(351h) Sulfur Poisoning of the NH3-SCR Reaction over Cu-SAPO-34

Sulfur is a common automotive catalyst poison and this also holds true for the newer metal-exchanged small pore zeolite selective catalytic reduction (SCR) catalysts. Sulfur poisoning modes likely involve formation of (NH₄)HSO₄, (NH₄)₂SO₄ and/or metal sulfates.

In this study, we investigated the poisoning effects of SO₂ on the SCR reaction over a Cu-SAPO-34 catalyst. NH₃-SCR activity tests were carried out in a reactor specifically designed to evaluate monolith-supported emissions catalysts. Performance regeneration was also evaluated via sample exposure to high temperatures (500-800°C) in N₂. Temperature programmed desorption (TPD) was also used to characterize the samples after exposure to NH₃ and NO_x in the presence of sulfur.

SO₂ exposure mainly affected low temperature activity, < 350^oC, and there was also an increase in performance in the mid-temperature range. With high temperature exposure to air, the SO₂-poisoned catalyst activity could be recovered. Moreover, at temperatures lower than 350°C, SO₂ poisoned the NH₃ oxidation as the main SCR side reaction. NH₃ oxidation kinetic analysis data confirmed that the poisoning was due to the formation of ammonium sulfate deposited on monomeric copper present in the CHA framework.

TPD results demonstrated that more NH₃ adsorbed in the presence of SO₂ and more sulfur adsorbed in the presence of NH₃. The extra NH₃ amount was in a 2:1 molar ratio with the extra sulfur that adsorbed, clearly demonstrating that ammonium sulfate formed⁴. Results also showed that SO₂ did not adsorb significantly at low temperatures when NO or NH₃ was not present in the feed.

A set of designed transient experiments enabled us to distinguish between the effect of ammonium sulfate and metal sulfate or sulfite species on NH₃-SCR activity. Results showed that ammonium sulfate was responsible for more than 70% of poisoning effect. With increasing temperature, more stable sulfur species formed. Although the poisoning effect of these species was not as severe as ammonium sulfate, they required higher temperature in order to be removed from the surface.       

Overall, two common modes of deactivation were observed. First ammonium sulfate formed upon exposure to NH₃ and SO₂, which could block zeolite pores thus limiting reactivity. This can be considered a primary SO₂ poisoning effect on low temperature performance, since the ammonium sulfate decomposes at temperature higher than 350°C. Secondly, metal sulfates can also form, and these require more severe conditions, i.e. higher temperature, for decomposition.

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