팔라듐 기반 상용 삼원촉매의 활성저하에 대한 반응속도론적 연구

Abstract

The automotive catalytic performance of TWC commonly depends on the catalyst mileages under real driving condition and the contents of noble metal such as Pd, Pt and Rh. When a TWC converter was operated under commercial operating condition, the deactivation of TWCs attributed to the exhaust gas temperature (up to 1000 oC) occurred with noble metal sintering. It may be another extensively affected by the contents of the metals included in TWC due to the important role for automotive catalytic efficiency and its durability. Although a number of TWC kinetics have been proposed to describe the catalyst activity, no systematic study for developing the deactivation kinetics of TWC has been reported yet with respect to the catalyst mileages. In the present study, it has been focused on the development of deactivation kinetics for predicting the TWC performance with respect to the catalyst mileages and Pd metal contents under realistic feed stream. In order to elucidate the effect of the catalyst mileages and Pd metal contents on the deactivation kinetics of Pd-based TWC, the commercial Pd-based TWCs provided by General Motors and Hyundai Motor Company, included on the systematic variations of the catalyst mileages aged by customers from 4k to 100k miles and Pd contents from 2g/L to 10g/L have been examined. The activity of Pd-based TWCs has been investigated using a packed-bed reactor system under the realistic feed stream. In addition, the alteration of the catalyst physicochemical characteristics has been extensively determined by pulse CO-chemisorption apparatus. The metal dispersion of Pd included in TWC with respect to the catalyst mileages has been decreased as the catalyst mileage increases. It seems that the TWC deactivation can be affected by loss of the active sites, strongly related with the Pd metal dispersion under real driving condition. With the characteristic property, the deactivation kinetics of commercial Pd-based TWCs has been developed on the basis of a simple empirical model as a function of the catalyst mileages. Particularly, the alteration of the metal dispersion of Pd included in TWC with respect to the catalyst mileages has been well described by the 2nd order deactivation kinetics, regardless of the metal contents. The kinetic parameters of stabilized 4k-Pd TWCs employed as fresh catalysts have been obtained for developing a primary kinetics by using the detailed TWC reaction kinetics previously reported. The primary kinetics well predicts the TWC performance of 4k- Pd TWCs whose Pd contents varied from 2g/L to 10g/L over the reactor space velocities covering 50,000 to 170,000 h-1 by the alteration of the frequency factors of reaction rate constants. Eventually, overall kinetics on the basis of the deactivation kinetics developed in the present study well describes the alteration of TWC performance, regardless of the Pd contents. On the other hand, the effect of Pd metal contents on TWC activity has been investigate to confirm the specific reaction property of both the oxidation and reduction during the course of TWC reaction on the surface of Pd TWCs. The enhancement effect of NO reduction activity as the increasing Pd metal contents has been observed over 4k-Pd TWCs, but that of the CO-oxidation activity was not. To indentify the effect of Pd metal contents on TWC activity, the various catalyst characterizations such as the BET surface area, NO-TPD and CO-chemisorption have been conducted over stabilized 4k-Pd TWCs with respect to the Pd metal contents varied from 0.7 (Pd 5 g/L) to 1.4 wt.% (Pd 10 g/L). The physicochemical properties including the BET surface area and the amount of NO adsorbed are hardly influenced by the alteration of the Pd contents (Pd 5, Pd 7 and Pd 10) employed in the present study. However, around 2-fold increase in the Pd metal particle size of 4k-Pd TWCs was observed, whereas Pd metallic surface area hardly changes with Pd metal contents increased from 0.7 wt.% to 1.4 wt.% by the pulse CO-chemisorption. Based upon the result observed, the specific reaction rates expressed by turnover number (TON) for both the CO-oxidation and NO-reduction have been estimated to compare the structural dependency of the reactions occurred over the Pd-TWC. The turnover number (TON) of NO-reduction increases with increasing the particle size of Pd at both temperatures of 170 and 180 oC while, that of CO-oxidation was quite similar at both temperatures of 190 and 200 oC. In conclusion, CO-oxidation is structure insensitive and the NO-reduction reaction on the surface of TWC attribute to the structure sensitive reaction with alteration of Pd particle size under the realistic feed stream. The deactivation kinetics of commercial Pd-based TWCs developed in present study will be a guideline for predicting the life of TWC which has long been an issue for automakers. In addition, the effect of Pd metal contents concerned in the present study may be good information for the commercial manufacturing of TWC in view of industrial cost.