1,720,967 research outputs found
DFT simulation of Mg/Al hydrotalcite with different intercalated anions: Periodic structure and solvating effects on the iodide/triiodide redox couple
No full textThe periodic structure of layered hydrotalcite (HT) with Mg/Al 2:1 ratio is optimized with various intercalated anions, namely mono- and di-hydrated chloride, nitrate, carbonate, iodide and triiodide. The geometrical parameters are in good agreement with the available experimental data, so that the triiodide structure, not yet characterized experimentally, provides a reliable estimate of the expected interlayer distance. The change of the triiodide formation energy and iodide/triiodide redox potential upon intercalation in HT are also estimated, to evaluate the possible use of this environment as contact electrolyte in dye-sensitized solar cells. © 2010 Elsevier B.V. All rights reserved
Monte Carlo modeling of carbon dioxide adsorption in porous aromatic frameworks
No full textThe adsorption isotherms of CO2 in several porous aromatic frameworks (PAFs) have been simulated with Grand Canonical Monte Carlo technique, to support the synthesis of new materials for efficient carbon dioxide capture and storage. The simulations covered the 0-60 bar pressure range and were repeated at 273, 298, and 323 K. The force field employed in the simulations was optimized to fit the correct behavior of the free gas and to reproduce the CO2-phenyl interactions computed at high quantum mechanical level. PAFs are based on the diamond structure, with polyaromatic chains inserted in C-C bonds. We examined four PAF-30n (n being the number of phenyl rings in the aromatic linkers), finding that PAF-302 is overall the best performing, although PAF-301 provides higher adsorbed densities at very low pressure. The CO2 adsorption then was simulated in a number of modified PAF-302, with different functional groups (aminomethane, toluene, pyridine, and imidazole) attached to the phenyl chains; different degrees of substitution (25%, 50%, and 100% derivatized rings) were considered. The effects of functionalization and the dependence on the substitution degree are carefully discussed, to determine the most promising materials at low, intermediate, and high pressures. © 2014 American Chemical Society
First principles study of 2D layered organohalide tin perovskites
Full text linkThis article describes the structure and the electronic properties of a series of layered perovskites of a general formula (A+)2(SnX4)-2 where X = I, Br and A+ is an organic cation, either formamidinium, 1-methylimidazolium, or phenylethylammonium. For each system, two conformations are considered, with eclipsed or staggered stacking of the adjacent inorganic layers. Geometry optimizations are performed at the density functional theory level with generalized gradient approximation (GGA) functional and semiempirical correction for dispersion energies; band profiles and bandgaps are computed including both spin orbit coupling (SOC) and correlation (GW) effects through an additive scheme. The theoretical procedures are validated by reproducing the experimental data of a well known 3D tin iodide perovskite. The results, combined with the calculations previously reported on PbI4 analogues, allow us to discuss the effect of cation, metal, and halide substitution in these systems and in particular to explore the possibility of changing the electronic bandgap as required by different applications. The balance of SOC and GW effects depends on the chemical nature of the studied perovskites and strongly influences the value of the simulated bandgap
Density Functional Theory Modeling of PbSe Nanoclusters: Effect of Surface Passivation on Shape and Composition
No full textAccurate Evaluation of the Dispersion Energy in the Simulation of Gas Adsorption into Porous Zeolites
No full textThe force fields
used to simulate the gas adsorption in porous
materials are strongly dominated by the van der Waals (vdW) terms.
Here we discuss the delicate problem to estimate these terms accurately,
analyzing the effect of different models. To this end, we simulated
the physisorption of CH4, CO2, and Ar into various
Al-free microporous zeolites (ITQ-29, SSZ-13, and silicalite-1), comparing
the theoretical results with accurate experimental isotherms. The
vdW terms in the force fields were parametrized against the free gas
densities and high-level quantum mechanical (QM) calculations, comparing
different methods to evaluate the dispersion energies. In particular,
MP2 and DFT with semiempirical corrections, with suitable basis sets,
were chosen to approximate the best QM calculations; either Lennard-Jones
or Morse expressions were used to include the vdW terms in the force
fields. The comparison of the simulated and experimental isotherms
revealed that a strong interplay exists between the definition of
the dispersion energies and the functional form used in the force
field; these results are fairly general and reproducible, at least
for the systems considered here. On this basis, the reliability of
different models can be discussed, and a recipe can be provided to
obtain accurate simulated adsorption isotherms
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Porous dipeptide crystals as selective CO2 adsorbents: experimental isotherms vs. grand canonical Monte Carlo simulations and MAS NMR spectroscopy
No full textMolecular crystals of dipeptides containing open channels can selectively absorb CO2 over N-2 and CH4, as shown by experimental and simulated isotherms. The efficient CO2 capture enabled separation from methane and allowed the direct observation by 2D MAS NMR of CO2 sequestered in the peptide-based biozeolites
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