Synthesis and Activation of Co–Mg–Al Layered Double Hydroxides

Author： Ribet S. Tichit D. Coq B. Ducourant B. Morato F.

ISSN： 0022-4596

Source： Journal of Solid State Chemistry, Vol.142, Iss.2, 1999-02, pp. : 382-392

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Abstract

Several Co/Mg/Al layered double hydroxides (LDH) with (Mg²⁺+Co²⁺)/Al³⁺=2 and 3 and Mg²⁺/Co²⁺ ranging from 0 to 4 have been synthesized by coprecipitation at pH=9 and NO^-₃ as main compensating anions. All samples present well-crystallized layered structures. From the evolution of the lattice a parameter with respectively the Al content and the Mg²⁺/(Mg²⁺+Co²⁺) ratio, it could be concluded that Mg²⁺, Co²⁺, and Al³⁺ belong to the same lattice, which is confirmed by X-ray diffraction (XRD), where no excess phases are detected. Distinct structures, related to the composition of the samples, are detected when calcination is performed up to 773 K. For the cobalt rich samples a spinel-like phase is formed whose amount decreases when the magnesium content increases. At high magnesium content a MgO-type phase is found. The thermal stabilities of intercalated NO^-₃ provided by the synthesis salts and of CO^2-₃ provided by the surrounding atmosphere increase with the magnesium content of the samples. Specific areas are also stabilized by Mg. TPR experiments provide evidence of two distinct reducible cobalt species whatever the sample. The species below 623 K are assigned to Co₃O₄. Those reduced around 970–1030 K must belong to a spinel-like phase whose composition largely depends on the Mg and Al content of the LDH. These two types of phases are quite indistinguishable by XRD due to their similar patterns.