Nov 282010
Production of ammonia 1946–2007
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Scientists at the University of Cambridge are working on ways to improve the efficiency of the ammonia synthesis process. With between 3-5% of the world’s natural gas used to create artificial fertilizers, the new research could have major implications for both the agricultural and energy sectors.

Ammonia (NH3) is one of the most important chemicals in the modern world, due mainly to its use in the manufacture of artificial fertilisers. Ammonia synthesis (via the “Haber” or “Haber-Bosch” process) is vital to the production of 100 million tons of fertiliser per year, responsible for sustaining one-third of the Earth’s population.

In the natural world, ammonia is generated by plants (predominantly the legumes) 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 3-5% of the world’s natural gas production referred to earlier is consumed in the Haber-Bosch process, amounting to around 1-2% of the world’s man-made energy supply.

Dr Steve Jenkins, of the Department of Chemistry at the University of Cambridge, one of the co-authors of the paper reporting the research, said: “The Haber-Bosch process was developed in the early twentieth century but has changed little since that time. Clearly, given the massive scale of worldwide ammonia production, even a tiny improvement in the efficiency of the ammonia synthesis process can have massive implications, not only for the economics of fertiliser production, but also for global energy 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. (A catalyst is defined as a substance that improves the speed or selectivity of a chemical reaction, whilst not being consumed or produced in the reaction itself.)

Great efforts have been expended over many decades on the problems of understanding how the iron catalyst does its job, why the addition of certain elements such as potassium can improve the catalysts, and whether any of the lessons learnt thus far can help us predict a better catalyst that is economically viable.

The Cambridge group’s findings, reported in the journalProceedings of the National Academy of Sciences, address some of these problems and pave the way for a more efficient way to produce fertilisers.

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