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

Cu/SSZ-13 catalysts with Si/Al = 6 and various Cu/Al ratios 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 (NH 3 -SCR) reactions. Prior to full dehydration of the zeolite catalysts, hydrated Cu 2+ 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 O 2 . For NH 3 oxidation on samples with low to intermediate Cu loadings, transient Cu-dimers are the low-temperature (⩽300 °C) active centers, while these dissociate to monomers at 350 °C and above and become active centers. For the much more complex standard SCR reaction, transient Cu-dimers are the active sites for reaction temperatures <250 °C at very low Cu loadings (Cu/Al ⩽ 0.016). Between ∼250 and 350 °C, these Cu-dimers become less stable causing SCR reaction rates to decrease. At temperatures ⩾350 °C, Cu 2+ monomers that had migrated to faces of 6-membered rings are the active sites. At intermediate Cu loadings, monomeric Cu 2+ ions are also active in SCR in the low-temperature regime; these are proposed to be located within CHA cages and next to 8-membered rings, likely in the form of [Cu(OH)] + . At high Cu loadings (i.e., more than one Cu 2+ ion in each unit cell), stable Cu-dimers form and these do not dissociate at temperatures above 350 °C. These moieties effectively occupy CHA cage space and block pore openings causing decreased efficiency of the catalysts. Also these moieties are highly active in catalyzing the NH 3 oxidation reaction thus causing SCR selectivities to decrease above ∼450 °C. Finally, our kinetics results strongly support a redox mechanism for standard SCR. [ABSTRACT FROM AUTHOR]