Characterizaiton of Calcined and Reduced Multi-Component Co-Ni-Mg-Al-Layered Double Hydroxides

摘要

Several Co-Ni-Mg-Al-layered double hydroxides (LDH) have been synthesized by coprecipitation at constant pH = 9. All samples present well crystallized, layered structures, no excess phases were detected by X-ray diffraction, and the lattice a parameter followed the Vegard correlation. The cations thus belong to the same, brucite-like, lattice. Calcined samples exhibit XRD patterns of the crystallographic phase preferentially adopted by the major cation in the LDH: i.e. a mixed oxide of the rock salt type in the case of Ni or Mg, and a spinel-like structure for Co. Surface areas reached maximum values in the range of 200-300 m~2g~(-1) after calcination at 573 K. They were stabilized with increasing Ni or Mg content, as higher amounts of mixed oxide phase were formed. Programmed temperature reduction showed two domains of H_2 consumption, assigned to reduction of Co_(3-x)Ni_xO_4 species below 720 K, and to NiMg(Al)O and Co(3-x-y)Ni_xMg_yAlO_6, of rock salt type and spinel-like structures respectively, above this temperature. These assignments are in agreement with the XRD patterns and show that Ni and Co are associated in the same crystallographic phases.