(165e) Sulfur Poisoning of the NH3-SCR Reaction Over Cu-SAPO-34: SO2 Vs. SO3

Abstract

Sulfur poisoning is still a durability issue for base metal/zeolite selective catalytic reduction (SCR) catalysts used for abatement of NO_x. Most studies have been based on SO₂. However, as diesel oxidation catalysts (DOCs) are employed upstream of the SCR catalyst, it is likely that some of the SO₂ is oxidized into SO₃. Therefore, investigating the impact of SO₃on Cu/zeolite SCR catalyst is also critical.

           Literature reveals that all zeolite-based SCR catalysts are susceptible to sulfation with low temperature SO₃ exposure. Major sulfation modes most likely involve formation of (NH₄)HSO₄, (NH₄)₂SO₄ or CuSO₄. It has been proposed that SO₂ and SO₃ have different sulfur poisoning mechanisms; however, a detailed mechanism has not been determined. In this study, we have investigated the poisoning effects of SO₂ and SO₃on the SCR reaction over a Cu-SAPO-34 catalyst.

             NH₃-SCR activity tests over the Cu-SAPO-34 were carried out in a fixed bed reactor in presence of SO₂ or SO₃. Regeneration was also studied by exposing the sample to high temperature (600 and 700^oC) in air. Surface species formed during exposure of the catalyst to sulfur, NO_x and NH₃ were characterized with in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). Temperature programmed desorption (TPD) was used to characterize the samples after exposure to NH₃ and NO_x in the presence of SO₂ or SO₃.

            Activity test results show that both SO₂ and SO₃ affected the NH₃-SCR activity with SO₂ having a smaller impact than SO₃. It has been found that 200 ppm SO₂ has a similar poisoning effect as that of lower concentrations, 50 ppm. In addition, SO₂ affected mainly low temperature activity, < 300^oC, while SO₃ affected the activity of NH₃-SCR in the whole temperature range (100~600^oC). High temperature de-sulfation experiments show that both SO₂ and SO₃ poisoned catalysts could be recovered and most activity regained via high temperature exposure. After exposing to 600^oC overnight in air, 90% of the activity was regained, but some low temperature activity was still lost, while exposure to an even higher temperature of 700^oC overnight, resulted in most activity regained.

            In situ DRIFTS of sulfur adsorption shows that both SO₂ and SO₃ adsorbed on the surface of the Cu-SAPO-34 catalyst. However, the surface ad-species were different. SO₂ interacted with the OH groups and was adsorbed mainly as HSO₃ ad-species. No surface sulfates were observed during SO₂ adsorption, and during TPD almost all the adsorbed SO₂ desorbed by ~300^oC which explains its limited impact on activity at higher temperature (>300^oC). For SO₃adsorption, some surface sulfates formed on the surface.

             DRIFTS results of co-adsorbed of SO₂ and NH₃ shows NH₃ adsorbed on Bronsted acid and Lewis acid sites and weakly adsorbed SO₂ was also evident, but no other new ad-species were observed. In addition, TPD experiments with the sample exposed to both NH₃ and SO₂ shows that both SO₂ and NH₃ could be adsorbed on the surface of catalyst, and desorbed in different temperature ranges from the surface of the catalyst, which indicates that SO₂ adsorption does not react with the adsorbed NH₃. However, the co-adsorption DRIFTS result for SO₃ and NH₃ shows that some surface sulfate species formed, and might due to the formation of ammonium sulfate or bisulfate. TPD experiments with the catalyst exposed to both NH₃ and SO₃ shows that no SO₃ desorbed from the surface of catalyst, but some SO₂ and NH₃ desorbed in the same temperature range (> 400^oC). This indicates that the adsorbed NH₃ and SO₃reacted and formed ammonium sulfate or bisulfate and then decomposed at higher temperatures.

            In order to elucidate the reason why SO₂ affected the SCR activity at low temperature, we further investigated the effect of SO₂ on NO_x adsorption since SO₂ adsorption did not affect the NH₃ adsorption. In situ DRIFTS of NO_x adsorption shows that NO_x was adsorbed mainly as bridging nitrate and bidentate nitrate species. However, the adsorbed bidentate nitrate disappeared in the presence of SO₂. This bidentate nitrate has previously been reported by our group as an important nitrate ad-species and favors the formation of a key intermediate (ammonium nitrate) for the NH₃-SCR reaction at low temperature. This result proves that SO₂ adsorption affected the NO_x adsorbed as the bidentate nitrate and influenced the formation of the intermediate, affecting the low temperature activity accordingly.