삼원촉매상에서의 반응속도론 연구 및 모노리스 반응기 모델 개발
- 삼원촉매상에서의 반응속도론 연구 및 모노리스 반응기 모델 개발
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- Three-Way Catalysts (TWCs) containing noble metals such as platinum (Pt), palladium (Pd), and rhodium (Rh) have been commonly employed for reducing the emissions of air pollutants in the exhaust gas stream from conventional automotive gasoline engines. TWCs have been designed to simultaneously convert all three major air pollutants including CO, HC and NOx into CO2, H2O and N2 and are highly effective under the stoichiometric exhaust gas conditions. For the development and operation of the effective TWC, the fundamental reaction kinetics of TWC has been developed. Although a variety of kinetic models, including those of Voltz et al. and Subramaniam and Varma, have been developed to reduce the time and resources required for the development and implementation of catalytic converters with improved emission control performance, more accurate TWC reaction kinetics based on reliable reaction mechanism in realistic exhaust streams is needed to improve the emission performance prediction capability of catalytic converter models.
In the present study, the detailed reaction kinetics based on reliable reaction mechanism has been established to predict the TWC performance under realistic feed stream conditions. On the basis of this effort, the monolith reactor models have been developed to simulate the performance of a modern three-way catalytic converter.
In order to examine the catalytic activity under the realistic feed conditions, the TWC including Pd and Pt/Rh/Ce aged by an engine-dynamometer from 4k to 100k miles has been provided by GM. A molten-salt bath reactor system was designed and built to develop the steady-state reaction kinetics over the TWC under near-isothermal conditions. The Pd catalyst was effective for removing CO and C3H6 at low temperatures, while the Pt/Rh/Ce catalyst was most efficient for NO reduction attributed to Rh existed on the catalyst surface. The light-off temperatures (LOTs
T50) of the simultaneous oxidations of CO, C3H6 and H2, and NO reduction shifted to higher reaction temperatures as the catalyst mileage increased from 4k to 100k miles. As the catalyst mileage increased, the BET surface area and the metal dispersion of the catalyst decreased as determined by the catalyst characterization study. However, no alteration of the activity ranking has been observed in the order of H2 ≒ NO > CO > C3H6, regardless of the both Pd and Pt/Rh/Ce catalyst and their mileages. The promotional effects of water and hydrogen on the three-way catalytic activity were examined using in-situ surface FTIR for the oxidation reactions over the both Pd and Pt/Rh/Ce catalysts. Water improved the CO oxidation activity by the formation of reaction intermediate including carboxylate and carbonate. Hydrogen also promoted the CO oxidation reaction similar to water. In addition, the presence of H2 in the feed stream containing NO significantly enhanced the TWC activity mainly due to the H-assisted dissociation reaction of NO.
Moreover, a series of variable-composition experiments, in which the feed composition was systematically varied to elucidate the differences in the reactivity of CO, C3H6 and H2 toward O2 or NO (partitioning experiment), were conducted to determine and verify the unified kinetic parameters for a given reaction between 300 and 450 oC. Also, by the isothermal experiments, the enhancement and inhibition of CO conversion activity have been examined as key components were added to and removed from the feed under a systematic steady-state isothermal reaction condition at 275 oC for 45h.
Based upon the variety of the kinetic results and the catalyst characterization, a detailed reaction kinetic model to predict the activity of the modern TWC has been developed by incorporating all the important reactions on the basis of the Langmuir-Hinshelwood mechanism. The enhancement effects of H2O and H2 on the oxidation reactions and H-assisted NO dissociation reaction have been included in the kinetic model. In particular, the secondary reactions such as NH3 oxidation and NH3-SCR reactions were also considered for predicting N2O and NH3 as the reaction byproducts formed over the both Pd and Pt/Rh/Ce catalysts.
The model predictions adequately describe the general trend of the measured conversions of CO, C3H6, NO and H2. N2O and NH3 formed during the TWC reaction can also be predicted by the model developed in the present study. Moreover, the decrease in activity of the 100k mile-aged catalyst can be well described by decreasing the frequency factors of the rate constants estimated for their 4k mile-aged counterparts, without alteration of the activation energies and the adsorption equilibrium constants estimated under the realistic full feed conditions.
The resulting detailed reaction kinetics developed using the powder samples was incorporated into a monolith reactor model to simulate the performance of a commercial modern TWC converter. The reactor model directly employed the kinetic parameters estimated from the present reaction kinetic study without any parameter adjustment. Initially, a one-dimensional (1D) reactor model developed based on the mass balance for the bulk gas phase and the washcoat layer within the monolith channels was used to predict species concentrations as a function of the average reactor temperature along the monolith reactor length. However, the model was not capable of describing the catalytic performance of the both Pd and Pt/Rh/Ce monolith reactors.
To resolve the drawback of the 1D model, a 2D non-isothermal reactor model has been developed to more accurately predict the conversion performance and thermal behavior of the catalytic monolith. The 2D model with the both heat and mass transfers and detailed reaction kinetics well describe the TWC performance, including the gas compositions and the temperature distribution as a function of both axial and radial positions for the single-bed monolith containing either Pd or Pt/Rh/Ce catalyst as well as the dual-bed monolith system including both Pd (front) and Pt/Rh/Ce (rear) catalysts. The model was further validated by predicting the TWC performance of the dual-bed reactor under the steady-state sweep test (st-ST) conditions (A/F=14.23 ~ 15.03).
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