$$4NH_3(g) + 5O_2(g) → 4NO(g) + 6H_2O(g)$$
$$ 2NO(g) + O_2(g) → 2NO_2(g)$$
$$3NO_2(g) + H_2O(l) → 2HNO_3(aq) + NO(g)$$
The overall reaction is:
$$4NH_3(g) + 6O_2(g) → 4HNO_3(aq) + 2H_2O(g)$$
This process is carried out in three steps:
1. Ammonia Oxidation: In this step, ammonia ($NH_3$) is oxidized to nitric oxide ($NO$) and water vapor ($H_2O$) in the presence of a platinum-rhodium gauze catalyst at a temperature of about 850°C.
2. Nitrogen Dioxide Oxidation: In the second step, nitric oxide ($NO$) is further oxidized to nitrogen dioxide ($NO_2$) in the presence of oxygen ($O_2$) at a lower temperature of about 200°C.
3. Nitric Acid Absorption: In the final step, nitrogen dioxide ($NO_2$) is absorbed in water to form nitric acid ($HNO_3$) and nitric oxide ($NO$), which is recycled back to the second step.
