PROCESS CHEMISTRY


N2 S


ince its first industrial use in 1913, the Haber process has changed the world. Nitrogen fertilisers made from its


ammonia enabled the population explosion of the 20th century and doubled the amount of reactive nitrogen in circulation. In 2015, production reached 150m t/year, and it 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 – ca500°C – to make the extremely stable molecule nitrogen reactive. It also needs high pressure – ca200bar (20,000kPa) – to shift the equilibrium towards the desired product. The process is also sensitive to oxygen, meaning that nitrogen and hydrogen must be introduced as purified elements, rather than as air and water.


These requirements in


combination 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 t of CO2


to half the UK’s emissions. However, Nature demonstrates


that there is another way, as nitrogen fixation in bacteria starts from air and water as reagents and operates at ambient pressure and temperature. The natural process relies on the


globally, corresponding 36 10 | 2017


NO QUICK FIX


The Haber process currently helps feed more than half the world, but might there be better, more energy-efficient ways of producing ammonia? Michael Gross investigates


Global amount of CO2 emissions from the ammonia industry in 2010, corresponding to half the UK’s emissions


245m t


In 2013, researchers in Scotland demonstrated for the first time the production of ammonia from air and water, at ambient temperature and pressure, using an electrochemical approach


150m t


highly complex enzyme nitrogenase, with active centres containing iron and molybdenum ions. 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 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,


Amount of ammonia produced globally for use in fertilisers and other applications


allowing lower process temperatures, while optimised catalyst formulations yield significant improvements in activity. Perhaps even more exciting are new approaches in processing, eg by electrochemistry, or simply running the reaction in an electric field, or bringing plasmas into play.’ In 2013, Shanwen Tao, 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.1


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,’ says Tao who is now at the University of Warwick, UK. ‘This could be a key technology for a possible “ammonia economy”,’ where ammonia replaces or complements hydrogen as an energy carrier – a concept that Tao and colleagues elaborated in an earlier review paper (see Box).2 Separate efforts using different


routes are on the way at several institutions in Japan, where a long research tradition in ammonia synthesis is traced back to Setsuro Tamaru, who worked with Fritz Haber in the crucial early phase


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