Highly Efficient Hyperbranched CNT Surfactants: Influence of Molar Mass and Functionalization

End-group-functionalized hyperbranched polymers were synthesized to act as a carbon nanotube (CNT) surfactant in aqueous solutions. Variation of the percentage of triphenylmethyl (trityl) functionalization and of the molar mass of the hyperbranched polyglycerol (PG) core resulted in the highest measured surfactant efficiency for a 5000 g/mol PG with 5.6% of the available hydroxyl end-groups replaced by trityl functions, as shown by UV–vis measurements. Semiempirical model calculations suggest an even higher efficiency for PG5000 with 2.5% functionalization and maximal molecule specific efficiency in general at low degrees of functionalization. Addition of trityl groups increases the surfactant–nanotube interactions in comparison to unfunctionalized PG because of π–π stacking interactions. However, at higher functionalization degrees mutual interactions between trityl groups come into play, decreasing the surfactant efficiency, while lack of water solubility becomes an issue at very high functionalization degrees. Low molar mass surfactants are less efficient compared to higher molar mass species most likely because the higher bulkiness of the latter allows for a better CNT separation and stabilization. The most efficient surfactant studied allowed dispersing 2.85 mg of CNT in 20 mL with as little as 1 mg of surfactant. These dispersions, remaining stable for at least 2 months, were mainly composed of individual CNTs as revealed by electron microscopy

Heterogeneous TiO₂/V₂O₅/Carbon Nanotube Electrodes for Lithium-Ion Batteries

Vanadium pentoxide (V₂O₅) is proposed and investigated as a cathode material for lithium-ion (Li-ion) batteries. However, the dissolution of V₂O₅ during the charge/discharge remains as an issue at the V₂O₅–electrolyte interface. In this work, we present a heterogeneous nanostructure with carbon nanotubes supported V₂O₅/titanium dioxide (TiO₂) multilayers as electrodes for thin-film Li-ion batteries. Atomic layer deposition of V₂O₅ on carbon nanotubes provides enhanced Li storage capacity and high rate performance. An additional TiO₂ layer leads to increased morphological stability and in return higher electrochemical cycling performance of V₂O₅/carbon nanotubes. The physical and chemical properties of TiO₂/V₂O₅/carbon nanotubes are characterized by cyclic voltammetry and charge/discharge measurements as well as electron microscopy. The detailed mechanism of the protective TiO₂ layer to improve the electrochemical cycling stability of the V₂O₅ is unveiled

Chabazite: Stable Cation-Exchanger in Hyper Alkaline Concrete Pore Water

To avoid impact on the environment, facilities for permanent disposal of hazardous waste adopt multibarrier design schemes. As the primary barrier very often consists of cement-based materials, two distinct aspects are essential for the selection of suitable complementary barriers: (1) selective sorption of the contaminants in the repository and (2) long-term chemical stability in hyperalkaline concrete-derived media. A multidisciplinary approach combining experimental strategies from environmental chemistry and materials science is therefore essential to provide a reliable assessment of potential candidate materials. Chabazite is typically synthesized in 1 M KOH solutions but also crystallizes in simulated young cement pore water, a pH 13 aqueous solution mainly containing K⁺ and Na⁺ cations. Its formation and stability in this medium was evaluated as a function of temperature (60 and 85 °C) over a timeframe of more than 2 years and was also asessed from a mechanistic point of view. Chabazite demonstrates excellent cation-exchange properties in simulated young cement pore water. Comparison of its Cs⁺ cation exchange properties at pH 8 and pH 13 unexpectedly demonstrated an increase of the <i>K</i>_D with increasing pH. The combined results identify chabazite as a valid candidate for inclusion in engineered barriers for concrete-based waste disposal

PdPb-Catalyzed Decarboxylation of Proline to Pyrrolidine: Highly Selective Formation of a Biobased Amine in Water

Amino acids have huge potential as platform chemicals in the biobased industry. Pd-catalyzed decarboxy­lation is a very promising route for the valorization of these natural compounds derived from protein waste or fermentation. We report that the highly abundant and nonessential amino acid l-proline is very reactive in the Pd-catalyzed decarboxy­lation. Full conversions are obtained with Pd/C and different Pd/MeO_<i>x</i> catalysts; this allowed the identification of the different side reactions and the mapping of the reaction network. Due to the high reactivity of pyrrolidine, the selectivity for pyrrolidine was initially low. By carefully modifying Pd/ZrO₂ with Pb in a controlled mannervia two incipient wetness impregnation stepsthe selectivity increased remarkably. Finally, a thorough investigation of the reaction parameters resulted in an increased activity of this modified catalyst and an even further enhanced selectivity under a low H₂ pressure of 4 bar at 235 °C in water. This results in a very selective and sustainable production route for the highly interesting pyrrolidine

Conceptual Frame Rationalizing the Self-Stabilization of H‑USY Zeolites in Hot Liquid Water

The wide range of liquid-phase reactions required for the catalytic conversion of biomass compounds into new bioplatform molecules defines a new set of challenges for the development of active, selective, and stable catalysts. The potential of bifunctional Ru/H-USY catalysts for conversions in hot liquid water (HLW) is assessed in terms of physicochemical stability and long-term catalytic performance of acid sites and noble metal functionality, as probed by hydrolytic hydrogenation of cellulose. It is shown that zeolite desilication is the main zeolite degradation mechanism in HLW. USY zeolite stability depends on two main parameters, viz., framework and extra-framework aluminum content. The former protects the zeolite lattice by counteracting hydrolysis of framework bonds, and the latter, when located at the external crystal surface, prevents solubilization of the zeolite framework which is the result of its low water-solubility. Hence, the hot liquid water stability of commercial H-USY zeolites, in contrast to their steam stability, increased with decreasing Si/Al ratio. As a result, mildly steamed USY zeolites containing a high amount of both Al species exhibit the highest resistance to HLW. During an initial period of transformations, Al-rich zeolites form additional protective extra-framework Al species at the outer surface, self-stabilizing the framework. A critical bulk Si/Al ratio of 3 was determined whereby USY zeolites with a lower Si/Al ratio will self-stabilize over time. Besides, due to the initial transformation period, the accessibility of the catalytic active sites is extensively enhanced resulting in a material that is more stable and drastically more accessible to large substrates than the original zeolite. When these findings are applied in the hydrolytic hydrogenation of cellulose, unprecedented nearly quantitative hexitol yields were obtained with a stable catalytic system

Electronic Coupling between Graphene and Topological Insulator Induced Anomalous Magnetotransport Properties

It has been theoretically proposed that the spin textures of surface states in a topological insulator can be directly transferred to graphene by means of the proximity effect, which is very important for realizing a two-dimensional topological insulator based on graphene. Here we report the anomalous magnetotransport properties of graphene–topological insulator Bi₂Se₃ heterojunctions, which are sensitive to the electronic coupling between graphene and the topological surface state. The coupling between the p_<i>z</i> orbitals of graphene and the p orbitals of the surface states on the Bi₂Se₃ bottom surface can be enhanced by applying a perpendicular negative magnetic field, resulting in a giant negative magnetoresistance at the Dirac point up to about −91%. An obvious resistance dip in the transfer curve at the Dirac point is also observed in the hybrid devices, which is consistent with theoretical predictions of the distorted Dirac bands with nontrivial spin textures inherited from the Bi₂Se₃ surface states

Asymmetric Modulation on Exchange Field in a Graphene/BiFeO₃ Heterostructure by External Magnetic Field

Graphene, having all atoms on its surface, is favorable to extend the functions by introducing the spin–orbit coupling and magnetism through proximity effect. Here, we report the tunable interfacial exchange field produced by proximity coupling in graphene/BiFeO₃ heterostructures. The exchange field has a notable dependence with external magnetic field, and it is much larger under negative magnetic field than that under positive magnetic field. For negative external magnetic field, interfacial exchange coupling gives rise to evident spin splitting for <i>N</i> ≠ 0 Landau levels and a quantum Hall metal state for <i>N</i> = 0 Landau level. Our findings suggest graphene/BiFeO₃ heterostructures are promising for spintronics