Exhaust gas purification apparatus of an internal combustion engine

A reducing agent addition method in an exhaust gas purification apparatus of an internal combustion engine which is provided with:

• a selective reduction type NOx catalyst that is arranged in an exhaust passage of the internal combustion engine;
• and
• a reducing agent addition part that is arranged in said exhaust passage at an upstream side of said selective reduction type NOx catalyst, and adds a reducing agent for supplying NH3 to said selective reduction type NOx catalyst;
• wherein
• said selective reduction type NOx catalyst has an active spot which purifies NOx by the use of NH3, and an adsorption site which adsorbs NH3;
• a vicinity site, which is located in the vicinity of said active spot, and a distant site, which is located distant from said active spot, exist in said adsorption site;
• said vicinity site tends to deliver NH3 to said active spot more easily than said distant site does, and the desorption rate of NH3 in said vicinity site is faster than the desorption rate of NH3 in said distant site;
• and
• the addition of the reducing agent from said reducing agent addition part is controlled based on the desorption rate of NH3 in said vicinity site so as to continue to cause the NH3 adsorbed to said vicinity site to exist.