce3+ may be facilitated by Pr and Cu. In Cu-promoted CeO2 systems both Ce4+ and Ce3+ can be present to assist in reducibility.41 Overall, vacancies are created through both increasing the temperature and increasing the Cu content, allowing these materials to uptake oxygen. In the present case, after the weight increase that occurred when the gas was switched to air, a weight loss was observed when the gas was switched back to N2. This is primarily attributed to the Pr4+ ↔ Pr3+ conversion. The exhibited O2 uptake of these materials, however, is not completely reversible. For Ce0.65Pr0.20Cu0.15O2−δ at 600 °C, the second N2 segment has a weight higher than that at the end of the first N2 segment, indicating that after air is introduced and cations are oxidized that a portion maintain their 4+ oxidation state (Figure S1 in the Supporting Information). Immediately after the gas was switched back to N2, 75% of the adsorbed oxygen was removed, and after 150 min, 88% of the adsorbed oxygen was removed. Our results do nevertheless indicate that both Pr and Cu can act as oxygen uptake promoters in CeO2. In Situ XRD. The mixed oxide exhibiting the highest oxygen uptake (Ce0.65Pr0.20Cu0.15O2−δ) was selected for the in situ XRD study. The evolution in lattice parameter as a function of temperature and time was determined in gas switching experiments (N2−air−N2) that mirrored those carried out using TGA. Figure 3 shows the plot of accumulated in situ XRD data, viewed down the intensity axis and with temperature

plotted vs 2θ. Expansion of the unit cell size corresponded to a shift to lower 2θ of the cubic fluorite reflections. At 25 °C under N2, the lattice parameter was 5.412 Å and was marginally larger in comparison to 5.411 Å of pure CeO2 (cubic fluorite, space group Fm3m ̅ ) (ICDD No. 00-034-0394). The unit cell of pure PrO2−δ (PrO1.83) is larger (5.47 Å, ICDD No. 00-0060329)42 as a portion of the cations exist in the 3+ oxidation state.43 As the sample was heated to 600 °C, the unit cell increased from 5.412 Å at 25 °C to 5.454 Å at 600 °C (65 min), as shown in Figure 4. With heating there is also a greater increase in the

Figure 4. Variation of the lattice parameter of Ce0.65Pr0.20Cu0.15O2−δ as a function of temperature (in the range 25−600 °C), during the initial temperature ramp under N2 gas.

thermal expansion for Cu-doped CeO2 in comparison to CeO2.36 In the range 25−600 °C the lattice expanded by a total of 0.77% from a combination of thermal expansion and cation reduction.22,44 Below ∼400 °C this increase is linear indicating that lattice cell expansion is primarily due to thermal expansion. Above this temperature it is hypothesized that the exponential increase in lattice parameter is primarily due to the reduction of Pr4+ cations rather than Ce4+. The pO2 value required to switch between Pr3+ and Pr4+ is pO2 ≳10−7 atm at 700 °C,6 and between Ce3+ and Ce4+ it is 10−30 atm at room temperature.7 Therefore, under a flow of N2 the reduction of Pr4+ is considered to be more likely. It also needs to be considered that the generation of vacancies may also decrease the degree of lattice expansion. In the work by Chatzichristodoulou et al.45 it was determined that the radius of an oxygen vacancy was smaller than that of the oxide ion in a metal oxide fluorite structure. During cation reduction, oxygen is evolved, which was evident when a pure PrO2−δ sample was heated under a flow of N2 (Figure S2 in the Supporting Information). Weight losses centered at ∼250, 380, and 460 °C were observed before a larger and more rapid loss at ∼550 °C.46 When 600 °C was reached, the Cu-doped Pr-CeO2 lattice continued to expand (Figure 4) at a rate of 1.5 × 10−4 Å min−1 due to further cation reduction under the flow of N2. After 167 min it reached 5.470 Å. In comparison to a system without vacancies, an increase in lattice parameter occurs with an increase in vacancy concentration.36 When the gas is switched from N2 to air, Figure 5 shows that there is a noticeable change in the lattice parameter concurrent with the weight increase observed in the TGA studies. Here the oxidation of Pr3+ cations results in the lattice contracting because of the smaller size of the generated Pr4+. Over 4 min, a decrease in unit cell size from 5.470 Å (167

Figure 3. Accumulated in situ XRD data, viewed down the intensity axis for Ce0.65Pr0.20Cu0.15O2−δ during the isothermal regime at 600 °C and temperature ramp from 25 to 600 °C. The (220), (311), and (222) cubic fluorite type reflections are labeled. D

DOI: 10.1021/acs.inorgchem.6b01499 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Variation of the lattice parameter of Ce0.65Pr0.20Cu0.15O2−δ as a function of time during the N2−air−N2 gas switching experiment.

Figure 6. Output of Rietveld refinement of room-temperature neutron diffraction data collected for 5% Cu doped PrCeO2. Experimental data are shown as “+” signs; the calculated pattern is denoted as a red solid line, the difference pattern the orange solid line below, and the green line the background model. The vertical tick marks below are the Bragg reflection markers.

min) to 5.461 Å (171 min) can be observed, corresponding to the lattice contracting by 0.16% at a rate of 2.1 × 10−3 Å min−1. From the TGA data this corresponded to 120 μmol g−1 of oxygen taken into the lattice as exposure to an oxidizing atmosphere can readily oxidize both Pr3+ and, if present, Ce3+. When N2 is reintroduced, the lattice parameter gradually increases back to 5.470 Å (292 min) over a 15 min period at a rate of 9.9 × 10−5 Å min−1. The rate of reduction when N2 was reintroduced is lower than the rate of oxidation. Ionic conductivity and diffusion increases with increasing vacancy concentration, until a decrease from oxygen vacancy ordering and vacancy−vacancy repulsion inhibits diffusion.47,48 A decrease in the ionic conductivity can also be due to the effects of dopant−vacancy associations.49 A higher amount of dopant−vacancy interactions can increase the defect cluster concentration, which trap ions and lower the ionic conductivity. In a work by Li et al.,50 the binding energy of vacancy−dopant defect clusters was higher for Yb-CeO2 than for Sm-CeO2, and a lower conductivity was also calculated for Yb-CeO2 in comparison to Sm-CeO2. At low dopant concentrations, larger cations (La3+, Pr3+) were found to prefer to occupy a second neighbor site position relative to the vacancy while smaller cations (Yb3+, Y3+, Dy3+, Gd3+, Sm3+) preferred to occupy a first neighbor site position. They suggested that smaller binding energies may be the result of cations located farther away from vacancies. Oxygen migration through the bulk can be affected by temperature,51 the presence of grain boundaries,52 and edge dislocations.53 Over the ∼100 min when air continues to be passed over the sample, the size of the lattice appears relatively constant at 5.455 Å. Under these experimental conditions, relatively slow lattice expansion occurred over the remainder of the experiment as Pr4+ cations were reduced and oxygen was gradually removed from the lattice. During reduction, oxygen migrates from the bulk to the surface and vacancies are formed as two electrons from the oxygen simultaneously locate in the 4f band of two cerium atoms.54 Oxygen removal is driven by the difference in concentration gradients between the oxygen in the atmosphere surrounding the particles and that at the particle surface. Furthermore, the oxygen removal rate in the Cu-doped Pr-CeO2 materials can be increased by carrying out reduction at a lower pO2, improving the reduction kinetics.55 Oxygen Occupancies: Neutron Diffraction. The output of the Rietveld refinement of the neutron diffraction data collected for 5% Cu-doped Pr-CeO2−δ is shown in Figure 6, and the refined structural parameters for CeO2, Pr-CeO2, and 5% Cu-doped Pr-CeO2−δ are shown in Table 1. The starting model consisted of Ce, Pr, and Cu sharing the 4a (0,0,0) sites,

and the stoichiometric amounts of each cation were input as occupancies into the model which were not refined: i.e., for Ce0.80Pr0.15Cu0.05O2−δ, the Ce, Pr, and Cu occupancies were 0.8, 0.15, and 0.05, respectively. Oxygen anions were put in the 8c (1/4,1/4,1/4) sites. Anion Frenkel defects (i.e., atoms displaced off normal lattice sites into interstitial sites) were considered to be the primary types of defects at room temperature as vacancies, which form through oxygen loss, electron transfer and Ce3+ generation; these would likely be generated at higher temperatures. These have been shown to have the lowest disorder formation energy in comparison to Frenkel or Shottky defects and form from oxygen anion displacement from the tetrahedral 8c site.56 Raman spectroscopy defect bands have also been attributed to Frenkel defects within the CeO2 lattice by Wu et al.57 Frenkel anion pairs were allowed to form by oxygen anions moving into the 48i (0.5, y, y) site.58 Constraining oxygen anions to move into the Fm3̅m 32f (x,x,x) site was also trialed but resulted in the refinement diverging.59,60 Refinement of the 8c site oxygen occupancy of the pure CeO2 returned a value of 1.002, indicating that these sites are fully occupied. The addition of 15% Pr decreased the oxygen occupancy to 0.983, indicating a vacancy increase in the tetrahedral 8c site. Defects are induced by Pr introduction. At room temperature lattice disorder is likely due to displaced oxygen anions that move into an oxygen interstitial site, forming anion Frenkel defects. Although vacancies are unlikely to be formed at room temperature from oxygen desorption to create Ce3+ or Pr3+ cations, a portion of these 3+ cations are considered to still be present due to the nonstoichiometric nature of PrO2−δ and the lower thermal stability of the Pr4+/Pr3+ redox couple.6 Only the refinement details of the 5% Cu containing material are reported as the presence of a second phase can be observed in the 15% Cu pattern with reflections matching those of CuO (space group C2/c, ICDD 00-048-1548; Figure S3 in the Supporting Information). In the latter case, Rietveld-based quantitative phase analysis indicated that the CuO was present at a level of 2.5 wt %. In addition, while CuO reflections were not evident in the neutron diffraction data collected for the 10% Cu sample, SEM revealed the presence of small amounts of faceted crystals (Figure S4 in the Supporting Information). Segregation of Cu into CuO was, however, not detrimental to E

DOI: 10.1021/acs.inorgchem.6b01499 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 1. Rietveld-Refined Structure Parameters of CeO2, Ce0.85Pr0.15O2−δ, and Ce0.80Pr0.15Cu0.05O2−δ from Neutron Diffraction Data sample a (Å) Uiso Ce, Pr, Cu (Å2) Uiso O (Å2) frac coord y in 48i situ Occ (O1) Occ (O2) 48i Rwp R(F2) χ2

CeO2

Ce0.85Pr0.15O2−δ

Ce0.80Pr0.15Cu0.05O2−δ

5.4115(2) 0.0083(5) 0.0117(3) 0.3(2) 1.002(8) 0.000(1) 0.0366 0.0241 0.5369

5.4193(2) 0.0089(5) 0.0124(3) 0.44(2) 0.983(6) 0.003(1) 0.0362 0.0390 0.5407

5.4164(2) 0.0119(5) 0.0125(3) 0.40(2) 0.975(8) 0.003(1) 0.0386 0.0271 0.5125

48i site of CeO2. The highest 8c site oxygen occupancy was obtained for CeO2, followed by Ce0.85Pr0.15O2−δ and then Ce0.80Pr0.15Cu0.05O2−δ. The addition of Pr increased the vacancy concentration and encouraged oxygen exchange and diffusion in CeO2, increasing the oxygen uptake. Uptake was aided by Cu addition through generating vacancies and distorting the lattice as Cu cations try to achieve a square-planar geometry in the 8fold coordination of Ce atoms of CeO2. Despite the presence of vacant tetrahedral sites at room temperature, our results indicate that a minimum temperature must be reached in order for the materials to take oxygen into the lattice. In situ XRD gas switching experiments revealed the change in lattice parameter as a function of temperature and under N2−air−N2 atmospheres. When the samples were heated from 25 to 600 °C under a flow of N2 the unit cell size initially increased linearly due to thermal expansion. As oxygen loss occurred from approximately ∼400 °C, an exponential increase in lattice parameter was then observed. Switching the gas between oxidizing (air) and reducing (N2) atmospheres showed the lattice contracting and expanding as cations were oxidized and reduced. When a constant temperature was maintained under a flow of N2, the lattice continued to increase slowly and was attributed to the reduction of cations. Inspection of the 15% Cu neutron diffraction pattern showed the presence of a secondary phase with reflections matching those of CuO. SEM also revealed that the surface of the 10% Cu material was embedded with faceted crystals of a segregated phase. Despite the presence of this segregated phase, no decrease in the uptake was apparent when gas switching experiments were carried out at 600 °C; the 10% and 15% Cu materials exhibiting segregation had uptakes higher than that of the 5% Cu material without segregation. Understanding the fine structural changes and kinetics of adsorption/desorption is an essential requirement for the use of CeO2-based materials as effective catalysts, SOFC, gas sensors, and oxygen-selective membranes. More specifically, as the exhibited oxygen uptakes indicate that they possess good ionic conductivity, their feasibility as oxygen-selective membranes may be improved by increasing their electrical conductivity. By development of these existing materials using knowledge from current advances in membrane technology, higher conductivities and permeability may be realized. Overall, our results reveal the oxygen uptake/release kinetics of a Cu-doped PrCeO2 system which can be used to develop and engineer materials with higher OSC.

the oxygen uptake of the 10% and 15% Cu doped Pr-CeO2; both had higher oxygen uptakes in comparison to the 5% Cu. It is likely that the solubility limit of Cu in the Pr-CeO2 lattice was exceeded; however, the observed segregation may also be a result of the calcination conditions. Knauth et al.61 found the segregation of CuO between CeO2 grains at 350 °C and pO2 = 20 kPa and the formation of metallic Cu at 450 °C and under 5% H2/H2O/N2. Overall, the 8c site occupancy of 5% Cu doped Pr-CeO2 was 0.975, which was lower than that of the materials without Cu. Despite the presence of vacant 8c tetrahedral sites which our results suggest, increasing the temperature to 420 °C does not lead to oxygen uptake for the 15% Pr materials. These results imply that a specific temperature needs to be reached to generate a substantial amount of 3+ before oxygen uptake is possible. From the in situ XRD data of Ce0.65Pr0.20Cu0.15O2, the generation of 3+ cations commenced at temperatures ≳300− 400 °C. The temperature at which this occurs for the 15% Pr materials is predicted to be higher because of the greater proportion of Ce as bulk CeO2 has a higher reduction enthalpy than PrO2−δ.62 Reduction of Pr4+ occurs readily via heating of the samples, which creates vacancies through oxygen loss and generation of 3+ cations. A lower amount of cations in the 3+ state are present at room temperature, as shown using in situ XRD by the smaller lattice parameter of Ce0.65Pr0.20Cu0.15O2 at room temperature, in comparison to the value when the sample is heated. Oxygen uptake does not occur at 420 °C for the 15% Cu doped PrCeO2 but is possible for the 20% Cu doped Pr-CeO2. For both 15% and 20% Cu doped Pr-CeO2 materials uptake is achieved at 600 °C. For the uptake of oxygen to occur, oxygen needs to be removed from the lattice and vacancies are required to be created via temperature treatment.

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CONCLUSIONS This study established the structural effects of doping CeO2 with Pr and Cu as well as relationships between structure and oxygen uptake. Oxygen uptakes were determined in N2−air− N2 gas switching experiments using TGA, which showed that the materials did not uptake oxygen at 420 °C, only at 600 °C. The highest uptake was measured for Ce0.65Pr0.20Cu0.15O2−δ (120 μmol g−1). At 600 °C both Pr and Cu were found to act as promoters to facilitate the oxygen uptake of CeO2. Neutron diffraction experiments at room temperature were used to determine the effect of doping on the vacancy concentration. Data refinement was carried out by allowing oxygen anions to move from the 8c site toward the interstitial F

DOI: 10.1021/acs.inorgchem.6b01499 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01499. TGA profiles of Ce0.65Pr0.20Cu0.15O2−δ and PrO2−δ, neutron diffraction pattern, and SEM images showing segregation (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for A.L.C.: alan.chaff[email protected]. ORCID

Alan L. Chaffee: 0000-0001-5100-6910 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 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. The authors also thank the Bragg Institute, Australian Nuclear Science and Technology Organisation (ANSTO), in providing the neutron research facilities, Dr. Vanessa Peterson for collecting our neutron diffraction data used in this work, and the facilities within the Monash Centre for Electron Microscopy.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b01499 Inorg. Chem. XXXX, XXX, XXX−XXX