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A BADER’S TOPOLOGICAL APPROACH FOR THE CHARACTERIZATION OF PRESSURE INDUCED PHASE TRANSITIONS
No full textPressure stability field of Mg-perovskite under deep mantle conditions: A topological approach based on Bader's analysis coupled with catastrophe theory
Full text linkThe pressure stability field of the Mg-perovskite phase was investigated by characterizing the evolution of the electron arrangement in the crystal. Ab initio calculations of the perovskite structures in the range 0–185 GPa were performed at the HF/DFT (Hartree-Fock/Density Functional Theory) exchange–correlation terms level. The electron densities, calculated throughout the ab-initio wave functions, were analysed by means of the Bader's theory, coupled with Thom's catastrophe theory. To the best of our knowledge the approach is used for the first time. The topological results show the occurrence of two topological anomalies at P~20 GPa and P~110 GPa which delineate the pressure range where Mg-perovskite is stable.
The paper accomplishes the twofold objectives of providing a contribution in shading light into the behaviour of the dominant component of the Earth's lower mantle across the D’’ layer and of proposing a novel approach in predicting the stability of a compound at extreme conditions
Synthetic and natural chromium bearing spinels: an optical spectroscopy study
No full textFour samples of synthetic chromium-bearing spinels of (Mg, Fe2+)(Cr, Fe3+)2O4 composition and four samples of natural spinels of predominantly (Mg, Fe2+)(Al, Cr)2O4 composition were studied at ambient conditions by means of optical absorption spectroscopy. Synthetic endmember MgCr2O4 spinel was also studied at pressures up to ca. 10 GPa. In both synthetic and natural samples, chromium is present predominantly as octahedral Cr3+ seen in the spectra as two broad intense absorption bands in the visible range caused by the electronic spin-allowed 4A2g → 4T2g and 4A2g → 4T1g transitions (U- and Y-band, respectively). A distinct doublet structure of the Y-band in both synthetic and natural spinels is related to trigonal distortion of the octahedral site in the spinel structure. A small, if any, splittingof the U-band can only be resolved at curve-fitting analysis. In all synthetic high-chromium spinels, a couple of relatively narrow and weak bands of the spin-allowed transitions 4A2g → 2Eg and 4A2g → 2T1g of Cr3+, intensified by exchange-coupled interaction between Cr3+ and Fe3+ at neighboring octahedral sites of the structure, appear at ~14,400 and ~15,100 cm−1. A vague broad band in the range from ca. 15,000 to 12,000 cm−1 in synthetic spinels is tentatively attributed to IVCr2+ + VICr3+ → IVCr3+ + VICr2+intervalence charge-transfer transition. Iron, mainly as octahedral Fe3+, causes intense high-energy absorption edge in near UV-range (ligand–metal charge-transfer O2− → Fe3+, Fe2+ transitions). As tetrahedral Fe2+, it appears as a strong infrared absorption band at around 4,850 cm−1 caused by electronic spin-allowed 5E → 5T2 transitions of IVFe2+. From the composition shift of the U-band in natural and synthetic MgCr2O4 spinels, the coefficient of local structural relaxation around Cr3+ in spinel MgAl2O4–MgCr2O4 system was evaluated as ~0.56(4), one of the lowest among (Al, Cr)O6 polyhedra known so far. The octahedral modulus of Cr3+ in MgCr2O4, derived from pressure-induced shift of the U-band of Cr3+, is ~313 (50) GPa, which is nearly the same as in natural low-chromium Mg, Al-spinel reported by Langer et al. (1997). Calculated from the results of the curve-fitting analysis, the Racah parameter B of Cr3+ in natural and synthetic MgCr2O4 spinels indicates that Cr–O-bonding in octahedral sites of MgCr2O4 has more covalent character than in the diluted natural samples. Within the uncertainty of determination in synthetic MgAl2O4 spinel, B
does not much depend on pressure
Combining photocatalytic and adsorption units for the partial oxidation of tyrosol to hydroxytyrosol
Full text linkPhotocatalytic partial oxidation of tyrosol (Tyr) to hydroxytyrosol (Htyr) which is a valuable antioxidant compound, was carried out in a fully recirculated flow photoreactor under UVA light irradiation by using as photocatalyst bare or fluorinated TiO2 immobilized on glass beads. The maximum selectivity towards Htyr obtained under optimized conditions in the presence of fluorinated TiO2 at pH 8.7 was approximately 12 %. Reducing the residence time did not result in any further improvement of reaction selectivity due to the fast overoxidation of Htyr. The photocatalytic reactor was then coupled with an adsorption unit containing a home-made adsorbent exposing amine-stabilized boronate moieties, which showed negligible interaction with Tyr but was capable of selectively binding Htyr on its surface. This allowed for the complete recirculation of the reactant Tyr to the reactor, while the desired product Htyr could be selectively removed from the reaction medium, significantly limiting its parasitic overreaction. Consequently, performing the reaction in the coupled system doubled the selectivity towards Htyr for all configurations, providing a maximum selectivity of 25 % under optimized conditions, which is twice the highest value ever reported for this photocatalytic reaction. Finally, the kinetic experimental data were modeled using the Copasi software package to elucidate the reaction pathway and estimate the apparent kinetic parameters of the photocatalytic process, both alone and in combination with the adsorption unit. The simulated results showed excellent agreement with the experimental data
Paper consolidation with Halloysite nanotubes. TGA, DMA and contact angle investigations
No full textOlive mill wastewaters decontamination based on organo-nano-clay composites
No full textGreen composites for environmental applications were successfully prepared by intercalation of the biosurfactant Quillaja saponin onto montmorillonite mineral clay on varying pH and surfactant/clay ratio. Equilibrium adsorption isotherms were constructed and the system was characterized by performing TGA and XRD analyses. The efficiency of the surfactant-modi ed clay in the removal of the organic content present in olive mill wastewaters (OMW) was evaluated by means of spectrophotometric measurements. The interest for this cogent issue comes from the consideration that, despite their high pollutant content, OMW can be considered as a potential resource of several organic compounds which can be recovered for a wide array of pharmaceutical and industrial applications. Two different strategies were proposed. In the first one, dried surfactant/clay hybrids were added to the batch samples under continuous stirring, while in the second alternative approach the organoclays were packed in chromatography column lled with multiple alternate layers of sand and organoclay. These studies revealed the efficacy of the methods used and suggested that the modification of the montmorillonite clay substrate significantly improves the performance of the clay
WOOD TREATMENT BASED ON HALLOYSITE/HYDROXYPROPYL CELLULOSE COMPOSITE. THERMOMECHANICAL AND POROSITY CHARACTERIZATION
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OUTLIER RECOGNITION AND ROBUST WEIGHTING PROCEDURES APPLIED IN CATION ORDERING-DISORDERING KINETIC DATA PROCESSING
No full textCharacterization of the pressure induced Ringwoodite to Mg-perovskite+Mg-wuestite phase transition by Catastrophe Theory
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