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.