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Reaction Path Analysis and Modeling on NOx Purification in a Cu-Chabazite SCR Catalyst Considering Cu Redox Chemistry and Reversible Hydrolysis of Cu Sites
ISSN: 0148-7191, e-ISSN: 2688-3627
To be published on September 15, 2020 by SAE International in United States
In this study, reaction path analysis and modeling on NOx purification phenomena by selective catalytic reduction with NH3 over Cu-Chabazite catalyst were carried out, taking into account changes in the valence state of Cu sites and the local structure due to differences in ligands to Cu sites. In a Cu-Chabazite catalyst, NOx was mainly reduced by adsorbed NH3 on divalent Cu sites, and the valence state of Cu also changes from divalent to monovalent during NOx reduction. On the other hand, it is known that the activation energy of NOx reduction of Cu-Chabazite catalyst changes between the low-temperature range of 200 °C or lower and high-temperature range of 300 °C or higher. In order to express this phenomenon, a reversible hydrolysis based on the difference in the coordination state of the hydroxyl group (OH-) to the Cu sites at low and high temperatures was introduced into the model. As a result, we showed that the NOx purification phenomenon can be expressed over a wide temperature range by using the activation energy specific to Cu-Chabazite catalyst. There are two types of Cu sites composed of ion-exchanged Cu in a Cu-Chabazite: single Al sites that balance with one Al and paired Al sites that balance with two Al in the zeolite structure. We also showed that the Cu at the single Al sites forms a Cu dimer in the middle to high temperature range and becomes a reaction site for NH3 oxidation by XAFS analysis. As a result, we showed the possibility that the NOx purification phenomenon can be expressed by the detailed kinetic model using rate parameters specific to the catalyst material. And, the necessity to consider the difference of single Al and paired Al sites was also showed to develop the detailed kinetic model including side reaction.