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Material World
Object

Catalytic converter

A ceramic honeycomb coated with platinum, palladium and rhodium — the single largest use of the platinum group metals, and the reason city air stopped getting worse.

A catalytic converter is a block of ceramic honeycomb, coated with a rough oxide washcoat, on which a few grams of platinum-group metal are dispersed as particles a few nanometres across. Exhaust gas passes through it at speed and leaves with most of its carbon monoxide, unburnt hydrocarbons and nitrogen oxides converted to carbon dioxide, water and nitrogen.

The three-way converter does all three reactions at once, which is harder than it sounds: two of them are oxidations and the third is a reduction, and they only run together within a very narrow band of air-fuel ratio. Holding the engine inside that band is what the oxygen sensor and the engine management computer exist to do — the converter is the reason modern engines are electronically controlled at all.

Economic significance

Autocatalysts are the largest single use of platinum, palladium and rhodium, and the metals are used in a specific division of labour: palladium and rhodium in petrol exhaust, platinum in diesel, which is why the price of the two has repeatedly crossed over as emissions rules and engine mixes have shifted.

The metal is genuinely recovered. Recycling rates for autocatalysts are among the highest of any industrial product, because a few grams of rhodium is worth enough to pay for collection — and that same fact has made converter theft an organised crime in most wealthy countries.

Requiring converters also required unleaded petrol, since lead poisons the catalyst irreversibly. The public health benefit of removing lead from fuel is generally reckoned to exceed the benefit of the converters themselves, and it happened as a side effect of installing them.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

is made of

  • Platinum — element · in diesel exhaust, where it tolerates the conditions palladium does not
  • Palladium — element · in petrol exhaust, and the largest single use of the metal in the world
  • Rhodium — element · a fraction of a gram, doing the nitrogen oxide reduction that neither of the others does well — and worth enough on its own to make converter theft organised crime
  • Technical ceramic — material · the honeycomb monolith the metals are dispersed across, which supplies the surface area and survives the thermal cycling

is used as

  • Catalysis — application · the largest catalytic application by metal value there is

Sources

  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
  • Material World
    Our own writing

Questions this page answers

Where it comes from, and what it becomes

Follow Catalytic converter back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Catalytic converter → is made of (in diesel exhaust, where it tolerates the conditions palladium does not) → Platinum → is produced by (collected in the slimes, where the copper's own refining concentrates it) → Electrorefining → takes as input (as impure smelted anodes, around 99% copper, which is not pure enough for wire) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (roasted to drive off sulfur, then reduced to copper) → Chalcopyrite
  • Catalytic converter → is made of (a fraction of a gram, doing the nitrogen oxide reduction that neither of the others does well — and worth enough on its own to make converter theft organised crime) → Rhodium → is produced by (from the same slimes and the same platinum-group concentrate, and rarer in them than palladium by an order of magnitude) → Electrorefining → takes as input (as impure smelted anodes, around 99% copper, which is not pure enough for wire) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (roasted to drive off sulfur, then reduced to copper) → Chalcopyrite
  • Catalytic converter → is made of (the honeycomb monolith the metals are dispersed across, which supplies the surface area and survives the thermal cycling) → Technical ceramic → is composed of (alumina is the most-used engineering ceramic) → Aluminium oxide → is sourced from (dissolved out with hot caustic soda in the Bayer process) → Bauxite → is sourced from (the residue of prolonged tropical weathering, which strips the silica and alkalis out of an aluminosilicate rock and leaves the aluminium behind — basalt is one common parent among several) → Basalt → is composed of (an early-crystallising constituent of basaltic magma) → Olivine
  • Catalytic converter → is made of (in petrol exhaust, and the largest single use of the metal in the world) → Palladium → is sourced from (obtained largely as a by-product of nickel mining) → Nickel → is produced by (reduced from roasted pentlandite concentrate, with the platinum-group metals following into the refinery) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation
  • Catalytic converter → is made of (the honeycomb monolith the metals are dispersed across, which supplies the surface area and survives the thermal cycling) → Technical ceramic → is composed of (alumina is the most-used engineering ceramic) → Aluminium oxide → is produced by (crystallised from the liquor and calcined, ready for the smelter) → Bayer process → takes as input (the ore, dissolved selectively so the iron and silica are left behind) → Bauxite → is sourced from (the residue of prolonged tropical weathering, which strips the silica and alkalis out of an aluminosilicate rock and leaves the aluminium behind — basalt is one common parent among several) → Basalt
  • Catalytic converter → is made of (the honeycomb monolith the metals are dispersed across, which supplies the surface area and survives the thermal cycling) → Technical ceramic → is composed of (alumina is the most-used engineering ceramic) → Aluminium oxide → is produced by (crystallised from the liquor and calcined, ready for the smelter) → Bayer process → takes as input (hot concentrated caustic soda dissolves the aluminium hydroxides) → Sodium hydroxide → is sourced from (electrolysis of brine, which produces chlorine in fixed proportion) → Halite

These are the most distinct paths back. Catalytic converter can be traced through others besides.

Downstream — what it becomes