The Ostwald process is a
chemical process used for making
nitric acid (HNO3).[1] The Ostwald process is a mainstay of the modern
chemical industry, and it provides the main raw material for the most common type of fertilizer production.[2] Historically and practically, the Ostwald process is closely associated with the
Haber process, which provides the requisite raw material,
ammonia (NH3).
Reactions
Ammonia is converted to nitric acid in 2 stages.
Initial oxidation of ammonia
The Ostwald process begins with burning
ammonia. Ammonia burns in
oxygen at temperature about 900 °C (1,650 °F) and pressure up to 8 standard atmospheres (810 kPa)[3] in the presence of a
catalyst such as
platinum gauze, alloyed with 10%
rhodium to increase its strength and NO yield, platinum metal on fused silica wool, copper or nickel to form
nitric oxide (nitrogen(II) oxide) and
water (as steam). This reaction is strongly
exothermic, making it a useful heat source once initiated:[4]
4NH3 + 5O2 → 4NO + 6H2O (ΔH = −905.2 kJ/mol)
Side reactions
A number of side reactions compete with the formation of nitric oxide. Some reactions convert the ammonia to N2, such as:
4NH3 + 6NO → 5N2 + 6H2O
This is a secondary reaction that is minimised by reducing the time the gas mixtures are in contact with the catalyst.[5]
Another side reaction produces
nitrous oxide:
4NH3 + 4O2 → 2N2O + 6H2O (ΔH = −1105 kJ/mol)
Platinum-Rhodium Catalyst
The platinum and rhodium catalyst is frequently replaced due to decomposition as a result of the extreme conditions which it operates under, leading to a form of degradation called
cauliflowering[6]. The exact mechanism of this process is unknown, the main theories being physical degradation by hydrogen atoms penetrating the platinum-rhodium lattice, or by metal atom transport from the centre of the metal to the surface[6].
Secondary oxidation
The nitric oxide (NO) formed in the prior catalysed reaction is then cooled down from around 900˚C to roughly 250˚C to be further oxidised to nitrogen dioxide (NO2)[7] by the reaction:
also occurs once the nitrogen dioxide has formed.[10]
Conversion of nitric oxide
Stage two encompasses the absorption of nitrous oxides in water and is carried out in an
absorption apparatus, a
plate column containing water[citation needed]. This gas is then readily absorbed by the water, yielding the desired product (nitric acid in a dilute form), while
reducing a portion of it back to nitric oxide:[4]
3NO2 + H2O → 2HNO3 + NO (ΔH = −117 kJ/mol)
The NO is recycled, and the acid is concentrated to the required strength by
distillation.
This is only one of over 40 absorption reactions of nitrous oxides recorded,[10] with other common reactions including:
3N2O4 + 2H2O → 4HNO3 + 2NO
And, if the last step is carried out in air:
4NO2 + O2 + 2H2O → 4HNO3 (ΔH = −348 kJ/mol).
Overall reaction
The overall reaction is the sum of the first equation, 3 times the second equation, and 2 times the last equation; all divided by 2:
Alternatively, if the last step is carried out in the air, the overall reaction is the sum of equation 1, 2 times equation 2, and equation 4; all divided by 2.
Without considering the state of the water,
NH3 + 2O2 → H2O + HNO3 (ΔH = −370.3 kJ/mol)
History
This section needs expansion. You can help by
adding to it. (May 2024)
^Thiemann, Michael; Scheibler, Erich; Wiegand, Karl Wilhelm (2005). "Nitric Acid, Nitrous Acid, and Nitrogen Oxides". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH.
doi:
10.1002/14356007.a17_293.
ISBN978-3-527-30673-2.
^Kroneck, Peter M. H.; Torres, Martha E. Sosa (2014). The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment. Dordrecht: Springer. p. 215.
ISBN978-94-017-9268-4.
^GB 190200698,
Ostwald, Wilhelm, "Improvements in the Manufacture of Nitric Acid and Nitrogen Oxides", published January 9, 1902, issued March 20, 1902
^GB 190208300,
Ostwald, Wilhelm, "Improvements in and relating to the Manufacture of Nitric Acid and Oxides of Nitrogen", published December 18, 1902, issued February 26, 1903