Experimental studies of NOx storage and reduction on Lean NOx Traps

Stricter emission standards have driven the research and development of several emission aftertreatment technologies like selective catalytic reduction (NH3-SCR) and lean NOX trap (LNT) technology. On an NH3-SCR catalyst, NH3 is injected into the exhaust, which then selectively reduces NOx to N2. During the LNT process, NOX is stored on an alkali earth oxide during the lean phase, forming surface nitrite/nitrate species, which are then reduced to N2 over precious metals during periodic “rich” operation. A combined LNT-SCR hybrid system promises to be more cost-effective and operates by utilizing NH3 formed during the rich phase operation of LNT to reduce NOx that breakthroughs LNT on the SCR catalyst downstream. An experimental set-up was built to monitor the transient reactor temperatures and effluent gas concentrations. This work systematically investigated the production of NH3 on typical Pt-Rh/BaO/Al2O3 LNT monolithic catalysts during the reduction of NOx by CO in the presence of excess water without any molecular H2 being fed to the system; under both steady-state and cyclic operation conditions. The objective was to determine the effect of various operating parameters and the involvement of intermediate species on the mechanism leading to NH3 formation and NOx reduction. Under steady-state conditions, H2 formed by wgs reaction plays a dominant role in reducing NOx to NH3. However, under cyclic operation conditions, hydrolysis of intermediate isocyanate species is shown to be the leading route to NH3 formation. An extensive study of the NOX storage mechanism and the impact of CO2 were conduced at low temperatures (< 300 ˚C). It was determined that the local minima in NOx conversion observed at 200 ˚C, during the reduction of NOx by H2 in the presence of CO2 was a result of the competition posed by CO2 for the two kinds of storage sites involved. Also, at 200 ˚C carbonates are relatively more stable on the Ba sites located far from Pt and were hard to replace by nitrates due to diffusion limitations. SpaciMS was employed to investigate NOx reduction mechanism by C3H6 in LNT systems. Spatial and temporal profiles of concentrations and temperatures were developed along the catalyst length.