Wilder Carrillo-Cabrera

ZSM-5 Zeolite Single Crystals with b-Axis-Aligned Mesoporous Channels as an Efficient Catalyst for Conversion of Bulky Organic Molecules

The relatively small and sole micropores in zeolite catalysts strongly influence the mass transfer and catalytic conversion of bulky molecules. We report here aluminosilicate zeolite ZSM-5 single crystals with b-axis-aligned mesopores, synthesized using a designed cationicamphiphilic copolymer as a mesoscale template. This sample exhibits excellent hydrothermal stability. The orientation of the mesopores was confirmed by scanning and transmission electron microscopy. More importantly, the b-axis-aligned mesoporous ZSM-5 shows much higher catalytic activities for bulky substrate conversion than conventional ZSM-5 and ZSM-5 with randomly oriented mesopores. The combination of good hydrothermal stability with high activities is important for design of novel zeolite catalysts. The b-axis-aligned mesoporous ZSM-5 reported here shows great potential for industrial applications.

Hydrogels and aerogels from noble metal nanoparticles.

Aerogels are fine inorganic superstructures with enormously high porosity and are known to be exceptional materials with a variety of applications, for example in the area of catalysis. The chemistry of the aerogel synthesis originated from the pioneering work from the early 1930s and was further developed starting from the 1960s. Attractive catalytic, thermoresistant, piezoelectric, antiseptic, and many other properties of the aerogels originate from the unique combination of the specific properties of nanomaterials magnified by macroscale self-assembly. Currently, the most investigated materials that form fine aerogel superstructures are silica and other metal oxides together with their mixtures. Recently, the possibility of creating aerogels and even light-emitting monoliths with densities 500 times less than their bulk counterparts from colloidal quantum dots and clusters of metal chalcogenides has attracted attention. These developments may open opportunities in areas such as semiconductor technology, photocatalysis, optoelectronics, and photonics. Quite a number of different approaches have focused on modifying oxide-based aerogels (silica, titania, alumina, etc.) with metal nanoparticles (such as of platinum) to carry the catalytic properties from the metal 15] into the porous structures of the aerogels. 16,17] Fine mesoporous assemblies of catalytically active metal nanoparticles were also created by using artificial opals and fungi as templates. Other superstructural materials derived from metal nanoparticles include mesoporous platinum–carbon composites, gold nanoparticles interlinked with dithiols, necklace nanochains of hybrid palladium–lipid nanospheres, electrocatalytically active nanoporous platinum aggregates, foams, and highly ordered twoand three-dimensional supercrystals. The creation of non-supported metal aerogels has however not been reported to date. Recently, the formation of highly porous spherical aggregates (“supraspheres”) of several hundred nanometers in diameter, where nanoparticles from one or two different metals were cross-linked with dithiols, was reported. 31] The metal aerogels presented herein exhibit an average density two orders of magnitude lower than that of the reported foams. Their primary structural units match the size range of single nanoparticles (5–20 nm), which is an order of magnitude smaller than that of the self-assembled supraspheres. Moreover, in the present case, no chemical cross-linkers are involved in the self-assembly process. The formation of such noble-metal nanoparticle-based mesoporous monometallic and bimetallic aerogels is an important step towards self-supported monoliths with enormously high catalytically active surfaces. Considering that metal nanoparticles possess very specific optical properties owing to their pronounced surface plasmon resonance, aerogels from metal nanoparticles may also find future applications in nanophotonics, for example, as advanced optical sensors and ultrasensitive detectors. In terms of size, shape, and composition control, the synthesis of colloidal metallic nanoparticles is nowadays a well-developed research field. For gel formation, various methods of slow destabilization, developed previously for quantum-dot-based gels, were systematically applied to aqueous colloidal solutions of gold, silver, and platinum nanoparticles. Supercritical drying of the hydrogels with liquid CO2 finally produces aerogels. Aqueous colloidal metal solutions are normally very stable in the dilute as-prepared state (below ca. 10 m particle concentration). To gelate these sols, efficient destabilization is initiated by concentrating the sols (see the Supporting Information). Gel formation is achieved by, for example, the addition of ethanol or hydrogen peroxide to the concentrated colloids. Different morphologies of the gels can be obtained depending on the type and amount of destabilizer, and also on the metal colloid. Figure 1 shows scanning electron microscopy (SEM; A and B) and transmission electron microscopy (TEM) images (C and D) of an aerogel manufactured from platinum nanoparticles with the use of ethanol as [*] A.-K. Herrmann, M. Vogel, Dr. N. Gaponik, Prof. Dr. A. Eychm ller Physical Chemistry/Electrochemistry, TU Dresden 01062 Dresden (Germany) Fax: (+ 49)351-37164 E-mail: Alexander.eychmueller@chemie.tu-dresden.de Homepage: http://www.chm.tu-dresden.de/pc2/index.shtml

The Samson phase, β-Mg2Al3, revisited

Co-Authors: Michael Feuerbacher, Carsten Thomas, Julien P. A. Makongo, Stefan Hoffmann, Wilder Carrillo-Cabrera, Raul Cardoso, Yuri Grin, Guido Kreiner, Jean-Marc Joubert, Thomas Schenk, Joseph Gastaldi, Henri Nguyen-Thi, Nathalie Mangelinck-Noël, Bernard Billia, Patricia Donnadieu, Aleksandra Czyrska-Filemonowicz, Anna Zielinska-Lipiec, Beata Dubiel, Thomas Weber, Philippe Schaub, Günter Krauss, Volker Gramlich, Jeppe Christensen, Sven Lidin, Daniel Fredrickson, Marek Mihalkovic, Wieslawa Sikora, Janusz Malinowski, Stefan Brühne, Thomas Proffen, Wolf Assmus, Marc de Boissieu, Francoise Bley, Jean-Luis Chemin, Jürgen Schreuer Abstract. The Al−Mg phase diagram has been reinvestigated in the vicinity of the stability range of the Samson phase, β-Mg2Al3 (cF1168). For the composition Mg 38.5 Al 61.5, this cubic phase, space group Fd-3m (no 227), a = 28.242(1) Å, V = 22526(2) Å3, undergoes at 214 °C a first-order phase transition to rhombohedral β′-Mg2Al3(hR293), a = 19.968(1) Å, c = 48.9114(8) Å, V = 16889(2) Å3, (i.e. 22519 Å3 for the equivalent cubic unit cell) space group R3m (no 160), a subgroup of index four of Fd-3m. The structure of the β-phase has been redetermined at ambient temperature as well as in situ at 400 °C. It essentially agrees with Samsons model, even in most of the many partially occupied and split positions. The structure of β′-Mg2Al3is closely related to that of the β-phase. Its atomic sites can be derived from those of the β-phase by group-theoretical considerations. The main difference between the two structures is that all atomic sites are fully occupied in case of the β′-phase. The reciprocal space, Bragg as well as diffuse scattering, has been explored as function of temperature and the β- to β′-phase transition was studied in detail. The microstructures of both phases have been analyzed by electron microscopy and X-ray topography showing them highly defective. Finally, the thermal expansion coefficients and elastic parameters have been determined. Their values are somewhere in between those of Al and Mg.

New Monoclinic Phase at the Composition Cu2SnSe3 and Its Thermoelectric Properties

A new monoclinic phase (m2) of ternary diamond-like compound Cu2SnSe3 was synthesized by reaction of the elements at 850 K. The crystal structure of m2-Cu2SnSe3 was determined through electron diffraction tomography and refined by full-profile techniques using synchrotron X-ray powder diffraction data (space group Cc, a = 6.9714(2) Å, b = 12.0787(5) Å, c = 13.3935(5) Å, β = 99.865(5)°, Z = 8). Thermal analysis and annealing experiments suggest that m2-Cu2SnSe3 is a low-temperature phase, while the high-temperature phase has a cubic crystal structure. According to quantum chemical calculations, m2-Cu2SnSe3 is a narrow-gap semiconductor. A study of the chemical bonding, applying the electron localizability approach, reveals covalent polar Cu-Se and Sn-Se interactions in the crystal structure. Thermoelectric properties were measured on a specimen consolidated using spark plasma sintering (SPS), confirming the semiconducting character. The thermoelectric figure of merit ZT reaches a maximum value of 0.33 at 650 K.

Crystal structure and transport properties of Ba8Ge43□3

The single phase clathrate-I Ba(8)Ge(43)square(3) (space group Ia3d (no. 230), a = 21.307(1) A) was synthesized by quenching the melt between cold steel plates. Specimens for physical property measurements were characterized by microstructure analysis and X-ray diffraction on polycrystalline samples as well as single crystals. Transport properties including thermopower, electrical resistivity, thermal conductivity and specific heat were investigated in a temperature range of 2-673 K. The electrical resistivity exhibits a metal-like temperature dependence below 300 K turning into a semiconductor-like behaviour above 300 K. The analysis of the specific heat at low temperature indicates a finite density of states at the Fermi level, thus corroborating the metallic character below 300 K. The temperature dependence of the specific heat was modelled assuming Einstein-like localized vibrations of Ba atoms inside the cages of the Ge framework. A conventional crystal-like behaviour of the thermal conductivity with a low lattice contribution (kappa(l)(300 K) = 2.7 W m(-1) K(-1)) has been evidenced.

Are type-I clathrates Zintl phases and 'phonon glasses and electron single crystals'?

Abstract We discuss to which extent the concepts of Zintl phases and of ‘phonon glasses and electron single crystals’ apply to type-I clathrates. In (β-) Eu 8 Ga 16 Ge 30 the presence of residual charge carriers appears to be related to a slight off-stoichiometry of the samples pointing to the validity of the Zintl concept in stoichiometric samples. The low and almost stoichiometry independent mobilities of (β-) Eu 8 Ga 16 Ge 30 , Sr 8 Ga 16 Ge 30 , and Ba 8 Ga 16 Ge 30 seriously question the validity of the ‘electron single crystal’ concept for type-I clathrates. The temperature dependence of the thermal conductivity of a Ba 8 Ga 16 Ge 30 single crystal indicates that tunneling states play a central role in producing ‘phonon glass’-like thermal conductivities.

Shape development and structure of a complex (otoconia-like?) calcite-gelatine composite

A large number of recent publications deal with control of the size and shape of calcium carbonate in its calcite modification by organic additives acting as growth modifiers or templates. Other reports focus not only on shape control but also on the control of the calcium carbonate modification formed. Only a few publications concentrate on calcite specimens showing a habit that is more or less close to the shape of biogenic (calcite) otoconia (see Figure 1), charac-

The Cluster Anion Si94

Solely on the basis of Raman spectra and quantum chemical calculations, the previously unknown cluster anion Si94- (structure shown) was characterized and its structure determined. The anion is formed as a component of solid phases by the thermal decomposition of alkali metal monosilicides.

BaGe5: A New Type of Intermetallic Clathrate

BaGe(5) constitutes a new type of intermetallic clathrate obtained by decomposition of clathrate-I Ba(8)Ge(43)(3) at low temperatures. The crystal structure consists of characteristic layers interconnected by covalent bonds. BaGe(5) is a semiconducting Zintl phase.

EuGa2±xGe4∓x: preparation, crystal chemistry and properties

Abstract The title compound was prepared from a mixture of the elements by melting in a glassy carbon crucible (HF furnace, argon atmosphere). EuGa2±xGe4∓x crystallizes in a new structure type (orthorhombic symmetry, space group Cmcm (no. 63), a=4.1571(3) A, b=11.268(1) A, c=13.155(1) A, V=616.2(2) A3, Z=4, Pearson symbol oC28) and is characterized by a 3D framework of 4-fold bonded (4b) E atoms (E=Ga,Ge) with channels parallel to the short a-axis. The europium atoms lie in the E18 holes of those channels. The structure can also be described as an intergrowth of segments from BaAl2Si2 and from a reconstructed diamond type of structure. The (Ga,Ge)24 framework of the title compound is equivalent to that of the Si(Al) atoms in the zeolite structure of CsAlSi5O12. Magnetic susceptibility measurements show that EuGa2±xGe4∓x undergoes a transition from a paramagnetic to an antiferromagnetic ordered state at about 9 K. In the paramagnetic region, an oxidation state of 2+ for europium (μeff≈8 μB/Eu) was obtained. The crystal chemical formula of EuGa2Ge4 is Eu2+[(4b)Ga1−]2[(4b)Ge0]4 according to a charge balanced Zintl phase. This was confirmed by quantum chemical calculations (TB-LMTO, ELF). However, like several other similar compounds, e.g. alkaline earth metal clathrates, EuGa2±xGe4∓x has a metal-like temperature dependence of the electrical resistivity with a low charge-carrier concentration in agreement with the low value of the calculated density of states.