Why is ammonia not oxidized with oxygen?



The Ostwald method is used for the large-scale production of nitric acid by oxidation of ammonia, which is preferably obtained by the Haber-Bosch process. It goes back to the German chemist Wilhelm Ostwald.

The process consists of three sub-steps:

In the first step the ammonia (NH3) with oxygen (O2) in the presence of a platinum-rhodium catalyst (in newer systems, a special recovery network is used to recover the expensive platinum) at 600 to 700 ° C to water (H.2O) and nitrogen monoxide (NO) implemented. The gas mixture may only touch the catalytic converter for a very short time - only about a thousandth of a second - since otherwise the nitrogen monoxide will break down again into the elements.

4 NH3 + 5 O2 → 4 NO + 6 H2O

In step 2, the temperature of the nitrogen monoxide (NO) drops to below 50 ° C and another oxidation with oxygen (O2) to nitrogen dioxide (NO2) from.

2 NO + O2 → 2 NO2

In the third step, this is converted into nitric acid (ENT) in trickle towers with water3) implemented. Nitrogen monoxide (NO) and nitrous acid (ENT) are formed as intermediate products2).

2 NO2 + H2O → ENT3 + ENT2
3 ENT2 → ENT3 + 2 NO + H2O
2 NO + O2 → 2 NO2(like step 2)

The 3rd step can be summarized as follows:

4 NO2 + O2 + 2 H.2O → 4 ENT3(aq)

An undesirable side reaction that also takes place without a catalyst is the oxidation of ammonia to elemental nitrogen with the formation of water:

4 NH3 + 3 O2 → 2 N2 + 6 H.2O

The reaction is suppressed by the highest possible network temperature and low pressure.

literature

  • F. Kuhlmann: Justus Liebigs Ann. Chem. 29, 272-279 (1839)

See also:

Category: Chemical-technical process