Engine exhaust gas feedstream NOx emissions aftertreatment includes a three-way catalytic device connected upstream of an ammonia-selective catalytic reduction device including a base metal. Engine operation can be modulated to generate an engine-out exhaust gas feedstream that converts to ammonia on the three-way catalytic device. The ammonia is stored on the ammonia-selective catalytic reduction device, and used to reduce NOx emissions in the exhaust gas feedstream.