Abstract: The effects of calcium substitution for lanthanum on the catalytic activity and stability of CaxLa1-x-MnAl11O19hexaaluminate catalyst prepared by co-precipitation were investigated. The samples were characterized byX-raydiffraction, N2 adsorption, temperature-programmed reduction, and transmission electron microscopy (TEM). It was observed that Ca could not be totally incorporated into the hexaaluminate lattice for a high doping amount (x=0.8). In this case, a separate CaAl4O7 phase was formed. Ca0.2La0.8MnAl11O19possessed the best catalytic activity with light-off temperature of 723 K and the highest BET surface area of 26.1 m2/g after calcination at1373K. The TEM results revealed that the substitution of La3+by Ca2+led to a thinner hexaaluminate planer crystal, which improved the sintering resistance, increased the surface area, and raised the amount of reactive Mn3+species and the reducibility of the hexaaluminate. The enhanced catalytic activity was attributed to the combined effect of higher specific surface area and Mn3+content. Compared to Sr2+- and K+-substituted La-hexaaluminates, the Ca2+-substituted samples demonstrated the lowest light-off temperature in methane combustion.

Key words: calcium, hexaaluminates, methane, catalytic combustion, thermal stability