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

Ton V.W.  Janssens, Umicore Denmark ApS

Peter S. Hammershøi^a, Anker D. Jensen^b, Ton V.W. Janssens^a

^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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