ACTIVATION UNDER OXIDIZING AND REDUCING ATMOSPHERES OF NI-CONTAINING LAYERED DOUBLE HYDROXIDES

Layered double hydroxides (LDHs) of the Ni2+/Mg2+/Al3+ type were prepa red in a large range of compositions as well as pure takovite (Ni/Al) and hydrotalcite (Mg/Al) samples. All of them have a well crystallized lamellar structure and contain NO3- and CO32- compensating anions. Th eir thermal stability and reducibility were followed by TG, in situ XR D, mass spectrometry, volumetry and TPR experiments. The thermal stabi lity of the hydrotalcite is higher than for the takovite structure and increases with the Mg content in the mixed Ni2+/Mg2+/Al3+ compounds. All samples are decomposed into a mixed oxide phase of the MgO or NiO type upon calcination. On the other hand, an excess magnesium aluminat e spinel-type phase is only detected in the hydrotalcite or in the Ni2 +/Mg2+/Al3+ samples containing the higher amounts of Mg.NO3- decompose d in two steps ar a higher temperature than CO32- species, for both an ion a simultaneous water release is observed. The reducibility of Ni d ecreases with both the Al content of the takovite samples and with the Mg content of the Ni2+/Mg2+/Al3+ samples. Whatever the sample Ni spec ies are less reducible when the calcination temperature increases. Dur ing TPR experiments, CO32- is decomposed to CO2 while NO3- is evolved as NO and N2O between 700 and 800 K. Above 800 K, the H-2 consumption corresponds to the reduction of Ni2+ into Ni-0, as shown by the XRD ex periment.