Fast Cycling NOx Storage and Reduction: Experiments and Modeling
Michael Harold, University of Houston
An experimental and modeling study is described of fast cycling NOx storage and reduction (NSR) with H2 and C3H6 as reductants on a Pt/BaO/CeO2/Al2O3 monolith catalyst for emission control of lean burn vehicles. The study provide mechanistic insight and deeper understanding of Toyota’s Di-Air (Diesel NOx Aftertreatment by Adsorbed Intermediates) technology. Differentiation is made between two NOx conversion enhancement mechanisms, the conventional NSR mechanism with improved NOx storage utilization and a hydrocarbon-intermediates mechanism. A comparison of cycle-average NOx conversion using mechanistic discrimination is accomplished using H2 and C3H6 as reductants at the same lean/rich stoichiometry level. Both aerobic + anhydrous and anaerobic + anhydrous propylene-containing feeds enable an estimate of the contribution of H2 generating water-gas shift and propylene steam reforming reactions. The observed NOx conversion enhancement of up to 45% (absolute) with 10x faster cycling is independent of reductant type and is attributed to improved NOx storage utilization. Exothermic heat effects with temperature rise as high as 275 oC and significant spatial gradients are encountered with H2 during slow cycling, which makes H2 less effective than C3H6. The hydrocarbon intermediate pathway is responsible for a smaller NOx conversion encountered during fast cycling. Involvement of a HC-intermediate NOx reduction pathway is indicated by secondary peaks of N2O, N2, and CO2 formed by the apparent oxidation of surface intermediates by O2 during the switch from a rich to lean phase. Adsorbed intermediate reactivity measurements provide further evidence. The experiments and modeling also suggest that this pathway mitigates the NOx puff resulting from stored NOx decomposition, which is the major NOx slip pathway during fast cycling. A comprehensive monolith model including all of the key chemical and physical processes predicts the experimental trends and is essential in reaching the mechanistic conclusions. Implications of these findings for the use of fast cycling NSR will be discussed.