Metal oxide synthesis calcinations

Fig. 20.18 Grain size for different concentration of precursors in solution for metal oxide synthesis vs. growth and calcinations temperature, (a) Sn02 was synthesized using a method based on the pyrolytic reaction of SnCl -SH O dissolved in methanol. In contrast to the conventional spray pyrolysis technique, pyrolytic reaction does not take place during deposition on the surface of nanocrystals. The treatment in the range of 400-900 °C was carried out after drop deposition on the substrate (Adapted with permission from Cirera et al. 1999, Copyright 1999 Elsevier), (b) Sn02 powders were synthesized by hydrothermal method (Adapted with permission from Baik et al. 2000a, b, c. Copyright 2000 John Wiley Sons)...

Metal organic decomposition (MOD) is a synthesis technique in which metal-containing organic chemicals react with water in a nonaqueous solvent to produce a metal hydroxide or hydrous oxide, or in special cases, an anhydrous metal oxide (7). MOD techniques can also be used to prepare nonoxide powders (8,9). Powders may require calcination to obtain the desired phase. A major advantage of the MOD method is the control over purity and stoichiometry that can be achieved. Two limitations are atmosphere control (if required) and expense of the chemicals. However, the cost of metal organic chemicals is decreasing with greater use of MOD techniques. 

The molecular design of supported metal oxide catalysts requires that we specify the synthesis method, oxide support, catalyst composition, calcination... 

In the preparation and activation of a catalyst, it is often the case that the chemical form of the active element used in the synthesis differs from the final active form. For example, in the preparation of supported metal nanoclusters, a solution of a metal salt is often used to impregnate the oxide support. The catalyst is then typically dried, calcined, and finally reduced in H2 to generate the active phase highly dispersed metal clusters on the oxide support. If the catalyst contains two or more metals, then bimetallic clusters may form. The activity of the catalyst may depend on the metal loading, the calcination temperature, and the reduction temperature, among others. 

Figure 3 Synthesis of mesoporous metal oxides (a) hexagonal assemblies of micellar systems are used as templates for the hydrolysis/condensation of M(OR)2 alkoxides (b) after calcination, the corresponding oxide exhibits an hexagonal array of mesopores...

As shown in Table 4 most of the metal oxides have a strong tendency to form lamellar structures, except Sb, W, Pb. The formation pathway of these materials depends on the charge of the surfactant and that of the inorganic ion involved in the synthesis. Different mesostructured Sb and W oxides were synthesized at room temperature by controlling the pH of the system. Regardless of their structures, all the resulting materials collapsed upon calcination. 

For the preparation of inorganic materials with well-defined morphologies, liquid phase syntheses are preferred. These synthetic reactions proceed at relatively lower temperatures and therefore require lower energies. The sol-gel (alkoxide) method is one of these methods - however, this method usually gives amorphous products, and calcination of the products is required to obtain crystallized products. In this chapter, solvothermal methods are dealt with, which are convenient for the synthesis of a variety of inorganic materials. General considerations for solvothermal reactions are discussed first and then the solvothermal synthesis of metal oxides is reviewed. 

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