6 results
Search Results
Now showing 1 - 6 of 6
- ItemEffects of Promoter on Structural and Surface Properties of Zirconium Oxide-Based Catalyst Materials(Basel : MDPI AG, 2020) Borovinskaya, E.S.; Oswald, S.; Reschetilowski, W.Ternary mixed oxide systems CuO/ZnO/ZrO2 and CuO/NiO/ZrO2 were synthesized by one-pot synthesis for a better understanding of the synthesis-property relationships of zirconium oxide-based catalyst materials. The prepared mixed oxide samples were analysed by a broad range of characterisation methods (XRD, N2-physisorption, Temperature-Programmed Ammonia Desorption (TPAD), and XPS) to examine the structural and surface properties, as well as to identify the location of the potential catalytically active sites. By XPS analysis, it could be shown that a progressive enrichment of the surface composition with copper takes place by changing from ZnO to NiO as a promoter. Thus, by addition of the second component, not only electronic but also the geometric properties of active sites, i.e., copper species distribution within the catalyst surface, can be affected in a desired way.
- ItemEntropy of conduction electrons from transport experiments(Basel : MDPI AG, 2020) Pérez, N.; Wolf, C.; Kunzmann, A.; Freudenberger, J.; Krautz, M.; Weise, B.; Nielsch, K.; Schierning, G.The entropy of conduction electrons was evaluated utilizing the thermodynamic definition of the Seebeck coefficient as a tool. This analysis was applied to two dierent kinds of scientific questions that can-if at all-be only partially addressed by other methods. These are the field-dependence of meta-magnetic phase transitions and the electronic structure in strongly disordered materials, such as alloys. We showed that the electronic entropy change in meta-magnetic transitions is not constant with the applied magnetic field, as is usually assumed. Furthermore, we traced the evolution of the electronic entropy with respect to the chemical composition of an alloy series. Insights about the strength and kind of interactions appearing in the exemplary materials can be identified in the experiments.
- ItemSyntheses, crystal structure and magnetic properties of Tl9RETe6 (RE = Ce, Sm, Gd)(Basel : MDPI AG, 2020) Isaeva, A.; Schönemann, R.; Doert, T.The three compounds Tl9RETe6 with RE = Ce, Sm, Gd were synthesized from the elements at 1020 K. Their isostructural crystal structures are ordered derivatives of the Tl5Te3 type with rare-earth metal and thallium occupying different Wyckoff positions. The structures can be understood as charge-ordered in accordance with the Zintl-Klemm concept: 9 Tl+ + RE3+ + 6 Te2-. DFT calculations for Tl9GdTe6, however, result in a low, but finite density of states at the Fermi level. Magnetic data confirm trivalent Gd, but indicate a small amount of Ce4+ in Tl9CeTe6; no indications for long-range magnetic order was found down to T = 2 K.
- ItemFilled carbon nanotubes as anode materials for lithium-ion batteries(Basel : MDPI AG, 2020) Thauer, E.; Ottmann, A.; Schneider, P.; Möller, L.; Deeg, L.; Zeus, R.; Wilhelmi, F.; Schlestein, L.; Neef, C.; Ghunaim, R.; Gellesch, M.; Nowka, C.; Scholz, M.; Haft, M.; Wurmehl, S.; Wenelska, K.; Mijowska, E.; Kapoor, A.; Bajpai, A.; Hampel, S.; Klingeler, R.Downsizing well-established materials to the nanoscale is a key route to novel functionalities, in particular if different functionalities are merged in hybrid nanomaterials. Hybrid carbon-based hierarchical nanostructures are particularly promising for electrochemical energy storage since they combine benefits of nanosize effects, enhanced electrical conductivity and integrity of bulk materials. We show that endohedral multiwalled carbon nanotubes (CNT) encapsulating high-capacity (here: conversion and alloying) electrode materials have a high potential for use in anode materials for lithium-ion batteries (LIB). There are two essential characteristics of filled CNT relevant for application in electrochemical energy storage: (1) rigid hollow cavities of the CNT provide upper limits for nanoparticles in their inner cavities which are both separated from the fillings of other CNT and protected against degradation. In particular, the CNT shells resist strong volume changes of encapsulates in response to electrochemical cycling, which in conventional conversion and alloying materials hinders application in energy storage devices. (2) Carbon mantles ensure electrical contact to the active material as they are unaffected by potential cracks of the encapsulate and form a stable conductive network in the electrode compound. Our studies confirm that encapsulates are electrochemically active and can achieve full theoretical reversible capacity. The results imply that encapsulating nanostructures inside CNT can provide a route to new high-performance nanocomposite anode materials for LIB.
- ItemSystematic investigations of annealing and functionalization of carbon nanotube yarns(Basel : MDPI AG, 2020) Scholz, M.; Hayashi, Y.; Eckert, V.; Khavrus, V.; Leonhardt, A.; Büchner, B.; Mertig, M.; Hampel, S.Carbon nanotube yarns (CNY) are a novel carbonaceous material and have received a great deal of interest since the beginning of the 21st century. CNY are of particular interest due to their useful heat conducting, electrical conducting, and mechanical properties. The electrical conductivity of carbon nanotube yarns can also be influenced by functionalization and annealing. A systematical study of this post synthetic treatment will assist in understanding what factors influences the conductivity of these materials. In this investigation, it is shown that the electrical conductivity can be increased by a factor of 2 and 5.5 through functionalization with acids and high temperature annealing respectively. The scale of the enhancement is dependent on the reducing of intertube space in case of functionalization. For annealing, not only is the highly graphitic structure of the carbon nanotubes (CNT) important, but it is also shown to influence the residual amorphous carbon in the structure. The promising results of this study can help to utilize CNY as a replacement for common materials in the field of electrical wiring.
- ItemInfluence of MoS2 on activity and stability of carbon nitride in photocatalytic hydrogen production(Basel : MDPI AG, 2019) Sivasankaran, R.P.; Rockstroh, N.; Kreyenschulte, C.R.; Bartling, S.; Lund, H.; Acharjya, A.; Junge, H.; Thomas, A.; Brückner, A.MoS2/C3N4 (MS-CN) composite photocatalysts have been synthesized by three different methods, i.e., in situ-photodeposition, sonochemical, and thermal decomposition. The crystal structure, optical properties, chemical composition, microstructure, and electron transfer properties were investigated by X-ray diffraction, UV-vis diffuse reflectance spectroyscopy, X-ray photoelectron spectroscopy, electron microscopy, photoluminescence, and in situ electron paramagnetic resonance spectroscopy. During photodeposition, the 2H MoS2 phase was formed upon reduction of [MoS4]2− by photogenerated conduction band electrons and then deposited on the surface of CN. A thin crystalline layer of 2H MoS2 formed an intimate interfacial contact with CN that favors charge separation and enhances the photocatalytic activity. The 2H MS-CN phase showed the highest photocatalytic H2 evolution rate (2342 µmol h−1 g−1, 25 mg catalyst/reaction) under UV-vis light irradiation in the presence of lactic acid as sacrificial reagent and Pt as cocatalyst.