Mechanism for SO2 poisoning of Cu-CHA during low temperature NH3-SCR

Density Functional Theory (DFT) calculations are used to investigate low temperature SO 2 deactivation of Cu-CHA during ammonia assisted selective catalytic reduction of NO (NH 3 -SCR).SO 2 is found to adsorb onforming a copper sulfate complex.NO and NH 3 react over the sulfate complex forming N 2 , H 2 O and H 2 SO 4 .H 2 SO 4 undergoes an acid-base reaction with NH 3 yielding SO 4 (NH 4 ) 2 and HSO 4 (NH 4 ), where HSO 4 (NH 4 ) is thermodynamically preferred during typical reaction conditions.The SO 2 -derived species are bulky and have considerable barriers for inter-cage diffusion.Moreover, the presence of HSO 4 (NH 4 ) species reduces the probability of having two ½Cu I ðNH 3 Þ 2 þ complexes in one cage, which is a requirement for O 2 activation.The results suggest that the key mechanism for low temperature SO 2 deactivation is of physical origin and that the catalyst can be regenerated by exposure to high temperatures where HSO 4 (NH 4 ) decomposes.The suggested mechanism agrees with experimental observations and provides atomistic understanding of sulfur poisoning of Cu-CHA during NH 3 -SCR.