Long-term SO2-poisoning of a Cu-CHA catalyst for NH3-SCR.

Ton V.W.  Janssens, Umicore Denmark ApS

Peter S. Hammershøia, Anker D. Jensenb, Ton V.W. Janssensa

aUmicore Denmark ApS, Nøjsomhedsvej 20, DK-2800 Kgs. Lyngby, Denmark.

bDepartment of Chemical and Biochemical Engineering, Technical University of Denmark, Søltofts Plads B229, 2800 Kgs. Lyngby, Denmark

The impact of long-term SO2 exposures of a Cu-CHA catalyst for NH3-SCR has been studied. A total of 48 catalyst samples were produced by exposing catalyst samples to 50 ppm SO2 for 1, 5, 15, 30, 65, and 120 h, at 200, 300, 400 and 500 °C. A part of each sample was regenerated at 550 °C for 6 h in an SO2 free atmosphere. The longest SO2 exposure corresponds to that of a Cu-CHA catalyst in a typical exhaust aftertreatment system during the entire life time of the catalyst. The deactivation by SO2 was evaluated from the SCR activity of the fresh, sulfated and regenerated catalyst samples. All SO2 exposures result in a significant deactivation of the low-temperature activity, i.e. 160-350 °C, while the NOx conversion above 400 °C remains essentially the same. Most of the deactivation at low temperature takes place during the first 15 h of SO2 exposure to a total loss of activity of 85-95%, dependent on the SO2 exposure temperature. After regeneration, the activity of the catalysts could be restored to about 80 % of its original level, independent of the SO2 exposure conditions, which suggests that regeneration is feasible for handling SO2-poisoning in exhaust systems.

The sulfur content in the samples, measured with ICP-OES, also show a fast increase in the S/Cu ratio within the first 15 h of exposure, similar to the observed trend in the deactivation. With longer SO2 exposures, the S/Cu ratio continue to increase slowly to a level in the range 0.5-1, indicating S adsorption on the Cu ions in the CHA zeolite. The activity of the regenerated catalysts matches the measured S/Cu ratios of maximum 0.2, pointing to a deactivation by loss of active sites. For the sulfated catalysts, the loss of activity is generally higher than the S/Cu ratio, which can be related to a loss of mobility of the Cu in the sulfated catalysts. The different impact of the sulfur on the deactivation of the sulfated and regenerated catalysts is also reflected in a gradual change in activation energy for SCR from 65 kJ/mol to about 25 kJ/mol with increasing deactivation levels for the sulfated catalysts, while it remains unchanged at 65 kJ/mol for the regenerated catalysts.

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