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. Ben Smye at materials search engine Matmatch, explains how.
The PGM family consists of the densest known metals and comprises six transitional metal elements that, structurally and chemically, are very similar. They are: platinum, palladium, rhodium, iridium, ruthenium and osmium. With high durability and longer lifecycles, PGMs are relied upon in a variety of demanding applications.
One of the most popular uses for PGMs is in catalytic converters, used to reduce nitrogen oxide and 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.
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 catalyst, which contains platinum and palladium. Cleaner exhaust then exits through the car’s exhaust pipe.
What are the most popular catalyst materials?
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.
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.
Reducing carbon emissions with precious metals
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, nitrogen and water vapour.
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.
What are the benefits of platinum?
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.
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 Celsius 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.
Finally, another advantage of platinum is that it can be efficiently recycled.