Selective catalytic reduction (SCR) is a technology used to reduce harmful nitrogen oxides (NOx) emissions from diesel engines. It involves injecting a urea-based solution, often referred to as diesel exhaust fluid (DEF), into the exhaust stream. The heat from the exhaust converts the urea into ammonia, which then reacts with the NOx in the presence of a catalyst, transforming them into harmless nitrogen and water vapor. This process is commonly employed in various applications, including automobiles, industrial machinery, and power plants, to meet stringent environmental regulations. By effectively lowering NOx emissions, SCR helps improve air quality and reduce the impact of diesel engines on the environment.