Tale of two atoms. A low-carbon leap in ammonia production

Nitrogen exists in nature as molecules of two atoms; the (triple) bond between them is extremely hard to break. To make ammonia, the bedrock of fertilizers and many other chemicals, you need a single atom of nitrogen to combine with three of hydrogen to form the NH₃ molecule. The Haber-Bosch process, invented in 1909, still remains the singular method of producing industrial scale ammonia. While ammonia has been the panacea for the world’s food requirements, the Haber-Bosch method of making it has a deleterious flipside — it requires a heck of a lot of heat, mostly obtained from burning stuff that emits carbon dioxide, mankind’s biggest enemy today. The bonding energy of the N₂ molecule is 945 KJ/mol — in other words, it takes burning 1.4 kg of coal to split one kg of molecular nitrogen into single atom nitrogen.

For decades, scientists have been scratching their heads over an alternative for Haber-Bosch, yet in vain.

And now, scientists at the Lawrence-Berkeley lab in California, USA, have announced that they have figured out a room-temperature alternative to separate the two atoms of a nitrogen molecule. This, if taken to the industry, can be the holy grail that mankind has been searching for.

What the L-B lab scientists did is truly the stuff of Harry Potter. Short of the swish of a magic wand, it has everything — powders, potions, gas and all. The group of five scientists, led by Polly Arnold, Director, Chemical Sciences Division, used rare earth metals, potassium and chemicals called phenolates to break the bond between two nitrogen atoms that form the nitrogen molecule.

How it works is something like this. It has been known for some decades that rare earth metals (of the lanthanide series in the periodic table, or even zirconium and titanium) can combine with molecular nitrogen — this has something to do with a peculiar way the electrons fill in their orbitals. Arnold and her team put this knowledge to work. They took phenolates and used them as a sort of a glue to link two rare earth metals at the molecular level to form a sort of a rectangle, or a ‘complex’. Then they diffused molecular nitrogen (N2) into the cavity of the rectangle. When they did this, each of the two atoms of the nitrogen molecules got linked to different molecules of the rare metals on either side, thereby “activating” the nitrogen (weakening the N-N bond). Then they introduced the villain that broke the love between the two nitrogen atoms — potassium. Post their divorce, the nitrogen atoms were free to join hands with someone else. When hydrogen becomes the new suitor, you get ammonia or amines.

At the heart of the whole process (which, by the way, happens at room temperature) is the trick employed to make the rectangular complexes with rare earth metals, into the cavity of which nitrogen molecules could be trapped and their bonds cleaved. “This new family of complexes upturned 90 years of accepted wisdom by proving that rare earths can bind and reduce dinitrogen,” says a paper by Arnold et al, published in Chem Catalysis.

In the process, potassium was used as a source of electrons. In a write-up published on L-B lab’s website, Arnold has said that her next step would be to use electrodes instead of potassium as a source of electrons, as these electrodes can supply electrons from, say, solar cells.

So, does all this mean that the world is ready to dump the century-old Haber-Bosch process and seize Polly Arnold’s method? Not at all. One chemistry professor at IIT-Madras, told quantum, that he suspects the yield rate would be poor in the L-B method. Even the L-B website does not believe that this could be so instantly transformative. However, the L-B route does open up an interesting pathway that could one day lead mankind to low-cost, low-carbon ammonia. The L-B scientists indeed have said that they are willing to licence the technology.