귀금속 함량 및 촉매 마일리지에 따른 삼원촉매 활성 예측을 위한 Reaction kinetics 개발

Abstract

Three-way catalyst (TWC) has been widely employed as the most efficient catalytic system to simultaneously remove all three major air pollutants including CO, HCs and NOx from the automotive exhaust gas, mainly those emitted from gasoline engine. Moreover, the modern commercial TWC consists of the bimetallic Pd/Rh catalyst, instead of the traditional Pt/Rh catalyst, due to economic interests. In addition, the washcoating methodology of current bimetallic Pd/Rh TWC monolith has shifted from the conventional single-layered catalytic system to the double-layered one to avoid the catalyst deactivation caused by the formation of Pd-Rh metal alloy upon thermal aging. The development of a kinetic model describing the alteration of the TWC performance with respect to the catalyst metal loading and time-on-stream has been a long-lasting task in automotive reaction engineering. Indeed, the acvive metal surface area (MSA) representing the number of active reaction sites formed on the catalyst surface plays a critical role in determining the TWC activity, and strongly depends on the catalyst noble metal loading and the degree of catalyst aging. A simple activity function concept based on the physicochemical characteristics including the MSA of novel metals may be one way for possibly predicting the TWC performance varied by both the noble metal loading and the catalyst mileage. In the present study, 3D activity functions for each Pd and Rh catalyst have been independently developed on the basis of the active metal surface area (MSA) of Pd or Rh, respectively, which can be used to predict the TWC performance as a function of Pd or Rh loadings and the catalyst mileage. To develop the activity function for the Pd catalyst, the commercial Pd-based TWCs (Pdc1) with a wide range of the Pd loading from 20 to 240 g/ft3 were obtained from GM R&D. Another series of commercial Pd-based TWCs (Pdc2) with the Pd loading from 140 to 280 g/ft3 were employed for broadening the applicability of the activity function developed in this study. In addition, a series of the model Pd TWCs (Pdm) with the Pd loading from 50 to 200 g/ft3 was prepared to further validate the activity function derived. Based upon the alteration of the Pd MSA examined by CO chemisorption, the 3D activity function for the Pd catalyst has been developed over the wide range of the Pd loadings from 20 to 280 g/ft3 and the catalyst mileage from 4k to 100k miles by integrating the second-order deactivation kinetics with the initial condition at 4k miles described as the activity function for 4k reference catalysts. The nonlinear catalytic activity of two series of commercial Pd-based TWCs (Pdc1 and Pdc2) as a function of the Pd loading and catalyst mileage was reasonably well predicted by the 3D activity function developed for the Pd catalyst. It has also proven to be universally applicable for the lab-prepared model Pdm catalysts with respect to the catalyst Pd loading and mileage. Moreover, the catalyst mileage of the Pdc3 catalyst aged by engine dynamometer was correctly estimated by the new 3D mileage function, which can be used in predicting the life expectancy of any Pd-based TWCs. The 3D activity function for the Rh catalyst on the basis of the alteration of the Rh MSA has been also developed by integrating the second-order deactivation kinetics. The activity function for the Rh catalyst again well predicted the linear dependence of the catalytic activity with the increasing Rh loading from 2.5 to 15 g/ft3 as well as the nonlinear decreasing trend of the catalytic activity with the increasing of the catalyst mileage from 4k to 100k miles. Based upon the detailed TWC reaction mechanism postulated, the reaction kinetics for each 4k Pd and 4k Rh catalysts employed as a reference catalytic system has been developed to predict the TWC performance of each Pd and Rh catalysts as a function of the Pd or Rh loading and the catalyst mileage. For the Pd catalyst, the overall reaction kinetic model combined with the activity function and the reference reaction kinetics reasonably well predicted the TWC performance of the commercial Pdc1 and Pdc2 and lab-prepared Pdm catalysts as a function of both the catalyst Pd loading from 20 to 280 g/ft3 and mileage from 4k to 100k miles. In addition, the TWC performance of the commercial Rh catalyst as a function of the Rh loading from 2.5 to 15 g/ft3 and the catalyst mileage from 4k to 100k miles was reasonably well described by the reaction kinetics combined with the activity function for the Rh catalyst. To develop the intrinsic reaction kinetics for double-layered bimetal Pd/Rh TWC monolith catalyst, the individual reaction kinetics derived for the Pd and Rh catalysts was combined with no further adjustment of the kinetic parameters obtained from individual Pd and Rh catalyst. The TWC performance of the lab-prepared double-layered Pd80/Rh2.5 monolith reactors was well captured by the combined reaction kinetics incorporated into the 2D non-isothermal monolith reactor model, regardless of the location of the catalysts washcoated. The combined kinetic model developed was also capable of predicting the TWC performance of the commercial double-layered Pd80/Rh2.5 monolith reactor (Rh on the top layer and Pd in the bottom layer). Consequently, the overall reaction kinetics for the double-layered Pd/Rh TWC monolith reactor as a function of the catalyst metal loadings and mileage has been derived by incorporating the activity functions developed for the Pd and Rh catalysts into the detailed reaction kinetics of the reference 4k Pdc1-240 and 4k Rh15 catalysts. The long-term catalytic performance (up to 100k miles) of the commercial Pdc1-80/Rh10 TWC monolith reactor as a function of the catalyst mileage was remarkably well described by the overall reaction kinetics combined with the individual reaction kinetics of the reference catalysts and their corresponding activity functions.