A chemical process for treating exhaust gas from a vehicular internal combustion engine with a lean/rich cycle using a selective catalytic reduction (SCR) catalyst, which chemical process comprising exposing the SCR catalyst in a NOx adsorber catalyst (NAC)+(downstream)SCR catalyst system to at least one reactant including nitrogen oxides and reducing the nitrogen oxides with NH 3 nitrogenous reductant generated over the NAC during periodic rich regeneration events, wherein the SCR catalyst comprises copper and a small pore molecular sieve having a maximum ring size of eight tetrahedral atoms having the AEI Framework Type Code.