Impregnation-induced memory effect of thermally activated layered double hydroxides

The memory effect of calcined Cu–Co–Zn–Al layered double hydroxides (LDH) upon impregnation with K-containing solutions was studied. A sample of nominal composition CuCoZn2Al4Ox was prepared by coprecipitation at controlled pH. The resulting hydrotalcite-like (HT) precipitate was calcined at 673, 773, or 873 K. The products were impregnated with different K-containing solutions [K2CO3(aq), KOH(aq) and KOH(alc)]. Reconstruction of the HT phase upon impregnation was followed by X-ray diffraction. The HT recrystallization was favored by impregnating calcined LDH samples with K2CO3(aq) by wet impregnation. The memory effect was reduced by increasing the calcination temperature of the parent layered double hydroxide. It is proposed that the reconstruction of the HT phase occurs via a retro-topotactic transformation from the Al3+ and divalent cations located both in the octahedral sites of the oxide matrix. The increase of the calcination temperature causes the solid-state diffusion of divalent cations into tetrahedral positions which results in the progressive formation of stable normal spinels. After calcination at 873 K, all the divalent cations occupy tetrahedral sites and the memory effect disappears.