Phase-dependent Al leaching and crystal structure stability of CoAl₂O₄ in alkaline media

Surface changes have attracted great interest in solid synthesis, catalysis and corrosion. Herein, we demonstrate a distinct surface Al/Co ratio-dependent reconstruction behavior in alkaline media for sol-gel-derived CoAl₂O₄ particles. Our results indicate that the nominal Al/Co = 2 represents an average composition for particles formed at low temperatures, with spinel Al-excess surfaces and Al-poor cores. Annealing at high temperature homogenizes the cation distribution by the progressive incorporation of surface-Al into the core. Upon exposure to alkaline solution, the Al-excess surface phases are selectively leached and stabilize at a new spinel-type Al-rich surface. In contrast, no selective leaching is observed in normal spinel CoAl₂O₄ with a homogeneously single phase. These unexpected findings highlight the surface-composition dependent phase dissolution and the progressive incorporation of atoms into specific crystal structures during multi-metal oxide synthesis, hence drawing attention to the neglected relay-like phase formation of each single compound before the final-phase formation of mixed metal compounds.