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ABSTRACT: Mesoporous Re–Si–Al, Re–Al, and Re–Si mixed oxides were obtained via a one-step non-hydrolytic sol–gel route followed by calcination. The texture and surface properties of the resulting catalysts were characterized by a combination of EDX, XRD, N2-physisorption, XPS, ToF-SIMS, and NH3-TPD. The loss of rhenium during calcination, the texture and the acidity of the catalysts depended on their composition. Migration of rhenium toward the surface occurred during the calcination treatment, as evidenced by XPS and ToF-SIMS. After calcination, ToF-SIMS showed the presence of well-dispersed ReOx surface species. The influence of the composition of Re–Si–Al catalysts on their performances in the cross-metathesis of ethene and butene to propene was investigated. The specific activity of the Re–Si–Al catalysts was much higher than that of Re–Al catalysts, whereas the Re–Si sample was not active. The best Re–Si–Al catalysts displayed excellent specific activities (up to 45 mmol g−1 h−1) and apparent TOF values (98 × 10−3 s−1).
Journal of Catalysis. 01/2013; 301:233-241.
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ABSTRACT: The segregation of As + ions implanted into thin Er films deposited on n-Si substrates is studied after ErSi 2-x formation. The same lowering of the effective Schottky barrier height (SBH) below 0.12 eV is obtained at moderate annealing temperatures, regardless of the redistribution of As dopants at the ErSi 2-x /Si interface. On the other hand, if the implanted dose is slightly enhanced, the annealing temperature required to reach sub-0.12-eV effective SBH can be further reduced. This process enables the formation of very low effective SBH ErSi 2-x /n-Si contacts with a low thermal budget. a) Electronic mail : nicolas.reckinger@uclouvain.be.
Applied Physics Letters 07/2011; 99:012110. · 3.84 Impact Factor
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ABSTRACT: The segregation of As+ ions implanted into thin Er films deposited on n-Si
substrates is studied after ErSi2-x formation. The same lowering of the
effective Schottky barrier height (SBH) below 0.12 eV is obtained at moderate
annealing temperatures, regardless of the redistribution of As dopants at the
ErSi2-x/Si interface. On the other hand, if the implanted dose is slightly
enhanced, the annealing temperature required to reach sub-0.12-eV effective SBH
can be further reduced. This process enables the formation of very low
effective SBH ErSi2-x/n-Si contacts with a low thermal budget.
Applied Physics Letters 07/2011; · 3.84 Impact Factor
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ABSTRACT: MoO3/SiO2–Al2O3 catalysts are prepared via flame spray pyrolysis and evaluated in the self-metathesis of propene to ethene and butene. Their specific surface area ranges between 100 and 170 m2 g−1 depending on the MoO3 loading (1–15 wt.%, corresponding to Mo surface density between 0.3 and 6.1 Mo atoms per nm2). The catalysts were characterized by N2-physisorption, X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectroscopy (ToF-SIMS). The silica–alumina matrix condenses first in the flame and forms non-porous spherical particles of 5–20 nm, followed by the dispersion of Mo oxide at their surface. Depending on the MoO3 loading, different MoOx species are stabilized: dispersed and amorphous molybdates (mono- and oligomeric) at low loadings (<5 wt.%, <1.5 Mo nm−2) and crystalline MoO3 species at higher loadings. Raman spectroscopy suggests the presence of monomeric species for surface densities of 0.3, 0.5 and 0.8 Mo nm−2. The formation of Mosingle bondOsingle bondMo bonds is, however, clearly established by ToF-SIMS from surface densities as low as 0.5 Mo nm−2. At 1.5 Mo nm−2, crystallites of β-MoO3 (2–3 nm) are detected and further increasing the loading induces the formation of bigger α- and β-MoO3 crystals (around 20 nm). The speciation of Mo proves to have a marked impact on the metathesis activity of the catalysts. Catalysts with high Mo loading and exhibiting MoO3 crystals are poorly active, whereas catalysts with low Mo loading (<5 wt.%) perform well in the reaction. The catalyst loaded with only 1 wt.% of MoO3 (0.3 Mo nm−2) is the most active, reaching turn over frequencies seven times higher than reference catalysts reported in the literature. Moreover, the specific metathesis activity is clearly inversely correlated to the degree of condensation of the molybdenum oxide phase (as evaluated by ToF-SIMS). The latter finding indicates that monomeric MoOx species are the main active centres in the olefin metathesis.
http://www.sciencedirect.com/science/article/pii/S0021951710003805
Journal of Catalysis. 01/2011; 277(2):154-163.
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ABSTRACT: web: http://dx.doi.org/10.1016/j.jcat.2010.11.003
MoO3/SiO2-Al2O3 catalysts are prepared via flame spray pyrolysis and evaluated in the self-metathesis of propene to ethene and butene. Their specific surface area ranges between 100 and 170 m² g-1 depending on the MoO3 loading (1 to 15 wt.%, corresponding to Mo surface density between 0.3 and 6.1 Mo atoms per nm²). The catalysts were characterized by N2-chemisorption, X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectroscopy (ToF SIMS). The silica-alumina matrix condenses first in the flame and forms non-porous spherical particles of 5-20 nm, followed by the dispersion of Mo oxide at their surface. Depending on the MoO3 loading, different MoOx species are stabilized: dispersed and amorphous molybdates (mono- and oligomeric) at low loadings (<5 wt.%, < 1.5 Mo nm-²) and crystalline MoO3 species at higher loadings. Raman spectroscopy suggests the presence of monomeric species for surface densities of 0.3, 0.5 and 0.8 Mo nm-2. The formation of Mo-O-Mo bonds is, however, clearly established by ToF SIMS from surface densities as low as 0.5 Mo nm-². At 1.5 Mo nm-² crystallites of β-MoO3 (2-3 nm) are detected and further increasing the loading induces the formation of bigger α- and β-MoO3 crystals (around 20 nm). The speciation of Mo proves to have a marked impact on the metathesis activity of the catalysts. Catalysts with high Mo loading and exhibiting MoO3 crystals are poorly active whereas catalysts with low Mo loading (< 5 wt.%) perform well in the reaction. The catalyst loaded with only 1 wt.% of MoO3 (0.3 Mo nm-2) is the most active, reaching turn over frequencies 7-times higher than reference catalysts reported in the literature. Moreover, the specific metathesis activity is clearly inversely correlated to the degree of condensation of the molybdenum oxide phase (as evaluated by ToF SIMS). The latter finding indicates that monomeric MoOx species are the main active centres in the olefin metathesis.
J. Catal. 01/2011; 277:154-163.
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ABSTRACT: The enhancement of the static secondary ion mass spectrometry (SIMS) signals resulting from the injection, closely to the sample surface, of H(2)O vapor at relatively high-pressure, was investigated for a set of organic materials. While the ion signals are generally improved with increasing H(2)O pressure upon 12 keV Ga(+) bombardment, a specific enhancement of the protonated ion intensity is clearly demonstrated in each case. For instance, the presence of H(2)O vapor induces an enhancement by one order of magnitude of the [M + H](+) static SIMS intensity for the antioxidant Irgafos 168 and a ∼1.5-fold increase for polymers such as poly(vinyl pyrrolidone).
Journal of the American Society for Mass Spectrometry 12/2010; 21(12):2005-10. · 4.00 Impact Factor
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Applied Catalysis B-Environmental. 01/2010; 94(1-2):38-45.
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Chemistry of Materials. 01/2009; 21(13):2817-2824.
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ABSTRACT: Most of the first-row transition-metal oxides, M(A)O(B) (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) were examined by static secondary ion mass spectrometry (s-SIMS) and laser ablation/ionization Fourier transform ion cyclotron resonance mass spectrometry (LA-FTICRMS). Positive and negative ions show strong correlation between the studied oxide and the detected cluster ions. Specific M(x)O(y) (+/-) species were systematically observed with both MS techniques for each investigated M(A)O(B) transition-metal oxide. Moreover, the ion composition and ion distribution are greatly dependent on the ionization technique. Laser ablation (LA)/ionization leads to larger cluster ions (ionic species with nearly hundred atoms were in particular detected for Sc2O3 and Y2O3 oxides), whereas hydrogenated, dihydrogenated, and sometimes trihydrogenated species were observed in s-SIMS. However, the ion distribution for a given M(x)O(y) (+/-) ion series (i.e. ions including the same number of metal atoms M) generally presented important similarities in both techniques.Finally, it was demonstrated that the chemical state of metal atoms in the observed ionic species is closely dependent on the metal electronic valence shell. High valence states (+III, +IV, +V, and +VI) are favored for metals with a less-than-half full valence shell configuration, whereas for other first-row transition metals (manganese, iron, cobalt, nickel, copper and zinc) lower metal valence states (0, +I or, +II) are involved.
Journal of Mass Spectrometry 05/2006; 41(4):527-42. · 3.27 Impact Factor
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ABSTRACT: This study deals with the secondary ion yield improvement induced by using C 60 + primary ions instead of Ga + to enhance the detection thresholds of the organic contaminations at the Si wafer surfaces by ToF-SIMS. For that purpose, a piece of Si wafer has been analysed with both ion sources. A large improvement is observed for the detection of hydrocarbon contaminants with C 60 + primary ions as compared to Ga + ions. A similar improvement for organic contaminations, such as phthalates and aliphatic amines, is observed in both secondary ion polarities. The Si atomic ion constitutes a minor peak with C 60 + ions while it dominates the spectrum in the case of Ga + ions. However, with the C 60 + source, inorganic combination peaks with the elements Si and O, are observed in the positive spectra (i.e. Si 2 O 2 H +), while they are marginal with the Ga + source. Furthermore, a series of negative silicon oxide clusters, Si n O (2n+1) H À , is observed up to n = 16 (977m/z) in the case of C 60 + ions. With Ga + ions, the largest negative silicon oxide cluster corresponds to n = 4 (257m/z). The detection of backscattered C 60 fragments is evoked to explain the origin of some hydrocarbon peaks with low H content. On average, for a comparable number of primary ions per spectrum, the C 60 + ion source gives intensities between two and four orders of magnitude higher than the Ga + one.
04/2006;
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ABSTRACT: This study deals with the secondary ion yield improvement induced by using C 60 + primary ions instead of Ga + ones to characterize human hair surfaces by ToF-SIMS. For that purpose, a bunch of hair fibres has been analysed with both ion sources. A high improvement is observed for the detection of amino acids with C 60 + primary ions as compared to Ga + ions. As an example, a yield enhancement factor greater than 3000 is found for the CNO À peak. A similar gain is observed for the positive secondary ions characteristic of the amino acids. Most of the atomic ions, such as Ca + , O À and S À , constitute minor peaks with C 60 + ions while they often dominate the spectrum in the case of Ga + ions. However, with the C 60 + source, a series of inorganic combination peaks with the elements Ca, S and O are observed in the positive spectra (i.e. HCaSO 4 +), while they are marginal with the Ga + source. For the mass range beyond 100 m/z and in both polarities, the hair fingerprints are similar with both sources. In average, for a comparable number of primary ions per spectrum, the C 60 + ion source gives intensities between two and three orders of magnitude higher than the Ga + one.
04/2006;
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ABSTRACT: Since the discovery of the Phillips catalysts, there still is much uncertainty concerning their activation, their molecular structure, the nature of the active chromium sites, and the polymerization mechanisms. Surface techniques are not easy to be used for such study according to the nonconductive behavior of the support. Therefore, model Phillips catalyst is elaborated by spin coating a trivalent chromium precursor on a silicon wafer. The surface characterization of this model catalyst is conducted by laser ablation mass spectrometry (LA-MS), laser desorption/ionization mass spectrometry (LDI-MS), and static secondary ion mass spectrometry (s-SIMS), at different steps of its preparation. To validate our approach, a comparison is also made between the model and the real Philips catalyst. Moreover, the model catalyst efficiency for polyethylene synthesis is evaluated and allows us to discuss the validity of the mechanisms previously proposed to explain the catalytic process. The characterization of Phillips model catalyst by mass spectrometry allows us to better understand the activation processes of such catalyst. Depending on the activation temperature, chromium oxide species are formed and anchored at the support surface. They consist mainly in mono-chromium sites at high temperature. The chromium valence is hexavalent. This model catalyst is active for the polymerization of ethylene. A pseudo-oligomer molecular weight distribution is observed by LA-MS, whereas s-SIMS allows us to elucidate the anchorage of the polymer at activate chromium surface sites.
Journal of the American Society for Mass Spectrometry 04/2006; 17(3):406-14. · 4.00 Impact Factor
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ABSTRACT: Pb–Ti–O thin films, lead titanate powder, lead monoxide, titanium dioxide and a stoichiometric PbO–TiO2 mixture were investigated by time-of-flight (ToF) SIMS. The nature of the numerous PbxOy+/−, TixOy+/− and PbxTix,Oy+/− ionic species generated by 15 keV Ga+ primary ion bombardment analysed by ToF-SIMS allowed the identification of the involved metal–oxygen system. The detection of PbxTix,Oy+/− mixed metal ions was specific to lead titanate (film and powder). Indeed, these species were not observed when the PbO–TiO2 stoichiometric mixture was examined. Consequently, they allowed unambiguous molecular identification of the Pb–Ti–O investigated system. Copyright © 2002 John Wiley & Sons, Ltd.
Surface and Interface Analysis 09/2002; 34(1):754 - 758. · 1.18 Impact Factor
