The relationship between thermodynamic Brønsted basicity and reactivity of basic sites of MgO samples was investigated by means of methanol deprotonation followed by IR and the conversion of ...2-methylbut-3-yn-2-ol (MBOH), respectively. The relative distribution of basic oxide ions, O
2−
LC, in low coordination (where LC
=
3C, 4C, and 5C refer to tri-, tetra-, and penta-coordinated oxide ions, respectively) was modulated using different preparation routes. The resulting samples were classified on the basis of the relative distributions of O
2−
LC ions determined by photoluminescence. The influence of the coordination of O
2−
LC ions on the basic properties was studied for clean surfaces obtained after high-temperature (⩾1023 K) evacuation of CO
2 and water; the lower the coordination of O
2−
LC ions, the higher the deprotonation ability and the reactivity of basic sites of the catalyst. The hydroxylation of clean MgO surfaces was studied and its influence on Brønsted basicity determined. Despite a low deprotonation ability, hydroxylated surfaces are more reactive than clean surfaces. The direct influence of OH groups on reactivity of basic sites was evidenced by correlating the latter and the amount of isolated OH groups evaluated by in situ diffuse reflectance Fourier transform infrared spectroscopy. It can be inferred that the peculiar reactivity of OH groups compared with O
2−
LC ions is due to the variable stability of the alcoholate intermediate formed on both kinds of basic sites. On hydroxylated surfaces, because OH groups are poor Brønsted bases, the number of alcoholate species is lower than on clean surfaces, but these intermediates are less stabilized and so are more reactive.
•Identification of PT30 resin degradation products by TGA-IR and TGA-MS.•Proposal of a degradation mechanism for PT30 resin in a nitrogen atmosphere.•Identification of the initiation degradation ...reaction based on the results of Density Functional Theory (DFT) calculations which highlight the less energetic network bonds.•Description of each step in the chain scissions of the PT30 resin.•Description of each step in the creation of degradation products.
The non-isothermal thermal degradation of the cured phenolic triazine thermoset resin (PT30) was considered under nitrogen atmosphere. Thermogravimetric analyses (TGA) showed that the degradation mechanism was decomposed in three main steps occurring at 440°C, 550°C, and 720°C. To understand the chemical reactions that happen, TGA experiments were coupled with the analyses of the gaseous products by Fourier transform infrared detection (TGA/FTIR) and by mass spectrometry (TGA/MS). A degradation mechanism is proposed based on these experimental results and on Density Functional Theory (DFT) calculations. It was found that carbon dioxide is the main degradation product generated by the oxygen-carbon chain scissions. In the second step, triazine rings’ rupture led to the formation of cyanide hydrogen and probably cyanic/isocyanic acid. Then, various aromatic compounds are produced from chain scissions and molecules’ recombination. Analyses of these degradation products contributed to proposing assumptions of thermal degradation mechanisms of the PT30 resin.
The interaction of water and methanol with MgO samples with different distributions of oxide ions of low coordination has been investigated by physical techniques, particularly in situ ...photoluminescence. First, the three photoluminescence fingerprints of oxide ions vs their coordination number have been obtained for samples outgassed at 1273 K. By a pseudo quantitative approach, the relative distribution of the oxide ions of low coordination O2- LC (where LC = 3C, 4C, and 5C refer to tri-, tetra-, and pentacoordinated oxide ions, respectively) was determined and correlated with the shape and size of MgO particles determined by TEM and XRD. The photoluminescence of surfaces of MgO obtained after outgassing at increasing temperature or after interaction of water or methanol with a clean surface, i.e., obtained by outgassing at 1273 K, was then studied and evidenced three other photoluminescent species assigned to surface OH groups. The nature and mechanism of formation of the hydroxyls groups responsible for these new luminescent species are discussed in relation with their thermal stability and FTIR experiments.
Technical improvement of the photoluminescence cell and its coupling to a dynamic vacuum system allow to avoid quenching phenomena. These results in emission spectra of MgO, which are better-resolved ...and more stable in time, leading to a better characterization of the very reactive oxide ions, i.e., those in low coordination. Apart from a more precise assignment of the spectra, new developments are proposed to evaluate the relative distribution of ions of low coordination. Discrimination of the oxide ions with different coordination numbers is also possible by studying in situ their interaction with propyne as a probe molecule.
Dans les villes polyglottes du Nord de Paris, des personnes de langue maternelle française issues de milieux défavorisés rencontrent des difficultés analogues, dans les rapports sociaux, à celles de ...personnes d'origine étrangère. L'amélioration des perceptions sociales des uns et des autres passe en partie, au-delà de la maîtrise du français, par la reconnaissance "des" bilinguismes. Toutefois, depuis 2004, un signal contraire a été envoyé, l'aide à la formation linguistique faisant dorénavant l'objet d'une offre publique de marché. Les associations de quartier, qui assuraient une offre de proximité, ne sont plus financées pour de la formation linguistique.
The deprotonation ability of clean (i.e., carbonate- and hydroxyl-free) MgO surfaces of various morphologies towards protic molecules, such as propyne and methanol, is evaluated by following the ...adsorption modes and amount of dissociated species by FTIR. Photoluminescence is used to identify the nature of the oxide ions involved in deprotonation. The same trend is observed for propyne and methanol: (i) the amount of dissociated species increases with the relative concentration of oxide ions of low coordination OLC 2, (ii) MgLC 2+ also are involved in deprotonation. Only 0.5% of OLC 2 are implied in propyne deprotonation. Quantitative adsorption and IR measurements suggest that the deprotonating Mg2+O2 pairs involve 3- and 4- coordinated ions: Mg3CO4C and Mg4CO3C. The pairs able to lead to deprotonation have to combine a strong Brønsted basic site OLC 2, able to abstract a proton and a strong acidic Lewis site able to stabilize the anion generated by deprotonation. The deprotonation equilibrium position is related to the thermodynamic Brønsted basicity. The choice of protic molecules used to identify the catalytically active basic sites was justified a posteriori by the correlation between the relative amount of dissociated propyne and methanol species and the catalytic conversion of 2-methylbut-3-yn-2-ol (MBOH).
Parler d’insertion ou d’intégration, s’il n’y a pas maîtrise de la langue, semble illusoire.
Une réflexion conduite par différents professionnels sur les pratiques linguistiques dans un département a ...permis de repérer les difficultés rencontrées, les besoins et les attentes, ainsi que quelques réponses possibles.
Conjointement à la nécessaire appropriation par tous de la langue commune, la reconnaissance du plurilinguisme effectif des migrants ou de leurs enfants et la valorisation des langues d’origine semblent être une des conditions d’une meilleure intégration.
Bailly Laurence. Les langues dans la ville. Échos d’un groupe de travail de Seine-Saint-Denis. In: VEI enjeux, n°130, 2002. Pratiques langagières urbaines. Enjeux identitaires, enjeux cognitifs. pp. 230-237.
Menus are used for exploring and selecting commands in interactive applications. They are widespread in current systems and used by a large variety of users. As a consequence, they have motivated ...many studies in Human-Computer Interaction (HCI). Facing the large variety of menus, it is difficult to have a clear understanding of the design possibilities and to ascertain their similarities and differences. In this article, we address a main challenge of menu design: the need to characterize the design space of menus. To do this, we propose a taxonomy of menu properties that structures existing work on visual menus. As properties have an impact on the performance of the menu, we start by refining performance through a list of quality criteria and by reviewing existing analytical and empirical methods for quality evaluation. This taxonomy of menu properties is a step toward the elaboration of advanced predictive models of menu performance and the optimization of menus. A key point of this work is to focus both on menus and on the properties of menus, and then enable a fine-grained analysis in terms of performance.
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