Catalyst Deactivation via Rhodium–Support Interactions under High-Temperature Oxidizing Conditions: A Comparative Study on Hexaaluminates versus Al2O3

Undesired solid-state reactions between rhodium (Rh) oxide and γ-Al 2 O 3 are a major cause of catalyst deactivation under high-temperature-oxidizing atmosphere, given that the Rh 3+ species incorporated into the bulk Al 2 O 3 structure are not easily reduced to active Rh metal species. To overcome this problem, the present study focused on replacing Al 2 O 3 with hexaaluminates as the thermostable supports for Rh. The hexaaluminate compounds, LaAl 11 O 18, LaMgAl 11 O 19, and La 0.8 Sr 0.2 MgAl 11 O 19, were found to successfully mitigate the deactivation of Rh catalysts following thermal aging at 900 °C in air and to have the capacity to preserve the catalytic activities for a model NO–CO–C 3 H 6 –O 2 reaction superior to that of Rh/Al 2 O 3 . The hexaaluminate structure consists of stacking a La–O monoatomic interlayer and a spinel block, the ionic arrangement of which is similar to the cation-deficient spinel structure of γ-Al 2 O 3 . Here, Rh 3+ ions are considered to replace the Al 3+ site in these spinel-based structures near the subsurface. However, the presence of the La–O interlayer in the hexaaluminates blocks the penetration of Rh 3+ and keeps this cation near the subsurface, as revealed via X-ray photoelectron spectroscopy, X-ray absorption fine structure, and H 2 temperature-programmed reduction analysis. Because the thin planar morphology of hexaaluminate particles spreads along the (001) plane, the basal planes account for a large portion of the exposed surface area. This surface will provide the most efficient blocking effect because the La–O interlayer also runs parallel to the (001) plane.