Catalytic combustion of toluene over manganese-substituted hexaaluminate catalysts

Manganese-substituted hexaaluminate catalysts (LaMn_xAl_12-xO₁₉) were prepared by coprecipitation method. X-ray diffraction (XRD) characterization indicated that the LaMn_xAl_12-xO₁₉ catalysts calcined at 1 200 ℃ has only hexaaluminate phase, while no inactive LaAlO₃ or LaMnO₃ perovskite phase, when the amount of manganese substituted (x) is 2.0～2.5. The UV-vsible diffuse reflection spectra, H₂-TPR profiles and BET specific surface area measurements showed that Al sites substituted by Mn³⁺ are mainly the octahedral ones in hexaaluminate crystal at x ≥ 1.5. With the increase of the amount of manganese substituted, the Mn³⁺/Mn²⁺ ratio in the catalyst is increased and the specific surface area is decreased instead. O₂-TPD profiles showed that the amount of lattice oxygen by chemisorption is increased and the amount of molecular oxygen by physisorption is decreased instead, with the increase of the value x. The catalytic tests for toluene combustion showed that the manganese-substituted hexaaluminate catalysts exhibits excellent low temperature activity and the preferable x value is 2.0～2.5. The catalytic combustion of toluene over LaMn_xAl_12-xO₁₉ may follow the Mars-van Krevelen mechanism, in which Mn³⁺ and Mn²⁺ cooperate to promote the mobility of lattice oxygen.