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• Alkali co-cations Na+ and K+ stabilize SSZ-13 during hydrothermal aging. • Alkaline earth co-cation Ca2+ destabilizes SSZ-13 during hydrothermal aging. • Repulsive interactions between Cu ions and co-cations prevents Cu overloading. • At optimized co-cation and Cu ratios, both SCR performance and catalyst stability improve substantially. Cu,M/SSZ-13 (M = Na+, K+ and Ca2+) selective catalytic reduction (SCR) catalysts with Si/Al = 6 and M/Cu ratios varying from 0.1 to 1.0 are prepared by solution ion exchange, followed by slurry drying and hydrothermal aging at 800 °C. The catalysts are characterized with X-ray diffraction and N 2 adsorption isotherms to probe their textural properties; 27Al and 29Si nuclear magnetic resonance and NH 3 temperature-programmed desorption to probe their acidity properties and levels of dealumination; and electron paramagnetic resonance and H 2 temperature-programmed reduction to quantitatively explore the nature of Cu moieties in the catalysts. These studies are followed by density functional theory calculations to elucidate Cu and co-cation interactions at an atomic level and SCR reaction tests to reveal correlations between the chemical and physical properties of the catalysts and their SCR performance. Through these comprehensive investigations, it is discovered that alkali metal co-cation addition can be used for the synthesis of highly stable, and highly active and selective Cu/SSZ-13 catalysts with relatively high Al content. In catalysts with optimal alkali incorporation, dealumination of the SSZ-13 substrate is largely inhibited, allowing high concentrations of SCR-active isolated Cu ions. At the same time, repulsive interactions between Cu ions and alkali metal co-cations preclude excessively high isolated Cu-ion loadings. Interplay between and optimization of these two factors is considered the underlying origin for this successful catalyst synthesis strategy. On the other hand, the alkaline earth co-cation Ca2+ fails to demonstrate any beneficial effects, since it destabilizes isolated Cu ions via site competition. [ABSTRACT FROM AUTHOR]

• Short perspective on the new science of commercial Cu/SSZ-13 SCR catalysts. • Brief historical context on catalytic vehicle emission control also provided. • Short discussion of future challenges for the vehicle emission control community. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Controllable preparation of Cu/SSZ-13 with various particle sizes was realized by adjusting seeds amounts and pH values; • The morphology and physiochemical properties of Cu/SSZ-13 are not affected by particle size; • The NH 3 -SCR reaction over Cu/SSZ-13 is not controlled by intracrystalline diffusion limitation. Abstract In this work, we report a facile technique to prepare crystalline Cu/SSZ-13 particles with uniform and controllable sizes. Samples with varying particles sizes (0.4–2 µm) with 1–2 wt% Cu were synthesized by carefully controlling the amount of zeolite seeds and by adjusting the pH values, with larger seed amounts and higher pH values resulting in smaller particle sizes. Various techniques (SEM, XRD, BET, H 2 -TPR etc.) were used to characterize these samples and the results confirm that despite the varying particle sizes, these Cu/SSZ-13 catalysts have similar Si/Al ratio, acidity and Cu distributions. Hence, these catalysts provided the perfect sample sets to probe the role of intra-crystalline diffusion in NH 3 SCR reactions. The kinetic experiment results indicate that turnover frequencies (TOFs) of the catalysts with different particle sizes and copper loadings are identical, therefore NH 3 -SCR reaction rate over Cu/SSZ-13 catalysts is not limited by intra-crystalline diffusion. [ABSTRACT FROM AUTHOR]

Cu/SSZ-13 catalysts with three Si/Al ratios of 6, 12 and 35 were synthesized with Cu incorporation via solution ion exchange. The implications of varying Si/Al ratios on the nature of the multiple Cu species that can be present in the SSZ-13 zeolite are a major focus of this work, as highlighted by the results of a variety of catalyst characterization and reaction kinetics measurements. Specifically, catalysts were characterized with surface area/pore volume measurements, temperature programmed reduction by H 2 (H 2 -TPR), NH 3 temperature programmed desorption (NH 3 -TPD), and DRIFTS and solid-state nuclear magnetic resonance (NMR) spectroscopies. Catalytic properties were examined using NO oxidation, ammonia oxidation, and standard ammonia selective catalytic reduction (NH 3 -SCR) reactions on selected catalysts under differential conditions. Besides indicating the possibility of multiple active Cu species for these reactions, the measurements are also used to untangle some of the complexities caused by the interplay between redox of Cu ion centers and Brønsted acidity. All three reactions appear to follow a redox reaction mechanism, yet the roles of Brønsted acidity are quite different. For NO oxidation, increasing Si/Al ratio lowers Cu redox barriers, thus enhancing reaction rates. Brønsted acidity appears to play essentially no role for this reaction. For standard NH 3 -SCR, residual Brønsted acidity plays a significant beneficial role at both low- and high-temperature regimes. For NH 3 oxidation, no clear trend is observed suggesting both Cu ion center redox and Brønsted acidity play important and perhaps competing roles. [ABSTRACT FROM AUTHOR]

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]