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A simple three-way catalyst (TWC) activity function based upon the alteration of the Pd metallic surface area (MSA) of Pd-only commercial monolith TWCs has been developed to describe the change of the catalytic performance with respect to the Pd metal loading varying from 5 to 10 g/L and the field-aged catalyst mileage from 4K (stabilized) to 98K miles. Particularly, the change of the Pd MSA of TWC with respect to the catalyst mileage has been well-correlated by the second-order sintering kinetics, regardless of the Pd content of the TWC. The TWC activity function was incorporated into the primary reaction kinetics developed for the stabilized 4K Pd-only TWCs to predict the deactivation of the Pd-only TWC performance with respect to the field-aged mileage. The overall reaction kinetic model with the TWC activity function developed in the present study is capable of describing the alteration of TWC performance with respect to the catalyst mileage, regardless of the catalyst metal content. ' INTRODUCTIONThe development of the catalyst activity function is useful for predicting the life of the catalyst for designing a commercial catalytic reactor, since more than 90% of the heterogeneous catalytic process experiences a decay of the reaction rate with respect to the reactor on-stream time. 1 The activity function can be obtained by correlating the rates at any time to time zero, although it requires exhaustive time and effort including the collection and test of the deactivated catalyst samples under practical operating conditions with respect to the duration of their uses. Another approach may be the independent development of an activity function based upon the alteration of the physicochemical properties of the catalyst including the content of the deactivation precursors, such as coke, sulfur, and contaminants, the metal dispersion, and the catalyst surface area. 2À4 The deactivation of the activity and durability of three-way catalyst (TWC) installed into gasoline driven cars is apparent as the catalyst mileage increases. 5 Furthermore, the technical and market demands for developing the deactivation model to predict the life of TWC becomes inevitable for the design of the TWC catalytic converter to meet the ever-tightening emission regulations throughout the world and the mileage guarantee by automakers. 6 However, a TWC activity function directly describing the alteration of TWC performance has been rarely reported, mainly due to the difficulty in collecting of the catalyst samples with respect to the catalyst mileage and the complexity of the deactivation of commercial TWC under the real driving condition.Ekstr€ om et al. predicted the decline of the catalytic activity of TWC aged under accelerated engine bench condition by the simple adjustment of the frequency factor of the rate constant included in the primary kinetics. 7 Baba et al. developed a TWC deactivation kinetics based upon the change of the frequency factor attributed to both sintering of Pt metal and phosphorus poisoning under accelerated engin...

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