Methodology of mechanical characterization of coated spherical materials

The aim of this work was to develop a methodology for the mechanical characterization of catalyst beads with a core-shell structure and more especially for coated spherical granules. Supports composed of an alpha alumina core coated by gamma alumina shell were shaped by pan coating to this purpose. The proposed methodology started with the characterization of the microstructure of the coating and the highlight of potential macro defects within. Thereafter three tests simulating mechanical stress, such as impact, compression (bulk crushing test), and shear (drum attrition test) are used. The operational parameters of these tests were also optimised in order to stress preferentially the shell of the coated materials. Among the evaluated tests, drum attrition seems to be most efficient for characterizing coated spherical granules

Effect of zeolite topology and reactor configuration on the direct conversion of CO2 to light olefins and aromatics

The direct transformation of CO2 into high-value-added hydrocarbons (i.e., olefins and aromatics) has the potential to make a decisive impact in our society. However, despite the efforts of the scientific community, no direct synthetic route exists today to synthesize olefins and aromatics from CO2 with high productivities and low undesired CO selectivity. Herein, we report the combination of a series of catalysts comprising potassium superoxide doped iron oxide and a highly acidic zeolite (ZSM-5 and MOR) that directly convert CO2 to either light olefins (in MOR) or aromatics (in ZSM-5) with high space–time yields (STYC2-C4= = 11.4 mmol·g–1·h–1; STYAROM = 9.2 mmol·g–1·h–1) at CO selectivities as low as 12.8% and a CO2 conversion of 49.8% (reaction conditions: T = 375 °C, P = 30 bar, H2/CO2 = 3, and 5000 mL·g–1·h–1). Comprehensive solid-state nuclear magnetic resonance characterization of the zeolite component reveals that the key for the low CO selectivity is the formation of surface formate species on the zeolite framework. The remarkable difference in selectivity between the two zeolites is further rationalized by first-principles simulations, which show a difference in reactivity for crucial carbenium ion intermediates in MOR and ZSM-5

Fe-MOF Materials as Precursors for the Catalytic Dehydrogenation of Isobutane.

We investigate the use of a series of iron-based metal-organic frameworks as precursors for the manufacturing of isobutane dehydrogenation catalysts. Both the as-prepared and spent catalysts were characterized by PXRD, XPS, PDF, ICP-OES, and CHNS+O to determine the physicochemical properties of the materials and the active phases responsible for the catalytic activity. In contrast to the previous literature, our results indicate that (i) the formation of metallic Fe under reaction conditions results in secondary cracking and coke formation; (ii) the formation of iron carbide only contributes to coke formation; and (iii) the stabilization of the Fe2+ species is paramount to achieve stable and selective catalysts. In this sense, promotion with potassium and incorporation of titanium improve the catalytic performance. While potassium is well known to improve the selectivity in iron-catalyzed dehydrogenation reactions, the unprecedented effect of titanium in the stabilization of a nanometric titanomaghemite phase, even under reductive reaction conditions, results in a moderately active and highly selective catalyst for several hours on stream with a remarkable resistance to coke formation

Elaboration, mise en forme et résistance mécanique de bi-matériaux sphériques (application en catalyse)

Cette étude est consacrée à l'élaboration, la mise en forme et à la caractérisation de la résistance mécanique de bi-matériaux sphériques. Ce type de matériaux peut être utilisé en extrême périphérie. Ainsi, il est proposé de remplacer le support classique par des matériaux à base d'alumine comportant une couche de grande surface spécifique d'épaisseur contrôlée, supportés sur des billes de très faible surface spécifique. La surface spécifique importante de la couche permet un dépôt sélectif de la phase métallique dans cette même couche lors de l'imprégnation de pre curseurs. A l'issur de cette étude, nous avons établi qu'il est possible d'obtenir des bi-matériaux sphériques par deux voies. La première fait intervenir un enrobage des coeurs d'alumine alpha par granulation des charges d'alumine gamma en utilisant un sol de boehmite en tant que liant. L'autre voie consiste en la pulvérisation de codispersions &queuses de boehmite et d'alumine gamma ou encore de sols de boehmite dans un lit fluidisé chaud de particules d'alumine alpha. Les résultats obtenus ont montré que les contraintes générées par le retrait après gélification de la boehmite au cours du séchage conditionnent les propriétés des couches formées. Ainsi, l'utilisation de sol de boehmite gélifiant avec une teneur en eau réduite, le choix d'une taille de cristallite de boehmite, le couplage d'une opération de séchage à la mise en forme, l'ajout d'une composante ductile au gel de boehmite (alcool polyvinylique) permettent d'améliorer la qualité des couches obtenues après séchage. Ces bi-matériaux, ont conduit à l'obtention des catalyseurs à base de palladium pour lesquels la phase métallique était essentiellement déposée dans l'épaisseur de la couche. L'évaluation de ce type de solide en hydrogénation sélective a montré des activités et des sélectivités très élevées.This study deals with the elaboration, shaping and mechanical strength charactérization of spherical core-shell materials. Such materials can be used as supports for the preparation of catalysts with the active phase distributed at the extreme outer layer. Thus, it is proposed to replace typical supports by low surface alumina area beads coated with a high specific surface area alumina layer of controlled thickness. The high specific surface area alumina layer allows a selective deposition of the metallic phase in this layer via impregnation of precursors. From this study, we have conclued that it is possible to form spherical core-shell materials by two routes. The first one involves the coating of alpha alumina cores by granulation of gamma alumina fillers using a boehmite sol as binder. The other route consists in pulverisation of aqueous co-dispersions of boehmite and gamma alumina or boehmite sols in a hot fluidized bed of alpha alumina beads. The results showed that the stress generated by shrinkage of boehmite gelation during drying governs the properties of the formed layers. Thus, the use of boehmite sol gelling with reduced water content, the choice of boehmite crystallite size, the coupling of a drying operation to shaping, the introduction of a ductile component to the boehmite gel (polyvinylic alcohol) allow to improve the layer quality after drying. These core-shell materials led to palladium based catalysts on which the mettalic phase was mainly deposited in the shell thickness. The evaluation of this kind of solids in selective hydrogenation showed high activity and selectivity.LYON-ENS Sciences (693872304) / SudocSudocFranceF

On the reconstruction of NiMo electrocatalysts by operando spectroscopy

Dissolved MoO 4 2− from NiMo electrodes during hydrogen evolution redeposits during high energy spectroscopy, providing misleading but important insights into Mo behavior

Establishing Efficient Cobalt-Based Catalytic Sites for Oxygen Evolution on a Ta₃N₅ Photocatalyst

In a photocatalytic suspension system with a powder semiconductor, the interface between the photocatalyst semiconductor and catalyst should be constructed to minimize resistance for charge transfer of excited carriers. This study demonstrates an in-depth understanding of pretreatment effects on the photocatalytic O₂ evolution reaction (OER) activity of visible-light-responsive Ta₃N₅ decorated with CoO_<i>x</i> nanoparticles. The CoO_<i>x</i>/Ta₃N₅ sample was synthesized by impregnation followed by sequential heat treatments under NH₃ flow and air flow at various temperatures. Various characterization techniques, including X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), scanning transmission electron microscopy (STEM), and X-ray photoelectron spectroscopy (XPS), were used to clarify the state and role of cobalt. No improvement in photocatalytic activity for OER over the bare Ta₃N₅ was observed for the as-impregnated CoO_<i>x</i>/Ta₃N₅, likely because of insufficient contact between CoO_<i>x</i> and Ta₃N₅. When the sample was treated in NH₃ at high temperature, a substantial improvement in the photocatalytic activity was observed. After NH₃ treatment at 700 °C, the Co⁰–CoO_<i>x</i> core–shell agglomerated cobalt structure was identified by XAS and STEM. No metallic cobalt species was evident after the photocatalytic OER, indicating that the metallic cobalt itself is not essential for the reaction. Accordingly, mild oxidation (200 °C) of the NH₃-treated CoO_<i>x</i>/Ta₃N₅ sample enhanced photocatalytic OER activity. Oxidation at higher temperatures drastically eliminated the photocatalytic activity, most likely because of unfavorable Ta₃N₅ oxidation. These results suggest that the intimate contact between cobalt species and Ta₃N₅ facilitated at high temperature is beneficial to enhancing hole transport and that the cobalt oxide provides electrocatalytic sites for OER

Stable Cr-MFI Catalysts for the Nonoxidative Dehydrogenation of Ethane: Catalytic Performance and Nature of the Active Sites

The nonoxidative catalytic dehydrogenation of ethane allows the production of ethylene at lower temperatures than those applied in steam crackers. This, however, requires stable catalysts that minimize coke production. Here, we report a single-component, promoter-free, low-loading, Cr-based catalyst exhibiting high activity, long-term stability, and improved regeneration properties for the direct dehydrogenation of ethane to ethylene. According to our detailed operando X-ray absorption spectroscopic analysis, the use of all-silica MFI zeolite as support promotes the stabilization of CrII(−O–Si≡)2 species with high coke resistance, even when the dehydrogenation is carried out under high ethane partial pressures (1.5 bar).ISSN:2155-543

Pure silica-supported transition metal catalysts for the non-oxidative dehydrogenation of ethane: confinement effects on the stability

Designing robust catalysts for high-temperature applications has always been a critical task for chemical industries. As an example, the non-oxidative dehydrogenation of alkanes is an important chemical process that requires thermally stable metal catalysts with high resistance to metal sintering. The main obstacle is to maintain the high dispersion of the active metal centres under reaction and regeneration conditions. In an attempt to overcome this issue, here we use all-silica zeolite as a support to make nanometric and single-site metal catalysts with enhanced stability for the non-oxidative dehydrogenation of ethane. Preliminary screening of different metal catalysts suggests that Co has the highest intrinsic activity while Cr and V are highly stable against sintering and display a moderate activity. The high stability of Cr and V could be attributed to their high Gibbs energy of reduction under reaction conditions. Operando X-ray absorption spectroscopy revealed that Cr based catalysts remain as single-site monomeric species during the reaction, making it possible to increase the loading and therefore productivity. In the case of Co, we established the optimum parameters to achieve the highest activity by evaluating the effects of support, metal loading, promoter, and synthesis process.ISSN:2050-7488ISSN:2050-749

PdZn/ZrO2+SAPO-34 bifunctional catalyst for CO2 conversion: Further insights by spectroscopic characterization

&lt;p&gt;Supplementary material: &nbsp;atomic concentrations calculated from XPS, XPS spectra&lt;/p&gt