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

Abstract

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 (NH3-SCR) reactions. Prior to full dehydration of the zeolite catalysts, 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 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, Cu2+ monomers that had migrated to faces of 6-membered rings are the active sites. At intermediate Cu loadings, monomeric Cu2+ 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 Cu2+ 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 NH3 oxidation reaction thus causing SCR selectivities to decrease above ∼450°C. Finally, our kinetics results strongly support a redox mechanism for standard SCR.