Role of A- and B-cations in catalytic property of substituted hexaaluminate (ABAl11O19−α) for high temperature combustion

Abstract

Cation-substituted hexaaluminate compounds, ABAl 11 O 19−α , were investigated for application to high temperature catalytic combustion. Two series of modifications of the compounds was made by cation substitution; substitution of large cations in the mirror plane with lanthanides ions, and substitution of transition metals for Al site in the spinel block. In a series of AMnAl 11 O 19−α (A=La, Pr, Sm and Nd), surface area and catalytic activity increased with an increase in ionic radius of lanthanides. La 3+ is superior as the large cation in the mirror plane of the hexaaluminate to other tri-valent cations with small ionic radii. The catalytic activities of LaBAl 11 O 19−α (B=Cr, Mn, Fe, Co, Ni, and Cu) were enhanced when Mn and Cu were employed as the B-site substituents. Althoug Mn and Cu were also effective substituents for enhancing catalytic activity in Ba-based hexaaluminate compounds, their activity was low as compared with the La-based catalysts. These results indicate that the redox cycle of transition metal in hexaaluminate lattice and catalytic activity appears to be affected sensitively with the electronic or structural effect of large cation in the mirror plane.