Global Reaction Kinetics for Oxidation and Storage in Diesel Oxidation Catalysts.

Abstract

Realizing the need for effective kinetic models that could be used over wide operating regimes, oxidation and storage kinetics for a diesel oxidation catalyst (DOC) were developed in this work. As a first step towards kinetics development, a simple catalyst formulation including only Platinum was chosen. Kinetics were generated by assuming that propylene was representative all the hydrocarbons (HCs) in the exhaust. A systematic methodology was formulated which consisted of (1) careful choice of concentration/temperature domain (2) measurement of reactor conversions of aged catalyst samples at chosen test points using a high space velocity integral reactor (3) developing a simplified 1D reactor model (4) defining an objective function which is critically sensitive to the differences between model predictions and experiments at all conversions (5) generating proper initial guesses and finally (6) modifying Langmuir-Hinshelwood rate expressions to arrive at the final rate forms. This methodology can be used to generate steady state global kinetics in general. Comparison of model predictions with light-off curves generated using a 1.7L Isuzu diesel engine revealed that propylene is not representative of all the HCs in the diesel exhaust. As a next step towards oxidation kinetics development, a commercially available DOC catalyst was used with HCs in the diesel exhaust speciated as propylene, representing partially oxidized HCs, and diesel fuel, representing unburnt fuel component in diesel exhaust. The systematic methodology developed previously was successfully used to generate oxidation kinetics for all the species of interest. Light-off curves comparison revealed excellent agreement between model predictions and engine data. Finally, reaction kinetics were developed for capturing hydrocarbon adsorption/desorption processes on zeolite. For this study the fuel components in the exhaust were further speciated as n-dodecane and toluene. A minimum of four experiments were found to be sufficient to generate the necessary kinetic constants for each adsorbable HC species. Studies on simplified warm-up process using a 1D adiabatic reactor model that incorporated both the oxidation and storage kinetics indicated that the storage component reduces the overall cold start HC emissions by at least a factor of 2 if the warm-up rate achieves 45-65C/min, a range commonly observed during start-up.