Nitrogen Fixation: Why It Is Hard and How It Is Done

The problem

Dinitrogen makes up most of the atmosphere yet is almost unreactive. The reason is the triple bond, which is among the strongest known and has no dipole, no low-lying vacant orbitals readily attacked, and no polarity to offer a nucleophile or electrophile a handle.

Converting it to ammonia is thermodynamically favourable at ordinary temperature but kinetically impossible without help — which is the standard illustration that thermodynamic favourability says nothing about rate.

The industrial compromise

The synthesis of ammonia from its elements is exothermic and reduces the number of gas molecules. Le Chatelier reasoning therefore says low temperature and high pressure favour the product.

Removing ammonia as it forms shifts equilibrium further toward product and allows unreacted gases to be recycled, which raises overall conversion well above the single-pass equilibrium yield. Mentioning recycling is what distinguishes a complete answer.

How the catalyst works

The catalyst provides a surface on which dinitrogen adsorbs and its bond is weakened. Dissociative adsorption — the bond breaking on the surface to give adsorbed nitrogen atoms — is generally accepted as the rate-determining step.

Once the atoms are on the surface, successive additions of adsorbed hydrogen build up ammonia, which then desorbs. The surface has therefore replaced the impossibly high barrier to breaking the free molecule with a series of much lower ones.

Promoters are added to the catalyst to improve its activity and stability, and the catalyst is poisoned by certain impurities, so the feed gases must be purified — a practical constraint worth knowing.

Biological fixation

Certain microorganisms fix nitrogen at ambient temperature and pressure using the enzyme nitrogenase, which contains iron and molybdenum in a complex cluster.

The contrast with the industrial process is stark and is a favourite discussion question: the enzyme achieves at ambient conditions what industry needs high pressure and elevated temperature to do. The trade-off is that the biological process consumes a large amount of chemical energy per molecule fixed, so it is not free — it substitutes chemical energy for physical conditions.

Understanding the enzyme well enough to imitate it remains an open problem, which is why the industrial process still operates as it does.

Coordination chemistry of dinitrogen

Dinitrogen can act as a ligand, binding end-on to a metal. Back-donation from the metal into its antibonding orbitals weakens the triple bond, which is the same mechanism by which carbon monoxide binding is described.

The extent of that weakening can be followed by infrared spectroscopy, since the nitrogen stretching frequency falls as back-donation increases — exactly parallel to the carbonyl case. Complexes showing substantially reduced frequencies are the ones in which the bond has been meaningfully activated.

Frequently asked questions

Why is dinitrogen so unreactive?

Because its triple bond is extremely strong and the molecule is non-polar with no readily accessible orbitals, so there is no easy point of attack.

Why not simply use a lower temperature for better yield?

Because the rate would be far too slow to be useful. The chosen temperature balances an acceptable rate against an acceptable equilibrium position.

Does the catalyst improve the yield?

Not the equilibrium yield. It increases the rate so that equilibrium is approached in a practical time, which raises the yield actually obtained in a given period.

How does the enzyme manage without high temperature?

By binding and activating the molecule at a metal cluster and supplying energy chemically rather than thermally. It substitutes chemical energy for the physical conditions industry uses.