Investigation of the light-off characteristics and secondary pollutants production mechanism in Pd/Rh catalyst for natural gas engines

Abstract

To investigate the influence mechanism of the light-off characteristics and secondary pollutants (N2O and NH3) of the Pd/Rh catalytic converter for natural gas engines, a global model of the catalytic converter and a more comprehensive catalytic reaction mechanism were first established. Further, the model and the catalytic reaction mechanism were validated by using experimental data. Then, the effects of exhaust temperature and exhaust composition (H2O, O2, and H2 concentrations) on the catalytic converters were investigated. The results showed that a higher temperature reduced the light-off time t50 of NO and CO. The production of N2O decreased to 0 at high temperatures due to the reduction reactions of R14 (N2O+H2 = H2O+N2) and R16 (N2O+CO = CO2 + N2). NH3 would form at the temperature above 825 K, and the required H2 originated from SR and WGS reactions. Increasing the H2 concentration promoted the conversion of NO and CO. When the temperature exceeds 600 K, increasing the H2 concentration decreases the production of N2O. H2O has the opposite effect on the production of N2O and NH3 at high temperatures. Increasing the content of H2O increased the conversion of NO and inhibited the conversion of CO. O2 concentration at 4000 × 10−6 is beneficial to reduce secondary pollutants of N2O and NH3. At the condition of 730–820 K and 4000 × 10−6 O2, the productions of N2O and NH3 were reduced to 0. This study contributes to the efficient application of Pd/Rh catalytic converters and further enhances the emission reduction of natural gas engines.