Exhaust gas control system for internal combustion engine

An exhaust gas control system for an internal combustion engine operable at a lean air-fuel ratio, the exhaust gas control system comprising:

• an NOx selective catalytic reduction catalyst provided in an exhaust passage of the internal combustion engine, the NOx selective catalytic reduction catalyst being configured to adsorb ammonia and reduce NOx using the adsorbed ammonia as a reducing agent;
• and
• an electronic control unit configured to
• i) change an air-fuel ratio in the internal combustion engine,
• ii) after a request to stop the internal combustion engine has been issued, until an air-fuel ratio in the NOx selective catalytic reduction catalyst becomes lower than or equal to a stoichiometric air-fuel ratio, operate the internal combustion engine at the stoichiometric air-fuel ratio or lower, and
• iii) after that, execute stop control to stop supply of fuel to the internal combustion engine.