The present invention relates to a system for ammonia combustion and exhaust gas treatment, the system comprising: an internal combustion engine for combusting ammonia; and an exhaust system comprising an intake for receiving an exhaust gas from the combustion engine, an upstream injector for adding hydrogen gas to the exhaust gas and a downstream catalyst article, wherein the downstream catalyst article comprises a three-way catalyst (TWC) composition. The invention further relates to a method for the treatment of an exhaust gas from an ammonia internal combustion engine.