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Abstract: Synchrotron radiation X-ray fluorescence microscopy is frequently used to investigate the spatial distribution of elements within a wide range of samples. Interrogation of heterogeneous samples that contain large concentration ranges has the potential to produce image artefacts due to the profile of the X-ray beam. The presence of these artefacts and the distribution of flux within the beam profile can significantly affect qualitative and quantitative analyses. Two distinct correction methods have been generated by referencing the beam profile itself or by employing an adaptive-thresholding procedure. Both methods significantly improve qualitative imaging by removing the artefacts without compromising the low-intensity features. The beam-profile correction method improves quantitative results but requires accurate two-dimensional characterization of the X-ray beam profile.

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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: A windowless electrochemical cell for the spectroscopic investigation of the liquid–liquid interface, using a dual droplet configuration, has been designed. The setup permits in situ probing of the bulk solutions and the interfacial region by fiber-optic UV–vis spectroscopy, microfocus X-ray fluorescence (XRF) elemental mapping, and microfocus X-ray absorption near-edge structure (?XANES) spectroscopy. The electrodeposition of Au, induced by ion transfer of the tetrachloroaurate complex from a halogenated solvent (containing a weak reducing agent) to the aqueous phase, has been monitored by a combination of the three techniques. The reaction can be followed in situ by UV–vis spectroscopy by detecting the oxidized form of the reducing agent. Voltammetric evidence suggests the formation of interfacial Au(I) species, whereas ?XANES detect the presence of metallic Au(0).

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Abstract: One of the major obstacles in replacing the widely used zinc dialkyldithiophosphate (ZDDP) antiwear additive with a more environmentally friendly one is the difficulty of time-resolving the surface species resulting from its decomposition mechanism under high contact pressure and temperature. To tackle this issue, a newly developed miniature pin-on-disc tribotester was coupled with synchrotron X-ray absorption spectroscopy (XAS) to perform in situ tribological tests while examining the composition of the formed triboreactive films. The results showed that in the case of bare steel surfaces the initial decomposition products are mainly zinc sulfate species, which with further shearing and heating are reduced to zinc sulfide mixed with metal oxides. The mixed base layer seems to enhance the tenacity of the subsequently formed zinc phosphate layers composing the main bulk of the protective triboreactive film. This base layer was not observed in the case of coated substrates with hydrogenated diamond-like carbon (a-C:H DLC) coating, which results in the formation of less durable films of small volume barely covering the contacting surfaces and readily removed by shear. Comprehensive decomposition pathways and kinetics for the ZDDP triboreactive films are proposed, which enable the control and modification of the ZDDP triboreactive films.

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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