Chapter 8. NOx Storage and Reduction: Effects of Pt Dispersion, Reductant Type, and Cycle Timing

Abstract

Studies of the lean NOx trap (LNT) using NOx storage and reduction (NSR) technology on several aspects are addressed. First, the effect of catalyst composition – precious metal loading and dispersion – on the performance of a LNT is examined. A model is described to predict the cycle-averaged NOx conversion and selectivity into N2 and NH3 as a function of Pt loading and dispersion. Second, the impact of cycle time and non-isothermal effects are examined by both experiments and modeling using H2 as the reductant. The mechanism of fast cycling features better utilization of NOx storage sites, resulting in a higher NOx conversion and lower NH3 selectivity. Significant exotherms are detected and modeled under traditional ∼1 min cycle, resulting in increased NOx slip, whereas reduced NOx slip is encountered during near-isothermal, sub-10 s cycle time operation (fast cycling). Third, the spatio-temporal concentration and temperature profiles are measured using C3H6 as the reductant, providing insight into NOx reduction pathways with hydrocarbons, including the water–gas shift reaction and steam reforming. Lastly, the effects of reductant type during fast cycling are experimentally studied. The stronger thermal effect with H2 compared to C3H6 may be due to its higher molecular diffusivity. Comparisons between C3H6 and H2 show that improved NOx storage site utilization is the main factor responsible for NOx conversion enhancement during fast cycling with the HC-intermediate pathway being a secondary factor.