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Lambert J.-F.,CNRS Surface Science Lab | Bergaya F.,French National Center for Scientific Research
Developments in Clay Science | Year: 2013

Smectites were the first clay minerals applied to CPN synthesis, with research in the field undergoing an explosive development after the first success of montmorillonite-nylon nanocomposites as structural materials in 1993. The most important domains of applications of smectite-polymer nanocomposites were discussed: structural materials, elastomer reinforcement, barrier properties, green chemistry, biomedical applications, optics, ionic conductors. The preparation procedures are systematically presented, including direct intercalation, compatibilization, use of organoclays, one-pot synthesis, in situ polymerization and polymer-templated clay mineral synthesis. Finally, offered is an overview of the main techniques available for the evaluation of nanocomposite formation mechanisms and the molecular structure. © 2013 Elsevier Ltd.

Tielens F.,CNRS Surface Science Lab | Santos E.,University of Ulm | Santos E.,National University of Cordoba
Journal of Physical Chemistry C | Year: 2010

The bonding of propanethiol molecules on a Au(111) surface is investigated using period DFT calculations within the framework of our model for chemical bond breaking that was recently proposed. The S-H bond breaking and the Au-S bond formation are analyzed through the evolution of the density of states. The energetics confirms the complexity of the reaction emerging from the interthiol chain interaction. The formation of a self-assembled monolayer is explained through a two-step mechanism, S-H bond breaking and Au-S bond formation. The production of H2 is found to be more favorable than the formation of Au-H species. The bonding and antibonding electronic states of the S-H bond have been identified and their evolutions during the process of bond breaking carefully analyzed. The corresponding bonding and antibonding states for the C-S bond are practically not affected during this process, indicating that the bond is preserved. The s orbital of the hydrogen atom strongly interacts with the gold surface and finally a Au-H bond is formed. © 2010 American Chemical Society.

Guesmi H.,CNRS Surface Science Lab | Massiani P.,CNRS Surface Science Lab
Catalysis Today | Year: 2011

The location of the divalent nickel cation exchanged in low amount in dehydrated NaX is identified by a combination of experimental (XAS) and theoretical (DFT) approaches. The Ni-O and Ni-T distances as well as the coordination numbers evaluated from experiments are in good agreement with the predicted models obtained from calculations. After dehydration, the migration of Ni2+ towards the hexagonal prism is confirmed and three distinct possible nickel sites are predicted. The results show the tendency of Ni 2+ to get closer to the most basic framework oxygen atoms. © 2011 Elsevier B.V. All rights reserved.

Albarazi A.,CNRS Jean Le Rond d'Alembert Institute | Beaunier P.,CNRS Surface Science Lab | Da Costa P.,CNRS Jean Le Rond d'Alembert Institute
International Journal of Hydrogen Energy | Year: 2013

In this work, the effects of doping Ni-based SBA-15 catalysts with Ceria-Zirconia mixed oxide (CZ) on the activity and stability of these catalysts during syngas production by methane dry reforming (MDR) were investigated and compared with the activity and stability of unmodified Ni/SBA-15. The above catalysts were prepared by incipient wetness impregnation (IWI) with different impregnation strategy. The samples were characterized by nitrogen physisorption, X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), temperature programmed reduction (TPR) and H2 chemisorption. The results indicated that the unmodified Ni/SBA-15 showed clear deactivation especially in the first period of the stability test and between 600°C and 630°C during the activity test whereas the CZ modified samples had better stability. © 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

The addition of minute amounts of H2 to the selective catalytic reduction of nitrogen oxides (NOx) by hydrocarbons (HC-SCR) has been shown to promote the reduction of NOx at remarkably lower reaction temperatures on Ag/Al2O3 catalysts. Despite extensive investigations, the origin of this remarkable hydrogen effect is still being debated in the literature. To provide further insights into this, the desorption-reactivity of nitrates preadsorbed on a Ag (1.6wt%)/Al2O3 catalyst is reported for the first time under desorbing feeds of increasing complexity. The results of these transient experiments together with those of steady-state experiments under selected gas compositions confirm some of the numerous roles of H2 suggested previously such as being responsible for (i) changes in the distribution of Ag species, (ii) promoted HC activation and (iii) lower nitrates coverage because of their lower stability in the presence of H2. In particular, it was found that nitrates decompose at temperatures as low as 120°C in the presence of H2 in the desorbing feed, instead of around 300°C in the absence of H2. The transient experiments in which the reactivity of the nitrates was studied under desorbing feeds containing propene (C3H6) as a reductant, allowed for the monitoring of the formation-decomposition of organo-NOx species (R-NOx). From these experiments, it can therefore be identified that another potential role of H2 is to promote the formation-decomposition-reaction of organo-NOx species at lower temperatures coinciding with the temperature of the onset of detection of N2 under both transient and steady-state conditions, in line with the "chemical effect" of H2 suggested earlier. © 2014 Elsevier B.V..

Tielens F.,CNRS Surface Science Lab | Dzwigaj S.,CNRS Surface Science Lab
Catalysis Today | Year: 2010

Ab initio periodic DFT calculations in combination with experimental FTIR of adsorption pyridine investigations have been used to study the acid-base properties of vanadium doped zeolite materials. It is evidenced that VO-H groups of V(V) and V(IV) framework sites are more acidic than SiO-H groups present in siliceous zeolites. It is proposed for the first time that the protonation of the penta coordinated V site can lead to the formation of a stable vanadyl group containing site through proton exchange,which is expected to be the inter-conversion path between both sites. We report in this paper protonation and deprotonation energies of different vanadium sites, and a geometry of experimentally observed Lewis and Brønsted acid sites is proposed. © 2009 Elsevier B.V. All rights reserved.

Vedrine J.C.,CNRS Surface Science Lab
Applied Catalysis A: General | Year: 2014

This short review article aims at revisiting the description of active sites in heterogeneous catalysis on solid surfaces and their role in catalyst activity and selectivity. Special emphasis is brought to: (i) structure of the solid surface; (ii) importance of active site isolation on the surface; (iii) dynamic behaviour under reaction conditions and (iv) importance of active phase-support interaction. After a general view on active sites in heterogeneous catalyst, this article describes different case studies of metal oxide catalysts, namely MoO3 and VPO catalysts, to exemplify the concepts, including active site description, structure and/or electronic sensitivity, synergy effects and dynamic surface behaviour during catalytic reaction. Static and dynamic models are discussed considering the many and different views expressed during the years and all considering active sites as atoms or ensembles of atoms as suggested by Taylor in 1925 and instable catalytic reaction intermediates as proposed by Sabatier in 1913. However, with the years, complex structures and dynamic behaviour with interaction between the active sites and the adsorbates (reactants and products) during catalytic reactions have been identified and characterised. In the latter case surface metalloinorganic or metalloorganic entities, designated as chemadphase, constitute the basis of reaction intermediates and are submitted to continuous oscillations on the surface under steady state. Recent developments in physical techniques used to characterise catalysts, in particular during catalytic reaction, and theoretical approach and modelling are also presented and discussed in view of improved description of active sites. © 2013 Elsevier B.V.

Vedrine J.C.,CNRS Surface Science Lab
Research on Chemical Intermediates | Year: 2015

In this overview, we focus on the different and current methods of acid-base characterization of heterogeneous catalysts and on their potential relation with catalytic properties in gas and liquid phases. Special emphasis is drawn on the main techniques currently employed such as the use of Hammett's indicators, the use of basic or acidic probes of different strength for adsorption measurements in microcalorimetry and of their thermal programmed desorption, the use of other techniques such as FTIR, ESR, NMR, photoluminescence, Raman, UV-Vis, and XPS, to identify and describe acid-base sites, the use of model catalytic reactions, and of theoretical/modeling approaches. Relationships between such a characterization and catalytic properties in different acid or base reactions are discussed, and implemented for different catalysts and different reactions. The concept of thermodynamic acidity/basicity (determined via chemical physical characterization) versus reactivity (determined via catalytic properties) of acid-base sites is presented, and explains why relationships between acid-base properties and catalytic properties are very often met but are difficult to be established unambiguously and cannot be generalized. The case of acid-base properties in liquid phase "polar" (water, alcohol) and "apolar" (cyclohexane) as "effective" and "intrinsic" properties, respectively, and their relation with catalytic properties in liquid phase are also discussed.

On the basis of the critical analysis of the NO x-TPD data obtained on WO x-ZrO 2 samples [Law, H. Y.; Blanchard, J.; Carrier, X.; Thomas, C. J. Phys. Chem. C2010, 114, 9731], this work provides convincing arguments for the first time upon which calculation of W surface density should be done by correcting the specific surface area of the samples for the content of W as WO 3, rather than by the straightforward manner used in most studies reported to date. The use of this particular calculation method allows reconciling some of the literature data, which, at first sight, seem to be incoherent. Consequently, authors must be aware that comparisons with literature data should be made with the greatest concern, regarding the method used to calculate W surface density in particular. © 2011 American Chemical Society.

Guesmi H.,CNRS Surface Science Lab | Tielens F.,CNRS Surface Science Lab
Journal of Physical Chemistry C | Year: 2012

The Cr/SiO 2 system is investigated using periodic DFT. The model represents the amorphous character of the silica surface and allows the investigation of the effect of hydration on the Cr(VI) monomers. First, the geometry and energetics are discussed and compared with experimental data. The phase diagram plotted from an atomistic thermodynamics model confirms the higher stability of mono-oxo and dioxo chromium, in comparison with species containing Cr-OH groups. In addition, the effect of the siloxane ring size on the spectroscopic signature of chromium is analyzed. A preliminary study is presented on the surface doping effect by Ti on the structure and stability of chromium species. The results reveal that the charge transfer process between Ti and Cr can explain the observed change in the reactivity of chromium species. © 2011 American Chemical Society.

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