(440e) Deactivation Model for Commercial Pd-Only TWCs

When properly defined and correctly combined with the primary reaction kinetics, the catalyst activity function can be a very useful parameter for predicting the performance of a catalyst by the numerical simulation of a reactor model, especially when the catalyst undergoes a deactivating process.¹  However, few such simulation models that can reliably predict the TWC performance as a function of the noble metal loading and the catalyst mileage have been reported, probably due to the paucity of catalyst samples with a desired range of the catalyst mileages accumulated under realistic driving conditions.  Ekström et al. predicted the decline of the catalytic activity of TWC aged under an accelerated engine bench condition by a simple adjustment of the frequency factor of the rate constant in the primary kinetics.²  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 the accelerated engine bench condition.³  They reported that the sintering of noble metal is the primary cause for the deactivation of TWCs due to the decrease of noble metal surface area (MSA).  Recently, a simple 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 142 to 283 g/ft³ and the field-aged catalyst mileage ranging from 4k (stabilized) to 98k miles.⁴

             In the present study, the TWC activity function developed previously has been further validated in describing the deactivation behavior of a commercial Pd-only TWC with a low Pd loading of 80 g/ft³ (Pd80) in the lab-aging program.  In a comparative aging study, the TWC activity over the Pd catalyst containing 142 g/ft³ of Pd (Pd142) aged by the lab-aging program was found to be quite similar to that over an identical catalyst aged by the field-aging program over a wide range of the catalyst mileage from 4k to 98k miles.  Given this well-matched TWC activity of the Pd142 catalyst between the lab-aged and the field-aged samples, the Pd80 catalysts were aged for 4k, 20k, 50k, 100k and 150k mile-equivalent by using the lab-aging program.  The initial deactivation of the TWC activity over the commercial Pd80 catalyst aged by the lab-aging program was similar to that over the filed-aged catalyst in the low catalyst mileage from 4k to 20k-mile.  The change of the Pd MSA of TWC as a function of the catalyst mileage has been well correlated by the 2^nd order sintering kinetics for all levels of the Pd loading of the catalyst.  The TWC activity function was then incorporated into the primary reaction kinetics developed for the stabilized 4k Pd80 TWCs to predict the deactivation of the Pd-only TWC as a function of the catalyst mileage.  The overall reaction kinetic model coupled with the TWC activity function developed in the present study has proven to be capable of describing the alteration of TWC performance as a function of the catalyst mileage, regardless of its metal loading and aging modes – field-aged or lab-aged.

References

1. J. A. Moulijn, A. E. van Diepen, F. Kapteijn, Appl. Catal. A:Gen., 212 (2001) 3.

2. F. Ekström, F. Wallin, A. Fathali, G. Russ, Top. Catal., 52 (2009) 1915.

3. N. Baba, K. Yokota, S. Matsunaga, S. Kojima, K. Ohsawa, T. Ito and H. Domyo, SAE Technical Paper Series 2000-01-0214 (2000).

4. S. B. Kang, H. J. Kwon, I.-S. Nam, Y. I. Song, S. H. Oh, Ind. Eng. Chem. Res., (2011) DOI: 10.1021/ie200083f.