Nitrogen oxides, commonly referred to as NOx, represent a group of compounds composed solely of nitrogen and oxygen atoms. These gases are naturally present in Earth’s atmosphere, with nitrogen (N₂) and oxygen (O₂) making up the vast majority of the air we breathe. Despite their abundance, these two elements do not typically combine under normal atmospheric conditions. Understanding the specific circumstances that cause these stable atmospheric gases to react and form NOx.
The Building Blocks of NOx
Atmospheric nitrogen exists primarily as dinitrogen (N₂), a molecule characterized by an exceptionally strong triple covalent bond between its two nitrogen atoms. This bond requires a substantial amount of energy to break, rendering N₂ largely unreactive under typical environmental conditions. Similarly, atmospheric oxygen exists as dioxygen (O₂), which, while more reactive than N₂, also possesses a strong double bond that contributes to its relative stability.
High Temperatures and NOx Formation
The primary factor enabling nitrogen and oxygen to combine and form NOx is the presence of extremely high temperatures. These elevated temperatures provide the significant energy input necessary to overcome the inherent stability of the N₂ and O₂ molecules. Specifically, the intense thermal energy is absorbed by the molecules, causing their strong chemical bonds to vibrate more vigorously until they eventually break apart. This bond breakage transforms the stable molecular forms into highly reactive individual atoms or fragments.
For instance, the triple bond in N₂ requires approximately 945 kilojoules per mole (kJ/mol) to break, while the double bond in O₂ needs about 498 kJ/mol. Such large energy requirements are only met in environments where temperatures reach thousands of degrees Celsius.
How the Reaction Happens
Once sufficiently high temperatures are achieved, the N₂ and O₂ molecules undergo a process called thermal dissociation. This means they break down into individual nitrogen (N) and oxygen (O) atoms, or sometimes into highly reactive molecular fragments known as radicals. These newly formed atoms and radicals are far more reactive than their molecular counterparts because they possess unpaired electrons, making them chemically unstable and eager to form new bonds.
The unbonded nitrogen and oxygen atoms then readily collide and combine to form various nitrogen oxides. The initial and most common product of this high-temperature reaction is nitric oxide (NO). For example, a free nitrogen atom might react with an oxygen molecule (N + O₂ → NO + O), or two free atoms could combine (N + O → NO). This nitric oxide can subsequently react with additional oxygen in the atmosphere to form nitrogen dioxide (NO₂), which is another common form of NOx. The sequence of these reactions, often referred to as the Zeldovich mechanism, describes how these highly energetic atoms rearrange into new chemical structures.
Common Environments for NOx Formation
High-temperature conditions necessary for NOx formation are frequently encountered in human-made processes. Internal combustion engines, found in vehicles like cars and trucks, are significant sources, as the combustion of fuel within their cylinders reaches temperatures ranging from 1,500°C to 2,500°C (2,700°F to 4,500°F). Similarly, large-scale power generation plants that burn fossil fuels operate at temperatures well above 1,000°C (1,800°F), creating ideal conditions for nitrogen and oxygen in the air to combine.
Industrial furnaces and boilers also generate the intense heat required for these reactions to occur. Beyond human activities, natural phenomena like lightning strikes provide the extreme temperatures needed for NOx formation. A lightning bolt can heat the air along its path to temperatures exceeding 20,000°C (36,000°F), causing atmospheric nitrogen and oxygen to instantly combine and form various nitrogen oxides.