The Molecularly Controlled Synthesis of Ordered Bi‐dimensional C60 Arrays

Much of the research effort concerning the nanoscopic properties of clays has focused on its mechanical applications, for example, as nanofillers for polymer reinforcement. To broaden the horizon of what is possible by exploiting the richness of clays in nanoscience, herein we report a bottom‐up approach for the production of hybrid materials in which clays act as the structure‐directing interface and reaction media. This new method, which combines self‐assembly with the Langmuir–Schaefer technique, uses the clay nanosheets as a template for the grafting of C₆₀ into a bi‐dimensional array, and allows for perfect layer‐by‐layer growth with control at the molecular level. In contrast to the more‐common growth of C₆₀ arrays through nanopatterning, our approach can be performed under atmospheric conditions, can be upscaled to areas of tenths of cm², and can be applied to almost any hydrophobic substrate. Herein, we report a detailed study of this approach by using temperature‐dependent X‐ray diffraction, spectroscopic measurements, and STM.     

Synthesis and characterization of γ-Fe2O3/carbon hybrids and their application in removal of hexavalent chromium ions from aqueous solutions

Magnetic Fe(2)O(3)/carbon hybrids were prepared in a two-step process. First, acetic acid vapor interacted with iron cations dispersed on the surface of a nanocasted ordered mesoporous carbon (CMK-3). In the second step, the primarily created iron acetate species underwent pyrolysis and transformed to magnetic iron oxide nanoparticles. X-ray diffraction, Fourier-transform infrared, and Raman spectroscopies were used for the chemical and structural characterization of the hybrids, while surface area measurements, thermal analysis, and transmission electron microscopy were employed to determine their physical, surface, and textural properties. These results revealed the preservation of the host carbon structure, which was homogenously and controllably loaded (up to 27 wt %) with nanosized (ca. 20 nm) iron oxides inside the mesoporous system. M?ssbauer spectroscopy and magnetic measurements at low temperatures confirmed the formation of γ-Fe(2)O(3) nanoparticles exhibiting superparamagnetic behavior. The kinetic studies showed a rapid removal of Cr(VI) ions from the aqueous solutions in the presence of these magnetic mesoporous hybrids and a considerably increased adsorption capacity per unit mass of sorbent in comparison to that of pristine CMK-3 carbon. The results also indicate highly pH-dependent sorption efficiency of the hybrids, whereas their kinetics was described by a pseudo-second-order kinetic model. Taking into account the simplicity of the synthetic procedure and possibility of magnetic separation of hybrids with immobilized pollutant, the developed mesoporous nanomaterials have quite real potential for applications in water treatment technologies.     

Kinetic study of oxygen adsorption over nanosized Au/γ-Al₂O₃ supported catalysts under selective CO oxidation conditions

O? adsorption is a key process for further understanding the mechanism of selective CO oxidation (SCO) on gold catalysts. Rate constants related to the elementary steps of O? adsorption, desorption and surface bonding, as well as the respective activation energies, over a nanosized Au/γ-Al?O? catalyst, were determined by Reversed-Flow Inverse Gas Chromatography (RF-IGC). The present study, carried-out in a wide temperature range (50-300 °C), both in excess as well as in the absence of H?, resulted in mechanistic insights and kinetic as well as energetic comparisons, on the sorption processes of SCO reactants. In the absence of H?, the rate of O? binding, over Au/γ-Al?O?, drastically changes with rising temperature, indicating possible O? dissociation at elevated temperatures. H? facilitates stronger O? bonding at higher temperatures, while low temperature binding remains practically unaffected. The lower energy barriers observed, under H? rich conditions, can be correlated to O? dissociation after hydrogenation. Although, H? enhances both selective CO reactant's desorption, O? desorption is more favored than that of CO, in agreement with the well-known mild bonding of SCO reactant's at lower temperatures. The experimentally observed drastic change in the strength of CO and O? binding is consistent both with well-known high activity of SCO at ambient temperatures, as well as with the loss of selectivity at higher temperatures.     

Porous membrane structures as stationary phase for capillary electrochromatography

This work presents the application of membrane technology for the fabrication of stationary phase for CEC columns using the technique based on phase inversion of polymer solution. A blend of polyimide P84 and sulphonated poly(ether ether ketone was processed via immersion precipitation dry‐wet spinning into small‐bore porous fiber. The morphology, zeta potential, and performance of the porous structure in the CEC separation were investigated. Noncharged molecules (as markers of the electroosmotic flow) and small organic compounds were injected into the column, driven under the application of voltage, and detected on the electropherogram. The proof of concept of applying porous membrane structure as stationary phase for CEC was shown and possible optimization to improve efficiency and selectivity was suggested.     

Cluster Expansion in the Canonical Ensemble

We consider a system of particles confined in a box ${\Lambda \subset \mathbb{R}^d}$ interacting via a tempered and stable pair potential. We prove the validity of the cluster expansion for the canonical partition function in the high temperature - low density regime. The convergence is uniform in the volume and in the thermodynamic limit it reproduces Mayer??s virial expansion providing an alternative and more direct derivation which avoids the deep combinatorial issues present in the original proof.     

Aerodynamic and aeroacoustic analysis of a pulsating internal flow by a multidomain weak collocation spectral method

A numerical methodology is presented for the coupled aerodynamic and aeroacoustic analysis of time dependent laminar viscous flows in general two-dimensional geometries. The overall procedure is constituted firstly by the solution of the incompressible Navier-Stokes equations and secondly by the solution of a linear evolution equation of second order in space and time which originates from Lighthill's acoustic analogy. A semi-implicit projection method is utilized for the time integration of the incompressible Navier-Stokes system, while a choice between a second order implicit Newmark scheme and a fourth order explicit Runge-Kutta-Nyström method is offered for the temporal discretization of the wave equation. The multidomain weak Legendre collocation spectral method on quadrilateral subdomain topologies is employed for the spatial approximation of both the fluid dynamic and the acoustic problems. A specific pulsating internal flow inside a plane constricted channel is selected as a representative application for the assessment of the capabilities of the proposed discrete algorithm. Numerous results are presented and discussed, so as to thoroughly demonstrate the behavior of the numerical procedure.     

Immobilization of η-cyclopentadienyltris(dimethylamido)zirconium polymerization catalysts on a chlorosilane- and HMDS-modified mesoporous silica surface: a new concept for supporting metallocene amides towards heterogeneous single-site-catalysts

The modification of a mesoporous silica surface with Si(Ind)(CH₃)₂Cl and the immobilization of CpZr(NMe₂)₃ on this surface was studied via IR-spectroscopy. To reduce side reactions, the indenyl-modified silica was reacted with hexamethyldisilazane (HMDS) under IR-control before the CpZr(NMe₂)₃-immobilization. The role of the hydroxyl group protection with HMDS is discussed. The surface modifications have been repeated via Schlenk technique at the same conditions and the surface modifications were studied with ¹³C CP MAS–NMR, ¹H MAS–NMR, elemental-, SEM- and BET-analysis. The surface species of the resulting catalysts are discussed. The precatalysts have been treated with methylaluminoxane (MAO) (Al:Zr (mol:mol)=500:1) and the resulting Zr contents (leaching-effect) are discussed. All catalysts have been tested in ethylene and propylene polymerization.     

Maximum and minimum solutions for nonlinear parabolic problems with discontinuities

In this paper we examine nonlinear parabolic problems with a discontinuous right hand side. Assuming the existence of an upper solution φ and a lower solution ψ such that ψ ≤ φ, we establish the existence of a maximum and a minimum solution in the order interval [ψ, φ]. Our approach does not consider the multivalued interpretation of the problem, but a weak one side “Lipschitz” condition on the discontinuous term. By employing a fixed point theorem for nondecreasing maps, we prove the existence of extremal solutions in [ψ, φ for the original single valued version of the problem.     

Mechanism and energetics of dimerization of SiH2 radicals on H-terminated Si(0 0 1)-(2×1) surfaces

The mechanism and energetics are presented of the dimerization of two adsorbed surface SiH₂ groups on the H-terminated Si(0 0 1)-(2 × 1) surface to form Si₂H₄ species during the initial stages of growth in plasma deposition of hydrogenated amorphous silicon (a-Si:H) films. The reactions are observed during classical molecular-dynamics (MD) simulations of a-Si:H film deposition from SiH₂ radical precursors impinging on an initially H-terminated Si(0 0 1)-(2 × 1) surface and substrate temperature, T, over the range 500T700 K. The Si₂H₄ species resulting from the surface SiH₂ dimerization reactions undergo surface conformational changes resulting in either a non-rotated (NRD) or a rotated dimer (RD) configuration. The RD configuration is found to be the energetically favorable one. The MD simulation results for the structure of the NRD and RD surface Si₂H₄ configurations corroborate with ab initio calculations of optimized adsorption configurations of SiH₂ radicals on crystalline Si surfaces, as well as results of STM imaging of the thermal decomposition of disilane on Si(0 0 1).