NOx is actually composed of two oxides of nitrogen: nitric oxide (NO) and nitrogen dioxide (NO2). Nitric oxide consists of one nitrogen atom bonded to an oxygen atom, while nitrogen dioxide has one nitrogen atom bonded to two oxygen atoms. NOx is formed from the nitrogen in air during high-temperature combustion in excess oxygen. The two species exist in equilibrium. The formation of NO is favored at high temperature and NO2 is favored at low temperature. Sources of NOx include stationary diesel engines and stationary gas engines for power generation, coal and gas turbines for electric utilities, and other industrial, commercial, and residential sources that burn fuels.
Nitrogen oxides have several adverse effects on the human body such as inflammation of the respiratory system. Additionally, they help contribute to the formation of ground level ozone, which also causes health issues. For this reason, NOx emissions from combustion sources are regulated by several local and national governments around the world. One of the most effective technologies for reducing NOx emissions is the Selective Catalytic Reduction (SCR) of NOx with ammonia. Johnson Matthey supplies SCR systems as well as SINOx® honeycomb SCR catalysts and plate SCR catalysts that nearly eliminate NOx emissions from industrial and power generating sources.