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Chemical synthesisHigh temperatureIndustrial scaleCO₂-emittingNobel Prize

Ostwald process

Catalytic oxidation of ammonia (from Haber-Bosch) on a platinum-rhodium gauze to produce nitric acid. Coupled with Haber-Bosch, it's the backbone of the fertilizer and explosives industry.

Molecular synthesis through controlled chemical reactions

Key reaction

4 NH₃ + 5 O₂ → 4 NO + 6 H₂O ; 2 NO + O₂ → 2 NO₂ ; 3 NO₂ + H₂O → 2 HNO₃ + NO

Operating conditions

Temperature
850-950 (gauze) ; 30-50 (absorption)°C
Pressure
4-12bar
Catalyst
Pt-Rh (90/10 ou 95/5) gauze tissée
Phase
gas + liquid

How it works

Ostwald process diagram: catalytic oxidation of NH₃ over Pt-Rh gauze at 850-950 °C, cooling, NO₂ absorption in a water tower to produce HNO₃.
>95 % selectivity over the Pt-Rh gauze depends on a sub-millisecond contact time — beyond that, NH₃ reverts to N₂.

How it works

The Ostwald process, patented in 1902 by Wilhelm Ostwald (Nobel 1909), oxidizes ammonia to nitric acid in three sequential steps, all run in a single integrated unit. Its power comes from the elegance of the catalyst: a simple woven 'gauze' of platinum-rhodium alloy, exposed to an NH₃ + air gas flow, selectively oxidizes NH₃ to NO without burning it to N₂. (1) Catalytic combustion: NH₃ and air are blended (10 vol % NH₃) and pass over the Pt-Rh gauze at 850-950 °C, under 4-12 bar: 4 NH₃ + 5 O₂ → 4 NO + 6 H₂O. Selectivity exceeds 95 % for the target NO against 5 % toward parasitic N₂ + N₂O. The gauze-gas contact lasts < 1 millisecond — beyond that, kinetics flip back toward N₂. (2) Oxidation and absorption: outgoing gases at 900 °C are cooled in a waste-heat boiler (producing HP steam), then enter a stainless steel absorption tower. On cooling, NO + ½ O₂ → NO₂ (slow at 30-50 °C), then 3 NO₂ + H₂O → 2 HNO₃ + NO. NO formed as a byproduct returns to the top of the tower to be re-oxidized. The tower is 30-50 m tall, 3-4 m in diameter, with 30-50 trays or structured packing, run under pressure (4-12 bar) to favor solubility. (3) Concentration: outgoing acid is diluted to ~60-65 wt %. For common uses (fertilizer), that's enough. For technical uses (explosives, organic chemistry), it must be concentrated to 98-99 % by extractive distillation with H₂SO₄ or Mg(NO₃)₂. The process is globally exothermic (ΔH = −906 kJ/mol for the full chain) and each tonne of acid produces ~6 GJ of recoverable steam. World production ~75 Mt/year (2022), ~80 % dedicated to fertilizers (ammonium nitrate, urea-nitrate). Parasitic N₂O (5-10 g per tonne acid), a greenhouse gas 265× more potent than CO₂, has become the main environmental issue — hence the systematic addition of N₂O abatement catalysts downstream.

Key components

The role of each main part, and the elements / compounds it involves.

  • Pt-Rh gauze

    Selective oxidation catalyst NH₃ → NO. The heart of the process.

    Fine metal weave (wire Ø ~75 µm) of Pt-Rh alloy (90/10 or 95/5 depending on operating pressure), stacked as 5-30 overlaid grids. Specific surface ~1 m²/g. Selectivity > 95 % for NO. Pt losses by evaporation: ~0.1-0.3 g Pt per tonne acid produced — captured downstream on Pd gauzes that recover ~80 % of volatile Pt.

    Pt-Rh 90/10 ou 95/5 · 5-30 grilles · pertes Pt 0,1-0,3 g/t · récupération Pd

    See also :nh3

  • NO waste-heat boiler

    Cools NO gases from 900 °C to 200 °C while producing HP steam.

    Tubular heat exchanger in austenitic stainless steel 304L or 316L, designed to resist moist NOₓ corrosion. Steam produced ~40 bar / 380 °C. Gradual cooling to avoid premature water condensation (which would dissolve NO into unstable HNO₃ and cause local corrosion).

    Inox 304L/316L · vapeur 40 bar / 380 °C · refroidissement contrôlé

  • NO₂ absorption tower

    Converts cooled NOₓ to HNO₃ by counter-current contact with water.

    Cylindrical tower 30-50 m tall, 3-4 m in diameter, stainless steel, pressurized at 4-12 bar. Valve trays or structured packing (Sulzer Mellapak™) to maximize NOₓ/H₂O transfer. Water in at the top, NOₓ gas at the bottom, concentrated acid out at the bottom (60-65 wt %). External coil cooling to stay < 50 °C (NO₂ solubility falls exponentially with T).

    30-50 m · 4-12 bar · acide sortant 60-65 % · cooling externe

    See also :hno3

  • N₂O abatement catalyst

    Decomposes parasitic N₂O (strong greenhouse gas) before atmospheric release.

    Secondary catalytic bed (Fe-zeolite or mixed Co-Ce-Al oxide) downstream of the gauze, at 800-900 °C. Decomposes 2 N₂O → 2 N₂ + O₂ at 90-99 % efficiency. Made mandatory in the EU and under Kyoto (each tonne N₂O avoided counts as 265 t CO₂-eq carbon credit). Example: Yara Porsgrunn eliminated > 30 kt N₂O/year as early as 2009.

    Fe-zéolite ou Co-Ce-Al · 800-900 °C · efficacité 90-99 %

Physical and chemical principles

The fundamental laws that make this process possible — and the constraints they impose.

  • Short-contact selectivity (millisecond chemistry)

    On the Pt-Rh gauze, two competing reactions consume NH₃: oxidation to NO (desired) and combustion to N₂ (parasitic, irreversible). At 900 °C, kinetics toward NO are faster during the first millisecond of contact, then NO starts reducing back to N₂. The thin gauze (low mass, low volume) ensures residence time < 1 ms — the reason for choosing a woven grid over a granular fixed bed.

    Applies to components :gauze-pt-rh

  • Gas/liquid oxidation cascade

    Three equilibria stack up in the absorption tower: NO + ½ O₂ ⇌ NO₂ (gas, slow), 2 NO₂ ⇌ N₂O₄ (gas, fast), 3 NO₂ + H₂O → 2 HNO₃ + NO (gas/liquid interface). Regenerated NO rises and re-loops. High pressure (4-12 bar) accelerates all three steps. The engineer's art lies in dimensioning heights and temperatures to minimize NOₓ losses.

    Applies to components :tour-absorption-no2

Compounds involved

Input

Output

World production

75 Mt/yr
2022

Main applications

  • Fertilizers (ammonium nitrate NH₄NO₃, urea-nitrate)80 %
  • Explosives (TNT, nitroglycerin, ANFO)8 %
  • Organic chemistry (aromatic nitration)6 %
  • Metallurgy (passivation, etching), miscellaneous6 %

Fugitive N₂O and the cost of Pt

Before systematic abatement (post-2005), an HNO₃ plant emitted 6-15 g of N₂O per tonne of acid — about 5 % of global anthropogenic N₂O emissions. With secondary abatement, it has fallen below 0.5 g/t — one of the fastest industrial climate wins of the 21st century. The Pt issue remains: ~150 t of Pt are tied up in gauzes worldwide, with ~30 t/year evaporative losses of which only 24 are recovered on Pd grids. The process remains structurally dependent on platinum prices.
  • Catalyseurs d'abattement N₂O secondaire (Fe-zéolite) — −95 % d'émissions
  • Grilles de récupération Pd en aval — +5-10 % de Pt récupéré
  • Optimisation pression / sélectivité (procédés mono- et duo-pression)
  • Couplage HNO₃ vert avec NH₃ vert (Haber-Bosch électrolytique)

Similar or competing processes

Related industrial processes — alternative chemistry, alternative technology.

  • haber-bosch

    Mandatory upstream source — Ostwald NH₃ comes 100 % from Haber-Bosch.

History and discovery

Discovery year1902
First industrial deployment1908
Wilhelm Ostwald· Allemagne
Nobel Prize: 1909
Sources
  • Ullmann's Encyclopedia of Industrial Chemistry — Nitric Acid
  • EFMA — Best Available Techniques for Nitric Acid Production
  • IFA Nitric Acid Statistics
  • Thiemann, M. — Nitric Acid, Nitrous Acid and Nitrogen Oxides (Wiley)
Processes