Calcination Decomposition, Solid-state

Calcination The rate of heat schedule as well as the final temperature employed are important variables. A high enough temperature must be employed to ensure decomposition of the salts used in the preparation. At sufficiently high temperature, some solid-state reactions are engendered. Further, differences can be obtained whether using forced... 

After a mixture of oxides is produced by the thermal decomposition of the freeze dried and calcined powder, fiorther solid-solid reactions can take place. Solid—solid reactions operate by the mechanisms discussed in Chapter 5, which include solid state diffusion and chemical reaction. Diffusion in ceramic solids is always ionic in nature and depends on defect or hole diffusivity, as well as electron conductivity. Once the ionic reactants are in close association, chemical reactions can take place, giving mixed oxides like BaTiOs and PbZrOs. 

This phase seems to be predominating in Zi, the sample obtained by solid state decomposition in presence of urea. This sample could have additional porosity, due to loss of CO2 during synthesis of and/or calcination. This could possibly result in exposure of additional catalytically active centers, making it highly active catalyst. The data in Table 1 illustrates the relative catalytic activity of various zinc oxide towards decomposition of propan-2-ol at 653"K at a contact time of 1.6 seconds. Under the above conditions all the catalyst showed dehydrogenation activity only the reactivity following the order Zi>Z2 4 Z5>Z3. 

Table 20.4 lists examples of salt precursors and their decomposition temperatures. The decomposition of salts to oxides is an example of a solid-state reaction. These reactions are often referred to as calcination and are frequently governed by kinetics rather than thermodynamics. As a consequence, they may be carried out at temperatures much greater than those necessary based on thermodynamic calculations. A feature of the decomposition reactions is that they often result in the production of extremely fine particles. 

Catalytic N2O decomposition was investigated over FeZSM-5 prepared by a novel exframework method. This method comprises the introduction of Fe in the MFI framework, followed by calcination and steam treatment to extract the iron to extra-framework positions. The ex-framework method induces superior activity for direct N2O decomposition, compared to FeZSM-5 catalysts prepared by solid and aqueous ion-exchange methods. The extraordinary catalytic performance of ex-FeZSM-5 is attributed to the highly dispersed state of the Fe in the zeolite matrix and the Fe(III)/Fe(II) redox behaviour of a significant fraction of the iron centers in the catalyst. 

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