Cracking ammonia

Scientists unravel the precise steps of ammonia synthesis

By Shalini Srivastava
Published: Friday 31 December 2010

imageIT IS the prime ingredient of fertilisers that help sustain one-third of the world’s population. Though it is commonly released as a pungent gas from decomposing organic matter, harnessing ammonia from nature requires an intensive industrial procedure.

Industry has been following a century- old Haber-Bosch process to produce ammonia, which requires high temperature and pressure. In such extreme conditions and in the presence of a catalyst, preferably iron, nitrogen breaks down into atoms and binds with hydrogen atoms to form ammonia (NH3). But the exact chemistry that takes place on the surface of the catalyst remained unknown, until now.

Scientists from University of Cambridge in the UK recently unfolded the process. They created an experimental setup of ultrahigh vacuum and exposed a single-crystal iron sample to nitrogen and hydrogen molecules. In ultrahigh vacuum the reaction proceeds at a slow rate which enables scientists to observe and analyse the steps of reaction.

But since the atoms require high pressure to remain stuck to the catalyst, the scientists kept flipping the setup to high pressure condition. They observed that nitrogen atom first settled on the catalyst surface and then combined with three hydrogen atoms one at a time.

Once hydrogenation was over and the molecule was formed, ammonia jumped off the catalyst. The scientists found the rate at which hydrogenation occurs on the catalyst surface is the slowest in the entire process of ammonia formation. Accelerating the step could enhance the reaction rate and thus the synthesis of ammonia, they noted in the November 8 issue of PNAS.

They repeated the experiment using potassium, which is used as a promoter in ammonia synthesis. They found when potassium is added to the surface of iron catalyst in specific ratio, the reaction was 20 per cent faster, thereby increasing ammoia synthesis.

Prasenjit Ghosh, chemistry professor at IIT Bombay said, “The experiment offers a valuable insight into the Haber-Bosch process. The 20 per cent rise in the rate of reaction could be significant for industry if the same technology can be successfully scaled up for bulk production.” Many other catalytic reactions could be analysed and explained using this experiment, he said.

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