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A cutaway catalytic converter that you can pass around so that students can see the honeycomb structure that increases surface area. Great for heterogeneous catalysis and redox. 

Curriculum Notes 

This demo is usually used during a unit on catalysis, but it could be used during a thermodynamics unit where you could investigate why the reactions only occur spontaneously at high temperatures. It's also a great consumer chemistry demo. Allow about 10 minutes for this demo.

One day of lead time is required for this project.

Discussion 

Most modern cars are equipped with three-way catalytic converters. "Three-way" refers to the three regulated emissions it helps to reduce -- carbon monoxide, VOCs (volatile organic compounds), and NOx (the various nitrogen oxides) molecules. The converter uses two different types of catalysts, a reduction catalyst and an oxidization catalyst. Both types consist of a ceramic structure coated with a metal catalyst, usually platinum, rhodium and/or palladium. The idea is to create a structure that exposes the maximum surface area of catalyst to the exhaust stream, while also minimizing the amount of catalyst required (they are very expensive). 

The reduction catalyst is the first stage of the catalytic converter. It uses platinum and rhodium to help reduce the NO_x emissions. When an NO or NO₂molecule contacts the catalyst, the catalyst rips the nitrogen atom out of the molecule and holds on to it, freeing the oxygen in the form of O₂. The nitrogen atoms bond with other nitrogen atoms that are also stuck to the catalyst, forming N₂. For example: 

$\ce{2NO -> N2 + O2}$ or $\ce{2NO2 -> N2 + 2O2}$

The oxidation catalyst is the second stage of the catalytic converter. It decreases the unburned hydrocarbons and carbon monoxide emissions by burning (oxidizing) them over a platinum and palladium catalyst. This catalyst aids the reaction of the CO and hydrocarbons with the remaining oxygen in the exhaust gas. For example: 

$\ce{2CO + O2 -> 2CO2}$

All of these reactions only occur at high temperatures after the engine has warmed up.¹

¹HowStuffWorks.com

Materials 

cutaway catalytic converter

Procedure 

Display the catalytic converter and explain how it works. You might want to point out how the honeycomb structure increases surface area.

Safety Precautions 

Don't drop it on your toe!