Transformations pseudomorphic

We can use our results to predict the conditions favorable for the transformation of gaylussite to aragonite. The porous nature of the pseudomorphs and the small amounts of calcium available in the lake water (Table 24.2) suggest that the replacement occurs by the incongruent dissolution of gaylussite, according to the... 

Dynamic atomic-resolution ETEM and diffraction studies provide fundamental insights into the catalyst precursor transformation mechanism. The studies reveal that the temperature regimes are critical to the formation of active catalysts. They show that the nature of the VHPO -> VPO transformation is topotac-tic. Topotaxy is defined as the conversion of a single crystal to a pseudomorph... 

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... 

The TPR method has now been applied to TlN O [52], VN [23], and NbC [53]. Although the TPR method produces high surface area materials, the pore structure of these is usually not controllable. Often, the pores are in the micropore regime (< 3 nm). However, a number of the solid state transformations that lead to carbides and nitrides are topotactic and exhibit pseudomorphism (retention of external particle size and shape) This provides a possible means of engineering the pore structure by preparing oxide precursors with the desired external morphology. 

The exothermic effects associated with some dehydrations [18] may result from delayed reciystallization, or the oxidation of a product of dehydration. Dehydration of magnesium carbonate trihydrate, followed by heating in carbon dioxide, resulted in the exothermic transformation of an anhydrous pseudomorph of the reactant into magnesite, MgCOj. 

Single crystals of MoOj.HjO ("a-molybdic acid") were transformed [130] on heating at 433 K into perfect pseudomorphs composed of MoOj crystalhtes. This reactant to product transition is explained by a topotactic reaction in which the M0O3 structure was formed by comer linking of isolated double chains after water... 

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... 

The material of this composition was initially exposed to simulated body fluid (SBF) for one month and the cross section of the specimen is shown in Figure 5. Two HA-like layers with different morphologies were found to form below and on the surface of the material, by two different and consecutive mechanisms. At first the material reacts with the surrounding SBF by dissolving the psW phase and forming a porous structure of HA-like phase that mimic porous bone, in a pseudomorphic transformation of the a-TCP, according to the reaction ... 

Pseudomorphic mineral replacement events consist in the transformation of a mineral phase, which is out of equilibrium into a more thermodynamically stable phase, involving dissolution and reprecipitation subprocesses. This natural phenomenon is characterized by the preservation of the shape and dimensions of the replaced parent phase whenever the kinetics of its dissolution are coupled with the kinetics of nucleation and crystallization of the new phase. The initiation and spatiotemporal harmonization of these re-equilibration reactions rely on parameters that are controllable in the laboratory. Reboul et al. combined sol-gel process and pseudomorphic replacement to introduce organic elements into a preshaped dense metal oxide phase. In the presence of an organic ligand solution and under microwave conditions, the dissolution of the metal oxide sacrificial phase provides the... 

In these syntheses the surface area increases with the progress of the reaction via development of pores as the density of the precursor increases without much change in exterior volume. This phenomenon, where the overall size and shape of a material remains constant over the course of a solid state transformation, is called pseudomorphism (<5). Another related phenomena that affects the development of surface area is topotaxy. Here the transformation proceeds in a manner such that the crystallographic planes of the final product bear a relationship with the planes of starting material throughout the bulk (8). 

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. 

It is often the case that the first zeolite to appear is not stable when left for a longer time at the reaction temperature in contact with its mother liquor. The first-formed crystals slowly dissolve a new type of zeolite nucleates and grows, thus consuming the first. Indeed this process may be repeated more than once, so that the second zeolite is consumed at a later time by yet another. Because the second zeolite usually has a different morphology and crystal size, rather than being a pseudomorph of the first, the transformation of the first into the second can reasonably be ascribed to a solution mechanism rather than to a solid-state rearrangement of the lattice of the first zeolite to give that of the second. 

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