Structure/Activity Relationships in Cu-CHA-Based NH3 Selective Catalytic Reduction Catalysts
Charles Peden, Pacific Northwest National Laboratory
Selective catalytic reduction (SCR) of NOx with ammonia using metal-exchanged molecular sieves with a Chabazite (CHA) structure has recently been commercialized on diesel vehicles. Catalysts with outstanding SCR performance (activity, hydrothermal stability, etc.) include the aluminosilicate zeolite, SSZ-13, and the silico-alumino-phosphate, SAPO-34, with ion-exchanged Cu. Apart from the commercial success, detailed catalyst structures, reaction mechanisms, and structure-activity relationships are still lacking. In this presentation, we will describe results of recent DOE/VT/CLEERS-funded studies of these Cu-CHA materials that address a number of aspects of the reactivity of these catalysts, as well as structural studies that account for the relative performance of these materials. Understanding the location and the chemical environment of the catalytically active Cu ions, as well as the adsorbed species present on (in) the catalyst under practical operation conditions are critical in developing a viable reaction mechanism. Transmission FTIR of adsorbed probe molecules and DRIFT spectroscopy under quasi operando conditions were used to determine the adsorption properties of these materials, as well as to follow the fate of adsorbed NH3 upon its exposure to oxidizing gases of different composition, respectively. We also use a number of characterization methods including x-ray absorption (XAS) electron paramagnetic resonance (EPR) spectroscopies, techniques ideally suited for Cu2+ exchanged zeolites, to study hydrated Cu-SSZ-13 catalysts at various Cu loadings in order to gain further insights into their locations. Finally, NH3-SCR and NH3 oxidation kinetics are investigated over these catalysts at high space velocity conditions for the development of structure-activity relationships. The implications of these results for understanding the NH3 SCR reaction mechanisms will also be discussed.
Acknowledgement: This work was supported by the U.S. DOE/EERE/Vehicle Technologies Office.