Claudia Weidenthaler

Spatially and Size Selective Synthesis of Fe-Based Nanoparticles on Ordered Mesoporous Supports as Highly Active and Stable Catalysts for Ammonia Decomposition

Uniform and highly dispersed γ-Fe(2)O(3) nanoparticles with a diameter of ∼6 nm supported on CMK-5 carbons and C/SBA-15 composites were prepared via simple impregnation and thermal treatment. The nanostructures of these materials were characterized by XRD, Mössbauer spectroscopy, XPS, SEM, TEM, and nitrogen sorption. Due to the confinement effect of the mesoporous ordered matrices, γ-Fe(2)O(3) nanoparticles were fully immobilized within the channels of the supports. Even at high Fe-loadings (up to about 12 wt %) on CMK-5 carbon no iron species were detected on the external surface of the carbon support by XPS analysis and electron microscopy. Fe(2)O(3)/CMK-5 showed the highest ammonia decomposition activity of all previously described Fe-based catalysts in this reaction. Complete ammonia decomposition was achieved at 700 °C and space velocities as high as 60,000 cm(3) g(cat)(-1) h(-1). At a space velocity of 7500 cm(3) g(cat)(-1) h(-1), complete ammonia conversion was maintained at 600 °C for 20 h. After the reaction, the immobilized γ-Fe(2)O(3) nanoparticles were found to be converted to much smaller nanoparticles (γ-Fe(2)O(3) and a small fraction of nitride), which were still embedded within the carbon matrix. The Fe(2)O(3)/CMK-5 catalyst is much more active than the benchmark NiO/Al(2)O(3) catalyst at high space velocity, due to its highly developed mesoporosity. γ-Fe(2)O(3) nanoparticles supported on carbon-silica composites are structurally much more stable over extended periods of time but less active than those supported on carbon. TEM observation reveals that iron-based nanoparticles penetrate through the carbon layer and then are anchored on the silica walls, thus preventing them from moving and sintering. In this way, the stability of the carbon-silica catalyst is improved. Comparison with the silica supported iron oxide catalyst reveals that the presence of a thin layer of carbon is essential for increased catalytic activity.

Controllable Synthesis of Mesoporous Peapod‐like Co3O4@Carbon Nanotube Arrays for High‐Performance Lithium‐Ion Batteries

Transition metal oxides are regarded as promising anode materials for lithium-ion batteries because of their high theoretical capacities compared with commercial graphite. Unfortunately, the implementation of such novel anodes is hampered by their large volume changes during the Li(+) insertion and extraction process and their low electric conductivities. Herein, we report a specifically designed anode architecture to overcome such problems, that is, mesoporous peapod-like Co3O4@carbon nanotube arrays, which are constructed through a controllable nanocasting process. Co3O4 nanoparticles are confined exclusively in the intratubular pores of the nanotube arrays. The pores between the nanotubes are open, and thus render the Co3O4 nanoparticles accessible for effective electrolyte diffusion. Moreover, the carbon nanotubes act as a conductive network. As a result, the peapod-like Co3O4 @carbon nanotube electrode shows a high specific capacity, excellent rate capacity, and very good cycling performance.

Co3O4-SiO2 nanocomposite: a very active catalyst for CO oxidation with unusual catalytic behavior

A high surface area Co(3)O(4)-SiO(2) nanocomposite catalyst has been prepared by use of activated carbon as template. The Co(3)O(4)-SiO(2) composite, the surface of which is rich in silica and Co(II) species compared with normal Co(3)O(4), exhibited very high activity for CO oxidation even at a temperature as low as -76 °C. A rather unusual temperature-dependent activity curve, with the lowest conversion at about 80 °C, was observed with a normal feed gas (H(2)O content ~3 ppm). The U-shape of the activity curve indicates a negative apparent activation energy over a certain temperature range, which has rarely been observed for the heterogeneously catalyzed oxidation of CO. Careful investigation of the catalytic behavior of Co(3)O(4)-SiO(2) catalyst led to the conclusion that adsorption of H(2)O molecules on the surface of the catalyst caused the unusual behavior. This conclusion was supported by in situ diffuse reflectance Fourier transform infrared (DRIFT) spectroscopic experiments under both normal and dry conditions.

Investigation of hydrogen discharging and recharging processes of Ti-doped NaAlH4 by X-ray diffraction analysis (XRD) and solid-state NMR spectroscopy

Abstract The processes occurring in the course of two sequential hydrogen discharging and recharging cycles of Ti-doped sodium alanate were investigated in parallel using XRD analysis and solid-state NMR spectroscopy. Both methods demonstrate that in hydrogen storage cycles ( Eq. (1) ) the majority phases involved are NaAlH 4 , Na 3 AlH 6 , Al and NaH. Only traces of other, as yet unidentified phases are observed, one of which has been tentatively assigned to an Al–Ti alloy on the basis of XRD analysis. The unsatisfactory hydrogen storage capacities heretofore observed in cycle tests are shown to be due entirely to the reaction of Na 3 AlH 6 with Al and hydrogen to NaAlH 4 ( Eq. (1) , 2nd hydrogenation step) being incomplete. Using XRD and NMR methods it has been shown that a higher level of rehydrogenation can be achieved by adding an excess of Al powder.

Pseudomorphic Transformation of Highly Ordered Mesoporous Co3O4 to CoO via Reduction with Glycerol

In this report, we describe the exploration of possibilities for a pseudomorphic reduction of ordered mesoporous metal oxides by high temperature treatment with glycerol, which was most thoroughly studied as an example of the reaction of Co3O4 to CoO. It was found that the glycerol process is a gentle reduction procedure, which maintains the framework topology on the mesoscale while changing the oxidation state and the structure at the atomic scale.

Solid-state hydrogen storage for mobile applications: Quo Vadis?

In times of severe shortage of fossil fuels new strategies have to be developed to assure future mobility. Fuel cell driven automotives with hydrogen as an energy carrier is one alternative discussed for the substitution of gasoline in the long term. Both the generation as well as the storage of hydrogen are technical challenges which have to be solved before hydrogen technology can be a real alternative for mobile applications. This perspective paper highlights the state-of-the art in the field of hydrogen storage, especially in solids, including the technical limitations. New potential research fields are discussed which may contribute to future energy supply in niche applications.

Regioselectively Controlled Synthesis of Colloidal Mushroom Nanostructures and Their Hollow Derivatives

In this study, a facile and controllable synthetic route for the fabrication of mushroom nanostructures (Fe(x)O(y)@PSD-SiO(2)) and their hollow derivatives has been established. The synthesis consists of partial coating of Fe(x)O(y) (Fe(3)O(4) or Fe(2)O(3)) with polymer spheres, followed by attaching silica hemispheres. The surface-accessible Fe(x)O(y) nanoparticles on the Janus-type Fe(x)O(y)@PSD nanospheres are key for directing the growth of the silica hemisphere on the Fe(x)O(y)@PSD seeds. The size and the porosity of the silica hemispheres are tunable by adjusting the amount of TEOS used and addition of a proper surfactant in a Stober-type process. After the iron oxide cores were leached out with concentrated HCl, mushroom nanostructures with hollow interiors were obtained, where the morphology of the hollow interior faithfully replicates the shape of the iron oxide core previously filling this void. This synthetic strategy provides a controllable method for the large-scale preparation of asymmetric colloidal nanostructures which could serve as building blocks for the assembly of new types of nanostructures.

High‐Temperature Stable, Iron‐Based Core–Shell Catalysts for Ammonia Decomposition

High-temperature, stable core-shell catalysts for ammonia decomposition have been synthesized. The highly active catalysts, which were found to be also excellent model systems for fundamental studies, are based on α-Fe(2)O(3) nanoparticles coated by porous silica shells. In a bottom-up approach, hematite nanoparticles were firstly obtained from the hydrothermal reaction of ferric chlorides, L-lysine, and water with adjustable average sizes of 35, 47, and 75 nm. Secondly, particles of each size could be coated by a porous silica shell by means of the base-catalyzed hydrolysis of tetraethylorthosilicate (TEOS) with cetyltetramethylammonium bromide (CTABr) as porogen. After calcination, TEM, high-resolution scanning electron microscopy (HR-SEM), energy-dispersive X-ray (EDX), XRD, and nitrogen sorption studies confirmed the successful encapsulation of hematite nanoparticles inside porous silica shells with a thickness of 20 nm, thereby leading to composites with surface areas of approximately 380 m(2)  g(-1) and iron contents between 10.5 and 12.2 wt %. The obtained catalysts were tested in ammonia decomposition. The influence of temperature, iron oxide core size, possible diffusion limitations, and dilution effects of the reagent gas stream with noble gases were studied. The catalysts are highly stable at 750 °C with a space velocity of 120,000 cm(3)  g(cat)(-1)  h(-1) and maintained conversions of around 80 % for the testing period time of 33 h. On the basis of the excellent stability under reaction conditions up to 800 °C, the system was investigated by in situ XRD, in which body-centered iron was determined, in addition to FeN(x), as the crystalline phase under reaction conditions above 650 °C.

Highly Ordered Mesoporous Cobalt-Containing Oxides: Structure, Catalytic Properties, and Active Sites in Oxidation of Carbon Monoxide

Co3O4 with a spinel structure is a very active oxide catalyst for the oxidation of CO. In such catalysts, octahedrally coordinated Co(3+) is considered to be the active site, while tetrahedrally coordinated Co(2+) is assumed to be basically inactive. In this study, a highly ordered mesoporous CoO has been prepared by H2 reduction of nanocast Co3O4 at low temperature (250 °C). The as-prepared CoO material, which has a rock-salt structure with a single Co(2+) octahedrally coordinated by lattice oxygen in Fm3̅m symmetry, exhibited unexpectedly high activity for CO oxidation. Careful investigation of the catalytic behavior of mesoporous CoO catalyst led to the conclusion that the oxidation of surface Co(2+) to Co(3+) causes the high activity. Other mesoporous spinels (CuCo2O4, CoCr2O4, and CoFe2O4) with different Co species substituted with non/low-active metal ions were also synthesized to investigate the catalytically active site of cobalt-based catalysts. The results show that not only is the octahedrally coordinated Co(3+) highly active but also the octahedrally coordinated Co(2+) species in CoFe2O4 with an inverse spinel structure shows some activity. These results suggest that the octahedrally coordinated Co(2+) species is easily oxidized and shows high catalytic activity for CO oxidation.

Nanostructured Ti-catalyzed MgH2 for hydrogen storage

Nanocrystalline Ti-catalyzed MgH(2) can be prepared by a homogeneously catalyzed synthesis method. Comprehensive characterization of this sample and measurements of hydrogen storage properties are discussed and compared to a commercial MgH(2) sample. The catalyzed MgH(2) nanocrystalline sample consists of two MgH(2) phases-a tetrahedral β-MgH(2) phase and an orthorhombic high-pressure modification γ-MgH(2). Transmission electron microscopy was used for the observation of the morphology of the samples and to confirm the nanostructure. N(2) adsorption measurement shows a BET surface area of 108 m(2) g(-1) of the nanostructured material. This sample exhibits a hydrogen desorption temperature more than 130 °C lower compared to commercial MgH(2). After desorption, the catalyzed nanocrystalline sample absorbs hydrogen 40 times faster than commercial MgH(2) at 300 °C. Both the Ti catalyst and the nanocrystalline structure with correspondingly high surface area are thought to play important roles in the improvement of hydrogen storage properties. The desorption enthalpy and entropy values of the catalyzed MgH(2) nanocrystalline sample are 77.7 kJ mol(-1) H(2) and 138.3 J K(-1) mol(-1) H(2), respectively. Thermodynamic properties do not change with the nanostructure.