Effect of SO₂ and SO₃ Exposure to Cu-CHA on Surface Nitrate and N₂O Formation for NH₃-SCR

We report effects of SO₂ and SO₃ exposure on ammonium nitrate (AN) and N₂O formation in Cu-CHA used for NH₃-SCR. First-principles calculations and several characterizations (ICP, BET, XRD, UV-vis-DRS) were applied to characterize the Cu-CHA material and speciation of sulfur species. The first-principles calculations demonstrate that the SO₂ exposure results in both (bi)sulfite and (bi)sulfate whereas the SO₃ exposure yields only (bi)sulfate. Furthermore, SOx adsorption on framework-bound dicopper species is shown to be favored with respect to adsorption onto framework-bound monocopper species. Temperature-programmed reduction with H₂ shows two clear reduction states and larger sulfur uptake for the SO₃-exposed Cu-CHA compared to the SO₂-exposed counterpart. Temperature-programmed desorption of formed ammonium nitrate (AN) highlights a significant decrease in nitrate storage due to sulfur species interacting with copper sites in the form of ammonium/copper (bi)bisulfite/sulfate. Especially, highly stable sulfur species from SO₃ exposure influence the NO₂-SCR chemistry by decreasing the N₂O selectivity during NH₃-SCR whereas an increased N₂O selectivity was observed for the SO₂-exposed Cu-CHA sample. This study provides fundamental insights into how SO₂ and SO₃ affect the N₂O formation during ammonium nitrate decomposition in NH₃-SCR applications, which is a very important topic for practical applications.