Identification of the Crystallographic Sites of Ir in BaIr0.2FeAl10.8O19 Hexaaluminate

Abstract

Noble metal ions in hexaaluminate structure are generally regarded as active centers for a variety of reactions, but their chemical states are still not clear for lack of effective characterization techniques. In this paper, in comparison with BaFeAl11O19, the crystallographic site of Ir in BaIr0.2FeAl10.8O19 hexaaluminate with Ba-beta(1)-Al2O3 structure was identified by Rietveld refinement and Fe-57 Mossbauer spectroscopy. In the BaIr0.2FeAl10.8O19 hexaaluminate structure, Fe occupied both the symmetric tetrahedral Al(2) sites in the spinel block and the distorted tetrahedral interstitial Al(5) sites in the mirror plane. In contrast to Fe, the framework Ir ions only occupied the distorted tetrahedral interstitial Al(5) sites in the loosely packed mirror plane, which originated from Ir ions in oxidic entities dispersed on the Ba-modified gamma-Al2O3 in the precursor. Ir ions in the Al(5) sites were highly active for N2O decomposition.