Synthesis and characterization of new hydrotalcite-like compounds containing Ni(II) and Mn(III) in the hydroxide layers and of their calcination

Two nickel-manganese hydroxycarbonates with Ni/Mn ratios close to 2/1 (Ni2Mn) and 3/1 (Ni3Mn) have been synthesized by coprecipitation under an air atmosphere from aqueous solutions of Ni(II) and Mn(II). Characterization by powder Xray diffraction, transmission electron microscopy, and differential thermal and thermogravimetric analyses confirm a hydrotalcite-like structure for these brown colored compounds, indicating that Mn(II) is at least partially oxidized to Mn(III) during the synthesis process. Thermal decomposition in air at 450 degrees C leads to formation of NiO as the major product in both samples, At 700 degrees C, the Ni/Mn ratio determines the nature of the nickel-manganese mixed oxides present, in addition to NiO. Temperature-programmed reduction of calcined samples indicate that the major mixed oxide in sample Ni2Mn calcined at 700 degrees C is ilmenite, NiMnO3, while in Ni3Mn it is the spinel NiMn2O4. At 1000 degrees C, formation of NiO and NiMn2O4 spinel occurs whichever the Ni/Mn ratio in the starting material. (C) 1996 Academic Press, Inc.