Conventionally, a NOx storage reduction catalyst been known as a catalyst which reduces and purifies nitrogen compound (NOx) in exhaust gases discharged from an internal combustion engine. The NOx storage reduction catalyst adsorbs NOx included in exhaust gases when the exhaust gases are in a lean atmosphere, while when the exhaust gases are in a rich atmosphere, the NOx storage reduction catalyst reduces and purifies the adsorbed NOx with hydrocarbons contained in the exhaust gases into harmless gas for discharge.
In addition, the NOx storage reduction catalyst also adsorbs sulfur oxides (hereinafter, referred to as SOx) contained in exhaust gases. The increase in the amount of adsorption of SOx causes a problem that the NOx purification capacity of the NOx storage reduction catalyst is reduced. Because of this, in the event that the SOx adsorption amount reaches a predetermined amount, in order to restore the NOx storage reduction catalyst from the sulfur poisoned state by releasing the adsorbed SOx from the NOx storage reduction catalyst, a so-called SOx purging needs to be carried out periodically in which unburned fuel is supplied to the oxidation catalyst disposed upstream of the reduction catalyst through post injection or exhaust pipe injection to increase the temperature of exhaust gases to the SOx desorption temperature (for example, refer to Patent Literature 1).
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