Surface speciation and interactions between adsorbed chloride and water on cerium dioxide

Sophie Sutherland-Harper1, Robin Taylor2, Jeff Hobbs3, Simon Pimblott1, Richard Pattrick1, Mark Sarsfield2, Melissa Denecke1, Francis Livens1, Nikolas Kaltsoyannis1, Bruce Arey4,

Libor Kovarik4, Mark Engelhard4, John Waters1, Carolyn Pearce1, 4

1University of Manchester, UK; 2National Nuclear Laboratory, Central Laboratory, Sellafield, Cumbria, UK; 3Sellafield Ltd., Cumbria, UK; 4Pacific Northwest National Laboratory, Washington, USA

Graphical Abstract

AbstractCeria particles with different specific surface areas (SSA) were contaminated with chloride and water, then heat treated at 500 and 900 °C to investigate sorption behaviour of these species on metal oxides. Results from x-ray photoelectron spectroscopy and infrared spectroscopy showed chloride and water adsorption onto particles increased with surface area and that these species were mostly removed on heat treatment (from 6.3 to 0.8 at% Cl-

on high SSA and from 1.4 to 0.4 at% on low SSA particles). X-ray diffraction  revealed  that chloride was not incorporated into the bulk ceria structure, but crystal size increased upon contamination. Ce LIII-edge x-ray absorption spectroscopy confirmed that chloride was not present in the first co-ordination sphere around Ce(IV) ions, so was not bonded to Ce as chloride in the bulk structure. Sintering of contaminated high SSA particles occurred with heat treatment at 900 °C, and they resembled low SSA particles synthesised at this temperature. Physical chloride-particle interactions were investigated using electron microscopy and energy dispersive x-ray analysis, showing that chloride was homogeneously distributed on ceria and that reduction of porosity did not trap surface-sorbed chloride inside the particles as surface area was reduced during sintering. This has implications for stabilisation of chloride-contaminated PuO2 for long term storage.

Highlights Chloride is homogeneously distributed within pores and on the surface of ceria

particles. Ceria crystal size increases due to chloride adsorption. Cerium oxidation state remains +4 after chloride-contamination. Chloride and water adsorption mechanisms are linked. Heat treating contaminated ceria removes chloride.

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Reduction of porosity during heat treatment does not trap chloride within ceria particles.

Keywords 4f/5f metal oxides Chloride and water adsorption Physical and chemical entrapment mechanisms Heat treatment Desorption processes Particle morphology, porosity and surface area

1. IntroductionInterim storage of civil plutonium (as plutonium dioxide, PuO2) in the UK has been necessary since the inception of large scale nuclear materials processing more than fifty years ago. PuO2 produced from reprocessing spent fuel from UK and overseas nuclear reactors is currently stored in sealed canisters at the Sellafield nuclear fuel reprocessing and decommissioning site in Cumbria, UK. Approximately 5% of the PuO2 inventory is known to be contaminated with chloride, as the thermal and radiolytic degradation of polyvinyl chloride (PVC), which was originally used as an intermediate liner between inner and outer metal containers, released HCl(g) leading to the adsorption of chloride onto the PuO2. The chloride-contaminated PuO2 must now be retrieved, treated and repackaged before it can be put into safe and secure long term storage, awaiting reuse as mixed oxide (MOX) fuel in future reactors or disposal in a geological disposal facility (GDF). During repackaging, the chloride-contaminated PuO2 will need to be heat treated to dry the powder to meet the conditions for acceptance for moisture content for storage in welded cans. Heat treatment will have additional benefits in reducing the levels of chloride contamination on the PuO2 powder, but this needs to be quantified to underpin the design of an efficient heat treatment process flowsheet and plant equipment.

The exact mechanism of chloride-retention by the PuO2 has not yet been established and requires a detailed understanding of the PuO2 interaction with chloride and other species present on the surface. Important factors to consider include PuO2 particle morphology, porosity and surface area, as well as surface-sorbed water [1–3]. Even in a dry atmosphere, PuO2 has at least 1-2 monolayers of chemisorbed H2O associated with it and experiments have shown that this only desorbs at temperatures of 600 to 900 °C. Parfitt et al. have proposed a mechanism for the adsorption of HCl on TiO2, whereby hydroxide and chloride ions attach to adjacent titanium ions, followed by the reaction of HCl with surface hydroxide ions to produce water. The water is co-ordinatively bound to Ti4+ and is associated with a chloride ion [4]. It can be hypothesised that an analogy can be drawn between this reaction mechanism and plutonium oxide.

Plutonium is both toxic and radiotoxic and therefore challenging to work with; thus, the experiments decribed here have been undertaken with ceria (CeO2), which is recognised as a suitable non-radiotoxic metal oxide analogue, see refs. [5][6][7][8][9] [5-9], before embarking on a systematic study of PuO2. CeO2 has the same +4 oxidation state and Fm3m crystal structure. Both Ce and Pu also occur in compounds in the +3 oxidation state, which

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affects their chemistry. Ce4+ and Pu4+ also have similar ionic radii (0.97 and 0.96 Å, respectively for 8 co-ordinate structures) [7,10,11]. The difference observed in redox behaviour between the two metal oxides originates from the small 5f and 6d orbital energy level gap in Pu and the larger 4f and 5d orbital gap in Ce [12]. The CeO2 samples described here have been calcined at 500 and 900 °C, in order to demonstrate the contrast in behaviour between metal oxides calcined over a broad range of temperatures, as a result of the difference in specific surface area. The CeO2 powders were also calcined in air with mixed crystallographic orientation, to reflect the synthetic conditions of stored PuO2, and to investigate how adsorbed impurities, such as water, affect the sorption of Cl-.

This paper describes the characterisation of low specific surface area (LSSA) and high specific surface area (HSSA) CeO2 powders, before and after heat treatment at varying temperatures to remove chloride contamination, using electron microscopy (scanning electron microscopy-focussed ion beam, SEM-FIB and transmission electron microscopy-energy dispersive x-ray analysis, TEM-EDX) combined with element mapping, x-ray photoelectron spectroscopy (XPS – a surface specific technique and ideal for studying the interactions of chloride ions and water molecules with the surface of CeO2), x-ray absorption spectroscopy (XAS), infrared (IR) spectroscopy and x-ray diffraction (XRD) [11,13]. The importance of using complimentary surface and bulk analysis techniques to investigate the structure and valence state of ceria particles has been noted previously [14]. The combination of these techniques allows both the chemical and physical nature of HCl interaction with 4f/5f metal oxides to be studied systematically for the first time. Results presented here show that: (i) more chloride and water are adsorbed to HSSA than LSSA ceria particles and, thus, more of these species are subsequently removed by heat treatment; (ii) chloride-contamination causes the crystals to increase in size without forming new crystal phases; and (iii) heat treating the HSSA particles at 900 °C causes them to sinter as they are heated above their calcination temperature. This paper provides a comprehensive, coherent study of chloride-contaminated and heat treated CeO2 with thorough characterisation, using complimentary analysis techniques, and demonstrates how CeO2 can act as an analogue, to gain a fundamental understanding of how the physical and chemical properties of metal oxides, including PuO2, change with time under different environmental conditions.

2. Experimental

2.1 CeO2 synthesisCerium nitrate hexahydrate (99.5%), oxalic acid dehydrate (AR Analysis grade) and nitric acid (68%, SG 1.42, Cl impurity = 0.5 ppm) were purchased from Fisher Scientific, UK. CeO2

was synthesised by the oxalate precipitation and calcination method. Solutions of 1.26 L of cerium nitrate hexahydrate (0.0523 mol, 0.0416 mol L-1) in nitric acid (0.252 mol, 0.2 mol L-1) and 0.22 L of oxalic acid dehydrate (0.15 mol, 0.68 mol L-1) in water were mixed in a precipitator at a constant flow rate of 3.47 and 0.60 mL min-1 respectively. The mixture was stirred at 400 rpm with a temperature of 25 ± 0.5 °C for 6 hours 2 minutes. The resulting cerium oxalate precipitate formed (white powder) was filtered in vacuo once it had reached the overflow tube of the precipitator, using a 0.22 µm pore size PVDF membrane, washed with nitric acid (50 mL, 0.01 mol L-1) and water (50 mL) and subsequently air dried overnight before being heated to 150 °C at 2 °C min-1 to desorb water and ensure uniform heating,

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then 340 °C at 5 °C min-1 and held at temperature for 1 hour to decompose the oxalate and calcined in air at 500 °C or 900 °C at 5 °C min-1 and held at temperature for 2 hours. Calcining the powders at 500 °C gives HSSA particles and 900 °C gives LSSA particles [15]. The effect of specific surface area of the products on their sorption properties was investigated. The polycrystalline products were contaminated with chloride by exposure to HCl vapour at ambient temperature, which had been drawn over CaCl2 to dry it, in glass apparatus, evacuated to 0.2 bar absolute and left for 7 days, with occasional agitation (see graphical abstract). After chloride-contamination, the samples were heated in a quartz tube to either 500 or 900 °C in air.

2.2 CeO2 characterisationBrunauer-Emmett-Teller (BET) specific surface area analysis, using a Mircomeritics Gemini 2365 BET, having been purged with He at 10 psi with a Mircomeritics Flowprep 060 unit, showed that the average particle surface area was 58 m2 g-1 (HSSA) and 2 m2 g-1 (LSSA). Both HSSA and LSSA samples were examined both prior to and post chloride-contamination and heat treatment.

The XPS data were acquired on a Kratos Analytical x-ray photoelectron spectrometer equipped with a monochromatic x-ray source of Al Kα radiation (1486.7 eV, 15 mA, 14 kV). Survey scan analyses were carried out with an analysis area of 300 x 700 µm2 and a pass energy of 160 eV. High resolution analyses were carried out with an analysis area of 300 x 700 µm2 and a pass energy 40 eV. The instrument work function was calibrated to give a binding energy (BE) of 83.96 eV for the Au 4f7/2 line for metallic gold and the spectrometer dispersion was adjusted to give a BE of 932.62 eV for the Cu 2p3/2 line of metallic copper. The Kratos charge neutraliser system was used on all specimens. Charge referencing was performed by shifting the binding energy scale by referencing the O1s line at 529.8 eV for CeO2 using CasaXPS Version 2.3.16 PR 1.6. The compositional results reported in Table 1 were obtained using standard sensitivity factors in the Phi MultiPak Version 9.1 software package, using peak area intensities after a Shirley background subtraction.

Ce LIII-edge XAS data were collected on the INE-Beamline at the ANKA synchrotron facility using a photon beam monochromatised using Si(111) crystals and calibrated against the first inflection point of a vanadium foil K-edge XAS, defined as 5465 eV. Spectra were recorded in transmission mode using air in the ionisation chambers, with the monochromator crystals detuned 70% from maximal intensity during data collection to supress higher harmonic wavelength contamination. Spectra were measured three times for each sample and averaged before being background subtracted and normalised.

XRD was performed on a Brüker D8 diffractometer (CuKα1, λ = 1.5406 Å, 40 kV, 40 mA, scan speed = 0.2, increment = 0.02 with rotation) to measure the size of the crystals and establish whether or not contamination with chloride affects the crystal phase or lattice spacings.

Samples for TEM were prepared using an FEI Helios 600 Nanolab Dual Beam instrument. Samples were lifted out for analysis using standard FIB-TEM lamella procedures. All samples were given a low keV polishing cleanup to minimise the amorphous damage from the gallium beam. TEM analysis was performed using the FEI Titan 80–300 operated at 300 kV. The microscope is equipped with CEOS aberration corrector for the probe-forming lens, which enables sub-angstrom image resolution in scanning transmission electron microscopy

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(STEM) mode. Compositional analysis was performed with an EDX Si(Li) electron dispersive spectrometry (EDS) detector. When performing EDS mapping analysis, the electron dose rate impinging on the sample was minimised by sub-pixel scanning, and artificially defocusing the probe to reduce electron density until no significant damage was observed on the sample. For quantitative (EDS) analysis, k factors provided by FEI TIA software were used.

IR spectra were recorded on a Thermo scientific Nicolet iS5 spectroscope to detect functional groups, especially those associated with water-contamination.

3. Results and Discussion

3.1 X-Ray Photoelectron SpectroscopyNo significant difference between the XPS spectra of uncontaminated HSSA versus LSSA CeO2 particles is observed (Fig. 1). As can be seen in Figure 1, a small amount of chloride-contamination was present on the surface of the starting material, likely due to Cl impurities in the nitric acid used to wash the cerium oxalate during synthesis. Fig. 2 shows that, upon exposure to HCl gas, the oxidation state of Ce in both samples remains +4 at the particle surface (probe depth ~20 atomic layers) [16,17]. By contrast, upon exposure to HCl gas (Fig. 3), a difference in chloride content between the HSSA and LSSA samples is observed as well as lower Cl 2p binding energies, suggesting a more reduced Cl- species, for the LSSA sample than HSSA (Table 1). These binding energies are low enough to indicate that the adsorbed species is the Cl- anion [18–23]. In two sample sets, more Cl- adsorbed to HSSA CeO2 than to LSSA, likely due to the higher surface area (Table 2). It is difficult to ascertain whether this chloride is physisorbed or chemisorbed to the CeO2 surface. Investigations to differentiate these species are ongoing. After the chloride-contaminated samples were heat treated at 900 °C, the observed amount of Cl- on the surface of both CeO2 samples reduced to nearly the same level. The area under the Cl 2p1/2 and 2p3/2 peak after a Shirley background subtraction was used to determine the surface concentration of Cl - following contamination and heat treatment (Fig. 4). Heat treating either contaminated HSSA or LSSA CeO2 at 500 °C removes less Cl- from the surface than heat treatment at 900 °C; thus more Cl- is volatilised at the higher temperature. After heat treatment at 900 °C, the Cl -

concentration is observed to be even lower than the small amount of Cl-contamination present prior to exposure to dry HCl vapour. The O 1s peaks of the HSSA CeO2 in Fig. 5 (upper right) show the lattice oxide peak at ~529.2 eV and an adsorbed OH peak at ~531.4 eV, from exposure to air after synthesis, in the uncontaminated sample (black) [17]. Upon chloride-contamination, the OH peak in the HSSA sample decreases in intensity, indicating that the chloride adsorption mechanism has a significant effect on the surface hydroxyl groups. Multiple spots on the HSSA CeO2 surface were analysed and showed the same trend with representative spectra in Figure 5 (upper right). This effect was not as obvious in the LSSA samples (Fig. 5, lower right), as the lower surface area to volume ratio of the sample gives a lower initial hydroxyl contribution. This supports the fact that interactions between HCl/Cl- and H2O/OH- species at the surface, governed by available surface area, are key to understanding the behaviour of these materials upon contamination, highlighting the need for surface sensitive techniques such as XPS to determine metal oxide surface chemistry and reactivity. Following heat treatment at 900 °C (green), the adsorbed OH peak reappears upon exposure to an air atmosphere at the same intensity as before

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contamination. This indicates that the chloride volatilises at the higher heat treatment temperature and is removed from the surface. The atomic % concentration of carbon on the samples is consistently ~20 %, which is in agreement with values previously reported for powder samples exposed to an air atmosphere (Fig. 5 left) [24].

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Figure 1: XPS spectra of (a) HSSA and (b) LSSA CeO2 showing counts per second (c/s) against binding energy.

Figure 2: High energy resolution XPS of Ce 3d peaks in Cl --contaminated HSSA (top) and LSSA (bottom) CeO2. Peak designations from Burroughs et al [16].

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HSSA

c/s

Binding Energy (eV)

4x 105

4.5

4

3.5

3

2.5

2

1.5 940 930 920 910 900 890 880

LSSA

c/s

Binding Energy (eV)

4x 10

2.8

2.6

2.4

2.2

2

1.8

1.6

1.4

1.2940 930 920 910 900 890 880

Ce4+ 3d3/2 Ce4+ 3d5/2 4f0

Ce4+ 3d5/2 4f1

Ce4+ 3d5/2 4f2

Ce4+ 3d3/2

Ce4+ 3d5/2 4f0

Ce4+ 3d5/2 4f1

Ce4+ 3d5/2 4f2

Figure 3: High energy resolution XPS spectra showing the Cl 2p peak of Cl--contaminated HSSA (a) and LSSA (b) CeO2 before (red) and after (green) heat treatment at 900 °C.

Sample Cl 2p3/2 / eV Cl 2p1/2 / eVHSSA + Cl--contamination 198.55 200.19

HSSA + Cl--contamination + 900 °C 198.67 200.24

LSSA + Cl--contamination 198.33 199.89LSSA + Cl--contamination + 900

°C 198.36 199.79Table 1: Cl 2p peak positions and splittings obtained from the XPS shown in Fig. 3.

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a

b

Sample Cl 2p concentration / at% (± 5 %)HSSA 1.2 1.2

HSSA + Cl--contamination 6.1 6.4HSSA + Cl--contamination + 500 °C 4.0 4.0HSSA + Cl--contamination + 900 °C 0.9 0.7

LSSA 0.6 0.4LSSA + Cl--contamination 1.3 1.4

LSSA + Cl--contamination + 500 °C 0.9 0.9LSSA + Cl--contamination + 900 °C 0.4 0.4

Table 2: Surface concentrations of Cl- acquired from two spots on HSSA and LSSA samples of CeO2

before and after Cl--contamination and heat treatment at 500 and 900 °C, obtained using XPS from the Cl 2p peak area.

Figure 4: Atomic concentration (±5 %) of Cl– on LSSA and HSSA CeO2 before and after contamination and with heat treatment at 500 and 900 °C.

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Figure 5: High energy resolution XPS spectra showing the O 1s and C 1s peaks of HSSA (upper panel, right and left respectively) and LSSA (lower panel, right and left respectively) CeO2 (black), Cl--contaminated CeO2 (red) and Cl--contaminated CeO2 heat treated at 900 °C (green).

3.2 X-ray Absorption SpectroscopyCe LIII edge x-ray absorption near edge structure (XANES) was used to corroborate the XPS results which indicate that the cerium oxidation state in the CeO2 samples is invariant to chloride or heat treatment. The extended x-ray absorption fine structure (EXAFS) was used to ascertain if chloride could be identified in the Ce4+ cation first co-ordination sphere.

Figure 6: XANES (a), EXAFS (b) and its Fourier Transform envelope function (c) for the LSSA CeO2 as prepared (red), Cl--contaminated CeO2 (green) and Cl--contaminated CeO2 heat treated at 500 °C (blue).

The Ce LIII XANES and EXAFS of the LSSA (Fig. 6) and HSSA (Fig. 7) samples retain the general spectral features following contamination with chloride and subsequent heat

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treatment. Only minor variations in the main XANES absorption intensities in the LSSA CeO2

samples at ~5732 and 5740 eV are observed, being greatest for CeO2 pre-contamination. As this sample did not exhibit measureable changes in crystal size with chloride exposure (cf next section), we assume the observed intensity changes are due to a decrease in photoelectron final state densities caused by the introduction of chloride into the sample. No shift in the energy of the XANES is observed, which supports results obtained from the XPS spectra of the surface, where no change in oxidation state occurs in the chloride-contaminated samples. The Ce LIII EXAFS spectra for the LSSA samples show only small differences in their first Fourier Transform (FT) peak intensities. If chloride ions were associated with the Ce(IV) cation first co-ordination shell (~1.8 Å peak in the FT), a phase shift would be expected. The imaginary part of the FT (not shown) shows no phase shift for non-contaminated, chloride-contaminated or heat treated samples, indicating that the first co-ordination shell in all cases is occupied by the same type of co-ordinating element, oxygen.

Figure 7: XANES (a), EXAFS (b) and Fourier Transform envelope (c; k transform range= 2.2 – 10.2 Å-1, using a Hanning window) of HSSA CeO2 (green), Cl--contaminated CeO2 (blue), Cl--contaminated CeO2

heat treated at 500 °C (red) and Cl--contaminated CeO2 heat treated at 900 °C (purple). The sharp EXAFS feature at 8.65 Å-1 results from multi-electron excitations.

The XANES and EXAFS for the HSSA CeO2 samples show similar trends (Fig. 7). The XANES main absorption peak intensities decrease following heat treatment. As for the LSSA samples, the first FT EXAFS peak does not exhibit any observable phase shift, suggesting that only oxygen atoms and no Cl- ions are in the Ce(IV) first co-ordination sphere. Also similar to LSSA, the first co-ordination sphere FT peak magnitude decreases with chloride-contamination. The changes following heat treatment are larger, however. This likely reflects changes in sample crystallite size and crystallinity (cf next section). The sample treated at 900 °C exhibits the largest magnitude, indicating the greatest ordering and suggesting this sample has the highest crystallinity.

3.3 X-Ray DiffractionX-ray powder diffraction was carried out to measure the size of the crystals and establish whether or not contamination with chloride affects the crystal phase or lattice spacings. The diffraction patterns for HSSA CeO2 before and after chloride-contamination and heat treatment (Fig. 8) all correspond to the cubic Fm3m phase expected for CeO2 [25]. Also as expected, upon chloride-contamination and heat treatment no phase changes in the bulk structure are observed. However, peak heights and widths do vary, indicative of changes in crystallite size. While this might be expected for the heat treated samples, an increase in crystallite size is also observed following chloride-contamination. This effect is not due to ageing, as the diffraction patterns of uncontaminated CeO2 monitored over 200 days showed no significant changes in crystal size. We conclude that exposure to dry HCl vapour leads to an increase in crystallite size from 18.6 nm for the synthesised sample to 46.1 nm following chloride-contamination, without incorporation of the contaminant into the bulk lattice

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structure; further investigation is required in order to establish the mechanism of this incorporation. Heat treating the contaminated particles to 500 °C reduces the amount of chloride in the CeO2, causing the crystallite size to shrink to 23.1 nm, but heat treatment at the higher temperature of 900 °C leads to a further increase in crystallite size to 98.7 nm, due to the crystals sintering above their original calcination temperature (Fig. 8d).

The micro-crystallites of LSSA CeO2 were all >500 nm, above which size analysis using the Scherrer equation becomes inaccurate. The LSSA CeO2 XRD patterns (Fig. 9) also confirm the synthesis of CeO2 product and that Cl- ions do not incorporate into the bulk of the structure, as all the peaks remain at the same 2θ. The LSSA particles were initially calcined at 900 °C, therefore the crystallite size does not change significantly with further heat treatment up to 900 °C.

Figure 8: XRD patterns of HSSA Cl--contaminated CeO2 particles with corresponding crystal sizes (a=CeO2, b=Cl--contaminated CeO2, c=Cl--contaminated CeO2 with calcination afterwards at 500 °C and d=Cl--contaminated CeO2 with calcination afterwards at 900 °C).

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d = 98.7 ± 5.1 nm

c = 23.1 ± 1.1 nm

b = 46.1 ±11.3 nm

a = 18.6 ± 1.5 nm

Figure 9: XRD patterns of LSSA Cl--contaminated CeO2 particles (a=CeO2, b=Cl--contaminated CeO2, c=Cl--contaminated CeO2 with calcination afterwards at 500 °C and d=Cl--contaminated CeO2 with calcination afterwards at 900 °C).

3.4 SEM-FIB and TEM-EDXThe generally larger particle size of as-prepared LSSA CeO2 (>15 µm) compared with HSSA CeO2 (<15 µm) is also observed in SEM micrographs (Fig. 10). The general acicular rod morphology of both samples is similar, so the adsorption and desorption of chloride and water on CeO2 should only be affected by surface area.

FIB-TEM lamella were extracted from an HSSA chloride-contaminated sample particle to compare its surface, bulk and micro- to nano-sized structures. The FIB extraction procedure and a section of the sample extracted are shown in the top images of Fig. 11. High resolution TEM analysis of the particle sections (Fig. 12) reveals these to be composed of nano-crystallites, ranging from around 10nm to 100 nm with hexagonal morphologies (circled). This hexagonal morphology has also been observed in industrial PuO2 samples [26]. The crystallite size range reflects the average crystallite sizes determined from XRD analysis. TEM images (Fig. 12) of FIB sections of chloride-contaminated particles before and after heat treatment at 900 °C show how crystallites increase in size with heating, as also shown by XRD. These individual crystals are too small to establish a mechanism for chloride and water adsorption and interaction with the grain boundaries.

The upper right image of Fig. 13 displays EDS results for Cl K fluorescence intensities of a chloride-contaminated HSSA CeO2 particle edge, measured by rastering the same area shown to its left. When CeO2 becomes contaminated with Cl-, one might expect the anions to adsorb preferentially to the surface of the particles. Comparison of the Cl- distribution across

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d

c

b

a

the particle and the Ce and Cl- spatial distributions both on and under the surface of the CeO2 particle shows that Cl- is actually spread homogeneously over the exterior and interior of the particles. Comparison of the Ce, O and Cl- distributions along a particle cross-section displayed in Fig. 14 shows that chloride ions are also found in the particle interior, with the distribution closely matching that of Ce and O intensity.

EDX spectra measured at three different sample areas indicated in Fig. 14 are displayed in Fig. 15. Comparison of relative peak intensities shows little variation, which indicates total surface coverage by Cl- with accessibility to all of the interior pore spaces. The increase in particle size indicates that sintering of the HSSA CeO2 particles did occur as a result of heat treatment (Fig. 14). However, the reduction in porosity did not physically trap surface-sorbed Cl- at the grain boundaries within particles as surface area was reduced during the sintering of CeO2. This is supported by EDX analysis of the contaminated HSSA particles after 900 °C heat treatment (shown in Fig. 16). If significant physical entrapment had occurred, areas of higher Cl- concentrations would be observed at grain boundaries within the sintered particles but, in fact, no Cl K signal was detected anywhere on the heat treated samples (Fig. 16), therefore the concentration of Cl- was below the limit of detection ( 0.1 wt%).

Figure 10: SEM images of CeO2 (left=HSSA, right=LSSA).

Figure 11: SEM images of Cl--contaminated HSSA CeO2 sectioned via FIB, showing where the section is taken from the sample (left), the section before removal from the sample with the carbon layer on top (middle) and after placing it on the TEM mount (right).

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Figure 12: TEM images of Cl--contaminated HSSA CeO2 before (top) and after (bottom) heat treatment at 900 °C with an individual hexagonal crystallite circled.

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Figure 13: TEM images at the surface of Cl--contaminated HSSA CeO2 with elemental mapping of Cl-

(orange), Ce (blue) and O (yellow).

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Figure 14: TEM images of Cl--contaminated HSSA CeO2 and elemental maps of Cl- (orange), Ce (blue) and O (yellow) over the same area. EDX was measured over the three areas indicated (top left).

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Ce La

Cl Ka

O Ka1 2Ce La

Cl Ka

O Ka

Ce La

Cl Ka

O Ka 3

Figure 15: EDX spectra of Cl--contaminated HSSA CeO2 measured over three areas labelled in Fig. 14, indicating the O Ka, Cl Ka and Ce La peaks.

Figure 16: EDX spectrum of Cl--contaminated HSSA CeO2 post heat treatment at 900 °C.

3.5 Infrared SpectroscopyThe IR spectra of HSSA CeO2 particles (Fig. 17) were used to confirm the presence of water in the sample before and after chloride-contamination and decontamination by heat treatment at 500 and 900 °C [27]. The lack of O-H vibration peaks observed at ~3,300 and 1,600 cm-1 for the dry, uncontaminated CeO2 sample (Figure 17a) contrasts with the IR spectrum for CeO2 after contamination with Cl- with significant O-H peak intensities (b). These peaks are greatly reduced after heat treatment at 500 °C (c) and disappear after 900 °C heat treatment (d). The group of small peaks at ν = 2,800 to 2,950 cm-1 are due to C-H bond vibrations and the broad peak at ν = 1,537 cm-1 in uncontaminated CeO2 (Fig. 17a) is due to the asymmetric νC=O stretching vibrations from carbonate, indicating that not all of the cerium oxalate has oxidised to form ceria upon calcination at 500 °C, as these peaks disappear after 900 °C heat treatment and are not seen in uncontaminated LSSA CeO2 (Fig. 18a), which has been calcined at the higher temperature [28,29]. De Almeida et al. have reported that the conversion of Ce2

III(C2O4)3·8H2O to CeIVO2 occurs at 450 °C under air [5]. Thus calcination at a higher temperature than 500 °C is needed to completely oxidise cerium oxalate. No IR peaks indicative of surface sorbed water at ~3,300 and 1,600 cm-1 for the chloride-contaminated LSSA CeO2 samples studied (Fig. 18b) are observed, meaning that a reduced amount of water is adsorbed by the lower SSA particles. No change in spectrum is observed after either heat treatment method (Fig. 18c and d). There was not enough powder to perform thermogravimmetric analyses in order to quantify the water and oxalate still present and deduce the stoichiometry of each sample. Loring et al. have shown that the presence of water increases reactivity in minerals, which could explain the increase in crystal size upon chloride-contamination seen in Fig. 8 [30]. This relationship between the amount of water and concentration of chloride in the HSSA CeO2 samples suggests that water adsorption during storage is linked to the amount of chloride present as a result of dry contamination and will be investigated in future studies.

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Cl Ka

O Ka Ce La

Figure 17: IR spectra of HSSA CeO2 particles (a=CeO2, b=Cl--contaminated CeO2, c=Cl--contaminated CeO2 heat treated at 500 °C and d=Cl--contaminated CeO2 heat treated at 900 °C).

Figure 18: IR spectra of LSSA CeO2 particles (a=CeO2, b=Cl--contaminated CeO2, c=Cl--contaminated CeO2

heat treated at 500 °C and d=Cl--contaminated CeO2 heat treated at 900 °C).

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d

c

b

a

C-Hν O-H

δ O-H

d

c

b

a

C-H

δ O-Hν O-

Hν C=O

ν C=O

4. ConclusionsHere, the results from a range of experimental techniques to assess the adsorption of chloride and water on LSSA and HSSA CeO2 treated at 500 oC and 900 oC are presented. XPS analysis shows a distinct difference between the chloride-sorption capabilities of CeO2

particles with high and low SSA. The advantage of heat treating the particles to a higher temperature of 900 oC is highlighted, as more Cl- is volatilised from the surface, especially for HSSA CeO2. The sorption of water to the particles follows a similar pattern, as shown by IR spectroscopy, with more adsorption taking place on HSSA than LSSA particles, despite chloride-contamination taking place in a dry atmosphere. As expected, the 900 °C heat treatment temperature removes more water than treatment at 500 °C. The XRD patterns before and after chloride-contamination show that chloride does not incorporate into the bulk of the CeO2 crystal structure, as no extra phases are detected, regardless of surface area. However, the addition of chloride does increase the average crystal size of HSSA CeO2 by an as yet unidentified mechanism. Heat treatment results in sintering and an increase in crystal size for the HSSA CeO2 particles but no change could be observed for the micro-sized LSSA CeO2 crystals. The TEM images of the CeO2 particles show that on the nano-scale the crystallites sinter after treatment at 900 °C to give hexagonal morphologies reminiscent of PuO2, but SEM images show differences in micro-scale morphologies between the two types of particles. Elemental mapping of particle edges and within pores demonstrates a homogeneous distribution of chloride throughout CeO2, such that it is not confined to the particle surface. Depth profiling from the CeO2 particle surface into the interior using FIB-SEM, combined with atomic scale chemical imaging using TEM-EDX, allows us to show for the first time that chloride is removed from not just the surface, but from within the particle interior after heat treatment at 900 °C, and does not become physically entrapped. XANES results show that contaminating the CeO2 samples with chloride does not change the cerium oxidation state. The lack of any phase shift, as an indicator for the presence of atoms or ions other than oxygen, in the nearest EXAFS FT peak show that Cl- ions are not incorportated into the Ce4+ first co-ordination sphere. The EXAFS data indicate an increase in crystallinity of HSSA CeO2 following heat treatment at 900 °C, reaffirming the hypothesis of sintering taking place upon heat treatment at a higher temperature. Overall, these results suggest that exposure to `dry` HCl(g) leads to a rather non-specific adsorption of chloride (as the anion) onto the CeO2 surface without any incorporation into the bulk, phase change or reduction of Ce(IV) and that the main controlling factor is the surface area of the CeO2. Heat treatment removes chloride and co-adsorbed water efficiently, particularly at the higher temperature, with morphological changes only occurring for the HSSA CeO2 when heated above the temperature at which it had been initially calcined. A key finding of this work is that a reduction in porosity did not result in physical entrapment of surface-sorbed chloride inside the particles as surface area was reduced during sintering of CeO2. This has implications for employing calcination in air to reduce the chloride content of stored PuO2 to acceptably low values, as it addresses the concern that chloride might remain physically entrapped within intergrain spaces in the PuO2

and subsequently become mobile, causing corrosion problems in the stored package. Thus, these results will be used as a basis for experimental work on chlorinated PuO2, to inform its behaviour and properties throughout various heat treatment processes.

AcknowledgementsWe thank the EPSRC and Sellafield Ltd. for a studentship (to SSH). FIB-SEM, TEM-EDX

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and XPS measurements were performed using EMSL, a national scientific user facility sponsored by the U.S. Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (Rapid Access EMSL User Proposal 48811). Pacific Northwest National Laboratory (PNNL) is a multi-program national laboratory operated for DOE by Battelle.

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