Manufacturing AAl₂O₄ (A=Cu, Co, Ni, Zn) Spinel Catalysts for Toluene Combustion: Elucidating the A-site Replacement Effect on the Reactivity

To decode the A-site substitution effect on the reactivity of Al-based spinel complex oxides, a series of AAl(2)O(4) (A=Cu, Co, Ni, Zn) compounds were manufactured and tested by toluene combustion. It was confirmed that a pure spinel crystalline phase has been successfully synthesized for all the catalysts. The toluene and CO oxidation activity are sequenced as CuAl2O4>CoAl2O4>NiAl2O4>ZnAl2O4. As discovered, both active surface O-2(-) and acid sites are the two critical factors to determine the reactivity, which can adsorb and activate the reactants effectively to form active intermediates, possibly following a Mars-van-Krevelen pathway. The activity improves positively with the amount increasing of these two kinds of active sites, thus showing an evident A-site replacement effect. CuAl2O4 owns the biggest numbers of these two kinds of active sites, thereby exhibiting the optimal catalytic activity. Moreover, it also shows good stability in a dry feed, and considerable ability to resist water vapor and sulfur poisoning. In-situ DRIFTS results have indicated that benzyl, benzoate, benzaldehyde and benzoquinone might be the major reaction intermediates.