Reaction and conditions
N₂ + 3H₂ ⇌ 2NH₃ is exothermic and reduces gas molecules from 4 to 2. By Le Chatelier's principle, low temperature and high pressure favor yield, but low temperature makes the reaction too slow. Plants therefore compromise at roughly 400-500°C with an iron catalyst promoted by potassium and aluminum oxides, and recycle unreacted gas.
Role of the catalyst
A catalyst does not shift the equilibrium; it lowers activation energy so equilibrium is reached faster. Nitrogen molecules adsorb on the iron surface and dissociate; Gerhard Ertl won the 2007 Nobel Prize in Chemistry for clarifying this surface mechanism.
History and impact
- Fritz Haber achieved the synthesis in the lab in 1909; Carl Bosch at BASF scaled it up in 1913.
- Synthetic nitrogen fertilizer is estimated to feed about half the world's population (Erisman et al., 2008).
- Via nitric acid, the same technology supplied explosives in World War I, a recurring theme in debates on the social responsibility of scientists.
Environmental issues
Because hydrogen comes mostly from natural gas, ammonia production is a major energy user and CO₂ source, and excess fertilizer causes water pollution and nitrous oxide emissions. Green hydrogen from electrolysis is being explored.
References
- Smil, V. (2001). Enriching the Earth: Fritz Haber, Carl Bosch, and the Transformation of World Food Production. MIT Press.
- Erisman, J. W., Sutton, M. A., Galloway, J., Klimont, Z., & Winiwarter, W. (2008). How a century of ammonia synthesis changed the world. Nature Geoscience, 1, 636-639.
- Ertl, G. (2008). Reactions at surfaces: From atoms to complexity (Nobel Lecture). Angewandte Chemie International Edition, 47(19), 3524-3535.
- Atkins, P., & de Paula, J. (2014). Physical Chemistry (10th ed.). Oxford University Press.