Thermal Activation of Layered Hydroxide-Based Catalysts

Keywords Mixed oxides, thermal decomposition, thermal history, photocatalysts 

Atul Tiwari and Baldev Raj, Reactions and Mechanisms in Thermal Analysis of Advanced Materials, (483-514) 

Structure (example for MgAl-LDH) due to the presence of anions and water molecules in the interlayer the basal spacing increases to the value of approximately 0.77 nm. Brucite consists of hydroxyl ions that are hexagonal closely packed around magnesium ions and has neither positive nor negative charge. On the contrary, due to the isomorph substitution of some divalent ions by trivalent ions, positive charge is attained in the LDH structure that is compensated by the mentioned anions in the interlayer. 

For the formation of LDHs many divalent and trivalent cations can be used in the brucite-like layer with the only requirement being that their radii in the case of octahedral coordination does not largely differ from those of Mg and Al [H]. Table 20.2. Although different combinations of divalent and trivalent metal ions have been constantly used for the LDH synthesis, the possibility of incorporating tetravalent cations in the brucite-like layer is recently often mentioned [4,11,12]. So far, there is no generally accepted opinion about this possibility, although there has been published work on this subject. 

The range of anions that can be incorporated into the interlayer is also broad [2]  

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Thermal Activation of Layered Hydroxide-Based Catalysts 485... 

As described previously, the cations in LDHs are evenly distributed in thebrucite-like layers. Thus, in principle, the catalytic activity of LDHs can be well controlled by varying the cation ratio and incorporating different cations. Catalytically active constituents of LDH include the hydroxide groups and the metal ions themselves, especially if these are redox active. The introduction of catalytically active anions, such as polyoxometalates (POMs), can further modify the properties of LDHs. Thermal decomposition (calcination) of LDH gives mixed basic oxides of high surface area and catalytic activity. Finally, the reduction of LDH can give rise to finely divided catalytically active metal and to the prospect of metal/base bifunctional catalyst. 

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