Hexaaluminate materials application

Hexaaluminate (HA) materials containing transition metal ions in the structure have been extensively investigated for GT applications, in view of their excellent thermal stability and catalytic activity [9, 99-101]. 

Solid State Reaction - Hexaaluminates have been long known as interesting materials for many applications (electrical ceramics, matrices for permanent immobilization of radioactive elements from nuclear wastes and refractory cement and concrete). For many years ceramists prepared hexaaluminates via solid state reaction for both crystallographic and application purposes.6... 

To prepare hexaaluminates for ceramic applications a slightly different sol-gel process was proposed by Debsikbar.19 Ba-hexaaluminates were prepared via hydrolysis of Al di(isopropoxide) acetoacetic ester chelate and anhydrous Ba acetate obtained by reaction between BaC03 and glacial acetic acid. The substitution of Al(i-OC3H7)3 with the alkoxy ester was intended to control the chemical polymerization during gel formation. The reaction was performed in 1-butanol. The formation of the gel slowly occurred at room temperature in about 10 h. To obtain the final phase the gel precursor was dried at 70 °C for about 2 weeks, ground and calcined at 1200°C for 2 h. However no data on the morphology of the final materials were reported by the author. 

Effective preparation methods of hexaaluminates for catalytic applications, such as the hydrolysis of alkoxides and the co-precipitation in aqueous medium, ensure high interspersion of the constituents in the precursor. This allows the formation of single phase materials with layered-alumina structure at reasonably low temperature (1100-1200 °C) and with high surface area. The hydrolysis of alkoxides was extensively studied and used for the industrial scale-up in the production of catalysts in the monolith shape. However, the co-precipitation in aqueous medium has much potential in view of the possible commercialization of these materials due to its simplicity and low cost. 

Hexaaluminates represent a class of materials that are highly resistant to sintering at high temperature. Therefore, these materials have attracted the attention of researchers who are involved in developing catalysts for high temperature applications. Hexaaluminates have been used as supports as well as the active material in catalytic combustion reactions [121-142]. Hexaaluminates can be represented by the formula AAI12O19 where A is an alkaline or alkaline-earth metal. They consist of a lamellar structure and both A cation and A1 can be partly substituted by other cations. Incorporation of other cations drastically modify the catalytic activity of these materials. However, such modifications are rather limited compared to the possibilities existing in perovskite type oxides. 

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