LDH Nanocomposites with Different Guest Entities as Precursors of Supported Ni Catalysts

Abstract

Nanocomposites built by intercalation of negatively charged Ni hydroxy citrate species [Ni(C6O7H3)x(OH)]yy(3x−1)− and NiCl42− guest entities in the interlayer space of host Mg/Al layered double hydroxides (LDHs) have been obtained by anionic exchange. The sizes of the Ni particles in the Ni/Mg(Al)O samples formed by thermal reduction in H2 of these precursors as well as the reconstruction abilities of the mixed oxides in water have been examined. The Ni complexation ratio x = [C6O7H3]/[Ni] in the [Ni(C6O7H3)x(OH)]yy(3x−1)− clusters has been varied from 0.5 to 2 giving rise to a series of LDH composites with different Ni contents. As x increases, the mean negative charge per Ni atom of the intercalated anionic species indeed increases, and the Ni content in the nanocomposites concurrently decreases from 15 to 7 wt %. The mean size of the Ni0 crystallites varies with the reduction temperature and the composition of the guest entities in the [Ni(C6O7H3)x(OH)]yy(3x−1)−−LDH nanocomposites as shown by XRD and TEM analyses. The mean size of the Ni0 crystallites decreases from approximately 10 to 5 nm after reduction at 600 °C when x increases from 0.6 to 2. This is consistent with the decrease of the average size of the Ni hydroxy citrate colloids in solution. It is shown that the size of the colloids in the precursors allows the size of the Ni0 particles in the final Ni0/Mg(Al)O material to be controlled. When NiCl42− is used as the guest entity, Ni particles obtained after reduction are larger. The Mg(Al)O supports behave very differently in the two types of reduced nanocomposites upon reconstruction in water. The layered structure is restored when the guest entity is NiCl42− in the precursor, but in contrast Mg(Al)O is highly stabilized when it is [Ni(C6O7H3)x(OH)]yy(3x−1)−. A strong interaction between the embedded Ni particles and Mg(Al)O prevents reconstruction in the latter case. The segregation of large particles in the former makes Mg(Al)O rehydration easier.