Anne-Sophie Mamede

Composition-Dependent Morphostructural Properties of Ni–Cu Oxide Nanoparticles Confined within the Channels of Ordered Mesoporous SBA-15 Silica

NiO and NiO-CuO polycrystalline rodlike nanoparticles were confined and stabilized within the channels of ordered mesoporous SBA-15 silica by a simple and viable approach consisting in incipient wetness impregnation of the calcined support with aqueous solutions of metal nitrates followed by a mild drying step at 25 °C and calcination. As revealed by low- and high-angle XRD, N2 adsorption/desorption, HRTEM/EDXS and H2 TPR analyses, the morphostructural properties of NiO-CuO nanoparticles can be controlled by adjusting their chemical composition, creating the prerequisites to obtain high performance bimetallic catalysts. Experimental evidence by in situ XRD monitoring during the thermoprogrammed reduction indicates that the confined NiO-CuO nanoparticles evolve into thermostable and well-dispersed Ni-Cu heterostructures. The strong Cu-Ni and Ni-support interactions demonstrated by TPR and XPS were put forward to explain the formation of these new bimetallic structures. The optimal Ni-Cu/SBA-15 catalyst (i.e., Cu/(Cu+Ni) atomic ratio of 0.2) proved a greatly enhanced reducibility and H2 chemisorption capacity, and an improved activity in the hydrogenation of cinnamaldehyde, as compared with the monometallic Ni/SBA-15 or Cu/SBA-15 counterparts, which can be associated with the synergism between nickel and copper and high dispersion of active components on the SBA-15 host. The unique structure and controllable properties of both oxidic and metallic forms of Ni-Cu/SBA-15 materials make them very attractive for both fundamental research and practical catalytic applications.

Operando resonance Raman spectroscopic characterisation of the oxidation state of palladium in Pd/γ-Al2O3 catalysts during the combustion of methane

Resonance Raman spectroscopy is used in order to investigate the behaviour of Pd/γ-alumina (2 wt.%) in the catalytic combustion of methane. Simultaneous spectroscopic and catalytic measurements allow an operando spectroscopic investigation of the catalyst under operational conditions. Temperature cycles were performed by alternately heating and cooling the system under the reaction conditions after various in situ gaseous pre-treatments of the catalyst. Our results show that neither the Raman features nor the conversion of methane are influenced by initial thermal activation treatment under reducing or oxidizing atmospheres. Pure metallic palladium was found to be inactive for the catalytic combustion of methane. Under the reaction conditions, the Pd/γ-Al2O3 catalyst is always in an oxide PdO form with its surface in an intermediate state between surface lacunary PdO and crystalline PdO species.

Structural, textural and acid–base properties of carbonate-containing hydroxyapatites

Carbonate-containing hydroxyapatites with different Ca/P ratios and optionally containing Na+ cations were successfully synthesized using a precipitation method. The solids were extensively characterized by XRD, LEIS, XPS, IR, TGA and NMR. Further, their acid–base properties were determined by NH3-TPD, PEA-XPS, CO2-TPD and by pulsed liquid chromatography using benzoic acid as a probe. The so-obtained structural, textural and acid–base properties could be finely correlated to give a clear picture of the system. The acidic properties of hydroxyapatites were attributed to Ca2+, surface HPO42− and OH− vacancies and the basic properties were attributed to PO43−, OH− and CaO species. The fine-tuning of the amount, of the nature and the strength of acid–base properties derived by varying the carbonate content in hydroxyapatites can find applications in catalysis, which was illustrated by isopropanol reactivity.

Adsorption of surfactin produced from Bacillus subtilis using nonwoven PET (polyethylene terephthalate) fibrous membranes functionalized with chitosan.

This article deals with an alternative method for bio-separation of surfactin produced by Bacillus subtilis using sorption method on nonwoven PET (polyethylene terephthalate) fibrous membranes functionalized with chitosan. In the first part of the study, surface functionalization of the PET nonwoven fibrous membranes is carried out with aqueous 65% deacetylated chitosan solution with or without a prior surface activation using air-atmospheric plasma treatment. Very small modification of the PET fibrous nonwoven air-permeability confirms the functionalization of PET fibre surface with little reduction of membrane porosity. The functionalized membranes are then characterized by physico-chemical methods: X-ray Photoelectron Spectroscopy (XPS), Wettability and zeta potential. Chitosan increases drastically the zeta potential of PET at all pH values though a prior plasma treatment of the PET membrane reduces slightly the increase in zeta potential values. Sorption of surfactin quantified by HPLC shows that the extent of surfactin sorption on PET nonwovens depends on the surface functionalization method. Surface functionalization with chitosan results in immediate sorption of the entire quantity of surfactin. A prior surface activation by air atmospheric plasma treatment of the PET membranes before chitosan application retards the sorption of entire surfactin which takes place after 1.5h, only. Increased zeta potential and increased hydrophobic behavior in the presence of chitosan without plasma activation would explain the interesting surfactin sorption results.

A well-defined silica-supported dinuclear tungsten(III) amido species: synthesis, characterization and reactivity.

Grafting of [W(2)(NMe(2))(6)] onto dehydroxylated silica affords the well-defined surface species [([triple bond, length as m-dash]Si-O)W(2)(NMe(2))(5)], characterized by elemental analysis, and infrared, Raman and NMR spectroscopies, and the catalytic reactivity of this supported tungsten(III) d(3)-d(3) dimer and of its alkoxide derivatives towards alkynes has been probed.

Nisin adsorption on hydrophilic and hydrophobic surfaces: evidence of its interactions and antibacterial activity

Study of peptides adsorption on surfaces remains a current challenge in literature. A complementary approach, combining X‐ray photoelectron spectroscopy (XPS) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) was used to investigate the antimicrobial peptide nisin adsorption on hydrophilic and hydrophobic surfaces. The native low density polyethylene was used as hydrophobic support and it was grafted with acrylic acid to render it hydrophilic. XPS permitted to confirm nisin adsorption and to determine its amount on the surfaces. ToF‐SIMS permitted to identify the adsorbed bacteriocin type and to observe its distribution and orientation behavior on both types of surfaces. Nisin was more oriented by its hydrophobic side to the hydrophobic substrate and by its hydrophilic side to the outer layers of the adsorbed peptide, in contrast to what was observed on the hydrophilic substrate. A correlation was found between XPS and ToF‐SIMS results, the types of interactions on both surfaces and the observed antibacterial activity. Such interfacial studies are crucial for better understanding the peptides interactions and adsorption on surfaces and must be considered when setting up antimicrobial surfaces. Copyright

Oxygen transport kinetics of the misfit layered oxide Ca3Co4O9+d

The oxygen transport kinetics of the misfit-layered cobaltite, Ca3Co4O9+δ, known for its thermoelectric properties, was investigated by combined application of 18O/16O isotope exchange and electrical conductivity relaxation techniques. Although oxygen diffusion is found to be two orders of magnitude lower than in well-investigated lanthanum nickelates, e.g., La2NiO4+δ, the mixed ionic–electronic conductor Ca3Co4O9+δ is found to exhibit fast surface exchange kinetics (k* = 1.6 × 10−7 cm s−1 at 700 °C to be compared to 1.3 × 10−7 cm s−1 for the nickelate), rendering it a promising electrode for application as an air electrode in solid oxide cells. In parallel, the chemical nature of the outermost surface of Ca3Co4O9+δ was characterized by means of Low Energy Ion Scattering (LEIS) spectroscopy. The absence of cobalt at the samples outermost surface suggests that the Ca2CoO3−δ rock salt layers in the structure may play a key role in the oxygen exchange mechanism.

Synergy between XANES Spectroscopy and DFT to Elucidate the Amorphous Structure of Heterogeneous Catalysts: TiO2‐Supported Molybdenum Oxide Catalysts

The properties of heterogeneous catalysts are directly correlated to the molecular structure of the active sites which often consists of nanometer-scale particles of transition metals (in a metallic, oxide or sulfide form) dispersed on an oxide support. The complexity of physico-chemical phenomena occurring during the catalyst synthesis/activation stages often leads to the formation of unknown supported phases featuring new ill-defined sites. Their identification requires an indepth characterization at the molecular scale of the catalyst with the use of various spectroscopic tools. Despite the valuable information so-obtained, these techniques sometimes are not able to provide the overall structure of the active species responsible for the catalytic activity. The use of theoretical tools to establish direct correlation between spectroscopic fingerprints and structural/electronic properties of catalysts is a powerful and quite new approach for unraveling the structure of catalysts. Thanks to its chemical and electronic (orbital) selectivity, X-ray absorption near-edge structure (XANES) spectroscopy is the technique of choice for molecular-scale characterization of catalysts. Furthermore, in the XANES spectra, the long mean free path of the photoelectron, induced by its small kinetic energy (Ec< 50–100 eV), provides a high contribution of multiple scattering events of the photoelectron allowing to probe the three-dimensional structure (3D) around the absorbing atom. XANES spectroscopy is however highly dependent on electronic parameters that make its fine interpretation difficult. This often leads to a restricted use of XANES spectra for the determination of the oxidation state of an absorber atom or/and basic consideration on its local symmetry. It is however possible to obtain a finer interpretation of spectra by calculation of XANES transitions. Indeed, the multiple scattering (MS) theory is well adapted to reproduce the XANES transitions observed at K edges of elements heavier than Li and at L2,3 edges of elements heavier than Cd. We propose herein to use MS simulations based on structural models predicted by DFT calculations for interpreting the MoK edge fingerprints observed for TiO2-supported molybdenum oxide catalysts which are widely used for olefin metathesis, selective oxidation reactions, and hydrotreatment. The structure of the oxomolybdate species formed during the catalyst activation is still unknown even though it has been extensively characterized by various spectroscopies. A three-step methodology was followed to unravel the catalyst structure: first simulations of XANES spectra of Mo reference compounds (ammonium heptamolybdate (hereafter noted AHM), (NH4)3[Al(OH)6Mo6O18] (noted AlMo6), a-(NH4)4[Mo8O26] (noted Mo8O26)) were performed to obtain direct correlation between spectroscopic fingerprints and structural properties of known structures. Then, these fingerprints are identified in the spectrum of the supported catalyst to generate catalyst structures for DFT optimization. Finally, the optimized geometry is used for modeling XANES spectra using MS theory. Figure 1A presents MoK edge experimental spectra of the activated catalyst (350 8C in oxygen), labeled hereafter 7.5 MoTi, together with four reference compounds in which

Surface Raman spectroscopic study of NO transformation over Pd-based catalysts

Correlations between in situ Raman spectroscopic and catalytic measurements on palladium based three-way catalysts during the reduction of NO by CO have been tentatively established. Particular attention to the selectivity towards the transformation of NO into N2O has been paid in order to explain why N2O is the main N-containing product during the engine cold start. A comparative study of the selectivity from temperature-programmed and steady-state experiments on bulk and supported palladium catalysts shows that bulk Pd is significantly more selective towards the production of N2 than Pd/Al2O3 at low conversion and low temperature. In addition, the subsequent reduction of N2O by CO occurs more readily on bulk palladium. In parallel to this catalytic information, Raman spectra recorded in comparable experimental conditions reveal different spectral features relative to the nature of chemisorbed species, and to the development of surface PdO islands mainly on Pd/Al2O3. Both differences have been compared and discussed in the light of a previous mechanism proposed earlier in the literature.

Ammoxidation of allyl alcohol – a sustainable route to acrylonitrile

The ammoxidation of allyl alcohol was demonstrated over antimony–iron oxide catalysts with a Sb/Fe ratio of 0.6 and 1. Both catalysts showed high performance with 83 and 84% yield of acrylonitrile, respectively, whereby the main difference was found in the initial performance. This was ascribed to the in-operando formation of the SbFeO4 mixed oxide on the catalyst surface under reaction conditions, as proven by XPS analysis.