Modeling of Nox Storage and Reduction in Catalytic Monolith Reactors
Jin Xu, Michael P. Harold, and Vemuri Balakotaiah. Department of Chemical Engineering, University of Houston, 4800 Calhoun Road, Houston, TX 77204
Abstract – The NOx trap monolithic catalytic reactor is emerging as a promising technology for meeting the increasingly stringent restrictions on vehicular emissions from lean burn and diesel engines. The NOx trap is a multi-functional catalytic (Pt/BaO/Al2O3) reactor operated periodically between lean and rich conditions. During the lean operation, the NOx is stored on an alkali earth oxide as nitrites/nitrates and reduced during the rich operation period, which is usually much shorter than the lean period. Successful design and operation of the NOx trap requires understanding the fundamental mechanisms and kinetics of storage and reduction and their coupling with the transport processes. In this study, a one-dimensional, two-phase short monolith model has been developed to investigate (using detailed kinetics) the synergy between the oxidation and reduction (Pt) and storage (BaO) components and the influence of different reductants on the efficiency of the NOx trap. The short monolith model allows us to include detailed chemistry describing the adsorption, desorption, storage and surface reaction steps for the various species. The model is used to generate bifurcation diagrams of monolith temperature/exit compositions versus feed temperature for different operating conditions. The effect of using hydrogen, carbon monoxide and hydrocarbons as reductants is investigated by determining the light-off temperature and the region of multiple steady states with feed temperature as the independent variable. The model predictions will be compared with experimental observations to confirm the reaction mechanisms and to extract reaction rate constants. The extension of the model to simulate the performance of an integral monolith reactor with strong axial concentration and temperature gradients will also be discussed.