B18-Core EXAFS
I18-Microfocus Spectroscopy
|
Diamond Proposal Number(s):
[7452, 5576, 3771, 6300]
Abstract: Europium LIII-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of synthetic glasses and melts equilibrated over a range of oxygen fugacities (fO2s, from -14 to +6 logarithmic units relative to the quartz-fayalite-magnetite, QFM, buffer) and temperatures (1250 – 1500 °C). Eu3+/ΣEu (where ΣEu = Eu2+ + Eu3+) values were determined from the spectra with a precision of ±0.015. Eu3+/ΣEu varies systematically with fO2 from 0 to 1 over the range studied, increases with decreasing melt polymerisation and temperature, and can be described by the empirical equation: Eu3+/ΣEu = 1/[1+10^(-0.25*logfO2 - 6410/T - 14.2Λ - 10.1)], where Λ is
the optical basicity of the melt and T is the temperature in K. Eu3+/ΣEu in glasses and melts equilibrated at the
same conditions are in excellent agreement for Fe-free systems. For Fe-bearing compositions the reaction
Eu2+ + Fe3+= Eu3+ + Fe2+ occurs during quenching to a glass and the high temperature value of Eu3+/ΣEu
is not preserved on cooling; in situ measurements are essential for determining Eu3+/ΣEu in natural melts.
|
Jul 2015
|
|
I18-Microfocus Spectroscopy
|
Diamond Proposal Number(s):
[8861]
Open Access
Abstract: Electrochemical gold nucleation and nanoparticle growth at a liquid/liquid interface were examined in situ by combined X-ray fluorescence (XRF) mapping with a microfocus X-ray beam and X-ray absorption fine structure (XAFS) spectroscopy. Gold deposition was achieved by reduction of [AuCl4]− with tri-p-tolylamine at a water/1,2-dichlorobenzene interface using a windowless liquid/liquid interface system formed from the contact of aqueous and organic phase droplets. The combination of XRF and XAFS, with a spatial resolution of approximately 70 μm, provided chemical speciation information near the interface under gold deposition conditions. Analysis of the X-ray absorption near-edge structure (XANES) reveals evidence for the presence of Au(I) species as a reduction intermediate, concomitant with a buildup of metallic gold at the interface. Cyclic voltammetry indicates the presence of two ion transfer peaks at the liquid/liquid interface, which are assigned to the transfer of [AuCl4]− and [AuCl2]−. Finally ex situ TEM analysis shows that the resultant nanoparticles have an average size between 3 and 4 nm. In line with this particle size, the XAFS indicates bulk-like structure.
|
Jul 2015
|
|
I18-Microfocus Spectroscopy
|
Diamond Proposal Number(s):
[11043]
Open Access
Abstract: Heterogeneous catalysis performed in the liquid phase is an important type of catalytic process which is rarely studied in situ. Using microfocus X-ray fluorescence and X-ray diffraction computed tomography (μ-XRF-CT, μ-XRD-CT) in combination with X-ray absorption near-edge spectroscopy (XANES), we have determined the active state of a Mo-promoted Pt/C catalyst (NanoSelect) for the liquid-phase hydrogenation of nitrobenzene under standard operating conditions. First, μ-XRF-CT and μ-XRD-CT reveal the active state of Pt catalyst to be reduced, noncrystalline, and evenly dispersed across the support surface. Second, imaging of the Pt and Mo distribution reveals they are highly stable on the support and not prone to leaching during the reaction. This study demonstrates the ability of chemical computed tomography to image the nature and spatial distribution of catalysts under reaction conditions.
|
Jul 2015
|
|
I13-2-Diamond Manchester Imaging
I18-Microfocus Spectroscopy
|
Steven
Street
,
W.
Xu
,
M.
Amri
,
Liya
Guo
,
Sarah
Glanvill
,
Paul
Quinn
,
Fred
Mosselmans
,
Joan
Vila-comamala
,
Christoph
Rau
,
Trevor
Rayment
,
Alison
Davenport
Diamond Proposal Number(s):
[7414, 8810]
Open Access
Abstract: Current oscillations were observed during one-dimensional pitting corrosion of 304 L stainless steel in neutral 1 M NaCl solutions with varying NaNO3 concentrations. Synchrotron X-ray diffraction was used to identify the salt layer at the corrosion front. It was found that, although current oscillations were induced in solutions with higher concentrations of NaNO3, the salt species in the pit did not change and a nitrate-free salt was present in all solutions. At higher NaNO3 concentrations, a change of salt crystal morphology was detected. Electrochemical oscillations were seen to coincide with secondary pitting on the pit surface indicating that two corrosion regimes were operating in parallel. Synchrotron radiography was used on artificial pits to measure the change in corrosion front and material loss in situ. Before nitrate was added, the corrosion front showed non-uniform material loss across the interface when beneath the salt layer. Nitrate addition induced a local region of passivation that propagated across the pit surface. Surface roughness was quantified using R-values and seen to vary without a clear trend until passivation, after which it stayed constant. A mechanism is suggested in which partial passivation occurs in these systems, where passivated areas are undercut as the corrosion front moves, generating surges in current.
|
Jun 2015
|
|
I18-Microfocus Spectroscopy
|
Diamond Proposal Number(s):
[7837]
Abstract: Current bioavailability models, such as the free ion activity model and biotic ligand model, explicitly consider that metal exposure will be mainly to the dissolved metal in ionic form. With the rise of nanotechnology products and the increasing release of metal-based nanoparticles (NPs) to the environment, such models may increasingly be applied to support risk assessment. However, it is not immediately clear whether the assumption of metal ion exposure will be relevant for NPs. Here using an established approach of oral gluing we have conducted a toxicokinetics study to investigate the routes of Ag NP and Ag+ ion uptake in the soil dwelling earthworm Lumbricus rubellus. Results indicated a significant part of the Ag uptake in the earthworms is through oral/gut uptake for both Ag+ ions and NPs. Thus, sealing the mouth reduced Ag uptake by between 40-75%. An X-ray analysis of the internal distribution of Ag in transverse sections confirmed the presence of increased Ag concentrations in exposed earthworm tissues. For the Ag NPs but not the Ag+ ions, high concentrations were associated with the gut wall, liver-like chloragogenous tissue and nephridia, which suggest a pathway for Ag NP uptake, detoxification and excretion via these organs. Overall our results indicate that Ag in ionic and NP form is assimilated and internally distributed in earthworms and that this uptake occurs predominantly via the gut epithelium and less so via the body wall. The importance of oral exposure questions the application of current metal bioavailability models, which implicitly consider that the dominant route of exposure is via the soil solution, for bioavailability assessment and modelling of metal-based NPs. Copyright Wiley Interscience
|
Apr 2015
|
|
I18-Microfocus Spectroscopy
|
Open Access
Abstract: The effect of nitrate on the iron dissolution in artificial corrosion pits in chloride-containing solutions has been studied with in-situ
synchrotron X-ray diffraction and ex-situ Raman spectroscopy. A black layer containing Fe3C and carbon was found in the pit in
pure 1 M HCl at 0.1 V(Ag/AgCl), but at higher voltages up to 1 V(Ag/AgCl), the black layer was not observed and Fe3C was not
detected. The dissolution of Fe3C at low potentials may be inhibited by a carbon layer. In the presence of nitrate, the black layer
containing Fe3C was present at all the voltages tested, and current fluctuations were observed and found to be potential-dependent,
with greater fluctuations at higher voltages. After dissolution in 1 M HCl, the metal surface appeared slightly roughened with some
small cavities, but the surface became crystallographically etched after dissolution in 1 M HCl and 50 mM NaNO3.
|
Feb 2015
|
|
I18-Microfocus Spectroscopy
|
Open Access
Abstract: The effect of nitrate on the salt layers in iron artificial corrosion pits in acidic chloride solutions has been studied using in-situ synchrotron X-ray diffraction. During dissolution in 1 M HCl, there is a salt layer of FeCl2.4H2O on the electrode surface, which is isotropic. With addition of trace nitrate, the salt layer remains FeCl2.4H2O and no nitrate phase is observed, but the diffraction pattern becomes anisotropic, consistent with the formation of platelets with (1 2 0) planes settling horizontally. In nitrate solution containing trace of chloride (0.1 M HNO3 + 10 mM HCl), a salt layer is formed that is isostructural with Co(NO3)2.6H2O, and therefore assumed to be Fe(NO3)2.6H2O. This is the first reported crystal structure of ferrous nitrate. The salt layer is also found to give an anisotropic diffraction pattern, consistent formation of platelets with (0 2 0) planes settling horizontally.
It is well known that salt layers can form at the bottom of growing corrosion pits due to supersaturation of metal salts,14 and that the presence of salt layers is important for continued pit growth.5 This information is significant both in the field of electrochemical machining (ECM), where nitrate solutions are often used,68 and in corrosion of steel in radioactive waste solutions.911
These salt layers are a slurry of crystallites that form on a dissolving metal surface12 when the rate of metal ion production (dissolution) is greater than the rate that they can diffuse from the interface, leading to supersaturation and thus crystallite nucleation. The equilibrium thickness of the layer is determined by a self-regulating process.13 The formation of the salt layer leads to a resistance to ion flow in the electrolyte since ions can only flow in channels between the crystallites. This resistance decreases the interfacial potential, decreasing the dissolution rate. The steady state thickness of the salt layer is such that the rate of metal ion production is equal to the rate of escape.
Investigations of salt layers in pits can only be carried out in situ, since they dissolve as soon as the interfacial potential driving dissolution is removed. Studies are often carried out in artificial corrosion pits, in which a wire or foil is embedded in resin and dissolved back to give a one-dimensional cavity,1315 which is simpler to study and model.
Electrochemical impedance measurements on the salt layer of iron in a chloride solution have led to the suggestion that the salt layer is duplex, comprising a compact semiconducting inner film and a porous outer film.1618 Rayment et al.12 carried out an in-situ synchrotron study on salt layers on Fe and stainless steel in HCl-containing artificial pits, and in both systems, the salt layers were found to be FeCl2.4H2O, but the size of the crystallites on Fe was much smaller than that on stainless steel.
However, despite their practical importance, salt layers in nitrate-containing solutions have received scant attention except in electrochemical machining (ECM) studies. Surface brightening was reported to involve salt precipitation on an iron surface in a nitrate-containing electrolyte.19 It was proposed that, during ECM processing of iron in NaNO3 (potential above 2 V, current density up to 80 A/cm2), a salt layer with a duplex structure was present on iron surfaces, comprising a solid oxide inner film and a supersaturated outer layer, which is a meta-stable viscous solution or molten salt of Fe(NO3)3.9H2O/Fe(NO3)2.6H2O due to Joule heating at high current densities.7,20,21 The lack of free water in the outer layer was presumed to suppress oxygen evolution.8
In this work, the salt layer composition and structure in nitrate-containing solutions have been characterized using in-situ synchrotron X-ray diffraction combined with electrochemical measurements on iron artificial pits
|
Feb 2015
|
|
B18-Core EXAFS
|
Diamond Proposal Number(s):
[4901]
Open Access
Abstract: Radioactive caesium (chiefly 137Cs) is a major environmental pollutant. The mobility of Cs in temperate soils is primarily controlled by sorption onto clay minerals, particularly the frayed edges of illite interlayers. This paper investigates the adsorption of Cs to illite at the molecular scale, over both the short and long term. Transmission electron microscopy (TEM) images showed that after initial absorption into the frayed edges, Cs migrated into the illite interlayer becoming incorporated within the mineral structure. Caesium initially exchanged with hydrated Ca at the frayed edges, causing them to collapse. This process was irreversible as Cs held in the collapsed interlayers was not exchangeable with Ca. Over the long term Cs did not remain at the edge of the illite crystals, but diffused into the interlayers by exchange with K. Results from extended X-ray absorption fine structure spectroscopy (EXAFS) and density functional theory modelling confirmed that Cs was incorporated into the illite interlayer and revealed its bonding environment.
|
Feb 2015
|
|
B18-Core EXAFS
B22-Multimode InfraRed imaging And Microspectroscopy
I06-Nanoscience
I10-Beamline for Advanced Dichroism
I11-High Resolution Powder Diffraction
I12-JEEP: Joint Engineering, Environmental and Processing
I18-Microfocus Spectroscopy
I22-Small angle scattering & Diffraction
|
Open Access
Abstract: Diamond Light Source Ltd celebrated its 10th anniversary as a company in December 2012 and has now accepted user experiments for over 5 years. This paper describes the current facilities available at Diamond and future developments that enhance its capacities with respect to the Earth and environmental sciences. A review of relevant research conducted at Diamond thus far is provided. This highlights how synchrotron-based studies have brought about important advances in our understanding of the fundamental parameters controlling highly complex mineralfluidmicrobe interface reactions in the natural environment. This new knowledge not only enhances our understanding of global biogeochemical processes, but also provides the opportunity for interventions to be designed for environmental remediation and beneficial use.
|
Jan 2015
|
|
B18-Core EXAFS
|
Open Access
Abstract: We used a combination of optically-detected x-ray absorption spectroscopy with molecular dynamics simulations to explore the origins of light emission in small (5 nm to 9 nm) Ge nanoparticles. Two sets of nanoparticles were studied, with oxygen and hydrogen terminated surfaces. We show that optically-detected x-ray absorption spectroscopy shows sufficient sensitivity to reveal the different origins of light emission in these two sets of samples. We found that in oxygen terminated nanoparticles its the oxide-rich regions that are responsible for the light emission. In hydrogen terminated nanoparticles we established that structurally disordered Ge regions contribute to the luminescence. Using a combination of molecular dynamics simulations and optically-detected x-ray absorption spectroscopy we show that these disordered regions correspond to the disordered layer a few Å thick at the surface of the simulated nanoparticle.
|
Dec 2014
|
|