Exhaust gas purification apparatus and exhaust gas purification method

An exhaust gas purification apparatus comprising:

• an oxidation reduction catalytic converter disposed in a main flow passage through which exhaust flows from an internal combustion engine that is configured to combust ammonia to obtain a driving force, the oxidation reduction catalytic converter being configured to provide oxidation catalysis and reduction catalysis;
• a selective reduction catalytic converter disposed downstream of the oxidation reduction catalytic converter in the main flow passage and configured to absorb ammonia having flowed through the oxidation reduction catalytic converter;
• a temperature acquirer configured to acquire a temperature of a region related to a temperature of the selective reduction catalytic converter;
• a mixing ratio acquirer configured to acquire mixing ratios of the exhaust upstream and downstream of the selective reduction catalytic converter, wherein the mixing ratios indicate a relationship of ammonia and oxygen;
• and
• a controller configured to perform a first control or a second control based on the mixing ratios of the exhaust acquired by the mixing ratio acquirer when the temperature acquired by the temperature acquirer is higher than a predetermined temperature that indicates activation of the selective reduction catalytic converter, the controller being configured to perform the first control when the mixing ratio of the exhaust downstream of the selective reduction catalytic converter is greater than the mixing ratio of the exhaust upstream of the selective reduction catalytic converter so that the first control changes the mixing ratio of the exhaust upstream of the oxidation reduction catalytic converter to change the mixing ratio of the exhaust upstream of the selective reduction catalytic converter from stoichiometric ratio to lean mixture ratio, the controller being configured to perform the second control when the mixing ratio of the exhaust downstream of the selective reduction catalytic converter is smaller than the mixing ratio of the exhaust upstream of the selective reduction catalytic converter so that the second control changes the mixing ratio of the exhaust upstream of the oxidation reduction catalytic converter to change the mixing ratio of the exhaust downstream of the selective reduction catalytic converter from stoichiometric ratio to lean mixture ratio.