Mechanism of reconstitution of hydrotalcite leading to eggshell-type Ni loading on MgAl mixed oxide

The mechanism of eggshell-type Ni loading on Mg-Al mixed oxide particles has been carefully studied in connection with the structure of the mixed oxides as the catalyst supports. The pore distribution, surface morphology, and crystal and coordination structure of the mixed oxides were studied by XRD, MAS-NMR, TG-DTA, SEM, TEM, ICP, and N2 and H2 adsorption methods. The Mg-Al mixed oxide was prepared as a powder by calcining Mg-Al hydrotalcite and pressing it into the particles. When the particles were dipped in an aqueous solution of Ni2+ nitrate, reconstitution of Mg-Al hydrotalcite took place in the surface layer of the particles, and simultaneously Ni2+ replaced a part of the Mg2+ sites by a “memory effect” of the hydrotalcite structure. The reconstitution by memory effect took place for periclase Mg(Al)O that formed from Mg-Al hydrotalcite by the calcination at low temperature and at a low rate of heating. The memory effect proceeded by a dissolution-recrystallization mechanism on the microporous phase and formed “worm-like” structures, which finally constituted a dense layer and covered the surface of the particles. The dense layer hindered the further penetration of Ni2+ nitrate solution into the cores of the particles, resulting in eggshell-type Ni loading. A balance between the rate of reconstitution of Mg-Al hydrotalcite and the rate of penetration of the aqueous solution of nickel nitrate determined the loading type of Ni. Eggshell-type loaded Ni catalyst showed an enhanced activity per unit amount of Ni due to the surface enrichment of active Ni species, since measurement of the effectiveness factor of the catalyst showed that the intraparticle mass transfer limitation exists in steam reforming of CH4 over the catalyst at 1073 K.