Selective catalytic reduction (SCR) is a technology used in the control of nitrogen oxide (NOx) emissions from combustion processes, such as those in power plants, vehicles, and industrial facilities. The process involves the use of a catalyst to convert NOx molecules into nitrogen, water, and carbon dioxide, which are less harmful to the environment. SCR typically involves injecting a reducing agent, such as ammonia or urea, into the exhaust gas stream, where it reacts with the NOx over a catalyst, usually made of metal oxides like vanadium or titanium. This reaction results in the formation of nitrogen gas and water vapor, effectively reducing the NOx emissions. SCR is known for its high efficiency and effectiveness in reducing NOx emissions, making it a widely used technology in the industry. It is also considered a cost-effective method for meeting stringent emission regulations and improving air quality.