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.

In situ time-resolved diffuse reflectance spectroscopy was first applied to supported Rh catalysts (0.4 wt % Rh/ZrO 2 and Rh/ZrP 2 O 7 ) under dynamic three-way catalysis conditions fluctuating between fuel-lean and fuel-rich gas atmospheres. The optical absorption at 650 nm was found to decrease upon lean-to-rich switching of the gas feed, which led to the reduction of Rh oxide (Rh 3+ ) to metallic Rh (Rh 0 ), followed by a reversible increase upon back switching rich-to-lean. The kinetic analysis suggested that the reduction of Rh 3+ to Rh 0 was faster than the reoxidation over Rh/ ZrP 2 O 7 , whereas the reduction was comparable with or slower than the reoxidation over Rh/ZrO 2 . The activation energy of Rh/ZrP 2 O 7 for the reduction, 13.6 kJ mol −1 , was smaller than that for the oxidation, 48.7 kJ mol −1 , which contrasted with those of Rh/ZrO 2 (21.4 and 34.1 kJ mol −1 , respectively). These results were closely associated with the higher NO reduction activity of Rh/ZrP 2 O 7 than Rh/ZrO 2 under a lean-gas atmosphere because Rh was more active in the metallic state than in the oxide state. Applying fast lean−rich perturbation of the gas feed with 1 s intervals led to an immediate and significant drop of the optical absorption intensity, suggesting that the reduction of Rh substantially penetrated to deeper layers under the surface. This study provided the first in situ evidence for the formation of active metallic Rh species under high-frequency lean−rich oscillations.

No abstract