Detailed Mechanism Of Nitrogen Oxides Abatement In Automotive Exhaust

Diesel engines have drawn great attention due to their good economic property and low COt emissions. Since diesel engines work under lean-burn conditions and produce high level oxygen exhaust, traditional three way catalysts (TWCs) are not functional for NOx purification. NOx storage and reduction (NSR) and selective catalytic reduction (SCR) are two effective technologies for diesel NOx control. SCR has already been highly commercialized and mostly used for heavy-duty applications. NSR can be used for medium-and light-duty engines. However these aftertreatment technologies need to be further improved out of environmental concern and stringent legislations. Aiming at elucidating NOx reduction properties, detailed mechanism for catalytic reduction of nitric oxide by carbon monoxide and by hydrogen over precious metal catalysts were studied.Detailed mechanisms of CO-NO and H2-NO reacitons were established. The former consists of5gas spieces,5surface spieces and11elementary steps and the latter consists of6gas spieces,10surface spieces and25elementary steps. The concentrations of reaction speices have been well predicted with the proposed mechnisms for both of the two reactions. Reaction profiles such as coverages, reaction path and sensitivity were further analyzed. N2O performs an crucial role in both CO and H2cases. Due to the facts that H2is far more less compative compared to NO in adsorbing onto Rh surface and adsorbed NO is not easy to desorb, H2did not illustrate obvious low temperature activity for the case studied herein.On the basis of CO-NO and H2-NO mechanisms, N2O reduction by CO and by H2were analyzed by kinetic modeling. Calculation results of N2O convertion versus temperature profiles agree well with experimental data. The different characteristics exhibited by CO and H2cases can be explained by the properties of adsorbed CO and H.The impacts of inlet H2concentration on the selectivity of N2O, N2and NH3for NO reduction by H2over platinum were also modeled via a detailed H2-NO reduction mechanism. The mechanism consists of6gas spieces,10surface spieces and28elementary steps. The selectivity of production depends heavily on the inlet NO/H2ratio. Taking270℃as an example, the phenomenon was analyzed from the angle of chemical kinetics. As H2concentration increases, more adsorbed H atoms accumulate on Pt and the catalytic surface starts to exhibit reductive atmosphere where production of low N valence compounds is favored.