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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.

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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.

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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

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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,1–4 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,6–8 and in corrosion of steel in radioactive waste solutions.9–11 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,13–15 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.16–18 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

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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 mineral–fluid–microbe 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.