We begin our new series breaking down key innovations in agriculture with the Haber-Bosch process, which enabled large-scale agriculture worldwide.
Ammonia – a compound of nitrogen and hydrogen – is therefore a key ingredient in fertilisers, allowing farmers to replenish the soil with nitrogen at will. As well as fertilisers, ammonia is used in pharmaceuticals, plastics, refrigerants, explosives, and in numerous industrial processes.
But how is it made? At the turn of the 20th Century, ammonia was mostly mined from deposits of niter (also known as saltpetre – the mineral form of potassium nitrate), but the known reserves would not satisfy predicted demands. Researchers had to find alternative sources.
Fritz Haber (left) and Carl Bosch (right) created and commercialised the process.
Atmospheric nitrogen, which makes up almost 80% of air, was the obvious feedstock – its supply, to all intents and purposes, being infinite. But reacting atmospheric nitrogen, which is exceptionally stable owing to its strong triple bonds, posed a challenge for chemists globally.
In 1905, German chemist Fritz Haber cracked the riddle of fixing nitrogen from air. Using high pressure and an iron catalyst, Haber was able to directly react nitrogen and hydrogen gas to create liquid ammonia.
His process was soon scaled up by BASF chemist and engineer Carl Bosch, becoming known as the Haber-Bosch process, and this would lead to the mass production of agricultural fertilisers and a phenomenal increase in the growth of crops for human consumption.
The Haber-Bosch process is conducted at a high pressure of 200 atmospheres and reaction temperatures of 450°C. It also requires a large feedstock of natural gas, and there is a global research and development effort to replace the process with a more sustainable alternative – just as the Haber-Bosch process replaced niter mining over a century ago.
The Haber process currently helps feed more than half the world, producing 150m tonnes of ammonia a year. This is forecast to rise further, in line with the food demand of a growing world population.
And yet, it has serious drawbacks. In its traditional form, the process requires high temperatures – around 500°C – to make the extremely stable molecule nitrogen reactive.
The Haber process takes place at extremely high temperatures, similar to that of an average fire.
It also needs high pressure to shift the equilibrium towards the desired product. The process is sensitive to oxygen, meaning that nitrogen and hydrogen must be introduced as purified elements, rather than as air and water.
These requirements together make the process extremely energy-hungry; estimated to consume between 1% and 2% of global primary energy production. In 2010, the ammonia industry emitted 245m tonnes of CO2 globally, corresponding to half the UK’s emissions.
The Haber process was developed by Carl Bosch (left) and Fritz Haber (right) in the early 20th century. Image: Wikimedia Commons
In nature, the process relies on the highly complex enzyme nitrogenase, operating at an ambient pressure and temperature. But using the entire biological system would not be economical for large-scale industrial synthesis, and thus the search for an inorganic system that matches the performance of the biological has become an important challenge.
In recent years, novel electrochemical approaches and new catalysts have yielded promising results suggesting that, at least for small-scale synthesis, other ways may have a future.
The chemical reaction that feeds the world. Video: TED-Ed
‘The last [few] years brought some spectacular results on ammonia synthesis research,’ comments Hans Fredriksson from Syngaschem at Eindhoven, Netherlands.
‘On the catalyst side, there is the discovery of ‘super promoters’, helping N2 dissociation, allowing lower process temperatures, while optimised catalyst formulations yield significant improvements in activity.
‘Perhaps even more exciting are new approaches in processing, for example by electrochemistry, or simply running the reaction in an electric field, or bringing plasmas into play,’ he said.
In 2013, Shanwen Tao, then at the University of Strathclyde, Glasgow, UK, and colleagues demonstrated for the first time the production of ammonia from air and water, at ambient temperature and pressure, using a proton-conducting Nafion membrane in an electrochemical approach.
Nafion, a Teflon-like material that conducts cations but neither electrons nor anions, is also used in fuel cells.
‘Electrochemical synthesis of ammonia is an important new approach for efficient synthesis of ammonia using green renewable electricity as the energy source. This could be a key technology for a possible ‘ammonia economy’,’ where ammonia replaces or complements hydrogen as an energy carrier, says Tao.
Researchers hope new approaches will be supported by renewable energy, reducing CO2 emissions. Image: Pexels
Separate efforts using different routes are being developed in Japan, with a particular focus on ruthenium as an efficient catalyst. One approach is to apply super promoters to provide electrons that destabilise nitrogen by weakening the triple bond and making the molecule more reactive for ammonia synthesis.
This was first reported in 2012 by Hideo Hosono’s group at the Tokyo Institute of Technology, who used ruthenium catalysts in combination with ‘electrides’ – a new class of ionic materials where electrons serve as the anions.
The method operates at atmospheric pressure and temperatures between 250 and 400°C, and hydrogen poisoning of ruthenium catalysts is no longer a problem.
Ruthenium is a type of metal in the platinum group. Image: Metalle-w/ Wikimedia Commons
‘This catalyst exhibits the highest activity and excellent long-term stability,’ says Hosono, who sees the future of his methods in distributed, small-scale applications of ammonia synthesis.
Hans Niemantsverdriet, director of SynCat@Beijing, China, acknowledges the rapid progress being made, but also strikes a note of caution.
‘In spite of interesting discoveries, I find it hard to imagine that these improvements will be able to replace the current large-scale and fully optimised technology,’ he says. ‘In the fertiliser area, novel technology will at best become a niche market for very special situations. Also, the CO2 footprint is hardly diminished.’
Ammonia is a core component of fertiliser, feeding nitrogen to plants for photosynthesis. Image: Maurice van Bruggen/Wikimedia Commons
In the long term, Niemantsverdriet has hope for the ammonia economy as championed by Tao and others, providing carbon-free hydrogen from renewable energies.
‘I strongly believe that there will be scope for large industrial parks where this technology can be cleverly integrated with gasification of coal in China, and perhaps biomass elsewhere,’ he says. ‘If dimensioned properly, this has the potential to reduce the carbon footprint in the future.’