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To improve the alkali metal resistance of commercial catalyst Cu/SSZ-13 for ammonia selective catalytic reduction (NH3-SCR) reaction, a simple method to synthesize Cu/SSZ-13 with a core–shell like structure was developed. Compared with smaller-sized counterparts, Cu/SSZ-13 with a crystal size of 2.3 μm exhibited excellent resistance to Na poisoning. To reveal the influence of the crystal size on Cu/SSZ-13, physical structure characterization (XRD, BET, SEM, NMR) and chemical acidic distribution (H2-TPR, UV-Vis, Diethylamine-TPD, pyridine-DRIFTs, EDS) were investigated. It was found that the larger the crystal size of the molecular sieve, the more Cu is distributed in the crystal core, and the less likely it was to be replaced by Na to generate CuO. Therefore, a 2.3 μm sized Cu/SSZ-13 well-controlled the reactivity of the side reaction NH3 oxidation and the generation of N2O. The result was helpful to guide the extension of the service life of Cu/SSZ-13.

Lean nitric oxide (NOx)-trap (LNT) and selective catalytic reduction (SCR) are efficient systems for the abatement of NOx. The combination of LNT and SCR catalysts improves overall NOx removal, but there is a risk that the SCR catalyst will be exposed to high temperatures and rich exhaust during the LNTs sulfur regeneration. Therefore, the effect of exposure to various rich conditions and temperatures on the subsequent SCR activity of a Cu-exchanged chabazite catalyst was studied. CO, H2, C3H6, and the combination of CO + H2 were used to simulate rich conditions. Aging was performed at 800 °C, 700 °C, and, in the case of CO, 600 °C, in a plug-flow reactor. Investigation of the nature of Cu sites was performed with NH3-temperature-programed desorption (TPD) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) of probe molecules (NH3 and NO). The combination of CO and H2 was especially detrimental to SCR activity and to NH3 oxidation. Rich aging with low reductant concentrations resulted in a significantly larger deactivation compared to lean conditions. Aging in CO at 800 °C caused SCR deactivation but promoted high-temperature NH3 oxidation. Rich conditions greatly enhanced the loss of Brønsted and Lewis acid sites at 800 °C, indicating dealumination and Cu migration. However, at 700 °C, mainly Brønsted sites disappeared during aging. DRIFT spectroscopy analysis revealed that CO aging modified the Cu2+/CuOH+ ratio in favor of the monovalent CuOH+ species, as opposed to lean aging. To summarize, we propose that the reason for the increased deactivation observed for mild rich conditions is the transformation of the Cu species from Z2Cu to ZCuOH, possibly in combination with the formation of Cu clusters.

To reveal the role of SO3 poisoning in Cu/SSZ-13 NH3-SCR catalysts, fresh and sulfated Cu/SSZ-13 catalysts were prepared in the presence or absence of SO3 flux. The deactivation mechanism is probed by the changes of structural, copper species, and selective catalytic reduction (SCR) activity. The variations concentrate on the changes of copper species as the Chabazite (CHA) framework of Cu/SSZ-13 catalysts could keep intact at high ratios of SO3/SOx. The thermal gravimetric analyzer (TGA) results reveal that the copper sulfate formed during sulfation and the amounts of sulfate species increased with an increase in the SO3/SOx ratio. In contrast to the changing trend of copper sulfate, temperature program reduction (H2-TPR), and electron paramagnetic resonance (EPR) results manifest that, since the number of active copper ions declines with an increase of the SO3/SOx ratio, the active sites transform to these inactive species during sulfation. Due to the combination of NH3-SCR activity and the kinetic tests, it is shown that the decreased number of active sites is responsible for the declined SCR activity at low temperature. As Cu/SSZ-13 catalysts show excellent acid-resistance ability, our study reveals that the Cu/SSZ-13 catalyst is a good candidate for NOx elimination, especially when SO3 exists.

To reveal the role of impregnated sodium (Na) ions in Cu/SSZ-13 catalysts, Cu/SSZ-13 catalysts with four Na-loading contents were prepared using an incipient wetness impregnation method and hydrothermally treated at 600 °C for 16 h. The physicochemical property and selective catalytic reduction (SCR) activity of these catalysts were studied to probe the deactivation mechanism. The impregnated Na exists as Na+ on catalysts and results in the loss of both Brönsted acid sites and Cu2+ ions. Moreover, the high loading of Na ions destroy the framework structure of Cu/SSZ-13 and forms new phases (SiO2/NaSiO3 and amorphous species) when Na loading was higher than 1.0 mmol/g. The decreased Cu2+ ions finally transformed into CuxO, CuO, and CuAlOx species. The inferior SCR activity of Na impregnated catalysts was mainly due to the reduced contents of Cu2+ ions at kinetic temperature region. The reduction in the amount of acid sites and Cu2+ ions, as well as copper oxide species (CuxO and CuO) formation, led to low SCR performance at high temperature. Our study also revealed that the existing problem of the Na ions’ effect should be well-considered, especially at high hydrothermal aging when diesel particulate filter (DPF) is applied in upstream of the SCR applications.

Cu/SSZ-13 Selective Catalytic Reduction (SCR) catalysts have been extensively studied for the past five-plus years. New and exciting fundamental and applied science has appeared in the literature quite frequently over this time. In this short review, a few topics specifically focused on a molecular-level understanding of this catalyst are summarized: (1) The nature of the active sites and, in particular, their transformations under varying reaction conditions that include dehydration, the presence of the various SCR reactants and hydrothermal aging; (2) Discussions of standard and fast SCR reaction mechanisms. Considerable progress has been made, especially in the last couple of years, on standard SCR mechanisms. In contrast, mechanisms for fast SCR are much less understood. Possible reaction paths are hypothesized for this latter case to stimulate further investigations; (3) Discussions of rational catalyst design based on new knowledge obtained regarding catalyst stability, overall catalytic performance and mechanistic catalytic chemistry.