Characteristics and Applications of Nanostructured Nitrides Synthesized by Vapor Phase Reactions
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Characteristics and Applications of Nanostructured Nitrides Synthesized by Vapor Phase Reactions Gerald Ziegenbalg, Carsten Pätzold, Ute Šingliar, Rico Berthold TU Bergakademie Freiberg, Institute of Technical Chemistry, Leipziger Strasse 29, 09599 Freiberg, Germany ABSTRACT 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 m2/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. INTRODUCTION Nitrides and oxynitrides are of growing interest for catalysis. Silicon nitride and silicon oxynitride were proposed as basic catalyst supports [1-5]. Transition metal nitrides are known as material with a catalytic activity comparable to noble metals such as platinum or palladium [6-8]. As for all catalysts, high specific surface, defined particle size and the availability of a simple and inexpensive synthesis way are preconditions for large scale using of a material. Traditionally, nitrides or oxynitrides are synthesized via solid-gas reactions. Examples are the direct nitridation of metals such as silicon or aluminum or the carbothermal reduction of metal oxides in nitrogen atmosphere. Other ways base on the thermal decomposition of metal organic precursors or temperature programmed reduction of metal oxides in an ammonia atmosphere. In most cases, milling is necessary to obtain powders with high specific surfaces and low particle size. It is well known that vapor phase reactions lead to nanoscale materials characterized by high purity and defined particle size. Nanoscale SiO2 and TiO2 powders are produced in large amounts worldwide. Little information, however, is present on the synthesis of nanoscale nitrides and oxynitrides. In the past, both substance classes were investigated mainly with respect to their use in the field of ceramics. The paper gives an overview about the use of gas phase ammonolysis reactions for the production of nanaoscale nitrides and oxynitrides. Their catalytic activity in dehydrogenation reactions will be discussed. EXPERIMENTAL DETAILS Powder synthesis was carried out in indirectly heated tube reactors based on silicon reinforced silicon carbide. The reactor had a length of 100 cm and an inner diameter of 5.2 cm. The gases were brought separately into the reaction chamber. Mixing was achieved
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