Direct decomposition of nitric oxide over stannate pyrochlore oxides: relationship between solid-state chemistry and catalytic activity

Abstract

Rare-earth stannate pyrochlores were synthesized and their NO decomposition activity was investigated in relation with the solid-state property. For pyrochlores free from transition metal substituents, Ln 2Sn 2O 7 (Ln=Y, La, Nd, Sm, Gd) and La 2− x Ln x ′Sn 2O 7 (Ln′=Nd, Sm), the strength of an Sn–O bond evaluated from its stretching IR band decreased almost linearly with increasing cubic cell parameter or the size of rare earth ions. The catalytic activity for the NO decomposition changed significantly with the sort of rare-earth ions, and oxides having the intermediate Sn–O bond strength showed the highest NO decomposition activity. For La 2Sn 1.9M 0.1O 7 (M=3d transition metals, TM), the redox property of TMs was responsible for the NO decomposition activity, and the Co-substituted oxide showed the highest activity. Changes in activity for both the TM-free and TM-substituted systems were discussed on the basis of the redox-type reaction mechanism in which oxide ion vacancies are active sites.