Impact of sulfation and desulfation on NO_x reduction using Cu-chabazite SCR catalysts

This bench reactor study investigates the impact of gaseous sulfur on the NO_x reduction activity of Cu-chabazite SCR (Cu-CHA) catalysts at SO₂ concentrations representative of marine diesel engine exhaust. After 2 h of 500 ppm SO₂ exposure at 250 and 400 °C in the simulated diesel exhaust gases, the NO_x reduction activity of the sulfated Cu-CHA SCR catalysts is severely degraded at evaluation temperatures below 250 °C; however, above 250 °C the impact of sulfur exposure is minimal. EPMA shows that sulfur is located throughout the washcoat and along the entire length of the sulfated samples. Interestingly, BET measurements reveal that the sulfated samples have a 20% decrease in surface area. Furthermore, the sulfated samples show a decrease in NO_x/nitrate absorption during NO exposure in a DRIFTS reactor which suggests that Cu sites in the catalyst are blocked by the presence of sulfur. SO₂ exposure also results in an increase in NH₃ storage capacity, possibly due to the formation of ammonium sulfate species in the sulfated samples. In all cases, lean thermal treatments as low as 500 °C reverse the effects of sulfur exposure and restore the NO_x reduction activity of the Cu-CHA catalyst to that of the fresh condition.