Evolution of Fe Crystallographic Sites from Barium Hexaaluminate to Hexaferrite

The substituted metal ions in the hexaaluminate structure are regarded as active centers, but their very limited substituted content prevents the obtainment of higher activity. Fe-substituted hexaaluminates exceptionally overcome such a limit. In this paper, we have studied the Fe crystallographic sites in BaFe x Al 12– x O 19 ( x = 1–12) and proposed the unique mechanism of stabilization of Fe ions in hexaaluminate structure at a high substitution level by employment of Rietveld refinement and 57 Fe Mössbauer spectroscopy. When x = 1–4, the site occupancy of Fe 3+ in both β I -Al 2 O 3 and newly formed magnetoplumbite phases was kept constant, and the increased Fe 3+ ions were accommodated by the increased amount of magnetoplumbite phase. When x = 5–12, the site occupancy of Fe 3+ in the magnetoplumbite phase continuously increased, and thus the increased Fe 3+ ions were accommodated by the magnetoplumbite structure. BaFe x Al 12– x O 19 catalysts exhibited different N 2 O decomposition properties under conventional furnace heating and microwave heating modes.