As of today (2024.06.20), Wikipedia tells:
Nitrogen fixation is a chemical process by which molecular dinitrogen ($\ce{N2}$) is converted into ammonia ($\ce{NH3}$)
Obviously, this is only half of the truth.
In hot combustion engines, nitrogen is also fixed as $\ce{NO_x}$. Is the path via $\ce{NH3}$ only traditional or has the oxygenation path major disadvantages?
The nitrogen molecule is absurdly stable. To the point that it took us a long time to understand that it was a substance like the others and not a neutral fluid, left over from the air when we removed that oxygen linked to phlogiston...
"Fixing" nitrogen by creating an oxide may happen by thermal oxidation. I put "fixing" in quotes because fixing should mean bringing a component from the gas phase into the solid phase and the thermal oxidation of nitrogen only occurs at temperatures so high that everything is gas. Thermal NOx formation is highly temperature-dependent, with significant formation occurring at temperatures above 1300°C (2372°F). The rate of NOx formation increases exponentially with temperature, making high-temperature combustion processes particularly prone to generating NOx.
Another way of artificial fixing the nitrogen is the Electric Arc Method: In this older method, an electric arc is passed through air to produce nitrogen oxides. It was historically used for nitric acid production before the development of the Ostwald process. You may get nitrogen monoxide, another gas, and will have a long way until this N is brought to the solid phase.
So came nature, to humble us chemists. Some bacteria do it at atmospheric pressure, room temperature and with almost no input of energy by the process called diazotrophy.
It actually brings to soil gaseous nitrogen of the atmosphere, essential to protein formation.
Diazotrophy is a biological process through which certain microorganisms convert atmospheric nitrogen (N₂) into a form that can be utilized by living organisms, such as ammonia (NH₃). This process is crucial for the nitrogen cycle in ecosystems because it makes nitrogen available to plants, which cannot use atmospheric nitrogen directly.
The key enzyme involved in diazotrophy is nitrogenase. This enzyme facilitates the reduction of atmospheric nitrogen (N₂) to ammonia (NH₃) in a reaction that requires a significant amount of energy, typically in the form of ATP.
The simplified reaction catalyzed by nitrogenase is:
$\ce{N2 + 8H+ + 8e− −> 2NH3 + H2}$
Some bacteria are free-living diazotrophs:
Azotobacter: Found in soil, these bacteria can fix nitrogen while living independently.(see figure on top of page) Cyanobacteria: Also known as blue-green algae, these organisms can fix nitrogen in aquatic environments.
Other bacteria live as symbiotic diazotrophs:
Rhizobia: These bacteria form symbiotic relationships with leguminous plants (e.g., peas, beans). They colonize plant root nodules where they fix nitrogen, benefiting the plant with usable nitrogen compounds. Frankia: These bacteria form symbiotic relationships with certain non-leguminous plants, such as alder trees.
Because of these bacteria, when people think about fixing nitrogen from the air, no one thinks about heavy industrial processes at very high temperatures or the use of electric arcs to start processes that will depend on many steps and separations. You simply put these small bacteria to work and the result is the production of ammonia, ready to form biochemical substances.
