Pseudomorphic synthesis

Generally, metal nitride and oxynitride catalysts have been prepared by the temperature programmed ammonolysis method. This technique was originally applied by Volpe and Boudart to M0O3 precursors to produce y-Mo2N which was pseudomOrphic with the precursor, with surface areas up to 225 m g The resultant surface area of materials is known to be a strong function of the synthesis parameters as demonstrated in Table 1. ... 

When iron catalysts are exposed to FT synthesis reaction environments, the catalysts first transform from hematite into magnetite. The transformation into magnetite is rapid and occurs pseudomorphically where the shape of the hematite crystals is retained including their swiss-cheese morphology. The transformation from magnetite to carbide is slow and is affected... 

Particles with spherical morphology are easily prepared under alkaline conditions. For instance, Huo et al. (279) reported the preparation of hard transparent spheres from an emulsion at room temperature. Griin and coworkers (154,280) modified the Stdber synthesis (281) of monodisperse spheres in the presence of ethanol and ammonium hydroxide and successfully prepared monodispersed mesoporous MCM-41 and MCM-48 spheres (Fig. 9.26c). Furthermore, pseudomorphic transformation of... 

In this section the salient features of carbide and nitride synthesis will be presented first. The role of ammonia and methane/hydrogen as reductant in the development of surface area will be discussed next, with references to the significance of pseudomorphic and topotactic phenomena in these transformations. 

The temperature programmed reaction synthesis of carbides and nitrides of V, Mo, and Nb, is presented under same reaction conditions and allows direct comparison of the temperatures at which various intermediates and products are formed. In general, the nitrides are formed at lower temperatures than the carbides for the same metal, because of the greater reducing-nitriding strength of ammonia over hydrogen-methane mixtures. The transformations of oxides to carbides or nitrides under TPR synthesis conditions are pseudomorphic, and in some cases of nitride synthesis are topotactic. 

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