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 Al 2 O 3 -based oxides lose their three-way catalytic activity as a result of unfavorable interface interactions that allow Rh 3+ to diffuse into the support structure and occupy Al 3+ sites. This study showed that the incorporated Rh 3+ ions were not easily reduced to active Rh metal species and caused substantial thermal deactivation. The deactivation was most obvious for γ-Al 2 O 3 and MgAl 2 O 4 with a spinel-type structure but much less for hexaaluminate (LaMgAl 11 O 19 ) 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 LaMgAl 11 O 19 showed that the diffusion along the c axis (∥c) was significantly suppressed compared to that normal to the c axis (⊥c). The diffusion of Rh 3+ was faster and nearly isotropic in a MgAl 2 O 4 single crystal, which was used as a model crystal for γ-Al 2 O 3 . These results show that the layered structure of hexaaluminate influences the Rh 3+ 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 Rh 3+ ions from the surface and preserves their smooth reduction to active metallic Rh nanoparticles.

Stability of PdxOy Particles Supported on Strontium Titanate Perovskite under Three-Way Catalyst Operating Conditions: Implications for Sintering Resistance

“…However, the thermal instability originated from the agglomeration (sintering) of metal nanoparticles (NPs) over time under operating condition is the major concern, especially for the catalyst with metal NPs in small sizes . Self-regenerable metal NPs on the oxide surface show great potential to overcome this problem. − Metal oxide NPs are highly anchored on the oxide support under oxidative conditions with strong metal–support interaction (MSI) leading to the sintering resistance, while they are subsequently reduced back to active metallic NPs under reductive conditions. − This approach is often employed to prepare the ultrastable TWCs in which the platinum-group metal (PGM) NPs are exposed to the high-temperature reduction–oxidation (redox) fluctuations of exhaust gas. , PGM-O–M bonds (M is the metal of the support surface) formed in oxidative atmospheres can suppress sintering effectively. ,,− …”

“…However, a very strong PGM-O-M bonding can lead to another deactivation process, i.e., a solid-solution reaction (SSR) under oxidative condition, − where the PGM ions are incorporated into the metal sites of oxide support. If the PGM cannot be reduced back to metallic NPs even under reductive conditions, the catalyst is deactivated. − For example, strong Rh–O–Al bonds in Rh/γ-Al 2 O 3 effectively inhibit the sintering, ,, but the SSR between Rh 2 O 3 and γ-Al 2 O 3 deactivates the catalyst under the TWC oxidative condition. Replacing γ-Al 2 O 3 with LaMgAl 11 O 19 can help mitigate SSR because the La–O layer of LaMgAl 11 O 19 ,, suppresses the penetration of Rh 3+ and preserves Rh 3+ near the surfaces.…”

“…If the PGM cannot be reduced back to metallic NPs even under reductive conditions, the catalyst is deactivated. − For example, strong Rh–O–Al bonds in Rh/γ-Al 2 O 3 effectively inhibit the sintering, ,, but the SSR between Rh 2 O 3 and γ-Al 2 O 3 deactivates the catalyst under the TWC oxidative condition. Replacing γ-Al 2 O 3 with LaMgAl 11 O 19 can help mitigate SSR because the La–O layer of LaMgAl 11 O 19 ,, suppresses the penetration of Rh 3+ and preserves Rh 3+ near the surfaces. Consequently, it is important to inhibit the direct PGM ion penetration from the support surface to the bulk region and to allow a smooth redox cycle of PGM NPs by TWC atmospheric fluctuation. − …”

“…Replacing γ-Al 2 O 3 with LaMgAl 11 O 19 can help mitigate SSR because the La–O layer of LaMgAl 11 O 19 ,, suppresses the penetration of Rh 3+ and preserves Rh 3+ near the surfaces. Consequently, it is important to inhibit the direct PGM ion penetration from the support surface to the bulk region and to allow a smooth redox cycle of PGM NPs by TWC atmospheric fluctuation. − …”

“…If the PGM cannot be reduced back to metallic NPs even under reductive conditions, the catalyst is deactivated. 12−15 For example, strong Rh−O−Al bonds in Rh/γ-Al 2 O 3 effectively inhibit the sintering, 12,13,15 region and to allow a smooth redox cycle of PGM NPs by TWC atmospheric fluctuation. 12−17 Among the numerous candidates for the TWC support materials, titanate perovskite (ATiO 3 ) and the related materials have received considerable research attention because of their chemical stability, strong anchoring effect, and low SSR.…”