Fabian Gramm

Structure of the polycrystalline zeolite catalyst IM-5 solved by enhanced charge flipping

Despite substantial advances in crystal structure determination methodology for polycrystalline materials, some problems have remained intractable. A case in point is the zeolite catalyst IM-5, whose structure has eluded determination for almost 10 years. Here we present a charge-flipping structure-solution algorithm, extended to facilitate the combined use of powder diffraction and electron microscopy data. With this algorithm, we have elucidated the complex structure of IM-5, with 24 topologically distinct silicon atoms and an unusual two-dimensional medium-pore channel system. This powerful approach to structure solution can be applied without modification to any type of polycrystalline material (e.g., catalysts, ceramics, pharmaceuticals, complex metal alloys) and is therefore pertinent to a diverse range of scientific disciplines.

Complex zeolite structure solved by combining powder diffraction and electron microscopy

Many industrially important materials, ranging from ceramics to catalysts to pharmaceuticals, are polycrystalline and cannot be grown as single crystals. This means that non-conventional methods of structure analysis must be applied to obtain the structural information that is fundamental to the understanding of the properties of these materials. Electron microscopy might appear to be a natural approach, but only relatively simple structures have been solved by this route. Powder diffraction is another obvious option, but the overlap of reflections with similar diffraction angles causes an ambiguity in the relative intensities of those reflections. Various ways of overcoming or circumventing this problem have been developed, and several of these involve incorporating chemical information into the structure determination process. For complex zeolite structures, the FOCUS algorithm has proved to be effective. Because it operates in both real and reciprocal space, phase information obtained from high-resolution transmission electron microscopy images can be incorporated directly into this algorithm in a simple way. Here we show that by doing so, the complexity limit can be extended much further. The power of this approach has been demonstrated with the solution of the structure of the zeolite TNU-9 (|H9.3&Al9.3Si182.7O384]; ref. 10) with 24 topologically distinct (Si,Al) atoms and 52 such O atoms. For comparison, ITQ-22 (ref. 11), the most complex zeolite known to date, has 16 topologically distinct (Si,Ge) atoms.

Amyloid-mediated synthesis of giant, fluorescent, gold single crystals and their hybrid sandwiched composites driven by liquid crystalline interactions

We report for the first time on the templating effect of β-lactoglobulin amyloid-like fibrils to synthesize gold single crystals of several decades of μm in dimensions. The gold single crystals were produced by reducing an aqueous solution of chloroauric acid by β-lactoglobulin amyloid protein fibrils. Atomic force microscopy, conventional and scanning transmission electron microscopy, electron diffraction and optical microscopy techniques were combined to characterize the structure of the gold crystals. The single-crystalline features of these macroscopic gold crystals are witnessed by their distinctive hexagonal and triangular shape and are confirmed by selected area electron diffraction (SAED). UV-vis absorption spectrum, recorded after a reaction time of 6h at the heating temperature of 55°C showed a surface plasmon resonance peak at 540 nm. With the increase of reaction time to 24h, the absorption spectrum peaks shift to a very broad and higher wavelength region extending up to near infrared region. Remarkably, these single crystalline gold crystals show auto fluorescence when illuminated to UV lamp. Further increase in β-lactoglobulin amyloid fibrils concentration above the isotropic-nematic transition, drives the formation of gold single crystals microplates stacking together and self-assembling into new hierarchical, layered protein-gold hybrid composites.

Structure determination of the zeolite IM-5 using electron crystallography

Abstract The structure of the complex zeolite IM-5 (Cmcm, a = 14.33(4) Å, b = 56.9(2) Å, c = 20.32(7) Å) was determined by combining selected area electron diffraction (SAED), 3D reconstruction of high resolution transmission electron microscopy (HRTEM) images from different zone axes and distance least squares (DLS) refinement. The unit cell parameters were determined from SAED. The space group was determined from extinctions in the SAED patterns and projection symmetries of HRTEM images. Using the structure factor amplitudes and phases of 144 independent reflections obtained from HRTEM images along the [100], [010] and [001] directions, a 3D electrostatic potential map was calculated by inverse Fourier transformation. From this 3D potential map, all 24 unique Si positions could be determined. Oxygen atoms were added between each Si–Si pair and further refined together with the Si positions by distance-least-squares. The final structure model deviates on average 0.16 Å for Si and 0.31 Å for O from the structure refined using X-ray powder diffraction data. This method is general and offers a new possibility for determining the structures of zeolites and other materials with complex structures.

Room-temperature transverse-electric polarized intersubband electroluminescence from InAs/AlInAs quantum dashes

We report the observation of transverse electric polarized electroluminescence from InAs/AlInAs quantum dash quantum cascade structures up to room temperature. The emission is attributed to the electric field confined along the shortest lateral dimension of the dashes, as confirmed by its dependence on crystallographic orientation both in absorption measurements on a dedicated sample and from electroluminescence itself. From the absorption, we estimate a dipole moment for the observed transition of 〈x〉 = 1.7 nm. The electroluminescence is peaked at around 110 meV and increases with applied bias. Its temperature dependence shows a decrease at higher temperatures limited by optical phonon emission.

Synthesis factors affecting the catalytic performance and stability of Ru/C catalysts for supercritical water gasification

Catalytic supercritical water gasification of isopropanol (450 °C, 30 MPa) over Ru/C catalysts was carried out in a fixed-bed plug flow reactor. In the absence of Ru, isopropanol decomposed to solid carbon (coke), and H2 over the carbon support. The Ru/C catalyst was able to gasify efficiently 10 wt% isopropanol over a period of 96 h at WHSVgRu = 1228 gOrg gRu−1 h−1 with the gas composition close to the calculated thermodynamic chemical equilibrium. The catalyst lifetime was affected by the decomposition of isopropanol to solid carbon (coke) over the carbon surface that progressively filled up the pores of the activated carbon and this resulted in a covering of the Ru nanoparticles (NPs). The Ru dispersion (D) was found to be a relevant parameter. The 0.5% Ru/C (D = 0.26) was more active than the 2% Ru/C (D = 0.14). The influence of the solvent (acetone vs. water) used during the catalyst impregnation was studied and the turnover frequency (TOF) was twice as high for the Ru/C catalyst prepared with acetone. The higher Ru dispersion and the lower content of residual chloride obtained for the catalyst prepared with acetone were both responsible.

TNU-9: a novel medium-pore zeolite with 24 topologically distinct tetrahedral sites

The synthesis, characterization, and catalytic properties of the new medium-pore zeolite TNU-9 are described.

Enhanced current injection from a quantum well to a quantum dash in magnetic field

Resonant tunneling injection is a key ingredient in achieving population inversion in a putative quantum dot cascade laser. In a quantum dot based structure, such resonant current requires a matching of the wavefunction shape in k-space between the injector and the quantum dot. We show experimentally that the injection into an excited state of a dash structure can be enhanced tenfold by an in-plane magnetic field that shifts the injector distribution in k-space. These experiments, performed on resonant tunneling diode structures, show unambiguously resonant tunneling into an ensemble of InAs dashes grown between two AlInAs barrier layers. They also show that interface roughness scattering can enhance the tunneling current.

Zeolite structure determination using electron crystallography

Abstract The structures of zeolite beta polymorph B and of IM-5 have been determined by combining selected area electron diffraction (SAED) patterns and high resolution transmission electron microscopy (HRTEM) images. The zeolite beta polymorph B structure could be solved using just a single HRTEM image while that of IM-5 required three images taken along the main crystallographic directions. The average deviations of the IM-5 model from that refined using X-ray powder diffraction data are about 0.16 A for Si and 0. 31A for O.

Effect of carbon surface functional groups on the synthesis of Ru/C catalysts for supercritical water gasification

A carbon support was treated with HNO3 to create surface functional groups (e.g. –COOH, –OH), which were then characterized by TGA, TPD, CNS elemental analysis, and Boehm titration. HNO3 modified the carbon surface properties by adding a high amount of carboxylic groups, improved the thermal stability of the carbon support, and reduced ca. 50% of the ash. The thermal pre-treatment (723 K under He) following the HNO3 pre-treatment successfully removed the carboxylic groups. 4% Ru/C catalysts were synthesized using the surface-modified carbon supports and characterized by H2-TPR, CO pulse chemisorption, N2-physisorption and HAADF-STEM. The Ru dispersion was increased in the presence of the carboxylic groups. Catalytic supercritical water gasification (CSCWG) of 10 wt.% isopropanol over the 4% Ru/C catalysts was carried out at 723 K and 30 MPa for 50 hours to assess the performance of the catalysts. It was found that the Ru/C catalyst prepared involving a pre-treatment with HNO3 did not exhibit a higher catalytic activity than the catalyst whose carbon support was not pre-treated with HNO3. Hence, the activity and the selectivity during CSCWG were not influenced by the pre-treatment of the catalyst support with HNO3.