Ostwald process
/ OST-vahlt /
Nitric acid is the chemical behind both the fertilizers that feed the world and the explosives that have shaped its wars. To make it on an industrial scale, you start from ammonia — itself made by the Haber-Bosch process — and burn it in a very controlled way. The Ostwald process is the industrial route that turns ammonia into nitric acid, and its key step uses a heterogeneous catalyst made of platinum alloyed with a little rhodium, in the form of fine woven gauze.
The process runs in three stages. First, the catalytic step: ammonia and air are blown through a stack of hot platinum-rhodium gauzes at around 900 degrees Celsius, where ammonia is oxidized to nitric oxide and water: 4 NH3 plus 5 O2 gives 4 NO plus 6 H2O. The catalyst here is doing something subtle and beautiful — without it, ammonia burned in air would simply go all the way to harmless nitrogen gas (N2) and water; the platinum gauze selectively steers the oxidation to stop at NO, the useful intermediate, which is a striking example of catalytic selectivity choosing one product over another more stable one. The contact time on the gauze is mere fractions of a millisecond, which is part of how the selectivity is achieved. Second, the nitric oxide is cooled and reacts with more oxygen to form nitrogen dioxide: 2 NO plus O2 gives 2 NO2. Third, the nitrogen dioxide is absorbed in water, where it disproportionates to give nitric acid and regenerate some nitric oxide: 3 NO2 plus H2O gives 2 HNO3 plus NO, and the recovered NO is recycled.
The Ostwald process matters because, together with Haber-Bosch, it completes humanity's mastery of the nitrogen cycle for industry: atmospheric nitrogen becomes ammonia, ammonia becomes nitric acid, and from there come ammonium nitrate fertilizers and a vast range of nitrogen chemicals. It is a prime example of a catalyst chosen for selectivity rather than mere speed — the whole trick is making the ammonia stop at NO instead of combusting to N2. The platinum-rhodium gauze is precious, and although the catalyst is regenerated chemically, platinum is slowly physically lost from the gauze in the hot gas stream (it volatilizes and reorganizes), so part of the engineering is catching and recovering the eye-wateringly valuable metal. An honest note: this is a high-temperature combustion harnessed and tamed, not a gentle reaction, and the nitrogen oxides involved are exactly the pollutants that other catalysts (in cars) are designed to destroy.
Blow ammonia and air through glowing platinum-rhodium gauze for a fraction of a millisecond and the ammonia is oxidized cleanly to nitric oxide, not all the way to harmless nitrogen — the catalyst chooses the useful but less stable product on the way to nitric acid.
Selectivity over speed: the catalyst stops the oxidation at the useful NO instead of the more stable N2.
Without the platinum catalyst, burning ammonia in air just gives nitrogen gas and water — useless here. The catalyst's job is selectivity, steering the oxidation to stop at nitric oxide. And though chemically regenerated, the platinum gauze slowly loses metal physically and must be recovered.