Importance of Pd and Pt excited states in N2O capture and activation: A comparative study with Rh and Au atoms

Nitrous oxide (N₂O) is an intermediate compound formed during catalysis occurring in automobile exhaust pipes. In this work, the N₂O capture and activation by Pt and Pd atoms in the ground and excited states of many multiplicities are studied. Pt and Pd + N₂O reactions are studied at multireference second‐order perturbation level of theory using C_s symmetry. The PtN₂O (¹A′, ⁵A′, and ⁵A″) species are spontaneously created from excited states. Only the ⁵A′ and ⁵A″ states exhibit N₂O activation reaction paths when N₂O approaches Pt end‐on by the N or O atoms side or side‐on yielding NO or N₂ as products, respectively. Pt⁺ cations ground and excited states, capture N₂O, although only Pt⁺ (⁶A′ and ⁶A″) states show N₂O activation yielding O and N₂ as products. In the Pd atom case, PdN₂O (¹A′ and ⁵A″) species are also spontaneously created from excited states. The ⁵A″ state exhibits N₂O activation yielding N₂ + O as products. Pd⁺ cations in both ground and excited states capture N₂O; however, only the [PdN₂O]⁺ (⁴A′, ⁴A″, ⁶A′, and ⁶A″) states in side‐on approaches and (⁶A′) in end‐on approach activate the N₂O and yield the N₂ bounded to the metal and O as product. The results obtained in this work are discussed and compared with previous calculations of Rh and Au atoms. The reaction paths show a metal–gas dative covalent bonding character. Löwdin charge population analyses for Pt and Pd active states show a binding done through charge donation and retrodonation between the metals and N₂O. © 2013 Wiley Periodicals, Inc.     

Thermal Decomposition of Ternary Sodium Graphite Intercalation Compounds

Graphite intercalation compounds (GICs) are often used to produce exfoliated or functionalised graphene related materials (GRMs) in a specific solvent. This study explores the formation of the Na-tetrahydrofuran (THF)-GIC and a new ternary system based on dimethylacetamide (DMAc). Detailed comparisons of in situ temperature dependent XRD with TGA-MS and Raman measurements reveal a series of dynamic transformations during heating. Surprisingly, the bulk of the intercalation compound is stable under ambient conditions, trapped between the graphene sheets. The heating process drives a reorganisation of the solvent and Na molecules, then an evaporation of the solvent; however, the solvent loss is arrested by restacking of the graphene layers, leading to trapped solvent bubbles. Eventually, the bubbles rupture, releasing the remaining solvent and creating expanded graphite. These trapped dopants may provide useful property enhancements, but also potentially confound measurements of grafting efficiency in liquid-phase covalent functionalization experiments on 2D materials.     

Study of the sulfur atom as hydrogen bond acceptor in N(2)-pyridylmethyl-N′-arylthioureas

The hydrogen acceptor capability of the sulfur atom in the biologically relevant N-2-pyridylmethyl-N-arilthioureas was explored. N-2-Pyridylmethyl thioreas were selected to avoid the formation of intramolecular six-membered hydrogen-bonded ring. The compounds studied were N-2-pyridylmethyl-N-phenylthiourea (1), N-2-pyridylmethyl-N-2-methoxythiourea (2), N-2-pyridylmethyl-N-4-methoxyphenylthiourea (3), and N-2-pyridylmethyl-N-4-bromophenylthiourea (4). 1 crystallizes in the monoclinic space group P2₁/c, with a = 7.419(1) Å, b = 18.437(2) Å, c = 9.656(1) Å, = 106.277(6)°, V = 1267.8(3) ų, Z = 4. 2 crystallizes in the monoclinic space group P2₁/c, with a = 8.064(2) Å, b = 18.382(7) Å, c = 9.865(5) Å, = 97.81(3)°, V = 1448.8(11) ų, Z = 4. 3 crystallizes in the monoclinic space group P2₁/c, with a = 11.472(1) Å, b = 13.520(1) Å, c = 10.088(1) Å, = 112.60(1)°, V = 1444.5(2) ų, Z = 4. 4 crystallizes in the triclinic space group P-1, with a = 4.583(3) Å, b = 10.263(3) Å, c = 14.396(3) Å, = 77.92(2)°, = 88.55(4)°, = 80.02(4)°, V = 652.1(5) ų, Z = 2. Both thiourea N–H groups form intermolecular hydrogen bonds, one with the thione sulfur atom and the other with the pyridine nitrogen atom but the H-bonding schemes are not the same maybe due to the flexibility of the molecules.     

Carbon nanotubes as heat dissipaters in microelectronics

We review our recent modelling work of carbon nanotubes as potential candidates for heat dissipation in microelectronics cooling. In the first part, we analyze the impact of nanotube defects on their thermal transport properties. In the second part, we investigate the loss of thermal properties of nanotubes in presence of an interface with various substances, including air and water. Comparison with previous works is established whenever is possible.     

A FT-IR Study of the Hydrolysis of Tetraethylorthosilicate (TEOS).

Abstract

The hydrolysis and polymerization of a non-catalyzed silica sol was investigated by Fourier Transform Infrared Spectroscopy (FT-IR) and deconvolution of the infrared spectra. The hydrolysis was followed by the 1168 and 812 cm^?1 bands which have been observed that decrease continously with the reaction time, and they dissapear showing the complete hydrolysis. The bands located at 1200 and 1147 cm^?1 are assigned to polymerization of Si-OH groups forming Si-O-Si bonds in cyclic or linear structures respectively. Both bands increase with time and are present simultaneously in the spectra showing that both kind of cross-linking of Si-OH groups are taking place in the sol to form the gel. In the gel the presence of Si-OH groups and Si-O^? free broken bonds have been detected by means of the bonds located at 960 and 920 cm^?1 respectively.     

Damage spreading in a driven lattice gas model

We studied damage spreading in a Driven Lattice Gas (DLG) model as a function of the temperature T$T$, the magnitude of the external driving field E$E$, and the lattice size. The DLG model undergoes an order–disorder second-order phase transition at the critical temperature _Tc(E)$T_c\left(E\right)$, such that the ordered phase is characterized by high-density strips running along the direction of the applied field; while in the disordered phase one has a lattice-gas-like behavior. It is found that the damage always spreads for all the investigated temperatures and reaches a saturation value _Dsat$D_{sat}$ that depends only on T$T$. _Dsat$D_{sat}$ increases for T<_Tc(E=∞)$T<T_c(E=\infty )$, decreases for T>_Tc(E=∞)$T>T_c(E=\infty )$ and is free of finite-size effects. This behavior can be explained as due to the existence of interfaces between the high-density strips and the lattice-gas-like phase whose roughness depends on T$T$. Also, we investigated damage spreading for a range of finite fields as a function of T$T$, finding a behavior similar to that of the case with E=∞$E=\infty$.     

Acid-induced cloud point extraction and preconcentration of polycyclic aromatic hydrocarbons from environmental solid samples

Liquid chromatography coupled to electrospray-mass spectrometry (LC-ES-MS) with positive ion detection was evaluated for the determination of tributyltin and triphenyltin in water samples using tripropyltin as internal standard. The separation was performed in the isocratic mode on a silica-based C18 column with a mobile phase containing 0.02% trifluoroacetic acid in acetonitrile-water (50:50, v/v). The optimum LC-ES-MS conditions were established and quantification was performed on the basis of the [M]+ ions. Limits of detection for standard solutions were 100 and 200 pg Sn injected for triphenyltin and tributyltin, respectively, and good reproducibility was observed. Solid-phase extraction was carried out on C18 cartridges to preconcentrate the analytes from natural water samples, with recoveries ranging from 80 to 110%. Limits of detection for SPE-LC-ES-MS were in the range of low ng l(-1), which demonstrates the suitability of the method for environmental samples.     

Further Insights into the Porous Structure of TEOS Derived Silica Gels

The porous structure of TEOS derived silica gels was studied using nitrogen adsorption at 77 K. Silica gels were prepared using TEOS, H₂O and ethanol for different molar ratios. No catalyst was used in this study. Silica gels were also heat treated up to 1000°C. The nitrogen sorption isotherms were analyzed by two models: Fractal and Percolation Theories. Using the fractal analysis approach, the surface roughness of the porous structure of silica gels was determined. The surface fractal dimension depends on the hydrolysis conditions and heat treatment. The surface fractal dimension decreases with increasing H₂O/TEOS molar ratio or heating temperature. For the silica gels studied, the surface fractal dimension changed from 2.6 to 2.5 after heating the gels, and from 2.4 to 2.6 with decreasing H₂O/TEOS ratio.Using the Percolation theory, we have determined the connectivity of the porous structure of silica gels. The extent of sorption hysteresis of the nitrogen isotherms reflects the connectivity of the pore network. The mean coordination number (connectivity) Z, and the linear dimension of the network, L, have been calculated from the hysteresis of the isotherms. For the as-prepared silica gels, Z was about 8 and L close to 2. On heating the gels, Z decreases to 4 and L increases to 7, results which are in accordance with the collapse of the porous network.