Effect of Ir crystallographic site on the catalytic performance of Ir-substituted barium hexaferrites for N 2O decomposition

Abstract

Ir-substituted barium hexaferrites (BaIr x Fe 12− x O 19, x ≤ 0.6) were prepared using the carbonates route and investigated for high-concentration N 2O decomposition. It was found that BaIr x Fe 12− x O 19 offered a better dispersion of iridium by incorporating Ir into the hexaferrite structure. The crystallographic sites of Ir in BaIr x Fe 12− x O 19 were identified through the analysis of Fe occupancy in the matrix by sensitive 57Fe Mössbauer spectroscopy. Framework iridium preferentially occupied the octahedral sites in the order of 2a, 12k, and 4f 2 as increasing x value. When x = 0.1 and 0.4, the substitution of Ir for Fe 3+ occurred in the 2a and 12k sites in the rigid spinel block with a comparable N 2O decomposition activity, while this substitution in the 4f 2 sites in the mirror plane at x = 0.6 resulted in a remarkable enhancement of activity. Ir ions in the octahedral 4f 2 sites in the loosely packed mirror plane were highly active for N 2O decomposition.