Peer-reviewed Publications |
Amiaud, L., Dulieu, F., Fillion, J. H., Momeni, A., & Lemaire, J. L. (2007). Interaction of atomic and molecular deuterium with a nonporous amorphous water ice surface between 8 and 30 K. J. Chem. Phys., 127(14), 12 pp.
Résumé: Molecular and atomic interactions of hydrogen on dust grains covered with ice at low temperatures are key mechanisms for star formation and chemistry in dark interstellar clouds. We have experimentally studied the interaction of atomic and molecular deuterium on nonporous amorphous water ice surfaces between 8 and 30 K, in conditions compatible with an extrapolation to an astrophysical context. The adsorption energy of D(2) presents a wide distribution, as already observed on porous water ice surfaces. At low coverage, the sticking coefficient of D(2) increases linearly with the number of deuterium molecules already adsorbed on the surface. Recombination of atomic D occurs via a prompt reaction that releases molecules into the gas phase. Part of the newly formed molecules are in vibrationally excited states (v=1-7). The atomic recombination efficiency increases with the presence of D(2) molecules already adsorbed on the water ice, probably because these increase the sticking coefficient of the atoms, as in the case of incident D(2). We have measured the atomic recombination efficiency in the presence of already absorbed D(2), as it is expected to occur in the interstellar medium. The recombination efficiency decreases rapidly with increasing temperature and is zero at 13 K. This allows us to estimate an upper limit to the value of the atom adsorption energy E(a)similar to 29 meV, in agreement with previous calculations. (C) 2007 American Institute of Physics.
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Amiaud, L., Fillion, J. H., Baouche, S., Dulieu, F., Momeni, A., & Lemaire, J. L. (2006). Interaction of D-2 with H2O amorphous ice studied by temperature-programed desorption experiments. J. Chem. Phys., 124(9), 9 pp.
Résumé: The gas-surface interaction of molecular hydrogen D-2 with a thin film of porous amorphous solid water (ASW) grown at 10 K by slow vapor deposition has been studied by temperature-programed-desorption (TPD) experiments. Molecular hydrogen diffuses rapidly into the porous network of the ice. The D-2 desorption occurring between 10 and 30 K is considered here as a good probe of the effective surface of ASW interacting with the gas. The desorption kinetics have been systematically measured at various coverages. A careful analysis based on the Arrhenius plot method has provided the D-2 binding energies as a function of the coverage. Asymmetric and broad distributions of binding energies were found, with a maximum population peaking at low energy. We propose a model for the desorption kinetics that assumes a complete thermal equilibrium of the molecules with the ice film. The sample is characterized by a distribution of adsorption sites that are filled according to a Fermi-Dirac statistic law. The TPD curves can be simulated and fitted to provide the parameters describing the distribution of the molecules as a function of their binding energy. This approach contributes to a correct description of the interaction of molecular hydrogen with the surface of possibly porous grain mantles in the interstellar medium. (c) 2006 American Institute of Physics.
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Bedford, N., Dablemont, C., Viau, G., Chupas, P., & Petkov, V. (2007). 3-D structure of nanosized catalysts by high-energy X-ray diffraction and reverse Monte Carlo simulations: Study of Ru. JOURNAL OF PHYSICAL CHEMISTRY C, 111(49), 18214–18219.
Résumé: Ruthenium exhibits a high catalytic activity that is further enhanced when the material is used as nanosized particles. The origin of the enhanced performance lies in the highly increased surface to volume ratio and, often, to the surface/environment-driven structural relaxation taking place at the nanoscale. In this paper, we show how high-energy X-ray diffraction, atomic pair distribution function analysis, and reverse Monte Carlo simulations may be used to determine the 3-D structure of nanoparticle catalysts such as Ru with sizes less than 5 nm. Ruthenium particles that are 4 nm in size are found to possess a hexagonal close packed-type structure, similar to that found in bulk Ru. Particles that are only 2 nm in size are heavily disordered and consist of a Ru core and a Ru-S skin due to the usage of thiol-based capping agents. This work is the first application of an approach for determining the atomic-scale structure of nanosized catalysts based entirely on experimental diffraction data. The new structural information is a starting point for a better understanding of the structure-property relationship and, hence, for the design of improved nanosized catalysts, including Ru.
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Bini, F., Rosier, C., Saint-Arroman, R. P., Neumann, E., Dablemont, C., de Mallmann, A., Lefebvre, F., Niccolai, G. P., Basset, J. M., Crocker, M., & Buijink, J. K. (2006). Surface organometallic chemistry of titanium: Synthesis, characterization, and reactivity of ( Si-O)(n)Ti(CH(2)C(CH(3))(3))(4-n) (n = 1, 2) grafted on aerosil silica and MCM-41. ORGANOMETALLICS, 25(15), 3743–3760.
Résumé: The reaction of tetrakisneopentyl titanium, TiNp(4) (1), with the surface of partially dehydroxylated Aerosil silica and MCM-41 and the reactivity of the resultant supported titanium alkyl product with water, alcohols, and oxygen are reported. Two methods of preparation have been investigated and compared for the grafting of TiNp(4): (i) reaction of the support with the vapor of the sublimed complex and (ii) impregnation of the support with a solution of the complex. The second method appeared to be more reliable for “larger scale” preparations. The surface species thus obtained were characterized by infrared spectroscopy, solid state NMR, XAFS, elemental analysis, and various test reactions. Whereas on an Aerosil silica partially dehydroxylated at 500 degrees C, SiO(2-(500)), the surface complex is a monopodal titanium trisalkyl complex, SiO-Ti[CH(2)C(CH(3))(3)](3), 2a, a bipodal titanium bisalkyl complex, ( SiO)(2)Ti[CH(2)C- CH(3))(3)](2), 2b, is obtained as the major species (ca. 65%) with 2a on MCM-41((500)). The reason for this difference in behavior is discussed on the basis of the surface structure. The results obtained from hydrolysis confirmed the structure proposed for the supported alkyl complexes. For the reaction of the alkyl surface complexes with alcohols (MeOH, EtOH, (t)BuOH), the surface compounds obtained were characterized by the same techniques and by XPS and UV-vis. The results are consistent with the formation of monosiloxytrisalkoxy titanium complexes on SiO(2-(500)), SiO-Ti(OR)(3), 3a(OR), and of SiO-Ti(O(t)Bu)(3), 3a(OtBu), and ( SiO)(2)Ti(O(t)Bu)(2), 3b(OtBu), on MCM-41((500)), after reaction with (t)BuOH. The supported titanium alkyl, 2a, also reacts with oxygen, leading mainly to SiO-Ti[OCH(2)C(CH(3))(3)](3), probably via an unstable surface compound such as SiO-Ti[OCH(2)C(CH(3))(3)](2)[OOCH(2)C(CH(3))(3)], resulting from the incorporation of two molecules of oxygen in 2a.
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Bobrov, K., & Guillemot, L. (2007). Nanostructure formation by reactions of H2O with pre-adsorbed O on a Ag(110) surface. SURFACE SCIENCE, 601(15), 3268–3275.
Résumé: We present results of an STM investigation of water interaction with an oxygen covered Ag(l 10) in the case of the 0(4 x 1) reconstructed surface. Regarding the formation of one-layer-thick silver nanostructures previously demonstrated, they point to the key role of the surface temperature at which the water dosing is made. Indeed we measure silver nanostructuring for dosing temperatures lower than 235 K. We follow, in real time during the water dosing, the modifications induced at the surface for two temperatures of 200 K and 240 K. Drastic differences are exhibited. At 200 K, after an initial stage of formation of molecular assembly strips along the [0 0 1], the reactive process leading to the conversion to an OH layer occurs clearly going along with the appearance and development of quasi- rectangular silver nanostructures. At 240 K, no such initial phase is evidenced. The complete conversion to an OH row structure of the scanned area occurs with no concomitant silver nanostructure formation. The dynamical behaviour of the reaction front allows the unravelling of the key role of the developing OH row ends intersecting the remaining Ag-O rows as particular reactive adsorption sites for the completion of the OH layer. (c) 2007 Elsevier B.V. All rights reserved.
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Borisov, A. G., Sidis, V., Roncin, P., Momeni, A., Khemliche, H., Mertens, A., & Winter, H. (2003). F- formation via simultaneous two-electron capture during grazing scattering of F+ ions from a LiF(001) surface. PHYSICAL REVIEW B, 67(11), 115403.
Résumé: For slow F+ ions (v<0.05 a.u.) scattered from a clean and flat LiF(001) surface under a grazing angle of incidence, large fractions of negative F- ions have recently been observed in the reflected beam, while for neutral F-0 projectiles no negative F- ions are produced in the same velocity range [P. Roncin , Phys. Rev. Lett. 89, 043201 (2002)]. From detailed studies on projectile energy loss and charge fractions, the conclusion was drawn that the F- ions are formed from F+ via a simultaneous capture of two electrons from adjacent F- sites at the surface. We present a theoretical description of the double-electron-capture process leading to F- formation from F+ projectiles grazingly scattered from the LiF(001) surface. We use quantum chemistry calculations to determine the relevant Hamiltonian matrix and close-coupling solution of the time-dependent Schrodinger equation. The theoretical results are in good agreement with experimental observations.
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Dablemont, C., Hamaker, C. G., Thouvenot, R., Sojka, Z., Che, M., Maatta, E. A., & Proust, A. (2006). Functionalization of heteropolyanions – Osmium and rhenium nitrido derivatives of Keggin- and Dawson-type polyoxotungstates: Synthesis, characterization and multinuclear (W-183, N-15) NMR, EPR, IR, and UV/Vis fingerprints. CHEMISTRY-A EUROPEAN JOURNAL, 12(36), 9150–9160.
Résumé: Reaction of K-10[alpha(2)-P2W17O61] or K-10[alpha(1)-P2W17O61] or [Bu4N][OsCl4N] in a water/methanol mixture, and subsequent precipitation with (BU4N)Br provided [alpha(2)-P2W17O61{(OsN)-N-VI}](7-) and [alpha(1)-P2W17O61{(OsN)-N-VI}](7-) Dawson structures as tetrabutylammonium salts. Reactions of [(Bu4N),][alpha-H3PW11O39] with either [ReCl3(N2Ph2)(PPh3)(2)] or [Bu4N][ReCl4N] are alternatives to the synthesis of [(BU4N)(4)] [alpha-PW11O39-{ReVIN}]. W-183 and N-15 NMR, EPR, IR, and UV-visible spectroscopies and cyclic voltammetry have been used to characterize these compounds and the corresponding [(Bu4N)(4)][alpha-PW11O39_ {(OsN)-N-VI}] Keggin derivative.
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Dablemont, C., Proust, A., Thouvenot, R., Afonso, C., Fournier, F., & Tabet, J. C. (2004). Functionalization of polyoxometalates: From Lindqvist to Keggin derivatives. 1. Synthesis, solution studies, and spectroscopic and ESI mass spectrometry characterization of the rhenium phenylimido tungstophosphate [PW11O39{ReNC6H5}](4-). INORGANIC CHEMISTRY, 43(11), 3514–3520.
Résumé: Reaction of [Bu4N](4)[H3PW11O39] with [Re(NPh)Cl-3(PPh3)(2)], in acetonitrile and in the presence of NEt3, provided the first Keggin-type organoimido derivative [Bu4N](4)[PW11O39{ReNPh}] (Ph = C6H5) (1). The functionalization was clearly demonstrated by various techniques including H-1 and N-14 NMR, electrochemistry, and ESI mass spectrometry. Conditions for the formation of 1 are also discussed.
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Dablemont, C., Proust, A., Thouvenot, R., Afonso, C., Fournier, F., & Tabet, J. C. (2005). Investigation of the reactivity of arylamines, organo-hydrazines and tolylisocyanate towards [PW12-xMxO40](n-) Keggin anions. DALTON TRANSACTIONS, (10), 1831–1841.
Résumé: Reaction of K-7[A,α-PW9Mo2O39] with Na2MoO4&BULL; 2H(2)O in a mixture of water/dioxane/hydrochloric acid and further precipitation with (Bu4N)Br provided (Bu4N)(3)[A,α-PW9Mo3O40] (3). Analogous reaction with K7-xNax[α-PW11O39] is an alternative to the synthesis of (Bu4N)(3)[α-PW11O39{(MoO)-O-VI}] (2). Multinuclear NMR and ESI mass spectrometry have been used to interpret the reaction of (Bu4N)(x)[α-PW11O39{ReO}] (x = 3 1; x = 4 1(I)), (Bu4N)(x)[α-PW11O39{MoO}] (x = 3 2; x = 4 2(I)) and (Bu4N)(3)[A,α-PW9Mo3O40] (3) by organohydrazines, arylamines, tolylisocyanate and tetraphenylphosphine imide.
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Dulieu, F., Amiaud, L., Baouche, S., Momeni, A., Fillion, J. H., & Lemaire, J. L. (2005). Isotopic segregation of molecular hydrogen on water ice surface at low temperature. Chem. Phys. Lett., 404(1-3), 187–191.
Résumé: Experimental studies of adsorption and desorption of molecular hydrogen on an amorphous porous solid water ice surface between 10 and 35 K reveal a very efficient isotopic segregation process. A statistical model, which take into account thermodynamic aspects of adsorption sites and isotopic competition, is proposed to understand the enhancement of deuterium fractionation. This mechanism could play a key role in chemistry at the surface of interstellar dust grains. (C) 2005 Elsevier B.V. All rights reserved.
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Guillemot, L., & Bobrov, K. (2007). On the formation of OH ordered layers by dissociation of H2O on an oxygen covered Ag(110) surface: An STM investigation. SURFACE SCIENCE, 601(3), 871–875.
Résumé: We present results of an STM investigation of water interaction with an oxygen covered Ag(110) on the example of the 0(4 x 1) reconstructed surface. In agreement with numerous previous experimental works, using diffraction techniques, we found that a structure of OH(1 x 2) type, displaying rows in the [1-10] direction, is formed. The new features revealed by this local probe study, is the presence of quasi rectangular islands evenly distributed across the terraces, with a density of 0.22 +/- 0.03 and a mean area of 90 +/- 15 nm(2) at 220 K. They are imaged at an apparent height of 0.14 nm. It is remarkable that the same OH row structure is present on the whole terrace “on top” and “in between” the islands. These features are attributed to silver islands of mono-atomic height, formed by clustering of silver ad-atoms released during reaction of the 0 atoms with the water molecules. These findings point to a more complex behaviour of the reaction dynamics than previously described. They emphasise the key role of the silver ad-atoms, present in the added rows of the initial Ag(110)-O(4 x 1) surface, in the formation of the nanostructures. In turn it is concluded that the rows evidenced by this STM and previous diffraction studies, are formed by OH chains. (c) 2006 Elsevier B.V. All rights reserved.
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Khemliche, H., Borisov, A. G., Momeni, A., & Roncin, P. (2002). Exciton and trion formation during neutralization of Ne+ at a LiF(001) surface. NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 191, 221–225.
Résumé: The grazing angle interaction of 2 keV Ne+ projectiles with a LiF(0 0 1) surface is studied with the combination, in coincidence, of projectile and electron time-of-flight spectroscopy. The measurements reveal that besides the standard Auger neutralization process that leads to electron ejection, there is another neutralization mechanism that does not result in electron emission. The latter process has been identified as the formation of an electron-bihole complex termed trion. We report here the detailed study of the scattering angle dependence of these two neutralization channels, with comparison with the process leading to population of surface excitons. (C) 2002 Published by Elsevier Science B.V.
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Khemliche, H., Villette, J., Borisov, A. G., Momeni, A., & Roncin, P. (2001). Electron bihole complex formation in neutralization of Ne+ on LiF(001). PHYSICAL REVIEW LETTERS, 86(25), 5699–5702.
Résumé: Neutralization of low keV Ne+ ions at a LiF(001) surface is studied in a grazing incidence geometry. The combination of energy loss and electron spectroscopy in coincidence reveals two neutralization channels of comparable importance. Besides the Anger process, the Nef neutralization can proceed via peculiar target excitation, corresponding to the formation of an electron bihole complex termed trion.
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Khemliche, H., Villette, J., Roncin, P., & Barat, M. (2000). Surface exciton population in proton impact with LiF(100). NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 164, 608–613.
Résumé: We have recently shown that the large energy loss experienced by slow protons interacting with a LiF surface is primarily caused by the population of surface excitons [11]. These states are most probably populated by electron transfer from H- ions formed at halogen sites, whereas secondary electron emission results from direct detachment of the negative ions in collisions with halogen sites. We report here on the impact energy dependence of the charge-state of the scattered projectile, its energy loss and the associated electron yield. From these complete measurements, the transfer probability to the surface excitons and its dependence upon impact energy is derived in the range 0.6-10 keV. (C) 2000 Elsevier Science B.V. All rights reserved.
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Kwen, H., Tomlinson, S., Maatta, E. A., Dablemont, C., Thouvenot, R., Proust, A., & Gouzerh, P. (2002). Functionalized heteropolyanions: high-valent metal nitrido fragments incorporated into a Keggin polyoxometalate structure. CHEMICAL COMMUNICATIONS, (24), 2970–2971.
Résumé: Three examples of nitrido-functionalized polyoxometalate species are reported, namely (n-Bu4N)(4)[PW11O39(OsN)] (1), (n-Bu4N)(4)[PW11O39(ReN)] (2), and (n-Bu4N)(3)[PW11O39-(ReN)] (3), which feature the incorporation of [Os-VI=N](3+), [Re-VI=N](3+) and [Re-VII=N](4+) fragments, respectively, into the framework of a Keggin-type heteropolyanion.
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Lidgi-Guigui, N., Dablemont, C., Veautier, D., Viau, G., Seneor, P., Van Dau, F. N., Mangeney, C., Vaures, A., Deranlot, C., & Friederich, A. (2007). Grafted 2D assembly of colloidal metal nanoparticles for application as a variable capacitor. ADVANCED MATERIALS, 19(13), 1729–+.
Résumé: A combination of sputtering and colloidal chemistry is employed to prepare [Co/Al2O3//Ru nanoparticles//Al2O3/Co] junctions (see figure). These junctions are applied as variable capacitors relying on Coulomb blockades in a 2D assembly of nanoparticles. AC measurements show a significant capacitance variation as a function of applied DC voltage with a maximum of relative variation value that is proportional to the particle density embedded in the dielectric layer and is in good agreement with the theoretical model.
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Oulie, P., Teichert, J., Vendier, L., Dablemont, C., & Etienne, M. (2006). Aromatic interactions in hydrotris(3-methylindazolyl) borate organoniobium complexes: control of an alkyne ligand orientation in the crystal. NEW JOURNAL OF CHEMISTRY, 30(5), 679–682.
Résumé: Reaction of NbCl3(MeOCH2CH2OMe)(PhC=CMe) with KTp(4Bo,3Me) in THF produces a 68% yield of the dichloro – phenylpropyne complex Tp(4Bo,3Me)NbCl(2)(PhC=CMe) [Tp(4Bo,3Me) = hydrotris(3-methylindazol-1-yl) borate]. As observed by solution NMR spectroscopy, the four-electron donor alkyne sits in the molecular mirror plane and restricted rotation of the alkyne ligand allows the observation of an equilibrium between two rotamers. The conformation of the alkyne ligand in the major isomer is such that the phenyl group is proximal to Tp(4Bo,3Me). Unexpectedly, the minor rotamer in solution, that with the distal phenyl group, is observed in the crystal. Analysis of the possible interactions suggest that aromatic interactions are responsible for this unusual observation.
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Rhers, B., Lucas, C., Taoufik, M., Herdtweck, E., Dablemont, C., Basset, J. M., & Lefebvre, F. (2006). Synthesis and characterization of new aryloxy containing tungsten complexes. COMPTES RENDUS CHIMIE, 9(9), 1169–1177.
Résumé: Disubstituted phenols react with the W(equivalent to CC(CH3)(3))(-CH2C(CH3)(3))(3) tungsten carbyne complex, leading to the formation of new complexes where one, two or three neopentyl ligands have been replaced by aryloxy ones. Upon addition of four equivalents of phenol a W(=CHC(CH3)(3))(-OAr)(4) carbenic species is obtained. The structure of the complex with three 2,6-diphenylphenol aryloxy ligands is described.
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Roncin, P., Borisov, A. G., Khemliche, H., Momeni, A., Mertens, A., & Winter, H. (2002). Evidence for F- formation by simultaneous double-electron capture during scattering of F+ from a LiF(001) surface. PHYSICAL REVIEW LETTERS, 89(4), 043201.
Résumé: Slow F+ ions (v<0.1 a.u.) scattered from a clean and flat LiF(001) surface under a grazing angle of incidence exhibit a high probability for forming F- ions in the reflected beam, whereas no negative ions are found for neutral F-0 projectiles. From detailed studies of projectile energy loss and charge transfer, we find evidence for a correlated double-electron capture process in the formation of the F- ions.
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Roncin, P., Villette, J., Atanas, J. P., & Khemliche, H. (1999). Energy loss of low energy protons on LiF(100): Surface excitation and H- mediated electron emission. PHYSICAL REVIEW LETTERS, 83(4), 864–867.
Résumé: Impact of 600 eV protons at grazing incidence on LiF(100) is studied with a new coincidence technique combining energy loss and electron spectroscopy. Correlation between the secondary electrons and the charge state of the scattered projectiles demonstrates the role of the H- ions formed on the surface as precursors for electron emission. However, the main channel for energy loss is not associated with electron emission but is interpreted as the population of surface excitons.
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Rousseau, P., Khemliche, H., Borisov, A. G., & Roncin, P. (2007). Quantum scattering of fast atoms and molecules on surfaces. PHYSICAL REVIEW LETTERS, 98(1), 016104.
Résumé: We present evidence for the diffraction of light keV atoms and molecules grazingly scattered on LiF(001) and NaCl(001) surfaces. At such energies, the de Broglie wavelength is 2 orders of magnitude smaller that the mean thermal atomic displacement in the crystal. Thus, no coherent scattering was expected and interaction of keV atoms with surfaces is routinely treated with classical mechanics. We show here that well-defined diffraction patterns can be observed indicating that, for grazing scattering, the pertinent wavelength is that associated with the slow motion perpendicular to the surface. The experimental data are well reproduced by an ab initio calculation.
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Soignier, S., Taoufik, M., Le Roux, E., Saggio, G., Dablemont, C., Baudouin, A., Lefebvre, F., de Mallmann, A., Thivolle-Cazat, J., Basset, J. M., Sunley, G., & Maunders, B. M. (2006). Tantalum hydrides supported on MCM-41 mesoporous silica: Activation of methane and thermal evolution of the tantalum-methyl species. ORGANOMETALLICS, 25(7), 1569–1577.
Résumé: The Ta(=CHtBu)(CH(2)tBU)(3) Complex 1 reacts with the OH groups of a MCM-41 mesoporous silica dehydroxylated at 500 degrees C to form the monosiloxy surface species [(equivalent to SiO)Ta(=CHtBu)(CH(2)tBu)(2)] 2, with evolution of 1 equiv per Ta of neopentane. Complex 2 leads to a mixture of supported tantalum hydrides [(equivalent to SiO)(2)Ta(H)(x)] (x = 1, 3) 3, by treatment under hydrogen at 150 degrees C. These surface complexes were characterized by the combined use of several techniques such as IR and EXAFS spectroscopies as well as H-1 MAS, C-13 CP/MAS, 2D H-1-C-13 HETCOR, and J-resolved solid-state NMR and mass balance analysis. The surface tantalum hydrides evolve reversibly to the monohydride species (equivalent to SiO)(2)Ta-H by heating at 150 degrees C under vacuum; they lead progressively to the complete formation of the supported trisiloxy tantalum complex (equivalent to SiO)(3)Ta by heating under hydrogen (600 Torr) up to 500 degrees C. They can activate at 150 degrees C the C-H bond Of CH4 to form first the surface tantalum methyl species [(=SiO)(2)Ta(CH3)(x)] with liberation of H-2. The initially rapid decrease of the v(Ta-H) bands followed by a slower rate indicates the presence of a distribution of Ta-H sites of various reactivity. The combined use of C-13 CP/MAS solid-state NMR and 100% C-13-labeled methane affords the observation of methylidene and methylidyne species on a few tantalum sites, which indicates the occurrence of an alpha-H elimination process. In parallel, a progressive transfer of methyl groups from tantalum to neighboring siloxane bridges was also evidenced, which grows with temperature; this process is reasonably accompanied by the formation of the trisiloxy tantalum complex (equivalent to SiO)(3)Ta.
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Thomas, A., Dablemont, C., Basset, J. M., & Lefebvre, F. (2005). Comparison of H3PW12O40 and H4SiW12O40 heteropolyacids supported on silica by H-1 MAS NMR. COMPTES RENDUS CHIMIE, 8(11-12), 1969–1974.
Résumé: The H3PW12O40 and H4SiW12O40 heteropolyacids supported on silica were studied by H-1 MAS NMR as a function of both the coverage and the dehydroxylation temperature. The signal at ca. 8 ppm attributed to the highly acidic protons of the anhydrous form of the heteropolyacid was never observed in the case of the phosphotungstic polyanion supported on silica while it was clearly present in the case of the silicotungstic species. These results explain the different activities of the two supported heteropolyacids in acid reactions involving strong acid sites.
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Tosin, G., Santini, C. C., Baudouin, A., De Mallman, A., Fiddy, S., Dablemont, C., & Basset, J. M. (2007). Reactivity of silica-supported hafnium tris-neopentyl with dihydrogen: Formation and characterization of silica surface hafnium hydrides and alkyl hydride. ORGANOMETALLICS, 26(17), 4118–4127.
Résumé: Surface organometallic chemistry represents an approach to the preparation of well-defined single sites for catalysis, the possibility of observing some elementary reaction steps, and the development of a fundamental basis for the synthesis of tailor-made catalysts. Silica-supported metal hydrides are an important class of new catalysts for alkane metathesis, methanolysis of alkanes, Ziegler-Natta depolymerization, alkane hydrogenolysis, etc. Understanding their mechanism of formation and aging is crucial. In the work presented here, the reaction of the well-defined silica surface organometallic complex [( Sio)Hf(CH(2)tBu)(3)], 1, (( SiO) = silica surface ligand) with dihydrogen has been performed at different temperatures (theta). At theta <= 100 degrees C, there is formation of a stable hafnium neopentyl dihydride, [( SiO)-Hf(CH(2)tBu)(H)(2)], 2. For 100 <= theta <= 200 degrees C, 2 affords, via a succession of beta-methyl transfer and subsequent hydrogenolysis of the resulting Hf-alkyl bonds, the formation Of [( SiO)(2)Hf(H)2], 3, and [( SiO)(3)SiH] with evolution of methane (C-1) and ethane (C-2). For 150 <= theta <= 300 degrees C, 3 is totally converted into [( SiO)(3)Hf(H)], 4, and [( SiO)(2)Si(H)(2)]. For theta >= 300 degrees C, [( SiO)(3)Hf(H)], 4, is transformed into [( SiO)(4)Hf], 5, and [( SiO)(2)Si(H)(2)] into [( SiO)(3)SiH]. At this temperature, [( SiO)(3)SiH] is the only hydride remaining on the surface. All these species have been characterized with a multitude of techniques such as elemental analysis and infrared, H-1 solid-state NMR, H-1 DQ solid-state NMR, and EXAFS spectroscopies. The results elucidate a complete mechanism of surface organometallic chemistry by which one observes the stepwise transformation of a hafnium tris-neopentyl to hafnium neopentyl hydrides, hafnium mono- and bis-hydrides, silicon bis-hydride with the ultimate formation of tetrasiloxy surface species [( SiO)(4)Hf)], and silicon mono-hydride, the only hydride stable at very high temperature. It is suggested that the formation of these surface silicon hydrides is responsible for the aging of such catalysts in any reaction involving dihydrogen.
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Valdes, J. E., Vargas, P., Guillemot, L., & Esaulov, V. A. (2007). Effect of oxygen adsorption on the energy losses in grazing scattering of hydrogen ions on Ag(110). NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 256(1), 81–85.
Résumé: In this work we present a study of the effect of oxygen adsorption on the energy loss of 4 keV hydrogen ions, which are scattered off an Ag (110) single-crystal surface with varying coverages of oxygen. In this case oxygen adsorption leads to an added row reconstruction of the surface. We performed measurements for grazing angles as a function of crystal azimuthal orientation, which show large differences in energy losses. Experimental results are discussed in the light of trajectory calculations of protons scattered under grazing incidence conditions on the surface. Using non-linear models for stopping power, ab initio crystal structure calculations of the electronic density and semi-classical simulations, we obtain data that is in very good agreement with experimental results. These simulations in particular allow us to properly take into account the variations of the surface electronic density and hence obtain an accurate description of the energy loss processes for ion scattering along various azimuthal orientations of the target. (c) 2006 Elsevier B.V. All rights reserved.
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Villette, J., Borisov, A. G., Khemliche, H., Momeni, A., & Roncin, P. (2000). Subsurface-channeling-like energy loss structure of the skipping motion on an ionic crystal. PHYSICAL REVIEW LETTERS, 85(15), 3137–3140.
Résumé: The skipping motion of Ne+ ions in grazing scattering from the LiF(001) surface is studied for velocity below 0.1 a.u. with a time-of-flight technique. It is demonstrated that suppression of electronic excitation and dominance of optical phonon excitation in the projectile stopping results in an odd 1,3, 5,... progression of the energy loss peaks, a feature usually ascribed to subsurface channeling. The experimental findings are well reproduced by parameter-free model calculations where thermal vibrations are the dominant cause for the ion trapping and detrapping.
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Actes de Conférences |
Roncin, P., Khemliche, H., Momeni, A., & Borisov, A. G. (2001). Translational spectroscopy in grazing collisions on insulators. The importance of the transient negative ion and of target excitations. In PHOTONIC (pp. 571–579).
Résumé: Energy loss spectroscopy is applied to grazing collisions of keV ions on insulator surfaces. For W projectiles, analyzing in coincidence the energy loss, the final charge state and the secondary electrons, the role of the intermediate negative ions formed on the surface is outlined. The study is extended to other projectiles to probe the influence of the projectile affinity level.
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Rousseau, P., Gugiu, M., Khemliche, H., & Roncin, P. (2005). Neutralization of noble gas ions on ionic insulators: Auger neutralization or double-electron capture. In NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS (Vol. 230, pp. 361–368).
Résumé: The neutralization of noble gas singly charged ions on large band gap ionic insulators may take place through two distinct mechanisms involving two valence band electrons. One is a kinetically assisted Auger neutralization, and the other a simultaneous double electron capture. Both lead to the same final states, that is at grazing incidence angles a scattered neutral projectile connected to a surface excited state (trion) or an ejected electron. So it is virtually impossible to experimentally identify the neutralization process. From comparison to systems whose neutralization mechanism is clearly identified, the incidence angle dependence of the trion population relative to the total neutral fraction appears as a robust parameter for distinguishing the primary capture process. (c) 2004 Elsevier B.V. All rights reserved.
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Rousseau, P., Khemliche, H., & Roncin, P. (2007). Auger rates on NaCl(001), effect of the final state and modeling via an effective length. In NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS (Vol. 258, pp. 13–17).
Résumé: Neutralization of keV He+, Ne+ and F+ ions colliding on NaCl(001) at grazing incidence is studied by energy loss in coincidence with emitted electrons. Three closely related Auger-like mechanisms are identified. In all processes, two electrons are removed from the valence band, one is captured on the ground state of the neutral projectile whereas the remaining electron is found either in the electron continuum, in the conduction band or in a surface excited state. To help comparing the three different processes, a procedure is proposed that allows a first order correction of so-called trajectory effect's on the observed neutralization fraction without a priori knowledge of the detailed trajectory. (C) 2006 Elsevier B.V. All rights reserved.
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Villette, J., Atanas, J. P., Khemliche, H., Barat, M., Morosov, V., & Roncin, P. (1999). Grazing collision of keV protons on LiF correlation between energy loss and electron emission. In NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS (Vol. 157, pp. 92–97).
Résumé: Grazing interaction of low energy protons with LiF(1 0 0) is studied using a new coincidence technique combining energy loss, charge state analysis and electron spectroscopy. Correlation between the scattered projectile energy loss and the number of emitted electrons points to an energy loss mechanism not leading to electron emission. Detailed analysis of the energy loss spectra, which show well-resolved structures, suggests that this mechanism corresponds to the population of surface excitons. Moreover the correlation between the projectile final charge stare and the number of emitted electrons sheds new light on the major role played by the negative ion at the surface. Electron removal from the valence band proceeds mainly through formation of H- at halogen sites, whereas electron emission results from the detachment of these negative ions at subsequent F- sites. (C) 1999 Elsevier Science B.V. All rights reserved.
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