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Ammonia synthesis is the production of ammonia from the combination of hydrogen and nitrogen gases. Most of the ammonia produced is used as fertilizer, although it can be used for other purposes, such as making explosives. The process was developed in Germany in the early 1900s.
The atmosphere is full of nitrogen gas, but it is unreactive and generally does not combine with other elements. The basic strategy for making ammonia, NH3, is to combine hydrogen gas, H2, with nitrogen gas, N2, at a high temperature and under high pressure. Chemically, this is a difficult reaction, so it requires an agent to speed up the process.
A catalyst is a compound that makes a reaction proceed more quickly. In ammonia synthesis, the catalyst used is generally iron. The type of iron used is reduced magnetite ore. There are other chemicals that can be used as catalysts, but iron is the most common.
The source of the hydrogen gas is usually natural gas, also known as methane, CH4. This describes the basic process of ammonia synthesis. Many other steps take place in an ammonia synthesis plant. Sulfur compounds are first removed from the natural gas by reacting with zinc oxide, which is transformed to zinc sulfide. This leaves free methane, which goes through several more transformations to generate hydrogen gas.
The temperature used is about 1482° F (400° C). The pressure used is less than what is optimal for the reaction. For safety reasons, pressure of around 200 atmospheres (atm) is used. The yield is about 10-20% under these conditions. As the mixture leaves the reactor, it is cooled, so that the ammonia will be become a liquid. The heat is captured and re-used to heat the incoming gases.
This ammonia synthesis method is known as the Haber process, because it was created by German chemist Fritz Haber, who developed the conditions for the chemical reaction. He developed it during a shortage of nitrogen-containing fertilizers in the early 1900s, and because Germany needed explosives for World War I. Currently, ammonia is produced on a large scale throughout the world.
An alternative way to make the hydrogen for the reaction is through the electrolysis of water. Electrolysis uses electricity to break down compounds. In this case, water is broken down to hydrogen and oxygen. This has been done in conjunction with electricity production from hydroelectric plants. As far back as 1911, the electrolysis of water has been used to provide hydrogen for the synthesis of ammonia.
Some microorganisms are able to use nitrogen gas from the air to produce ammonia. This process is called nitrogen fixation. In this case the catalysts are complex enzymes known as nitrogenases. The bacteria that carry out this process live on the roots of legumes, such as pea plants. Nitrogen fixation has significantly improved the nutrient status of many soils.