Fast Cycling NOx Storage and Reduction on Lean NOx Trap Catalytic Reactor

An experimental and modeling study describes fast cycling NOx storage and reduction (NSR) with H2 and C3H6 as reductants on a Pt/BaO/CeO2/Al2O3 lean NOx trap (LNT) catalyst for emission control of lean burn vehicles. The results provide insight and deeper understanding of the impact of cycle frequency on the LNT performance and underlying NSR mechanism. Experiments and modeling reveal NOx conversion enhancement for cycle times less than 10 s (fast cycling) compared to the longer cycle times of ~1 min, typical of conventional NSR. The more frequent storage and regeneration of fast cycling increases the utilization of stored NOx capacity by decreasing NOx breakthrough during the lean phase. A comparison of cycle-average NOx conversion using H2 and C3H6 as reductants at the same lean/rich stoichiometry level provides revealing information. 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. Exotherm with aerobic rich feed with H2 results in lower NOx conversion resulted from higher temperature rise, due to higher molecular diffusivity. Comparison of the temporal dependence of the effluent composition when using H2 and C3H6 as reductants enables an assessment of the possible intermediate mechanism. Involvement of a surface N-containing oxygenate pathway is indicated by the appearance of peaks of N2O, N2, and CO2 during the rich to lean switch. Adsorbed intermediate reactivity measurements provide further evidence. Double peaks are possibly products mainly from oxidation of intermediates. In order to investigate previous experimental results in detail, a global kinetic model incorporated into a non-isothermal 1-D monolith reactor is built and is capable of capturing most of the experimental trends. Differences in NOx conversion under fast cycling with anaerobic feed between H2 and C3H6 are mostly due to higher molecular diffusivity of H2. Proposed HC-intermediate (selected as C2H3NCO) pathway can predict the experimental double peaks of N2, while formation of which consumes ~2.5 times C3H6 according to the stoichiometry number, resulting in less NOx reduction efficiency than traditional C3H6 reduction pathway. In sum, through both experiments and modeling the traditional NOx reduction pathway is the most effective and the better utilization of the storage site is the most important factor in NOx reduction enhancement during fast cycling.