팔라듐 기반 삼원촉매의 초기입자크기가 소결속도에 미치는 영향 연구

Abstract

Three-way catalyst (TWC) has been widely employed as the most efficient after-treatment catalytic system for gasoline engine vehicles to simultaneously remove three major air pollutants such as carbon monoxide (CO), unburned or partially oxidized hydrocarbons (HCs) and nitrogen oxides (NOx). However, the sintering of noble metal attributable to the high exhaust gas temperature typically above 600 oC leads to the deactivation of TWC. Although the fundamental sintering behavior of the noble metals has been widely examined with respect to the time on stream, a systematic study for the effect of the initial particle size on the sintering rate and sintering kinetic parameter in the Ostwald-ripening mechanism has been hardly investigated, particularly over Pd-based TWCs. In the present study, the effect of the initial Pd particle size on the sintering rate of Pd-based TWC catalysts has been examined over both the commercial Pd catalyst obtained from the GM R&D and the lab-prepared model Pd catalyst with a wide range of the initial Pd particle size as a function of the catalyst mileage from 4k to 100k miles. The average particle size of Pd has been systematically analyzed by a variety of catalyst characterization tools including CO-chemisorption, XRD and TEM analysis. The initial Pd particle size of the 4k-stabilized commercial and lab-prepared model Pd catalyst increased with the increasing Pd loading from 10 to 400 g/ft3. The sintering rate of both catalysts strongly depended on their initial particle sizes, revealing a slower sintering rate of the Pd catalyst with a larger initial Pd particle size. After the 20k-miles equivalent aging, the average size of Pd particles over the Pd catalyst decreased as the Pd loading increased from 10 to 400 g/ft3. This sintering trend of the Pd catalyst was maintained when the catalyst mileage increased from 20k to 100k miles. Moreover, the TWC performances of the 100k commercial Pd240 and Pd80 catalysts have been compared, while maintaining an identical total amount of Pd in both reactors; the 100k Pd240 commercial catalyst exhibited a higher TWC activity than the 100k Pd80 catalyst. To further confirm the sintering trend of Pd, the effect of the initial Pd particle size on the sintering rate has also been investigated over the model Pd80 catalyst supported on the fresh γ-Al2O3 or the pre-aged γ-Al2O3. The initial particle size of the model Pd80 catalyst was controlled by pre-aging the γ-Al2O3 before impregnating Pd onto the Al2O3 support. As expected, the 4k stabilized Pd80 catalyst supported on the pre-aged γ-Al2O3 exhibited a lower TWC performance and a larger initial Pd particle size, compared to that supported on fresh γ-Al2O3. After the 100k miles-equivalent aging of the catalyst, the Pd80 catalyst supported on the pre-aged γ-Al2O3 sintered more slowly than that supported on the fresh γ-Al2O3. To identify the critical factor affecting the sintering rate of the Pd catalyst, the sintering kinetics of the Pd catalyst has been investigated on the basis of the Ostwald-ripening mechanism. The sintering equation based upon the Ostwald-ripening mechanism reasonably well predicted the alteration of the Pd particle size as a function of the aging time and the initial particle size, correctly capturing the moderation of the sintering rate of the Pd catalyst with a larger initial Pd particle size. In addition, the alteration of the sintering kinetic parameter (KI) for both the commercial and the model Pd catalysts has been successfully described by an identical sintering correlation, indicating that the sintering rate of Pd strongly depends on the initial particle size, regardless of the catalyst preparation method. Furthermore, both surface tension (γm) and the rate constant of Pd monomer detachment (Dt) have played important roles in determining the sintering kinetic parameter (KI) of the Pd catalyst. The surface tension of Pd particles calculated by using a numerical simulator (Forcite module, Material studio 7.0) exponentially decreased from 3.17 to 1.77 J/m2 with the increasing initial Pd particle size from 4.0 to 17.0 nm. The rate constant of Pd monomer detachment (Dt) was also evaluated from the sintering kinetic parameter (KI), revealing the decrease of Dt as the initial Pd particle size increased. Specifically, the rate constant of Pd monomer detachment (Dt) exponentially decreased from 3.07x1019 to 4.70x1018 m-2s-1 as the initial Pd particle size increased from 4.0 to 17.0 nm. The decrease of the rate constant of Pd monomer detachment (Dt) with a larger Pd particle might be readily elucidated by the higher activation energy (E) for detaching a Pd monomer from the Pd crystallite.