Étude de l'oxydation de solides carbonés ex-cellulose et de leurs interactions avec des sels de cuivre supportés

This work was carried out inside the European Research Group (GDRE-CNRS) "Adsorbants carbonés et environnement". This fundamental study is devoted to the influence of some carbon physico-chemical properties (and especially carbon surface chemistry) on the preparation of supported-catalysts. Following the aim, it's necessary to have at one's disposal both functionnalized carbons and powerful analytical tools. Cellulose can be used as a well-defined carbonaceous precursor in order to prepare a great variety of supports in simple and reproducible conditions. The pyrolysis parameters were optimized, and the char obtained at 450 °C in a sealed reactor has the following characteristic ; a great content of elemental carbon (82%). Supplementary pretreatments at 250 °C were found to expand considerably the initially low specific surface area of the char. The solid surface chemistry was greatly modified by an oxidative post-treatment under air at 280 °C. Functionnal evolution during oxidation is mainly related to the progressive conversion of carboxyle goups into anhydrides structures. Acidic groups are beneficial in terms of rendering a more uniform distribution of the cooper precursor on the support during impregnation. The supported copper oxide is reduced during the activation under inert atmosphere at 400 °C in a two steps redox mechanism, in which carbon behaves as the reducing agent. Functionnal groups and low cooper loadings act like promoting factors for the reduction of the active phase. The connections between supports properties and active sites were further investigated with the help of a model reaction, the catalytic degradation of alcohols. The development of new methodologies for DRIFT spectroscopy has allowed us to monitor the functionnal evolution of the solids during oxidation, and also to study the interactions between cooper salts and the supports both during impregnation and activationLe contexte général de ce travail (qui s'inscrit dans le cadre du groupe de recherche européen (GDRE) du CNRS "Adsorbants carbonés et environnement") est l'étude fondamentale du rôle joué par les caractéristiques physico-chimiques des supports carbonés (et notamment de la chimie de surface) pendant la préparation des catalyseurs supportés. Dans ce cadre, il est nécessaire de disposer à la fois de supports hautement fonctionnalisés, ainsi que d'outils d'investigation adaptés. Une large variété de supports a été élaborée dans des conditions simples et reproductibles à partir de cellulose, précurseur carbone bien défini. Le résidu ex-cellulose obtenu après la pyrolyse en réacteur ferme à 450 °C est caractérisé par un teneur importante en carbone (82%) et un rendement de carbonisation élevé (33%). Les prétraitements réalisés à 250 °C permettent d'améliorer considérablement la surface spécifique des solides. La chimie de surface du carbonisat a été modulée par un post-traitement oxydant sous air à 280 °C. L'évolution fonctionnelle du solide pendant l'oxydation est reliée principalement à la conversion progressive des fonctions carboxyles en fonctions anhydrides. La distribution du sel de cuivre sur le support pendant l'imprégnation est favorisée par la présence de fonctions acides. L'oxyde CuO supporté est réduit pendant l'activation sous atmosphère inerte à 400 °C selon un mécanisme redox en deux étapes, le carbone jouant le rôle de réducteur. Les processus de réduction de la phase active sont favorisés sur les supports hautement fonctionnalisés, ainsi que par la diminution de la charge catalytique. Les relations support-site actif ont été précisées dans le cadre d'une réaction modèle, la dégradation catalytique des alcools. La mise au point de nouvelles méthodologies en DRIFT a permis d'étudier l'évolution fonctionnelle du solide pendant l'oxydation, ainsi que la nature des interactions entre les els de cuivre et les supports pendant l'imprégnation et l'activatio

Élimination sélective d'un mélange d'hydrocarbures imbrûlés Diesel par adsorption sur des matériaux zéolithiques

Actuellement, environ 80% de la pollution par les hydrocarbures (HC) imbrûlés est émise durant la période de démarrage à froid des moteurs (période dite de "cold-start"), d'une durée approximative de 120 s. Pour les véhicules Diesel, une méthode particulièrement prometteuse pourrait consister à piéger les HC sur un adsorbant à des températures inférieures à 200-250 C, pour les oxyder ensuite à plus haute température grâce à un catalyseur d'oxydation conventionnel. Cependant, peu d'études dans la littérature traitent de l'adsorption/désorption d'un mélange complexe d'HC, notamment en présence d'inhibiteurs. Dans cette étude, nous avons cherché à évaluer dans quelle mesure les propriétés structurales et chimiques d'adsorbants zéolithiques pouvaient affecter les caractéristiques d'adsorption et désorption d'un mélange modèle d'HC (propène, toluène, décane) en absence/présence d'inhibiteurs (NOx, H2O, COx..). Dans ce but, 3 séries de tests ont été menées pour mesurer les influences respectives: (i) de la composition du mélange d'HC (binaire/ternaire) ainsi que la présence d'eau sur la zéolithe HY de structure faujasite (pour différents rapports Si/Al); (ii) de l'incorporation d'un cation métallique (Cu, Pt, Ce, Cs) pour la zéolithe HY (rapport Si/Al = 5); (iii) de la topologie et de la porosité de zéolithes protonées (structures FAU, BEA, MOR, MFI, FER, LTA). Les propriétés texturales, structurales et chimiques des zéolithes étudiées ont été systématiquement caractérisées par porosimétrie à l'azote, DRX, FTIR/ATR, DRS-UV-Vis, FTIR du CO, NO et de la pyridine adsorbée. D'une manière générale, les courbes de percée obtenues avec différentes compositions indiquent que la diffusion de chaque HC est fortement affectée par la présence d'autres composés présents simultanément dans le réseau poreux. Pour les zéolithes Y protonées, les faibles rapports Si/Al favorisent la sélectivité pour l'adsorption des insaturés alors que les zéolithes plus hydrophobes (Si/Al élevés) adsorbent préférentiellement le décane. Parmi les différents inhibiteurs testés (CO, NOx, H2O), seul l'eau impacte fortement les capacités d'adsorption aux faibles rapports Si/Al, en raison d'une compétition avec les HC sur les sites acides. L'incorporation d'espèces métalliques dans la zéolithe HY-5 améliore la sélectivité d'adsorption pour les insaturés, notamment les plus légers (propène). Les expériences de TPSR sous atmosphère réactive ont montré que la présence de nouveaux sites acides de Lewis Cu+ et/ou Cu2+ en position d'échange améliore fortement la réduction des NOx par les HC pré-adsorbés. La zéolithe modifiée au Pt a un comportement typique d'un catalyseur d'oxydation, l'intégralité des composés carbonés (HC, CO,...) étant totalement convertie en CO2 à partir de 300C. L'étude des caractéristiques d'adsorption/désorption des HC pour différentes structures zéolithiques a permis de montrer que la topologie ainsi que la force des sites acides sont également des paramètres déterminants pour le piégeage des HC. L'ensemble de ces résultats permet d'envisager l'utilisation de certaines des formulations testées pour une application "cold-start"Currently, about 80% of the pollution from unburnt hydrocarbons (HC) is emitted during the cold-start period, for an approximate duration of 120 s. For Diesel vehicles, a promising approach might be to trap the HC onto an adsorbent below 200-250 C, and then to oxidize them at higher temperatures over a conventional oxidation catalyst. However, few publications in the literature deal with the adsorption / desorption of a complex mixture of HC, especially in the presence of inhibitors. In this study, we tried to assess in which extent structural and chemical properties of zeolite adsorbents could affect the adsorption and desorption characteristics of a model mixture of HC (propene, toluene, decane) in the absence/presence of inhibitors (NOx, H2O, COx ..). For this purpose, three sets of tests were performed in order to measure the respective influences of: (i) the composition of the HC mixture (ternary/binary) and the presence of water on the HY zeolite (with the faujasite structure) for several Si/Al ratios; (ii) the incorporation of a metallic cation (Cu, Pt, Ce, Cs) in the HY zeolite (Si/Al = 5); (iii) the topology and porosity of protonated zeolite (FAU, BEA, MOR, MFI, FER, LTA structures). The textural, structural and chemical properties of the studied zeolites were systematically characterized by nitrogen porosimetry, XRD, FTIR / ATR, DRS-UV-Vis, FTIR of CO, NO and adsorbed pyridine. In general, the breakthrough curves obtained with different compositions indicate that the distribution of each HC is strongly affected by the presence of other compounds present simultaneously in the porous network. For protonated Y zeolites, low Si/Al ratios promote selectively the adsorption of unsaturated HC while the more hydrophobic zeolites (for higher Si/Al ratios) preferentially adsorb decane. Among the various tested inhibitors (CO, NOx, H2O), only water has a strong influence on the adsorption capacities at low Si/Al ratios, due to a competitive adsorption with HC on acid sites. The incorporation of metallic species in the HY-5 zeolite enhances the selectivity for the adsorption of unsaturated hydrocarbons and especially the light ones (propene). TPSR experiments under reactive atmosphere showed that the presence of new Lewis acid sites Cu+ and/or Cu2+ on an exchange site strongly improves the NOx reduction by pre-adsorbed HC. The impregnated Pt-zeolite has a typical behaviour of an oxidation catalyst, all of the carboneous compounds (HC, CO,...) being completely converted to CO2 from 300 C. The study of adsorption/desorption characteristics of HC among different zeolitic structures show that the topology and the strength of acid sites are also critical for the HC trapping. All these results allow to consider using some of these tested formulations for a "cold-start" applicationNANCY-INPL-Bib. électronique (545479901) / SudocSudocFranceF

Structure–activity relationships between the state of silver on different supports and their i2 and ch3i adsorption properties

International audienceIn this study, the performances of silver-impregnated adsorbents prepared from different host supports (SBA-15, alumina, ceria, and faujasite Y zeolite) and calcined or not at 500 °C (1 h) were compared for the capture of I2 and CH3I. By keeping the silver content rather similar (about 15–17 wt %) among the sorbents, it was possible to assess the effect of silver dispersion and speciation on the adsorption capacities measured for both adsorbates. In a first part, several characterization techniques (XRD, DRS-UV-Vis, TEM, etc.) were used to probe the state of silver in the calcined and non-calcined materials. It was found that the characteristics of silver species are strongly influenced by the thermal treatment, the presence or absence of exchange sites, and the stability of the supports. Silver agglomeration was enhanced after calcination at 500 °C especially for supports bearing no exchange sites (SBA-15) or no ordered pores (alumina and ceria). Then, the adsorption performances of the studied silver sorbents were discussed in relation with their physicochemical characteristics. After-test characterizations were useful to assess the proportion of silver species that have reacted with CH3I and I2 to yield AgI precipitates. Depending on the adsorbate, different trends were obtained. I2 adsorption/reaction with silver sites was found to be quantitative (I/Ag ≈1), whatever the silver speciation and dispersion on the support. By contrast, a high proportion of cationic silver species was found essential to increase CH3I adsorption (I/Ag about 0.6–0.7 against 0.2–0.3 for Ag agglomerated species

Reactivity of ceria-zirconia catalysts for the catalytic wet peroxidative oxidation of azo dyes: reactivity and quantification of surface Ce(IV)-peroxo species

International audienceBACKGROUNDDoping nanoceria with zirconia often leads to improved catalytic properties due to enhanced concentration of structural defects in the resulting materials. This study aims to identify the parameters and mechanisms governing the reactivity of well-characterized commercial ceria-zirconia (CexZr1-xO2) nanocatalysts with various Ce contents (x=1, 0.80, 0.50, 0.21, 0) or surface area towards the degradation of Orange II dye in the presence of H2O2. RESULTSDiscoloration and mineralization kinetics were found to be strongly related both to the composition and surface area of the single or mixed oxides. Under standard conditions (23 degrees C, dark, stoichiometric concentration in H2O2), the best discoloration and mineralization performances were achieved for pure ceria (100% of color removal in 2h and 65% mineralization in 8h), but this arises mainly from its higher specific area by comparison with Zr-doped materials. For the first time, the amount of Ce-IV(O-2)(2-) surface species, intermediates in the production of hydroxyl radicals and generated from the reaction of H2O2 with Ce(III) sites, was quantitated using TPD-MS in order to provide a rational basis to explain the catalytic performance. CONCLUSIONDoping ceria with Zr increases the surface density of defective Ce(III) sites that can be converted to Ce-IV(O-2)(2-) species in the presence of H2O2, but other parameters, such as the specific surface area and surface Ce content of the catalysts, are also important. An in-depth mechanism for the reaction of H2O2 on the ceria-zirconia surface is discussed based on the experimental results. (c) 2015 Society of Chemical Industr

Performances of pretreated ceria-zirconia nanomaterials/H<SUB align="right">2</SUB align="right">O<SUB align="right">2</SUB align="right"> system for the degradation of Orange II dye

International audienceIn this work, the parameters affecting the reactivity of non-sulfated and sulfated commercial ceria-zirconia (CexZr1&minus;xO2) nanomaterials with varying Ce content (x = 1, 0.80, 0.50, 0.21, 0) towards the degradation of Orange II dye have been evaluated. Characterisation of the catalysts was performed using nitrogen adsorption, XRD, Raman, TGA thermogravimetry and DR-UV-Vis spectroscopy. Briefly, it can be deduced that the sulfation treatment mostly affects the Ce rich catalysts by increasing the crystallite size and lowering the specific surface area. It is shown that the H2O2 dissociates on Ce (III) surface sites to yield peroxide-like species. Such species played a key role in enhancing the discoloration of the dye under dark and UV-Vis conditions. By contrast, the sulfation treatment is unfavorable for the adsorption of H2O2. Sulfates interfere in the formation and the chemical stability of the surface peroxide species via pre-chelating the cerium surface centers, resulting in reducing the catalytic activity of catalysts, especially for CeO2

Evaluation of Silver Zeolites Sorbents Toward Their Ability to Promote Stable CH₃I Storage as AgI Precipitates

In this study, up to 13 different silver zeolites sorbents were prepared by repeated ion exchange from their parent structures (FAU X and Y, MOR, *BEA, MFI, FER), characterized, and evaluated for their ability to capture methyl iodide in the context of a nuclear severe accident. A novel methodology was implemented to establish structure–activity relationships between sorbent properties and iodine trapping stability. After saturation of the zeolite bed with CH₃I during a dynamic breakthrough experiment at 100 °C, a two-step quantitative desorption method was elaborated with the aim to quantify separately the CH₃I fractions trapped by physisorption, chemisorption, or reacted as AgI precipitates. Besides, an analysis of the mechanisms involved in CH₃I sorption and decomposition processes was also carried out. Overall, Ag/Y zeolites displayed the highest fractions trapped as stable AgI precipitates, thanks to the presence of high amounts of dispersed silver species at accessible locations in the large supercages, and their low sodium content

Dynamic sorption properties of Metal-Organic Frameworks for the capture of methyl iodide

International audienceIn this work, methyl iodide (CH3I) sorption properties of Metal-Organic Frameworks (MOFs) were experimentally studied. CH3I adsorption capacities were determined at 35 °C for the different prototypical selected MOFs, MIL-53(Al), MIL-100(Al), MIL-120(Al), CAU-1(Al), UiO-66(Zr), HKUST-1(Cu) and ZIF-8(Zn) from fixed-bed reactor measurements by using infrared spectroscopy. Quantitative exploitation of breakthrough curves reveals higher sorption performances (Qsat up to 425 mg/g) for materials showing pore size and geometry well-adjusted to CH3I dimension, such as MIL-120(Al), MIL-53(Al) and HKUST-1(Cu). In situ DRIFTS (Diffuse Reflection Infrared Fourier Transform Spectroscopy) experiments were also carried out in order to further investigate the interaction modes of physisorbed CH3I species within MIL-53 and MIL-120 compounds. For MIL-120, the confinement of adsorbed CH3I molecules within the pores is strengthened by specific interactions with the ligand and hydroxyl groups. For a breathing compound like MIL-53(Al), the adsorption measurement confirmed a swelling related to CH3I adsorption

Evolution of unburnt hydrocarbons under "cold-start" conditions from adsorption/desorption to conversion: On the screening of zeolitic materials

International audienceThe general purpose of this work is to examine the relative ability of some well-selected zeolitic materials for the reduction of HC emissions generated within the Diesel "cold-start" period, i.e. when the work temperature of the Diesel Oxidation Catalyst (DOC) has not been reached. More peculiarly, this study is focused on the chemical, textural and structural parameters of zeolites influent on the elimination, namely by adsorption, of unburnt HC (propene, toluene and decane) in presence of potential inhibitors (H2O, CO, NO). Simulated "cold-start" conditions consisted in the rapid heating of the pre-treated zeolite sorbent/catalyst under the whole gas mixture from 35 to 530 C. The quantity of trapped HC and those converted to CO, by oxidation were measured in function of the temperature, as well as the amount of NOx converted by the HC-SCR reaction. The interpretation of the HC emission profiles in close relation with the porous and acidic (through FTIR of adsorbed pyridine) properties of the corresponding zeolites allowed to gain insight onto the relative contributions of the pore topology, the pore size and the acid strength. For some selected zeolites, several consecutive cold-start cycles were performed in order to assess their stability

Effects of Zeolitic Parameters and Irradiation on the Retention Properties of Silver Zeolites Exposed to Molecular Iodine

We have investigated the iodine retention behavior of silver zeolite sorbents under a variety of conditions. First, a preliminary study was achieved for the liquid phase in order to determine (i) the main kinetic and thermodynamic characteristics pertaining to the adsorption behavior and (ii) the influence of some zeolitic parameters (nature of the framework, silver content, preparation method, etc.) on the adsorption capacities. Thanks to the online monitoring of gas-phase I₂ concentrations by UV–vis spectroscopy, the iodine dynamic adsorption/desorption behavior of some selected sorbents was then studied at 100 °C. It was found that faujasite sorbents with silver content higher than 20 wt % could maintain a high working capacity (up to 330 mg/g) and present a high ability for irreversible trapping. Finally, the stability of trapped iodine species was evidenced using additional tests performed at 120 °C in the presence of irradiation and humidity