What is actually inside a catalytic converter

Three parts, one of which matters

Cut a converter open and there are three things in it. There is a stainless steel shell, which is worth its weight in stainless steel and no more. There is a substrate — a honeycomb, usually ceramic cordierite, sometimes a rolled metallic foil — which is worth almost nothing on its own. And there is the washcoat, a porous alumina layer sprayed onto that honeycomb, in which the platinum-group metals are dispersed as particles a few nanometres across.

The washcoat is a small fraction of the unit’s mass. It is essentially all of its value. This is why converters are traded by unit and by assay rather than by weight, and why a heavy unit is not automatically a good one.

What each metal is doing

A three-way catalyst runs two opposite reactions at once, which is the reason it needs more than one metal.

• Platinum (Pt) oxidises carbon monoxide and unburnt hydrocarbons into carbon dioxide and water. It tolerates sulphur better than palladium, which is why it stayed dominant in diesel applications long after petrol moved on.
• Palladium (Pd) does the same oxidation work and, for about twenty years, did it more cheaply. Most petrol converters built since the late 1990s are palladium-heavy for that reason alone.
• Rhodium (Rh) reduces nitrogen oxides back to nitrogen. Very little else will do this job in an exhaust stream, there is very little rhodium in the world, and those two facts together are why a few tenths of a gramme of it can be the largest single line in a settlement.

Why the badge tells you very little

Loading is set by the engine the converter was certified against — its displacement, its emissions standard, whether it runs petrol or diesel — and by what platinum and palladium cost in the year the part was specified. Two cars of the same model and year, one built for a market with a tighter standard, can carry materially different loadings.

The categories that actually predict content are the ones we sort to: ceramic or metallic, petrol or diesel, two-wheeler or four-wheeler, and whether the unit is a diesel oxidation catalyst or a full three-way. Even inside those, the spread is wide enough that any serious lot is settled on assay.

What this means for a supplier

If you are being quoted a flat price per kilo for mixed converters, you are being quoted a price that has to be low enough to cover the seller’s worst case. Sorting your material and settling it on assay moves that risk off your side of the table. Tell us the type and the count and we will tell you what to expect; the catalytic converter page sets out how a lot is actually settled.