Patent ID: 8671660

Claim:
A catalyst degradation determination device for an exhaust purification system of an internal combustion engine equipped with a selective reduction catalyst that is provided in an exhaust channel of the internal combustion engine, and reduces NOx in exhaust flowing through the exhaust channel under the present of a reducing agent, and a reducing agent supply means for supplying a reducing agent or an additive serving as a source of the reducing agent to the selective reduction catalyst, the device comprising: a slip determination means for determining occurrence of reducing agent slip in the selective reduction catalyst, wherein an amount of the reducing agent stored in the selective reduction catalyst is defined as a storage amount, and an amount of the reducing agent that is stored by the selective reduction catalyst defined as a storage capacity; a storage amount fluctuating means for reducing, in a selective reduction catalyst in a state in which the storage amount is a maximum, the storage amount thereof by a predetermined detection reduced-amount portion by way of controlling the reducing agent supply means, and then increasing the storage amount thereof until it is determined by way of the slip determination means that reducing agent slip has occurred; and a catalyst degradation determination means for determining degradation of the selective reduction catalyst based on determination results of the slip determination means when fluctuating the storage amount by way of the storage amount fluctuating means, wherein the detection reduced-amount portion is set to a value that is larger than the storage capacity of the selective reduction catalyst in a predetermined degradation state, and smaller than the storage capacity of the selective reduction catalyst in a predetermined normal state.