Catalytic Converters

Introduction

Catalytic converters are an integral part of converting much of the pollution that comes out of a car's exhaust. Almost every car since the year 1970 has come with a catalytic converter in order to reduce harmful emissions. I chose to explain the function and chemistry behind the catalytic converter because I have an interest in cars and I wanted to know more about them.

Background

In order to understand how catalytic converters work, one must understand that air and fuel burn in a car's engine best at a specific ratio. The ratio for gasoline burning vehicles is approximately 14.7 pounds of air to one pound of gasoline. When there is more than 14.7 pounds of air for each pound of gas, the mixture is running lean. When the opposite occurs, meaning less that 14.7 pounds of air for each pound of gas, the mixture is rich.

The exhaust from the combustion in a cars engine is comprised of six main ingredients. Nitrogen gas, carbon dioxide, and water vapor are three of the main emissions. These gases do not cause damage to the atmosphere like the other gases do. Carbon Monoxide, other hydrocarbons, and Nitrogen Oxides result in a majority of the pollution caused by cars. Carbon Monoxide is a colorless and odorless gas that can kill you if too much is inhaled. Hydrocarbons come out of an engine's exhaust from unburned fuel. These hydrocarbons can be broken down by the sun, creating ground level ozone, also known as smog. Nitrogen oxides can cause acid rain. Catalytic converters are designed to reduce these last three emissions.

How it Works

Most new cars are equipped with what is called a three-way catalytic converter. They are called this because they try to cut down on the Carbon Monoxide, hydrocarbons, and Nitrogen Oxides. There are two main parts to the catalytic converter. They both function in the same way but remove different pollutants. The structure inside the catalytic converter is a honeycomb to allow for a maximum surface area for the reactions to occur while requiring minimal amounts of the catalysts required for the reactions to take place because these catalysts are expensive.

Reduction Catalyst

The reduction catalyst is the first step that the exhaust goes through in the catalytic converter. The catalysts used in this part are platinum and rhodium to break now NO and NO2 molecules in to N2 and O2 gases which are non-polluting. This process occurs using either of the following two chemical equations:

For NO:

2NO => N2