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

We use dry CO oxidation to probe CuII 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 % O2/He at 550 °C for 1 hour; (ii) pre-oxidation in 8 % O2/He at 550 °C for 1 hour followed by NH3 saturation at 200 °C in O2. Pre-adsorbed NH3 was necessary to observe a significant CO2 evolution, the rate of CO2 formation being second order in the CuIIOH fraction of the CuII sites. This is direct evidence that NH3, not involved in the CO oxidation chemistry, favours the formation of CuII dimers with removable oxygen by ligating with and mobilizing isolated CuIIOH ions, otherwise inactive. These findings are relevant for the low-temperature NH3−SCR redox mechanism over Cu−CHA, in which CuII dimers may play a role in parallel or in alternative to the isolated CuII sites so far proposed for the reduction half cycle of standard SCR.