Gerald Ziegenbalg

Propane dehydrogenation over supported platinum silicon nitride catalysts

Amorphous silicon nitrides were prepared by gas phase ammonolysis of tetrachlorosilane followed by thermal treatment in nitrogen or ammonia atmosphere. After deposition of platinum on the materials the obtained catalysts proved to be catalytically active and selective for propane dehydrogenation. Incorporation of boron, carbon and titanium in the supports influences significantly the catalytic properties. It is suggested that the comparable high catalytic activity, propene selectivity and stability of the catalysts are due to the basic properties of the catalyst surface.

Synthesis of α-silicon nitride powder by gas-phase ammonolysis of CH3SiCl3

Abstract An alternative to silicon tetrachloride mostly used for silicon nitride production by gas phase ammonolysis is CH 3 SiCl 3 . While powders resulting from ammonolysis of CH 3 SiCl 3 at room temperature have a constant composition, high temperature reactions result in materials showing a strong dependence on the reaction conditions. Increasing reaction temperature leads to powders with increasing silicon content, while the chlorine content decreases. Mixtures of non stoichiometric, chlorine and carbon containing silicon nitride intermediates and ammoniumchloride are obtained. Gaseous reaction products are HCl, SiCl 4 , CH 4 and H 2 . Quantitative mass spectrometric analysis of the exhaust gases allowed to balance the reactions. 29 Si and 13 C CP-MAS-NMR powder characterisation was used to deduce the reactions taking place in the gas phase. An overlapping of radical forming and substitution reactions is probable. Dechlorination of the powders in ammonia at 900°C followed by a crystallization step at 1500°C results in crystalline α-Si 3 N 4 which is equivalent to powders obtained by ammonolysis of SiCl 4 .

Gas-phase synthesis of amorphous silicon nitride — reaction paths and powder characteristics

The gas phase reaction between SiCl4 and NH3 is investigated in the temperature range between 525 and 800°C at atmospheric pressure and at conditions typical for powder synthesis. By means of mass spectrometric in-situ measurements it was possible to detect the gaseous compounds H2NSiCl3, H2NSiCl2NH2, Cl3SiNHSiCl3, NH2Cl2SiNHSiCl3, (SiCl2NH)3 and Si3(NH)3Cl5NH2. The reactions taken place in the gas phase are very fast and result in the formation of a fine, chlorine containing product. Powders sampled at a reaction temperature of 800°C have an average molar ratio Si : N : Cl of 1 : 1, 33 : 0.28. Based on the proved gaseous intermediates and the composition of the powders reaction paths resulting in the formation of powders are derived. α-Si3N4 powders with a high sintering activity are obtained after thermal dechlorination of the synthesis products in ammonia atmosphere followed by a crystallization process between 1200 and 1500°C.

Porous Silicon Nitride Materials as Basic Catalysts

Silicon nitride powders characterized by a high specific surface area were prepared by gas-phase ammonolysis of tetrachlorosilane. The compositions as well as the structural and textural properties of the materials were varied by the reaction conditions and the aftertreatment of the synthesis products. The Knoevenagel condensation of benzaldehyde and malonitrile served as a test reaction to characterize the catalytic properties of the materials. The experiments showed that amorphous silicon nitride powders have better catalytic properties than high surface magnesium oxide. However, treatment of the amorphous silicon nitride powders with water leads to a loss of nitrogen as ammonia and a decrease in the resulting catalytic activity.

A novel technology for sealing and immobilization — the use of precipitation processes from supersaturated solutions

Mineral forming solutions can be prepared by using of special precipitation inhibitors. These are compounds allowing the mixing of solutions which are incompatible under normal conditions, for example BaCl2 and Na2SO4 solutions or lime suspensions and diluted sulfuric acid. Clear, temporary stable solutions are obtained. If these are used as grout, directed precipitation takes place in the flow paths. The paper summarizes the fundamentals of the technology and gives an overview about the characteristics of gypsum and BaSO4 forming solutions. Their immobilization effect was proven by column tests, which are discussed in detail.

The use of BaSO4 supersaturated solutions for in-situ immobilization of heavy metals in the abandoned Wismut GmbH uranium mine at Königstein

The former uranium ISL-mine at Konigstein (Germany) is presently being flooded. To support the flooding process, a new technology to reduce contaminant potential in the source was developed and applied. The application based on the injection of supersaturated BaSO4-solutions to precipitate solved contaminants and to cover reactive mineral surfaces. Since 2002 the technology is applied in the southern part of the mine in order to immobilize contaminants in highly polluted areas before flooding. The article describes the fundamentals of the technology and the full-scale application.

Characteristics and Applications of Nanostructured Nitrides Synthesized by Vapor Phase Reactions

Gas phase ammonolysis of volatile metal chlorides at elevated temperatures is a favorable way to produce nitride or oxynitride nanopowders. Their composition as well as the physico-chemical properties is determined by reaction temperature, molar ratio of the reactants and the residence time of the gases in the reaction zone. Both single and multi component powders can be obtained. Typical particle sizes are in the range of 50 to 350 nm. The specific surface can reach values up to 300 m 2 /g. Microporous analysis revealed the presence of pores with a diameter between 0.6 and 0.7 nm in amorphous silicon nitride. The powders can be used, depending on the characteristics, as catalyst or basic catalyst support. The paper gives an overview about vapor phase synthesis of single and multi component nitrides as well as the use of amorphous silicon nitride as a basic catalyst support for dehydrogenation of propane.