Dirk Demuth

Surfactant controlled preparation of mesostructured transition-metal oxide compounds

The use of surfactants as templates allows the synthesis of mesostructured transition-metal oxides by a cooperative, interface controlled condensation process.

Synthesis of large optically clear silicoaluminophosphate crystals with AFI structure

The optimization of the hydrothermal SAPO-5 synthesis using the system Al2O3:P2O5:SiO2:TEA (triethylamine): H2O resulted in the formation of large crystals (average size of up to 220 × 50 μm) of hexagonal rod-like morphology and high optical quality in high yields only, if an aluminum oxide hydrate sol or a pseudohydrargillite was used as aluminum source. It was possible to control the morphology and the crystal size of SAPO-5 by the silica source used. Further, it was found that the silica in the synthesis gel drastically influences the rate of crystal growth in the crystallographic c-axis. The amount of pyrogenic silica acid used controls the morphology of the crystals, while the reactivity of the silica determines the crystal size obtained. With powder X-ray diffraction (XRD) and thermogravimetric analysis-differential thermal analysis (TGA-DTA) it was demonstrated that silicon predominantly substitutes for phosphorus sites if a silicon molar fraction (SiAl + P + Si) of <0.06 is used in the synthesis gel.

Copper (sub)oxide formation: a surface sensitive characterization of model catalysts

Model studies on the catalytic methanol oxidation over single and polycrystalline copper have been performed. The catalytic activity was investigated by means of temperature-programmed techniques (thermal desorption and temperature-programmed reaction spectroscopy, TDS and TPRS, respectively). The TPRS experiments call for the existence of chemically inequivalent species of atomic oxygen accessibly for catalytic processes on the copper surface. The surface morphological changes after the combined action of oxygen and methanol were observed by using atomic force (AFM) and scanning electron miscroscopy (SEM) and indicate the participation of not only the surface but to a great extend also the bulk. Furthermore, ex situ X-ray absorption spectroscopy (XAS) at the O K-edge shows that a copper suboxide phase of Cu(x2.5)O is formed at the surface/near-surface region up to a depth of about 100 A. Core-level (XPS) and valence band (UPS) photoemission suggests that the suboxide phase can be viewed as an oxygen-deficient copper(I) oxide phase exhibiting an increased density-of-states at the Fermi level pointing to an electrically conducting phase. The depth-selective recording of X-ray absorption spectra gives clear evidence of the formation of a protective copper(I) oxide film underneath the suboxide layer covering the bulk metal phase.

Element distribution and growth mechanism of large SAPO-5 crystals

Abstract The spatial distribution of silicon as T-atom substituent in large optically clear crystals with the AFI structure was studied by electron microprobing. The silicon concentration in the center of the crystals is lower by a factor of two to three than in the other part of the crystals. This is the second example of a microporous material, for which an inhomogeneous heteroelement distribution was observed. This corresponds to the findings of our kinetic studies and previous findings concerning the correlation between silicon content and crystal size. During the early stages of AFI crystal growth we found ‘pencil type’ crystals which develop into hexagonal prisms by flattening of the tips during crystal growth. Increasing silicon content accelerates the morphological change and inhibits lengthwise growth of the crystallites.

Surfactant based synthesis of oxidic catalysts and catalyst supports

The preparation of mesostructured metal oxides based on the surfactant controlledsynthesis of MCM-41 is presented. It can be expected that the new metal oxide materials will exhibit a high surface area of nearly 1000 m 2 /g and could have a possible use as catalysts, especially in partial oxidation reactions and as hosts for quantum sized material, if the surfactant can be removed without destroying the structure.