Patented by Wilhelm Ostwald in 1902, the Ostwald process involves the catalytic oxidation of ammonia for large-scale production of nitric acid (HNO3). During this process, ammonia is heated with oxygen in the presence of a platinum rhodium gauze catalyst at 800-950 °C to form water and nitrogen monoxide (NO) in a rapid reaction. During cooling in heat exchangers, the nitrogen monoxide converts to nitrogen dioxide (NO2) at temperatures < 50 °C with the existing as well as additionally supplied oxygen in a second reaction step. Then as cooling continues, the nitrogen dioxide partially dimerizes to dinitrogen tetroxide (N2O4). By adding water, the mixture of NO2 and N2O4 is converted to nitric acid under pressure in spray towers.
Ammonium nitrate (NH4NO3), a main component of many fertilizers, is formed by neutralizing ammonia with nitric acid.
When large-scale production first started, only woven gauze made of fine platinum wire was used as a catalyst. In modern systems, woven or knitted gauze with a platinum rhodium alloy (PtRh5) has become the standard, and are characterized by high selectivity and service life.