Unusual Redox Behavior of Rh/AlPO₄ and Its Impact on Three-Way Catalysis

The influence of the redox behavior of Rh/AlPO₄ on automotive three-way catalysis (TWC) was studied to correlate catalytic activity with thermal stability and metal–support interactions. Compared with a reference Rh/Al₂O₃ catalyst, Rh/AlPO₄ exhibited a much higher stability against thermal aging under an oxidizing atmosphere; further deactivation was induced by a high-temperature reduction treatment. In situ X-ray absorption fine structure experiments revealed a higher reducibility of Rh oxide (RhO_<i>x</i>) to Rh, and the metal showed a higher tolerance to reoxidation when supported on AlPO₄ compared with Al₂O₃. This unusual redox behavior is associated with an Rh–O–P interfacial linkage, which is preserved under oxidizing and reducing atmospheres. Another effect of the Rh–O–P interfacial linkage was observed for the metallic Rh with an electron-deficient character. This leads to the decreasing back-donation from Rh <i>d</i>-orbitals to the antibonding π* orbital of chemisorbed CO or NO, which is a possible reason for the deactivation by high-temperature reduction treatments. On the other hand, surface acid sites on AlPO₄ promoted oxidative adsorption of C₃H₆ as aldehyde, which showed a higher reactivity toward O₂, as well as NO, compared with carboxylate adsorbed on Al₂O₃. A precise control of the acid–base character of the metal phosphate supports is therefore a key to enhance the catalytic performance of supported Rh catalysts for TWC applications

Rhodium Nanoparticle Anchoring on AlPO₄ for Efficient Catalyst Sintering Suppression

Rhodium catalysts exhibited higher dispersion with tridymite-type AlPO₄ supports than with Al₂O₃ during thermal aging at 900 °C under an oxidizing atmosphere. The local structural analysis via X-ray photoelectron spectroscopy, transmission electron microscopy, X-ray absorption fine structure, and infrared spectroscopy suggested that the sintering of AlPO₄-supported Rh nanoparticles was significantly suppressed because of anchoring via a Rh–O–P linkage at the interface between the metal and support. Most of the AlPO₄ surface was terminated by phosphate P–OH groups, which were converted into a Rh–O–P linkage when Rh oxide (RhO_<i>x</i>) was loaded. This interaction enables the thin planar RhO_<i>x</i> nanoparticles to establish close and stable contact with the AlPO₄ surface. It differs from Rh–O–Al bonding in the oxide-supported catalyst Rh/Al₂O₃, which causes undesired solid reactions that yield deactivated phases. The Rh–O–P interfacial linkage was preserved under oxidizing and reducing atmospheres, which contrasts with conventional metal oxide supports that only present the anchoring effect under an oxidizing atmosphere. These experimental results agree with a density functional theory optimized coherent interface RhO_<i>x</i>/AlPO₄ model

Tuning the Electron Density of Rh Supported on Metal Phosphates for Three-Way Catalysis

The automotive three-way catalysis (TWC) performance of Rh supported on alkaline-earth and rare-earth phosphates was studied in comparison to that of Rh on aluminum phosphate (AlPO₄). The anchoring of Rh via interfacial Rh–O–P bonding in Rh/AlPO₄ leads to efficient Rh sintering suppression. However, the electron-withdrawing nature of the phosphate affords electron-deficient Rh, which has a negative impact on its catalytic activity under a reducing atmosphere due to a decrease in back-donation from the Rh <i>d</i>-orbitals to the antibonding π* orbitals of adsorbed CO and NO molecules. Notably, the extent of this electron deficiency could successfully be reduced by replacing AlPO₄ with alkaline-earth or rare-earth phosphates, and the Rh oxide formed on these phosphate supports was readily reduced to metallic Rh. This behavior is in complete contrast to that of corresponding metal oxide supports, because the higher basicity of these supports yields Rh oxides that are more difficult to reduce. Among the phosphate-supported catalysts investigated in the present study, Rh/LaPO₄ demonstrated the highest TWC performance after thermal aging under both oxidizing and reducing atmospheres. The effect of the higher basicity of LaPO₄ compared to that of AlPO₄ is most obvious in its improved catalytic activity for elementary CO–O₂, CO–H₂O, and CO–NO reactions. Importantly, this improvement is achieved while maintaining the activity toward C₃H₆ as an advanced feature of phosphate supports

Rh/ZrP₂O₇ as an Efficient Automotive Catalyst for NO_<i>x</i> Reduction under Slightly Lean Conditions

The three-way catalyst performances of honeycomb-coated Rh catalysts were studied on several metal phosphate supports (AlPO₄, YPO₄, ZrP₂O₇, and LaPO₄) using various simulated exhausts with different air-to-fuel ratios (<i>A</i>/<i>F</i>). These supports significantly improved the NO_<i>x</i> purification (deNO_<i>x</i>) efficiency under slightly lean conditions (14.6 < <i>A</i>/<i>F</i> ≤ 15.3) as compared with conventional Rh/ZrO₂ catalysts. The deNO_<i>x</i> activity exhibited the following sequence of increasing the mean electronegativity of the supports: ZrO₂ < YPO₄ < LaPO₄ < AlPO₄ < ZrP₂O₇. Although both NO–CO and NO–C₃H₆ reactions contributed to the deNO_<i>x</i> process, the latter reaction was more favored on Rh/ZrP₂O₇ than on Rh/ZrO₂. Partially oxidized C₃H₆ was adsorbed on Rh/ZrP₂O₇ in the form of reactive aldehyde species, in contrast to the less-reactive carboxylate species adsorbed on Rh/ZrO₂. Furthermore, Rh oxide was more easily reduced to the active metallic state on ZrP₂O₇ compared with Rh/ZrO₂ when the atmosphere was changed from lean (<i>A</i>/<i>F</i> > 14.6) to rich (<i>A</i>/<i>F</i> < 14.6). Metallic Rh formed on ZrP₂O₇ was only slowly oxidized on exposure to excess O₂, whereas Rh on ZrO₂ was readily oxidized to less-active Rh₂O₃. The high activity of Rh/ZrP₂O₇ toward C₃H₆ oxidation via aldehyde species as well as the resistance of metallic Rh against reoxidation are considered to be possible reasons for the enhanced deNO_<i>x</i> performance of this catalyst in a slightly lean region