Selective catalytic reduction (SCR) is an industrial process for controlling nitrogen oxides (NOx) after combustion, in which ammonia is mixed with the flue gas in the presence of a vanadium catalyst on a titanium oxide support. Alternatively, tungsten-based catalysts can also be used. The SCR stage is usually followed by total oxidation.

Nitrogen oxides are produced during combustion in stationary engines and are contained in the exhaust gases of thermal power stations and industrial plants. These pollutants consist of nitrogen monoxide and nitrogen dioxide (also known as nitrous gases) and are considered harmful to health. In higher concentrations, they also act as strong respiratory poisons.

Selective catalytic reduction or SCR involves several chemical reactions in which nitrogen oxides are converted into diatomic nitrogen and water using ammonia as a reducing agent. As such, these reaction products can be released into the environment without concern. Anhydrous ammonia, aqueous ammonia or a urea solution can be used as a reducing agent. The optimal process temperature is in the range of 320 to 500 °C.

The advantages of SCR are its high efficiency in removing nitrogen oxides (NOx) and its low ammonia slip. Another advantage is the comparatively simple integration of an SCR system into existing systems.