Anisotropic Rh3+ Diffusion in Layered Hexaaluminate Mitigates Thermal Deactivation of Supported Rhodium Catalysts

Abstract

When exposed to a high-temperature oxidizing environment, Rh catalysts supported on Al2O3-based oxides lose their three-way catalytic activity as a result of unfavorable interface interactions that allow Rh3+ to diffuse into the support structure and occupy Al3+ sites. This study showed that the incorporated Rh3+ ions were not easily reduced to active Rh metal species and caused substantial thermal deactivation. The deactivation was most obvious for γ-Al2O3 and MgAl2O4 with a spinel-type structure but much less for hexaaluminate (LaMgAl11O19) with a layered structure consisting of alternative stacking of a spinel block and a La–O monolayer. After annealing at 900 and 1000 °C for 100 h in the air, Rh-deposited single crystals were studied by dynamic secondary ion mass spectrometry to analyze the Rh depth profile. The Rh depth profile in LaMgAl11O19 showed that the diffusion along the c axis (∥c) was significantly suppressed compared to that normal to the c axis (⊥c). The diffusion of Rh3+ was faster and nearly isotropic in a MgAl2O4 single crystal, which was used as a model crystal for γ-Al2O3. These results show that the layered structure of hexaaluminate influences the Rh3+ diffusion, i.e., the La–O interlayer between closely packed spinel blocks running at every approximately 1.1 nm interval is likely the diffusion barrier. This barrier effectively blocks further penetration of the incorporated Rh3+ ions from the surface and preserves their smooth reduction to active metallic Rh nanoparticles.