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-Al₂O₃ are a major cause of catalyst deactivation under high-temperature-oxidizing atmosphere, given that the Rh³⁺ species incorporated into the bulk Al₂O₃ structure are not easily reduced to active Rh metal species. To overcome this problem, the present study focused on replacing Al₂O₃ with hexaaluminates as the thermostable supports for Rh. The hexaaluminate compounds, LaAl₁₁O₁₈, LaMgAl₁₁O₁₉, and La_0.8Sr_0.2MgAl₁₁O₁₉, 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–C₃H₆–O₂ reaction superior to that of Rh/Al₂O₃. 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-Al₂O₃. Here, Rh³⁺ ions are considered to replace the Al³⁺ site in these spinel-based structures near the subsurface. However, the presence of the La–O interlayer in the hexaaluminates blocks the penetration of Rh³⁺ and keeps this cation near the subsurface, as revealed via X-ray photoelectron spectroscopy, X-ray absorption fine structure, and H₂ 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.