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 gamma-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 La0.8Sr0.2MgAl11O19, were found to successfully mitigate the deactivation of Rh catalysts following thermal aging at 900 degrees C in air and to have the capacity to preserve the catalytic activities for a model NO-CO-C3H6-O-2 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.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 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.

Automated summary

The study found that replacing Al2O3 with hexaaluminates can effectively mitigate the deactivation of Rh catalysts under high-temperature oxidizing atmosphere, with the hexaaluminate structure able to block the diffusion of Rh3+ species and keep them near the surface. This structure shows promise in maintaining catalytic activity and outperforming Al2O3 as a support material for Rh catalysts.