Ostwald method

A laboratory setup of the Ostwald process

The Ostwald process is used for the large-scale production of nitric acid by oxidation of ammonia , which is preferably obtained by the Haber-Bosch process .

history

It goes back to the German-Baltic chemist Wilhelm Ostwald , who received the patent for the process in 1902 . The basic chemical reaction of ammonia with air at the platinum contact, however, was patented by Frédéric Kuhlmann as early as 1838 .

Process description

The procedure consists of three sub-steps. One starting material is ammonia gas produced by the Haber-Bosch process. Other starting materials are oxygen and water.

Heterogeneously catalyzed combustion of ammonia gas

In the first step, ammonia (NH ₃ ) with oxygen (O ₂ mixed) and in the presence of a platinum - rhodium - catalyst at 800 ° C to 900 ° C to water (H ₂ O) and nitrogen monoxide (NO) is reacted (On newer systems a special recovery network is used to recover the expensive platinum). The gas mixture may only touch the catalyst for a very short time - only about a thousandth of a second - because otherwise the nitrogen monoxide, which is unstable at the reaction temperature, will break down into the elements N ₂ and O ₂ .

${\displaystyle \mathrm {4\ NH_{3}(g)\ +\ 5\ O_{2}(g)\ {\xrightarrow[{Pt/Pt-Rh}]{800-900\ ^{\circ }C}}} }$ ${\displaystyle \mathrm {4\ NO(g)\ +\ 6\ H_{2}O(g)\ \ \Delta } H^{0}=-906{,}11~{\rm {kJ/mol}}}$

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

${\displaystyle \mathrm {4\ NH_{3}(g)\ +\ 3\ O_{2}(g)\ \longrightarrow \ 2\ N_{2}(g)\ +\ 6\ H_{2}O(g)} }$

Another undesirable side reaction is the formation of laughing gas:

${\displaystyle \mathrm {4\ NH_{3}(g)\ +\ 4\ O_{2}(g)\ \longrightarrow \ 2\ N_{2}O(g)\ +\ 6\ H_{2}O(g)} }$

According to the Le Chatelier principle, the side reactions are suppressed by the highest possible network temperature and low pressure.

Generation of nitrogen dioxide

In step 2, the temperature of the nitrogen monoxide (NO) is lowered to below 50 ° C and it is mixed with air. Another oxidation with oxygen (O ₂ ) to nitrogen dioxide (NO ₂ ) takes place, which then dimerizes to nitrous tetraoxide:

${\displaystyle \mathrm {2\ NO(g)\ +\ O_{2}(g)\ \rightleftharpoons \ 2\ NO_{2}(g)\ \ \Delta } H^{0}=-114{,}22~{\rm {kJ/mol}}}$

${\displaystyle \mathrm {2\ NO_{2}(g)\ \rightleftharpoons \ N_{2}O_{4}(g)\ \ \Delta } H^{0}=-57{,}23~{\rm {kJ/mol}}}$

Reaction in the oxidation and absorption column

In the third step, the nitrogen oxides are converted into nitric acid (HNO ₃ ) in trickle towers with water :

${\displaystyle \mathrm {2\ N_{2}O_{4}(g)\ +\ O_{2}(g)\ +\ 2\ H_{2}O(l)\ \longrightarrow \ 4\ HNO_{3}(aq)} }$

Nitrogen monoxide (NO) and nitrous acid (HNO ₂ ) are formed as intermediate products , because the reaction takes place via the following intermediate steps:

${\displaystyle \mathrm {N_{2}O_{4}(g)\ +\ H_{2}O(l)\ \longrightarrow \ HNO_{3}(aq)\ +\ HNO_{2}(aq)} }$

${\displaystyle \mathrm {3\ HNO_{2}(aq)\ \longrightarrow \ HNO_{3}(aq)\ +\ 2\ NO(g)\uparrow +\ H_{2}O(l)} }$

${\displaystyle \mathrm {2\ NO(g)\ +\ O_{2}(g)\ \rightleftharpoons \ 2\ NO_{2}(g)} }$ (like step 2)

Products

The approx. 60 percent nitric acid obtained with the Ostwald process can only be concentrated to 68.5 percent, as this composition is then an azeotrope with a maximum boiling point of 122 ° C. A higher concentration can be achieved by countercurrent dehydration with drying agents such as phosphorus pentoxide or sulfuric acid .

literature

• Frédéric Kuhlmann: Treatise on the formation of saltpeter. New generation of nitric acid and ammonia . In: Annals of Pharmacy . tape 29 , no. 1 , 1839, ISSN  0365-5490 , p. 272-279 , doi : 10.1002 / jlac.18390290305 .
• Erwin Riedel : Inorganic Chemistry. 6th edition. de Gruyter, Berlin 2004, ISBN 3-11-018168-1 .

Individual evidence

1. ↑ Patent GB190200698 : Improvements in the Manufacture of Nitric Acid and Nitrogen Oxides. Registered January 9, 1902 , published March 20, 1902 , inventor: Wilhelm Ostwald. ‌
2. ↑ patent FR11331: Pour la fabrication de l'acide nitrique et des nitrates. Published on December 22nd, 1838 , inventor: Frédéric Kuhlmann. ‌
3. ^ J. Graham Smith: Frédéric Kuhlmann, pioneer of platinum as an industrial catalyst . In: Platinum Metals Review . tape 32 , 1988, pp. 84-90 ( PDF file ).
4. ↑ Prof. Dr. Nils Wiberg: Textbook of Inorganic Chemistry . 102nd edition. de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 730 .
5. ^ Charles E. Mortimer: Chemistry . 10th revised edition. Thieme, Stuttgart 2010, ISBN 978-3-13-484310-1 , p. 444 .