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.

The hydrothermal stability of Cu/SSZ-13 SCR catalysts has been extensively studied, yet atomic-level understanding of changes to the zeolite support and the Cu active sites during hydrothermal aging are still lacking. In this work, via the utilization of spectroscopic methods including solid-state 27Al and 29Si NMR, EPR, DRIFTS, and XPS, together with imaging and elemental mapping using STEM, detailed kinetic analyses, and theoretical calculations with DFT, various Cu species, including two types of isolated active sites and CuOx clusters, were precisely quantified for samples hydrothermally aged under varying conditions. This quantification convincingly confirms the exceptional hydrothermal stability of isolated Cu2+-2Z sites and the gradual conversion of [Cu­(OH)]+-Z to CuOx clusters with increasing aging severity. This stability difference is rationalized from the hydrolysis activation barrier difference between the two isolated sites via DFT. Discussions are provided on the nature of the CuOx clusters and their possible detrimental roles on catalyst stability. Finally, a few rational design principles for Cu/SSZ-13 are derived rigorously from the atomic-level understanding of this catalyst obtained here.

Pd/zeolite passive NO x adsorber (PNA) materials were prepared with solution ion-exchange between NH 4 /zeolites (Beta, and PdCl 2 solutions. The nature of Pd (dispersion, distribution, and oxidation states) in these materials was characterized with Na + ion exchange, TEM imaging, CO titration with FTIR, and in situ XPS. The NO x trapping and release properties were tested using feeds with different compositions. It is concluded that multiple Pd species coexist in these materials: atomically dispersed Pd in the cationic sites of zeolites and PdO 2 and PdO particles on the external surfaces. While Pd is largely atomically dispersed in ZSM-5, the small pore opening for SSZ-13 inhibits Pd diffusion such that the majority of Pd stays as external surface PdO 2 clusters. NO x trapping and release are not simple chemisorption and desorption events but involve rather complex chemical reactions. In the absence of CO in the feed, cationic Pd(II) sites with oxygen ligands and PdO 2 clusters are reduced by NO to Pd(I) and PdO clusters. These reduced sites are the primary NO adsorption sites. In the presence of H 2 O, the as-formed NO 2 desorbs immediately. In the presence of CO in the feed, metallic Pd, "naked" Pd 2+ , and Pd + sites are responsible for NO adsorption. For Pd adsorption sites with the same oxidation states but in different zeolite frameworks, NO binding energies are not expected to vary greatly. However, NO release temperatures do vary substantially with different zeolite structures. This indicates that NO transport within these materials plays an important role in determining release temperatures. Finally, some rational design principles for efficient PNA materials are suggested.

Using a traditional aqueous solution ion exchange method under a protecting atmosphere of N2, a series of Fe/SSZ-13 catalysts with various Fe loadings were synthesized. UV–vis, EPR, and Mössbauer spectroscopic methods, coupled with temperature-programmed reduction and desorption techniques, were used to probe the nature of the Fe sites. The major Fe species are extraframework Fe­(III) species: [Fe­(OH)2]+ (monomeric) and [HO–Fe–O–Fe–OH]2+ (dimeric). Larger oligomers with unknown nuclearity, poorly crystallized Fe oxide particles, together with isolated Fe2+ ions, are minor Fe-containing moieties. Reaction rate and Fe loading correlations, and temperature and Fe loading effects on SCR selectivities, suggest that isolated Fe3+ ions are the active sites for low-temperature standard SCR, and dimeric sites provide the majority of reactivity at higher temperatures. For NO oxidation, dimeric sites are the active centers. NH3 oxidation, on the other hand, is catalyzed by sites with higher nuclearity.

Using a three-step aqueous solution ion-exchange method, cocation modified Cu/SSZ-13 SCR catalysts were synthesized. These catalysts, in both fresh and hydrothermally aged forms, were characterized with several methods including temperature-programmed reduction by H 2 (H 2 -TPR), temperature-programmed desorption of NH 3 (NH 3 -TPD), and 27 Al solid-state nuclear magnetic resonance (NMR) and diffuse reflectance Infrared Fourier Transform (DRIFT) spectroscopies. Their catalytic performance was probed using steady-state standard NH 3 -SCR. Characterization results indicate that cocations weaken interactions between Cu-ions and the CHA framework making them more readily reducible. By removing a portion of Brønsted acid sites, cocations also help to mitigate hydrolysis of the zeolite catalysts during hydrothermal aging as evidenced from 27 Al NMR. Reaction tests show that certain cocations, especially Li + and Na + , promote low-temperature SCR rates while others show much less pronounced effects. In terms of applications, our results indicate that introducing cocations can be a viable strategy to improve both lowand high-temperature performance of Cu/SSZ-13 SCR catalysts.