The effect(s) of SO 2 on the two types of active sites on Cu-SSZ-13 NH 3 −SCR catalysts, Z2Cu and ZCuOH, were investigated. Two Cu-SSZ-13 catalysts with Si:Al ratios of 6 and 30 were synthesized, and they provide very different distributions of these two active sites. Inductively coupled plasma optical emission spectroscopy (ICP-OES), H 2 temperature-programmed reduction (H 2 -TPR), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were utilized to characterize catalyst samples and quantify the amounts of total Cu, Z2Cu and ZCuOH. In situ DRIFTS results show that Z2Cu and ZCuOH responses to low-temperature (<200 °C) SO 2 poisoning were site-dependent. Results of SO 2 and SO 2 + NH 3 temperatureprogrammed desorption (TPD) and DRIFTS experiments, supplemented with DFT calculations, revealed that the different observed responses correspond to different sulfur intermediates that form. On Z2Cu sites, SO 2 only adsorbs when it is cofed with NH 3 via formation of ammonium sulfate, with its fingerprint TPD feature at 380 °C. However, low-temperature interaction between SO 2 and ZCuOH leads to copper bisulfite species formation, which can be further oxidized to form copper bisulfate with increasing temperature. In terms of low-temperature SCR functionality, the activity of both Cu-SSZ-13 samples were found to be significantly inhibited by SO 2 . However, in terms of regeneration (i.e., desulfation) behavior, Cu-SSZ-13 with a Si:Al = 30 (higher ZCuOH compared to Z2Cu) seemed to require higher desulfation temperatures (>550 °C). Therefore, compared with Z2Cu, ZCuOH sites are more susceptible to severe low-temperature SO 2 poisoning because of the formation of more stable bisulfite and ultimately bisulfate species.

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SO2 poisoning of NH3-SCR over Cu-SAPO-34 was studied, specifically to evaluate the forms/states of stored S and the effect of such species on low-temperature NO x reduction activity. Two primary sulfur species types were observed and were found to be interchangeable depending on whether NH3 was available or not. In one case both ammonium sulfate and Cu sulfate species could be present and in the other only Cu sulfate species. Cu sulfate, in the absence of ammonia, was found in three different states/forms, identified by three desorption features during temperature-programmed desorption (TPD) experiments. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of NO adsorption was used to investigate the nature and accessibility of the Cu species before and after sulfate formation, without the interference of ammonium sulfate; these data revealed that the Cu2+ inside the six-membered rings was completely blocked by sulfur and that the nature of the [CuOH]+ close to the eight-membered ring changed. In comparing the effect of different forms of S on low-temperature NO x reduction activity, ammonium sulfate had the greatest impact on performance loss. Interestingly, the results also show that ammonium sulfate can actually play a role as a SCR reactant, likely analogous to the mechanism involving ammonium nitrate. Ammonium sulfate decomposes at temperatures as low as ∼300–350 °C, whereas higher temperatures (>480 °C) were needed to desorb other S-containing species. This appears favorable, as NH3 can react with preadsorbed sulfur on the catalyst to form ammonium sulfate, which decomposes at lower temperatures in comparison to the other sulfate forms.