A system includes a controller. The controller is configured to monitor a catalytic activity within an ASC assembly that converts ammonia (NH3) within a fluid received from a three-way catalyst (TWC) assembly into nitrogen (N2) to determine whether the catalytic activity in the ASC assembly has aged. The controller is configured to adjust a temperature of the fluid flowing into an inlet of the ASC assembly by controlling an amount of oxidant injected via an oxidant injection system into the fluid upstream of the inlet of the ASC assembly based on a determination that the catalytic activity in the ASC assembly has aged.