Anne Ponchel

Studies of the cerium-metal–oxygen–hydrogen system (metal=Cu, Ni)

Abstract In situ X-ray diffraction (XRD), in situ X-ray photoelectron spectroscopy (XPS), electron microscopy and thermogravimetric techniques were used to characterize CeM x O y mixed oxides in the oxidized and H 2 reduced state. The processes occurring during the hydrogen treatment, and the amounts of hydrogen occluded in the solids when treated at different temperatures under flowing hydrogen have also been determined. The CeM x O y oxides can be described as a mixture of copper oxide or nickel oxide and of ceria modified by the insertion of a part of copper or nickel in its lattice. The size of the copper oxide or nickel oxide varies considerably from clusters to a crystallized material, depending on the x value and on the experimental conditions. In the partially reduced state anionic vacancies able to receive hydrogen, probably in a hydridic form, are created in the bulk and at the surface of the solid and a reduction mechanism partly based on heterolytic dissociation of H 2 is proposed (O 2− +□+H 2 →H − +OH − ). Moreover, it appears that different kinds of active sites which differ from each other in terms of the environment of nickel or copper can exist.

Characterization and catalytic behavior of VOx-CeO2 catalysts for the oxidative dehydrogenation of propane

A series of ceria-supported vanadium catalysts was prepared via impregnation of the support with an ammonium metavanadate solution. The 723 K calcined samples were tested for propane oxydehydrogenation (ODH) activity and selectivity. The sample exhibiting the highest propane conversion was found to be the ceria support material itself, although this showed essentially no selectivity towards propene. An optimum propene yield of 4.2% was obtained at 673 K for the 6 wt% V2O5-CeO2 sample. Conversion decreased with increasing V loading which was attributed to the formation of cerium vanadate (CeVO4). This phase was found in all samples after calcination, its abundance rising in proportion to the V loading. In the 6 wt% V2O5 catalyst hydrated surface VOx species were present, although they underwent conversion to CeVO4 at temperatures above 573 K. The low reducibility of these surface vanadates was linked to the oxidation activity. It is inferred that surface polyvanadate species are responsible for the selective ODH of propane with V-O-V and/or V-O-Ce being the active oxygen species.

Remediation technologies using cyclodextrins: an overview

Nowadays, the human activity and the modern way of life are responsible for the increase of the environmental pollution. Industrial processes generate a variety of molecules that may pollute air, water, and soils due to negative impacts for ecosystems and humans. The development of innovative remediation technologies has thus emerged as a significant environmental priority. Within this scope, supramolecular chemistry, which is a recent discipline, could provide solutions. In particular, cyclodextrins (CDs) are a family of cyclic oligosaccharides having a low-polarity cavity in which organic compounds of appropriate shape and size can form inclusion complexes. This unique property makes them suitable for application in environmental protection. Here, we review the use of cyclodextrins and cyclodextrin derivatives in remediation technologies. Accordingly, the present review shows the advantages of using CDs in soil, groundwater, wastewater, and atmosphere remediation. Resulting processes are highly versatile, since the complexing ability of CD is applicable to a wide range of pollutants. They may also been referred to green processes, according to the CD innocuity. Moreover, as inclusion phenomena correspond to reversible equilibriums, a major trend in the CD environmental application field is to develop methods, which combine supramolecular chemistry and irreversible processes, as advanced oxidation or biodegradation. Such processes might lead to a complete remediation of pollutants and eventually to the CD recycling.

Cyclodextrins as Mass Transfer Additives in Aqueous Organometallic Catalysis

During the past fifteen years, the use of chemically modified cyclodextrins (CDs) in aqueous organometallic catalysis has sig- nificantly contributed to enlarge the application field of biphasic processes in chemistry. In this paper, we describe how these su- pramolecular receptors became one of the most efficient solutions to solve mass transfer problems in aqueous organometallic catalysis. The scientific gaps that have been cleared to explain the exact role of the CDs in these biphasic systems are especially emphasized. In particular, the impact of supramolecular interactions between chemically modified CDs and substrates, water soluble ligands or or- ganometallic catalysts is addressed for a better understanding of the recognition processes involved in the catalytic reactions.

Effect of the sequence of potassium introduction to V2O5/TiO2 catalysts on their physicochemical properties and catalytic performance in oxidative dehydrogenation of propane

Abstract Two series of K-promoted V 2 O 5 /TiO 2 catalysts were prepared by: (a) deposition of vanadia on K-doped TiO 2 support (TiK V preparations), and (b) deposition of K on vanadia-titania catalysts (TiV K preparation). They were characterized by 51 V NMR, XPS, surface potential (work function) techniques and isopropanol decomposition, a probe reaction for the acid-base properties, and tested in oxidative dehydrogenation of propane (ODH). It has been found that the sequence of the K introduction in the preparation step is preserved in the calcined preparations, with more potassium being present on the surface of TiV K than TiK V catalysts. The vanadium species on TiK V samples include V 2 O 5 and polymeric [VO x ] γ species. The TiV K sequence leads to the formation of potassium vanadates (KV 3 O 8 and possibly KVO 3 ), and to the decrease in the amount of V 2 O 5 . The TiV K catalysts are more active and selective in the ODH of propane, and more active in isopropanol dehydrogenation to the acetone (thus more basic) than the TiK V samples.

CeNixOy and CeAlzNixOy solids studied by electron microscopy, XRD, XPS and depth sputtering techniques

Mixed oxides based on nickel, cerium and aluminum, effective in the catalytic hydrogenation of unsaturated compounds, were prepared by a coprecipitation route and studied by XRD, TEM, XPS and ion sputtering techniques. The CeNixOy mixed oxides (0⩽x⩽1) can be described as a mixture of nickel oxide and ceria modified by the insertion of nickel cations in its lattice. Ion sputtering followed by XPS analysis has been very useful for estimating the size of NiO clusters present in the compounds, too small to be detected by XRD and TEM. Among the different CeNixOy catalysts studied, the CeNi0.5Oy solid presents the highest number of interactions between Ce4+ cations and Ni2+ cations, located either in the solid solution of cerium–nickel and/or at the NiO/CeO2 interface. In the CeAlzNixOy series (0<x⩽5 and 0⩽z⩽1), the addition of aluminum leads to a disorganization of the previous structure with favored interactions between Al3+ cations and Ni2+ cations.

Cyclodextrins as growth controlling agents for enhancing the catalytic activity of PVP-stabilized Ru(0) nanoparticles

Cyclodextrins act as growth controllers in the synthesis of PVP-stabilized Ru(0) nanoparticles, leading to enhancement of the catalytic activity in the hydrogenation of furfural.

Effect of potassium addition to the TiO2 support on the structure of V2O5/TiO2 and its catalytic properties in the oxidative dehydrogenation of propane

Vanadium oxide has been deposited by a grafting technique onto TiO2 anatase, both pure and doped with potassium [(1.2 and 2.5) atoms nm–2]. The V content varied between 0.1 and 20 atoms nm–2[0.01–2 V2O5 monolayers (ML)]. The prepared samples were characterized by X-ray photoelectron spectroscopy (XPS), 51V magic-angle spinning (MAS) NMR and a surface potential (SP) technique and tested as catalysts in the oxidative dehydrogenation (ODH) of propane and propan-2-ol decomposition, a probe reaction for acid–base properties. From the XPS and SP data it has been inferred that VOx are located beside the K centres on the bare surface of TiO2 with the lower K content sample, whereas they cover the K-doped fraction of the surface for the sample with higher K content. Monomeric and polymeric VOx species and V2O5 were detected by 51V NMR on pure and K-doped catalysts. For the K-doped samples the polymeric species were observed only at high V content and new tetrahedral VOx species and traces of KVO3 appeared. It has been found that the presence of K on the TiO2 surface leads to (a) a decrease in the reducibility of the vanadia phase at low V content; (b) a decrease in the surface potential (electronic work function); (c) a decrease in acidity and increase in basicity and (d) a decrease in the total activity for ODH of propane. The pattern of the activity and selectivity changes with the total V content depends on the amount of K on the support surface: with K 1 ML) are required to obtain the same catalytic performance. Polymeric [VOx] species seem to be more active and selective in the ODH of propane than monomeric species or bulk V2O5.

Storage of reactive hydrogen species in CeMxOy (M = Cu, Ni; 0≤x≤1) mixed oxides

Abstract Interaction of hydrogen with a series of mixed oxides CeM x O y (M = Cu, Ni; 0 ⩽ x ⩽ 1) has been studied in the 300–1073 K temperature range. A dynamic method of titration has been used to provide evidence for reactive hydrogen H∗ present in the solids. This method of titration involves hydrogenation at 423 K of isoprene under helium flow in the absence of gaseous hydrogen. The hydrogen reservoir capacity depends on the pretreatment temperature under H 2 and is related to the creation of anionic vacancies in the solid. All the CeM x O y mixed oxides are large catalytic hydrogen reservoirs with marked diffusion properties of hydrogen species (H∗, OH − ). In the series studied, the highest amount of hydrogen H∗ (29.5 10 −3 mol g −1 ) is stored by CeNi 0.7 O y pretreated under H 2 at 523 K (H 6.8 CeNi 0.7 O y ). In the conditions in which the incorporation of hydrogen in the solid occurs, X-ray diffraction analysis under H 2 shows shifts depending on the treatment temperature and corresponding to non-negligeable expansion of the ceria-based lattice attributed to the insertion of hydrogen species of hydridic nature in the anionic vacancies.

Self‐Assembled Metastable γ‐Ga2O3 Nanoflowers with Hexagonal Nanopetals for Solar‐Blind Photodetection

Metastable γ-Ga2O3 nanoflowers assembled from hexagonal nanopetals are successfully constructed by the oxidation of metallic Ga in acetone solution. The nanoflowers with a hollow interior structure exhibit a short response time and a large light-current-dark-current ratio under a relatively low bias voltage, suggesting an especially important potential application in solar-blind photodetection.