Mechanism Study Of Nitrogen Oxides Reduction In Automotive Exhaust Gases

People enjoy the convenience of automotive, but the exhaust gases emitted from the pipe cause damage to the natural environment and human health simultaneously. To meet increasingly more stringent emission standards, the automotive aftertreatments are implemented inevitably. The diesel engine, working under lean burn conditions, faces the enormous challenge to control the emission NOx. The NOx storage reduction coupled with in-situ selective catalytic reduction (NSR-SCR) is a combination system, which provides the benefits of simple structure, no external source of NH3, low precious metal consumption, and high NOx abatement efficiency. Aiming at elucidating the by-products NH3and N2O, the mechanism of NOx catalytic reduction was studied.The microkinetic model of NO/H2over Pt catalyst was proposed, and the mechanism comprised6gas species,10surface species and28reaction steps. The model was validated by reproducing the experimental data quantitatively. The trends of N-containing species versus temperature were investigated under different NO/H2feed ratios. The results implicated the selectivity to the by-product NH3was highly sensitive function of feed composition. The mechanism of NH3formation was further studied, and the results implied the addition of the second hydrogen to N(s) is of more effect on the NH3formation than the others. Sensitivity and coverage analyses results showed the NH(s) is a predominant intermediate, and the NH2(S) hydrogenation is rate limiting.The microkinetic model of NO/CO over Pt catalyst, sharing the same NOx mechanisms, with the NO/H2model, was developed. The mechanism consisted of5gas species,5surface species and11reaction steps. The predicted trends were in good agreement with the experimental data. The generation of the side product N2O was favored at low temperatures and low NO conversion, while as the temperature rise N2O decomposed gradually and N2became the main product. The reaction pathway analysis for N2results showed the reaction rate of the N-N recombination path (N(s)+N(s)→N2) was much higher than the N2O decomposition path (N2O(S)→N2+O(S)) under low temperatures and low N2production. As the N2production increased, the latter become the predominant reaction pathway. The proposed model predicated the CO self-inhibition phenomenon qualitatively. The NO conversion was lowed sharply with increasing CO feed, and the inhibition effect could be lessened by increasing temperature.