Activation under oxidizing and reducing atmospheres of Ni-containing layered double hydroxides

Abstract

Layered double hydroxides (LDHs) of the Ni2+/Mg2+/Al3+ type were prepared in a large range of compositions as weil as pure takovite (Ni/Al) and hydrotalcite (Mg/Al) samples. All of them have a well crystallized lamellar structure and contain NO-3 and CO2-3compensating anions. Their thermal stability and reducibility were followed by TG, in situ XRD, mass spectrometry, volumetry and TPR experiments. The thermal stability 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 aluminate spinel-type phase is only detected in the hydrotalcite or in the Ni2+/Mg2+/Al3+ samples containing the higher amounts of Mg. NO-3 decomposed in two steps at a higher temperature than CO2-3 species, for both anion a simultaneous water release is observed. The reducibility of Ni decreases with both the Al content of the takovite samples and with the Mg content of the Ni2+/Mg2+/Al3+ samples. Whatever the sample Ni species are less reducible when the calcination temperature increases. During TPR experiments, CO2-3 is decomposed to CO2 while NO-3 is evolved as NO and N2O between 700 and 800 K. Above 800 K, the H2 consumption corresponds to the reduction of Ni2+ into Ni0, as shown by the XRD experiment.