Michael Hirscher

Planar Metamaterial Analogue of Electromagnetically Induced Transparency for Plasmonic Sensing

We experimentally demonstrate a planar metamaterial analogue of electromagnetically induced transparency at optical frequencies. The structure consists of an optically bright dipole antenna and an optically dark quadrupole antenna, which are cut-out structures in a thin gold film. A pronounced coupling-induced reflectance peak is observed within a broad resonance spectrum. A metamaterial sensor based on these coupling effects is experimentally demonstrated and yields a sensitivity of 588 nm/RIU and a figure of merit of 3.8.

Desorption of hydrogen from blowing agents used for foaming metals

Abstract Hydrogen desorption from TiH2, ZrH2, and MgH2 was studied by thermal desorption spectroscopy (TDS), differential thermal analysis (DTA) and thermogravimetric analysis (TGA). Loose hydride powders as well as powder compacts of zinc and various hydrides were studied. It was found that during the powder compaction process free surfaces on the hydride powder particles were created. As a consequence, the desorption temperature of the hydride in the precursor was lowered in comparison to loose powder exposed to air. Foam expansion of zinc was highest for TiH2 which also exhibits the highest desorption rate at the melting point of zinc, followed by ZrH2 and MgH2 which decompose at lower temperatures and are therefore less effective for foaming. The desorption kinetics of Al and AlSi7 compacts containing TiH2 were also studied for matters of comparison. The much lower foam expansions compared to Zn foams could be explained by higher hydrogen losses at temperatures below the melting point of Al and AlSi7.

Heat of Adsorption for Hydrogen in Microporous High‐Surface‐Area Materials

The limited resources of fossil fuels will soon require a change to renewable energies. The ideal energy carrier for mobile applications is hydrogen, but the problem of an adequate and safe storage system is still unsolved. One of the possibilities for hydrogen storage is physisorption in porous materials. The big advantage of molecular hydrogen storage is that short refuelling times can be realized due to the extremely fast kinetics. Additionally, no extra heat management is needed as the heat of adsorption is lower than for other storage processes, for example in metal hydrides. On the other hand, owing to this low heat of adsorption, a cryosystem is needed to reach high storage capacities. Microporous materials possessing a high specific surface area (SSA), for example carbon nanotubes, activated carbon, zeolites, and coordination polymers or metal–organic frameworks (MOFs), show a high hydrogen uptake at low temperatures, typically 77 K. Characteristic for this cryo-adsorption in porous materials is that the maximum hydrogen uptake at high pressures depends linearly on the SSA of the material. At low pressures distinct differences in the hydrogen uptake exist for the different materials. Therefore, the pressure to reach, for example, 80% of the maximum storage capacity, differs strongly between the materials. For all materials the hydrogen uptake decreases with increasing temperature. The strength of this decrease as well as the hydrogen uptake at low pressures are governed by the heat of adsorption. Typically, the isosteric heat of adsorption is calculated from the adsorption isotherms measured at 77 K and 87 K since these temperatures can easily be realized by liquid nitrogen and liquid argon, respectively. This small temperature range leads to a very high uncertainty in the heat of adsorption. Only in a few publications is the isosteric heat of adsorption determined with higher accuracy from several isotherms measured at various temperatures. Herein, we present hydrogen adsorption isotherms measured over a wide temperature (77–298 K) and pressure ACHTUNGTRENNUNG(0–20 bar) range. This allows the determination of the heat of adsorption for a wide range of surface coverage with very high accuracy. For the first time different microporous materials have been investigated systematically and their heats of adsorption are correlated to the structures of the materials. Two activated carbon samples, Norit R0.8 and Takeda 4A, with BET SSAs of 1384 mg 1 and 397 mg 1 respectively, and four different metal–organic frameworks, MOF-5, Cu-BTC, MIL-53 and MIL-101, with SSAs between 902 mg 1 and 3293 mg 1 have been investigated. The hydrogen uptake was measured with an automated Sieverts’ apparatus (PCTPro2000, HyEnergy, USA). Figure 1 shows the dependence of the hydrogen uptake on the pressure as an example for Cu-BTC (for other materials, see the Supporting Information). For all materials the isotherms at

Desorption Studies of Hydrogen in Metal–Organic Frameworks†

The diameter is decisive: Adsorption sites for hydrogen in the metal-organic frameworks CuBTC, MIL-53, MOF-5, and IRMOF-8 could be identified by using thermal desorption spectroscopy at very low temperatures (see graph). The correlation between the desorption spectra and the pore structure of these MOFs shows that at high hydrogen concentrations the diameter of the cavity determines the heat of adsorption.

Hydrogen desorption properties of mechanically alloyed MgH2 composite materials

Abstract Composite H storage materials were produced by mechanical alloying of MgH 2 with metallic additives La(Ni 0.7 Fe 0.3 ) 5 , Pd 3 Fe and (Fe 0.8 Mn 0.2 )Ti and with the non-metallic additive Si. The H desorption properties of these alloys were investigated by thermal desorption spectroscopy. The desorption spectra yielded maximum H desorption rates at about 500 K for MgH 2 +La(Ni 0.7 Fe 0.3 ) 5 , between 510 K and 560 K for MgH 2 +Pd 3 Fe and at about 570 K for MgH 2 +(Fe 0.8 Mn 0.2 )Ti, compared to a maximum at about 720 K for pure MgH 2 powder. These results showed that composite materials with metallic additives reveal enhanced desorption kinetics. In contrast, no shift of the desorption maximum to lower temperatures was obtained by mechanically alloying MgH 2 +Si. Scanning electron microscopy investigations showed similar microstructures for all composite materials, with MgH 2 covering the additive particles like a thin film of irregular thickness. Owing to these results, a model for the H desorption process was developed with a catalytic process at the MgH 2 –additive interface playing a major role.

Hydrogen Storage by Cryoadsorption in Ultrahigh‐Porosity Metal–Organic Frameworks

Limited fossil fuel resources and the environmental impact of their use require a change to renewable energy sources in the near future. Owing to the fluctuating supply of renewable energy, the key problem to be solved for this change is energy storage. For applications in transportation, in particular, an efficient energy carrier is needed that can be produced and used in a closed cycle. Presently, hydrogen is the only energy carrier that can be produced easily in large amounts and on an appropriate timescale. Electric energy, from either solar or wind power, or from future fusion reactors, can be used to produce hydrogen from water by electrolysis. The combustion of hydrogen in either an internal combustion engine or a fuel cell generates only water, and the cycle is closed (for a comprehensive overview see Ref. [1]). Hydrogen has the highest gravimetric energy density of all chemical fuels; however, the volumetric density is very low since hydrogen is gaseous under normal conditions down to its boiling point at 20 K. An efficient and safe means of hydrogen storage is thus the bottleneck for the commercialization of fuel-cell-driven vehicles since storage in either liquid form or under high pressure has severe disadvantages. Ideal would be the storage of hydrogen in lightweight solids. There are two principal approaches: 1) the chemical bonding of hydrogen as a hydride, in other words, chemisorption, and 2) the adsorption of hydrogen molecules on surfaces, in other words, physisorption. Owing to the formation of either metallic, ionic, or covalent bonds in hydrides, the interaction energy for chemisorbed hydrogen is typically quite high, leading to a high heat evolution during absorption, which limits fast refueling. Furthermore, hydrides are either too heavy or require high temperatures for hydrogen release. The physisorption of hydrogen molecules is a rapid process; however, owing to the weak van der Waals forces, high storage capacities can be achieved only at low temperatures. Typically these are cryogenic temperatures between 60 and 120 K and, therefore, this kind of hydrogen storage by physically adsorbed hydrogen molecules on a porous material is called cryoadsorption. Nevertheless, from the viewpoint of reversibility and fast refueling times this cryoadsorption has great potential to be used in hydrogen-storage devices. One key to reaching high storage capacity by cryoadsorption is a high specific surface area. The maximum hydrogen uptake at low temperature was found to be linearly dependent on the specific surface area of carbonaceous materials. The best of these carbonaceous materials are activated carbons with a surface area of slightly over 3000 m g . For a further improvement of the storage capacity, materials with even higher surface areas accessible for hydrogen molecules are needed. Metal–organic frameworks (MOFs) are a new class of crystalline materials exhibiting extremely high porosity, which immediately attracted great attention as potential gas-storage materials. MOFs are crystalline solids composed of inorganic subunits, for example, metal oxide clusters, and rigid organic linkers. These building blocks can be used to design an almost infinite variety of frameworks with tunable and well-defined pore structures, extremely high specific surface areas, and no dead volume, in contrast to zeolites. Soon after the first synthesis of these novel porous materials, some high-surfacearea MOFs were reported to display hydrogen-storage capacities similar to the best activated carbons. Two research groups independently found that also in the case of MOFs a linear correlation exists between the hydrogen uptake at 77 K and the specific surface area. 5] Therefore, one means of increasing the storage capacity is to generate larger specific surface areas. Last year the group of Kaskel succeeded in synthesizing a new mesoporous framework by joining {Zn4O(CO2)6} units through 4,4’,4’’-benzene-1,3,5-triyl-tribenzoate (BTB) 2,6-naphthalenedicarboxylate (NDC) linkers; the new framework material, DUT-6, was named after the Dresden University of Technology. A recent publication by Yaghi et al. described a further step towards even higher specific surface areas. They prepared a whole series of new mesoporous MOFs, including DUT-6 (renamed MOF-205). In this new series the highest Brunauer–Emmett–Teller (BET) specific surface area of 6240 m g 1 is exhibited by MOF-210, which is composed of {Zn4O(CO2)6} units and 4,4’,4’’-(benzene-1,3,5-triyl-tris(ethyne-2,1-diyl))tribenzoate (BTE) and biphenyl-4,4’-dicarboxylate (BPDC) linkers. This extremely high surface area may be very close to the ultimate limit possible for porous structures. Therefore, MOF-210 shows the highest excess hydrogen uptake of 86 mgg 1 ever observed for physisorption [*] Dr. M. Hirscher Max-Planck-Institut f r Metallforschung Heisenbergstrasse 3, 70569 Stuttgart (Germany) Fax: (+ 49)711-689-1952 E-mail: hirscher@mf.mpg.de

A fluorene based covalent triazine framework with high CO2 and H2 capture and storage capacities

Porous organic polymers have come into focus recently for the capture and storage of postcombusted CO2. Covalent triazine frameworks (CTFs) constitute a nitrogen-rich subclass of porous polymers, which offers enhanced tunability and functionality combined with high chemical and thermal stability. In this work a new covalent triazine framework based on fluorene building blocks is presented, along with a comprehensive elucidation of its local structure, porosity, and capacity for CO2 capture and H2 storage. The framework is synthesized under ionothermal conditions at 300–600 °C using ZnCl2 as a Lewis acidic trimerization catalyst and reaction medium. Whereas the materials synthesized at lower temperatures mostly feature ultramicropores and moderate surface areas as probed by CO2 sorption (297 m2 g−1 at 300 °C), the porosity is significantly increased at higher synthesis temperatures, giving rise to surface areas in excess of 2800 m2 g−1. With a high fraction of micropores and a surface area of 1235 m2 g−1, the CTF obtained at 350 °C shows an excellent CO2 sorption capacity at 273 K (4.28 mmol g−1), which is one of the highest observed among all porous organic polymers. Additionally, the materials have CO2/N2 selectivities of up to 37. The hydrogen adsorption capacity of 4.36 wt% at 77 K and 20 bar is comparable to that of other POPs, yet the highest among all CTFs studied to date.

Metal@COFs: Covalent Organic Frameworks as Templates for Pd Nanoparticles and Hydrogen Storage Properties of Pd@COF‐102 Hybrid Material

Three-dimensional covalent organic frameworks (COFs) have been demonstrated as a new class of templates for nanoparticles. Photodecomposition of the [Pd(η(3)-C(3) H(5))(η(5)-C(5)H(5))]@COF-102 inclusion compound (synthesized by a gas-phase infiltration method) led to the formation of the Pd@COF-102 hybrid material. Advanced electron microscopy techniques (including high-angle annular dark-field scanning transmission electron microscopy and electron tomography) along with other conventional characterization techniques unambiguously showed that highly monodisperse Pd nanoparticles ((2.4±0.5) nm) were evenly distributed inside the COF-102 framework. The Pd@COF-102 hybrid material is a rare example of a metal-nanoparticle-loaded porous crystalline material with a very narrow size distribution without any larger agglomerates even at high loadings (30 wt %). Two samples with moderate Pd content (3.5 and 9.5 wt %) were used to study the hydrogen storage properties of the metal-decorated COF surface. The uptakes at room temperature from these samples were higher than those of similar systems such as Pd@metal-organic frameworks (MOFs). The studies show that the H(2) capacities were enhanced by a factor of 2-3 through Pd impregnation on COF-102 at room temperature and 20 bar. This remarkable enhancement is not just due to Pd hydride formation and can be mainly ascribed to hydrogenation of residual organic compounds, such as bicyclopentadiene. The significantly higher reversible hydrogen storage capacity that comes from decomposed products of the employed organometallic Pd precursor suggests that this discovery may be relevant to the discussion of the spillover phenomenon in metal/MOFs and related systems.

MFU‐4 – A Metal‐Organic Framework for Highly Effective H2/D2 Separation

The metal-organic framework, MFU-4, possessing small cavities and apertures, is exploited for quantum sieving of hydrogen isotopes. Quantum mechanically, a molecule confined in a small cavity shows an increase in effective size depending on the particle mass, which leads to a faster deuterium adsorption from a H(2)/D(2) isotope mixture.

A Cryogenically Flexible Covalent Organic Framework for Efficient Hydrogen Isotope Separation by Quantum Sieving

Conventional molecular sieves are often utilized in industryfor the purification of gas mixtures consisting of differentmolecular sizes. However, separation of hydrogen isotopesrequires special efforts because of their identical size, shape,and thermodynamic properties. Separation of isotope mix-tures is only possible with limited techniques, such ascryogenic distillation, thermal diffusion, and the Girdlersulfide process, but these methods have a low separationfactor and are excessively time- and energy-intensive.