A topological analysis of void spaces in tungstate frameworks:assessing storage properties for the environmentally important guest molecules and ions: CO₂, UO₂, PuO₂, U, Pu, Sr²+, Cs+, CH₄, and H₂

This is the author accepted manuscript. The final version is available from ACS via http://dx.doi.org/10.1021/acssuschemeng.5b00369The identification of inorganic materials, which are able to encapsulate environmentally important small molecules or ions via host-guest interactions, is crucial for the design and development of next-generation energy sources and for storing environmental waste. Especially sought after are molecular sponges with the ability to incorporate CO2, gas pollutants, or nuclear waste materials such as UO2 and PuO2 oxides or U, Pu, Sr2+ or Cs+ ions. Porous framework structures promise very attractive prospects for applications in environmental technologies, if they are able to incorporate CH4 for biogas energy applications, or to store H2, which is important for fuel cells e.g. in the automotive industry. All of these applications should benefit from the host being resistant to extreme conditions such as heat, nuclear radiation, rapid gas expansion, or wear and tear from heavy gas cycling. As inorganic tungstates are well known for their thermal stability, and their rigid open-framework networks, the potential of Na2O-Al2O3-WO3 and Na2O-WO3 phases for such applications was evaluated. To this end, all known experimentally-determined crystal structures with the stoichiometric formula MaM?bWcOd (M = any element) are surveyed together with all corresponding theoretically calculated NaaAlbWcOd and NaxWyOz structures that are statistically likely to form. Network descriptors that categorize these host structures are used to reveal topological patterns in the hosts, including the nature of porous cages which are able to accommodate a certain type of guest; this leads to the classification of preferential structure types for a given environmental storage application. Crystal structures of two new tungstates NaAlW2O8 (1) and NaAlW3O11 (2) and one updated structure determination of Na2W2O7 (3) are also presented from in-house X-ray diffraction studies, and their potential merits for environmental applications are assessed against those of this larger data-sourced survey. Overall, results show that tungstate structures with three-nodal topologies are most frequently able to accommodate CH4 or H2, while CO2 appears to be captured by a wide range of nodal structure types. The computationally generated host structures appear systematically smaller than the experimentally determined structures. For the structures of 1 and 2, potential applications in nuclear waste storage seem feasible.J. M. C. is indebted to the Fulbright Commission for a UK-US Fulbright Scholar Award hosted by Argonne National Laboratory where work done was supported by DOE Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357

Multiple rare-earth ion environments in amorphous (Gd2O3)(0.230)(P2O5)(0.770) revealed by gadolinium K-edge anomalous x-ray scattering

A Gd K-edge anomalous x-ray scattering (AXS) study is performed on the rare-earth (R) phosphate glass, (Gd2O3)0.230(P2O5)0.770, in order to determine Gd⋯Gd separations in its local structure. The minimum rare-earth separation is of particular interest given that the optical properties of these glasses can quench when rare-earth ions become too close to each other. To this end, a weak Gd⋯Gd pairwise correlation is located at 4.2(1)Å, which is representative of a metaphosphate R⋯R separation. More intense first-neighbor Gd⋯Gd pairwise correlations are found at the larger radial distributions, 4.8(1), 5.1(1), and 5.4(1)Å. These reflect a mixed ultraphosphate and metaphosphate structural character, respectively. A second-neighbor Gd⋯Gd pairwise correlation lies at 6.6(1)Å which is indicative of metaphosphate structures. Meta- and ultraphosphate classifications are made by comparing the R⋯R separations against those of rare-earth phosphate crystal structures, R(PO3)3 and RP5O14, respectively, or difference pair-distribution function (ΔPDF) features determined on similar glasses using difference neutron-scattering methods. The local structure of this glass is therefore found to display multiple rare-earth ion environments, presumably because its composition lies between these two stoichiometric formulae. These Gd⋯Gd separations are well-resolved in ΔPDFs that represent the AXS signal. Indeed, the spatial resolution is so good that it also enables the identification of R⋯X(X=R, P, O) pairwise correlations up to r∼9Å; their average separations lie at r∼7.1(1), 7.6(1), 7.9(1), 8.4(1), and 8.7(1)Å. This is a report of a Gd K-edge AXS study on an amorphous material. Its demonstrated ability to characterize the local structure of a glass up to such a long range of r heralds exciting prospects for AXS studies on other ternary noncrystalline materials. However, the technical challenge of such an experiment should not be underestimated, as is highlighted in this work where probing AXS signal near the Gd K edge is found to produce inelastic x-ray scattering that precludes the normal AXS methods of data processing. Nonetheless, it is shown that AXS results are not only tractable but they also reveal local structure of rare-earth phosphate glasses that is important from a materials-centered perspective and which could not be obtained by other materials characterization methods.J.M.C. is grateful to the Royal Commission of the Great Exhibition 1851 for a 2014 Design Fellowship hosted by Argonne National Laboratory (ANL) where work done was supported by the U.S. Department of Energy (DOE) Office of Science, Office of Basic Energy Sciences, and X-ray 1-BM beam line of the Advanced Photon Source, which is a DOE Office of Science User Facility, all under Contract No. DE-AC02-06CH11357. J.M.C. and R.J.N. are also indebted to the Engineering and Physical Sciences Research Council Grant No. GR/L41035 for funding

Reprocessing with GANEX:Methodology for Ligand Radiation Tolerance Testing

Results demonstrating the methodology for testing the radiation tolerance of organic ligands are presented. A high activity sealed source was used to irradiate samples which were sequentially removed and analysed using a sensitive mass spectrometer. The degradation of a candidate ligand for a new reprocessing process “GANEX” was found to be around 50% after 567 kGy exposure to gamma from Cs-137.<br/

Effects of rare-earth co-doping on the local structure of rare-earth phosphate glasses using high and low energy X-ray diffraction

Rare-earth co-doping in inorganic materials has a long-held tradition of facilitating highly desirable optoelectronic properties for their application to the laser industry. This study concentrates specifically on rare-earth phosphate glasses, (R2O3)x(R'2O3)y(P2O5)1-(x+y), where (R, R') denotes (Ce, Er) or (La, Nd) co-doping and the total rare-earth composition corresponds to a range between metaphosphate, RP3O9, and ultraphosphate, RP5O14. Thereupon, the effects of rare-earth co-doping on the local structure are assessed at the atomic level. Pair-distribution function analysis of high-energy X-ray diffraction data (Qmax = 28 Å-1) is employed to make this assessment. Results reveal a stark structural invariance to rare-earth co-doping which bears testament to the open-framework and rigid nature of these glasses. A range of desirable attributes of these glasses unfold from this finding; in particular, a structural simplicity that will enable facile molecular engineering of rare-earth phosphate glasses with 'dial-up' lasing properties. When considered together with other factors, this finding also demonstrates additional prospects for these co-doped rare-earth phosphate glasses in nuclear waste storage applications. This study also reveals, for the first time, the ability to distinguish between P-O and PO bonding in these rare-earth phosphate glasses from X-ray diffraction data in a fully quantitative manner. Complementary analysis of high-energy X-ray diffraction data on single rare-earth phosphate glasses of similar rare-earth composition to the co-doped materials is also presented in this context. In a technical sense, all high-energy X-ray diffraction data on these glasses are compared with analogous low-energy diffraction data; their salient differences reveal distinct advantages of high-energy X-ray diffraction data for the study of amorphous materials

Topological Analysis of Void Space in Phosphate Frameworks: Assessing Storage Properties for the Environmentally Important Guest Molecules and Ions: CO₂, H₂O, UO₂, PuO₂, U, Pu, Sr²⁺, Cs⁺, CH₄, and H₂

The entrapment of environmentally important materials to enable containment of polluting wastes from industry or energy production, storage of alternative fuels, or water sanitation, is of vital and immediate importance. Many of these materials are small molecules or ions that can be encapsulated via their adsorption into framework structures to create a host–guest complex. This is an ever-growing field of study and, as such, the search for more suitable porous materials for environmental applications is fundamental to progress. However, many industrial areas that require the use of adsorbents are fraught with practical challenges, such as high temperatures, rapid gas expansion, radioactivity, or repetitive gas cycling, that the host material must withstand. Inorganic phosphates have a proven history of rigid structures, thermal stability, and are suspected to possess good resistance to radiation over geologic time scales. Furthermore, various experimental studies have established their ability to adsorb small molecules, such as water. In light of this, all known crystal structures of phosphate frameworks with meta- (P₃O₉) or ultra- (P₅O₁₄) stoichiometries are combined in a data-mining survey together with all theoretically possible structures of Ln<i>_a</i>P<i>_b</i>O<i>_c</i> (where <i>a</i>, <i>b</i>, <i>c</i> are any integer, and Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, or Tm) that are statistically likely to form. Topological patterns within these framework structures are used to assess their suitability for hosting a variety of small guest molecules or ions that are important for environmental applications: CO₂, H₂O, UO₂, PuO₂, U, Pu, Sr²⁺, Cs⁺, CH₄, and H₂. A range of viable phosphate-based host–guest complexes are identified from this data-mining and pattern-based structural analysis. Therein, distinct topological preferences for hosting such guests are found, and metaphosphate stoichiometries are generally preferred over ultraphosphate configurations

Topological Analysis of Void Spaces in Tungstate Frameworks: Assessing Storage Properties for the Environmentally Important Guest Molecules and Ions: CO₂, UO₂, PuO₂, U, Pu, Sr²⁺, Cs⁺, CH₄, and H₂

The identification of inorganic materials, which are able to encapsulate environmentally important small molecules or ions via host–guest interactions, is crucial for the design and development of next-generation energy sources and for storing environmental waste. Especially sought after are molecular sponges with the ability to incorporate CO₂, gas pollutants, or nuclear waste materials such as UO₂ and PuO₂ oxides or U, Pu, Sr²⁺, or Cs⁺ ions. Porous framework structures promise very attractive prospects for applications in environmental technologies, if they are able to incorporate CH₄ for biogas energy applications or to store H₂, which is important for fuel cells, e.g., in the automotive industry. All of these applications should benefit from the host being resistant to extreme conditions such as heat, nuclear radiation, rapid gas expansion, or wear and tear from heavy gas cycling. As inorganic tungstates are well known for their thermal stability and their rigid open-framework networks, the potential of Na₂O–Al₂O₃–WO₃ and Na₂O–WO₃ phases for such applications was evaluated. To this end, all known experimentally determined crystal structures with the stoichiometric formula M_aM′_bW_cO_d (M = any element) are surveyed together with all corresponding theoretically calculated Na_aAl_bW_cO_d and Na_<i>x</i>W_<i>y</i>O_<i>z</i> structures that are statistically likely to form. Network descriptors that categorize these host structures are used to reveal topological patterns in the hosts, including the nature of porous cages, which are able to accommodate a certain type of guest; this leads to the classification of preferential structure types for a given environmental storage application. Crystal structures of two new tungstates NaAlW₂O₈ (<b>1</b>) and NaAlW₃O₁₁ (<b>2</b>) and one updated structure determination of Na₂W₂O₇ (<b>3</b>) are also presented from in-house X-ray diffraction studies, and their potential merits for environmental applications are assessed against those of this larger data-sourced survey. Overall, results show that tungstate structures with three-nodal topologies are most frequently able to accommodate CH₄ or H₂, while CO₂ appears to be captured by a wide range of nodal structure types. The computationally generated host structures appear systematically smaller than the experimentally determined structures. For the structures of <b>1</b> and <b>2</b>, potential applications in nuclear waste storage seem feasible

An investigation of the reaction of uranium with nitrogen - A thermogravimetric analysis study

In this work, the heating corrosion reaction of uranium with dry air and with dry nitrogen gas was investigated. Metallic uranium samples of different specific surface area and mass were reacted in a thermogravimetric analysis system at temperature ramping from 323 K to 1273 K at 15 K.min −1 . Ignition was achieved during all heating profiles of the samples reacting with air, with the reported ignition temperature decreasing with increasing specific surface area. Heating cycles were conducted for the uranium and dry nitrogen gas system to investigate if ignition could be attained. These experiments were conducted by varying the specific surface area, sample mass, initial oxide thickness, nitrogen flow rate, and heating ramp rate. None of these experiments showed signs of clear ignition under dry nitrogen. It is hence suggested that uranium ignition under a dry nitrogen atmosphere is not readily attainable and would require a combination of extreme conditions, such as high heating ramp rate for a non-oxidised sample of high surface area and mass