Oxidation by CO2 of Au0 species on La2O3-supported gold clusters

The IR spectra that characterize La(2)O(3)-supported gold clusters show that the original Au(0) species can be oxidized by CO(2) during the catalytic CO oxidation reaction, indicating that CO(2) is the actual gold oxidizing agent.     

Dynamic interactive membranes with pressure-driven tunable porosity and self-healing ability

When pressure is applied to dynamic interactive membranes consisting of micelles composed of a triblock copolymer, their morphologies can be fine-tuned. Membranes with a range of porosities are accessible which can regulate and thereby control filtration performance and also display effective autonomous healing.     

Polysiloxane‐Backbone Block Copolymers in a One‐Pot Synthesis: A Silicone Platform for Facile Functionalization

Block copolymers consisting exclusively of a silicon–oxygen backbone are synthesized by sequential anionic ring‐opening polymerization of different cyclic siloxane monomers. After formation of a poly(dimethylsiloxane) (PDMS) block by butyllithium‐initiated polymerization of D3, a functional second block is generated by subsequent addition of tetramethyl tetravinyl cyclotetrasiloxane (D4^V), resulting in diblock copolymers comprised a simple PDMS block and a functional poly(methylvinylsiloxane) (PMVS) block. Polymers of varying block length ratios were obtained and characterized. The vinyl groups of the second block can be easily modified with a variety of side chains using hydrosilylation chemistry to attach compounds with Si—H bond. Conversion of the hydrosilylation used for polymer modification was investigated.     

Metallodynameric membranes--toward the constitutional transport of gases

The adequate selection of macromonomers, dialdehyde core connectors and of coordinating metal ions makes possible the generation of metallodynameric materials, allowing the fine modulation of the gas transport through rubbery membranes.     

A short, rigid, structurally pure carbon nanotube by stepwise chemical synthesis

The inaccessibility of uniform-diameter, single-chirality carbon nanotubes (CNTs) in pure form continues to thwart efforts by scientists to use these ultrathin materials in innovative applications that could revolutionize nanoscale electronics. Stimulated by the challenge to address this long-standing problem, we and other organic chemists have envisioned a new production strategy involving the controlled elongation of small hydrocarbon templates, such as hemispherical nanotube end-caps, prepared by bottom-up chemical synthesis; the diameter and rim structure encoded in the template would dictate the diameter and chirality of the resulting CNT. Toward that objective, a short [5,5] CNT has now been synthesized by stepwise chemical methods. This C(50)H(10) geodesic polyarene has been isolated, purified, crystallized, and fully characterized by NMR spectroscopy, UV-vis absorption spectroscopy, high resolution mass spectrometry, and X-ray crystallography.     

Effect of Cr and Pt promoters on the surface properties of tungstated zirconia: FTIR spectroscopy of probe molecules (CO and NO)

Adsorption of IR probe molecules (CO and NO) has been used for characterization of a Pt-Cr(2)O(3)/WO(3)-ZrO(2) sample. For better assignments of the results obtained 'blank' experiments with WO(3)-ZrO(2), Pt/WO(3)-ZrO(2) and Cr(2)O(3)/WO(3)-ZrO(2) were also performed. The Cr(2)O(3)/WO(3)-ZrO(2) sample was prepared via sulfate route and surface sulfates were detected on the activated sample. These species keep ca. 50% of their initial concentration even after reduction by hydrogen at 773 K. The amount of Lewis acid sites on the activated sample, as measured by CO adsorption, is relatively low and is associated mainly with coordiantively unsaturated (c.u.s.) Zr(4+) cations. Reduction of the sample generates Cr(3+) ions and the concentration of the Lewis acid sites continuously increases with the reduction temperature up to 773 K. At this reduction temperature, however, the strength of the acid sites decreases due to partial removal of the sulfates. While CO is not an efficient probe to discriminate between Zr(4+) and Cr(3+) sites, NO is selectively strongly adsorbed on Cr(3+) cations which allows a more precise monitoring of the evolution of the Cr(3+) sites during reduction. The activated Pt-Cr(2)O(3)/WO(3)-ZrO(2) sample also shows presence of sulfates but these species are practically absent after reduction with hydrogen at 773 K. It was found that platinum promotes the reduction of chromium and tungsten species as well. CO adsorption on activated sample reveals the existence of cationic platinum. However, reduction with hydrogen even at room temperature generates metallic Pt. Its amount continuously increases with the reduction temperature up to 573 K and then, due to the SMSI effect and/or agglomeration, the amount of platinum accessible to adsorption decreases. Comparison with the results obtained with the Pt/WO(3)-ZrO(2) sample shows that chromium and/or sulfates stabilize the cationic form of platinum.     

Reduction of bowl-shaped hydrocarbons: dianions and tetraanions of annelated corannulenes

[structure: see text] The reduction of several annelated corannulene derivatives was undertaken using lithium and potassium metals. It was found that annelation affects the annulenic character of corannulene by changing its charge distribution; the dianions of derivatives that are annelated with six-membered rings have less annulenic character and are less paratropic than corannulene dianion. This effect is even more pronounced in corannulenes that are peri-annelated with five-membered rings. The alkali metal used in the reduction process has a great influence on the outcome, especially on the degree of reduction. Most derivatives get reduced to tetraanions only with potassium, and not with lithium, the exception being systems that can stabilize the tetraanion with lithium by special means, such as aggregation or dimerization. One such system is cyclopenta[bc]corannulene (acecorannulylene), which gives a coordinative dimer that consists of two cyclopentacorannulene tetraanions, bound together in a convex-convex fashion by lithium cations. The points of contact in this dimer are two rehybridized carbons from each cyclopentacorannulene unit, which are bridged together by two lithium cations.     

TURAN'S EXTREMAL PROBLEM FOR POSITIVE DEFINITE FUNCTIONS ON GROUPS

We study the following question: given an open set , symmetricabout 0, and a continuous, integrable, positive definite functionf, supported in and with f(0) = 1, how large can f be? Thisproblem has been studied so far mostly for convex domains inEuclidean space. In this paper we study the question in arbitrarylocally compact abelian groups and for more general domains.Our emphasis is on finite groups as well as Euclidean spacesand ^d. We exhibit upper bounds for f assuming geometric propertiesof of two types: (a) packing properties of and (b) spectralproperties of . Several examples and applications of the maintheorems are shown. In particular, we recover and extend severalknown results concerning convex domains in Euclidean space.Also, we investigate the question of estimating f over possiblydispersed sets solely in dependence of the given measure m :=||of . In this respect we show that in and the integral is maximalfor intervals.     

Periodicity of the Spectrum of a Finite Union of Intervals

A set Ω, of Lebesgue measure 1, in the real line is called spectral if there is a set Λ of real numbers such that the exponential functions e _λ (x)=exp (2πiλx), λ∈Λ, form a complete orthonormal system on L ²(Ω). Such a set Λ is called a spectrum of Ω. In this note we present a simplified proof of the fact that any spectrum Λ of a set Ω which is finite union of intervals must be periodic. The original proof is due to Bose and Madan.     

Universal mechanism of dissipation in Fermi superfluids at ultralow temperatures

We show that the vortex dynamics in Fermi superfluids at ultralow temperatures is governed by the local heating of the vortex cores creating the heat flux carried by nonequilibrium quasiparticles emitted by moving vortices. This mechanism provides a universal zero temperature limit of dissipation in Fermi superfluids. For the typical experimental conditions realized by the turbulent motion of ^{3}He-B, the temperature of the vortex cores is estimated to be of the order 0.2 T(c). The dispersion of Kelvin waves is derived, and the heat flow generated by Kelvin cascade is shown to have a value close to that experimentally observed.