PREPARATION AND THERMAL REACTIVITY OF NICKEL CHROMIUM AND NICKEL ALUMINUM HYDROTALCITE-TYPE PRECURSORS

Nickel/chromium and nickel/aluminium hydrotalcite-type precipitates, with nickel-to-M (III) ratios ranging from 2.0 to 3.0, are prepared by coprecipitation at pH = 8.0 +/- 0.1 from the nitrate salts of the elements. On the basis of X-ray diffraction (XRD) and infrared (IR) analyses, it is observed that the level of supersaturation and/or a following hydrothermal treatment influence only the crystallinity of the precipitates, but do not modify their nature and/or the characteristics of the mixed oxides obtained by thermal decomposition. The thermogravimetric (TG) and differential scanning calorimetric (DSC) analyses of the precipitates show that their decomposition involves an initial loss of the interlayer water followed by elimination of the carbonate and hydroxide ions, with some differences directly related to the crystallinity of the samples. The nickel/aluminium samples obtained by thermal decomposition are more stable than the nickel/chromium samples, and are characterized by a small crystal size and reducibility of the nickel oxide particles, forming the spinel phase NiAl2O4 only at the highest temperature. Furthermore, the nickel/aluminium samples show very narrow distributions of the porosity, which is strongly modified in the nickel/chromium samples by the formation of NiCrO4 and/or NiCr2O4. For all the samples investigated the formation of a spinel phase has a negative effect, giving rise to remarkable increases in both nickel oxide crystal size and reducibility. On the basis of these results, the thermal stability of the nickel oxide particles obtained by decomposition of nickel/aluminium hydrotalcite-type precipitates seems to be related mainly to the nature of the trivalent ion present and, as a consequence, so is the behaviour of the oxide obtained by thermal decomposition.