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

Studies of the lean NO_x trap (LNT) using NO_x 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 NO_x 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 NO_x storage sites, resulting in a higher NO_x conversion and lower NH₃ selectivity. Significant exotherms are detected and modeled under traditional ∼1 min cycle, resulting in increased NO_x slip, whereas reduced NO_x 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 NO_x 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 NO_x storage site utilization is the main factor responsible for NO_x conversion enhancement during fast cycling with the HC-intermediate pathway being a secondary factor.