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Portrait of The Haber process

Science · Chemistry

The Haber process

1909 (laboratory); 1913 (first industrial plant, at Oppau)

Chemistry — industrial catalysis

The Haber process fixes nitrogen from the air as ammonia: nitrogen and hydrogen are passed over an iron catalyst at roughly 400–500 °C and 150–250 atmospheres, and the ammonia is condensed out while the unreacted gases are recycled. Fritz Haber demonstrated it on a bench in 1909 with Robert Le Rossignol; Carl Bosch turned it into industrial plant for BASF, which began production at Oppau in 1913. It is arguably the most consequential chemical process ever devised. Synthetic fertiliser made from its ammonia now supports roughly half the world's population — and the same ammonia, oxidised to nitric acid, gave Germany the explosives to continue the First World War after the Allied blockade cut off Chilean nitrate.

How to spot them

  • ▸Nitrogen and hydrogen over an iron catalyst at high temperature and pressure
  • ▸'Bread from air' — the industrial synthesis of ammonia
  • ▸Roughly half the nitrogen atoms in your body have passed through it
  • ▸It freed Germany from imported Chilean nitrate in the First World War
  • ▸Its inventor also directed Germany's poison gas programme

Key facts

Nitrogen and hydrogen combined over a promoted iron catalyst at about 450 °C and 200 atmospheres

N2 + 3H2 ⇌ 2NH3, exothermic at about −92 kJ per mole of nitrogen.

Often called 'bread from air': the reaction that makes ammonia on an industrial scale

The nitrogen is simply taken from the atmosphere, which is 78 per cent N2.

Roughly half the nitrogen atoms in your body were fixed by this reaction rather than by bacteria

Vaclav Smil's estimate; without it the planet could feed perhaps three to four billion people.

Carl Bosch engineered it up to industrial scale for BASF at Oppau in 1913, and won his own Nobel Prize in 1931 (1913)

The real difficulty was metallurgical: steel vessels that would not be embrittled by hot hydrogen at 200 atmospheres.

By making explosives from air, it let Germany fight on after the Allied blockade cut off Chilean nitrate

Ammonia oxidised to nitric acid, and nitric acid to TNT and other propellants.

Consumes something like one to two per cent of the world's entire energy supply

Most of it goes into making the hydrogen from natural gas by steam reforming, which is also why the process is a large source of carbon dioxide.

Its inventor, who also directed Germany's chlorine gas programme, was given the 1918 Nobel Prize in Chemistry for it (1918)

Announced in 1919 and bitterly contested by scientists in the Allied countries.

Key ideas

  • It is a textbook exercise in Le Chatelier's principle and compromise. High pressure favours ammonia because four molecules of gas become two; low temperature favours it because the reaction is exothermic — but at low temperature the reaction is far too slow, so industry accepts a hot catalyst and recycles the unconverted gas.
  • Only about 10–20 per cent of the gas converts on each pass. The economics depend entirely on separating the ammonia by cooling and returning the rest to the reactor.

Quick-fire facts

  • •The catalyst is iron derived from magnetite, promoted with potassium oxide, alumina and calcium oxide.
  • •Fritz Haber received the 1918 Nobel Prize in Chemistry; Carl Bosch shared the 1931 prize with Friedrich Bergius for high-pressure chemical methods.
  • •Haber supervised the first large-scale chlorine attack at Ypres in April 1915; his wife Clara Immerwahr, herself a chemist, shot herself days later.
  • •Haber was Jewish by birth and a fervent German patriot; he was forced out by the Nazi race laws in 1933 and died in Basel in 1934.
  • •His institute developed the pesticide Zyklon A; the later Zyklon B was used to murder relatives of his at Auschwitz.
  • •World ammonia production now exceeds 150 million tonnes a year, around 80 per cent of it for fertiliser.
  • •Haber had a famous wager with Walther Nernst, who had calculated that the yields would be uselessly small.
  • •Common misconception: the catalyst does not increase the yield at equilibrium, only the speed at which equilibrium is reached. The yield comes from the pressure, the cooling and the recycling.

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