(165e) Sulfur Poisoning of the NH3-SCR Reaction Over Cu-SAPO-34: SO2 Vs. SO3
Sulfur poisoning is still a durability issue for base metal/zeolite selective catalytic reduction (SCR) catalysts used for abatement of NOx. 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 700oC) in air. Surface species formed during exposure of the catalyst to sulfur, NOx 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 NOx 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, < 300oC, while SO3 affected the activity of NH3-SCR in the whole temperature range (100~600oC). 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 600oC 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 700oC 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 ~300oC which explains its limited impact on activity at higher temperature (>300oC). 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 (> 400oC). 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 NOx adsorption since SO2 adsorption did not affect the NH3 adsorption. In situ DRIFTS of NOx adsorption shows that NOx 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 NOx adsorbed as the bidentate nitrate and influenced the formation of the intermediate, affecting the low temperature activity accordingly.