Platinum's industrial value rests on catalysis. A catalyst provides a surface on which reacting molecules are held in the right orientation and their bonds weakened, so that a reaction proceeds at a temperature at which it otherwise would not, while the catalyst itself is left unchanged. Platinum is extraordinarily good at this for reactions involving hydrogen and oxygen.
The largest single application is the vehicle catalytic converter, introduced in the United States from 1975, in which platinum together with palladium and rhodium converts carbon monoxide, unburned hydrocarbons and nitrogen oxides into carbon dioxide, water and nitrogen. Roughly half of platinum demand is automotive. The Ostwald process, which oxidises ammonia to nitric acid over a platinum-rhodium gauze, supplies the world's nitrates for fertiliser and explosives. Hydrogen fuel cells use platinum electrodes, and the cost and scarcity of the metal is one of the central obstacles to their wider adoption.
Platinum is chemically almost inert, unaffected by acids other than aqua regia, and its melting point is high, which made it the material of choice for laboratory crucibles and for physical standards. From 1889 until 2019 the kilogram was defined as the mass of a particular cylinder of platinum-iridium alloy kept at Sevres near Paris, and the metre was likewise defined by a platinum-iridium bar until 1960. The kilogram prototype was the last physical artefact to define an SI unit, and it was retired because it was observed to drift relative to its own official copies by a few tens of micrograms over a century.
Platinum's scarcity is real: about five parts per billion in the crust, and roughly three quarters of world supply comes from a single geological formation, the Bushveld Complex in South Africa.