Intercluster porous nanocomposites from combination of [AlO₄ Al₁₂(OH)₂₄(H₂O)₁₂]⁷⁺ cation and Anderson-type [Al₁₋ₓ Crₓ Mo₆O₂₄H₆]³⁻ anion: XRD, TG-DTA, SEM-EDAX and EPR studies

Abstract

The synthesis of intercluster porous nanocomposites obtained from polyoxometallate compounds such as the [AlO₄Al₁₂(OH)₂₄(H₂O)₁₂] ⁷⁺ cation (named Al₁₃) and the Anderson-type [Al₁₋ₓ Crₓ Mo₆O₂₄H₆ ]³⁻ anion [Al₁₋ₓ Crₓ Mo₆O₂₄H₆] has been performed in order to study the interaction between the two cluster ions, the stability of the XMo₆ planar configuration, the products obtained after thermal treatment, the structure and the local symmetry of the Cr³⁺ species. Chemical, thermal, structural and spectroscopic characterizations of the original and thermally treated phases have been followed by different techniques such as TG-DTA, XRD, SEM-EDAX, and mainly by EPR. All the results have shown that the structure of the intercluster nanocomposites (Al₁₃)(Al₁₋ₓ Crₓ Mo₆)₂ precursors is similar to that reported by Son et al. for the chromium-free (Al₁₃)(AlMo₆)₂ intercluster nanocomposite [Son et al., J. Am. Chem. Soc. 122 (2000) 7432]. After thermal treatment in air at several temperatures of the (Al₁₃)(Al₁₋ₓ Crₓ Mo₆)₂ nanocomposites the following phases have been observed and characterised: (i) at 400 ºC an amorphous phase containing dispersed Cr³⁺ ions; (ii) at 700 C a crystalline phase corresponding to Cr₂(MoO₄)₃/Al₂(MoO₄)₃ solid solutions; (iii) at 950 ºC αAl₂O₃/Cr₂O₃ solid solutions with a random dispersion of the Cr³⁺ ions.