Influence of the compensating anions of Ni Al and Ni/Mg/Al layered double hydroxides on their activation under oxidising and reducing atmospheres

The activities and selectivities in the hydrogenation of acetonitrile on catalysts obtained from Ni/Al and Ni/Mg/Al layered double hydroxides (LDHs) precursors depend on the nature of their compensating anions: CO32-,NO3- or Cl-. The latter gives rise to sample with low activity and the less selective to primary amine. Studies by XRD, TG-DSC, mass spectrometry and TPR experiments show that the anion influences the thermal stability and the reducibility of the Ni-containing LDHs. The thermal decomposition of LDH into the mixed oxide form involves the dehydration, then the dehydroxylation of the layers with the concurrent decomposition of CO32- and NO3- which occurs at similar temperature in air or H-2 atmosphere. In TPR experiments the reduction of Ni2+ to Ni-0 takes place at above 800 K. A specific behaviour is observed when Cl- is the compensating anion, with a better reducibility of Ni2+ and the existence of Ni species reduced at low temperatures, not observed with CO32- and NO3- as anions. In addition, all the mixed oxides are poorly reconstructed in water, and a clear segregation of bayerite occurs on the chloride samples. This last phenomenon is enhanced in the Mg-containing compound. It is proposed that the presence of both Cl and Mg favours the extraction of aluminium and makes the accessibility to a significant amount of Ni2+ cations easier, thus increasing their reducibility. In this case, Ni-0 particles with an average size around 65 nm are detected after reduction at 723 K, while the other samples, poorly reduced, still contain the mixed oxide structure. (C) 1999 Elsevier Science B.V. All rights reserved.