Claudia Hoffmann

Preparation and application of cellular and nanoporous carbides

A tutorial review on cellular as well as nanoporous carbides covering their structure, synthesis and potential applications. Especially new carbide materials with a hierarchical pore structure are in focus. As a central theme silicon carbide based materials are picked out, but also titanium, tungsten and boron carbides, as well as carbide-derived carbons, are part of this review.

Nanocasting Hierarchical Carbide-Derived Carbons in Nanostructured Opal Assemblies for High-Performance Cathodes in Lithium–Sulfur Batteries

Silica nanospheres are used as templates for the generation of carbide-derived carbons with monodisperse spherical mesopores (d=20-40 nm) and microporous walls. The nanocasting approach with a polycarbosilane precursor and subsequent pyrolysis, followed by silica template removal and chlorine treatment, results in carbide-derived carbons DUT-86 (DUT=Dresden University of Technology) with remarkable textural characteristics, monodisperse, spherical mesopores tunable in diameter, and very high pore volumes up to 5.0 cm3 g(-1). Morphology replication allows these nanopores to be arranged in a nanostructured inverse opal-like structure. Specific surface areas are very high (2450 m2 g(-1)) due to the simultaneous presence of micropores. Testing DUT-86 samples as cathode materials in Li-S batteries reveals excellent performance, and tailoring of the pore size allows optimization of cell performance, especially the active center accessibility and sulfur utilization. The outstanding pore volumes allow sulfur loadings of 80 wt %, a value seldom achieved in composite cathodes, and initial capacities of 1165 mAh gsulfur(-1) are reached. After 100 cycle capacities of 860 mAh gsulfur(-1) are retained, rendering DUT-86 a high-performance sulfur host material.

Polymer-derived nanoporous silicon carbide with monodisperse spherical pores

The synthesis of polymer-derived nanoporous silicon carbide with monodisperse spherical pores is described. An incipient wetness method was used to fill the interparticle voids of microemulsion-derived silica nanospheres with the polycarbosilane SMP-10. The spheres have a very narrow diameter distribution in the mesoscale that could be replicated as pores of the silicon carbide materials by performing pyrolysis in an inert atmosphere and subsequent HF etching. Using a pyrolysis temperature between 973 K and 1573 K control of the pore sizes, the specific surface areas as well as the silicon carbide structure was achieved. Shrinkage of the system due to crystallization and structure transformations seems to occur. Even for temperatures as high as 1573 K SiC with uniform spherical pores and specific surface areas up to 433.1 m2 g−1 could be synthesized. This class of silicon carbides (named DUT-45, DUT = Dresden University of Technology) is characterized by unique nitrogen physisorption performance and small angle X-ray scattering curves.

Nanoporous and Highly Active Silicon Carbide Supported CeO2-Catalysts for the Methane Oxidation Reaction

CeOx @SiO2 nanoparticles are used for the first time for the generation of porous SiC materials with tailored pore diameter in the mesopore range containing encapsulated and catalytically active CeO2 nanoparticles. The nanocasting approach with a preceramic polymer and subsequent pyrolysis is performed at 1300 °C, selective leaching of the siliceous part results in CeOx /SiC catalysts with remarkable characteristics like monodisperse, spherical pores and specific surface areas of up to 438 m(2) ·g(-1) . Porous SiC materials are promising supports for high temperature applications. The catalysts show excellent activities in the oxidation of methane with onset temperatures of the reaction 270 K below the onset of the homogeneous reaction. The synthesis scheme using core-shell particles is suited to functionalize silicon carbide with a high degree of stabilization of the active nanoparticles against sintering in the core of the template even at pyrolysis temperatures of 1300 °C rendering the novel synthesis principle as an attractive approach for a wide range of catalytic reactions.

Nanoporous silicon carbide as nickel support for the carbon dioxide reforming of methane

Fumed silica is used as a template in the nanocasting approach towards nanoporous silicon carbide, and it can then be applied as a catalyst support. By varying the pyrolysis temperature between 1000 and 1500 °C, the structural parameters of the resulting silicon carbide materials DUT-87 (DUT = Dresden University of Technology) can be controlled. A specific surface of 328 m2 g−1 is obtained. Furthermore, the oxidation behaviour of such nanoporous SiCs is investigated. The materials are distinguished by an impressive thermal stability at 900 °C for at least 12 h, which is allowed by the presence of a passive oxidation even for such highly porous SiCs. Hence, nickel (10 wt%) was supported on the fresh DUT-87 as well as controlled oxidized DUT-87preox samples, and the influence of the different support properties on the characteristics of the catalyst samples which are used in the carbon dioxide reforming of methane was investigated. The SiO2 layer on the SiC for the DUT-87preox samples could prevent the formation of nickel silicide to a large extent at temperatures up to 850 °C. This resulted in higher activities during the dry reforming of methane at 800 °C and the performance of the siliceous supports was significantly exceeded, emphasizing the beneficial effect of SiC. Effective methane reaction rates of 1.2 mmol g−1 s−1 were obtained for KPK1ox which was based on DUT-87 pyrolysed at 1300 °C and oxidative treatment prior to nickel insertion. Furthermore, a stable conversion level was reached over the whole time on stream of 8 h.

Pore Structure and Surface Acidity Effects of Ordered Mesoporous Supports on Enantioselective Hydrogenation of Ethyl Pyruvate

The enantioselective hydrogenation of ethyl pyruvate is studied using cinchonidine‐modified platinum supported on ordered mesoporous materials as a catalyst. The influence of different pore structures and hence, textural properties of the support on the enantiodifferentiation is investigated. The selected representative model supports are SBA‐15, SBA‐16, and KIT‐6. Depending on the pore structure, varying enantioselectivities are obtained. Furthermore, SBA‐15 is post‐synthetically aluminated to produce materials with different Si/Al ratios. An increase in Al content leads to an increased support surface acidity and enhanced enantioselectivity. The catalyst samples are characterized by small‐angle X‐ray scattering (SAXS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), temperature‐programmed desorption of ammonia (TPAD), and N2 and CO sorption techniques. Applying optimized reaction conditions resulted in enantiomeric excesses of (R)‐ethyl lactate of 90–94 % ee at total conversion.