Synthesis and activation of Co-Mg-Al layered double hydroxides

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 copreci pitation at pH = 9 and NO3- as main compensating anions, All samples presen t 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 magn esium content increases. At high magnesium content a MgO-type phase is foun d, The thermal stabilities of intercalated NO3- provided by the synthesis s alts and of CO32- provided by the surrounding atmosphere increase with the magnesium content of the samples. Specific areas are also stabilized by Mg. TPR experiments prov ide 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 composit ion 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. ( C) 1999 Academic Press.