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Platinum crystals

Platinum crystals (Jurii, Wikimedia Commons)

STEM in Context

Catalytic Converters

Lars Rose, Sherry Boodram & Let's Talk Science

Summary

Catalytic converters use noble metals as catalysts to speed up chemical reactions. This makes vehicle emissions less harmful.

Do you have any friends who are studying for their driving test? Or maybe you’re the one mastering the rules of the road. But how much do you really know about your car? For example, did you that noble metals help clean the engine’s exhaust?

Misconception Alert

Noble metals and precious metals aren’t the same thing. Precious metals have a high monetary value. Noble metals are highly resistant to corrosion and oxidation. However, some precious metals are also noble metals.

What comes out of a car’s exhaust pipe?

Car exhaust is also called vehicle emissions. It contains many substances. Some are more harmful than others. 

Your car’s engine probably uses gasoline as fuel. Gasoline is a hydrocarbon. Your car mixes this fuel with air before burning it. This process is called combustion, and it produces many chemical by-products.

Some of these by-products are perfectly safe. For example, air consists of 78% nitrogen gas (N2). Some of this nitrogen reacts with oxygen during combustion. However, most of it gets released as N2 in the engine’s exhaust. Engine exhaust also includes water (H2O). You’ll often see water dripping from exhaust pipes during the winter. 

Car engines also emit a lot of harmful substances. Some of these can cause acid precipitation. This is the case with carbon dioxide (CO2), nitrogen oxides (NOx) and sulphur oxides

Other vehicle emissions can cause health problems like cardiovascular disease and cancer. This is the case with unburnt hydrocarbons, particulates (carbon particles), and volatile organic compounds (VOCs)

Car engines also release carbon monoxide (CO). This poisonous gas can replace oxygen in your bloodstream. If you breathe enough of it, it can even suffocate you!

That all sounds very dangerous, doesn’t it? Fortunately, catalytic converters help make engine emissions less harmful. Here’s how.

What is a catalytic converter?

The catalytic converter was invented around 1950 by Eugène Houdry. He was a French mechanical engineer. He designed the catalytic converter to clean up automobile exhaust. 

Widespread use of catalytic converters began around 1975. At that time, governments started trying to reduce air pollution from cars. But back then, a lot of vehicles used leaded gasoline. Lead (Pb) can keep a catalytic converter from functioning properly. That’s because lead can coat the surface that normally reacts with the exhaust gases.  

Did you know?

Imagine you used the same amount of fuel in an SUV with a catalytic converter and a lawnmower without one. The lawnmower would emit around 100 times more pollution!

How do catalytic converters work?

On a car, the catalytic converter is attached to the exhaust pipe. A metal casing contains a ceramic honeycomb. The honeycomb is coated with a mix of platinum (Pt), palladium (Pd) and rhodium (Rh). These noble metals are good at resisting oxidation, corrosion and acid. That means they can stand up to bad weather and all the chemicals released by a car engine. 

The noble metals in catalytic converters act as catalysts. Catalysts are compounds that can trigger a chemical reaction without being affected themselves. The honeycomb structure inside a catalytic converter maximizes the surface area where reactions can take place. 

Platinum, palladium and rhodium catalysts
Catalytic converters use elements like Platinum (Pt), Palladium (Pd), and Rhodium (Rh) as catalysts (Let’s Talk Science using photographs by Periodictableru [CC BY], Hi-Res Images of Chemical Elements [CC BY and Alchemist-hp (talk) www.pse-mendelejew. derivative work: Purpy Pupple [CC BY-SA 3.0] Wikimedia Commons (Pt, Pd, Rh)).

Did you know? 

Today, about 98% of all new vehicles sold worldwide contain a catalytic converter.

What chemical reactions happen in a catalytic converter?

Catalytic converters use reduction and oxidation (redox) reactions to reduce harmful emissions.

They use a reduction catalyst composed of platinum and rhodium. It helps reduce nitrogen oxides (NOx) by removing nitrogen atoms from nitrogen oxide molecules (NO and NO2). This lets the free oxygen form oxygen gas (O2). Then, the nitrogen atoms attached to the catalyst react with each other. This reaction creates nitrogen gas (N2).

Reduction reactions for nitric acid and nitrogen dioxide
Reduction reactions for nitric acid and nitrogen dioxide (© 2019 Let’s Talk Science).

 

Image - Text Version

Nitric acid and nitrogen dioxide are reduced to form nitrogen gas and oxygen gas.

 

Catalytic converters also use an oxidative catalyst composed of platinum or palladium. It helps reduce hydrocarbons (HC) and carbon monoxide (CO). To start with, carbon monoxide and oxygen combine to form carbon dioxide (CO2). Then, unburnt hydrocarbons and oxygen combine to form carbon dioxide and water.

Oxidation reactions for carbon monoxide and unburned hydrocarbons
Oxidation reactions for carbon monoxide and unburned hydrocarbons (© 2019 Let’s Talk Science).

 

Image - Text Version

Carbon monoxide and oxygen combine to form carbon dioxide. Unburnt hydrocarbons and oxygen combine to form carbon dioxide and water.

 

Modern catalytic converters also use oxygen sensors. They’re sometimes called lambda sensors. They control how much extra oxygen gets pumped into the exhaust stream. Maintaining the correct amount of oxygen makes the reduction and oxidation reactions more efficient.

Did you know?

A car engine produces the most pollution right after you turn it on. That’s because catalytic converters can take a few minutes to kick in. This is a great reason to walk if you only need to travel a short distance!

Researchers are looking at whether gold could be used in catalytic converters. That might sound expensive. But gold is actually cheaper than many other noble metals. And there’s more of it! In fact, we could run out of metals like platinum in the next couple of decades. In some places, people even steal catalytic converters just to get at the precious noble metals inside!

Starting Points

Connecting and Relating
  • Would you purchase a car if you knew it didn’t have a catalytic convertor? Why or why not?
  • Do you try to limit the amount of pollution you produce? Explain.
  • Have you experienced smog or heavy air pollution? Describe the effects.
  •  
Connecting and Relating
  • Would you purchase a car if you knew it didn’t have a catalytic convertor? Why or why not?
  • Do you try to limit the amount of pollution you produce? Explain.
  • Have you experienced smog or heavy air pollution? Describe the effects.
  •  
Relating Science and Technology to Society and the Environment
  • What are some examples of noble metals used as catalysts in catalytic converters? Should governments invest more into the production of these metals? Why or why not?
  • Should we continue to study the use of gold as a catalyst in catalytic convertors. Or should we use more environmentally friendly cars? Explain.
  •  
Relating Science and Technology to Society and the Environment
  • What are some examples of noble metals used as catalysts in catalytic converters? Should governments invest more into the production of these metals? Why or why not?
  • Should we continue to study the use of gold as a catalyst in catalytic convertors. Or should we use more environmentally friendly cars? Explain.
  •  
Exploring Concepts
  • What is a catalyst? What catalysts are commonly used in catalytic converters?
  • What role does the catalyst play in reducing harmful emissions from car exhaust? What chemical reactions occur during this process?
  • Catalytic converters still allow carbon dioxide to be produced during combustion. Carbon dioxide is a major contributor to climate change. Is the production of nitrogen oxides and sulphur oxides worse than the emission of carbon dioxide for the environment? Explain.
  • What is the function of the oxygen sensor in a vehicle’s exhaust system?
  •  
Exploring Concepts
  • What is a catalyst? What catalysts are commonly used in catalytic converters?
  • What role does the catalyst play in reducing harmful emissions from car exhaust? What chemical reactions occur during this process?
  • Catalytic converters still allow carbon dioxide to be produced during combustion. Carbon dioxide is a major contributor to climate change. Is the production of nitrogen oxides and sulphur oxides worse than the emission of carbon dioxide for the environment? Explain.
  • What is the function of the oxygen sensor in a vehicle’s exhaust system?
  •  
Teaching Suggestions:
  • This article can be used to support teaching and learning of Chemistry, Environmental Science, Pollution and Technology & Engineering related to air quality, catalysts, combustion, redox reactions, metals and hydrocarbons. Concepts introduced include gasoline, hydrocarbon, combusting, carbon dioxide, nitrogen oxides (NOx), sulfur oxides, acid precipitation, volatile organic compounds (VOCs), carbon monoxide, catalytic converter, catalysts, compounds, oxidation, reduction, reduction catalyst, Nitric acid, Nitrogen dioxide, oxidative catalyst and oxygen sensor.
  • After reading the article, teachers could have students complete a Key Ideas Round Robin learning strategy to summarize the key ideas from the article. Ready-to-use Key Ideas Round Robin reproducibles are available in [Google doc] and [PDF] formats.
  • For teaching about catalysts in Chemistry, teachers could use this article as an example of an application of catalysts that are used in everyday life.
  • To go further, teachers could have students debate the use of gold as a catalyst in catalytic converters using a Pros & Cons Organizer learning strategy. Ready-to-use Pros & Cons reproducibles in [Google doc] and [PDF] formats.
  • To have students investigate the issues surrounding transportation and fossil fuels and learn about the carbon footprint associated with different modes of transportation in society, teachers could have students participate in the Energy4Travel Action Project. 
Teaching Suggestions:
  • This article can be used to support teaching and learning of Chemistry, Environmental Science, Pollution and Technology & Engineering related to air quality, catalysts, combustion, redox reactions, metals and hydrocarbons. Concepts introduced include gasoline, hydrocarbon, combusting, carbon dioxide, nitrogen oxides (NOx), sulfur oxides, acid precipitation, volatile organic compounds (VOCs), carbon monoxide, catalytic converter, catalysts, compounds, oxidation, reduction, reduction catalyst, Nitric acid, Nitrogen dioxide, oxidative catalyst and oxygen sensor.
  • After reading the article, teachers could have students complete a Key Ideas Round Robin learning strategy to summarize the key ideas from the article. Ready-to-use Key Ideas Round Robin reproducibles are available in [Google doc] and [PDF] formats.
  • For teaching about catalysts in Chemistry, teachers could use this article as an example of an application of catalysts that are used in everyday life.
  • To go further, teachers could have students debate the use of gold as a catalyst in catalytic converters using a Pros & Cons Organizer learning strategy. Ready-to-use Pros & Cons reproducibles in [Google doc] and [PDF] formats.
  • To have students investigate the issues surrounding transportation and fossil fuels and learn about the carbon footprint associated with different modes of transportation in society, teachers could have students participate in the Energy4Travel Action Project. 

Learn more

Catalytic Converter: How It Works (2018) 

Video (5:23 min.) by Donut Media explaining how catalytic converters work.

Shortages of rare earth elements could limit clean energy development (2018) 

Accessible article from Laura Mast of Massive Science discussing the shortages of rare earth elements (which include noble metals). 

What is the world’s scarcest material? (2014) 

An overview from Rachel Nuwer of the BBC on limited rare earth elements, our current uses for them, and what might happen when we run out of them.

The Periodic Table of Elements: Precious Metals 

Infographic from AccuPlan  showing the location of each precious metal on the periodic table of elements, and also providing some information about the properties and uses or each.

References

Helmenstine, A. M. (2018, September 19). Noble metals list and properties. ThougthCo.

International Platinum Group Metals Association. (n.d.). Catalytic converters.

Nice, K., & Bryant, C. W. (2000, July 11). How catalytic converters work. How Stuff Works.