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Localization and migration of highly mobile and extremely efficient bismuth promoter in iron Fischer–Tropsch catalysts were elucidated using in situ methods., Liquid metals are a new emerging and rapidly growing class of materials and can be considered as efficient promoters and active phases for heterogeneous catalysts for sustainable processes. Because of low cost, high selectivity and flexibility, iron-based catalysts are the catalysts of choice for light olefin synthesis via Fischer–Tropsch reaction. Promotion of iron catalysts supported by carbon nanotubes with bismuth, which is liquid under the reaction conditions, results in a several fold increase in the reaction rate and in a much higher light olefin selectivity. In order to elucidate the spectacular enhancement of the catalytic performance, we conducted extensive in-depth characterization of the bismuth-promoted iron catalysts under the reacting gas and reaction temperatures by a combination of cutting-edge in situ techniques: in situ scanning transmission electron microscopy, near-atmospheric pressure X-ray photoelectron spectroscopy and in situ X-ray adsorption near edge structure. In situ scanning transmission electron microscopy conducted under atmospheric pressure of carbon monoxide at the temperature of catalyst activation showed iron sintering proceeding via the particle migration and coalescence mechanism. Catalyst activation in carbon monoxide and in syngas leads to liquid bismuth metallic species, which readily migrate over the catalyst surface with the formation of larger spherical bismuth droplets and iron–bismuth core–shell structures. In the working catalysts, during Fischer–Tropsch synthesis, metallic bismuth located at the interface of iron species undergoes continuous oxidation and reduction cycles, which facilitate carbon monoxide dissociation and result in the substantial increase in the reaction rate.

International audience; The concept of interfacial coordination chemistry was one of the innovation for which Michel Che became internationally known during his tenure as full professor at the Université Pierre et Marie Curie. Isolated metal ions on oxide surfaces behave like coordination compounds to which the surface contributes not only via neutral oxo bridges but also via hydroxyl groups. The latter being pH sensitive, the surface can be charged either positively or negatively, allowing selective adsorption of transition metal ions of opposite charge. In the early eighties, examples exploiting these properties for the preparation of oxide/metal supported catalysts were still rare. This review presents Michel Che's early studies, mostly on molybdenum and nickel ions, on which he built, developed and promoted the concept of interfacial coordination chemistry, now widely used in the field of heterogeneous catalysis. This allowed him and his close collaborators to achieve a molecular-scale understanding of several catalyst preparation procedures, such as impregnation, selective adsorption, grafting, depositionprecipitation, and zeolite functionalization. The impact and legacy of the concept of Interfacial Coordination Chemistry on the current design of improved catalytic formulations is also reviewed.

International audience; A monometallic Ni and a bimetallic Ni-Fe catalyst were used in the aqueous phase hydrogenation of xylose in batch conditions (T = 50–150 °C, PH2 = 10–30 bar, xylose mass fraction in water = 3.7–11.0 wt.%) to evidence the benefits of promoting Ni by Fe. The activity of the catalysts increased with temperature, but a temperature of 80 °C allowed minimizing nickel leaching at full conversion. The presence of reduced Fe at the surface of the bimetallic nanoparticles increased both the first-order apparent rate constant and the adsorption constant of xylose. The catalytic activity of Ni/SiO2 strongly declined and no deactivation was found for the Ni-Fe catalyst. A restructuring of the bimetallic nanoparticles took place, as the size of the metal particles and the Fe proportion in the surface layers increased, suggesting a flattening and coalescing of the particles over the silica surface.

International audience; Copper is active in hydrogenation reactions, but it is a base metal difficult to disperse inside mesostructured supports, with adverse consequences on the catalytic properties. Incorporating aluminium into mesoporous SBA-15 silicas allows solving both problems of Cu dispersion and activity. The incorporation of 20 wt.% Al2O3 in the SBA-15 host structure leads to an increase of the copper metallic surface area by a factor 6 compared to the pure siliceous SBA-15-supported catalyst. The support also proves to be stable upon exposure to the impregnation aqueous solution used for copper introduction. The incorporation of Al provides Lewis acidic sites that favour the transformation of cinnamaldehyde into cinnamyl alcohol, a feature usually assigned to larger metal particles. As a consequence, the catalytic activity of Cu catalysts is enhanced compared to silica-supported Cu catalysts, while the chemoselectivity towards the unsaturated alcohol appears to be similar to that of a mesoporous alumina-supported system.

The selective hydroconversion of 5‐hydroxymethylfurfural (HMF) to biofuels is currently highly sought‐for. While the literature has demonstrated that this reaction is possible on promoted Ni catalysts, we show here that a monometallic, non‐promoted Ni/SBA‐15 catalyst, prepared by incipient wetness impregnation, can convert HMF to 2,5‐dimethylfuran (DMF) and to 2,5‐dimethyltetrahydrofuran (DMTHF) at 180 °C, in a consecutive way. Through a control over reaction time, high yields to DMF (71 %, at conversion of 93 %) or DMTHF (97 %, at conversion of 100 %) can be achieved. Kinetic modelling suggests a preferential route to DMF via 5‐methylfurfural (MFFR) as intermediate, though the route via 2,5‐bis(hydroxylmethyl)furan (BHMF) is also present. The favored route in the experimental conditions involves the hydrogenolysis of the hydroxyl group of HMF as first step, followed by the hydrogenation of the aldehyde function, to methylfurfuryl alcohol (MFOL). It is suggested a higher reaction rate of hydrogenation or hydrogenolysis of the side group is linked to the presence of a methyl group in the molecule. No hydrogenation of the furan ring is detected on the intermediates. 12;7

International audience; Supported Fe-Ni catalysts have been reported for their activity and selectivity in the hydrogenation of unsaturated organic molecules. However, the control of the size and composition of the bimetallic nanoparticles remains a bottleneck when oxide-supported catalysts are prepared by impregnation, and alternative procedures should be investigated. Starting with Ni(II) and Fe(II) sulfates as precursor salts, deposition-precipitation with urea (DPU) on SiO2 in an inert atmosphere initially leads to the formation of an ill-crystallized Fe-containing Ni(II) 1:1 phyllosilicate, which reduces under hydrogen at 700 °C into bimetallic fcc Fe-Ni nanoparticles of 5.4 nm in average. Compared with the composition of the DPU solution (50 Fe at %, 50 Ni at %), an excess of Ni is detected on the catalyst (38 Fe at %, 62 Ni at %), due to the preferential reaction of Ni2+ ions with silica. In situ X-ray absorption spectroscopy and 57Fe Mössbauer spectroscopy show that the reduction of Fe ions to the metallic state is triggered by the formation of reduced Ni centers above 350 °C, and, from then, proceeds progressively, resulting in an excess of Fe in the outer shells of the bimetallic particles. The composition of individual Fe-Ni particles evidences a standard deviation of 8%. The bimetallic Fe-Ni/SiO2 catalyst gives high yields in furfuryl alcohol in the hydrogenation of furfural, in contrast with an analog Ni/SiO2 catalyst that favours side-reactions of etherification, hydrogenolysis and hydrogenation of the furan ring.

Melt infiltration (MI) has been described as an effective way to disperse transition metals (TM) in ordered mesoporous supports. The alternative method described here is based on the infiltration of molten precursors into the support pores in the presence of a support polymer porogen. The resulting materials contain metal oxide particles (metallic particles after reduction) smaller than 2 nm, dispersed within the support micropores, which prove to be stable upon reduction up to 900 °C and to exhibit improved catalytic performances for the hydrogenation of cinnamaldehyde due to the high fraction of the active metal exposed.

Successfully modeling the behavior of catalytic systems at different scales is a matter of importance not only for a fundamental understanding but also for a more rational design of catalysts and a more precise definition of the kinetic laws used as inputs in chemical engineering. We have developed here a multiscale modeling of the dehydration of isopropyl alcohol to propene and diisopropyl ether on γ-alumina catalysts, which clearly evidences and explains the central character of cooperative effects between coadsorbates in the kinetic network. The evolution of partial pressures with contact time was simulated using an original DFT-based microkinetic model based on a “macro site” centered on the main active site located on the (100) planes of alumina and comprising several neighboring adsorption sites. The formation of isopropyl alcohol–isopropyl alcohol or water–isopropyl alcohol dimers on the surface was required to correctly simulate the production of the minor product, diisopropyl ether, and the evolu...

Furfural and 5-hydroxymethylfurfural stand out as bridges connecting biomass raw materials to the biorefinery industry. Their reductive transformations by hydroconversion are key routes toward a wide variety of chemicals and biofuels, and heterogeneous catalysis plays a central role in these reactions. The catalyst efficiency highly depends on the nature of metals, supports, and additives, on the catalyst preparation procedure, and obviously on reaction conditions to which catalyst and reactants are exposed: solvent, pressure, and temperature. The present review focuses on the roles played by the catalyst at the molecular level in the hydroconversion of furfural and 5-hydroxymethylfurfural in the gas or liquid phases, including catalytic hydrogen transfer routes and electro/photoreduction, into oxygenates or hydrocarbons (e.g., furfuryl alcohol, 2,5-bis(hydroxymethyl)furan, cyclopentanone, 1,5-pentanediol, 2-methylfuran, 2,5-dimethylfuran, furan, furfuryl ethers, etc.). The mechanism of adsorption of the reactant and the mechanism of the reaction of hydroconversion are correlated to the specificities of each active metal, both noble (Pt, Pd, Ru, Au, Rh, and Ir) and non-noble (Ni, Cu, Co, Mo, and Fe), with an emphasis on the role of the support and of additives on catalytic performances (conversion, yield, and stability). The reusability of catalytic systems (deactivation mechanism, protection, and regeneration methods) is also discussed.

International audience; This chapter gives an overview of the current preparation strategies for the synthesis of bulk and supported metal oxide catalysts, in order to give rational guides to understand the synthetic challenges associated with the preparation of these catalytic systems developed in the successive chapters. Bulk and mixed oxides syntheses for catalysis are reviewed with a presentation of the main synthesis strategies and of novel routes implemented, with a focus on the control of the textural properties of the materials for simple oxides and on two classes of mixed oxides: perovskites and hexaaluminates, that are particularly interesting for high temperature reactions. The final section concentrates on the synthesis of supported metal oxide catalysts, with a particular emphasis on the discussion of the interfacial chemistry occurring at the solid–liquid interface during the first steps of the deposition of an active oxide phase on a high-surface area support.

Understanding how alumina degradation to boehmite AlOOH can be prevented under hydrothermal conditions is key to design more stable catalysts supported on alumina for Fischer–Tropsch synthesis. We compare the effect of four inorganic dopants to inhibit γ-alumina chemical weathering under hydrothermal conditions: three metal ions: Mg2+, Zr4+, Ni2+, introduced by impregnation, and nonmetal Si, introduced by the grafting of tetraethyl orthosilicate. Boehmite is formed by a mechanism of dissolution–precipitation. A significant decrease of alumina weathering to boehmite is evidenced for all dopants. For metal ions, the thermal conversion of doped γ-Al2O3 into an alumina-rich mixed oxide or the coverage of the alumina surface by particles of a poorly soluble oxide such as NiO contribute to the stabilization of the solid. Alumina dissolution and degradation are only inhibited fully through Si grafting. IR spectra in the OH stretching region suggest that the most reactive alumina sites toward dissolution are basic Al−OH sites located on lateral facets that are blocked upon Si introduction. Fischer–Tropsch reactivity shows that Co catalysts prepared on Mg2+- and Si-doped supports are less active or, at best, as active as a reference Co/γ-Al2O3 catalyst, which is explained by a lower reducibility of CoO on doped supports. The Co catalyst prepared on Mg2+-doped γ-Al2O3 calcined at 900 °C exhibits the best combination of catalytic activity, ease of preparation, and hydrothermal stability.

Copper (oxide) nanoparticles with dispersion and size ranging from highly dispersed nanoparticles confined within the intra-wall pores to nanoparticles confined within the main mesopores can be simply produced via incipient wetness impregnation by using mesoporous silica supports containing the structure directing agent P123 partially occluding the intra-wall pores of SBA-15. Remarkably high copper dispersions of 69.5 down to 25.9% for metal loadings from 5 to 20 wt% were achieved as a result of the dual effect of the presence of surface silanol groups and the unique confining space between the silica walls and residual P123 in the intra-wall pores. The copper catalysts demonstrated outstanding activities in the hydrogenation of cinnamaldehyde, with TOF values ranging from 5.1 × 10−3 to 22.1 × 10−3 s−1, which are much higher than those of other supported copper catalysts reported elsewhere and prepared using conventional approaches.

The alpha-dicarbonyl contents of commercial honey samples from different botanical origins were analyzed as their quinoxaline derivatives using HPLC-DAD, HPLC-MS, HPLC-MS/MS, and HPLC-TOF-MS. A total of nine such compounds were detected, of which five were previously reported in honey (glucosone, 3-deoxyglucosone, glyoxal, methylglyoxal, and 2,3-butanedione) and three were reported only from sources other than honey [3-deoxypentulose, 1,4-dideoxyhexulose, and 3,4-dideoxyglucoson-3-ene (3,4-DGE)]. An unknown alpha-dicarbonyl compound was also tentatively identified as an oxidation product of 3,4-DGE and was termed 3,4-dideoxyglucosone-3,5-diene (3,4-DGD). Only glyoxal (0.3-1.3 mg/kg), methylglyoxal (0.8-33 mg/kg), and 2,3-butanedione (0-4.3 mg/kg) were quantified in all honey samples. Furthermore, analysis of the alpha-dicarbonyl profile of various honey samples indicated that certain alpha-dicarbonyl compounds are found in specific honey samples in much higher proportions relative to the average amounts. The free radical scavenging activity as measured by DPPH method has also indicated that the darker honey samples such as buckwheat, manuka, blueberry, and eucalyptus had higher antioxidant properties compared to lighter-colored samples.

Ni (15 wt%)/Al 2 O 3 catalysts prepared by decomposition of supported Ni(II)–ethylenediamine complexes in inert atmosphere, followed by post-treatment in hydrogen, are able to catalyze carbon monoxide hydrogenation at temperatures lower than for a catalyst prepared from nickel nitrate and containing a fraction of surface nickel aluminate. Unlike the latter, the catalyst prepared from Ni(II)–en complexes exhibits mostly surface metallic sites and is more stable under reaction despite the absence of nickel aluminate. Deactivation occurs during the reaction, probably by carbon deposition, but less strongly than for an ex-nickel nitrate catalyst.

Ni (7 wt%)/Al2O3 catalysts prepared by decomposition of Ni(II)-ethylenediamine complexes in inert atmosphere initially contain a mixture of metallic and oxidized nickel. X-ray photoelectron spectroscopy shows that after a hydrogen treatment at 500 °C, the system contains more metallic nickel than catalysts prepared from the usual precursor, nickel nitrate. Carbonaceous species resulting from the partial oxidation of ethylenediamine are also eliminated. The catalyst post-treated in hydrogen exhibits a high metallic surface area accessible to reactants and is able to catalyze CO methanation.

Hydration of gamma-Al2O3 is often reported to occur via the superficial transformation of the alumina surface into aluminum hydroxide-like layers. However, very little evidence has been given so far to support this hypothesis. It is demonstrated here by X-ray diffraction, TEM, electron diffraction, and solubility studies that a second process of hydration takes place that involves the dissolution of alumina and subsequent precipitation of well-shaped Al(OH)3 particles from supersaturated alumina aqueous solution. This process can be observed on a macroscopic scale (XRD, TEM) for any pH5, provided that the contact time between alumina and water exceeds 10 h. The least thermodynamically stable phase of aluminum hydroxide, bayerite, becomes favored compared with gibbsite when the pH of the solution is increased. It is assumed that the rate of formation of bayerite germs is greater than that of gibbsite due to variations in aluminum speciation in solution as a function of pH.

International audience; Alcohol dehydration is of prominent relevancein the context of biomass conversion. This reaction can beefficiently catalyzed by alumina surfaces, but the nature ofactive sites, the mechanisms involved, and the key parametersto tune both the activity and the alkene/ether selectivityremain a matter of debate. In the present paper, isopropanoldehydration to propene and diisopropylether over γ-alumina,δ-alumina, and sodium-poisoned γ-alumina was investigatedthrough a combined experimental and theoretical study. Theexperimental kinetic study shows that dehydration occursfollowing the same reaction mechanism on all materials, although γ-alumina activated above 450 °C exhibits the highest densityof active sites and the highest global activity. Results suggest that all the reaction pathways involved in dehydration require thesame set of adjacent active sites located on the (100) facets of γ-alumina. DFT transition-state calculations of the formation ofpropene and diisopropylether on the main terminations of alumina, (110) and (100), were also performed. The less activatedpathways for both the formation of the olefin (E2 mechanism) and the formation of the ether (SN2 mechanism) were found on aAlV Lewis acidic site of the (100) termination, with calculated activation enthalpies (125 and 112 kJ·mol−1 for propene anddiisopropylether formation, respectively) in good agreement with the experimental values (128 and 118 kJ·mol−1, respectively).The higher or lesser selectivity toward propene or ether appears to originate from significantly different activation entropies. Theeffect of coadsorbed sodium on the reaction is linked to the poisoning of Al sites by neighboring, Na-stabilized OH groups, butno influence of sodium on distant sites is evidenced. Reaction temperature is identified as the main key parameter to tune alkene/ether selectivity rather than morphology effects, which in turn affect drastically the number of available active sites, and thuscatalytic activity.

International audience; Amphiphilic organo-polyoxometalates (POMs) used in the radical emulsion polymerization of styrene allowed the preparation in aqueous medium of stable 50-100 nm polystyrene-POM composite latexes. Thanks to the presence of a trithiocarbonate group in the POM amphiphile, POMs could be covalently linked to the polymer particle surface. The chemical and catalytic integrity of the POMs was confirmed, and the POM-mediated surface photoactivity of the latexes was demonstrated by the spatially controlled nucleation of silver nanoparticles at the periphery of the composites.

When CO2+/ZSM5 catalysts are prepared by solid state exchange between H-ZSM5 and cobalt acetate, CO2+ ions form layered cobalt phyllosilicate at the surface of the zeolite grains, bringing about a reassessment of the chemistry involved in the preparation of catalytic systems by solid state exchange.

International audience; Platinum gauze catalysts are used extensively in the production of nitricacid from ammonia, where they are subject to harsh operating conditions combiningelevated temperatures and oxidizing environments. These cause significant loss of metalspecies as volatile oxides. How different metallic species behave in these environments ata fundamental, atomic-scale level is not well understood. In this work, we study the earlystages of oxidation of a Pt−Rh−Pd gauze at temperatures of 873−1273 K. Using acombination of advanced experimental methods, we explore how the oxidation behaviorcan strongly influence the surface and near-surface gauze microstructure. We show thatRh and Pd can segregate on different areas of the same surface and discuss how suchatomic migration can be linked to mechanisms of metal loss from such alloys.

International audience; tThe introduction of organic additives, such as polyols, sugars or polyacids, in impregnation solutionshas been suggested to better control the size of nanoparticles on supported catalysts. The present paperfocuses on the roles played by three of these additives, sorbitol, glucose and citric acid, in the preparationof Ni/Al2O3catalysts. ATR-IR, circular dichroism spectroscopy and viscosity measurements show thatthe additives act as modifiers of the physical properties of the solutions, by increasing their viscosity viahydrogen bonding, rather than as stable ligands toward Ni2+ions. Recrystallization of the nickel precursorsalt is prevented upon drying. As a consequence, no population of large particles is evidenced by XRDand TPR on the catalysts prepared with additives, unlike reference catalysts prepared by conventionalimpregnation of nickel nitrate. Adding these organic molecules in the impregnation solution is thusan easy way to narrow the size distribution of NiO nanoparticles without significantly affecting theirreducibility.

International audience; A molecular-scale investigation of the interaction at room temperature between gaseous trichlorosilane (HSiCl3), used as a passivating agent, and surfaces of aluminum alloy AA6061 in a polished or hydroxylated state is conducted. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) provide information on the topography and morphology of AA6061 before and after hydroxylation and surface passivation, while surface chemistry has been investigated by Polarization Modulation Infrared Reflection-Absorption Spectroscopy (PM-IRRAS) and X-ray photoelectron spectroscopy (XPS). Oxidation and hydroxylation of the polished alloy surface in boiling water strongly modifies the roughness of the surface, with formation of platelets and needles of oxyhydroxide AlOOH. PM-IRRAS and XPS reveal that, upon adsorption, HSiCl3 dissociates and mainly forms HSiOHn(OAl)3−n, HSi(OSi)n(OAl)3−n and condensed HSiOx species, by reaction with single bondOH groups from the AlOOH surface phase. The amount of deposited Si-containing species is larger on the rough surface of the hydroxylated alloy and this deposit is accompanied by a decrease of the amount of free single bondOH groups evidenced by PM-IRRAS. These results can find applications in the field of functionalization of aluminum alloys. It is suggested that a homogeneous oxidation of the alloy surface prior to exposure to gaseous HSiCl3 may enhance the adsorption of the passivating agent.

International audience; The paper focuses on the impact of sorbitol addition on the genesis of cobalt active phases in silica-supported catalysts and on their performance in Fischer-Tropsch synthesis. The catalysts were characterized by ex situ and in situ diffuse reflectance spectroscopy in the ultraviolet and visible regions, differential thermal and thermogravimetric analyses combined with mass spectroscopy, in situ Quick-X-ray absorption, X-ray diffraction, H2-temperature programmed reduction, in situ magnetic measurements and propene chemisorption. Sorbitol retards the decomposition of cobalt ionic complexes into Co3O4 during thermal treatment in air, leading to higher cobalt oxide dispersion. The smaller cobalt oxide particles in sorbitol-derived samples exhibit a much lower reducibility than those prepared without the additive. Fischer-Tropsch reaction rate is proportional to the number of cobalt metal sites, which is a function of both cobalt dispersion and cobalt reducibility. It depends in a complex manner on the initial sorbitol content.

Three Pt catalysts exhibiting similar metal particles size ( C bond was favored with Pt/SBA-15 and Pt/Al-SBA-15, with significantly higher reaction rates on the latter catalyst. The acidic support changes the adsorptive properties of the platinum nanoparticles, modifying their surface electron density and possibly their morphology. In contrast, Pt/Al2O3 was the most selective catalyst of the series toward cinnamyl alcohol, with C C and C O hydrogenations taking place at the same rate. The presence of Lewis acidic sites close to the particles is assumed to favor the adsorption of molecules via their polar C O bond. Even if cinnamaldehyde hydrogenation does not require acidic sites to be performed, the composition and acidity of the support significantly influence the reaction rate and selectivity.

67-1 FIELD Section Title:Catalysis, Reaction Kinetics, and Inorganic Reaction Mechanisms 66 Laboratoire de Reactivite de Surface (UMR 7197 CNRS), Universite Pierre et Marie Curie, Paris, Fr. FIELD URL: written in English; Nickel(II) nitrate is the nickel precursor generally selected to prep. oxide-supported catalysts, due to its low cost, high soly. in water and decompn. at moderate temps. However, its decompn. produces NOx, and it is known to melt and redistribute over the support upon heating, leading to poorly dispersed metal particles after redn. and to their partial expulsion from the porous support system. The benefits of using NiII acetate as an alternative to NiII nitrate are explored by investigating a series of model Ni catalysts (1-5 wt %) prepd. from the mesoporous siliceous supports MCM-41 and Al-MCM-41. The problems encountered with NiII nitrate, in terms of particle size and redistribution of nickel, are not obsd. with NiII acetate. In the case of MCM-41-supported systems, metal particles are well dispersed and remain located within the porous support. The higher pH of the impregnation soln., favoring electrostatic interactions between nickel ions and the support, may account for the higher metal dispersion. The formation of silicate-like species in strong interaction with the support does not appear prominent on siliceous MCM-41 when nickel acetate is the precursor, but seems to be enhanced for higher Ni contents. Finally, Al-MCM-41 supports are more sensitive to the exposure to nickel acetate than their siliceous analogs because acetate ions are able to ext. Al ions from the support, resulting in a partial disruption of the support organization and to the formation of nickel silicate species. [on SciFinder(R)]

Mesoporous aluminas can be prepared using organic additives as porogens even if these are not surfactants, as is the case for glucose. It is shown here that porosity arises from glucose entrapped in the material precipitated at the beginning of the synthesis, and subsequently caramelized. After impregnation by nickel(II) salts and calcination, more difficult-to-reduce nickel aluminate is detected on mesoporous aluminas than on commercial aluminas, because the surface area exposed to water is larger in the former case. The presence of citrates in the impregnating solution helps to lower the proportion of nickel aluminate, but may also lead to larger NiO particles.

EXAFS spectroscopy is shown as a tool of prime importance to probe the formation of metal–oxygen–support bonds and unravel the surface molecular structure in oxide-supported systems through two examples: (i) a molecular metal complex (Ni(II) bisglycinate) characterized after impregnation and drying on Al2O3, and (ii) a tungsten oxide nanophase characterized after deposition on zirconia and high temperature thermal treatment (tungstated zirconia catalysts, i.e. WOx/ZrO2). Unlike other spectroscopic techniques, EXAFS at the Ni K-edge proves that a modest thermal activation during the impregnation step triggers the grafting of nickel(II) bisglycinate onto the support: Al next-nearest neighbours are detected when the impregnation is carried out at 60 °C instead of room temperature. Characterization of WOx/ZrO2 catalysts shows the presence of W next-nearest neighbours around tungsten, with W–W distances distinctive of edge-shared WO6 octahedra only. The WOx overlayer can thus be described as bidimensional, nanometric slabs of 4 to 5 WO6 units on each side. In these slabs, W octahedra are interconnected to form a more condensed structure than the one present in bulk WO3 (in which linkage through corners exists). Moreover, EXAFS results conclusively demonstrate that the WOx overlayer is directly anchored to the ZrO2 surface by means of W–O–Zr bonds with a W–Zr distance of 3.14 A.

Differences between the acidic properties of silicoaluminic Pt-containing catalysts and those assessed on their parent supports have been reported in the literature and attributed to the presence of the metal nanoparticles and to their influence on the acid sites. It is shown here that for mesoporous materials containing various types of Al species, an alternative explanation can be proposed. 27Al NMR spectroscopy, FTIR of adsorbed CO and probe catalytic tests suggest the redistribution of aluminium atoms upon contact of the parent support with the aqueous solution containing the Pt precursor. Upon contact with water and thermal treatment, strong and mild Bronsted sites (Si–O(H)–Al) transform into strong Lewis sites (isolated tetracoordinated Al atoms). As a consequence, it may not be straightforward to deduce the acidic properties of metal-containing catalysts supported on Al-containing mesoporous materials from those of the bare support, because the surface species may differ significantly.

International audience; EXAFS spectroscopy is shown as a tool of prime importance to probe the formation of metal–oxygen–support bonds and unravel the surface molecular structure in oxide-supported systems through two examples: (i) a molecular metal complex (Ni(II) bisglycinate) characterized after impregnation and drying on Al2O3, and (ii) a tungsten oxide nanophase characterized after deposition on zirconia and high temperature thermal treatment (tungstated zirconia catalysts, i.e. WOx/ZrO2). Unlike other spectroscopic techniques, EXAFS at the Ni K-edge proves that a modest thermal activation during the impregnation step triggers the grafting of nickel(II) bisglycinate onto the support: Al next-nearest neighbours are detected when the impregnation is carried out at 60 °C instead of room temperature. Characterization of WOx/ZrO2 catalysts shows the presence of W next-nearest neighbours around tungsten, with W–W distances distinctive of edge-shared WO6 octahedra only. The WOx overlayer can thus be described as bidimensional, nanometric slabs of 4 to 5 WO6 units on each side. In these slabs, W octahedra are interconnected to form a more condensed structure than the one present in bulk WO3 (in which linkage through corners exists). Moreover, EXAFS results conclusively demonstrate that the WOx overlayer is directly anchored to the ZrO2 surface by means of W–O–Zr bonds with a W–Zr distance of 3.14 Å.

In the present paper are compared the physicochemical properties and stability of mesoporous aluminas synthesized following five routes in which water is the solvent. TEM shows that high pHs lead to heterogeneous materials because of the initial formation of bayerite, which is converted to η-alumina upon calcination and may reappear upon contacting the calcined oxide with water. The most promising procedures use neutral organic additives at moderate pH; materials are homogeneous, with specific surface areas of 350 ± 20 m2 g−1. The similarity of results obtained for the different routes, whatever the organic additive, suggests that the formation of the precipitated phase occurs not around micelles, but through the formation and sticking of boehmite nanoparticles, mediated by the organic agent present in a molecular form or as aggregates. The different ways to overcome the limits imposed by this mechanism are discussed. The alumina prepared with the addition of glucose is checked to be suitable as support for a Pt/Al2O3 catalyst prepared by impregnation with a basic solution.

The catalytic properties and structural characterization of catalysts derived from hypercrosslinked polystyrene (HPS), a polymer exhibiting mesoporosity and loaded with ruthenium in various amounts are investigated in the oxidation of d -glucose to d -gluconic acid. Transmission electron microscopy, X-ray fluorescence analysis, nitrogen physisorption measurements, diffuse reflectance infrared Fourier transform spectroscopy of adsorbed CO, EXAFS, XANES and catalytic studies show the presence of catalytically active nanoparticles of mixed composition (metal/oxide) with a mean diameter of 1.0–1.2 nm. The most active catalyst is found to contain 0.74 Ru wt.%. The highest selectivity to d -gluconic acid is 99.8% for a conversion of d -glucose of 99%; the specific activity of the catalyst is measured to be 1.13 × 10 −3 mol mol −1 Ru s −1 . The high-catalytic activity is attributed to the presence of pores of various sizes which facilitate mass transport and the high stability is attributed to the presence of small mesopores that have high-sorptional capacity and hinder the migration of the nanoparticles.

Synthesis of neutral complexes of Ni{sup 2+} with amino acids has often been reported on a qualitative basis, with a lack of information on the parameters involved in the dissolution of the nickel-containing solid precursor. This paper reports on a systematic study of the reactivity of Ni(OH){sub 2} toward glycine in aqueous solution. The crystallinity and size of hydroxide particles are found to be key parameters in the rapid glycine-promoted dissolution of the hydroxide and synthesis of [Ni(glycinate){sub 2}(H{sub 2}O){sub 2}]. These parameters derive from the nature of the salt used to prepare the hydroxide. Ni(II) chloride leads to the most reactive solid precursor, because of the presence of defects in the Ni(OH){sub 2} sheets arrangements, assigned to the substitution of Cl{sup -} ions to OH{sup -} ions at the edges of the particles. The reaction between this hydroxide and glycine at 80 deg. C is quantitative after 7 min and similar rates of dissolution are obtained with other amino acids, alanine or histidine, the reaction with serine being slower. When the hydroxide contains nitrate or carbonate ions, a glycinato complex with composition similar to [Ni(glycinate){sub 2}(H{sub 2}O){sub 2}], but with a different crystal structure, is also formed. Spectroscopic resultsmore » may suggest a structure involving bridging ligands. - Graphical abstract: Ni(OH){sub 2} solid precursors can be readily selected on the basis of their defects, themselves stemming from the nickel salt chosen for precipitation, to rapidly synthesize neutral Ni(II)-aminoacid complexes by ligand-promoted dissolution.« less

The thermal reduction of Co/Al2O3 catalysts prepared by impregnation of Co(II) nitrate generally leads to migration of cobalt ions into alumina with formation of cobalt aluminate. In order to prevent the latter, it is attempted to reduce cobalt by decomposition of supported precursor salts containing ethylenediamine (en), a procedure successfully applied earlier to nickel. Alumina is impregnated with an aqueous solution containing Co(II) nitrate and two or three equivalents of en. After drying, several cobalt compounds are detected on the support by X-ray diffraction, diffuse reflectance UV-vis spectroscopy and X-ray absorption spectroscopy: crystals of [Co(III)(en)3](NO3)3 whatever the initial en/Co stoichiometry, Co(II) and Co(III)-en complexes, grafted or not, as well as a Co(II)(OH)2-like phase. The composition of these phases reveals that dramatic evolutions in cobalt speciation have taken place in the impregnating solution, driven by the oxidation of complexed cobalt by dissolved oxygen and by the precipitation of the Co(III) salt. Decomposition of these phases in an inert gas atmosphere does not lead to metal particles exclusively, as is the case of nickel, but to a mixture of oxidic and metallic phases. A treatment in hydrogen at 500 °C completes cobalt reduction, leading to 6-20 nm metal particles. This rather broad size distribution is linked to the wide range of cobalt species on the support, which stems itself from the coordination chemistry of cobalt bonded to ethylenediamine in aqueous solutions.

The evolution of nickel speciation during the successive preparation steps of Ni-SiO(2) catalysts is studied by UV-Vis-NIR, FT-IR, DTG, TPR and TEM. The study focuses on the effect of the number of chelating ligands in the precursor complexes [Ni(en)(x)(H(2)O)((6-2x))](2+) (en = ethylenediamine, x = 1, 2, 3) on the adsorption on silica, and on nickel speciation after thermal treatment. When the en:Ni ratio in solution increases from 1 to 3, the most abundant complex is [Ni(en)(H(2)O)(4)](2+) (64% of all Ni complexes), [Ni(en)(2)(H(2)O)(2)](2+) (81%) and [Ni(en)(3)](2+) (61%), respectively. Equilibrium adsorption of [Ni(en)(x)(H(2)O)((6-2x))](2+) on SiO(2) results in the selective grafting of [Ni(en)(H(2)O)(4)](2+) and [Ni(en)(2)(H(2)O)(2)](2+), through the substitution of two labile H(2)O ligands by two surface SiO(-) groups. The surface [Ni(en)(H(2)O)(2)(SiO)(2)] complex formed by the grafting of [Ni(en)(H(2)O)(4)](2+) onto silica tends to transform into NiO and nickel phyllosilicate after calcination, which consequently leads to large and heterogeneously distributed metallic Ni particles upon reduction. In contrast, [Ni(en)(2)(SiO)(2)], resulting from the grafting of [Ni(en)(2)(H(2)O)(2)](2+) onto silica, no longer has aqua ligands able to react with other nickel complexes or silicium-containing species. Calcination transforms these complexes into isolated Ni(2+) ions, which are reduced into small metallic Ni particles with a more homogeneous size distribution, even at higher Ni loading.

This contribution presents the molecular approach used to investigate the different preparation steps of important catalytic systems, based on Mo, Ni, and Co. The first preparation step consists of impregnating oxide supports by aqueous solutions of transition-metal complexes (TMCs), called precursor complexes, and involves the liquid-solid (wet) interface. The following steps consist of thermal activation treatments, involving the gas-solid (dry) interface. The molecular approach is based on (i) the use of the TMC to monitor and probe the nature of its own interactions with the oxide support, (ii) the selection of the most appropriate techniques to monitor such interactions, and (iii) the design of experiments to identify and isolate the key parameters controlling the "TMC-oxide support" interactions. Physical techniques play a key role in the characterization of catalyst preparation both at the macroscopic [temperature-programmed reduction (TPR), X-ray diffraction (XRD)] and molecular [Raman, X-ray absorption near-edge spectroscopy-extended X-ray absorption fine structure (XANES)-EXAFS, diffuse reflectance UV-vis] levels, particularly when they are used simultaneously. They give some insight into the structure of the species existing on the surface, with the oxide support behaving as a solid ligand, when exposed for a short time to the precursor complex solution or after thermal treatment, or, for longer contact times, as a reactant.

Physicochemical and catalytic properties of cobalt-containing ZSM-5 zeolites prepared from cobalt acetate by solid-state ion-exchange are compared as a function of the Co/Al atomic ratio. When Co/Al=1/2, cobalt is present both as ions in cationic sites and oxide particles on the zeolite; for Co/Al=1 or 3/2, TEM and EXAFS show that a cobalt phyllosilicate phase is present on the outer surface of the zeolite, but Lewis acid sites (Co 2+ ions) are still accessible as revealed by pyridine adsorption. Catalyst prepared with Co/Al=1 exhibits high activity and selectivity toward acetonitrile in the ammoxidation of ethane and ethylene. Co 2+ ions inside the zeolite and on the phyllosilicate surface seem to be the active species.

1.5 Ni wt %/Al2O3 catalysts have been prepared by incipient wetness impregnation using [Ni(diamine)x(H2O)(6-2x)]Y2 precursors (diamine = 1,2-ethanediamine (en) and trans-1,2-cyclohexanediamine (tc); x = 0, 1, and 2; Y = NO3- and Cl-), to avoid the formation, during calcination, of difficult-to-reduce nickel aluminate. N2 was chosen for thermal treatment to help reveal and take advantage of the reactions occurring between Ni2+, ligands, counterions, and support. In the case of [Ni(en)2(H2O)2]Y2 salts used as precursors, in situ UV-vis and DRIFT spectroscopies show that after treatment at 230 degrees C Ni(II) ions are grafted to alumina via two OAl bonds and that the diamine ligands still remain coordinated to grafted nickel ions but in a monodentate way, bridging the cation with the alumina surface. With Y = Cl-, the chloride counterions desorb as hydrogen chloride, and hydrogen released upon decomposition of the en ligands is able to reduce a fraction of nickel ions into metal as evidenced by XPS. In contrast, with Y = NO3-, compounds such as CO or NO are formed during thermal treatment, indicating that nitrate ions burn the en ligands. After thermal treatment at 500 degrees C, a surface phase containing Ni(II) ions forms, characterized by XPS and UV-vis spectroscopy. Temperature-programmed reduction shows that these ions can be quantitatively reduced to the metallic state at 500 degrees C, in contrast with the aluminate obtained when the preparation is carried out from [Ni(H2O)6]2+, which is reduced only partly at 950 degrees C. On the other hand, a total self-reduction of nickel complexes leading to 2-5-nm metal particles is obtained upon thermal treatment via the hydrogen released by a hydrogen-rich ligand such as tc, whatever the Y counterion. An appropriate choice of the ligand and the counterion allows then to obtain selectively Ni(II) ions or a dispersed reduced nickel phase after treatment in N2, as a result of the reactions occurring between the chemical partners present on alumina.

Co2+/ZSM5 catalysts (Co/Al = 1, overstoichiometric ratio) have been prepared by solid-state reaction at 500 °C in argon from H-ZSM5 and four Co(II) precursors: formate, nitrate, chloride and acetate. Co3O4 particles supported by the zeolite grains are the only cobalt-containing phases detected when formate is used. When cobalt nitrate is the precursor, Co3O4 particles appear to be independent from the grains. A fraction of cobalt has nevertheless migrated into the zeolite; this is linked with [Co(H2O)6](NO3)2 low melting point and wetting of the zeolite grains by the molten salt at the beginning of the thermal treatment. Catalysts prepared from cobalt chloride contain both nanometric Co3O4 particles on the support grains and cobalt ions inside the zeolitic channels. Ion exchange is possible, at least partly, due to the fact that unlike nitrate and formate, cobalt chloride has not decomposed before reaching the temperature at which the exchange process is activated. This process is chemically driven by the release of HCl in the gas phase. Finally, the major phase detected when cobalt acetate is the precursor is a poorly organized cobalt phyllosilicate surrounding zeolite grains. Acetate ions attack the zeolitic framework from outside during the solid-state reaction; cobalt ions are inserted in the silicate matrix and progressively migrate deeper in the grains upon thermal activation.

In this work, texture, structure, acidity and activity for ethylene ammoxidation of Co-ZSM-5 catalysts with various Co contents prepared by aqueous exchange, solid-state-exchange and impregnation are studied. ICP, nitrogen physisorption, XRD, NH3 TPD and n-hexane isomerisation test are the main techniques used for this investigation. Significant textural and structural modifications are observed after the introduction of cobalt in the H-ZSM-5 zeolite. The acidity strength of the catalysts varies with the preparation method and the Co/Al ratio. A good correlation between the acidity and the catalytic activity is observed. The strength of ammonia bonding to the catalyst appears to have a crucial effect on the activity of Co-ZSM-5 in ethylene ammoxidation.

A Ni0 (15 wt %)/Al2O3 catalyst is prepared by impregnation using [Ni(en)2(H2O)2](NO3)2 as precursor salt containing the chelating ligand ethylenediamine (en). Due to the low solubility of this salt and the high viscosity of its solution, large crystals of the salt deposit on the support, clogging the porosity of alumina. These crystals split during the thermal treatment in argon; hydrogen produced in situ from organic residues reduces quantitatively nickel (II) to monodisperse small Ni0 particles, compared with only one half of nickel reduced by hydrogen when catalysts are prepared from [Ni(H2O)6](NO3)2. To cite this article: F. Negrier et al., C. R. Chimie 6 (2003).

This communication discusses some of the phenomena occurring during the thermal transformation of oxide-supported platinum catalysts. Based on previously established molecular understanding of the deposition step, the reactivity of several surface platinum species is followed through the later steps of catalyst preparation : drying, calcmation (or thermal activation under inert atmosphere) and H2 reduction. The phenomena observed include Pt complex self-reduction, H spillover to various support sites and formation of strongly held “residual chlorine”. Their consequences on metal dispersion and catalytic activity are briefly presented.

Methane oligomerization to alkanes can be accomplished on supported platinum via a two-step procedure: formation of carbonaceous species on the metallic surface by methane adsorption, followed by hydrogenation of these species. Temperature-programmed oxidation (TPO) experiments performed after hydrogenation steps of various durations show that the hydrogenation of a carbonaceous deposit obtained at 300 C on the reference Pt/SiO{sub 2} catalyst EUROPt-1 is not a fast process. Two groups of surface carbonaceous species have been characterized through their different reactivities toward oxygen, but at 300 C their reactivities toward hydrogen are similar. Among alkanes up to C{sub 5}, methane is the main product of hydrogenation, corresponding to one-half of the surface carbon reactive toward hydrogen; linear and branched alkanes are produced from the other half of the reactive carbonaceous species. On EUROPt-1, mainly ethane and n-pentane are produced during the first minutes of reaction, while on a sintered catalyst the initial production in n-pentane is negligible. The release of n-pentane during an intermediate purge with inert gas on EUROPt-1 shows that C-C bonds can form already during methane adsorption, leading to C{sub 5} precursors on specific active sites of this catalyst. A model of formation of C{sub 5} precursors is proposedmore » by analogy with the organometallic chemistry of molecular hydrocarbon platinacycles. The subsequent production of alkanes (C{sub 2} > C{sub 3} > C{sub 4} > C{sub 5}) could be described through a statistical model of dynamic coupling between carbonaceous species involving hydrogen, rather than by hydrogenolysis of heavier carbonaceous species. However, this latter mechanism is likely to predominate for the production of C{sub 6}-C{sub 8} compounds.« less

Hydrocarbons can be produced from methane through a two-step procedure on Pt/SiO2 catalyst EUROPt-1: (1) adsorption of methane and dehydrogenation in flow conditions; (2) subsequent hydrogenation of the carbonaceous deposit. Methane adsorption was performed at 300 °C; it was shown by TPO experiments that only 55% of the carbonaceous species deposited were hydrogenated at that temperature after 15 minutes in hydrogen. Hydrogen reacted with one half of the reactive carbonaceous species to give back methane and with the other half to form higher alkanes. The first two minutes of hydrogenation mainly lead to ethane and n-pentane. Between 2 and 30 minutes the rate of production in C2-C5 alkanes appeared to be steady with time and their instantaneous selectivity decreased when the number of carbon atoms in the chain increased. Both steps of the nonoxidative conversion of methane contribute to the creation of C-C bonds between carbonaceous species, with distinct slates in alkanes.

Residual chlorine ions on a Pt/Al2O3 catalyst surface prepared from chlorine-containing precursors appear to inhibit the total oxidation of methane. At 450°C, as chlorine is eliminated with time on stream, the reaction rate increases despite the sintering of the platinum particles. The steady state reaction rate which is reached after 60 h is identical to that obtained with a catalyst prepared from a precursor containing no chlorine. Whether chlorine is present or not in the initial state of the catalyst does not appear to have an influence on the evolution of the platinum particle size.