B18-Core EXAFS
I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[14239, 13284]
Abstract: The rhodium(III) hydrogarnets Ca3Rh2(OH)12 and Sr3Rh2(OH)12 crystallize as polycrystalline powders under hydrothermal conditions at 200 °C from RhCl3·3H2O and either Ca(OH)2 or Sr(OH)2 in either 12 M NaOH or KOH. Rietveld refinements against synchrotron powder X-ray diffraction (XRD) data allow the first crystal structures of the two materials to be determined. If BaO2 is used as a reagent and the concentration of hydroxide increased to hydroflux conditions (excess NaOH), then single crystals of a new complex rhodium hydroxide, BaNaRh(OH)6, are formed in a phase-pure sample, with sodium included from the flux. Structure solution from single-crystal XRD data reveals isolated octahedral Rh centers that share hydroxides with 10-coordinate Ba and two independent 8-coordinate Na sites. 23Na magic-angle spinning NMR confirms the presence of the two crystallographically distinct Na sites and also verifies the diamagnetic nature of the sample, expected for Rh(III). The thermal behavior of the hydroxides on heating in air was investigated using X-ray thermodiffractometry, showing different decomposition pathways for each material. Ca3Rh2(OH)12 yields CaRh2O4 and CaO above 650 °C, from which phase-pure CaRh2O4 is isolated by washing with dilute nitric acid, a material previously only reported by high-pressure or high-temperature synthesis. Sr3Rh2(OH)12 decomposes to give a less crystalline material with a powder XRD pattern that is matched to the 2H-layered hexagonal perovskite Sr6Rh5O15, which contains mixed-valent Rh3+/4+, confirmed by Rh K-edge XANES spectroscopy. On heating BaNaRh(OH)6, a complex set of decomposition events takes place via transient phases.
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Aug 2018
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[11070]
Abstract: The templated zeolite-analogue GaPO-34 (CHA structure type) crystallises from a gel precursor Ga2O3:2H3PO4:1HF:1.7SDA:70H2O (where SDA = structure directing agent), treated hydrothermally for 24 hours at 170 °C using either pyridine or 1-methylimizadole as SDA and one of either poorly crystalline ε-Ga2O3 or γ-Ga2O3 as gallium precursor. If the same gels are stirred for periods shorter than 2 hours but treated under identical hydrothermal conditions, then a second phase crystallises, free of GaPO-34. If β-Ga2O3 is used as a reagent only the second phase is found to crystallise, irrespective of gel aging time. The competing phase, which we denote GaPO-34A, has been structurally characterised using synchrotron powder X-ray diffraction for the pyridine material, GaPO-34A(pyr), and using single-crystal X-ray diffraction for the 1-methylimiazole material, GaPO-34A(mim). The structure of GaPO-34A(pyr), P1 , a = 10.22682(6) Å, b = 12.09585(7) Å, c = 13.86713(8) Å, α = 104.6531(4) °, β = 100.8111(6) °, γ = 102.5228(6) °, contains 7 unique gallium sites and 6 phosphorus sites, with empirical formula [Ga7P6O24(OH)2F3(H2O)2].2(C5NH6). GaPO-34A(mim) is isostructural but is modelled as a half volume unit cell, P1 , a = 5.0991(2) Å, b = 12.0631(6) Å, c = 13.8405(9) Å, α = 104.626(5) °, β = 100.346(5) °, γ = 101.936(4) °, with a gallium and a bridging fluoride partially occupied and two partially occupied SDA sites. Solid-state 31P and 71Ga NMR spectroscopy confirms the structural complexity of GaPO-34A with signals resulting from overlapping lineshapes from multiple Ga and P sites, while 1H and 13C solid-state NMR spectra confirm the presence of the protonated SDA and provide evidence for disorder in the SDA. The protonated SDA is located in 14-ring one-dimensional channels with hydrogen bonding deduced from the SDA nitrogens to framework oxygen distances. Upon thermal treatment to investigate SDA removal, structure collapse occurs, which may be due the large number of bridging hydroxides and fluorides in the as-made material, and the unequal amounts of gallium and phosphorus present.
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Nov 2017
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I11-High Resolution Powder Diffraction
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Samuel A.
Morris
,
Giulia P. M.
Bignami
,
Yuyang
Tian
,
Marta
Navarro
,
Daniel S.
Firth
,
Jiří
Čejka
,
Paul S.
Wheatley
,
Daniel M.
Dawson
,
Wojciech A.
Slawinski
,
David S.
Wragg
,
Russell E.
Morris
,
Sharon E.
Ashbrook
Abstract: The assembly–disassembly–organization–reassembly (ADOR) mechanism is a recent method for preparing inorganic framework materials and, in particular, zeolites. This flexible approach has enabled the synthesis of isoreticular families of zeolites with unprecedented continuous control over porosity, and the design and preparation of materials that would have been difficult—or even impossible—to obtain using traditional hydrothermal techniques. Applying the ADOR process to a parent zeolite with the UTL framework topology, for example, has led to six previously unknown zeolites (named IPC-n, where n = 2, 4, 6, 7, 9 and 10). To realize the full potential of the ADOR method, however, a further understanding of the complex mechanism at play is needed. Here, we probe the disassembly, organization and reassembly steps of the ADOR process through a combination of in situ solid-state NMR spectroscopy and powder X-ray diffraction experiments. We further use the insight gained to explain the formation of the unusual structure of zeolite IPC-6.
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Apr 2017
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B18-Core EXAFS
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Laura
Mitchell
,
Patrick
Williamson
,
Barbora
Ehrlichová
,
Amanda E.
Anderson
,
Valerie R.
Seymour
,
Sharon E.
Ashbrook
,
Nadia
Acerbi
,
Luke
Daniels
,
Richard
Walton
,
Matthew L.
Clarke
,
Paul
Wright
Diamond Proposal Number(s):
[8708]
Abstract: The trivalent metal cations Al3+, Cr3+, and Fe3+ were each introduced, together with Sc3+, into MIL-100(Sc,M) solid solutions (M=Al, Cr, Fe) by direct synthesis. The substitution has been confirmed by powder X-ray diffraction (PXRD) and solid-state NMR, UV/Vis, and X-ray absorption (XAS) spectroscopy. Mixed Sc/Fe MIL-100 samples were prepared in which part of the Fe is present as α-Fe2O3 nanoparticles within the mesoporous cages of the MOF, as shown by XAS, TGA, and PXRD. The catalytic activity of the mixed-metal catalysts in Lewis acid catalysed FriedelCrafts additions increases with the amount of Sc present, with the attenuating effect of the second metal decreasing in the order Al>Fe>Cr. Mixed-metal Sc,Fe materials give acceptable activity: 40 % Fe incorporation only results in a 20 % decrease in activity over the same reaction time and pure product can still be obtained and filtered off after extended reaction times. Supported α-Fe2O3 nanoparticles were also active Lewis acid species, although less active than Sc3+ in trimer sites. The incorporation of Fe3+ into MIL-100(Sc) imparts activity for oxidation catalysis and tandem catalytic processes (Lewis acid+oxidation) that make use of both catalytically active framework Sc3+ and Fe3+. A procedure for using these mixed-metal heterogeneous catalysts has been developed for making ketones from (hetero)aromatics and a hemiacetal.
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Dec 2014
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B18-Core EXAFS
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Abstract: Perovskites of nominal composition NaCe1?xLaxTi2O6 (0?x?1) crystallise directly under hydrothermal conditions at 240 °C. Raman spectroscopy shows distortion from the ideal cubic structure and Rietveld analysis of powder X-ray and neutron diffraction reveals that the materials represent a continuous series in rhombohedral space group R3?c. Ce LIII-edge X-ray absorption near edge structure spectroscopy shows that while the majority of cerium is present as Ce3+ there is evidence for Ce4+. The paramagnetic Ce3+ affects the chemical shift and line width of 23Na MAS NMR spectra, which also show with no evidence for A-site ordering. 2H MAS NMR of samples prepared in D2O shows the inclusion of deuterium, which IR spectroscopy shows is most likely to be as D2O. The deuterium content is highest for the cerium-rich materials, consistent with oxidation of some cerium to Ce4+ to provide charge balance of A-site water.
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Oct 2013
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[6108]
Abstract: The crystal structure of NaNbO3 has been studied in detail in the temperature regime 360 < T < 520 °C using a combination of high-resolution neutron and synchrotron X-ray powder diffraction, supported by first-principles calculations. A systematic symmetry-mode analysis is used to determine the presence of the key active distortion modes that, in turn, provides a small and an unambiguous set of trial structural models. A unique model for Phase S (480 < T < 510 °C) is elucidated, having a 2 × 2 × 4 superlattice of the aristotype perovskite structure, space group Pmmn. This unusual and unique structure features a novel example of a compound octahedral tilt system in a perovskite. Two possible structural models for Phase R (370 < T < 470 °C) are determined, each having a 2 × 2 × 6 superlattice and differing only in the nature of the complex tilt system along the ‘long’ axis. It is impossible to identify a definitive model from the present study, although reasons for preferring one over the other are discussed. Some of the possible pitfalls in determining such complex, pseudosymmetric crystal structures from powder diffraction data are also highlighted.
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Jun 2012
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I11-High Resolution Powder Diffraction
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Abstract: The scandium analogue of the flexible terephthalate MIL-53 yields a novel closed pore structure upon removal of guest molecules which has unusual thermal behaviour and stepwise opening during CO2 adsorption. By contrast, the nitro-functionalised MIL-53(Sc) cannot fully close and the structure possesses permanent porosity for CO2.
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Jan 2012
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I11-High Resolution Powder Diffraction
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A. Lorena
Picone
,
Stewart J.
Warrender
,
Alexandra M. Z.
Slawin
,
Daniel M.
Dawson
,
Sharon E.
Ashbrook
,
Paul A.
Wright
,
Stephen P.
Thompson
,
Lucia
Gaberova
,
Philip L.
Llewellyn
,
Beatrice
Moulin
,
Alexandre
Vimont
,
Marco
Daturi
,
Min Bum
Park
,
Sam Kyung
Sung
,
In-sik
Nam
,
Suk Bong
Hong
Abstract: Copper cyclam (cyclam = 1,4,8,11-tetraazacyclotetradecane) and tetraethylammonium (TEA+) act as co-templates for the hydrothermal crystallisation of the silicoaluminophosphate SAPO STA-7, as determined by UV–visible, ESR and solid-state MAS NMR spectroscopies, powder and single crystal X-ray diffraction and chemical analysis. Calcination of the as-prepared solid in flowing oxygen removes all organics to leave Cu(II),H-SAPO STA-7 (hereafter Cu-SAPO STA-7) in which the presence of bridging hydroxyl groups is confirmed by IR and the presence of multiple environments for Cu2+ is shown by IR using NO as a probe molecule. Rietveld analysis of synchrotron X-ray powder diffraction data collected over the temperature range 293–673 K locates Cu2+ cations distributed over four sites: above six membered rings (6MRs) and in the three different 8MR windows of the STA-7 structure. Cu-SAPO STA-7 is a very good catalyst for the selective catalytic reduction of NO with NH3, in the presence or absence of water vapour, so that this approach represents an efficient and effective route to copper-containing SAPO catalysts that obviates the need for an aqueous Cu2+ ion exchange step during preparation.
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Dec 2011
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I11-High Resolution Powder Diffraction
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Abstract: The crystal structure of the small pore scandium terephthalate Sc2(O2CC6H4CO2)3 (hereafter Sc2BDC3, BDC = 1,4-benzenedicarboxylate) has been investigated as a function of temperature and of functionalization, and its performance as an adsorbent for CO2 has been examined. The structure of Sc2BDC3 has been followed in vacuo over the temperature range 140 to 523 K by high resolution synchrotron X-ray powder diffraction, revealing a phase change at 225 K from monoclinic C2/c (low temperature) to Fddd (high temperature). The orthorhombic form shows negative thermal expansivity of 2.4 × 10–5 K–1: Rietveld analysis shows that this results largely from a decrease in the c axis, which is caused by carboxylate group rotation. 2H wide-line and MAS NMR of deuterated Sc2BDC3 indicates reorientation of phenyl groups via π flips at temperatures above 298 K. The same framework solid has also been prepared using monofunctionalized terephthalate linkers containing -NH2 and -NO2 groups. The structure of Sc2(NH2-BDC)3 has been determined by Rietveld analysis of synchrotron powder diffraction at 100 and 298 K and found to be orthorhombic at both temperatures, whereas the structure of Sc2(NO2-BDC)3 has been determined by single crystal diffraction at 298 K and Rietveld analysis of synchrotron powder diffraction at 100, 298, 373, and 473 K and is found to be monoclinic at all temperatures. Partial ordering of functional groups is observed in each structure. CO2 adsorption at 196 and 273 K indicates that whereas Sc2BDC3 has the largest capacity, Sc2(NH2-BDC)3 shows the highest uptake at low partial pressure because of strong -NH2···CO2 interactions. Remarkably, Sc2(NO2-BDC)3 adsorbs 2.6 mmol CO2 g–1 at 196 K (P/P0 = 0.5), suggesting that the -NO2 groups are able to rotate to allow CO2 molecules to diffuse along the narrow channels.
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Sep 2011
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I11-High Resolution Powder Diffraction
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Abstract: Sodium niobate (NaNbO3) has a particularly complex phase diagram, with a series of phase transitions as a function of temperature and pressure, and even at room temperature a number of different structural variations have been suggested. Recent work has demonstrated that bulk powders of NaNbO3, prepared using a variety of synthetic approaches, contain a mixture of perovskite phases; the commonly reported Pbcm phase and a second, polar phase tentatively identified as belonging to space group P21ma. The two phases exhibit very similar 23Na MAS NMR spectra, although high-resolution MQMAS spectra were able to distinguish between them. Here, we investigate whether different perovskite polymorphs can be distinguished and/or identified using a variety of 93Nb NMR methods, including MAS, MQMAS and wideline experiments. We compare the experimental results obtained for these more common perovskite materials to those for the metastable ilmenite polymorph of NaNbO3. Our experimental results are supported by first-principles calculations of NMR parameters using a planewave pseudopotential approach. The calculated NMR parameters appear very different for each of the phases investigated, but high forces on the atoms indicate many of the structural models derived from diffraction require optimisation of the atomic coordinates. After geometry optimisation, most of these perovskite phases exhibit very similar NMR parameters, in contrast to recent work where it was suggested that 93Nb provides a useful tool for distinguishing NaNbO3 polymorphs. Finally, we consider the origin of the quadrupolar coupling in these materials, and its dependence on the deviation from ideality of the NbO6 octahedra.
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Mar 2011
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