Synthesis of Cu-Al spinels and its non-isothermal formation kinetics analysis

Cu-Al spinels were synthesized by a solid phase method using Cu(OH)₂ and pseudo-boehmite as the raw materials. The effects of synthesis temperature, synthesis time and Cu/Al molar ratio on the formation and properties of Cu-Al spinels were fully investigated by the thermogravimetry(TG/DTG), X-ray diffraction(XRD), H₂ temperature programmed reduction(H₂-TPR). The non-isothermal kinetics of Cu-Al spinel formation process were analyzed using Coats-Redfern method and two diffusion-controlled kinetic models. Characterization results showed that the Cu-Al surface spinels with unsaturated coordination formed easily at the temperature as low as 400℃, and the content of these surface spinel decreased sharply with the synthesis temperature rising. The hardly-reducible spinel Cu²⁺ species and easily-reducible spinel Cu²⁺ species were identified at the synthesis temperature of 700 and 800℃, respectively. The spinel content increased gradually with the synthesis temperature increasing, leading to the formation of Al-rich spinel solid solutions with different Cu/Al molar ratios. At a higher temperature of 1200℃, however, the formation of stoichiometric CuAl₂O₄ spinel was observed. Hence, the spinel reducibility varied dramatically with the synthesis temperature as illustrated by the drastic change of the molar ratio of hardly-reducible spinel Cu²⁺ species and easily-reducible spinel Cu²⁺ species. An appropriate excess of Al³⁺(Cu/Al=1:3) could result in the formation of spinel solid solution with more hardly-reducible spinel Cu²⁺ species, while an excess of Cu²⁺ would lead to the formation of delafossite-type CuAlO₂. Both samples owned low reducibility as compared to the stochiometric CuAl₂O₄ spinel. Besides, a longer synthesis time would favor the spinel formation as well but to a limited extent. Non-isothermal kinetics analysis showed that the formation process of Cu-Al spinel owned three kinetic regions in terms of synthesis temperature, namely 700-850, 850-950 and 950-1200℃, and the apparent activation energies were determined to be 85.2, 304.4 and 38.1 kJ/mol, respectively. The diffusion of reactants via product layer could be considered as an one-dimensional diffusion below 950℃, whereas it was more likely to be a three-dimensional diffusion above 950℃, indicating that the product layer became much thicker.