Cu-SSZ-13 catalysts with three Si/Al ratios, at 6, 12 and 35, are synthesized with solution ion exchange. Catalysts are characterized with surface area/pore volume measurements, temperature programmed reduction (TPR), and electron paramagnetic resonance (EPR) spectroscopy. Catalytic properties are examined using NO oxidation, ammonia oxidation, and standard ammonia selective catalytic reduction (NH3-SCR) reactions. By varying Si/Al ratios and Cu loadings, it is possible to synthesize catalysts with one dominant type of isolated Cu2+ ion species. Prior to full dehydration of the zeolite catalyst, hydrated Cu2+ ions are found to be very mobile as judged from EPR. NO oxidation is catalyzed by O-bridged Cu-dimer species that form at relatively high Cu loadings and in the presence of O2. For NH3 oxidation and standard SCR reactions, transient Cu-dimers even form at much lower Cu loadings; and these are proposed to be the active sites for reaction temperatures = 350 �C. These dimer species can be viewed as in equilibrium with monomeric Cu ion complexes. Between ~250 and 350 �C, these moieties become less stable causing SCR reaction rates to decrease. At temperatures above 350 �C and at low Cu loadings, Cu-dimers completely dissociate to regenerate isolated Cu2+ monomers that then locate at ion-exchange sites of the zeolite lattice. At low Cu loadings, these Cu species are the high-temperature active SCR catalytic centers. At high Cu loadings, on the other hand, both Cu-dimers and monomers are highly active in the high temperature kinetic regime, yet Cu-dimers are less selective in SCR. Br�nsted acidity is also very important for SCR reactivity in the high-temperature regime.

Understanding ammonia selective catalytic reduction kinetics over Cu-SSZ-13 from motion of the Cu ions

248th National Meeting (2014)

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