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August 18, 2008 Volume 86, Number 33 Web Exclusive

Nitrogen-Fixing The World

The Haber-Bosch Reaction: An Early Chemical Impact On Sustainability

Ammonia production helped boost global agriculture but also led to unsustainable population growth

Now that the concept of sustainability has grabbed global attention, chemists are contemplating new strategies for developing chemical products and processes in a more sustainable way. But chemists have been taking steps toward sustainability for a long time. One famous example is the Haber-Bosch nitrogen-fixation reaction, which uses a simple iron catalyst to make ammonia and other nitrogen-containing compounds from nitrogen and hydrogen (C&EN, July 28, page 53).

Shutterstock FERTILE GROUND An Asian farmer spreads fertilizer by hand on a rice field.

Ammonia production via the Haber-Bosch process ramped up in the 1910s just before and during World War I and quickly became a key player in global sustainability, although not initially in the social and environmental contexts that dominate today's sustainability discussions. The compound was in high demand in Germany as the country entered the war and needed it to make nitrates for gunpowder and explosives. Prior to the war, animal wastes were a primary source of fertilizers, and global nitrate supplies came mostly from controlled composting of plant matter and from saltpeter deposits in Chile that were mined almost exclusively by British companies. As the war got under way with Britain and Germany on opposite sides, Germany's supply line to the vital chemical was significantly reduced. In fact, without the Haber-Bosch process Germany may not have been able to fight in the war for as long as it did.

The rise of industrial ammonia production had other repercussions. For one thing, it caused the Chilean saltpeter industry to eventually collapse, leaving ghost towns in the Atacama Desert in northern Chile. But the major influence of the Haber-Bosch process on sustainability is that ammonia production made inexpensive fertilizers available to the world, allowing many people to grow a steady abundance of food for the first time.

Invention of the Haber-Bosch ammonia process also had an unintended consequence: It sparked a global population boom that has yet to slack off. With fertilizers boosting food production, world population has climbed from about 1.6 billion people in 1900 to about 6.7 billion people today.

This population growth and the resulting growth in resource consumption and waste production are behind the growing sense of urgency for global sustainability. It's possible to grow even more food and for the world's population to be significantly higher than it is now, but people living in parts of Africa and other developing regions can't always afford or don't have access to fertilizers (C&EN, Jan. 28, page 74). Ammonia produced via the Haber-Bosch process???at a rate of 275 billion lb per year???and its derivative products ammonium nitrate and urea are among the highest volume chemicals currently made. This one process consumes up to 5% of the world's annual natural gas production to make hydrogen and generate heat to run the reaction, and it consumes about 2% of the world's annual energy production.

Chemical fertilizers contribute about half of the nitrogen input into global agriculture, while biological nitrogen-fixation taking place in leguminous plants contributes the other half. That means about half of the nitrogen atoms in the body of an average person living in a developed country once passed through a chemical plant and participated in the nitrogen-to-ammonia Haber-Bosch reaction. Perhaps no other human invention has had a more dramatic impact on Earth than Haber-Bosch chemistry.