Mechanistic Insights into Cs-Ion Exchange in the Zeolite Chabazite from In Situ Powder X-Ray Diffraction.

Zeolites contain extraframework cations that are exchangeable under favorable aqueous conditions; this is the fundamental feature for their application in water purification and necessary to produce cation forms for other applications such as catalysis. Optimization of the process is common, but there is little fundamental understanding based on real-time experiments of the mechanism of exchange for most zeolites. The sodium and potassium forms of zeolite chabazite selectively uptake Cs+ by ion exchange, leading to its application in removing radioactive 137Cs+ from industrial nuclear waste streams, as well as from contaminated environments in the aftermath of the Fukushima and Three Mile Island accidents. In this study, in situ synchrotron powder X-ray diffraction patterns have been collected on chabazite as it undergoes Cs-ion exchange. Applying Rietveld refinement to these patterns has revealed the time-resolved structural changes that occur in the zeolite as exchange progresses, charting the changes in the spatial distribution of the extraframework cations and water molecules in the structure during the reaction. Ultimately, a detailed mechanistic understanding of how this dynamic ion-exchange reaction occurs has been obtained.

Crystal Structures of a Cubic Tin(II) Germanate, α-Sn6GeO8, and a Tetragonal Tin(II) Silicate, γ-Sn6SiO8.

A cubic tin(II) germanate, α-Sn6GeO8 (space group F4̅3m, a = 10.52521(2) Å, and Z = 4), has been synthesized by both regular hydrothermal and microwave-assisted hydrothermal methods, and the crystal structure of this material has been solved by Rietveld refinement of synchrotron powder X-ray diffraction (PXRD) data. The crystal structure is analogous to α-Sn6SiO8 and is therefore related to the zinc blende structure comprising a face-centered cubic array of [Sn6O8]4- anionic clusters with Ge4+ cations occupying half of the tetrahedral holes. Variable-temperature PXRD has revealed that tin(II) germanate has high thermal stability: remaining stable at 950 K and mostly decomposing over the range 984-1034 K. The tin(II) germanate has been further characterized by X-ray fluorescence (XRF), Raman, and diffuse reflectance (DR) UV-vis spectroscopies. In addition, variable-temperature PXRD studies have revealed the formation of a tetragonal tin(II) silicate polymorph, γ-Sn6SiO8 (space group I4̅, a = 7.30414(6) Å, c = 10.53731(6) Å, and Z = 2), at temperatures below 170 K. The crystal structure of γ-Sn6SiO8 has been elucidated by Rietveld refinement. While a transition to a tetragonal polymorph is observed upon cooling α-Sn6SiO8, no corresponding transition is observed for α-Sn6GeO8, which retains its cubic structure over the probed temperature range.

Evolution of the Local Structure in the Sol-Gel Synthesis of Fe3C Nanostructures.

The sol-gel synthesis of iron carbide (Fe3C) nanoparticles proceeds through multiple intermediate crystalline phases, including iron oxide (FeOx) and iron nitride (Fe3N). The control of particle size is challenging, and most methods produce polydisperse Fe3C nanoparticles of 20-100 nm in diameter. Given the wide range of applications of Fe3C nanoparticles, it is essential that we understand the evolution of the system during the synthesis. Here, we report an in situ synchrotron total scattering study of the formation of Fe3C from gelatin and iron nitrate sol-gel precursors. A pair distribution function analysis reveals a dramatic increase in local ordering between 300 and 350 °C, indicating rapid nucleation and growth of iron oxide nanoparticles. The oxide intermediate remains stable until the emergence of Fe3N at 600 °C. Structural refinement of the high-temperature data revealed local distortion of the NFe6 octahedra, resulting in a change in the twist angle suggestive of a carbonitride intermediate. This work demonstrates the importance of intermediate phases in controlling the particle size of a sol-gel product. It is also, to the best of our knowledge, the first example of in situ total scattering analysis of a sol-gel system.

Nuclear wastewater decontamination by 3D-Printed hierarchical zeolite monoliths.

The selective removal of radioactive cationic species, specifically 137Cs+ and 90Sr2+, from contaminated water is critical for nuclear waste remediation processes and environmental cleanup after accidents, such as the Fukushima Daiichi Nuclear Power Plant disaster in 2011. Nanoporous silicates, such as zeolites, are most commonly used for this process but in addition to acting as selective ion exchange media must also be deployable in a correct physical form for flow columns. Herein, Digital Light Processing (DLP) three-dimensional (3D) printing was utilized to form monoliths from zeolite ion exchange powders that are known to be good for nuclear wastewater treatment. The monoliths comprise 3D porous structures that will selectively remove radionuclides in an engineered form that can be tailored to various sizes and shapes as required for any column system and can even be made with fine-grained powders unsuitable for normal gravity flow column use. 3D-printed monoliths of zeolites chabazite and 4A were made, characterized, and evaluated for their ion exchange capacities for cesium and strontium under static conditions. The 3D-printed monoliths with 50 wt% zeolite loadings exhibit Cs and Sr uptake with an equivalent ion-capacity as their pristine powders. These monoliths retain their porosity, shape and mechanical integrity in aqueous media, providing a great potential for use to not only remove radionuclides from nuclear wastewater, but more widely in other aqueous separation-based applications and processes.

Sn6SiO8, a Tin(II) Silicate with a Zinc Blende Related Structure and High Thermal Stability.

The crystal structure of a novel cubic tin(II) silicate, Sn6SiO8 (space group F4̅3m, a = 10.40708(2) Å, and Z = 4), synthesized by microwave-assisted hydrothermal synthesis has been solved by Rietveld refinement of the powder X-ray diffraction (PXRD) data. The structure, analogous to zinc blende, comprises a face-centered-cubic array of [Sn6O8]4- anions, with Si4+ occupying half of the tetrahedral holes. The tin(II) silicate has been further characterized by variable-temperature PXRD, demonstrating stability of the structure and resistance to SnII oxidation up to ∼600 °C, when the compound begins to thermally decompose.

A Potential Wasteform for Cs Immobilization: Synthesis, Structure Determination, and Aqueous Durability of Cs2TiNb6O18.

Cs2TiNb6O18 is a potential ceramic wasteform for the long-term immobilization of radioactive cesium. Cs2TiNb6O18 was synthesized using the aqueous precursor method and a solid-state reaction, and its crystal structure was determined from the Rietveld refinement of synchrotron X-ray and neutron powder diffraction data. The structure is a pyrochlore analogue, space group P3̅m1 with Cs in 9-fold coordination. The calculated bond valence sum from analysis of neutron diffraction data of 0.84 and high coordination number suggest that Cs has a strong bonding environment. The chemical aqueous durability was investigated using the MCC-1 and PCT-B standard test methods. The measured Cs leach rates of 3.8 × 10-3 and 2.1 × 10-3 g m-2 day-1 obtained via the MCC-1 and PCT-B methods, respectively, demonstrate good promise of a safe long-term immobilization material comparable to, if not better than, hollandite-the material in the multiphase titanate ceramics (Synroc) targeted for cesium sequestration.

Combined PDF and Rietveld studies of ADORable zeolites and the disordered intermediate IPC-1P.

The disordered intermediate of the ADORable zeolite UTL has been structurally confirmed using the pair distribution function (PDF) technique. The intermediate, IPC-1P, is a disordered layered compound formed by the hydrolysis of UTL in 0.1 M hydrochloric acid solution. Its structure is unsolvable by traditional X-ray diffraction techniques. The PDF technique was first benchmarked against high-quality synchrotron Rietveld refinements of IPC-2 (OKO) and IPC-4 (PCR) - two end products of IPC-1P condensation that share very similar structural features. An IPC-1P starting model derived from density functional theory was used for the PDF refinement, which yielded a final fit of Rw = 18% and a geometrically reasonable structure. This confirms the layers do stay intact throughout the ADOR process and shows PDF is a viable technique for layered zeolite structure determination.

Sol-gel preparation of low oxygen content, high surface area silicon nitride and imidonitride materials.

Reactions of Si(NHMe)4 with ammonia are effectively catalysed by small ammonium triflate concentrations, and can be used to produce free-standing silicon imide gels. Firing at various temperatures produces amorphous or partially crystallised silicon imidonitride/nitride samples with high surface areas and low oxygen contents. The crystalline phase is entirely α-Si3N4 and structural similarities are observed between the amorphous and crystallised materials.

In-situ high-pressure powder X-ray diffraction study of α-zirconium phosphate.

The high-pressure structural chemistry of α-zirconium phosphate, α-Zr(HPO4)2·H2O, was studied using in-situ high-pressure diffraction and synchrotron radiation. The layered phosphate was studied under both hydrostatic and non-hydrostatic conditions and Rietveld refinement carried out on the resulting diffraction patterns. It was found that under hydrostatic conditions no uptake of additional water molecules from the pressure-transmitting medium occurred, contrary to what had previously been observed with some zeolite materials and a layered titanium phosphate. Under hydrostatic conditions the sample remained crystalline up to 10 GPa, but under non-hydrostatic conditions the sample amorphized between 7.3 and 9.5 GPa. The calculated bulk modulus, K0 = 15.2 GPa, showed the material to be very compressible with the weak linkages in the structure of the type Zr-O-P.

Ultrasound-driven preparation and pair distribution function-assisted structure solution of a copper-based layered coordination polymer.

Nanoparticles of a copper-based layered coordination polymer, STAM-2, have been prepared via an ultrasound mediated transformation from a layered metal-organic framework, STAM-1. The structure of the material was then solved using pair distribution function analysis to identify the structural units present and the final structural model refined against the pair distribution function data.

The effect of extra framework species on the intrinsic negative thermal expansion property of zeolites with the LTA topology.

X-Ray powder diffraction studies have been used to measure the thermal expansion properties of three zeolites with the LTA topology and provide details of the underlying structural changes. Siliceous ITQ-29 and dehydrated and hydrated silver zeolite A have large negative, moderate negative and positive thermal expansion coefficients, respectively.

Metal-organic frameworks for the storage and delivery of biologically active hydrogen sulfide.

Hydrogen sulfide is an extremely toxic gas that is also of great interest for biological applications when delivered in the correct amount and at the desired rate. Here we show that the highly porous metal-organic frameworks with the CPO-27 structure can bind the hydrogen sulfide relatively strongly, allowing the storage of the gas for at least several months. Delivered gas is biologically active in preliminary vasodilation studies of porcine arteries, and the structure of the hydrogen sulfide molecules inside the framework has been elucidated using a combination of powder X-ray diffraction and pair distribution function analysis.

Characterization of cerium oxide nanoparticles-part 1: size measurements.

The present study gives an overview of some of the major aspects for consideration in the characterization of nanomaterials (NMs). Part 1 focuses on the measurement of particle size and size-related parameters using several analytical techniques such as transmission electron microscopy, atomic force microscopy, dynamic light scattering, X-ray diffraction, and Brunauer, Emmett, and Teller surface area measurements as applied to commercially available cerium oxide nanoparticles (NPs) and microparticles (MPs). Part 2 (see companion paper) considers nonsize-related characterization and analysis. The results are discussed in relation to the nature of the sample and preparation, and the analytical principles, limitations, and advantages of each technique. Accurate information on the particle size of the different fractions of a sample can be obtained by using a combination of different types of microscopy, spectroscopy, separation, and other techniques; this should inform ecotoxicological and environmental studies. The good agreement between the measured primary particle size of the NPs (~15 nm) by atomic force microscopy, transmission electron microscopy, X-ray diffraction, and Brunauer, Emmett, and Teller suggests that the primary particles are formed of semispherical single crystals. For MPs, all measurements agree that they are large particles in the range above the NPs (100 nm), with some difference between the measured sizes, possibly as a result of polydispersity effects. Additionally, our findings suggest that atomic force microscopy and transmission electron microscopy prepared by centrifugation methods provide consistent data at low concentrations when dynamic light scattering fails.

Characterization of cerium oxide nanoparticles-part 2: nonsize measurements.

Part 1 (see companion paper) of the present study discussed the application of a multimethod approach in characterizing the size of cerium oxide nanoparticles (NPs). However, other properties less routinely investigated, such as shape and morphology, structure, chemical composition, and surface properties, are likely to play an important role in determining the behavior, reactivity, and potential toxicity of these NPs. The present study describes the measurement of the aforementioned physicochemical properties of NPs (applied also to nanomaterials [NMs]) compared with micrometer particles (MPs). The authors use a wide range of techniques, including high resolution-transmission electron microscopy, scanning electron microscopy, atomic force microscopy, X-ray diffraction, X-ray energy dispersive spectroscopy, electron energy loss spectroscopy, X-ray photoelectron spectroscopy, and electrophoresis, and compare these techniques, their advantages, and their limitations, along with recommendations about how best to approach NM characterization, using an application to commercial cerium oxide NPs and MPs. Results show that both cerium oxide NPs and MPs are formed of single polyhedron or truncated polyhedron crystals. Cerium oxide NPs contain a mixture of Ce(3+) and Ce(4+) cations, whereas the MPs contain mainly Ce(4+) , which is potentially important in understanding the toxicity of cerium oxide NPs. The isoelectric point of cerium oxide NPs was approximately pH 8, which explains their propensity to aggregate in aqueous media (see companion paper).

High-pressure and -temperature ion exchange of aluminosilicate and gallosilicate natrolite.

The simultaneous application of high pressure and high temperature has been used to achieve direct ion exchange of large cesium cations for the small sodium cations found in the zeolite natrolite by putting it into a superhydrated state with increased pore size. The larger cations remain trapped upon pressure release, and thus, this method is a means of producing new cationic forms of zeolites.

Pair distribution function analysis of pressure treated zeolite Na-A.

Pair distribution function studies using X-ray scattering data from zeolite Na-A samples treated at pressure up to 8 GPa indicate a pressure-induced amorphisation mechanism involving loss of crystallographic order of the aluminosilicate framework but retention of the local sodium to oxygen bonding.

Hydrostatic low-range pressure applications of the Paris-Edinburgh cell utilizing polymer gaskets for diffuse X-ray scattering measurements.

The use of a polymeric Torlon (polyamide-imide) gasket material in a Paris-Edinburgh pressure cell for in situ high-pressure X-ray scattering measurements is demonstrated. The relatively low bulk modulus of the gasket allows for fine control of the sample pressure over the range 0.01-0.42 GPa. The quality of the data obtained in this way is suitable for Bragg and pair distribution function analysis.

High-pressure neutron diffraction study of superhydrated natrolite.

Neutron powder diffraction data were collected on a sample of natrolite and a 1:1 (v/v) mixture of perdeuterated methanol and water at a pressure of 1.87(11) GPa. The natrolite sample was superhydrated, with a water content double that observed at ambient pressure. All of the water deuterium atoms were located and the nature and extent of the hydrogen bonding elucidated for the first time. This has allowed the calculation of bond valence sums for the water oxygen atoms, and from this, it can be deduced that the key energetic factor leading to loss of the additional water molecule upon pressure release is the poor coordination to sodium cations within the pores.

Synchrotron X-ray powder diffraction and computational investigation of purely siliceous zeolite Y under pressure.

High-pressure synchrotron X-ray powder diffraction measurements of a sample of purely siliceous zeolite Y (faujasite) were carried out up to 8.0 GPa at room temperature using a diamond anvil cell. Measurements using silicone oil as the pressure-transmitting medium show compression of the zeolite followed by a loss of long-range ordering at 2.2 GPa. The experimentally determined bulk modulus, 38(2) GPa, is, within experimental error, identical to that of quartz. When using a methanol:ethanol:water mixture (16:3:1) as the pressure-transmitting medium, two distinct compressibility regions are observed with a dramatic change in the compression mechanism at 4 GPa. Rietveld refinement analysis of the powder patterns provides a detailed description of the underlying chemistry, with sequential pore filling the main response up to 4 GPa and framework distortions at higher pressures.

An ordered array of cadmium clusters assembled in zeolite A.

We report the assembly of a cubic array of cationic cadmium clusters encapsulated in zeolite A, and its characterization through Rietveld analysis of powder synchrotron X-ray diffraction data and high resolution transmission electron microscopy.