226,934 research outputs found
Molecular dynamics as a tool to study heterogeneity in zeolites - Effect of Na cations on diffusion of CO and N in Na-ZSM-5
No full textZeolites typically contain extra-framework cations to charge-compensate for trivalent Al atom substitutions in the SiO framework. These cations, such as Na, directly interact with quadrupolar guest molecules, such as CO and N, which move through their micropores, causing energetic heterogeneity. To assess the effects of heterogeneity in Na-ZSM-5 on diffusion of CO and N, molecular dynamics (MD) simulations are carried out. In silicalite-1, the pure-silicon form of ZSM-5, the self-diffusivity exhibits a monotonic decrease with molecular loading, while the corrected diffusivity shows a relatively constant value. In contrast, the Na cations cause a maximum or a flat profile over molecular loading for the self- and corrected diffusivities of CO at T=200 and 300K, while the cations only have minimal impact on the diffusivity of N. The MD simulations allow us to identify energy basins or sites at which guest molecules spend a relatively long time, and construct a coarse-grained lattice representation for the pore network. Average residence times at these sites are calculated for both species. The trends observed in the residence times correlate to the trends observed in the diffusivity. The residence times for CO at T=200K are long at low loading, but decrease with loading as additional CO molecules compete to stay close to a cation. In contrast, the residence times for N are relatively insensitive to the cations, only mildly increasing near a cation. This difference in behavior can be associated to the quadrupole moments of these molecules
Pnictogénium cations (P, As, Sb) et germylènes à ligands N,N’ et N,O-chélatants : synthèse, étude structurale et réactivité
No full textCe travail concerne la stabilisation d'espèces à basse coordinence du groupe 15 (P, As et Sb) et du germanium par des substituents N,N' et N,O chélatants comportant un système pi-conjugué comme les aminotroponiminate et aminotroponate. Le premier chapitre porte sur la mise au point des voies de synthèse de composés dichlorés ou de chloropnictogénium cations. L'étude par RMN multinoyaux (1H, 13C et 31P) des pnictogénium cations a montré une bonne délocalisation de la charge positive sur le cycle insaturé à 7 chaînons ce qui a été confirmé par l'étude de leur structure par diffraction des rayons X et des calculs DFT. Dans le deuxième chapitre, nous avons réalisé des réactions caractéristiques comme des réactions d'oxydation. Les phosphénium cations conduisent très facilement aux composés hydroxylés correspondants. Une réaction inattendue d'élimination de HCl a conduit à des dioxo- et thiooxo-phosphoranes qui constituent les premiers exemples d'aminométa phosphonate (et de son équivalent soufré) stabilisés par complexation intramoléculaire. Les réactions de cycloaddition avec une o-quinone ont également permis d'accéder à de nouveaux cycloadduits. La dernière partie de ce chapitre porte sur l'aptitude de ces phosphéniums cations à former des complexes avec les métaux de transition. Nous avons pu ainsi isoler le premier hydroxyphosphénium stabilisé par complexation avec le tungstène. Dans le troisième chapitre, nous avons développé l'étude de nouveaux systèmes chélatants: les di-amonitroponimines et di-aminotropones pontées. Nous avons ensuite réalisée une extension de ces réactions au germanium et pu ainsi accéder à de nouveaux bis-germylènes. Ces espèces bien que tri-coordinnées conservent leur caractère divalent comme le montrent leurs réactions de cycloaddition avec une o-quinone. Enfin, un bis-germylène complexé par le tungstène a pu être isolé et sa structure déterminée par diffraction des rayons X.This thesis entitled "N,N'- and N,O-chelated pnictogenium cations (P, As, Sb) and germylenes: syntheses, structural studies and reactivity" is structured in three chapters. In the first chapter an easy and direct route to the first phosphenium and arsenium cations supported by N,N' or N,O-chelation derived from a tropolone scaffold was presented. These new compounds were fully characterized by various spectroscopic methods, X-ray analyses and DFT calculations. The expected Lewis amphoterism which is illustrated on the one hand, by the high-lying phosphorus lone-pair orbital (HOMO-1), and on the other hand, by the delocalized positive charge opens up new perspectives for the involvement of these ambiphilic species as valuable ligands for new catalysts. The second chapter concerns the reactivity of these new pnictogenium cations. A variety of reactions including oxidation (with dimethylsulfoxide, sulfur or selenium), cycloaddition and halide ion extraction were studied. The first example of an aminometaphosphonate (and its sulfur equivalent) stabilized by intramolecular complexation was reported. For the first time, a hydroxyphosphenium cation stabilized by complexation with pentacarbonyltungsten was prepared and its structure elucidated by X-ray single-crystal diffraction study. The last chapter refers to the synthesis and characterization of bridged bis(pnictogenium) cations with an extension to divalent germanium compounds. Among the most noteworthy result is the stable bis-germylene pentacarbonyl tungsten complex
Corrosion inhibition effects of metal cations on SUS304 in 0.5 M Cl- aqueous solution
Full text linkThe corrosion characteristics of SUS304 exposed to 0.5 M Cl- aqueous solution containing different metal cations were studied with immersion tests, surface analysis and electrochemical tests. The mechanism of corrosion with metal cations was clarified by the XPS analysis results together with the hard and soft acid and base (HSAB) concept and the passive films structure. It is supposed that metal cations with large hardness make a layer by chemical bonding with the passive films. The passive films are protected by the metal cation layer from Cl- attack, and consequently corrosion reactions are inhibited
Sorption of ionisable organic compounds by field soils. Part 2: cations, bases and zwitterions
No full textSorption of a range of permanent cations, bases and zwitterionic compounds was measured as a function of pH in two soil types. Pyridinium cations were more strongly sorbed (K(d) 10- > 1000) than aliphatic cations (K(d) 6. However, an additional mechanism of sorption was observed for those zwitterions capable of chelation (e.g. picolinic acid and alanine) which gave rise to high K(d) values at pH values near neutral. Inorganic phosphate was strongly sorbed (K(d) > 140) save at very low pH. Glyphosate and inorganic phosphate were sorbed very strongly at pH values near to 4 (K(d) > 200). The very strong sorption was attributed to ligand exchange interaction. Sorption of picolinic acid was similar when measured in water or calcium chloride solution (0.01 M). However, sorption decreased with increasing concentration of calcium chloride up to 1 M, probably because the protonated form of picolinic acid was displaced by calcium ions
Immobilization of (Aqueous) Cations in Low pH M-S-H Cement
Full text linkIncorporation of heavy metal ions in cement hydrates is of great interest for the storage and immobilization of toxic, hazardous, and radioactive wastes using cementitious matrix. Magnesium silicate hydrate (M-S-H) is a low pH alternative cementitious binder to commonly used Portland cement. Low pH cements have been considered as promising matrix for municipal and nuclear waste immobilization in the last decades. It is however crucial to assure that the incorporation of secondary ions is not detrimental for the formation of the hydration products. Herein, we investigate the early stages of formation of M-S-H from electrolyte solutions in presence of a wide range of metal cations (LiI, BaII, CsI, CrIII, FeIII, CoII, NiII, CuI, ZnII, PbII, AlIII). The final solid products obtained after 24 h have been characterized via powder X-ray diffraction (PXRD), attenuated total reflectance-Fourier transformed infrared spectroscopy (FTIR-ATR), elemental analysis via energy-dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HR-TEM). In all the experiments, the main precipitated phase after 24 h was confirmed to be M-S-H with a ratio (total metal/Si) close to one. The obtained M-S-H products showed strong immobilization capacity for the secondary metal cations and can incorporate up to 30% of the total metal content at the early stages of M-S-H formation without significantly delaying the nucleation of the M-S-H. It has been observed that presence of Cr, Co, and Fe in the solution is prolonging the growth period of M-S-H. This is related to a higher average secondary metal/total metal ratio in the precipitated material. Secondary phases that co-precipitate in some of the experiments (Fe, Pb, Ni, and Zn) were also effectively trapped within in the M-S-H matrix. Barium was the only element in which the formation of a secondary carbonate phase isolated from the M-S-H precipitates was detected.publishe
Radical ions and dehydro cations in detection of 2,3 '-bisindolylmethanes using electrospray mass spectrometry
No full textA series of 2,3'-bisindolylmethanes were analyzed using electrospray ionization quadrupole time-of flight and radical ions ([M](+center dot)) and dehydro cations ([M-H](+center dot)) were detected in positive-ion mode. The formation pathways of [M](+center dot) were proposed confirmed by D-labeling, free radical trapping and CID-MS/MS experiments. It was found that [M](+center dot) ion was formed in droplets by the direct loss of an electron and the processes forming [M](+center dot) and IM-Hr ions were independent. The fragmentation pathways of the [M-H](+center dot) and [M](+center dot) ions were studied by CID-MS/MS. The dominant fragmentation pattern of [M](+center dot) ions is the loss of benzyl radical comparing to that of [M-H](+center dot) ions. (C) 2016 Elsevier B.V. All rights reserved
The role of organic cations as additives in photovoltaic perovskites.
No full textThe design of additives for perovskite-based solar cells seeks to improve the balance between stability and power conversion efficiency. Organic molecules such as theophylline, theobromine and caffeine (xanthines) have proved to be a good engineering solution. As an alternative, we present a first-principle study of the use of organic cations as additives. These cations are got when the free nitrogen of the imidazole unit of the aforementioned molecules is quaternized. We have found that the interaction between the organic cations and the MAPbI3 perovskite surface is stronger compared to the organic molecules. The Pb-O and I-H bonds of the interface dominated these interactions. In addition, organic cations showed higher charge transfer through the interface and shallow states that are harmless and could improve the charge carrier mobility. These characteristics show that quaternized xanthines should be a promising additive for perovskite materials in photovoltaic applications
Single stage electrochemical exfoliation method for the production of few-layer graphene via intercalation of tetraalkylammonium cations
Full text linkWe present a non-oxidative production route to few layer graphene via the electrochemical intercalation of tetraalkylammonium cations into pristine graphite. Two forms of graphite have been studied as the source material with each yielding a slightly different result. Highly orientated pyrolytic graphite (HOPG) offers greater advantages in terms of the exfoliate size but the source electrode set up introduces difficulties to the procedure and requires the use of sonication. Using a graphite rod electrode, few layer graphene flakes (2 nm thickness) are formed directly although the flake diameters from this source are typically small (ca. 100–200 nm). Significantly, for a solvent based route, the graphite rod does not require ultrasonication or any secondary physical processing of the resulting dispersion. Flakes have been characterized using Raman spectroscopy, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS)
Amplifying dementia as a global public health problem: A cross-country comparison of the impact of COVID-19 pandemic
Full text linkBackground:
Emerging evidence describes impacts of the COVID-19 pandemic upon people living with dementia and their informal carers, however without evidence-based global comparisons to date. The aim of this international study was to explore and compare the impact of COVID-19 and associated public health restrictions on the lives of people living with dementia and informal carers and access to dementia care across five countries.
Methods:
People with dementia and informal carers from the UK, Australia, Italy, India, and Poland participated in remote semi-structured interviews. Participants were asked about their experiences of the pandemic and how restrictions have impacted on their lives and care. Transcripts were analysed by researchers in each country using thematic analysis, then combined across sites.
Results:
Fifteen people living with dementia and 111 informal carers participated across the five countries. Four themes emerged: (1) Limited access and support; (2) Technology and issues accessing remote support; (3) Emotional impact; and (4) Decline of cognitive and physical health reported by carers. Whilst variations were noted, the pandemic has indirectly affected people with dementia and carers across all five countries. The pandemic removed access to social support services and thus increased carer burden. Remote services were not always provided and were limited in benefit and usability for those with dementia. Carers frequently described noticeably deteriorating cognitive and physical health in people with dementia.
Conclusions:
The pandemic has amplified dementia as a global public health problem, and both people affected by the condition ad their carers need support to better access vital support services to live well. This is even more important with emerging new waves and new variants of the virus affecting different countries, in our globally connected world.No Full Tex
The selectivity coefficients () of various interfering cations (M<sup>n+</sup>).
No full text<p>The selectivity coefficients () of various interfering cations (M<sup>n+</sup>).</p
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