Clean speed ahead with catalysts
Box 2 | The Haber-Bosch process
Although abundant in the atmosphere, nitrogen (N2) is a very stable, unreactive molecule: the triple bond between the two nitrogen atoms is one of the strongest in nature. Without a catalyst, ammonia (NH3) can only be manufactured at a temperature of around 3,000ºC under very high pressure. In 1909, Fritz Haber patented a process in which a catalyst was used for a reaction between hydrogen and nitrogen to produce ammonia at the much more moderate temperature of 525ºC and 20MPa. In this process, nitrogen and hydrogen molecules first bind to (are adsorbed onto) the surface of fine iron oxide (magnetite) particles, this catalyst weakens the bonds of both molecules so they break apart into individual atoms. The resulting 'free' hydrogen and nitrogen atoms are then able to combine to form ammonia breaking away from the magnetite surface as they do so. The process is summarised in the following equation:
Carl Bosch helped to commercialise the process and, almost overnight, the industrial production of ammonia boomed, making possible a huge increase in the production of food (as well as munitions) worldwide. Ammonia-based fertiliser is said to now sustain one-third of the world's population. It is the principal invention that led to the near-eradication of mass-starvations and, together with antibiotics, the exponential population growth of recent times.
The Haber-Bosch process marked a dramatic increase in the efficiency of ammonia production but, because of the relatively high temperatures and pressures still required, it consumes about 1 per cent of the world's total energy budget. Scientists are trying to improve the process by learning from nature. Many bacteria fix nitrogen from the air at normal temperatures and atmospheric pressure using a family of enzymes called nitrogenases as catalysts. Research is underway at the University of New South Wales to develop new catalysts to fix nitrogen for production of ammonia. Scientists at the Massachusetts Institute of Technology have developed a catalyst that acts like an 'artificial' nitrogenase to produce ammonia on an experimental scale. If this process could be scaled up and made more efficient, it could have a large impact on global energy consumption and the cost of food production.
Related sites
Fritz Haber: Ammonia synthesis, poison gas and the Great War (Chemistry in Australia)
The Haber process (Chemguide, UK)
The Nobel Prize in Chemistry 2007
(Nobelprize.org, Sweden)
Fritz Haber: The Nobel Prize in Chemistry 1918
(Nobelprize.org, Sweden)
External sites are not endorsed by the Australian Academy of Science.
Posted November 2008.