In situ time-resolved characterization of Au-CeO2 and AuOx-CeO2 catalysts during the water-gas shift reaction: presence of Au and O vacancies in the active phase

Synchrotron-based in situ time-resolved x-ray diffraction and x-ray absorption spectroscopies were used to study the behavior of nanostructured {Au+AuO(x)}-CeO(2) catalysts under the water-gas shift (WGS) reaction. At temperatures above 250 degrees C, a complete AuO(x)-->Au transformation was observed with high catalytic activity. Photoemission results for the oxidation and reduction of Au nanoparticles supported on rough ceria films or a CeO(2)(111) single crystal corroborate that cationic Au(delta+) species cannot be the key sites responsible for the WGS activity at high temperatures. The rate determining steps for the WGS seem to occur at the gold-ceria interface, with the active sites involving small gold clusters (<2 nm) and O vacancies.     

Perylene-based profluorescent nitroxides for the rapid monitoring of polyester degradation upon weathering: An assessment

A profluorescent nitroxide possessing an isoindoline nitroxide moiety linked to a perylene fluorophore was developed to monitor radical mediated degradation of melamine-formaldehyde crosslinked polyester coil coatings in an industry standard accelerated weathering tester. Trapping of polyester-derived radicals (most likely C-radicals) that are generated during polymer degradation leads to fluorescent closed-shell alkoxy amines, which was used to obtain time-dependent degradation profiles to assess the relative stability of different polyesters towards weathering. The nitroxide probe couples excellent thermal stability and satisfactory photostability with high sensitivity and enables detection of free radical damage in polyesters under conditions that mimic exposure to the environment on a time scale of hours rather than months or years required by other testing methods. There are indications that the profluorescent nitroxide undergoes partial photo-degradation in the absence of polymer-derived radicals. Unexpectedly, it was also found that UV-induced fragmentation of the NO–C bond in closed-shell alkoxy amines leads to regeneration of the profluorescent nitroxide and the respective C-radical. The maximum fluorescence intensity that could be achieved with a given probe concentration is therefore not only determined by the amount of polyester radicals formed during accelerated weathering, but also by the light-driven side reactions of the profluorescent nitroxide and the corresponding alkoxy amine radical trapping products. Studies to determine the optimum probe concentration in the polymer matrix revealed that aggregation and re-absorption effects lowered the fluorescence intensity at higher concentrations of the profluorescent nitroxide, but too low probe concentrations, where these effects would be avoided, were not sufficient to trap the amount of polyester radicals formed upon weathering. The optimized experimental conditions were used to assess the impact of temperature and UV irradiance on polymer degradation during accelerated weathering.     

Heteronuclear Nickel‐Iron Complexes and the Crystal Structure of [Fe2(CO)6(μ3‐S)2{Ni(dppe)}]

A series of heteronuclear nickel‐iron complexes [Fe₂(CO)₆(μ‐SH)(μ₃‐S){NiCl(PPh₃)₂}] ( 1 ), [Fe₂(CO)₆(μ‐SH)(μ₃‐S){NiCl(dppe)}] ( 2 ), [Fe₂(CO)₆(μ₃‐S)₂{Ni(PPh₃)₂}] ( 3 ), [Fe₂(CO)₆(μ₃‐S)₂{Ni(dppe)}] ( 4 ) and [Fe₂(CO)₆(μ‐SPh)(μ₃‐S){NiCl(dppe)}] ( 5 ) have been prepared. The structure of 4 has been determined by X‐ray crystallography. The central metal‐sulfur core of 4 has a trigonal bipyramidal shape with a NiFe₂ base plane with two axial sulfur atoms. Each iron atom is 5‐coordinate forming a distorted square pyramid; the nickel is square planar coordinated by two sulfur atoms and two phosphorus atoms.     

Efficient synthesis of an imidazole-substituted delta-amino acid by the integration of chiral technologies

Two methods to produce (2S)-5-amino-2-(1-n-propyl-1H-imidazol-4-ylmethyl)-pentanoic acid were investigated. Diastereoisomeric salt resolution, using the quinidine salt, gave the desired intermediate in 98% ee and 33% yield. Asymmetric hydrogenation of various substrates gave high conversions, with up to 83% ee. Integration of these two approaches via asymmetric hydrogenation of a quinidine salt substrate followed by crystallization provided the desired intermediate in 94% ee and 76% yield.     

Edge Capping of 2D‐MXene Sheets with Polyanionic Salts To Mitigate Oxidation in Aqueous Colloidal Suspensions

MXenes have shown promise in myriad applications, such as energy storage, catalysis, EMI shielding, among many others. However, MXene oxidation in aqueous colloidal suspensions when stored in water at ambient conditions remains a challenge. It is now shown that by simply capping the edges of individual MXene flakes, Ti₃C₂T_z and V₂CT_z, by polyanions such as polyphosphates, polysilicates or polyborates, it is possible to quite significantly reduce their propensity for oxidation even when held in aerated water for weeks. This breakthrough resulted from the realization that the edges of MXene sheets are positively charged. It is thus an example of selectively functionalizing the edges differently from the MXene sheet surfaces.     

Mechanical deformations of a liquid crystal elastomer at director angles between 0° and 90°: Deducing an empirical model encompassing anisotropic nonlinearity

Despite the wealth of studies reporting mechanical properties of liquid crystal elastomers (LCEs), no theory can currently describe their complete mechanical anisotropy and nonlinearity. Here, we present the first comprehensive study of mechanical anisotropy in an all‐acrylate LCE via tensile tests that simultaneously track liquid crystal (LC) director rotation. We then use an empirical approach to gain a deeper insight into the LCE's mechanical responses at values of strain, up to 1.5, for initial director orientations between 0° and 90°. Using a method analogous to time–temperature superposition, we create master curves for the LCE's mechanical response and use these to deduce a model that accurately predicts the load curve of the LCE for stresses applied at angles between 15° and 70° relative to the initial LC director. This LCE has been shown to exhibit auxetic behavior for deformations perpendicular to the director. Interestingly, our empirical model predicts that the LCE will further demonstrate auxetic behavior when stressed at angles between 54° and 90° to the director. Our approach could be extended to any LCE; so it represents a significant step forward toward models that would aid the further development of LCE theory and the design and modeling of LCE‐based technologies. © 2019 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1367–1377     

SULPHUR ORGANIC COMPOUNS AS PROTECTORS FOR RADIATION

Abstract

The development of the scientific principles of the protective action of various compounds and in particular sulphur organic compounds for irradiation is one of the most actual problem in the modern radiation chemistry and radiobiology. With that in view the radiolysis of the various sulphur organic compounds in stationary and pulse conditions both in pure state and in model systems has been studied and the main regularities of their radiolytic behaviour are established. The primary processes of the radiolysis of sulphurorganic compounds containing SHY -S-, NH2, COOH, C H functional groups are investipated and the radical products of the radiolysis- anion- andcation-radicals, thiyl and alkyl radicals are identified and their reactivity is studied in irradiation field.     

Phase transitions,optical, dielectric and viscoelastic properties of colloidal suspensions of BaTiO3 nanoparticles and cyanobiphenyl liquid crystals

We report experimental studies on the phase transitions and physical properties of colloidal suspensions of BaTiO₃ nanoparticles and two cyanobiphenyl liquid crystals (4-pentyl-4?-cyanobiphenyl and 4-octyl-4?-cyanobiphenyl). From the differential scanning calorimetric measurements, we show that the nanoparticles have antagonistic effect on the isotropic to nematic and nematic to smectic-A phase transitions. The birefringence, dielectric anisotropy and splay elastic constant remain almost unchanged, whereas the bend elastic constant and rotational viscosity decrease considerably. The experimental results are discussed based on the possible contribution of BaTiO₃ nanoparticles and free surfactant molecules in the suspensions.     

Development of a trace gas stable isotope capture system in a mobile laboratory for temporal and spatial sampling of field and laboratory experiments

We describe the development of a novel mobile field laboratory, purposely designed for the automated capture and subsequent stable isotopic analyses of multiple gas samples. The multiple capture system is integrated into a mobile laboratory that is fully capable of measuring the concentration of carbon dioxide, methane and nitrous oxide trace gases in a flow-through system connected to a gas chromatograph fitted with both electron capture and flame ionisation detectors. The capture of gases is achieved by routing samples through a series of 135 mL gas flasks that are sealed by micro-solenoid valves triggered by a timing system. Trace gas light stable isotope ratio mass spectrometry can then be carried out on gas samples collected by the system (NERC (15)N Stable Isotope Facility). The excitingly unique potential of the system to the ecological research field is that it will allow the collection of cyclical data for three different trace gases both in real-time and in situ. We present data arising from the validation of this mobile system as well as a preliminary experimental assessment of this technique. This technique was used to measure delta(13)C in CO(2) and CH(4) in soil gases released from waterlogged cores and delta(13)C-CH(4) values were significantly depleted in wet cores compared with dry ones (p < 0.001).