Around 1900 the world was running out of fertilizer. The nitrogen it needed was all around — in the air — but locked in a molecule that refuses to react. The Haber–Bosch process broke that lock and became one of the most important chemical reactions ever put to work.
This article explains why nitrogen is hard to use, how Fritz Haber made ammonia in the laboratory, how Carl Bosch scaled it up, the equilibrium chemistry behind the operating conditions and the consequences, good and bad, for the modern world.
The nitrogen problem
Nitrogen is in every protein and in DNA, and crops need a lot of it. The air is 78% nitrogen, but as N₂: two atoms joined by a triple bond that takes about 945 kJ/mol to break. Plants cannot use it. They need fixed nitrogen — ammonia or nitrate in the soil — which in nature comes mainly from bacteria and, in small amounts, from lightning.
In the 19th century farmers added fixed nitrogen from guano (seabird droppings) and from Chilean saltpetre (sodium nitrate). Both were limited. In 1898 the British chemist William Crookes warned that wheat production would soon fall behind population unless chemists learned to fix nitrogen from the air.
Haber’s tabletop ammonia
Fritz Haber, at the University of Karlsruhe, studied the equilibrium between nitrogen, hydrogen and ammonia. The yields at normal pressure were tiny. With the engineer Robert Le Rossignol, he built a small apparatus that worked at about 200 atmospheres and 500 °C, with an osmium catalyst and a loop that recycled the unreacted gas.
In July 1909 the apparatus produced a steady flow of liquid ammonia in front of representatives of the chemical company BASF. The chemistry was solved; the engineering was not.
Bosch scales it up
BASF gave the job to Carl Bosch. He faced problems nobody had solved before:
- The catalyst. Osmium was rare and expensive. Alwin Mittasch and his team ran thousands of tests on different materials and found that iron, with small amounts of aluminium oxide and potassium oxide as promoters, worked well and cheaply. Industry still uses this type of catalyst.
- The reactor. At high pressure and temperature, hydrogen diffused into the steel walls and made them brittle until they burst. Bosch designed a reactor with a thin inner lining of soft iron inside a strong steel outer tube with vent holes, so hydrogen that leaked through could escape.
- The gases. The plant needed huge amounts of pure hydrogen and nitrogen, and of compressors and valves for high pressure.
The first factory opened at Oppau, Germany, in 1913. High-pressure chemistry had become an industry.
The chemistry of the conditions
The reaction is a reversible equilibrium, so its conditions are chosen with Le Chatelier’s principle: a system at equilibrium shifts to oppose a change.
The trade-off between yield and speed is a classic exam question: lower temperatures would give a better equilibrium yield, but the reaction would be far too slow, even with the catalyst. Learn more about reversible reactions in What is a Chemical Reaction.
Today most of the hydrogen comes from natural gas by steam reforming (CH₄ + H₂O → CO + 3 H₂), and the nitrogen comes straight from the air.
Feeding the world
About 80% of the ammonia made today goes into fertilizers, such as urea and ammonium nitrate. Studies estimate that Haber–Bosch nitrogen supports food for roughly half of the world’s population, and that about half of the nitrogen atoms in a typical human body passed through an ammonia plant. Few chemical reactions have changed more lives.
Haber received the Nobel Prize in Chemistry for 1918, and Bosch shared the 1931 prize with Friedrich Bergius for high-pressure chemical methods.
The darker side
Ammonia can be oxidized to nitric acid (the Ostwald process), the starting point for explosives. During the First World War, when Germany was cut off from Chilean nitrate, Haber–Bosch ammonia kept its munitions factories running.
Haber himself directed Germany’s chemical weapons programme and oversaw the first large chlorine gas attack, at Ypres in April 1915. Years later, as a scientist of Jewish origin, he was forced out by the Nazi government and left Germany in 1933; he died in Basel in 1934.
The process also has environmental costs. Making ammonia uses roughly 1–2% of the world’s energy and produces a similar share of global CO₂ emissions. Excess fertilizer washes into rivers and seas, causing algal blooms and low-oxygen “dead zones”, and part of it escapes as nitrous oxide, a powerful greenhouse gas. Researchers are working on “green ammonia” made with hydrogen from electrolysis powered by renewable electricity.
Timeline
- 1898 — Crookes’ warning. Wheat supplies will fall behind without nitrogen fixation.
- 1909 — Haber’s demonstration. Liquid ammonia from nitrogen and hydrogen on a laboratory bench.
- 1913 — Oppau. BASF opens the first industrial ammonia plant.
- 1918 — Nobel Prize. Haber is awarded the prize for chemistry for the synthesis of ammonia.
- 1931 — Nobel Prize. Bosch shares the prize for high-pressure chemistry.
- 2023 — Over 150 million tonnes a year. Ammonia is one of the most produced chemicals in the world.
Frequently asked questions
What is the Haber–Bosch process?
It is the industrial method that combines nitrogen from the air with hydrogen to make ammonia, N₂ + 3H₂ ⇌ 2NH₃, using an iron catalyst at roughly 400–500 °C and 150–300 bar. Most of the ammonia becomes fertilizer.
Why is nitrogen from the air so hard to use?
The two atoms in an N₂ molecule are held by a triple bond, one of the strongest bonds in chemistry, so the molecule is very unreactive. Plants cannot use it directly and depend on nitrogen that has been “fixed” into compounds like ammonia or nitrate.
Why does the process use high pressure?
Four molecules of gas (one N₂ and three H₂) become two molecules of ammonia. By Le Chatelier’s principle, high pressure shifts the equilibrium toward the side with fewer gas molecules, giving more ammonia.
How many people depend on the Haber–Bosch process?
Studies estimate that fertilizer made with Haber–Bosch ammonia supports food production for roughly half of the world’s population.
References and further reading
- Smil, V. — Enriching the Earth: Fritz Haber, Carl Bosch, and the Transformation of World Food Production. MIT Press, 2001.
- Erisman, J. W. et al. — How a century of ammonia synthesis changed the world. Nature Geoscience 1, 636–639 (2008).
- The Nobel Prize — Fritz Haber (Chemistry 1918) and Carl Bosch (Chemistry 1931).
- Royal Society — Ammonia: zero-carbon fertiliser, fuel and energy store (2020).