Homing in on a cheaper Haber-Bosch process

Synthetic cycle of the formation of ammonia from azide and hydrogen with x-ray crystal structure of the ruthenium(IV) nitrido intermediate in the centre © Bjorn Askevold

A cheaper alternative to the Haber-Bosch process could have moved a step closer thanks to a new ruthenium-based catalyst complex developed by chemists in Germany.

Each year the Haber-Bosch process produces millions of tonnes of ammonia for the fertiliser industry by direct hydrogenation of nitrogen with hydrogen gas over a catalyst. However, this process needs temperatures of around 450°C and pressures of 300 bar, consuming vast amounts of energy.

Now, a team led by Sven Schneider at the University of Erlangen-Nürnberg and Max Holthausen at the University of Frankfurt have shown how a ruthenium complex with a nitrogen-metal triple-bond can split molecular hydrogen to produce high yields of ammonia at atmospheric pressure and temperatures of only around 50°C. The finding could point the way to solving the ‘second half’ of the Haber-Bosch process, the activation of hydrogen and hydrogenation of a nitrogen atom to create ammonia.

The German researchers synthesised a planar molecule with a ruthenium centre clamped by a nitrogen and two bulky flanking phosphine groups – known as a PNP pincer ligand. A nitrido ligand triply bonded to the ruthenium can then be introduced with an azide. This ligand combine with the hydrogen to form ammonia. More here Homing in on a cheaper Haber-Bosch process.

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