Selective Catalytic Reduction: The NOx Removal Method of Choice from Passenger Cars to Power Plants

There is continued focus on controlling the emissions of pollutants from engines around the world. The Three Way Catalyst (TWC) has been successfully removing carbon monoxide (CO), hydrocarbon (HC) and nitrogen oxide (NOx) emissions from gasoline engines since the early 1970s. These catalysts operate under so-called stoichiometric conditions, in which there is a balance between the respective concentrations of oxidising and reducing species in the vehicle exhaust. This means that both oxidation (CO to CO2 and HC to CO2 and water) and reduction (NOx to nitrogen) reactions can be carried out simultaneously, leading to the very high conversions (over 90%) seen in these systems.