Hexaaluminate materials exhibit remarkable thermal stability due to their peculiar layered structure. The Al³⁺ ions in the hexaaluminate lattice can be substituted by transition or noble metals, giving rise to redox centers for a variety of reactions. Oxygen in the mirror plane of hexaaluminate is loosely packed, making it a preferential diffusion route of oxygen. All of these favor the application of hexaaluminate in high-temperature oxygen-involved reaction. In this review, the structure of hexaaluminate is firstly introduced. Furthermore, the effect of structure type(magnetoplumbite and β-Al₂O₃) and metal substitution on the microstructure of hexaaluminate (especially metal chemical state) are carefully described. Then we discuss recent advances of hexaaluminate in high-temperature oxygen-involved reactions, such as, catalytic combustion of CH₄, process-gas N₂O abatement, decomposition of N₂O as a propellant, CH₄ chemical looping combustion and reforming, with a special emphasis on the relationship between the microstucture and reaction performance. At last, a brief summary and an outlook are given.
Contents
1 Introduction
2 Crystal structure of hexaaluminate
2.1 Structural type and metal substitution
2.2 Anisotropic crystal growth and preferential diffusion of oxygen
3 Effect of metal substitution on microstructure of hexaaluminate
3.1 Substitution of large cations in the mirror plane
3.2 Substitution of Al³⁺ ions by transition metal ions
3.3 Substitution of Al³⁺ ions by noble metal ions
4 High temperature application of hexaaluminate
4.1 Catalytic combustion of methane
4.2 Catalytic decomposition of nitrous oxide
4.3 Chemical looping combustion and reforming of methane
5 Conclusion and outlook