The largest single use of platinum group metals is in vehicle exhaust systems. The chemistry of that application determines how much metal is needed and which metal is used.
The converter accelerates reactions it does not join
A catalytic converter passes exhaust gas over a ceramic structure coated with a thin layer of platinum group metal dispersed as fine particles.
The metal provides a surface on which harmful gases react into less harmful ones, and it is not consumed by those reactions.
Because the metal only works at its surface, dispersing it as small particles across a large area is what determines how little of it is required.
Different fuels favour different metals
Diesel engines run with excess air, and platinum performs the required oxidation reliably in that environment.
Petrol engines operate near a chemically balanced mixture where palladium performs comparably and, for a long period, more cheaply.
Rhodium handles the reduction of nitrogen oxides, a reaction the other two do less well, which is why it is used in small quantities that are difficult to replace.
Substitution is possible but slow
The metals are chemically similar enough that formulations can be shifted between them when relative prices move far apart.
Reformulating requires testing and regulatory validation, because a converter must meet emissions standards over the vehicle's life rather than only when new.
The lag between a price signal and a change in metal consumption is therefore measured in years, which allows large price gaps to persist.
Tightening standards raise loadings
Each successive tightening of emissions limits has generally required more catalyst activity, which means more metal per vehicle or more sophisticated deployment of it.
Manufacturers work continuously to reduce loadings, since the metal is among the more expensive components in the exhaust system.
Demand per vehicle is the outcome of those two opposing pressures rather than a fixed quantity.
Electrification changes the demand profile
Battery electric vehicles have no exhaust and therefore no autocatalyst, so their growth reduces the metal required per vehicle sold.
Hybrids retain a converter and can require careful catalyst design, because the engine starts and stops and the catalyst must work when cold.
Hydrogen fuel cells use platinum in their electrodes, which is why platinum's long term demand picture differs from palladium's despite their shared history in exhaust systems.