Low‐T CO Oxidation over Cu−CHA Catalysts in Presence of NH₃: Probing the Mobility of Cu^II Ions and the Role of Multinuclear Cu^II Species

Abstract

Abstract We use dry CO oxidation to probe Cu II dimers in Cu−CHA catalysts in the presence of ammonia. Samples with different Cu/Al and Si/Al ratios were exposed to CO in He at 200 °C after either one of two catalyst pre‐treatments: (i) pre‐oxidization in 8 % O 2 /He at 550 °C for 1 hour; (ii) pre‐oxidation in 8 % O 2 /He at 550 °C for 1 hour followed by NH 3 saturation at 200 °C in O 2 . Pre‐adsorbed NH 3 was necessary to observe a significant CO 2 evolution, the rate of CO 2 formation being second order in the Cu II OH fraction of the Cu II sites. This is direct evidence that NH 3 , not involved in the CO oxidation chemistry, favours the formation of Cu II dimers with removable oxygen by ligating with and mobilizing isolated Cu II OH ions, otherwise inactive. These findings are relevant for the low‐temperature NH 3 −SCR redox mechanism over Cu−CHA, in which Cu II dimers may play a role in parallel or in alternative to the isolated Cu II sites so far proposed for the reduction half cycle of standard SCR.