Effect of SO 2 and SO 3 Exposure to Cu-CHA on Surface Nitrate and N 2 O Formation for NH 3 -SCR.

We report effects of SO 2 and SO 3 exposure on ammonium nitrate (AN) and N 2 O formation in Cu-CHA used for NH 3 -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 SO 2 exposure results in both (bi)sulfite and (bi)sulfate whereas the SO 3 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 H 2 shows two clear reduction states and larger sulfur uptake for the SO 3 -exposed Cu-CHA compared to the SO 2 -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 SO 3 exposure influence the NO 2 -SCR chemistry by decreasing the N 2 O selectivity during NH 3 -SCR whereas an increased N 2 O selectivity was observed for the SO 2 -exposed Cu-CHA sample. This study provides fundamental insights into how SO 2 and SO 3 affect the N 2 O formation during ammonium nitrate decomposition in NH 3 -SCR applications, which is a very important topic for practical applications.