Kinetic Modeling of NOx Storage/Reduction on Pt/BaO/Al2O3 Monolith Catalysts

Abstract

A balance of the complexity of the reactor model and the chemical reaction mechanism has to be made in order to predict the dynamic nature of NOx storage/reduction processes in real time. In this work, a one-dimensional, two-phase model is used to simulate the transient behavior of a monolithic Pt/BaO/Al2O3 catalyst for NOx storage/reduction. The following aspects of the process are discussed: (i) kinetics of NO and NO2 adsorption on BaO sites, (ii) effects of CO2 and H2O on NO/NO2 adsorption, and (iii) reduction of surface nitrates using H2. NOx adsorption with excess oxygen involves two kinetic routes, namely, NO2 disproportionation and direct NO adsorption, both of which form nitrates on the catalyst at 300 °C. A model with two time scales was found to be necessary to describe NO2 adsorption on the 20 wt % BaO catalyst. The model and parameters required to fit the NOx breakthrough curves suggest that CO2 and H2O in the feed reduce the number of sites for NO adsorption by changing the surface morphology of the Ba phase. The rate constants for both fast and slow NO2 uptake are decreased in the presence of CO2 and H2O, but the total capacity remains the same. Under reaction conditions, H2 reduction of surface NOx is limited by the supply of the reductant; that is, the rate of surface NOx removal is limited by the flux of inlet H2. NH3 serves as the reducing intermediate/H carrier during the H2 reduction process. The confined reduction front moving along the channel localizes the heat generation, thus leading to a surface temperature in the reduction front about 35 °C higher than the inlet gas temperature for our reaction conditions.