Low-temperature synthesis of spinels MAl₂0₄ (M=Mg, Co, Ni, Cu, Zn) prepared by a sol-gel method

Abstract

High-surface-area spinels of the general formula MA1₂O₄, where M = Mg, Co, Ni, Cu and Zn have been successfully prepared at low temperature (600 °C) from precursor solutions containing the nitrate salts and the surfactant cetyltrimethylammonium bromide as the gelating agent. Thermal analysis (thermogravimetry/differential thermogravimetry/differential thermal analysis) of the precursors dried at 100 °C showed an exothemal decomposition around 250–260 °C and no mass loss above 600 °C under airflow. The solids were heated at 600, 800 and 1000 °C and at each step the X-ray diffraction spectra were obtained in order to check the development of the spinel MAl₂O₄ crystal phase. For M = Mg, Co and Zn samples treated at 600 °C, the MgAl₂O₄, CoAl₂O₄ and ZnAl₂O₄ crystal phases are formed. Nitrogen porosimetry for the MAl₂O₄ samples, heated at 600 °C, revealed mesoporous solids of specific surface areas from 106 to 190 m² g⁻¹ depending on the M cation. At 1000 °C the MgAl₂O₄ spinel possesses a specific surface area of 57 m² g⁻¹ and ZnAl₂O₄ has a specific surface area of 75 m² g^-1.