Hydrotalcite-derived aluminum-doped cobalt oxides for catalytic benzene combustion: Effect of calcination atmosphere

Al-doped Co3O4 and CoO are prepared from Co‒Al hydrotalcite by calcination in air and N2 atmospheres, and characterized by TG-DTA, XRD, SEM, N2 adsorption, Raman, XPS, and H2-TPR. Co‒Al hydrotalcite is oxidized to Co(Co,Al)2O4 in air accompanying with the collapse of layered structure, while in N2 it decomposes to Co(Al)O that remains the plate morphology of hydrotalcite. In both cases, Al3+ is doped into the lattice of cobalt oxides, leading to marked changes in the crystal size, surface state, and reducibility. Especially, Al-doped CoO shows smaller crystal size (4.1 nm), larger surface area (172 m2 g‒1), and higher degree of structural disorder than Al-doped Co3O4. Meanwhile, the Al-doped cobalt oxides show different reducibilities to those of Co3O4 and CoO, indicating a strong interaction between cobalt and aluminum. The N2-calcined sample exhibits higher activity for benzene combustion than the air-calcined sample as well as a relatively good stability during heating/cooling cycles and a good long-time durability. Under the reaction atmosphere, Co(Al)O is transferred to Co(Co,Al)2O4. The resulting Co(Co,Al)2O4 remains the original plate morphology and textural property and presents abundant surface adsorbed oxygen species, which accounts for its high activity.