EFFECT OF STRUCTURAL MODIFICATION ON THE CATALYTIC PROPERTY OF MN-SUBSTITUTED HEXAALUMINATES

The effects of structural modification on the catalytic properties of Mn-substituted hexaaluminates were investigated with a view toward developing a high-temperature combustion catalyst. The surface areas and catalytic activities of Mn-substituted hexaaluminates are significantly enhanced when the mirror plane cations are partially replaced by aliovalent cations of the same size. In the Sr1-xLaxMnAl11O19-α system calcined at 1300°C, Sr substitution gave rise to the maximum surface area, 23.8 M2/g, and the maximum combustion activity (T10% = 500°C) at x = 0.2. Transmission electron microscopy revealed that the substitution thinned the planar crystals of hexaaluminate and thus increased the surface area. Although the increase in catalytic activity corresponds to that in surface area, Sr substitution also affects the oxygen sorption property and the oxidation state of Mn ions in the hexaaluminate lattice. In particular, oxygen species desorbed at high temperatures (650-800°C) appear to play an important role in catalytic reaction. These oxygen species are related to the oxidation/reduction of Mn ion, which is brought into the highest oxidation state by the partial substitution. © 1990.