SO₂ Poisoning of the NH₃-SCR Reaction over Cu-SAPO-34: Effect of Ammonium Sulfate versus Other S-Containing Species

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Abstract

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 NOx 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 NOx 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.

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