Technical University of Berlin

About: Technical University of Berlin is a education organization based out in Berlin, Germany. It is known for research contribution in the topics: Laser & Catalysis. The organization has 27292 authors who have published 59342 publications receiving 1414623 citations. The organization is also known as: Technische Universität Berlin & TU Berlin.

Rational Extension of the Family of Layered, Covalent, Triazine‐Based Frameworks with Regular Porosity

Abstract: 2010 WILEY-VCH Verlag Gmb Porous solids are an important class of materials in sorption and chromatography applications and more recently they have also become relevant for applications in catalysis (as active catalyst or support) and storage (gas storage in microporous, batteries). Controlled construction of such extended porous frameworks from suitable molecular building blocks paves the way for establishing a connection betweenmolecular and solid properties in a well-defined, pre-ordered chemical environment. Rational synthesis of extended arrays of organicmatter in bulk, in solution, in crystals, and in thin films has always been a paramount goal in chemistry. The classical synthetic tools to obtain long-range regularity are, however, limited to non-covalent interactions, in contrast to the structurally more random character of covalent polymerization reactions. The most challenging hurdle in the synthesis of extended, yet precisely defined 2D and 3D structures based on covalent chemistry is widely believed to be the requirement that the reaction linking individual organic constituents should be reversible, allowing the scaffold to arrange to the thermodynamic, well-ordered product rather than the kinetic, amorphous structure. Reports by Yaghi et al. have shown that two kinds of condensation reactions yielding planar, six-membered boroxine rings (B3O3) and five-membered BO2C2 rings meet this criterion and thus can be utilized as covalent linkers between organic units to generate 2D and 3D covalent organic frameworks (COFs). Successful synthesis of COF materials from light, non-metallic, molecular building blocks is appealing for applications such as gas storage and catalysis – in particular because they hold the promise of being completely inert with respect to water, unlike many (metal organic) frameworks known from literature. Additionally, the large number of inexpensive organic compounds offers modularity, in which desirable features such as surface functionalization and tunable pore sizes can conceptually be easily achieved. Previously, we showed that the polytrimerization reaction of 1,4-dicyanobenzene in zinc chloride is an alternative way towards covalently linked, covalent, triazine-based frameworks (CTF-1). Although other porous, organic networks based on the triazine linker have been synthesized since, CTF-1 remained unique in particular because it not only exhibited permanent microand mesoporosity but also crystallinity. In the following, we shall report on a secondmember in the class of covalent, triazine-based frameworks (termed CTF-2), which was obtained from the ionothermal condensation of 2,6-naphthalenedinitrile (Fig. 1). This find should corroborate our previous assertion that the triazine ring (C3N3) is a fruitful and modular linking-unit in the synthesis of extended and ordered layered frameworks. In planning the synthesis, we drew on our previous experiences with the cyclotrimerization of dinitrile compounds in a zinc chloride salt melt, and we chose 2,6-naphthalenedicarbonitrile, because of its rigidity and its relative chemical stability resulting from its extended aromatic p-system. The later trait was of special significance, since the reaction conditions involve both elevated temperatures ( 400 8C) and the partaking of a catalytically active species, opening up potential pathways to many undesired decomposition and condensation reactions. One such pathway of aromatic nitrile decomposition involves C H bond cleavage, but also the oxidative halogenation of the aromatic unit in the presence of a metal halide. Homolytic cleavage of the carbon nitrile bond at temperatures above 400 8C and the associated depletion of nitrogen content within the structure were observed previously for a set of studied model compounds in this type of reaction. The framework CTF-2 was synthesized heating zinc chloride and 2,6-naphthalenedicarbonitrile in a quartz glass ampoule at 400 8C for 40 h. The set-up in a closed system was chosen since the monomer starts to sublimate at temperatures around 220 8C. The typical yields of this reaction are in the range of 80–85%, suggesting that some of the side reactions outlined above do occur. In the FTIR spectrum we see that the intensity of the carbonitrile band at 2225 cm 1 decreases significantly after 20 h and diminishes further following the course of the reaction, while bands at 1535 and 1315 cm , indicative of aromatic C N stretching and breathing modes in the triazine unit, appear (c.f. Supporting Information). The C solid-state NMR (CP/MAS) experiment performed on CTF-2 further confirmed the presence of sp carbons from the triazine unit ( 169 ppm) and the naphthalene (133, 128 ppm) as well as sp carbons from unreacted carbonitriles (110 ppm), which are most likely situated at the terminal edges of the condensed material (Supporting Information). Elemental analysis revealed a molecular ratio of C36.0H18.8N5.4, which is close to the theoretical composition of a perfectly condensed, infinite framework, namely C36H18N6. The washing procedure outlined in the experimental section left a

Electrolysis of low-grade and saline surface water

Abstract: Powered by renewable energy sources such as solar, marine, geothermal and wind, generation of storable hydrogen fuel through water electrolysis provides a promising path towards energy sustainability. However, state-of-the-art electrolysis requires support from associated processes such as desalination of water sources, further purification of desalinated water, and transportation of water, which often contribute financial and energy costs. One strategy to avoid these operations is to develop electrolysers that are capable of operating with impure water feeds directly. Here we review recent developments in electrode materials/catalysts for water electrolysis using low-grade and saline water, a significantly more abundant resource worldwide compared to potable water. We address the associated challenges in design of electrolysers, and discuss future potential approaches that may yield highly active and selective materials for water electrolysis in the presence of common impurities such as metal ions, chloride and bio-organisms. Production of hydrogen fuel by electrolysis of low-grade or saline water, as opposed to pure water, could have benefits in terms of resource availability and cost. This Review examines the challenges of this approach and how they can be addressed through catalyst and electrolyser design.

The Slashdot Zoo: Mining a Social Network with Negative Edges

Abstract: We analyse the corpus of user relationships of the Slashdot technology news site. The data was collected from the Slashdot Zoo feature where users of the website can tag other users as friends and foes, providing positive and negative endorsements. We adapt social network analysis techniques to the problem of negative edge weights. In particular, we consider signed variants of global network characteristics such as the clustering coefficient, node-level characteristics such as centrality and popularity measures, and link-level characteristics such as distances and similarity measures. We evaluate these measures on the task of identifying unpopular users, as well as on the task of predicting the sign of links and show that the network exhibits multiplicative transitivity which allows algebraic methods based on matrix multiplication to be used. We compare our methods to traditional methods which are only suitable for positively weighted edges.

Direct importance estimation for covariate shift adaptation

Abstract: A situation where training and test samples follow different input distributions is called covariate shift. Under covariate shift, standard learning methods such as maximum likelihood estimation are no longer consistent—weighted variants according to the ratio of test and training input densities are consistent. Therefore, accurately estimating the density ratio, called the importance, is one of the key issues in covariate shift adaptation. A naive approach to this task is to first estimate training and test input densities separately and then estimate the importance by taking the ratio of the estimated densities. However, this naive approach tends to perform poorly since density estimation is a hard task particularly in high dimensional cases. In this paper, we propose a direct importance estimation method that does not involve density estimation. Our method is equipped with a natural cross validation procedure and hence tuning parameters such as the kernel width can be objectively optimized. Furthermore, we give rigorous mathematical proofs for the convergence of the proposed algorithm. Simulations illustrate the usefulness of our approach.