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

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 NO x 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 NO x 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.

Site selective adsorption and relocation of SOx in deactivation of Cu–CHA catalysts for NH3-SCR

“…The impact of SO2 at temperatures above 350 °C is clearly less, indicating that deactivation by SO2 is mainly relevant for the low-temperature performance, in agreement with earlier observations. [13][14][15][16][17][18][19][20][21][22][23][24] The low-temperature activity after SO2 exposure is partially restored by heating the Cu-CHA catalysts to 550 C in SO2-free gas ( Figure 1B). 13,14,17 The changes in the deactivation of NH3-SCR activity with time of SO2 exposure are illustrated in Figure 2A, which shows the deactivation, according to Eq.…”

“…Ultra-low sulfur diesel contains less than 10 wt ppm (Europe) or 15 wt ppm (US) sulfur, and the SO2 concentration in the exhaust gas typically reaches a few ppmv, which is nevertheless sufficient to reduce the low-temperature activity of the Cu-CHA catalysts significantly. [13][14][15][16][17][18][19][20][21][22][23][24] To be able to preserve the good lowtemperature activity, it is important to understand the impact of SO2 on the NH3-SCR activity of Cu-CHA based catalysts. We have recently shown that deactivation of Cu-CHA catalysts depends on the total SO2 exposure, which is the product of the SO2 concentration and the exposure time.…”

Importance of the Cu oxidation state for the SO2-poisoning of a Cu-SAPO-34 catalyst in the NH3-SCR reaction

“…It has been argued that the effect of NO and NH3 on SO2-poisoning is the formation of ammonium sulfate, which may infer mass transfer limitations by pore-blocking [14][15][16]. However, ammonium sulfate decomposes at about 350 °C, and can therefore feasibly be removed [16].…”

“…It has been argued that the effect of NO and NH3 on SO2-poisoning is the formation of ammonium sulfate, which may infer mass transfer limitations by pore-blocking [14][15][16]. However, ammonium sulfate decomposes at about 350 °C, and can therefore feasibly be removed [16]. Moreover, the presence of NO and NH3, or release of NH3 from ammonium sulfate, has a suggested beneficial effect on the regeneration of SO2-poisoned catalysts, due to the reducing properties of the SCR gas mixture and NH3 [8,16].…”

H2O and/or SO2 Tolerance of Cu-Mn/SAPO-34 Catalyst for NO Reduction with NH3 at Low Temperature

“…Wijayanti et al [23] studied the SO 2 poisoning effects and found that the main reason for the deactivation is the formation of copper sulfates, resulting in the loss of redox properties. Jangjou et al [24] reported the S species formed on Cu 2+ at 6MR by DRIFT study. On the basis of such observations, the formation of ammonium sulfates in a complex with Cu is claimed as the main mechanism for the loss of low-temperature deNO x activity.…”