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 x ) 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 d-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 catalysts exhibited higher dispersion with tridymite-type AlPO 4 supports than with Al 2 O 3 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 AlPO 4supported 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 AlPO 4 surface was terminated by phosphate P−OH groups, which were converted into a Rh−O−P linkage when Rh oxide (RhO x ) was loaded. This interaction enables the thin planar RhO x nanoparticles to establish close and stable contact with the AlPO 4 surface. It differs from Rh−O−Al bonding in the oxide-supported catalyst Rh/Al 2 O 3 , 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 x / AlPO 4 model.

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 (A/F). These supports significantly improved the NO x purification (deNO x ) efficiency under slightly lean conditions (14.6 < A/F ≤ 15.3) as compared with conventional Rh/ZrO2 catalysts. The deNO x 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 x 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 (A/F > 14.6) to rich (A/F < 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 x performance of this catalyst in a slightly lean region.

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 (AlPO 4 ). The anchoring of Rh via interfacial Rh−O−P bonding in Rh/AlPO 4 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 dorbitals to the antibonding π* orbitals of adsorbed CO and NO molecules. Notably, the extent of this electron deficiency could successfully be reduced by replacing AlPO 4 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/LaPO 4 demonstrated the highest TWC performance after thermal aging under both oxidizing and reducing atmospheres. The effect of the higher basicity of LaPO 4 compared to that of AlPO 4 is most obvious in its improved catalytic activity for elementary CO−O 2 , CO−H 2 O, and CO−NO reactions. Importantly, this improvement is achieved while maintaining the activity toward C 3 H 6 as an advanced feature of phosphate supports. ■ INTRODUCTIONRh is a key element of current three-way catalyst (TWC) technology due to its specificity for the catalysis of NO reduction to N 2 even at lower concentrations than those required for Pd and Pt catalysts. Rh is indispensable for NO dissociation and the recombination of N atoms to yield N 2 , whereas Pd and Pt are responsible for the oxidation of CO and hydrocarbons to CO 2 and H 2 O. 1−5 However, among the platinum-group metals, Rh has the highest cost and the lowest availability, and thus reduction of its use in TWCs is necessary. Several attempts to address this issue have involved the use of metal−support interactions to suppress sintering and extend catalyst life. 6−18 In conventional TWCs, the interactions at the interface between Rh oxide (Rh 2 O 3 ) and metal oxide supports play a key role. For instance, Tanabe et al. 15 reported that an Nd 2 O 3 -enriched ZrO 2 surface efficiently stabilizes Rh nanoparticles through Rh−O−Nd interfacial bonding. This behavior is analogous to anchoring mechanisms reported for Pt and Pd supported on CeO 2 , 10−13,17,19−21 which are due to electrostatic interactions at the metal/oxide support interface under an oxidizing atmosphere.We previously reported another type of anchoring mechanism in Rh/AlPO 4 in which the Rh nanoparticles are stabilized against sintering via Rh−O−P interfacial bonding. 22−25 This Rh−O−P bonding is preserved when the reaction conditions are changed from oxidizing to reducing atmosphere and vice versa 24 and is useful for stabilizing Rh under oscillating redox ...

A rhodium catalyst supported on AlPO4 exhibited a much higher resistance to sulfur and phosphorus poisoning compared with a reference catalyst (Rh/Al2O3). The acidic surface of AlPO4 was effective in preventing the adsorption of sulfur oxides (SO2), whereas Lewis acid/base sites on Al2O3 favored SO2 adsorption followed by the formation of sulfite, leading to deterioration of the activity of Rh/Al2O3 for the model NO–CO–C3H6–O2 reaction. Similarly, the AlPO4 support suppressed the extent of phosphorus poisoning caused by dimethylphosphite (DMP) (CH3O)2POH, which was used as a model phosphorus source. A greater amount of inactive phosphate overlayers were deposited from the gas feed containing DMP and O2 on Rh/Al2O3 than Rh/AlPO4 because of the reaction between P2O5 vapors and Al2O3. Consequently, the active Rh surface was covered to a greater extent for Rh/Al2O3 than Rh/AlPO4.