The Ostwald Process

The Ostwald process is a chemical process for producing nitric acid, which historically and practically is closely associated with the Haber process, providing the requisite raw material, ammonia.

In the Ostwald process ammonia is converted to nitric acid in two stages. In the first stage ammonia is catalytically oxidized by atmospheric oxygen on a Platinum based gauze catalyst to form nitric oxide (NO). This step is strongly exothermic, making it a useful heat source once initiated:

4 NH₃ (g) + 5 O₂ (g) → 4 NO (g) + 6 H₂O (g)     (ΔH = −950 kJ/mol)

In the oxidative environment the initially nitric oxide is further oxidised to yield nitrogen dioxide (NO₂)

2 NO (g) + O₂ (g) → 2 NO₂ (g)     (ΔH = −114 kJ/mol)

In the second stage this gas is readily absorbed by water, yielding the desired product,

3 NO₂ (g) + H₂O (l) → 2 HNO₃ (aq) + NO (g)     (ΔH = −117 kJ/mol)

while reducing a portion of it back to NO which is recycled and the acid is concentrated to the required strength by distillation.

Typical conditions for the first stage are pressures between atmospheric pressure and 14 atmospheres and temperatures between 780 and 950°C. The first stage contributes to an overall yield of about 93 to 98%, depending on the operating parameters and load of the individual process.

Numerous studies have been conducted to elucidate the mechanism of catalytic ammonia oxidation over platinum based catalyst gauzes for the purpose to reduce yield abating side-reactions revert the nitrogen back to N₂

4 NH₃ (g) + 3 O₂ (g) → 2 N₂ (g) + 6 H₂O (g)     (ΔH = −1.266 kJ/mol)

and secondary-reactions that form N₂O

4 NO + 4 NH₃ + 3 O₂ → 4 N₂O + 6 H₂O    (ΔH = −1.104 kJ/mol)

The production of N₂ is associated with the highest heat of reaction and causes a measurable increase in temperature of the auto thermal operated industrial reactor when the NO selectivity decreases due to catalytic aging.

The initial step in the reaction sequence is the adsorption of ammonia and oxygen on the surface of the Platinum based gauze catalyst.




The adsorption of ammonia is followed by its dehydrogenation by means of surface oxygen. In a further reaction step the remaining sorbed nitrogen atoms dimerisate to give N₂ or react with further surface oxygen atoms to yield NO. The product-selectivity towards the desired product NO depend on the dilution of the adsorbed nitrogen atoms with surface oxygen atoms, in other words on the O₂/NH₃ ratio of the initial gas feed.
Increasing O₂/NH₃ ratios affect higher NO yields.

The temperature dependency of the yield reflected in the amount of formed N₂O. N₂O is formed in a secondary reaction step between adsorbed NO molecules and sorbed N atoms on the catalyst surface.



Low catalyst temperatures decelerate the desorption velocity of the formed NO molecules from the catalyst surface and causes higher N₂O emissions from the catalyst.