Novel and effective surface enrichment of active species in Ni-loaded catalyst prepared from Mg-Al hydrotalcite-type anionic clay

Egg shell-type Ni loaded catalysts have been prepared by adopting the “memory effect” of Mg-Al hydrotalcite and have been successfully used in the steam reforming of CH4. Mg-Al(3/1) mixed oxide particles were first prepared as the catalyst support by the thermal decomposition of Mg-Al hydrotalcite. When the particles were dipped in aqueous solution of Ni(II) nitrate, Mg-Al hydrotalcite was reconstituted in the surface layer of the particles, and simultaneously Ni(II) substituted for the Mg(II) sites, leading to the formation of egg shell-type Ni-loading. The conditions of both preparation of Mg-Al(3/1) mixed oxide and dipping in Ni(II) nitrate solution were varied, and their influences on egg shell-type Ni-loading and on the catalytic activity were studied. The shape of egg shell-type Ni-loading was significantly affected by the rate of heating, the final calcination temperature of Mg-Al(3/1) mixed oxide, the pH of Ni(II) nitrate aqueous solution, and the dipping time of the mixed oxide in the aqueous solution. The smaller heating rate, the lower temperature calcination, and a dipping in the aqueous solution at the lower pH were preferable for the Ni-loading in egg shell-type. Under the mild heating conditions, periclase-like MgO structure was formed, on which the reconstitution of Mg-Al hydrotalcite was preferentially provoked in the surface layer of the particle by “memory effect” during the dipping at the lower pH. It is concluded that the activity of egg shell-type Ni-loaded catalyst was enhanced by the surface enrichment of well-dispersed Ni metal particles.