Ammonia Production

Ammonia (NH₃) is one of the most important chemicals in the modern world, mostly due to its use in the manufacture of artificial fertilisers. The Haber, or Haber-Bosch process, is used to produce ammonia and is vital to the production of 100 million tons of fertiliser per year, responsible for sustaining one-third of the Earth's population.

Ammonia is generated naturally by plants and certain bacteria, which extract nitrogen from the atmosphere in a process known as nitrogen fixation. Natural nitrogen fixation occurs at ambient temperatures and pressures, but artificial nitrogen fixation via the Haber-Bosch process requires high pressures (150-250 atmospheres) and high temperatures (300-550 degrees Celsius) to produce the vast quantities of ammonia necessary to satisfy global demand.

The key to the Haber-Bosch process is an iron catalyst which encourages the dissociation of N₂ molecules, and provides a platform on which the resulting N atoms can be successively hydrogenated to yield NH, NH₂ and finally NH₃.

Scientists at the University of Cambridge exposed their iron sample to nitrogen ions, in order to readily build up a coverage of nitrogen atoms on the surface (to a density of just over one nitrogen atom per two top-layer iron atoms at the surface). Under uhv conditions, they can utilise Auger Electron Spectroscopy (AES) to quantify the amount of nitrogen on the surface. Then, they expose the sample to 0.6 mbar H₂ gas for a period of several minutes. This pressure is still very low compared with industrial conditions, but it allows the reaction to proceed sufficiently rapidly for them to take meaningful measurements over a timescale of minutes. If they used only uhv pressures of H₂, the reaction would be so slow that it would take hours, during which time contamination would build up on the surface and ruin the experiment.
After an exposure of several minutes, they rapidly evacuate the experimental chamber to return to uhv conditions and use AES to evaluate how much nitrogen is left on the surface, then expose to H₂ again and repeat. By doing this several times, they can measure the drop in surface nitrogen (corresponding to production of ammonia) as a function of time and temperature.

Their results suggest that, under certain conditions, namely when the ammonia pressure is kept low, the hydrogenation steps (from N to NH to NH₂ to NH₃) may actually be the most important.

Journal Reference:

1. Poobalasuntharam Iyngaran, David C. Madden, Stephen J. Jenkins, David A. King. Hydrogenation of N over Fe{111}. Proceedings of the National Academy of Sciences, 2010; DOI: 10.1073/pnas.1006634107

Further Reading
Haber Process
Nitrogen Cycle

Study Questions

1. Write a balanced chemical equation for the production of ammonia from hydrogen and nitrogen gas.
2. Predict the effect of high pressure in the reaction vessel on the yield of ammonia.
3. The Haber Process is an exothermic reaction. Explain what is meant by the term exothermic.
4. Explain what would happen to the yield of ammonia if the reaction vessel were cooled.
5. It is estimated that between 3 and 5% of the world's natural gas production is used in the production of ammonia. What would the natural gas be used for in the is process?
6. The Haber process typically produces an ammonia yield of between 10 and 20%. Describe 4 ways that this yield could be improved.
7. In the article above it is said that measuring the drop in surface nitrogen corresponds to measuring production of ammonia. Explain why this is true.
8. Describe another way you could measure the production of ammonia.