Materials & Applications

Platinum Group Metals in Catalytic Converters

Platinum Group Metals in Catalytic Converters

In this article we’ll cover:

  • What are the platinum group metals?
  • Platinum group metals as catalytic converters.
  • Platinum group metals for automotive emission control.
  • How PMGs reduce carbon emissions.
  • Advantage of platinum over other catalytic materials.
  • Cleaner roads, brighter future with PMGs.

There is an increasing number of vehicles on our roads yet, at the same time, international emissions regulations are tightening. It poses a conundrum to manufacturers and consumers alike, but one answer lies in achieving less pollution-per-vehicle.

This may seem impossible but, fortunately, exotic platinum group metals (PGMs) can play a key role. Here, our head of growth Ben Smye explains how. 

What are the platinum group metals?

The platinum group metals family consists of the densest known metals and comprises six transitional metal elements that, structurally and chemically, are very similar. They are platinumpalladiumrhodiumiridiumruthenium and osmium. With high durability and longer lifecycles, PGMs are relied upon in a variety of demanding applications.  

Platinum group metals as catalytic converters

One of the most popular uses for PGMs is in catalytic converters, used to eliminate nitrogen oxide, carbon monoxide emissions or hydrocarbon emissions from unburnt fuel. Catalytic converters contain metals and chemicals, specifically the catalysts that remove or neutralise harmful pollutants from the car exhaust

catalytic converter
Inside the converter, the gases flow through a dense honeycomb structure made from a ceramic and coated with the catalysts. The honeycomb structure means the gases touch a bigger area of catalyst at once, so they are converted more quickly and efficiently.

Dirty exhaust flows through two chambers. The first chamber, the reduction catalyst, contains platinum plus rhodium to eliminate the harmful nitrogen oxide (NO). This leaves only harmless nitrogen and oxygen. The exhaust flows for the second chamber, the oxidation catalystwhich contains platinum and palladium. Cleaner exhaust then exits through the car’s exhaust pipe. 

Platinum group metals for automotive emission control

PGMs are good automotive emission control catalysts because they are useful in catalysing NO to nitrogen and to oxidise carbon monoxide (CO) and hydrocarbons (HC)

Platinum is one of the more popular catalysts due to its capabilities as a good oxidisation catalyst. Palladium and rhodium are also widely used in converters alongside metals like cerium, copper, iron, manganese and nickel — and it is of note that not all of these are precious metals.  

catalytic converter
There are between 3-7 grams of platinum group metals in a standard catalytic converter, but the amount varies based on manufacturer and model.

In terms of the extent to which they are used, there are generally around 3 to 7 grams of PGMs in a standard catalytic converter. However, the amount varies depending on the size of the unit. Small catalytic converters may contain only half, or less, of the platinum found in larger industrial converters. Larger units are becoming more required for construction and agricultural equipment in many markets.  

The growth of the PGM market will be driven primarily by automotive production and consumption, particularly in Europe and Asia-Pacific (APAC) regions.  

How PMGs reduce carbon emissions 

Platinum’s popularity is not only down to its properties, but also the critical role it can play in helping to reduce vehicle carbon emissions.  

The International Platinum Group Metals Association (IPA) estimates that catalytic converters fitted inside a car exhaust pipe can convert more than 90 per cent of HC, CO and NO from an engine into less harmful carbon dioxide (CO2), nitrogen and water vapour.  

Catalytic converter or “cat” converts the harmful emissions from the engine’s combustion chamber into water vapour and carbon dioxide. Oxidation converts carbon monoxide into carbon dioxide and hydrocarbons into carbon dioxides and water. Although these gases are still harmful to the atmosphere, they are less detrimental than gases produced by the engine.

For petrol-powered vehicles, where there is a balance between reductants and oxidants in the exhaust gas, platinum and palladium can be equally effective and the choice between these materials is normally determined by cost. 

Rhodium is generally used in addition to these two elements as part of a three-way catalyst that’s able to reduce NO to nitrogen, as well as oxidise CO and HC.  

Advantage of platinum over other catalytic materials

While other transition elements can catalyse oxidation reactions, platinum has several advantages over these

Firstly, its high melting point. Although this might not seem relevant because platinum will never come close to its melting temperature in the catalytic converter, this property provides an indication of the material’s overall thermal durability. 

The melting point of a material is always crucial when choosing whether it should be used in high-temperature applications. This temperature is often taken to be half the material’s melting point. 

This gives platinum an advantage over other catalytic materials like silver and gold. This is evident in the latter material’s Tammann temperature. The Tammann temperature of a material is half its melting point. When a metal approaches this temperature, it’s no longer solid but not-yet liquid and its particles can coalesce with other materials in a process called sintering.  

Cross section of catalytic converter
Catalytic converters for the auto industry use 33% of the world’s platinum while jewellery accounts for 29%. The remaining 8% is made up of platinum coins, bars and ingots used for investment purposes.

Both silver and gold have a Tammann temperature that’s well below the average temperature produced by exhaust gas. For example, this would typically be 600 to 700 degrees centigrade for a petrol car being driven on a motorway.  

To reliably claim this, however, it’s vital that engineers ensure they have as much information about a PGM’s thermal properties as possible. Historically, this has proven challenging, leading to many automotive manufacturers working with specific suppliers. With the digitisation of materials data, supported by materials databases like Matmatch, this is changing and it’s possible to easily access verified, trustworthy material data. 

Further to this, metals like silver and copper tend to react with poisonous sulphur-containing molecules to form compounds, such as metal sulphates or sulphides. Such interactions leave less metal available for useful catalytic reactions to take place. In contact, platinum tends not to become totally or irreversibly poisoned.  

Pieces of broken catalytic converter
Pieces of a broken catalytic converter.

Finally, another advantage of platinum is that it can be efficiently recycled

Cleaner roads, brighter future with PMGs

Environmental concerns will majorly drive demand for PGMs. As a vital component of automotive catalytic converters, they will prove critical in reducing vehicle carbon emissions — especially with more stringent government rules in this regard. As emissions regulations become for stringent, this could be the time for platinum, as well as other PGMs, to shine as demand grows. 

Design engineers can use Matmatch’s free online database to explore the unique chemical and physical properties of PGMs and match the right materials and applications. Our database features all the PGMs with high purity, high melting points and excellent resistance to corrosion — plus, of course, their exceptional catalytic properties. New suppliers and materials are added regularly. 

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