Matthias Leistner

Carbide-Derived Carbon Monoliths with Hierarchical Pore Architectures†

Porous carbon materials are crucial components in catalysis, gas storage, electronics, and biochemistry. A hierarchical pore architecture in these materials is essential to achieve high surface areas combined with advanced mass transport kinetics. Widely used approaches for the generation of microor mesopores are activation and nanocasting. In contrast, macroporous carbon materials are primarily obtained by carbonization of polymeric precursor gels or replication of larger templates. A relatively new class of microand mesoporous carbon material with tunable porosity are carbide-derived carbon materials (CDCs). High-temperature chlorination of carbides leads to selective removal of metalor semi-metal atoms and allows control over the pore size of the resulting CDCs in a subngstrcm range by changing synthesis conditions or the carbide precursor. These materials have been studied for applications in gas storage and as electrode materials in supercapacitors because of their high specific surface areas. Recently, metal etching from pyrolyzed pre-ceramic components (polysilsesquioxanes or polysilazanes) was found to be a useful route towards carbide-derived carbon materials with enhanced porosity and gas-storage properties. A significant step towards ultrahigh specific surface area combined with a hierarchical mesoporous–microporous system was achieved using nanocasting of silica templates (SBA-15 or KIT-6) with polycarbosilane precursors and subsequent chlorine treatment of the resulting ordered mesoporous silicon carbides. These ordered mesoporous CDCs offer specific surface areas as high as 2800 mg 1 and total pore volumes of up to 2 cmg . Their mesostructure can be easily controlled by changing the silica hard template, resulting in excellent performance in protein adsorption, gas storage, and as electrodes for supercapacitors. However, such carbon materials are available only as nonstructured micrometer-sized powders and cannot be shaped into films without the addition of binders or the use of high mechanical stress, leading to structural deformation. Chlorine treatment of mechanically mixed Si/SiC precursors was found to be a useful route towards monolithic CDC with a hierarchical pore system. The presence of a free metal phase in the precursor system provides the opportunity to introduce a secondary macroporosity of 3 mm sized channels with a volume of 0.23 cmg 1 along with the microporous carbide-derived carbon material system. The introduction of large transport pores in polymerbased CDCs might be an alternative way to form materials that combine high surface areas with efficient fluid transport. The current literature describes a variety of routes for the production of highly macroporous ceramics from precursor polymers with controllable cell and window sizes. In particular, direct blowing of polycarbosilanes was found to be a useful approach for the generation of silicon carbide foams that might be suitable materials for the production of hierarchical CDCs. In the following, we describe a novel synthesis route for monolithic carbide-derived carbon materials, including micro-, meso-, and macroporous structures with extremely high specific surface area. They can be obtained by hightemperature chlorination of macroporous polymer-derived silicon carbide (SiC-PolyHIPE). A soft-templating approach starting from a high internal phase emulsion (HIPE) was used with an external oil phase consisting of liquid polycarbosilane SMP-10 and the cross-linker paradivinylbenzene. Using Span-80 as surfactant to stabilize the internal water phase, the application of oxidic or carbon hard templates and the corresponding template removal under harsh conditions is no longer necessary. After cross-linking the polymer chains, the resulting PolyHIPEs were pyrolyzed to silicon carbides at maximum temperatures of 700, 800, and 1000 8C and subsequently converted into CDCs by chlorine treatment at the maximum pyrolysis temperature (Supporting [*] M. Oschatz, L. Borchardt, Dr. I. Senkovska, N. Klein, Dr. R. Frind, Prof. Dr. S. Kaskel Department of Inorganic Chemistry Dresden University of Technology Bergstrasse 66, 01062 Dresden (Germany) E-mail: stefan.kaskel@chemie.tu-dresden.de

Integration of accessible secondary metal sites into MOFs for H2S removal

2,2′-Bipyridine-5,5′-dicarboxylic acid (H2bipy) was used for the synthesis of a zirconium based metal–organic framework with the aim to integrate an additional N-donor coordination functionality into the framework. The resulting MOF (UiO-67(bipy)) with a structure isoreticular to UiO-67 (UiO – University of Oslo) has a BET surface area up to 2500 m2 g−1. The chelating bipyridine moiety was used for postsynthetic functionalization of the MOF with different metal salts (MIIXy; M = Cu2+, Ni2+, Co2+; X = Cl−, NO3−, SO42−, acac2−) to give the corresponding metal_salt@UiO-67(bipy) materials. Due to the highly dispersed metal ions, metal_salt@UiO-67(bipy) shows good performance in toxic hydrogen sulfide capture. Especially, the copper loaded samples have high capacities of up to 7.8 wt%.

Evolution of porosity in carbide-derived carbon aerogels

Carbide-derived carbon (CDC) aerogel monoliths with very high porosity are synthesized starting from polymeric precursors. Cross-linking by platinum-catalyzed hydrosilylation of polycarbosilanes followed by supercritical drying yields preceramic aerogels. After ceramic conversion and silicon extraction in hot chlorine gas, hierarchically porous carbon materials with specific surface areas as high as 2122 m2 g−1 and outstanding total pore volumes close to 9 cm3 g−1 are obtained. Their pore structure is controllable by the applied synthesis temperature as shown by combined nitrogen (−196 °C) and carbon dioxide (0 °C) measurements coupled with electron microscopic methods. The combination of large micropore volumes and the aerogel-type pore system leads to advanced adsorption properties due to a combination of large storage capacities and effective materials transport in comparison with purely microporous reference materials as shown by thermal response measurements.

Advanced structural analysis of nanoporous materials by thermal response measurements.

Thermal response measurements based on optical adsorption calorimetry are presented as a versatile tool for the time-saving and profound characterization of the pore structure of porous carbon-based materials. This technique measures the time-resolved temperature change of an adsorbent during adsorption of a test gas. Six carbide and carbon materials with well-defined nanopore architecture including micro- and/or mesopores are characterized by thermal response measurements based on n-butane and carbon dioxide as the test gases. With this tool, the pore systems of the model materials can be clearly distinguished and accurately analyzed. The obtained calorimetric data are correlated with the adsorption/desorption isotherms of the materials. The pore structures can be estimated from a single experiment due to different adsorption enthalpies/temperature increases in micro- and mesopores. Adsorption/desorption cycling of n-butane at 298 K/1 bar with increasing desorption time allows to determine the pore structure of the materials in more detail due to different equilibration times. Adsorption of the organic test gas at selected relative pressures reveals specific contributions of particular pore systems to the increase of the temperature of the samples and different adsorption mechanisms. The use of carbon dioxide as the test gas at 298 K/1 bar provides detailed insights into the ultramicropore structure of the materials because under these conditions the adsorption of this test gas is very sensitive to the presence of pores smaller than 0.7 nm.

Direct synthesis of carbide-derived carbon monoliths with hierarchical pore design by hard-templating

Carbide-derived carbon Monoliths (CDC-Ms) containing a multimodal arrangement with high volumes of micro- meso- and macropores are prepared by direct nanocasting of silica monoliths with polycarbosilane precursors. CDC-Ms show well-defined pore structures along with specific surface areas of more than 2600 m2 g−1 and overall pore volumes as high as 3.14 cm3 g−1. They exhibit advanced gas filtration properties compared to purely microporous materials due to enhanced storage capacities and kinetics as demonstrated by thermal response measurements based on InfraSORP technology.

In-Line Plasma-Chemical Etching of Crystalline Silicon Solar Wafers at Atmospheric Pressure

Atmospheric pressure plasma technologies are a potential substitution for wet chemical and vacuum processes in the production of crystalline silicon solar cells, leading to a simplified in-line processing chain. In this contribution, a plasma chemical etching technology is presented as a basic step for a future continuous production process. A linearly extended DC arc discharge is used for activation of the etching gases. An Ar-N2 mixture is fed through the plasma source; etch gases are injected into the afterglow plasma, near the substrate. Controlled purge gas systems prevent the contamination of the reaction zone with air or moisture as well as the release of reaction products. The plasma source was studied by a noninvasive in-line monitoring of the DC arc phenomena. Silicon etching rates can be controlled by the etch gas composition and the plasma conditions. Fourier Transform infrared spectroscopy of the waste gas was applied to monitor the current etching rates. First industrial tests confirmed standard efficiencies of the cells after edge isolation by atmospheric pressure plasma etching.