Haber–Bosch process

The process combines nitrogen gas (N₂) with hydrogen gas (H₂) to form ammonia (NH₃): N₂ + 3H₂ ⇌ 2NH₃. Nitrogen makes up most of Earth's atmosphere, but its molecules are stable and do not readily react. The industrial reaction therefore uses high pressure, elevated temperature and a catalyst, commonly iron-based, to produce ammonia at a practical rate. Operators balance temperature and pressure because conditions that increase reaction speed do not all maximize the equilibrium yield. Removing ammonia as it forms and recycling unreacted gases help sustain production.

Fritz Haber demonstrated laboratory synthesis of ammonia from its elements in the early 1900s. The process was not the work of one person alone: earlier experiments by other chemists, including Walther Nernst, contributed to the understanding of the reaction, and accounts of scientific priority distinguish those contributions from Haber's later results. At BASF, engineer Carl Bosch and his colleagues developed equipment and materials capable of operating the chemistry at industrial scale; the name Haber–Bosch reflects this combination of laboratory chemistry and engineering. BASF's Oppau plant began operating in 1913, marking an early commercial application of the process.

Ammonia is an important feedstock for nitrogen fertilizers, which supply crops with nitrogen in a form they can use. Industrial fixation of atmospheric nitrogen greatly expanded the amount of reactive nitrogen available to agriculture and supported large-scale fertilizer production. Ammonia can also be converted into nitric acid and other chemicals. In the process's early history, these uses had both agricultural and military importance, since nitric acid was used to make explosives. The Haber–Bosch process is consequently regarded as a major technological change in food production and the chemical industry, as well as a technology with consequential social and political effects.

Conventional ammonia plants commonly obtain hydrogen from natural gas through steam methane reforming, a process that consumes energy and releases carbon dioxide. The Royal Society identifies hydrogen production as the main source of emissions in this route. One proposed alternative is green ammonia: hydrogen is made by electrolysing water with renewable electricity, while nitrogen is separated from air; the gases are then reacted using the Haber–Bosch process. This approach could reduce production emissions, although it depends on access to low-carbon electricity.