Investigation of Engine Exhaust Conversion and N2O/NH3 Generation on Pd-Based Catalyst

Natural gas (NG) engine catalysts face unique challenges in emission control due to their distinct raw emission characteristics. This study investigates the exhaust conversion and by-product generation of a Palladium-based catalyst of an NG engine through small-sample catalyst experiments, mainly focusing on the effect of feed gas composition on the conversion efficiency and N2O/NH3 emissions. Results show that N2O is generated via NO reduction by H2 (80~275 °C) and CO (275~400 °C) in the temperature range of 80~400 °C. NH3 generation occurs at 175~550 °C, mainly via NO reduction by H2 (supplied from the water–gas shift (WGS) reaction) and CO below 425 °C and exclusively by H2 (supplied from the steam reforming (SR) reaction) above 425 °C. An increase (0.9705~1.0176) in lambda enhances CO and CH4 conversion while reducing N2O and NH3 emissions, but it inhibits NO conversion and promotes NO2 formation. A lambda of 0.9941 achieves high conversion efficiency (≥90%) for CO, CH4, and NO, with reduced N2O and zero NH3 emissions. An increase in H2O (8~16%) accelerates the WGS and SR reactions, improving pollutant conversion. However, it aggravates N2O and NH3 emissions, with peak levels rising by 54% and 31%, respectively. Increased H2 (500~1500 ppm) preferentially consumes NO and reversely shifts the equilibrium of the WGS and SR reactions, reducing CO and CH4 conversion while improving NO conversion. And it promotes N2O selectivity at high temperature and NH3 selectivity at low temperature and peak emissions, with peak concentrations increasing by 58% and 15%, respectively. These findings reveal the by-product formation mechanism in the catalyst, providing valuable insights for the emission control of NG-fueled engines.