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 NOx in the presence of sulfur.
SO2 exposure mainly affected low temperature activity, < 350oC, 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.
- Y. Cheng, C. Montreuil, G. Cavataio, and C. Lambert, SAE International 2009-01-0898.
- Y. Cheng, C. Lambert, D.H. Kim, J.H. Kwak, S.J. Cho, and C.H.F. Peden, Catalysis Today 151(2010)266.
- B. Ramachandran, R.G. Herman, S. Choi, H.G. Stenger, C.E. Lyman, and J.W. Sale, Catalysis Today 55(2000)281.
- L. Zhang, D. Wang, T. Liu, K. Kamasamudram, J. Li and W.S. Epling, Applied Catalysis B: Environmental 156-157(2014)371.