Exhaust gas purification apparatus for an internal combustion engine

An exhaust gas purification apparatus for an internal combustion engine comprising:

• an NOx selective catalytic reduction catalyst that is arranged in an exhaust passage of the internal combustion engine and selectively reduces NOx in an exhaust gas by using ammonia as a reducing agent;
• an addition valve that is arranged in said exhaust passage at the upstream side of said NOx selective catalytic reduction catalyst and configured to add an additive agent, which is ammonia or a precursor of ammonia, into the exhaust gas;
• a temperature sensor configured to acquire a temperature of said NOx selective catalytic reduction catalyst;
• and
• a controller configured to carry out equivalent ratio control in which an amount of the additive agent corresponding to an amount of NOx flowing into said NOx selective catalytic reduction catalyst and an equivalent ratio is added from said addition valve;
• wherein in cases where the temperature acquired by said temperature sensor is equal to or higher than a predetermined temperature at which ammonia is oxidized, and in cases where an NOx reduction rate in said NOx selective catalytic reduction catalyst is smaller than a predetermined lower limit reduction rate, when the addition of the additive agent from said addition valve is carried out at least twice by changing the equivalent ratio with respect to the amount of NOx flowing into said NOx selective catalytic reduction catalyst, said controller configured to make the equivalent ratio in said equivalent ratio control larger in the case where an amount of change of an amount of reduction of NOx accompanying a change of the equivalent ratio is larger than an amount of change of the amount of oxidation of ammonia accompanying the change of the equivalent ratio, than in the case where the amount of change of the amount of reduction of NOx accompanying the change of the equivalent ratio is smaller than the amount of change of the amount of oxidation of ammonia accompanying the change of the equivalent ratio.