Reaction Analysis and Modeling of Fast SCR in a Cu-Chabazite SCR Catalyst Considering Generation and Decomposition of Ammonium Nitrate

In this study, reaction path analysis and modeling of NOx reduction phenomena by fast SCR reaction on a Cu-chabazite catalyst were conducted, considering the formation and decomposition of ammonium nitrate (NH4NO3). White crystals of NH4NO3 decompose at temperatures < 200 °C. Thus, the reaction behavior changes at 200 °C under fast SCR reaction conditions. NH4NO3 formation can occur on both Cu sites and Brønsted acid sites, which are active sites for NOx reduction in the Cu-chabazite catalyst, but it is unclear where NH4NO3 accumulates on the catalyst. Analyses using catalyst test pieces with different active sites were performed to estimate this accumulation. The results suggested that NH4NO3 accumulation does not depend on the presence of either Cu sites or Brønsted acid sites. Therefore, it is assumed that NH4NO3 can be accumulated everywhere on the catalyst, including on the zeolite framework. This phenomenon was included in the model as formation/accumulation sites S'. As a result, a model considering the fast SCR reaction behavior changes under 200 °C was developed. Validation results confirmed that the model could accurately predict NOx reduction phenomena for a wide range of temperatures and NO2/NOx ratios. The results also suggested that the model was applicable under transient conditions of NH3 concentration. The model performed well under a wide range of conditions by considering active sites and reactions on the catalyst.