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Tse JS, Grant J, Skelton JM, Gillie LJ, Zhu R, Pesce GL, Ball RJ, Parker SC, Molinari M. Location of Artinite (Mg 2CO 3(OH) 2·3H 2O) within the MgO-CO 2-H 2O system using ab initio thermodynamics. Phys Chem Chem Phys 2023. [PMID: 37377444 DOI: 10.1039/d3cp00518f] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/29/2023]
Abstract
The MgO-CO2-H2O system have a variety of important industrial applications including in catalysis, immobilisation of radionuclides and heavy metals, construction, and mineralisation and permanent storage of anthropogenic CO2. Here, we develop a computational approach to generate phase stability plots for the MgO-CO2-H2O system that do not rely on traditional experimental corrections for the solid phases. We compare the predictions made by several dispersion-corrected density-functional theory schemes, and we include the temperature-dependent Gibbs free energy through the quasi-harmonic approximation. We locate the Artinite phase (Mg2CO3(OH)2·3H2O) within the MgO-CO2-H2O phase stability plot, and we demonstrate that this widely-overlooked hydrated and carbonated phase is metastable and can be stabilised by inhibiting the formation of fully-carbonated stable phases. Similar considerations may apply more broadly to other lesser known phases. These findings provide new insight to explain conflicting results from experimental studies, and demonstrate how this phase can potentially be stabilised by optimising the synthesis conditions.
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Affiliation(s)
- Joshua S Tse
- Department of Chemistry, University of Huddersfield, Queensgate, Huddersfield, HD1 3DH, UK.
| | - James Grant
- Department of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY, UK.
| | - Jonathan M Skelton
- Department of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, UK
| | - Lisa J Gillie
- Department of Chemistry, University of Huddersfield, Queensgate, Huddersfield, HD1 3DH, UK.
| | - Runliang Zhu
- CAS Key Laboratory of Mineralogy and Metallogeny, and Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, P. R. China
| | - Giovanni L Pesce
- Department of Mechanical and Construction Engineering, Northumbria University, Newcastle upon Tyne, NE1 8ST, UK
| | - Richard J Ball
- Department of Architecture and Civil Engineering, University of Bath, Bath, BA2 7AY, UK
| | - Stephen C Parker
- Department of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY, UK.
| | - Marco Molinari
- Department of Chemistry, University of Huddersfield, Queensgate, Huddersfield, HD1 3DH, UK.
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2
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El Jamal G, Gouder T, Eloirdi R, Tereshina-Chitrova E, Horákd L, Jonsson M. Mixed H 2O/H 2 plasma-induced redox reactions of thin uranium oxide films under UHV conditions. Dalton Trans 2021; 50:12583-12591. [PMID: 34286754 DOI: 10.1039/d1dt01020d] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
X-ray photoelectron spectroscopy (XPS) has been used to study the effect of mixed H2O/H2 gas plasma on the surfaces of UO2, U2O5 and UO3 thin films at 400 °C. The experiments were performed in situ under ultra-high vacuum conditions. Deconvolution of the U4f7/2 peaks into U(iv), U(v) and U(vi) components revealed the surface composition of the films after 10 min plasma exposure as a function of H2 concentration in the feed gas of the plasma. Some selected films (unexposed and exposed) were also analysed using grazing-incidence X-ray diffraction (GIXRD). The XPS results show that U(v) is formed as a major product upon 10 minutes exposure of UO3 by a mixed H2O/H2 plasma in a fairly wide H2 concentration range. When starting with U(v) (U2O5), rather high H2 concentrations are needed to reduce U(v) to U(iv) in 10 minutes. In the plasma induced oxidation of UO2, U(v) is never observed as a major product after 10 minutes and it would seem that once U(v) is formed in the oxidation of UO2 it is rapidly oxidized further to U(vi). The grazing incidence X-ray diffraction analysis shows that there is a considerable impact of the plasma and heating conditions on the crystal structure of the films in line with the change of the oxidation state. This structural difference is proposed to be the main kinetic barrier for plasma induced transfer between U(iv) and U(v) in both directions.
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Affiliation(s)
- Ghada El Jamal
- School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Department of Chemistry, Applied Physical Chemistry, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden.
| | - Thomas Gouder
- European Commission, Joint Research Centre, Postfach 2340, DE-76215 Karlsruhe, Germany
| | - Rachel Eloirdi
- European Commission, Joint Research Centre, Postfach 2340, DE-76215 Karlsruhe, Germany
| | - Evgenia Tereshina-Chitrova
- Institute of Physics, ASCR, Prague, Czech Republic and Faculty of Mathematics and Physics, Charles University, 12116 Prague, Czech Republic
| | - Lukáš Horákd
- Faculty of Mathematics and Physics, Charles University, 12116 Prague, Czech Republic
| | - Mats Jonsson
- School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Department of Chemistry, Applied Physical Chemistry, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden.
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Abstract
Our study has far-reaching implications for the safe use of nuclear materials around the world. The strong oxidative tendency of the actinides drives contamination of groundwater near waste storage sites. A key finding of our study is that excess oxygen from the environment can be incorporated at far greater levels than previously thought, while still preserving the nominal cubic crystal structure of the widely used nuclear fuel UO2. This insight, enabled by atomic-resolution spectroscopy and theory calculations, will allow us to develop better, more reliable models for nuclear waste storage and disposal. Oxygen defects govern the behavior of a range of materials spanning catalysis, quantum computing, and nuclear energy. Understanding and controlling these defects is particularly important for the safe use, storage, and disposal of actinide oxides in the nuclear fuel cycle, since their oxidation state influences fuel lifetimes, stability, and the contamination of groundwater. However, poorly understood nanoscale fluctuations in these systems can lead to significant deviations from bulk oxidation behavior. Here we describe the use of aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy to resolve changes in the local oxygen defect environment in UO2+x surfaces. We observe large image contrast and spectral changes that reflect the presence of sizable gradients in interstitial oxygen content at the nanoscale, which we quantify through first-principles calculations and image simulations. These findings reveal an unprecedented level of excess oxygen incorporated in a complex near-surface spatial distribution, offering additional insight into defect formation pathways and kinetics during UO2 surface oxidation.
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Liu T, Li S, Gao T, Ao B. Theoretical prediction of some layered Pa2O5 phases: structure and properties. RSC Adv 2019; 9:31398-31405. [PMID: 35527940 PMCID: PMC9072604 DOI: 10.1039/c9ra06735c] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/26/2019] [Accepted: 09/19/2019] [Indexed: 11/21/2022] Open
Abstract
Density functional theory (DFT) was used to predict and study protactinium pentoxide (Pa2O5), which presents a fluorite and layered protactinium oxide-type structure. Although the layered structure has been observed with the isostructural transition Nb and Ta metal pentoxides experimentally, the detailed structure and properties of the layered Pa2O5 are not clear and understandable. Our theoretical prediction explored some possible stable structures of the Pa2O5 stoichiometry according to the existing M2O5 structures (where M is an actinide Np or transition Nb, Ta, and V metal) and replacing the M ions with protactinium ions. The structural, mechanical, thermodynamic and electronic properties including lattice parameters, bulk moduli, elastic constants, entropy and band gaps were predicted for all the simulated structures. Pa2O5 in the β-V2O5 structure was found to be a competitive structure in terms of stability, whereas Pa2O5 in the ζ-Nb2O5 structure was found to be the most stable overall. This is consistent with Sellers's experimental observations. In particular, Pa2O5 in the ζ-Nb2O5 structure is predicted to be charge-transfer insulators. Furthermore, we predict that ζ-Nb2O5-structured Pa2O5 is the most thermodynamically stable under ambient conditions and pressure. Density functional theory (DFT) was used to predict and study protactinium pentoxide (Pa2O5), which presents a fluorite and layered protactinium oxide-type structure.![]()
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Affiliation(s)
- Tao Liu
- Institute of Atomic and Molecular Physics
- Sichuan University
- Chengdu
- China
- School of Electronic and Communication Engineering
| | - Shichang Li
- Institute of Atomic and Molecular Physics
- Sichuan University
- Chengdu
- China
| | - Tao Gao
- Institute of Atomic and Molecular Physics
- Sichuan University
- Chengdu
- China
| | - Bingyun Ao
- Science and Technology on Surface Physics and Chemistry Laboratory
- Jiangyou
- China
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Direct observation of pure pentavalent uranium in U 2O 5 thin films by high resolution photoemission spectroscopy. Sci Rep 2018; 8:8306. [PMID: 29844333 PMCID: PMC5974404 DOI: 10.1038/s41598-018-26594-z] [Citation(s) in RCA: 34] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/18/2018] [Accepted: 05/14/2018] [Indexed: 11/16/2022] Open
Abstract
Thin films of the elusive intermediate uranium oxide U2O5 have been prepared by exposing UO3 precursor multilayers to atomic hydrogen. Electron photoemission spectra measured about the uranium 4f core-level doublet contain sharp satellites separated by 7.9(1) eV from the 4f main lines, whilst satellites characteristics of the U(IV) and U(VI) oxidation states, expected respectively at 6.9(1) and 9.7(1) eV from the main 4f lines, are absent. This shows that uranium ions in the films are in a pure pentavalent oxidation state, in contrast to previous investigations of binary oxides claiming that U(V) occurs only as a metastable intermediate state coexisting with U(IV) and U(VI) species. The ratio between the 5f valence band and 4f core-level uranium photoemission intensities decreases by about 50% from UO2 to U2O5, which is consistent with the 5f 2 (UO2) and 5f 1 (U2O5) electronic configurations of the initial state. Our studies conclusively establish the stability of uranium pentoxide.
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