Formation and Decomposition of Ammonium Nitrate on an Ammonia Oxidation Catalyst

Achieving high NO_x conversion at low-temperature (T ≤ 200 °C) is a topic of active research due to potential reductions in regulated NO_x emissions from diesel engines. At these temperatures, ammonium nitrate may form as a result of interactions between NH₃ and NO₂. Ammonium nitrate formation can reduce the availability of NH₃ for NO_x conversion and block active catalyst sites. The thermal decomposition of ammonium nitrate may result in the formation of N₂O, a regulated Greenhouse Gas (GHG). In this study, we investigate the formation and thermal and chemical decomposition of ammonium nitrate on a state-of-the-art dual-layer ammonia oxidation (AMOX) catalyst. Reactor-based constant-temperature ammonium nitrate formation, temperature programmed desorption (TPD), and NO titration experiments are used to characterize formation and decomposition. N₂ adsorption and diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) experiments are also conducted to elucidate the physical and chemical impacts of ammonium nitrate formation on the AMOX catalyst. The insights provided herein support the diesel aftertreatment communities’ ongoing efforts to understand low-temperature chemical processes such as ammonium salt formation and their impact on emissions.