The Impact of Ammonium Nitrate Species on Low Temperature NOx Conversion Over Cu/CHA SCR Catalyst

Cu/CHA catalysts have been widely used in the industry, due to their desirable performance characteristics including the unmatched hydrothermal stability. While broadly recognized for their outstanding activity at or above 200°C, these catalysts may not show desired levels of NOx conversion at lower temperatures. To achieve high NOx conversions it is desirable to have NO₂/NOx close to 0.5 for fast SCR. However even under such optimal gas feed conditions, sustained use of Cu/CHA below 200°C leads to ammonium nitrate formation and accumulation, resulting in the inhibition of NOx conversion. In this contribution, the formation and decomposition of NH₄NO₃ on a commercial Cu/CHA catalyst have been investigated systematically. First, the impact of NH₄NO₃ self-inhibition on SCR activity as a function of temperature and NO₂/NOx ratios was investigated through reactor testing. Second, the formation of NH₄NO₃ was qualitatively measured through various adsorption and model reactions as well as its decomposition reactions by the temperature-programed desorption (TPD). In addition, the transient formation and accumulation of NH₄NO₃ under relevant process conditions were tracked by in situ DRIFTS spectroscopy to identify its formation and accumulation mechanisms. Last, a kinetic model was built to estimate the NH₄NO₃ inhibition impact on NOx conversion. This work provides insights into the limiting processes and SCR reactions at low temperatures that are important for addressing future low temperature NOx conversion needs.