Nitrogen oxides (NOₓ) consist of a mixture of nitrogen monoxide (NO) and nitrogen dioxide (NO₂) and are among the most significant pollutants. They are produced during combustion processes in engines at high temperatures. They have a harmful effect on the environment and human health. NOₓ contributes to the formation of ground-level ozone, a key component of photochemical smog, and is a major contributor to the formation of acid rain. On a human level, these gases promote respiratory diseases, worsen asthma and can increase the risk of cardiovascular disease.

Given the serious impacts on ecosystems and public health, technological measures have been developed to reduce NOₓ emissions. One of the most effective methods is selective catalytic reduction (SCR).

SCR is a chemical process in which nitrogen oxides are converted into the harmless products nitrogen (N₂) and water (H₂O) using a reducing agent. Ammonia (NH₃) is usually used as a reducing agent, which comes into contact with the exhaust gases in a temperature range of around 250–400 °C. The reaction is made possible by a catalyst based on vanadium oxide (V₂O₅).

Despite the efficiency of SCR, challenges such as controlling the ammonia slip rate and maintaining the optimal operating temperature are the focus of current research. Selective catalytic reduction remains the most important process for reducing NOₓ and thus a key contribution to improving air quality and protecting the environment.