Model-based monitoring for selective catalytic reduction device in aftertreatment assembly

An aftertreatment assembly includes a selective catalytic reduction (SCR) device having a catalyst and configured to receive an exhaust gas. A controller is operatively connected to the SCR device. The controller having a processor and a tangible, non-transitory memory on which is recorded instructions for executing a method of model-based monitoring of the SCR device. The method relies on a physics-based model that may be implemented in a variety of forms. The controller is configured to obtain at least one estimated parameter, and at least one threshold parameter based at least partially on a catalyst degradation model. The catalyst degradation model is based at least partially on a predetermined threshold storage capacity (ΘT). A catalyst status is determined based on a comparison of the estimated and threshold parameters. The operation of the assembly is controlled based at least partially on the catalyst status.