Microscopic work function anisotropy and surface chemistry of 316L stainless steel using photoelectron emission microscopy

International audienceWe have studied the variation in the work function of the surface of sputtered cleaned 316L stainless steel with only a very thin residual oxide surface layer as a function of grain orientation using X-ray photo-electron emission microscopy (XPEEM) and Electron Backscattering Diffraction. The grains are mainly oriented [1 1 1] and [1 0 1]. Four distinct work function values spanning a 150 meV energy window are measured. Grains oriented [1 1 1] have a higher work function than those oriented [1 0 1]. From core level XPEEM we deduce that all grain surfaces are Cr enriched and Ni depleted whereas the Cr/Fe ratio is similar for all grains. The [1 1 1] oriented grains show evidence for a Cr 2 O 3 surface oxide and a higher concentration of defective oxygen sites

Direct and co-catalytic oxidative aromatization of 1,4-dihydropyridines and related substrates using gold nanoparticles supported on carbon nanotubes

International audienceA heterogeneous catalyst was assembled by stabilization of gold nanoparticles on carbon nanotubes. The resulting nanohybrid was used in the catalytic aerobic oxidation of 1,4-dihydropyridines. The system proved very efficient on the investigated substrates either directly or in the presence of a quinone co-catalyst. Pyridines have found applications in various domains such as in the synthesis of drugs, 1 herbicides, 2 or insecticides. 3 In addition, the pyridine scaffold plays a central role in living systems since nicotinamide adenine dinucleotide (NAD + / NADH) and nicotinamide adenine dinucleotide phosphate (NADP + /NADPH) are key pyridine-incorporating co-factors involved in oxido-reduction processes. 4 Among the various methods developed for the synthesis of pyridine derivatives, 5 the oxidation of Hantzsch 1,4-dihydropyridines is likely one of the most straightforward approaches. The Hantzsch pyridine synthesis is a multi-component process which involves an aldehyde, two equivalents of a β-keto ester, and a nitrogen atom source. The initial condensation affords a dihydro-pyridine intermediate which can be oxidized, in a second step, into the corresponding pyridine. This reaction was reported for the first time in 1881 by Arthur Hantzsch. 6 Numerous reagents have been reported to promote the second step-oxidative aromatization reaction and include, for example , copper bromide, 7 ferric chloride, 8 palladium on carbon, 9 manganese dioxide, 10 and tert-butylhydroperoxide. 11 However, the oxidation of dihydropyridines is not always easy when substrates bear sensitive functional groups and there is still the quest for mild and general protocols. Also, some of the reported procedures lead to the formation of by-products which can be difficult to remove from the reaction mixture. Therefore, the development of milder, sustainable, and more effective methods for the oxidative aromatization of 1,4-dihydropyridines is sound. In the present article, we report the assembly and use of a recyclable CNT-supported gold catalyst for the selective and mild aerobic oxidation of 1,4-dihydropyridines (DHPs) and related substrates (Fig. 1). The CNT-gold catalyst was assembled using a layer-by-layer approach according to a previously reported procedure Fig. 1 Overview of the catalytic assembly and catalyzed oxidation process studied in the present work

Mécanisme d'oxydation / dissolution du dioxyde d'uranium

Conférence du 02 au 06 juillet 2007. Communication par affiche

Uranium (VI) interaction with pyrite (FeS2): chemical and spectroscopic studies.

International audienceThe mechanism of uranium(VI) interaction with pyrite was studied by solution chemistry and X-ray Photoelectron Spectroscopy (XPS). Natural pyrite was characterized by X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). After equilibration in 10(-2) moI L-1 NaNO3, pyrite was reacted with uranium(VI) by the batch method in an anoxic glove box (P-O2 < 1 ppm) at ambient temperature. The reaction products of uranium, iron and sulphur were characterized (oxidation state, chemical environment) by XPS. Quantitative analysis revealed that only a few atomic percent of uranium is retained at the pyrite surface. The U4f core level binding energies are consistent with the coexistence of an uranium(VI) species and of uranium in a reduced form. No sulphur oxidation products were observed by XPS, but spectral decomposition of the Fe 2p lines revealed the presence of iron(III) oxide or (oxy)hydroxide. These results seem to point to a redox reaction between uraniurn(VI) and pyrite

Interaction de l'uranium (VI) et de la pyrite (FeS2)

Conférence les 10 et 11 mars 2005. Communication par affiche

Iodination of humic acid samples from different origins

http://www.radiochimacta.deInternational audienceThe iodination of Gorleben and synthetic humic substances (HS) was studied complementarily by UV-Visible and X-ray Photoelectron Spectroscopy (XPS). The I2(aq) consumption kinetics could neither be linearly correlated to [H+] nor to HS concentration as already observed in the literature. Nevertheless, the electrophilic substitution mechanism was further evidenced by a production of both I3- and I-, leading to a covalent bonding. The XPS analysis of the iodinated samples confirmed the covalent bonding between iodine and carbon of HS for all the samples

Rôle du silicium sur le comportement en oxydation d'un acier austénitique en milieu acide nitrique

National audienceL’enrichissement en silicium (de 3,7 à 4,5%m) d’un acier austénitique bascarbone a permis d’élever fortement sa résistance en corrosion dans le milieuHNO$_3$ chaud. Cette étude a été conduite pour analyser le comportement encorrosion de l’acier dans un large domaine de potentiels incluant les domainesactif, passif et transpassif. D’une part, une étude paramétrique a montrél’influence des différents paramètres sur le comportement en corrosion de l’acieren milieu HNO$_3$ : l’influence de l’acidité du milieu (jouant sur la stabilité desespèces), de la charge nitrate (impactant le caractère oxydant du milieu), de latempérature (modifiant les cinétiques), des produits de réduction comme HNO$_3$ etd’oxydation pouvant catalyser la cinétique de réduction. La littérature ne permetpas en effet de découpler les effets des différents paramètres. D’autrepart, la cinétique de corrosion et la nature de la couche passive ont été observéessur l’ensemble des potentiels du domaine anodique. Des méthodesélectrochimiques ont été utilisées (voltammétrie, chronoampérométrie,spectroscopie d’impédance) ainsi que des suivis de perte de masse et desméthodes d’analyses élémentaires (ICP-AES). Le couplage entre suivimassique/ICP-AES et électrochimie s’est révélé particulièrement intéressant pourdécoupler les contributions anodiques et cathodiques de la courbe de polarisationet a mis en évidence le domaine actif de l’acier (masqué par une contributioncathodique majoritaire). Ces résultats ont en particulier permis de montrer que laprésence de silicium permet à l’acier d’être polarisé dans son domaine passif loinde son domaine transpassif en milieu acide nitrique (contrairement aux aciersexempts de silicium), comme il avait été pressenti dans des travaux antérieurs.L’utilisation de microscopes (MO, MEB) et de l’XPS a permis de caractériser lamorphologie et la chimie de surface de l’acier en fonction du potentiel. Un lien apu ainsi être établi entre les propriétés de surface et le comportement enoxydation de l’acier, mettant en évidence le rôle particulier du siliciu

Transpassive elemental dissolution of a silicon-rich austenitic stainless steel in acidic media investigated through atomic emission spectroelectrochemistry

International audienceBecause of their good resistance to corrosion in acidic media, austenitic stainless steels are largely used for equipment of the spent nuclear fuel reprocessing plants. Still, they might be exposed to an intergranular type of corrosion. The intergranular corrosion appears in the transpassive domain of the steel, for example when the chemical environment becomes sufficiently oxidizing. However, when few wt.% of silicon are added to a 18Cr-10Ni stainless steel, the alloy loses its sensitivity to intergranular attack. This work compares the transpassive corrosion behavior of two similar stainless steel that are differentiated by the addition of silicon in their composition. Using the in-situ atomic emission spectroelectrochemistry (AESEC), chronoamperometries were performed in the transpassive domain of each steel. During the potentiostatic transpassive dissolution of each sample, an inductive coupled plasma atomic emission spectrometer connected to a three electrode flow cell enabled to quantify each element's dissolution rate. Then, scanning electron micrographs were taken to evaluate the surface's morphology after the experiment. As expected, the steel without silicon displayed both a selective dissolution behavior according to AESEC measurements and a locally corroded surface, very sensitive to the flow rate, according to SEM micrographs. On the other hand, the silicon-rich stainless steel displayed a non-selective dissolution behavior and a homogeneous surface morphology. Transmission electron microscopy was then used in addition to this work, to understand better the dissolution mechanisms in which the silicon is involved from its metallurgical characteristics in the bulk and in the passive layer. Comparing the total current measured and the elemental dissolution rates, the valence of each element dissolving was determined. In this work, in-situ and ex-situ techniques lead to the suggestion of a transpassive dissolution mechanism for each stainless steel as a function of silicon enrichment

Interaction de l'uranium (VI) avec la pyrite (FeS2).

Communications Orale

Uptake of uranium and trace elements in pyrite (FeS₂) suspensions

International audiencePyrite dissolution and interaction with Fe(II), Co(II), Eu(III) and U(VI) have been studied under anoxic conditions by solution chemistry and spectroscopic techniques. Aqueous data show a maximal cation uptake above pH 5.5. Iron (II) uptake can explain the non-stoichiometric [S]aq/[Fe]aq ratios often observed during dissolution experiments. Protonation data corrected for pyrite dissolution resulted in a proton site density of 9&nbsp;±&nbsp;3&nbsp;sites&nbsp;nm−2. Concentration isotherms for Eu(III) and U(VI) sorption on pyrite indicate two different behaviours which can be related to the contrasted redox properties of these elements. For Eu(III), sorption can be explained by the existence of a unique site with a saturation concentration of 1.25&nbsp;×&nbsp;10−6&nbsp;mol&nbsp;g−1. In the U(VI) case, sorption seems to occur on two different sites with a total saturation concentration of 4.5&nbsp;×&nbsp;10−8&nbsp;mol&nbsp;g−1. At lower concentration, uranium reduction occurs, limiting the concentration of dissolved uranium to the solubility of UO2(s).Scanning electron microscopy and micro-Raman spectrometry of U(VI)-sorbed pyrite indicate a heterogeneous distribution of U at the pyrite surface and a close association with oxidized S. X-ray photoelectron spectroscopy confirms the partial reduction of U and the formation of a hyperstoichiometric UO2+x(s). Our results are consistent with a chemistry of the pyrite surface governed not by Fe(II)-bound hydroxyl groups, but by S groups which can either sorb cations and protons, or sorb and reduce redox-sensitive elements such as U(VI)