Reaction Path Analysis and Modeling of NOx Reduction in a Cu-chabazite SCR Catalyst Considering Cu Redox Chemistry and Reversible Hydrolysis of Cu Sites

In this study, reaction path analysis and modeling of NOx reduction phenomena by selective catalytic reduction (SCR) with NH₃ over a Cu-chabazite catalyst were conducted considering changes in the valence state of Cu sites and local structure due to differences in ligands to the Cu sites. The analysis showed that in the Cu-chabazite catalyst, NOx was mainly reduced by adsorbed NH₃ on divalent Cu sites accompanied by a change in valence state of Cu from divalent to monovalent. It is known that the activation energy of NOx reduction on a Cu-chabazite catalyst changes between low temperatures ≤ 200 °C and mid to high temperatures ≥ 300 °C. To express this phenomenon, a reversible hydrolysis reaction based on the difference in coordination state of hydroxyl groups (OH⁻) to Cu sites at low and high temperatures was introduced into the model. The results showed that NOx reduction phenomena can be expressed over a wide temperature range by using the activation energy specific to the Cu-chabazite catalyst. There are two types of ion-exchanged Cu in Cu-chabazite: single Al sites that balance with one Al and paired Al sites that balance with two Al in the zeolite structure. We showed by XAFS analysis that Cu at the single Al sites forms a Cu dimer at mid to high temperatures ≥ 300 °C and serves as a reaction site for NH₃ oxidation. Thus, NOx reduction can be expressed by a detailed kinetic model using rate parameters specific to the catalyst material. The difference between the single Al and paired Al sites was also important for developing a detailed kinetic model including side reactions.