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The catalytic activity of Rh/ZrO 2 for NO-CO-C 3 H 6 -O 2 reaction under stoichiometric conditions was significantly improved by addition of rare earth. Among the catalysts tested here, Y-doped Rh/ZrO 2 (Rh/Y/ZrO 2 ) showed the highest three-way catalytic performance. The catalytic activity of rare earth-doped Rh/ZrO 2 was strongly related to the Rh dispersion, which was improved by using rare earth-doped ZrO 2 with large amount of surface basic sites. The role of rare earth additive was considered to control the surface basicity of ZrO 2 and stabilize the Rh species in high dispersion state. Y-stabilized ZrO 2 was found to be effective support for Rh catalyst in NO-CO-C 3 H 6 -O 2 reaction. The site geometry and the surface electric state of Rh can be altered by interacting with Y 2 O 3 , leading us to consideration that the presence of suitable Rh-Y 2 O 3 interaction is responsible for high NO reduction activity of Rh/Y-ZrO 2 .

The effect of crystal structure of Y-doped ZrO 2 as support on the three-way catalytic performance of Rh catalyst was investigated. Rh catalyst supported on 3Y-ZrO 2 with tetragonal phase showed higher activity than that on 3Y/ZrO 2 with monoclinic phase. The difference in the Rhsupport oxide interaction was suspected from the measurements of temperature-programmed reduction by CO (CO-TPR). The use of Y-doped ZrO 2 as support was found to contribute to the formation of easily reducible Rh species, resulting in high catalytic activity. Rh species can more strongly interact with 3Y/ZrO 2 than with 3Y-ZrO 2 , resulting in the formation of catalytically active reducible Rh species in Rh/3Y-ZrO 2 , which showed the highest activity. From Fourier transform infrared (FT-IR) spectroscopy following CO adsorption, no qualitative difference in the surface valence state of Rh particles irrespective of crystallite structure of Y-doped ZrO 2 was observed. On the other hand, a large fraction of the surface of Rh particles in Rh/3Y-ZrO 2 was found to be Rh 0 sites rather than Rh + sites. This is related to the interaction of Rh particles with Y species in tetragonal 3Y-ZrO 2. We concluded that the tetragonal structure of 3Y-ZrO 2 with which Rh can interact via Y species is a key-factor to obtain highly active Rh catalysts.

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